Biochar is amended to agricultural soils to increase water retention, provide nutrient release, control soil pH, and to remediate heavy metals and organics contamination.  The geochemistry and mineralogy of agricultural soils is heterogeneous, hence developing flexible methods to predict the impacts of biochar amendment on metals distribution in soils are needed to inform the type of biochar used and the amendment level.  In this study, we test if a component additivity surface complexation modelling approach can predict the speciation of Cd(II) and Se(VI) in two agricultural soils amended with biochar.  To do so, potentiometric titrations were conducted for each sorbent and the data subsequently used to calculate the proton-active surface functional group concentrations and acidity constants.  Following, Cd(II) and Se(VI) metal adsorption experiments to individual sorbents were performed as a function of pH (from 2 to 8), and the data modeled to calculate metal-surface stability constants for each sorbent.  The combined constants and site concentrations were then used to predict Cd(II) and Se(VI) speciation in multi-component soil systems containing either both metals, a biochar amendment, or both.  Generally, the predictive models match well with analogous multi-component experimental adsorption data.  Our results indicate that both the biochar and the two soils tested strongly adsorb Cd(II) and Se(VI), and that the biochar has a higher proton reactivity and metal adsorption capacity on a per-gram basis.  Thus, biochar amendment should enhance the overall concentration of proton-active sites upon amendment into these soils.  However, the leaching of organic acids from soils, which can bind both metals, may play a key role in solubilizing Cd(II) and Se(VI) into solution and thereby reduce metal uptake to soil and biochar. This aspect is the focus of future studies.

Impact of biochar amendment on trace metals speciation in agricultural soils

253rd National Meeting (2017)

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