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 sulfadi...