Extensive dark production of hydroxyl radicals from oxygenation of polluted river sediments

•The dark OH was produced from polluted river sediments oxygenation.•The cumulative OH concentrations were 57–1479 μmol/kg.•Oxygenation of river sediments was an important source of OH production.•Sediment Fe(II) was the main contributor to OH production.•Production of OH was important for carbon cycling and micropollutant fate.