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Journal of Hazardous Materials 283 (2015) 643–656

Contents lists available at ScienceDirect

Journal of Hazardous Materials

j o ur nal ho me pa ge: www.elsev ier .com/ locate / jhazmat

ynthesis, characterization and sorption properties of silica modifiedith some derivatives of �-cyclodextrin

leksandra Shvets ∗, Lyudmila Belyakovahuiko Institute of Surface Chemistry of National Academy of Sciences of Ukraine, 17 General Naumov Str., Kiev 03164, Ukraine

i g h l i g h t s

�-Cyclodextrin-containing silicashave been synthesized.Effect of salts of water hardness onsorption of cadmium (II) was studied.Organosilicas show high affinity tocadmium (II) at sorption from multi-component solutions.Cations interaction with functionalorganosilicas consistent with HSABtheory.

g r a p h i c a l a b s t r a c t

r t i c l e i n f o

rticle history:eceived 29 May 2014eceived in revised form6 September 2014ccepted 12 October 2014vailable online 22 October 2014

eywords:ilica-Cyclodextrinadmium nitrate

a b s t r a c t

Nanoporous �-cyclodextrin-containing silicas which differ by functional substituents of wide edge ofattached cyclic oligosaccharide molecules (alcohol, bromoacetyl, thiosemicarbazidoacetyl groups) havebeen synthesized. The structure and chemical composition of the surface, porosity of obtained materials,their chemical and thermal stability have been characterized by scanning electron microscopy, IR spec-troscopy, thermogravimetry, nitrogen ad-desorption, elemental and chemical analyses of solid surface.Sorption of trace amounts of cadmium (II) in the presence of ten- and hundred-fold excess of hardnesssalts by synthesized organosilicas has been studied. It has been demonstrated that the sorption equi-librium is reached after 30 min. The sorption of trace amounts of cadmium (II) from multi-componentsolutions does not decrease, but even increases in the presence of hardness salts, simulating soft andhard water. Coefficients of distribution and selectivity as well as the sorption parameters of Langmuir and

ardness saltsorption

Freundlich equations have been calculated. It was found that the driving force of cadmium (II) sorptionon the surface of functional �-cyclodextrin-containing silicas is the formation of inclusion complexes “�-cyclodextrin–nitrate-anion”. It has been proved the formation of supramolecular structures on the surfaceof synthesized organosilicas as a result of cadmium (II) sorption. Chemical composition of supermoleculesdepends on the structure of surface active centers.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Cadmium and its compounds are nonbiodegradable highly toxicubstances with maximum allowable concentration in drinking

∗ Corresponding author. Tel.: +38 0444229691; fax: +38 0444243567.E-mail addresses: shvec sasha@mail.ru (O. Shvets), mila.belyakova@gmail.com

L. Belyakova).

ttp://dx.doi.org/10.1016/j.jhazmat.2014.10.012304-3894/© 2014 Elsevier B.V. All rights reserved.

water of 0.003 mg/L [1]. Cadmium compounds could accumulate inhuman body and cause anemia, lungs dysfunction, lever and kid-ney impairment [2,3]. Sorption methods are used most often forelimination of ions of toxic metals [4–17]. Organic resins and inor-ganic ion exchange materials provide good results at extraction of

heavy metals’ compounds, for example, from wastes of metallur-gical, chemical and petro-chemical industries [18–25]. Chelatingorganic polymers are used for concentration, elimination and quan-titative analysis of trace amounts of heavy metals [11,12,16,26].

6 azard

Htmapv

ctisre

smo

((

(

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t�sIoigaeta

2

2

0os(dcpp

44 O. Shvets, L. Belyakova / Journal of H

owever, swelling of functional resins and, as a result, poor sorp-ion kinetics are significant limitations of the most polymeric

aterials [26,27]. Therefore, inorganic sorbents with complexingbility are preferred, and among them oxide materials, for exam-le, highly disperse amorphous silicas with surface modified witharious organic reagents [4,28–34].

Adsorption and surface chemical reactions are used for modifi-ation of silica materials. The first method is simpler. At the sameime synthesized sorbents can be used only once, since modifiers desorbed from the carrier surface in contact with solutions. Theorbents obtained by chemical modification of surface (chemicaleactions in surface layer) are suitable for reuse after their regen-ration.

Efficiency of usage of complexing organosilicas materials fororption, extraction and express-analysis of trace amounts of heavyetals compounds in water and aqueous solutions depends, first

f all, on the following factors:

1) affinity of functional groups of organosilicas to absorbed ions;2) hydrolytic, chemical, thermal and mechanical stability of mod-

ified silicas;3) parameters of porous structure, which do not impede access of

absorbed ions to active sites of organosilicas;4) absence of functional groups in surface layer of modified silicas

which reduce the specificity of sorption;5) simple and efficient methods of regeneration without loss of

sorbent’s capacity.

Therefore, not only aspects of chemical design of selective sorp-ion centers on surface of highly disperse silicas, but also creation ofhemically attached supramolecular structures which are comple-entary with respect to absorbed substances becomes promising.

tructures like that could be macrocyclic compounds with invari-ble geometry of molecules, for example, crown-ethers, calixarensr cyclodextrins [35,36]. Considerable number of scientific studies35–38] has been devoted to sorption of ions by crown-ethers andalixarens, as well as materials based on them. Recently, articleshich describe the interaction of cyclodextrins with ions of toxicetals are published [39–45].In the present paper the results of synthesis of three func-

ional organosilicas chemically modified with derivatives of-cyclodextrin are described. Grafted �-cyclodextrins differ in theide functional groups of wide edge of immobilized molecules.n the first case these are secondary alcohol groups, in the sec-nd case – bromoacetyl and residual secondary alcohol groups,n the third case – residual alcohol and thiosemicarbazidoacetylroups. Detailed study of the structure and chemical composition ofttached functional layer of organosilicas was carried out in order tolucidate the contribution of various active sites of surface to sorp-ion of cadmium (II), calcium (II), and magnesium (II) from singlend multi-component solutions.

. Experimental

.1. Materials

Nanoporous amorphous silica–silochrome C-120 with.4 mmol/g of isolated silanol groups, average pore diameterf 40 nm and specific surface area of 118 m2/g was a startingilica matrix for the synthesis of organosilicas. �-Cyclodextrin�-CD, from Fluka, purity 99%), (3-aminopropyl)triethoxysilane,

imethylchlorosilane, p-toluenesulfonyl chloride and thiosemi-arbazide (from Merck, purity 99%), were used without additionalurification. Toluene, pyridine and acetone (all from Reakhim,ure analytical) were distilled and dried for 72 h by use of activated

ous Materials 283 (2015) 643–656

4 A molecular sieves. Cadmium (II), calcium (II), and magnium(II) nitrates (Merck, purity 98%) were used for sorption studieswithout additional purification.

