Adsorption and transport of arsenate in carbonate-rich soils: Coupled effects of nonlinear and rate-limited sorption Irfan Yolcubal * , Nihat Hakan Akyol Department of Geological Engineering, University of Kocaeli, Umuttepe Kampüsü, TR-41380 Kocaeli, Turkey article info Article history: Received 13 April 2008 Received in revised form 2 July 2008 Accepted 5 July 2008 Available online 20 August 2008 Keywords: Arsenic Calcareous soil Transport modeling Sequential extraction Column experiment Turkey abstract The transport and fate of arsenate in carbonate-rich soil under alkaline conditions was investigated with multiple approaches combining batch, sequential extraction and column experiments as well as trans- port modeling studies. Batch experiments indicated that sorption isotherm was nonlinear over a wide range of concentration (0.1–200 mg L 1 ) examined. As(V) adsorption to the calcareous soil was initially fast but then continued at a slower rate, indicating the potential effect of rate-limited sorption on trans- port. Column experiments illustrated that transport of As(V) was significantly retarded compared to a non-reactive tracer. The degree of retardation decreased with increasing As(V) concentration. As(V) breakthrough curves exhibited nonideal transport behavior due to the coupled effects of nonlinear and rate-limited sorption on arsenate transport, which is consistent with the results of modeling studies. The contribution of nonlinear sorption to the arsenate retardation was negligible at low concentration but increased with increasing As(V) concentration. Sequential extraction results showed that nonspecif- ically sorbed (easily exchangeable, outer sphere complexes) fraction of arsenate is dominant with respect to the inner-sphere surface bound complexes of arsenate in the carbonate soil fraction, indicating high bioavailability and transport for arsenate in the carbonate-rich soils of which Fe and Al oxyhydroxide fractions are limited. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Arsenic enters into the subsurface from both natural processes (e.g., mineral weathering, geothermal processes) and anthropo- genic activities such as mining activities and applications of pesti- cides and herbicides. Contamination of groundwater by arsenic from soils and aquifers is a widespread and serious problem world- wide (Smedley and Kinniburgh, 2002). It is well-documented that arsenic is a known carcinogen and causes a number of adverse health effects, such as black foot disease, and skin legions (WHO, 2001; Bissen and Frimmel, 2003). It is therefore, of utmost impor- tance to improve the understanding of arsenic retention and transport behavior in soils and aquifers to assess the risk of con- tamination to groundwater. Arsenic in soils and groundwater occurs predominantly in the oxidation states of As(III) and As(V) and is commonly found in the form of inorganic oxyanions. As(V) (arsenate) oxyanions ðH 3 AsO 4 ; H 2 AsO  4 ; HAsO 2 4 Þ are the dominant arsenic species under aerobic conditions and interact strongly with soil matrix, while As(III) (arsenite) oxyanions ðH 3 AsO 3 ; H 2 AsO  3 ; HAsO 2 3 Þ are stable under anaerobic conditions and more toxic and mobile than the arsenate (Smedley and Kinniburgh, 2002; Bissen and Frimmel, 2003). Speciation of arsenic in the soils and groundwater is largely controlled by Eh and pH conditions of the environment. Bioavailability and mobility of arsenic in soils and aquifers are determined by several complex and coupled processes, includ- ing oxidation–reduction, precipitation–dissolution, adsorption– desorption, and biochemical methylation. Among these processes, adsorption and desorption reactions mainly control the movement of arsenic in soils and aquifers. Arsenic adsorption in soils and aquifers is affected by chemical factors, such as soil mineralogy, pH, Eh and the presence of competing ions in soil solution (Sadiq, 1997). Most studies of arsenic adsorption have focused on oxide minerals, especially Fe, Al, and Mn oxides and hydroxides. It is well documented that Fe and Al oxyhydroxides (e.g., goethite, hematite, ferrihydrite, gibbsite) have a high affinity to arsenate (Fuller et al., 1993; Waychunas et al., 1993; Manning and Goldberg, 1996b; Fendorf et al., 1997; Jain and Loeppert, 2000; O’reilly et al., 2001). Arsenate oxyanions are strongly retained on these surfaces by forming inner-sphere surface complexes via a ligand exchange mechanisms (Waychunas et al., 1993; Sun and Doner, 1996; Fendorf et al., 1997; Grossl et al., 1997; Goldberg and Johnston, 2001; O’Reilly et al., 2001; Goldberg, 2002; Sherman and Randall, 2003; Alexandratos et al., 2007). Surfaces of clay minerals and Mn oxides are generally negatively charged in neutral–alkaline soil pHs and may play an effective role in As adsorption only in acidic soils (Anderson et al., 1976; Frost and Griffin, 1977; Anderson and 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.07.013 * Corresponding author. Tel.: +90 262 3033167; fax: +90 262 3033003. E-mail address: yolcubal@kocaeli.edu.tr (I. Yolcubal). Chemosphere 73 (2008) 1300–1307 Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere