Tight Ultrafiltration Ceramic Membrane for Separation of Dyes and Mixed Salts (both NaCl/Na2SO4) in Textile Wastewater Treatment

Commercial nanofiltration (NF) membranes have been used to separate dyes and salts in industry; however, NF membrane's high rejection to divalent salts (i.e., Na2SO4) leads to a reduction of salt recovery. In this study, a tight ultrafiltration (t-UF) ceramic membrane (MWCO 8800 Da) is proposed to fractionate dyes and mixed salts (NaCl/Na2SO4) for textile wastewater treatment. Performance of the t-UF ceramic membrane and DK polymeric membrane (from GE) has been compared regarding to permeability, retention of reactive dyes, and permeation of salts. The t-UF ceramic membrane presents better permeability, competitive rejection of dye molecules (>98%), and reduced rejection of NaCl (<10%) and Na2SO4 (<30%) in comparison with DK membrane; the pure water permeability of t-UF membrane is at least 6 times that of DK membrane. In particular, the operation parameters (TMP, temperature, and pH) and solution environment (concentration and charges) have been intensively evaluated for dye/dual-salts separation efficiency in the membrane process. It also reveals that the t-UF ceramic membrane has performed negative rejection to chloride ions at low operating pressure in the dye and NaCl/Na2SO4 solution due to the electrostatic Donnan effect. Concentration of salt, valence of inorganic ions, and charge of dyes are found as having a significant effect on membrane separation performance. In conclusion, the strong retention of dyes and free permeation of salts (i.e., low retention) by the t-UF ceramic membrane proves that it can be applied to desalinate dyeing wastewater of high salinity and recover dyes and salts separately.