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Journal of Hazardous Materials 167 (2009) 1215–1221

Contents lists available at ScienceDirect

Journal of Hazardous Materials

journa l homepage: www.e lsev ier .com/ locate / jhazmat

reparation and evaluation of thiol-functionalized activated aluminaor arsenite removal from water

umin Hao, Mei-Juan Han, Xiaoguang Meng ∗

enter for Environmental Systems, Stevens Institute of Technology, Hoboken, NJ 07030, USA

r t i c l e i n f o

rticle history:eceived 30 July 2008eceived in revised form 27 December 2008ccepted 28 January 2009vailable online 7 February 2009

eywords:

a b s t r a c t

Arsenic species such as arsenite [As(III)] and arsenate [As(V)] are known human carcinogens. Though lotsof metal oxide adsorbents have been developed for removal of As(V), they are much less effective forAs(III) adsorption. In this study, various inorganic–organic hybrid adsorbents bearing thiol groups havebeen prepared by modifying activated alumina (AA) with mercaptopropyl-functionalized silica underdifferent experiment conditions. Raman spectra demonstrated the successful functionalization of AAand verified the formation of As–S complexes after As(III) adsorption. Batch experiments were applied

rsenitehiolctivated aluminaybrid adsorbentdsorption

to evaluate the As(III) adsorption performance of the hybrid adsorbents. Compare with AA, the hybridadsorbents exhibited enhanced adsorption abilities for As(III) due to the introduction of thiol groups,and as the thiol loading increased, the uptake of As(III) increased. Experimental results indicated thatthe hybrid adsorbents still maintained the merit of the AA for As(V) adsorption. Based on the results,one hybrid adsorbent referred to as BL(AA)30(MPTS)3.3 has been selected by consideration of not only theadsorption capacity but also its environmentally friendly and cost-effective production. The investigation

brid a

has indicated that the hy

. Introduction

Arsenic (As) is one of the most toxic contaminants found in thenvironment. Arsenic contamination has been receiving increasedttention and emerged as a major concern on a global scale inecent years [1,2]. Chronic intake of arsenic has been associatedith increased risk of cancers, diabetes, developmental and repro-uctive problems, and cardiovascular disease [2–5]. To minimizehese risks, the World Health Organization (WHO) has set a pro-isional guideline limit of 0.01 mg L−1 for arsenic as the drinkingater standard [6]. The United States Environmental Protectiongency (USEPA) announced its ruling in 2001 to lower the max-

mum contaminant level (MCL) from 50 to 10 ppb, and this newegulation has become effective from January 2006 [7,8].

The more stringent drinking water standard has a significantmpact on the management of arsenic contaminated sites and has

otivated researchers and water treatment industries to developnnovative arsenic removal technologies. The current water treat-

ent technologies, such as coagulation/precipitation, ion exchange,ime softening, and metal oxide adsorption, are effective for remov-ng arsenate [As(V)], but much less for arsenite [As(III)] [9–12]

ainly because As(III) is a hydrophilic, neutral species below pH

∗ Corresponding author. Tel.: +1 201 216 8014; fax: +1 201 216 8303.E-mail address: xmeng@stevens.edu (X. Meng).

304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.jhazmat.2009.01.124

dsorbents are very promising for As(III) removal from water.© 2009 Elsevier B.V. All rights reserved.

9 and is not accessible to the major removal mechanisms of anionsorption and anion exchange [11]. Compared with As(V), As(III),which is present predominantly in anoxic groundwater, is moretoxic and more mobile [13–16]. To enhance As(III) removal, a pre-oxidization of As(III) to As(V) is usually involved using oxidizingagents or photocatalytic oxidation on TiO2 [9,11,17–22]. However,the application of a pre-oxidation step causes operational complex-ity, increases cost of water treatment, and especially diminishes theoverall viability of the fixed-bed process [23]. These problems havehighlighted the urgent necessity for the development of direct andcost-effective As(III) remediation technologies without the need foradditional pre-oxidization.

