Zr, Hf and REE distribution in river water under different ionic strength conditions

Abstract

The Platani River flowing in south-central Sicily, interacting with evaporite rocks, generates a wide range of ionic strength in the water catchment from 0.1 to 5.0molkg-1. We sampled 38 river sites and analysed the composition for the dissolved fraction filtered through 0.45μm, the truly dissolved fraction obtained through ultrafiltration (10kDa) and the relative included colloidal fraction. This study was focused on the recognition of Zr, Hf and REE behaviour under changing ionic strength conditions, since this is one of parameters responsible for colloid stability in natural waters. In turn, this phenomenon leads to REE release from the colloidal fraction and their scavenging onto surfaces of suspended particles or sediment, or their complexation with dissolved ligands. Our results indicated that in both dissolved and ultra-filtrated fractions REE increases either in the middle (Sm - Dy) or in the heavier (Ho - Lu) part of the PAAS-normalised distribution, while the Zr/Hf ratio value ranges from sub-chondritic to super-chondritic. Scanning Electron Microscopic and Energy Dispersive X-ray Spectrometric (SEM-EDS) analyses and dissolved Mg, Al and Fe concentrations suggested that the studied colloids consist of aggregations of Al-oxyhydroxides, carbonate nanoparticles and clays where organic traces were not found. The studied colloids showed greater affinity with dissolved Zr than Hf determining Zr/Hf values larger than the chondritic values. The largest Zr/Hf values were found in colloidal fractions from waters with ionic strength larger than 0.6molkg-1. These Zr/Hf values up to 280 (w/w) are provided by the faster removal of Hf relative to Zr from coagulating colloids and its preferential scavenging onto authigenic Fe-oxyhydroxides in bottom sediment. Further studies are needed to clarify is this suggested process can represent a suitable starting point for the Zr-Hf decoupling observed in seawater.