Arsenate Adsorption by Amorphous Titanium (Hydr)Oxides: Governing Roles of Medium-Range Ordering and Surface Disorder

Amorphous metal (hydr)oxides are used for oxyanion removal from water, yet the structural features responsible for their adsorption performance remain poorly understood. This paper systematically investigated how the ordering range and associated structural characteristics of amorphous titanium (hydr)oxides influence the adsorption of arsenate, used here as a model oxyanion. Six materials were synthesized with ordering ranges spanning approximately 4 to 43 Å. As the ordering range increased, a transition from microporous–mesoporous structures to predominantly mesoporous architectures was observed. Characterizations revealed progressive structural ordering with increasing synthesis aging temperature. The most amorphous sample exhibited stronger water binding, providing indirect evidence of fewer surrounding hydration clusters than in more ordered samples. However, despite possessing the highest specific surface area, the most amorphous material did not exhibit the highest arsenate uptake. Instead, the material aged at 140 °C achieved the greatest adsorption capacity, exceeding that of the fully amorphous material by >20%. This adsorption maximum coincided with an intermediate ordering range, a transitional structural state, and a mesopore-dominated pore architecture. The results indicate that arsenate adsorption is not governed by surface area alone. Instead, the ordering range provides a useful descriptor linking atomic-scale structure, pore architecture, hydration behavior, and adsorption performance in amorphous metal (hydr)oxides.