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Adsorption of Glyphosate by Humic Substances

A. Piccolo,* G. Celano, a n d P . C on t e

Dipar timento di Scienze Ch imico-Agra rie, U niversit a di Na poli F ederico II , Via U niversit a 100,80055 P ortici, I ta ly

Four humic substa nces were extra cted from a peat (HA1) and a volcan ic soil (HA2), a n oxidized

coa l (HA3), an d a lignite (HA4). The four humic ma teria ls presented distinct d ifferences in theirch e mica l a n d p h y sica l-ch e mica l ch a ra ct e ris t ics a s a s s e s se d by ch e mica l me t h ods , 13C-NMRspectroscopy, and high-performa nce size exclusion chroma togra phy. The intera ctions betw een th esewell-chara cterized an d purified hum ic substa nces a nd t he w idely used glyphosate [N-(phosphono-methy l)glycine] herbicide were st udied by mea ns of adsorption isotherms. Adsorption of glyphosa tewa s found to be surprisingly high a nd followed the order HA1 > H A2 > H A3g HA4. Humic extractsfrom soil adsorbed glyphosate even more tha n clay minera ls, thereby indicat ing tha t th e interactionswith humic substances, in either a solid or dissolved form, ar e far more importa nt tha n previouslybelieved. Adsorption is explained by th e multiple hydrogen bondings w hich ca n occur am ong theva rious acidic and oxygen-conta ining groups of both m olecules. How ever, the order of ad sorptiondid not simply follow the order of acidity but rather that of increasing aliphaticity and molecularsize of humic substa nces. In fa ct, the least a dsorbing humic ma teria ls from nonsoil sources showeda high content of ar omatic structures and sma ll molecular dimensions. These results reveal t ha tthe extent of glyphosate adsorption on humic substances varies considerably with their macro-

molecular structur e and dim ension a nd th a t is favored by a h igh degree of stereochemica l flexibilitycombined with a large molecular size.

Keywords: Adsorpti on; glyphosate; hu mi c substances

INTRODUCTION

Glyphosate,N-(phosphonometh yl)glycine, is t he moste xt e n s iv ely u s ed h e rbicide in a g ricu l t u re a n d ot h e rf ie lds f o r t h e c o n t ro l o f ma n y a n n u a l a n d p e re n n ia lweeds. One can find a sufficiently large litera ture onthe chemistry of glyphosate, its mode of action, and itseffects on pla nt meta bolism (Ma lik et a l., 1989), wh ereasrelatively less information is a vailable on the glyphosat e

beha vior in soils (Torsten sson et al., 1985). G lyphosa teis reported to be rapidly inact ivated in soil (Sprankleet al. , 1975a ). How ever, its inactiva tion appears not tobe permanent since residual activity was found capableof injuring some plant species (Salazar and Appleby,1982). B owmer (1982) indicat ed tha t the a mount ofg ly ph os a t e lef t in s olu t ion in irr ig a t io n w a t e rs , a f t e radsorption by deep silty sediments, was not insignifi-c a n t . G ly p h os a t e bin din g t o s oi l wa s a t t r ibut e d t o t h ephosphonic acid moiety rea cting w ith polyvalent cationsadsorbed on clay an d, generally, on soil organ ic matt er(Spra nkle et a l., 1975b; Ha nce, 1976). Other w orks havealso related glyphosate adsorption to the clay contenta n d t h e ca t io n e x ch a n g e ca p a c i t y o f t h e s oi ls (G la s s ,1987).

Detailed studies on the role of soil components onglyphosat e a dsorption were limited t o cat ion-sat ura tedclay minerals, and no investigat ions on the adsorbingc a pa c i t y of h u mic s u bs t a n c es h a v e bee n p rev iou s lyreported in the literature, despite recognition that soilhumic molecules adsorb most pesticides in soils (Webera nd Miller, 1989). Furt hermore, wa ter-soluble humicsubstan ces ar e regarded as important carriers of orga nic

con t a min a n t s in t h e s oi l e n v iron me n t (Ch iou e t a l . ,1986). In fact , adsorption of xenobiotics to dissolvedhumic materia l may serve as a mecha nism for enhan cedmobility, even in homogeneous isotropic porous m edia(M cC a r t h y a n d Za c h a r a , 1989). B i n d in g t o h u m icsubstances ha s a ddit ional environmental implicat ions,including effects on biodegradation, volatilization, hy-drolysis, photolysis, an d bioaccumula tion of polluta nts.

