Reagent-Free Electromembrane Process for Decarbonization of Natural Water

Abstract

The reagent-free electromembrane process of removing carbonates, bicarbonates, and carbonic acid from softened natural carbonate water using an electrodialysis synthesizer EDS-01 with a two-cell unit cell formed by a bipolar membrane and a cation-exchange membrane has been studied. MB-2M membranes modified with an ionopolymer containing phosphoric acid groups catalytically active in a water-splitting reaction have been used as bipolar membranes, while heterogeneous membranes Ralex CMH (Mega a.s., Czech Republic) have been used as cation-exchange membranes. The decarbonization process has been carried out in two stages. At the first stage, a reagent-free correction of pH of the solution treated has been carried out. The value of pH in acid compartments has been adjusted to be 2.5–4.0. At the second stage, this solution has been deaerated with air purified from carbon dioxide. For a quantitative description of the process, a previously developed model has been adapted to describe the electrodialysis process with bipolar and cation-exchange membranes. It is shown that the electrodiffusion transfer of anions through the cation-exchange membrane and bipolar membrane is practically absent, and the change in the concentrations of carbonate ions, bicarbonates, and carbonic acid is due to the quasi-equilibrium chemical reactions. The deaeration of acidified softened water reduces the total carbon content from 5 to 1 mmol/L. The decarbonization of softened water is accompanied by a decrease in the concentration of sodium cations and total mineralization. With an EDS-01 electrodialysis synthesizer performance of 100 L/h, the specific energy consumption is in the range from 0.16 to 6.12 kW h/m3 depending on the current density.