Response to Comment on “Prediction of Soil Sorption Coefficients

Correspondence/Rebuttal pubs.acs.org/est

Response to Comment on “Prediction of Soil Sorption Coefficients Using Model Molecular Structures for Organic Matter and the Quantum Mechanical COSMO-SAC Model” ABSTRACT: In a comment on our article,1 Rayne has identified 60 compounds that would be expected to ionize in natural aquatic systems and soils, and for which the experimental KOC values would be pH dependent. pH dependency of experimental KOC values was not considered in our article, whichas statedwas intended to include only nonionic organic compounds. Therefore, these 60 compounds should not have been included in the data set used to evaluate the model performance. However, the model was not calibrated using the experimental data; therefore, elimination of these compounds has no impact on the model presented. Furthermore, elimination of these compounds from the data set has little impact on the results and conclusions presented in our article.

methods is slightly reduced to 1.32 and 0.62, respectively, for the 380 compounds compared to the original 440. The overall accuracy of the predictions using the SPARC method4 is more significantly affected by the elimination of the 60 compounds; RMSEs for the SPARC log KOC predictions decreased to between 0.92 and 1.94, representing an improvement of 0.03−0.18 over the RMSEs for the 440 compound data set. However, the original conclusion is unchanged: by comparison with the COSMO-SAC results, the RMSEs in the SPARC predictions are always higher when using the same model for OM (Figure 1). For terrestrial HA, SPARC performs almost as well as COSMO-SAC. Rayne has also commented on the use of neutral molecules as model OM. In using these structures, we assumed that the HA and FA structures were fully protonated. At high solids to water ratio, the fraction of deprotonated sites would be expected to be small because only a small number of sites need

For the 380 diverse, environmentally relevant, nonionic organic compounds remaining in the data set after the elimination of the compounds identified by Rayne, the log KOC values are predicted using the quantum mechanics-based COSMO-SAC model with a root-mean-square error (RMSE) of 0.82−1.06, depending on which model humic acid (HA) or fulvic acid (FA) molecular structure is used to represent organic matter (OM) (Figure 1). This represents a reduction of 0.02−0.03 in the RMSE for predictions using each individual model HA or FA compared to the results presented for the original 440 compound data set. The COSMO-SAC KOC predictions corresponding to the Suwanee River FA model OM SR-FA-a (i.e., model OM number 12 in Figure 1) agree most closely with the experimental data, as was also indicated by the original results. Similarly, the RMSE in log KOC predictions made using the Winget at al.2 and Klamt et al.3 quantum mechanics-based

Figure 1. Comparison of root-mean-square errors in the log KOC predictions based on different model HA and FA molecular structures as representations of OM. RMSEs are calculated using experimental data from the Winget et al.2 compilation. Black bars: COSMO-SAC predictions for 380 solutes. Gray bars: SPARC predictions for 378 solutes. Models 1−16 for OM are described in ref 1.

© XXXX American Chemical Society

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dx.doi.org/10.1021/es402076b | Environ. Sci. Technol. XXXX, XXX, XXX−XXX

Environmental Science & Technology

Correspondence/Rebuttal

to deprotonate to achieve the equilibrium pH. Additionally, cations in solution would be expected to neutralize the charge that develops. This would result in near-neutral molecules, consistent with our model. For nonionic solutes, the model HA and FA are expected to provide reasonable predictions of KOC, as we concluded in our article.

Kathy L. Phillips† Dominic M. Di Toro*,‡ †

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Geosyntec Consultants, 289 Great Road, Suite 105, Acton, Massachusetts 01720, United States ‡ Department of Civil and Environmental Engineering, DuPont Hall, University of Delaware, Newark, Delaware 19716, United States

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Phillips, K.; Di Toro, D.; Sandler, S. Prediction of soil sorption coefficients using model molecular structures for organic matter and the quantum mechanical COSMO-SAC model. Environ. Sci. Technol. 2011, 45, 1021−1027. (2) Winget, P.; Cramer, C. J.; Truhlar, D. G. Prediction of soil sorption coefficients using a universal solvation model. Environ. Sci. Technol. 2000, 34, 4733−4740. (3) Klamt, A.; Eckert, F.; Diedenhofen, M. Prediction of soil sorption coefficients with a conductor-like screening model for real solvents. Environ. Toxicol. Chem. 2002, 21, 2562−2566. (4) Karickhoff, S. W.; Carreira, L. A.; Hilal, S. H. SPARC, version 4.5, September 2009. http://ibmlc2.chem.uga.edu/sparc/.

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dx.doi.org/10.1021/es402076b | Environ. Sci. Technol. XXXX, XXX, XXX−XXX