A novel hetero-exopolysaccharide for the adsorption of methylene blue from aqueous solutions: Isotherm, kinetic, and mechanism studies

Abstract Dye pollution in water poses severe health and ecological risks to humans and the environment. Thus, we aim to develop an effective adsorbent for the removal of methylene blue (MB) and to study the adsorption process and the adsorption mechanism through both experimental data and mathematical modeling of isotherms and kinetics. Here, a negatively charged hetero-exopolysaccharide, exopolysaccharide-R040 (EPS-R040) as an effective adsorbent for the removal of MB from aqueous solutions was prepared from a newly identified Lactobacillus plantarum. EPS-R040 exhibited a rough porous network structure with holes or caves and it was composed of galactose, glucose, mannose, and glucuronic acid with several modifications, including carbonylation, acylation, sulfation, and carboxylation, and a highly negative charge. The adsorption ability was affected by the contact time, temperature, the initial MB content, and pH. Maximum adsorption capacity of 1853.0 mg/g was observed with the equilibrium adsorption process fitting the Langmuir isotherm and following the pseudo-second-order and Elovich models. The Weber-Morris model proved that the adsorption was a chemisorption process coupled with intraparticle and boundary layer diffusions. EPS-R040 also exhibited excellent desorption ability and can be reused for three cycles without a significant loss of adsorption ability. Moreover, hydrogen binding and electron interaction were proved to be the main adsorption mechanism for MB. All these findings reveal the potential application of EPS-R040 in removal of MB from aqueous solutions due to its good thermal stability, excellent adsorption performance, simple preparation, safety and no synthesis required.