2.2. Methods

The morphology of organosilicas surface was investigatedusing JEOL JSM-6400 scanning electron microscope (magnification1 × 103–5 × 104, operating at 3–30 keV).

The parameters of porous structure of starting and mod-ified silicas were calculated from low-temperature nitrogenad-desorption isotherms measured using Sorptometer KELVIN1042. Organosilicas were preliminarily evacuated at 463 K, 20 h.Specific surface area SBET (m2/g) was calculated according toBrunauer–Emmett–Teller equation [46] from values of low-temperature nitrogen adsorption at P/P0 = 0.30–0.35. Total porevolume V� (cm3/g) was determined from nitrogen adsorptionat P/P0 ≈ 0.9. The calculation of pore size distribution was real-ized according to Barrett–Joyner–Halenda (BJH) method [47] fromdesorption branch of isotherm at P/P0 = 0.4–0.9. Average pore diam-eter of organosilicas was determined from maxima at pore sizedistribution curves.

The concentration of silanol groups in the investigated materialswas determined by chemisorption of dimethylchlorosilane [48] andpotentiometric titration (Ionometer I-120.1) [27].

The amount of functional groups on the surface of organosili-cas was determined by potentiometric titration, elemental analysis(Elemental Analyzer EA 1110) and thermogravimetry (Derivato-graph Q-1500 D).

Infrared transmission spectra were registered in the range from4000 to 400 cm−1 using Thermo Nicolet NEXUS FT-IR spectropho-tometer. To record IR spectra materials were pressed in plates(∼30 mg) under the pressure of 108 Pa.

Total thermal analysis of functional organosilicas (batch weight150–180 mg) was performed in temperature range 293–1273 K atheating rate of 10 K/min. Registration of TG, DTG and TGA curveswas realized simultaneously with sensitivity 500, 500, 100 mcV,respectively. Thermal analyses were carried out in air ensuringcomplete burning out of organic groups attached to silica surface.The quantity of grafted functional groups was calculated from TGand DTG curves, and activation energies of dehydration (Eact) fromDTA curves according to equation [49]:

Eact = RT20

b· 1

�(1)

where R is universal gas constant, 8.314 J/mol K; T0 is temperatureof dehydration beginning, K; b is heating rate, K/min; � is time,when maximal dehydration rate is achieved, min.

2.3. Synthesis of functional derivatives of ˇ-cyclodextrin

2.3.1. Synthesis of mono-(6-O-(toluenesulfonyl))-ˇ-cyclodextrinThe synthesis of mono-(6-O-(toluenesulfonyl))-�-cyclodextrin

was carried out by reacting of �-CD (1 mmol) dried at 378 K during6 h with solution of p-toluenesulfonyl chloride (9.58 mmol) in drypyridine at 277 K for 4 h, and then for 24 h at room temperature.Further, the reaction mixture was vacuum-dried at 313 K up to theformation of a crystalline precipitate. The precipitate was washedwith diethyl ether followed by recrystallization from hot water anddried in air.

In the IR spectrum of �-cyclodextrin (Fig. 1, curve 1) there arefollowing absorption bands: the band of the valence vibrations

of the O–H bond in the secondary hydroxyl groups connected byhydrogen bonds (3375 cm−1), the band of the valence vibrations ofthe C–H bond in the methylene groups (2940 cm−1), the band ofthe deformation vibrations of the O–H bonds in the COH groups

O. Shvets, L. Belyakova / Journal of Hazard

4000 3500 3000 2500 2000 1500 1000 500

6801755

1365

129014251495

1610

16352940

3375

1

2

3

Transmittance

Wavenumber (cm-1)

Fig. 1. IR spectra of �-cyclodextrin (1), mono-(6-O-(toluenesulfonyl))-�-CD (2) andbromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CD (3).

4000 3500 3000 2500 2000 1500 1000 500

1470

1435

154 0

1540

680

1755

16101635

3290

146015902880

2950

3290

145528802955

1570

33103375

Transmittance

Wavenumber (cm-1)

1

2

3

4

5

Fc

atabrotic[n2at(

temperature. Afterwards, the solid phase was transferred on aporous glass filter, washed with toluene until the absence of silane

TE

ig. 2. IR spectra of initial silica (1), aminopropylsilica (2), and �-cyclodextrin-ontaining silicas 1–3 (curves 3–5).

nd/or of the water molecules (1635 cm−1). After chemical reac-ion between �-cyclodextrin and p-toluenesulfonyl chloride thebsorption bands belonging to toluenesulfonyl group, namely, theands of the valence vibrations of the C C bond in the benzeneing (1610, 1495 cm−1), the bands of the deformation vibrationsf the C–H bonds in the hydrocarbon groups (1425, 1290 cm−1),he band of the asymmetric valence vibrations of the S O bondn the toluenesulfonyl groups (1365 cm−1) are observed (Fig. 1,urve 2) [50,51]. These data are consistent with the 1H NMR results52]. For modified �-cyclodextrin the protons’ signals of tolue-esulfonyl group with chemical shifts ı (ppm) were registered:.433 (s) assigned to methyl radical of the toluenesulfonyl group,nd also the signals at 7.452 (d) and 7.747 (d) attributed to pro-

ons of the benzene ring at the positions 3, 5 and 2, 6 respectivelyFig. 2). Hence, the reaction of electrophilic substitution of proton in

able 1lemental analysis of �-cyclodextrin and its derivatives.

Analyzed substance Co

H

�-Cyclodextrin 62Mono-(6-O-(toluenesulfonyl))-�-cyclodextrin 60Heptakis-(6-O-(toluenesulfonyl))-�-cyclodextrin 52Bromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-cyclodextrin 36

ous Materials 283 (2015) 643–656 645

alcoholic groups of �-CD (first of all of the primary groups at posi-tion 6) by toluenesulfonyl groups takes place.

According to the data of elemental analysis (Table 1) only onealcoholic group of �-cyclodextrin molecule takes part in the reac-tion with p-toluenesulfonyl chloride (Scheme 1).

The yield of mono-(6-O-(toluenesulfonyl))-�-cyclodextrinequals 38%.

