Environ. Sci. Technol. 2000, 34, 120-124
Chlorothalonil in a Quartz Sand Soil: Speciation and Kinetics DONALD S. GAMBLE,* ALDO G. BRUCCOLERI, ELSPETH LINDSAY, AND COOPER H. LANGFORD Department of Chemistry, University of Calgary, Calgary, Alberta T2N 1N4, Canada GREGORY A. LEYS Ricerca Inc., 7528 Auburn Road, P.O. Box 1000, Painesville Ohio 44077-1000
processes as surface sorption, desorption, and bound residue formation by intraparticle diffusion. Mass balance losses are known to have a number of possible causes, including bound residues. When the other possibilities have been experimentally eliminated, then intraparticle diffusion is known to have caused bound residues (3). The concept of “bound residue” is operationally defined here as that portion of an organic chemical that is not recovered by the online HPLC microextraction method as previously described (2, 9). The practical reason is that this corresponds to the particular portion which under field conditions is protected from both immediate leaching and immediate biodegradation. This is not true of harsher laboratory extraction methods that are sometimes used for bound residues. The kinetic information produced permitted subsequent investigations of equilibria.
Theory The distribution of chlorothalonil among the dissolved, labile sorbed, and bound residue states was monitored during an 18 day period in an aqueous slurry of an analyzed quartz sand soil from Simcoe, ON, Canada. The Simcoe soil is 90.-95.% quartz sand. The online HPLC microextraction method was used for this purpose, because it is the only available technique that can resolve the total amount of a pesticide in a soil into its dissolved, labile sorbed, and bound residue components. The processes for which the molecular level kinetics were determined included labile surface sorption and desorption and bound residue formation. At a reaction time of 14 days, the solution concentration of 0.75 × 10-6 M was 43.3% of the total chlorothalonil, 26.2% was in the labile sorbed state, and 30.5% was a bound residue. There were no chemical reactions and no biodegradation during the 18 day period. The kinetics of mass transfer among the three states were determined and are consistent with intraparticle diffusion. Although the amounts are small, it is suspected that the 5.10.% nonquartz materials in the Simcoe soil contribute most of the sorption and bound residue effects.
Introduction The fungicide chlorothalonil has been used on a field site in North Carolina, and its 4-hydroxy derivative is an important environmental issue. The behavior of chlorothalonil in the soil is directly relevant to the on-site production, persistence, and leaching of its 4-hydroxy derivative. Kalkhoff et al. have recently made this point for another case (1). The characteristics of such a system are largely influenced by the chemical structure of the organic compound and the types and amounts of chemical materials in the soil (2). A sandy soil from a field site near Simcoe, ON was used for the first part of an investigation of the interactions of chlorothalonil and its 4-hydroxy derivative with quartz sandy soils. Although quartz would not be expected to contribute much to such phenomena as labile “surface” sorption, bound residue formation, and catalyzed chemical reactions, there remains the question of the possible interactions with the nonquartz components of the soil (3-8). The objective was to determine the molecular level kinetics of chlorothalonil in the Simcoe quartz sand soil for such * Corresponding author phone: (403)220-3927, (902)667-1984; fax: (403)289-9488; e-mail:
[email protected], dgamble@ns. sympatico.ca. 120
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 1, 2000
Although the kinetics that describe the interaction processes are quite conventional, they are outlined here to define symbols and to explain the basis of the calculations (2, 3, 9, 10). At t ) 0, reaction time before any sorption or desorption processes have begun, the solution concentration is M0 (mol/ L). Sorption experiments generally have an initial fast drop in solution concentration. The mass balance for total labile surface sorption sites is eq 1. θC is the labile surface sorption capacity in (mol/g).
θ C ) θ0 + θ1
(1)
θ0 and θ1 (mol/g) are empty and filled surface sorption sites. It is frequently useful to tabulate χ1 ) θ1/θC, the mole fraction of filled sites. The rate of removal from solution is described by the second-order kinetics in eq 2.
Rf ) (dM1/dt)f ) - kB1θ0M1
(2)
M1 is the solution concentration in molarity after the start of the experiment. There are two sets of conditions under which this process can become pseudo-first-order. When the solution concentration is relatively large and the ratio of solid to solution is sufficiently small, then M1 can remain approximately constant, while θ0 decreases significantly in accordance with eq 1. The removal from solution then becomes pseudo-first-order with the apparent rate constant in eq 3.
kS1 ) kB1M1
(3)
The ratio of solid to solution required to satisfy eq 3 depends on the numerical value of θC. In the other set of experimental conditions, the solution concentration is relatively low, and the ratio of solids to solution is sufficiently high so that θ1 is quite small. That is, θ1 , θ0. Then θ0 remains approximately constant, while M1 decreases significantly. The quite different pseudo-first-order rate constant in eq 4 then becomes apparent.
kS1 ) kB1θ0
(4)
The numerical value of θC is again an important factor for the manifestation of pseudo-first-order kinetics. The design of the experiments reported here satisfied eq 4 for the second set of conditions, especially during the early parts of the experiments that were used for the calculations. These calculations included the initial rate calculation of kS1 using eqs 2 and 4. The surface desorption is described by the first10.1021/es990273y CCC: $19.00
2000 American Chemical Society Published on Web 12/02/1999
TABLE 1. Analysis of the Soil from Simcoe Ontario method elemental analysis
chemical analysis
symbols and formulas
component carbon hydrogen nitrogen iron
phosphorus X-ray diffraction quartz calcite albite brushite
C H N Fe P SiO2 CaCO3 NaAlSi3O8 CaPO3(OH)‚2H2O
TABLE 2. Chlorothalonol in Simcoe Soil: Kinetics Rate Constantsa
analysis results
process
1.4 wt % 0.14 wt % 0.03 wt % 16 607.5 mg/kg 469.5 mg/kg 90-95 wt % 0.9-5.9 wt %
X-ray diffraction lepidocrocite FeO(OH) >24 180 and chemical mg/kg analysis dufrenite Fe5(PO4)3(OH)5‚2H2O
