Contrasting impacts of chemical and physical ageing on hydrochar properties and sorption of norfloxacin with coexisting Cu2+

The conversion of agricultural biomass into hydrochar has enormous potential to improve soil quality. In particular, hydrochar particles introduced into the natural environment readily bind environmental pollutants. The interaction of hydrochar and pollutants will, however, be impacted by long term natural ageing in the earth surface. The adsorption performance and the associated mechanisms that could be affected by physical or chemical ageing are not yet fully understood. To elucidate the influence of different types ageing on the physicochemical properties and sorption capacity of hydrochar, we systematically characterized the elemental composition, specific surface area, total organic carbon, and functional groups of fresh and aged hydrochar. Norfloxacin (NOR), a typical antibiotic as a model in this study, was used for the sorption performance of different aged hydrochars in the presence or absence of Cu2+. The various artificial accelerated ageing methods have been employed such as H2O2 oxidation, HNO3/H2SO4 acidification, high temperature, and freeze-thaw cycles. The results showed that ageing could increase hydrochar polarity and surface functional groups, which both increased NOR hydrophobic partition and H-bonding interaction on hydrochars. The chemical ageing largely increased the abundance of CO than physical ageing. H-bonding dominated NOR sorption on hydrochars after acidification, high temperature, and freeze-thaw cycles. The hydrophobic partition was the main sorption mechanism of NOR on oxidative aged hydrochars. The coexisting Cu2+ inhibited NOR sorption on most aged hydrochars (acidification, high temperature, and freeze-thaw cycles), whereas specially for oxidative ageing, Cu2+ increased hydrophobic sorption sites on hydrochars surface and enhanced the sorption capacity for NOR. The results from this study are likely to reveal the mechanisms of pollutant adsorption on hydrochars and their different susceptibilities under various ageing environment, suggesting us to comprehensively consider the reciprocal effects of natural ageing and coexisting pollutants on a long-term use of hydrochar in the field.