Immobilization of phosphate by a Technosol spolic silandic: kinetics, equilibrium and dependency on environmental variables

PURPOSE: Phosphorus is an essential element that at high concentrations generates eutrophication of aquatic systems. In this study, we used batch and continuous tests to evaluate the efficiency of a Technosol to retain the phosphorus present (as phosphate) in aqueous samples. MATERIALS AND METHODS: Phosphate sorption on Technosol was studied through batch and continuous experiments. Sorption kinetics and isotherms were investigated at different phosphate loadings and pH. In batch tests, we have determined how the presence of different anions (bicarbonate, sulphate, chloride, chromate and molybdate) affected phosphate retention. In continuous flow systems, phosphate immobilization was assessed as a function of flow rate, pH and Technosol concentration. Finally, the potential reutilization of the column was evaluated using consecutive sorption-desorption cycles. RESULTS AND DISCUSSION: Phosphate sorption follows a pseudo-second-order kinetics model and a Langmuir isotherm model. The maximum sorption capacity ranged from 7.1 to 18.5 mg g⁻¹, with larger values obtained at the highest pH. The main mechanisms involved in the sorption process were precipitation (as Ca-P minerals) and surface adsorption. In the column experiments, we observed a sorption reduction from 6.19 ± 0.06 to 2.37 ± 0.06 mg g⁻¹ as the flow rate increased from 1.5 to 5.0 mL min⁻¹. In addition, the retention capacity decreased by 14% when the height of the reactive layer was halved. Finally, the retention capacity of the Technosol spolic silandic recovered well after several sorption-desorption cycles, reaching 40% of the original value after first and second cycles. CONCLUSIONS: The material effectively retained phosphate in batch and continuous flow systems. The Technosol spolic silandic is considered an efficient sorbent to remove the excess of phosphate from the soil solution and the aqueous system. This material may be a useful tool to mitigate or minimize two important environmental problems: eutrophication and the scarcity of natural sources of phosphate. The Technosol can thus be recycled as a phosphate-rich amendment and the leachates can be used to produce liquid fertilizer.