Insight Mechanism of Humic Acids on Cr(VI) Detoxification and Methane Production Improvement during Anaerobic Digestion of Leachate

Humic acids (HAs) produced during the landfill on Cr(VI) fate and their combined impact on the anaerobic digestion (AD) process has not yet been fully elucidated. This study systematically evaluated the interactions and mechanism of HAs on the AD of Cr(VI)-contaminated leachate. Results indicated that high concentrations of HAs (i.e., 200, 500, 1000 mg/L) detoxified the Cr(VI) on AD and the maximum methane production of 1011.63 ± 248.01 mL/g COD was obtained at 500 mg/L HAs, representing an 18.4% increase compared to the control group. Mechanistic studies demonstrated that high concentrations of HAs (i.e., C4-C5) detoxified the Cr(VI) on AD via adsorption and reduction, and the enhanced direct interspecies electron transfer (DIET) played an important role in methane production. The adsorption capacity (32.89 mg Cr(VI)/g HAs) was significantly higher than the granular sludge (3.29 mg Cr(VI)/g TS) and adsorbed Cr(VI) was reduced to less toxic Cr(III) due to the presence of functional groups of HAs (e.g., hydroxyl and phenolic groups), achieving detoxification of Cr(VI) on AD. The increase of viable microbial cells proved the detoxification of high HAs concentrations. Moreover, the increase in the abundance of Syntrophomonas and Methanosaeta has led to the higher methane production at high HAs concentration.