Amendment of biochar reduces the release of toxic elements under dynamic redox conditions in a contaminated floodplain soil

• Pre-set redox conditions affect release dynamics of metals via changes of pH.
• Amendment of biochar decreased release of metals under dynamic redox conditions.
• Contaminated floodplain soil (non-treated) and treated with biochar behave similarly.
• Addition of biochar to soil seems to have little effect on redox processes.

Biochar (BC) can be used to remediate soils contaminated with potential toxic elements (PTEs). However, the efficiency of BC to immobilize PTEs in highly contaminated floodplain soils under dynamic redox conditions has not been studied up to date. Thus, we have (i) quantified the impact of pre-definite redox conditions on the release dynamics of dissolved aluminum (Al), arsenic (As), cadmium (Cd), copper (Cu), nickel (Ni), and zinc (Zn) in a highly contaminated soil (CS) (non-treated) and in the same soil treated with 10 g kg−1 biochar based material (CS + BC), and (ii) assessed the efficacy of the material to reduce the concentrations of PTEs in soil solution under dynamic redox conditions using an automated biogeochemical microcosm apparatus. The impact of redox potential (EH), pH, dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), iron (Fe), manganese (Mn), and sulfate (SO42−) on dynamics of PTEs was also determined. The EH was lowered to +68 mV and afterwards increased stepwise to +535 mV. Significant negative correlation between EH and pH in CS and CS + BC was detected. The systematic increase of EH along with decrease of pH favors the mobilization of PTEs in CS and CS + BC. The material addition seems to have little effect on redox processes because pattern of EH/pH and release dynamics of PTEs was basically similar in CS and CS + BC. However, concentrations of dissolved PTEs were considerably lower in CS + BC than in CS which demonstrates that BC is able to decrease concentrations of dissolved PTEs even under dynamic redox conditions.