Arsenic (III) adsorption on iron acetate coated activated alumina: thermodynamic, kinetics and equilibrium approach
© Das et al.; licensee BioMed Central Ltd. 2013
Received: 14 August 2012
Accepted: 29 September 2013
Published: 20 December 2013
The adsorption potential of iron acetate coated activated alumina (IACAA) for removal of arsenic [As (III)] as arsenite by batch sorption technique is described. IACAA was characterized by XRD, FTIR, EDAX and SEM instruments. Percentage adsorption on IACAA was determined as a function of pH, contact time and adsorbent dose. The study revealed that the removal of As (III) was best achieved at pH =7.4. The initial As (III) concentration (0.45 mg/L) came down to less than 0.01 mg/L at contact time 90 min with adsorbent dose of 1 g/100 mL. The sorption was reasonably explained with Langmuir and Freundlich isotherms. The thermodynamic parameters such as ΔG 0 , ΔH 0 , ΔS 0 and E a were calculated in order to understand the nature of sorption process. The sorption process was found to be controlled by pseudo-second order and intraparticle diffusion models.
KeywordsArsenic Iron acetate Activated alumina Adsorption Kinetic Equilibrium
Arsenic contamination in natural water is the worldwide problem. There have been widespread reports of arsenic poisoning, in the major parts of Ganga delta in West Bengal , Brahmaputra basin , in northern eastern part of India, particularly Golaghat district of Assam  and other low-lying areas in Bangladesh . The provisional standard guideline for concentration of arsenic is fixed at 10 ppb (0.01 mg/L) in the drinking water.
In natural water arsenic exists in inorganic forms with the oxidation  states −3, 0, +3 and +5. Arsenic is uniquely sensitive to mobilization (at pH 6.5-8.5) under both oxidizing and reducing conditions . Predominantly, the species arsenite [As (III)] and arsenate [As (V)] exist in ground and surface water respectively. This is why trivalent arsenite predominates in moderately reducing anaerobic environments such as groundwater  and pentavalent species are stable in oxygen rich aerobic environments. It is reported that As (III) is more toxic to biological systems than As (V) . Inorganic species of arsenic represents a potential threat to environment, human and animal health due to their carcinogenic and other effects. Long term drinking water exposure causes skin, lung, bladder and kidney cancer as well as pigmentation changes, skin thickening (hyperkeratosis) neurological disorders, muscular weakness, and loss of appetite . It is very essential, therefore, to remove arsenic from water. Usually, a removal technique of arsenic from aqueous system should be: (i) safe in operation with respect to the maximum contaminant level, (ii) highly efficient, (iii) easy for application and (iv) low cost . Conventional water treatment processes remove toxic metal ions through mechanism such as sorption and particle removal. Advanced water treatment techniques, which can be used as either a primary treatment or post treatment, involve ion exchange, reverse osmosis, adsorption, coagulation, precipitation, adsorption-co precipitation with hydrolyzing metals  etc. Now a day, the adsorption process is getting the best preference over other treatment processes. Available literature demonstrated that arsenic removal can be achieved by adsorption process using activated alumina  iron oxide-coated sand  iron oxide-coated cement  and activated red mud . There is also report of arsenic removal by coagulation method using ferric chloride . The removal of arsenic from drinking water using activated alumina (AA) is found to be the best removal adsorbent as per reports . However, for As (III) removal, both the rate of adsorption as well as low adsorption capacity of As (III) limits the use of AA . But most of the oxides of iron and manganese are available only as fine powders or are generated in aqueous suspension like hydroxides or gels . Adsorbents in powder form have practical limitations, including difficulty in solid/liquid separation, low hydraulic conductivity and leaching of the metal/metal oxide along with treated water .
Present study was carried out to evaluate the performance of iron acetate coated activated alumina (IACAA) for As (III) removal. The process parameters such as effect of adsorbent dose, pH, initial concentration and contact time were investigated. The Langmuir and Freundlich isotherm models were tested for their applicability. Thermodynamic parameters for the process were also calculated to complete the investigation for efficacy of IACAA in removal of arsenic from contaminated water.
