The Effects of Low-Molecular-Weight Organic Ligands on Gallium and Aluminum Mineral Weathering and Soil Sorption

Aluminum (Al) is a significant component of primary and secondary aluminosilicate minerals, which serve as the scaffolding of a soil. Minerals such as kaolinite and feldspars are only two examples of Al-bearing solids commonly found in the Critical Zone. As the third most abundant element in the Earth’s crust, Al plays a crucial role in biogeochemical cycling. Its presence within soils is essential for soil formation; however, a slight change in acidity can cause detrimental effects to life. Aluminum toxicity is defined as when excess Al has been introduced to plant tissues from a minor change in acidity leading to growth stagnation and mortality by inhibiting root growth mechanisms (Poschenrieder et al., 2008). Previous research has found the production of low-molecular-weight organic ligands from plants and decaying organisms has provided a natural mitigation for soil acidity. Since Al toxicity varies for different species, the transport and uptake of Al within the Critical Zone has become of recent interest. Using the paradigm to classify the uptake and mobilization of metals by using a stable isotope ratio, such as 65Cu/63Cu, 65Ca/63Ca, and 7Li/5Li has proven extremely useful in recent years (Wiederhold et al., 2015). Unfortunately, a consequence of using this method applied to Al is its lack of multiple stable isotopes existing in nature. Consequently, another method is required to classify Al movement and uptake within the Zone (Derry and Richardson, 2016). Previous research has highlighted the observation in which Gallium (Ga) tends to substitute for Al in solid minerals phases. Moreover, historical data shows the proportion in which Ga substitutes to Al which is approximately 0.1 mM/M Ga/Al, as reported by Shiller and Frilot (1996). Although they behave similarly in the solid phase, Shiller and Frilot’s study noted the deviation in Ga/Al concerning stream waters in California. A proposed mechanism of fractionation of this ratio is the effect of complexation and sorption of low-molecular-weight organic ligands fractionating the Ga/Al ratio. Low-molecular-weight organic ligands produced by biota in soils may play an essential role in the sorption and potentially fractionation of Ga/Al ratio in stream waters (Shiller and Frilot, 1996). To further validate Ga/Al as a geochemical tracer for Al, we conducted two batch reactor experiments and a column leaching experiment examining several gaps in the knowledge in the geochemical signature of the Ga/Al ratio in aquatic systems. As a means to gain insight as to why Ga/Al is fractionated when weathered from bulk rock samples with a known Ga/Al of ~ 0.1 mM/M, two different organic acid treatments, and a control were used. Varying the material, a Luquillo soil, quartz diorite, and kaolinite were treated with low-molecular-weight organic ligands to test the hypothesis put forth by Shiller and Frilot, 1996 stating that the Ga/Al was influenced by to low-molecular-weight organic ligands. Our first objective in this study was to evaluate the effect of low-molecular-weight organic ligands in the mobilization of Al, Fe3+, and 71,69Ga within a highly weathered Al-rich soil. Secondly, we sought to evaluate the effect of pH, ligand, and material type to evaluate the leaching behavior of Ga with respect to Al and Fe from primary and secondary mineral sources within soils. Lastly, we wanted to determine how these Al, Fe3+, and 71,69Ga sorb to different minerals to understand their transport during the weathering process. Obtained results show the effect of low-molecular-weight organic ligands is pronounced effect regarding Al and Ga dissolution. Generally, materials treated with citrate caused the Ga/Al to decrease with increasing pH, whereas the catechol and control treatments tended to increase the Ga/Al ratio with increasing pH. The relationship complexed Ga, and Al suggests the effect of organic low-molecular-weight organic ligands varies with the type of material, stability of the metal complex, and decreases with increasing pH