Structure-activity relationships of the papermill sludge-based flocculants in different dye wastewater treatment

The molecular structures of the flocculants determine their application performances, but seldom researches have focused on the structure-activity relationships (SAR) of the papermill sludge-based flocculants (PSBFs). Here, four flocculants with different charge densities (CD), molecular weights (Mw) and chain architectures were synthesized from papermill sludge. The characterization results of the PSBFs confirmed the success of chemical modification and the structural diversities of the four flocculants. The series of decolorization experiments showed that branched flocculants were more efficient and economical in dye removals than linear ones, the maximum color removal was above 95.0%. The flocs formed by branched flocculants were large, strong, loose and easy to settle, but those by linear ones were not. However, branched flocculants with high Mw levels were limited in decolorization efficiencies due to the steric hindrance effects. The abundant side chains of the high-Mw flocculants intertwined with each other, thereby impeding the contact between the contaminants and flocculants. Positively charged flocculants could remove contaminants mainly through charge neutralization and bridging action. Branched flocculants with abundant side chains were more pH-insensitive than linear ones due to the strong adsorption bridging effects. Also, the branched PSBFs were efficient even with the coexisting humic acid (HA) and kaolin particles in both disperse red and reactive blue dye removals. The flocculation mechanisms and SAR of the PSBFs were discussed in this manuscript. (C) 2020 Elsevier Ltd. All rights reserved.