Biochar immobilizes soil-borne arsenic but not cationic metals in the presence of low-molecular-weight organic acids

Highlights: • Biochar was protonated by low-molecular-weight organic acids (LMWOAs). • This disfavours adsorption of the cationic metals mobilized by LMWOAs. • But As was re-immobilized by the protonated biochar effectively. • Increased metal levels due to release of biochar-borne metals under LMWOA attack • Biochar could enhance reductive dissolution of iron and manganese oxides. A batch experiment was conducted to examine the effects of biochar on the behaviour of soil-borne arsenic and metals that were mobilized by three low-molecular-weight organic acids. In the presence of citric acid, oxalic acid and malic acid at a molar concentration of 0.01 M, the surface of biochar was protonated, which disfavours adsorption of the cationic metals released from the soil by organic acid-driven mobilization. In contrast, the oxyanionic As species were re-immobilized by the protonated biochar effectively. Biochar could also immobilize oxyanionic Cr species but not cationic Cr species. The addition of biochar increased the level of metals in the solution due to the release of the biochar-borne metals under attack by LMWOAs via cation exchange. Biochar could also have the potential to enhance reductive dissolution of iron and manganese oxides in the soil, leading to enhanced release of trace elements bound to these oxides. The findings obtained from this study have implications for evaluating the role of biochar in immobilizing trace elements in rhizosphere. Adsorption of cationic heavy metals on biochar in the presence of LMWOAs is unlikely to be a mechanism responsible for the impeded uptake of heavy metals by plants growing in heavy metal-contaminated soils.