Aging reduces the bioavailability of copper and cadmium in soil immobilized by biochars with various concentrations of endogenous metals

Biochar is widely used for environmental remediation. However, the effects of aging on the bioavailability of trace metals in biochar-amended soil remain largely unknown, especially for the biochars with various concentrations of endogenous metals. In this study, three biochars marked as BB, MB, and HB were produced from the straws of Pennisetum sinese grown in background soil, moderately-, and highly-polluted soils by trace metals, respectively. We distinguished the effects of dry-wet (DW) and freeze-thaw (FT) aging on the bioavailability of copper (Cu) and cadmium (Cd) from soil particles, the biochar interior, and the surface of biochar. The adsorption capacities of Cu2+ and Cd2+ followed the order of BB > MB > HB. DW and FT aging both increased the adsorption capacity of Cu2+, but decreased that of Cd2+ in the three biochars, resulting in a reduction in Cu bioavailability and in-crease in Cd bioavailability in the biochars after the saturated adsorption of Cu2+ and Cd2+. The incorporation of the three biochars decreased Cu bioavailability compared to the control after incubation for 30d, while the ad-dition of MB increased Cd bioavailability. DW and FT aging decreased Cu bioavailability in biochar-amended soil by decreasing the bioavailability of Cu adsorbed on the biochar surface and immobilized by soil particles. Mean-while, aging decreased Cd bioavailability by decreasing the bioavailability of Cd immobilized by soil particles. Overall, environmental risk would be increased by the application of biochars with high endogenous Cd. The major implications are that biochar dosage and environmental risk should be carefully assessed before large-scale, continuous application, especially for biochars containing high contents of endogenous trace metals. (C) 2021 Published by Elsevier B.V.

Automated summary

The study found that dry-wet and freeze-thaw aging both affect the adsorption capacity of biochar for copper and cadmium, resulting in decreased bioavailability of Cu and increased bioavailability of Cd. Different biochars have different adsorption capacities, leading to varied effects on Cu and Cd. Careful assessment of biochar dosage and environmental risk is necessary before large-scale application.