INVESTIGATING THE ROLE OF SOLUTION CHEMISTRY AND HYDROXIDE PRECIPITATION ON GALLIUM ALUMINUM RATIOS IN THE CRITICAL ZONE

Understanding biogeochemical cycling of aluminum (Al) in the Critical Zone may be enhanced using the chemically similar element gallium (Ga) as a pseudo-isotopic tracer for weathering processes. We investigated the role of different low molecular weight organic ligands (LMWOLs) and hydroxide precipitation on the effect of Ga/Al ratios and concentrations. Batch reactor experiments were set up using dilute hydrochloric acid, 0.01 M citric acid, 0.01 M oxalic acid, and 0.01 M catechol solutions at pH 4, 5, and 6. Six aluminosilicate minerals, as well as B and C soil horizons from four Critical Zone Observatories, were weathered for eight weeks total. Decanted solutions from 2 week runs were analyzed for elemental concentrations. Initial batch reactor results indicated that solution pH was not a driving factor of element release from minerals, but LMWOLs, especially citric acid, were the dominant control of elemental dissolution. Gallium concentrations in the decanted solutions from citric acid treatments ranged from 1.54 µg per two weeks from feldspar dissolution to 5.16 µg per two weeks from muscovite dissolution in the first set of batch reactor experiments. Aluminum concentrations ranged from 417 µg per two weeks from feldspar to 2,088 µg per two weeks from the muscovite. However, the dilute hydrochloric acid solution led to the highest Ga/Al concentrations for each mineral type and pH, from 30 mmol/mol Ga/Al from mafic mineral dissolution to 553 mmol/mol Ga/Al from muscovite dissolution. To investigate secondary oxide phase fractionation of Ga/Al ratios, bayerite and goethite were synthesized via forced hydrolysis and spiked with varying amounts of Ga by weight, creating a control, low Ga, and high Ga conditions. Initial mineral synthesis XRD patterns show little change between control minerals and those spiked with Ga for both Al and iron minerals, possibly signaling substitution of Ga into the mineral lattices without changes to the unit cell.