Recent spectroscopic investigat ions on the glypho-s a t e-humic substances interact ions have led Piccoloan d Cela no (1994) to demonstra te tha t glyphosate bindsto humic substances through a hydrogen-bonding mech-anism, and thus the herbicide may be adsorbed on ortra nsported by humic substan ces. Moreover, glyphosat ew a s f ou n d t o b e a d s or bed on a n i ron-h u mic a c idc o mp le x t o a la rg e r e x t e n t t h a n t h a t re p o rt e d in t h elitera ture for clay s a nd soils (P iccolo et a l. , 1995). Thisdegree of adsorption wa s a ttr ibuted to a ligand exchan gemechanism whereby one or more hydroxyls of the ironhydrat ion sphere can be exchanged by the glyphosatephosphono group.

The aim of this work wa s to investigate the intera c-t ions of glyphosat e with hum ic substances. In par t icu-

lar , the extent of adsorption of the herbicide on purifiedhumic substances extracted from different sources wasassessed, an d t he effect of the hum ic substances chemi-cal properties on glyphosate adsorption was examined.

MATERIALS AND METHODS

Humic Substances. H um i c sub st anc es w e r e e xt r act e dfrom th e following: (1) a v olca nic soil (Typic Dyst ra ndept) fromthe lake of Vico, near Rome; (2) a peat soil (Dystric Histosol)near Lucca; (3) an oxidized coal (Eniricerche, SpA); (4) a NorthDa kota Leonardite (Mamm oth Chem. Co.). The extra ction wa scarried out by sha king the origina l mat erials overnight in 0.5N N a O H u n d e r a N 2 a t m osphe r e b y st and ar d pr oc e d ur e s(Stevenson, 1994). The hum ic acids (HAs) were precipitat ed

Dipa rtiment o di S cienze C himico-Agra rie. ResearchContribution 131.

Pr esent a ddress: Dipart imento di Produzione Veg-etale, Universit a della Ba silicata , Via Naza rio Sauro,P o t e n za , I t a ly .

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fr om t he al kal i ne ext r ac t s b y l ow e r ing t he pH t o 1 w i t h 6 NH C l . The H As w e r e t he n pur i f ie d b y a d oub le ac i d-b asedissolution-precipita tion procedure, a 0.5% (v/v) H Cl-H Ftrea tment for 36 h, and a dialysis in Spectra pore 3 tubes (3500MW cutoff) against disti l led w at er unt i l chloride-free. Thehumic samples were t hen freeze-dried an d st ored in a desic-cator. The four humic extracts (HA1, HA2, HA3, a nd H A4,respectively) were analyzed for their elemental content usinga Perkin-Elmer 40C microanalyzer and for total , carboxylic,and phenolic acidities and ash content by standard methods(Stevenson, 1994).

Molecular Weight Distribution. Molecular weight dis-

t r ib u t io n of h u m ic m a t e r ia l s w a s r ecor d ed w i t h a h i gh -performance size exclusion chromatography (HPSEC) system.The system consisted of a Perkin-Elmer LC-200 pump and aP erkin-Elmer LC-295 UV-visible detector, equipped with tw ocolumns, TSK G 3000SW (600 mm) a nd TSK G 4000SW (300mm), in series, preceded by a TSK Gua rd-Column, an d by a0.2 m inlet filter. Columns ca libration wa s obta ined by usingglobular proteins of known molecular weight (Pha rma cia kit ,20-500 kDa). Elution wa s performed with 0.06 N NaNO 3a tpH 7 containing 0.3 g L-1 Na N3. Freeze-dried humic ma terialsw e r e f i r st d i ssolved i nt o t he NaNO 3 e lut i ng sol ut ion a t aconcentra tion of 0.5 gL-1, passed through a 0.2m fil ter, andi nje ct e d i n t o t h e H P S E C s y s t em t h r ou g h a 1 00 L loop.Ca lculation of the weight-averaged (Mw), a nd n umber-avera ged(Mn ) molecular weights, and polydispersity (Mw/Mn ) for t hehumic sa mples were done by t he P erkin-Elmer-Nelson Turbo-

chrom 4-SEC software, using the method described by Yau eta l. (1979).

