Correspondence Comment on “Competitive Sorption between Atrazine and Other Organic Compounds in Soils and Model Sorbents” SIR: The recent contribution by Xing et al. (1) presented some new and interesting data concerning competitive sorption of nonpolar and moderately polar organic molecules on soil organic matter (SOM) and other sorbents. We would like, however, to make several comments concerning their interpretations for Langmuir sorption sites in SOM. (i) Xing et al. (1) suggested that sorption to SOM can be described by a dual-mode sorption model with a partition domain contribution and a Langmuir isotherm term, where the Langmuir term represents a composite of specific sorption sites. So, for example, they estimated the relative contribution of these two sorption modes to the total atrazine sorption isotherm on Pahokee part by determining the contribution of the partition term independently. Estimation of the partition term was based on competition between prometon and atrazine for specific Langmuir sites, whereby atrazine sorption in the presence of a large concentration of successful competitor was through partitioning alone. In this way they estimated (1; Figure 7) that Langmuir-type “hole” sorption comprised 37-54% of total atrazine sorption in peat in the absence of competing molecules. Estimated parameters for the dual-mode sorption of atrazine on Pahokee peat were Kp (partition coefficient) ) 39.4 mL/g and Langmuir isotherm capacity term (S°) ) 161 µg/g. They speculated that the Langmuir sites are “internal to the matrix rather than on the external SOM-bulk liquid interface” (p 2439). This suggestion was based on two lines of reasoning: (a) isotherms for 1,3-dichlorobenzene, metolachlor, and 2,4-dichlorophenol on Pahokee peat and soils become increasingly nonlinear with increasing contact time (2), suggesting rate-limiting diffusion into interior specific sites; and (b) the N2 BET external surface area measured for peat is “small”. We take issue with rationale (a) because nonlinearity expressed in isotherms determined after 30180 days (2) is not relevant for nonlinearity in isotherms determined after only 2 days (1). More importantly, concerning rationale (b), we suggest that a simple calculation of the maximum possible capacity of peat external surface area for atrazine adsorption can be instructive: The reported N2 BET measured external surface area for Pahokee peat is 0.88 m2/g (1). The density of atrazine is reported to be 1.187 g/cm3 (3), while the molecular weight (MW) of atrazine is 215.7. For simplicity, we assume that the atrazine molecule is a sphere: (1) From atrazine density, MW, and the assumption of sphericity, it is possible to calculate the radius of one atrazine molecule (r) such that r ) 4.16 × 10-10 m. (2) The cross-sectional area (A) of an atrazine molecule can then be obtained such that A ) 54.4 × 10-20 m2. We calculate the surface area covered by 1 mol of atrazine (MA) to be 3.3 × 105 m2/mol. (3) To calculate the maximal possible concentration of atrazine spheres on external surface area (maximal capacity), we divide the N2 BET surface area by the surface area covered by 1 mol of atrazine (MA), giving 2.7 × 10-6 mol/g. Converting
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1997 American Chemical Society
concentration to weight units, we obtain a possible maximum surface area coverage by atrazine of 582 µg/g peat. This calculated maximal possible atrazine concentration on external surface area of 582 µg/g peat is substantially greater than the Langmuir capacity term obtained by Xing et al. (1) of 161 µg/g peat. Thus the surface area is sufficiently capacious for sorption of atrazine with no need of recourse to “interior matrix holes” for explaining the nature of Langmuir sites. (ii) Xing et al. (1) used the dual-mode model to fit atrazine isotherms in both the Pahokee peat (44.6% C) and Cheshire soil (1.4% C). They obtained nearly co-equal contributions for partitioning and Langmuir terms in both sorbents. When we express Langmuir capacity (S°) on an organic carbon (OC) basis, we see that S° is 1229 µg/g OC for the soil versus only 361 µg/g OC for the peat. If sites are internal to the matrix, normalization of Langmuir capacity to organic carbon should result in similar capacities. The difference in capacities thus may be better interpreted as sorption on the external surface area, as befits the differences in soil organic matter versus bulkier peat organic matter. The affinity terms (b) for peat and soil are 0.288 and 0.064 mL/µg, respectively, such that the total Langmuir contributions (S° b) to the distribution coefficients at low concentrations are similar for the two sorbents. The apparent similarity between the soil and peat is thus in part a result of compensation between affinity and capacity terms in the Langmuir isotherm. (iii) We recently obtained a sorption isotherm for atrazine from hexane on lyophilized Pahokee peat (unpublished data). The isotherm was fit to the same dual-mode model, and the Langmuir capacity term was found to be 448 ( 55 µg/g peat. This coverage is similar to the maximal possible coverage calculated above (i). The difference between atrazine coverage obtained for sorption from hexane and sorption from water (1) may thus be a function of competition for surface sorption sites by water molecules. Our discussion is predicated on the assumption that external surface area measured for N2 is the same as that for atrazine. This is the same assumption made by Xing et al. (1). It is beyond the scope of this letter to discuss the ramifications if this assumption is untrue. We wish to point out, however, that we do not a priori dismiss the possibility of specific internal sorption sites, rather the evidence brought by (1) does not justify this conclusion. This was not the major focus of their otherwise interesting and elucidating work.
Literature Cited (1) Xing, B.; Pignatello, J. J.; Gigliotti, B. Environ. Sci. Technol. 1996, 30, 2432-2440. (2) Xing, B.; Pignatello, J. J. Environ. Toxicol. Chem. 1996, 15, 12821288. (3) The Pesticide Manual, 9th ed.; Worthing, C. R., Hance, R. J., Eds.; The British Crop Protection Council: 1991; pp 1141.
M. D. Borisover and E. R. Graber* Institute of Soils and Water Agricultural Research Organization Volcani Center P.O. Box 6 Bet Dagan 50250, Israel ES9700111
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