Unexpected favorable role of Ca2+ in phosphate removal by using nanosized ferric oxides confined in porous polystyrene beads.

Polystyrene-based nano-ferric oxide composite is a representative nanomaterial successfully applied in scale-up water decontamination for arsenic and phosphorous. However, little is available on the effect of solution chemistry (for instance, the coexisting Ca2+) on 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 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 co-precipitation 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 co-precipitation, 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 co-precipitation 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 towards 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.