THERMODYNAMIC STABILITY OF PYROMORPHITE PB5(PO4)3CL AT 6, 22 AND 45°C

The purpose of this study was experimental determination of pyromorphite (PY) solubility changes with temperature. 70 mg of crushed natural PY was dissolved for 4 weeks in 500 mL of 0.1 M KNO₃, pH = 3.5, at 6, 22, and 45°C. Experiments were open to the air. Solutions were periodically syringe-sampled and analyzed for Pb using AAS.

The equilibrium was attained after 10 days. Solubility of PY measured with [Pb] increases with temperature: equilibrium [Pb] averaged from triplicates increases from 8.04±0.16 mg/L for 6°C to 10.65±0.10 mg/L for 45°C. The results indicate that dissolution of PY is endothermic. This contradicts theoretical calculations. Based on thermodynamic data from Robie et al. (1978) and Shevade et al. (2001) the reaction is exothermic with the enthalpy of dissolution reaction:

Pb₅(PO₄)₃Cl ↔ 5Pb²⁺ + 3PO₄^3- + Cl^-

equal to: ΔH_rxn = 5ΔH_Pb + 3ΔH_PO4 + ΔH_Cl – ΔH_PY = 5(-1.7) + 3(-1259.55) + (-167.08) - (-3754.8) = -199.43 kJ/mol

Apparent solubilities at 6, 22, and 45°C were determined using experimental results. Measured [Pb] recalculated to activities was used as input. [PO₄] and [Cl] were calculated assuming stoichiometric dissolution and using speciation modeled with PHREEQC with the thermodynamic database from MINTEQA2. Log(K_sp) equals to –79.6, -79.0, and –77,3 for 6, 22, and 45°C, respectively. The dependence of log(K_sp) on the inverse temperature is close to linear (r²=0.96) suggesting that the enthalpy of the reaction is constant over the temperature range used in the experiments. Linear fit of the experimental results to the function:

log(K_sp) = A • 1/T + B

where A = - ΔH_rxn°/ln(10)R; B = ΔS_rxn°/ln(10)R; R = gas constant

allows to estimate the enthalpy of the dissolution reaction ΔH_rxn = 102 kJ/mol.

This research was partially financed by MEN grant nr 2P04D 01329 and by AGH grant "Lead apatites in environment".