TM TTBa loi phng php c th c s dng m t s bit ha kim loi, to phc v phn on trong cc loi t: (i) l thuyt a ha hc, (ii) mt s hiu bit tt v cc tnh cht v iu kin c nh hng n bin i kim loi t, v (iii) phn tch trong phng th nghim, bao gm c vic s dng t nh tun t. Phng php tip cn a ha tnh ton s bit ha kim loi trong dung dch t lin quan n hai phng php nhit ng lc khc nhau, s dng cc hng s cn bng, hoc nng lng Gibbs min ph. Mt s v d v trng thi cn bng cc tnh ton lin tc c minh ha, ch ra cch k thut nh l c s ca m hnh ha ca cc ion kim loi bit ha, s dng cc m hnh nh GEOCHEM v SOILCHEM. Qu trnh chnh lin quan n vic lu gi v di ng ca kim loi trong t l (I) phong, (2) gii th v kh nng ha tan, (3) lng ma,(4) s hp thu ca cc nh my, (5) khng c ng ca sinh vt t, (6) trao i trn cc trang web trao i cation ca t, (7) hp ph v chemisorption c th, (8) thi, v (9) ra tri. Cc yu t chnh nh hng n tan ca kim loi trong t l pH, cht hu c ha tan v oxi ha kh. i vi hu ht cc kim loi vi lng, ngoi Cd v Zn, trao i cation khng c th l mt c ch duy tr t quan trng hn l hp ph c th ca Fe v Mn oxit v cc cht hu c trong t. t qu trnh lu gi bng kim loi thng quan trng hn nhiu so vi quy trnh chit xut kim loi. Topsoils chu nc thi u vo b sung bn hoc nhim kim loi trn khng c xu hng tch t kim loi. Nguy c chit xut kim loi groundwaters ni chung l nh. Mt trong nhng k thut chnh c s dng rng ri xc nh s lng cc phn s kim loi khc nhau trong t l khai thc tun t. Mt lot cc phng n khai thc theo n hng c trnh by v s thnh cng ca d on nh my sn c kim loi t nh c nhn c nh gi.3.1 GII THIUS phn ca cc kim loi c hi trong t khng ch ph thuc vo cc iu kin mi trng v th nhng nh pH, ngp ng v hm lng cht hu c, m cn v hnh thc ha hc ban u ca kim loi v cc loi thc vt v ng vt trong h thng. Ngun gc v hnh thc ban u ca cc kim loi c hi trong t c a ra trong Chng 1 v phytoavailability cc kim loi c hi c tho lun trongChng 4. Chng ny l c lin quan vi cc khoang t: rn, trao i v pha dung dch nc ca cc kim loi c hi v tan ca n, bin i ha hc, to phc, hp ph v c tnh di ng. Ba phng php chnh c sn cho chng ti cc c tnh ca ion kim loi c bit, phc kim loi v cc thnh phn kim loi khc nhau trong t b nhim:(1) l thuyt a ha vi m hnh ha da trn my tnh;(2) s hiu bit r rng v quy trnh v iu kin kim sot cc phn ng v bin i ca cc loi kim loi v do nh hng n tnh di ng kim loi t; v(3) phn tch trong phng th nghim ca cc phn s kim loi khc nhau trong t b nhim, s dng mt chui cc extractants ha.C nhn, mi trong ba phng php tip cn cung cp mt du hiu ca qu trnh lin kt vi cc kim loi c hi trong t v retentions tng i v linh ng. Mc d trong nhng nm gn y Sposito v cng s ca ng c gng so snh ca phng th nghim phng php tip cn vi nhng t s dng vi tnh m phng da trn s dng l thuyt ha hc (mt s v d c a ra bi Sposito, 1983), c ng k vi v d, nu c, ca nghin cu h sinh thi v t s dng mt s kt hp ca ba phng php ny trong mt n lc cung cp nhng gii thch chi tit hn v cc qu trnh kim loi trong t v trong mt bc tranh r rng hn v s phn ca mt v lu di ca h. Rt nhiu nghin cu, c bit l cc ng dng nng nghip v sinh thi, s dng extractants ha nh lng cc phn phn on kim loi t khc nhau. Mt s nh cc nghin cu c gng m t phytoavailability kim loi tng ng bng phn s kim loi c th t extract- vi s hp thu kim loi thc vt. Nhng cch tip cn phng th nghim v nh knh th nghim s c tho lun trong Phn 3.4. Mc ch trong phn tip theo l xem xt ba phng php nu trn v nh gi phm vi v gii hn ca h trong vic d on v s phn ca cc kim loi trong cc h sinh thi khc nhau.3.2 L THUYT a ha ca kim loi bit ha TRN TTrc khi xem xt nh hng ca kim sot mi trng, c bit l th nhng, v s phn ca cc kim loi trong h thng t-thc vt, n l hu ch xem xt li mt cch ngn gn cc nguyn tc l ha c bn kim sot s bit ha kim loi trong h thng mi trng.S dng l thuyt ha hc c bn v thng tin t cc bng tun hon, mt khung n gin c th c xy dng cho php d on s b v cch thc cc loi khc nhau ca cc kim loi c th phn ng trong h thng mi trng. Tnh ton ca cc loi hnh ny c s dng kh rng ri tho lun v s bit du vt kim loi trong cc vng nc ngt v nc mn (v d nh Sibley v Morgan, 1975; Stumm v Morgan, 1981; Andreae, 1986; Turner, 1987; Fergusson, 1991). Cng vic tng t trong cc gii php ca t v t c Laken mt thi gian xut hin. Morgan (1987) v Sposito (1986a) i mt chng ng di trong phc tho nhng nguyn tc c bn a ha cho s bit ha kim loi trong h thng mi trng, thng da trn l thuyt ha hc m c sn trong hn 50 nm. y l c s ca phng php tip cn m hnh a ha hc da trn my tnh c trnh by bi Sposito v Mattigod (1980) v c pht trin bi Sposito v Coves (1988) c bit cho t. K thut phn on ha hc, s dng cc gii php chit ha hc c thit k loi b cc phn phn on kim loi t c th v mang tnh cht cung cp d liu thc nghim kim tra u ra ca tnh bit ha a ha. Sposito v cc ng nghip pht hin ra mi quan h tt gia nghin cu v m phng my tnh ca h. Cc kch bn ny bt u gip trong vic d on s bit du vt kim loi, c bit l trong t m bn thi c p dng.Mc d phn ln cc n lc hin ti trong l thuyt a ha l hng vo vic ci thin cc m hnh hin tnh im cn bng loi trong cc gii php ca t, cc loi ha cht ny "quy tc" s dng trong tnh ton phc tp nh vy c th gip cc nh khoa hc t trc tip trong tin on m loi kim loi mong i trong mi trng khc nhau v h thng t. Mt s nguyn tc c bn a ha arc nu di y.3.2.1 NGUYN TC HA HC CA KIM LOI bit ha3.2.1.1 "Ni quy" chi phi s hnh thnh v n nh ca khu phc hp kim loiCc s bit ha ca cc kim loi trong mi trng c th c d on t s hiu bit ca ba loi chnh ca cc cu trc in t, h hnh thnh:(I) Cc phn ng phi hp, lin quan n vic chia s ca cc electron, v d:A + B = AB(Ii) Cc phn ng chuyn in t, chng hn nh cc phn ng oxi ha kh, trong cc electron c chuyn giao gia cc cp oxi ha-kh:Oxi + Red | = 0x2 + Red2(Iii) Cc phn ng gc t doCc nh ha hc phn loi kim loi theo cu hnh ca h nguyn t vo cc nh ti tr electron i (cn c Lewis) v nhn electron i (axit Lewis). Pearson (1968a, b) sau xc nh "mm" "cng" v cc nh ti tr v cc cht nhn trn c s ca s hnh thnh ca cc hp cht n nh. Kim loi ion dng ca nhm Ia v IIa trong bng tun hon (bao gm c Na +, K +, Ca2 *, Mg2 *, nhng cng Mn, +, Al3 +, Fe3 +, vv) c coi nh l cht nhn "cng" (c phn loi nh Mt lp kim loi bng Nieboer v Richardson, 1980), trong khi kim loi cation nh Cu, Ag +, Cd2f, Pt4 *, Au *, v Hg2 + c coi l cht nhn "mm" (kim loi lp B ca Nieboer v Richardson, 1980) (Bng 3.1). Nhiu ngi trong s cc kim loi nh du l ng bin gii, khng phi mm cng khng kh. Nieboer v Richardson (1980) t Cd vo loi ng bin gii cng vi hu ht cc kim loi vi lng khc. acceptors cng c c trng bi polariz-kh nng thp, m in thp v mt in tch dng ln (trng thi xi ha cao v bn knh nh .), trong khi cc nh ti tr cng c phn cc thp, in m cao v mt in tch m cao Cc characteristics ngc li l ng s tht ca acceptors mm v cc nh ti tr Trong s cc kim loi ng bin gii, "mm" hay c im lp B tng theo th t:. MnJt> Zn2 *> Ni2 *> Fe2 * ^ Cd2 *> Cu2 +> Pb2 + (Nieboer v Richardson, 1980) cc kim loi mm to thnh phc cht bn vng vi cc nh ti tr electron nng t hng th ba, th t v th nm ca bng tun hon:. P, S, Cl , Br v I, trong khi kim loi cng to thnh phc cht bn vng vi cc nh ti tr electron t '.he hng th hai ca bng tun hon: N, O v F. i vi khu phc hp kim loi cng, n t hng ca s thch mi quan h n nh v do l:CO32-> Noj ~P0. '"> S042'> Cl 07trong khi i vi cc phc kim loi mm, cc n t hng ca s thch mi quan h v do stability l:I> Br> Cl *> F Se> S O As> P> NS> N> Oiu ny gii thch insolubilities ln ca "mm" sulfua kim loi, v d, so vi hydroxit, cacbonat hoc pht pht v cng gii thch l do ti sao mt s kim loi c tm thy trong lp v ca tri t ch yu l qung sunfua (Pb, Zn, Mg), trong khi nhng ngi khc c tm thy ch yu nh oxit v cacbonat (v d: Al, Ca, Mg). Mt cuc tho lun y hn v nhng nguyn tc ny c a ra bi Morgan (1987).3.2.1.2 Ionic tim nng v kh nng ha tanTim nng ion ca cc nguyn t (t l ph ion (Z) v bn knh ion (r) l mt du hiu hu ch ca tnh tan tng i ca cc ion, vi gi tr thp cho thy tnh tan cao hn. Trong hnh 3.1, cc cation kim (Na, K) c t l Z / r thp hn 30 v tim nng ion cao. H rt ha tan, d dng vt to thnh cation ngm nc v d dng b ra tri t t. Cc ion kim loi chuyn tip (Mn, Fe, Al) v mt s kim loi c hi c kh nng, vi t l Z / r ln hn 30 v tim nng ion trung gian, khng phi l rt d ho tan v khi phong c xu hng kt ta nh oxyhydroxides (Bohn el al., 1985). Hai phng php nhit ng lc khc nhau c th c s dng tnh ton s bit ha ion: s dng hng s cn bng, hoc s dng cc ngun nng lng Gibbs min ph. C hai u l i tng cc iu kin cn bng v cn bng khi lng. Cn bng ha hc xc nh iu kin n nh nht cho mt b nht nh ca cht phn ng v sn phm v c th c tnh ton t hng s cn bng nng lng v min ph. Cn bng khi lng ra lnh rng tng cc nng ca cc ion v hp phi bng tng nng . V l thuyt, cc tnh ton nhit ng lc hc tng t c th c p dng cho c hai khu phc hp kim loi v c v hu c. Trong thc t, khng c hiu bit v c cu trc v c ch rng buc ca cc phi t hu c v chng ti khng c d liu nhit ng lc ng tin cy cho cc loi hu t thc t.Tnh ton ca hnh thnh ion s dng hng s cn bngPhng trnh cn bng tan ca mt kim loi hai thnh phn rn c th c a ra nh:Ma Lho) = aM "aq) + BL (, q)Trong :M = du vt kim loi L = liganda, b = h s t lng m, l = valencies ca kim loi v phi t tng ngHng s cn bng cho phn ng ny (Kr) l:(Mm * Y (L '~) b(Me Lho)Biu thc ny c th c vit li di dng nht k cho cc kim loi:ng nhp M '"* = - \] og (MaLho)) - b log) + ng nhp Kr] (3.3)mtv cho thy hai yu t iu tit hot ng ca cc cation kim loi trong t gii php c th l cc hot ng ca MaLh v L ~. S ng gp ca L '~ l mt yu t quan trng k t khi L c th tham gia vo mt lot cc qu trnh kt ta, hp ph v phc trong t. M trong mt lot cc hp kim rn c th l nhiu kh nng hnh thnh trong iu kin t ai nht nh, v v th m giai on rn c nhiu kh nng kim sot hot ng ca cc cation kim loi trong dung dch, c th c d on s dng th t l hot ng. Nhng m mu log [(pha rn) / (min ph cation kim loi)] chng li bt k characteristic t quan trng, chng hn nh pH hoc PCO, ni du ngoc () quy cho hot ng nhit ng lc hc. Mt v d v cc th tc dn phosphate pha rn c gii thch bi Sposito (1983).Dissolution reactions for three lead phosphates at standard temperature and pressure are given below, with the calculations for the dissolution constant and the activity ratio given in steps 2 and 3: Step I: Dissolution reactions for three lead phosphates (at 298.15 K) arc given by: (log K values are calculated from Lindsay, 1979)Pb,(P04>2 + 4H* = 3Pb2+ + 2H2PO4log K -1.80(3.4a)Pb,(P04)iCI + 6H + * 5Pb2+ + 3112P04 + Cl'-5.06(3.4b)Pb AI,(P04)2(0H)5.H20 + 9H+ =Pb2 * + 3AI,+ + 2H2P04 + 6H2O9.74(3.4c)

Step 2: The dissolution constant for each reaction is calculated (the example below is for equation (3.4a))lPb2,]' |H;PO;|2 iPb,(Po.,)2]. i H * r' Step 3: The activity ratio is calculated for each part of equation (3.4) (the example below is for equation (3.4a))log K = 3 log Pb2 + + 2 log H2POT - log Pb,(P04h + 4pH

To plot an activity ratio diagram, one of the soil solution parameters must be chosen as the independent variable. The others must be held constant by inserting in (he equation values which represent a typical soil solution concentration for the desired species. Levels of H2PO4 in soil are controlled by the presence of iron phosphates (strengite) at low pH and by calcium phosphates at high pH. Thus for H2PO4, concentrations in the soil solution over pH 3-7 range from I0~5 5 to 106 5 Molar. Using a concentration of 10'6 M, a value of -6 is inserted in place of log H2PO4. Thus, equation (3.6) becomes:log[(Pb(PQ4)n.67)/(Pb2*)] - -2.2 + 4/3pH(3.7a)A constant of 1 is commonly used for the activity of water, and Sposito (1983) uses a concentration of I0~2,M for Cl', and the dissolution reaction of kaolinite (ALSLCMOHH) to calculate A1J+ activity. Thus, he calculateslog(AL,+ ) = 7.48 - 3pHUsing these insertions, equations (3.4b) and (3.4c) similarly become:log|(Pb(P04)n.ftCln.2)/(Pb2*)) =0.88 + ^ pH(3.7b)log[(Pb AI,(P04)2(0H),.H20)/Pb2+)] =0.70Phn ng gii cho ba pht dn nhit v p sut tiu chun c a ra di y, vi nhng tnh ton cho hng s gii th v t l hot ng nht nh trong bc 2 v 3:- Bc I: Gii th phn ng cho ba pht ch (ti 298,15 K) h quang cho bi: (log K gi tr c tnh ton t Lindsay, 1979)Pb, (P04> 2 + 4H * = 3Pb2 + + 2H2PO4 log K -1,80 (3.4a)Pb, (P04) ICI + 6H + + + * 5Pb2 3112P04 + Cl '-5,06 (3.4b)Pb AI, (P04) 2 (0h) 5.H20 + 9H + =Pb2 + * 3AI, + + 2H2P04 '+ 6H2O 9,74 (3.4c)

