The role of water in extraction and separation of rare earth elements in supercritical carbon dioxide

•Supercritical carbon dioxide and tributyl phosphate were used to extract neodymium and holmium nitrate.•Metal and water concentrations were measured in situ with UV–vis and near-IR spectroscopy.•Holmium was preferentially extracted over neodymium.•The largest separation occurred at low water and tributyl phosphate concentrations.