Magnetic binary oxide particles (MBOP): A promising adsorbent for removal of As (III) in water

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Magnetic binary oxide particles (MBOP): A promising adsorbent for removal of As (III) in water Rajesh M. Dhoble b, Sneha Lunge a, A.G. Bhole c, Sadhana Rayalu a,* a

Environmental Materials Division, National Environmental Engineering Research Institute, Nagpur, M.S., India Civil Engineering Department, Priyadarshini Indira Gandhi College of Engineering, Nagpur, M.S., India c Civil Engineering Department, Visvesvaraya National Institute of Technology, Nagpur, M.S., India b

article info

abstract

Article history:

Magnetic binary oxide particles (MBOP) synthesized using chitosan template has been

Received 7 March 2011

investigated for uptake capacity of arsenic (III). Batch experiments were performed to

Received in revised form

determine the rate of adsorption and equilibrium isotherm and also effect of various rate

10 June 2011

limiting factors including adsorbent dose, pH, optimum contact time, initial adsorbate

Accepted 16 June 2011

concentration and influence of presence cations and anions. It was observed that uptake of

Available online 28 June 2011

arsenic (III) was independent of pH of the solution. Maximum adsorption of arsenic (III) was w99% at pH 7.0 with dose of adsorbent 1 g/L and initial As (III) concentration of 1.0 mg/L at

Keywords:

optimal contact time of 14 h. The adsorption equilibrium data fitted well to Langmuir and

Arsenic (III)

Freundlich isotherm. The maximum adsorption capacity of adsorbent was 16.94 mg/g.

Batch adsorption

With increase in concentration of Ca2þ, Mg2þ from 50 mg/L to 600 mg/L, adsorption of As

Isotherm

(III) was significantly reduced while for Fe3þ the adsorption of arsenic (III) was increased

Magnetic adsorbent

with increase in concentration. Temperature study was carried out at 293 K, 303 K and 313 K reveals that the adsorption process is exothermic nature. A distinct advantage of this adsorbent is that adsorbent can readily be isolated from sample solutions by application of an external magnetic field. Saturation magnetization is a key factor for successful magnetic separation was observed to be 18.78 emu/g which is sufficient for separation by conventional magnate. ª 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Arsenic is one of the several common occurring toxic metals found in environment and designated by United State Environmental Protection Agency (USEPA) under clean water acts (Martinson and Reddy, 2009). The sources of arsenic and many other trace elements in ground water are from geochemical reactions, weathering of rocks, industrial wastewater discharge, agriculture uses of arsenical pesticides, discharges from coal fired thermal power plants, herbicides and fertilizers etc. Arsenic concentrations are very low in major rock-forming silicates, 0.05e2.3 mg kg1, and in carbonates, 1e8 mg/kg. The

highest arsenic concentrations (20e200 mg/kg) are typically observed in organic and sulphide-rich shales, sedimentary ironstones, phosphatic rocks, and some coals (Smedley and Kinniburgh, 2002). In rural areas of India and Bangladesh, ground water is the main source for drinking water through dug well and tubewells. It has been reported that in India many districts in West Bengal are suffering from arsenic problem. Recent studies carried out in northeastern England revealed arsenic enrichment within the urban and industrially affected rivers. Arsenic concentration in rural area averaged between 0.6 and 0.9 mg/L and in between 3.2 and 5.6 mg/L for the rivers influenced by industrial discharges (Escudero et al., 2009).

* Corresponding author. E-mail addresses: [email protected] (R.M. Dhoble), [email protected] (S. Lunge), [email protected] (S. Rayalu). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.06.016

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Arsenic occurs in different forms and inorganic trivalent form of arsenic (III) is the most toxic among the various forms of arsenic present in natural water. The common valencies of geogenic arsenic in ground water sources are As (III) (arsenite) and As(V) (arsenate). The inorganic hydrolysed species are 2 3 and H3AsO4, present as H3AsO3, H2 AsO 3 , HAsO3 , AsO3  2 3 H2 AsO4 , HAsO4 and AsO4 (Gupta et al., 1997). As per United State Environmental Protection Agency (USEPA) and International Agency for Research on Cancer, arsenic is classified as a human carcinogen. Arsenic in drinking water indicates that arsenic could cause liver, lung, kidney and skin cancer (Smith et al., 1992). An acute high dose of arsenic by oral intake cause gastrointestinal irritation resulting in difficultly in swallowing, thirst, abnormal high blood pressure and convulsions (Pontius et al., 1994). It has been reported that arsenic may cause neurological damage to those who drink water contaminated with slightly greater than 0.1 mg/L of As (III). The lowest arsenic concentration in water sample producing dermatosis was found to be 0.2 mg/L (Chakraborti and Saha, 1987). The total quantity of arsenic consumed per day and the duration of exposure are very important factors. WHO provisional guideline value for arsenic in drinking water is 10 mg/l (Who, 2004). Various physio-chemical treatment methods have been adopted to remove arsenic from drinking water both in laboratory and field condition below the MCL (maximum contamination level). Among a variety of technologies (including precipitation, coagulation, membrane separation, ion exchange, lime softening and adsorption), adsorption and coagulation are believed to be the cost effective method. Although coagulation with iron and aluminium salts is more effective, the requirement of skilled operator and the introduction of contaminants into the water limit its application in small community and household levels. Since solid adsorbents are easy to handle and are appropriate for use in country side where high arsenic ground water mostly occurs, adsorption has received much attention on As removal. Iron containing substances have been widely investigated to remove arsenic from aqueous solution due to their high specific surface area, including Mn-substituted Fe oxyhydroxide (Lakshmipathiraj et al., 2006), granular ferric hydroxide (Banerjee et al., 2008), ferrihydrite (Jessen et al., 2005), goethite (Sun and Doner, 1998), zero valent iron (Nikolaidis et al., 2003), Ce(IV)- doped Fe oxide (Zhang et al., 2003), copper oxide incorporated alumina (Pillewan et al., 2011), natural hematite and natural siderite (Guo et al., 2007). This study investigates the feasibility of the magnetic adsorbent for trivalent arsenic As(III) removal from aqueous solution. The main objectives are (i) to understand the As(III) adsorption kinetics, (ii) to evaluate the influence of temperature, pH and coexisting anions on the As(III) removal capacities; and (iii) to describe and explain some important thermodynamic parameters.

