A comparative evaluation of mercury(II) adsorption equilibrium and kinetics onto silica gel and...

A Comparative Evaluation of Mercury(II) Adsorption Equilibrium andKinetics onto Silica Gel and Sulfur‐Functionalised Silica GelsAdsorbents

Khairiraihanna Johari,1 Norasikin Saman1 and Hanapi Mat1,2*1. Advanced Materials and Process Engineering Laboratory, Faculty of Chemical Engineering, Universiti Teknologi Malaysia,

81310 UTM Skudai, Johor, Malaysia

2. Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia

This paper presents a comparative evaluation of mercury(II) adsorption equilibrium and kinetics onto silica gel and sulfur‐functionalised silica geladsorbents. The silica gel was synthesised using tetraethyl orthosilicate (TEOS) as a precursor, while the sulfur‐functionalised silica gel adsorbents wereprepared through a co‐condensation method using TEOS as a precursor with bis(triethoxysilylpropyl)tetrasulfide (BTESPT) and 3‐mercaptopropyltrimethoxysilane (MPTMS) as sulfur ligands. The synthesised adsorbents were characterized by using SEM, FTIR, nitrogen adsorption/desorption(NAD), and EDX. A batch adsorption experiment was employed to evaluate the removal of Hg(II) by using the adsorbents. The study revealed that theHg(II) adsorption capacitywas found to be dependent on the initial Hg(II) concentration, agitation time, and pHof the solution. TheHg(II) adsorptioncapacities obtained for the SG‐TEOS, SG‐BTESPT and SG‐MPTMS were 40.95, 93.32 and 102.37mg/g, respectively. The Hg(II) adsorption was wellfitted to the Langmuir isotherm models (R2>0.935) and obeyed the pseudo‐second order kinetics model (R2>0.991). A further analysis of thekinetics suggested that the Hg(II) adsorption process was governed by both intra‐particle and externalmass transfer processes, in which film diffusionwas the rate‐limiting step.

Keywords: sol‐gel synthesis, organosilane, silica gel, adsorption, mercury ion

INTRODUCTION

Mercury is considered as a hazardous toxic heavy metal toboth humans and the ecosystem because of its hightoxicity to the central nervous system and the tendency to

bio‐accumulate in a human body.[1] Mercury compounds areemitted into the environment by various anthropogenic andnatural sources. The existing mercury in the environment isgenerally from industrial facilities such as metal plating, miningoperations, fertilizer industry, tanneries, and textile industries.[2] Itwas reported that heavymetals, includingmercury, are dischargedinto the environment via their waste effluents, which need to beremoved. Generally, mercury exists at low concentration in waterand sediments. Several technologies are reported to be effective inremoving mercury from liquid hydrocarbon feeds and wastestreams, which include adsorption, ion exchange, chemicalprecipitation, membrane filtration, photoreduction and electro-coagulation.[3–6] Among all these methods, adsorption process isone of the effective techniques that have been successfullyemployed for metal removal from wastewater.[7] The successfulapplication of the adsorption process for the heavy metal removalprocess depends, among others, on the adsorbent characteristicssuch as high surface area with favourable surface chemistry.

Generally, heavy metals, such as mercury in particular, showvery high affinity towards sulfur. It had been reported byresearchers that adsorbents containing sulfur in their structureor adsorbent modified with organic compounds containing sulfurpresent a good adsorption capacity and a high removal efficiency ofheavy metals, namely Hg, Ag, Zn and Cd.[8,9] Thus, in this study,sulfur was chosen as a functionalisation agent for adsorbentsmodification as a mercury atom/molecule tends to bind with anymolecule present that has either sulfur or sulfur hydrogencombination in its structure. The binding involved mercury, while

a soft Lewis acid prefers to bind with a pair of electron from Lewisbase (sulfur), which acts as an electron pair donor, based on theacid‐base theory.[10] Many researchers have recently focused theirstudy on ligands containing sulfur, which usually come in theforms of thiol, thioether, thiocarbamate, sulfate and sulfide groups,especially for mercury removal.[11]

The purpose of functionalisation into the organic and inorganicsolid supports is to modify the materials with a certain target offunctional groups, which can be exploited for further applica-tions.[12] The inorganic solid supports like silicas (e.g. silica gelsand mesoporous silicas) have been one of the most widely used,due to thewell‐known surface chemistry and the broad spectrumoforganosilanes, which are available tomodify the silica surfacewithfunctional groups to give it desired properties.[11,13,14] Structurally,they are stable and rigid. The silanol groups on the surface of thesilicas can be functionalised inmanyways,which then can serve asthe point of attachment of the chelating agents as supportingmatrices[15] and can be chemically modified with functionalcompounds.[10]

Alcantara et al.[10] studied the modification of silica gel by usingthe attachment of 2‐mercaptobenzimidazole (MBI) in removingHg(II) from the aqueous phase. The presence of soft and hardbasics centered in a pendant structure is an ideal method to removemetal ions, namely mercury, from water. Moreover, a research by

*Author to whom correspondence may be addressed.E‐mail address: [email protected]. J. Chem. Eng. 92:1048–1058, 2014© 2013 Canadian Society for Chemical EngineeringDOI 10.1002/cjce.21949Published online 3 December 2013 in Wiley Online Library(wileyonlinelibrary.com).

