A review on production of lignin-based flocculants: Sustainable feedstock and low carbon footprint applications

Lignin, which can be recycled from bioethanol and paper production wastes, is the second largest renewable resource. However, studies and applications on sustainable upgrading of lignin are extremely limited and most of them only focus on its combustion to obtain energy. The abundance of reactive groups in lignin makes it a potential feedstock for the production of sustainable low carbon footprint value-added materials. Among them, the preparation of flocculants from lignin has attracted much attention. This paper systematically reviews recent progress of low carbon footprint lignin-based flocculants. The modification methods for the production of lignin-based flocculants, Mannich, sulfonation, carboxylation, crosslinking and graft copolymerization, and the flocculation mechanisms in wastewater treatment are discussed emphatically. Moreover, the factors affecting the flocculation performance of lignin-based flocculants, including internal factors, such as charge density, molecular weight, conformation, and external factors, such as dosage, treatment time, wastewater pH, and temperature are discussed in detail, which are conducive to design, production and application of lignin-based flocculants for lower carbon footprint. Furthermore, we critically point out the current deficiencies of the lignin-based flocculants and the challenges facing commercialization, then we propose the corresponding possible solutions. The production and application of high-efficiency lignin-based flocculants are of great significance for resource conservation, low carbon footprint, and wastewater reuse.