Sequential reduction-oxidation degradation of alkaline nitrophenol wastewater by a novel Al⁰/O₂ system

Alkaline nitrophenol (NP) wastewater is highly toxic and resistant to direct oxidation. Although reductive treatment can enhance NP degradability, the alkaline environment complicates remediation, especially for zero-valent iron (Fe⁰) that readily undergoes passivation. To address these challenges, this study developed a sequential reduction-oxidation system employing carbon nanotube-modified micron-sized zero-valent aluminum (CNTs@mAl⁰) to activate O₂ for p-nitrophenol (PNP) degradation under alkaline conditions. In the reduction stage, CNTs@mAl⁰ achieved complete conversion of PNP to p-aminophenol (PAP). Upon subsequent O₂ sparging, the generated PAP underwent further oxidation and mineralization by •OH and ¹O₂ derived from O₂ activation, achieving > 99.9% overall PNP removal and 66.1% TOC removal. Electrochemical analyses and material characterizations (SEM-EDS and XPS) revealed that CNTs not only act as conductive bridges to facilitate electron transfer in mAl⁰, but also facilitate O₂ diffusion and activation through their high specific surface area, hydrophobic porous structure, and π*-electron-rich carbon framework, boosting reactive oxygen species (ROS) generation and enhancing PAP mineralization. Notably, this method effectively removed 98.3% of diazodinitrophenol (DDNP) from real alkaline NP explosive wastewater, while also removing 49.5% TOC, confirming its practical efficacy. Overall, this study offers a promising strategy for the treatment of alkaline wastewater contaminated with refractory nitroaromatic pollutants.