1048 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING VOLUME 92, JUNE 2014

Khan et al.[12] on the modification of 1,5‐diphenylcarbazide (DPC)functionalised sol‐gel silicas for the extraction of Hg(II) in theaqueous phase showed high stability towards themedium andwaseasy to regenerate. There was also a report on the extraction ofHg(II) from aqueous solution by using silica functionalised withthiol via grafting procedures.[13] Themodification of new silylationagent, which was anchored onto silica surface, presented a highaffinity for mercury due to the presence of two basic sulfur centresin the structure. Furthermore, the thiol‐functionalised silica gel,obtained via a surface imprinting technique, was applied for aselective solid‐phase extraction of Hg(II). It showed that ion‐imprinted functionalised adsorbent had a higher selectivity andadsorption capacity towards Hg(II) compared to non‐imprintedadsorbent.[10] Ladhe et al.[16] showed that the thiol‐functionalisedsilica‐mixed membranes contained free thiol (SH) groups whichhave the potential to capture silver from aqueous solutions.

In order to enhance adsorption capacity and selectivity, thechoice of ligands is important. For instance, sulfur ligands such asMPTMS and BTESPT have been widely used to functionalisemesoporous silica, which was observed having high adsorptioncapacity and selectivity towards Hg(II) due to its thiol (MPTMS)and thioether (BTESPT) content.[11,17–23] Table 1 shows the Hg(II)adsorption capacity of MPTMS and BTESPT functionalisedadsorbents, as reported by several researchers. So far, onlyWang et al.[11] had studied Hg(II) onto MPTMS functionalisedsilica gel that gave a maximum adsorption capacity (qmax) of44.13mg/g. So far, there has been no comparative study of sulfurligands functionalised silica gel for Hg(II) adsorption. In this study,we thus synthesised sulfur‐functionalised silica gel (SFSG)adsorbents using BTESPT and MPTMS as sulfur ligands via in‐situ procedure for mercury adsorption in the aqueous phase. Thechoice of silica gel as a support may offer a better alternative to theexisting adsorbents due to its good physical, chemical andmechanical properties. The Hg(II) adsorption equilibrium andkinetics of these sulfur‐functionalised silica gels, as well as a silicagel, were studied. In addition, the adsorbent regeneration potentialof the selected adsorbent was also demonstrated.

MATERIALS AND METHODS

Materials

The synthesis and functionalised procedures used the followingchemicals: tetraethyl orthosilicate [TEOS] (99%, Merck,Germany), 3‐mercaptopropyl trimethoxysilane [MPTMS] (ACROS,Belgium), and bis(triethoxysilylpropyl) tetrasulfide [BTESPT](Power Chemical Corporation, China) (Merck, Germany) inFigure 1 were used as received. Other chemicals such as ethanol

[EtOH] (99%), ammonia solution [NH3] (25%), sodium hydroxide[NaOH] (99%), nitric acid [HNO3] (25.0%), potassium iodide [KI](99%), hydrochloric acid [HCl] (36.5–38.0%), andmercury nitratemonohydrate [Hg(NO3)2] were obtained from Merck (Germany),Goodrich Chemical Enterprise, and J.T. Baker (Europe). Deionisedwater used throughout the experiments was obtained from a PuriteWater Systemmodel Select Analyst HP40 (U.K.) that is available inour laboratory.

Preparation of SG‐TEOS

The preparation of silica gel using a sol‐gel process was carried outaccording to the modified method by Stöber et al.[24] It wasprepared via three steps: (i) hydrolysis of TEOS; (ii) gelation andaging; and (iii) drying. Firstly, the homogenous solution ofethanol, water and aqueous ammonia (base catalyst) wasprepared. 6mL of TEOS was added drop‐wise into the ethanol/water/ammonia solution while stirring at 600 rpm for 10min, andthe stirring was continued for another 2 h. The hydrolysis andaging of gels were carried out at 308C and 508C, respectively. After2 days of aging, the produced precipitatewasfiltered,washed usingdeionised water and dried at ambient temperature (30� 18C). Thefinal product was then ground to produce a fine powder before itunderwent characterisation and adsorption studies. This silica gelsample was denoted as SG‐TEOS.

