The impacts of aging on the mobility of cadmium and free radicals from biochars

Biochar is widely used for environmental remediation, but the impacts of aging on the mobility of endogenous pollutants from biochars remains unclear. This study investigated the cadmium (Cd) mobility in biochars produced from two rice straws (BJG, BJN), a sludge (BSL), and a sediment (BSE) before and after aging using toxicity characteristic leaching procedure (TCLP) and sequential extraction. The free radicals and physicochemical properties of biochars were analyzed by electron paramagnetic resonance spectrometer, XRD, FTIR, and XPS. The total Cd and the release ratio of Cd in the four biochars ranged from 481 µg·kg^-1 to 3520 µg·kg^-1 and from 6.17 % to 31.6 %, respectively. Aging increased surface areas by 32.3 % and 20.5 % in the straw biochars of BJN and BSL and reduced surface crystal structure for the four biochars. Aging reduced Si-O-Si and CC/CO bonds for BJG and BJN, but increased oxygen-containing groups for all biochars. Aging decreased the pH by 0.27-0.38 units in BJG, BJN, and BSL, and reduced TCLP-extractable Cd by 30.1 %-49.6 % for all biochars. The acid soluble Cd concentrations were decreased by 16.4 %-56.5 % after aging, showing higher percentages in BJG and BJN than in BSL and BSE. Free radicals were increased by 0.43-1.34 times after aging, showing were two orders of magnitude higher in BJN and BJG than BSL and BSE. This study revealed that aging decreased the Cd mobility but increased free radicals for all biochars, and straw biochars pose higher soil environmental risks than those from sludge and sediment.