2.

Materials and methods

2.1.

Reagents

All chemicals were analytical grade. All stock and fresh solutions were prepared in deionized water for entire study.

Standards for calibration were prepared from As (III) standard reference sodium (Meta) arsenite. Stock solution (1000 mg/L) was prepared from sodium arsenite and frozen to prevent oxidation. Solutions of As (III) of 100 mg/L were prepared in every fortnight and working solutions of As (III) were prepared according to experiment requirements. pH was adjusted by standard acid and base solutions of 0.1 N HCl and 0.1 N NaOH respectively. For the study of effect of adsorption due to presence of background ions in ground water, Ca(NO3)4$H2O, MgSO4.7H2O, FeSO4.7H2O, NaCl, Na2SO4, NaNO3and Na2HPO4 (Merck India) salts were used. Effect of temperature on arsenite adsorption was studied by varying temperature.

2.2.

Synthesis

The Magnetic binary oxide particles (MBOP) were synthesized by template method. 27 g of chitosan was dissolved in 900 ml of 5% acetic acid with constant stirring on mechanical stirrer for 1 h. In one beaker 84.78 g of aluminium nitrate was dissolved in 100 ml of distilled water and in another beaker 97.89 g of ferrous nitrate was dissolved in 100 ml of distilled water. Aluminium nitrate and ferrous nitrate solutions added to the chitosan gel with stirring for 180 min. The resulting Al-Fe-chitosan slurry was added drop wise into NH4OH solution (50% v/v) under vigorous stirring, using a syringe pump. The gel macro spheres formed were allowed to stabilize in NH4OH solution for 60 min. The beads were separated from the NH4OH solution and washed with deionised water and dried at 70  C for 24 h in oven. The dried beads were calcined at 450  C for 6 h in muffle furnace. Finally the calcined product was subjected to multiple washing with deionised water and dried at 80  C.

2.3.

Methods

MBOP was characterized by using different techniques like Xray diffraction, Scanning electron microscopy (SEM), Wave length energy dispersive analysis of X-ray (WDAX), and BET surface area analysis. Magnetic properties of adsorbent were examined by Vibrating sampler magnetometer (VSM). The Xray pattern of adsorbent was recorded on Rigaku X-ray diffractometer. SEM was performed by using JEOL-6380A for analysing the surface morphology of the material. Composition of material was determined by wave length energy dispersive analysis of X-ray (WDAX). Surface area of adsorbent was measured by Brauner, Emmett and Teller (BET) method using micromeritics ASAP 2010 surface area analyzer.

2.4.

Batch study

Batch adsorptions were carried out by shaking 50 ml of arsenic (III) samples in a controlled rotary shaking machine (Model no. CIS-24, Remi Instruments, Mumbai, India) at a speed of 150 rpm. The solution was taken in glass stopper bottles of 125 ml capacity. The dose of adsorbent and arsenite concentration was varied within feasible parameter range. The solution was then filtered through Whatman filter paper no. 42 and the filtrate was analyzed for residual arsenic after adsorption in Atomic absorption spectrophotometer hydride vapour generator (AASHVG-1) 6300 Shimadzu Japan 2007. All adsorption experiments were conducted at a room

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 7 6 9 e4 7 8 1

temperature of 30  2  C to investigate the effect of various parameters like adsorbent dose, initial arsenic concentration, presence of interfering ions and pH etc. The specific amount of arsenic adsorbed was calculated from the following equation qe ¼ ðC0  Ce Þ 

V W

(1)

and %Removal ¼

ðC0  Ce Þ  100 Ce

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NaOH. According to the WHO guidelines, the normal range of pH in drinking water lies in the range 6.0e8.5, and is mostly dependent upon the geological characteristics of the soil and weather conditions. Also, pH of the solution is one of the major factors which significantly affect the As (III) adsorption. Hence, it is necessary to study the effect of pH on removal of As (III) from water. The effect of background ions i.e. cations and anions commonly present in ground water was also studied with different proportions.

(2)

where qe is the adsorption amount (mg/g) in the solid at equilibrium; Co, Ce are initial and equilibrium concentrations of arsenic (mg/L), respectively; V is volume (ml) of the aqueous solution and W is the mass (g) of adsorbent used in the experiments. The effect of pH on As (III) removal was studied by adjusting the pH of the solution using 0.1 N HCl and 0.1 N

2.5.

Kinetic study

In order to estimate equilibrium adsorption rate for the uptake of As (III) by MBOP, time dependent sorption studies were conducted. Adsorption kinetics was monitored by adding known weight of MBOP into 50 ml of 1 mg/L arsenic solution at 293 K, 303 K and 313 K stirred at 150 rpm. A portion of solution

Fig. 1 e (a) XRD of MBOP, (b) SEM of MBOP, (c) SEM of MBOP after arsenic (III) adsorption, (d) FTIR of MBOP, (e) VSM magnetization curves for MBOP, (f) Photographs of MBOP attracted by magnetic bar a) before adsorption b) after adsorption.

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Fig. 1 e (continued).

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was withdrawn from the vessel at predetermined time intervals and filtered. The filtrate as analyzed for residual concentration of As (III) using Atomic absorption spectrophotometer.

that the MBOP is attracted by the magnetic bar. This further confirms that MBOP is magnetic nature.

