One-step combustion synthesis of undoped c-ZrO₂for Cr(VI) removal from aqueous solutions

The active practical application of materials based on cubic zirconium dioxide (c-ZrO₂) for catalysis, luminescence, and sorption of heavy metals demands the development of methods for its preparation in a nanostructured form. In this work, nanoparticles of undoped cubic zirconia were obtained by solution combustion method, the features of their structure and morphology were investigated, and the efficiency of their use as a basis for sorbents for the removal of hexavalent chromium Cr(VI) from aqueous solutions was evaluated. Based on XPS, it was established that the stabilization of the high-temperature cubic phase of c-ZrO₂occurred due to oxygen vacancies which were formed during the synthesis by glycine-nitrate combustion. From the results of PXRD and Raman spectroscopy cubic structure of the obtained zirconium dioxide nanoparticles is concluded, the average crystallite size was approximately 2 nm. Adsorption structural analysis and SEM indicated aggregation of c-ZrO₂nanocrystals into primary (45-95 nm) and secondary (submicron) agglomerates. The specific surface of the nanopowder determined by the Brunauer-Emmet-Teller method was 25.4 m²g^-1, the pore volume was 0.1670 cm³g^-1, the major part of which is associated with interparticle porosity. Using kinetic pH-metry, it was found that on the surface of synthesized c-ZrO₂, rapidly hydrated aprotic Lewis acid centers predominated, and the point of zero charge was 5.8. The results of the sorption of Cr(VI) from aqueous solutions with a concentration of 48-242 mg l^-1at 25 °C and pH = 5 are described by the Freundlich isotherm (R² = 0.971), which corresponds to multilayer adsorption. The maximum adsorption capacity according to Langmuir was 33 mg g^-1or 1.34 mg m^-2per unit area. These results allow us to consider the obtained undoped zirconium dioxide as a promising base for sorbents of heavy metals.