Modeling kinetics of Cu(II) release during Fe oxide transformation under the influence of humic substances

Abstract

The dynamic behavior of Cu(II) in soil is usually affected by Fe oxides and humic substances, both of which are capable of Cu(II) binding. However, Fe oxides may undergo transformation processes and the presence of humic substances may complicate the Fe oxide transformation and Cu(II) binding. There is still a lack of quantitative understanding on how varying Fe oxide compositions and humic substances affect Cu(II) kinetic behavior. In this study, we developed a unified quantitative model for Cu(II) release from Fe oxides or Fe oxide-humic substance composites during the Fe oxide transformation based on the experimental data from our previous studies. The kinetic data were collected in a stirred-flow reactor system, in which Cu(II) release kinetics was studied during the Fe oxide transformation processes. Our model successfully reproduced the temporal changes of effluent Cu(II) concentrations with only one fitting parameter while other parameters were derived from the thermodynamic constraints or from the literatures. The modeling results indicated that Cu(II) release rates decreased with Fe oxide aging processes, while the presence of humic substances increased Cu(II) mobility. Our model was able to quantitatively assess the roles of different adsorbents in controlling the Cu(II) release rates, which highlighted the importance of accounting for the variations of Fe oxide transformation and the impact of humic substances in kinetic modeling. Our model has provided a quantitative tool for quantifying the kinetic behavior of heavy metals in multi-adsorbent systems, which is helpful for predicting the geochemical cycling of heavy metals in the environment.