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Adsorption and removal of arsenic (V) using crystallinemanganese (II,III) oxide: Kinetics, equilibrium, effect ofpH and ionic strengthKamel Babaeivelnia, Amid P. Khodadousta & Dorin Bogdana

a Department of Civil and Materials Engineering, University of Illlinois at Chicago, Chicago,Illinois, USAPublished online: 19 Aug 2014.

To cite this article: Kamel Babaeivelni, Amid P. Khodadoust & Dorin Bogdan (2014) Adsorption and removal of arsenic (V)using crystalline manganese (II,III) oxide: Kinetics, equilibrium, effect of pH and ionic strength, Journal of EnvironmentalScience and Health, Part A: Toxic/Hazardous Substances and Environmental Engineering, 49:13, 1462-1473, DOI:10.1080/10934529.2014.937160

To link to this article: http://dx.doi.org/10.1080/10934529.2014.937160

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Adsorption and removal of arsenic (V) using crystallinemanganese (II,III) oxide: Kinetics, equilibrium,effect of pH and ionic strength

KAMEL BABAEIVELNI, AMID P. KHODADOUST and DORIN BOGDAN

Department of Civil and Materials Engineering, University of Illlinois at Chicago, Chicago, Illinois, USA

Manganese (II,III) oxide (Mn3O4) crystalline powder was evaluated as a potential sorbent for removal of arsenic (V) from water.Adsorption isotherm experiments were carried out to determine the adsorption capacity using de-ionized (DI) water, a syntheticsolution containing bicarbonate alkalinity, and two natual groundwater samples. Adsorption isotherm data followed the Langmuirand Freundlich equations, indicating favorable adsorption of arsenic (V) onto Mn3O4, while results from the Dubinin–Radushkevich equation were suggestive of chemisorption of arsenic (V). When normalized to the sorbent surface area, the maximumadsorption capacity of Mn3O4 for arsenic (V) was 101 mg m¡2, comparable to that of activated alumina. Arsenic (V) adsorptiononto Mn3O4 followed pseudo–second-order kinetics. Adsorption of arsenic (V) was greatest at pH 2, while adsorption at pH 7–9 waswithin 91% of maximum adsorption, whereas adsorption decreased to 32% of maximum adsorption at pH 10. Surface chargeanalysis confirmed the adsorption of arsenic (V) onto the acidic surface of the Mn3O4 sorbent with a pHPZC of 7.32. The presence ofcoexisting ions bicarbonate and phosphate resulted in a decrease in arsenic (V) uptake. Comparable adsorption capacities wereobtained for the synthetic solution and both groundwater samples. Overall, crystalline Mn3O4 was an effective and viable sorbent forremoval of arsenic (V) from natural water, removing greater than 95% of arsenic (V) from a 1 mg L¡1 solution within 60 min ofcontact time.

Keywords:Manganese oxide, arsenic (V), arsenate, adsorption, pH, ionic strength.

Introduction

Arsenic is a naturally occurring metalloid that is presentthroughout the environment in water, soil, air and food.[1]

The consumption of high levels of arsenic concentrationby humans has been linked to the cancer of the skin, blad-der, lung and kidney and the neurological and cardiovas-cular systems.[2] Considering the severe effect of arsenic onhuman health and to limit the exposure to arsenic, theWorld Health Organization (WHO) as well as the U.S.Environmental Protection Agency (EPA) recommend amaximum contaminant level (MCL) of arsenic in drinkingwater of 10 mg L¡1.[3,4] Many countries are affected byhigh levels of arsenic in surface water and ground water.

Contamination through soil leaching, industrial waste-waters, hydrothermal activities and mining are the mainsource of contamination.[5,6] The presence of various con-centrations of arsenic within the shallow zones of groundwater has been reported in many countries. Concentrationof arsenic in some regions in Bangladesh is as high as1000 mg L¡1.[7]

Arsenic may exist in water in different oxidation states(C5,C3,0,¡3),[8] but As(III) [arsenite] and As(V) [arsenate]are the predominant valence states of inorganic arsenicspecies. As(III), the most toxic form, exists as anuncharged species (H3AsO3) or anionic species (H2AsO3

¡)in a moderately reducing environment, whereas As(V) spe-cies are H2AsO4

¡ or HAsO42¡ anions in natural waters.[9]

In natural surface waters, As(V) is the dominant species,while natural groundwaters contain mainly As(III) due tothe dominant reducing conditions.[10]

The technologies employed to remove arsenic fromwater should be simple, effective, selective, and not removeall ions present in water.[11] Several methods have beenused for removal of arsenic but they usually suffer fromone or more drawbacks, scope of application and limita-tions.[12] However, adsorption [13] has been recognized as a

Address correspondence to Amid P. Khodadoust, Departmentof Civil and Materials Engineering, University of Illlinois atChicago, Chicago, IL 60607, USA; E-mail: akhodado@uic.edu;kbabae2@uic.edu; dbogda2@uic.eduReceived February 14, 2014.Color versions of one or more of the figures in the article can befound online at www.tandfonline.com/lesa.

