Preparation and Coagulation Performance of Carboxypropylated and Carboxypentylated Lignosulfonates for Dye Removal

In this work, 1-carboxypropyled (1-CPRLS) and 5-carboxypentyled lignosulfonates (5-CPELS) were synthesized using 2-chlorobutanoic acid and 6-chlorohexanoic acid as carboxylate group donors via S(N)1 and S(N)2 mechanisms, respectively. 1-Carboxypropyl and 5-carboxypentyl lignosulfonates with the charge densities of −3.45 and −2.94 meq g(−1) and molecular weights of 87,900 and 42,400 g·mol(−1) were produced, respectively, under mild conditions. The carboxylate content and degree of substitution (DS) of the 1-CPRLS product were 2.37 mmol·g(−1) and 0.70 mol·mol(−1), while those of 5-CPELS products were 2.13 mmol·g(−1) and 0.66 mol·mol(−1), respectively. The grafting of carboxypropyl and carboxypentyl groups to lignosulfonate was confirmed by Fourier transform infrared (FT-IR) and nuclear magnetic resonance ((1)H-NMR and (13)C-NMR) spectroscopies. In addition, 1-CPRLS and 5-CPELS were applied as coagulants for removing ethyl violet (EV) dye from a simulated solution, and their performance was related to their charge densities and molecular weights. Furthermore, fundamental discussion is provided on the advantages of (1) producing 1-CPRLS and (2) the superior properties and performance of 1-CPRLS to carboxyethylated lignosulfonate.