Collaborative Research: The Effects of Aquatic Photochemistry on the Proton and Metal Binding Properties of Dissolved Organic Matter in Freshwater Systems

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0087695ZafiriouFreshwater dissolved organic matter (DOM) strongly affects many important processes, particularlyin organic-rich systems. Recent reports show that (1) the major term in the proton balance of the highlycolored Satilla River is "organic alkalinity," and (2) that photoreactions in this water, catalyzed in part byFe, consume O2, bleach color, and form CO, CO2 (the major product), and Fe(II). We hypothesized thatCO2 arises by Fe-catalyzed "photodecarboxylation":DOM-COO-H + O2 + hv_______ FeDOM-H + CO2a well-known reaction for simple molecules. The implied massive loss of the principal organic ionizablegroups should profoundly influence many important properties of photo-altered DOM (hv-DOM): e.g.metal-binding capacity, acid-base properties, hydrophobic binding, adsorption/coagulation,bioavailability, optical properties, and perhaps behavior in water purification (e.g. chlorination).However, an initial study determining and modeling Satilla water's whole pH titration curves before vs.after photolysis (solar simulator) falsified the hypothesis above: for every CO2 formed only ~ 0.1 acidicgroup was lost; pH decreased (stronger acids formed than were consumed). Furthermore, Fe was involvedin the changes observed.Objectives: Hence we are proposing (1) better characterizing the relationships among CO2 evolution,proton binding sites (concentrations and affinities), and other variables: [Fe], pH, [O2], light dose, andwavelength; (2) assessing the generality and environmental variability of these effects by studying otherriverwaters, sampled in different seasons; (3) examining the effect of these changes on an importantphysico-chemical property of hv-DOM: its copper binding strength and capacity, and (4) formulating abetter heuristic scheme than the hypothesis above to rationalize the results chemically. Clearly to someextent photo-generated CO2 does not originate simply from -COO-groups. Acidic groups, if lost, seem tobe replaced by similar but not identical new acids from DOM photo-oxidation.Work Plan and Methods: This is a collaborative effort among a postdoctoral associate and two PIs withexperience in organic/inorganic alkalinity-pH-CO2 properties, and DOM photochemistry. We are usingoptimizations of methods demonstrated to apply to this problem, and well-known Cu-binding studymethods. The Altamaha, a more typical second river chosen for study, is also DOM-rich. Its DOM isthought to be derived from both allochthonous and autochthonous sources, in contrast to the Satilla'smainly allochthonous, highly colored DOM. Suwannee River water and well-known isolates of its humicand fulvic acids (SRFA, SRHA) are also being studied, since in organic-rich colored waters thesefractions comprise a major portion of the DOM, organic alkalinity, and light-absorbing material.Education: This project contributes to training in environmental science by involving an independent-workingpostdoctoral associate and graduate students at UGA and WHOI.Societal Impacts: This study increases our understanding of processes potentially affecting watertransparency, potability and treatment (e.g. chlorination), aquatic biota, and also the binding, transport,and transformations of agrichemicals and pollutants (especially Cu).