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Environmental fate & pathways

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Cesium fluoroaluminate is a soluble metal salt consisting of cesium cations and fluoroaluminate anions and thus liberates cesium cations and fluoroaluminate anions upon dissolution. Since cesium and fluoroaluminate ions behave differently in the environment, their fate is assessed separately. Fluoroaluminate ions are moderately stable but will eventually disintegrate resulting in aluminium cations and fluoride anions that participate in prevalent geochemical speciation processes.

 

The assessment of the partitioning of aluminium and cesium between freshwater and sediments is based on Kp values derived from monitoring data for elemental Al and Cs concentrations in water and corresponding sediments provided by the FOREGS Geochemical Baseline Mapping Programme that aimed to provide high quality, multi-purpose homogeneous environmental geochemical baseline data for Europe. Paired samples, i.e. samples with the same coordinates for the sampling location of stream water (filtered to < 0.45 µm) and sediment (wet sieved in the field to <0.15 mm) were processed (Salminen et al. 2005) and results correspond to steady-state conditions, independent of environmental speciation. Sampled stream water and sediments cover a wide range of environmental conditions. Water parameters such as pH, hardness and organic carbon concentrations cover several magnitudes. Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe, the European median values are selected as representative sediment-water partition coefficients of Europe.Sediment-water partition coefficients of fluoride cannot be calculated since the FOREGS dadabase does only contain streamwater but not sediment concentrations. A conservative estimate is applied based on peer-reviewed literature.

Aluminium has a low mobility under most environmental conditions, although below pH 5.5 its solubility increases. Because of its amphoteric nature, aluminium may also be mobilised in anionic form under strongly alkaline conditions at pH > 8. The speciation of aqueous Al is dependent on pH and the presence and nature of complexing ligands. In aqueous solutions, unhydrolyzed Al (3+) (aq) does not remain as free ion, but is surrounded by six molecules of water Al(H2O)6 (3+). As solution pH increases, protons are removed from the coordinated waters giving a series of hydrolysis products: Al(H2O)5(OH) (2+); Al(H2O)4(OH)2 (+); Al(H2O)3(OH)3; Al(H2O)2(OH)4 (-); Al(H2O)(OH)5 (2-). Thus, in the absence of significant concentrations of complexing ligands, the dominant form of dissolved Al below about pH 4.5 is Al(H2O)6 (3+). Above pH 4.5, the hydrolysed forms Al(H2O)5(OH) (2+) and Al(H2O)4(OH)2 (+) are predominant. At low pH, organic ligands, including humic and fulvic acids, and inorganic ligands, such as fluoride, readily complex with dissolved aluminium. As a result of organic complexation, particularly through chelation, a significant proportion of Al is usually in colloidal or particulate forms. Concentrations of dissolved Al are generally low in most natural waters (Salminen et al. 2015 and references therein). Median aluminium concentrations across Europe amount to 5.45 mg/kg sediment and 0.017 mg/L stream water, respectively. Regarding the partitioning of aluminium in the water column, a European median log Kp value of 2.39 L/kg is derived for sediment-water partitioning.

 

Cesium is a relatively rare element. Released cesium is rapidly and strongly adsorbed by soil, especially clay, and therefore does not tend to be present in the aqueous surface environment. An European median total concentration in topsoil is reported with 3.71 mg/kg and a global with 3 mg/kg W (Salminen et al. 2005 and references therein). Median cesium concentrations across Europe amount to 3 mg/kg sediment and 0.01 microg/L stream water, respectively. Regarding the partitioning of cesium in the water column, a European median log Kp value of 5.70 L/kg is derived for sediment-water partitioning. Regarding the partitioning of cesium in the water column, a European median log Kp value of 1.95 L/kg is derived for sediment-water partitioning.

 

Fluoride ions in solution form strong complexes with other ions, particularly Ca2+, Al3+, Fe3+, PO43-and B(OH)4-. The concentration of fluoride ions in solution is often controlled by the solubility of fluoride; and the concentration inversely proportional to that of Ca2+. Fluoride also adsorbs to mineral surfaces such as gibbsite, kaolinite, halloysite, and freshly precipitated amorphous Al(OH)3. Sorption to these solid phases may be favoured at lower pH (Salminen et al. 2005 and references therein). The transport, distribution and transformation of fluoride in soil are influenced by pH and the formation of predominantly aluminium and calcium complexes. Adsorption to the soil solid phase is stronger at slightly acidic pH (5.5–6.5). Fluoride is not readily leached from soils. Median fluoride concentrations across Europe amount to 0.1 mg/L stream water. According to a review conducted by SCHER (2011), the electronegativity of the fluoride ion results in little partitioning to solids and thus, sewage sludge and soils are unlikely to become contaminated (SCHER, 2011). Further, for a substance produced at 100,000 - 1 000,000 tpa, soil levels of fluoride were found to be „negligible compared to background concentrations“ (EU RAR, 2001). Consequently, due to the electronegativity of the fluoride ion, it may be assumed that there is little partitioning to solids in soils, sediment and suspended matter. Conservatively, an estimate of 0.01 for the solids-water partition coefficient soil, solids-water partition coefficient sediment (freshwater) and solids-water partition coefficient suspended matter (freshwater) is applied.

Reference

European Union Risk Assessment Report (EU RAR) (2001) on hydrogen fluoride, CAS No.: 7664-39-3; EINECS No.: 231-634-8; European Commission, Joint Research Centre.

SCHER, 2009: Scientific Committee on Health and Environmental Risks (SCHER). Critical review of any new evidence on the hazard profile, health effects, and human exposure to fluoride and the fluoridating agents of drinking water.

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