Pulsed oxygen supplementation for toluene biodegradation in groundwater with coexisting nitrate: Kinetics and...

Pulsed air sparging can supplement oxygen to effectively stimulate biodegradation of toluene in groundwater. However, nitrate, commonly coexisting with toluene, can compete with oxygen for electron donors, while this specific influence on toluene degradation kinetics and metabolic pathway is still unclear. In this study, the influence of nitrate on toluene degradation during pulsed oxygen supplementation was investigated at different pulsed levels with the initial headspace concentration of 5% (O5), 10% (O10) and 21% (O21). Results showed that the average first-order toluene biodegradation rate coefficients in O5, O10 and O21 were 0.14, 0.50 and 0.73 h^-1, respectively, suggesting that oxygen greatly enhanced toluene biodegradation. Nitrite was initially accumulated in O10, which was further consumed after toluene supplementation. A numerical model was developed to simulate the degradation kinetics of toluene, demonstrating that the degradation rate coefficient of toluene by oxygen was >10 times higher than nitrate. Additionally, when the supplemented oxygen was insufficient (O5), nitrate and nitrite acted as important electron acceptors. Under such conditions, toluene might be anaerobically oxidized to benzoyl-CoA, which underwent ring-opening reactions by the regulation of badDEFG, bamBC and boxAB. When the supplemented oxygen was sufficient (O10 and O21), toluene might be degraded aerobically into catechol and dihydroxybenzoic acid, which further underwent ring-opening reactions associated with dmpB, catAE, pcaGH, chqB and ligAB. The proposed degradation pathway was supported by the detection of selected intermediates including o-cresol, benzoic acid and hydroxybenzoic acid. These findings provide insights into the toluene degradation pathway and mechanism during pulsed oxygen supplementation.