Acid-treated sodium alginate-based composite gel microspheres toward methylene blue removal: Fabrication and mechanistic insights

Dye-contaminated wastewater from the textile and dyeing industries poses severe environmental and health risks due to the persistence and toxicity of synthetic dyes. To address this issue, sodium alginate (SA)-based composite gel microspheres were fabricated via ionic crosslinking with calcium chloride followed by acid exchange to enhance structural stability and adsorption performance. Two types of functionalized adsorbents were synthesized and optimized: SA modified, and with stearyltrimethylammonium chloride (SA/S-H) and SA co -functionalized with poly(sodium 4-styrenesulfonate) (SA/S/PS-H). Comprehensive characterization using scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed the successful preparation and compositional uniformity of the adsorbents. Adsorption kinetics for methylene blue (MB) followed the pseudo-second-order model, reaching equilibrium within 6 h， while isotherm data fitted the Dubinin-Radushkevich (D-R) model. Notably, the incorporation of PSSS significantly enhanced the adsorption capacity; at 45 °C, SA/S/PS-H achieved a remarkable maximum adsorption capacity of 900.91 mg/g, compared to 612.61 mg/g for SA/S-H. The dominant adsorption mechanisms were identified as electrostatic attraction, micropore filling and hydrogen bonding. Furthermore, the microspheres showed excellent reusability despite the interference of co-existing cations. This study presents a simple and scalable approach for constructing high-performance bio-based adsorbents with significant potential for sustainable and practical treatment of dye-laden wastewater.