Adsorption of As(III) from Aqueous Solution onto Iron Oxide Impregnated Activated Alumina

Water Qual. Res. J. Canada, 2004

•

Volume 39, No. 3, 258–266

Copyright © 2004, CAWQ

Adsorption of As(III) from Aqueous Solution onto Iron Oxide Impregnated Activated Alumina Shugi Kuriakose, Tony Sarvinder Singh and Kamal K. Pant* Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi, 110 016, India

The optimal parameters affecting the adsorption of arsenic ions As(III) on iron oxide impregnated activated alumina (IOIAA) were determined by conducting batch and column experiments. The adsorption of As(III) was strongly dependent on pH, temperature and initial adsorbate concentration. The adsorption process satisfied the Langmuir and Freundlich isotherms. Equilibrium studies were conducted to obtain the thermodynamic parameters and data showed the endothermic nature of adsorption. Kinetics studies showed that a pseudo first-order rate equation successfully described the adsorption process. Equilibrium was attained within 10 h and the time taken to attain equilibrium was independent of initial arsenite concentration. Column studies showed that adsorption was strongly dependent on empty bed contact time. Column design parameters such as the time taken for the establishment of primary adsorption zone, fractional capacity, length of primary adsorption zone and the percentage saturation at breakpoint were calculated to be in the range of 18.3 to 70.4 h, 0.39 to 0.63, 3.0 to 3.85 cm and 69.6 to 81.5%, respectively. The observations mentioned above provide a direct relationship between the length of the adsorption zone (δ) and percent saturation at break point. Key words: activated alumina, iron oxide, adsorption, arsenite, breakthrough curve

Introduction

namely, aluminum compounds, lanthanum compounds and iron compounds (Tokunaga et al. 1997). Several studies are available in the literature on goethite, hematite, manganese greensand, iron oxide coated sand (Subramanian et al. 1997; Fendorf et al. 1997; Grossl et al. 1997; Singh et al. 1996; Fe loaded coral limestone, granular ferric hydroxide, pyrite (Ohki et al. 1996; Driehaus et al. 1998; Zouboulis et al. 1992; ferrihydrite (Manceau 1995), activated carbon (2002), hydrotalcite (Manju et al. 2000), and ferruginous manganese ore (Chakravorty et al. 2002). However the arsenic species present in water limits the performance of most of these adsorbents, as most of the available adsorbents are more suited for As(V) removal than As(III). Efforts are also being made to develop a suitable adsorbent for the removal of As(III) from groundwater. Adsorption onto activated alumina has been one of the promising methods for As(V) removal from drinking water but As(III), which is a more mobile and toxic form of arsenic is sometimes difficult to remove by simple activated alumina. Iron(III) oxides have been effectively used for removal of both As(V) and As(III) from aqueous solutions. The occurrence of ferric oxides in the form of fine powder limits the use of ferric oxide in column operations. Adsorption of As(III) by iron oxide coated sand, iron oxide coated granulated activated carbon (Petrusevski et al. 2002), and MnO2 coated sand (Bajpai and Cahudhari 1999) has been reported in the literature. Vaishya and Aggarwal (1993) showed that sand from the Ganga River (India), which presumably is rich in iron coatings, could remove arsenite from solution, with a reported capacity

Contamination of drinking water by arsenic has become one of the key environmental health problems of the twenty-first century with new cases of arsenic contamination reported every year. Because of the rapidly accumulating toxicity data, WHO has revised the maximum contaminant limit of arsenic from 50 to 10 µg/L and many countries are following this. Arsenic occurs in groundwater as As(V) (H3AsO4, H3AsO41-, HAsO42-) or As(III) (H 3AsO 3, H 2AsO 31-, HAsO 32-). Of these two forms, As(III) is more toxic, difficult to remove and is found in reducing conditions. Most of the developed technologies are effective only when As(V) is the predominant species (Singh et al. 2001). Removal of As(III) has been carried out by converting it to As(V) by oxidation (Driehaus et al. 1995). Oxidizing agents being used for this purpose include manganese oxide (Driehaus et al. 1995), ozone (Kim and Nriagu 2000), and iron (Daus et al. 2000); however residual concentration of these oxidizing agents in water may have toxic effects on human health, thereby limiting their application. Traditionally co-precipitation using lime, ferric salts and alum has been used for arsenic removal but the sludge generation/disposal is a problem. Thus, there is an urgent need for the development of a simple economical and efficient arsenic removal technology. Most of the adsorbents used for the removal of arsenic can be classified into three major groups, * Corresponding author; [email protected] 258

