Photochemistry of Hydrochar: Reactive Oxygen Species Generation and Sulfadimidine Degradation

Biochar, mainly including pyrochar produced via pyrolysis of biomass at moderate temperatures of 350-700 °C and hydrochar formed by hydrothermal carbonization in a range of 150-350 °C, has received increasing attention because of its significant environmental impacts. It is known that pyrochar can generate reactive oxygen species even in the dark owing to the presence of persistent free radicals, but hydrochar is far less studied. In this study, we systematically investigate the photochemistry of hydrochar and check its effects on the sulfadimidine degradation. Different from pyrochar derived from the same biomass, hydrochar could generate much more H2O2 and •OH under daylight irradiation, which could enhance the sulfadimidine degradation rate six times more than that found in the dark. Raman spectroscopy, Fourier transform infrared spectroscopy, electron paramagnetic resonance, and X-ray photoelectron spectroscopy were employed to elucidate this interesting phenomenon. Characterization results revealed that the higher reactive oxygen species generation ability of hydrochar under solar light irradiation was attributed to its abundant photoactive surface oxygenated functional groups. This study clarifies the differences of pyrochar and hydrochar on organic pollutant degradation, and also sheds light on environmental effects of hydrochar.