Iron-loaded magnetic alginate-chitosan double-gel interpenetrated porous beads for phosphate removal from water: Preparation, adsorption behavior and pH stability

Phosphate removal from water relies mainly on the effective adsorbent. Iron-loaded magnetic alginate-chitosan double-gel interpenetrated porous beads (M-IACBs) were prepared from waterworks iron sludge, magnetic nanoparticles, sodium alginate, and chitosan where magnetic nanoparticles were also synthesized from iron sludge. The interpenetrating network constructed by sodium alginate and chitosan improves the stability of the beads, while iron sludge acts as the main functional body for phosphate adsorption. M-IACBs with uniform size (~ 2 mm) and strong saturation magnetization intensity (~ 15.0 emu/g) maintain good stability in the pH range of 4–8. They have good selectivity for phosphate in the presence of competing ions. The phosphate adsorption by the beads followed the Langmuir model, indicating that the adsorption was dominated by monolayer adsorption, and the fitting yielded a maximum phosphate adsorption capacity of 18.5 mg/g. The pseudo-second-order model better agrees with the experimental data. The adsorption properties of iron sludge and beads were compared. Granulation was found to enhance the availability of the adsorbent but slow down the adsorption kinetics. The adsorption mechanisms of phosphate are ligand exchange and electrostatic attraction. This study provides a reference pathway for phosphate removal and resource utilization of iron sludge in waterworks. • Iron sludge was recycled to prepare magnetic granular adsorbent for phosphate removal. • The maximum adsorption capacity of the beads is as high as 18.5 mg/g. • The double gel network formed by sodium alginate and chitosan improves the pH stability of the beads. • The adsorption mechanism includes electrostatic attraction and ligand exchange.