Iron-manganese modified corncob biochar for fluoride removal from groundwater: Insights into adsorption mechanisms

Abstract

Biochar, as an efficient, effective, and potential soil improver, has broad application prospects in the field of defluoridation. This study aimed to evaluate the defluoridation potential of iron (Fe) and manganese (Mn) co-modified biochar from groundwater. The varied Fe/Mn molar ratio (2:1 and 1:2) modified biochar was prepared by corncob with the pyrolysis temperature of 300°C, 400°C, and 500°C. Batch experiments for fluoride (F^‒) removal were performed by corncob biochar before and after Fe–Mn modified. Their composition, structure, and performance were analyzed by multiple characterization techniques to clarify F^‒ removal mechanisms. Our results indicated that unmodified corncob biochar produced at 400 °C (BC400) exhibited the highest F^‒ adsorption efficiency (87.3%) among three unmodified samples, attributable to its largest specific surface area (2.55 m²/g). Notably, F^‒ removal amounts by Fe-Mn modified BC400 were 2 times higher than BC400. The enhanced F^- removal performance of Fe–Mn modified biochar can be attributed to several mechanisms: (1) the modification produced rougher surface textures, resulting in an increased specific surface area (about 3.50 m²/g); (2) newly formed Fe–O and Mn–O bonds on the biochar surface facilitated the formation of complexes with F^‒; and (3) the adsorption results fitted well with pseudo-second-order and Freundlich models (R²>0.98), indicating that the removal process involved physicochemical adsorption. These findings demonstrate that Fe–Mn modified biochar is a highly efficient and cost-effective material for F^‒ remediation and holds significant potential for application in contaminated groundwater and soil systems.