Water Qual. Res. J. Canada, 2006 • Volume 41, No. 2, 130–139Copyright © 2006, CAWQ

130

Removal of Arsenic(III) from Groundwater using Low-Cost Industrial By-products—Blast Furnace Slag

Sushil Raj Kanel,1 Heechul Choi,1* Ju-Yong Kim,1

Saravanamuthu Vigneswaran2 and Wang Geun Shim3

1Department of Environmental Science and Engineering, Gwangju Institute of Science and Technology (GIST), 1 Oryong-dong, Buk-gu, Gwangju 500-712, Korea

2Environmental Engineering Group, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia

3Faculty of Applied Chemistry, Chonnam National University, Gwangju 500-757, Korea

Blast furnace slag (BFS), a steel industrial by-product, was tested for the removal of As(III), which is a highly toxic, mobileand predominant species in anoxic groundwater. Batch adsorption experiments were performed to determine the feasibilityof BFS as an adsorbent for removing As(III) from groundwater as As(III) concentration and the pH of water were varied.The maximum As(III) adsorption capacity by BFS was 1.40 mg As(III)/g of BFS at 1 mg/L As(III) initial concentration, at25oC, which was calculated using the Langmuir isotherm. The homogeneous surface diffusion model (HSDM) was success-fully applied to predict the sorptive removal of As(III) onto the BFS. Kinetic studies indicated that the film diffusion as wellas surface diffusion of As in the BFS was involved. It was found that the film diffusion coefficient (kf) was 5.27 × 10-5 to4.06 × 10-6 m/s and surface diffusion coefficient (Ds) was 2.31 × 10-14 to 7.13 × 10-14 m2/s for the initial As(III) concentra-tions of 0.1 to 100 mg/L. Oxidation of As(III) to As(V) and its adsorption/precipitation onto BFS is involved during theAs(III) removal mechanism. It was also found that H4SiO4

0, PO43-, NO3

-, SO42- and HCO3

- are potential interferences in theAs(III) adsorption reaction. Results suggest that 99.9% As(III) at 1 mg/L can be removed by 10 g/L BFS, which can be usedas a permeable reactive barrier (PRB) material to remove As(III) from groundwater. Details of As(III) adsorption and co-precipitation systems and interferences of As(III) molecular interactions were also studied.

Key words: blast furnace slag, film diffusion, homogeneous surface diffusion, film diffusion

* Corresponding author; hcchoi@gist.ac.kr

Introduction

Arsenic, a common toxic element, is present in both inor-ganic and organic forms in water (Thirunavukkarasu et al.2002). Arsenic (As) contamination is a global environmen-tal issue (Islam et al. 2004) which mainly exists in ground-water as arsenite [As(III)] and arsenate [As(V)] species(Ferguson and Gavis 1972). More attention is required forthe removal of As(III) due to its high toxicity (Cullen andReimer 1989) and mobility than As(V) (Manning andMartens 1997) at neutral pH. This also correlates with thelower removal efficiency of As(III) by conventional watertreatment processes (Chiu and Hering 2000).

Many different methods including precipitation-coagulation, co-precipitation, ion exchange, electro-coag-ulation, oxidation and adsorption are being used for Asremediation (Bissen and Frimmel 2003). Among them,the adsorption method has received more attention dueto its high efficiency and cost-effectiveness, and is consid-ered the most suitable technology for developing coun-tries (Jiang 2001). Metal oxides and hydroxides of ironor alumina (Bissen and Frimmel 2003) are the most com-

mon adsorbents studied for the removal of As from waterand wastewater. However, there is an urgent need todevelop low-cost alternatives due to the arising seriousproblem of As contamination in groundwater in manydeveloping countries such as Bangladesh, India andNepal (Islam et al. 2004).

BFS is a steel industrial by-product, having FeO andCaO as the main components and one of the cheapestmaterials widely available (Proctor et al. 2000). On theother hand, iron oxide and calcium oxide are some of themost important adsorbents being used, due to strongbond formation such as FeAsO4 with As (Manning et al.2002; Kim et al. 2003) and to stabilize sludge producedafter As adsorption (Twidwell et al. 1999), respectively. Inthis context, BFS plays a significant role for the adsorp-tion of As owing to coexistence of both of these importantcompounds as major chemical components. In addition,BFS recovered from high temperatures is environmentallysafe since metals present are tightly bound to its matrixand do not leach easily (Su and Suarez 1995). Hence, wehave chosen BFS as a model adsorbent for treatment of Asfrom groundwater. Additionally, to ensure the safety of itsuse, we have prepared nonconventional adsorbent, acid-treated blast furnace slag (ABFS) and applied for real

groundwater treatment of Bangladesh and Nepal.Recently, steel slag was studied as a permeable reactivebarrier material for the removal of As at high concentra-tions (final treated groundwater was 500 µg/L) for minetailing leachate (Ahn et al. 2003). Special attention isrequired to decrease the final concentrations of As belowthe WHO limit (10 µg/L) (WHO 1993).

To the best of our knowledge, the application ofBFS and ABFS to the removal of As from groundwaterin developing countries such as Bangladesh and Nepalhas not been reported. As contamination in these coun-tries has recently been detected not only in drinkingwater (Mukherjee and Bhattacharya 2001), but also inagricultural products such as rice (Meharg and Rahman2003). There is an urgent need to study blast furnaceslag as an adsorbent for the removal of arsenic in devel-oping countries. The main objective of this research is toinvestigate the effectiveness of removal of As from aque-ous solutions and delineate adsorption phenomena. Thespecific objectives of this study are:

1. To remove As from groundwater at low concentra-tions, which exist commonly in groundwater inmost developing countries.

2. To investigate the sorption mechanism of arsenic ontoBFS using a homogeneous surface diffusion model.

3. To delineate the groundwater chemistry such as theeffect of pH and foreign anions during As adsorption.

Experimental—Materials and Methods

Materials

Dry BFS was obtained from Pohang Steel Industry,Pohang, Korea. It was treated with hydrogen peroxide at60ºC for 24 h to oxidize the adhering organic matterbefore use. After grounding, it was washed with distilledwater to remove fine particles, and was dried at 100ºCfor 12 h (Choi et al. 2002). It was then screened to getdifferent geometrical sizes of 0 to 75, 75 to 180, 180 to425, 425 to 600 and 600 to 1700 µm for the adsorptionstudy. ABFS was prepared by adding hydrochloric acidonto the BFS using the reported method (Apak et al.1998) and the material was stored in vacuum desiccatorsfor further use. In some experiments, groundwater fromBangladesh was used (pH 6.5) with total alkalinity, dis-solved organic carbon (DOC), iron (Fe2+), sulfate(SO4

2-), silica (Si2+) and phosphate (H2PO42-) levels of

209, 10.0, 7.1, 8.52, 18.0 and 0.35 mg/L, respectively(Mukherjee and Bhattacharya 2001). A groundwatersample from Nepal (pH 7.1) was also used with alkalin-ity, DOC, Fe2+, SO4

2-, Si2+ and H2PO42- levels of 320,

1.0, 2.1, 8.5, 41.0 and 0.2 mg/L, respectively (Kanel etal. 2003). There was no arsenic in the groundwaterobtained from Nepal, whereas Bangladesh groundwatercontained 20 µg/L. The chemical reagents used in the

study (NaAsO2, HCl, NaOH, NaH2PO4, KI and NaBH4)were reagent grade obtained from Aldrich ChemicalCompany. All the chemicals used are of analyticalreagent grade unless specified otherwise.

For all the materials, the leaching potential of pollu-tants using water was determined by a Korean standardtest (Ahn et al. 2003). In this test, 500 mL of deionizedwater (pH 5.8–6.2) was added to 50 g of each materialand mechanically shaken for 24 h. The concentrations ofAs and dissolved metals in the leachate were determinedby inductively coupled plasma-atomic emission spec-trometry (ICP-AES, Jobin Yvon 138 Ultrace).

