Adsorption of arsenate on synthetic goethite from aqueous solutions

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Abstract

Goethite was synthesized from the oxidation of ferrous carbonate precipitated from the double decomposition of ferrous sulfate doped with sodium lauryl sulfate (an anionic surfactant) and sodium carbonate in aqueous medium. The specific surface area and pore volume of goethite were 103 m2 g−1 and 0.50 cm3 g−1. Batch experiments were conducted to study the efficacy of removal of arsenic(V) using this goethite as adsorbent for solutions with 5–25 mg l−1 of arsenic(V). The nature of adsorption was studied by zeta-potential measurements. The adsorption process followed by Langmuir isotherm and diffusion coefficient of arsenate was determined to be 3.84 × 1011 cm2 s−1. The optimum pH of adsorption was found to be 5.0. The kinetics of adsorption was evaluated with 10 mg l−1 and 20 mg l−1 of As(V) solutions and activation energy of adsorption, as calculated from isoconversional method was in the range of 20 kJ mol−1 to 43 kJ mol−1. This suggests that the adsorption process is by diffusion at the initial phase and later through chemical control. FT-IR characterization of arsenic treated goethite indicated the presence of both Assingle bondOsingle bondFe and Assingle bondO groups and supported the concept of surface complex formation.

Introduction

Arsenic is known to cause various ailments to humans ranging from skin rashes to carcinoma and its presence in exceedingly higher concentrations in drinking water poses threat to millions of people in West Bengal of India and Bangladesh [1]. United States Environmental Protection Agency (USPEA) has revised the maximum contaminant level for arsenic in drinking water from 50 μg l−1 to 10 μg l−1 because of its proven toxic effects on human health. Under reducing conditions, arsenic occurs in trivalent form (arsenious acid species) whereas the pentavalent state (arsenic acid species) is common in oxidizing conditions in aqueous systems [2]. In order to combat the problem of arsenic contamination, various treatment methods are proposed. The most common methods include the application of aluminum and iron salts that are used in water purification [3]. The removal of arsenic is accomplished by adsorption techniques wherein adsorbents such as amorphous ferric hydroxide [4], ferrihydrite [5], natural iron ores [6], ferruginous manganese ores [7], iron oxide coated polymers [8], lanthanum compounds [9], zero valent iron [10], silica containing iron oxide [11], activated carbon [12], have been recommended. In another study, it has been demonstrated that arsenic in the form of arsenate could be removed up to 95% by adsorption on to hardened paste of Portland cement [13]. Iron filings (40 mesh) immersed in water was used to remove arsenic in batch type adsorption and a removal efficiency of 90% was reported [14]. The arsenic uptake from aqueous solutions by iron bearing minerals such as goethite, lipidocrocite, mackinawite and pyrite was studied and found to be better [15]. In a comparative study [16], that evaluated the arsenic removal from water using a variety of adsorbents, viz., zirconia-impregnated activated carbon, AM3 (a commercial adsorbent comprising calcite, fluorite and iron oxide) and granules iron hydroxide (GIH), the sorption capacity was found to be 2.8 mg, 2.0 mg and 2.3 mg of arsenic per gram of adsorbent. It is observed that in most of these cases, the adsorption efficiency is dependent on the surface properties of the adsorbent such as specific surface area, surface charge, pore volume and pore sizes.

Among the various adsorbents, iron bearing minerals especially goethite was observed to be more effective and economically viable. Various synthesis methods of goethite [17] were reviewed and found that the particle size, shape and surface area of goethite depend on the Fe(III):OH ratio, the rate of base titration of iron salt, temperature of neutralization and time of crystallization. Most of the synthesis methods were based on neutralization of ferric nitrate with an alkali and subsequent aging spanning from 20 h to 336 h. The specific surface area of goethite specimens obtained by the above method ranges from 11 m2 g−1 to 150 m2 g−1. In another method, goethite nano crystals with mean size ranging from 1 nm to 10 nm and specific surface area around 300 m2 g−1 by hydrolysis of aqueous solutions of ferric salts followed by membrane purification and freeze drying was reported [18].

In the present investigation, the synthesis of goethite was carried out by oxidation of ferrous carbonate precipitated from ferrous sulfate solution doped with sodium lauryl sulfate and sodium carbonate solution, wherein CO32−/Fe2+molar ratio of the resultant solution was maintained at 1.0. The interaction of arsenate with goethite surface, the kinetics of adsorption process and the activation energy of adsorption by isoconversional method were also studied. An attempt was made to elucidate the mechanism of arsenic(V) adsorption on goethite synthesized from the oxidation of ferrous carbonate.

Section snippets

Reagents

Electrolytic grade (99.9% purity) iron powder was treated with sulphuric acid to prepare 0.5 M ferrous sulfate solution. 2.0 M stock solutions of sodium carbonate and sodium hydroxide were prepared by using analar grade chemicals obtained from E. Merck sodium lauryl sulfate obtained from British Drug House (BDH) was used to prepare 1% (w/v) solutions. Arsenic(V) test solutions were prepared from H3AsO4 (E. Merck-NIST Certified: 1000 ppm As) standard reference solution. The test solutions were

Effect of pH on arsenic adsorption

The effect of pH on the adsorption of arsenic at goethite surface was studied with an initial concentration of 10 mg l−1 of arsenic solution. One hundred milligrams of goethite was added to each of the test solution and equilibrated for 1 h. After equilibration, unadsorbed arsenic and the final pH of the solution were measured and the results are shown in Table 1.

From the results, it is apparent that maximum adsorption of arsenic on goethite was observed around pH 5.0. In general, arsenate removal

FT-IR studies

Fig. 7 shows FT-IR spectra in which spectrum (a) is for pure goethite and spectrum (b) is for goethite treated with 15 mg l−1 arsenic at pH 5.0 at 30 °C. The bands at 797 cm−1 and 893 cm−1 in spectrum (a) are characteristic of goethite and arise due to γ and δ single bondOH bending modes of out and in plane modes. The intense band at 3170 cm−1 is due to the bulk single bondOH stretching. The symmetric stretching of Fesingle bondO is indicated by a band at 613 cm−1 [31]. In spectrum (b), bands at 862 cm−1 and 830 cm−1 could be assigned

Conclusion

Goethite with high surface area was synthesized from ferrous sulfate treated with sodium lauryl sulfate and sodium carbonate. Batch experiments of adsorption were conducted to study the efficacy of removal of arsenic(V) from aqueous solutions. The optimum pH of adsorption was determined to be 5.0. The zeta-potential measurements have indicated that the arsenate is adsorbed on goethite by chemisorption. The adsorption data showed that Langmuir isotherm was the best fit and using the plot, the

Acknowledgements

The authors thank Prof. S.P. Mehrotra, Director, National Metallurgical Laboratory, Jamshedpur for permission to publish this paper. The financial support from Department of Biotechnology, Ministry of Science and Technology, New Delhi is gratefully acknowledged.

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