REACTIVE

Reactive & Functional Polymers 66 (2006) 1014–1020

www.elsevier.com/locate/react

&FUNCTIONALPOLYMERS

Functionalized sol–gel material for extraction of mercury (II)

Aslam Khan a,*, Farzana Mahmood b, M. Younas Khokhar b, Shujaat Ahmed a

a Nuclear Chemistry Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistanb Department of Chemistry, Bahauddin Zakariya University, Multan, Pakistan

Received 6 December 2004; received in revised form 18 January 2006; accepted 21 January 2006

Available online 24 February 2006

Abstract

1,5 Diphenylcarbazide (DPC) functionalized sol–gel silicas (DPCSG) were prepared and investigated for the solid phaseextraction of Hg(II) from aqueous solution by a batch equilibrium technique. The metal sorption properties of DPCSGwere examined to evaluate their performance toward metal extraction under different controlling factors, such as pH ofsolutions, equilibration shaking times, effect of foreign ions and complexing agents. The equilibration was fast and wasachieved within 30 min. Maximum extraction capacity of Hg(II) ions onto the DPCSG from non-competitive solutionswas 5.6 mg/g (0.028 mmol/g) at 28 ± 2 �C and the enhancement factor was found to be 560 at optimized pH 6 when com-pared with control and commercial silica gel. The adsorption data obtained as a function of mercury concentration werefitted to a Langmuir adsorption model. The stripping and regeneration of the sorbent were reported and it was furtherobserved that extraction and regeneration were indicated by the dark violet colouration and decoloration of the extractant,respectively. It is concluded from the extraction data that DPCSG is a promising material for the extraction of mercury.� 2006 Elsevier B.V. All rights reserved.

Keywords: Sol–gel; Diphenylcarbazide; Mercury; Extraction

1. Introduction

The use of organically functionalized solid sup-port extractants for separation–extraction of toxicmetals from aqueous media is gaining popularity[1–18] and reduced waste generation by the use offunctionalized solid support materials for extractionis the major advantage of such materials and it is rec-ognized that the method is an alternative to the well-known solvent extraction method. Functionalizationof the inorganic or organic solid support is usually

1381-5148/$ - see front matter � 2006 Elsevier B.V. All rights reserved

doi:10.1016/j.reactfunctpolym.2006.01.009

* Corresponding author. Tel.: +92 519290231; fax: +92519290275.

E-mail address: aslamkhan@pinstech.org.pk (A. Khan).

aimed at modifying the material with certain targetfunctional groups that can be exploited for furtherapplication, such as liquid chromatography, extrac-tion–separation and preconcentration. The inorganicsolid supports include silica gel, alumina, zirconiaand other oxide species of which silica gel is the mostwidely used solid support. To circumvent some limi-tations of immobilized silica gel, modified sol–gelmaterials have been discovered recently. This mate-rial has given rise to new prospects for adsorbentdesign. The sol–gel materials can be physically orchemically functionalized. The physical functionali-zation can be achieved by incorporation of organicreagent in the matrix when it is solidified. The ligandsin these hydrophilic solids are in the matrix of sol–gel.

.

A. Khan et al. / Reactive & Functional Polymers 66 (2006) 1014–1020 1015

Sol–gel sorbents functionalized with different organicreagents have been prepared and applied for extrac-tion and determination of metals [19–23].

In the current study, we have targeted the well-known toxic mercury (II) extraction by DPC dopedsol–gel silica (DPCSG). Our sol–gel silica sorbentwas found to catch Hg(II) from aqueous solutions.The adsorbed mercury stripped easily with 0.5 MHNO3 solution and the regenerated gels could beused in subsequent cycles for Hg(II) removal. In thispaper, the physico-chemical parameters, such as pH,kinetics of uptake and removal, possible mechanism,effect of diverse ions on the extraction, regenerationof sorbent and comparison of proposed sorbent withsome reported sorbents have been discussed.

2. Experimental

2.1. Reagents and chemicals

The chemicals in this study including tetraethoxy-silane (TEOS) 98.5% (Aldrich, USA), ethanol andammonium fluoride (Merck, Germany) and DPC(Fluka, Switzerland) were used as received. Doubledistilled water was used in the preparation of solu-tions, sol–gel reactions and for the washing of sor-bents. All other chemicals were of analytical grade.

