The role of Pb(II) complexes in hydrothermal mass transfer: An X-ray absorption spectroscopic study

We studied the nature and geometry of Pb(II) complexes in chloride-, bromide-, and bisulfide-bearing solutions in situ at pressures of 600 and 800 bar and temperatures up to 489 °C using in situ XAS spectroscopy combined with ab initio molecular dynamics (MD) simulations. In halide-free acidic solutions, Pb(II) complexes to approximately seven water ligands (predominantly [Pb(H₂O)₇]²⁺) at 30 °C; this hydration number reduces to approximately six ligands at 191 °C. Due to the influence of a stereochemically active electron lone pair, Pb(II) halide complexes are characterised by a broad bond distance distribution, highly flexible structures, and a distinct gap for excess electron density. Residual maps (based on R-factor) of parameter space confirmed that many possible parameter combinations resulted in equally satisfactory fits of the EXAFS data. Thus, EXAFS interpretation was constrained using MD simulations and results of experimental speciation and solubility studies available in the literature. The results indicate that in chloride-bearing solutions, at T > 200 °C, Pb(II) has a maximum coordination of four to five ligands, and the Pb-complex is always coordinated to a combination of water and chloride; even at 400 ≤ T ≤ 500 °C in an acidified solution containing ~10 m Cl⁻, the predominant Pb(II) complex is [Pb(H₂O)_1-2Cl₃]⁻. The highest order bromide complex similarly contains three halide ligands ([Pb(H₂O)_0.5Br₃]⁻ in a ~4 m Br⁻ acidified solution above 400 °C), but the hydration number is lower than in the case of chloride. With regard to bisulfide complexing, the solubility of galena in the presence of bisulfide was lower than expected from extrapolations using available low temperature solubility data, but the formation of Pb(II) bisulfide complexes was confirmed by the increase in solubility with increase in bisulfide concentration. The new data show that in terms of coordination geometry, Pb(II) behaves like semi-metals such as Sb(III), As(III), Bi(III), or Te(IV), which are affected by a stereochemically active lone pair, rather than like divalent first row transition metals such as Zn. Pb(II)-chloride complexes with a maximum of three chloride ligands are the dominant complexes responsible for Pb transport. In contrast, Zn(II) forms tetrahedral chloride complexes, and ZnCl₄ ²⁻ is stable in brines up to temperatures ≥300 °C. These differences in coordination chemistry, combined with the differences in solubilities of Zn and Pb minerals, account for Zn/Pb fractionation in hydrothermal fluids.