Combined effects of fulvic acid and phosphate on FeII oxidation and Fe precipitation

The chemical transformation of ferrous Fe (FeII) under aerobic conditions is a key process governing Fe cycling in soil and is strongly influenced by coexisting constituents such as phosphate and organic matter. Transient redox fluctuations result in the simultaneous presence of FeII and O₂. However, the combined effects of phosphate and fulvic acid (FA) on FeII oxidation in the presence of O₂ and Fe precipitation remain poorly understood. In this study, we used kinetic experiments and solid-phase characterizations to unravel the underlying mechanisms. Our results demonstrate that FA impeded both FeII oxidation and Fe precipitation. This inhibition might be attributed to FeII complexation with FA and the subsequent adsorption of FA onto Fe mineral surfaces. When phosphate and FA coexisted, phosphate partially decreased the inhibitory FA effects, which is likely due to the formation of Fe hydroxyphosphate and competitive adsorption between phosphate and FA on Fe mineral surfaces. In the absence of phosphate, goethite was the predominant ferric (FeIII) product. However, phosphate addition favored the formation of lepidocrocite and Fe hydroxyphosphate. Increasing phosphate concentrations led to greater phosphate incorporation into Fe minerals. When both phosphate and FA were present, the crystallinity of lepidocrocite and Fe hydroxyphosphate decreased compared to the sole phosphate system. Overall, this study reveals that FA simultaneously inhibited FeII oxidation and Fe precipitation, and that phosphate mitigated these effects via Fe hydroxyphosphate formation and competitive interactions. These findings provide an insight into how coexisting organic and inorganic ligands regulate Fe redox kinetics and Fe precipitation, and underscore the importance of incorporating such interactions into future studies of Fe cycling under fluctuating redox conditions in soil.