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

Abstract

Hydroxyl radical (OH) is the most powerful reactive oxygen species that plays a critical role in the redox transformation of elements and contaminants. While recent advances have recognized the dark production of hydroxyl radical (OH) from oxygenation of natural waters and subsurface sediments at anoxic-oxic interfaces, much less is known regarding the dark production of OH from river sediments, the important component in environments. In this study, we collected 22 polluted river sediments spanning a large gradient and area along the main stem of Maozhou river watershed of southern China. Using a combination of field measurements, bench-scale experiments, and complementary characterization techniques, we provided robust evidence that the transportation of river sediment from anoxic to oxic conditions triggered the extensive production of OH. The cumulative OH concentrations produced within 48 h across all sediments ranged from 57 to 1479 μmol/kg (or 88–2294 μM in sediment pore water), much higher than those reported previously from subsurface sediments and anoxic waters. Sediment Fe(II) is the main contributor to OH production, although organic carbon may also contribute to a degree. Because river sediments contained abundant Fe(II) and organic carbon, and the anoxic-oxic interfaces are ubiquitous in river sediments, we expect that the extensive dark production of OH from oxygenation of river sediments is widespread and represents a previously under-appreciated source of OH production. Findings of this study provide novel insights for the understanding of carbon cycling and micropollutant fate, which are valuable for designing efficient sediment remediation strategies.