Influences of Dissolved Organic Carbon, Iron, and Aluminum Photochemistry on Phosphorus Cycling in Fresh Surface Waters

Publication Year:
2010
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Repository URL:
https://digitalcommons.library.umaine.edu/etd/750
Author(s):
Johnson, Dustin J.
Tags:
Organic water pollutants; Phosphorus cycle; Biogeochemistry; Earth Sciences
thesis / dissertation description
Dissolved organic carbon (DOC) provides a mechanism for the export of terrestrially-derived iron (Fe) and aluminum (Al) to surface waters. Fe and Al particles are important in surface water chemistry because they adsorb dissolved phosphorus (P), a limiting nutrient to phytoplankton. Phosphorus cycling is an essential component in controlling the trophic dynamics and overall water quality in freshwater environments. Surface waters exposed to ultraviolet radiation (UVR) undergo photochemical reactions involving DOC-Fe or DOC-A1 complexes that, in turn, influence how trace metals and nutrients, including P, interact in first- and second-order streams and lakes. We conducted laboratory photochemical experiments to assess the influence of DOC, Fe and Al concentrations on P cycling, and to quantify the size distribution of aqueous Fe, Al, and P particles induced by irradiation. Batch solutions were irradiated and analyzed for concentrations of DOC, organically-bound dissolved Fe or Al, and four distinct size fractions of Fe or Al and P using ultrafiltration. The size fractions were determined by passing solution sequentially through 0.45 urn, 100 KDa (~30 nm), and 5 KDa (~3 nm) filters to evaluate the relationship between similarly sized Fe, Al, and P particulate, colloidal, and dissolved fractions, respectively. DOC concentration and the size distribution of Fe and Al particles are negatively correlated. As the initial DOC concentration is increased, the mass-averaged Fe and Al particle size decreases, reaching lower size limits of 26 and 34 nm, respectively. Furthermore, a positive correlation exists between the mass-averaged Fe and Al particle size and the mass-averaged P particle size distribution. The Fe or Al size fraction present at the highest concentration during irradiation is associated with similar sized P in solution. These data suggest that dissolved P (Pd) is adsorbed to Fe- or Al-(oxy)hydroxide particles. The adsorption of Pd falls between or below the range of maximum sorption capacity for ferrihydrite, suggesting that the maximum sorption capacity of suspended colloids was reached. In the majority of irradiation experiments, the Pd concentration decreased as the fractions of P associated with particulate and colloidal Fe and Al increased. These results highlight the importance of characterization of photosensitive complexes and particles in understanding how UVR influences the cycling of P in surface fresh water systems.