Multi-surface modeling of Ni(II) and Cd(II) partitioning in soils: Effects of salts and solid/liquid ratios Chaoting Zhang a , Xueyuan Gu a, ⁎, Cheng Gu a , Les J. Evans b a State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Xianlin Ave. 163, Nanjing 210023, PR China b School of Environmental Sciences, University of Guelph. Guelph, Ontario N1G 2W1, Canada HIGHLIGHTS • A multi-surface model predicts Ni/Cd desorption in soils with wide S/L ratios. • The model's performance has an order of 0.01 M CaCl 2 N 0.01 M NaNO 3 N water. • Adding soil background cations signifi- cantly promote the model's prediction. • The model has potential applications to solid/liquid ratio at field conditions. GRAPHICAL ABSTRACT abstract article info Article history: Received 14 February 2018 Received in revised form 12 April 2018 Accepted 14 April 2018 Available online 24 April 2018 Editor: Jay Gan Metal partitioning in soils is a key process controlling metal bioavailability and mobility and is greatly influenced by the solid/liquid ratio. However, metal partitioning is difficult to describe either by a simple partition coefficient or by isotherm adsorption equations. This study investigated the solubility of Ni(II) and Cd(II) in 19 soils as a function of three extraction reagents (water, 0.01 M NaNO 3 , and 0.01 M CaCl 2 ), five solid/liquid ratios (5–400 g/L) and field condition extracted by Rhizon samplers. Thermodynamically based multi-surface models (MSMs) that included generic parameters were used to describe metal partitioning under the studied conditions. The results showed that Ni/Cd solubility depended on the soil type, extraction reagent, and solid/liquid ratio. Soil major background cations (especially Ca 2+ , Mg 2+ , Fe 3+ and Al 3+ ) had a significant effect on the model's predic- tion ability. The MSM was able to predict the extractable metal in 0.01 M CaCl 2 in various soils at different solid/ liquid ratios when soil background cations were included in the calculation; without the background cations, the model was able to predict metal partitioning only at solid/liquid ratios of b100 g/L. In addition, the model failed to predict water-extracted and 0.01 M-NaNO 3 -extracted Ni/Cd if background cations were not included, but could reasonably do so if they were included. More importantly, after the background cations were included, MSMs rel- atively well predicted the Ni/Cd content in soil pore water under 80% field capacity conditions with water as the solution matrix. © 2018 Elsevier B.V. All rights reserved. Keywords: Multi-surface model Suspension density Pore water Field condition 1. Introduction The bioavailability and mobility of heavy metals in soils and sedi- ments are largely determined by the solid/liquid partitioning of the Science of the Total Environment 635 (2018) 859–866 ⁎ Corresponding author. E-mail addresses: xygu@nju.edu.cn, (X. Gu), chenggu@nju.edu.cn, (C. Gu), levans@uoguelph.ca. (L.J. Evans). https://doi.org/10.1016/j.scitotenv.2018.04.191 0048-9697/© 2018 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv