Coagulation behaviors and in-situ flocs characteristics of composite coagulants in cyanide-containing wastewater: Role of cationic polyelectrolyte

In this paper, composite coagulants (PFS, PFSC05, PFSC1 and PFSC5), prepared by mixing polyferric sulfate (PFS) and cationic polyelectrolyte (CP) coagulants with different weight percent (W _p) of CP (W _p = 0%, 0.5%, 1% and 5%, respectively), were adopted to treat cyanide-containing wastewater. PFSC5 exhibited superior coagulation performances at optimal conditions: the removal of total cyanide (TCN) and chemical oxygen demand (COD) was 95%–97% and 50%–55%, respectively. The effects of CP on the properties and structure of flocs were investigated by laser diffraction instrument and small-angle laser light scattering (SALLS), respectively. The results show that the flocs of PFSC5 have higher growth rate, higher strength factor and lower recovery factor than other flocs. They are also much denser and more uniform owing to the higher fractal dimension (D_f) and less microflocs (10–100 μm). Furthermore, the dense structure of the PFSC5 flocs can be restored after shear and is more resistant to hydraulic conditions. Particularly, detailed morphology evolution of the flocs was in-situ detected by on-line particle imaging. Due to strong ionic strength in wastewater, the CP in PFSC5 plays a significant role of adsorption, while the main mechanism of CP is electrostatic patch aggregation during the PFSC05 systems.