Kinetics, equilibrium and thermodynamics studies of arsenate adsorption from aqueous solutions onto iron hydroxide T. Mahmood a , S.U. Din a , A. Naeem a, *, S. Tasleem a , A. Alum b , S. Mustafa a a National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar 25120, Pakistan b Arizona State University, School of Sustainable Engineering and the Built Environment, Tempe, AZ 85287, USA 1. Introduction The presence of arsenic in drinking water is a serious environmental problem. Soluble arsenic is present in natural waters in the forms of arsenate [As(V)] and arsenite [As(III)]. Arsenate (V) is the most stable surface water species under aerobic conditions and exists as deprotanated oxyanions of arsenic acid (H 2 AsO 4  , HAsO 4 2 ). Arsenite (III) exists in neutral forms at natural pH values. Removal of As(III) from water is difficult as compared to As(V). Generally, As(III) remains undissociated and is neutral and therefore exhibits only limited adsorption sites as compared to As(V). It is necessary to oxidize As(III) to As(V) for its effective removal from drinking water [1]. Natural processes such as volcanic exhalations, weathering reactions, and biological activi- ties are responsible for the mobilization of arsenic. The anthropo- genic sources responsible for the release of arsenic to the environment are mining activities, smelting operations, fertilizers, combustion of fossil fuels and arsenic pesticides [2]. Arsenic uptake through drinking water causes various chronic diseases such as epidermal, cardiovascular, respiratory, neurolog- ical and hematological and renal diseases. Arsenic uptake also causes cancer of lung, bladder, liver, kidney and prostate [3–7]. In addition, the notorious black foot disease which results in the amputation of feet, arm and other parts of the body is also one of the toxicological effects of arsenic contaminated water. The use of arsenic contaminated water may also result in diabetes mellitus. Arsenic intake in large quantity may lead to hallucination, impaired memory, emotional disorder and perplexity [5,7]. The World Health Organization (WHO), the US Environmental Protection Agency (USEPA) and European Commission (EC) have laid the guidelines [4] for arsenic in ground water to be 10 mg L 1 . Arsenic concentration is higher than the permissible limit of WHO in some parts [5,6] of Mexico, India, Argentina, USA, Bangladesh, Chile and Pakistan. Different technologies such as precipitation–coagulation, membrane separation, ion exchange, and adsorption have been used for the removal of toxic elements from water. Adsorption is considered to be one of the most preferred techniques due to its high efficiency, economy and easy operation [7–9]. Adsorbents like activated alumina, activated carbon, natural laterite, orange juice residues, mixed rare earth oxides, lanthanum compounds and plant biomasses have been tested for the adsorption of arsenic from drinking water [10]. Many of them have low adsorption affinity for the removal of arsenic from water. On the other hand, iron containing compounds have been found more promising, efficient and versatile for the adsorption of both As(III) and As(V) [5]. Several researchers have carried out kinetics investigation on the adsorption of arsenic onto metal oxides. Fuller et al. [11] examined the kinetics of arsenate adsorption on ferrihydrite in coprecipitation and post-synthesis adsorption experiments in the pH range from 7.5 to 9.0. Ping et al. [12] investigated the kinetics of arsenic adsorption onto oxides of iron and aluminum which was influenced by the contact time and the surface coverage. Ping and Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx A R T I C L E I N F O Article history: Received 14 March 2013 Accepted 2 December 2013 Available online xxx Keywords: Sorption Arsenate Metal oxide Film diffusion A B S T R A C T This paper reports the equilibrium, kinetics and thermodynamic studies of arsenate adsorption onto freshly precipitated iron hydroxide. Adsorption of arsenate onto iron hydroxide depends on arsenate concentration, contact time and temperature. The intrapartical diffusion model indicates that both the film and intrapartical diffusion control arsenate adsorption on iron hydroxide. The values of activation energies (E a ) indicate the chemical nature of adsorption accompanied by diffusion controlled processes as the rate limiting step. The endothermic nature of the adsorption process suggests that adsorption reaction consumes energy. The negative values of DS # can be assigned to decreased randomness at the solid liquid interface. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +92 91 9218480; fax: +92 91 9216671. E-mail address: naeeem64@yahoo.com (A. Naeem). G Model JIEC-1733; No. of Pages 9 Please cite this article in press as: T. Mahmood, et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2013.12.004 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry jou r n al h o mep ag e: w ww .elsevier .co m /loc ate/jiec 1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.004