Distribution of rare earth elements in anionic, cationic and particulate fractions in boreal humus-rich streams affected by acid sulphate soils.

The abundance, fractionation and physicochemical forms of rare earth elements (REEs) were determined in five boreal humus-rich streams (dissolved organic carbon, DOC = 14-40 mg/l) affected by acid sulphate soils. The sampling was carried out during high-water flow in autumn when the acid sulphate soils are extensively flushed. The analytical procedures included ion-exchange experiments in field and ICP-MS determination. There was a general decrease in pH (range 4.5-6.2) and increase in the REE concentrations (La range 0.82-23 microg/l) as the proportion of the catchment cover of acid sulphate soils increased, explained by high amounts of REEs in the acidic runoff from such soils. In each stream, four different REE fractions were identified: (1) A cationic fraction, which is dominant in the REE-rich runoff from the acid sulphate soils and which is depleted in HREEs due to hydrochemical and/or geochemical processes, (2) an anionic fraction identified as humus-REE complexes, which in general is more abundant the higher the DOC concentrations and which also becomes increasingly abundant across the lanthanide series, (3) a fraction having a well-developed MREE enrichment, presumably consisting of colloidal REEs, and (4) a minor uncharacterised particle-associated fraction. The REE pool in the streams thus consists of several coexisting and contrasting REE species. The identification and quantification of such species is a prerequisite for the precise and accurate characterisation of the REE hydrochemistry of the streams.