Methods

As(III) adsorption studies. Stock solutions of 1000 mg/LAs(III) and As(V) were prepared by dissolving NaAsO2

and Na2HAsO4.7H2O, respectively, in deionized (DI)water. Batch adsorption of As(III) was studied in 50-mLpolypropylene copolymer centrifuge tubes containing200 mg BFS (unless otherwise specified) in 20 mL ofAs(III) reaction solution as described previously (Kanelet al. 2005). The pH of the solution was not controlledfrom the beginning and monitored during the experi-ments. Equilibrium experiments were conducted for 72 hat 25ºC (unless otherwise specified) in a shaken waterbath (185 rpm) kept in the dark by covering with alu-minum foil. The 72-h reaction time was shown to beadequate to attain equilibrium. After 72 h, the super-natant solution was filtered through a 0.45-µm mem-brane filter (Millipore), with a disposable syringe, andanalyzed by hydride generation atomic absorption spec-trophotometry (HGAAS; Perkin-Elmer 5100 PC) with adetection limit of 1 µg L-1 As. As(III) was the initial formof As in all experiments (unless otherwise specified) andtotal As (AsT) was measured after adsorption with slag.As(III) and AsT concentrations were also measured incertain experiments using an anion exchange cartridgemethod, and competing anionic experiments and specia-tion of As(III) studies were performed as described inour previous report (Kanel et al. 2005).

Instrumentation. The chemical composition of the BFSwas determined by the x-ray fluorescence (X-RF) analy-sis. The BFS material was characterized by powder x-raydiffraction (XRD) using a Rigaku diffractrometer andmonochromatized CuKα radiation (generator tension =40 kV, current = 40 mA). Diffractograms were recordedfrom 5 to 85o (2θ) with a step size of 0.02º and a counttime of 5 s per step. Morphological analysis of the sam-ples was performed by field emission scanning electronmicroscopy (FE-SEM) using a Hitachi S-4700 micro-scope (at 15 kV). Powder samples were prepared bymounting on carbon tape followed by platinum coating.TEM-EDX was used to find out the chemical composi-tion of BFS and its treated products with As. The specific

Removal of Arsenic(III) from Groundwater by Blast Furnace Slag 131

surface area (SBET) of BFS was measured by BET surfacearea analysis (ASAP 2020, Micrometritics). The isoelec-tric point of BFS in aqueous solution was measured by alight scattering instrument (ELS-8000, Photal, OtsukaElectronics, Japan).

Adsorption isotherms. To determine the mechanisticparameters associated with As adsorption on BFS, theequilibrium data were analyzed using well-known Fre-undlich (equation 1) and Langmuir (equation 2) sorptionisotherms, which are as follows:

q = KFC1n (1)

q = KLbC (2)1 + bC

where q (mg/g) is the sorption capacity, C (mg/L) isthe equilibrium solute concentration, KF and n are theFreundlich constants, KL (mg/g) is the maximum sorp-tion capacity and b is the Langmuir constant. The Fre-undlich and Langmuir parameters were obtained bynonlinear least-square regression analysis (Reddad et al.2002). Thermodynamic parameters were also calculatedbased on equations 3 to 5 and reported in Table 3(Ajmal et al. 1998).

ΔG = -RT lnK (3)

lnK = ΔH + C (4)RT

ΔG = ΔH - TΔS (5)

where ΔG is the Gibbs free energy change, R is the idealgas constant (4.187 J/mol K), T is temperature (Kelvin), Kis the Langmuir or Freundlich isotherm constant, ΔH isthe enthalpy change and ΔS the entropy change.