Preparation of SG‐BTESPT and SG‐MPTMS

The functionalisation of silica gel with organic sulfur was carriedout via co‐condensation process (in‐situ procedure). The solutionsof ethanol, water and ammonia were mixed, forming an ethanol‐water‐ammonia homogenous solution. 6mL of BTESPT/TEOS orMPTMS/TEOS mixture was added drop‐wise into the ethanol‐water‐ammonia solution while stirring at 600 rpm for 2 hours. Theconcentration of BTESPT/TEOS or MPTMS/TEOS used was 10%(v/v). The hydrolysis and aging of gels were carried out at roomtemperature (30� 18C) and in the oven at 50� 0.58C, respectively.Finally, after 2 days of aging, the resulting products were filtered,washed and dried. Both adsorbents synthesised using BTESPT/TEOS and MPTMS/TEOS were denoted as SG‐BTESPT and SG‐MPTMS, respectively.

Characterisation of Adsorbents

The surface morphology of the fractured particle of SG adsorbentswas analysed using a scanning electron microscope (SEM) modelJEOL JSM‐6390LV. The elemental composition (weight percent-age) of the SG adsorbents was determined using energy dispersiveX‐ray (EDX) diffractometer model JSM‐6701F. The FTIR spectra ofSG adsorbents were analysed using a Perkin Elmer Model 2000, inorder to determine the existence of functional groups on the SG

Table 1. The maximum adsorption capacity for Hg(II) ions of different adsorbents reported in literatures.

Adsorbents Sulfur ligands Maximum adsorption capacity, q (mg/g) Refs.

Geduran (G 60) MPTMS 194.57 [17]

Kiesek gel 40 (K 60) MPTMS 176.52 [17]

Silica gel MPTMS 44.13 [11]

MCM‐41 MPTMS 118.35 [18]

SBA‐15 (654m2/g) BTESPT 1280.00 [19]

SBA‐15 (242m2/g) BTESPT 2710.00 [19]

Diatom silica MPTMS 185.20 [20]

MCM‐41 MPTMS 249.00 [21]

Zeolite MPTMS 89.10 [22]

Diatom particle MPTMS 9.00–13.40 [23]

VOLUME 92, JUNE 2014 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING 1049

surfaces. Themeasurements were carried out over 4 000–400 cm�1

using the KBr discmethod. The surface area and pore volume of theSG adsorbents were determined by using nitrogen adsorption/desorption (NAD) method using Micromeritics ASAP 2000.

Hg(II) Adsorption Experiment

Hg(II) adsorption experiments were carried out according to thebatch adsorption method. The mercury stock solutions fromHg(NO3)2 salts were prepared for each experiment. In a typicalequilibrium adsorption experiment, 25mg of adsorbent wassuspended in a 125mL Erlenmeyer flask containing 25mL ofaqueous Hg(II) solution with a concentration of 50mg/dm3. Theinitial pH of the Hg(II) solution was varied from pH 2.0 to 14.0using HNO3 and NaOH solutions. The mixture was then agitated at200 rpm in a temperature‐controlled orbital shaker for 2 days at30� 0.58C. Two days of agitation was sufficient for the mixture toachieve equilibrium conditions. The solution was filtered usingnylon membrane filters (0.45mm) to remove the adsorbent andthe concentration of Hg(II) solutions was determined using anatomic absorption spectrophotometer (AAS) (model Perkin ElmerPrecisely HGA 900). The effects of initial Hg(II) concentrations(25–200mg/dm3), agitation time (1–1 440min) and temperature(30, 40 and 608C) were also studied.

The amount of Hg(II) adsorbed at equilibrium was calculatedusing the following equation:

qe ¼ðCo � CeÞV

W

� �ð1Þ

where qe is the amount of Hg(II) adsorbed (mg/g), Co and Ce arethe concentrations of the aqueous phase before and afterequilibrium (mg/dm3), respectively, W is the weight of adsorbent(g), and V is the suspension volume (mL). In the adsorption timedependence study, the amount of the Hg(II) adsorbed at time t, qt is

given by:

qt ¼ðCo � CeÞV

W

� �ð2Þ

where Ct is the Hg(II) concentration at time, t. As shown in Table 2,the isotherm models of the Langmuir, Freundlich, Dubinin‐Radushkevich, and Tempkin were used to model the isothermadsorption data, while the pseudo‐first‐order, pseudo‐second‐order, Elovich, Weber and Morris, Boyd plots, and film diffusionmodels were used to model the kinetic adsorption data.

Adsorbent Regeneration Experiment

A batch regeneration experiment was conducted using mercuryloaded SG‐MPTMS adsorbent using the same procedure as used in

Figure 1. Chemical structure of precursors: tetraethylorthosilicate (TEOS), bis(triethoxysilylpropyl)tetrasulfide (BTESPT) and 3‐mercaptopropyltrimethoxysilane (MPTMS).

Table 2. Non‐linear forms of isotherm and kinetic (chemical reactionand diffusion) models.

