Closing the Phosphorus Loop by Recovering Phosphorus From Waste Streams With Layered Double Hydroxide Nanocomposites and Converting the Product into an Efficient Fertilizer

Abstract:

Phosphorus (P) recovery has been frequently discussed in recent decades due to the uncertain availability and uneven distribution of global phosphate rock reserves. Sorption technology is increasingly considered as a reliable, efficient and environmentally friendly P removal method from aqueous solution. In this study, a series of Mg-Al-based layered double hydroxide nanocomposites and their corresponding calcined products were fabricated and applied as phosphate adsorbents. The prepared samples were with average size at ~50 nm and self-assembled into larger particles in irregular shapes. The results of batch adsorption experiments demonstrated that calcination significantly enhanced the adsorption ability of LDHs for phosphorus, and the maximum adsorption capacity of calcined Mg-Al-LDH was as high as 100.7 mg-P/g. Furthermore, incorporation of Zr⁴⁺ and La³⁺ into LDH materials increases the sorption selectivity as well as sorption amount of phosphorus in LDHs, which was confirmed by experiments operated in synthetic human urine. For the first time ammonia (NH₄OH) and potassium hydroxide (KOH) solutions were employed to desorb the P-loaded LDH. Identification of solids derived from two eluting solutions showed that struvite (MgNH₄PO₄•6H₂O, MAP) was precipitated in ammonia solution while most phosphate was desorbed into liquid phase in KOH system without crystallization of potassium struvite (MgKPO₄•6H₂O) due to its higher solubility. Quantitative X-ray diffraction technique was used to determine struvite contents in obtained solids and the results revealed that ~ 30% of adsorbed P was transferred into struvite form in the sample extracted by 0.5M NH₄OH. Leaching tests suggested that the phosphorus releasing kinetics of ammonia treated LDH was comparable to that of pure struvite product, indicating that postsorption Mg-Al-LDH desorbed with ammonia could serve as a slow-releasing fertilizer in agriculture (see Figure 1).