Sorption characteristics of phosphate by bauxite residue in aqueous solution

Abstract

This study was conducted to evaluate the phosphate sorption properties of red and brown muds, with particular focus on the reaction time. The amount of phosphate adsorbed by these two muds correspond better with the Freundlich isotherm and pseudo-second-order models than with the Langmuir isotherm and pseudo-first-order models. The maximum sorption capacities of phosphate by red and brown mud were 17.0 mg/g and 43.1 mg/g, respectively, which indicated a 2.5 times higher sorption capacity of phosphate by brown mud compared to red mud. The phosphate content, extracted by each extraction solution, of the total amount of adsorbed phosphate was in the order of Ca2-P (60.0%) > Ca8-P (26.4%) > Fe-P (6.5%) > O-P (2.9%) > Ca10-P (2.2%) > Al-P (0%) for red mud and Ca2-P (57.0%) > Ca8-P (33.5%) > Ca10-P (5.0%) > O-P (3.1%) > Fe-P (1.4%) > Al-P (0%) for brown mud. In particular, the sorption of phosphate by Ca-P in the red and brown mud corresponded to 88.6% and 95.5% of the total sorption amount, respectively. A comprehensive review of the above results showed that the removal of phosphate by red and brown mud was time dependent, which was dominantly influenced by Ca-P formed through the reaction between Ca ions and phosphates in each mud. The results of SEM-EDS mapping and FTIR clearly demonstrated the presence of phosphorus in both muds after phosphate sorption.