Interception of volatile organic compounds through CNT electrochemistry of electrified membrane surface during membrane distillation

Abstract

The unimpeded transport of volatile organic compounds (VOCs) in the membrane distillation (MD) greatly restricts its application in treating industrial wastewater treatment. Herein, an electrically MD process (E-MD) was conducted by coupling electrified carbon nanotube (CNT) membrane for the VOCs intercept test. This work systematically investigated membrane characteristic, the intercept efficiency and mechanism of VOCs in the E-MD. The results showed that CNT significantly promoted conductivity and permeate performance of the membrane. The transport of 7 VOCs across the membrane was highly correlated with their molecular weights and hydrophobicity. By applying a low voltage of 3.0 V on the membrane surface, the distilled VOCs concentrations were efficaciously reduced. Meanwhile, the distilled phenol concentration was only 0.15 mg/L in the E-MD, 62 times lower than that in standard MD. Moreover, the cathodic CNT and anodic CNT modes were adopted, which exhibited different intercept efficiencies. Compared with the cathodic CNT, the anodic CNT mode was superior in intercepting VOCs but with poor stability due to electrochemical byproducts induced CNT fouling. As revealed by mechanism investigation, direct electrooxidation and electropolymerization contributed to the VOCs intercept under the anodic CNT mode. By contrast, the favorable intercept efficiency under the cathodic CNT mode was mainly ascribed to the locally elevated pH near the CNT surface achieved through water electrolysis. At an elevated pH, phenol compounds were substantially ionized, and were therefore more favorably intercepted by the electronegative CNT networks of membrane surface. This study emphasizes the application potential of electrified MD membranes in reducing noxious VOCs of the cold stream.