Redox-reactive contaminant removal using biochar-coated metals: The role of electrochemical properties

Abstract

The reductive removal of redox-reactive contaminants, including 2,4-dinitrotoluene, 4-chlorophenol, chromate, and selenate, using biochar-coated metals and the relationship between reduction kinetics and electrochemical properties of biochar-coated metals were examined. To investigate the role of electrochemical properties, one-electron-transfer electrochemical reduction and oxidation mediators were used to determine electron-accepting capacities (EACs) and electron-donating capacities (EDCs) of biochar, zero-valent iron [Fe(0)], FeS, zero-valent zinc [Zn(0)], biochar-coated Fe(0), biochar-coated FeS, and biochar-coated Zn(0). Kinetic analysis showed that biochar coating resulted in decreasing the reductive transformation of contaminants by biochar-coated metals, suggesting that the sorbed molecules were further reduced to reduction products. Biochar, metals, and biochar-coated metal EDCs were an order of magnitude higher than their EACs, ranging between 11.01 and 38.61 mmol e−/g. The linear relationship between the EDCs and kinetic rates revealed that electron transfer from the core metal to sorbed contaminant oxidation is responsible for the reductive removal. A signifcant and irreversible EDC decrease generates electrons for the reductive transformation. To our knowledge, it is the frst attempt to examine the electrochemical properties of metal-biochar composites and their relationship with reduction kinetics of various contaminants. Our results indicate that biochar-coated metals can be used as a sorbent and reductant for redox-reactive contaminants in subsurface environments.