2.3.2. Synthesis of bromoacetyl derivative ofheptakis-(6-O-(toluenesulfonyl))-ˇ-cyclodextrin

Synthesis of bromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CD requires obtaining ofheptakis-(6-O-(toluenesulfonyl))-�-CD and its interaction withbromoacetyl bromide (Scheme 2).

In the IR spectrum of heptakis-(6-O-(toluenesulfonyl))-�-cyclodextrin after interaction with bromoacetyl bromide(Scheme 2), besides the characteristic absorption bands of �-cyclodextrin and toluenesulfonyl groups, the bands of the valencevibrations of the C O (1755 cm−1) and C–Br (680 cm−1) bondsin the bromoacetyl groups appear (Fig. 1, curve 3) [50]. Basedon the data of elemental analysis (Table 1) it is believed that allsecondary OH-groups of heptakis-(6-O-(toluenesulfonyl))-�-CD atthe position 2 and two hydroxyl groups at the position 3 took partin the chemical reaction with bromoacetylbromide. The yield ofbromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CDis equal to 42%.

2.4. Synthesis of ˇ-cyclodextrin-containing silicas

Synthesis of �-cyclodextrin-containing silicas was carried outthrough consecutive two- and three-step chemical reactions insurface layer of starting silica. The first step was a chemicalgrafting of amino groups on silica surface through reaction ofelectrophilic substitution between surface silanol groups and(3-aminopropyl)triethoxysilane [30]. Then, grafted aminopropylgroups were involved in the chemical reaction with mono-(6-O-(toluenesulfonyl))-�-CD (organosilica 1) or bromoacetylderivative of heptakis-(6-O-(toluenesulfonyl))-�-CD (organosil-ica 2). Organosilica 3 with thiosemicarbazidoacetyl groups wasobtained from organosilica 2 by nucleophilic substitution reactionbetween side bromoacetyl groups of wide edge of grafted �-CDmolecules and thiosemicarbazide.

2.4.1. AminopropylsilicaSilochrome C-120 dried up at 473 K for 6 h was placed into

a three-necked reactor supplied with a stirrer and a reflux con-denser, and suspended in a small amount of toluene during 40 minat room temperature. Then (3-aminopropyl)triethoxysilane (three-fold excess in relation to the content of silanol groups on silicasurface taken for modification) was added. The reaction mixturewas stirred at 383 K for 6 h. It was then left overnight at room

(absence of a violet color on addition of ninhydrin), then acetone,distilled water (for hydrolysis of ether groups) and again acetone.

ntent of chemical elements, mg/g Chemical formula of�-cyclodextrin

C Br S

.5 445.0 – – C42H70O35

.1 456.5 – 24.1 C49H76O37S

.3 495.1 – 100.5 C91H112O49S7

.9 396.4 217.7 68.0 C109H121O58S7Br9

646 O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656

Scheme 1. Synthesis of mono-(6-O-(toluenesulfonyl))-�-cyclodextrin.

Scheme 2. Synthesis of bromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-cyclodextrin.

Scheme 3. Synthesis of aminopropylsilica.

O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656 647

esis o

TT

tta1t[

2bi

mhrsotduC(T3fid

m3

Scheme 4. Synth

he resulting aminopropylsilica was dried in air at 423 K during 6 h.he aminopropylsilica synthesis is shown in Scheme 3.

In the IR spectrum of aminopropylsilica (Fig. 2, curve 2) charac-eristic absorption bands at 3375, 3310 and 1570 cm−1 belonging tohe valence and deformation vibrations of the N–H bond in graftedmino groups and the absorption bands at 2955, 2880 and 1455,415 cm−1 corresponding to the valence and deformation vibra-ions of the C–H bonds in the hydrocarbon groups are registered50,51].

.4.2. Mono-(6-O-(toluenesulfonyl))- andromoacetyl-heptakis-(6-O-(toluenesulfonyl))-ˇ-cyclodextrinsmmobilization on the silica surface

The interaction of aminopropylsilica withono-(6-O-(toluenesulfonyl))-�-CD (Scheme 4) or bromoacetyl-

eptakis-(6-O-(toluenesulfonyl))-�-CD (Scheme 5) has beenealized under optimal conditions of the reaction of electrophilicubstitution of proton in aminopropyl groups with �-cyclodextrinnes. A batch of air-dried aminopropylsilica (2 g) was placed intohe three-necked reactor supplied with a stirrer and a reflux con-enser, and suspended in dry pyridine (20 mL) at room temperaturender continuous stirring. Then, mono-(6-O-(toluenesulfonyl))-�-D (0.2 g) or bromoacetyl-heptakis-(6-O-(toluenesulfonyl))-�-CD0.5 g) was added. The reactor was placed into a hot water bath.he modification of aminopropylsilica surface was carried out at33 K for 4 days. Then solid phase was moved on the porous glasslter, washed by pyridine (25 mL × 3), acetone (25 mL × 3) and

ried in air.

In the IR spectrum of aminopropylsilica after interaction withono-(6-O-(toluenesulfonyl))-�-CD (organosilica 1, Fig. 2, curve

) the absorption bands of the valence vibrations of the O–H bonds

Scheme 5. Synthesis o

f organosilica 1.

in the secondary alcohol groups (3290 cm−1), the C–H bonds of�-CD (2950, 2880 cm−1), the deformation vibrations of the N–Hbonds (1590, 1540 cm−1) in the primary and secondary aminogroups, and the C–H bond (1460 cm−1) in the methylene groupswere registered [51]. It is an evidence of partial participation ofthe primary aminopropyl groups in chemical grafting of mono-(6-O-(toluenesulfonyl))-�-CD. In the IR spectrum of organosilica1 (Fig. 2, curve 3) the absorption bands of the valence vibrations ofthe C C bond of benzene ring and asymmetric valence vibrationsof S O bond in the R1–O–SO2–R2 groups were absent. Hence, thereaction of electrophilic substitution on aminopropylsilica surfaceruns with participating of toluenesulfonyl group of mono-(6-O-(toluenesulfonyl))-�-CD (Scheme 4).

In the IR spectrum of aminopropylsilica modified withbromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CD(Fig. 2, curve 4) the absorption bands belonging to bromoacetyl-heptakis-(6-O-(toluenesulfonyl))-�-CD, namely, the band of thevalence vibrations of the O–H bond in the secondary hydroxylgroups of �-CD (3290 cm−1), the band of the deformation vibra-tions of the O–H bonds in COH groups of �-CD and/or in watermolecules (1635 cm−1), the band of the valence vibrations of theC C bond in benzene ring of toluenesulfonyl groups (1610 cm−1),the band of the valence vibrations of the C O bond (1755 cm−1),the C–Br bond (680 cm−1) in bromoacetyl groups and also theabsorption band at 1540 cm−1 of the secondary amino groups wereregistered. Chemical grafting of bromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CD onto aminopropylsilica surface is

presented in Scheme 5.

