Rare earth element fractionation during the precipitation and crystallisation of hydrous ferric oxides from anoxic lake water

Abstract

Abstract An enrichment of light rare earth elements (LREE) is characteristic for most of the acidic, Fe- and SO 4 -rich pit lakes and groundwaters in the lignite mining area of Lower Lusatia (Germany). One of these acidic lakes – the pit lake “RL 1223” – has a strong thermal and chemical stratification. The upper water layer (0–9 m) shows pH values of about 3 during all seasons. The monimolimnion (10–17 m) of the lake is anoxic and has pH values of about 7. The rare earth element (REE) patterns of the upper lake water show enriched LREE (La N /Yb N = 1.6) whereas the opposite patterns (depletion of LREE, La N /Yb N = 0.4) are found in the anoxic water of the monimolimnion. Experiments were conducted to observe the behaviour of REE during Fe oxidation in water from the monimolimnion (depth 14 m). The sampled monimolimnion water was placed in plastic bottles, and the changing water chemistry was observed for 40 weeks after sampling. Due to the initial anoxic conditions almost all Fe precipitated in the investigated water, and the pH value decreased from about 7 to 3 during the oxidation. The Fe precipitates are identified as ferrihydrite which is transformed into goethite within the oxidation process. Stable pH conditions (pH 3.0) were reached after about 10 weeks of oxidation. The original REE patterns of the investigated water are generally reflected in the Fe precipitates collected at the beginning of the experiment as well as after up to 40 weeks of oxidation. However, in the corresponding water LREE were temporally enriched with a maximum La N /Yb N ratio of 1.0 and a maximum La N /Sm N ratio of 2.3 after 6 weeks of oxidation time (pH 3.8–4.9). Although complex geochemical changes took place between the start and the end of the experiment REE patterns observed at these points in time are nearly identical. These differences of the REE pattern can be explained by the sampling procedure. The experimental findings can be transmitted to the mining dump aquifers of the study area where geochemical conditions comparable to the experimental oxidation time from 3 to 6 weeks are found and hydrous ferric oxides are precipitating. Groundwater passing through the mining dumps can preferentially desorb LREE from the Fe precipitates and display the typical LREE enrichment and carry it to the epilimnion of the acidic pit lakes in Lower Lusatia.