- Bc 2: Hng s gii cho mi phn ng c tnh ton (v d di y l dnh cho phng trnh (3.4a))lPb2,] '| H; PO; | (. Po,) 2 IPB, 2]. i H * r '- Bc 3: T l hot ng c tnh cho mi mt phn ca phng trnh (3.4) (v d di y l dnh cho phng trnh (3.4a))ng K = 3 log Pb2 + + 2 log H2POT - ng nhp Pb, (P04h + 4pH

v mt s t l hot ng, mt trong nhng thng s dung dch t phi c chn l cc bin c lp. Nhng ngi khc phi c t chc lin tc bng cch chn vo (ng gi tr phng trnh m i din cho mt dung dch t concentration in hnh cho cc loi mong mun. Mc H2PO4 trong t c iu khin bi s hin din ca st pht pht (strengite) pH thp v bng pht pht canxi pH cao. V vy cho H2PO4, nng trong dung dch t trn phm vi pH 3-7 t I0 ~ 05-ngy 10 thng nm "6 5 mol. S dng nng 10'6 M, mt gi tr -6 c chn vo v tr ca ng nhp H2PO4 Nh vy, phng trnh (3.6) tr thnh.:log [(Pb (PQ4) n.67) / (Pb2 *)] - -2.2 + 4 / 3ph (3.7a)Mt hng s ca 1 thng c s dng cho cc hot ng ca nc, v Sposito (1983) s dng nng I0 ~ 2, M cho Cl ', v cc phn ng gii th kaolinit (ALSLCMOHH) tnh ton A1J + hot ng. V vy, ng tnh tonlog (AL, +) = 7,48 - 3phS dng nhng phn thm vo, phng trnh (3.4b) v (3.4c) tr thnh tng t:ng nhp | (Pb (P04) n.ftCln.2) / (Pb2 *)) = 0.88 + ^ pH (3.7b)log [(Pb AI, (P04) 2 (0h) ,. H20) / Pb2 +)] = 0.70Equations (3.7a), (3.7b) and (3.7c) are plotted in Figure 3.2. For a chosen value of the soil characteristic under examination, for example, soil pH, and assuming that solid phases are in the Standard State, the solid in a soil mixture which produces the largest log[(solid)/(free)] value is the most stable and hence the only one that will form at equilibrium. The activity ratio is largest when the activity of the trace metal cation is the smallest. For the conditions stated in the calculations of equations (3.7a) and (3.7b), Figure 3.2 shows that chloropyroinorphite (Pbj(P04)jCI) is the most stable solid across the pH range of 3-7.Calculation of ion speciation using Gibbs free energiesA second method for assessing the stability of compounds and the dissolution sequence of mixtures of compounds is to calculate the change in Gibbs free energy for selected processes. For an equation such as that in equation (3.1) above, the free energy for the reaction is calculated as:AGr= AG[products] - AG[reactants](3.8)If the result of this calculation is negative, then the reactants are unstable and the reaction will occur spontaneously. If the AG,is positive, the reaction is less likely to occur without other additions to the system. The relationship between K (reaction activity constant) and AGr (change in free energy for the reaction) is given by:AG= - RT In K(3.9)where R = universal gas constant and T = absolute temperature (K).At 25 C, equation (3.9) reduces to:A G,= -1.364 log Kor, more usefully,8r0')1.364This equation is useful for calculating equilibrium constants for chemical weathering reactions in which thermodynamic data are difficult to measure by conventional methods. Lindsay (1979) uses this method to calculate the log A' values of an extremely large number of likely and unlikely trace metal reactions in soil. Lindsay (1979) also develops qualitative diagrams which provide graphic summaries of mineral sequences which might be expected to occur if equilibrium were attained.These types of thermodynamic calculations are useful tools for predicting the general sequence of groups of reactions, but are considered unreliable since they suffer from several important problems: (a) AG calculations are based on chemical species in the Standard State and these conditions arc rarely achievedPhng trnh (3.7a), (3.7b) v (3.7c) c v trong hnh 3.2. i vi mt gi tr c chn ca cc c tnh t di, kim tra, v d, pH ca t, v gi nh rng giai on rn l bang chun, cht rn trong hn hp t trong sn xut cc bn ghi ln nht [(rn) / (min ph)] gi tr l n nh nht v do ch c mt m s hnh thnh trng thi cn bng. T l hot ng l ln nht khi cc hot ng ca cc cation kim loi vi lng l nh nht. i vi cc iu kin quy nh trong cc tnh ton ca phng trnh (3.7a) v (3.7b), Hnh 3.2 cho thy chloropyroinorphite (Pbj (P04) JCI) l n nh nht rn trn phm vi pH 3-7.Tnh ton ca hnh thnh ion s dng nng lng Gibbs min phMt phng php th hai nh gi s n nh ca cc hp cht v trnh t gii th ca cc hn hp ca cc hp cht l tnh ton s thay i nng lng t do Gibbs cho qu trnh la chn. i vi mt phng trnh nh trong phng trnh (3.1) trn, nng lng t do ca phn ng c tnh nh sau:AGR = AG [sn phm] - AG [Ho] (3.8)Nu kt qu tnh ton ny l tiu cc, sau cc cht phn ng khng n nh v phn ng s xy ra mt cch t nhin. Nu AG, l tch cc, phn ng l t c kh nng xy ra m khng cn b sung khc vo h thng. Mi quan h gia K (phn ng hot ng lin tc) v AGR (thay i nng lng t do ca phn ng) c cho bi:AG = - RT Trong K (3.9)trong R = hng s kh ph qut v T = nhit tuyt i ( K).Ti 25 C, phng trnh (3.9) gim ti:A G, = -1,364 log K hay, hu ch hn, 8r0 ')1,364Phng trnh ny rt hu ch cho vic tnh ton hng s cn bng cho phn ng phong ha ha hc trong d liu nhit ng lc hc l kh o lng bng cc phng php thng thng. Lindsay (1979) s dng phng php ny tnh ton cc log A ' gi tr ca mt s lng rt ln cc phn ng du vt kim loi c kh nng v khng trong t. Lindsay (1979) cng pht trin s tnh m nu tm tt ha ca chui khong sn m c th c d kin s xy ra nu cn bng c thnh tu.Nhng loi tnh ton nhit ng l cng c hu ch d on trnh t chung ca cc nhm phn ng, nhng c coi l khng ng tin cy v chng b mt s vn quan trng: (a) tnh ton AG ang da vo cc loi ha hc bang chun v cc iu kin cung him khi t c Figure 3.2. Activity ratio diagram for lead phosphate solid phases in soil, under the conditions: (H2PO..) = 10'*, (Cr)=10'J, (H20)=I, and control of (Al*4) by kaolinite with (Si(OH))= I O'45 Reproduced by permission of Academic Press Ltd from Sposito (1983)Hnh 3.2. S t l hot ng cho phosphate ch pha rn trong t, di cc iu kin: (H2PO ..) = 10 '*, (Cr) = 10'J, (H20) = I, v kim sot (Al * 4) bng vi kaolinit (Si (OH) ") = I O'45 sao chp vi s cho php ca Academic Press Ltd t Sposito (1983)

in field conditions; (b) there are difficulties in writing equations which realistically represent complicated reactions and interactions occurring in field soils; (c) the continually changing chemical and physical conditions of field soils and the fact that many mineral transformation processes proceed extremely slowly mean that a true equilibrium is rarely achieved; (d) for many trace metals in soils, reactions involve organic complexes and often very little is known about their formation, structure or interactions with inorganic metal ligands and free ions; and (e) soil organics are continually being produced and decomposed and hence are in a constant state of flux. Despite these drawbacks, thermodynamic calculations provide at least some estimate of element speciation and concentrations in complex systems, and also in cases where concentrations of metal ions and ligands are below detection levels for conventional analytical methods such as atomic absorption spectrophotometry (AAS). Such calculations are also the basis of geochemical modelling strategies outlined below. There are currently more reliable and available equilibrium constant data than free energy values and while Gibbs free energy approaches may be adequate for simple systems, equilibrium constant approaches are preferred for large complex systems such as the soil (Nordstrom el al., 1979).3.2.2MODELLING METAL SPECIATION IN SOILSComputer models have been developed over the last decade to calculate the equilibrium partitioning of soluble metal ions, metal ligands and organo- metallic complexes in aqueous solutions and in soils. All are based on standard chemical thermodynamic data. Several publications have tabulated thermodynamic data in standard states: dissociation constants, solubility products, free energies and enthalpies for different solute forms of trace metals (e.g. Feitknecht and Schindler, 1963; Sillen and Martel), 1964; Hogfeldt, 1979; Robie el al., 1979) but problems in using geochemical modelling as a tool for simulating trace metal speciation are sometimes due to inconsistencies in the quoted values of these data. Other problems arise through the need to characterise realistically the amounts and speciation of the major elements in solution to allow correction for the ionic strength activity of the solution. For variable valence elements (especially many trace metals), a knowledge of redox potential is also required in order to determine their valency states.Geochemistry models for aqueous systems have been available for some time, including WATF.Q (Truesdell and Jones, 1974), SOLMNEQ (Kharaka and Barnes, 1973) and REDEQL2 (McDuff and Morel, 1973). These have had the capacity to calculate the concentrations and particular forms of major and some trace elements in solution and to estimate the effects of changing solution conditions, such as pH, Eh, ionic strength, CO2 pressure, or the concentrations of particular ions, on the solubility and speciation of a chosen chemical element in solution. Development of models capable of handling the geochemistry of soil solutions in contact with solid soil particles requires simulation of processes such as solubility equilibria, including precipitation and dissolution reactions, specific adsorption on oxide-like surfaces and cation exchange. First trong iu kin thc a; (B) c nhng kh khn trong vit phng trnh m thc t i din cho cc phn ng phc tp v tng tc xy ra trong lnh vc t ai; (C) cc iu kin ha hc v vt l lin tc thay i ca t trng v thc t l qu trnh chuyn i nhiu khong sn tin hnh rt chm c ngha l mt trng thi cn bng thc s l rt him khi t c; (D) cho nhiu kim loi vi lng trong t, cc phn ng lin quan n phc hp hu c v thng rt t c bit v s hnh thnh, cu trc hoc tng tc vi cc ligand kim loi v c v cc ion t do ca h; v (e) cc cht hu c trong t vn tip tc c sn xut v phn hy v do ang trong mt trng thi lin tc ca thng lng. Mc d c nhng hn ch, tnh ton nhit ng lc cung cp t nht mt s c tnh ca cc phn t bit ha v nng trong cc h thng phc tp, v cng trong trng hp nng ca cc ion kim loi v phi t l di mc pht hin cc phng php phn tch thng thng nh quang ph hp th nguyn t (AAS). Tnh ton nh vy cng l c s ca cc chin lc xy dng m hnh a ha nu di y. Hin ang c trng thi cn bng d liu lin tc v ng tin cy hn so vi gi tr nng lng c sn min ph v trong khi cch tip cn nng lng t do Gibbs c th cho h thng n gin, trng thi cn bng phng php tip cn lin tc c a thch cho cc h thng ln v phc tp nh t (Nordstrom el al., 1979).3.2.2 M HNH HA METAL bit ha TRN TM hnh my tnh c pht trin trong thp k qua tnh ton cc phn vng cn bng ca cc ion kim loi ho tan, cc ligand kim loi v hp kim organo- trong dung dch nc v trong t. Tt c u da trn d liu nhit ng lc tiu chun ha. Mt s n phm lp bng d liu thermodynamic trong tiu chun quc gia: hng s phn ly, sn phm ha tan, nng lng min ph v enthalpies cho cc hnh thc khc nhau ca cc cht tan kim loi du vt (v d Feitknecht v Schindler, 1963; Sillen v Martel), 1964; Hogfeldt, 1979; Robie el al., 1979), nhng vn trong vic s dng m hnh a ha nh mt cng c m phng cc du vt