Intrinsic activity of organic acids controlling photochemical behavior and transformation of schwertmannite in acid mine drainage

Occurrence and transformation of schwertmannite (Sch) widely influence the speciation and distributions of iron and sulfur as well as pollutants in acid mine drainage (AMD)-impacted ecosystems. Despite extensive research on the biogeochemical fate of Sch in terrestrial systems, the mechanisms underlying its phase transformation mediated by redox processes in acidic waters remain inadequately understood. This study investigates how the intrinsic activity of naturally abundant organic acids in AMD-impacted waters affects the photochemical behavior and transformation of Sch under oxic and anoxic conditions through comprehensive characterizations. Solid product characterization results showed that the addition of oxalic acid (OA) and tartaric acid (TA), rather than formic acid (FA), significantly accelerated photochemical transformation of Sch into goethite (Gt) and magnetite (Mt), increasing by 29%–−47% and 35% under anoxic condition, respectively. Comparison analyses suggested the photoactivated interfacial electron transfer could be accelerated by the organic acids with the stronger complexing and electron-donating abilities, further enhancing photoreductive dissolution of structural Fe(III) to initiate Fe(II)-catalyzed transformation of Sch. Such transformation pathway of Sch was inhibited due to oxygenation of Fe(II) and only 21% Gt newly formed in the Sch/TA system under oxic conditions. It is further found that carbon-centered radicals (CCR•), derived from organic acids containing electron-withdrawing groups with lower dissociation enthalpy, efficiently protected Fe(II) from oxygenation by competing with oxidants, thus enhancing Sch transformation. The study provides new insights into the expanded transformation pathways of Sch, advancing the understanding of iron cycling and reactive species production in the euphotic zone of acidic waters.