Production and prediction of hydroxyl radicals in distinct redox-fluctuation zones of the Yellow River Estuary

Hydroxyl radicals (·OH) produced in subsurface sediments play an important role in biogeochemical cycles. One of the major sources of ·OH in sediments is associated with reduced compounds (e.g., iron and organic matter) oxygenation. Moreover, the properties of iron forms and dissolved organic matter (DOM) components varied significantly across redox-fluctuation zones of estuaries. However, the influence of these variations on mechanisms of ·OH production in estuaries remains unexplored. Herein, sediments from riparian zones, wetlands, and rice fields in the Yellow River Estuary were collected to systematically explore the diverse mechanisms of ·OH generation. Rhythmic continuous ·OH production (82–730 μmol/kg) occurred throughout the estuary, demonstrating notable spatial heterogeneity. The amorphous iron form and humic-like DOM components were the key contributors to ·OH accumulation in estuary wetlands and freshwater restoration wetlands, respectively. The crystalline iron form and protein-like DOM components influenced the capabilities of iron reduction and continuous ·OH production. Moreover, the orthogonal partial least squares models outperformed various multivariate models in screening crucial factors and predicting the spatiotemporal production of ·OH. This study provides novel insights into varied mechanisms of ·OH generation within distinct redox-fluctuation zones in estuaries and further elucidates elemental behavior and contaminant fate in estuarine environments. Environmental implication Given that estuaries serve as sinks for anthropogenic pollutants, various organic pollutants (e.g., emerging contaminants such as antibiotics) have been widely detected in estuarine environments. The production of ·OH in sediments has been proven to affect the fate of contaminants. Therefore, the varied mechanisms of ·OH in estuarine environments, dominated by diverse iron forms and DOM components, were explored in this study. MLR and OPLS models exhibited good performance in screening crucial factors and predicting ·OH production. Our work highlights that in estuarine subsurface environments, the presence of ·OH potentially leads to a natural degradation of pollutants.