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 (•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 •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 •OH during the anoxic–oxic transformation of both abiogenic and biogenic goethite (Gtchem and Gtbio). In anoxic phase, greater Fe(III) bioreduction rate and Fe(II) adsorption ability were obtained, especially with the Gtbio systems in the presence of HA (4.6–46 mg C/L). Thus, when the anoxic conditions were subsequently perturbed by O2, a far higher level of •OH production was observed with the Gtbio/HA systems (47.1–71.3 μM) than that of the Gtchem/HA systems (17.0–29.2 μM). However, coexisting HA led to higher extents of increase in cumulative •OH with Gtchem (40.5%-141.3%) than Gtbio (1.7%-54.0%). Adsorbed Fe(II) exerted a strong impact on •OH generation and contributed to at least 72.8% and 54.4% of cumulative •OH in the Gtchem/HA and Gtbio/HA systems, respectively. During four continuous redox cycles, more •OH would be accumulated with a higher redox fluctuation frequency. Additionally, the generation of •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.