kim loi s bit l i khi do mu thun trong cc gi tr nim yt ca cc d liu. Cc vn khc ny sinh qua cu m t thc t cc s liu v s bit ha ca cc yu t chnh trong gii php cho php hiu chnh i vi cc hot ng cng ion ca cc gii php. i vi nguyn t ha tr bin (c bit l nhiu kim loi vi lng), mt kin thc v tim nng oxi ha kh cng c yu cu xc nh trng thi ha tr ca h.M hnh a ha hc cho cc h thng dch nc c sn sng cho mt s thi gian, bao gm WATF.Q (Truesdell v Jones, 1974), SOLMNEQ (Kharaka v Barnes, 1973) v REDEQL2 (McDuff v Morel, 1973). Nhng iu ny c nng lc tnh ton nng v hnh thc c th ca chnh v mt s nguyn t vi lng trong dung dch v c lng nh hng ca vic thay i iu kin, gii php, chng hn nh pH, Eh, sc mnh ion, p sut CO2, hoc nng ca cc ion c bit, trn ha tan v bit ha ca mt nguyn t ha hc c la chn trong gii php. Pht trin cc m hnh c kh nng x l cc a ha ca cc gii php t tip xc vi cc ht t rn i hi m phng cc qu trnh nh cc im cn bng tan, bao gm kt ta v gii th phn ng, c th hp ph trn b mt "oxide-like" v trao i cation. u tin GEOCHEM (Sposito and Mattigod, 1980) and subsequently SOILCHEM (Sposito and Coves, 1988) are models written specifically to calculate the speciation of chemical elements among the aqueous solution, solid and adsorbed forms in soil. GEOCHEM and SOILCHEM are later developments of the REDEQL model (Morel and Morgan, 1972) which is based on the equilibrium constant approach. GEOCHEM is an iterative program which uses stability constants corrected for ionic strength to calculate the metal and ligand species present. Its main advantages over previous models is its inclusion of a large number of metalligand complexes. GEOCHEM uses a large database of 2000 aqueous species and 889 organic species. In addition, routines have been added to the model to handle ion exchange, adsorption/ resorption processes and clay mineral solubility. Since geochemical simulations are based on thermodynamic calculations, they suffer from the same limitations and benefits .as expressed in Section 3.2.1 above. Simulating concentrations of metal ions and ligands which are below instrument detection levels is advantageous while the problems of non-equilibrium are limiting. Although soils do not reflect equilibrium conditions, it is possible to consider steady state for some of these soil components that are known to react with one another on a short-term and nearest-neighbour basis. This steady slate can be characterised by knowing (i) the soil solids that affect mineral solubility on a short-term basis, and (ii) the type and quantity of organic ligands that are present in macroamounts. For instance, the addition of citric and oxalic acids to soils should, at least temporarily, transform some water-insoluble Al and Fe into water-soluble Al and Fe.The main purpose of geochemical models is the prediction of species composition and concentrations in defined soil conditions. Several applications of the GEOCHEM model have indicated its relatively accurate prediction of metal speciation in various soil contamination conditions. Mattigod and Sposito (1979), for example, modelled the effects on soil chemistry of geothermal brine spillage. In another application, Sposito and Bingham (1981) showed an overall relationship between total soil Cd, and Cd uptake by sweetcorn. They then computed Cd speciation in saturated soil extracts and found a good correlation between plant Cd uptake and concentrations of CdCI* in the soil solution. No relationship was found between Cd in the plant and Cd2* in the soil solution.Lighthart el al. (1983) used GEOCHEM to predict the types and concentrations of Cd and Cu species in soil and the levels that inhibit soil microbial respiration. The model predicted four soil Cd fractions (Cd2*, adsorbed, organic and other) and five soil Cu fractions (Cu2*, hydroxides, carbonates, organic, adsorbed). At a soil total Cd concentration of 0.5 mmol kg"1, the Cd21 concentration was calculated to be I0~5 M, and just within Ihe range considered to be inhibitory to microbial respiration (Figure 3.3(a)). Increasing free Cd2* in solution correlated well with decreasing respiration. The predominant Cd phase in soil was thought to be CdCO.i, which accounted for 40% of Ihe Cd at the 0.5 mmol kg'1 total soil Cd. The speciation of soil Cu is controlled by organic complexation and a strong adsorption affinity. Cu did not have an inhibitory effect until total soil Cu concentrations of 5-50 mmol kg"1 were reached. These levels were computed to have solution Cu2* concentrations of 0.01-0.1 /iM respectively (Figure 3.3(b)).GEOCHEM (Sposito v Mattigod, 1980) v subse xuyn SOILCHEM (Sposito v Coves, 1988) c m hnh bng vn bn c th tnh ton s bit ha ca cc nguyn t ha hc trong dung dch nc, rn v cc hnh thc hp ph trong t. GEOCHEM v SOILCHEM ang pht trin sau ny ca m hnh REDEQL (Morel v Morgan, 1972) m l da trn cch tip cn hng s cn bng. GEOCHEM l mt chng trnh lp i lp li trong s dng cc hng s n nh c hiu chnh cng ion tnh ton cc kim loi v cc loi ligand hin. u im chnh ca n trn m hnh trc l s bao gm ca mt s lng ln cc hp kim loi-phi t. GEOCHEM s dng mt c s d liu ln ca 2.000 loi dch nc v 889 loi hu. Ngoi ra, thi quen c thm vo m hnh x l trao i, hp ph / qu trnh ti hp thu ion v ha tan khong st. K t simulations a ha c da trn cc tnh ton nhit ng lc hc, u gp phi nhng hn ch tng t v li ch .as by ti mc 3.2.1 trn. M phng nng ca cc ion kim loi v phi t l di mc pht hin c l thun li trong khi cc vn ca khng cn bng c hn ch. Mc d t khng phn nh iu kin cn bng, c th xem xt "trng thi n nh" cho mt s cc thnh phn t m c bit phn ng vi nhau trn mt on ngn hn v c s gn nht xm. Slate n nh ny c th c c trng bi bit (i) cc cht rn t c nh hng n kh nng ha tan khong sn trn c s ngn hn, v (ii) cc loi v s lng ca cc phi t hu c c mt trong macroamounts. V d, vic b sung cc citric v acid oxalic cho t nn, t nht l tm thi, chuyn i mt s nc khng ha tan Al v Fe vo Al ha tan trong nc v Fe.Mc ch chnh ca cc m hnh a ha hc l nhng d on ca composition loi v nng trong iu kin t ai c xc nh. Mt s ng dng ca m hnh GEOCHEM ch ra d on tng i chnh xc ca s bit ha kim loi trong iu kin nhim t khc nhau. Mattigod v Sposito (1979), v d, m hnh ha cc tc ng v ha hc t trn nc mui a nhit. Trong mt ng dng khc, Sposito v Bingham (1981) cho thy mt mi quan h tng th gia tng Cd t, v hp th Cd bng ht bp. Sau h tnh Cd bit ha trong dch chit t bo ha v tm thy mt mi tng quan tt gia s hp thu Cd cy v nng cc CdCI * trong dung dch t. Khng c mi quan h c tm thy gia Cd trong nh my v Cd2 * trong dung dch t.Lighthart el al. (1983) c s dng d on GEOCHEM cc loi v concen-trations ca loi Cd, Cu trong t v mc c ch vi sinh vt t h hp. M hnh d on bn phn s Cd t (Cd2 *, hp ph, hu c v khc) v nm t C s thp phn (Cu2 *, hydroxit, cacbonat, hu c, hp th). mt nng tng Cd t 0,5 mmol kg "1, nng Cd21 c tnh ton l I0 ~ 5 M, v ch trong phm vi IHE coi l c ch h hp ca vi sinh vt (Hnh 3.3 (a)). Tng cng t do Cd2 * trong gii php tng quan vi gim h hp. Cc giai on Cd predominant trong t c cho l CdCO.i, chim 40% ca IHE Cd 0,5 mmol kg'1 tng Cd t. Cc s bit ha ca t C c iu khin bi to phc hu c v mt i lc hp ph mnh. Cu khng c tc dng c ch cho n khi nng Cu tng s t ca 5-50 mmol kg "1 t c. Cc mc ny c tnh ton c gii php Cu2 * nng 0,01-0,1 / iM (Hnh 3.3 (b)). detailed information on factors controlling metal speciation and soil retention processes is necessary before best use can be made of existing soil information systems.In this section the soil processes associated with metals in soils will be reviewed, outlining in each case the influence of changes in environmental conditions and soil properties.Below are listed some of the main processes associated with toxic metals in soils:(i)weathering of in situ parent material(ii)dissolution and solubility of minerals and complexes, accompanied by precipitation and co-precipitation of inorganic insoluble species, such as carbonates and sulphides(iii)uptake by plant roots and immobilisation by soil organisms (some current ideas about uptake are discussed in Chapters 2 and 4)(iv)exchange onto cation exchange sites of clays or soil organic matter(v)specific chemisorption and adsorption/desorption on oxides and hydroxides of iron, aluminium and manganese(vi)chelation and complexation by different fractions of soil organic matter(vii)leaching of mobile ions and soluble organo-metallic chelatesWhether any one of the above processes dominates over any other in controlling soil solution metal speciation and concentration, depends entirely on the toxic metal in question, its speciation, and a whole range of soil properties and conditions, including soil pH, redox, organic matter amount and composition, clay content and Fe, Mn and A1 oxide content. It is worth clarifying the use of terminology here. The term metal fractionation will be used for the soil fractions in which the metal is located (e.g. easily exchangeable, chemisorbed, or organically bound), while the term metal speciation will be used for the different chemical forms or species in which the metal can exist, such as different oxidation states or the hydroxide or sulphide ligands. In this section the aim will be to review each of the possible processes associated with heavy metals in soil, in light of the major influencing environmental and soil factors.3.3.1 WEATHERING OF PARENT MATERIALSGenerally the trace metal input to soil from in situ weathering of parent rock is low and only likely to produce potentially toxic metal concentrations locally in areas of oxide-rich deposits, ores and other lithologies high in trace metals, such as ultramafic rocks, including serpentine. In soils developed on serpentine, for example, extremely high concentrations of nickel (up to 500-1000 fig Nig 1 total Ni) have been recorded (see Chapter 12, this volume). Such metal-enriched soils are often characterised by very typical plants, including hyperaccumulator species, which can have foliar Niconcentrations as high as 1000/tgg'1 (see Chapter 12, this volume). The mineral chemical weathering processesdissolution, hydration, hydrolysis, oxidation, reduction and carbonationare described by Ollier (1984). The chemical thng tin chi tit v cc yu t kim sot s bit kim loi v cc qu trnh gi t l cn thit trc khi s dng tt nht c th c thc hin trong h thng thng tin t ai hin hnh.Trong phn ny, cc quy trnh lin quan n t vi kim loi trong t s c xem xt, a ra trong mi trng hp nh hng ca nhng thay i trong iu kin mi trng v tnh cht ca t.Di y l lit k mt s cc qu trnh chnh kt hp vi cc kim loi c hi trong t:(I) thi tit ca vt cht situ m(Ii) gii th v kh nng ha tan cc khong cht v phc hp, km theo ma v ng kt ta khng tan ca loi v c, chng hn nh cacbonat v sunfua(Iii) s hp thu ca r cy v c ng ca sinh vt t (mt s tng hin ti v s hp thu s c tho lun Chng 2 v 4)(Iv) trao i trn cc trang web trao i cation ca t st hoc cht hu c ca t(V) chemisorption c th v hp ph / gii hp trn oxit v hydroxit st, nhm v mangan(Vi) thi v to phc ca cc phn phn on khc nhau ca vt cht hu c ca t(Vii) thm thu ca cc ion di ng v chelates organo-kim loi ha tanCho d bt k mt trong cc qu trnh trn chim u th hn bt k khc trong vic kim sot dch t bit ha kim loi v nng , ph thuc hon ton vo cc kim loi c hi trong cu hi, s bit ha ca n, v mt lot cc tnh cht ca t v iu kin, bao gm c t c pH, oxy ha kh, cht hu c s lng v thnh phn, hm lng st v Fe, Mn v ni dung oxit A1. l gi tr lm r vic s dng cc thut ng y. Thut ng "phn on kim" s c s dng cho cc phn t m cc kim loi c t (v d nh d dng exchangeable, chemisorbed, hoc