Organo-mineral associations and sequestration mechanisms impacting carbon cycling in diverse terrestrial and aquatic systems

• Main Author: Sowers, Tyler Dale
• Format: Thesis
• Language: English
• Published: University of Delaware 2019
• Subjects: permafrost
• Online Access: http://udspace.udel.edu/handle/19716/24969; https://doi.org/10.58088/296g-yk78

Sowers, Tyler Dale Sequestration of organic matter (OM) in environmental systems is critical to mitigating climate change. Organo-mineral associations, especially those with iron (Fe) oxides, drive the chemistry of OM sequestration and stability in soils. In the past 20 years, research exploring the sequestration of OM to Fe oxides has intensified. Poorly crystalline Fe oxides, such as ferrihydrite, demonstrate a high affinity for OM in binary systems. Calcium commonly co-associates with OM and Fe oxides in soils, though the bonding mechanism (e.g., cation bridging) and implications of the coassociation for OM sequestration remain unresolved. The biogeochemical cycling of OM in systems containing biogenically-produced Fe oxides and in permafrost conditions is also poorly understood. In an effort to gain an environmentally comprehensive understanding of C cycling in chemically heterogeneous systems, we explored the effect of calcium (Ca2+), biogenic Fe oxides, and permafrost conditions on the cycling of OM. Extensive batch and advanced spectroscopic experiments were used to investigate the occurrence and importance of Fe, Ca, OM ternary associations. OC sorption extent to ferrihydrite in the presence of Ca2+ increased across all tested pH values, especially at pH >7. Sorbed OC concentration at pH 9 increased from 8.72 to 13.30 mmol OC g-1 ferrihydrite between treatments of no added Ca2+ and 30 mM Ca2+ addition. Batch experiments were paired with spectroscopic studies to probe OC speciation and mechanism of sorption complexes. ATR-FTIR spectroscopy analysis revealed that carboxylic functional moieties were the primary sorbed OC species and suggested an increase in Fe-carboxylate ligand exchange in the presence of Ca at pH 9. STXM-NEXAFS was used to spatially resolve Fe, Ca, and OC relationships and to probe the effect of Ca on sorbed OC speciation. Organic carbon was found to highly associate with Ca (R2 = 0.91). Carboxylic acid moieties were dominantly sequestered; however, Ca facilitated the additional ...