The radiotracer 203Hg was prepared by neutronirradiation of spec-pure mercuric oxide in theresearch reactor PARR-1 of PINSTECH at a fluxof 7 · 1013 ns�1. The known amount of mercuryoxide was sealed in a polythene capsule before irradi-ation. The irradiated material was dissolved in con-centrated HNO3 acid, the acid was removed byrepeated washing with distilled water and diluted to25 ml. Further dilution was made from this stocksolution. The radiochemical purity of the tracer waschecked using a 25 cm3 Ge (Li) detector coupled witha 4k series of 85 Canberra, USA multichannelanalyzer.

Buffer solutions of 1–10 pH with an ionicstrength of 10 mmol were prepared by using appro-priate volumes of solutions of KCl and HCl (pH 1–2), sodium acetate–acetic acid (pH 3–6), sodiumdi-hydrogen phosphate–di-sodium hydrogen phos-phate (pH 7–8), and sodium hydrogen carbonate–sodium carbonate (9–10).

2.2. Apparatus/instruments

All pH measurements were made on a Metrohm-605 pH meter. The shaking was performed in a

30 ml pyrex culture vial having teflon lined screw-type polythene cap. Gross gamma-ray measure-ments were made on a Tennelec, USA countingassembly equipped with a well type 25 cm3 NaI(Tl)crystal.

2.3. Preparation of ligand doped and control sol–gel

The ligand doped sol–gels were prepared bymixing 40.0 ml of tetraethoxysilane, 40.0 ml ofdouble distilled water and 100 ml of ethanol inthe presence of 10�2 M ammonium fluoride as acatalyst; the gel started to appear in about 5 min.The prepared gel was placed at room temperature25 �C ± 2 for 24 h to complete the gelation andthen placed in an oven at 48 �C. During drying,shrinkage of the gel occurred and the gel crackedinto pieces. After 7 days, pieces of a transparenthard gel were obtained. The hard gel pieces werecrushed, ground and sieved to eliminate any poten-tial variations in adsorption properties due to par-ticle sizes. Control sol–gel silica was also preparedfor comparison according to the above procedurewithout DPC.

2.4. Conditioning of ligand-doped gels

The dry powder was soaked in water for 3 daysto leach the undoped reagent. In this process, someof the free reagent leached out of the sol–gel. Nofurther leaching was observed after 3 days soaking.After the leaching process, the sol–gel powder wasoven dried at 48 �C for 48 h and stored in a vac-uum desiccator. The sorbent was conditioned inappropriate buffer before it was used for theremoval of metals. The amount of doped reagentwas determined by spectrophotometric determina-tion of leached dye in the washings. About0.055 mmol of DPC was entrapped per gram ofsorbent.

2.5. Procedure for extraction of mercury ions by

sorbent

Hg(II) extraction from the single metal aqueoussolutions was investigated in batch adsorption-equi-librium experiments at room temperature(28 ± 2 �C). Sorption isotherm of mercury (II) ionwas studied by equilibrating 50 mg of the sorbentwith 5 ml of the solution of certain pH containingan appropriate quantity of tracer alone or tracerplus carrier of mercury. After equilibration, the vials

1016 A. Khan et al. / Reactive & Functional Polymers 66 (2006) 1014–1020

were centrifuged and 1 ml supernatant aqueousphase was radio-assayed for Hg203 by gammascintillation counter after checking for any type ofprecipitation or turbidity. Control trials were per-formed for experiments as and when required. Theconcentration of adsorbed metal ions (Q) was calcu-lated using the following equation:

Q ¼ ½ðCi � CfÞ=ma�V ðmg=LÞHere, Q is the amount of metal ion adsorbed

(mg g�1 sol–gel), where Ci and Cf are the concentra-tion of aqueous phases before and after equilibra-tion, respectively (mg L�1); ma is the quantity ofthe adsorbent (g); and V is the suspension volume(ml).

2.6. Effect of pH

The mechanism involved at a solid/solutioninterface can be explained with the help of a pHdependence study and such data can further helpin indicating the appropriate pH of effluent/wastewater for achieving a maximum efficiency in theremoval of ions by a sorbent. In view of this, thestudy was extended to incorporate pH dependencethus by varying the solution pH from 1 to 8 at afixed initial sorbate concentration.

The following equation was used to compute thepercentage removal:

% Sorption ¼ Ci � Cf

Cf� 100

where Ci is the initial activity of the solution and Cf

the final activity of the solution.

2.7. Kinetic experiments

The kinetic study was performed by followingdiscontinuous method. A series of screw-cap testtubes, each containing the same weighed amountof sorbent, were used for each kinetic run. Themoment when a known volume (typically 5 ml) ofa solution containing tracer was introduced intothe test tubes having sorbent and stirring wasstarted was taken as the time zero of the experi-ments. At various time intervals, one of the test tubewas quickly centrifuged at high speed, 1 ml of thesupernatant aliquot was assayed for metal ion anda plot of amount of metal ions retained on the sor-bent as a function of time was drawn, where eachexperimental point is the result of the measurementsperformed on a single test tube.

