Effect of Heavy Metal (Zn) on Redox Property of Hydrochar Produced from Lignin, Cellulose, and D-Xylose

To provide fundamental information for environmentally sound utilization of hyperaccumulator, hydrothermal carbonization (HTC) of Zn-loaded lignin, cellulose, and D-xylose was carried out to explore the effect of Zn(II) on chemical structure and the redox property of hydrochar. By batch redox reaction as well as I-2/NaBH4 titration, we quantified the reducing and oxidizing capacities of three kinds of hydrochar. Results indicated that all hydrochar samples are redox-active with a reducing ability that is more extensive than the oxidizing ability. Reducing capacities of hydrochar (by I-2 titration) are in the order of lignin (1.021 mmol e(-)/g) >> cellulose (0.300 mmol e(-)/g) > D-xylose (0.142 mmol e(-)/g). Zn(II) loading increases the reducing capacities of lignin (up to 3.340 mmol e(-)/g) and cellulose (up to 1.089 mmol e(-)/g) hydrochar while it decreases the reducing capacity of D-xylose hydrochar. Redox ability is highly correlated with surface phenolic hydroxyl/benzoquinonyl groups of hydrochar. We propose that Zn(II) affects the redox ability of hydrochar by inhibiting the decomposition of phenolic hydroxyl group during hydrothermal carbonization of biomass.