An efficient and robust flow-through electrochemical Ti4O7 membrane system for simultaneous Cr(VI) reduction and Cr immobilization with membrane cleaning by a periodic polarity reversal strategy

Abstract

As for the Cr(VI) reduction and subsequent Cr immobilization from the neutral and basic water, acidification and subsequent alkalization are necessitated, which usually suffers from the cumbersome operation, high chemical cost and increased water salinity. In this study, a porous Ti4O7 reactive electrochemical membrane (REM) was utilized as both the flow-through cathode and microfiltration membrane for the reduction of Cr(VI), and precipitation and interception of Cr(OH)3 particles synchronously, where the electrolysis cell was divided by a polytetrafluoroethylene (PTFE) membrane. As compared with the flow-by mode, the flow-through mode exhibited a 3-fold increase in the mass transfer rate constant and therefore nearly 100% of Cr(VI) was reduced when passing through the Ti4O7 REM cathode. The divided electrolysis cell was operated to generate a high alkaline environment (pH ∼ 11.0) throughout the whole volume of the cathodic chamber regardless of the initial pH changing from 5.0 to 10.0, which favored the formation of Cr(OH)3 particles and its interception by Ti4O7 REM. Thus, increasing initial pH from 5.0 to 10.0 exhibited negligible effects on the Cr(VI) removal efficiency and Cr(total) immobilization efficiency. Under the optimum condition (initial Cr(VI) concentration 500 ppb, current density 1 mA cm−2, flux 287 L m-2 h−1), the Cr(total) removal efficiency could achieve at as high as 95% with residual Cr(total) concentration of 27 μg L-1. As the reaction progressed, Cr(OH)3 passivated the membrane gradually, decreasing the Cr(VI) reduction efficiency to 84% at 30 h. Interestingly, the fouled Ti4O7 REM could be recovered completely after 30 min’s membrane cleaning operation just by periodically reversing the polarity at current density of 5 mA cm−2. Generally, this process can be considered as a promising and suitable treatment method for Cr(VI) detoxification from distributed water with a very low specific energy consumption of 0.109 kWh m−3.