Chemical reactions on silica surface were proved by IR spec-troscopy, and also by elemental and chemical analyses of reactionproducts and modified silicas (Tables 2 and 3).

f organosilica 2.

648 O. Shvets, L. Belyakova / Journal of Hazard

Table 2Elemental analysis of surface layer of synthesized organosilicas.

Organosilica Content of chemical elements, mg/g organosilica

H C N Br S

Aminopropylsilica 4.5 10.08 2.80 – –Organosilica 1 7.0 32.08 2.81 – –Organosilica 2 6.0 33.31 2.80 7.01 1.98Organosilica 3 6.5 34.05 6.62 – 5.03

Table 3Chemical composition of silicas surface.

Organosilica Content of functional groups, mmol/g organosilica

Silanol Aminopropyl �-Cyclodextrin

Initial silica 0.40a – –0.40b

0.40c

Aminopropylsilica 0.20a 0.20a –0.20c

0.20d

Organosilica 1 0.20a 0.18a –0.20c 0.024c (on C)0.18d 0.02d

Organosilica 2 0.20a 0.19a –0.20c 0.010c (on C)

0.009c (on Br)0.19d 0.010c (on S)

0.01d

Organosilica 3 0.20a 0.19a –0.20c 0.009c (on C)

0.010c (on S)0.19d 0.01d

a Potentiometric titration.b Chemisorption of dimethylchlorosilane.

2t

tt

tka

Freundlich [53]:

c Elemental analysis.d Thermogravimetry.

.4.3. Functionalization of ˇ-cyclodextrin-containing silica withhiosemicarbazide

The replacement of bromoacetyl groups of organosilica 2 onhiosemicarbazidoacetyl groups was done under optimal condi-ions of surface reaction of nucleophilic substitution.

Three-fold excess of thiosemicarbazide in relation to the con-

ent of bromoacetyl groups was dissolved in 20 mL dry toluene andept at 383 K during 6 h. Then the solid phase was transferred to

glass filter, washed with toluene (25 mL × 3), acetone (25 mL × 3)

Scheme 6. Synthesis o

ous Materials 283 (2015) 643–656

and dried in air. The conversion degree of bromoacetyl groups tothiosemicarbazidoacetyl groups was 100% (Table 3).

Chemical immobilization of thiosemicarbazide with breaking ofthe C–Br bond was confirmed by the disappearing of absorptionband of the valence vibrations of the C–Br bond at 680 cm−1 inthe IR spectrum of organosilica 3 (Fig. 2, curve 5). Moreover, theintensity of the absorption band of the secondary amino groups(1540 cm−1) significantly increases, and also the absorption bandsof the valence vibrations of the C S bond (1435 cm−1) and the–N–C–N–bond (1470 cm−1) appear [51]. These changes in the IRspectrum indicate on the chemical reaction of nucleophilic substi-tution between side bromoacetyl groups of wide edge of grafted�-cyclodextrin molecules and thiosemicarbazide (Scheme 6).

2.5. Sorption procedure and calculations

The sorption of ions was studied from aqueous nitrate solu-tions with composition Cd:(Ca + Mg) = 1:10 and 1:100 at 295 Kunder static conditions. Concentrations of solutions were var-ied within the ranges: 3.2 × 10−4–4.4 × 10−3 M for Cd(NO3)2,3 × 10−3–1.6 × 10−2 M (soft water) and 3 × 10−2–1.6 × 10−1 M(hard water) for Ca(NO3)2 and Mg(NO3)2 [1]. Weighed amountsof organosilicas were brought into contact with nitrate solutions.The obtained suspensions were kept in a JULABO SW22 waterthermostat at 295 K with shaking for 4 h. The amounts of Cd(II)in the initial and equilibrium solutions were determined spec-trophotometrically, and the sum of Ca(II) and Mg(II) – by meansof complexometric back titration. Equilibrium adsorption of Cd(II),Ca(II) and Mg(II) was calculated according to the equation:

qe = (C0 − Ce) · V

W(2)

where qe is the equilibrium adsorption, mol/g; C0 and Ce are theinitial and equilibrium concentration of adsorptive in a solution,mol/L; V is the solution volume, L; m is the batch of organosilica, g.

Relative contents of various species of cadmium (II), calcium (II),and magnesium (II) in aqueous solutions were determined using aprogram CHEAQS (Chemical Equilibria in Aquatic System by WilkoVerweij, 1999–2006).

Isotherms of sorption have been analyzed by equations of

lgqe = lgKF + 1n

· lgCe (3)

f organosilica 3.

azardous Materials 283 (2015) 643–656 649

wsct

wtca

R

wm

K

K

wc

ˇ

wKs

2ˇ

2c2tT4wo

dScfwwardd

3

3

3s

mwCs

the volume and diameter of the pores change just slightly (Table 4).At prolonged modification of silica surface as, for example, in thecase of organosilica 2 synthesis (Scheme 5), partial dissolution

Table 4Porosity parameters of synthesized organosilicas.

Organosilica Specificsurfacearea, m2/g

Volume of pores,cm3/g

Averagediameter ofpores, nm

VBET VBJH

Initial silica 118 0.80 40Aminopropylsilica 114 0.75 0.72 33

O. Shvets, L. Belyakova / Journal of H

here qe is the equilibrium adsorption, mg/g; KF is Freundlich con-tant (adsorption capacity), mg/g; 1/n is Freundlich constant whichharacterizes intensity of adsorption; Ce is the equilibrium concen-ration of adsorptive, mg/L, and Langmuir [54]:

Ce

qe= 1

KL · Qm+ 1

Qm· Ce (4)

here Ce is the equilibrium concentration of adsorptive, mg/L; qe ishe equilibrium adsorption, mg/g; KL is Langmuir constant whichharacterizes energy of adsorption, L/mg; Qm is the capacity ofdsorption monolayer, mg/g.

Separation factor RL [55] was calculated by the formula:

L = 1(1 + KL · C0)

(5)

here C0 is the initial concentration of adsorptive, mg/L; KL is Lang-uir constant, L/mg.Distribution coefficients of ions between solid and liquid phases

d (L/g) [26] were determined as:

d = qe

Ce(6)

here qe is the equilibrium adsorption, mol/g; Ce is the equilibriumoncentration of adsorptive, mol/L.

