Humic acids promote hydroxyl radical production during transformation of biogenic and abiogenic goethite under redox fluctuation

Different studies have separately documented the production of hydroxyl radicals (center dot OH) upon the oxygenation of reduced iron (oxyhydr)oxides (Fe oxides) or humic acids (HAs). While HA generally coexists with Fe oxides and can alter the kinetics of Fe oxide transformation in anoxic environments, little was known about center dot OH production with coexisting HA and Fe oxide during anoxic-oxic transition. In this study, it was found for the first time that HA could improve the generation of center dot OH during the anoxic-oxic transformation of both abiogenic and biogenic goethite (Gt(chem) and Gt(bio)). In anoxic phase, greater Fe(III) bioreduction rate and Fe(II) adsorption ability were obtained, especially with the Gt(bio) systems in the presence of HA (4.6-46 mg C/L). Thus, when the anoxic conditions were subsequently perturbed by O-2, a far higher level of center dot OH production was observed with the Gt(bio)/HA systems (47.1-71.3 mu M) than that of the Gt(chem)/HA systems (17.0-29.2 mu M). However, coexisting HA led to higher extents of increase in cumulative center dot OH with Gt(chem) (40.5%-141.3%) than Gt(bio) (1.7%-54.0%). Adsorbed Fe (II) exerted a strong impact on center dot OH generation and contributed to at least 72.8% and 54.4% of cumulative center dot OH in the Gt(chem)/HA and Gt(bio)/HA systems, respectively. During four continuous redox cycles, more center dot OH would be accumulated with a higher redox fluctuation frequency. Additionally, the generation of center dot OH decreased the average molecular weight, bleached the chromophores and increased the oxygen-containing functional group contents of HA. This study provides new insights into biogeochemical cycles of Fe/C and potentials for in-situ remediation at anoxic-oxic interfaces.

Automated summary

This study shows that the presence of humic acids can enhance the generation of hydroxyl radicals during the anoxic-oxic transition, particularly in biogenic goethite systems. Adsorbed Fe(II) plays a significant role in the generation of hydroxyl radicals. Additionally, the generation of hydroxyl radicals can lead to changes in the molecular weight and composition of humic acids, providing insights into biogeochemical cycles and in-situ remediation at anoxic-oxic interfaces.