Membrane electrolysis for separation of cobalt from terephthalic acid industrial wastewater

Recovery of valuable metals from wastewaters containing both metals and organics is challenging with current technologies, in part due to their interactions. Typical approaches are chemical intensive. Here, we developed a membrane electrolysis system coupled to an acidic and alkaline crystallizer to enable separate precipitation of the organics and metals without additional chemicals. The target industrial wastewater contained mainly purified terephthalic acid (PTA), benzoic acid (BA), p-Toluic acid (PA), cobalt (Co), and manganese (Mn). We examined the removal and recovery efficiency of PTA and cobalt from two types of synthetic stream and the real process stream using several configurations. The acidic crystallizer reached a removal efficiency of PTA of 98.7 +/- 0.2% (Coulombic efficiency 99.71 +/- 0.2%, pH 3.03 +/- 0.18) in batch tests of the simple synthetic stream. The alkaline crystallizer achieved a cobalt recovery efficiency of 94.51 +/- 0.21% (Coulombic efficiency 87.67 +/- 0.31%, pH 11.37 +/- 0.21) in batch tests of the simple synthetic stream (TPA and Co). Then, the system was operated continuously with complex synthetic stream (TPA, BA, PA, Co and Mn). The alkaline crystallizer achieved a cobalt recovery efficiency of 97.78 +/- 0.02% (Coulombic efficiency 90.45 +/- 0.17%)at pH 11.68 +/- 0.02. The acidic crystallizer obtained a PTA removal efficiency of 61.2 +/- 0.1% (Coulombic efficiency 62.3 +/- 0.2%) over 144 h (pH 3.71 +/- 0.03). A real stream was tested over 5 h runs in batch showing 31.1 +/- 1.0% PTA (Coulombic efficiency 26.5 +/- 0.2%) and 82.92 +/- 0.22% cobalt removal (Coulombic efficiency 75.27 +/- 0.31%) at pH 2.71 +/- 0.12 and 8.07 +/- 0.02, respectively. However, micron-scale precipitates were generated from real stream tests. To conclude, the membrane electrolysis cell coupled with acidic and alkaline crystallizers enabled simultaneous separation of PTA and cobalt as solid precipitates from a complex stream with no chemical addition. The efficiencies were lower with the real stream than the synthetic streams, showing the impact of matrix effects and the need to optimize the performance of the crystallizers.