Selectivity coefficients ̌ [26] were calculated by the formula:

= Kd Cd

Kd (Ca+Mg)(7)

here Kd Cd is the distribution coefficient for cadmium (II) cations;d (Ca+Mg) is the distribution coefficient for calcium (II), and magne-ium (II) cations at their sorption from binary solutions.

.6. Chemical stability and regeneration of-cyclodextrin-containing silicas

For determination of the chemical stability of organosilicas0 mL of 5 M HNO3 was added to 0.1 g of �-CD-containing sili-as. The suspensions were kept in contact with nitric acid during4 h at room temperature. Organosilicas were separated from solu-ions by filtration, rinsed with distilled water and dried in air.hen, organosilicas were repeatedly brought into contact with.4 × 10−3 M Cd(NO3)2 solutions for 4 h. Values of Cd(II) sorptionere determined and compared with initial static capacities of

rganosilicas.The regeneration of organosilicas was performed at static con-

itions. 6% solution of thiourea in 1 M HNO3 was used as eluent.amples of �-CD-containing silicas (0.1 g) after sorption of Cd(II)ations were brought into contact with 20 mL of eluent and keptor 20 min at room temperature under stirring. Then liquid phaseas separated by filtration and organosilicas were again pouredith CS(NH2)2 solution. The procedure was repeated three times

nd then organosilicas were transferred onto a glass filter, carefullyinsed with distilled water until neutral reaction of rinsing water,ried in air and static capacity on Cd(II) was determined as it isescribed in the chapter 2.5.

. Results and discussion

.1. Characterization of ˇ-cyclodextrin-containing silicas

.1.1. Surface structure, chemical composition and porosity ofynthesized organosilicas

The data of scanning electron microscopy illustrate nonuniform

acroporous globular structure of starting silica and confirm itside pore size distribution (Fig. 3a). After chemical grafting of �-D derivatives the relief of surface of �-cyclodextrin-containingilicas has a smaller roughness (Fig. 3b).

Fig. 3. Scanning electron micrographs of silica surface before (a) and after (b) itsmodification with functional derivatives of �-cyclodextrin.

Low-temperature nitrogen ad-desorption isotherms fororganosilicas 1–3 (Fig. 4a) are IV type isotherms according to theIUPAC classification [56]. On the isotherms of organosilicas steepraising at P/P0 = 0.90–0.99 and narrow hysteresis loop of H1 typeare observed, indicating macroporous structure. Modification ofsurface of initial silica with �-CD and its functional derivativescauses, as a rule, decrease in specific surface area, at the same time

Organosilica 1 114 0.88 0.87 32Organosilica 2 125 (98)a 0.93 0.94 18 (24)*

Organosilica 3 94 0.93 0.93 31

a Synthesis during 4 days (1 day).

650 O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

Relative pressure (P/P0)

Volumeofpores(cm3 /g)

1

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

Volumeofpor es(cm3 /g )

Relative pressure (P/P0)

2

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

Relative pressure (P/P0)

Volumeofpor es(cm3 /g)

3

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

700

Volumeofpores( cm3 /g)

Relative pressure (P/P0)

4

0.0 0.2 0.4 0.6 0.8 1.00

100

200

300

400

500

600

700

tive p

Volumeofpores(cm3 /g )

5

propy

olsmic

opsnrg

Rela

Fig. 4. Nitrogen ad-desorption isotherms for initial silica (1), amino

f highly disperse silicon dioxide in acidic medium [29] couldead to increasing of specific surface area and decreasing of poreize (Table 4). Nevertheless, �-CD-silicas remain highly disperseaterials with nanoscale pores. Therefore, it can be supposed that

ons sorption onto �-cyclodextrin-containing silicas will not beomplicated by inner diffusion.

Elemental analysis of organosilicas (Table 2) shows the increasef carbon content in organosilicas after the interaction of amino-ropylsilica with functional �-cyclodextrins. The absence of

ulfur in organosilica 1 is evidence of participation of tolue-esulfonyl groups of mono-(6-O-(toluenesulfonyl))-�-CD in theeaction of electrophilic substitution with surface aminopropylroups. For organosilicas 2 (or 3) the presence of bromine

ressure (P/P0)

lsilica (2), and �-cyclodextrin-containing silicas 1–3 (curves 3–5).

(or sulfur) proves chemical immobilization of bromoacetyl- andthiosemicarbazidoacetyl-�-cyclodextrins, correspondingly. Thedegree of immobilization of �-cyclodextrin functional derivativesonto silica surface was calculated from the data of elemental anal-ysis (Table 3). Obtained results coincide well with the data ofpotentiometric titration, chemical and thermogravimetric analysis(Table 3).

3.1.2. Thermal stability of ˇ-cyclodextrin-containing silicas

On DTA curve of �-CD (Fig. 5a) four thermoeffects are observed:

one endothermic effect with maximum at 383 K and three exother-mic effects (573–603, 628, 773 K) which are accompanied by weightloss on TG and DTG curves. Weight loss up to 473 K equals 11%,

O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656 651

400 600 800 1000 1200100

80

60

40

20

0

Weight(%)

Тemperature (K)

603

383

773DTG

TG

(a)

400 600 800 1000 1200100

80

60

40

20

0

Exo

Endo

Тem

pera

ture

Тemperature (K)

603

773628

573

383

DTA

400 600 800 1000100

80

60

40

20

0

Weight(%)

Тemperature (K)

(b)

TG

DTG

818

648

573

488

363

400 600 800 1000100

80

60

40

20

0

Exo

Тem

pera

ture

Endo

Тemperature (K)

DTA

803

453

omoa

i(e�t(tmtgessrm7

bcgta(

ocd

aiwh�

Fig. 5. Curves of thermal analysis of �-cyclodextrin (a) and br

n the range of 473–723 K it is 74% and higher than 723 K – 15%.See Table S1 in Supporting Information for details.) Endothermicffect corresponds to water removal from the inner cavities of-cyclodextrin molecules. Its quantity, according to the litera-

ure data [38,57], is 8–11 molecules. The experimental resultsweight loss of 11%) agree well with calculated data (11.26%):here are 8 molecules of water in the cavity of �-CD. The exother-

ic effect with two maxima (573, 603 K) is logically attributedo oxidative destruction of side primary and secondary alcoholroups (Fig. 5a). However, weight loss in the range of 473–723 Kquals 74% that significantly exceeds the content of primary andecondary alcohol groups of �-CD molecule (Tables S1 and S2). Con-equently, it can be argued that the disruption of glucopiranoseings begins already in this temperature interval (exoeffect withaximum at 628 K) and ends at 873 K (exoeffect with maximum at

73 K).Removal of water molecules from the inner cavity of

romoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-yclodextrin and decomposition of residual secondary alcoholroups occur, mainly, before 473 K. Oxidative destruction ofoluenesulfonyl and bromoacetyl groups takes place, mostly,t 473–723 K. Glucopiranose rings break up higher than 723 KFig. 5b).

