Due to compositional and spatial heterogeneity of soils, it is always a challenge to predict the risks of trace elements in terrestrial system. While, the solid-solution partitioning of metals is a key factor controlling their accumulation, mobility, and bioavailability in soils, it is essential to predict metal partitioning in soil/solution to assess the potential risk of metal leaching and its bioavailability to biomass. Recently, assemblage or multi-surface models were found to be a promising tool to predict trace-element partitioning in soils. The models assume that trace-element adsorption in heterogeneous soils can be described by the sum of the individual reactive surfaces. While due to the model parameters are thermodynamically based, it can be extrapolated to various soils compared with empirical models. In this talk, we will introduce two case study using the multi-surface speciation model to predict metal partitioning and bioavailability.

1) Cr(VI) partitioning in various soils

We investigated the adsorption of Cr(VI) on 12 soils (previously depleted of organic matter) as a function of pH and the loading concentration of Cr(VI). A simple mechanism-based multi-surface complexation model is proposed to predict Cr(VI) partitioning. The retention of Cr(VI) in soils was considered to be attributed to two reactive oxide surfaces, goethite, and hydrous ferric oxide (HFO); however, modelling results showed that the best prediction was obtained with goethite alone, whereas the addition of HFO resulted in an overestimation of adsorption in some soils. Cr(VI) adsorption onto goethite could be described using our previously proposed CD-MUSIC model. The available phosphate in soils was identified as a strong competitor for Cr(VI) adsorption; thus, for soils with low Fe/P ratio (<1) its effect on Cr(VI) retention should not be neglected.

2) Prediction of partition and bioavailability of Cd(II) and Ni(II) to wheat

An improved soil multi-surface speciation model was developed to predict the partition of Cd(II) and Ni(II) in soils. Nineteen selected soils from China with a wide range of varieties spiked with Cd(II) and Ni(II) were used to test the model, while wheat was used as test plant. Adsorption to soil organic matters was evaluated using the NICA-Donnan model and to iron oxides using CD-MUSIC and a two-site DDL model with published model parameters. The partition of Cd(II) and Ni(II) in soil-water phase was evaluated with 0.01M CaCl2 extraction. The phytoaccumulation of Cd(II) and Ni (II) in wheat roots and shoots was evaluated using several methods, such as 0.01M CaCl2, 0.43M HNO3, soil pore water, DGT extraction and modelling. In addition, a set of data from field including soil properties and Cd accumulated in wheat seeds were also used to evaluate the model prediction. The results from both lab and field experiments confirmed that the multi-surface model was an efficient method for metal phyto-accumulation assessment.

Acknowledgement

The authors thank National Key R&D Programs of China(2018YFC1800602) the Natural Science Foundation of China (No. 21577062 and 21876080).