2.8. Distribution ratios (Rd)

Distribution ratios (Rd) were calculated as

Rd ¼ Ci � Cf

Cf

� VW

ml g�1

where Ci and Cf are the concentration of aqueousphases before and after equilibration, respectively,W is the weight of dry sol–gel silica (g) and V isthe volume of the aqueous phase (ml).

3. Results and discussion

3.1. Diphenylcarbazide (DPC) as a reagent

NH N H C

O

NH

1,5 Diphenylcarbazide (DPC) is the complexing or-ganic reagent. It was selected for this study becauseit has functional groups (–N–, =O) for ion exchangeand/or complexation of metal ions, removal-com-plexation of mercury [24] and its molecule is largeenough that can be securely entrapped inside thepores of sol–gel.

3.2. Effect of pH

The major factors that govern the binding ofmetal ions to chelate compound either in solutionor immobilized on solid support are the availabilityof metal ions and the nature of the donor atomsthat are mainly oxygen, nitrogen or phosphorus.As these adsorption controlling factors are influ-enced by pH value, therefore, to evaluate the stabil-ity of the newly synthesized sorbent for metal ionsextraction, we studied the effect of pH on theadsorption of mercury by equilibrating 5 ml of2.487 · 10�4 M mercuric ion solution with 50 mgsorbent and the results are depicted in Fig. 1. Thisfigure shows that the extraction behavior of metalions was sensitive to pH changes. In the case ofcontrol sol–gel and commercial silica gel, the effecton extraction is negligible. While in the case ofDPCSG, below pH 4, the protonated ligand siteshave no reactivity towards the positively chargedmercuric ions; when the pH of the solution is

0

20

40

60

80

100

0 42 6 8 10pH

Hg

(II)

rem

ove

d (

%)

Fig. 1. Removal of Hg(II) as a function of pH of solution(Hg(II) = 5.0 ppm, volume = 5 ml, sorbent = 50.0 mg, shakingtime = 30 min). ¤, DPC doped sol–gel silica; ·, commercialsilica gel; s, control sol–gel silica.

A. Khan et al. / Reactive & Functional Polymers 66 (2006) 1014–1020 1017

increased the extraction increases and attains itsmaximum value at pH 6. The observed phenome-non of high extraction at pH 6.0 may be attributedto more availability of deprotonated donor activesites responsible for the co-ordination of the softmetal mercury ion.

3.3. Equilibration time

The effect of shaking time on the extraction ofmetal ions is considered to be of significant impor-tance to study the extracting behavior of sorbent.Fig. 2 shows equilibrium adsorption times of themetal ions on 50 mg DPCSG from 5 ml of9.95 · 10�5 M mercuric solutions at pH 6.0. It is

50

60

70

80

90

100

0 10 20 30 40 50 60Time (minutes)

Hg

(II)

ext

ract

ed (

%)

Fig. 2. Extraction of Hg(II) vs. equilibration time (Hg(II)9.95 · 10�5 M, DPCSG = 50 mg, volume = 5 ml at pH 6).

evident from the figure that adsorption whichwas very rapid initially became slower with lapseof time. About 90% of the total extraction tookplace within the first 10 min and the colour ofthe sorbent turned dark violet after that; the addi-tional removal is slow. This may be attributed tothe fast complex formation between the metal ionsand DPC molecules near the surface of gel net-work. As reported earlier [21], at the initial stage,the process of adsorption was fast due to theavailability of abundant free sites near the surfacewhere the approach of metal ions faced less hin-drance. With the lapse of time and decrease inthe availability of active sites near the surface,the metal ions had to diffuse into the pores ofthe silica through the interconnected three-dimen-sional network of pores and channels before theyreacted with the entrapped reagent molecules andwere retained inside the pores.