Adsorption kinetic model. The dynamics of adsorptionof As has been predicted using a homogeneous surfacediffusion model (HSDM). The overall mass balance forthe closed batch experiments is as follows (Traegner andSuidan 1989; Vigneswaran and Moon 1998):

V dCb = –M dqave (6)dt dt

where Cb is the bulk liquid As concentration (mg/L),qave is the average mass fraction of As on the slag (mg/g),M is the total mass of slag in the reactor (mg) and V isthe total liquid volume in the reactor (L). The diffusioninside a spherical porous particle is:

(7)

Initial condition is:

t = 0 ; q = 0 (8)

Two boundary conditions are:

r = 0 ; ∂q = 0 (9)∂r

r = rp ; Dsρp∂q = kf (Cb – Cs) (10)∂r

The average mass fraction of As on the slag is:

where r is the radial coordinate with an origin at thecentre of the particle (m), q is the mass fraction of As alongthe inner surface of the particle (mg/g), rp is particle radius(m), Ds is the surface diffusion coefficient (m2/s), ρp is parti-cle density (g/cm3), kf is film diffusion coefficient (cm/s) andCs is solid-liquid interface As concentrations (mg/L). Biotnumber is a dimensionless parameter, which represents theexistence of film diffusion and surface diffusion accordingto its value. Biot number is defined using the followingequation to find out the limiting diffusion mechanism:

Biot number, Bi = kf rpC0 (12)Dsρpq0

where qo is solid phase As concentration (mg/g),which is in equilibrium with a liquid phase As concen-tration, Co, and Bi is the Biot number (dimensionless).

The governing partial differential equations cannotbe solved analytically. Therefore, numerical methodshave been generally employed. In this study, this compli-cated set of partial differential equations was first dis-cretized by an orthogonal collocation method (OCM) toform a set of first-order ordinary differential equations(ODEs) (Villadesen and Stewart 1967). The resulting setof ODEs was solved using the subroutine double preci-sion variable ordinary differential equation (DVODE)(Brown et al. 1989). The DVODE program employsGear’s method with variable order and step size.

Results and Discussion

Characterization of the Adsorbent

The chemical composition of BFS by XRF analysis is listedin Table 1. BFS is a complex heterogeneous materialmainly composed of total iron (43.14%), CaO (35.43%),SiO2 (10.08%), Al2O3 (3.24%), MnO (2.52%) and MgO(2.06%). The XRD study also confirmed that the maincomponents of slag were oxides of iron and calcium (Fig.1). The BET results showed that the specific surface areaand pore size of BFS is SBET = 12.56 m2/g and 794 nm,respectively. The isoelectric point of BFS in aqueous solu-tion was measured by a light scattering instrument, whichwas found to be 3.2. Lopez-Delgado et al. (1998) alsoreported the isoelectric point of blast furnace sludge was

132 Kanel et al.

∂q = Ds∂2q + 2 ∂q

∂t ∂r2 r ∂r

⎞⎟⎠⎞

⎟⎠

qave = 3 rp qr2dr (11)r3p

0∫

3.23. The morphological appearance of the BFS usingSEM is shown in Fig. 2A and 2B. In both samples, ahexagonal shaped structure was clearly seen, which wasdue to the presence of iron oxide. Finally TEM-EDXanalysis was performed on As-adsorbed BFS (Fig. 3B) andits control (Fig. 3A) to further confirm the As adsorptionon BFS. The spectrum showed that there was no As on thecontrol sample (Fig. 3A), however, the As-treated BFS con-tained different elements such as As, O, Fe, Ca, Al, Si, S, K,Cu and C (Fig. 3B), where the appearance of a C and Cupeak arose from the TEM grid (carbon coated coppergrid). FTIR was used to find out the adsorption of As(III)on BFS (Fig. 4). It was found that as the concentration ofAs(III) increased from 0 to 1000 mg/L, the peaks at 426,474 and 510 cm-1 were shifted to 428, 477 and 516 cm-1 ,respectively, whereas the peaks at 2359 and 2520 cm-1

were shifted to 2330 and 2359 cm-1, respectively.

Mechanism of Arsenic Removal by BFS (Acid-Treated and Natural)

Several experiments were conducted to find out the As(1 mg/L) removal mechanism by BFS (10 g/L) at pH 12.

