Adsorption, desorption, and redox reactions at iron oxide nanoparticle surfaces

Abstract: Iron oxide nanoparticles are involved in several important biogeochemical processes. The interfaces between aqueous solutions and iron oxide nanoparticle surfaces are found everywhere in nature, and the chemical and microbial processes occurring at these complex interfaces control e.g. nutrient and contaminant availability and transport. Recently, it has been shown that certain ectomycorrhizal (ECM) fungi decompose dissolved organic matter (DOM) via non-enzymatic reactions involving the attack by reactive oxygen species (ROS), particularly the hydroxyl radical (•OH) generated via the Fenton reaction (Fe2+ + H2O2 → Fe3+ + •OH + OH-). Previous studies on fungal non-enzymatic DOM decomposition included soluble Fe3+ complexes that were reduced by metabolites to Fe2+ in order to initiate the Fenton reaction. However, the mechanisms of •OH formation in soil environments where solid and low-solubility iron oxide nanoparticles are the predominating iron source, are very much unknown.This thesis focused on the reactions between ferrihydrite or goethite nanoparticles and the model metabolite 2,6- dimethoxyhydroquinone (2,6-DMHQ) or DOM modified by the ECM fungus P. involutus. These reactions were studied at different experimental conditions, primarily varying pH and the O2 level. In order to accomplish the set aims, this PhD project included batch experiments combined with conventional wet-chemical analyses as well as the Simultaneous Infrared and Potentiometric Titration (SIPT) method to monitor the reactions at the water-mineral interfaces in real-time.The overall results showed that iron oxide nanoparticles were reductively dissolved by 2,6-DMHQ at pH 4.0 and 4.5. Under aerobic conditions these reactions produced both Fenton reagents and •OH was generated. The extent of •OH generation was sensitive to the reduction potential (EH) of the iron oxide, the O2 concentration and the competitive adsorption of organic and inorganic anions. The reactions between 2,6-DMHQ and the iron oxides at pH 7.0 and under aerobic conditions generated low amounts of •OH. At these conditions reductive dissolution was of minor importance. Instead, catalytic oxidation of 2,6-DMHQ produced H2O2 that partly was degraded by the iron oxide surfaces into •OH. Co-adsorbed anions further promoted this process,DOM modified by P. involutus increased the reductive dissolution of iron oxides at pH 4.0 as compared to fresh DOM. Reactions between the Fe2+ produced and H2O2 generated •OH that preferentially oxidized some of the DOM components. These results suggested that modified DOM contained secondary metabolites that possibly serve both as iron reducers and antioxidants.This thesis has provided new knowledge on the complex reaction mechanisms between iron oxide nanoparticles and redox-active organic compounds. It has increased the understanding of non-enzymatic •OH generation, and the knowledge obtained will help to understand the role of this process in organic matter decomposition. Finally, the research has identified possible mechanisms behind toxicity of iron oxide nanoparticles.