Effective removal of sulfur dyes from water by biosorption and subsequent immobilized laccase degradation on crosslinked chitosan beads

In this study, sulfur dyes were effectively removed from water samples through biosorption and subsequent immobilized laccase degradation on glutaraldehyde-crosslinked chitosan beads (GA-CBs). The Langmuir equation described the biosorption isotherms of the single dyes, sulfur blue 15 (SB15) and sulfur brown GD (SBGD), well. The decreased biosorption of one dye was found in binary mixtures by increasing the concentration of the second dye, and the extended Freundlich equation could reasonably describe such isotherms. Laccase immobilized on GA-CBs showed a high catalytic ability and provided reusability even at low laccase concentrations. In the treatment of single dyes at an initial concentration of 200 mg L⁻¹, for example, over 81.6% and 70.8% of SB15 and SBGD, respectively, were removed at a pH of 6.5 after the successive biosorption and enzymatic degradation with a biosorbent dose of 3.6 g L⁻¹, an initial laccase activity of 2.24 U L⁻¹, and 2.66 × 10⁻⁴ mg (g support)⁻¹ of immobilized laccase. In the treatment of binary mixtures under the same conditions, the efficiency of each dye decreased. The overall removals of SB15 and SBGD from binary mixtures containing 100 mg L⁻¹ of SB15 and 100 mg L⁻¹ of SBGD became 78.2% and 52.6%, respectively.