Dry-wet and freeze-thaw aging activate endogenous copper and cadmium in biochar

Abstract

Abstract While biochar is useful for environmental remediation, the effects of aging on the activation of endogenous trace metals in biochar remain unclear. In this study, three biochars with different concentrations of copper (Cu) and cadmium (Cd) were produced from the straws of Pennisetum sinese grown in moderately-polluted (MB), highly-polluted (HB), and clean (CB) soils. The effects of aging on biochar properties and bioavailability of endogenous Cu and Cd were investigated by Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Boehm titration, and chemical extraction before and after dry-wet (DW) and freeze-thaw (FT) aging. DW and FT aging increased the specific surface areas and abundance of oxygen-containing functional groups in the biochars. Aging decreased the biochar pH, degree of graphitization, and stability. The leachability of Cu and Cd were higher after FT aging than after DW aging, and the bioavailability of Cu and Cd were higher in MB and HB than those in CB. Aging increased the bioavailability of Cu in all biochars, whereas it only increased the bioavailability of Cd in MB. Aging increased the mobility of Cu and Cd by 2.31–12.2 and 0.22–1.31 times. The trends of the mobility and leachability of Cu were not consistent with those of total Cu in the biochars, while mobility and leachability of Cd and total Cd had the same trends. The activation of endogenous Cu was primary associated with the compositions of organic functional groups in the biochars. However, Cd activation was mostly related to the changes in the inorganic components and pH of the biochars. This study implies that aging can activate endogenous metals in biochars, and the potential environmental risk of a biochar should be carefully evaluated to ensure its safe and sustainable application.