Simultaneous removal of Cr(VI), Cd(II) and As(III) in groundwater by ZVI-biochar based composite: Synergy, performance and mechanism

Abstract

The efficient remediation of multi-heavy metals contaminated groundwater by permeable reactive barrier (PRB) is a global concern due to its importance for human health and ecosystems. However, the development of PRB materials that are high efficiency, low cost and environmentally friendly is still limited. In this paper, a novel composite material consisting of ZVI/biochar/scoria was developed for simultaneous removal of Cr(VI), Cd(II) and As(III) from groundwater, and the batch and column experiments were conducted to assess the synergistic effect, removal efficiency, adsorption mechanism of the composite material. The results showed that when the optimum ratio of ZVI/biochar/scoria was 4:2:1, the simultaneous removal rate of Cr(VI), Cd(II), and As(III) reached 99.25 %, 98.97 % and 95.18 %, respectively. The adsorption of heavy metal ions by ZVI-biochar based composite displayed competitive and selective behaviour in a descending order of Cd(II)>Cr(VI)>As(III). After 44 days of continuous operation, the column adsorption experiments achieved a removal rate of 99.95 %, 90.76 % and 86.72 % for Cr(VI), Cd(Ⅱ) and As(Ⅲ). The composite material exhibited broad pH adaptability (pH 5–9) and resistance to common ions (Cl⁻, SO₄²⁻, Fe^3 +, Mn²⁺, Na⁺ and K⁺), though HCO₃^⁻ significantly inhibited Cr/As removal. The characterization results revealed that the removal mechanisms of the composite for three heavy metals were dominated by reduction and precipitation (Cr(VI)), electrostatic attraction and surface complexation (Cd(II)), and oxidation and co-precipitation (As(III)), respectively. The simultaneous removal of Cr(VI), Cd(II) and As(III) was significantly enhanced by the synergistic effect of Fe-C microelectrolysis, biochar functional groups and scoria adsorption.