3.2. 2.6.

Thermodynamics

Thermodynamic parameters of adsorption including standard free energy change (ΔG ), standard Enthalpy change (ΔH ), and standard entropy change (ΔS ) were calculated at 293 K, 303 K and 313 K temperature.

3.

Results and discussion

3.1.

Characterization of MBOP

The prepared MBOP is reddish brown coloured granular adsorbent. The X-ray diffraction spectrum pattern of the MBOP did not show any sharp peak (Fig. 1a), thereby indicating the amorphous nature of the product. The surface morphology of MBOP before and after arsenic adsorption was studied from SEM. It can be seen from Fig. 1b the adsorbent has expected large number of porous structure which indicates the adsorbent may have a high surface area and high adsorption capacity. These large pores are formed by the elimination of chitosan template during calcinations step of synthesis. After arsenic adsorption the surface morphology of MBOP remains unchanged suggesting physical adsorption of arsenic (Fig. 1c). The BET surface area, average pore size and total pore volume of MBOP was observed to be 123.28 m2/g, 61.59 Ǻ, and 0.1732 cm3/g, respectively. The radius of arsenite (0.58 Ǻ) is much smaller than the pore size of the MBOP. This may facilitate increased dispersion of arsenite in the inner layer of the granular MBOP. The chemical analysis of the product gave iron, aluminium and oxygen contents as 42.6%, 16.69%, and 34.21%, respectively as analyzed by WDAX. The FTIR spectrum of MBOP (Fig. 1d) indicates the presence of predominant peaks at 3512.38, 3321.24 cm1 (eOH and eNH stretching vibrations), 2900.67and 2342.24 cm_1 (eCH stretching vibration, 1648.20 cm1 (eNH bending vibration in eNH2)), 1378.91 cm1 (eNH deformation vibration in eNH2). The low intensity band at 1062.40 cm1 is attributed to FeeOH structural vibration. The band between 400 and 450 cm1 could be due to the superposition of the characteristic stretching bands of aluminium oxide. The bands observed between 1100 and 500 cm1 could be characteristic vibrations of aluminium oxide.

3.1.1.

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Effect of dose of MBOP

The influence of adsorbent dose on As (III) removal at a fixed initial arsenic concentration of 1 mg/L and neutral pH is shown in Fig. 2. It was noticed that percentage removal of As (III) increased from 27% to 99% with an increase in adsorbent dose from 0.01 g/L to 2.0 g/L respectively which is due to the higher active site/As (III) ratio. However, it was noticed that after a dosage of 0.5 g/L, there was no significant change in the percentage removal of As (III). Usually a point of intersection in this graph is considered as the optimum dose as this point represents balance between % As (III) removal and adsorption capacity. The intersection point is at 0.03 g/L however dose of 1 g/L has been selected as it was required to bring down the arsenic level below 0.01 mg/L (10 ppb) as per WHO guidelines. Adsorbent dose of 1 g/L was used for further study.

3.3.

Effect of pH on As (III) uptake

pH plays significant role in adsorption-based water treatment processes, because proton concentration can strongly modify the redox potential of sorbates and sorbents, enhance dissolution of the adsorbent material and modify chemical speciation of the adsorbates as well as surface charge of adsorbent (Escudero et al., 2009). The effect of pH on arsenic removal by MBOP was studied over a broad pH range of 3e11 with adsorbent dose of 1 g/L; initial concentration 1 mg/L, shaking speed of 150 rpm and contact time of 24 h. The effect of pH on arsenic (III) adsorption is shown in Fig. 3. pH of the arsenic contaminated ground water is normally reported between 7 and 9 and there is drastic reduction in the uptake capacity of most of the adsorbents in the pH above 7. It is evident from Fig. 3 that there was no significant effect of pH on As (III) adsorption over a wide range of pH 3 to 9 which is highly advantageous for practical operation. This may be due to the specific chemical reaction interaction between adsorbate and adsorbent surface. Arsenic and (III) exists in non-ionic (H3AsO3) and anionic (H2 AsO1 3 ) form in the pH range 2e7 and 7.5e9 respectively. In the HAsO2 3

Magnetic property of MBOP

A distinct advantage of MBOP is that adsorbent can readily be isolated from solution by application of an external magnetic field. Fig. 1e shows the VSM magnetization curves for MBOP at room temperature. MBOP exhibited typical superparamagnetic behaviour, characterized by strong magnetic susceptibility. Saturation magnetization is a key factor for successful magnetic separation. Ma et al. observed that saturation value of 16.3 emu/g was sufficient for magnetic separation with a conventional magnet (Ma et al., 2005). Thus, the saturation magnetization value achieved with MBOP was high (18.78 emu/g) enough for magnetic separation. Fig. 1f shows

Fig. 2 e Effect of adsorbent dose on As(III) removal.

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Fig. 3 e Effect of pH on As (III) adsorption by MOA.

acidic range, when non-ionic As (III) species comes in contact with the adsorbent, the adsorbent surface is charged positively which helps in conversion of non-ionic arsenic to its anionic form, which in turns assists in the adsorption process (Kundu and Gupta, 2006). However, slight decrease in arsenic (III) adsorption capacity was observed above pH 9 which may be due to i) competition of excessive OH ˉ ions for adsorption, ii) negative surface charge of adsorbent at alkaline conditions and iii) negatively charged H2 AsO 3 species starts dominating. When neutral species of H3AsO3 exists then the maximum removal of arsenic (III) occurs. In alkaline medium, the negatively charged H2 AsO1 3 species start dominating and surface also tends to acquire negative charges. This tendency of adsorbate species and adsorbent surface will continue to increase with increase of pH causing a gradual increase in repulsive forces between the surface and adsorbate species resulting in a decrease of adsorption (Rajan et al., 2009). The results obtaned are nearly similar to those described by Kundu and Gupta. (2006). Keeping in view practical operating conditions and drinking water standard for pH, pH 7.0 appears to be optimal and has been used in the entire study.

