Low-grade magnesium alloy scraps: An efficient and cost-effective reducing agent for the removal of Cr(VI) as a model contaminant

The lack of feasible technologies to recycle low-grade Mg alloy scrap (LGS) has led to its accumulation in landfills and manufacturing/recycling plants. To address this issue, we examined the feasibility of harnessing LGS as a powerful and cost-efficient reductant for hexavalent chromium (Cr(VI)) using AZ91 alloy scrap (Mg-AZS) as a representative LGS. After a preliminary wash with acetone to remove the lubricant oil retained in the scraps, Mg-AZS achieved 96.3 % Cr(VI) removal efficiency ([Cr(VI)]₀ = 19.4 μM) within 48 h. The surface-area-normalized reactivity of Mg-AZS was at least comparable to, or even higher than, that of pure Mg(0), suggesting its potential as a cost-effective alternative. This enhanced reducing power was attributed to the composition of Mg-AZS, which inherently contains trace metal impurities (e.g., 0.24 wt%Fe and 0.12 wt%Cu), making it an intrinsic mixed-metal catalyst. Upon sonication before use, its reactivity increased 3-fold, achieving 97.5 % Cr(VI) removal within 6 h due to the effective removal of residual oil. Additionally, sonication alleviated the passivation of Mg-AZS caused by the surface precipitation of Cr(OH)₃, enabling continuous reuse for at least 6 cycles. Finally, Mg-AZS achieved 100 % Cr(VI) removal in contaminated groundwater ([Mg(0)]₀ = 0.25 g/L, [Cr(VI)₀ = 500 μM]) within 40 min. These results demonstrate the potential of LGS for treating Cr(VI)-contaminated groundwater and reducing Mg waste.