Comparative analysis of humic acid, lignin and lignite-based hydrogels for methylene blue dye removal

Abstract

This study investigates the application of hydrogels incorporating natural polymers - ammonium humate, lignosulfonate, and ammonium lignite to adsorb methylene blue (MB) from aqueous solutions. The hydrogels were synthesized via free radical polymerization and characterized using FTIR, ¹³C-NMR, SEM, TGA, and XRD. The synthesized hydrogels demonstrated exceptional swelling capabilities. The influence of various parameters, including adsorbent amount (0.1-0.4 g), temperature (293.15-323.15 K), and initial dye concentration (5-20 mgL^-1) on MB removal efficiency was systematically evaluated using the Taguchi method. Under optimized conditions, HA-g-SAH achieved a maximum removal efficiency of 93.7% ±1.1% at 323.15 K, significantly outperforming LS-g-SAH (92.4% ±1.3%), Lt-g-SAH (82.4 ±1.4%), and the control (ctrl) hydrogel (67.9 ±0.9%). According to ANOVA results, the change of biopolymer proved to be an effective factor in the improved dye removal efficiency of hydrogels. Thermodynamic parameters (ΔG°, ΔH°, and ΔS°) were evaluated to assess the temperature effect on adsorption. The adsorption isotherm data were analyzed by applying the Langmuir and Freundlich models. The findings highlight the effectiveness of natural polymer-based hydrogels as sustainable and efficient adsorbents for removing dyes from wastewater.