Journal of Environmental Science and Health, Part A (2014) 49, 1462–1473Copyright © Taylor & Francis Group, LLCISSN: 1093-4529 (Print); 1532-4117 (Online)DOI: 10.1080/10934529.2014.937160

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superior method for removal of arsenic owing to its lowercost, flexibility and simplicity of design, high efficiency,easy handling and high selectivity. Different sorbents havebeen used as promising media for removal of arsenic fromwater.[14]

Manganese oxides have been used in mixed-oxide sorb-ents such as iron-manganese [15] and cerium–manganese[16] or as loaded/substituted material for enhancement ofarsenic sorption from aqueous solutions. Manning et al.[17]

have suggested that birnessite (MnO2) used at near neutralpH in drinking water treatment and environmental reme-diation may play a beneficial role as both an efficient oxi-dant of As(III) and a sorbent for As(V). A low costferruginous manganese ore (FMO) with the major mineralphases of pyrolusite and goethite has been studied for theremoval of arsenic from groundwater.[8] The adsorption ofarsenate has also been studied by natural manganeseoxides by monocomponent column experiments.[18] Themaximum arsenate adsorption of 2.23 mmol g¡1 wasobtained from a solution of 0.67 mM sodium arsenate(50 mg L¡1 arsenic) and 1 mM sodium chloride at pH 7.9.Thirunavukkarasu et al.[19] applied manganese “green-sand” in column studies to examine the removal of arsenicin drinking water. A MnO2 -loaded polystyrene resin (R-MnO2) has been demonstrated as a strong solid sorbentfor arsenic retention where the R-MnO2 maximal capaci-ties were 0.7 mmol/g and 0.3 mmol/g towards As(III)and As(V), respectively.[5]

Deschamps et al.[20] investigated the adsorption of arse-nic onto some soils enriched with manganese and iron.They claimed that the presence of naturally occurringmanganese oxides promotes the effective oxidation of As(III) to As(V). Also, the manganese minerals in their studyshowed a significant uptake of both the As(III) and As(V).Wu et al.[21] showed that using a manganese sand carrierinstead of a quartz sand carrier for arsenic adsorption toiron-oxide-coated sand can improve the adsorption capac-ity of the sorbent. Manganese-substituted iron oxyhydrox-ide (MIOH) has been used as sorbent for the adsorption ofAs(III) and As(V) from water.[22]

Manganese oxides affect the mobility and fate ofmany pollutants in the environment and can easily par-ticipate in different redox reactions and strongly adsorbnumerous ions.[23] Manganese oxides are widely presentaround the globe and are available in countries facingarsenic groundwater contamination. The addition ofmineral oxides to small volumes of water has been iden-tified by U.S. EPA as a feasible process to remove arse-nic from water.[24] In this study, crystalline manganese(II,III) oxide (Mn3O4) powder was evaluated as a rela-tively inexpensive sorbent for removal of As(V). Theaim of this study was to investigate the effectiveness ofMn3O4 for adsorption and removal of As(V) from waterand the effect of different parameters like contact time,sorbent dosage, pH, ionic strength and co-existing ionson the removal of As(V).

Materials and methods

Materials

All chemicals were of analytical reagent grade and no fur-ther purification was carried out. All solutions were pre-pared with de-ionized (DI) water with a resistance greaterthan 18 MV. The manganese (II,III) oxide powder(Mn3O4, 97% purity, density of 4.8 g mL¡1 at 25�C) withan average particle size of less than 325 U.S. mesh(44 mm) was obtained from Sigma-Aldrich (St. Louis,MO). Sodium arsenate (Na2HAsO4.7H2O, 99% purity,ACS grade) was purchased from Sigma-Aldrich. The otherchemicals were obtained from Fisher scientific (Fair Lawn,NJ). The calcium chloride (CaCl2, 98.8% purity, ACSgrade), sodium sulfate (Na2SO4, 99.3% purity, ACS grade)and sodium bicarbonate (NaHCO3, 100% purity, ACSgrade) were used to prepare a synthetic solution represen-tative of natural waters with bicarbonate alkalinity andtotal hardness considering the effect of solution ionicstrength. The stock solution of As(V) was prepared withsodium arsenate in DI water.

Sorbent characterization

X-ray powder diffraction(XRD) patterns were obtainedon a Siemens D5000 with Cu Ka radiation, λ D 1.5418 A

�,

at 40 KV and 30 mA. The pattern were obtained from 15�to 65� 2u with a step size of 0.02� 2u, counting for 10 sstep¡1 to better resolve the peaks. The BET surface area ofthe Mn3O4 was measured with an Accelerated SurfaceArea and Porosimetry system, ASAP 2010 (MicromeriticsInstrument Corporation, Norcross, GA), using nitrogenadsorption/desorption isotherms. The zeta potential of thesorbent was measured by a Zeta-meter system 3.0 (Zetameter Inc.; VA). A suspension of 100 mg L¡1 Mn3O4 in1 mMNaCl solution was prepared. The pH of the sampleswere adjusted from 3 to 10 using 0.1 M HCl and 0.1 MNaOH solutions.

Batch adsorption experiments

An initial arsenic concentration of 1 mg L¡1 was chosen todetermine the adsorption capacity of the sorbent. Theexperiments were performed with dry sorbent suspensionsof different dosages in DI water and the synthetic solution.The synthetic solution was prepared from the stock solu-tion of As(V), and contained 2.5 mM sodium bicarbonate(NaHCO3), 1 mM calcium chloride (CaCl2) and 0.5 mMsodium sulfate (Na2SO4). The solid sorbent was added to50 mL of As(V) solution in high density polyethylene(HDPE) bottles. After the addition of sorbent to the bot-tles, the bottles were shaken in a rotating tumbler at16 rpm in the dark, and the suspensions were left to equili-brate for 24 h at 25�C. After the shaking period, the sus-pensions were then centrifuged to separate the aqueous

