Elucidating the redox-driven dynamic interactions between arsenic and iron-impregnated biochar in a paddy soil using geochemical and spectroscopic techniques

Iron (Fe)-modified biochar, a renewable amendment that synthetizes the functions of biochar and Fe materials, demonstrates a potential to remediate arsenic (As)-contaminated soils. However, the effectiveness of Fe-based biochar to immobilize As in paddy soils under varying redox conditions (Eh) has not been quantified. We tested the capability of the raw (RBC) and Fe-impregnated (FeBC) biochars to immobilize As in a paddy soil under various Eh conditions (from -400 to +300 mV) using a biogeochemical microcosm system. In the control, As was mobilized (686.2–1535.8 μg L - 1 ) under reducing conditions and immobilized (61.6–71.1 μg L - 1 ) under oxidizing conditions. Application of FeBC immobilized As at Eh < 0 mV by 32.6%–81.1%, compared to the control, because of the transformation of As-bound Fe (hydro)oxides (e.g., ferrihydrite) and the formation of complexes (e.g., ternary As-Fe-DOC). Application of RBC immobilized As at Eh < -100 mV by 16.0%–41.3%, compared to the control, due to its porous structure and oxygen-containing functional groups. Mobilized As at Eh > +200 mV was caused by the increase of pH after RBC application. Amendment of the Fe-modified biochar can be a suitable approach for alleviating the environmental risk of As under reducing conditions in paddy soils. • Iron modification process enhanced the efficacy of biochar for As immobilization. • Raw biochar stabilized As due to its porous structure and oxygen functional groups. • Iron-impregnated biochar immobilized As under reduced conditions. • Redox-induced transformation of iron compounds dominated As (im)mobilization. • Biochar designed by iron is a promising way for mitigating As toxicity in soils.