The solubility of tellurium dioxide in water vapor at 250°C

Tellurium is a critical mineral of increasing importance in green energy technologies. Numerical simulations using previous thermodynamic data predict up to ~0.1 ppm in epithermal vapor with H 2 Te(g) as the dominant vapor species [1]. However, fluid inclusion studies show up to hundreds of ppm Te in vapor inclusions from epithermal ore deposits [2]. In this study, we measured experimentally the solubility of tellurium in hydrothermal vapors to determine the hydrated tellurium speciation in water vapor. Hydration is the effect of water vapor molecules binding to a metal, greatly increasing its solubility [3]. Experiments were conducted in batch-type Ti Parr reactors at 250°C and a range of water vapor pressures (P H2O ) using several different oxygen buffers (e.g., MoO 2 -MoO 3 , WO 2 -WO 3 and Ni-NiO). Kinetic experiments were conducted between 1-25 days at 250°C and 20 bar and at different redox conditions. At oxidizing conditions, equilibrium conditions were reached after ~10 days with 1.33 ±0.01 ppm dissolved Te, whereas in N 2 -degassed experiments equilibrium was reached after ~22 days with 0.669 ±0.004 ppm dissolved Te, indicating slower reaction kinetics and reduced solubility at lower redox conditions. Experiments at 250°C and log f O 2 of -24 (MoO 2 -MoO 3 buffer), show increasing Te solubility with increasing P H2O ranging from 1-3 ppm between 15-25 bar to 12.27±0.01 ppm Te at 35 bar. The MoO 2 -MoO 3 buffered experiments overlap in concentrations with results from the kinetic series at oxidizing conditions. Experiments at 250°C and log f O 2 of -39.37 (WO 2 -WO 3 buffer), show an increase in Te solubility between 15-35 bar from 14.51 ppm up to 15.45±0.01 ppm. Previous experimental work has shown enhanced solubility of TeO 2 in water vapor due to the formation of TeO 2 * x H 2 O with x = 1 and 2 [1,4-5], whereas in this study Te shows higher hydration numbers similar to other metals [3]. Our results demonstrate the significant role of P H2O on Tesolubility in low density fluids as wellas a strong redox control based on results from the WO 2 -WO 3 buffered experiment. The experimental data generated can be applied in thermodynamic models to discern tellurium mobility in hydrothermal systems.