Enhanced Removal of Aqueous Phosphorus by Zr–Fe-, Mn–Fe-, and Mn–Zr–Fe-Modified Natural Zeolites: Comparison Studies and Adsorption Mechanism

Novel adsorbents [Zr–Fe-, Mn–Fe-, and Mn–Zr–Fe-modified natural zeolites (MNZs)] were prepared by a green synthetic method and used for aqueous phosphorus removal. The MNZs were characterized by scanning electron microscopy, Brunauer–Emmett–Teller (BET), X-ray powder diffraction, X-ray photoelectron spectroscopy, Fourier-transform infrared spectrometer, and magnetic property. The results showed that the BET-specific surface areas of the synthesized novel adsorbents (92.599–279.660 m2/g) were much higher compared with natural zeolites (17.494 m2/g). Strong magnetism was observed for the synthesized novel adsorbents. At 35 mg/L phosphorus and 308 K, the equilibrium adsorption amounts of Mn–Fe-, Zr–Fe-, and Mn–Zr–Fe-modified zeolites were 11.8, 8.8, and 20.9 mg/g, respectively, which were 3.7, 2.8, and 6.5 times that of natural zeolites (3.2 mg/g). The pseudo-second-order kinetics model (R2 > 0.98) and Langmuir model (R2 > 0.96) were more suitable to describe the adsorption process. The intraparticle diffusion rates of the modified zeolites were in the order of Mn–Zr–Fe-, Mn–Fe-, and Zr–Fe-modified zeolites, respectively. Adsorption of phosphorus onto the modified zeolites was an endothermic process (ΔH0 > 97 kJ/mol). Acidic conditions contributed to the adsorption of phosphorus. The adsorption capacity of the synthesized novel adsorbents decreased with increasing coexisting ionic strength, and Cl− and HCO3− had the minimum and maximum impact on the adsorption of phosphorus, respectively. The ligand exchange and electrostatic attraction played key roles in the adsorption of phosphorus onto the modified zeolites. Excellent regeneration ability for the modified zeolites was observed. The synthesized novel adsorbents have great potential applications in removing phosphorus from eutrophic water.