Interpretation of obtained results is consistent with the dataf Table S2. Calculated values of thermal degradation fragmentsoincide with the data of thermogravimetric analysis. The mainecomposition of �-cyclodextrins occurs at 473–723 K.

After chemical immobilization of functional �-cyclodextrins onminopropylsilica surface the type of �-CD destruction, in general,

s retained. At the same time, there are some peculiarities. Firstly,

eight loss for organosilicas 1–3 (from 298 to 473 K) is significantlyigher than quantity of water in the inner cavities of immobilized-cyclodextrin molecules. It can be associated with a condensation

cetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CD (b).

of the secondary silanol groups which are formed as a result ofhydrolysis of side ether groups of grafted aminopropyl radicals. Fororganosilica 1 (Fig. 6a) in interval from 473 to 723 K decomposi-tion of the primary and secondary alcohol groups of �-CD takesplace, and disruption of glucopiranose rings begins. At 723–1073 Kdecomposition of chemically immobilized aminopropyl groups isalso observed [30].

For organosilica 2 (Fig. 6b) removal of water from the cavitiesof grafted molecules of bromoacetyl derivative of heptakis-(6-O-(toluenesulfonyl))-�-CD, condensation of the secondary silanolgroups, partial decomposition of toluenesulfonyl and alcoholgroups of �-cyclodextrin occur up to 473 K. At 473–723 K destruc-tion of functional groups of wide side of immobilized molecules offunctionalized �-cyclodextrin (secondary alcohol and bromoacetylgroups) ends, and decomposition of glucopiranose rings begins.Within 723–1073 K the destruction of glucopiranose rings finishes,and also the destruction of aminopropyl radicals (Tmax = 863 K)takes place. Superposition of these two processes appears asexothermic effect with maximum at 863 K and smooth weight lossup to 1073 K.

Thermodestruction of organosilica 3 (Fig. 6c) occurs the sameway as for organosilica 2. Thiosemicarbazydoacetyl groups decom-pose at 473–723 K.

Calculated values of activation energy of water removal from �-cyclodextrins and �-cyclodextrin-containing silicas are higher thanthose for aminopropylsilica, which was used as initial carrier forsynthesis of �-CD-materials. Activation energies for �-CD-silicashave an intermediate values (Table 5). This may be due to removalof adsorbed water as well as water from the inner cavities of �-cyclodextrins on the �-CD-silicas surface.

Thus, synthesized �-cyclodextrin-containing silicas have high

thermal stability. The nature of their stepwise destruction dependson the chemical composition of grafted �-cyclodextrins.

652 O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656

400 600 800 1000 1200Тemperature (K)

Weight(%)

(a)

863

588

353

TG

DTG

400 600 800 1000 1200

Exo

Тem

pera

ture

Endo

Тemperature (K)

588 863

DTA

400 600 800 1000 1200

Weight(%)

Тemperature (K)

863

623

(b)

DTG

TG

353

400 600 800 1000 1200Тemperature (K)

Endo

Тem

pera

ture

Exo

623 863

DTA

400 600 800 1000 1200Тemperature (K)

Weight(%) 863

623

508343

TG

DTG

(c)

400 600 800 1000 1200

Endo

Тem

p era

ture

Exo

Тemperature (K)

863623

DTA

Fig. 6. Curves of thermal analysis of or

Table 5Activation energy (Eact.) of water removal from �-cyclodextrins and organosilicas.

Compound T0, K Tmax, K 1/�, min−1 Eact. , kJ/mol

�-CD 333 383 0.200 18.4Bromoacetyl-�-CD 332 363 0.200 18.3Aminopropylsilica 298 343 0.133 9.2Organosilica 1 320 353 0.154 13.1

Organosilica 2 320 353 0.152 12.9Organosilica 3 311 343 0.179 14.4

ganosilicas 1 (a), 2 (b), and 3 (c).

3.1.3. Chemical stability and regeneration of organosilicasThe values of static capacity (on cadmium (II)) of �-CD-

containing silicas before and after their contact with concentratednitric acid during a day are presented in Table 6. Synthesizedorganosilicas have high chemical stability and can be used for Cd(II)sorption.

The experiments on organosilicas 1–3 regeneration withthiourea solution have demonstrated the possibility of, practically,

complete desorption of adsorbed Cd(II) cations, maintaining capac-ity at the next sorption elimination of Cd (II) from nitrate solutions(Table 6).

O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656 653

Table 6Chemical stability and regeneration of �-cyclodextrin-containing silicas.

Organosilica Static capacity on Cd(II) ×102, mmol/g

Initial After contactwith 5 M HNO3

After contact with 6%solution of thiourea in 1 MHNO3

3

st

feef

cttoth

c

Fta

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.350

1

2

3

4

5

6

12

4

5

q e(molCd/molf unct.groups)

3

Ce Cd 102 (mol/L)

Organosilica 1 3.8 3.8 3.5Organosilica 2 4.7 4.4 4.6Organosilica 3 5.2 5.2 5.2

.2. Sorption properties of ˇ-cyclodextrin-containing silicas

Sorption of cadmium (II) was studied from individual nitrateolution and solution, modeling soft and hard water, at averageemperature of tap water (295 K).

Cadmium (II), calcium (II) and magnesium (II) nitrates are in theorm of non-hydrolyzed cations – Me2+, Me(NO3)+ (Fig. 7) underxperimental conditions. The formation of only cationic speciesxcludes their interaction to one another and creates conditionsor competitive sorption.

In preliminary tests it was showed that the sorption of cadmiumations by �-CD-containing silicas occurs much more efficientlyhan with starting silochrome and aminopropyl silica (Fig. 8), andhe sorption equilibrium is reached within 30 min (Fig. 9). Cationsf calcium and magnesium are poorly eliminated from nitrate solu-

ions by organosilicas 1 and 2, and organosilica 3 does not uptakeardness salts at all (Fig. 10, curve 4).