hu c b rng buc), trong khi thut ng "s bit ha kim loi" s c s dng cho cc hnh thc ha hc khc nhau hoc cc loi trong kim loi c th tn ti, chng hn nh trng thi oxy ha khc nhau hoc cc hydroxit hoc sulphide ligand. Trong phn ny, mc tiu s c xem xt tng cng on c th kt hp vi kim loi nng trong t, trong nh sng ca cc nh hng environmental v t yu t chnh.3.3.1 ma nng LIU PH HUYNHNi chung u vo du vt kim loi t t trong phong situ ca cha m l thp v ch c kh nng sn xut nng kim loi c hi c kh nng a phng trong lnh vc tin gi oxit giu, qung v cc loi t khc cao trong kim loi vi lng, chng hn nh cc siu mafic, bao gm serpentine . Trong t pht trin trn serpentine, v d, vi nng rt cao ca niken (ln n 500-1000 v Nig 1 tng Ni) c ghi nhn (xem Chng 12, trong s ny). t kim loi lm giu nh vy thng c c trng bi cc nh my rt in hnh, bao gm c "tch ly qu nhiu" loi, trong c th c l Ninng cao nh 1000 / tgg'1 (xem Chng 12, trong s ny). Cc khong sn phong ha ha hc qu trnh gii th, hydrat ha, thy phn, qu trnh oxy ha, gim v carbonat-c m t bi Ollier (1984). Cc cht ha hc weathering processessolution, reduction and oxidationare particularly important processes for trace metals in soils and will be discussed in more detail in the following sections. Background mineralogy and chemical theory relevant to chemical weathering and metal mobility are reviewed by Ollier (1984) and Baton (1978).3.3.2DISSOLUTION/PRECIPITATION, SOLUBILITY AND FREE IONS IN SOLUTIONPrecipitation/dissolution mechanisms and adsorption/desorption mechanisms (Section 3.3.3) are the main physico-chemical processes that control concentrations of metal species in the soil solution. Once in solution, trace metal ions, whether simple or complex, exhibit typical exchange behaviour on silicate clay minerals, with the strength of metal bonding dependant on ion charge and hydration characteristics. In the soil solution, trace metal cations such as Cd2 + , Pb2+ and Cu2+ compete with more abundant soil cations such as Ca2* and Na* for cation exchange sites. For this reason, strong partitioning of trace metals onto cation exchange sites is not normally seen. Instead, many trace metals are specifically adsorbed, or chemisorbed, onto amorphous oxides of Al, Fe and Mn, and also onto soil organic matter (see Section 3.3.3). The rales and direction of both precipitation/dissolution processes and adsorption/ desorption processes are strongly influenced by acidity and redox potential. Theoretically precipitation/dissolution processes should occur at a given pH, and, unlike adsorption and ion exchange processes, precipitation/dissolution is less dependent on the amount of reactant or the different mineral surfaces present in soil. Apart from Fe and Mn, trace metal solubility in soils is not controlled by the solubility product of a pure solid phase (Brummer el al., 1983). This is partly because adsorption of metal cations from the very low soil solution concentrations is able to maintain solution solubility at a level loo low for precipitation to occur. At high metal loadings, and in alkaline and calcareous soils, precipitation processes may begin to control metal ion concentrations in the soil solution. In acid mineral soils and in organic soils, precipitation of metals as hydroxides or carbonates is highly unlikely, even with high metal loadings. Brummer el al. (1983) point out the very close interrelations which exist between ion exchange/adsorption and precipitation/ dissolution processes in the generally low metal concentration environment of the soil.An extremely important breakthrough in understanding mineral and metal solubility in soil was made in 1979 with the publication of the seminal work on soil chemical equilibria by Lindsay (1979), in which he calculated the solubility relationships for a very large number of minerals and metal3.3.2 GII TH / lng ma, ha tan v min ph ion trong SOLUTIONC ch ma / gii th v c ch hp ph / gii hp (mc 3.3.3) l cc qu trnh l ha chnh l kim sot concentrations ca loi kim loi trong dung dch t. Mt khi trong dung dch, theo di cc ion kim loi, cho d n gin hay phc tp, th hin hnh vi giao lu in hnh v khong sn t st silicate, vi sc mnh ca kim loi lin kt ph thuc vo in tch ion v c tnh thy ha. Trong dung dch t, theo di cc cation kim loi nh Cd2 +, Pb2 + v Cu2 + cnh tranh vi cc cation t phong ph hn nh Ca2 * v Na * cho cc trang web trao i cation. V l do ny, phn vng mnh m ca kim loi nh du vo cc trang web trao i cation thng khng nhn thy. Thay vo , nhiu kim loi vi lng c hp ph c bit, hoc chemisorbed, ln oxit v nh hnh ca Al, Fe v Mn, v cng vo cht hu c trong t (xem Phn 3.3.3). Cc RALES v hng ca c ma / quy trnh gii th v hp ph / quy trnh gii hp b nh hng mnh m bi axit v oxi ha kh. V mt l thuyt cc qu trnh kt ta / gii th xy ra mt pH nht nh, v, khng ging nh cc qu trnh hp ph v trao i ion, lng ma / gii th t ph thuc vo lng cht phn ng hoc cc b mt khong cht khc nhau c trong t. Ngoi Fe v Mn, du vt kim loi ha tan trong t khng c kim sot bi cc sn phm ha tan ca mt cht rn tinh khit (Brummer el al., 1983). iu ny mt phn l do s hp ph ca cc cation kim loi t cc nng dung dch t rt thp c th duy tr gii php ha tan ti mt loo cp thp cho kt ta xy ra. Ti tri kim loi cao, v trong mi trng kim v t vi, qu trnh kt ta c th bt u kim sot nng ion kim loi trong dung dch t. Trong t khong v acid hu c trong t, lng ma ca cc kim loi nh hydroxit hoc cacbonat l rt kh, ngay c vi ti trng kim loi cao. Brummer el al. (1983) ch ra mi tng quan rt gn m tn ti gia trao i ion / hp ph v qu trnh kt ta / gii th trong mi trng nng kim loi ni chung thp ca t.Mt bc t ph v cng quan trng trong vic tm hiu khong sn v kim loi ha tan trong t c thc hin vo nm 1979 vi vic xut bn cc tc phm hi tho v im cn bng ha hc t ca Lindsay (1979), trong ng tnh ton cc mi quan h tan cho mt s lng rt ln cc khong sn v kim loi complexes in soils. He presents graphs of the solubilities of Al, Fe, Mn, Zn, Cu, Cd, Pb and Hg minerals, which help in predicting which minerals are likely to control the solubility of these metals in pure solutions of differing pH, redox and ionic composition. Some examples of these calculations are given later in this section. Their applicability to complex soil solutions is questionable, but an understanding of solubility theory is a vital precursor lo estimating how metal cation concentrations in the soil solution may vary in relation to other pedological factors.The kinetics of precipitation-dissolution reactions in soils have been little studied, particularly for metal-containing minerals. Much less is known about controls on rates of solubility processes than for adsorption processes and the lack of kinetic data for solubility processes is a real limitation for modelling metal speciation.3.3.2.1 Influence of acidity on metal solublity in soilOne of the most important factors controlling metal solubility in soils is acidity. Soils in humid temperate climates naturally become more acidic through time by leaching, unless lithological replenishment or anthropogenic inputs of mineral cations occurs. Acid precipitation exacerbates this problem. Other soil processes, particularly organic matter decomposition and root ion uptake, can also contribute, even if only locally, to soil acidification. A summary of the relative mobilities of trace metals in relation to pH and redox (Eh) is given in Table 3.3. According to Plant and Raiswell (1983), many metals are relatively more mobile under acid, oxidising conditions and are retained very strongly under alkaline and reducing conditions.Three types of evidence have been used to indicate how metal solubility increases with increased soil acidity. The first of these is direct measurement of metal ion activities in soils of different pH. Field and pot experiments extracting soil solutes from a range of soil types maintained at different pH conditions have shown that Zn, Cd, Cu and to a lesser extent Pb, are much more soluble at pH 4-5 than in the pH range 5-7 (Brummer and Herms, 1983). Solution metal concentrations increased in the order Cd > Zn > Cu > Pb with decreasing pH. The second type of evidence is provided by correlations between metal uptake in plants and pH of rhizosphere soil. Sarkar and Wyn Jones (1982) found that with increasing soil rhizosphere acidity, french beans took up increasing amounts of Zn, Fe and Mn. Acidity caused by rhizosphere metabolic products, such as H2CO3, was also suggested by Xian and Shokohifard (1989) as a possible reason for increased metal uptake of metal carbonates which become increasingly soluble as soil pH declines. Different plants have differing abilities to acidify their rhizospheres. Youssef and Chino (1991) found that soybean has a greater ability than barley to solubilise Zn, Mn and Fe in the rhizosphere. The third type of evidence is derived from chemical thermodynamic calculations of solid-solution metal equilibria in soils. These types of calculations are discussed in more detail below.phc trong t. ng trnh by th ca cc tnh tan ca Al, Fe, Mn, Zn, Cu, Cd, Pb v Hg khong cht, gip cho vic tin on c khong sn c kh nng kim sot tan ca cc kim loi trong cc gii php khc nhau ca tinh khit pH, oxy ha kh v thnh phn ion . Mt s v d v nhng tnh ton ny c a ra sau trong phn ny. ng dng ca h n cc gii php t phc tp l vn , nhng s hiu bit v l thuyt ha tan l mt tin thn lo sng cn c tnh c bao nng cation kim loi trong dung dch t c th thay i lin quan n cc yu t th nhng khc.ng hc ca phn ng kt ta gii th trong cc loi t c nghin cu rt t, c bit l i vi cc khong cht c cha kim loi. t c bit v iu khin trn gi ca cc qu trnh ha tan hn cho qu trnh hp ph v thiu d liu ng hc ca qu trnh ha tan l mt gii hn thc s cho s bit ha m hnh kim loi.3.3.2.1 nh hng ca nng axit trn solublity kim loi trong tMt trong nhng yu t quan trng nht kim sot kim loi ha tan trong t l axit. t vng kh hu n i m t nhin tr thnh c tnh axit hn qua thi gian bng cch thm thu, tr khi b sung thch hc hoc u vo con ngi ca cc cation khong xy ra. Lng ma axt lm trm trng thm vn ny. Qu trnh t khc, c bit l phn hy cc cht hu c v hp thu ion gc, cng c th ng gp, thm ch nu ch ti a phng, qu trnh axit ha t. Mt bn tm tt ca linh ng tng i ca cc kim loi du vt lin quan n pH v oxy ha kh (Eh) c a ra trong Bng 3.3. Theo Plant v Raiswell (1983), nhiu kim loi l in thoi di ng tng i nhiu hn di acid, iu kin oxy ha v c gi li rt mnh m di kim v cc iu kin gim.Ba loi bng chng c s dng ch ra cch kim loi ha tan tng vi tng chua ca t. Vic u tin l o trc tip ca hot ng ion kim loi trong t ca pH khc nhau. Dng v ni th nghim chit xut cc cht ho tan trong t t mt lot cc loi t c duy tr iu kin pH khc nhau ch ra rng Zn, Cd, Cu v n mt mc thp hn Pb, nhiu hn ha tan pH 4-5 hn trong khong pH 5-7 (Brummer v Herms, 1983). Nng dung dch kim loi tng trong Cd order> Zn> Cu> Pb vi gim pH. Loi th hai ca bng chng c cung cp bi cc mi tng quan gia s hp thu kim loi thc vt v pH ca t vng r. Sarkar v Wyn Jones (1982) thy rng vi s gia tng t vng r chua, u Php ln tng lng Zn, Fe v Mn. Tnh axit gy ra bi cc sn phm trao i cht vng r, nh H2CO3, cng c xut bi Xian v Shokohifard (1989) l mt l do c th cho tng cng hp th kim loi ca cacbonat kim loi m ngy cng tr nn ha tan nh pH t gim. Cc nh my khc nhau kh nng khc nhau lm chua rhizospheres ca h. Youssef v Chino (1991) pht hin ra rng u tng c kh nng ln hn so vi la mch ha tan Zn, Mn v Fe trong vng r. Loi th ba ca bng chng c ngun gc t cc tnh ton nhit ng ha hc ca rn gii php im cn bng kim loi trong t. Nhng loi tnh ton c tho lun chi tit hn di y.