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phase from the sorbent. The supernatant was filtered witha 0.45 mm cellulose acetate filter and the concentration ofarsenic in the filtrate was analyzed.For the kinetics experiments, 0.25 g of Mn3O4 was

mixed with 50 mL of a 1 mg L¡1 solution of As(V) at25�C. To investigate the effect of the solution pH on theadsorption of As(V), 0.25 g of Mn3O4 was added to50 mL of 1 mg L¡1 solution of As(V). The initial pH ofthe solution in DI water was adjusted to 2–10 using 0.1 MHCl and 0.1 M NaOH solutions. In order to find the effectof ionic strength and the effect of co-existing ions on theadsorption of arsenic onto Mn3O4, different concentra-tions of sulfate, phosphate, bicarbonate and calcium wereselected to evaluate the uptake of arsenic as function ofindividual ions added to DI water. The CaCl2, Na2SO4,NaHCO3 and Na2HPO4 were used as sources of calcium,sulfate, bicarbonate and phosphate, respectively. A 50 mLvolume of each co-existing ion solution with an initialarsenic concentration of 1 mg L¡1 was mixed with 0.25 gMn3O4, and the experiments were performed in similarfashion to the batch adsorption tests.

Analytical methods

The graphite furnace atomic absorption spectroscopy(GFAAS) with Zeeman background correction was usedto determine the arsenic concentration in the adsorptiontests. For arsenic analysis, all measurements were carriedout using an electrodeless lamp (EDL) at 193.7 nm andthe modifier used was palladium-magnesium modifier.The atomization and pre-treatment temperature were 2000and 1200�C, respectively. The detection limit of arsenicwas estimated to be 1 mg L¡1. The uptake of arsenic(V) onthe sorbents was determined as:

qD .C0 ¡Ce/

m£A (1)

where C0 and Ce are the initial and equilibrium concentra-tion of As(V) in the solution(mg L¡1), q is the adsorbed As(V) (mg kg¡1), m is the adsorbent dosage (kg) and A is thesolution volume (L).

Results and discussion

Characterization of sorbent

The chemical composition of manganese (II,III) oxidepowder was determined by X-ray diffraction (XRD). TheXRD pattern of the Mn3O4 sorbent is shown in Figure 1.The XRD pattern revealed good agreement with Mn3O4

(JCPDS card No 24-0734). The appearance of the narrowand strong peaks in the XRD pattern of the sorbent wasindicative of its crystalline structure. The BET surfacearea was determined to be 3.41 m2 g¡1.

Effect of sorbent dosage

The removal of arsenic was determined as function of sor-bent dosage by increasing the sorbent dosage from 1 to20 g L¡1 at an initial As(V) concentration of 1 mg L¡1

and a solution volume (50 mL) of DI water or syntheticsolution. The results presented in Figure 2 show that theuptake of arsenic from both DI water and synthetic solu-tion increased with increasing dosage of the sorbent. ThepH of the solutions was not kept constant in order todetermine how the presence of the sorbent affected thearsenic removal efficiency when the solution was naturallyexposed to different dosages of the sorbent. The solutionpH for different dosages of Mn3O4 was in the range of6.6–6.9 in DI water and 7.9–8.3 in the synthetic solution.A dosage of 20 g L¡1 Mn3O4 was applied for the syn-

thetic solution at pH 8.2 to achieve the WHO and U.S.EPA standards. A 20 g L¡1 of Mn3O4 decreased the As(V)concentration from 1000 mg L¡1 down to 6.01 mg L¡1.

2θ(deg)

10 20 30 40 50 60 70

Inte

nsity

(C

ount

s)

0

50

100

150

200

250

Fig. 1. XRD pattern of the Mn3O4.

Sorbent Dosage (g/L)

Ars

enic

Rem

oval

(%

)

0

10

20

30

40

50

60

70

80

90

100

110

120

DI watersynthetic solution

2 4 10 13 205

Fig. 2. Effect of sorbent dosage on the removal of As(V) usingMn304 from a 1 mg/L solution.

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However, in DI water, an adsorbent dosage of 10 g L¡1 atpH 6.6 was sufficient to decrease the arsenic concentrationfrom 1000 mg L¡1 down to 2.4 mg L¡1 (greater than 99%removal), which meets the WHO and U.S. EPA standards,indicating that the Mn3O4 sorbent has a greater adsorptioncapacity in DI water than in the synthetic solution. Theeffect of pH and competing ions on the adsorption of As(V) is discussed later.

Influence of contact time and adsorption kinetics

The effect of contact time on removal of As(V) from thesynthetic solution was determined at an initial As(V) con-centration of 1 mg L¡1 by taking samples at different timeintervals and measuring the residual concentration of As(V) in the solution. A sorbent dosage of 20 g L¡1 was usedat pH 8.2. The data presented in Figure 3 show that arse-nic adsorption increased over time, reaching equilibriumafter 6 h. Arsenic adsorption was high within the first60 min, but thereafter the adsorption decreased andapproached equilibrium. About 96% removal of arsenicwas achieved within the first 60 min of contact betweensorbent and solution, while only 1–3% of additionalremoval occurred in the following 17 h. Grossl et al.[25]

and Raven et al.[26] observed a similar trend while usingferrihydrite as sorbent. At equilibrium, about 99% of arse-nic was adsorbed and the equilibrium adsorption capacityfor As(V) was 49.8 mg kg¡1.The Lagergren pseudo–first-order model [27] and the

pseudo–second-order model [28] were employed to studythe adsorption kinetics. The results of the uptake of As(V)by Mn3O4 powder at different time interval were used tounderstand the dynamics of the adsorption process and toevaluate the performance of the sorbent with the time. Thepseudo–first-order model is given as:

dqt

dtD k1.qe ¡ qt/ (2)

where qe and qt are the adsorption capacities (mg kg¡1) atequilibrium and at time t, respectively, and k1 is theadsorption rate constant (min¡1) of pseudo–first-orderadsorption. The integrated form of the Lagergren pseudo–first-order is as follows:

ln.qe ¡ qt/D lnqe ¡ k1t (3)

