Size-dependent growth kinetics of struvite crystals in wastewater with calcium ions

Abstract

<jats:title>Abstract</jats:title><jats:p>Kinetic parameters describing continuous reaction crystallization of struvite from aqueous solutions containing also calcium ions (from 100 to 2000 mg Ca<jats:sup>2+</jats:sup>/kg) were estimated. Test results were compared with kinetic data of struvite manufactured from real cattle liquid manure. Kinetic model for ideal MSMPR (Mixed Suspension Mixed Product Removal) crystallizer was used assuming dependence of crystal growth rate <jats:italic>G</jats:italic> on its size <jats:italic>L</jats:italic> (size-dependent growth, SDG MSMPR model). Based on nonlinear regression and statistical analysis, one from within five considered <jats:italic>G</jats:italic>(<jats:italic>L</jats:italic>) models was selected – Rojkowski exponential (RE) model – rendering the experimental population density distributions the best. It was concluded, that calcium ions influenced all components of struvite manufacturing process disadvantageously. A rise of Ca<jats:sup>2+</jats:sup> concentration in a feed from 100 to 2000 mg/kg increased nucleation rate <jats:italic>ca</jats:italic>. 160-time, whereas growth rate of nuclei up to macroscopic size <jats:italic>G</jats:italic><jats:sub>0</jats:sub> decreased more than 10-time. Linear (larger) crystal growth rate <jats:italic>G</jats:italic><jats:sub>¥</jats:sub> was nearly two-times smaller: 1.71·10<jats:sup>–8</jats:sup> m/s (100 mg Ca<jats:sup>2+</jats:sup>/kg) – 9.10·10<jats:sup>–9</jats:sup> m/s (2000 mg Ca<jats:sup>2+</jats:sup>/kg). Resulting in a product with deteriorated quality. Mean size of the crystals decreased nearly two-times (to 18.4 μm), non-homogeneity within product population enlarged and calcium fraction in the product increased. The product, beside struvite MgNH<jats:sub>4</jats:sub>PO<jats:sub>4</jats:sub>·6H<jats:sub>2</jats:sub>O, also contained hydrated amorphous calcium phosphate(V) Ca<jats:sub>3</jats:sub>(PO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>·<jats:italic>n</jats:italic>H<jats:sub>2</jats:sub>O (ACP). It was observed, that 5-times smaller concentration of phosphate(V) ions in a feed and magnesium ions excess in relation to phosphate(V) and ammonium ions (1.2 : 1 : 1) influenced all kinetic parameters of continuous struvite reaction crystallization advantageously.</jats:p>

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

“…26 Unfortunately, these conditions are not found in actual HTL-AP product effluents, which contain significant concentrations of foreign ions and polar organic compounds, 7,25,27 higher ionic strength (0.2−0.4 M), 25,28 and nonequimolar Mg/NH 3 −N/P ratios. 29−32 Few studies have investigated the struvite crystallization thermodynamics and kinetics in real wastewaters, adopting different kinetic models, e.g., linear growth rate, 33 first-order kinetics, 26 population balance, 34 and size-dependent growth 35 models. To design a fluidized bed reactor for struvite production, a recent study 36 proposed three solid−liquid flow models, i.e., complete mixing of liquid and bed, plug flow of liquid and perfect classification of the bed, and plug flow of liquid and complete mixing of the bed, incorporated with reduced thermodynamic and growth kinetic models; the aim of the study was to improve previously proposed flow models by Rahaman et al 37 and Burns et al 38 However, those studies assume an ideal condition, neglecting the change of chemical thermodynamic properties at different temperatures and pH, e.g., ionic strength and effective concentration or activity of the reacting ions, during crystallization and the possibility of side reactions caused by foreign ions, e.g., hydroxyapatite formation due to Ca 2+ presence.…”

Effects of Physicochemical Parameters on Struvite Crystallization Based on Kinetics

“…This model is based on the population balance equation. It is a method used to estimate the kinetic parameters of the crystallization of struvite, which depend on the crystal size distribution (CSD) of the product [ 17 , 25 , 26 ]. The crystal growth rate and nucleation rate affect CSD in solution, which is the key to control the quality of struvite [ 27 ].…”

“…In a study conducted by Hutnik et al [ 26 ], the SDG model was used to determine how calcium ions affect the crystallization of struvite. When the concentration of calcium ions was increased from 100 mg/L to 2000 mg/L, the nuclear growth rate decreased significantly from 2.30 × 10 −11 to 2.09 × 10 −12 m/s.…”

Phosphorus recovery by struvite precipitation: a review of the impact of calcium on struvite quality

“…The size attained by struvite crystal during product precipitation depends on a two-step process: nucleation and crystal growth. The growth of the crystal is affected by a combination of several factors such as the pH (Matynia et al 2006), supersaturation, mixing turbulence (Matynia et al 2006), and presence of foreign ions (Le et al 2005;Liu & Wang 2019;Moragaspitiya et al 2019;Hutnik et al 2020). Le et al (2005) found that at higher concentrations of Ca (with regard to Mg) in the solution, the mean size of the struvite is decreased.…”

“…The presence of Ca 2þ during struvite precipitation has been shown to negatively affect characteristics of the struvite produced (Le et al 2005Liu & Wang 2019;Moragaspitiya et al 2019;Bayuseno & Schmahl 2020;Hutnik et al 2020). Precipitating struvite at high Ca concentration produces undesired amorphous product and reduces the proportion of P in the struvite (Desmidt et al 2013;Li et al 2016).…”