Geochemical theory (see Section 3.2.1) can be used to write equilibration reactions for the solubility of soil minerals in the form of equations. Illustrations of these equations in relation to controlling factors such as pH or pH + pe can allow estimation of which mineral solids might be important in controlling metal cation concentrations in the soil solution. The solubility product approach allows identification of the least soluble mineral able to precipitate from the soil solution. Theoretically, this least soluble mineral under the given soil conditions, is likely to control metal ion concentrations in the soil solution. The numerous examples given by Lindsay (1979) confirm that pH plays an important part in many of the reactions. The solubility product approach to estimating which solid phases control metal ion concentrations suffers from a few problems (McBride, 1989):(i)Soil metals are usually present in trace quantities which make it impossible to identify discrete mineral phases.(ii)It is not always possible to measure the free metal ion concentration in solution (separately from metal-ligand complexes) and hence it is not possible to accurately estimate the ion activity product.(iii)It is doubtful whether equilibrium is ever attained in complex field soils.(iv)Pure minerals may not control solution metal ion concentrations because mixed oxides are less soluble and may maintain low metal solubilities.(v)Co-precipitation of metals as impurities in Al, Fe and Mn oxides, sometimes with organic matter, complicates the simple solubility product model.Tattle 3.4. Activities of several metal cations in well-oxidised soilsMetalReactionlog KTransformed equationZnSoil-Zn + 211* * Zn1*5.8tog Zn2* = 5.8-2 pHCttSoil-Ctt + 2H * Cu!*2.Rlog Ctt = 2.8-2 pHFeSoil-Fe + 3H = Fe"2.7log Fe" = 2.7 = 3 pHCdSoil-Cd = Cd21-7.0log Cd2' = -7.0IbSoil-Pb Pb2'- R.50log Pb2* = -8.5< /// 7:CaSoil-Ca * Ca2-2.50log Ca2* = -2.5MgSoil-Mg * Mg2 4- 3.0log Mg2* = -3,0> pH 7:CaCaCOj + 2H ,* - Ca-' I CO, + H,09.72log Ca2* = 9.72-log CO,*MgMgCaCO, + 2H - Mg2 * + CO, l 11,0 r CaCO,8.70log Mg2* = 8.70- log CO," Source: Reproduced by permission front Lindsay (1979). Copyright John Wiley & Sons Inc. " log CO; at 0.00.1 alntos = -2.52; log CO, at 0.000.1 atmos - - 3.52.L thuyt a ho (xem Phn 3.2.1) c th c s dng vit cc phn ng cn bng cho tan ca cc khong cht trong t di dng phng trnh. Minh ha ca cc phng trnh lin quan n cc yu t kim sot nh pH hoc pH + pe c th cho php c lng trong cht rn khong sn c th l quan trng trong vic kim sot nng cation kim loi trong dung dch t. Cc cch tip cn sn phm ha tan cho php xc nh cc khong ha tan t c kh nng kt ta t dung dch t. V mt l thuyt, khong ha tan nht ny theo nhng iu kin t ai nht nh, c kh nng kim sot nng ion kim loi trong dung dch t. Rt nhiu v d c a ra bi Lindsay (1979) khng nh rng pH ng mt vai tr quan trng trong nhiu phn ng. Cc cch tip cn sn phm ha tan v vic c lng m giai on rn kim sot nng ion kim loi b mt vi vn (McBride, 1989):(I) cc kim loi t ny thng hin din vi s lng du vt m lm cho n khng th xc nh cc giai on khong ri rc.(Ii) N khng phi l lun lun c th o nng ion kim loi t do trong dung dch (tch bit khi khu phc hp kim loi-phi t) v do n khng th nh gi chnh xc cc sn phm hot ng ion.(Iii) Ngi ta nghi ng liu trng thi cn bng l bao gi t c trong lnh vc t ai phc tp.(Iv) cc khong cht tinh khit c th khng kim sot nng ion kim loi gii php, v cc oxit hn hp t tan v c th duy tr tnh tan kim loi thp.(V) Co-s kt ta ca cc kim loi nh cc tp cht trong Al, Fe v Mn oxit, i khi vi cc cht hu c, lm phc tp cc m hnh sn phm ha tan n gin.Ni to lao 3.4. Cc hot ng ca mt s cc cation kim loi trong t cng khng b oxy haPhng trnh phn ng kim loi log K ci binZn Soil-Zn + 211 * * * Zn1 5,8 tog Zn2 * = 5,8-2 pHCTT Soil-CTT + 2H * Cu! * 2.R ng CTT = 2,8-2 pHFe Soil-Fe + 3H '= Fe "2.7 log Fe" = 2,7 = 3 pHCd Soil-Cd = Cd21 -7,0 log Cd2 '= -7,0I'b Soil-Pb Pb2 '- R.50 log Pb2 * = -8,5 7: "= log CO2 ti 0.003 Atmos = - 2,52 b = log CO2 ti 0,0003 nguyn t = - 3,52..

Figures 3.5, 3.6 and 3.7 are graphic illustrations of the relationship between the metal cations Zn24, Pb2+, Cd24 and soil solution pH for a number of selected soil minerals. The log K values used to construct these graphs arc given in Table 3.5 (taken from Lindsay, 1979). Other conditions used in graph construction are given in the keys to each graph. Illustrations of this type can be used to suggest which soil minerals are likely to control soil solution metal ion activity over a given range of soil pH conditions and under the influence of certain controlling factors. For the formation of metal phosphates, for example, the factors which control soil solution HjPO* concentration will alsoHnh 3.5, 3.6 v 3.7 l nhng in hnh ha ca cc mi quan h gia cc cation kim loi Zn24, Pb2 +, Cd24 v gii php pH t cho mt s loi khong sn t c la chn. Cc log K gi tr s dng xy dng cc th arc c a ra trong bng 3.5 (ly t Lindsay, 1979). Cc iu kin khc c s dng trong xy dng th c a ra trong nhng cha kha mi th. Minh ha ca loi hnh ny c th c s dng ngh m t khong cht c kh nng kim sot hot ng ion kim loi dung dch t trn mt phm vi nht nh ca cc iu kin pH t v di nh hng ca cc yu t chi phi nht nh. i vi s hnh thnh ca cc dng photphat kim loi, v d, cc yu t kim sot dung dch t HjPO * tp trung cng sZinc control the formation of metal phosphates. At low pH, soil H2PO4 concentrations are controlled by iron phosphates, while at high pH, H2PO4 concentrations are controlled by calcium phosphates. The controls of soil iron and calcium phosphates on the stability of Zn, Pb and Cd phosphates and hence on metal ion activity in the soil solution, are illustrated in Figures 3.5, 3.6 and 3.7.The Zn hydroxides, oxides and carbonates are very soluble and will dissolve if added to the soil. In Figure 3.5, smithsonite (Zn carbonate) is the most soluble mineral illustrated. Willemite (Zn silicate) is of intermediate stability and franklinite (a Zn-ferric oxide) is very stable in soil and is probably the most important mineral to control Zn2t activity in soil (Sadiq, 1991). The solubility of Zn2 + in equilibrium with franklinite is controlled by Fe' + activity, which in turn is affected by the presence in soil of different iron minerals. In Figure 3.5 the solubility of franklinite is illustrated for five different forms of soil iron. The solubilities of all Z11 species illustrated in Figure 3.5 decrease 100-fold for every pH unit increase (Lindsay, 1979).In soil, Cd hydroxides, oxides, sulphates and silicates are all extremely soluble. In calculations of Cd stability in soil, Lindsay (1979) shows that CdCO3 (octavite) is the main Cd mineral to control Cd2+ activity in the soil solution (Figure 3.7). At elevated CO2 concentrations, octavite decreases Cd2< solubility 100-fold for every pH unit increase above pH 7.5. Cd phosphates may also influence Cd2t activity in calcareous soils. In the case of both lead and manganese, both pH and redox are important controls on ion solubility. In the range of pH experienced in soils, the most insoluble of the soil lead phosphate minerals, PbsfPC^bCl (chloropyromorphite) has the capability to control Pb2+ concentrations in solution kim sot s hnh thnh ca cc dng photphat kim loi. pH thp, H2PO4 t concen-trations c kim sot bi phosphat st, trong khi pH cao, nng H2PO4 c iu khin bi phosphat calcium. Cc iu khin ca st v canxi phosphat t vo s n nh ca Zn, Pb v Cd pht pht v do vo hot ng ion kim loi trong dung dch t, c minh ha trong hnh 3.5, 3.6 v 3.7.Cc Zn hydroxit, oxit v cacbonat rt d ho tan v s gii th nu c b sung vo t. Trong hnh 3.5, Smithsonit (Zn cacbonat) l khong ha tan nht minh ha. Willemite (Zn silicate) l s n nh ca trung gian v franklinite (mt oxit Zn-st) l rt n nh trong t v c l l khong cht quan trng nht kim sot hot ng Zn2t trong t (Sadiq, 1991). Kh nng ha tan ca Zn2 + trong trng thi cn bng vi franklinite c iu khin bi hot ng + Fe ', do b nh hng bi s hin din trong t ca cc khong cht st khc nhau. Trong hnh 3.5 tan ca franklinite c minh ha trong nm hnh thc khc nhau ca st t. Cc tnh tan ca tt c cc loi Z11 c minh ha trong hnh 3.5 gim 100 ln cho mi n v pH tng (Lindsay, 1979).Trong t, Cd hydroxit, oxit, sulphates v silicat u rt ha tan. Trong tnh ton ca Cd trong t n nh, Lindsay (1979) cho thy rng CdCO3 (octavite) l Cd khong sn chnh kim sot Cd2 + hot ng trong dung dch t (Hnh 3.7). nng CO2 cao, octavite gim Cd2 Zn > Cd > Sr > Pb Cu > Ni > Co > Pb > Cd > Zn > Mg > Sr Pb > Cd > Co > Cu > Ni > Zn > Sr > Mg Cu > Ni > Zn > Co > Mg > SrSource: Reproduced by permission of Springer-Verlag from McBride (1989).Table 3.10(b). Empirical heavy metal affinity series for soil componentsMaterialAffinity sequenceSourceAmorphous Al oxidesCu > Pb > Zn > Ni > Co > CdKinniburgh et al. (1976)Amorphous Al oxidesCu > Pb > Zn > CdLeckie et al. (1980)Amorphous Fe oxidesPb > Cu > Zn > Ni > Cd > CoKinniburgh et al. (1976)Amorphous Fe oxidesPb > Cu > Zn > CdLcckie et al. (1980)Ooetliite (FcOOH)Cu > Pb > Zn > CdForbes et al. (1974)Goelhitc (FcOOH)Cu > Pb > Zn > Co > Ni > MnMcKenzie (1980)HaematitePb > Cu > Zn > Co > Ni > MnMcKenzie (1980)Mn oxide (birnessite)Pb > Cu > Mn * Co > Zn > NiMcKenzie (1980)Mn oxidesCu > ZnMurray (1975)Mn-SiO;Pb > Cn > Zn > C'r > CdLeckie et al. (1980)Fulvic acid (pH 5)Cu > Pb > ZnSchnitzer and Skinner (1967)Humic acid (pH 4-7)Zn > Cu > PbVerloo and Cottcnic (1972)Humic acid (pH 4 6)Cu > Pb Cd > ZnStevenson (1977)

affinity for Cd1 than for 'Zn21. McBride concludes that transition metals classified as "hard" (sec Table 3.1) are bonded more strongly than soft transition metals. However, soft" non-transition metals (e.g. Pb2') are preferred over harder non-transition metals (e.g. Cd2 + , Mg2' ). Zn2 + displays intermediate behaviour.The kinetics of sorption processes arc not well understood but a little more evidence is available than for solubility reactions. Christensen (1984) found that Cd sorption from low Cd concentration conditions in soils, was extremely rapid, with 95% of the sorption occurring within the first 10 min of a sorption isotherm experiment. Other studies have indicated a two-step sorption process, with an initially rapid first step, representing adsorption onto highly accessible sites on the adsorbing surface, followed by a second, slower type of sorption, characteristic of modified surfaces, co-precipitation associated with Fe and Mn oxides, and solid state diffusion processes (Mattigod el al., 1981). These second stage sorption processes may occur over a period of days. Much further research is required to properly understand the sorption kinetics of different potentially toxic metals on different surfaces under different soil conditions.In addition to problems in understanding the rates of sorption reactions, other concurrent processes, which are closely associaled with metal adsorption, complicate the sorption story:(i)precipitation at the adsorbing surface(ii)formation of ternary complexes(iii)metal diffusion into the mineral surfacePrecipitationBoth adsorption and precipitation of metals may occur at mineral surfaces. These processes arc sometimes difficult to distinguish. Sposito (1986ft) has highlighted this problem for Mn and Fe oxides. Mn oxides, for example, promote the oxidation of Mn and Fe. The resulting close association of Mn with Fe and other heavy metals may be due to co-precipitation rather than adsorption at the oxide surface. Gleam and McBride (1986), working with rather unusual titanium oxides, showed (hat Cu and Mn adsorption and precipitation were pH-related. Al pH < 6, adsorption predominated while al pH > 6, precipitation predominated. A similar pattern was reported for Pb, with adsorption occurring below pH 6 and the precipitation of lead carbonates occurring al pH > 6 (Harter, 1979).Ternary complexesMetal sorption in soil can be complicated by the formation of stable