3.4. Adsorption isotherm and mechanism

The results collected from the extraction ofHg(II) as a function of equilibrium concentrationof mercuric ions in 5 ml aqueous phase to investi-gate the adsorption capacities of the metal ions on50 mg doped sol–gel by varying the initial concen-tration of mercury from 7.96 · 10�7 to4.05 · 10�3 M are given in Fig. 3. Metal ion extrac-tion capacity of the sorbent increased first withincreasing initial concentration of the ions, and thenit reached a saturation level. The maximum adsorp-tion capacities of the doped sol–gel silica were foundto be 5.6 mg/g (0.028 mmol g�1) at 28 ± 2 �C and

0

0.01

0.02

0.03

0 1 2 3

Equilibrium of [Hg(II)] mmolL-1

Hg

(II)

rem

ove

d m

mo

lg-1

4

Fig. 3. Isotherm for removal of Hg(II) by doped sol–gel silica(DPCSG) (sorbent = 50 mg, volume = 5 ml, shaking time = 30 -min, initial concentration of Hg(II) in solutions = 1.6, 4.8, 8, 16,32, 48, 80, 160.320, 480, and 816 ppm).

0

20

40

60

80

100

120

140

0 1 2 3 4

Ce (mM)L-1

Ce/

qe

(gL

-1)

Fig. 5. Langmuir adsorption isotherm for sorption of Hg(III) byDPCSG.

1018 A. Khan et al. / Reactive & Functional Polymers 66 (2006) 1014–1020

that of control and commercial silica gel was verylow at optimized conditions.

The extraction of mercury by the sol–gel silicamodified with DPC may be contributed by tworoutes: (a) the removal of mercury by the plainsol–gel silica without being doped with the reagent,i.e., ion exchange of mercury with silanol groups(–SiOH)– and/or adsorption of mercury by –SiO�

on the surface of silica; (b) the removal of mercuryby the entrapped reagent, i.e., ion exchange and/orcomplexation of mercury with DPC entrapped inthe sol–gel silica. The large difference in extractioncapacity levels between doped and control sol–gelclearly indicates that the adsorption of Hg(II) bythe sol–gel is mainly attributed by the entrappedDPC and ion exchange of DPC with silanol groups(–SiOH) and/or adsorption of Hg(II) by –SiO� onthe surface of silica and the matrix of silica playsa very minor role in the extraction of Hg(II). Largersurface area of DPCSG (388.8 m2/g) than blanksol–gel silica (334.6 m2/g) and very low uptake ofmercury by blank under experimental conditionsmainly support the idea that the removal of mercuryfollows the second route.

3.5. Effect of initial concentration of metal ions

The effect of Hg(II) concentration on its sorp-tion was examined over about 100-fold concentra-tion from 7.96 · 10�3 to 4.00 mmol L�1 using50 mg of DPC doped sol–gel at optimized condi-tions and the results are depicted in Fig. 4. Thedistribution ratio showed a sharp decrease up to0.23 mmol L�1 and then becomes almost asymp-totic in nature.

0

1

2

3

4

0 1 2 3 4 5

Hg (II) mmolL-1

Rd

X10

00 m

l g-1

Fig. 4. Variation of the distribution ratio (Rd) of Hg(II) as afunction of its concentration.

3.6. Langmuir isotherm

The extraction data were further analyzed by fit-ting the data to Langmuir isotherm, which is shownin Fig. 5. The Langmuir equation governing theamount of metal sorbed and its concentration insolution is given by

Ce

qe

¼ 1

Nbþ Ce

N

where Ce is the equilibrium concentration (mmol),qe is the amount of mercury adsorbed at equilib-rium (mmol g�1), N the maximum sorption capac-ity (mmol g�1) and b a Langmuir constant. Thelinear plot (Fig. 5) of Ce/qe against Ce yielded astraight line, indicating the validity of applyingLangmuir isotherm model on the presentsorption process with b = 33.9 L mmol�1 andN = 0.029 mmol g�1 which is very close to theexperimental value (0.028 mmol g�1).

3.7. Effect of foreign ions

The presence of foreign ions influences the sorp-tion of metal ions on to the solid surface because ofcompetition for the sorption sites, complex forma-tion, oxidation–reduction of targeted ions by for-eign ions, dissociation and precipitation. In orderto evaluate the selectivity of sorbent, the sorptionof mercury was measured in the presence of suchdiverse ions and the results are tabulated in Table1. Most of the anions and cations were added astheir sodium salt and nitrate salts, respectively.The concentration of mercury was kept at9.37 · 10�4 M, while the concentration of diverse

Table 1Effect of foreign ions (10 mg as compound) on the sorption ofHg(II) onto DPCSG after 30 min agitation time from acetatebuffer solution of pH 6

Foreign ion % Sorption of Hg+2

Nil 89.8Se+4 81.9Cd+2 82.0Y+3 85.7Ni+2 80.5Cr+3 39.6Pb+2 88.9Mn+2 69.0Cu+2 20.2Fe+2 9.2Ascorbic acid 66.6EDTA 29.4S2O�2

3 NilSodium citrate 77.9

A. Khan et al. / Reactive & Functional Polymers 66 (2006) 1014–1020 1019

ions was about 100-fold of mercury concentrationat optimized conditions. Se(IV), Cd(II), Y(III),Ni(II), and Pb(II) reduced the extraction by62.7%, while Mn(II), Cu(II), Fe(II) ascorbate andEDTA reduced the extraction to high degree andS2O�2

3 completely stopped the extraction. Theexperimental data suggest that DPCSG can be usedfor separation of mercury from bimetal solutions ofmercury with Se(IV), Cd(II), Y(III), Ni(II) andPb(II).

