Variations in redox properties of biochar and humic acid induced by interactive molecular exchange

Abstract

Biochar and humic substances are prevalent redox pools in the environment, which is critical to electron transfer in geochemical cycles and pollution control processes. The dissolution of biochar and the sorption of humic substances on biochar have the potential to redistribute redox substances, consequently altering the redox properties of solid and liquid phases. However, studies have often focused on either sorption or dissolution separately, overlooking the mutual effects and not involving redox properties. Herein, molecular interactions between biochar and humic acid (HA) and variations in their redox properties were elucidated using UPLC Orbitrap MS and mediated electrochemical measurement. The dissolution of biochar was far greater than the sorption of HA constituents, and HA enhanced the dissolution of biochar through molecular exchange. But preferential sorption of oxygenated aromatics to pine char (pi500) mainly by hydrogen bonding and higher saturated molecules to starch char (st700) mainly by hydrophobic interactions caused the oxygenated functional groups on biochar to increase (1 ~ 1.5 times). Thus the Electron exchange capacities (EEC) of pi500 and st700 turned to 1 ~ 3 times, and the EEC of HA decreased ca. 50%. Absorption (partition) caused more sorption of HA constituents to pi500 than to st700, while larger surface area resulted in higher EEC of st700 with sorbed HA. The enrichment of redox constituents on biochar is promising for its long-term use in waste reclamation and pollution control. The findings can aid in the understanding of variations in redox properties under interactions between pyrolytic and natural organic matter. Graphical Abstract: (Figure presented.)