Adsorption of As(III) from Aqueous Solution onto IOIAA

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of 0.024 mg/g. Joshi and Chaudhuri (1996) reported that iron oxide coated sand (IOCS) was able to remove both arsenite and arsenate. A simple fixed bed unit of IOCS was able to treat about 160 to 190 bed volumes of water containing 1000 µg/L arsenite and 150 to 165 bed volumes of water with 1000 µg/L arsenate. Difficulty in regeneration, non-porous nature, low surface area and high-pressure loss during column runs are some of the disadvantages of using these materials for the removal of arsenic. Generally, the published research work is focussed on the adsorption of As(III) in batch mode. The results of such research work are not strictly applicable for continuous operation. Moreover the adsorption capacity of the sorbents was also low. In the present study, iron oxide impregnated activated alumina was used for the adsorption of As(III) from water in batch and column operations. The objective of the research was to investigate the arsenic(III) removal efficiency of the iron oxide impregnated activated alumina. The effect of parameters such as pH, iron oxide impregnation, adsorbent dose, contact time, and initial solute concentration (C0) on As(III) adsorption were studied to determine the optimum conditions. Breakthrough studies were carried out to evaluate the performance of the adsorbent in continuous fixed bed operation through changing empty bed contact time by varying flow rate and bed height.

centrating the residual solution to get ferric oxide to precipitate (Thirunavukkarasu et al. 2001). The final iron oxide content on the AA pellets was 10% (by weight). These pellets were characterized for their surface area, pore volume and other properties. The surface area and pore volume measurements were carried out using BET surface area and pore volume analyzer (Micromeritics ASAP 2010 USA). The physico-chemical properties of AA before and after impregnation with iron oxide are shown in Table 1. It was found that there was a significant reduction in surface area and pore size as a result of iron oxide diffusion into the pores of the AA (Table 1).

Experimental

A comparative study for the effectiveness of iron oxide impregnated AA and AA (without impregnation) for the As(III) removal was conducted. Batch experiments were carried out in a temperature controlled mechanical shaker (Neolab instruments, Mumbai, India). Solution pH is an important factor for all water and wastewater treatment processes as it affects the speciation of the metals. Studies were conducted at different initial pH levels (3.0–12.0) to determine the effect on As(III) adsorption, 50 gL-1 of iron oxide impregnated AA was added to different flasks each containing As(III) solution. These samples were agitated in a mechanical shaker at 85 rpm and 298 K until equilibrium was reached. Samples were filtered through Whatman no.1 filter paper and the residual As(III) concentration was determined.

Materials All reagents were prepared using double distilled water. Stock solution of arsenite was prepared using analytical grade sodium arsenite (NaAsO 2, Merck, Germany). Working arsenic solutions of different concentrations were prepared by diluting stock arsenic solutions.

Preparation of Adsorbent Activated alumina (CATAL-AD-101) used in this study was procured from IPCL, Baroda, India. This material has been found effective for the removal of As(V) (Singh and Pant 2004). Ferric sulphate [Fe2(SO4)3.H20, AR Grade, Merck, Germany] was used for impregnation. One hundred grams of activated alumina (AA) pellets were kept in 0.12 M Fe2(SO4)3.H20 solution and placed in an oven at 378 K for 24 h, followed by heat treatment at 878 K for 2 h to set iron oxide coated activated alumina. The ferric sulphate solution was strongly acidic (pH 2.5) due to the hydrolysis of Fe3+ ions as shown below. Fe+3 + nH2O

Fe(OH)3-nn + nH+

(1)

During the heat treatment process water evaporated and sulphate was converted to sulphur trioxide, which escaped from the solution, neutralizing and con-

Arsenic Analysis Arsenic analysis was carried out using a Varian Spectra AA 880 graphite furnace atomic absorption spectrometer (GF-AAS Varian, Australia), with Zeeman background correction. All measurements were based on integrated absorbance and performed at 193.7 nm by using a hollow cathode lamp (Varian, Australia). Pretreatment temperature of the furnace was 1400 K and atomization temperature was 2500 K. The calibration range was 20 to 100 µg L-1 of arsenic.