�-Cyclodextrin-containing silicas hardly absorb cations of cal-ium and magnesium from multi-component solutions with molar

0

10

20

30

40

50

60

70

80

90

100

Relativecontentsofcations(%)

Cd (NO3)+

Cd2+

Ca(NO3)+

Ca2+

Mg2+

(a)

0

10

20

30

40

50

60

70

80

90

100

Relativecontentsofcations(%)

Cd2+

Cd(NO3)+

Ca2+

Ca(NO3)+

Mg2+

(b)

ig. 7. Distribution of Cd(II), Ca(II), and Mg(II) species in multi-component solu-ions: 0.02 M solution of Cd(NO3)2 and 0.2 M (a) or 2 M (b) solutions of Ca(NO3)2

nd Mg(NO3)2.

Fig. 8. Isotherms of cadmium (II) sorption from individual nitrate solutions on initialsilica (1), aminopropylsilica (2), and �-cyclodextrin-containing silicas 1–3 (curves3–5)

ratio Cd:(Ca + Mg) = 1:10. Cadmium (II) sorption capacity in thepresence of hardness salts decreases by 16% for organosilica1(Fig. 10a, curves 1 and 2), and capacity did not change fororganosilicas 2 and 3 (Fig. 10b and c, curves 1 and 2).

Noticeable decrease of affinity of �-CD-containing silicas to cad-mium (II) cations is observed only in the case of organosilica 1 at100-fold increasing of content of hardness salts in comparison withconcentration of cadmium cations (Fig. 10a, curve 3).

It was found that inner cavities of �-cyclodextrin molecules areoccupied by nitrate-ions [52] and therefore they cannot absorbmetal cations. Residual surface aminopropyl groups and functionalgroups of narrow edge of grafted cyclic oligosaccharides moleculesare sorption inactive. Consequently, only side groups of upper edgeof immobilized �-cyclodextrin molecules and its functional deriva-tives – secondary alcohol groups (organosilica 1), bromoacetyl andsecondary alcohol groups (organosilica 2), thiosemicarbazidoacetyland secondary alcohol groups (organosilica 3) – could be active sitesof cadmium, calcium or magnesium cations sorption.

Isotherms of sorption of Cd(II) cations from Cd:(Ca + Mg) = 1:10solutions are linearized well in coordinates of Freundlich equation(Fig. S1b) as for Cd(II) sorption from single-component solutions(Fig. S1a, Table 7). The Freundlich equation describes sorption pro-cesses on heterogeneous surfaces [53]. Surface “heterogeneity” forstudied �-cyclodextrin-containing silicas might be caused either

by Cd2+ (46%) and Cd(NO3)+ (43%) sorption on two different typesof centers of organosilicas surface or by their simultaneous sorp-tion, but with substantially dissimilar affinity toward organosilicas

0 20 40 60 80 100 120

0.01

0.02

0.03

0.04

0.05

0.06

3

Time (min)

1

2

q t(mmol/ g)

Fig. 9. Kinetics of cadmium (II) sorption from individual nitrate solutions on �-cyclodextrin-containing silicas 1–3

654 O. Shvets, L. Belyakova / Journal of Hazardous Materials 283 (2015) 643–656

Table 7Parameters of Freundlich and Langmuir equation for cadmium (II) sorption from single-(Cd(NO3)2) and multi-component (Cd:(Ca + Mg) = 1:10, Cd:(Ca + Mg) = 1:100) nitratesolutions on �-cyclodextrin-containing silicas.

Silica Freundlich isotherm Langmuir isotherm

Cd(NO3)2 Cd:(Ca + Mg) = 1:10 Cd:(Ca + Mg) = 1:100

1/n KF, mg/g R2 1/n KF, mg/g R2 KL × 10−4, L/mg Qm , mg/g RL R2

0.97 4.0 ± 0.4 0.62 ± 0.06 0.72 ± 0.07 0.960.98 125 ± 12 3.7 ± 0.4 0.010 ± 0.001 0.990.99 83 ± 8 4.0 ± 0.4 0.060 ± 0.006 0.99

soce

mtppc1cFcacirwmas

mesdoi�css7is

smsacs

c

Table 8Molar ratio of [Cd(II)]:[�-cyclodextrin] on the surface of �-cyclodextrin-containingsilicas after Cd(II) sorption from single- and multi-component solutions.

Silica Molar ratio of [Cd(II)]:[�-cyclodextrin] on the silica surface

Cd Cd:(Ca + Mg) = 1:10 Cd:(Ca + Mg) = 1:100

TP3

Organosilica 1 0.80 3.9 ± 0.2 0.99 1.30 3.8 ± 0.3Organosilica 2 1.00 4.3 ± 0.3 0.99 1.13 4.1 ± 0.3

Organosilica 3 1.25 5.5 ± 0.3 0.99 0.47 5.1 ± 0.4

urface. More likely it appears to be the second case, sincerganosilica 1 has only one type of active centers on surface, butadmium cations sorption is described by Freundlich isothermquation too.

Freundlich constants KF for cadmium cations sorption fromulti-component solutions (Table 7) are slightly inferior to sorp-

ion constants from individual solutions. This is an additionalroof of slight contribution of hardness salts to total sorptionrocess. Constant 1/n for cadmium (II) sorption from single-omponent nitrate solutions increases in the order: organosilica

< organosilica 2 < organosilica 3. In Cd(II) sorption from multi-omponent nitrate solutions change sequence in 1/n is opposite.or organosilicas 1 and 2 1/n values are even higher than those foradmium (II) sorption from individual solutions. This is, obviously,ssociated with significant increase of negative charge of �-yclodextrin-containing silicas (as a result of the formation ofnclusion complexes “�-cyclodextrin–nitrate-anion”) that is theeason for intensification of Cd(II) sorption. For organosilica 3,hich has the most active centers for complex formation with cad-ium cations, low value of constant 1/n, most likely, is due to

decrease of ions mobility with an increase in ionic strength ofolutions by two orders.

Sorption capacity of �-cyclodextrin-containing silicas for cad-ium (II) from Cd:(Ca + Mg) = 1:100 solutions is attained at lower

quilibrium concentrations (Fig. 10, curves 3) than that foringle-component and Cd:(Ca + Mg) = 1:10 solutions. This may beue to higher content of nitrate-anions in the surface layer ofrganosilicas. Hundred-fold excess of nitrate-anions in solutionsncreases probability of their interaction with inner cavities of-CD molecules, and negative surface charge of �-cyclodextrin-ontaining silicas promotes sorption of metal cations. Cadmium (II)orption capacity decreases (in comparison with single-componentolutions of cadmium nitrate) for organosilica 1 significantly (by5%), but for organosilicas 2 and 3 by 13% and 15%, correspond-

ngly (Fig. 10, curves 3). In addition, organosilicas 1 and 2 adsorbmall amount of hardness salts (Fig. 10a and b, curves 4).