It is well known that As(III) has an especially high affinity fortissue proteins by strongly binding to mercaptan (thiol) groupsexisting in biomolecules such as amino acids, peptides and proteins(including some enzymes), which explains the higher toxicity ofthis species and the metabolism of arsenic in mammals [5,24–26].Based on the above understanding, some thiol-based adsorbentsincluding polymer resins and silica gels have been developed forAs(III) removal [27–30], and some of them exhibited the selectiveremoval of As(III) and had a high adsorption capacity. Recently,McKimmy et al. [31] synthesized thiol-functionalized mesostruc-

tured silica using direct assembly methods for As(III) trapping, andthe As(III) concentration was reduced by up to 98% under batchequilibrium conditions. More interestingly, a biomass (treatedwaste chicken feathers) with a cysteine-rich protein has been usedfor selective As(III) adsorption up to 270 �mol g−1 of biomass [26].

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216 J. Hao et al. / Journal of Hazard

hese results suggested that materials bearing thiol functionalroups as As(III) adsorbents are very promising, and prompted uso develop new thiol-functionalized adsorbents for As(III).

Activated alumina (AA), as a low-cost material, has a high sur-ace area and a distribution of both macro- and micropores, and haseen extensively studied and used as arsenic adsorbent [2,32,33].he AA adsorption has been classified among the best availableechnologies (BAT) for arsenic removal from water [34]. How-ver, the conventional commercially available AA has ill-definedore structures, low adsorption capacities and exhibits slow kinet-

cs [35]. Moreover, the uptake of As(III) by AA is much less thanhat of As(V) in most pH conditions [32]. To improve its perfor-

ance, some modified types of AA such as Fe(OH)3-coated AA,ron oxide-impregnated AA, manganese-amended AA and biopoly-

er chitosan-coated AA, have been developed and proved to beore effective for As(III) and As(V) removal than virgin AA [36–41].owever, to our knowledge, there is no report on the use of

hiol-functionalized AA for arsenic removal up to present althoughhiol-modified alumina materials have been used to adsorb certaineavy metals like Hg, Pb, Cd, etc. [42,43]. The thiol-functionalizedA is expected to take advantage of the strength of the twoaterials.In this study, various novel hybrid adsorbents bearing

hiol groups were prepared by introducing mercaptopropyl-unctionalized silica onto the surface of the AA under differentonditions. The hybrid adsorbents were characterized by scanninglectron microscopy (SEM), Raman spectroscopy and Brunauer–mmett–Teller (BET) surface area analysis. Their adsorption prop-rties for As(III) were investigated and the effects of the variousactors on As(III) removal are discussed. It is also expected that theybrid adsorbents will still be effective for As(V) removal due to theresence of aluminol active sites remaining uncovered. The broadbjective of this investigation was to develop a cost-effective andnvironmentally friendly hybrid adsorbent for remedying ground-ater and drinking water contaminated by As(III), or both As(III)

nd As(V), which is also viable for a fixed-bed process withoutre-oxidation of As(III).

. Materials and methods

.1. Materials and characterization

(3-Mercaptopropyl)triethoxysilicane (MPTS, 80%, technical) andodium arsenite (NaAsO2, Certified) were purchased from AldrichMilwaukee, WI, USA) and Fisher Scientific (Fair Lawn, NJ, USA),espectively. All other chemicals were analytical grade and pur-hased from Aldrich or Fisher Scientific and used as received. Thectivated alumina (AA-400G-48 MESH) was obtained from Alcanpecialty Aluminas (Brockville, ON, Canada), and sieved with 70esh sieve to removal fine particles. As(III) and As(V) stock solu-

ions containing 1000 mg L−1 (ppm) of arsenic was prepared byissolving NaAsO2 and Na2HAsO4·7H2O in Milli-Q ultrapure water,espectively. The arsenic working solutions were freshly preparedy diluting the stock solution with aged tap water that had beenurged with oxygen-free nitrogen gas for at least 30 min prior tose. The aged tap water used in this study contained approximately1 ppm of calcium, 16 ppb of iron, 2.1 ppm of potassium, 7.4 ppm ofagnesium, 32 ppm of sodium, 150 ppb of phosphorus, 4.4 ppm of

ulfur, 1.9 ppm of silicon and 58 ppm of chlorin.The Raman spectra were taken with ∼4 cm−1 spectral resolution

n a Thermo Nicolet Almega XR Dispersive Raman Spectrometer

Thermo Fisher Scientific Inc., USA). The surface area and porosityf the adsorbents were determined with a Micromeritics ASAP 2010nalyzer (Micromeritics Instrument Corporation, Norcross, GA).he morphology and microstructure of the adsorbents were exam-ned using a field-emission scanning electron microscope (LEO 982,

aterials 167 (2009) 1215–1221

LEO Electron Microscopy Inc., Thornwood, NY) operated at an accel-erating voltage of 5 kV.

