Ion-exchange (IX): arsenic and chromium removal from brines and removal of inorganic contaminants by specialty resins

摘要

Although ion exchange is highly efficient in removing inorganic contaminants, similar to other water treatment technologies, ion exchange has some drawbacks that need to be studied further. Three issues related to drawbacks of ion-exchange resins in water treatment were addressed in this research. The first issue was the influence of anionic inorganic co-contaminants including nitrate, Cr(VI), Se(VI), and As(V) on the performances of nitrate and perchlorate specialty (selective) resins in water treatment. It was found that nitrate can be removed from waters using perchlorate specialty resins, but the resin is poorly regenerated. Perchlorate was not easily removed from either nitrate or perchlorate specialty resins. The results showed that simultaneous removal of nitrate and Cr(VI) is optimal when using nitrate specialty resin. Perchlorate/nitrate specialty resins were inefficient in removing As(V), but could exchange Cr(VI) or Se(VI). A major issue realized from this research is the accumulation of co-contaminants in specialty resins and their release during resin regeneration. Such a release may deem waste regenerant brines hazardous, significantly affecting disposal costs. The presence of the co-contaminant ions affected the run length and the brine composition when perchlorate or nitrate specialty resins were used. Brine treatment is a serious challenge for IX water industry when removing arsenic (V) or chromium (VI) from drinking water.Arsenic (V) removal from brines using ferric chloride was the second issue of this research. The optimum pH range for the process was found to be 4.5-6.5. Higher brine alkalinity affected coagulation because it commands larger amounts of acid to lower the pH to the desired level. Increasing ionic strength slightly enhanced the arsenic (V) removal efficiency. For arsenic (V) concentrations typical in ion exchange brines and to achieve a remaining As (V) concentration of 5 mg/L, Fe/As molar ratios varying from 1.3 to 1.7 are needed at operating pH values of 5.5 to 6.5. The Fe/As ratios needed to treat brines are significantly lower than those used to treat drinking waters. Solids concentration varying from 2 to 18 mg/L were found.The third issue of this research was chromium removal from IX brines. Optimum pH range for the process was found to be 8-10.3. The chromium removal efficiency improved only slightly when the ionic strength increased from 0.1 M to 1.5 M. For chromium (VI) concentrations typically found in IX brines, a CaS5/Cr(VI) molar ratio varying from 0.6 to 1.4 was needed to obtain a final chromium concentration below 5 mg/L. The maximum total chromium removal efficiencies were obtained at reducing conditions when oxidation reduction potentials of the brines were between -0.1 to 0 V. Solids concentrations varying from 0.2 to 1.5 g/L were found. The results of this research have direct application to the treatment of residual wastes brines containing chromium.