H2/Cl2 production performance of wastewater electrolysis from saline-alkali land treatment

Abstract: Solar and wind resources are usually abundant in saline-alkali lands of China. These renewable energies can be expected to partially replace fossil fuel energy for the carbon peaking and carbon neutrality goals in recent years. Specifically, energy utilization and resource regeneration can also be realized for the waste of solar and wind energy nearby to electrolyze wastewater from the saline-alkali land treatment. It is of practical significance to utilize renewable energy for the treatment of wastewater and the production of H2/Cl2. Correspondingly, the first step is to concentrate the wastewater, due to a very low salt concentration (about 0.2 mol/L) after sprinkler irrigation in the saline-alkali land. It is necessary to explore the appropriate concentration of saline water for the better stable production of Cl2. In addition, the existing purification cannot fully meet the high requirement so far, particularly in the complicated process with the high cost. This study aims to explore a low-carbon and economic process for the removal of impurities from saline wastewater. A feasible treatment technology was also proposed to reduce the impurity ions for the cost-saving purification of high-salt wastewater. A series of electrolysis experiments were carried out on the wastewater from saline-alkali land treatment. A systematic investigation was then implemented to clarify the effects of salt concentration and the impurity removal processes before/during electrolysis. The CO2 regulation cell was added in this case. The influence of Ca(OH)2 was explored in the process of impurity removal before electrolysis. The electrolytic experiment was set without or with the CO2 input flow of 44.3 mL/min. Results showed that there was a linear correlation of the current density with the production of H2/Cl2 in the wastewater electrolysis under different salt concentrations. More importantly, the rate of H2 production was slightly higher than that of Cl2 production. The side reaction of oxygen evolution took a small proportion in the anodic reaction. The suitable concentration of salt for the wastewater electrolysis was 3.5 mol/L, which contributed to the largest current density and cathode pH. In the optimal process, Ca(OH)2 was added to the wastewater followed by concentration. The concentrations of Ca2+, Mg2+ and SO42- were also achieved lower than 0.02, 0.1, and 0.2 mol/L, respectively. The reason was that the Mg2+ and Ca(OH)2 reacted first, when a certain amount of Ca(OH)2 was added to the wastewater. Subsequently, the Mg2+ was removed in the form of Mg(OH)2. The concentrations of Ca2+ and SO42- were then similar in the wastewater after the removal of Mg2+. Therefore, the Ca2+ and SO42- were both removed as CaSO4 by the concentration. Moreover, the continuous injection of CO2 during electrolysis can be expected to convert the hydroxide precipitation of Ca2+ and Mg2+ ions into bicarbonate with high solubility. As such, only a few adverse effects were caused by precipitation generation and accumulation in the process of electrolysis. In the end, the adverse effect of impurity ions was further reduced, with which the H2/Cl2 yield increased by about 10%.