2.2. Preparation of the adsorbents

The hybrid adsorbents were prepared via two different methodsdescribed as follows:

In situ (IS) method: the in situ hydrolysis–condensation of MPTSwas carried out in the presence of AA. Several drops of glacial aceticacid (HAcO) or 1 M HCl (in the case of dry toluene as a reactionmedium, no acid was used) was added into a mixture containing3.3 mL of MPTS and 100 mL of various liquid media (dry toluene, dryethanol, 95% or 70% aqueous ethanol and water as shown in Table 1).The mixture was purged with nitrogen gas for 30 min and then 30 gof AA was added. The sealed mixture was allowed to react for 24 hat 75 ◦C and then filtered. The solid product was washed severaltimes with 95% ethanol to remove any unbound ingredients. Afterair-drying overnight under a hood, the solid was dried at 75 ◦C for48 h under vacuum to yield the hybrid adsorbents of the IS series

Blending (BL) method: the hybrid adsorbents were prepared bysimply blending the AA with pre-synthesized thiol-functionalizedsilica sol. Briefly, 10 �L of 1 M HCl was added to a solution of1.0–5.0 mL of MPTS in 25 mL of 95% ethanol. The mixture waspurged with nitrogen gas for 30 min followed by shaking for 24 h at75 ◦C. After the mixture was cooled to room temperature, 30 g of AAwas added and mixed completely. The samples were air-dried undera hood overnight and then at 75 ◦C for 48 h under vacuum. Thus,the mercaptopropyl silica from the MPTS hydrolysis–condensationreaction was coated onto the AA to give the hybrid adsorbents ofthe BL series

All the hybrid samples were labeled as M(AA)30(MPTS)n, where“M” represented the preparation method, “(AA)30” and “(MPTS)n”indicated the composites and their quantities used in the prepara-tion. For example, the sample BL(AA)30(MPTS)3.3 means the samplewas prepared by the BL method, and 30 g of AA and 3.3 mL of MPTSwere used. All the hybrid adsorbents synthesized in this work arelisted in Table 1.

2.3. Batch adsorption tests

All the batch adsorption experiments were conducted inacid-washed high-density polyethylene bottles under a nitrogenatmosphere, and 1.0 g L−1 of AA or hybrid adsorbent was used.To a suspension containing 0.10 g of adsorbent in aged tap waterwas added a defined amount of arsenic stock solution to obtaina 100-mL of mixture with the desired initial arsenic concentra-tion (2–20 mg L−1). After the solution pH was adjusted to 7.0 ± 0.1by adding 0.1 M HCl and NaOH, the suspension bottle was placedin a rotary mixer and shaken at room temperature. Throughoutthe experiment, a nitrogen atmosphere and neutral pH value weremaintained. After 38 or 70 h of mixing, 1.5 mL of supernatant liquidwas taken and centrifuged for 10 min to separate the solution fromthe solid for analysis of residual arsenic concentration.

2.4. Chemical analysis

The arsenic concentration in the solution was determined witha graphite furnace atomic absorption spectrometer (GFAAS, VarianZeeman Spectra AA 220Z). In order to monitor and ensure that theAs(III) was not significantly oxidized during adsorption, speciationanalysis was conducted in some selected samples using disposable

speciation cartridges [44] immediately after the samples were col-lected, and the results indicated that only 10–15% of the solubleAs(III) was oxidized after 3 days of mixing. In all experiments, pHwas measured with a pH meter (Orion 920A+), which was calibratedwith three buffers (pH 4.0, 7.0 and 10.0) weekly.

