Effect of NaOH concentration on structure and phosphate adsorption of polymer-based hydrated ferric oxide composite adsorbents

Polymer-based hydrated ferric oxide (HFO) composite adsorbents had attracted extensive attention because of large phosphate adsorption capacity, rapid adsorption rate and high regeneration efficiency. Nevertheless, whether the same structure and phosphate adsorption could be performed by composite adsorbents prepared at different NaOH concentration was still unclear. In order to provide evidence for optimization of polymer-based hydrated ferric oxide composite adsorbents preparation, the structure and phosphate adsorption of composite adsorbents prepared by regulating NaOH concentration were studied. The results show that when NaOH concentration increases from 1 mol·L⁻¹ to 6 mol·L⁻¹, there are no significantly effect on the HFO loadings (approximately 16wt%, mass fraction in Fe) and crystal structure of composite adsorbents. However, the considerable self-agglomerate of HFO nanoparticles decrease, and HFO nanoparticles distribute evenly. Moreover, phosphate adsorption capacity is increased with NaOH concentration and then remains the same (18 mg·g⁻¹). Additionally, adsorption equilibrium times are 240 min, and the adsorption kinetic curves are fitted well with pseudo-first order kinetic model (R² > 0.99). The optimal pH value for phosphate adsorption is 6-8, and the influence degree of phosphate adsorption by co-existing ions is SO₄²⁻>Cl⁻>NO₃⁻ under the same concentration. The regeneration efficiencies of composite adsorbents approach 100% during 5 continuous adsorption regeneration cycles. The experiments show that the HFO nanoparticles distribute evenly with the increase of NaOH concentration, and the phosphate adsorption capacity of composite adsorbents is increased and then remains the same, while there is no significant difference in crystal structure, adsorption equilibrium time, optimal pH, effect of coexisting anions, and elution effect.