A Cr(VI)-free strategy through rapid-adsorption-and-strong-reduction in efficiently oxidative removal of Cr(III)-complexes by carbon-modified Al⁰/O₂ system

The removal of hazardous Cr(III) complexes via advanced oxidation processes can generate highly toxic Cr(VI), posing significant environmental risks. Therefore, there is an urgent need to develop alternative methods that completely suppress the generation of Cr(VI). In this study, with no detectable Cr(VI) in the solution (below the detection limit of 0.004 mg/L), the carbon nanotubes-modified micron-sized zero-valent aluminum (CNTs@mAl⁰) can effectively remove 99.50% of Cr(III)-EDTA and 83.34% of TOC under the typical acidic conditions of industrial chromium wastewater (pH = 3.00) by activating molecular oxygen. Then the following alkaline precipitation can easily remove the decomplexed free chromium, lowering total chromium concentration to 0.11 mg/L, which meets most national and regional standards, whereas direct alkali precipitation fails to meet them. Serving as a strong electron reservoir (E⁰ = - 1.662 V), micron-sized zero-valent aluminum (mAl⁰) synergizes with highly conductive carbon nanotubes (CNTs) to effectively activate O₂, facilitating the production of reactive oxygen species (primarily ¹O₂ and •OH) that drive the oxidative decomplexation of Cr(III)-EDTA. Furthermore, the organic ligands are efficiently mineralized, thus preventing Cr(III)'s re-complexation. More interestingly, once the Cr(VI) is possibly generated, it would be rapidly absorbed by CNTs and be immediately reduced by mAl⁰ with strong reducing power, thereby completely suppressing the generation of any Cr(VI) in bulk solution. In conclusion, this study presents an environmentally friendly and straightforward treatment system for Cr(III)-complexes, offering an effective solution to mitigate the environmental risks posed by chromium, particularly Cr(VI), in wastewater.