The overlooked role of carbonaceous supports in enhancing arsenite oxidation and removal by nZVI: Surface area versus electrochemical property

Carbonaceous supports (e.g. biochar) have been widely applied to enhance the performance of nanoscale zerovalent iron (nZVI) for removing heavy metals (e.g. arsenic; As) due to their large surface areas (SSA) to alleviate nZVI particles aggregation, while their electrochemical properties are largely overlooked. Herein, the role of these two characteristics in enhancing As(III) removal has been systematically investigated through designing two parallel experiments to identify the effects of the two characteristics of biochar. Our results suggest that biochar with varying surface areas (33.38-470.36 m(2)/g) supported nZVI exhibits a substantially higher the As (III) oxidation rate (29.7-34.3%) and removal [295.5-371.5 mg/(g.nZVI)] efficiency compared to that by nZVI [27.1%; 192.7 mg/(g.nZVI)]. Despite the importance of SSA, the biochar with extremely low SSA of 33.38 m(2)/g appears to have comparable capacity of enhancing nZVI's performance with a nZVI loading up to 50%. On the contrary, electrochemical analysis suggests that the oxidation and removal of As(III) are highly related to the electron accepting capacity of biochar, indicating a strong electrochemical effect of biochar in As(III) removal varying with pyrolysis temperature. Surface quinone moieties are found responsible for enhancing electron transfer between biochar [up to 0.25 mmol e.(g biochar)(-1)] and nZVI, promoting nZVI corrosion and the generation of reactive oxygen species (ROS; mainly (A) over cap center dot O-2(-)) for enhancing the As(III) oxidation and removal. These findings highlight the important role of electrochemical properties of carbonaceous supports rather than SSA in developing nZVI-based materials for further improving heavy metals removal.