Membrane capacitive deionization for the treatment of sulfate, thiosulfate, and thiocyanate from gold mining process water

This study investigated the removal of sulfate, thiosulfate, and thiocyanate (SO<sub>4</sub><sup>2-</sup> , S<sub>2</sub>O<sub>3</sub><sup>2-</sup> and SCN<sup>-</sup>) by membrane capacitive deionization (MCDI) process. An electrochemical cell was fabricated using an anion exchange membrane and a cation exchange membrane placed on a carbon paper electrode, which operated as the anode and cathode. Batch mode experiments were conducted by recirculating single salt solutions of SO<sub>4</sub><sup>2-</sup> , S<sub>2</sub>O<sub>3</sub><sup>2-</sup>, and SCN<sup>-</sup> to determine the maximum removal efficiency. The anode electrode removed 8600 mg SO<sub>4</sub><sup>2-</sup>, 9850 mg S<sub>2</sub>O<sub>3</sub><sup>2-</sup>, and 9350 mg SCN<sup>-</sup> per m2 of the electrode. Batch mode experiments were also performed in mixed salt solution containing 100 mg/L SO<sub>4</sub><sup>2-</sup>, S<sub>2</sub>O<sub>3</sub><sup>2-</sup>, and SCN<sup>-</sup> to observe the effect of co-ions on the removal efficiency. In the adsorption phase, salt removal efficiency percentages were determined to be 57% for SO<sub>4</sub><sup>2-</sup>, 57% for SCN-, and 88% for S<sub>2</sub>O<sub>3</sub><sup>2-</sup> at 1.2 V. In the desorption phase, the electrodes were regenerated by reversing the operating voltage to -5.0 V. The single-pass mode MCDI experiment showed that both S<sub>2</sub>O<sub>3</sub><sup>2-</sup>, and SCN<sup>-</sup> were removed with a total salt removal of 539.6 mg/m² and 510 mg/m², respectively, during the adsorption phase. In the desorption phase, the removal rates increased to 719 mg/m² for S<sub>2</sub>O<sub>3</sub><sup>2-</sup> and 643 mg/m² for SCN<sup>-</sup>, indicating the electro-oxidation of these ions.