Simultaneous removal of nitrate and hexavalent chromium in groundwater using indigenous microorganisms enhanced by emulsified vegetable oil: Interactions and remediation threshold values

Highlights: • Emulsified vegetable oil was used for simultaneous remediation of NO₃^- and Cr(VI). • NO₃^- promoted the rate of Cr(VI) removal while Cr(VI) inhibited NO₃^- reduction. • Threshold for NO₃^- and Cr(VI) single pollution was 1600 and 10 mg/L, respectively. • The dose of emulsified vegetable oil played an important role for the thresholds. • The maximum thresholds for NO₃^- and Cr(VI) combined pollution were predicted. Combined pollution in groundwater has become increasingly serious. Adding emulsified vegetable oil to an aquifer is an effective method to remediate multiple pollutants. However, the efficiency and threshold values for the remediation of groundwater contaminated by both nitrate and hexavalent chromium (Cr(VI)) stimulated by emulsified vegetable oil remain unclear. In this study, emulsified vegetable oil was used for the first time to simultaneously remediate nitrate and Cr(VI) in groundwater. The results suggested that the addition of emulsified vegetable oil could effectively remediate nitrate and Cr(VI), and there were interplay effects between nitrate and Cr(VI). Nitrate promoted Cr(VI) removal, while Cr(VI) inhibited nitrate reduction. The remediation thresholds for nitrate and Cr(VI) alone were 1600 mg/L and 10 mg/L, respectively (emulsified vegetable oil = 7 g/L). For combined pollution, the remediation threshold values were 868.10 mg/L for nitrate and 12.43 mg/L for Cr(VI) (emulsified vegetable oil = 7 g/L). The dose of emulsified vegetable oil played an important role in the threshold value. When the concentration of emulsified vegetable oil was 10.8 g/L, the maximum threshold values were 1379.79 mg/L for nitrate and 12.67 mg/L for Cr(VI). When the pollutant concentration was below the threshold value, the contaminant could be completely removed.