3.4.

Effect of initial concentration

Fig. 4 shows the effect of initial arsenic concentration on adsorption of As (III) by MBOP. It is observed that the percent removal of As (III) decreases while the equilibrium As (III) adsorption capacity increases with the increase in initial As (III) concentration. This decrease of percent arsenic (III) removal may be attributed to the fact that at higher As (III) concentration; the number of active sites on adsorbent surface is not enough to accommodate arsenic (III) ions. However, at low As (III) concentration, the ratio of surface active sites to total As (III) is high and therefore As (III) ions can interact with the active sites on adsorbent surface sufficiently.

3.5.

Effect of background ions

In ground water several ionic species are present and these ions may interfere in the uptake of arsenic by the adsorbent

Fig. 4 e Effect of initial As (III) concentration on As(III) removal by MBOP.

through the competitive binding or complexation (Bhaskar et al., 2006). Batch equilibrium experiments were conducted to find the individual effect of cations (Ca2þ, Mg2þ and Fe3þ) 3  and anions (SO2 4 , PO4 , Cl and NO3 ) on adsorption of As (III). The salts used in this study includes MgSO4$7H2O, Ca(NO3)2.4H2O, FeSO4$7H2O, Na2SO4, NaCl and Na2HPO4. The percentage removal of As (III) was compared with samples having no background ions. As it is evident from Fig. 5a, with increase in concentration of Ca2þ, Mg2þ from 50 mg/L to 600 mg/L, adsorption of As (III) was reduced significantly from 85.37 to 57.72% and 84.81 to 68.49% respectively and for similar concentration of Fe3þ the adsorption of arsenic (III) increased from 37% to 95%. However iron concentration in ground water is generally very low; hence effect on arsenic adsorption at low concentration was also studied. Increase in Fe3þ concentration from 2 mg/L to 25 mg/L resulted in increase of As (III) removal from 13.64% to 34.7% (Fig. 5b). Fig. 5c shows the effect of presence of anions. It was 2 ions has no observed that the presence of NO 3 and SO4 significant effect on As (III) adsorption in concentration range of 100e800 mg/L. In presence of Cl- ions, decrease in As (III) adsorption was observed compared to As (III) adsorption in blank (without cations and anions). It was observed that at Clconcentration of 100 mg/L, there is drastic decrease in As (III) adsorption than As (III) adsorption at Cl- concentration >100 mg/L. In presence of PO-4 ions, decrease in As (III) adsorption was observed. At lower concentration of 100 mg/L, it was observed that PO-4 does not interfere much in As (III) adsorption. However at PO-4 concentration > 100 mg/L, nega tive effect of PO 4 ions was observed up to 600 mg/L PO4 .  Further increase in PO4 concentration upto 800 mg/L has no significant effect on As (III) adsorption.

3.6.

Adsorption isotherms

In order to study the dominant adsorption mechanism and to compute various adsorption parameters three isotherm

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Fig. 5 e a: Effect of presence of Ca, Mg and Fe on arsenic removal. b: Effect arsenic removal in presence of Fe at low concentration. c: Effect of background anions on arsenic removal by MBOP.

models namely Langmuir, Freundlich and D-R isotherms were used. The Langmuir adsorption model can be represented in linear form as follows: 1 1 1 1  þ ¼ qe qmax b Ce qmax

(3)

Where qmax is the maximum amount of the arsenic ion per unit weight of adsorbent to form a complete monolayer on the surface bound at high Ce, while b is a constant related to the affinity of the binding sites. qe represent a particle limiting adsorption capacity when the surface is fully covered with solute. Langmuir parameters, qmax and b were calculated from the slope and intercept of the linear plots of 1/qe vs 1/Ce (Fig. 6a). The Freundlich model indicates the heterogeneity of the adsorbent surface and considers multilayer adsorption. The linear form of Freundlich adsorption model is as follows (Pillewan et al., 2011):


log qe ¼ log KF þ 1=nlog ðCe Þ

(4)

Where KF and 1/n are Freundlich constants, related to adsorption capacity and adsorption intensity (heterogeneity factor) respectively. The values of KF and 1/n were obtained from the slope and intercept of the linear Freundlich plot of log qe vs log Ce (Fig. 6b). In order to predict the adsorption efficiency of the process, the dimensionless quantity (r) was calculated by using the following equation. r¼

1 1 þ bC0

(5)

Where C0 and b are the initial concentration of arsenic and Langmuir isotherm constant. If the value of r < 1, it represents favourable adsorption while greater than 1.0 represents unfavourable adsorption.

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towards iron and also it is reported that metal oxide incorporation or coating increase the zeta potential to more positive values resulting in enhanced anion sorption (Maliyekkala et al., 2009). The mean free energy (E ) was of adsorption was calculated from the kads value using the following equation: 0:5

E ¼ ð2kÞ

(8)

As shown in Table 1, the E value is 5.0 kJ/mol for As (III) on the MBOP medium. The numerical value of mean free energy is in the range of 1e8 and 9e16 kJ/mol for physical and chemical adsorption respectively (Saeed, 2003). In the present study the E value is less than 8 kJ/mol which is within the energy range of physical adsorption which implies that the type of adsorption is physical.

3.7.

