Struvite precipitation from source-separated human urine: mineralogical characterization, phosphorus release kinetics, and heavy metal safety assessment

Abstract The recovery of phosphorus from human urine via struvite precipitation has emerged as a promising strategy for sustainable nutrient management. However, limited understanding of the materials' physicochemical properties, phosphorus speciation, and potential contamination by tracing heavy metals continues to hinder their safe application. This study addresses these gaps by systematically characterizing struvite precipitated from source-separated human urine collected from dry toilet facilities and stored at ambient temperature for six months, representing a scarcely explored effluent matrix in nutrient recovery research. The precipitation was driven by natural microbial urease activity and by the addition of magnesium oxide (MgO) without pH adjustment (Mg:P molar ratio of 1.71:2.21). The resulting solids were analyzed via X-ray diffraction (XRD), solid-state phosphorus-31 nuclear magnetic resonance 31 P NMR, inductively coupled plasma atomic emission spectrometry (ICP-OES), and the Measurement and Standard Testing (MTP) protocol. Struvite (MgNH₄PO₄∙6H₂O) was identified as the dominant crystalline phase (79.54%), with minor presence of newberyite (MgHPO₄∙3H₂O) revealed by 31 P NMR. Heavy metals, including arsenic, cadmium, lead, chromium, and mercury, were detected at concentrations well below regulatory thresholds established by Brazilian legislation. Phosphorus speciation analysis showed a predominance of inorganic forms (70.0 mg∙g⁻ 1 IP) within the product, which contained 10.2% P. Additionally, 61% of the total phosphorus content was solubilized within 10 days at neutral pH, with the kinetics excellently described by the Elovich model. These findings support the feasibility of converting human urine into a safe, phosphorus-rich fertilizer, advancing circular economy principles towards sustainable sanitation practices.