Time(hr)

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

C/C

0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Fig. 3. Effect of contact time on adsorption of As(V) from thesynthetic solution (pH D 8.2) onto 20 g/L Mn3O4.

t(min)

50 100 150 200 250Ln

(qe-

q t)-2.0

-1.5

-1.0

-0.5

0.0

0.5

R2 = 0.908

t(min)

0 100 200 300 400

t/qt

0

1

2

3

4

5

6

7

8

R2= 0.999

t1/2

6 8 10 12 14 16 18 20

q e(m

g/kg

)

48.6

48.8

49.0

49.2

49.4

49.6

49.8

R2 = 0.897

a

b

C

Fig. 4. Test of (a) pseudo–first-order kinetics model and (b)pseudo–second-order kinetics model (c) intraparticle diffusionmodel (pH D 8.2, T D 25�C, sorbent dosage D 5 g/L).

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The adsorption rate constant (k1) and the theoreticalvalue of the equilibrium adsorption capacity (qe) can beobtained from the slope and the intercept of the linear plotof ln(qe¡qt) versus t, respectively (Fig. 4a). The pseudo–second-order model and its linearized form are given inEq. 4 and Eq. 5, respectively:

dqt

dtD k2.qe ¡ qt/

2 (4)

t

qtD 1

k2

� �1q2e

� �C t

qe(5)

where k2 is the adsorption rate constant (g.mg¡1 min¡1) ofpseudo-second-order adsorption and can be obtained fromthe intercept of a linear plot of t/qt against t (Fig. 4b). Itshould be noted that all kinetic models were applied withinthe equilibrium time. The kinetic parameters of abovemodels are shown in Table 1. The pseudo–second-ordermodel provides the R2 value of 0.999 and the predictedequilibrium adsorption value for the pseudo–second-ordermodel was obtained as 50 mg kg¡1. The higher value ofcorrelation coefficient and the comparable theoretical andexperimental values of qe in the case of pseudo–second-order kinetics (Table 1) shows that the pseudo–second-order model was able to accurately describe the adsorptionkinetic data. The better fit of pseudo–second-order modelto the kinetic data suggests the existence of more than onerate-controlling step. [29]

The three main steps occurring in the process of adsorp-tion of arsenic (adsorbate) onto the Mn3O4 sorbent are:

1. Transport of adsorbate molecules from bulk solution tothe external surface of the sorbent by diffusion throughthe boundary layer (film diffusion);

2. Diffusion of the adsorbate from the external surfaceinto the pores of the sorbent (pore diffusion);

3. Adsorption of the adsorbate on the active sites on theinternal surface of the sorbent pores.

Determining the rate-limiting step helps elucidate theadsorption mechanism for adsorption of As (V) onto theMn3O4 sorbent. The last step (step 3) is very rapid andusually does not represent the rate-limiting step in theuptake of As(V).[30] Hence, either pore diffusion or filmdiffusion or a combination of both may be the rate-con-trolling steps. The experimental data can be modeled

based on Fick’s 2nd law to determine the sole existence ofintraparticle diffusion in the adsorption process as therate-controlling step:

qt D kidt1=2 C c (6)

where qt (mg kg¡1) is the adsorption capacity at time t(min), kid (mg.kg¡1.min¡1/2) is the rate constant of intra-particle diffusion and C (mg kg¡1) represents the boundarylayer effect. Larger values of C are indicative of larger val-ues of boundary layer thickness.[31] For qt versus t1/2, astraight line passing through the origin (C D 0) indicatesthat the intraparticle diffusion model is the main rate-determining step for the adsorption process.[32] The valuesof kid and C were determined in Figure 4c from the slopeand intercept of the plot of qt versus t

1/2, respectively. Thenon-zero value of C obtained from the intraparticle diffu-sion model for the present system (Fig. 4c) indicates thatmainly external mass transfer (film diffusion) was takingplace in the adsorption process, which was expected due tothe crystalline structure of the Mn3O4 sorbent.

Adsorption equilibrium isotherm experiments

Equilibrium adsorption isotherms can provide essentialphysiochemical data for evaluating the applicability of theadsorption process.[33] Figure 5 shows the adsorption iso-therm data for the adsorption of As(V) onto the Mn3O4

sorbent in DI water and the synthetic solution. Twoadsorption isotherm models, Langmuir and Freundlich,were employed to fit the equilibrium adsorption data at25�C using an initial As(V) concentration of 1 mg L¡1.The Dubinin–Radushkevich (D–R) adsorption equationwas also used to predict the nature of the adsorptionprocess.The Langmuir adsorption model describes adsorption

on homogeneous surfaces and assumes that the adsorbentsurface consists of active sites with a uniform energy andtherefore, the adsorption energy is constant. It can modelthe monolayer coverage of the adsorption surface and theadsorption that occurs through the same mechanism. TheLangmuir isotherm can be described as the followingequation:[34]

qe D qmKLCe

1CKLCe

(7)

Table 1. Pseudo–first-order and pseudo–second-order kinetic parameters for adsorption of As(V) onto Mn3O4.

