Enhanced phosphate removal with fine activated alumina synthesized from a sodium aluminate solution: performance and mechanism

Abstract

Fine activated alumina (FAA) acting as an adsorbent for phosphate was synthesized from an industrial sodium aluminate solution based on phase evolution from Al(OH)₃ and NH₄Al(OH)₂CO₃. This material was obtained in the form of γ-Al₂O₃ with an open mesoporous structure and a specific surface area of 648.02 m² g⁻¹. The phosphate adsorption capacity of the FAA gradually increased with increases in phosphate concentration or contact time. The maximum adsorption capacity was 261.66 mg g⁻¹ when phosphate was present as H₂PO₄⁻ at a pH of 5.0. A removal efficiency of over 96% was achieved in a 50 mg L⁻¹ phosphate solution. The adsorption of phosphate anions could be explained using non-linear Langmuir or Freundlich isotherm models and a pseudo-second-order kinetic model. Tetra-coordinate AlO₄ sites acting as Lewis acids resulted in some chemisorption, while (O)_nAl(OH)₂⁺ (n = 4, 5, 6) Brønsted acid groups generated by the protonation of AlO₄ or AlO₆ sites in the FAA led to physisorption. Analyses of aluminum-oxygen coordination units using Fourier transform infrared and X-ray photoelectron spectroscopy demonstrated that physisorption was predominant. Minimal chemisorption was also verified by the significant desorption rate observed in dilute NaOH solutions and the high performance of the regenerated FAA. The high specific surface area, many open mesopores and numerous highly active tetra-coordinate AlO₄ sites on the FAA all synergistically contributed to its exceptional adsorption capacity.