J. Hao et al. / Journal of Hazardous Materials 167 (2009) 1215–1221 1217

Table 1The adsorbents prepared in this work and their adsorption for As(III). Adsorption tests were conducted in suspensions containing 1.0 g L−1 adsorbent. Initial As(III) = 20 mg L−1,equilibrium pH 7.0 ± 0.1, equilibrium time = 38 h.

Entry Adsorbent Reaction medium Initial As (III)a (mg L−1) As (III) adsorbed on adsorbent % Removal

(mg g−1) (� mol g−1)

1 Activated alumina – 20 4.64 62 232 IS(AA)30(MPTS)3.3 Dry toluene 20 10.62 142 533 IS(AA)30(MPTS)3.3 Dry EtOH + HAcO 20 6.63 88 334 IS(AA)30(MPTS)3.3 95% EtOH + HAcO 20 7.11 95 365 IS(AA)30(MPTS)3.3 95% EtOH + HCl 20 10.55 141 536 IS(AA)30(MPTS)3.3 90% EtOH + HCl 20 9.92 132 507 IS(AA)30(MPTS)3.3 70% EtOH + HCl 20 9.78 131 498 IS(AA)30(MPTS)3.3 Water + HCl 20 9.28 124 469 IS(AA)30(MPTS)3.3 Water + HCl b 20 7.98 107 40

10 BL(AA)30(MPTS)1.0 95% EtOH + HCl 20 5.95 79 3011 BL(AA)30(MPTS)1.8 95% EtOH + HCl 20 7.00 93 3512 BL(AA)30(MPTS)2.5 95% EtOH + HCl 20 8.06 108 4013 BL(AA)30(MPTS)3.3 95% EtOH + HCl 20 9.89 132 4914 BL(AA)30(MPTS)4.2 95% EtOH + HCl 20 10.69 143 53

in pre

r(

q

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3

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adsorbents may be significantly altered due to the microstructurechange.

The physical characteristics of some adsorbents obtained fromBET measurements are listed in Table 2. As anticipated, the BET

Table 2Physical characteristics of some adsorbents.

Adsorbent BET surfacearea (m2 g−1)

Average porediameter (nm)

Total pore volume(cm3 g−1)

Activated alumina 237 5.24 0.33BL(AA) (MPTS) 226 5.09 0.29

15 BL(AA)30(MPTS)5.0 95% EtOH + HCl 20

a The volume of As(III) solution used in the batch experiments was 100 mL.b The volume of the reaction medium (water) is three times more than that used

The As(III) adsorbed on the adsorbents and the percentageemoval (R%) of As(III) were calculated according to Eqs. (1) and2), respectively, as shown below:

= V([As]0 − [As]eq)W

(1)

% = 100([As]0 − [As]eq)

[As]0(2)

here q is the concentration of the adsorbed arsenic in the unitf mg g−1, which is easily converted to �mol g−1; V is the solu-ion volume (L); W is the mass of the adsorbent (g); and [As]0nd [As]eq are the initial and equilibrium arsenic concentrationsmg L−1), respectively.

. Results and discussion

.1. Preparation of the hybrid adsorbents

The modification of solid surfaces by organosilanol chemistryr sol–gel process is routinely used for material functionalizationsffording new physico-chemical surface properties and enablingiverse applications. Various functionalized adsorbents for heavyetal adsorption have been developed by modifying porous

ubstrates or filtration membranes with organosilane reagents con-aining certain target functional groups such as mercapto, amino,nd carboxyl groups [45–48]. Here, the AA modification with MPTS,ffording hybrid adsorbents for arsenic removal, was performedsing two methods, i.e. IS and BL. There are two possible approacheso introduce thiol functional groups onto the AA surface associatedith the IS method. When the dry toluene (aprotic environments)as used as the reaction medium, a direct condensation reac-

ion between ethoxyl groups of MPTS and hydroxyl groups of AAurface occurred to create Al–O–Si covalent bonds. Thus, a self-ssembled monolayer (SAM) of mercaptopropyl-functionalizedilica was formed over the AA surface; On the other hand, whenhe modification of AA with MPTS was conducted in aqueous

edia with an acid as a reaction catalyst (protic environments),mercaptopropyl-functionalized silica multilayer would be con-

tructed. In the IS method involved in both approaches mentioned

bove, covalent bonding predominates. While in the BL method,PTS was first hydrolyzed and polymerized to create organosilica

ol in 95% aqueous ethanol with HCl as a catalyst, followed by gelormation and coating onto the AA surface. This is a sol–gel-likerocess, and physical coating took place.