i lc vi Cd'1 hn cho 'Zn21. McBride kt lun rng cc kim loi chuyn tip c phn loi nh l "cng" (sec Bng 3.1) c gn b hn cc kim loi chuyn tip "mm". Tuy nhin, "mm" kim loi chuyn tip (v d nh Pb2 ') c a thch hn "kh" kim loi chuyn tip (v d nh Cd2 +, Mg2'). Zn2 + hin th hnh vi trung gian.ng hc ca qu trnh hp ph Arc cha c hiu r, nhng mt cht bng chng hn l c sn hn cho cc phn ng ha tan. Christensen (1984) thy rng Cd sorption t iu kin nng Cd trong t thp, l cc k nhanh chng, vi 95% ca hp ph xy ra trong vng 10 pht u tin ca mt th nghim hp ph ng nhit. Cc nghin cu khc ch ra mt qu trnh hp ph hai bc, vi mt bc ban u nhanh chng u tin, i din cho hp ph ln cc trang web truy cp cao trn b mt hp ph, theo sau l mt th hai, g chm hn ca hp ph, c trng ca cc b mt bin i, ng kt ta kt hp vi Fe v Mn oxit, v cc qu trnh khuch tn trng thi rn (Mattigod el al., 1981). Cc qu trnh ny giai on hp ph th hai c th xy ra trong mt khong thi ngy. Nghin cu nhiu hn na l cn thit hiu ng ng hc hp ph cc kim loi c hi c kh nng khc nhau trn cc b mt khc nhau trong iu kin t ai khc nhau.Ngoi vn trong vic tm hiu gi ca phn ng hp ph, ng thi qu trnh khc, c associaled cht ch vi hp ph kim loi, lm phc tp nhng cu chuyn hp ph:(I) lng ma ti cc b mt hp ph(Ii) to phc ternary(Iii) kim loi khuch tn vo b mt khongS kt taC hai hp ph v kt ta ca cc kim loi c th xy ra b mt khong cht. Cc qu trnh h quang i khi rt kh phn bit. Sposito (1986ft) nu bt vn ny cho Mn v Fe oxit. Oxit Mn, v d, thc y qu trnh oxy ha ca Mn v Fe. Kt qu l s lin kt cht ch ca Mn vi Fe v kim loi nng khc c th l do ng kt ta hn l hp ph trn b mt oxit. Gleam v McBride (1986), lm vic vi cc oxit titan kh bt thng, cho thy (m Cu v Mn hp ph v ma cng c pH lin quan. Al pH 6, lng ma chim u th. Mt m hnh tng t c bo co cho Pb, vi hp ph xy ra khi pH di 6 v kt ta cacbonat ch xy ra al pH> 6 (Harter, 1979).Phc bc baHp ph kim loi trong t c th phc tp bi s hnh thnh n nh 99surface-metal-ligand (ternary) complexes. Sposito (1983) identifies four categories of ligand effects:(a)the ligand has a high affinity for the metal, forms a stable complex and the complex has either(i)a high affinity, or(ii)a low affinity for the adsorbing surface(b)the ligand has a high affinity for the adsorbing surface and is adsorbed, then the adsorbed ligand has either(i)a high affinity, or(ii)a low affinity for the metal.Categories (a(i)) and (b(i)) above would result in enhanced adsorption of the metal while categories (a(ii and (b(ii)) would result in decreased metal adsorption. Both organic and inorganic ligands can enhance metal adsorption in this way. An organic ligand example of (a(i)) above is provided by glutamic acid which increases Cu2+ adsorption on amorphous iron oxide (Davis and Leckie, 1978). Not all organic ligands promote metal adsorption. Farrah and Pickering (1977) found that EDTA prevented the adsorption of Cd on clay minerals over a wide pH range. Haas and Horowitz (1986) found a similar depression of Cd adsorption on kaolinite in the presence of EDTA, but Cd sorption was slightly promoted in the presence of alginic acid and humic acid (see Figure 3.11(e)). Chairidchai and Ritchie (1990) have subsequently shown that Zn sorption in soil is depressed in the presence of several organic ligands found in the rhizosphere. This effect may increase availability of Zn to plants. Phosphate and sulphate complexion with metals at the surface of soil minerals is thought to be an important adsorption process for Cd on Fe and Mn oxides (Benjamin and Leckie, 1982), Zn on Fe and Al oxides (e.g. Bolland el al., 1977; Shuman, 1986) and Cu on allophane (Clark and McBride, 1985). The effect of different levels of soil extractable P on Zn sorption is illustrated in Figure 3.11(f). Since sulphate and phosphate tend to be present in soils in higher concentrations than trace metals, they have the potential, through ternary complex formation and precipitation, of controlling the availability of trace metals to plants.Melal diffusion al the mineral surfaceMetal sorportion reactions may not be restricted to the mineral surface, but metal diffusion into the mineral structure can occur. Gerth and Brummer (1983) reported the diffusion of Zn, Ni and Cd into goethite, with the rates of these three metals decreased in the order Ni < Zn < Cd, paralleling their ionic radii of 0.35, 0.37 and 0.49 A respectively. These authors suggest that melal adsorption is determined by three different steps: surface adsorption, diffusion into the mineral and fixation at positions within the mineral.

b mt kim loi-phi t (ternary) phc. Sposito (1983) xc nh bn loi tc ligand:(A) phi t c i lc cao vi cc kim loi, to thnh mt phc hp n nh v phc tp c th(I) c i lc cao, hoc(Ii) c i lc thp i vi cc b mt hp ph(B) cc ligand c i lc cao vi cc b mt hp ph v c hp ph, sau phi t hp ph c th(I) c i lc cao, hoc(Ii) c i lc thp i vi kim loi.Danh mc (a (i)) v (b (i)) trn s dn n tng cng hp ph ca cc kim loi trong khi danh mc (a (ii v (b (ii)) s cho kt qu gim hp ph kim loi. C hai phi t hu c v v c c th tng cng hp ph kim loi theo cch ny. Mt v d ligand hu c (a (i)) trn c cung cp bi axit glutamic lm tng Cu2 + hp ph trn oxit st v nh hnh (Davis v Leckie, 1978). Khng phi tt c cc ligand hu c thc y s hp ph kim loi Farrah. v Pickering (1977) thy rng EDTA ngn cn s hp th ca Cd v khong sn t st trn mt phm vi pH rng. Haas v Horowitz (1986) tm thy mt trm cm tng t ca Cd hp ph trn kaolinit trong s hin din ca EDTA, nhng Cd sorption hi pht huy trong s hin din ca axit alginic v axit humic (xem hnh 3.11 (e)). Chairidchai v Ritchie (1990) sau ch ra rng Zn hp ph trong t l b trm cm trong s hin din ca mt s ligand hu c trong vng r. Hiu ng ny c th lm tng tnh sn c ca Zn cho cy trng. Phosphate v da sulphate vi cc kim loi b mt ca cc khong cht t c cho l mt qu trnh hp ph quan trng i vi Cd trn Fe v Mn oxit (Benjamin v Leckie, 1982), Zn vo Fe v Al oxit (v d: Bolland el al., 1977; Shuman, 1986) v Cu vo allophane (Clark v McBride, 1985). nh hng ca cc mc khc nhau ca t P chit trn Zn hp ph c minh ha trong hnh 3.11 (f). K t sulphate v phosphate c xu hng c mt trong t nng cao hn so vi cc kim loi vi lng, h c tim nng, thng qua hnh phc tp bc ba v lng ma, kim sot s sn c ca cc kim loi vi lng cho cy trng.Melal khuch tn al b mt khongPhn ng kim loi sorportion c th khng b hn ch vi b mt khong cht, nhng kim loi khuch tn vo cu trc khong sn c th xy ra. Gerth v Brummer (1983) bo co s khuch tn ca Zn, Cd v Ni vo goethite, vi t l ca ba kim loi gim theo th t Ni more strongly retained by many soil colloids Ilian are Cd, 7.n and Ni. Cd and 7.n have greater cation exchange abilities on layer silicates, but the relative strength of their adsorption on soil oxides and organic fractions (Table 3.10(b)) is less. More than the other divalent metal ions, Cu2* is strongly adsorbed by organic matter and soil oxides. McLaren el til. (1981), measuring Cu adsorption on a range of soil clay minerals, oxides and organic mailer, found that On sorption on Fe and Mn oxides and humic acids could be as much as five to six times higher than on soil clay minerals, while sorption on fulvic acids was only around two times higher (Figure 3.12).Table 3.11 is an attempt to summarise the salient characteristics of soil-metal adsorption for C'd, (Ti, Zn and Pb. Comparing the soil sorption characteristics of these four metals, there are some clear similarities and some dear differences in behaviour. First, the effect of soil pH on metal adsorption is strong for Cd, Zn and Pb, but less so for Cu. The general retention pattern for most trace metals in relation to soil pi I is similar to that described by Yong ci til. (1990) I'm lead:(i)cation exchange processes at low pH(pl-l 2-4), with differences due tovalence and ionic si/T.(ii)formation of soluble hydroxy species at intermediate pll (pH 4-6) which are adsorbed on clay surfaces, and(iii)when pH exceeds that required for hydroxide formation (pH>5), precipitation processes dominate.Ca1 has been shown to be important in inhibiting divalent metal cation sorption for Cd2'. Zn2'1 and Pb2'1. Theoretically, this could reduce metal sorption in calcareous soils, but in practice, the higher pll of such soils elevates metal retention overall and Cd (and possibly other metals) has been shown to be specifically sorbed by calcite (Alloway el at.. 1988). The sorption of metals by organic matter is a particularly important process for Cu and for Pb. Cu, Pb and Zn form stable soluble organic chelates in water, and in the soil solution and in sludge-treated soils, Neal and Sposito (1986) found that soluble organic chelates of Cd reduced metal sorption. When soluble sludge organics were removed from soil by washing, Cd adsorption on soil mineral materials was increased.Summarising the above information would lead us to suggest that Cd and Zn might be more mobile in soils than Cu or Pb since these two ions tend to be sorbed by cation exchange, and competing ions in the soil solution (including H+, through pH effects) are likely to successfully replace these ions on exchange complexes. Cd and Zn migration and mobility through soil may tints be enhanced by soil acidification. Soils high in Fe, Mn and Al oxides and in organic matter, such as brown forest soils or podsols. could potentially adsorb all divalent metals, but preferentially retain Cu, Pb and, to a lesser extent, Zn. In soils to which organic sludges have been applied, Cu, Pb and Zn would be the most likely organo-metallic soluble chelates in leachateTable 3.11. Summary of soil adsorption of trace metalsMetalMajor adsorption characteristicsC'dFactors influencing:increased pH = increased Cd sorption**'*increased CEC = increased Cd sorption* (associated with layer silicates)' increased OM = increased Cd sorption* increased CaCOr = increased Cd sorption'Competing cations: Ca!', Co3*, Cr3*. Ni3*, Zn3*. Pb2'can inhibit Cd sorption*Organic Cd complexes:Cd-ltmuic acid complexes are less stable than Pb or Cu'1 CttFactors inflneucing:pH changesless effect on Cu sorption at low concentrations than other metal ions'--'organic matter and Fe/Mn oxides arc the most important controls on Cu sorption'clay minerals aitd CEC arc less important for Cu sorption'Competing cations: Ca3* much less effective at releasing Cu3' into solution than for Cd3*-'Organic Cu complexes:humic and fulvic acids bind Cu3* strongly*soluble Cuchelates ate important Cu species in the soil solution'ZnFactors influencing:increased pll = increased 7.n sorption' -'increased CEC = increased Zn sorption"'increased clay and soil organic matter = increased Zn sorptionCompeting cations: Ca3* inhibits Zn3* sorption'"Phosphate enhances Zn sorption on variable charge colloids (Fe/Mn oxides)""Organic Zn complexes:Soluble Znfulvales are important Zn species in the soil solution''PbFactors influencing:increased pll = increased Pb retention, but probably mainly by precipitation as lead carbonate at high pH*or as adsorption or (he hydrolysed species: PbOH* at intermediate pll*Fe, Mn and Al oxidesall have strong affinity for Pb*-' increased CEC - increased Pb sorption at intermediate pl-ls'Competing cations: Ca3* inhibits Pb sorption al intermediated pl-ls' Organic 1*1) complexes:Stionger association of Pb with organic matter at high pHsHumic and fulvicPb complexes more stable al high pH*Soluble Pb-chelates are important Pb species in the soil solution"Sources: "Christensen (1984), 'Eriksson (1989), Harter (1983). "Levi-Minzi cl til (1976). 