Isotherm modelsLangmuir qe ¼ qmaxKLCe

1þKLCe

Freundlich qe ¼ KFC1=ne

Dubinin‐Radushkevich (D‐R) qe ¼ qmax � expð�b � e2ÞTempkin qe ¼ RT

bT� lnðKT � CeÞ

Chemical reaction based modelsPseudo‐first order qt ¼ qeð1� e�k1tÞPseudo‐second‐order qt ¼ q2e k2t

1þqek2t

Elovich qt ¼ 1bln ðabÞ þ 1

bln t

Diffusion based modelsWeber and Morris qt ¼ kidt0:5

Boyd plots B ¼ p2 D1a2

Film diffusion qtqe¼ 6 D2

pa2� �0:5

t0:5


the adsorption experiment. The regeneration experiment wascarried out by agitating the mercury loaded adsorbent with 25mLof 0.1M HCl and KI solutions as a desorption agent. The agitationrate was fixed at 200 rpm for 1 day to allow sufficient time forremoving the unadsorbed mercury ions. The mixture was thenfiltered, washed using deionised water and dried at ambienttemperature (30� 0.58C). These adsorption and desorption wereperformed for five times using SG‐MPTMS in order to assess theirregenerability and stability.

RESULTS AND DISCUSSION

Adsorbent Characterisations

In this study, the synthesis of silica gels was prepared according tothe Stöber method by using ammonia as a catalyst in basiccondition.[24] With the presence of co‐solvent (e.g. ethanol) inbasic condition, the morphology of silica gels tended to bespherical.[25] Figure 2a shows the SEM images of silica gelsynthesised using TEOS as a precursor. These images clearlyillustrate no significant difference in spherical particles sizes(0.27–0.29mm). Figures 2b,c exhibit the morphology and particlesizes of sulfur‐functionalised silica gels via co‐condensationprocess (in‐situ procedure). The SEM image of SG‐BTESPTdemonstrates that the particles were spherical, and some non‐spherical aggregates adhered on to the surfaces with particle sizes

in the range of 0.32–0.34mm. In contrast, fractured and irregularshapes with sizes of 1.37–3.95mm were observed for SG‐MPTMS.The description of the morphology and particle size estimation forthe synthesised adsorbents are listed in Table 3.

Nitrogen adsorption/desorption isotherms and pore sizedistributions of SG‐TEOS, SG‐BTESPT and SG‐MPTMS adsorbentsare shown in Figure 3. According to IUPAC classifications, theadsorption branch for SG‐TEOS, SG‐BTESPT corresponding to theType II isotherm and SG‐MPTMS adsorbent gave a Type IVstandard isotherm, respectively. Estella et al.[26] reported thesimilar trend of isotherm types of synthesised silica xerogels. Broadpore size distributionswere observed for SG‐TEOS and SG‐BTESPT,while a narrow pore size distribution was observed for SG‐MPTMSadsorbent. The total BET surface area and porosity of synthesisedadsorbents are summarised in Table 3. The results showed that thesurface area of SG‐MPTMS adsorbent was observed to be largerwhich might be due to the existing thiol functional groups in thesilica gel framework. This indicated that the higher BET surfacearea and micropore volume of the SG‐MPTMS might haveattributed to high adsorption capacity. A report by Im et al.[27]

implied that the high surface area and large pore sizes are believedto enhance the metal binding and uptake kinetics of porousmaterials. In contrast, the SG‐BTESPT showed similar physicalproperties of the SG‐TEOS.

The Si, O, C and S elemental compositionwasmeasured by usingEDX diffractometer, and the results are summarised in Table 3. All

Figure 2. SEM images of (a) SG‐TEOS, (b) SG‐BTESPT and (c) SG‐MPTMS adsorbents.


the synthesised adsorbents possessed a high content of silicon andoxygen elements. As expected, there were no sulfur and carbondetected for SG‐TEOS. The presence of sulfur and carbon elementsin SFSG adsorbents was due to the existence of sulfur in thestructure of BTESPT and MPTMS. The presence of tetrasulfide (S4)at the centre in BTESPT structure resulted in a higher sulfur loadingin the SG‐BTESPT compared to the SG‐MPTMS. The sulfur contentsof SG‐BTESPT and SG‐MPTMS were observed to be 2.07% and1.70%, respectively.

