Activation of endogenous cadmium from biochar under simulated acid rain enhances the accumulation risk of lettuce (Lactuca sativa L.).

Biochar has been widely applied to remediate heavy metal-contaminated soils, but the environmental risk of the endogenous pollutants in biochar remains unclear. Two biochars with different endogenous cadmium (Cd) concentrations were prepared from background soil (BCB) and contaminated soil (BCC), respectively. We studied the effects of simulated acid rain (SAR) on the activation mechanism of endogenous Cd in biochar and Cd uptake of Cd by lettuce from the biochar-amended soils. SAR aging significantly increased Cd bioavailability by 27.5 % and 53.9 % in BCB and BCC, respectively. The activation of Cd from biochar may be due to the decrease of biochar pH and persistent free radicals (PFRs) and the increase of specific surface area (SSA) and O-contained functional groups in biochars. Two biochars at dosages of 2 % and 5 % rates did not change soil pore water Cd, but BCB and BCC at 10 % increased pore water Cd by 17.3 % and 219 %, respectively after SAR aging. SAR aging significantly increased the bioavailability of Cd in BCB and BCC treated soils than those before SAR aging. BCB application enhanced the biomass of lettuce (Lactuca sativa L.) and decreased the uptake of Cd. However, BCC addition at 10 % decreased the biomass of lettuce and increased the accumulation of Cd. In summary, endogenous Cd in biochar from contaminated soils has a potential environmental risk to plants and human health and the negative effects of endogenous pollutants from the biochars should be further investigated.