Synthesis and solubility of hopeite Zn₃(PO₄)₂·4H₂O

To synthesize 10 g of hopeite, 14.4 g of Zn(CH₃COO)₂·2H₂O and 5.9 g of K₂HPO₄ were individually dissolved in 500 mL of redistilled water at 25°C. Using a peristaltic pump, these solutions were gradually and simultaneously mixed into a 3 L beaker filled with 1.5 L of continuously stirred redistilled water. The hopeite synthesis reaction proceeded according to the reaction: 1 3(Zn(CH3COO)2)⋅2H2O+2KH2PO4=Zn3(PO4)2⋅4H2O+2CH3COOK+6H2O $$\matrix{ {3\left( {{\rm{Zn}}{{\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COO}}} \right)}_2}} \right) \cdot 2{{\rm{H}}_2}{\rm{O}} + 2{\rm{K}}{{\rm{H}}_2}{\rm{P}}{{\rm{O}}_4} = {\rm{Z}}{{\rm{n}}_3}{{\left( {{\rm{P}}{{\rm{O}}_4}} \right)}_2} \cdot 4{{\rm{H}}_2}{\rm{O}}} \hfill \cr {\quad + 2{\rm{C}}{{\rm{H}}_3}{\rm{COOK}} + 6{{\rm{H}}_2}{\rm{O}}} \hfill \cr } $$

A white precipitate appeared immediately upon mixing. The suspension lasted four days after the Zn and PO₄ solutions were exhausted. It was then decanted, filtered, and washed with redistilled water and acetone. The residue was dried at 110°C for 12 h.

X-ray diffraction (XRD) analysis was performed on a SmartLab RIGAKU diffractometer using a copper X-ray tube in the angular range of 5–70°2θ with a 0.05°2θ measuring step. The PCPDFWIN version 1.30, formalized by JCPDSICDD, was used to identify mineral phases. Scanning electron microscopy (SEM) was performed using an FEI QUANTA 200 FEG microscope with an Energy Dispersive Spectrometer (EDS) on uncoated samples. Zn concentrations were measured by atomic absorption spectroscopy (AAS) method using a SavantAA GBC spectrometer. PO₄ concentrations were determined colorimetrically by the molybdenum blue method (Lenoble et al. 2003) using a Hitachi 1600 spectrophotometer at 870 nm wavelength.

Dissolution experiments were conducted at 25°C with initial pH values of 2.0. 200 mg aliquot of synthetic Zn₃(PO₄)₂·4H₂O was added to 250 mL of 0.05 M KNO₃, and the pH was adjusted to the desired value using 0.1 M HNO₃. The experiment was conducted in triplicate in unbuffered suspensions. In each case, the pH drifted freely after setting the initial value. To determine that equilibration had been reached, samples of the reaction solutions were taken after 1, 2, 4, and 8 hours and after 1, 2, 4, 7, 14, and 30 days. The suspension bottles were shaken twice a week and always after sampling. For each sample, the solids were allowed to settle before 7 mL of the supernatant was withdrawn, without refilling with the solution afterward. The samples were analyzed for pH (combined glass electrode), Zn_tot (AAS) and P(V)_tot (UV-Vis). Solubility product calculations were performed using the computer program PHREEQC with a modified MINTEQ. v4 thermodynamic database (Allison et al. 1991). The activities of ionic species were calculated from measured concentrations of elements by applying the Davies equation or the extended Debye-Huckel equation.

The white precipitate produced in the synthesis is identified as the hopeite Zn₃(PO₄)₂·4H₂O by comparing peak positions with those of hopeite reported in Joint Committee on Powder Diffraction Standards (JCPDS) cards 33-1474. All peaks produced by the precipitate are identified as hopeite peaks (Fig. 1).

Figure 1.

The SEM observations indicate that the precipitate is composed mainly of 10–100 μm grains. Among these grains, idiomorphic crystals in the form of a rectangular shape reach lengths up to 200 μm (Fig. 2). The surface of the grains is smooth. The crystals are composed of Zn, P, O as determined by EDS.

Figure 2.

In the dissolution experiments, most of the reaction occurred within two days with dissolution rates, as indicated by the increase in zinc and phosphate concentrations, declining with time. The evolution of the solution composition over time is shown in Figure 3. The hopeite dissolved stoichiometrically. Hopeite was assumed to be in equilibrium with the solution when the concentrations of Zn and PO₄ in three consecutive samples of the test solution were equal within an error measured by twice the standard deviation. To determine the congruence of the dissolution of hopeite, as a prerequisite for determining the solubility coefficient, it was necessary to determine the equilibrium state for Zn²⁺ and PO₄^3- concentrations. For this purpose, the last five concentration values for both ions within the error limit were considered. The equilibrium concentration for Zn²⁺ was 2.117 ± 0.04, and for PO₄^3- was 1.432 ± 0.012, i.e. the molar ratio of the two Zn : PO₄ ions is 1.48 ± 0.03. The calculated molar ratio of the ions in the aqueous solution studied was 3:2 and is consistent with the chemical formula of hopeite Zn₃(PO₄)₂·4H₂O. The dissolution of hopeite was congruent, meaning that the mineral dissolved without forming solid secondary phases.