The As removal was more than 99.9% at pH 12 withBFS dose of 10 g/L (Fig. 5). Whereas in ABFS at pH 7,even though the calcium oxide was removed during theacid treatment, the As(III) adsorption capacity remainedconstant (<2% of BFS) which may be due to favorablepH conditions (Fig. 5). The pH 12 was chosen because ofthe drift in pH after addition of slag. This was due to thedissolution of a high concentration of Ca2+

(2.4–6.7 mg/L) present in BFS (Dimitrova andMehandgiev 1998), for example:

=SiOCa + 2HOH ⇒ =SiOH + Ca2+ + 2OH- (13)

Since there was an absence of calcium oxide inABFS (removed during acid treatment), the pH wasautomatically buffered. The ABFS doses were 25.0 and17.5 g/L for groundwater from Nepal and Bangladesh(Fig. 6), respectively, where the As was spiked to get1.0 mg/L initial concentration of As in this groundwater.The AsT removal was more than 99.9% at 25 and

Removal of Arsenic(III) from Groundwater by Blast Furnace Slag 133

TABLE 1. Chemical composition of blast furnace slag (BFS)by XRF analysis

Analytical Analytical data data

Contents (% weight) Contents (% weight)

SiO2 10.08 MgO 2.06Al2O3 3.24 K2O 0.09Fe2O3 43.14 V2O5 0.26TiO2 0.48 P2O5 0.91ZnO 0.21 SrO 0.04SO3 1.35 Cr2O3 0.19MnO 2.52 CaO 35.43

Total 100

Fig. 1. X-ray diffraction patterns of BFS sample. Fig. 2. SEM photomicrographs of untreated BFS.

A

B

17.5 g/L ABFS for Nepal and Bangladesh groundwater,respectively. Results show that a higher amount of ABFSwas needed in real groundwater compared to As-spikedDI water. This is possibly due to the presence of anionsin the real groundwater, which compete with As duringits adsorption. Even though the calcium oxide wasremoved from ABFS during the acid treatment, the Asadsorption capacity remained constant (<2%) whichmay be attributed to the favorable pH conditions(Fig. 6). These results clearly indicate that ABFS has agreat potential to be used as one of the best adsorbentsfor removing As(III) from water.

Arsenic adsorption on BFS is an extremely complexprocess and it depends on the arsenic valence, as well ason the solution pH (Manju et al. 1998). Since BFS sur-faces are negatively charged in the experimental pHrange (Lopex-Delgado et al. 1998), specific adsorptionmust occur through ligand exchange, i.e., by replacingCa/Fe-coordinated surface OH or Ca-coordinated H2O.

Two different mechanisms may be responsible for bind-ing As on BFS. At higher pH (~12), calcium ions lead tocomplexation with As to give calcium arsenate as shownin the equation below (Ahn et al. 2003):

HAsO32- + Ca2+ + nH2O = CaHAsO3.nH2O (14)

At neutral pH, iron and iron oxide present in BFSform a bond with As by complex heterogeneous reac-tions. The removal of arsenic by ABFS may be due toadsorption, precipitation or complexation of As on ironoxide present in ABFS. Wikie and Hering (1996) alsomentioned that the adsorption of As(III) on hydrous fer-ric oxide over the pH range of 4 to 9 is not stronglydependent on pH. The BFS contains Fe2O3 (43.141%),which also contributes to the adsorption of As(III) atneutral pH. Kim et al. (2003) reported the sorption ofarsenic(III) on amorphous Fe(III) stabilizes the available

134 Kanel et al.

Fig. 3. Adsorbed phase TEM micrographs of BFS: (A) BFS without As, (B) BFS after adsorption of 1000 µg/L As(III) with10 g/L BFS at pH 12.

Fig. 4. Adsorbed phase FTIR spectra of BFS in the presence ofarsenic (0, 100, 500 and 1000 µg/L) with 10 g/L BFS at pH 12.

Fig. 5. Variation of AsT adsorption with respect to acidtreated and untreated BFS; initial As(III): 1000 µg/L at pH 12and 7.

