Insights into the differential mechanisms of copper immobilization at the interface of iron/aluminum oxides with fulvic acid

Iron/aluminum (Fe/Al) oxides and fulvic acid (FA) are omnipresent in nature, and their interaction determines the migration and immobilization of copper (Cu). In this study, we provide new insights into the understanding of the differential mechanisms for Cu(II) immobilization at the interaction interfaces of Fe/Al oxides with FA. The immobilization behaviors and mechanisms of Cu(II) at the Fe/Al oxides-FA interaction interfaces were elucidated under different coexistence levels of 3 wt% and 5 wt% FA, and at solution pH of 4.0 −6.0 based on batch adsorption experiments, liquid-phase Cu(II) species assessment by Cu ion-selective electrodes, the surface charge analysis by zeta potential measurement and potentiometric titration, attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS). The results showed that the Cu(II) adsorption capacity at the Fe/Al oxide-FA interaction interfaces followed: amorphous (Am)·Fe > goethite > Am·Al > gibbsite. Fe oxides showed greater Cu(II) immobilization capacity and affinity than Al oxides. The concentration of Cu(II) complexed with FA in the liquid phase followed: goethite > Am·Fe and gibbsite > Am·Al, which showed that the strong adsorption of FA by the oxides resulted in a lower concentration of Cu(II) complexed with FA in the liquid phase (FA adsorption followed: Am·Fe > Am·Al > goethite > gibbsite). The free Cu 2 + decreased with the increase of FA coexistence. BET and surface charge analyses showed that the larger specific surface area, less positive charge and higher binding sites and higher FA adsorption capacity of Am·Fe and goethite contributed to their greater Cu(II) immobilization capacity. ATR-FTIR and XPS analyses revealed the abundance of surface reactive hydroxyl and oxygen vacancy defects of Am·Fe and goethite significantly promoted their Cu(II) immobilization compared with Al oxides. Principal component analysis revealed that surface reactive hydroxyl and oxygen vacancy defects dominated the differences in surface charge and Cu(II) immobilization, and quantified their contributions.