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

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

0087695 Zafiriou Freshwater dissolved organic matter (DOM) strongly affects many important processes, particularly in organic-rich systems. Recent reports show that (1) the major term in the proton balance of the highly colored Satilla River is "organic alkalinity," and (2) that photoreactions in this water, catalyzed in part by Fe, consume O2, bleach color, and form CO, CO2 (the major product), and Fe(II). We hypothesized that CO2 arises by Fe-catalyzed "photodecarboxylation": DOM-COO-H + O2 + hv_______ Fe DOM-H + CO2 a well-known reaction for simple molecules. The implied massive loss of the principal organic ionizable groups 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 acidic group was lost; pH decreased (stronger acids formed than were consumed). Furthermore, Fe was involved in 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, and wavelength; (2) assessing the generality and environmental variability of these effects by studying other riverwaters, sampled in different seasons; (3) examining the effect of these changes on an important physico-chemical property of hv-DOM: its copper binding strength and capacity, and (4) formulating a better heuristic scheme than the hypothesis above to rationalize the results chemically. Clearly to some extent photo-generated CO2 does not originate simply from -COO - groups. Acidic groups, if lost, seem to be 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 with experience in organic/inorganic alkalinity-pH-CO2 properties, and DOM photochemistry. We are using optimizations of methods demonstrated to apply to this problem, and well-known Cu-binding study methods. The Altamaha, a more typical second river chosen for study, is also DOM-rich. Its DOM is thought to be derived from both allochthonous and autochthonous sources, in contrast to the Satilla's mainly allochthonous, highly colored DOM. Suwannee River water and well-known isolates of its humic and fulvic acids (SRFA, SRHA) are also being studied, since in organic-rich colored waters these fractions 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-working postdoctoral associate and graduate students at UGA and WHOI. Societal Impacts: This study increases our understanding of processes potentially affecting water transparency, potability and treatment (e.g. chlorination), aquatic biota, and also the binding, transport, and transformations of agrichemicals and pollutants (especially Cu).