Dorothy G. Richey (email@example.com) & Charles T. Driscoll (firstname.lastname@example.org)
Department of Civil and Environmental Engineering, 220 Hinds Hall,
Syracuse University, Syracuse, New York 13244, U.S.A.
The "Montreal Protocol on Substances That Deplete the Ozone Layer" in 1987, and the subsequent London Amendments of 1990 require a complete phase out of chlorofluorocarbons (CFCs) by the year 2000. In the United States, the 1990 Amendments to the Clean Air Act call for similar controls on CFCs. The search for environmentally acceptable replacements for CFCs has led to the use of partially halogenated compounds, hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs).
Trifluoroacetic acid (TFA) has been identified as an important breakdown product of the CFC replacement chemicals (Ball and Wallington, 1993). Trifluoroacetic acid will be produced in the atmosphere and transported via precipitation to the Earth's surface. The fate of TFA is uncertain. Three experimental components are underway to evaluate TFA transport in the environment (1) retention of TFA by different soil types; (2) reversibility of TFA retention by soils; and (3) determination of background concentrations of TFA in air, precipitation, water, vegetation and soil.
Preliminary laboratory studies suggested TFA was not significantly retained by soils (van Dijk, 1991). Soils were obtained from 15 terrestrial sites that are part of the National Science Foundation (NSF) Long-Term Ecological Research (LTER) Programme, three sites in Brazil, and two sites in the Czech Republic. Batch soil adsorption studies were conducted on each of these soils. Miscible displacement experiments were conducted on four soils to evaluate the reversibility of TFA adsorption and to simulate the movement of solutes under saturated soil conditions. Three soils that retained TFA in the batch equilibrium study were used. One additional soil and silicon dioxide that did not retain TFA were used as a reference. A pulse addition and a step-equilibrium study were completed on all five soils for a total of eight columns. Breakthrough curves for TFA and bromide (Br) were plotted as the fraction C/Co versus effluent volume. Intensive monitoring of TFA in air, water soil and vegetation will be conducted at Hubbard Brook Experimental Forest (HBEF), NH. A monthly monitoring programme at HBEF and Syracuse, NY was initiated in November, 1996 to monitor TFA concentrations in precipitation, throughfall, streamflow and soil water. Vegetation will be sampled at HBEF four times during the growing season.
There is considerable variability of TFA retention in soils. Most soils (43 of 54) did not retain TFA strongly (>25 %) (Richey et al., 1997). Retention ranged from a high of 260 µmol kg-1 to 25 µmol kg-1 (0-60 % of added TFA). Soils with high organic matter content (30-80 %) retained 20-60 % of added TFA. Trifluoroacetate retention increases with decreasing pH. Trifluoroacetate retention decreases with increasing inorganic anion concentration. The reversibility of TFA retention is critical to assessing plant-available concentrations of TFA.
Preliminary results of miscible displacement experiments indicate that TFA is readily desorbed from soils but may be more difficult to desorb from soils with high organic matter content. Breakthrough curves of TFA are similar to Br breakthrough curves. It is anticipated that increases in the emission of CFC replacement chemicals and atmospheric deposition of TFA will provide increasing inputs of TFA to terrestrial and aquatic ecosystems in the future. It is critical to determine the background concentrations of TFA before increasing inputs of TFA occur. Frank et al. (1996) suggests that the current concentrations of TFA in rain is not explained by known precursors of TFA and there are additional unknown sources. Clearly, it is important to conduct both intensive monitoring and extensive measurements of TFA in the environment.
Ball, J.C. & Wallington, T.J. J. Air & Waste Manag. 43, 1260-1262 (1993).
Frank, H., Klein, A. & Renschen, D. Nature 34, 382 (1996).
Richey, D.G., Driscoll, C.T. & Likens, G.E. Environ. Sci. & Tech. in press (1997).
van Dijk, N.R.M. Adsorption test with sodium trifluoroacetate (NaTFA). Solvay Duphar, B.V. Internal Doc. No. 56635/9191 (1991).
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