Efficiency of KOH-modified rice straw-derived biochar for reducing cadmium mobility, bioaccessibility and bioavailability risk index in red soil

Cadmium (Cd) pollution in agricultural soils has exerted a serious threat due to continuous application of pesticides, fertilizers, and wastewater irrigation. The present study aimed to test the efficiency of KOH-modified and non-modified rice straw-derived biochar (KBC and BC, respectively) for reducing Cd solubility and bioavailability in Cd-contaminated soil. Cadmium-contaminated soil was incubated for 60 d with 15 and 30 g kg(-1) BC and KBC. At the end of incubation, Cd mobility was estimated by the European Community Bureau of Reference sequential extraction and toxicity characteristic leaching procedure (TCLP), while bioavailability was determined using 1 mol L-1 NH4NO3 extraction. The bioavailability risk index and bioaccessibility, assessed by a simple bioaccessibility extraction test, of Cd were used to examine the potential effects of Cd on living organisms. The results indicated that application of both KBC and BC significantly increased soil pH, cation exchange capacity, nutrients, and organic carbon. The soluble fraction of Cd was significantly decreased by 30.3% and 27.4%, respectively, with the addition of KBC and BC at 30 g kg(-1) compared to the control (without biochar addition). Similarly, the bioaccessible Cd was significantly decreased by 32.4% and 25.2%, respectively, with the addition of KBC and BC at 30 g kg(-1) compared to the control. In addition, both KBC and BC significantly reduced Cd leaching in the TCLP and NH4NO3-extractable Cd in the amended soil compared to the control. The reduction in Cd solubility and bioaccessibility by KBC and BC may be due to significant increases in soil pH and surface complexation. Overall, KBC at an application rate of 30 g kg(-1) demonstrated positive results as soil amendment for Cd immobilization, and reduced bioaccessible Cd in contaminated soil.