The Mechanic Prediction Model Of Cr(Ⅵ) Partition In Soils

Chromium is an important element and is widely used in industrial processes. There are two oxidation states of Cr in the environment including Cr(Ⅲ) and Cr(Ⅵ). Because Cr(Ⅵ) has higher toxicity, solubility, and mobility than Cr(Ⅲ), it has much higher environmental risk. Previous studies show that the retention of Cr(Ⅵ) in soils is mainly influenced by iron oxides. In this study, the interaction between Cr(Ⅵ) and goethite surfaces were investigated by batch adsorption experiments as a function of pH, ionic strength, surface loading of Cr(Ⅵ), common cations and clay minerals in soils. Coupled with microscopic x-ray absorption spectroscopy results, a charge distribution multisite surface complexation model (CD-MUSIC model) was developed to describe the adsorption of Cr(Ⅵ) at the goethite-water interface. Based on this model, the adsorption behavior of Cr(Ⅵ) onto 12 selected soils across China were investigated. Mechanistic multi-surface speciation model was used to predict the adsorption of Cr(Ⅵ) in soils and various model frameworks were compared to obtain the best prediction results. The main conclusions are as follows:(1) The adsorption of Cr(Ⅵ) onto goethite surface were influenced by various factors. Cr(Ⅵ) adsorption was strongly affected by pH and decreased with increasing pH. Ionic strength showed little effect to Cr(Ⅵ) adsorption, suggesting an inner-sphere surface complexation. Phosphate could strongly inhibit the adsorption of Cr(Ⅵ), while Cr(Ⅵ) showed weak influence to phosphate adsorption. Bivalent metallic cations, such as Ca2+, Mg2+, Cu2+, Zn2+, showed almost no influence to Cr(Ⅵ) adsorption with 1:1 molar ratio. Kaolinite has almost no effect to Cr(Ⅵ) adsorption, while high loading of illite and montmorillonite inhibit Cr(Ⅵ) adsorption to some extent.(2) Cr(Ⅵ) adsorption at the goethite-water interface was successfully described by CD-MUSIC model. EXAFS evidence showed that the interaction between Cr(Ⅵ) and goethite surface mainly involved inner-sphere covalent bonds with bidentate and monodentate surface complex. The following two surface reactions were then used to describe the adsorption behavior of Cr(Ⅵ).Results showed that the model can well describe the competitive adsorption of Cr(Ⅵ) and phosphate, suggesting the reliability of the model parameters.(3) Cr(Ⅵ) adsorption on soils was mainly influenced by the content of iron oxides and pH values, i.e. higher iron oxides contained and lower pH result higher adsorption capacity of Cr(Ⅵ). The adsorption isotherm of Cr(Ⅵ) could be fitted by Langmiur model.(4) The partition of Cr(Ⅵ) in soils were predicted using mechanistic multi-surface complexation models and different model frameworks and model parameters were compared. Results showed that Cr(Ⅵ) retention on soils could be well modeled by adsorption of Cr(Ⅵ) onto goethites. Considering the competitive effect of phosphate could improve the prediction. The effective surface area of iron oxides in soils are in different. Using the effective surface area of soil goethite obtained based on the maximum adsorption capacity for Cr(Ⅵ) could well predict Cr(Ⅵ) adsorption. If the parameters could not be obtained, using the surface area optimized with adsorption data (45.8 m2/g) could also well predict Cr(Ⅵ) adsorption. We also compared the performance of mechanistic multi-surface complexation model and the empirical partition model and results showed that the former one is better than the latter one.In conclusion, the developed mechanistic partition model for Cr(Ⅵ) in soils has advantages on wide range of application, better prediction and simple framework. It provide a practical tool for Cr(Ⅵ) risk assessment in the environment.