NMR Spectra. Q u a n t it a t i v e 13C NM R spe c t r a w e r e ob -tained by suspending 100 mg of each HA in 1 mL of 0.5 MNaOH for 1 day at room temperature under N 2. The suspen-sion w as then fi l tered through a cid-wa shed glass w ool, whichw a s t h e n w a s h e d w i t h 1 m L o f D 2O (for a deuterium NMRlock signal). The fi l trat es were combined into a final volumeof appr oxim at e l y 2 m L . S olut i on 13C N M R s p ect r a w e r erecorded at 75.4 MHz on a Varian XL 300 spectrometer usingconditions suitable to obtain quanti tative intensity distribu-tions. Br iefly, these were by using inverse-gat ed decoupling,a 45 pulse, an acquisition time of 0.1 s, and a relaxation delay(decoupler off) of 1.9 s. The num ber of scans ran ged from80 000 to 100 000. The free induction deca ys w ere processedb y a ppl y ing fr om 20 t o 50 H z l ine b r oad e ning and b ase li ne

correction as a l lowed by the ava ilable Varian softwa re. Theche m ic al shi f t i s e xpr essed i n ppm on a scal e r e l at i ve t oexterna l sodium 3-(methylsilyl)propiona te (TSP ) at 0 ppm. Thespect r a w e r e d i vid e d i nt o t he foll ow i ng a r e as: 0-110 ppm,alipha tic C and subst itute C (amino acids, car bohydrat es, etc.);110-160 ppm, aromatic and phenolic C; 160-190 ppm, car -b oxy l C . Ar e as w e r e m e asur ed b y a n a ut om at i c int e g r at or .

GlyphosateAdsorption on Humic Substances. Samples(50 mg) of each of the four HAs were sha ken in a plastic screw-cap vials with 5 mL of increasing concentrations of glyphosate(25, 50, 75, 100, 200, 250 mg L-1) in a 0.01 M solution of CaC l2for 24 h. A shaking t ime of 24 h wa s previously assessed tobe sufficient to reach t he a dsorption thermodynamic equil ib-rium (Piccolo et a l . , 1995). At the end of sha king t ime, andfor both series of a dsorption, a fter centrifugation, the super-natant was removed and purified as follows, and its glyphosate

content w as determined. Glyphosate a dsorption experimentsw e r e m a d e i n d u pl ica t e a n d a t r oo m t e m pe ra t u r e . Th eglyphosat e-free a cid (MW) 169.08 and 99.8%of purity), usedin this study w as kindly supplied by Monsanto, I ta ly. The pHof the conta ct solution wa s tha t of glyphosat e-free acid in Ca Cl2(pH 3) and did not change appreciably after the shakings.

Purification of Glyphosate Extracts. To elimina te thec at i ons and d i ssol ve d or g ani c m at t e r t hat m ay have passe dinto the supernat ant after the sha king, and t hat interfere witht he sub se q ue nt H P L C d e t e r m i nat i on of g l y phosat e (G l ass,1984), the supernata nt w as furt her purified. A glass columnof 20 cm length and 1 cm diameter, containing glass wool atthe lower t ip, was packed first with a layer of 0.8 g of Dowex50 4-400 cation exchange resin a nd th en wit h a second la yerof 0.3 g of XAD-2 resin (Serva , resear ch gra de). This pa ckedcolumn, with a void volume of 1 mL, was washed with 0.5 N

NaOH, eluted to neutrali ty with disti l led water, washed with0.5 N HCl, eluted to neutrality with distilled water, and finallyeluted to a chloride-free point with Milli-Q grade purifiedwa ter. A 1 mL aliquot of the supernata nt solution conta iningg ly ph os a t e w a s t h en i n se rt e d i n t o t h e col um n , u n d er N2pressure (1.5 atm), only unti l disappearance into the resinlayer. A 5 mL a liquot of Mill i-Q grade purified wa ter w erethen pa ssed, under pressure, through th e column, and th e first2.5 m L t hat e lut e d out of t he c ol um n w a s c ol le ct e d . Thi spurified fraction, well shaken, was used for the HPLC deter-minat ion of glyphosat e. The use of known concentrat ions ofglyphosat e through t he described purifica tion column ensur edtha t the loss of herbicide wa s consistently less t ha n 1%.

Glyphosate Determination. Glyphosate wa s determinedby a HP LC appar at us (Perkin-Elmer, 400 series) employing a100 m L l oop, a UV d e t e c t or a t 190 nm , an ani on e xc hang ecolumn (P ar tisil 10 SAX, 25 cm), a nd a P erkin-Elmer-Nelson,Model 2100, integrat or equipment and softwa re. The eluentw a s a 0 .0 8 M p ot a s s iu m p h o sp h a t e b u ff er a t p H 2. 1 a n dpum pe d at an i soc r at i c f l ow r at e of 1. 6 m L mi n-1 giving ar e t ent i on t i m e of 2. 72 m i n for t he g l y phosat e peak. Thi sm et h od w a s a d a p t e d f r om t h a t r ep or t ed b y ot h e r a u t h or s(Lawrence and Leduc, 1978) and had a detection l imit of 5ppm.

