Sewage treatment for chromium(VI) disposal by combined reduction with sodium sulfite in the presence of solid adsorbents

Abstract

To improve the existing procedure for the purification of low concentration (50–150 mg/l) sewage from Cr(VI) using sodium sulfite, it was suggested that sewage should be passed through an adsorbent layer. An alternative method is to pass sewage through the adsorbent layer placed in the interelectrode space of an electrochemical cell. Both methods increase the degree of purification for sewage with pH ≥ 6 and allow pH of purified water to be maintained at 6.5–8.0 in accordance with sanitary requirements. It was shown that combined adsorption-reagent and electrochemical reduction can be used by reducing Cr(VI) with sodium sulfite on adsorbents such as Fe₂O₃, kaolin, and activated coal (AC) and passing electric current through an AC adsorbent layer. The degree of purification varied from 80 to 94% for combined processes. The theoretical rate constants were calculated for adsorbent-reagent reduction (4.0–4.7 min⁻¹), and the transfer coefficients were calculated for electrochemical reduction (1–4 mm/s⁻¹).