Adsorption of As(V) by magnetic alginate-chitosan porous beads based on iron sludge

Magnetic alginate-chitosan porous beads based on iron (Fe) sludge (M-ACFBs) were prepared by waterworks iron sludge, sodium alginate, chitosan, and magnetic nanoparticles. The magnetic nanoparticles were also synthesized using waterworks iron sludge through the co-precipitation method. In addition, a double gel network of sodium alginate and chitosan was constructed to improve the pH stability of the beads. At the same time, iron sludge served as the functional body for As(V) adsorption. The beads have uniform bead size (∼2 mm), high specific surface area (115.4 m2/g), distinguished mesopores (5.7 nm in size), and strong saturation magnetization (∼15.0 emu/g). The ratio of magnetic nanoparticles to iron sludge was optimized, and the effects of pH, contact time, temperature, and coexisting ions on As(V) adsorption effect were studied. Also, the adsorption kinetics and adsorption isotherms are thoroughly discussed. The adsorption mechanism of M-ACFBs on As(V) is concluded as ligand exchange and electrostatic attraction, and the maximum adsorption capacity is as high as 14.2 ± 0.4 mg/g. It is found that magnetite (Fe3O4) can form a maghemite (γ-Fe2O3) layer in an oxygen-rich environment, and at the surface of this γ-Fe2O3 layer, Fe(II), O2, and As(V) undergo complex redox reactions, leading to the appearance of As(III), which leads to a challenge for the disposal of arsenic-loaded adsorbents. This study provides a reference pathway for the resource utilization of backwash iron sludge and As(V) removal from water.