Thermodynamics and Kinetics of Struvite Crystallization from Hydrothermal Liquefaction Aqueous-Phase Considering Hydroxyapatite and Organics Coprecipitation

Struvite (MgNH4PO4 center dot 6H(2)O), a mineral containing bioavailable phosphorus and nitrogen, is a solid slow-release fertilizer that can be produced from the aqueous-phase coproduct of hydrothermal liquefaction (HTL-AP). However, if the struvite crystallization process is carried out at nonoptimal conditions, then the P in the HTL-AP can crystallize with Ca, forming an undesirable hydroxyapatite (Ca-10(PO4)(6)(OH)(2)) solid byproduct that also acts as an adsorbent for potentially phytotoxic organics. To maximize struvite yield and purity, a deeper understanding of struvite and hydroxyapatite crystallization as well as organics adsorption onto hydroxyapatite is critical. In this study, crystallization experiments varying temperature, time, pH, and Mg/Ca ratio were carried out. The experimental results informed a supersaturation-based reversible crystallization model for struvite and hydroxyapatite while the adsorption of organics was satisfactorily described by the classical Ritchie's and Langmuir equations. The Arrhenius and van 't Hoff equations were applied to describe the thermodynamics and kinetics of these processes. Struvite yield and purity are maximized at 25 degrees C, pH 8, and Mg/Ca ratio of 4, with a P-recovery rate of >90%. A less alkaline pH and higher Mg/Ca ratios maximize the reactivity among NH4+, HPO42-, and Mg2+, which increases struvite selectivity over hydroxyapatite and reduces organic physisorption onto hydroxyapatite by suppressing organics deprotonation. Lower temperatures reduce struvite endothermic dissolution and organics endothermic adsorption and favor hydroxyapatite exothermic dissolution. The thermodynamics and kinetics data and models from this study are useful in designing a more sustainable and economically feasible HTL-AP valorization route.