Arsenate removal from synthetic wastewater by adsorption onto fly ash

In this work, the removal of arsenate from synthetic wastewater by adsorption onto a coal combustion fly ash (CCA) has been experimentally studied. The possibility to reuse CCA as low-cost sorbent has been carefully checked by a multiple analysis that included the evaluation of metals releases in aqueous solution, the assessment of arsenate adsorption capacity and a possible beneficiation treatment. Following this pattern, a preliminary CCA characterization has been performed and significant releases of different metallic ions (mainly Ca, Al, K, Ba and Si) were observed, including arsenic itself whose total content on CCA resulted to be around 0.05 mg/g. Arsenate adsorption isotherms on raw CCA at constant temperature (20°C) have been determined in two synthetic arsenic polluted aqueous solutions, a distilled water and a mineral water simulating a groundwater. The results pointed out that in both the experimental conditions CCA shows almost the same arsenate adsorption capacities. In order to increase CCA adsorption capacity and to simultaneously remove the arsenic originally present on CCA, a HCl treatment of CCA sample has been also performed. The treated sample showed higher adsorption capacity when compared to the raw sample, that can be related to an increase of surface area and to the oxidation of CCA surface. Finally, the effect of equilibrium pH on arsenate adsorption on both raw and treated CCA samples has been investigated. Experimental results, interpreted in light of a speciation analysis of arsenate ions, revealed that for each pH investigated, the treated CCA sample showed the highest adsorption capacity, unless collateral phenomena (i.e. arsenic precipitation as calcium arsenate hexahydrate) take place.