Sorption isotherms of cadmium (II) from Cd:(Ca + Mg) = 1:100olutions are well described by Langmuir equation (Fig. S2) foronolayer adsorption on localized centers of energetically uniform

urface [54]. This means that differences in specificity of individualctive centers of negatively charged surface of �-cyclodextrin-

ontaining silicas at cations sorption from multi-component nitrateolutions with high ionic strength are leveled.

Monolayer capacity Qm on cadmium (II) for �-yclodextrin-containing silicas at sorption from solutions with

able 9arameters of cadmium (II) sorption on �-cyclodextrin-containing silicas (concentratio

× 10−2–1.6 × 10−1 M).

Silica Distribution coefficient Kd Selectivity coefficient ˇCd/(Ca

Cd, mL/g (Ca + Mg), mL/g

Organosilica 1 125 4 31

Organosilica 2 200 0.43 465

Organosilica 3 340 0.15 2266

Organosilica 1 1.0:1.0 0.9:1.0 0.3:1.0Organosilica 2 4.0:1.0 4.0:1.0 4.0:1.0Organosilica 3 5.0:1.0 5.0:1.0 4.4:1.0

Cd:(Ca + Mg) = 1:100 (Table 7) is lower than Freundlich constantKF values for solutions with Cd:(Ca + Mg) = 1:10 (Table 7). Contentof cadmium (II) in monolayer especially noticeably decreasesfor organosilica 1. These results are in complete agreement withdata on simultaneous sorption of cadmium, calcium, and magne-sium cations using organosilica 1 as well as chemical analysis oforganosilicas after cadmium (II) sorption from solutions of variouschemical composition (Table 8).

Distribution coefficients of cadmium (II) and hardness salts,and also selectivity coefficients of cadmium (II) sorption frommulti-component nitrate solutions with Cd:(Ca + Mg) = 1:100for studied �-cyclodextrin-containing silicas are presented inTable 9. Capacity of �-cyclodextrin-containing silicas on cad-mium (II), distribution and selectivity coefficients at sorptionfrom single- and multi-component nitrate solutions vary sym-batically with complexing ability of side functional groups ofgrafted �-CD molecules: secondary alcohol groups < bromoacetylgroups < thiosemicarbazidoacetyl groups. Sorption results can beexplained in terms of theory of hard and soft acids and bases[58–61]: Cd(II) cations are soft acids, which form, as a rule, sta-ble complexes with soft bases, but Ca(II) and Mg(II) cations ashard acids – preferably with hard bases. The “softness” of sorp-tion centers of �-cyclodextrin-containing silicas increases in order:secondary alcohol < bromoacetyl < thiosemicarbazidoacetyl groups[58,59]. Consequently, Cd(II) ions in the formation of surfacecomplexes will prefer bromoacetyl and thiosemicarbazidoacetylgroups, but Ca(II) and Mg(II) ions–secondary alcohol groups. Really,a small sorption of Ca(II) and Mg(II) ions from Cd:(Ca + Mg) = 1:100solutions is observed for organosilicas 1 and 2, whereas organosil-ica 3 with soft functional groups does not adsorb hard Ca(II) andMg(II) cations at all (Fig. 10, curves 4). This interpretation is consis-tent with the results obtained by elemental and chemical analysis

of organosilicas after cadmium (II) sorption. The chemical com-position of the surface supramolecular structures depends on theaffinity of the �-cyclodextrin-containing silicas to cadmium cations(Table 9). The apparent advantage synthesized organosilicas is their

n of Cd(NO3)2 is 3.2 × 10−4–4.4 × 10−3 M, total concentration of hardness salts is

+Mg) Chemical composition of surface supermolecules after Cd(II) sorption

C42H69O34·Cd(NO3)2

C102H114O55S6Br9·4Cd(NO3)2

C111H150O55S15N27·5Cd(NO3)2

O. Shvets, L. Belyakova / Journal of Hazard

0.05 0.10 0.15 0.20 0.25 0.30 0.350.0

0.2

0.4

0.6

0.8

1.0

1.2

Ce Cd 102 (mol/L)

Ce Ca+Mg 10 (mol/L)

q e(molCd,Ca,Mg/molfunct.groups)

2

4

3

1

(a)

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.350

1

2

3

4

5

q e(molCd,Ca,Mg/molfunct.groups)

Ce Ca+Mg 10 (mol/L)

Ce Cd 102 (mol/L)

4

3

21

(b)

0.05 0.10 0.15 0.20 0.25 0.30 0.350

1

2

3

4

5

6

Ce Cd 102 (mol/L)

q e(molCd /molfunct.groups) 2

3

1

(c)

Fig. 10. Cadmium (II) sorption on �-cyclodextrin-containing silicas 1 (a), 2 (b),and 3 (c) from single solutions (curves 1), multi-component solutions with ratioCd:(Ca + Mg) = 1:10 (curves 2) and 1:100 (curves 3). Sorption of Ca(II) and Mg(II)ions from multi-component solutions with ratio 1: 100 (curve 4).

[

[

[

[

[

ous Materials 283 (2015) 643–656 655

ability to absorb along with cadmium cations also toxic nitrate-anions.

4. Conclusions

Functional organosilicas with “hard” and “soft” side functionalgroups of wide edge of grafted �-cyclodextrin molecules have beensynthesized. The structure and chemical composition of the surface,porosity of obtained materials, their chemical and thermal stabil-ity have been characterized by scanning electron microscopy, IRspectroscopy, thermogravimetry, nitrogen ad-desorption, elemen-tal and chemical analysis of solid surface. Sorption of cadmium (II)cations from single- and multi-component aqueous nitrate solu-tions has been studied. Coefficients of distribution and selectivityas well as the sorption parameters of Langmuir and Freundlichequations have been calculated.

It was found that the sorption equilibrium is reached after30 min. The sorption of trace amounts of cadmium (II) from multi-component solutions does not decrease, but even increases in thepresence of hardness salts, simulating soft and hard water. Thedriving force of cadmium (II) sorption on the surface of functional�-cyclodextrin-containing silicas is the formation of inclusion com-plexes “�-cyclodextrin–nitrate-anion”.

It has been proved the formation of supramolecular structureson the surface of synthesized organosilicas as a result of cadmium(II) sorption. Chemical composition of supermolecules depends onthe structure of surface active centers.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jhazmat.2014.10.012.

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