Recovery of boron and zinc from wastewater via electrodialytic metathesis

With the development of industrial civilization, large amounts of wastewater containing boron and zinc are being produced. Such wastewater poses a great threat to human health if discharged without treatment. In this study, an integrated technology of electrodialytic metathesis (EDM) and a hydrothermal process was utilized to recover boron and zinc from wastewater in the form of zinc borate (Zn 2 B 6 O 11 ·3H 2 O). Experimental results showed that electrolyte concentration , current density , and initial boron and zinc concentration clearly affected the recovery ratios for B 4 O 7 2− and Zn 2+ and their corresponding specific energy consumption and current efficiencies . Under the optimal experimental conditions (an electrolyte concentration of 1.0 mol/L in the recycling compartment in the experimental setup, a current density of 15 mA/cm 2 , initial ZnSO 4 and Na 2 B 4 O 7 concentrations of 0.1 mol/L, and a flow rate of 20 mL/min), the recovery ratio, specific energy consumption, and current efficiency were 85.6 %, 6.5 kW·h/mol, and 12.3 %, respectively, for recovering B 4 O 7 2− and 80.6 %, 1.7 kW·h/mol, and 45.7 %, respectively, for recovering Zn 2+ . When one recovery compartment was employed, the current efficiencies for recovering B 4 O 7 2− and Zn 2+ were 12.3 % and 45.7 %, respectively, and these values were 11.4 % and 43.8 %, respectively, when two recovery compartment were used. After B 4 O 7 2− and Zn 2+ concentrated by EDM were treated via a hydrothermal process, a white precipitate was produced. The results of X-ray diffraction, Fourier transform infrared spectroscopy , and thermogravimetric analyses showed that this precipitate exhibited physicochemical properties similar to those of commercial Zn 2 B 6 O 11 ·3H 2 O. Thus, the integrated technology can be regarded as an effective method for recovering boron and zinc from wastewater.