Effective adsorption of cationic dyes by lignin sulfonate polymer based on simple emulsion polymerization: isotherm and kinetic studies

Abstract

This study describes the synthesis of a lignin sulfonate polymer based on a simple emulsion polymerization from lignin sulfonates derived from the accessible by-products of paper pulp and the adsorption properties of the lignin sulfonate polymer towards dyes. Fourier transform infrared spectroscopy (FT-IR) was employed to characterize the successful synthesis of the lignin sulfonate polymer. The lignin sulfonate polymer presents selective adsorption of cationic dyes in aqueous solution, with a percentage adsorption of malachite green exceeding 95% at pH 7. The effects of pH, sorbent amount, initial malachite green concentration and temperature on the adsorption properties have been discussed. 4 kinetic models and 3 isotherm models were employed to evaluate the experimental data. The results indicated that the adsorption kinetics data fit a pseudo second-order kinetic model and the Langmuir adsorption isotherm was applicable for the adsorption of malachite green onto the lignin sulfonate polymer. Furthermore, the thermodynamic analysis (the values of ΔG⁰ are negative and between −20 and 0 kJ mol⁻¹, the values of ΔH⁰ and ΔS⁰ are positive) demonstrates that the adsorption process of malachite green onto the lignin sulfonate polymer is endothermic, spontaneous and random. The results suggest that the lignin sulfonate polymer is a low-cost, alternative sorbent and has the potential to reduce the refractory chemical oxygen demand (COD) in effluents.