Batch Study

TABLE 1. Physical properties of adsorbents Properties

Activated alumina

Iron oxide impregnated activated alumina (IOIAA)

Particle form Colour Particle size (mm) Surface area (m2/g) Pore volume (cm3/g) Bulk density (g/cm3)

Spheres White 2.0 ± 0.2 365 ± 5 0.42 0.800

Spheres Reddish brown 2.0 ± 0.2 200 ± 10 0.27 0.850

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Equilibrium Study Different weights of adsorbent (0.1–5.0 g) were added to 500-mL conical flasks, each filled with 100 mL of As(III) solution (C0 1.4 mg/L) and allowed to equilibrate for 24 h to reach equilibrium at the desired temperature (298, 308 and 315 K). Residual arsenic concentration was determined in each sample. Kinetic studies were conducted with different initial As(III) concentrationsranging from 1.6 to 2.3 mg/L. Iron oxide impregnated AA pellets (5.0 g) were suspended in 100 mL of As(III) solution at pH 12.0 in different conical flasks in a shaker at 85 rpm. Samples withdrawn at regular time intervals were filtered and analyzed for residual As(III) concentration.

Column Experiments A Perspex column (45 cm length and 2.5 cm internal diameter) was used as a vertical fixed bed column. An overhead water reservoir with constant head and fitted with a flow control valve was placed over the column through which arsenic solution was allowed to pass. Fixed bed studies were conducted to evaluate column performance at optimum pH 12.0. Contact time was varied from 4 to 18 min by varying inlet flow rate and bed height. Samples were collected at regular intervals at the exit of the column and analyzed.

Results and Discussion Effectiveness of Iron Oxide Impregnation The performance of iron oxide impregnated AA (IOIAA) was compared with AA for the removal of As(III). The As(III) uptake plotted as a function of the equilibrium concentration is shown in Fig. 1. Iron oxide coating results in an increase in the percentage adsorption of solute. Compared to AA, the As(III) uptake increased from 0.158 to 0.286 mg/g with iron oxide impregnation. A higher value of qe for iron oxide impregnated AA compared to AA confirmed that impregnation resulted in an increased sorption of As(III). Higher adsorption was achieved most probably due to enhancement in the rate of reaction. Cation (available in solution) adsorption and columbic interaction play a major role in the adsorption of As(III) on IOIAA. Formation of ferric arsenite could be another mechanism responsible for this. Ferric arsenite formation by the following reaction has also been reported elsewhere (Katsoyiannis and Zouboulis 2002). M-FeOH +H3AsO3

M-Fe-H2AsO3 + H2O

Fig. 1. Effect of iron oxide coating on As(III) adsorption (pH 12.0, adsorbent dose 5 g/100 mL; C0 1.0 mg/L).

Fig. 2. It was observed that the percentage of As(III) removal was least affected (78–83%) in the pH range of 3 to 8. Increasing pH beyond 8.0, there was a significant increase in the As(III) removal and maximum As(III) removal (96.7%) was obtained at a pH of 12.0. The solubility of activated alumina and IOIAA used in the present study at pH 12 were less than 0.5% (by wt.) and 0.2% (by wt.), respectively. Several investigators reported maximum As(III) removal in an alkaline pH range by different sorbents such as red mud (Altundogan et al. 2000) and hydrotalcite (Manju and Anirudhan 2000). Manju et al. (1998) also reported that As(III) adsorption on coconut husk carbon increased from 16 to 86% as pH was increased from 3 to 12. In contrast to the results obtained for iron oxide impregnated AA, a pH value of 7.6 was found to be optimum for AA (Singh and Pant 2004). The difference in optimum pH in the case of IOIAA and AA for maximum As(III) removal could be attributed to the difference in surface characteristics of both adsorbents. The higher removal in the basic medium could be due to the presence of As(III) as anionic species H2AsO3-. Under the condition of pH less than 9.2, non-ionic H3AsO3 is the dominant species and therefore the only force acting between H3AsO3 and the surface is the weaker van der

(2)

Effect of Solution pH on As(III) Adsorption The percentage of As(III) removed at various initial pH levels for arsenic concentration of 1.1 mg/L is shown in

Fig. 2. Effect of pH on As(III) adsorption (C0 1.1 mg/L; adsorbent dose 50 g/L; temperature 298 K).