A B

As and can form insoluble iron arsenic compounds suchas FeAsO4. It is difficult to distinguish the amount ofadsorption by calcium oxide and iron oxide separately.Recent studies using x-ray absorption spectroscopyshowed that the products after reaction of As(III) andAs(V) with BFS were inner-sphere As(III) and As(V) sur-face complexation on Fe(III) oxides/hydroxide products(Chiu and Hering 2000). A desorption study confirmsthat As(III) transforms to As(V) due to the oxidationinduced by MnO present in BFS. This leads to adsorp-tion of As(V) onto BFS.

Effects of Sorbent Dose, Particle Size and Initial Concentration

Figure 5 shows the percentage removal of As (1 mg/L)with increase in BFS concentration (0–35 g/L). The Asremoval was more than 99.9% at pH 12 with BFS doseof 10 g/L. In the case of ABFS, even though the calciumoxide was removed during the acid treatment, the As(III)adsorption capacity remained constant (<2% of BFS)may be due to the favorable pH conditions. This wastaken as the optimum slag concentration in the subse-quent experiments unless otherwise specified.

To study the effect of BFS size, the experiments wereperformed using different particle sizes. For particle sizes0 to 75 µm and 75 to 180 µm, the sorption of AsT was100% within 10 h whereas for 180 to 425 µm and 425 to600 µm were 95 and 92%, respectively, in 72 h of reac-tion time. It was found that the removal of AsT decreasedwith the increase in particle size and the peak value wasobserved with the finest particles (0–75 µm). However,particle size of 75 to 180 µm was selected for furtheradsorption studies for the sake of easy preparation.

The uptake of As on BFS was studied with Assolution with initial As(III) concentrations (0.10–

100.00 mg/L). Figure 7 shows that the adsorption of Asis nearly complete (~100%) with lower As(III) concen-trations (0.10–1.00 mg/L). As the concentration of AsT

was increased from 0.10 to 100 mg/L, the removal effi-ciency of AsT decreased from 100 to 50.7%. This maybe attributed to the decrease in the active sites at the sur-face of BFS available for the adsorption of AsT.

Isotherm, Kinetics and Modelling of Arsenic Adsorption

To predict the equilibrium amount of As sorbed ontoBFS, the experimental data were fitted to the Langmuirand Freundlich equations. The Langmuir and Freundlichisotherm coefficients are summarized in Table 2. Themaximum adsorption of As on BFS calculated by theLangmuir isotherm (Fig. 8) was 1.4 mg As(III)/g BFS at25ºC. The As(III) adsorption process was endothermic(ΔH = -15.52 KJ/mol) (Table 3) since a decrease inAs(III) adsorption was observed with the increase intemperature (Atkins 1994). It was observed that thereaction was spontaneous due to its negative free energy.The negative value of adsorption shows the spontaneousaffinity of BFS for adsorbing AsT.

The kinetics of adsorption of As on BFS were pre-dicted using homogeneous surface diffusion model(HSDM) according to equations 3 to 8 and comparedwith experimental data (Fig. 7) (Traegner and Suidan1989). The model describes the experimental results suc-cessfully except As concentration of 0.1 mg/L. Thekinetic parameters such as film diffusion coefficient (kf)and surface diffusion coefficient (Ds) were determined

Removal of Arsenic(III) from Groundwater by Blast Furnace Slag 135

Fig. 6. Removal of As using acid treated BFS from groundwaterfrom Nepal and Bangladesh; initial As(III): 1000 µg/L, pH 7.

Fig. 7. Variation of AsT adsorption with respect to initialAs(III) concentration; initial As(III): 0.01 to 100 mg/L, slag10 g/L, pH 12.

based on the curve fitting result and presented in Table 4.The result from the table shows that the film diffusioncoefficient is greater than the surface diffusion coefficientof total As sorbed onto BFS. The film diffusion of As inthe BFS was found to be the main rate-limiting step dur-ing the sorption of total As onto BFS. We have also cal-culated the Biot number using equation 9, which wasfound to be 55.24 for 1 mg/L As(III) adsorbed onto BFS(10 g/L). This shows that both external mass transport aswell as the surface diffusion is important for the adsorp-tion process (Traegner and Suidan 1989).

