Response of organic acid-mobilized heavy metals in soils to biochar application

Abstract

Microcosm experiments were conducted to examine the chemical behaviour of selected heavy metals in the co-presence of biochar and low-molecular-weight organic acids. The results showed that except for Cd under citric acid treatment where the added biochar did not have effects on inhibiting the release of soil-borne Cd, the biochar could partially impede LMWOA-driven mobilization of soil-borne Cd, Cu and Pb. The reduction of LMWOA-driven mobilization of the heavy metals was in the range of 17–31% for Cd in malic acid treatment, 22–65% for Cd in the oxalic acid treatment, 11–36% for Cu in the citric acid treatment, 21–39% for Cu in the malic acid treatment, and 0–19% for Cu in the oxalic acid treatment. Biochar was only effective for Pb in citric acid treatment with a reduction rate of about 16%. Under the set experimental conditions, the soil pH remained <5 after biochar addition except for Cd in malic and oxalic acid treatments. This caused protonation of biochar surfaces and consequently disabled the biochar or reduced the capacity of biochar to adsorb cationic metals. It is likely that the liming effect of biochar played a more important role in the inhibition of LMWOA-driven mobilization of the heavy metals. The findings obtained from this study have implications for optimizing remediation methods for reducing uptake of soil-borne heavy metals by vegetables using biochar as a soil conditioner. However, the cost-effectiveness of using a high biochar application rate to achieve the remediation goal needs to be evaluated before a decision is made.