Experimental qe D 49.8 mg/kg

Order k1(1st), k2(2

nd), kid qe (mg/kg) C R2 value

1st 0.022 (min¡1) 13.6 — 0.9082nd 0.011 (kg/mg.min¡1) 50 — 0.999Intraparticle diffusion 0.7407 (mg/kg/min0.5) — 48.11 0.897

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where qe is the amount of arsenic adsorbed per unit weightof adsorbent (mg kg¡1), qm is maximum arsenic adsorbed(mg kg¡1) corresponding to complete coverage of avail-able sites, KL is Langmuir constant related to the freeenergy or net enthalpy of adsorption and Ce is the concen-tration of arsenic in solution at equilibrium (mg L¡1). Thelinear form of Langmuir model is as:

Ce

qeD Ce

qmC 1

KLqm(8)

where qm and KL can be calculated from the slope andintercept of the plot of Ce/qe versus Ce or 1/qe versus 1/Ce, respectively.The Freundlich empirical equation and its linearized

form are expressed as follows:

qe DKFC1=ne (9)

logqe D logKF C .1=n/logCe (10)

where KF is the Freundlich constant indicating the adsorp-tion capacity and 1/n is a constant that shows the adsorp-tion intensity or strength.[35] The Freundlich isothermmodel has been applied for describing both adsorption on

heterogeneous surfaces and multilayer sorption.[36] Thehigher value of KF indicates the higher level of adsorbateremoval while 1/n values less than one indicate goodadsorption intensity.[37] The Freundlich paramters KF and1/n can be determined from the intercept and slope of theplot of log qe against log Ce.To evaluate the nature of the sorption, the adsorption

equilibrium data were also fitted with Dubinin–Radushke-vich (D–R) adsorption equation. This model envisages theprocess mechanisms and determines the mean free energyof the sorption which is defined as the free energy changefor removing a molecule from its location in the sorptionspace to an infinite distance in solution from the sorptionsurface.[38] The D–R adsorption isotherm can be expressedas:

qe D qm exp.¡KDRe2/ (11)

and linearized as:

In qe D In qm ¡KDRe2 (12)

where qm is the D–R constant, e (Polanyi potential) is RTln(1 C 1/Ce) and KDR is a constant related to adsorptionenergy (mol2. kJ2)¡1. The qm and KDR can be determinedfrom the intercept and slope of the plot ln qe versus e2,respectively. The type of adsorption such as physisorptionor chemisorption can be determined by calculating themean free energy (E) in the system using the followingequation:[39]

ED 1ffiffiffiffiffiffiffiffiffiffiffiffiffiffi.2KDR/

p (13)

If the magnitude of E is less than 8 kJ mol¡1 the sorptionprocess is of physical sorption, while if E is in the range of8 to 16 kJ mol¡1, the sorption is supposed to proceed viachemisorption.The adsorption parameters obtained from the adsorp-

tion isotherm plots are listed in Table 2. The adsorptiondata for both DI water and the synthetic solution followedthe the Langmuir and Freundlich equations with high R2

values, indicating the favorable adsorption of As (V) ontothe Mn3O4 sorbent. The maximum adsorption capacity of

Ce(μg/L)

0 100 200 300 400 500 600 700 800

q e(m

g/kg

)

0

50

100

150

200

250

300

350

400

DI waterSynthetic solution

Fig. 5. Adsorption equilibrium isotherm data for DI water andsynthetic solution.

Table 2. Adsorption parameters for adsorption of As(V) onto Mn3O4.

Langmuir Freundlich D-R

DIwater

Syntheticsolution

DIwater

Syntheticsolution

DIwater

Syntheticsolution

qm(mg/kg) 344.8 256.4 KF (mg/kg)(L/mg)1/n 96.5 30.85 KDR (mol2/kJ2) 0.0034 0.0076KL(L/mg) 0.0366 0.0294 1/n 0.196 0.346 E(kJ/mol) 12.13 8.11R2 0.992 0.984 R2 0.992 0.996 R2 0.963 0.962

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the sorbent qm obtained from the Langmuir adsorptionequation was 345 mg kg¡1 and 256 mg kg¡1 in DI waterand synthetic solution, respectively. The values of Lang-muir parameter qm and the Freundlich parameter KF areboth greater for DI water than for the synthetic solution,indicating a greater adsorption capacity for DI water thanfor the synthetic solution. This result can be due to thepresence of the other ions in the synthetic solution whichcan compete with arsenic for the limited active sites on thesurface of the sorbent. The feasibility of isotherm criterioncan be determined from a dimensionless constant separa-tion, RL, obtained from the Langmuir isotherm.[40] TheRL can be expressed as:

RL D 11CKLC0

(14)

where KL is the Langmuir constant (L mg¡1) and C0 is theinitial arsenic concentration (mg L¡1). The value of RL

demonstrates the nature of adsorption as linear (RL D 1),irreversible (RL D 0), unfavorable (RL > 1), and favorable(0 < RL < 1). With an initial As (V) concentration of 1 mgL¡1, the calculated value of RL in DI water and the syn-thetic solution were 0.027 and 0.033, respectively, showingnearly irreversible adsorption of As (V) onto the Mn3O4

sorbent.The adsorption equilibrium data were also fitted with

Dubinin–Radushkevich (D-R) adsorption equation. Themean free energy of adsorption of As(V) onto Mn3O4

