A hybrid process of coprecipitation-induced crystallization-capacitive deionization-ion exchange process for heavy metals removal from hypersaline ternary precursor wastewater

A hybrid process of coprecipitation-induced crystallization-capacitive deionization-ion exchange (CPT-IC-CDI-IX) was developed for heavy metals removal from the hypersaline ternary precursor wastewater (HTPW), containing Ni(II) (262 mg/L), Co(II) (15.1 mg/L), Mn(II) (20.5 mg/L) and a great deal of total dissolved solids (TDS, in the main form of Na2SO4, 176 g/L). After coprecipitation (CPT) on basis of solubility product theory, the residues of Ni(II), Co(II) and Mn(II) in filtrate of primary sedimentation (FPS) were reduced to 1.25, 0.90 and 0.10 mg/L, respectively, suggesting that CPT was propitious for reutilization of heavy metals. The salinity was a key factor affecting heavy metal removal. Based on calculation of ionic association constant, HTPW should be desalted to below 10 g/L for eliminating the association effect between salt anions and trace heavy metal ions. Hence, 3 degrees C-induced crystallization (IC) was applied in desalination referring to theoretical crystallization temperature with recovery of Na2SO4. TDS in FPS was obviously removed from 191 to 68.7 g/L, while Ni(II) and Co(II) were decreased to 0.401 and 0.228 mg/L, respectively. A self-made capacitive deionization (CDI) stack followed the IC unit for deep Na2SO4 removal to 9.60 g/L. After eliminating ionic association effect, Ni(II) and Co(II) were further removed by ion exchange achieving the levels of 0.0412 and 0.0279 mg/L, respectively, which were below the emission standards of Ni(II) (0.5 mg/L) and Co(II) (0.1 mg/L). The heavy metals in HTPW can be efficiently removed from hypersaline wastewater by the hybrid technique with lower operating cost.