Adsorption of phosphate by acid-modified fly ash and palygorskite in aqueous solution: Experimental and modeling

Abstract

In this study, a class F fly ash and palygorskite have been acid-modified and then evaluated for the adsorption of phosphate in aqueous solution via bench-scale batch experiments. XRD, XRF, SEM, and FTIR were employed to characterize the acid-modified fly ash (MFA) and palygorskite (MPal). Both MFA and MPal show enhanced phosphate adsorption after the modification treatment. The effects of pH, adsorbent dosage, and co-ions on phosphate adsorption, as well as adsorption thermodynamics and kinetics, and leaching features of spent (used) adsorbents were also investigated. The isotherms data fit well with the Langmuir model rather than the Freundlich model, giving maximum capacities (298K) of 13.3mg P g−1 for MFA and 10.5mg P g−1 for MPal, respectively. Surface complexation modeling of P adsorption data with the nonelectrostatic generalized composite (GC) approach indicates that phosphate were directly bound to the metal centers by ligand exchange to form two monodentate complexes, ≡SHPO4− and ≡SPO42−. The GC model appears to be an easy and efficient tool to provide an insight into the mechanism of phosphate adsorption on complex adsorbents with limited model parameters. Leaching test results suggest that the spent adsorbents can be safely disposed or further reused.