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PHOTOCHEMICAL ALTERATION OF DISSOLVED ORGANIC MATTER: EFFECTS ON THE CONCENTRATION AND ACIDITIES OF IONIZABLE SITES IN DISSOLVED ORGANIC MATTER IN THE SATILLA RIVER OF GEORGIA, USA
Citation:
Xie, H., Y. Wang, W. J. Cai, O. C. Zafiriou, AND R G. Zepp. PHOTOCHEMICAL ALTERATION OF DISSOLVED ORGANIC MATTER: EFFECTS ON THE CONCENTRATION AND ACIDITIES OF IONIZABLE SITES IN DISSOLVED ORGANIC MATTER IN THE SATILLA RIVER OF GEORGIA, USA. Presented at 10th International Meeting of the International Humic Substance Society, Toulouse, France, July 24-28, 2000.
Impact/Purpose:
The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic resources to global change. The task will contribute experimental and modeling tools for assessments of the interactions of global climate and UV changes with coral reefs and selected watersheds and estuaries in the U.S. These activities are contributing primarily to two APGs in the ecosystems component of the Global Change Research Multiyear Plan: the 2006 APG (APG 3) on building the capacity to assess global change impacts on coastal aquatic ecosystems, including coral reefs and estuaries and the 2004 APG (APG 2) on building capacity to assess and respond to global change impacts on selected watersheds. One major task objective is to assess interactions of global warming and UV exposure that are contributing to the observed coral bleaching and disease. Our lab is working with scientists at the NHEERL Gulf Ecology Lab to characterize UV exposure and effects at several coral reef sites. Other research in this task is examining the interactions between UV-induced breakdown of refractory organic matter in estuaries and coastal areas that enhance UV penetration into the water and concurrently form biologically-labile nitrogen-, phosphorus- and carbon-containing substances that stimulate productivity and microbial activity. This task also involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). The objectives of this project are to assess the impacts of land use and climatic changes on soil nutrient cycles and microbiota, trace gas exchange and water quality in the Brazilian cerrado. This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. Other objectives of this task are to assess the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the Piedmont region of the southestern U.S.
Description:
The acid-base properties of humic substances, the major component of dissolved organic matter (DOM), area major control on the alkalinity, or acid neutralizing capacity of freshwater systems. Alkalinity is one of the fundamental parameters measured in aquatic sciences, and is an important index for water quality. At the molecular level, the number and pKa values of charged sites of humic substances profoundly influence their physico-chemical properties, such as aggregation and disaggregation, hydrophobic binding, trace metal binding, etc. It has been reported that major proton-binding sites in DOM consist of carboxyl, phenolic, and amine groups. Numerical models which describe the charge sites with these functional groups can reasonably simulate the titration curves of DOM and constrain the concentrations and pKa values of each charge group. Humic substances in natural waters are subjected to photochemical degradation by solar radiation, producing both reactive transient intermediates (e.g. hydroxyl radicals) and relatively long-lived photoproducts, such as carbon monoxide (CO), dissolved inorganic carbon (DIC), and hydrogen peroxide (HOOH). It has been suggested that photoproduction of DIC may be related to destruction of carboxylgroups in humic substances. Phenolic groups are well known for their high photochemical reactivity, and they may thus be chemically altered during photolysis of humic substances. Photo-degradation ofDOM also produces non-humic low-molecular-weight carboxylic acids, including oxalic, formic, acetic, and malonic acids. Therefore, photochemical processes in natural waters may significantly influence the proton and metal binding properties of charge sites in DOM. In this paper, we present the results of a preliminary study on the effects of photo-oxidation of DOM on the concentrations and acidities of proton-binding sites in DOM, employing acid-base titration and numerical modeling approaches. We also explored the role of iron in modifying the acid-base properties of DOM, since iron has been shown to be capable of catalyzing photo-decomposition of DOM.