Efficient removal of direct dyes and heavy metal ion by sodium alginate-based hydrogel microspheres: Equilibrium isotherms, kinetics and regeneration performance study

Improving the adsorption capacity of materials for pollutants by means of modification is an important direction in the research of water treatment technology. To improve the applicability of sodium alginate composites in the field of adsorption, magnetic sodium alginate-based hydrogel microsphere adsorbent material Fe₃O₄@SA/PEI-Fe (FSPF) was synthesized in a single step by using polyethyleneimine grafting modification of sodium alginate by sol-gel method. The material was used for the removal of direct blue GL (DB 200) and direct date red B (DR 13) from simulated wastewater, as well as Cu(II) and Pb(II) from simulated wastewater with heavy metal ions. Under the optimum preparation conditions, 0.025 g of FPSF could adsorb 2400 mg/L and 1700 mg/L of DB 200 and DR 13 dyes in 60 min, with removal rates of 92.9 % and 90.8 %, and adsorption quantities of 2229 mg/g and 1539 mg/g, respectively. For 300 mg/L and 400 mg/L pH 5.5 of the For 300 mg/L and 400 mg/L of Cu(II) and Pb(II) ions at pH 5.5, the FPSF reached adsorption equilibrium within 30 min, with removal rates of 95.8 % and 92.3 %, and adsorption quantities of 239 mg/g and 307 mg/g, respectively, which demonstrated highly efficient adsorption performance. In addition, the material has good magnetic responsiveness and can be quickly separated from the water column under an applied magnetic field. The adsorption mechanism follows the Pseudo-second-order kinetic model equation and Langmuir model, which is mainly monolayer adsorption. Thermodynamic analysis showed that the adsorption process for dye molecules was a spontaneous exothermic reaction, while the adsorption process for heavy metal ions was a spontaneous adsorption reaction. Cyclic regeneration experiments showed that after five rounds of adsorption-desorption cycles, the removal efficiencies of the material for the four pollutants in water were still maintained at about 80 % of the initial level.