Chromium removal from ion-exchange waste brines with calcium polysulfide

Chromium removal from ion-exchange (IX) brines presents a serious challenge to the water industry. Although chromium removal with calcium polysulfide (CaS5) from drinking waters has been investigated somewhat, its removal from ion-exchange brines has not been evaluated to date. In this study, a Central Composite Design as well as experimental coagulation tests were performed to investigate the influence of pH, CaS5/Cr(VI) molar ratio, alkalinity, and ionic strength in the removal of chromium from IX brines. The optimal pH range for the process was found to be pH 8–10.3 and brine alkalinity did not affect coagulation. The efficiency of chromium removal improved only slightly when the ionic strength increased from 0.1 M to 1.5 M; no significant difference was observed for an ionic strength change from 1.5 to 2.1 M. For chromium (VI) concentrations typically found in ion-exchange brines, a CaS5/Cr(VI) molar ratio varying from 0.6 to 1.4 was needed to obtain a final chromium concentration <5 mg/L. Maximum efficiency for total chromium removal was obtained when oxidation reduction potentials were between −0.1 and 0 (V). Solids concentrations (0.2–1.5 g/L) were found to increase proportionally with CaS5 dosage. The results of this research are directly applicable to the treatment of residual waste brines containing chromium.

► A CCD model developed can be used to treat IX brines with varying Cr concentrations.
► Chromium removal from IX brines by calcium polysulfide does not require pH adjustment.
► The high ionic strength of IX brines has no major effect on Cr removal by polysulfide.
► Addition of excessive polysulfide generates more sludge and lowers removal efficiency.
► Chromium removal efficiency is optimum at final ORP values between −0.1 and 0 V.