Optimization of Typical Constructed Wetland Substrate Conditions and Their Feasibility for Removing the Co-contamination of Benzene and Cr(VI)/ethanol

Constructed wetlands (CWs), as a green remediation technology, exhibit significant potential in treating benzene, toluene, ethylbenzene and xylene (BTEX)-contaminated urban runoff. The removal efficiency of BTEX is often influenced by environmental conditions in the substrate of CWs, such as dissolved oxygen (DO) and particulate organic matter (POM) concentrations. However, little is known about the co-influence of these factors on BTEX degradation. In this study, benzene was taken as the representative BTEX, central composite design-response surface methodology (CCD-RSM) results showed that DO concentration had a slight impact on anaerobic benzene degradation, high POM concentrations improved while high initial benzene concentrations decreased benzene degradation efficiency. POM concentration and initial benzene concentration had the most significant synergistic impact on benzene degradation. The optimal DO concentration, initial benzene concentration, and POM concentration were 0.3 mg/L, 90 µM, and 40 mg/L, respectively. Furthermore, Cr(VI) and ethanol occasionally coexist with BTEX in urban runoff, the addition of Cr(VI) and ethanol to the benzene degradation consortium led to a significant decline in benzene biodegradation (P < 0.05), possibly by inhibition of biomass growth and ATP production. Shinella spp. may be closely associated with Cr(VI) reduction, Delftia spp. likely played a key role in benzene biodegradation under ethanol stress. Community function prediction indicated that the abundances of anaerobic benzene biodegradation- and electron transfer-associated genes also decreased when Cr(VI) or ethanol was added. The results of this study will provide a theoretical basis for the design and application of constructed wetlands in remediating BTEX-contaminated urban runoff.