Solubility of Fe 2(OH) 3Cl (pure-iron end-member of hibbingite) in NaCl and Na 2SO 4 brines

Abstract

Pure-iron end-member hibbingite, Fe 2(OH) 3Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe 2(OH) 3Cl(s), the pure-iron end-member of hibbingite (Fe II, Mg) 2(OH) 3Cl(s), and Fe(OH) 2(s) in 0.04 m to 6 m NaCl brines has been determined. For the reaction Fe 2(OH) 3Cl(s) + 3H + ↔ 3 H 2O + 2 Fe 2+ + Cl −, the solubility constant of Fe 2(OH) 3Cl(s) at infinite dilution and 25 °C has been found to be log 10 K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reaction Fe(OH) 2(s) + 2H + ↔ 2 H 2O + Fe 2+, the solubility constant of Fe(OH) 2 at infinite dilution and 25 °C has been found to be log 10 K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe 2(OH) 3Cl(s) and Fe(OH) 2(s), the Na +–Fe 2+ pair Pitzer interaction parameter θ Na + / Fe 2+ has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH) 2(s) to Fe 2(OH) 3Cl(s). Additionally, when Fe 2(OH) 3Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.