Tailored design of food waste hydrochar for efficient adsorption and catalytic degradation of refractory organic contaminant

Custom-designed carbonaceous adsorbent/catalyst from ‘green’ sources with desired functionalities under science-informed conditions is indispensable to promote the sustainable industrial wastewater treatment. In this study, we prepared hydrochar by hydrothermal carbonization (HTC) of three types of pre-consumer food waste (i.e., lettuce, taro, and watermelon peel) with different components at various temperatures (i.e., 180–240 °C). The performance of food waste hydrochar was examined through the adsorptive removal and peroxymonosulfate (PMS)-initiated catalytic degradation of a representative, recalcitrant organic contaminant, 2,4-dichlorophenoxy acetic acid (2,4-D). The LHC_180−240 derived from fibre-rich lettuce manifested a substantial 2,4-D adsorption (77.4–88.4 mg g⁻¹) possibly due to intensive partitioning and/or chemisorption, which were dependent on the mesoporous carbon structure with low aromaticity and abundant C–O functional groups. In comparison, HTC of starch-rich taro at a relatively low temperature (200 °C) produced the THC₂₀₀ that displayed a superior catalytic ability (73.5 mg g⁻¹) probably owing to a highly graphitized C domain with low polarity and enriched ketonic (C[dbnd]O) functionality, which might facilitate radical/non-radical PMS activation. Interestingly, the WHC_180−240 produced from watermelon peel with moderate-level carbohydrates and low-fibre content presented an improved structure and functional groups (i.e., C–O and C[dbnd]O), but inhibited the PMS activation for 2,4-D degradation possibly due to interference by its inherent dissolved organic matter (1.13–2.26 wt.%). This study provided insightful guidance for tailoring future design of multifunctional hydrochar adsorbent/catalyst for sustainable remediation.