Modulating the Central Metal Redox-State Ratios of Amorphous Titanium (Hydr)Oxide by Incorporating Cerium Enhances Mixtures of Oxyanions Removal from Water

Amorphous metal (hydr)oxides exhibit superior oxyanion adsorption properties but remain understudied due to characterization challenges compared to their crystalline counterparts. Their structural disorder can alter the central metal redox states, enhancing oxyanion adsorption. Previously, we demonstrated that the reduced redox state of titanium in amorphous titanium (hydr)oxide (a-TiHO) increased arsenate and arsenite adsorption. Here, we show that incorporating cerium into a-TiHO further enhances redox modulation, improving the simultaneous adsorption of multiple oxyanions (arsenate, vanadate, tungstate, chromate, and selenate) from water. Using sol-gel synthesis, we produced cerium-titanium (hydr)oxides with varying Ce(III)/Ce(IV) and Ti(III)/Ti(IV) ratios, including a highly crystalline material via annealing. The best adsorption occurred in a 98% amorphous material with a 10:1 reduced-to-oxidized ratio of the central metal, achieving capacities of 22 and 19 μmol of oxyanion per millimole of metal at pH 6.5 and 8.5, respectively. Increasing the cerium content consistently improved oxyanion mixture adsorption. Advanced characterization, such as pair distribution function (PDF), combined with X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR), revealed that higher (III)/(IV) central metal ratios correlate with increased oxygen vacancies, which enhance oxyanion adsorption. These findings demonstrate the potential of tuning amorphous metal (hydr)oxides for advanced water remediation applications.