The FTIR spectra of the synthesised adsorbents were obtainedand compared as shown in Figure 4. The basic characteristics ofsynthesised SG‐TEOS in the range of 4 000–2800 cm�1, showed alarge and broad band near 3 400 cm�1, which was attributed to theOH bond stretching from silanol group (SiOH) and some adsorbedwaters. In general, this strong peak, related to the hydrolysis and

polycondensation of TEOS with water, could be described with theexistence of hydroxyl group on the surface of silica. This band wasoften accompanied by another adsorption peak at 1 634 cm�1

belonging to the OH vibrations of adsorbed water. The observedpeaks around 1 030–1 200 and 800 cm�1 were attributed toSi��O��Si asymmetric stretching and the bands around950 cm�1, which belonged to SiO vibrations, indicating theoccurrence of silica matrices. These results are similar to thosereported by previous researchers.[11,28,29,30] The FTIR spectra forSG‐BTESPT and SG‐MPTMS were observed to be similar with SG‐TEOS. However, an additional peak was observed in the spectradue to the introduction of sulfur ligands into the silica frameworks.The SG‐BTESPT showed a characteristic peak of CH groupstretching (2 950 cm�1) and SS, SC (568 cm�1) which wereassociated with functional groups in BTESPT which had been

Table 3. Morphology, elemental compositions, particle size estimation, and structural characteristics of adsorbents.

Adsorbents Morphology

Elemental composition (wt. %)

Particle size (mm) SBET (m2/g) Vp (cm3/g) Rm (nm)Si O C S

SG‐TEOS Spherical 46.040 53.960 — — 0.270–0.400 24.950 0.096 15.410SG‐BTESPT Spherical 44.000 53.230 0.700 2.070 0.320–0.470 24.940 0.003 22.058SG‐MPTMS Non‐spherical 44.040 52.720 1.530 1.700 1.370–3.950 448.230 0.013 10.690

Figure 3. Low temperature nitrogen adsorption/desorption isotherms and pore size distributions: (a) SG‐TEOS, (b) SG‐BTESPT and (c) SG‐MPTMSadsorbents.


successfully functionalised into the silica gel. Similar results werepreviously reported by Alcantara et al.,[10] Guo et al.[31] and Tenget al.[32] In the spectra of SG‐MPTMS, the SH stretching vibrationcould be found around 2 600–2 500 cm�1,[20] which could not beobserved in the spectrum of SG‐TEOS and SG‐BTESPT. Thisindicates that the MPTMS was chemically bound to the silicasurfaces.

Effect of pH

The pH of the aqueous solution is an important controllingparameter in the adsorption process.[7] Figure 5 presents the effectsof initial and equilibrium pH on the Hg(II) adsorption by threeadsorbents. It was observed that equilibrium pH increasedwith theinitial pH below 7, and the equilibrium pH tended to be lower thanthe initial pH above 7. The changes in pH of the Hg(II) solution

showed that the adsorption of Hg(II) increased with the increase ofthe initial pH up to pH 10, and it decreased thereafter. This could beexplained by the fact that at low pH values (pH< 4), the availableH3O

þ ions might have competed with Hg(II) ions for active sites onthe SG solid surfaces, which reduced the total amount of Hg(II)adsorbed.With an increasing pH, increased rates of Hg(II) sorptionwere observed due to the decreasing H3O

þ concentration and theformation of positively charged Hg(OH)þ. Furthermore, atinitial pH value >10, the adsorption of mercury decreased asHg(OH)2 became the dominant species in the solution. In this case,the repulsion electrostatic effect between the SG solid surfaces andnegative charge species might have occurred. Similar results werealso obtained by Asadi et al.,[33] where formations of negativelycharged hydrolysed ions decreased the adsorption of metal ions athigher pH. In addition, the high Hg(II) adsorption capacity of SG‐BTESPT and SG‐MPTMSwere observed at a low solution pH. Thesemight be due to several mechanisms such as physical adsorption,ion exchange, micro‐precipitation and chelation that involvedduring Hg(II) adsorption by both adsorbents. The behaviour ofHg(II) adsorption was similar with the report by Cestari andAiroldi,[34] where higher Hg(II) adsorption was achieved at a lowsolution pH using thiol‐silica adsorbent, due to the fact that SHgroup is more dissociated in aqueous medium.

Effect of Hg(II) Concentrations

Figure 6 shows the effects of initial mercury concentration in theaqueous solution of Hg(II) adsorption for SG‐TEOS, SG‐BTESPTand SG‐MPTMS. At initial pH 7.15, increasing the initialconcentration of Hg(II) solution from 2.70 to 155.10mg/dm3

increased the adsorption of Hg(II) by the adsorbents. Overallresults showed that the SG‐MPTMS gave the highest adsorptioncapacity compared to the SG‐TEOS and SG‐BTESPT due to thepresence of thiol functional group in the structure of MPTMS,which had a high affinity towards soft acid metals, namelymercury. These results were similar to those reported by previousresearchers.[11,13,35,36]

Table 4 shows the values of constant parameters and correlationcoefficients, R2 obtained from linear regression analysis. Theconstants obtained from this linear regression analysis were usedto plot the non‐linear regressive data (line) and compared with

Figure 4. FTIR spectra of synthesized adsorbents.

Figure 5. Effect of pH on Hg(II) adsorption onto silica gel adsorbents.Experimental conditions: [Hg(II)]¼50mg/dm3; agitation time¼2 days;temperature¼30�0.58C; and agitation speed¼200 rpm.

