Phosphate adsorption from sewage sludge filtrate using zinc–aluminum layered double hydroxides

A series of layered double hydroxides (LDHs) with different metal cations were synthesized to remove phosphate in waste sludge filtrate from a municipal wastewater treatment plant for phosphorus recovery and to help control eutrophication. The highest phosphate adsorption capacity was obtained by using Zn–Al-2-300, that is LDHs with Zn/Al molar ratio of 2 and calcined at 300 °C for 4 h. Circumneutral and mildly alkaline waters appeared suitable for the possible application of Zn–Al LDHs due to the amphoteric nature of aluminum hydroxide. Phosphate adsorption from the sludge filtrate by the LDHs followed pseudo-second-order kinetics, and the adsorption capacity at equilibrium was determined to be ∼50 mg P/g. Adsorption isotherms showed that phosphate uptake in this study was an endothermic process and had a good fit with a Langmuir-type model. The absorbed phosphate can be effectively desorbed (more than 80%) from LDHs particles by a 5 wt% NaOH solution. The regeneration rate of used LDHs was ∼60% after six cycles of adsorption–desorption–regeneration.