Figure 3.

The ion activity product (IAP) for the dissolution reaction of hopeite 2 Zn3(PO4)2⋅4H2O⇔3Zn2++2PO43−+4H2O∗ $${\rm{Z}}{{\rm{n}}_3}{\left( {{\rm{P}}{{\rm{O}}_4}} \right)_2} \cdot 4{{\rm{H}}_2}{\rm{O}} \Leftrightarrow 3{\rm{Z}}{{\rm{n}}^{2 + }} + 2{\rm{P}}{{\rm{O}}_4}^{3 - } + 4{{\rm{H}}_2}{{\rm{O}}^ * }$$ * activity for H₂O is 0, so it is not taken into account in further calculations can be written as 3 logIAP=3log{ Zn2+ }+2log{ PO43− } $$\log {\rm{IAP}} = 3\log \left\{ {{\rm{Z}}{{\rm{n}}^{2 + }}} \right\} + 2\log \left\{ {{\rm{P}}{{\rm{O}}_4}^{3 - }} \right\}$$ where brackets denote activities. At equilibrium, the IAP is equal to the solubility product, K_sp. Based on measured concentrations and pH at equilibrium, Zn²⁺ and PO₄^3- activities were calculated using the computer program PHREEQC. Based on the calculated activities, the log IAP was calculated using Eq. (3). The equilibrium log IAP from the experiments, represents the log K_sp at 25°C and equals –35.72 ± 0.03. This value agrees well with the log K_sp = –35.29 ± 0.1 Nriagu (1973) reported.

Based on the solubility product determined for hopeite in the experiments, the Gibbs free energy of formation ΔG°_f(hopeite) is calculated. The Gibbs free energy ΔG°_r of dissolution reaction (2) is expressed by the equation 4 ΔGr∘=3ΔGf∘(Zn2+)+2ΔGf∘(PO43−)+4ΔGf∘(H2O)−ΔGf∘ (hopeite) $$\matrix{ {{\rm{\Delta G}}_{\rm{r}}^ \circ } \hfill & { = 3{\rm{\Delta G}}_{\rm{f}}^ \circ \left( {{\rm{Z}}{{\rm{n}}^{2 + }}} \right) + 2{\rm{\Delta G}}_{\rm{f}}^ \circ \left( {{\rm{PO}}_4^{3 - }} \right) + 4{\rm{\Delta G}}_{\rm{f}}^ \circ \left( {{{\rm{H}}_2}{\rm{O}}} \right)} \hfill \cr {} \hfill & { - {\rm{\Delta }}G_f^ \circ {\rm{\;(hopeite)\;}}} \hfill \cr } $$ is, at equilibrium, related to the solubility product K_sp as follows: 5 ΔGr∘=−RTlnKsp $${\rm{\Delta G}}_{\rm{r}}^ \circ = - {\rm{RTln}}{{\rm{K}}_{{\rm{sp}}}}$$ where R is the gas constant (8.314472 J mol⁻¹ K⁻¹), and T is the temperature (298.15 K). This gives a value for ΔG°_r of 203.9 ± 0.1 kJ mol⁻¹. Calculation of the Gibbs free energy of the formation of hopeite is enabled by equation 3 as follows: 6 ΔGf∘ (hopeite) =3ΔGf∘(Zn2+)+2ΔGf∘(PO43−)+4ΔGf∘(H2O)+RTInKsp $$\matrix{ {{\rm{\Delta G}}_{\rm{f}}^ \circ {\rm{\;(hopeite)\;}} = 3{\rm{\Delta G}}_{\rm{f}}^ \circ \left( {{\rm{Z}}{{\rm{n}}^{2 + }}} \right) + 2{\rm{\Delta G}}_{\rm{f}}^ \circ \left( {{\rm{P}}{{\rm{O}}_4}^{3 - }} \right)} \hfill \cr {\quad + 4{\rm{\Delta G}}_{\rm{f}}^ \circ \left( {{{\rm{H}}_2}{\rm{O}}} \right) + {\rm{RTIn}}{{\rm{K}}_{{\rm{sp}}}}} \hfill \cr } $$

Based on available values of ΔG°_f(Zn²⁺) = −147.30 ± 0.2, ΔG°_f(PO₄³⁻) = −1001.60 ± 0.9, and ΔG°_f(H₂O) = −237.1 ± 0.1 kJ mol⁻¹ from Robie et al. (1978) and the value of K_sp = −35.72 ± 0.03 experimentally determined above, the Gibbs free energy of formation of hopeite may be calculated as ΔG°_f(hopeite) = −3597.4 ± 1.0 kJ mol⁻¹.