Adsorption of As(III) from Aqueous Solution onto IOIAA

Waals force. At a higher pH the dominant species is negatively charged leading to a higher adsorption. It should be noted that although optimal removal was achieved at pH 12.0, the treated water pH at the end of the experimental run was 7.3, which is suitable for drinking without any further post treatment.

Adsorption Isotherms Adsorption isotherm data are quantified to describe the interactions between the adsorbate and adsorbent and are critical in optimizing the use of adsorbent. The Langmuir equation is the most popular of all the nonlinear isotherm expressions; it is a two-parameter equation: qe =

bqmCe 1 + bCe

(3)

where constants b and qm relate to the energy of adsorption and adsorption capacity and their values were obtained from the slope and intercept of the plot of 1/qe versus 1/Ce (Fig. 3). The linear nature of the plot shows that the adsorption follows the Langmuir isotherm and the increase in temperature from 298 to 315 K resulted in an increase of about 17% in adsorption capacity (Table 2). The value of b, which is a measure of heat of adsorption, also increased with an increase in temperature. Lin et al. (2001) obtained similar results for arsenite adsorption on AA and found qm to be in the range 0.76 to 2.29 mg/g and b in the range 0.77 to 1.24 L/mg. The Freundlich isotherm equation for representing equilibrium data, is given by: 1

q e = kC e n

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and intensity of adsorption. Similar results have been reported for arsenite adsorption on red mud and coconut husk carbon (Altundogan et al. 2000; Manju et al. 1998). As(III) removal increased with an increase in temperature showing the endothermic nature of the process. Thermodynamic parameters were also evaluated to confirm the endothermic nature of the process. The thermodynamic constants, Gibb’s free energy (∆Gº), enthalpy change (∆Hº) and entropy change (∆Sº) were calculated to evaluate the thermodynamic feasibility of the process and to confirm the nature of the adsorption process. The Gibb’s free energy change of the process is related to equilibrium constant (k0) by equation 6:

∆ G o = − RT ln K o

(5)

The Gibb’s free energy change is related to the enthalpy change (∆Hº) and entropy change (∆Sº) as:

ln K o =

∆ S o ∆H o − R RT

(6)

where Ko is equilibrium constant (m3mol-1) which was calculated from the Langmuir constant b. The values of enthalpy change (∆Hº) and entropy change (∆Sº) were calculated from the slope and intercept of the plot of ln Ko versus 1/T (Fig. 5). The parameters as calculated are also reported in Table 2. The negative ∆Gº value confirmed the feasibility of the sorption process and the spontaneous nature of adsorption. The positive value of ∆Hº obtained indicated the endothermic nature of the process. The positive ∆Sº value indicated the affinity of the adsorbent for As(III).

(4)

Kinetics Study where k and n are Freundlich constants corresponding to adsorption capacity and adsorption intensity, respectively. The slope (1/n) and intercept (k) of a loglog plot of qe versus Ce as determined from Fig. 4 are tabulated in Table 2. An increase in temperature resulted in an increase in k and n indicating increased capacity

Fig. 3. Plot of Langmuir adsorption isotherm (pH 12.0; C0 1.4 mg/L; rpm 85).

The kinetics of As(III) adsorption on iron oxide impregnated AA was studied at different initial concentrations of As(III) (1.6–2.3 mg/L) as a function of time at 298 K and the results are shown in Fig. 6 which is a plot of residual As(III) concentration as a function of contact time. The uptake of As(III) increased with an increasing contact time, however the adsorption rate was rapid in the first four hours. Equilibrium was attained in approximately 10 h after which the amount of residual As(III) in solution remained practically unchanged. For an initial As(III) concentration of 1.6 mg/L the equilibrium uptake of As(III) was 30 mg/kg and increased to 44.0 mg/kg when the initial concentration was increased to 2.3 mg/L. In the present study, the maximum adsorption capacity with iron oxide impregnated AA was 378 mg/kg from a water having initial As(III) concentration of 1.4 mg/L. This is significantly higher than that estimated for iron oxide coated sand (18.3 µg/g) and ferrihydrite (285 µg/g) for arsenic removal (Thirunavukkarasu et al. 2001).