Figure 6. The effect of initial Hg(II) concentration on the Hg(II) adsorptioncapacity by SG‐TEOS, SG‐BTESPT and SG‐MPTMSadsorbents. Experimentalconditions: initial pH¼7.5; agitation time¼2 days;temperature¼30�0.58C; and agitation speed¼200 rpm.


experimental data as shown in Figure 7. The results showed thatthe Langmuir model gave the best fitting with higher R2 values forSG‐TEOS and SG‐MPTMS adsorbents. In the case of SG‐BTESPT,higher R2 values were observed for Langmuir, Freundlich, andDubinin‐Radushkevich; however, Langmuir gave the best fitting

for the experimental data (Figure 7). This can be further explainedby the “L” curve types observed for the non‐linear plots ofsynthesised adsorbents. The curves do not reach any plateau,suggesting that the solid did not show a clear limited adsorptioncapacity.[37] Hence, from the R2 values and curve plots, the

Table 4. Langmuir, Freundlich, Temkin and Dubinin‐Radushkevich (D‐R) isothermmodel parameters and correlation coefficients for Hg(II) adsorption ofSG‐TEOS, SG‐BTESPT and SG‐MPTMS adsorbents.

Isotherm models Parameters

Adsorbents

SG‐TEOS SG‐BTESPT SG‐MPTMS

Langmuir qmax (mg/g) 50.000 111.110 125.000KL (l/mg) 0.458 0.083 0.129R2 0.994 0.973 0.935Dqe (%) 25.442 23.588 14.417

Freundlich KF (l/mg) 2.713 10.783 15.8801/n 0.614 1.734 0.536R2 0.929 0.977 0.850Dqe (%) 30.946 30.363 31.417

Dubinin‐Radushkevich (D‐R) b (mol2/kJ2)�109 5.000 4.000 4.000qmax (mmol/g) 1.484 3.006 2.820R2 0.955 0.993 0.849Dqe (%) 30.720 24.686 28.173

Tempkin KT (dm3/g) 166.274 438.929 891.677bT (kJ/mol) 57.253 30.351 28.955R2 0.987 0.921 0.837Dqe (%) 25.934 28.965 20.359

Figure 7. The isotherm models analysis of Hg(II) sorption onto silica gel adsorbents adsorption isotherm data: a) SG‐TEOS; (b) SG‐BTESPT; and (c)SG‐MPTMS adsorbents. Experimental conditions: initial pH¼7.5; agitation time¼2 days; temperature¼30�0.58C; and agitation speed¼200 rpm.


Langmuir isotherm is more suitable for describing the Hg(II) ontoSG‐TEOS, SG‐BTESPT and SG‐MPTMS adsorbents. It is assumedthat the process occurred at substantially homogeneous functionalgroups and binding sites on the surface of the adsorbents up to amonolayer coverage (chemical adsorption). Similar adsorptionbehaviour was also reported by Yu et al.[20] on Hg(II) adsorptiononto MPTMS functionalised diatom silica.

Effect of Agitation Time

The results of the Hg(II) adsorption onto SG‐TEOS, SG‐BTESPT,and SG‐MPTMS adsorbents as a function of contact time are shownin Figure 8. It was observed that the amount of Hg(II) adsorbedincreased with an increase in agitation time. These resultsrevealed that the Hg(II) adsorption was rapid at the initial stagesof the contact period, and slowly reaching the equilibriumconditions. For all three adsorbents, almost 50% of Hg(II) wasadsorbed within the first 30min. This might be due to the fact thata large number of vacant surface sites were available during theinitial stage of the adsorption process. After that, the remainingvacant surface sites were difficult for the solution to absorb due torepulsive forces between the solute molecules on the solid andbulk phases.[38] The final equilibrium time showed that theadsorbents reached saturation at approximately 8 h. A furtheranalysis of the time dependence of the Hg(II) adsorption capacitydata should provide information about Hg(II) adsorption kineticand its mechanism.

The experimental data of Hg(II) adsorption kinetics wereanalysed by using a linear regression method, and the parametersobtained substituted the non‐linear equation and plotted, asillustrated in Figure 8. The values of parameters (slope andintercept) and R2 obtained from the linear regression are tabulatedin Table 5. The rate constant of SG‐MPTMS was observed to belarger than the rate constants of SG‐TEOS and SG‐BTESPTadsorbents. This might be due to the existence of SH functionalgroups from the SG‐MPTMS surface which had a stronger metal‐binding capability and allowed the adsorption to reach equilibriumfaster than other adsorbents.[20] In addition, the results indicatedthat the calculated qe values of the pseudo‐second order weregenerally closer to the experimental values compared to those fromthe pseudo‐first order and Elovich equation models. The goodagreement in experimental and calculated qe values indicated thatthe adsorption system obeyed the pseudo‐second order kineticmodel for the entire adsorption period. In addition, the kineticmodel predictions of the Hg(II) adsorption onto the threeadsorbents (Figure 8) exhibited the pseudo‐second‐order kineticmodel as the best fitting model compared to other models.Thus, the model assumption of chemical adsorption agreedwell with the correlation coefficient values for the second‐orderkinetic model obtained for all three adsorbents above 0.991,indicating the applicability of the second‐order kinetic model forthe entire adsorption process. Ramadhan et al.[39] and Yu et al.[20]