RESULTS

Humic Substance Properties. All h u mic s u b-sta nces showed an ash content less tha n 5%. Table 1also revealed that the elemental a nalyses for the humicma t e ria l o r ig in a t in g f ro m s o i ls ( H A 1 a n d H A 2) h a dv a lu es s imila r t o ot h e r s oi l h u mic a cids p rev iou s lyreported (Schnitzer, 1978), whereas humic substances

from oxidized coal and lignite (HA3 and HA4, respec-t iv e ly ) s h o we d h ig h e r C a n d H c o n t e n t bu t lo we r Ncontent.

Total acidity, measured by wet chemistry methods,increased in th e order H A1 < H A3 < H A4 < HA2. Thesame order was observed for the values found for thecarboxyl groups, wh ereas the order of phenolic groupcontent, obtained by difference between total acidity andcarboxyl acidity, varied consistently (Table 1).

Th e l iq u id-s t a t e N M R s pe ct ra of t h e f ou r h u micextracts revealed significant structural differences asillustra ted in Figure 1. The quant itat ive evaluation ofthe car bon distribution in t he NMR spectra (Table 2)showed that the content of aliphatic C w as in the orderH A1 ) H A2 > H A4 > HA3. The a romatic C contentfollowed a reverse order, with the maximum amount of72% bein g t h a t of h u mic ma t t e r e xt ra c t e d f ro m t h eoxidized coa l (HA3). The content of ca rboxyl C reflectedthe order found by th e wet chemistr y meth od except forthe value of humic substa nces from lignite (HA4) tha ta p p ea re d h ig h er t h a n t h e on e re port e d in Ta ble 1.Inconsistencies in carboxyl content determination be-tween NMR spectroscopy and traditional methods werepreviously indicat ed (Piccolo et a l. , 1990)

T h e H P SE C c h ro ma t o g ra ms o bt a in e d f o r t h e f o u rhumic extracts a re shown in Figure 2. The chromato-g ra ms re ve a led t h a t bot h h u mic ma t e ria ls f ro m s o ils(HA1, HA2) conta ined a high m olecular weight fra ctione lu t in g a t a ro u n d t h e v o id v o lu me , a s is s h o wn by a

Table 1. Elemental, Ash, and Acid Functional GroupContent of Humic Substances

acidities (mequivg-1)

sa mple %C %H %N % a sh t ot a l C OOH P hOH

H A1 51.6 2.4 4.1 4.2 6.1 2.7 3.4H A2 50.5 2.7 3.7 1.7 10.7 4.7 6.0H A3 62.9 3.0 1.9 2.7 9.0 3.4 5.6H A4 64.2 3.5 1.5 1.5 9.3 4.4 4.9

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noticea ble peak for H A1 and by a less resolved but st illvisible a bsorbing zone for HA2. Lower molecula r weightfract ions w ere present in t he humic mat erial extractedfrom the m ore humified ma teria ls of oxidized coa l (HA3)a n d l ig n i t e (H A4). I n p a rt icu la r , t h e H A4 ma t e ria lshowed a dist inct peak a t a higher retention t ime tha nfor HA3, thereby indicating that the former humic acidwa s composed of ma teria l of lower molecular dim ensiontha n the lat t er. When the weight-avera ged molecularweight , Mw, wa s ca lc ula t e d f rom t h e s ize e xclu sionchroma tograms using th e a vailable softwa re (Table 3),the order of molecular dimension of the four humicma teria ls wa s confirmed except for H A3, which showeda slightly lower Mw t h a n H A4. A si m il a r o r d er w a sfound for the number-avera ged molecular weight , Mn,with the humic mat erial from the peat soil showing t helar gest dimension. The polydispersit y, the M w/Mn ra t io,an index of the molecular size diversity present in ea chhumic extra ct, wa s higher for the soil humic substa ncestha n for the ma terial extra cted from both oxidized coaland lignite (Table 3), as for the order HA1 > H A2 >H A3 ) HA4.

