Temperature-induced modulation of mesopore size in hierarchically porous amorphous TiO2/ZrO2 beads for improved dye adsorption capacity

Abstract

Amorphous TiO₂/ZrO₂ matrices have received widespread interest in adsorption-based water remediation schemes owing to their high adsorption capacity potential in view of their high surface area. However, such potential is often limited by their intrinsic microstructure. Herein, temperature processes were examined in combination with a sol–gel templating technique for the fabrication of micro-meso-macroporous millimeter-sized amorphous TiO₂/ZrO₂ (64/36% w/w) beads with variable mesopore sizes (6–13 nm). The beads featured high surface areas (up to 273 m² g⁻¹) and high hydroxyl group densities (up to 4.1 OH nm⁻²). Unlike the conventional post-synthesis calcination temperature treatment, the temperature treatment applied to the template and the hybrid alginate/TiO₂/ZrO₂ beads allowed pore size modulation without compromising the amorphous nature of the TiO₂/ZrO₂ materials, thus retaining the high surface areas. The TiO₂/ZrO₂ beads displayed high adsorption capabilities (e.g., q_max > 40 mg g⁻¹ compared with <5 mg g⁻¹ for inorganic clays; 25.1 mg g⁻¹ for canola stalks; 43.5 mg g⁻¹ for α-Fe₂O₃) towards acid orange 7 (AO7), a typical water pollutant. The study demonstrated correlation between the mesopore dimensions and q_max. By enlarging the mesopore of the amorphous TiO₂/ZrO₂ beads, accessibility of the AO7 molecule to the abundant surface sites, as provided by the high surface areas and surface hydroxyl group densities, was improved. This led to enhanced AO7 adsorption capacities (up to 101 mg g⁻¹). The macroscopic dimension, spherical geometry, and uniform bead size of the TiO₂/ZrO₂ matrices are attractive attributes as plant-scale chromatography packing.