Optimization of phosphate recovery from urine by layered double hydroxides

Urine contains sufficient phosphorus (P) to consider P recycling form urine as an interesting strategy. In this study, the potential of MgAl or ZnAl layered double hydroxides (LDHs) to be used in such recovery was assessed. LDHs are anion exchangers with a high P selectivity, and P-loaded LDHs have demonstrated fertiliser potential. A critical factor for efficient P recycling with LDH is the stability of these materials, which can be compromised by urinary citrate, complexing aluminium (Al³⁺) and by the low pH of fresh urine dissolving the alkaline LDHs. Different phase pure ZnAl and MgAl LDHs were synthesised by coprecipitation in scenarios of varying synthesis pH and Mg/Al or Zn/Al ratios. The obtained materials were incubated in P solutions at different pH, with or without citrate in full factorial combinations, and in fresh and stored human urine. The P sorption capacities increased for LDHs synthesised at lower pH, at increasing Al content and for sorption solutions with lower pH. These trends are explained by an increased anion exchange capacity (AEC) and by P speciation (charge) in the LDHs, an interpretation supported by XRD measurements. The P capacity reached 61mg P/g LDH, which equals 85% of the theoretical LDH exchange capacity. Only 1g LDH is required to remove 90% of P from 1L urine and evidence is found that sorption, not struvite precipitation, is the P removal mechanism involved. The ZnAl LDHs were equally effective in P uptake compared to the MgAl LDHs, but the ZnAl materials showed more irreversible P sorption in contrast with the high desorption yields (53mg P/g) of the MgAl LDHs. Therefore, the large potential of MgAl LDHs for P recovery from urine is supported by this study.