The behaviour of REEs in Thailand's Mae Klong estuary: Suggestions from the Y/Ho ratios and lanthanide tetrad effects

The concentrations of Rare Earth Elements and yttrium (REY) were measured in dissolved phase, in suspended particulate matter (SPM) and in sediments in seven sampling stations in the Mae Klong estuarine system (Inner Thailand Gulf) in order to study their behaviour and distribution pattern. The analysed samples generally show high Rare Earth Element (REE) content in the dissolved phase, with high Medium Rare Earth Elements (MREEs) and Y enrichments in the shale-normalized pattern (versus PAAS). These chemical features are interpreted in terms of direct influences of weathering processes of REE-rich minerals (e.g., phosphates), which abundantly out-crop in the Mae Klong watershed. These large MREE enrichments also suggest that dissolved sulphate complexes play an important role in the dissolved REE fate close to the marine coastal system. The shale normalized REEs and Y/Ho ratios additionally indicate a direct REY co-precipitation with Fe-oxyhydroxides along the river path and consequent adsorption, as coating, onto FeOOH surfaces. The REY behaviour in SPM appears strongly influenced by Al, Fe and Mn content, suggesting that the formation of an authigenic Fe, Mn oxyhydroxides and aluminosilicate particulate leads to REY adsorption and/or co-precipitation. The observed variations in the Y/Ho ratios suggest that different Y–Ho fractionation processes take place in the studied estuarine system. Strong similarities among chondrite-normalized REE patterns in both sediments and granitic rocks out-cropping in the surrounding regions suggest a generic relationship between the two groups of materials.