(obtained from the D–R adsorption equation) in DI waterand synthetic solution was calculated as 12.13 kJ mol¡1

and 8.11 kJ mol¡1, respectively, which was suggestive ofthe chemisorption of As(V) onto the surface of Mn3O4.The adsorption parameters presented in Table 2 show

that the Freundlich parameter 1/n was smaller for DIwater than for synthetic solution, while the Langmuirparameter KL and the D–R parameter KDR were larger for

DI water than for synthetic solution. The smaller 1/nvalue, and the larger KL and KDR values for DI watershow that the binding of As(V) onto Mn3O4 was strongerin DI water than in the synthetic solution. The strongerbinding of As(V) for DI water may be attributed to the pHof DI water, which was lower than the pH of syntheticsolution, as discussed later as part of the effect of pH onadsorption.The maximum adsorption capacity of Mn3O4 in this

study (256–345 mg kg¡1) was compared to several repre-sentative metal oxide sorbents (Table 3). When theadsorption capacities are compared in terms of sorbentmass (mg of As(V) removed per kg of sorbent), some ofthe sorbents have a greater adsorption capacity (TiO2, goe-thite, activated alumina), while some others have a similaradsorption capacity (ferrihydrite, hematite, MnO2). Whenthe adsorption capacities are compared in terms of sorbentsurface area [mg of As(V) removed per m2 of sorbent], allother sorbents have adsorption capacities that are compa-rable to or lower than the surface normalized adsorptioncapacity of crystalline Mn3O4 sorbent (101.5 mg m¡2).

Effect of pH on adsorption of As(V)

The speciation of As(V) is pH dependent and the corre-sponding stability of species pH values for As(V) are:H3AsO4 (pH 0–2), H2AsO4

¡ (pH 3–6), HAsO42¡ (pH

7–11), and AsO43¡ (pH 12–14). [48] Hence, pH can play an

effective role for the adsorption of arsenic species. The pHdependence of As(V) adsorption has been reported forother sorbents.[49] To determine the optimum pH foradsorption of As(V) onto Mn3O4 powder, the uptake ofarsenic as a function of pH was studied. Figure 6 showsthe effect of pH on adsorption of arsenate in the pH rangeof 2.0–10.0 with an initial As(V) concentration of 1 mgL¡1. The initial pH before adsorption was the pH ofthe arsenic solution before the addition of Mn3O4. The

Table 3. Comparison of Mn3O4 with other sorbents for As(V) adsorption capacity.

Adsorbent pHConcentration

rangeSurface area

(m2/g)Capacity(mg/kg)

Capacity persurface area(mg/m2) Reference

Hydrous titanium dioxide 4.0 0.2–8.5 mg/L 280 33,400 119.3 [41]Goethite 9.0 0–60 mg/L 39 4,000 102.6 [42]Activated alumina 7.0 50 mg/m3 195 9,200 47.2 [43]Ferrihydrite (FH) — 325 mg/L 141 250 1.8 [44]MnO2 7.9 <1 mg/L 17 172 10.1 [18]micro-/nano-structured

MnO2 spheres— 0.1–0.8 mM 162.54 14,500 89.2 [45]

Hematite 4.2 133.49 mmol/L 14.4 200 13.9 [46]Fe/GAC — 237–362 mg/L 631.3 3,887.5 6.2 [47]Mn3O4 6.6-8.3 DI and

synthetic solution1 mg/L 3.4 256–345 75.3–101.5 Present work

Mn3O4 7.9–8.4 Groundwater 1 mg/L 3.4 220–240 64.7–70.6 Present work

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Mn3O4 sorbent most effectively adsorbed arsenate in thepH interval 2.0–8.0 with nearly 92–97% removal of arse-nic. Stronger arsenic adsorption onto Mn3O4 powder wasobtained at lower pH values. The adsorption of As(V)onto Mn3O4 decreased slightly in the pH range 2–9, whiledecreasing sharply when solution pH was greater than 9.The effect of pH on adsorption can be interpreted based

on the surface charge of the sorbent. In order to observethe surface charges of the Mn3O4 suspensions and under-stand the interactions at the solid-solution interface, thezeta potential values were measured in a 1 mMNaCl solu-tion. The point of zero charge (PZC) for Mn3O4 was deter-mined as 7.32 (Fig. 7a). Below the pHPZC of 7.32, theMn3O4 particles are predominantly positively charged andabove this pH they are negatively charged. Adsorption ofOH¡ or HC on the neutral �Metal-OH0 site results in thenet surface charge of Mn3O4 at the solid-solution interface.At the PZC or pHpzc, cations and anions may adsorb ontothe surface through the formation of outer-sphere com-plexes via van der Waals forces.As solution pH increases from lower pH to 7.32, the

lesser adsorption of As(V) can be due to the decreasingelectrostatic attraction between the anionic As(V) speciesand the less positive surface of Mn3O4. The lower adsorp-tion of As(V) at pH > 7.32 can be attributed to an increasein repulsion between the more negatively charged As(V)species and the negatively charged surface sites of Mn3O4.In addition, the decrease in the uptake of As(V) on Mn3O4

at higher pH could be due to an increase in competinghydroxyl ions (OH¡) for adsorption sites with increasingpH, which can cause a reduction in adsorption.[50] Similarresults were also obtained by Chandra et al.[51] and Luoet al.,[52] who investigated the adsorption of As(V) onmagnetite-reduced graphene oxide composites and magne-tite Fe3O4-reduced graphite oxide-MnO2 nanocomposites,respectively. The adsorption of As(V) onto Mn3O4 powder

is of a wide pH range, which should be of advantage forpractical operation.The specific adsorption of ions onto the surface can

change the PZC of the surface. The decrease in the PZC ofthe sorbent (Fig. 7b) clearly proves the adsorption of As(V) species onto the surface of Mn3O4. Also, the anionicH2AsO4

¡ and HAsO42¡ species are the predominant As

(V) species in the pH range 4–8, which can contribute neg-ative charge on the surface of the sorbent. Hence, the shiftin the PZC value to the acidic side can be attributed to theformation of negatively charged surface complexes by spe-cific adsorption of HxAsO4

x¡3 ions onto the surface ofMn3O4.