Adsorption kinetics

Kinetic models are used to examine the rate of the adsorption process and potential rate-controlling step. The capability of the pseudo-first-order and pseudo-second-order kinetic model was examined in this study. The pseudo-first-order equation of Lagergren is generally expressed as follows (Pillewan et al., 2011):
Kad t log qe  q ¼ log qe  2:303

Fig. 6 e Adsorption isotherm a) Langmuir fit, b) Freundlich fit for arsenic adsorption by MBOP.

Where qe and q (both in mg/g) are the amount of arsenic adsorbed per unit mass of adsorbent at equilibrium and time “t” respectively. The adsorption rate constant (Kad) for arsenic sorption was calculated from the slope of the linear plot log (qeq) vs time (t) as shown in Fig. 7a. The pseudo-second-order model is also commonly used to predict the kinetic parameters linear for of which can be written as (Pillewan et al., 2011)

The DeR isotherm model considers more common features of adsorption and is not based on homogenous monolayer adsorption like Langmuir. The linear form of DeR isotherm model is as follows (Pillewan et al., 2011):

and

ln qe ¼ ln Qm  kads 32

h ¼ kq2e

(6)

(9)

t 1 t ¼ þ qt h qe

(10)

(11)

and   1 3 ¼ RTln 1 þ Ce

(7)

Where Qm is the theoretical adsorption capacity (mg/g), kads is a constant related to adsorption energy, Ɛ is polyani potential, R is gas constant (kJ/mol. K), T is temperature (K ). Results of experimental data were fitted to these three isotherms models to determine which model most accurately described adsorption by the adsorbent. The results of various adsorption parameters obtained from these isotherm model are also presented in Table 1. On comparison of the fitness of the three isotherms it is evident that for As (III) the experimental data were well fitted to Langmuir model followed by Freundlich and D-R models signifying the monolayer adsorption of arsenic on uniform surface. The values of adsorption capacity for MBOP obtained from the Langmuir model was 16.94 mg/g. The significantly high adsorption capacity is probably due to increased affinity of arsenic

Table 1 e Adsorption isotherm parameters for As(III) adsorption by MBOP. Isotherm parameters

293 K

303 K

313 K

Langmuir isotherm qmax (mg/g) b (L/mg) R2

23.256 7.167 0.997

16.949 5.9 0.996

14.925 3.941 0.991

Freundlich Isotherm Kf (mg/g) 1/n R2

5.57 0.787 0.993

5.007 0.835 0.988

4.216 0.846 0.983

DeR Isotherm Kads Qm (mg/g) R2 E (KJ/mol)

-2E-8 24.16 0.948 5.0

-2E-8 19.7 0.931 5.0

-2E-8 15.64 0.928 5.0

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of pseudo-first-order (Eq. (5)) and pseudo-second-order (Eq. (7)) models are presented in Fig. 7b. The values of kad, k and h and correlation coefficients obtained from the linear plots are also presented in Table 2. It is apparent from the values of correlation coefficients that the pseudo-first-order kinetic model fitted well as compared to pseudo-second-order model. Sorption of a liquid adsorbate on porous solid adsorbent can be modelled by diffusion models, which can be particle diffusion and intra-particle pore diffusion model. The particle diffusion model can be written as (Pillewan et al., 2011): 

Ct Ce

ln

 ¼ kp t

(12)

Where kp is the particle diffusion coefficient (min1). The value of kp can be obtained by slope of the plot between ln (Ct/Ce) and t (Fig. 8a). The intra-particle pore diffusion model given by Weber and Morris is also commonly used to characterize the sorption data. In order to test the contribution of intra-particle pore diffusion on the adsorption process, the rate constant for intra-particle pore diffusion was obtained by using following equation. According to this model, if the rate limiting step is diffusion of adsorbate within the pores of adsorbent particle (intra-particle diffusion) a graph between amount of adsorbate adsorbed and square root of time should give a straight line passing through the origin. The equation can be written as (Pillewan et al., 2011): qt ¼ ki t1=2

Fig. 7 e Kinetic a) Pseudo-first order model, b) Pseudosecond order model for arsenic removal by MBOP.

Where qt is the amount of arsenic adsorbed at time t (mg/g), qe is the amount of arsenic adsorbed at equilibrium (mg/g), h is the initial sorption rate (mg/g min). The values of qe (1/slope), k (slope2/intercept) and h (1/intercept) can be calculated from the plots of t/qt versus t and given in Table 2. The linear plots

(13)

Where ki (mg/g min1/2) is the intra-particle pore diffusion rate constant, qt amount of arsenic adsorbed per unit mass of adsorbent at any time t, was plotted as a function of square root of time t1/2 (Fig. 8b). The plots of linear forms of particle diffusion and intra-particle pore diffusion models are given in Fig. 8 and b respectively for As(III) and the values of different parameters are given in Table 2. The values of R2 for intraparticle pore diffusion model are closer to unity indicating that intra-particle pore diffusion of adsorbate is contributing more towards rate determining step. However, in case of intra-particle diffusion model the lines are not passing through the origin, which reveals that the adsorption of arsenic on MBOP is a complex process involving surface

Table 2 e Various kinetic and diffusion parameters for As(III) adsorption by MBOP. Lagergren parameters T (K) 293 303 313

Pseudo-second-order parameters 1

1

2

Kad (min )

R

0.002303 0.002303 0.002303

0.99 0.99 0.99

k (g mg

293 303 313

min )

h (mg g1 min1)

R2

0.00591 0.005817 0.005599

0.99 0.98 0.97

0.004916 0.004243 0.003583

Particle diffusion model T (K)

1

Intra-particle pore diffusion model

Kp (min1)

R2

Ki (mg g1 min1/2)

R2

0.009 0.03 0.122

0.86 0.89 0.88

0.033 0.035 0.037

0.967 0.982 0.972

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3.9.

