Citric acid improved photocatalytic performance of iron-enriched sludge hydrochar towards organic Pollutants: Roles of iron species and dissolved organic matter

•Citric acid increased the hydrochar photodegradation for rhodamine B by 3.98 times.•Hydroxyl radical played a pivotal role in the hydrochar photocatalytic system.•Citric acid promoted the electron cycle of iron in hydrochar under radiation.•Fe-O-Si bonds formed in hydrochar enhanced solar radiation absorption.•Citric acid changed the conformation and photoactivity of dissolved hydrochar. The application of hydrochar in the field of photocatalysis is gradually attracting attention, nevertheless, the impact of coexisting substances on the photocatalytic activity of iron-based hydrochar remains insufficiently explored. The study converted iron-enriched sludge into hydrochar and thoroughly investigated the influence of citric acid (CA) on the photocatalytic activity of hydrochar for the degradation of rhodamine B (RhB). The addition of CA significantly enhanced the photocatalytic degradation rate constant (k) of RhB by hydrochar, increasing it from 0.062 ± 0.015 h−1 to 0.309 ± 0.007 h−1. Furthermore, the removal efficiency of RhB also increased from 21.6 ± 1.0 % to 93.6 ± 2.0 %. This enhancement was attributed to the formation of a Fe3+-CA complex within the hydrochar matrix. The complex activated molecular oxygen produced hydroxyl radicals for the effective degradation of RhB. The Fe-O-Si bonds in the hydrochar diminished the excitation energy and increased the solar absorption of the Fe3+-CA complex. The structure of dissolved organic matter (DOM) derived from hydrochar transformed from a dispersed to an aggregated state following the addition of CA, and the triplet state DOM (3DOM*) transferred energy to oxygen under light excitation to generate singlet oxygen. This study underscores that coexisting substances in the environment affect the photocatalytic process of hydrochar, deepening the understanding of the environmental behavior and potential application of hydrochar.