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TABLE 2. Adsorption isotherm and thermodynamic parameters

Temperature (K) 298 308 315

Langmuir isotherm constant b (L/mg) qm (mg/g) 0.554 0.584 0.646

Freundlich isotherm constant R

0.734 0.803 0.858

2

0.993 0.990 0.984

2

k (mg/g)

n

R

0.302 0.344 0.374

1.113 1.114 1.160

0.996 0.992 0.980

The nature of adsorption kinetics was determined using the Lagergren equation given below (Lagergren 1898):

Ko x 10-3 -∆Gº ∆Hº ∆Sº (m3 mol-1) (kJ mol-1) (kJ mol-1) (kJ mol-1 K-1) 41.5 43.8 48.4

26.34 27.36 28.25

6.82

0.111

where k ad1 is the rate constant of the first-order adsorption (h-1) and qe and qt are the amount of As(III) adsorbed per unit mass of adsorbent (mg/g) at equilibrium and at time, t, respectively. It was noted that a semi-log plot of (qe-qt) as a function of time yields a straight line (Fig. 7) showing that the adsorption process followed the first-order kinetics at lower As(III) concentration. The value of the adsorption rate constants, kad,

were obtained from the slope of Fig. 8 with the regression coefficients more than 0.95. The first-order rate constant (kad1) varied from 1.0 to 1.42 h-1 as concentration was increased from 1.6 to 2.3 mg/L. There was a considerable increase in the first-order rate constant at higher As(III) concentration, so a pseudo second-order rate constant was also applied which showed that the adsorption rate constant followed a pseudo second order at a higher concentration (Table 3). Besides adsorption on the outer surface of the adsorbent, there is also a possibility of transport of adsorbate ions from the solution into the pores of the sorbent due to the rapid stirring in the batch reactor. The experimental data could be used to study the rate-limiting step in the adsorption process. Since the particles are vigorously agi-

Fig. 4. Plot of Freundlich adsorption isotherm (pH 12.0; C0 1.4 mg/L; rpm 85).

Fig. 6. Residual As(III) concentration as a function of contact time (pH 12.0; adsorbent dose 50 g/L; temperature 298 K).

Fig. 5. Estimation of thermodynamic parameters (pH 12.0; C0 1.4 mg/L; rpm 85).

Fig. 7. Evaluation of first-order rate constants (pH 12.0; adsorbent dose 50 g/L; temperature 298 K).

log (q e − q t ) = −

k ad 1 t 2 .303

+ log (q e

)

(7)

Adsorption of As(III) from Aqueous Solution onto IOIAA

Fig. 8. Estimation of intra particle diffusion rate constants (pH 12.0; adsorbent dose 50 g L-1; temperature 298 K).

tated during the adsorption, it is reasonable to assume that the mass transfer from the bulk liquid to the particle external surface does not limit the rate. Hence, it can be postulated that the rate-limiting step may be intraparticle diffusion. As these act in series, the slower of the two will be the rate-limiting step. The rate constant for intraparticle diffusion is obtained using the equation (Al-Asheh et al. 2003): q = kpt 0.5

(8)

where kp (mg g-1 h-0.5) is the intraparticle diffusion rate constant. The dual nature of these plots (Fig. 8) may be explained as the initial curved portions are attributed to boundary layer diffusion effects while the final linear portions are due to intraparticle diffusion effects. The intraparticle diffusion rate constant, kp, at different initial concentrations (Table 3) were determined from the slope of the linear portion of the respective plots. The initial As(III) concentration of the solution did not influence the adsorption rate constants significantly (Table 3). The intraparticle diffusion rate constants were found to vary from 0.02 to 0.03 mg g-1 h -0.5 with an increase in initial As(III) concentration from 1.6 to 2.3 mg/L.