Thermodynamic parameters

Thermodynamic parameters of adsorption namely standard free energy change (ΔG ), standard enthalpy change (ΔH ), and standard entropy change (ΔS ) were calculated using standard methods. Standard free energy change (ΔG ) is given by the equation (Sekar et al., 2004): DG ¼ RT ln ðK0 Þ

(15)



Where ΔG standard free energy change of sorption (KJ/mol), T the temperature in Kelvin and R is universal gas constant (8.314 J/mol K) and K0 is the thermodynamic equilibrium constant equal to qmax  b of Langmuir isotherm (Sekar et al., 2004). The standard enthalpy change (ΔH ), and standard entropy change (ΔS ) was calculated using following equation (Sekar et al., 2004): ln ðK0 Þ ¼

DS DH  R RT

(16)

Where (ΔH ) is standard enthalpy change (KJ/mol) and (ΔS ) is standard entropy change (KJ/mol K). The values of ΔH and ΔS were obtained from the slope and intercept of the Vant Hoff’s plot of ln(K0) against 1/T (Fig. 10). These values are observed to be 39.674 kJ/mol and 0.0928 kJ/mol K respectively. The negative values of ΔH indicated the exothermic nature of the sorption process. Negative value of entropy change (ΔS ), indicate a greater order of reaction during the adsorption of As (III). It may be due to the fixation of As (III) to the exchanger sites resulting in a decrease in the degree of freedom of the system. The negative values of ΔG 12.46.798, 11.601, 10.603 kJ/mol at all temperatures studied indicated feasibility and spontaneity of the sorption reaction.

3.10. Fig. 8 e a) Particle diffusion model, b) WebereMorris plot for arsenic adsorption by MBOP.

Adsorption of As(V) and field trial

Most of the ground water or other water supplies contain both As species III and V. Hence it is necessary to study the removal

adsorption, inter-particle diffusion and intra-particle diffusion all contributing towards the rate of sorption.

3.8.

Mass transfer coefficient

Mass transfer analysis for the removal of arsenic (III) was carried out using the following equation:  ln

     Ct 1 MK 1 þ MK  ¼ ln  b$Ss$t C0 1 þ MK 1 þ MK MK

(14)

Where K is the constant obtained by multiplying Qmax and b (L/g), M is the mass of the adsorbent per unit volume of particle free adsorbate solution (g/L). Ss is the outer surface of adsorbent per unit volume of particle free slurry (L/cm) and b is the mass transfer coefficient (cm/min). ln((Ct/C0)1/ (1 þ MK)) versus t for the temperature of 293, 303 and 313 K gives the straight line of slope ((1 þ MK )/MK )bSs and the value of mass transfer coefficient b was calculated from the slope of the plots and was found to be as 1.871  105, 2.807  105and 3.743  105 cm/s, respectively (Fig. 9).

Fig. 9 e Estimation of mass transfer coefficient of MBOP for arsenic (III) removal.

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Table 3 e Physio-chemical parameters of field water before and after treatment with MBOP. Parameter

Fig. 10 e Vant Hoff’s plot for removal of arsenic by MBOP.

of As(V) by MBOP. Adsorption capacities were calculated using Langmuir isotherm model (Fig. 11). MBOP has adsorption capacity of 11.31 mg/g for As(V). Adsorption capacity of MBOP was calculated for arsenic III and V when these species exist together in equimolar ratio. For this, deionized water was spiked with arsenic III and V (w1 mg/L) and equilibrium concentration of arsenic III and V was monitored at different dose of MBOP. Adsorption capacity of 11.4 and 11.14 mg/g was obtained for arsenic III and V respectively which indicates that MBOP has equal affinity for arsenic III and V. Considering the practical application, MBOP was also tested for arsenic removal in ground water contaminated with arsenic III and V. The physico-chemical parameters of ground water before and after adsorption are given in Table 3. At an adsorbent dose of 1 g/L, MBOP has reduced the arsenic III and V concentration in ground water from 983.71 to 998.91 mg/L to 7.44 and 9.89 mg/L (992.6 mg/L to 9.81 mg/L), which is below WHO permissible limit for arsenic. Also there was no major change in the other water quality parameter of water after removal of arsenic by MBOP. The residual concentration of iron and aluminium ion in treated water was 0.206 mg/L and 0.0018 mg/L respectively, which is lower than the standard set by WHO for drinking water indicating that the MBOP can be

Fig. 11 e Isothem plot for adsorption of As(V), As (III) in (III D V) system, As (V) in (III D V).

pH Colour Odour Turbidity Alkalinity Total hardness Conductivity Chloride Sulphate TDS Ca2þ Mg2þ Fe2þ Aluminium As (V) Arsenic (III)

Unit

Before After treatment treatment

6.58 Hazans 0.4 Odourless NTU 3.9 mg/L 120 CaCO3 170 ms/cm 110 mg/L 33.74 mg/L 146 mg/L 350.58 mg/L 29 mg/L 19.2 mg/L 0.215 mg/L BDL ppb 998.91 ppb 983.71

6.67 0.2 Odourless 4.1 110 160 98 28.93 138 340.56 31 9.6 0.206 0.018 9.89 7.44

Permissible limit 6.5e8.5 5 Unobjectionable 5 200 300 250 200 500 75 30 0.3 0.03 10 10

used for treatment of arsenic III and V contaminated drinking water.

4.

Mechanism of arsenic adsorption

From XRD it is clear that the product is amorphous. It has been reported that the adsorption of arsenic on amorphous metal oxides is through formation of inner sphere surface complexes which are mainly attached as bidentate linkages with some monodentat linkages (Maliyekkala et al., 2009). It has also been reported that adsorption following formation of inner sphere complex are not much influenced by pH and ionic strength (Goldberg and Johnston, 2001). Removal of arsenic by MBOP remains almost constant at different pH (Fig. 3) which also indicates that the adsorption of arsenic on MBOP is through formation of inner sphere complex through formation of AseO linkages.