had also reported that the pseudo‐second‐order model is more

Figure 8. The kinetic models analysis of Hg(II) adsorption onto silica gel adsorbents experimental data: (a) SG‐TEOS; (b) SG‐BTESPT; and (c) SG‐MPTMSadsorbents Experimental data conditions: [Hg(II)]¼50mg/dm3; initial pH¼7.5; temperature¼30�0.58C; and agitation speed¼200 rpm.


suitable for the description of the Hg(II) adsorption kinetics ontoadsorbents.

The physical steps can be described by analysing the processusing diffusion based kinetic models, such as the intra‐particlediffusion model, external mass transfer model, and the Boyd plot,as summarised in Table 2. The diffusion kinetic plots fitting andcoefficients are shown in Figure 8 and Table 5, respectively. It wasnoticed that the plots did not provide an accurate fitting to the SGadsorbent kinetic data as compared to the pseudo‐second‐ordermodel. The calculated results indicated that the film diffusioncoefficients for SG‐TEOS, SG‐BTESPT and SG‐MPTMS adsorbents(1.564� 10�13, 0.191� 10�13 and 1.107� 10�13m2/min) wererelatively lower than the pore diffusion coefficients (5.032� 10�13,0.457� 10�13 and 4.574� 10�13m2/min). These supported thesuggestion that the Hg(II) transfer process occurred slower insidethe boundary film than in the pores within the adsorbents.

Furthermore, it is believed that this could confirm the dominatingnature of the film diffusion mechanism on the overall Hg(II)adsorption process by SG adsorbents.

Temperature Dependence of Thermodynamic Parameters

The effect of temperature on Hg(II) adsorption onto adsorbent wascarried out for temperatures of 30, 40 and 608C. The resultsrevealed that the adsorption capacity slightly increased when thetemperature increased from 30 to 608C. These results indicatedthat the Hg(II) adsorption onto SFSG adsorbents was endothermicin nature. The Gibbs free energy of the adsorption reaction,DG8 (kJ/mol) is given by Equation (3):

DG0 ¼ �RT lnKa ð3Þ

where Ka is considered to be the adsorption equilibrium constantwhich is obtained by: Ka¼Ce(adsorbent)/Ce(solution). The Gibbsfree energy change is also related to the change in enthalpy, DH0

(kJ/mol) and entropy, DS0 (kJ/molK) and at constant tempera-ture, as given by Equation (4):

DG0 ¼ DH0 � TDS0 ð4Þ

Combining Equations (3) and (4) gives the van’t Hoff equation(Equation 5):

lnKa ¼ �DG0

RT¼ �DS0

R��DH0

RTð5Þ

where T is the absolute temperature (K) and R is the universal gasconstant (8.314 J/mol.K). The values of DG0, DS0, and DH0 of theadsorbents were calculated, as shown in Table 6. The negativevalue of DG0 indicated a favourable and spontaneous nature ofthe Hg(II) adsorption onto the adsorbents. The values of DG0 forthe adsorbents indicated that more negative values of DG0 wereobserved from sulfur‐functionalised silica gels adsorbents (SG‐BTESPT and SG‐MPTMS), which implied a greater driving force ofHg(II) sorption compared to SF‐TEOS. The positive value of DS0

suggested that the randomness of the solid‐solution interfaceincreased during theHg(II) sorption process, and the positive valueof DH0 indicated that the sorption reaction was an endothermicprocess. These results showed that the favourable behaviour ofHg(II) sorption process agreed well with the literature as reviewedby Doke and Khan.[40]

Adsorbent Regeneration

The regeneration experiment was carried out by recycling theadsorbent for reuse in multiple cycles. In this study, HCl and KIsolutions were used as regenerating agents for SG‐MPTMS

Table 5. Kinetic parameters and correlation coefficients (R2) for Hg(II)adsorption of SG‐TEOS, SG‐BTESPT and SG‐MPTMS adsorbents at30�1oC.