'Zacliara el al. (1992), 'Alloway cl al. (1988), 'Christensen (1987), "Stevenson (1977), 'Mel aren cl al. (1981), 'Cavallaro and Melhide (1978), Senrsi ct al. (1989), 'Shuman (1985), "Kiekenv (1990), "nnllnml el al. (1977), "Gcrrilsc and van Oriel (1984), "Gccring and Hodgson 11969). "Griffin and Shiimp (1976), 'Harter (197*)), Kinniburgb cl al. (1976),.'McKenzie (1980), "Ciicgson and Alloway (1984), 'Schnilzcr and Skinner (1967)

gi li mnh m hn bng nhiu keo t Ilian l Cd, 7.n v Ni. C'd v 7.n c kh nng trao i cation ln hn trn lp silicat, nhng sc mnh tng i ca s hp ph ca h trn cc oxit t v phn hu c (Bng 3.10 (b)) l t. Nhiu hn cc ion ha tr hai kim loi khc, Cu2 * c hp th mnh bi vt cht v t oxit hu c. McLaren el til. (1981), o hp ph Cu vo mt lot cc khong cht t st t, oxit v bu phm hu c, pht hin ra rng Trn hp ph trn Fe v Mn oxit v axit humic c th c cng nhiu cng 5-6 ln cao hn v khong sn t st t, trong khi hp ph vo cc axit fulvic cao ch khong hai ln (Hnh 3.12).Bng 3.11 l mt n lc tm tt nhng c im ni bt ca t hp ph kim loi cho C'd, (Ti, Zn v Pb. So snh cc c tnh hp ph ca t bn kim loi, c mt s im tng ng r rng v mt s khc bit trong hnh vi thn yu. u tin , nh hng ca pH t qu trnh hp ph kim loi mnh m cho Cd, Zn v Pb, nhng km hn cho Cu. Cc m hnh lu chung ca hu ht cc kim loi vi lng trong mi quan h vi pi t ti l tng t nh m t bi Yong ci til. (1990 ) Ti dn:(I) qu trnh trao i cation pH thp (pl-l 2-4), vi s khc bit doha tr v ion si / T.(Ii) hnh thnh loi hydroxy ha tan ti PLL trung gian (pH 4-6) c hp ph trn b mt t st, v(Iii) khi pH vt qu yu cu cho s hnh thnh hydroxide (pH> 5), qu trnh kt ta chim u th.Ca'1 c chng minh l quan trng trong vic c ch kim loi ha tr hai cation hp ph cho Cd2 '. Zn2'1 v Pb2'1. V mt l thuyt, iu ny c th lm gim hp ph kim loi trong t vi, nhng trong thc t, vic pll cao ca t nh nng gi kim loi ni chung v Cd (v cc kim loi c th khc) c chng minh c sorbed c bit bi calcite (Alloway el ti .. 1988) . Vic hp ph kim loi bng cht hu c l mt qu trnh c bit quan trng i vi Cu v Pb. Cu, Pb v Zn thnh chelates hu c ha tan trong nc n nh, v trong dung dch t v trong t bn c x l, Neal v Sposito (1986) thy rng chelates hu c ha tan ca Cd gim hp ph kim loi. Khi bn hu c ha tan c g b t t bng cch ra, Cd hp ph trn vt liu khong sn t c tng ln.Summarising cc thng tin trn s dn chng ti cho thy rng Cd v Zn c th l in thoi di ng nhiu hn trong t hn Cu hoc Pb t hai ion ny c xu hng c sorbed bng cch trao i cation, v cc ion cnh tranh trong dung dch t (k c H +, thng qua hiu ng pH) c kh nng thay th thnh cng cc ion trn phc hp trao i. Cd v Zn di c v di ng thng qua cc sc thi mu t c th c tng cng bng axit ha t. t cao trong Fe, Mn v Al oxit v cht hu c, chng hn nh t rng nu hoc podsols. c kh nng hp th tt c cc kim loi ha tr II, nhng u tin gi li Cu, Pb, v mt mc thp hn, Zn. Trong t m bn hu c c p dng, Cu, Pb v Zn s l chelates ha tan hu c kim loi c kh nng nht trong nc r rcBng 3.11. Tm tt ca t hp ph kim loi du vtc im hp ph kim loi chnhCc yu t nh hng n C'd:pH tng = tng hp ph Cd * '*' *tng CEC = tng Cd sorption * (kt hp vi lp silicat) 'tng OM = tng hp ph Cd * tng CaCOr = tng C'd sorption'Cnh tranh cation: Ca ', CO3 *, Cr3 *. Ni3 *, Zn3 *. Pb2'-Cd c th c ch hp ph *Phc Cd hu c:Phc axit Cd-ltmuic t n nh hn so vi yu t Pb hoc Cu'1 CTT inflneucing:thay i t hn pH nh hng n Cu sorption nng thp hn so vi cc ion kim loi khc '-'cht hu c v Fe / Mn oxit arc cc iu khin quan trng nht trn Cu sorption 'khong st aitd arc CEC t quan trng i vi Cu sorption 'Cnh tranh cation: CA3 * t hiu qu hn qua vic pht hnh Cu3 'vo gii php hn cho CD3 * -'Hu c phc Cu:axit humic v fulvic bind Cu3 * mnh m *ha tan Cu-chelates n cc loi quan trng Cu trong dung dch t 'Zn yu t nh hng:tng PLL = tng 7.n sorption '-'tng CEC = tng Zn sorption "'tng st v cht hu c trong t = Zn tng hp phCnh tranh cation: CA3 * c ch * Zn3 sorption '"Phosphate tng cng Zn hp ph trn keo ph bin i (Fe / Mn oxit) ""Phc Zn hu c:Ha tan Zn-fulvales l loi quan trng Zn trong dung dch t ''Cc yu t nh hng n Pb:tng PLL = tng lu Pb, nhng c l ch yu l do lng ma nh ch cacbonat pH cao *hoc l hp ph hoc (ng thy phn loi: PbOH * PLL trung gian *Fe, Mn v Al oxit-tt c u c i lc mnh m cho Pb * - 'tng CEC - Pb tng hp ph ti trung pl-ls'Cnh tranh cation: CA3 * c ch hu c Pb sorption al trung gian pl-ls '1 * 1) phc:Stionger hip hi ca Pb vi cc cht hu c pH cao "Humic v fulvic-Pb phc al n nh hn pH cao *Tan Pb-chelates l loi quan trng Pb trong dung dch t "Ngun: ". Christensen (1984), '' Eriksson (1989)," Harter (1983) "Levi-Minzi cl til (1976). 'Zacliara el al. (1992), 'Alloway cl al. (1988), 'Christensen (1987), "Stevenson (1977)," Mel aren cl al. (1981),' Cavallaro v Melhide (1978), 'Senrsi ct al. (1989),' Shuman (1985), ' "Kiekenv (1990)," nnllnml el al. (1977), "Gcrrilsc v van Oriel (1984)," Gccring v Hodgson 11.969). "Griffin v Shiimp (1976)," Harter (197 *)), 'Kinniburgb cl al. (1976) ,. 'McKenzie (1980), "Ciicgson v Alloway (1984),' Schnilzcr v Skinner (1967) waters. The desorption processes associated with Cd, Pb, Zn and Cu, and their controlling factors, have been less studied than sorption processes. Information on metal sorption was sought to allow estimations of acceptable metal loadings to soils. Indications that metal desorption patterns arc different from sorption patterns confirms that implying desorption patterns and quantities from studies of metal adsorption would provide an inadequate estimation. Experimentation designed to produce information specifically on metal desorption is now required for predicting soil outputs and subsequent likely pollution inputs to adjacent watercourses.3.3.4 ORGANIC COMPLEXATION AND CHELATIONOrganic complexation of metals in soils and waters is thought to be one of the most important factors governing solubility and bioavailability of metals in soil-plant systems. It is important to differentiate between naturally occurring organic compounds in soil and those compounds derived from mans activities. Senesi (1992) divides into three main classes the organic compounds in soil which could form metal complexes:(i)naturally occurring soil organic molecules of known structure and chemical properties, including aliphatic acids, polysaccharides, amino acids, polyphenols;(ii)anthropogenically derived organic chemicals from agriculture, industrial and urban activities;(iii)humic and fulvic acid substances which accumulate in soil but whose structures are unknown in detail.By far the largest source of organic matter in soils is the decomposition of plant residues. Soil organic matter (SOM) is an incredibly complex mixture of different organic compounds whose detailed composition is never completely known, partly because of the difficulty in determining the exact molecular configurations of lignin and its aromatic decay products, and partly because organic matter decomposition continually changes its composition. In general, the composition of SOM is dominated by large molecular weight humin and humic acid (HA) compounds and lower molecular weight fulvic acids (FA). Schnitzer and Khan (1972) define humic acids as molecules having molecular weights of around 10 000-2000, and fulvic acids as having molecular weights around 2000-500. Organic compounds that are present in the smallest amounts are those which are broken down easilythe carbohydrates, such as cellulose, hemicelluloses and polysaccharides. Nitrogen-containing compounds, including proteins and amino acids, are also fairly easily decomposed. Plant- protecting compounds such as lignins, tannins and waxes, are broken down only with difficulty and thus remain in soil for longer periods. Apart from studies of simple organic ligands in plant root exudates, studies of organo- metallic complexation in soil have focused on the more persistent HA and FA fractions since these compounds potentially affect the longer term fate of metals in soil-plant systems.The first studies of metal-organic complexation in soils aimed to elucidate mechanisms of metal, particularly iron and aluminium, migration during podsolisation. The earliest studies implicated the role of polyphenols from tree foliar drip in chelating and mobilising soil Fe and Al (e.g. Bloomfield, 1957; Coulson el al., 1960; Malcolm and McCracken, 1968). A second school of thought, headed by Schitzer and his colleagues in the USA, emphasised the importance of humic and fulvic acids in Fe and Al mobility and translocation (e.g. Schnitzer and Desjardins, 1969). Subsequent investigators of toxic metals and their mobility in soils have analogised from these early studies. After a waning of interest from pedologists studying podsolisation, recently attention has swung back to the metal-chelating properties of fresh litter organics and of smaller molecular weight organic molecules. One of the reasons for this increased interest is the influences of acid rain and leaf litter acidification on metal mobility in soil. McColl and Pohlman (1986), for example, found that organic acids from Pinus ponderosa litter leached Al and other metals faster than mineral acids at comparable pH and concentration. They identified oxalic, malic, citric, protocatechuic and salicylic acids as chelating organic acids. In terms of binding and transporting capacity of potentially toxic metals, it is now generally agreed that fulvic acids are the most important organic fraction in soils. Humic acids do have important metal-binding capacity, but their molecular size and configuration means that they are generally less mobile in the soil pore-space and less likely to be leached down the soil profile. Humic acids are thus regarded more as the metal-immobilising fraction of soil organic matter.It would be wrong to underestimate the importance of organo-metallic complexation in maintaining in solution metal ions which under the more normal pH ranges of soils (around neutrality and in slightly neutral conditions) would normally be converted into insoluble compounds. Enhanced solubility of potentially toxic metals through organic chelation can cause environmental problems in, for example, leachates from disposal sites, or from sewage sludge applications. Soluble organic chelates provide the mobile, transport phase for metal migration, particularly contributing to the contamination of surface and groundwaters. Where organo-metallic complexes are resistant to biodegradation, their persistence in the environment makes them extremely important in the transport of metals. Francis el al. (1992) confirmed this suggestion even for very small molecular weight citrate-metal complexes which they found to be resistant to bacterial decay.As well as increasing metal mobility through enhanced solubility, metal chelates may also affect the bioavailability of toxic metals to plants. Piccolo (1989), for example, found that the addition of humic acid to both organic and mineral soils acted to immobilise soluble and exchangeable forms of potentially toxic metals. The effect was more pronounced in mineral soils I 10which had lower organic mater contents at the outset. To quantify amounts of bioavailable metals after HA treatment. Piccolo (1989) determined extract- able metals in DTIA (diethylenctriamincpcnlnacclic acid) extracts. The effectiveness of added HA in decreasing metal extractabilily followed the order: Pb > Cu > Cd > Ni > Zn. This is more or less the same order as the stability constants of humic substances: Cu > Pb Cd (Takamatsu and Yoshida. 