Simultaneous removal of Cd(II) and As(III) by graphene-like biochar-supported zero-valent iron from irrigation waters under aerobic conditions: Synergistic effects and mechanisms

Abstract

Irrigation water is commonly contaminated with cadmium and arsenic near mining regions, which significantly contributes to excessive heavy metals in rice grains. Herein, we have developed a novel graphene-like biochar (GB)-supported nanoscale zero-valent iron (nZVI) and the underlying mechanisms of synergistic effects between GB and nZVI for the simultaneous removal of Cd(II) and As(III) under aerobic conditions. The results show that GB/nZVI has a high removal capacity of 363 mg/g (nZVI) for As(III) at pH 4 and 92.8 mg/g (nZVI) for Cd(II) at pH 7. These values are significantly higher than GB and nZVI (1.7 times for Cd(II); 1.4 times for As(III)) alone, suggesting strong synergistic effects between GB and nZVI. GB promotes nZVI oxidation to form iron oxyhydroxides and causing 35 % of As(III) converting to As(V). Importantly, As(III) significantly enhance Cd(II) removal by GB/nZVI (i.e., 131.8 mg/g as nZVI). Coexisting ions such as phosphate and humic acid have a stronger inhibitory effect on the simultaneous removal of Cd(II) and As(III). Our results indicate that oxidation and surface complexation are the dominant mechanisms and electrostatic binding exists for As(III) removal, while surface complexation predominates for Cd(II) removal. These findings provide insight into developing an effective solution for removing Cd(II)/As(III) from irrigation waters.

Keywords: Adsorption; Arsenate; Heavy metals; Oxidation; Water treatment.