Fig. 12 e Desorption of MBOP using NaOH.

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Table 4 e Comparison between various adsorbent used for arsenic removal on the basis of adsorption capacity. Adsorbent Iron-impregnated chitosan granular Modified Native Cellulose Fibre Molybdate-impregnated chitosan beads (MICB) Iron oxide coated sponge Copper Oxide Incorporated Mesoporous Alumina Activated alumina Iron oxide coated sand Activated alumina Iron oxide impregnated activated alumina CuO Magnetic binary oxide particle (MBOP)

5.

pH

Initial concentration of As (III) (mg/L)

Ref.

8.0 8.0 5.0

1.007 10 10

6.48 8.96 1.98

Gang et al., 2010 Tian et al., 2011 Chen et al., 2008

7.3 7.0

1.0 1.0

3.85 2.61

Nguyen et al., 2010 Pillewan et al., 2011

7.0 7.5 7.6 12.0

e 0.4 1.0 1.4

3.5 0.029 0.18 0.734

Lin and Wu, 2001 Gupta et al., 2005 Singh and Pant, 2004 Shugi et al., 2004

8 7.0

0.1e100 1.0

Desorption study

Considering the practical applicability in field it is desirable that an adsorbent should be fully regenerated and reused so that it can be put into cyclic use in a cost effective manner. Regeneration of arsenic saturated adsorbents has been achieved using either alkali or strong acids including desorption of arsenic from MBOP. To study the regeneration, MBOP was first saturated with arsenic by shaking the adsorbent with initial arsenic concentration of 1 mg/L and adsorbent dose of 1 g/L for 24 h. This adsorption cycle was repeated till the adsorbent get saturated. Regeneration studies were conducted by shaking the required quantity of arsenic saturated MBOP with different concentrations of NaOH for 1 h. The results of regeneration studies are presented in Fig. 12. As evident from the results the amount of As (III) leached decreases at NaOH concentration of 5% and 10%. With NaOH concentration of 1% and 2% almost all the As (III) desorbed (more than 95% regeneration) in 1 h resulting in complete regeneration of MBOP and MBOP retained the original adsorption capacity after one complete adsorption desorption cycle, confirming the reusability of MBOP for arsenic removal. The results of regeneration studies suggest that the MBOP can be used in a continuous flow for removal of arsenic.

6.

Adsorption capacity qmax (mg/g)

Comparison with other adsorbents

A comparison has been made between MBOP and previously reported adsorbents for arsenic removal (Table 4). For comparison, Langmuir adsorption capacity was considered. An analytical comparison shows that MBOP is better than many other adsorbents except CuO in terms of adsorption capacity with additional feature of magnetic separation. Cost analysis has been done for the production of MBOP. The cost of MBOP works out to be Rs. 320 per Kg which includes cost of raw materials and process cost. The water treatment cost was calculated on the basis of dose of MBOP required to treat 100 L

26.9 16.94

Martinson and Reddy, 2009 Present work

of water with arsenic concentration 0.001e2.5 mg/L which works out to be Rs. 32. The cost MBOP and water treatment cost appears to be very economic.

7.

Conclusion

Magnetic adsorption process provides a cost effective and environmentally benign water treatment process. Consequently, a novel binary oxide with magnetic property was prepared, characterized and applied for in the removal arsenic as a model contaminant in water. The material was effective in arsenic removal in water. The arsenic uptake was rapid initially up to 3 h and then gradually slows down as it reaches to equilibrium. Arsenic uptake depends on the initial concentration, adsorbent dose and temperature and independent upon the pH. The adsorption kinetics follows pseudo-second order kinetic model. The adsorption process was exothermic in nature. The equilibrium data fitted well to the Langmuir and Freundlich model. Adsorption of As (III) was not affected remarkably by presence of other anions like chloride, nitrate, sulphate etc. Up to 95% of the adsorbed arsenic on MBOP was desorbed using 1% and 2% NaOH solution. The adsorbent retained the original adsorption capacity after one complete adsorption desorption cycle, confirming the reusability of MBOP for arsenic removal. Further tests are still required to determine the robustness of the material and fix bed column studies for practical application of material.

Acknowledgement We thank Director, NEERI for providing research facilities. We thankfully acknowledge to the Council of Scientific and Industrial Research (CSIR) as one of the author Mrs. Sneha Lunge is CSIR Senior Research Fellow (SRF). We thankfully acknowledge to Sophisticated Analytical Instrument facilities, IIT Chennai for VSM test.

