Nutrient recovery from landfill leachate as struvite and carnallite: environmental safety and agronomic performance in tropical soils

Abstract

Landfill leachate is a complex, nutrient-rich waste stream with high potential for resource recovery through chemical precipitation. This study assessed the environmental safety and agronomic performance of struvite (MgNH₄PO₄·6H₂O) and novel struvite–carnallite (KMgCl₃·6H₂O) mineral association, both being recovered from landfill leachate. A greenhouse experiment was conducted using lettuce grown in a highly P-fixing tropical soil Ultisol, comparing increasing doses of the recovered materials with a commercial NPK fertilizer. The double-dose struvite treatment (2S) corresponded to supplying twice the recommended P rate to assess the upper agronomic response. The 2S treatment produced the highest shoot dry mass to 13.7 g, representing a 105% increase relative to the NPK treatment, and resulted in the greatest foliar concentrations of P and N (6.55 and 50.4 g kg^-1). The struvite–carnallite treatment (2SC) resulted in the highest K concentrations in the shoots and roots (74.7 and 19.9 g kg^-1 ), demonstrating the agronomic relevance of carnallite formation during leachate treatment. Although Na accumulation reached 4337 mg kg⁻¹ in the roots, no visual phytotoxicity or biomass reduction was observed. Concentrations of toxic metals in plant tissues (Cd < 0.19 mg kg⁻¹; Pb < 1.13 mg kg⁻¹) remained below regulatory thresholds, confirming the purity and environmental safety of precipitation-based mineral recovery. A strong linear relationship between soil P availability and shoot dry mass (R² = 0.826, p = 0.002) indicates that the recovered minerals function effectively as slow-release P sources, capable of reducing P-fixation in weathered tropical soils. This study provides the first evidence of carnallite formation during nutrient recovery from landfill leachate, demonstrating that struvite-based fertilizers are sustainable, low-risk alternatives for tropical soil. These findings support waste valorization strategies to reduce dependence on imported mineral fertilizers in weathered soils.