IMPACTS OF CITRATE ON THE EVOLUTION OF MINERAL GRAINS IN A 15-YEAR WEATHERING COLUMN OF GRANITE AND BASALT

Laboratory weathering studies provide insights into many of Earth’s surface processes such as soil formation, biogeochemical cycling, and atmospheric CO₂ dynamics. Long-term studies of granitic material in columns have yielded long-duration (13.8 years) weathering rates, but long-duration basalt weathering rates have not yet been investigated. Additionally, in natural settings, biota produce organic acids such as citrate, that affect weathering by changing the solubility of minerals and increasing mineral dissolution rates. The role of these organic acids remains under characterized in published column weathering studies. Therefore, in this study we investigated the role of citrate in a >15-year laboratory weathering study of basalt and granite.

Four columns were set up using either powdered Tuolomne River Series granite or Columbia River Basalt both with and without 0.01M citrate. For the duration of the experiment, flow rates and effluent pH levels were recorded, and effluent from the columns was collected for major and trace elemental analysis. BET surface area was measured at the beginning and conclusion of the experiment. Reacted rock samples from multiple depths throughout each column at the end of the experiment were analyzed by XRD, SEM, and ICP-AES to determine mineralogy and bulk chemistry.

Elemental release rates largely decreased over time. In the citrate-containing columns, higher elemental release rates were maintained than in the non-citrate counterparts for the entire duration of the experiment. Reacted granite samples were found to be primarily composed of quartz, K-feldspar, and plagioclase minerals while the basalt was found to primarily consist of augite and plagioclase minerals. There was no significant difference in mineralogy between the inlet and outlet of the columns noted in the XRD spectra. Additionally, no neoformation of clays or carbonates were observed by XRD. However, in the presence of citrate, there is notably more elemental depletion with respect to parent material at the inlet of the columns than at the outlet. The continued enhancement of weathering rates in the presence of citrate highlights the importance of biogeochemical interactions on long time scales when developing a comprehensive understanding of rock weathering processes.