Synthesis, characterization of a novel lignin-based polymer and its behavior as a coagulant aid in coagulation/ultrafiltration hybrid process

• Product-LDA was characterized as a cationic biopolymer with broad structure.
• Addition of LDA enhanced coagulation performance and decreased membrane fouling.
• LDA addition resulted in flocs with open structure and strong anti-shear ability.
• LDA introduced charge neutralization and absorption bridging effect.
• The coagulation aid effect of LDA was not affected by the type of metal salts.

A novel lignin-based flocculant was introduced in recycling of the biomass existing in papermaking sludge. The product, lignin-diallyl dimethyl ammonium chloride-acrylamide (LDA) was characterized as a cationic macromolecule polymer with broad structure. To demonstrate its efficiency, LDA was used as coagulant aid with polyferric chloride (PFC) and polyaluminum chloride (PAC) to remove humic acid by coagulation–ultrafiltration processes. Results showed that coagulation aid effect of LDA was not affected by the type of metal salts and removal ratios of dissolved organic carbon were maximum 15.1% and 11.9% larger than single PFC and PAC coagulation, respectively. More concentrated flocs with larger size and open structure as well as low fractal dimension were formed by dual-coagulants. In PFC coagulation system, LDA could absorb negative charge on the surface of Fe(III) coagulated flocs efficiently and achieve the maximum floc strength accordingly. In the ultrafiltration experiment, cake layer fouling was demonstrated to be the foremost fouling mechanism. PAC combined used with LDA resulted in the minimum membrane fouling due to the formation of looser flocs as well as more concentrated floc size distribution. Overall, LDA brought in charge neutralization and absorption bridging effect and played a positive role in coagulation–ultrafiltration processes.