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

Abstract

Struvite (MgNH 4 PO 4 •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 (PO 4 ) 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 °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 NH 4 + , HPO 4 2− , and Mg 2+ , 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.

Sustainable Resource Recovery from Dairy Waste: A Case Study of Hydrothermal Co-liquefaction of Acid Whey and Anaerobic Digestate Mixture

“…Another benefit is that the acids contained in acid whey may facilitate reductive depolymerization of lignin and hydroxymethylation, cyclic acetalization, and pyran formation of lignin-derived oily monomers, enhancing lignin conversion into biocrude oil. 28 These acids may also solubilize mineral nutrients in the digestate, resulting in higher nutrient yield in HTL-AP and ultimately allowing post-HTL nutrient recovery via struvite (MgNH 4 PO 4 •6H 2 O) crystallization, 29 ammonia air stripping, 30 and membrane separation. 31 Moreover, the mixing of acid whey and digestates increases the polysaccharide-tolignin ratio in the HTL feedstock because of high lactose content in acid whey.…”

“…The regression analysis provides a measure of the statistical significance of the investigated independent variables. The response surface optimization employing the desirability function approach was performed to specify a range of optimal HTL operating conditions for various acid whey and manure digestate mixing ratios that yielded maximal biocrude energy recovery (i.e., a high oil yield with a high carbon content and low heteroatom content), low hydro-char yield, and maximal Mg, NH 3 -N, and P yields and minimal Ca yield in HTL-AP; these latter yields are attractive for the nutrient recovery process, for example, struvite production and membrane separation. , The minimal Ca distribution into the HTL-AP allows for higher struvite purity and prevents membrane fouling for an extended lifetime. The optimized reaction temperature and time were validated through additional triplicated HTL experiments (herein referred to as “validation experiment”) on feedstock with different mixture compositions.…”

Reactivity and Stability of Natural Clay Minerals with Various Phyllosilicate Structures as Catalysts for Hydrothermal Liquefaction of Wet Biomass Waste

“…The absorption bands at 1540 and 1450 cm −1 were integrated and used to calculate the amount of Bronsted and Lewis acidic sites, respectively, using eqs 9 and 10. 38 (9) (10) where C BA and C LA are the concentrations of Bronsted and Lewis acidic sites (mmol/g), respectively, IMEC BA and IMEC LA are the integration molar extinction coefficients (1.67 and 2.22 cm/μmol) for Bronsted and Lewis acidic sites, respectively, r is the disk radius (cm), w is the disk weight (g), and A 1450 and A 1540 are the integrated areas of bands at 1540 and 1450 cm −1 , respectively. 38 Subsequently, pyridine was evacuated at 100, 200, and 400 °C for 10 min followed by acquisition of the spectra and calculation of the area of bands at 1540 and 1450 cm −1 .…”

“…This target is according to the fact that nutrients in the aqueous phase exist in their bioavailable forms, e.g., N as NH 3 -N and P as orthophosphates. 8 With these bioavailable forms, their post-HTL recovery as marketable products is more efficient, e.g., ammonia stripping-acid scrubbing to recover NH 3 -N as ammonium-sulfate, 9 struvite crystallization to recover NH 3 -N, P, and Mg as solid slowrelease fertilizer, 10 and membrane separation for selective mineral purification. 11 The third target is minimizing the formation of hydrochar to prevent potential plugging of reactor's pressure control valves and piping systems, leading to failure of process safety instrumented systems.…”

Probing elemental speciation in hydrochar produced from hydrothermal liquefaction of anaerobic digestates using quantitative X-ray diffraction

“…Resource recovery from anaerobic digestates using hydrothermal liquefaction can effectively mitigate potential environmental pollution caused by direct land spreading of the digestates on farms and pastures for soil amendment or fertilizer, e.g., residual greenhouse gases emissions, water basin eutrophication and acidication due to excessive runoff of nutrients into river basins and lakes, and cross-contamination between farm and food production facility. 1 Hydrothermal liquefaction (HTL) processing at 280 to 360 °C and 10 to 20 MPa convert lignocellulosic carbon and nutrients contained in the digestates produced during anaerobic digestion into renewable biocrude oil and a nutrient-rich aqueous-phase (HTL-AP) coproduct that can be further processed for (NPK) recovery using a number of methods including struvite crystallization, 2 ammonia stripping, 3 and membrane separation. 4 In addition, HTL produces a solid carbonaceous coproduct called hydrochar with relatively similar carbon and energy content to brown coal (lignite) and comparable nutrient content to low-grade phosphate rock.…”