Degradation of methyl orange by zero-valent tungsten without external oxidants

Recently, zero-valent tungsten (W0) has been applied as a new zero-valent metal to degrade pollutants by the activation of H2O2, peroxydisulfate and peroxymonosulfate to generate •OH or SO4•–. However, the direct pollutant degradation by W0 without external oxidants has not been investigated yet. In this work, Methyl Orange was directly degraded by W0 without any external oxidants, even dissolved oxygen. The degradation followed first-order kinetics, and kobs showed an approximately linear correlation with W0 dosage. An acidic condition (pH 2) led to a higher degradation rate (0.162 min-1) and simultaneously a lower metal leaching (2.7 mg/L). SEM and XRD confirmed the unchanged morphology and crystalline structure of W0 after reaction, while the oxide shell was found more stable in the acidic condition, supported by higher W6+ content observed by XPS. Both •OH and O2•– were detected by ESR, but radical scavenging experiments excluded •OH as the main reactive species. By contrast, O2•– and electrons had a higher contribution. The O2•– was dependent on the dissolved oxygen while electrons were not, and thus the degradation occurred efficiently in the anaerobic condition. This work provides opportunities for a greener wastewater treatment process and supplementary understandings of the oxidant-involved processes.