Enhanced Organic Matter Sequestration and Phosphate Adsorption by Fe Oxides Generated from Abiotic Fe(II) Oxidation in the Presence of Heavy Metals

Fe(II) oxidation-precipitation affects the fate of organic matter (OM) and nutrients in soils/sediments, but the effects of commonly coexisting heavy metals (HMs) remain understudied. Herein, we investigated the individual and combined effects of HMs and OM on Fe(II) oxidation-precipitation at neutral pH, along with the associated HM and organic matter sequestration, as well as phosphorus adsorption by the resultant Fe oxides. Various HMs exerted different influences on Fe(II) oxidation, which were attributed to their intrinsic properties, such as hydrolysis constant, ionic charge, and radius. Specifically, Cu(II) or Cr(III) accelerated Fe(II) oxidation and facilitated the formation of poorly crystalline Fe oxides, with the rate of acceleration depending on their concentrations, whereas Mn(II), Co(II), Ni(II), or Zn(II) showed negligible effects. Meanwhile, the sequestration efficiency of HMs followed the trend of Cr(III) > Zn(II) > Cu(II) > Co(II) > Ni(II) > Mn(II). Either Cu(II) or Cr(III) enhanced organic matter sequestration via promoting aggregation of Fe-OM associations. Fe oxides generated in the presence of Cu(II), Cr(III), and/or OM exhibited elevated phosphate adsorption due to their small particle sizes. Our results highlight the previously underappreciated importance of HMs on Fe(II) oxidation and their potential effects on carbon and phosphate retention, which should be considered in predictive models of organic matter and phosphate sequestration.