[53]

In the outer-sphere complexes, no certain chemical reac-tion can occur between the adsorbate and the surface thatcould change the surface charge of the sorbent. Therefore,the decrease in the PZC value of the suspension can beattributed to the formation of inner-sphere complexes byAs(V) at the surface of Mn3O4.

[54]

pH

1 2 3 4 5 6 7 8 9 10 11

Ars

enic

Upt

ake

(q),

mg/

kg

0

20

40

60

80

100

120

140

160

180

200

220

240

Fig. 6. Effect of pH on adsorption of As(V) onto Mn3O4 (5 g/Lsorbent dosage) from a 1 mg/L solution.

pH

3 4 5 6 7 8 9 10 11

ZP

(mV

)

-30

-20

-10

0

10

20a

pH2 3 4 5 6 7 8 9 10 11

ZP

(mV

)

-20

-15

-10

-5

0

5

10

15

20b

Fig. 7. Zeta potential of 0.1 g/L Mn3O4 as a function of pH inwater (a): in the absence of arsenate; (b): in the presence ofarsenate.

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Due to the nature of the As(V) solution in DI water,which is not a buffered solution, the final pH of the solu-tion after adsorption could change considerably during theexperiments. Adding the sorbent to the solution isexpected to shift the pH towards the point of zero charge,i.e., lower the pH for pH > PZC and increase the pH forpH < PZC. The measurments of the final pH of thesolutions with different initial pH confirmed this claim.

Effect of co-existing ions on the uptake of As(V)

The presence of co-existing ions, such as sulfate, bicarbon-ate, phosphate and calcium which are usually present innatural waters may affect the removal efficiency of arsenicfrom water. An initial As(V) concentration of 1 mg L¡1

was used in all solutions with a sorbent dosage of 5 g L¡1.The effect of other ions on arsenic adsorption was investi-gated by increasing the concentration of HCO3

¡ from 1 to5.5 mM and increasing the concentrations of SO4

2¡, Ca2C

and PO43¡ from 0.5 to 2 mM. The effect of co-existing

ions on arsenic adsorption is shown in Figure 8. The pres-ence of bicarbonate and phosphate suppressed the adsorp-tion of As(V), where phosphate had the greatest effect onadsorption. The decrease in the adsorption of As(V) in thepresence of phosphate and bicarbonate could be attributedto competition between these anions and As(V) for the lim-ited available adsorption sites on the surface of Mn3O4.Calcium and sulfate did not show any noticeable influenceon the adsorption of As(V).The adsorption of As(V) decreased greatly at PO4

3¡

concentrations up to 1 mM. With a further increase in thePO4

3¡ concentration, adsorption of arsenic decreasedslightly. The removal of As(V) decreased to 11% at thePO4

3¡ concentration of 2 mM. The interference of phos-phate on the adsorption of As(V) can be attributed to thesimilar behavior of arsenate and phosphate in solution in

many aspects and the similar types of surface-complexeswhich can be formed by phosphate and arsenate.Zeng,[55] Jian et al.[50] and Li et al.[56] have observed a

negative effect and high interference of phosphate ions onthe removal percent of arsenate while using Fe(III)-SiBinary Oxide, ferrihydrite and hydrous cerium oxidenanoparticles as sorbents, respectively. Similarly, increas-ing the concentration of bicarbonate had negative effecton the adsorption of As(V). Increasing the concentrationof bicarbonate up to 5.5 mM, decreased the removal ofarsenic from 97% to 68%. Although the highest competi-tive effect was observed in the presence of PO4

3¡, HCO3¡

indicated a good competitive capacity.

Sorbent reuse

The sorbent can be a cost-effective sorbent if the sorbentcan be reused in multiple cycles of operation. The adsorp-tion capacity of the sorbent was tested by exposing thesame mass of sorbent (1 g) to the synthetic solution [1 mgL¡1 As(V)] in several consecutive cycles of adsorption.The percent uptake of As(V) by Mn3O4 from the syntheticsolution for each adsorption cycle was 99.5%, 75.6% and69.8% for the first, the second and the third cyclesof adsorption, respectively. The uptake of As(V) by 1 g ofsorbent for the first, the second and the third cycles ofadsorption corresponded to 40.6%, 30.9% and 28.5%of the total combined As(V) uptake (100%) achieved dur-ing the three cycles of adsorption, showing that nearly60% of the total uptake of As(V) was achieved during thesecond and third cycles of adsorption. The results indicatethat reusing the Mn3O4 sorbent from the first adsorptioncycle in two extra consecutive cycles was conducive to theremoval of substantial additional amounts of As(V) fromthe synthetic solution.