Glyphosate Adsorption. The equilibrium adsorp-tion isotherms of glyphosat e on the four different humics u bs t a n c es a re s h own in F ig u re 3. H A1, from p ea t ,revealed t he highest ra te of ad sorption, reaching moret h a n 7500 g of g ly ph os a t e /g of a d s or b en t a t t h e

herbicide concentration of 250 gmL-1. The secondh i gh es t a d s or b in g h u m ic m a t e r ia l w a s H A2, f r omvolcan ic soil , followed in order by HA3 and HA4. Acomparison of the adsorbing capacity of the various HAsis reported in Table 4 for t he initia l glyphosat e concen-tration of 100 gmL-1. At higher glyphosate concen-t ra t ion s , t h e a ds orbin g ca p a c i t ie s o f H A3 a n d H A4tended to become similar (Figure 3).

The ad sorption dat a over the ra nge of concentra tionsstudied here were used to f it both the Freundlich andLan gmuir equa tions. For the Freundlich equation, Ce) K

fC

w

1/n

, wh e re C

e i s t h e a mo u n t o f g ly p h o s a t e a d-sorbed (gg-1 of adsorbent), Cw i s t h e e q u il ibr iumconcentra tion of glyphosat e (gmL-1), an d Kfa n d na reconstants that give estimates of the adsorptive capacityan d intensity, respectively. For the Langmuir equat ion,Ce ) k1k2Cw/1 + k1Cw, Ce a nd Cw h a v e t h e s a m emeaning as in the Freundlich equation, and k1 a n d k2are indexes of binding strength and adsorption maxi-mum, respectively. From the linearized forms of botht h e F re u n dlic h a n d L a n g mu ir e q u a t io n s ( W h it e a n dZelazny, 1992), the above constants, Kf, n, a n d k1, k2,respectively, can be calculated as well as the relat ivecorresponding determination coefficients (r2). Theirv a lu es f or t h e a ds o rpt ion of g ly ph os a t e on t h e f ou rhumic substa nces a re shown in Ta ble 5. The ads orption

Figure 1. 13C NMR spectra of humic substances

Table 2. Distribution of C Intensity in Different Regions(ppm) of13C NMR Spectra of Humic Substances

relat ive intensity (%of total a rea)

sample0-110

(aliphat ic C)110-160

(aromatic C)160-190

(carboxyl C)

H A1 52.8 40.7 6.5H A2 53.6 37.0 8.4H A3 18.0 72.0 10.0

H A4 29.8 55.7 14.5

Figure 2. High-performance size exclusion chromatogramsof humic substa nces.

Table 3. Weight-Average (Mw) and Number-Average (Mn)Molecular Weights and Polydispersity (Mw/Mn) of HumicSubstances

sample Mw Mn Mw/Mn

H A1 1.1 105 6.3 104 1.7

H A2 8.2 104

5.2 104

1.6H A3 5.6 104 4.0 104 1.4H A4 7.8 104 5.4 104 1.4

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consta nts for both F reundlich and Lan gmuir equat ionsfollowed the same order observed for the adsorptionisotherms (HA1 > H A2 > H A3 > HA4). The intensityof binding, t he n constant in the Freundlich equation,showed tha t both humic substa nces from soils and fromox idized coa l a ds o rbed g ly ph os a t e wit h e q u iv a le n tstrength. Moreover, such binding intensity wa s highert h a n t h a t s h ow n b y t h e l ig n it e h u m ic m a t t e r . C on -versely, t he k1 Langmuir values failed to show differ-ences in binding strengths of glyphosate on the varioushumic materials.

D I S C U S S I O N

The four humic substances used in this study pre-sented significant differences in their chemical andphysical-chemical characteristics (Tables 1-3). HA1and HA2, both extracted from soils, contained more than50%C in aliphatic, peptidic, and carbohydrat ic struc-t u re s , wh e re a s t h e o lde r a n d mo re h u mif ie d o rg a n icma t t e r of l ig n i t e a n d ox idize d coa l p rodu ce d h u mics u bs t a n ce s w i t h a m u ch h ig h er a r o ma t i c C . S u chstructural diversity is r esponsible for their differentacidic reactivity, molecular dimension, and consequentglyphosat e adsorption.

