Hexavalent chromium reduction by atmospheric plasma bubbles

The effective removal of carcinogenic and mutagenic hexavalent chromium [Cr(VI)] from contaminated water is of key environmental and societal importance. However, it is challenging due to its recalcitrance and lack of treatment technologies. Non-thermal plasma (NTP) is gaining increasing attention as an alternative to conventional chemical methods for wastewater remediation. However, there is a lack of understanding regarding the mechanisms of action of plasma-liquid interactions, particularly for heavy metal remediation. In this study, an atmospheric plasma bubble (APB) reactor that combines two types of plasma discharge, namely a dielectric barrier discharge (DBD) and pulsed spark discharge, was developed. This design enables the efficient transfer of plasma species for Cr(VI) reduction. The performance of Cr(VI) reduction was found to be influenced by several factors, including feeding gas, applied voltage, initial Cr(VI) concentration, plasma exposure time, pH and radical scavenger additions. A reduction of over 98% was achieved when the initial pH was adjusted to below 3 or when ethanol (1%) was used as an oxidative species scavenger. This resulted in an energy efficiency of greater than 0.7 g/kWh. Cytotoxicity studies revealed that the APB treatment could effectively reduce the toxicity of Cr(VI)-containing wastewater, as evidenced by the increased viability in human cell lines. This study provides insights into the fundamental aspects of plasma oxidation/reduction chemistry and would support further applications of plasma-based water remediation.