11.53 154 58

paration of adsorbent 8.

Evidently, the resulting adsorbents are influenced by prepara-tion methods as well as a large number of experimental parameters.To optimize the preparation conditions, the IS method was appliedto prepare the adsorbents IS(AA)30(MPTS)3.3 with consistent MPTSdosage in diverse reaction media (dry toluene, dry ethanol, 95%and 70% aqueous ethanol and water). Using the BL method, theadsorbents BL(AA)30(MPTS)n with various MPTS dosage, i.e. dif-ferent n values varying from 1.0 to 5.0 mL, were prepared in 95%ethanol solutions to investigate the effect of MPTS dosage on adsor-bent properties. HCl or HAcO was used to catalyze the MPTShydrolysis–condensation reactions except in the toluene system.Table 1 lists the adsorbents prepared in this work and the virgin AA.

3.2. Characterization of the adsorbents

The morphologies of the AA and the hybrid adsorbents werecharacterized by SEM. Fig. 1 presents typical SEM images of AA(Fig. 1a and c) and the hybrid BL(AA)30(MPTS)3.3 (Fig. 1b and d). Thetop panel shows the large scale images, and the bottom panel showsthe high-magnification images. The large scale images showed thatthere was no obvious difference in adsorbent surface structures,indicating that the organosilica coating onto the AA surface wasuniform. The high-resolution micrographs revealed the effect ofthe coating on the microstructure. The surface of the virgin AA wascomposed of many small particles, which led to a rough surfaceand the presence of a porous structure. After coating, fewer smallparticles were observed and some bigger congeries appeared. Itcan be expected that the physicochemical properties of the hybrid

30 1.0

BL(AA)30(MPTS)1.8 224 5.00 0.29BL(AA)30(MPTS)2.5 207 4.91 0.25BL(AA)30(MPTS)3.3 202 4.78 0.25BL(AA)30(MPTS)4.2 186 4.94 0.23BL(AA)30(MPTS)5.0 172 4.87 0.21

1218 J. Hao et al. / Journal of Hazardous Materials 167 (2009) 1215–1221

and (

siCs2oi

osiah

Fa

Fig. 1. SEM images of the adsorbents: (a, c) AA

urface area, pore diameter and pore volume exhibited a decreas-ng trend as the initial dosage of MPTS increased from 1.0 to 5.0 mL.ompared with virgin AA (a surface area of 237 m2 g−1 was mea-ured), the specific surface areas of the hybrid adsorbents (from26 to 172 m2 g−1) decreased by approximately 5–30%. The changef structural characteristics and properties may exert an importantnfluence on the adsorption performance of the hybrid adsorbents.

In order to verify the mercapto organosilane functionalization

f AA and the binding of the As(III) to the mercapto groups, Ramanpectroscopy was utilized. The Raman spectra of the samplesncluding virgin AA and the hybrid adsorbent before and after As(III)dsorption are shown in Fig. 2. Compared with AA (spectrum a), theybrid adsorbent (spectrum b) clearly showed the characteristic

ig. 2. Raman spectra of (a) AA and (b) hybrid adsorbent before and (c) after As(III)dsorption.

b, d) the hybrid adsorbent BL(AA)30(MPTS)3.3.

bands of the mercaptopropyl organosilica [31,49] besides the bandsat 497 and 362 cm−1 observed in the AA spectrum [50]. The mer-captan S–H stretching appeared at 2566 cm−1 as a strong band, andthe C–H vibration bands of mercaptopropyl groups appeared in therange of 2927–2866 cm−1. The Raman peaks at 1296 and 1252 cm−1

assigned to CH2–Si and CH2–S, respectively, were observed clearly.The C–S stretching mode was found at 647 cm−1 with a higherintensity. The spectroscopy results are unambiguously indicative ofthe existence of the mercaptopropyl groups in the hybrid adsorbent.After As(III) adsorption, a strong new shoulder band (overlappedwith the band at 362 cm−1) appeared at 375 cm−1 in the Ramanspectrum (c), indicating the formation of the As–S bond. At the sametime, the S–H stretching showed a dramatic decrease in intensity.The Raman spectra change provided evidence for the binding ofAs(III) directly to the thiol groups. It should be pointed out that theresidual of the thiol band implied that not all the thiol groups wereaccessible for As(III) binding, which will be discussed in detail inthe following paragraphs.