Effect of Competing Individual Anions

Table 5 shows the effect of individual anions (HCO3-,

SO42-, NO3

-, H4SiO40, and H2PO4

2-) on the adsorption ofAs(III) on BFS. The sulfate and bicarbonate ions had littlehindering effect on the uptake up to 1 mM concentrationsof As(III) while nitrate and phosphate ion reduced theuptake of As(III) from 99.9 to 81.2 and 48.0% when theseions were present at 1 mM. However, when the concentra-tion of anions was increased further up to 10 mM, the

adsorption of As(III) on BFS decreased from 99.9 to 74.8,44.6, 42.2, 30.0 and 10.0% for nitrate, sulfate, bicarbon-ate, silicate and phosphate ions, respectively. The competi-tive uptake of these anions may pose a problem during theAs adsorption process, especially when these ions are pre-sent in high concentrations. Neverthless, BFS has a highcapacity to remove arsenic in the presence of anions, whichmay be due to the presence of 35.43% of CaO as shown inTable 1 (Kostura et al. 2005). These results are importantfor the in situ remediation using BFS due to the coexistenceof competing anions in natural groundwater.

Desorption and Speciation of As(III/V)

Experiments were also performed to determine the speci-ation of As(III/V) after adsorption of As(III) on BFS. Theresults of desorption and speciation of As(III/V) showedthat desorption of As(V) was 18.8, 13, 7.8, 5.0 and4.1% for BFS dosages of 5.0, 7.5, 10.0, 12.5 and15.0 g/L, respectively, whereas there was no desorptionof As(III) observed. It was interesting to note that thedesorption study showed oxidation of As(III) to As(V)

136 Kanel et al.

TABLE 2. The related parameters for the adsorption of total arsenic (AsT) on BFS at differenttemperatures

Freundlich constants Langmuir constants

Temperature (°C) R2 KFa nb R2 KL

c bd

25 0.996 0.502 1.71 0.869 1.40 1.1035 0.996 0.550 1.73 0.974 2.50 1.0045 0.996 0.620 1.78 0.996 3.10 0.90

aKF; Freundlich constant related to adsorption capacity.bn; Freundlich adsorption intensity.cKL; Langmuir constant related to maximum adsorption capacity.db; Langmuir constant related to energy of adsorption.

Fig. 8. Langmuir and Freundlich plots for the adsorption of AsT by BFS, initial As(III):1000 µg/L, slag 10 g/L, pH 12.

once adsorbed on BFS. The highest As(V) recovery wasabout 18.8% [whereas there was no As(III) desorption]in the 1.0 mg/L As(III) treatment with 5.0 g/L BFS.These data confirm that oxidation of As(III) to As(V) isinvolved in the BFS removal mechanism, which may bedue to the oxidation induced by the presence of 2.5%manganese oxides in BFS (Table 1) (Su and Puls 2001;Tournassat et al. 2002; Tani et al. 2004).

Leaching Test

The deionized water-leaching test showed strongly alkalineconditions (pH 12.2) and high dissolved Ca concentra-tions. The common mineral phase in the materials was cal-cium hydroxide (portlandite), which results from the addi-tion of CaO as a raw material in the iron-making process(Ahn et al. 2003). The dissolution of this calcium hydrox-ide thus caused high pH and Ca concentrations. The leach-ing of Fe was not significant, and the concentrations of As,Cr, Cu, Mn, Cd and Co were measured by ICP-AES andfound to be below the detection limits (0.01 mg/L) for allmaterials. In considering the solid to liquid ratio (1:10) ofthe test, trace metal levels in the field may be low due tothe excessive amounts of groundwater (Ahn et al. 2003).

