First Report on Separation of Sulfate Ions from Evaporated Residue of RO Reject Stream Generated from the Leather Industry

Reverse Osmosis (RO), the membrane separation process, is widely being used on the fresh water supply to industrial and domestic sectors with the generation of reject streams containing huge concentrations of cations and anions besides organic impurities. Hence, the reject stream is advised to evaporate in solar pans/multiple effect evaporators to reduce the volume of disposal. The evaporated residue consists of a high concentration of chloride and sulfate ions along with organic impurities. In our previous study, chloride ion was separated as sodium chloride, and in the present investigation, an attempt was made to separate sulfate ions as calcium sulfate from the saturated solution of reject stream generated from the leather industry. The optimum conditions for the precipitation of calcium sulfate were observed as follows: pH 4.0 and temperature 40 degrees C. The separated calcium sulfate was calcined at 700 degrees C to destruct the occluded organic impurities. The recovered calcium sulfate was characterized for surface morphology, chemical composition, thermal behavior and crystalline modification through scanning electron microscopy (SEM), electron dispersive X-ray (EDX), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) analyses. The recovered calcium sulfate was of fine particles with increase in surface area. The recovered calcium sulfate was tested for its adsorptive property toward aqueous leather dyes.