Column Studies An idealized breakthrough curve is expressed by plotting mass concentration of solute in the effluent, Ct, and the total mass quantity of solute-free water, Veff, which has passed a unit cross-sectional area of the adsorber. The breakthrough point was chosen at a C b value of 0.01 mg/L. Adsorption of As(III) was studied at the flow rates of 120 and 300 mL/h. The influent As(III) concen-

263

tration was maintained at 0.5 mg/L for these experiments. The breakthrough plots of Ct/C0 against volume of effluent treated at an arsenic concentration of 0.5 mg/L at different flow rates are shown in Fig. 9 and 10. As can be seen from Fig. 9 and 10, an increase in breakthrough time with a decrease in the flow rate was observed. This is probably due to the availability of sorption sites to capture metal ion around or within the IOIAA pellet. With the gradual occupancy of these sites, the uptake of As(III) became less effective. As the flow rate increased, the breakthrough curve becomes steeper. The empty bed contact time (EBCT) of 9.4 and 3.7 min corresponding to flow rates of 2 and 5 mL/min were taken for column runs. With a higher EBCT, metal ions had more time to contact with IOIAA, which resulted in higher removal of As(III) ions in fixed-bed columns. A high decrease in sorbate concentration can be attributed to the fact that residence time for the solute in the column was sufficiently high enough for adsorption equilibrium to be reached at that flow rate. At a higher flow rate, reduction in As(III) removal efficiency has been observed. The effect of residence time was also studied by varying the bed height under identical conditions. Experiments were conducted at a column bed height of 6 and 12 cm corresponding to residence times of 9.4 and 18.8 min, respectively. As(III) ions were passed through the column at a flow rate for 120 mL/h with an initial As(III) concentration of 0.5 mg/L. The results are shown in Fig. 10. At a lower contact time, the curve is steeper showing the faster exhaustion of the fixed bed.

Analysis of Breakthrough Data Breakthrough capacity of the fixed bed was determined by plotting exit As(III) concentration, Ct, against volume of water treated (V). Mass units for C and Ve were used to illustrate the concept of mass balance. Nature of the breakthrough curve between Vb and Vx and the quantities of effluent treated per unit cross section till breakthrough are primary parameters for the designing of a fixed bed adsorber (Gupta et al. 2000). The time required for movement of PAZ onwards in the column (tδ) can be calculated as:

tδ =

V x − Vb Fm

(9)

TABLE 3. Intraparticle diffusion and reaction rate constants adsorption of As(III) on IOIAA Initial As(III) conc. (mg/L)

kad1 (h-1)

R2

Kad2 (mg/g)-1 min-1

R2

kp (mg g-1 h-0.5)

1.6 2.1 2.3

1.4 1.3 1.0

0.934 0.981 0.986

0.62 0.28 0.28

0.98 0.987 0.987

0.022 0.032 0.035

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Fig. 9. Breakthrough curves at different flow rate (bed height 6 cm; C0 0.5 mg/L; pH 12).

Fig. 10. Effect of bed height on shape of breakthrough curve (2 mL min-1, C0 0.5 mg/L).

Fractional capacity, f, of IOIAA in the adsorption zone at breakpoint to continue to remove solute from solution under the limiting conditions is given by:

mation of PAZ varied from 18.3 to 70.4 h. The percent saturation at breakpoint also depended on EBCT and varied from 69.6 to 81.5. These observations provide a direct relationship between length of adsorption zone and percent saturation at breakpoint. The smaller the length of the adsorption zone, the lower the percentage saturation observed will be. These operation parameters may be helpful in designing a fixed bed adsorber for the removal of arsenic ions of a known concentration.

f=

Ms C O (Vx − Vb )

(10)

Thus, for a depth D of the IOIAA bed, the depth and time ratios may be equated as:

tδ δ = D tx − t f

(11)

where tf represented the time required for initial formation of the PAZ. The percent saturation was calculated as:

% saturation = [1 −

δ (1 − f ) ]↔ 100 ×100 D

Conclusions The following conclusions were drawn from the present study. 1.

(12) 2.

The values of Vb, Vx, Cb and Cx were obtained from breakthrough curves and the values are listed in Table 4. These values were used to calculate tx, tδ, tf, δ, f and % saturation (Table 5) at different EBCT at a bed height of 6 cm and an initial As(III) concentration of 0.5 mg/L. As can be seen in Table 5 the time taken for the initial for-

3.

