Equilibrium and kinetics studies of arsenate adsorption by FePO 4 M. Hamayun 1 , T. Mahmood 1 , A. Naeem ⇑ , M. Muska 1 , S.U. Din 1 , M. Waseem 1 National Centre of Excellence in Physical Chemistry, University of Peshawar, 25120 Peshawar, Pakistan highlights  FePO 4 is a potential adsorbent for anion-exchange removal of arsenate.  Adsorption mechanism involves the exchange of arsenate with OH ions of surface.  Both film diffusion and pore diffusion are involved in kinetics adsorption mechanism.  FTIR and EDX studies validate the adsorption of arsenate by FePO 4 . article info Article history: Received 15 July 2013 Received in revised form 12 October 2013 Accepted 14 October 2013 Available online 23 November 2013 Keywords: Adsorption Arsenic Sorption maxima Kinetics Ferric phosphate abstract The present work is focusing on removal of arsenate from aqueous solution using FePO 4 . The equilibrium study regarding the removal of arsenic by FePO 4 was carried out at 298, 308, 318 and 328 K. Langmuir parameters were found to increase with the increase in temperature indicating that the adsorption is favorable at high temperature. Kinetic study of arsenate adsorption on FePO 4 was also carried out at dif- ferent temperatures and at pH 6 and 8. Different kinetic models were used to the kinetic data amongst which pseudo second order model was best fitted. The mechanism of the adsorption kinetics was inves- tigated by employing intraparticle diffusion and Richenberg models. The energy of activation (E a ) was found to be 30 and 35.52 kJ mol 1 at pH 6 and pH 8, respectively, suggesting chemisorption nature of the adsorption process. The negative entropic values of activation signified the existence of entropy bar- rier while the positive DG # values indicated the existence of energy barrier to be crossed over for the occurrence of a chemical reaction. Both the spectroscopic studies and increase in equilibrium pH reveal the anion exchange removal of arsenate from aqueous solution to the solid surface. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Soils with high concentration of heavy metals affect ground water, surrounding ecosystem, human health and agriculture productivity (Houben et al., 2013). Arsenic is a metalloid which is a dangerous water pollutant throughout the world. Arsenic is mobilized by natural weathering reactions, biological activity, geo- chemical reactions, volcanic emissions and other anthropogenic activities. Soil erosion and leaching contribute to 612  10 8 and 2380  10 8 g year 1 arsenic, respectively, in dissolved and sus- pended forms in the oceans (Mohan and Pittman, 2007). Most environmental arsenic problems are the result of mobilization un- der natural conditions. However, mining activities, combustion of fossil fuels, use of arsenic pesticides, herbicides, crop desiccants and use of arsenic additives to livestock feed are the other sources of arsenic. It occurs in natural waters in both inorganic and organic forms, such as monomethyl arsenic acid (MMAA), dimethyl arsenic acid (DMAA), and arseno-sugars. The inorganic form of arsenic is more toxic than the organic form. Arsenic usually occurs in two va- lence states (Zhang et al., 2013), arsenite [As (III)] and arsenate [As (V)]. In natural waters, arsenite species primarily consist of arseni- ous acid (H 3 AsO 3 ), while arsenate species consist of H 2 AsO 1 4 and HAsO 2 4 (Jeong et al., 2007; Mohan and Pittman, 2007). The use of arsenic contaminated water for a long period gives rise to serious diseases in the human beings. The chronic health ef- fects associated with arsenic include cancer of lung, bladder, liver, kidney, skin pigmentation, nerve tissue injuries and cardiovascular diseases (Ansone et al., 2013). Several countries like Bangladesh, Taiwan, Argentina, Mexico, Chile, Mongolia, China, Hungary, Thai- land, USA, Germany, New Zealand, South Africa, Pakistan and India are at the risk due to arsenic contamination of water (Manjare et al., 2005). Considering its health and toxicological effects, many regulatory agencies have revised the maximum contaminant level (MCL) for arsenic in drinking water from 0.05 to 0.01 mg L 1 (Gregor, 2001). The US Environmental Protection Agency recom- mends that public water systems must comply with the standard of 10 lgL 1 for arsenic instead of 50 lgL 1 used for years (Yang et al., 2007; Gupta et al., 2012). 0045-6535/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2013.10.075 ⇑ Corresponding author. Tel./fax: +92 91 9216766. E-mail address: naeeem64@yahoo.com (A. Naeem). 1 Fax: +92 91 9216766. Chemosphere 99 (2014) 207–215 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere