Efficient recovery of carboxylates from the effluents treated by advanced oxidation processes using flow-electrode capacitive deionization in short-circuited closed-cycle operation

Abstract

• Acetate and oxalate can be effectively recovered from wastewater using FCDI. • Carbon flow-electrodes of FCDI facilitated the decrease of cell voltage. • FCDI in SCC operation showed high removal efficiency of anions, low energy consumption and stable desalination. • High selective recovery of oxalate could be realized by adjusting influent pH. Advanced oxidation processes (AOPs) are highly efficient novel methods for a wide range of organic substances. However, the decomposition of organic pollutants usually results in the formation of a series of carboxylates that are difficult to be further degraded by oxidants, which not only induces the excessive consumption of chemicals, but also leads to waste of carbon resources. Therefore, the recovery of small molecular carboxylates from effluents is a significant strategy. However, the excessive high voltage in conventional electrochemical technology for recovering small molecule acids can easily lead to organic matter degradation and high energy consumption. In present study, the removal and recovery of refractory acetate and liable oxalate by using FCDI system operated in short-circuited closed-cycle (SCC) operation was investigated. The results showed that Ac − and Ox − could be effectively accumulated in anode chamber from wastewater containing high-strength of Cl − . The significance of carbon flow-electrodes for the recovery performance and operating cost was evidenced. Higher carbon conditions induced an obvious decrease of cell voltage and thus the decomposition of organic compounds and energy consumption were decreased simultaneously. On the premise of the recovery rate, under the optimal conditions (carbon = 5 wt%, j = 5.7 A/m 2 , HRT = 7.45 min, pH i = 7.1 and t = 180 min), more than 80% of acetate and oxalate can be recovered from wastewater with a low energy consumption of 0.45 kWh/m 3 . The calculated average salt recovery rates (ASRR r ) for Ac − and Ox − were 0.186 and 0.45 μmol/(cm 2 ·min), respectively. The findings indicated that FCDI system with carbon particle flow-electrode operating in SCC mode is a promising strategy for continuously recovering aqueous carboxylates. In further research, combining FCDI operation with other techniques such as AOPs is possible to realize simultaneous oxidation and carboxylates recovery, in other words, in-situ product recovery, that if recovered will greatly contribute to the sustainability of the low carbon economy.