Method and methodology
Arsenic trioxide, iron acetate, hydrochloric acid and sodium chloride of analytical reagent grade were procured from E. Merck (India) Ltd and used as received. Activated alumina was obtained from Loba Chemie Pvt., India with size between 70 and 230 μm. Double distilled (DD) water was used throughout for preparing solution. All the instruments used for the experimental purpose were calibrated as per recommended procedure. The initial pH of the arsenic solutions was adjusted using NaOH (0.1 M) and/or HCl (0.1 M) solutions as and when necessary and measured by Cyberscan pH 510 (Eutech) instrument. The determination of concentration of arsenic was done using AAnalyst 200 Atomic Absorption Spectrophotometer (Perkin Elmer). All the measurements were based on integrated absorbance and performed at 193.7 nm by using a quartz tube analyser (Perkin Elmer) followed by the atomization temperature 2000 K. Scanning Electron Microscope (JEOL 6390LV) was used to study the morphology of the samples. The Energy Dispersive X-rays Analysis (EDAX) attached to the SEM was used to analyze the elemental constituents of the adsorbents. Mineral phases of activated alumina, iron acetate coated activated alumina and arsenic adsorbed iron acetate coated activated alumina were characterized by powder X-ray diffraction (Bruker D8). The measurement conditions were taken as follows: anode material = Cu; K-alpha, ג = a 1.5406 Ǻ. Fourier Transform Infrared Spectrophotometer (FTIR) spectrum, (NICOLET Impact I-410) was used to scan the elements.
Preparation of IACAA was carried out in two steps. In first step, 25 g of activated alumina was impregnated with 25 mL of 1.5 M (CH3COO)2Fe in a heat resistant dish and the mixture was heated to 110°C after thorough mixing, until it became dry. In the second step, the same mixture was calcined at 400°C for 3 hours, cooled to room temperature and washed with DD water until the washed water became clear. The washed samples were dried at 110°C for 8 hours and stored in air tight containers for further use.
Batch sorption experiments were conducted to obtain rate and equilibrium data. The reaction mixture consisting 100 mL of known concentration of As (III) solution and known quantity of IACAA was shaken in a temperature controlled orbital shaker at three different temperatures of 301 K, 306 K and 311 K. Spiked water arsenic concentration was fixed at 0.45 mg/L. The effect of adsorbent dose was studied by varying the adsorbent dose from 0.1 g/100 mL to 2.5 g/100 mL and maintaining pH of the solution at 7.4 with a constant contact time of 90 min. The study of the effect of initial pH of the solutions on arsenic uptake by the adsorbent was done by using fix dose of the adsorbent at varying pH of the solutions. The effect of contact time was studied with varying contact time from 30 to 180 min keeping pH of the solutions and dose of the adsorbent constant. The sorption isotherm was also performed by mixing 1 g of IACAA with 100 mL spiked arsenic concentration at different initial concentrations of arsenic. The kinetics and thermodynamic parameters were established by conducting the experiments at different reaction times and at three different temperatures respectively.
To determine the reusability of the IACAA samples adsorption/desorption cycles were repeated using the adsorbent sample. 100 mL solution of both NaOH and HCl (0.1, 0.3 and 0.5 M) and 1 g of the adsorbed adsorbent was used separately and agitated for about 2 hours at shaken speed of 165 rpm. The aqueous phase was then separated and concentration of arsenic in that phase was determined.
Where, q e is the equilibrium quantity adsorbed (mg/g), C e is the equilibrium concentration (mg/L), q m is the maximum adsorption capacity (mg/g) and b is the Langmuir constant.
Where, q e is adsorbed amount (mg/g), C e is equilibrium arsenic concentration (mg/L), k f (mg/g) is the Freundlich constant related to adsorption capacity and n is constant related to energy of intensity of adsorption.
Dimensionless equilibrium parameter (R L )
Where, b is the Langmuir isotherm constant and C o is the initial arsenic ion concentration (mg/L). The value of R L indicates the shape of the isotherms. If the value 0 < R L < 1 then the Langmuir isotherm is favourable, if R L = 0 it is irreversible, if R L = 1 it is linear and if R L > 1 the isotherm is unfavourable.
χ2 (Chi-square) analysis
Where, q e,m is equilibrium capacity obtained by calculating from the model (mg/g) and q e is the experimental data of the equilibrium capacity (mg/g). If the data from the models are similar to the experimental data, χ2 will be a small number, but if they differ then χ2 will be a bigger number .
