Cationic High Molecular Weight Lignin Polymer: A Flocculant for the Removal of Anionic Azo-Dyes from Simulated Wastewater

Abstract

The presence of dyes in wastewater effluents made from the textile industry is a major environmental problem due to their complex structure and poor biodegradability. In this study, a cationic lignin polymer was synthesized via the free radical polymerization of lignin with [2-(methacryloyloxy) ethyl] trimethyl ammonium chloride (METAC) and used to remove anionic azo-dyes (reactive black 5, RB5, and reactive orange 16, RO16) from simulated wastewater. The effects of pH, salt, and concentration of dyes, as well as the charge density and molecular weight of lignin-METAC polymer on dye removal were examined. Results demonstrated that lignin-METAC was an effective flocculant for the removal of dye via charge neutralization and bridging mechanisms. The dye removal efficiency of lignin-METAC polymer was independent of pH. The dosage of the lignin polymer required for reaching the maximum removal had a linear relationship with the dye concentration. The presence of inorganic salts including NaCl, NaNO₃, and Na₂SO₄ had a marginal effect on the dye removal. Under the optimized conditions, greater than 98% of RB5 and 94% of RO16 were removed at lignin-METAC concentrations of 120 mg/L and 105 mg/L in the dye solutions, respectively.

Keywords: COD; azo dye; flocculation; lignin modification; lignin-METAC.

Figure 1

Effect of pH on the dye removal of (a) RB5; (b) RO16 (from a dye concentration of 100 mg/L) using Sample 4.

Figure 2

Overall cationic charges of the dye solutions (dye concentration, 100 mg/L) as function of lignin-METAC polymer (sample 4) concentration in the solutions.

Figure 3

Structure of RB5 and RO16.

Figure 4

Effect of dye concentration on dye removal (a), RB5; (b) RO16 at pH 6 using sample 4.

Figure 5

Relationship between optimum lignin-METAC dosage and dye concentration.

Figure 6

Effect of dosage of lignin-METAC samples with different charge densities on dye removal (100 mg/L RB5 (a) and RO16 (b) dye solution, pH 6, 30 °C).

Figure 7

Relationship between charge density, MW, and optimum concentration of lignin-METAC copolymer in dye removal.

Figure 8

Effect of salt dosage on dye removal (100 mg/L RB5 dye solution, 110 mg/L lignin-METAC dosage (sample 4), pH 6, 30 °C).

Figure 9

Hydrodynamic diameter (Hy) of lignin-METAC (sample 4) and dye RB5 in solutions containing NaCl or Na₂CO₃, pH 6, 30 °C.

Figure 10

The COD removal of 100 mg/L RB5 and RO16 by 120 mg/L lignin-METAC dosage (sample 4) for RB5 and 105 mg/L lignin-METAC (sample 4) dosages for RO16 pH 6, 30 °C.