Iron-modified biochars and their aging reduce soil cadmium mobility and inhibit rice cadmium uptake by promoting soil iron redox cycling.

Iron (Fe) modified biochar has been widely used for cadmium (Cd) contaminated soil remediation. However, the accompanying anions introduced during the modification process potentially affect the behavior of Cd in soil. In this study, we investigated the distinct Cd immobilization mechanisms by Fe(SO) modified biochar (FSBC) and Fe(NO) modified biochar (FNBC) in a two-year pot experiment. Results showed that both FSBC and FNBC significantly reduced Cd concentrations in rice grains by 23%-42% and 30%-37% compared to pristine biochar (BC). Specifically, NFBC promoted the formation of amorphous Fe oxides by enhancing the NO-reducing Fe(II) oxidation process, which significantly increased Fe/Mn oxide-bound Cd and decreased soil CaCl-extractable Cd. For FSBC, the introduction of SO significantly promoted the formation of Fe plaques by enhancing the Fe(III) reduction process, which blocked the Cd transfer from the soil to the rice roots. More importantly, after two years of biochar application, an organo-mineral complex layer is formed on the biochar surface, which immobilized a large amount of Cd. The Cd immobilization on the surface of aged biochar could be due to the fixation by the secondary Fe oxides within the organo-mineral layer and the complexation by the surface functional groups. The result of laser ablation inductively coupled plasma mass spectrometry showed that the Cd content on aged FNBC and FSBC was 5.9 and 2.6 times higher than on aged BC. This might be attributed to the Fe-modified biochar's higher electron exchange capability (EEC), which promoted the development of organo-mineral complexes. Notably, the EEC of biochar was maintained during its aging process, which may keep the biochar surface active and facilitate continual Cd immobilization. This study revealed the complex mechanisms of soil Cd immobilization with Fe-modified biochar, providing new insights into sustainable biochar environmental remediation.