Removal of arsenic from natural groundwater spiked

with arsenic

Two groundwater samples were collected from Deer Parkin northeastern Illinois [Groundwater #1] and from theBloomington–Normal area in central Illinois [Groundwa-ter #2]. The composition of these two groundwaters isshown in Table 4. The real groundwaters were spiked with1 mg L¡1 As(V), without the addition of sodium bicarbon-ate, calcium chloride and sodium sulfate. A range of sor-bent dosage (2–40 g L¡1) was applied to the contaminatedgroundwaters for 24 h for removal of arsenic. The resultswere then compared with the DI water and the syntheticsolution.The removal of arsenic from the two groundwater sam-

ples as function of sorbent dosage is shown in Figure 9.The adsorption pH ranged from 8.07 to 8.45 for ground-water #1 and from 7.81–7.97 for groundwater #2. Thedata from Figure 9 show that greater than 99% removal of

Ion Concentration (mM)

0 1 2 3 4 5 6

Ars

enic

Rem

ova

l (%

)

0

20

40

60

80

100

120

SO42-

HCO3-

Ca2+

PO43-

Fig. 8. Effect of coexisting ions on adsorption of As (V) ontoMn3O4.

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aresenic from both groundwater samples was obtainedwith 20 g L¡1 of Mn3O4 sorbent, while greater than 95%removal of arsenic was obtained with 10 g L¡1 of the sor-bent. Comparable removal of arsenic was observed forgroundwater #1 and groundwater #2 at higher sorbentdosage (� 10 g L¡1), while at lower sorbent dosage(< 10 g L¡1), greater arsenic adsorption was achievedfrom groundwater #2 than from groundwater #1.Since the alkalinity (bicarbonate) concentration was

larger in groundwater #1 than in groundwater #2, thenbased on the results shown in Figure 8 for the effect ofbicarbonate on adsorption of arsenic onto Mn3O4, therewould be greater competition between arsenate and bicar-bonate in groundwater #1 for the smaller number of avail-able adsorption sites at lower dosages of sorbent, whereasthere was less competition between arsenate and bicarbon-ate for the greater number of adsorption sites at higherdosages of sorbent.As compared to the arsenic removal from DI water,

lower arsenic removal was obtained from the two ground-water samples and from the synthetic solution; this lowerremoval of arsenic may be attributed to the presence ofother anions competing with arsenic on the limited activeadsorption sites on the surface of Mn3O4 sorbent. Asshown in Figure 2 and Figure 9, comparable removal of

arsenic was achieved from the real groundwater samplesand the synthetic solution with Mn3O4 dosages of 5–40 gL¡1. The adsorption capacity of the sorbent for removalof arsenic from the two groundwater samples was deter-mined from adsorption isotherm experiments. The adsorp-tion isotherm data presented in Figure 10 show that theadsorption of As (V) onto the Mn3O4 sorbent from bothgroundwaters was favorable; adsorption from groundwa-ter #1 followed the Langmuir model, while adsorptionfrom groundwater #2 followed the Freundlich model. Theadsorption capacity of the Mn3O4 sorbent for adsorptionof As(V) from the two groundwaters was determined torange from 220 to 250 mg kg¡1 (Table 3), with the adsorp-tion capacity being slightly greater for groundwater #2than for groundwater #1. The adsorption data presentedin Figure 5 and Figure 10 show that the adsorption capac-ity of the Mn3O4 sorbent was comparable for naturalgroundwater and the synthetic solution.

Conclusion

This study showed the effectiveness of crystalline manga-nese oxide (II,III) powder (Mn3O4) for removal of As(V)from water. The surface charge analysis and zeta potentialmeasurements confirmed the adsorption of arsenic ontothe surface of Mn3O4. The Mn3O4 sorbent was able toremove greater than 99% of arsenic under the condition ofadsorbent dosage 20 g L¡1, pH of 6.6–8.3, temperature of25�C and contact time of 24 h. The adsorption processcould be explained by the pseudo–second-order kineticsmodel. The Langmuir, Freundlich and D–R adsorptionequations were applied to equilibrium adsorption isothermdata. The maximum adsorption capacity of the sorbent inDI water and synthetic solution was found to be 345 mgkg¡1 and 256 mg kg¡1, respectively.

Table 4. Characteristics of natural groundwaters.

ParameterGroundwater

#1Groundwater

#2

pH 7.65 7.58TDS (mg/L) 341 1290Alkalinity (mg/L as CaCO3) 202 150Total Hardness (mg/L as CaCO3) 273 796

Sorbent Dosage (g/L)

Ars

enic

Rem

oval

(%

)

0

10

20

30

40

50

60

70

80

90

100

110

120

Groundwater #1Groundwater #2

4020131052

Fig. 9. Arsenic removal from natural groundwater spiked withAs (V).

Ce (μg/L)

0 100 200 300 400 500 600 700 800 900

q e (m

g/kg

)

0

50

100

150

200

250

Groundwater #1Groundwater #2

Fig. 10. Adsorption equilibrium isotherm data for groundwaterspiked with As (V).

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Chemical adsorption (chemisorption) governed theadsorption of As(V) onto the surface of Mn3O4. Theremoval of arsenic decreased with increasing pH. Experi-ments on the influence of other existing ions in watershowed that adsorption of arsenic decreased in order ofions; phosphate > bicarbonate > calcium » sulfate. Thedata reported in this study contribute to a better under-standing of the effect of different factors like co-existingions, pH, sorbent dosage and surface charge on theremoval of arsenic by manganese oxides. The ability of theMn3O4 sorbent for the removal of arsenic from naturalgroundwater was evaluated for two real groundwaters,where the appreciable uptake of As(V) from both ground-waters was indicative of a practical sorbent for removal ofarsenic from natural water. Based on the results obtainedfrom this study, Mn3O4 powder was shown to be aneffective and suitable sorbent for removal of As(V) fromwater.

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