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Abstract
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 degrees C for 4h. 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 approximately 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 approximately
60% after six cycles of adsorption-desorption-regeneration.