Phosphate adsorption on amorphous aluminum hydroxide-yielded transition aluminas: Insights on fundamental chemistry and making engineering adsorbents

Transition aluminas (TAs) are evolved alumina polymorphs during mineral phase transformation from amorphous to crystalline structure, which have been extensively used as engineering adsorbents or platforms of functional materials for pollutants (e.g. eutrophication nutrient PO43−) mitigation in water remediation. A well-understanding of structure-binding activity of pollutants on transition aluminas is essential for developing promising water remediation materials. In this work, TAs were synthesized through annealing amorphous aluminum (oxy)hydroxide, and a decreased trend of phosphate (PO43−) adsorption was observed, which was not in pace with the changes of specific surface areas and site densities. The structure disorders of TAs deteriorated continuously peaked at 700 °C and TA at 700 °C has the largest extent of structural disorder, but does not present the highest adsorption capacity, which is not consistent with the consensus that oxides' structural disorder facilitate adsorption. Multiple techniques including pair distribution function, selective chemical extraction, 27Al NMR and quantum chemistry calculations were employed to reveal the underlying mechanism differences of phosphate adsorption on TAs. The results indicated the fraction of amorphous contents is the key feature governing PO43− adsorption, regardless of mineral phase, structural disorder, AlO4/AlO6 arrangement, and Gibbs free energy of adsorption. Three strategic measures are suggested on making promising alumina engineering adsorbents, including tuning transition temperature, introducing non-tetrahedral and non-octahedral nanostructured grains, and employing more potential methods for synthesizing amorphous nanocrystalline composite. This work sheds lights on understanding the fundamental chemistry of phosphate on transition alumina and producing prospective alumina remediation materials.