Flocculation-to-adsorption transition of novel salt-responsive polyelectrolyte for recycling of highly polluted saline textile effluents

Herein, a new salt responsive starch flocculant (SSF) via starch etherification with butyl glycidyl ether (BGE) and 2-chloro-4,6-diethylamino-[1,3,5]-triazine (CDAT) was reported as highly salt-responsive polymer. The resulting polymer exhibits a reversible switch from dissolution to precipitation in response to slight external salt stimuli due to the integration of highly charged CDAT moiety. The extent of phase separation, relative solubility and residual polymer in water solution could be precisely modulated by changing the salt concentration and the type of counter ions. The results demonstrated that dyes removal is salt dependent, and that the addition of inorganic salt leads to a pronounced broadening of flocculation window. The as-prepared polymers experience an obvious transition from flocculation to adsorption processes with an increasing salt concentration from 0 to 100 g/L due to the salt-triggered switchable solubility of SSF. Two different mechanisms combined the coagulation and adsorption may be involved in dye decolourization from saline reactive dyeing effluents. The beneficial effect of salt-sensitive flocculant was found to be more significant in reuse of saline wastewater for fabric dyeing, in which the flocculant residuals are significantly low (less than 10.9%). After flocculation and sedimentation, the clarified saline wastewater can be readily recovered and reused three times while retaining high-quality colour reproduction. The salt-driven switchable solubility of polymer from water-soluble flocculants to solid-state adsorbents offers a possibility to overcome the limitations of industrial flocculation processes, such as a narrow flocculation window and high polymer residuals at flocculant overdose.