Role of zeolite's exchangeable cations in phosphate adsorption onto zirconium-modified zeolite

To determine the role of zeolite's exchangeable cations such as Ca2 + and Na+ in the adsorption of phosphate on zirconium-modified zeolite (ZrMZ), the adsorption behaviors and mechanisms of phosphate on ZrMZ, NaCl-treated ZrMZ and CaCl2-treated ZrMZ in the absence and presence of ammonium were comparatively investigated in this study. Results showed that coexisting ammonium promoted the adsorption of phosphate on the raw, NaCl-treated and CaCl2-treated ZrMZs. In the absence of ammonium, the treatment of ZrMZ with NaCl inhibited the phosphate adsorption, but the treatment of ZrMZ with CaCl2 enhanced the adsorption of phosphate. In the presence of ammonium, the treatment of ZrMZ with NaCl suppressed the adsorption of phosphate, but the effect of CaCl2 treatment on the phosphate adsorption depended upon coexisting ammonium amount. In the presence of a certain amount of ammonium (5–30 mg N/L), the treatment of ZrMZ with CaCl2 improved the phosphate adsorption efficiency. The mechanism for phosphate adsorption onto ZrMZ was the exchange of surface hydroxyl groups with phosphate ions and the formation of phosphate inner-sphere coordination complexes, and the enhancement of phosphate adsorption onto ZrMZ by coexisting Ca2 + could be attributed to the formation of CaHPO40 species in the calcium/phosphate solution and the formation of phosphate-bridged ternary complex (Zr)(OPO3H)Ca on the adsorbent surface. The increased phosphate adsorption induced by CaCl2 treatment could be attributed to the increase in the amount of released Ca2 + from ZrMZ, and the decreased phosphate adsorption induced by NaCl treatment could be attributed to the decrease in the amount of released Ca2 + from ZrMZ. This work suggests that the exchangeable Ca2 + in ZrMZ plays an important role in the phosphate adsorption, and its release enhances the adsorption of phosphate.