Different performances and mechanisms of phosphate adsorption onto metal oxides and metal hydroxides: A comparative study

Abstract

BACKGROUND: Adsorption plays a significant role in phosphate removal and recovery. Metal (hydro)oxides are the most popular adsorbents for phosphate adsorption, but their performances differ greatly, with different adsorption mechanisms under different conditions. A comparative study of phosphate adsorption onto iron hydroxide and iron oxide doped activated carbon fiber (ACF-FeOH and ACF-FeO) was performed. RESULTS: The phosphate adsorption performance of ACF-FeOH and ACF-FeO with different initial pH, ion strength and competing ions was investigated. The contrasting results showed that the amount adsorbed by ACF-FeOH is higher than that by ACF-FeO, and phosphate adsorption on ACF-FeOH is more dependent on pH and ion strength than on ACF-FeO. The adsorption mechanisms were studied by zeta potential measurements, X-ray photoelectron spectroscopy (XPS) analysis and batch desorption tests. The significantly higher isoelectric point (IEP) of ACF-FeOH with respect to that of ACF-FeO resulted in a greater phosphate adsorption by ACF-FeOH than by ACF-FeO. XPS analysis showed that the concentration of surface hydroxyl groups on ACF-FeOH is also higher than that on ACF-FeO. CONCLUSION: ACF-FeOH performs better than ACF-FeO with higher adsorption capacity. Electrostatic interaction and ion exchange are the two main adsorption mechanisms both for ACF-FeOH and for ACF-FeO. This study should provide some guidance on the preparation and application of iron-based sorbents in phosphorus removal and recovery.