The amount of glyphosate adsorbed by humic sub-sta nces wa s found to be surprisingly high. The Freun-

dlich a dsorption consta nts, Kf, were significan tly highertha n t hose calculated for the glyphosat e a dsorption ont h re e s o ils of di ff ere n t t e xt u ra l p rop ert ies by G la s s(1987) and on four different European soils by Piccoloet al . (1994). Furthermore, the amount of glyphosatea ds o rbe d o n h u mic ma t e ria l w a s of t h e s a me o rder ofmagnitude indicated by Glass (1987) for adsorption onclay minerals and their cat ion-saturated forms, mont-morillonite and illite being stronger adsorbents thankaolinite. However, the Kfconsta nts reported by G lass(1987) for the three clay minerals were significantlylowe r t h a n t h o se f o u nd f or t h e s oi l h u mic ma t e ria ls ,H A1 a n d H A2. Th e Kf of cla y min era ls we re o n lycompara ble to the less a dsorbing humic ma terials of thisstudy, HA3 and HA4, which were extracted from nonsoilsources. These results indicate tha t h umic substa ncesisolated from soil are capable of a much higher glypho-sate adsorption than both the soil itself and the mineralsoil components.

In a previous study, the F reundlich Kf con s t a n t a n dt h e bin din g in t e n s i t y , n, we re s e n s ibly h ig h e r t h a nreported here when glyphosate wa s put in conta ct witha synthetic iron-humate complex that contained about8%chela ted iron (Piccolo et a l., 1995). The par ticula rlyh ig h a ds o rpt ion of g ly ph os a t e on a n iron -s a t u ra t e d

montmorillonite had been previously reported (Glass,1987). While the a dsorption on soil had been relatedto the clay content and cat ion exchange capacity, thesignificant interact ion with the cat ion-saturated clays(Glass, 1984) and with the iron-humate material (Pic-colo et a l. , 1995) had been expla ined by the recognizedcapacity of glyphosate to strongly complex diva lent a ndt r iv a len t ca t io n s. M cB ride (1991) s h owe d, by E SRspectroscopy, that a divalent cation such as copper wasbound to glyphosate in 1:1 complexes by the amine N,and in 2:1 complexes by both carboxylate and phos-phonat e groups. In a study of glyphosat e interact ionswith European soils containing different amounts of irona nd a luminum oxides, Piccolo et a l. (1994) indicat ed tha t

the higher the iron and aluminum content , the largerthe glyphosat e a dsorption w as on soil .

I n t h is s t u dy , h owe v er , t h e h u mic s u bst a n c e s we rethoroughly purified and contained less than 0.1%poly-valent ca tions. With such a low content of cations heldby t h e h u mic s u bs t a n ce s, me t a l comp le xin g ca n n o taccount for the significant adsorption of glyphosate.

Conversely, recent spectroscopic evidence (Piccolo andCelano, 1994) has proved that hydrogen bonding is amecha nism of interaction betw een glyphosate a nd puri-fied humic substa nces. Hydrogen bonding can thus bea dv oca t e d t o e xp la in t h e re le va n t g ly ph os a t e-humicma terial interactions reported in this study. In fact, theglyphosat e molecule conta ins various electronegativea t o ms , wh ic h c a n a c t a s bo t h a s h y dro g e n do n o r a n dhyd rogen a cceptor gr oups (Wa uchope, 1976; P iccolo andCela no, 1993), an d so does the hum ic mat ter due t o thelarg e content of oxygen-conta ining functions. However,other structura l a nd st ereochemical properties of humicsubstances must a lso play a role in glyphosat e adsorp-tion. In fa ct , had only hydrogen bonding been respon-sible for glyphosate adsorption, the order of humic-adsorbing capacity (HA1 > H A2 > H A3 g HA4) shouldh a v e b ee n cl os el y r el a t ed t o t h e con t en t of a c id icfunctions, w hich are likely to form h ydrogen bonds w iththe corresponding glyphosat e functions. That wa s notthe case, since the order of acidities was HA2 > H A4 >H A3 > H A1 (Ta ble 1). Th is orde r wa s g en e ra l lyconfirmed by the NMR results (Table 2), whereby the

Figure 3. Adsorption isotherms for glyphosate on humicsubstances.

Table 4. Adsorption of Glyphosatea by HumicSubstances

sample amount a dsorbed

b

(mg g-1) sa m ple amount adsorbed

b

(mg g-1)

H A1 4511 ( 14 H A3 1133 ( 5H A2 1504 ( 4 H A4 514 ( 6

a Init ia l concentrat ion of glyphosate of 100 mg m L-1. b Meansof two or more determinat ions and standard deviat ions.

