Origin of middle rare earth element enrichment in acid mine drainage-impacted areas

The commonly observed enrichment of middle rare earth elements (MREE) in water sampled in acid mine drainage (AMD)-impacted areas was found to be the result of preferential release from the widespread mineral pyrite (FeS₂). Three different mining-impacted sites in Europe were sampled for water, and various pyrite samples were used in batch experiments with diluted sulphuric acid simulating AMD-impacted water with high sulphate concentration and high acidity. All water samples independent on their origin from groundwater, creek water or lake water as well as on the surrounding rock types showed MREE enrichment. Also the pyrite samples showed MREE enrichment in the respective acidic leachate but not always in their total contents indicating a process-controlled release. It is discussed that most probably complexation to sulphite (SO₃ ^2?) or another intermediate S-species during pyrite oxidation is the reason for the MREE enrichment in the normalized REE patterns.