Coagulation and flocculation of dye-containing solutions using a biopolymer (Chitosan)

Chitosan, dissolved in acetic acid, was used for the coagulation-flocculation of an anionic dye (Reactive Black 5). In acidic solutions protonated amine groups of chitosan attract dye sulfonic groups. Increasing chitosan dosage increases dye removal up to a concentration resulting in complete neutralization of anionic charges; above, the excess of cationic charges leads to suspension re-stabilization. Process efficiency increases with decreasing the initial pH of dye solution: the molar ratio between dye molecules and amine groups ([n]) respects the stoichiometry between sulfonic functions and protonated amine groups at initial pH 5; at initial pH 3 a possible dye aggregation phenomenon results in higher molar ratio [n]. The coefficient [n] depends on both the pH and the molecular weight of chitosan. The main mechanism for dye coagulation with chitosan sounds to be charge neutralization at acidic pH.