Complexation of Zr and Hf monomers in supercritical aqueous solutions: Insights from ab initio molecular dynamics simulations

Abstract

The enhanced mobilization of high field strength elements (HFSE) by certain geological fluids at high pressures, P, and high temperatures, T, as indicated by field observations and recent solubility measurements is most likely related to the type of complex formation in the fluid phase. However, only a few in situ experimental studies have been performed so far to constrain HFSE speciation in supercritical aqueous fluids. Here, we complement these investigations by ab initio molecular dynamics simulations to explore the complexation of Zr4+ and Hf4+ monomers in aqueous solutions at T=1000K and P∼1GPa. Regardless of the type of fluid (basic, neutral, acidic) both Zr and Hf seem to prefer an octahedral coordination. By systematically changing fluid composition and coordinating anions, the structure of various [(Zr,Hf)Clk(OH)m(H2O)n]4−k−m(k=0–6,m=2–6,n=0–4) complexes is investigated. For each complex that is stable on the time scale of the simulation, theoretical X-ray absorption spectra are calculated and compared to experimental data. From this comparison, the most likely complexes in HCl solutions are [(Zr,Hf)Cl3(Cl,OH)2(H2O)]−. The differences between experimental and theoretical XANES spectra present in the case of NaOH solutions indicate HFSE speciation beyond monomers in this system. Charge-neutral [(Zr,Hf)(OH)4(H2O)1–2] complexes, which may be the dominant species in neutral fluids at very low HFSE concentrations, show the lowest coordination (5.2(1)) of all investigated species. Finally, differences in the complexation of Zr and Hf in aqueous solutions at ambient and supercritical conditions and the possible formation of more complex oligomeric species are discussed.