Articles | Volume 13, issue 19
https://doi.org/10.5194/bg-13-5557-2016
https://doi.org/10.5194/bg-13-5557-2016
Research article
 | 
06 Oct 2016
Research article |  | 06 Oct 2016

Surface complexation modeling of Cd(II) sorption to montmorillonite, bacteria, and their composite

Ning Wang, Huihui Du, Qiaoyun Huang, Peng Cai, Xingmin Rong, Xionghan Feng, and Wenli Chen

Abstract. Surface complexation modeling (SCM) has emerged as a powerful tool for simulating heavy metal adsorption processes on the surface of soil solid components under different geochemical conditions. The component additivity (CA) approach is one of the strategies that have been widely used in multicomponent systems. In this study, potentiometric titration, isothermal adsorption, zeta potential measurement, and extended X-ray absorption fine-structure (EXAFS) spectra analysis were conducted to investigate Cd adsorption on 2 : 1 clay mineral montmorillonite, on Gram-positive bacteria Bacillus subtilis, and their mineral–organic composite. We developed constant capacitance models of Cd adsorption on montmorillonite, bacterial cells, and mineral–organic composite. The adsorption behavior of Cd on the surface of the composite was well explained by CA-SCM. Some deviations were observed from the model simulations at pH < 5, where the values predicted by the model were lower than the experimental results. The Cd complexes of X2Cd, SOCd+, R-COOCd+, and R-POCd+ were the predominant species on the composite surface over the pH range of 3 to 8. The distribution ratio of the adsorbed Cd between montmorillonite and bacterial fractions in the composite as predicted by CA-SCM closely coincided with the estimated value of EXAFS at pH 6. The model could be useful for the prediction of heavy metal distribution at the interface of multicomponents and their risk evaluation in soils and associated environments.

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Short summary
We developed surface complexation model for Cd(II) adsorption on montmorillonite, B. subtilis, and their composite. The Cd adsorption behavior onto the composite can be described by a component additivity approach (CA-SCM). The Cd complexes of X2Cd, SOCd+, R-COOCd+, and R-POCd+ were the predominant species on the composite over the pH range of 3 to 8. The model could be useful for the prediction of heavy metal distribution at the interface of multicomponents and their risk evaluation in soils.
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