Kinetics of Ni reaction with organic matter-ferrihydrite composites: Experiments and modeling

The interactions between organic matter (OM) and minerals are among the most important environmental processes, which significantly affect the fate of carbon and contaminants in the environment. How the interactions between OM and ferrihydrite (Fh), one of the most reactive iron oxides, affect the kinetic reactions of heavy metals with both OM and iron oxides, however, are still largely unknown. In this study, two typical OM, humic acids (HA) and fulvic acids (FA), were used to synthesize OM-Fh co-precipitation composites. We investigated the interactions between OM and ferrihydrite and their impact on kinetic reactions of nickel (Ni) with OM-Fh composites using spherical aberration corrected scanning transmission electron microscopy (Cs-STEM), batch and kinetic experiments, and X-ray adsorption spectroscopy (XAS). Cs-STEM results, at both nano and subnano scales, showed three typical distributions of OM on ferrihydrite, adsorbed on ferrihydrite surfaces, accumulated as OM nanoparticles, and being blocked in tiny pores in composites. Batch experiments and modeling quantified the effects of OM and ferrihydrite interactions, which was supported by XAS results. The kinetics model developed in this study integrated both WHAM 7 and CD-MUSIC models and successfully predicted the kinetics of Ni adsorption and desorption on OM-Fh composites under the impact of OM-Fh interactions. The model can quantitatively assess the dynamic changes of Ni speciation on different OM and Fh binding sites during the kinetic processes. Our results contribute to developing a comprehensive model for predicting the dynamic metal behavior in the environment when multiple adsorbents are present.