Table 5. Freundlich and Langmuir Parameters withRespective Determination Coefficients (r2) forGlyphosate Adsorption on Humic Substances

F r eun dlich L a n gm uir

sample Kf 1/n r2 k1 k2 r2

H A1 454 0.55 0.95 0.010 11904 0.92H A2 179 0.54 0.92 0.009 4807 0.78H A3 98 0.51 0.84 0.016 1715 0.96

H A4 7 0.97 0.88 0.009 1108 0.73

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most glyphosate-adsorbing humic material, HA1, ap-peared as the least acidic compound.

A closer relat ion to the adsorbing capacity of humicsubstances was revealed by their content of aliphatic Ca n d t h e ir molecu la r s ize . Th e t w o mo st a ds o rbin ghumic materials, HA1 and HA2, derived from soils,showed a lower content of aromat ic C (Table 2) and alar ger molecular dimension (Ta ble 3) tha n t he HA3 andHA4 which were the least a dsorbing and most aromat icma terials. In part icular, HA1 revealed a molecula r sizeclose to double tha t of HA3 a nd HA4. These observa-t ions suggest that the larger the molecular size of thehumic substances, the grea ter is t he number of hydro-gen bondings occurring between th e herbicide an d thehumic molecule and, thus, glyphosa te adsorption. Fur-thermore, a lesser degree of stereochemical rigidity, dueto a lower content of aromatic rings, allows an easierpenetrat ion of the small herbicide molecule into theinner rea ctive sites of the h umic ma cromolecule, therebyfavoring glyphosat e a dsorption. The importa nce of thespecific chemical st ructure a nd m olecular dimension ofh u mic s u bs t a n c es in de t ermin in g t h e ir ca p a c i t y t ointeract with organic compounds in the environmentha d been alr eady indicated (Piccolo, 1994). Also in thecase of glyphosate, the occurrence of hydrogen bondings

between the herbicide and humic substances is con-trolled not only by the content of hydrogen donor a nda c ce pt o r g rou p s bu t a ls o by t h e molecu la r s ize a n dstereochemical flexibility of the humic material.

In summa ry, this work has shown that the adsorptionof glyphosate on purified humic substances, far frombeing negligible, is compar able to if not higher tha n t ha tobs erv ed f or cla y min era ls a n d s oi ls . H o we ve r , t h eextent of adsorption may be varia ble since is dependenton t h e ma c romolecu la r s t ru ct u re a n d dimen s ion ofhumic mat erial. Results showed that humic substancesfrom soils w ere capable of lar ge glyphosat e a dsorptionbecause of fa vora ble molecular chara cterist ics such ashigh molecular size an d high st ereochemica l flexibility.Th es e f in d in g s m a y w e ll h a v e i m pl ica t i on s i n t h e

prediction of residual g lyphosate t oxicities in soils w ithdifferent organ ic ma tter propert ies. In fa ct , part of theg ly ph os a t e s pre a d on s oi l ma y be o n ly t e mpora ri lybound to humic material by hy drogen bondings and ma ybe released after some t ime. Moreover, the herbicidemobility to deeper soil horizons ma y be controlled bythe formation of relat ively stable complexes betweenglyphosat e a nd wa ter-soluble humic substa nces.

LITERATURE CITED

B ow m e r , K . H . R e si d ue s of g l y phosat e i n i r r i g at i on w at e r .Pesti c. Sci. 1982, 13, 623-638.

Chiou, C. T.; Kile, D. E.; Binton, T. I.; Malcom, R. L.; Leenheer,J . A. A c om par i son of w at e r sol ubi li t y e nhanc em e nt of

organic solutes by aq uat ic humic mat erials an d commercialhumic a cids. E n vi r on. S ci . T echnol . 1986, 20, 502-508.Gla ss, R. L. Meta l complex formation by glyphosat e.J . A g r i c.

Food Chem. 1984, 32, 1249-1253.G l a s s , R . L . Ad s or p t ion of g ly p ho sa t e b y s oi ls a n d cl a y

minerals. J. A gri c. Food Chem. 1987, 35, 497-500.Hance, R. J . Adsorption of glyphosate by soils. Pestic. Sci. 1976,

7, 363-366.L aw r e nce , J . F . ; L e d uc , R . De t e r mi nat i on of g l y phosat e i n

formulations. A nal . Chem. 1978, 50, 1161-1166.

Malik, J .; B ar ry, G .; Kishore, G. The herbicide glyphosate. J .B i oFact ors1989, 2, 17-25.

