Biochar affects the dissolved and colloidal concentrations of Cd, Cu, Ni, and Zn and their phytoavailability and potential mobility in a mining soil under dynamic redox-conditions

Highlights: • Biochar (BC) treated soil showed a wider range of E_H and pH than the untreated soil. • Biochar increased dissolved Cd, Cu, Ni, and Zn, particularly under oxic conditions. • Cadmium was abundant in the colloidal and Cu in the dissolved fraction. • Biochar increased dissolved Cd, Ni, Zn and in particular Cu under oxic conditions. • The phytoavailability of the elements was higher than their potential mobility. There is a lack of knowledge on the effects of biochar (BC) on the release dynamics of potentially toxic elements (PTEs) in different phases of soil under systematic change of redox potential (E_H). We aimed to elucidate the impact of pre-definite E_H on the release dynamics of dissolved and colloidal concentrations of Cd, Cu, Ni, and Zn as well as their phytoavailability and potential mobility in the solid-phase of a mining soil treated with rice hull biochar (S + BC) compared to non-treated soil (S). The influence of E_H-dependent changes of soil pH, dissolved organic carbon (DOC), dissolved aromatic carbon (DAC), Fe, Mn, SO₄^{2 −}, and Cl⁻ on the elements release was also determined. The experiment was conducted stepwise from reducing (− 30 mV in S and − 12 mV in S + BC) to oxidizing (+ 218 mV in S and + 333 mV in S + BC) conditions using an automated biogeochemical microcosm system. Biochar-treated soil exhibited a wider range of E_H and a lower pH than the non-treated soil. Dissolved concentrations of Cd, Cu, Ni, Zn, Fe, Mn, SO₄^{2 −}, and DAC increased under oxic conditions in the non-treated and biochar-treated-soils, which might be due to the decline of pH, and/or sulfide oxidation. Cadmium was more abundant in the colloidal fraction, while Cu, Mn, and DOC were more abundant in the dissolved fraction. Nickel, Zn, and Fe distributed almost equally in both fractions. Biochar increased the dissolved concentration of Cd, Ni, Zn and in particular Cu under oxic conditions. However, the biochar did not significantly affect the colloidal fraction of Cd, Cu, Ni, and Zn. The phytoavailability of the studied elements was higher than the potential mobility. We conclude that increasing the dissolved concentrations of the elements under oxic conditions might increase their release and transfer into the groundwater and the food chain which should be harmful for the environment.