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 4 7 6 9 e4 7 8 1

references

Banerjee, K., Amy, G.L., Prevost, M., Nour, S., Jekel, M., Gallagher, P. M., Blumenschein, C.D., 2008. Kinetic and thermodynamic aspects of adsorption of arsenic onto granular ferric hydroxide (GFH). Water Research 42, 3371e3378. Bhaskar, P.B., Gupta, A.K., Ayoob, S., Kandu, S., 2006. Investigation of arsenic (V) removal by modified calcined bauxite. Colloids and Surfaces, 237e245. Chakraborti, A.K., Saha, K.C., 1987. Arsenical dermatosis from tube well water in West Bengal, India. Journal of Medical Research 85 (3), 326e334. Chen, C.-Y., Chang, T.-h., Kuo, K.-T., Chen, Y.-F., Chung, Y.-C., 2008. Characteristics of molybdate-impregnated chitosan beads (MICB) in terms of arsenic removal from water and the application of a MICB-packed column to remove arsenic from wastewater. Bioresource Technology 99, 7487e7494. Escudero, C., Fiol, N., Villaescusa, I., Bollinger, J.C., 2009. Arsenic removal by waste metal (hydr)oxide entrapped into calcium alginate beads. Journal of Hazardous Materials 164, 533e541. Gang, D., Deng, B., Lin, L., 2010. As (III) removal using an ironimpregnated chitosan sorbent. Journal of Hazardous Materials 182, 156e161. Goldberg, S., Johnston, C.T., 2001. Mechanisms of arsenic adsorption on amorphous oxides evaluated using macroscopic measurements, vibrational spectroscopy, and surface complexation modeling. Journal of Colloid and Interface Science 234 (1), 204e216. Guo, H.M., Stuben, D., Berner, Z., 2007. Adsorption of arsenic (III) and arsenic(V) from groundwater using natural siderite as the adsorbent. Journal of Colloid Interface Science 315, 47e53. Gupta, V.K., Saini, V.K., Jain, N., 2005. Adsorption of As (III) from aqueous solutions by iron oxide-coated sand. Journal of Colloid and Interface Science 288, 55e60. Gupta, A. Datta, Bandopadhyay, A., Pratip, 1997. Technologies and Options for Arsenic Removal IWWA, Annual Convention, Calcutta 1-9. Jessen, S., Larsen, F., Koch, C.B., Avin, E., 2005. Sorption and desorption of arsenic to ferrihydrite in a sand filter. Environmental Science and Technology 39, 8045e8051. Kundu, S., Gupta, A.K., 2006. Adsorptive removal of As (III) from aqueous solution using iron oxide coated cement (IOCC): evaluation of kinetics, equilibrium and thermodynamic models. Separation and Purification Technology 51 (2), 165e172. Lakshmipathiraj, P., Narasimhan, B.R.V., Prabhakar, S., Bhaskar, R.G., 2006. Adsorption studies of arsenic on Mnsubstituted iron oxyhydroxide. Journal of Colloid Interface Science 304, 317e322. Lin, T.F., Wu, J.K., 2001. Adsorption of arsenite and arsenate within activated alumina grains: equilibrium and kinetics. Water Research 35 (8), 2049e2057. Ma, Z.Y., Guan, Y.P., Liu, H.Z., 2005. Synthesis and characterization of micron-sizedmonodisperse supermagnetic polymer particles with amino groups. Journal of Polymer Science and Polymer Chemistry 43, 3433e3439.

4781

Maliyekkala, S.M., Philip, L., Pradeep, T., 2009. As(III) removal from drinking water using manganese oxide-coated-alumina: performance evaluation and mechanistic details of surface binding. Chemical Engineering Journal 153, 101e107. Martinson, C.A., Reddy, K., 2009. Adsorption of arsenic (III) and arsenic (V) by cupric oxide nanoparticles. Journal of Colloid and Interface Science 336, 406e411. Nguyen, T.V., Vigneswaran, S., Ngo, H., Kandasamy, J., 2010. Arsenic removal by iron oxide coated sponge: experimental performance and mathematical models. Journal of Hazardous Materials 182, 723e729. Nikolaidis, N.P., Dobbs, G.M., Lackovic, J.A., 2003. Arsenic removal by zerovalent iron: field, laboratory and modeling studies. Water Research 37, 1417e1425. Pillewan, P., Mukherjee, S., Roychowdhury, T., Das, S., Bansiwal, A., Rayalu, S., 2011. Removal of As(III) and As(V) from water by copper oxide incorporated mesoporous alumina. Journal of Hazardous Material 186, 367e375. Pontius, F.W., Brown, K.G., Chen, C.J., 1994. Health implication of arsenic in drinking water. Journal of American Water Work Association 86, 52e63. Rajan, D., Talat, M., Hasan, S.H., 2009. Biosorption of arsenic from aqueous solution using agriculture residue rice polish. Journal of Hazardous Materials 166, 1050e1059. Saeed, M.M., 2003. Adsorption profile nd thermodynamic paramerters of the preconcnentration of Eu (III) on 2thenoyltrifluoroacetone loaded plyurethane (PUR) foam. Journal of Radioanalysis and Nuclear Chemistry 256, 73. Sekar, M., Sakthi, V., Rengaraj, S., 2004. Kinetics and equilibrium adsorption study of lead(II) onto activated carbon prepared from coconut shell. Journal of Colloid Interface Science 279, 307e313. Shugi, K., Tony, S., Pant, K.K., 2004. Adsorption of As(III) from aqueous solution onto iron oxide impregnated activated alumina. Water Quality Research Journal of Canada 39 (3), 258e266. Singh, T.S., Pant, K.K., 2004. Equilibrium kinetics and thermodynamics studies for adsorption of As (III) on activated alumina. Separation and Purification Technology 36, 139e147. Smedley, P.L., Kinniburgh, D.G., 2002. A review of the source, behaviour and distribution of arsenic in natural waters. Applied Geochemistry 17, 517e568. Smith, A.H., Hopenhayn-Rich, C., Bates, M.N., Goeden, H.M., Piccoitto, I.H., Duggan, H.M., 1992. Cancer risk from arsenic in drinking water. Environmental Health Perspective 97, 259e267. Sun, X., Doner, H.E., 1998. Adsorption and oxidation of arsenite on goethite. Soil Science 163, 278e287. Tian, Ye., Wu, M., Liu, R., Wang, D., Lin, X., Liu, W., Ma, L., Li, Y., Huang, Y., 2011. Modified native cellulose fibersdA novel efficient adsorbent for both fluoride and arsenic. Journal of Hazardous Materials 185, 93e100. Who, 2004. Guideline for drinking water quality. In: Recommendation, third ed.. World Health Organization, Geneva. Zhang, Y.M., Yang, M., Huang, X., 2003. Arsenic (V) removal with a Ce (IV)-doped iron oxide adsorbent. Chemosphere 51, 945e952.