Interaction between phosphate and acid-activated neutralized red mud during adsorption process

•Interaction between phosphate and AaN-RM was first time investigated.•Phosphate adsorption was governed by heterogeneous processes.•Different phosphate complexes were formed on AaN-RM surface.•59.78% of phosphate was adsorbed by ion exchange and precipitation mechanisms.•40.22% of phosphate was adsorbed by surface deposition mechanism. Acid-activated neutralized red mud (AaN-RM) has become a promising adsorbent for phosphate adsorption. The maximum phosphate adsorption capacity of AaN-RM reached 492.46mgg−1 in this study, which was much higher than that of many other adsorbents. However, no study has specifically investigated how the phosphate reacted with AaN-RM. For the first time the interaction between phosphate and AaN-RM during adsorption process was investigated in this research. Kinetic models and isotherms were used to analyze the possible reaction pathways between AaN-RM and phosphate. Particularly, the phosphate complexes on AaN-RM surface, and the exact role of different adsorption mechanisms were systemically identified. The phosphate adsorption was well described by pseudo second-order kinetic model and Langmuir–Freundlich isotherm, which suggested that chemisorption occurred between the phosphate and AaN-RM, and the phosphate adsorption was governed by heterogeneous processes. Furthermore, the phosphate complexes of Fe–P, Al–P, Fe–P–H3PO4 and Al–P–H3PO4 were formed on AaN-RM surface through ion exchange, precipitation and surface deposition mechanisms. XPS analysis of P 2p peak showed that 59.78% of the phosphate was adsorbed through the ion exchange and precipitation with strong chemical bonds, and 40.22% was adsorbed through the surface deposition with weak chemical bonds.