Phosphorus recycling from urine using layered double hydroxides: A kinetic study

Abstract

Layered double hydroxides (LDH) are anionic clay minerals that have the potential to recycle phosphate (P) from urine and may yield efficient P fertilisers. The efficient P recovery requires P uptake to be relatively fast, ensuring small adsorption columns to reduce process costs. This study assesses the kinetics of P adsorption and desorption of three phase-pure MgAl LDH with varying Al content and a phase-pure ZnAl LDH, all prepared by coprecipitation. The adsorption kinetics (0–24 h) in agitated suspension showed that 90% of the capacity is reached within 5 h, with similar rate constants for model solution as for synthetic urine. The LDH with low Al content maintain structural properties during adsorption, while crystallinity reduces for LDH with high Al content. The combination of XRD analysis, speciation modelling and kinetic information suggest that P uptake by LDH includes a fast step of ligand exchange, followed by slower step of anion exchange in the interlayer. The latter process dominates in LDH with high Al content whereas the former dominates the LDH with low Al content. The XRD analysis and speciation modelling furthermore suggests precipitation as an additional P removal mechanism. The P desorption (0–2 weeks) showed that >95% of P could be desorbed from MgAl-LDH whereas only 51% of adsorbed P is desorbed from the ZnAl LDH. Continuous flow pilot-scale adsorption columns with pelletized MgAl LDH granules (1 mm diameter) recovered >70% of urine-P at residence times of only 1 min, up to point of 70% of exchange capacity beyond which the recovery decreased. These results show that the LDH technology for P recovery is efficient to recycle P into high potential fertilisers.