Phosphorus recycling from urine using layered double hydroxides

Phosphorus (P) is an essential nutrient for all organisms and is often the nutrient that limits primary production in aquatic ecosystems and agriculture. Emissions of P to aquatic systems induce water eutrophication with a number of undesirable effects. Phosphorus needs to be removed from wastewaters to limit eutrophication. Wastewater treatments typically use P removal techniques based on the addition of ferric iron (Fe(III)) or aluminium (Al) salts that induce precipitation of phosphates (PO4). These precipitates are too insoluble to be used as fertilisers, hence that procedure does not allow for recycling wastewater P in agriculture. Recycling P is increasingly needed because P is a limited commodity. For this reason, P removal techniques need to be found that capture P in a bioavailable and economically interesting form. An example of a technique that captures P in a bioavailable form is struvite (MgNH4PO4·6 H2O) crystallisation. This work was set up to test the use of layered double hydroxides (LDHs) for the recovery of PO4 from urine in agriculture as an alternative to struvite crystallisation. Globally, the annual flux of P in human urine is 14% of the annual P use in fertilisers and urine is a waste stream that can be selectively collected and that has a high fraction of P present as inorganic anionic PO4. The LDHs are clays that act as anion exchangers with a similar structure to brucite (Mg(OH2)). By substitution of divalent by trivalent cations in the layers, a charge deficit is introduced to the minerals, which needs to be compensated by anions in the interlayer. These materials are already known to have a high sorption capacity and selectivity for PO4. Furthermore, they have potential slow release properties, which could enhance their P fertilisation potential compared to conventional P fertilisers. At the outset, it was postulated that this technology is promising provided that the acidity of fresh urine (pH 6.0) and its high concentration of citrate that may dissolve the LDHs, do not hamper efficient recovery. Different types of LDHs were synthesised with the coprecipitation method. By varying synthesis conditions, LDH synthesis was optimised. Magnesium (Mg) Al LDHs were compared with zinc (Zn) Al LDHs using materials with M2+/M3+ ratios of 2, 3 and 4. The M2+/M3+ ratio of the materials directly affects the anion exchange capacity (AEC) of the materials. The complexity of the adsorption solutions was increased by adding citrate and by performing the adsorption test with synthetic and human urine. Furthermore, the kinetics of the PO4 adsorption were tested. The P sorption capacities increased with increasing Al content of the LDHs, for LDHs that are synthesised at lower pH and in sorption solutions with lower pH. These trends are explained by the AEC of the LDHs and by P speciation (charge) in the LDHs, an interpretation supported by X-ray diffraction (XRD) and Fourier transform-infrared (FTIR) measurements. The PO4 adsorption is optimal from fresh urine at a pH of 6 using MgAl LDHs synthesised at a pH of 10 with a Mg/Al ratio of 2 over a period of 5 hours. These experiments confirmed that the P adsorption is efficient and not affected by other components in the urine. Furthermore, the results between the synthetic and real urine are consistent, confirming the potential of LDHs for P recycling from real urine The P adsorption was equal for synthetic and real urine, indicating that the synthetic urine is a good model for real human urine. The P capacity reached 61 mg P/g LDH, which equals 85 % of the maximum theoretical LDH exchange capacity, assuming P binds as a divalent cation. Only 10 g LDH is required to remove 90 % of P from 1 L urine and evidence is found that no struvite precipitation is involved in the reaction. The ZnAl LDHs took up equal amount of PO4 compared to MgAl LDHs, but desorbed much less P afterwards, i.e. their release of P for use as a fertiliser was considered insufficient. In contrast, MgAl LDHs desorb up to 84% of the adsorbed P, making them useable as P fertilisers. These materials were then tested in a proof of concept continuous flow adsorption column. The continuous flow pilot-scale adsorption columns with pelletized MgAl LDH granules (1 mm diameter) recovered >80% 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. The PO4 loaded LDHs were analysed using multiple spectroscopic techniques: XRD, FTIR spectroscopy, Raman spectroscopy and nuclear magnetic resonance (NMR). The AEC of the materials affects the interlayer anion population and the selectivity of the materials. The as synthesised high AEC LDHs have less NO3- and more OH- than low AEC LDHs. This effect is explained as a higher selectivity of high AEC LDHs for higher charge density anions than low AEC LDHs. Similarly, after PO4 adsorption, high AEC LDHs have a larger proportion of higher valence PO4 in the interlayer instead of lower valence PO4, which is favoured by low AEC LDHs. The results further indicate that ligand exchange is the most important PO4 adsorption mechanism for all LDHs. This contradicts the generally accepted view that intercalation is the most important PO4 adsorption mechanism. Differences in the adsorption mechanisms between the materials are still present. In low AEC LDHs, ligand exchange appears to be almost completely responsible for PO4 adsorption. In phase pure high AEC LDHs however, intercalation contributes considerably to PO4 removal from solution. This can be detected by XRD which shows that the d-spacing shifts during adsorption. This effect is enhanced when adsorption is performed at a higher pH, as this shifts the PO4 speciation in solution to higher charges, which are favoured by these LDHs. These high AEC materials furthermore take up more PO4 via secondary phases than low AEC LDHs as high AEC LDHs are inherently less stable than low AEC LDHs. This effect is even more pronounced for ZnAl LDHs, explaining their low desorption yields. The PO4 loaded MgAl LDHs were finally tested to assess their P fertiliser potential. Ryegrass was grown in a pot trail using a P- and N-deficient soil with different urine derived fertilisers, i.e. LDH-P, stored urine and urine mixed with sewage sludge as a source of P. The treatments included mineral N and P reference doses and mineral N- and P-compensated urine fertilisers. The fertiliser use efficiency of urine and urine mixed with sludge was lower than that of mineral fertilisers at equivalent total nutrient input. For stored urine, this was related to lower N availability and for urine mixed with sludge this was related to lower P availability in the sludge. In contrast, the yield and P uptake of ryegrass grown on LDHs loaded with P from urine showed equal fertiliser P use efficiency as the mineral fertiliser. Interestingly, the residual soil P after harvest, i.e. the sum of isotopically exchangeable P in soil and the P uptake, was higher for LDH-P than for mineral P, confirming the slow release properties of LDHs that limit loss of P by fixation in the pH neutral soils. Taken together, MgAl LDHs are promising candidates to recycle P from waste streams containing urine. Centralised collection of urine is possible and the LDH synthesis and sorption of P from urine are relatively straightforward. Fertilisers with 6%P (or 14% P2O5) content can be produced. The LDH fertilisers lose less P fertiliser value by P fixation in soil compared to soluble ones over short growing times in pH neutral soil, suggesting potentially enhanced fertilisation efficiency over longer growing times. The upscaling of this technology is needed to assess the technology in pilot-scale under realistic conditions. It is unclear if this technique or struvite crystallisation is the most efficient one for recycling P from waste streams. However, all P recycling methods will need governmental support in order to compete with conventional fertilisers.

Jaar van publicatie:2020

Toegankelijkheid:Closed