Kinetic models

Adsorbents

SG‐TEOS SG‐BTESPT SG‐MPTMS

Pseudo‐first‐orderqe,exp (mg/g) 25.500 40.500 47.500qe,theory (mg/g) 13.243 28.744 19.409k1�103 (min�1) 2.000 2.000 7.000R2 0.934 0.988 0.965

Pseudo‐second‐orderqe,exp (mg/g) 25.500 40.500 47.500qe,theory (mg/g) 25.640 41.667 47.620k1�103 (g/mgmin) 5.300 0.300 1.840h (mg/gmin) 3.484 0.521 4.082R2 0.999 0.991 0.999

Elovicha (mg/gmin) 185.949 2.495 440.130b (g/mg) 0.465 0.185 0.269R2 0.786 0.954 0.919

Intra‐particle diffusion model (Weber‐Morris)Kid1 (mg/gmin0.5) 2.306 1.597 4.118R2 0.863 0.828 0.860Kid2 (mg/gmin0.5) 0.170 1.597 0.259R2 0.761 0.946 0.812

Pore diffusion model (Boyd plot)D1�1013 (m2/min) 5.032 0.457 4.574R2 0.766 0.929 0.937

Film diffusion modelD2�1013 (m2/min) 1.564 0.191 1.107R2 0.802 0.964 0.956

Table 6. Thermodynamic parameter data for the Hg(II) adsorption onto silica gel adsorbents.

Adsorbents Temperature (K) q (mg/g) DG8 (kJmol�1) DHo (kJmol�1) DS8 (kJmol�1 K�1)

SG‐TEOS 303 25.988 �0.180 1.680 0.006313 26.725 �0.410333 26.853 �0.400

SG‐BTESPT 303 42.476 �4.190 34.860 0.130313 43.240 �5.210333 47.185 �8.010

SG‐MPTMS 303 47.798 �7.320 103.690 0.370313 50.367 �14.540333 48.980 �18.980


adsorbent. In addition, to assess the reusability of this adsorbent,the adsorption‐desorption was repeated for four cycles. The resultsof the regeneration studies are shown in Figure 9. It was observedthat, the percentage of Hg(II) amount adsorbed was almostconstant when using both desorption agents. Jeon and Park[41]

reported similar results for the desorption of Hg(II) ions usingaminated chitosan beads.Moreover, the SG‐MPTMS adsorbentwasfound to be able to remove about 95.59% and 97.35% Hg(II) forHCl and KI solution respectively, through five adsorption cycles.However, as shown in Figure 9, the pattern of the Hg(II) desorptionindicates the increased percentage of adsorption while using the KIsolution as desorbing agent. This might be due to the formation ofrelatively more soluble iodide (I�) complexes compared to thechloride complexes.[42] The desorption performance of theregenerated adsorbent, SG‐MPTMS was found to be very close tothe freshly prepared adsorbent. This indicates that the SG‐MPTMSadsorbent can be regenerated and reused repeatedly for at least fourtimes. Moreover, this adsorbent also showed stability towardsHg(II). Therefore, it can be concluded that the SG‐MPTMS canpotentially be used economically in the actual process.

CONCLUSIONS

Silica gel adsorbentswere successfully synthesised according to themodified Stöber method using TEOS and organosilanes (BTESPTand MPTMS) as a precursor and functionalisation agents,respectively. The sulfur‐functionalised silica gel (SFSG) adsorbentsprepared via a co‐condensation method were observed to havehigher sulfur content, larger surface area and pore diametercompared to pure silica gel.

The present results show that the SFSG adsorbents had a betteradsorption capacity than silica gel due to the fact that sulfur ligandshave high a affinity towards mercury. The SG‐MPTMS showed thehighest adsorption capacity, followed by SG‐BTESPT and SG‐TEOS,observed at pH between 7.0 and 8.5. The isotherm data of theSG‐TEOS, SG‐BTESPT and SG‐MPTMS adsorbents fitted theLangmuir isotherm model with maximum Hg(II) adsorptioncapacities of 50.00, 111.10 and 125.00mg/g, respectively. Inaddition, the kinetic data fitted well into the pseudo‐second orderkinetic model, in which the film diffusion was the rate‐limitingstep. The regeneration studies carried out on SG‐MPTMS showedthat the adsorption capacity of the recycled adsorbents was

maintained at 90% after 5 cycles of regeneration process,indicating that the SG‐MPTMS can be a potential adsorbent forthe mercury removal process because of its high adsorptioncapacity, favourable fast kinetics and good regenerability.

ACKNOWLEDGMENT

The financial supports from MOSTI under the eScience ResearchProgram (Project Vot. 79410), MOHE under the FundamentalResearch Grant Scheme (Project Vot. 78602), the 2009 ExxonMobilResearch Grant, ExxonMobil Exploration and ProductionMalaysia, Inc. (Project Vot. 73339) and the University ResearchGrant (Project Vot. GUP 00H63) are gratefully acknowledged.

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Manuscript received April 04, 2013; revised manuscript receivedJuly 02, 2013; accepted for publication July 05, 2013.


A comparative evaluation of mercury(II) adsorption equilibrium and kinetics onto silica gel and...

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