Iron oxide impregnated activated alumina was found to be an effective adsorbent for the removal of As(III). Compared to activated alumina there was significant enhancement in adsorption capacity. Adsorption was strongly dependent on initial pH and initial As(III) concentration. Maximum adsorption (96.7%) was achieved at pH 12. The adsorption process followed first-order kinetics at the lower As(III) concentrations (up to 2.1 mg/L). At higher As(III) concentration, the behaviour was accurately explained by pseudo second-order rate kinetics.

TABLE 4. Observed parameters for fixed bed adsorption of As(III) on IOIAA EBCT (s) 222 564 1128

C0 (mg/L)

Cx (mg/L)

Cb (mg/L)

Vx (g/cm2)

Vb (g/cm2)

Vx-Vb (g/cm2)

Fm (g/h/cm2)

D (cm)

0.5 0.5 0.5

0.49 0.49 0.49

0.01 0.01 0.01

3971 4521 5499

2138 2444 3177

1833 2077 2322

61.0 24.4 12.2

6 6 6

TABLE 5. Calculated parameters for fixed bed adsorption of As(III) on IOIAA EBCT (s)

tx (h)

tδ (h)

f

tf (h)

δ (cm)

222 564 1128

65.0 185.2 450.7

30.0 85.1 190.3

0.39 0.55 0.63

18.3 38.2 70.4

3.85 3.19 3.0

% Saturation 69.6 76.0 81.5

Adsorption of As(III) from Aqueous Solution onto IOIAA

4.

5.

Experimental data satisfactorily fit Langmuir and Freundlich isotherms. Higher adsorption was observed at higher temperatures indicating the endothermic nature of adsorption. The breakthrough curves for adsorption of As(III) showed the mutual effects of adsorption capacity and adsorption rate. The shape of breakthrough curve obtained was a strong function of flow rate and bed height of the sorbents bed. The percent saturation was lower for the smaller length of adsorption zone. The parameters obtained from this study can be used to establish the time required for breakthrough to occur and additional solution required per unit cross-sectional area of adsorber in the complete exhaustion of the bed.

Acknowledgement We are thankful to the UGC (University Grants Commission), New Delhi, India, for providing financial support for the research.

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Received: October 23, 2003; accepted: June 14, 2004.

Nomenclature b C C0 Cb Ce Cx D δ f Fm ∆G ∆H ∆S k K kad1 kad2 Ko kp Ms n q qe qm qt R t T tδ tf tx

V Vb Vx W

Langmuir constant (L/mg). Arsenic concentration in solution (mg/L). Initial As(III) concentration (mg/L). Effluent sorbate concentration at breakthrough (mg/L). Equilibrium As(III) concentration (mg/L). Effluent sorbate concentration at bed saturation (mg/L). Bed height (cm). Length of primary adsorption zone (cm). Fractional capacity of IOIAA column. Mass flow rate (g cm2 h-1). Gibb’s free energy change (kJ/mol). Enthalpy change (kJ/mol). Entropy change (kJ mol-1K). Freundlich constant (mg/g). Temperature (Kelvin). First-order rate constant (h-1). Second-order rate constant (mg g-1 min-1). Equilibrium constant (m3/mol). Intraparticle diffusion coefficient (mg g-1 min-0.5). Amount of solute adsorbed by sorbent in PAZ from breakthrough to exhaustion (g). Freundlich constant (dimensionless). Amount of As(III) adsorbed per unit mass of adsorbent (mg/g). Amount of As(III) adsorbed per unit mass of adsorbent at equilibrium (mg/g). Maximum monolayer capacity (mg/g). Amount of As(III) adsorbed per unit mass of adsorbent at time t (mg/g). Gas constant (kJ mol-1). Time (s). Temperature (K). Time required for movement of PAZ downward the column (h). Time required for initial formation of PAZ (h). Total time taken for primary adsorption zone (PAZ) to establish and move down length of column (h). Volume of water sample per unit cross-sectional area of the bed (g cm-2). Volume of effluent treated till breakthrough per unit cross-sectional area of the bed (g cm-2). Volume of effluent treated per unit crosssectional area of the bed till saturation (g cm-2). Weight of adsorbent (g).