McBride, M. B. Electron spin resonance study of copper ioncomplexation by glyphosate and related l igands. S oi l S ci .S oc. A m. J . 1991, 55, 979-985.

M cC a r t h y , J . F . ; Za c h a r a , J . M . S u b su r fa c e t r a n s p or t ofc ont am i nant s . E nvi ron. S ci . Technol . 1989, 23, 469-502.

P iccolo, A. Int eractions between humic substa nces and orga nicpollutant s in th e environment. In H umi c S ubst ances i n t h eG l o b a l E n v i r o n m e n t a n d I m p l i c a t i o n s o n H u m a n H e a l t h ;Senesi, N., Miano, T. M., Eds.; Elsevier: Amsterda m, 1994;pp 961-980.

Piccolo, A.; Celano, G. Modification of infrared spectra of theherbicide glyphosat e induced by pH va riation. J . E n v i r o n .Sci. Heal t h 1993, 28, 447-457.

P iccolo, A.; Ca mpan ella, L.; P etronio, B. M. Ca rbon-13 nuclearmagnetic resonance spectra of soil humic substances ex-tracted by different mechanisms. Soil Sci. Soc. Am. J . 1990,54, 750-756.

P iccolo, A.; Celano, G . Hyd rogen bonding intera ctions betw eent he he r bi ci d e g l y phosat e and w a t e r -solub le hum i c sub -stances. E n vi r on. Toxi col . Chem. 1994, 13, 1737-1741.

Piccolo, A.; Celano, G.; Arienzo, M.; Mirabella, A. Adsorptionand desorption of glyphosate in some European soils. J .E nvi ron. S ci . Heal t h 1994, 29, 1105-1115.

P iccolo, A.; Ga tta , L.; Campa nella, L. I ntera ctions of glyphosat ehe r bi ci d e w i t h a hum i c ac i d a nd i t s i r on com pl ex. A n n .Chi m . (Rome)1995, 85, 31-40.

Schnitzer, M. Humic substances: chemistry a nd reactions. InS oi l O r g a n i c M a t t er ; K h a n , S . U . , S c h n i t z e r , M . , E d s . ;Elsevier: Amsterda m, 1978; pp 1-78.

Spra nkle, P. ; Meggit , W. F.; P enner, D. Ra pid inactivation ofglyphosat e in t he soil. Weed Sci. 1975a, 23, 224-228.

Spran kle, P . ; Meggit , W. F.; P enner, D. Adsorption, m obilityand microbial degradation of glyphosate in soil . Weed Sci.1975b, 23, 229-234.

Stevenson, F. J .H um us Chemi stry: Genesis, Composition, andReactions, 2nd ed.; Wiley Int erscience: New York, 1994.

Tor st e nsson, L . B e havi our of g l y phosat e i n soi ls and i t sd e gr ad a t i on. I n Th e Herbi ci des Gl yphosat e; G r ossb ar d , E . ,Atkinson, D., Eds. ; B utterw orth: London, 1985; pp 137-150.

Wauchope, D . Acid dissociat ion consta nts of a rsenic acid,methylarsonic acid (MAA), dimethylarsinic acid (cacodylic

acid) and N-(phosphonomethyl)glycine (glyphosate). J . A g - ric. Food Chem. 1976, 24, 717-721.

Weber, J . B.; Miller, C. T. Organic chemical movement overand t hr ough soi l. I n React i ons and M ovement of Or gani cC h em i c a l i n S oi l s ; S how ne y , B . L . , B r ow n, K . , E d s. ; S oi lScience Society of America Inc. : Madison, WI, 1989; pp305-334.

White, G . N.; Zela zny , L. W. Charge properties of soil colloids.I n S oi l P h ysi cal Chemi st ry; S par ks, D. L . , E d . ; C R C P r e ssInc.: Boca Ra ton, FL, 1992; pp 39-82.

Yau , W. W.; Kirkla nd, J . J .; Bly , D. D . M odern Size ExclusionChromat ography; Wiley Int erscience: New York, 1979; pp318-326.

Received for review September 14, 1995. Accepted May 2,

1996.X

This work was partial ly supported by the EuropeanU nion contra ct S TEP -CT-90-0025: Effects of m onocultur epractices on soil organic matter st at us an d qua li ty in relat ionto interactions in soil and transport to groundwater of selectedherbicides.

J F950620X

X Abstr a ct published inA d va n ce A CS A b str a cts,J uly1, 1996.

2446 J. Agric. Food Chem.,Vol. 44, No. 8, 1996 Piccolo et al.