3.3. Effects of preparation conditions on As(III) removal

To optimize the preparation conditions, the hybrid adsorbentswere prepared using the IS method in diverse reaction media andthe BL method with different MPTS dosages. As shown in Table 1,all the adsorbents prepared under various conditions exhibitedenhanced adsorption capacities for As(III) by 30–150% comparedwith AA under the same adsorption conditions. Accordingly, theAs(III) removal percentages were much higher than that of AA.For the adsorbents 2–9 prepared by the IS method with consistentMPTS dosage, the experimental results indicated that the As(III)adsorption performance depended on the preparation conditions

including reaction media, catalysts (acids), and water content. Thedry toluene reaction medium endowed the adsorbent with thehighest adsorption capacity for As(III), probably due to the forma-tion of a uniform MPTS monolayer with a high degree of coverage,which enabled the highest level of the available –SH active sites.

J. Hao et al. / Journal of Hazardous Materials 167 (2009) 1215–1221 1219

Table 3As(III) uptake by the hybrid adsorbents with different –SH loading. Adsorption tests were conducted in suspensions containing 1.0 g L−1 adsorbent. Initial As(III) = 20 mg L−1,equilibrium pH 7.0 ± 0.1, equilibrium time = 38 h.

Adsorbent Dosage of MPTS (n value) (mL per 30 g of AA) SH loadinga (�mol g−1) As(III) adsorbed (�mol g−1) As (III)/–SH molar ratiob

BL(AA)30(MPTS)1.0 1 135 79 0.47:1BL(AA)30(MPTS)1.8 1.8 240 93 0.31:1BL(AA)30(MPTS)2.5 2.5 330 108 0.26:1BL(AA)30(MPTS)3.3 3.3 430 132 0.25:1BL(AA)30(MPTS)4.2 4.2 540 143 0.21:1B

ing alf As(I

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exhibited an ascending trend as the feed As(III) amounts declined.When the initial As(III) concentration was higher than 14 mg L−1,the adsorption reached a plateau and a maximum of about 9 mg g−1

was obtained. When the initial As(III) concentration was lower thanabout 5 mg L−1, the As(III) removal was more than 90%; for the sys-

L(AA)30(MPTS)5.0 5 635

a The loading concentrations were obtained by the theoretical calculations assumb The adsorbed As(III)/–SH molar ratios were computed by dividing the amount o

t was found that the adsorbent prepared in 95% aqueous ethanolith HCl as a catalyst showed a similar capacity. HCl as the catalyst

avored a higher adsorption capacity than HAcO, which can be seeny comparison among the adsorbents 3–5. Examination of the rolef the water content in the reaction media (see adsorbents 5–9)mplied that the adsorbent performance decreased with increasedater content. The increase of water content in the reaction media

esulted in decreased solubility of MPTS in aqueous ethanol or insol-bility in water, which might lead to a lower coating rate and higher

nhomogeneity.Comparing adsorbents 5 and 13 prepared by different methods

ut in the same reaction media, we found that these two adsor-ents showed high and comparable adsorption abilities for As(III),

ndicating that the preparation method (IS vs. BL) did not have a sig-ificant effect on the adsorbent performance. However, it should beointed out that there are several advantages associated with theL method compared with the IS method, such as easier operation,

ower damage to the AA structure, and use of low toxic (or non-oxic) and low-cost agents (ethanol and water) rather than toluene.herefore, the BL method using aqueous ethanol or water as a reac-ion medium is clearly a cost-effective and environmentally benignpproach to achieve the hybrid adsorbents.

