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

Recently, zero-valent tungsten (W-0) has been applied as a new zero-valent metal to degrade pollutants by the activation of H2O2, peroxydisulfate and peroxymonosulfate to generate (OH)-O-center dot or SO4 center dot-. However, the direct pollutant degradation by W-0 without external oxidants has not been investigated yet. In this work, Methyl Orange was directly degraded by W-0 without any external oxidants, even dissolved oxygen. The degradation followed first-order kinetics, and k(obs) showed an approximately linear correlation with W-0 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 W-0 after reaction, while the oxide shell was found more stable in the acidic condition, supported by higher W6+ content observed by XPS. Both (OH)-O-center dot and O-2(center dot)- were detected by ESR, but radical scavenging experiments excluded (OH)-O-center dot as the main reactive species. By contrast, O-2(center dot)- and electrons had a higher contribution. The O-2(center dot)- 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.