One-pot synthesis of MgAl layered double hydroxides (LDH) from weathered tropical soils

Nutrient recycling is imperative for moving towards a circular economy. Layered double hydroxides (LDH) have been suggested as materials of interest for phosphorus (P) recycling, as they can selectively recover P from liquid waste streams and can be used as efficient P fertilisers in agriculture. Local P recycling with LDH can be particularly valuable for smallholder farmers with limited access to fertilisers, e.g. in tropical regions. However, conventional LDH production relies on external sources of Al³⁺ for the synthesis of MgAl LDH. The present work proposed a new fertiliser technology based on LDH partly synthesized at room temperature with weathered soil as a local and omnipresent source of Al, supplemented with MgO and NaOH. The successful synthesis of LDH via this approach was confirmed by X-ray diffractometry (XRD). The capacity of this LDH to recover P was determined with synthetic P solutions and human urine as a waste model. A chemical desorption test demonstrated the available nature of P sorbed onto the soil-based LDH, which largely outperformed the availability of P sorbed onto the untreated soil. Finally, the fertiliser potential of the soil-based LDH loaded with recycled P was confirmed in a pot trial with rice (Oryza sativa L.) in weathered soil, indicating the combined P fertiliser and liming effect. Taken together, this study presented the proof-of-concept of a novel technology for local P recycling in tropical regions.