.4. Effect of MPTS dosage on As(III) removal

The hybrid adsorbents BL(AA)30(MPTS)n prepared by the BLethod were used to investigate the effects of MPTS dosage ons(III) removal. Table 3 lists the MPTS dosage (n value) in each adsor-ent and the corresponding –SH loading concentration calculatedheoretically. The amount of As(III) adsorbed on each adsorbentnd the molar ratios of the adsorbed As(III) to –SH loading con-entration are also summarized in Table 3. From the results, withhe elevation of the value of n from 1.0 to 5.0 (–SH loading from35 to 635 �mol g−1), the uptake of As(III) increased from 79 to54 �mol g−1. At the same time, we noted that the As(III)/–SHolar ratio decreased, which means that the percentage of active

ites accessible to As(III) decreased as the –SH loading quantityncreased. In other words, the higher the –SH loading, the greaterhe As(III) uptake: however, the lower is the accessibility of the –SHroups. Elevating –SH loading in the adsorbents definitely favorshe capture of more arsenite due to more active sites being avail-ble. On the other hand, the elevation of –SH loading (MPTS dosage)esulted in a thickened coating layer and thus decreased surfacerea for the adsorbents. Thus, more –SH groups will be embed-ed in the coating, and the percentage of the active –SH groupsxposed to As(III) species will decline, which consequentially leadso a decrease in the As(III)/–SH molar ratio. The fact mentionedbove implies that increasing the MPTS dosage to enhance As(III)

dsorption capacity is not a cost-effective way. Furthermore, asentioned at the beginning, one of our goals is to develop an adsor-

ent suitable for arsenic removal including both As(III) and As(V). Aigh degree of MPTS coating will enclose the active aluminol groupsAl-OH) of the AA surface and impair the intrinsic merit of AA for

154 0.19:1

l the mercaptopropyl silica synthesized from MPTS has been introduced onto AA.II) (�mol g−1) adsorbed by the –SH loading (�mol g−1) for each adsorbent.

As(V) uptake, which will be experimentally demonstrated in Sec-tion 3.6. Therefore, there exists a significant issue concerning theoptimum loading amount of MPTS in the preparation of the hybridadsorbents.

In order to ascertain the appropriate MPTS dosage, both theadsorbed As(III) and As(III)/–SH molar ratio were plotted vs. MPTSdosage (n value) or –SH loading concentration, respectively, as illus-trated in Fig. 3. As the dosage of MPTS decreased from 5 to 1 mL,the As(III)/–SH molar ratio exhibited a stepped-up increase whenthe dosage of MPTS was less than 2.5 mL; however, in this rangethe As(III) adsorption capacity was very low. In order to balancethese two contrary factors, an MPTS dosage of 3.3 mL per 30 g ofAA may represent the best tradeoff among the overall propertiesof the hybrid adsorbents. Thus BL(AA)30(MPTS)3.3 was selected forfurther investigation.

3.5. Effect of initial As(III) concentration on As(III) removal

BL(AA)30(MPTS)3.3 was used for the investigation of the effectof the initial As(III) amount in the feed solution on the adsorp-tion behavior of the hybrid adsorbent. Fig. 4 shows the adsorbedAs(III) and the corresponding removal percentage as a functionof initial As(III) concentrations. As expected, with the decline ofthe feed As(III) amount, the As(III) adsorbed onto the adsorbentdecreased. Moreover, such decrease appeared steeper in the lowerconcentration range. However, the removal percentage of As(III)

Fig. 3. Removal of As(III) and adsorbed As(III)/–SH molar ratio as functions of MPTSdosage (or –SH loading amount) in the suspensions containing 1.0 g L−1 adsorbent.Initial As(III) = 20 mg L−1, equilibrium pH 7.0 ± 0.1, equilibrium time = 38 h.

1220 J. Hao et al. / Journal of Hazardous M

Fig. 4. Removal of As(III) as a function of the initial As(III) concentration in thesuspensions containing 1.0 g L−1 adsorbent. Equilibrium pH 7.0 ± 0.1, equilibriumtime = 70 h.

