The development of a multi-surface soil speciation model for Cd (II) and Pb (II): Comparison of two approaches for metal adsorption to clay fractions Xueyuan Gu a,⇑ , Jian Sun a , Les J. Evans b a State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, 163, Xianlin Ave., Nanjing, Jiangsu 210023, China b School of Environmental Sciences, University of Guelph, Guelph, Ontario N1G 2W1, Canada article info Article history: Available online 2 June 2014 Editorial handling by M. Kersten abstract The mobility of toxic metals in soils or sediments is of great concern to scientists and environmentalists since it directly affects the bioavailability of metals and their movement to surface and ground waters. In this study, a multi-surface soil speciation model for Cd (II) and Pb (II) was developed to predict the par- tition of metals on various soil solid components (e.g. soil organic matter (SOM), oxide mineral, and clay mineral). In previous study, the sorption of metal cations on SOM and oxide minerals has been evaluated by thermodynamically based surface complexation model. However, metal binding to soil clay fractions was normally treated in a simplistic manner: only cation exchange reactions were considered and exchange coefficient was assumed unity. In this study, the binding of metals onto clays was described by a two-site surface sorption model (a basal surface site and an edge site). The model was checked by predicting the adsorption behavior of Cd (II) and Pb (II) onto three selected Chinese soils as a function of pH and ionic strengths. Results showed that the proposed model more accurately predicted the metal adsorption on soils under studied condition, especially in low ionic strength condition, suggesting that adsorption of metals to soil clay fractions need to be considered more carefully when modeling the par- tition of trace elements in soils. The developed soil speciation model will be useful when evaluating the movement and bioavailability of toxic metals in soil environment. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The partition of toxic metals in soils or sediments is of great concern to scientists and environmentalists since it directly influ- ence the bioavailability and mobility of metals (Reeder et al., 2006). Adsorption and desorption processes are one of the most critical geochemical processes that affect their speciation distribu- tion and movement. Many components of soils and sediments, such as soil organic matters (SOM), oxide minerals and clay miner- als, have a great influence on the retention of metallic cations (Evans, 1989; Sparks, 2003; Sposito, 2008). In the last few decades, thermodynamic process based surface complexation models (SCM) have made great progress in evaluating the sorption reactions of ions on the surfaces of soil reactive phases, such as SOM (Tipping, 1998; Kinniburgh et al., 1999; Gustafsson, 2001), oxides (Dzombak and Morel, 1990; Hiemstra and van Riemsdijk, 1996) and clay minerals (Evans et aL., 2010). Based on that, assemblage or multi-surface models have been proposed, which assume that the adsorption properties of whole soils can be described by the sum of the individual adsorption properties of each of its constitu- ents (Davis et al., 1998; Weng et al., 2001; Cancès et al., 2003; Khai et al., 2008; Hiemstra et al., 2010a,b). Each particle surface is assigned one specific surface complexation model with defined variables and parameters to describe sorption on that surface: sur- face components, surface species, surface site densities and differ- ent model parameters. In previous studies, metal binding to SOM and ferric oxides was carefully considered using various SCM models. However, binding to soil clay fractions were treated in a simpler way: only cation exchange reactions were accounted in those models and adsorption onto the edge sites of clay minerals was neglected. Adsorption of metal ions on clay fractions is one the most important processes in soils because of its large surface areas and surface charges properties (Kraepiel et al., 1999; Sparks, 2003). Numerous studies have shown that typical 1:1 or 1:2 phyl- losilicate clay minerals bear two types of surface sites: perma- nently negatively charged siloxane surface site and variable charged edge site. Metallic cations can be bound to clay mineral http://dx.doi.org/10.1016/j.apgeochem.2014.05.014 0883-2927/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel./fax: +86 25 89680361. E-mail addresses: xygu@nju.edu.cn (X. Gu), jsun-nju@163.com (J. Sun), levans @uoguelph.ca (L.J. Evans). Applied Geochemistry 47 (2014) 99–108 Contents lists available at ScienceDirect Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem