Unexpected Favorable Role of Ca 2+ in Phosphate Removal by Using Nanosized Ferric Oxides Confined in Porous Polystyrene Beads

Polystyrene-based nanoferric oxide composite is a representative nanomaterial successfully applied in scale-up water decontamination for arsenic and phosphorus. However, little is available on the effect of solution chemistry (for instance, the coexisting Ca 2+ ) on the long-term performance of the nanocomposite. In this study, we carried out 20 cyclic runs of phosphate adsorption–desorption on a polymer-supported ferric nanocomposite [email protected] Unexpectedly, an enhanced phosphate removal was observed in the presence of Ca 2+, which is quite different from its adverse effect on phosphate capture by granular ferric oxide. Further mechanistic studies revealed that enhanced phosphate removal was mainly realized via the Ca–P coprecipitation inside the networking pores of [email protected] as well as the possible formation of the multiple Fe–P–Ca-P complex. The complex formation led to a distinct increase in P adsorption, and the coprecipitation, driven by the accumulated OH – in confined pores during phosphate adsorption and alkaline regeneration, favored P removal via the formation of amorphous calcium phosphate (ACP) and hydroxyapatite inside. TEM-EDS spectra indicated that coprecipitation did not occur on the surface of loaded nano-HFO, greatly mitigating its adverse effect on P adsorption on the surface of nano-HFO. Fixed-bed column study showed that the presence of Ca 2+ increased the effective treatable volume of [email protected] toward P-containing influents by ∼70%. This study is believed to shed new insights into the effect of solution chemistry on similar nanocomposites for advanced water treatment.