Thermodynamic parameters associated with adsorption viz. standard free energy change (ΔG o ), standard enthalpy change (ΔH o ), standard entropy change (ΔS o ), activation energy (E a ), were calculated as follows:
Where, ΔG o is the standard free energy change (kJ/mol), T is the temperature in Kelvin and R is the universal gas constant (8.314 J/mol/ K). The sorption distribution coefficient K o for sorption reaction was determined from the slope of the plot against C e at different temperatures and extrapolating to zero C e according to the method suggested by Khan and Singh .
Where, ∆H0 is the standard enthalpy change (kJ/mol) and ∆S0 is the standard entropy change (kJ/mol/K). The values of ∆H0 and ∆S0 can be obtained from the slope and intercept of a plot of 1n K 0 against respectively.
Where, C 0 and C e are the initial and equilibrium arsenic concentrations respectively. The plot of ln(1–θ) against will give a linear plot with intercept of lnS* and slope of .
Where q t is the amount of adsorbate on the surface of the adsorbent at time t (mg/g) and K ad is the equilibrium rate constant of pseudo-first-order sorption (min-1).
Where, , the amount of adsorbate on the surface of the adsorbent at any time, t amount (mg/g), k being pseudo-second-order rate constant (g/mg min), q e is the amount of adsorbate sorbed at equilibrium (mg/g)and the initial sorption rate, h = (mg/g min). The value of q e (1/slope), k (slope2/intercept) and h (1/intercept) of the pseudo-second-order equation were obtained experimentally by plotting against t for arsenic sorption at different temperatures.
Intraparticle diffusion model
Where, k i is the intraparticle rate constant (mg/g min 0.5). The slope of plot of q t 0versus t1/2 gives the value of the intraparticle rate constant.
Results and discussion
Effect of adsorbent dose
Effect of pH
Effect of contact time
Effect of initial concentration
A comparison of Langmuir and Freundlich isotherm parameters obtained at different temperatures
Langmuir isotherm parameters
Freundlich isotherm parameters
q m (mg/g)
k f (mg 1-1/nL 1/ng -1)
In case of Freundlich isotherm, the value of k f and n (Table 1) are obtained from the linear plot of ln q e vs ln C e Figure 9(b). Value of n lying between 1 and 10 also indicates the favourable conditions for adsorption isotherms as well.
The R L and Chi-square values of IACAA
R L values
χ 2values for isotherms
R2 values for Langmuir isotherm (0.991, 0.984 and 0.982) at different temperatures 301 K, 306 K and 311 K are presented in Table 1. It was found out that Langmuir adsorption model is better fitted than Freundlich model. The χ2 values calculated using equation (4) is given in Table 2. In case of Langmuir isotherm, the χ2 values are found to be much lower than that of Freundlich isotherm and hence the adsorption of arsenic on IACAA follows preferably Langmuir isotherm.
Thermodynamic parameters obtained at three different temperatures during arsenic sorption on IACAA
ΔG o (KJ/mol)
ΔH o (kJ/mol)
ΔS o (kJ /mol/K)
Lagergren constants for sorption of arsenic on IACAA at different temperature
Pseudo-second-order kinetic parameters of IACAA
It was observed that no desorption of As (III) was detected under normal condition. The experimental results revealed that eluent NaOH are found to be more effective to desorp arsenic in comparison to that of HCl. The trend of desorption percentage of different concentration of NaOH is as follows: 0.1 M < 0.3 M < 0.5 M. The maximum desorption of arsenic was found to be 34.4% with 0.5 M NaOH solution.
The overall study reveals that the adsorption of arsenic onto IACAA is found to be dependent on pH, adsorbent dose and contact time. Best removal of As (III) is achieved at pH = 7.4. The initial As (III) concentration (0.45 mg/L) comes down to less than 0.01 mg/L with the minimal adsorbent dose (1 g/100 mL) at contact time 90 minutes. The thermodynamic studies of sorption of arsenic on IACAA show that the reaction is spontaneous and endothermic process. The equilibrium data are fitted to both Langmuir and Freundlich adsorption isotherm. But it is found that Langmuir isotherm model fitted well followed by Freundlich. The pseudo-second order kinetic model is found to be the best correlation of the data for sorption of arsenic on IACAA. The kinetic of the reaction follows intraparticle diffusion model.
The authors are thankful to DRDO HQs, New Delhi for providing necessary financial assistance to carry out the research work.
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