Cost Estimation

The by-product from the steel industry, BFS, is availablefreely and adding the expenses for transportation, chem-icals, etc., the final product would cost approximatelyUS$12 per tonne. This is in contrast to the commercially

available carbons, the cheapest variety of which costsapproximately US$1000 per tonne (Gupta 1998). Thecost of BFS is comparatively cheaper (US$12 per tonne)than other industrial waste such as bassage fly ash(US$15 per tonne), red mud (US$25 per tonne), ferrugi-nous manganese ore (US$50–56 per tonne) (Chakravartiet al. 2002). Therefore, the development of low-cost,widely available, worldwide and efficient BFS for arsenicadsorption would be a good replacement for commer-cially available costly adsorbents.

Conclusions

We have presented evidence that As(III) can be removedefficiently without any pretreatment by adsorption onlow-cost and widely available BFS. This study indicatesthat the BFS could be used as a PRB to remove As(III) asan alternative to the pump and treat method in in situconditions. The effects of competing anions showedHCO3

-, NO3-, SO4

2- H4SiO40 and PO4

3- (>10 mM) arepotential interferences in the As(III) adsorption reaction.When the concentration of anions was increased furtherup to 10 mM, the adsorption of As(III) on BFS decreasedfrom 99.9 to 74.8, 44.6, 42.2, 30.0 and 10.0% fornitrate, sulfate, bicarbonate, silicate and phosphate ions,respectively. Arsenic removal tests with real groundwa-ter from Nepal and Bangladesh showed promisingresults, indicating that it has great potential to be used as

Removal of Arsenic(III) from Groundwater by Blast Furnace Slag 137

TABLE 4. Calculation of film (kf) and surface diffusion (Ds)coefficients of total arsenic (AsT) on BFS

Initial concentration of As(III) (mg/L) Ds (m2/s) Kf (m/s)

0.1 2.31 × 10-14 5.27 × 10-05

0.5 2.42 × 10-15 1.15 × 10-05

1.0 2.65 × 10-14 2.04 × 10-05

5.0 8.75 × 10-14 4.17 × 10-06

10.0 1.23 × 10-13 2.56 × 10-06

15.0 2.31 × 10-13 9.68 × 10-06

25.0 1.50 × 10-13 3.74 × 10-06

50.0 1.66 × 10-13 4.27 × 10-06

100.0 7.13 × 10-14 4.06 × 10-06

TABLE 5. Percentage of As(III) removal in presence of competitive anions

Concentration % Uptake of As(III) in the presence of aniona,b

of anions (mM) NO3- SO4

2- HCO3- H4SiO4

o PO43-

0.0 99.0 99.0 99.0 99.0 99.00.1 95.5 93.2 98.2 99.0 84.01.0 81.2 92.5 96.6 99.0 48.010.0 74.8 44.6 42.2 30.0 10.0

an = 3, average of triplicate results where the standard deviation is less than 5%. banions in mM.

TABLE 3. Thermodynamic parameters for the adsorption ofAsT on BFS at different temperatures (Langmuir isotherm)

Temperature ΔGa ΔSb ΔHc

(°C) lnK (KJ/mol) (KJ/K/mol) (KJ/mol)

25 -0.32 -0.42 -0.01535 -0.92 -1.18 -0.017 -15.5245 -1.12 -1.48 -0.018

aΔG; Free energy.bΔS; Entropy of activation.cΔH; Enthalpy.

an effective adsorbent for groundwater treatment indeveloping countries. In spite of its great potential, fur-ther research such as column studies in the presence ofother scavengers (heavy metals and anions) are requiredto better understand its potential for in situ as well as exsitu applications.

Acknowledgements

This work was supported by Grant B10 from the Sus-tainable Water Resources Research Center (SWRRC) ofthe 21st century frontier R&D program through the Cen-ter for Water Research (CWR) at Gwangju Institute ofScience and Technology (GIST) and the NationalResearch Laboratory Program of the Korea Science andEngineering Foundation.

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Received: September 22, 2005; accepted: February 7, 2006.

Removal of Arsenic(III) from Groundwater by Blast Furnace Slag 139