Bio-refractory organics removal and floc characteristics of poly-silicic-cation coagulants in tertiary-treatment of coking wastewater

Poly-silicic-cation coagulants are increasingly being used to remove the bio-refractory organic compounds from industrial wastewaters, yet the research on the interactions between the coagulants and organics remains limited. In this study, three composite coagulant poly-aluminum-silicate-chloride (PASiC), poly-ferric-silicate-chloride (PFSiC) and poly-aluminum-ferric-silicate-chloride (PAFSiC) were prepared and examined as tertiary-treatment coagulants for coking wastewater. Excitation-emission fluorescence (EEM) spectra and gas chromatograph/mass spectroscopy (GC/MS) were applied to characterize composition of the organics, and the floc characteristics were monitored using a laser diffraction particle size analyzer and a particle vision system. The results show that a board range of organics including microbial metabolites, fulvic acid and humic acid-like fluorophores and polymeric aromatic hydrocarbons, can be removed by PAFSiC. This is attributed to the synergetic removal of organic compounds by aluminum and iron based poly-silicic salts. Compared with PASiC and PFSiC, the resultant flocs of PAFSiC have the moderate size and porous inner structure, with more branch-like units but less agglomeration. The structure is favorable for capture, adsorption and enmeshment of the bio-refractory organic compounds from the bio-treated coking wastewater. This study indicates that the organic removal is closely related to floc characteristics and PAFSiC can be used as an effective and promising coagulant for tertiary-treatment of coking wastewater.