Efficient and stable removal of phosphate from aqueous solutions by hollow microspheres of MgO/ZrO2 composite oxide

Excessive phosphorus is regarded as one of the main causes of eutrophication, destroying the balance of the original ecosystem of water bodies and endangering human health. In this study, a novel MgO/ZrO₂ composite oxide with a hollow structure (MgZr-CS) was prepared by a simple carbon template method, and served as an adsorbent for the efficient uptake of phosphate from aqueous solutions. For comparison, MgO/ZrO₂ bimetal oxide (MgZr) was also prepared by the conventional co-precipitation method. The effects of pH and coexisting anions on the adsorption of phosphate were studied. The adsorption performance of materials was described by the adsorption kinetic model and isothermal model. The adsorption capability of both MgZr-CS and MgZr was negligibly affected by coexisting ions (e.g., Cl⁻, NO₃⁻, HCO₃⁻ and SO₄²⁻) in water. The adsorption kinetic of both MgZr-CS and MgZr was better correlated with the pseudo-second-order kinetic model, while the adsorption isotherm followed the Langmuir model. However, the Langmuir maximum phosphate adsorption capacity of MgZr-CS was 149.3 mg/g at initial pH of 7, which was about twice that of MgZr. Remarkably, different from MgZr that only presented well phosphate adsorption performance under the acidic environment, MgZr-CS could maintain the excellent adsorption performance within a particularly wide pH range of 3–11. Compared to adsorbents reported in previous papers, MgZr-CS also exhibited larger adsorption capacity and higher pH stability. Moreover, MgZr-CS performed prominent in the treatment of actual domestic sewage, and its phosphate removal rate could reach 99.3%. The adsorption mechanism analysis revealed that the excellent phosphate performance of MgZr-CS was mainly due to the existence of oxygen vacancies providing more active adsorption sites. Results shown here indicated that MgZr-CS can be used as a potential adsorbent for phosphate.