Gallium isotope fractionation during adsorption on clay and oxide minerals: implications for using gallium isotopes as a geochemical tracer
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Increasing climate change leads to the imbalance between silicate, sulfide, and carbonate weathering fluxes, influencing the global carbon budget. Thus, understanding this balance is critical for understanding future global warming. Gallium (Ga) concentration is low in carbonates, moderate in silicates, and high in some sulfides (e.g., sphalerite). Other significant fluxes of Ga into the environment are acid mine drainage streams caused by sulfide mineral oxidation and waste effluents from the semi-conductor industry. Thus, Ga isotopes can be a promising environmental tracer. However, utilization of Ga isotopes as tracer requires a comprehensive understanding of its mobility and isotope fractionation processes. Thus, this study focused on adsorption and how this process fractionates the isotope composition of Ga. Laboratory experiments were conducted with clay (kaolinite and Ca-montmorillonite) and oxide minerals (silica, aluminum oxide, and goethite) to determine the effects of pH, ionic strength, Al competition, and low temperature on Ga adsorption. Experimental results together with Ga speciation and adsorption modeling provide insight onto the Ga adsorption mechanisms and associated Ga isotope fractionation process. Ga isotope ratio analyses were conducted for samples from the adsorption experiments on clay minerals and aluminum oxide, as a function of pH and ionic strength at room temperature (21°C) by Multi-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS). At low concentration (50 U+03BCg/L), Ga adsorption on kaolinite was highest (adsorbed Ga = 63%) at pH 4.8 and lowest at alkaline pH (adsorbed Ga = 0) at pH = 9 which was attributed to the formation of the SOU+2084GaU+00B9U+207B surface complex at all pH values. Conversely, during adsorption on Ca-montmorillonite, ionic exchange and surface complexation (SOHU+2082Ga(OH)U+2084 and SOGa(OH)U+2083U+00B9U+207B) caused high Ga sorption in acidic solutions, but adsorption also decreased with increasing pH, where only surface complexation of SOGa(OH)U+2083U+00B9U+207B occurred. At high concentrations (1000 U+03BCg/L and 3000 U+03BCg/L), Ga precipitation (α-GaOOH) was observed at both 21°C and 5°C. In contrast with clay minerals, adsorption on oxide minerals was highest at mildly acidic to circumneutral pH (4 – 7). Ga adsorption on silica was highest (adsorbed Ga = 98%) at pH 4 where SOGa(OH)U+2083U+00B9U+207B was formed, and lowest (adsorbed Ga = 44%) at pH 9 where SOH(Ga(OH)U+2084)U+2082U+00B2U+207B was formed. Conversely, Ga adsorption on goethite was greater than 95% at pH 4 – 7 and lowest at pH 3 (adsorbed Ga = 14%), due to a single surface complex (SOGa(OH)U+2082). Ga adsorption on aluminum oxide was highest (adsorbed Ga ≈ 80%) at circumneutral pH (4 – 7), and lowest (adsorbed Ga = 24%) at pH 3. For all oxides, moderate Ga adsorption (adsorbed Ga ≈ 45%) occurred at alkaline pH (9). Analysis of in-house reference standards for gallium isotopes demonstrated that there was a constant shift in the isotope data from the expected values. Analyzing the stock solution throughout the study shows a consistent value (2.04 ± 0.09‰, 2SD; n = 10). This is 0.6‰ higher than the δU+2077U+00B9Ga of the pure Ga solution used to prepare the experimental Ga stock solutions (δU+2077U+00B9Ga = 1.44 ± 0.09‰, 2SD; n = 21). This isotope fractionation is likely caused by the incomplete purification of the sample matrix and/or interfering elements that affect the accuracy of isotope ratio measurement. However, the degree of fractionation appears relatively constant within samples having similar matrix loading. Therefore, the isotopic data of the experimental samples can be interpreted with the assumption that the fractionation due to the isotopic analysis was somewhat consistent among experimental samples and in-house standards, given that their matrix element contents were also similar. For Ga adsorption on clay and aluminum oxide minerals, the lighter isotope (U+2076U+2079Ga) was preferentially adsorbed on the solid phase, leaving the solution enriched in the heavier isotope (U+2077U+00B9Ga). Ga isotope fractionation during adsorption on kaolinite, Ca-montmorillonite, and aluminum oxide followed the closed-system equilibrium model. For all minerals, fractionation factors (α) did not vary with solution pH, suggesting pH independent isotope fractionation. Ga isotope fractionation (Δsolution-solid) was greatest following adsorption on Ca-montmorillonite (1.70 ± 0.63‰, 2SD; n = 4) and was lowest after adsorption on aluminum oxide (0.62 ± 0.97‰, n = 6). Following Ga adsorption on kaolinite, Δsolution-solid was 0.94 ± 0.37‰, 2SD; n = 5. Ga adsorption on kaolinite at pH 3 decreased with increasing ionic strength, but ionic strength had negligible effects on δU+2077U+00B9Ga values (1.78 ± 0.06‰, 2SD; n = 6). In experiments testing influence of adsorption on Ga/Al ratios, under acidic conditions (pH 3 and pH 5), Ga/Al remained unchanged whereas Ga/Al increased significantly at pH 7 due to the preferential adsorption of Ga and precipitation of Al as gibbsite. This study demonstrates that Ga adsorption on common aquifer minerals such as clays and oxides induces a relatively large fractionation between δU+2077U+00B9Ga in the solution and solid (1.70‰ for Ca-montmorillonite and 0.62‰ for aluminum oxide). Future studies can focus on other processes such as dissolution of primary minerals and precipitation of secondary minerals to provide a more comprehensive understanding on Ga isotope fractionation by chemical weathering processes. Overall, Ga isotopes can potentially be used as a geochemical tracer of chemical weathering due to the association of Ga with rocks whose weathering directly affects the global carbon budget. Ga isotopes may also be useful for tracing point sources of Ga from the technology industry which consumes large quantities of Ga, whose wastes can contaminate the environment.
Cite this version of the work
John Ulkem (2022). Gallium isotope fractionation during adsorption on clay and oxide minerals: implications for using gallium isotopes as a geochemical tracer. UWSpace. http://hdl.handle.net/10012/18216