Unexpected Favorable Role of Ca2+ 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 Ca2+) on the long-term performance of the nano composite. In this study, we carried out 20 cyclic runs of phosphate adsorption-desorption on a polymer-supported ferric nano composite HFO@201. Unexpectedly, an enhanced phosphate removal was observed in the presence of Ca2+, 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 HFO@201 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 Ca2+ increased the effective treatable volume of HFO@201 toward P-containing influents by similar to 70%. This study is believed to shed new insights into the effect of solution chemistry on similar nanocomposites for advanced water treatment.