Previous studies showed that coating of titanium electrodes with manganese–molybdenum oxides instead of Ir MMO remarkably decreased the electrocatalytic activity towards formation of hypochlorite 8, 9, 10, 11. While the feasibility of simultaneous production of acid and caustic was demonstrated, the practical and economic feasibility is expected to be limited due to complex reactor configuration and large energy requirements of the system caused by the use of multiple membranes. bipolar membrane electrodialysis) was proposed for simultaneous acid and caustic generation from reverse osmosis concentrates 7. To avoid chlorine formation, a five-compartment electrochemical system (i.e. Consequently, these materials do not allow for the direct production of HCl from NaCl solutions. However, the currently commercially available anode materials such as mixed metal oxide (MMO) coated titanium and boron doped diamond (BDD) are prone to chlorine formation even at low chloride concentrations 4, 5, 6.
Protons (H +) and hydroxide ions (OH −) could be produced by electrolysis of water using a two-chambered electrochemical cell with anode being fed with NaCl containing water and cathode fed with clean water. On-site generation would also avoid the concentration step and thus reduce the overall energy consumption. As in most cases both compounds are used at relatively low concentrations by the water industry, there is a general interest in on-site generation of moderate strength HCl and NaOH solutions to avoid the aforementioned issues. However, the transport, storage and handling of concentrated HCl and NaOH come with serious occupational health and safety (OH&S) concerns for the water industry. Although HCl cannot be directly synthesized in this process, it can be formed by burning chlorine and hydrogen gas produced in the cathode 3. Caustic is mainly produced in the chlor-alkali process by the electrolysis of sodium chloride (NaCl) with concomitant chlorine production 2, 3. Hydrochloric acid (HCl) and caustic soda (NaOH) are both widely used chemicals for water and wastewater treatment 1, 2. In theory, artificial brine could be replaced by saline waste streams such as Reverse Osmosis Concentrate (ROC), turning ROC into a valuable resource. In this study, artificial brine was used as a source of sodium and chloride ions. Overall, this study showed the potential of simultaneous HCl and NaOH generation from NaCl and represents a major step forward for the water industry towards on-site production of HCl and NaOH. The reduction in CE for HCl generation was caused by proton cross-over from the anode to the middle compartment. HCl was anodically produced at moderate strengths at a CE of 65 ± 4% together with a CE of 89 ± 1% for cathodic caustic production. The results showed that protons could be anodically generated at a high Coulombic efficiency (i.e. ≥ 95%) with chlorine formation accounting for 3 ~ 5% of the charge supplied. This study presents an innovative technology simultaneously generating HCl and NaOH from NaCl using a Mn 0.84Mo 0.16O 2.23 oxygen evolution electrode during water electrolysis.
Direct anodic electrochemical HCl production by water electrolysis has not been successful as current commercially available electrodes are prone to chlorine formation. Hydrochloric acid (HCl) and caustic (NaOH) are among the most widely used chemicals by the water industry.
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