Table 4As(V) uptake by AA and the hybrid adsorbents with different –SH loading. Adsorp-tion tests were conducted in suspensions containing 1.0 g L−1 adsorbent. InitialAs(V) = 20 mg L−1, equilibrium pH 7.0 ± 0.1, equilibrium time = 38 h.

Adsorbent Initial As(V)(mg L−1)

As(V) adsorbed onadsorbent (mg g−1)

% Removal

Activated alumina 20 18.7 93.5BL(AA)30(MPTS)1.0 20 18.5 92.5BL(AA)30(MPTS)1.8 20 18.1 90.5BBBB

ttmcwl

3

aAitrnatt(tomstaa

4

d

[

L(AA)30(MPTS)2.5 20 17.8 89L(AA)30(MPTS)3.3 20 17.4 87L(AA)30(MPTS)4.2 20 16.8 84L(AA)30(MPTS)5.0 20 16.6 83

em with 2 mg L−1 of initial concentration, the residual arsenic inhe solution was measured to be as low as 6 �g L−1 (ppb), which

eans 99.7% of As(III) was removed. This value is lower than theurrent USEPA MCL of 10 �g L−1 for drinking water. In most ground-ater contaminated by As(III), the concentration of As(III) is far

ower than 2 mg L−1.

.6. As(V) adsorption

As stated in the introduction, although the hybrid adsorbentsimed at As(III) removal from water, they should also be effective fors(V) removal due to the presence of active aluminol sites remain-

ng uncovered. The hybrid adsorbents BL(AA)30(MPTS)n were usedo investigate the As(V) adsorption. The experiments were car-ied out under the same conditions as As(III) adsorption exceptitrogen atmosphere was not applied. Table 4 lists the As(V)dsorption capacities and the adsorption percentages by AA andhe hybrid adsorbents. The results in the table indicate that allhe hybrid adsorbents tested here exhibited high As(V) capacities18.5–16.6 mg L−1) and removal efficiencies (92.5–83%). Althoughhe As(V) adsorption capacity decreased gradually with the increasef –SH loading (MPTS dosage), the hybrid adsorbents still hadore than 83% of As(V) adsorption ability of the virgin AA. For the

elected BL(AA)30(MPTS)3.3, compared with AA, the As(V) adsorp-ion dropped by only 7% (from 18.7 to 17.4 mg g−1) while As(III)dsorption increased by more than 110% (from 4.64 to 9.89 mg g−1

s shown in Table 1).

. Conclusions

Thiol-functionalized organic–inorganic hybrid adsorbents forirect removal of As(III) from water have been prepared by means

aterials 167 (2009) 1215–1221

of introducing the reactive ingredients onto AA as substrate. Thehybrid adsorbents were characterized by SEM, Raman spectroscopyand BET surface area analysis. Based on the evaluation of the dif-ferent preparation methods and conditions, a cost-effective andenvironmentally benign approach has been developed to obtainthe optimum hybrid adsorbent with the best balance of overallproperties. Compared with AA, the functionalized hybrid adsor-bents have decreased surface areas, pore sizes and pore volumes,and exhibited enhanced adsorption abilities for As(III). As the thiolloading increased, the uptake of As(III) increased, while the effi-ciency decreased. For the selected adsorbent BL(AA)30(MPTS)3.3,a removal percentage up to 99.7% of As(III) was achieved using1.0 g L−1 adsorbent and 2 mg L−1 of initial As(III) concentration. TheAs(III) removal by the hybrid adsorbents synthesized in this study isa direct remediation process, eliminating the need for pre-oxidationof the As(III). Besides, the study on As(V) adsorption indicated allthe hybrid adsorbents were still highly effective for As(V) removal.These advantages make it a potentially attractive adsorbent forarsenic removal from water contaminated by As(III) or both As(III)and As(V).

Further investigations on the adsorption isotherm, kinetics, pHeffect, and column tests including lab and field filtrations have beenconducted to fully evaluate the properties of the hybrid adsorbent,which will be reported elsewhere.

Acknowledgements

We would like to thank Dr. Su and Dr. Chou in CES for their tech-nical support in the facilities, and the Ph.D. students Zhonghou Xuand Shiyou Xu for their help with BET and SEM measurements.

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