Inorganic composite coagulant for wool scouring wastewater treatment: Performance, kinetics and coagulation mechanism

Wool is perceived to be a clean and green natural fibre. However, raw wool contains substantial amounts of contaminants that must be removed in a wool scouring process before the wool can be used as a textile fibre. Coagulation is the first step for scouring wastewater treatment. But the role of inorganic coagulants in increasing sludge density for scouring wastewater treatment remains unclear. This work systematically prepared a series of inorganic composite coagulants (polyaluminium ferric silicate (PAFS)) and investigated their efficiency for treating wool scouring wastewater. Compared with conventional coagulants, polyaluminium chloride (PAC) and polyferric sulfate (PFS), PAFS showed improved coagulation performance over a wider pH range (3-7), achieving higher removal rates of 97 % and 99 % for chemical oxygen demand (COD) and turbidity, respectively. In addition, PAFS showed its advantages for practical wastewater treatment, including fast floc settling rate, increased sludge density (with a reduced sludge volume) and low floc moisture content. This optimized PAFS was successfully obtained in a scaled-up process. Zeta potential tests revealed that PAFS coagulated colloidal particles by the synergistic action of electro-neutralization and adsorption bridging. This work has practical implications for increasing the efficiency of wastewater treatment and reducing the environmental impact of wool scouring.