3.8. Recovery of mercury and regeneration of sorbent

A sorbent is considered to be highly efficient ifthe release of retained metal ions is fast and quan-titative. It was found that 0.5 N HNO3 was fairlyeffective for recovering metal ions from the sol–gel sorbent. The sorbent loaded with a knownamount of mercury was washed with a pH 6 solu-tion. The washings were discarded after analyzing

Table 2Comparison of DPCSG with some reported sorbents for mercury

Adsorbent Capacity(mmol/g)

Equilibrattime

Dithiocarbamate-incorporated monosizepolystyrene microspheres

0.16 30 min

Procion Brown MX5BR immobilizedpoly(hydroxyethylmethacrylate/chitosan) composite membranes

0.34 45 min

Dithiocarbamate grafted on silica gel 0.30 2 h1,5 Diphenylcarbazide functionalized

sol–gel material0.028 30 min

for mercury. The mercury was stripped recoveredwith a single wash of 10 ml of 0.5 N HNO3 solu-tion. The desorption of mercury was also indicatedby the disappearance of violet colour. After mer-cury was recovered from the sorbent, the sorbentwas regenerated by washing with enough distilledwater, neutralizing with NaHCO3 (pH 8.2) solu-tion again and washing with distilled water, condi-tioning in pH 6 buffer solution for 2 h. Thisregenerated sorbent was used again for the extrac-tion of mercury.

The cycle of extraction–recovery–regenerationwas repeated three times for one batch of sorbents.The uptake performance of regenerated sorbent wasfound to be very close to the freshly prepared sor-bents which indicates that the sol–gel silica sorbentcan be regenerated and reused repeatedly at leastfor three times.

4. Comparison with alternative sorbents

Although sorbents reported in the literature forthe adsorption of mercury, shown in Table 2, caneffectively achieve mercury concentration reduction,there is scope for improvement. The modified silicadescribed in this work shows fast exchange kinetics,regenerability, reasonable selectivity and does notrequire chemical activation of silica. In addition,this class of sorbents are of low cost, easy and sim-ple to prepare and have controlled porosity whichcan provide control capacities and high physicalstability.

5. Conclusions

Sol–gel sorbents offer several advantages to tradi-tional solid support metal ion extraction techniques.The prepared sorbent attains rapid equilibration,

ion Separation of mercury from: References

Cu(II), Cd(II) and Pb(II) [25]

Cd(II), Pb(II) [26]

Effect of metal ions not reported [27]Effect of foreign metal ions suggestsseparation of mercury from Se(IV),Cd(II), Ni(II) and Pb(II)

Table 3Properties of control and DPCSG

Sample Control sol–gel silica DPC doped sol–gel silica (DPCSG)

Ingredients 20 ml TEOS + 20 mlH2O + 50 ml EtOH

20 ml TEOS + 20 ml H2O + 50 ml0.01 M Diphenylcarbazide EtOH solution

Catalyst NH4F NH4FDrying period at 48 �C 7 days 7 daysWeight of dried sol–gel silica (g) approx. 5.9 6.0Particle size (ASTM mesh number) � 80 + 120 � 80 + 120Mercury removal capacity (mmol g�1) 5.4 · 10�3 0.028Surface area m2/g 334.6 388.8% Enhancement factor – 560Elution solution – 0.5 N HNO3

Regeneration tested 3 times

1020 A. Khan et al. / Reactive & Functional Polymers 66 (2006) 1014–1020

exhibits hybrid properties of inorganic silica andcomplexing reagent DPC, shows high stabilitytowards medium and its regeneration is easy. In addi-tion, it does not require organic solvent for extractionand chemical modification of DPC ligand for its prep-aration. The major features summarized in Table 3recommend DPC doped sol–gel silica (DPCSG), anew class of sorbents, as an alternative to the otherbetter developed metal ion extraction techniques,such as solvent and ligand-grafted sorbents.

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