1978). Since added organic substances influence both metal mobility and bioavailability, and the effects may be different, depending on the original amounts and fractions of soil organic matter, the additions of sewage sludges and other organic slurries and leachates to soils should be planned with great care and with suitable attention to in situ soil conditions.Soluble metal chelates can also reduce the concentrations of toxic metal ions in solution. Much attention has been focused on organo-Al complexes in soil since these arc likely to limit transport of toxic Al'* to groundwaters and adjacent watercourses, Bloom el at. (1979) has shown that Ihe chelation of Al1* by humic acids in acid soils controls the soil solution concentration of toxic Al'1. Similar results have been reported by Hue el al. (1986) who showed that Al'1 chelation with short chain carboxylic acids, such as citric, oxalic and tartaric acids, is an important Al detoxifying process in acid subsoils where deep roots would otherwise suffer Al1* toxicity. The complexing of Al1* with I IA in acid topsoils has even been shown to reduce the need for liming (Hoyt, 1977).As with inorganic ligands the classification of metals according to Pearson (1968 ) and Nicboer and Richardson (1980; see Table 3.1) indicates their organic ligand complexing tendencies. Seticsi (1992) has tabulated the preferred electrostatic interactions of class A, class B and intermediate metals (Table 3.12). Prom this (able it can be predicted that most potentially toxic metals, classified as borderline metals by Nieboer and Richardson, will have a stronger tendency to form organic complexes with N-containing amide and amino ligands: MnJ Cu> Cd> Ni> Zn. y l nhiu hn hoc t hn theo th t nh cc hng s n nh ca cht humic: Cu> Pb Cd (Takamatsu v Yoshida 1978.). K t khi cc cht hu c thm nh hng c tnh di ng bng kim loi v sinh kh dng, v cc hiu ng c th khc nhau, ty thuc vo s tin gc v cc phn phn on ca vt cht hu c trong t, nhng b sung ca cn nc thi v bn hu c khc v leachates cho t cn c quy hoch cn thn v vi s quan tm ph hp vi iu kin t situ.Chelates kim loi ha tan cng c th lm gim nng ca cc ion kim loi c hi trong dung dch. Nhiu s ch tp trung vo cc phc organo-Al trong t t cc h quang c kh nng hn ch vn chuyn cc cht c hi Al '* groundwaters v knh rch ln cn, Bloom el ti. (1979) ch ra rng IHE thi ca Al1 * bi axit humic trong t axit kim sot nng dung dch t ca c Al'1. Kt qu tng t cng c bo co bi Hue el al. (1986), ngi ch ra rng Al'1 thi vi axit cacboxylic chui ngn, chng hn nh citric, oxalic v axit tartaric, l mt qu trnh gii c quan trng Al trong axit subsoils ni r su nu khng s b Al1 * c tnh. Cc phc ca Al1 * I IA trong topsoils axit thm ch c chng minh l lm gim s cn thit phi bn vi (Hoyt, 1977).Nh vi cc ligand phn v c ca kim loi theo Pearson (1968 ) v Nicboer v Richardson (1980; xem Bng 3.1) cho thy xu hng phi t to phc hu c ca h. Seticsi (1992) lp bng tng tc tnh in a thch ca lp A, lp B v kim loi trung gian (Bng 3.12). Prom ny (c th n c th c d on rng hu ht cc kim loi c hi c kh nng phn loi l kim loi ng bin gii ca Nieboer v Richardson, s c mt xu hng mnh m hn to ra cc phc hu c vi N-cha amide v amino ligand: MnJ Cu2*> Zn24 > MnJ+> Ca2+> Mg24This sequence is very similiar to that generated by Tipping and Hurley (1992) who modelled metal binding on humic substances. Their affinity sequence of metals for undefined humic substances followed the order:Cu2+> Pb24> Zn2' = Ni24 > Co24 > Cd24 > Mn2f> Ca2' > Mg2 +These patterns of metal affinity agree very well with that reported in an empirical study of soil fulvic acid with an ionic strength of 0.1 and pH 3 (Schnitzer and Harmsen, 1970):Fe34 > Al34 > Cu24 > Ni24 > Co24> Pb24 = Ca24 > Mn24 > Mg24and agrees well with the theory that class B and borderline metals should show stronger binding affinities with soil organic ligands than should class A metals.3.3.4.2Spectrophotometric techniques for studying metal binding on organic matterA large number of spectroscopy techniques have been valuable in elucidating how metal complexation occurs in HA and FA fractions. Most important of these spectroscopic techniques are infrared (IR), electron spin resonance (ESR) and nuclear magnetic resonance (NMR). While the techniques will not be discussed here, some of the major results will be used to illustrate current understanding of complexation processes. It is valuable to review the mechanisms of metal binding to soil organic fractions since these provide information on the activities of functional groups, and on the exchangeability and potential mobility of metals. A comprehensive review of spectrophoto- metric and other techniques for assessing metal bonding in HA and FA from different environmental origins is given by Senesi (1992).IR spectroscopy has shown how important COOH groups on HA are in metal complexing. IR results confirmed the earlier work of Schnitzer and Skinner (1965) who chemically blocked the acid carboxyl (COOH) and the phenolic hydroxyl (OH) groups on soil fulvic acid and showed that both ligand types were important sites of metal binding during chelation. Piccolo and Stevenson (1982) found that the quantity of metal ion available for complexing determined the type of binding which occurred on the dissociated COO" of soil FA and HA fractions. Al low levels of Cu24, covalent bonds are preferentially formed, while at higher metal concentrations, bonding becomes increasingly ionic. Perhaps the biggest insights into these mechanisms of metal and HA ligand binding has come from studies using ESR spectrophotometry. These techniques have indicated how two levels of metal binding are possible:(i)inner sphere complexes in which metals form bonds of covalent character with HA ligands, and both are completely or partially dehydrated; and(ii)outer sphere complexes in which metals are electrostatically attracted to HA functional ligands, and the metal ion remains hydrated.Very simple examples of these two types of complexation are illustrated in Figure 3.14. Inner sphere complexes are properly called chelates since functional groups donate electron pairs to the metal ion. In outer sphere complexation, the ligand does not interact directly with the electrons of the metal ion. Authors have implied from these two types of complexation that metals bound to outer sphere complexes may be more readily available for exchange, and for plant and microbial uptake.Lakatos el at. (1977) has added more detail to understanding of inner and outer sphere binding mechanisms using ESR spectrophotometry. These authors classified HA ligands into three categories, depending on the predominant type of complexation that occurred in experiments with Mnu, V02+ and Cu2 + . Weak complexing groups, including sulphonic, phosphoric, carboxylic and aliphatic groups, formed weak outer sphere coordinations. Strong complexing ligands, including iminodiacetate acetylacetonate, peptide and porphyrin groups, formed inner sphere complexes. Lakatos el at. (1977) suggested that carboxyl groups are a third category, which are borderline in their completing abilities, since they tended to form inner sphere complexesmi quan h ca cc ion kim loi trong axit fulvic nh mt ton th c tm thy gim theo th t:Fe, 4> Cu2 *> Zn24> MnJ +> Ca2 +> Mg24Trnh t ny rt ging vi to bi Tipping v Hurley (1992), ngi mu kim loi rng buc v cht humic. T i lc ca kim loi dng cho khng xc nh "cht humic" theo th t:Cu2 +> Pb24> Zn2 '= Ni24> Co24> Cd24> Mn2f> Ca2'> Mg2 +Nhng m hnh ca kim loi mi quan h ph hp rt tt vi bo co trong mt nghin cu thc nghim ca t axit fulvic vi mt sc mnh ion l 0,1 v pH 3 (Schnitzer v Harmsen, 1970):Fe34> Al34> Cu24> Ni24> Co24> Pb24 = Ca24> Mn24> Mg24v cng ng vi l thuyt rng lp B v kim loi ng bin gii nn thy i lc gn kt cht ch hn vi cc ligand hu c t hn so vi kim loi nn lp Mt.3.3.4.2 k thut quang ph nghin cu kim loi rng buc v cht hu cMt s lng ln cc k thut quang ph c gi tr trong vic lm sng t cch to phc kim loi xy ra HA v FA phn s. Quan trng nht ca nhng k thut quang ph l (IR) hng ngoi, spin electron cng hng (ESR) v cng hng t ht nhn (NMR). Trong khi k thut ny s khng c tho lun y, mt s kt qu ch yu s c s dng minh ha cho s hiu bit hin ti ca cc qu trnh phc. N c gi tr xem xt cc c ch rng buc phn hu c trong t t nhng cung cp thng tin v hot ng ca cc nhm chc nng kim loi, v trn exchangeability v di ng tim nng ca cc kim loi. Mt nh gi ton din v spectrophoto- metric v cc k thut nh gi lin kt kim loi trong HA v FA t ngun gc mi trng khc nhau c a ra bi Senesi (1992).IR quang ph cho thy cch cc nhm COOH quan trng v HA ang to phc kim loi. Kt qu IR xc nhn vic trc y ca Schnitzer v Skinner (1965), ngi ha chn carboxyl axit (COOH) v nhm hydroxyl phenol (OH) trn t axit fulvic v cho thy rng c hai loi phi t l cc a im quan trng ca lin kt trong qu trnh thi kim loi. Piccolo v Stevenson (1982) thy rng s lng cc ion kim loi c sn cho phc xc nh cc loi rng buc m xy ra trn COO phn ly "ca t FA v HA phn s. Al mc thp Cu24, lin kt ha tr c hnh thnh preferentially, trong khi nng kim loi cao hn, lin kt ngy cng tr nn ion. C l nhng hiu bit ln nht vo cc c ch ca kim loi v HA ligand rng buc n t cc nghin cu s dng quang ph ESR.Nhng k thut ny ch ra lm th no hai "cp" ca kim loi lin kt c th l:(I) hp bn trong qu cu trong kim loi hnh thnh lin kt cng ha tr vi cc nhn vt HA ligand, v c hai u hon ton hoc mt phn mt nc; v(Ii) hp lp v ngoi bng kim loi