Correspondence/Rebuttal pubs.acs.org/est
Comment on “Prediction of Soil Sorption Coefficients Using Model Molecular Structures for Organic Matter and the Quantum Mechanical COSMO-SAC Model” n their article, Phillips et al.1 present what they term “[a] new method ... to predict KOC [organic carbon normalized soil−water equilibrium partition coefficient] for nonionic organic compounds that requires only molecular structures. No calibration is performed.” The authors go on to state that “[t]he experimental KOC data set ...was used to evaluate the accuracy of the KOC predictions. This data set contains KOC values for 440 nonionic organic compounds, some of which represent averages of measured KOC values from multiple soil and sediment types and different studies.” Of these 440 purportedly nonionic compounds presented in the Supporting Information of Phillips et al.,1 the following compounds have acidic and/or basic functional groups that would result in their substantial (and in many cases, effectively complete) ionization in natural surface aquatic systems and in soils and groundwaters (experimental or estimated pKa values are provided in parentheses): acetic acid (4.762); acridine (5.58 2 ); 4-aminobenzoic acid (4.78 3); aniline (4.87 2 ); anthracene-9-carboxylic acid (∼3.64); asulam (4.15); benzidine (4.652); benzo[f]quinoline (5.156), benzoic acid (4.202), 2,2′biquinoline (4.183); bromacil (9.307); 1-butylamine (10.602); chloramben (∼3.48); chlorimuro (2.033); 2-chlorophenol (8.56 2 ); 6-chloropicolinic acid (3.55 9 ); chlorsulfuron (∼3.410); 2,4-dichlorophenoxy acetic acid (2.64−3.3111); 2,5dichloro-6-methoxybenzoic acid (2.603); 3,4-dichlorophenol (8.6312); 2,3-dichlorophenol (7.7012); 3,6-dichlorosalicylic acid (1.9513); diflubenzuron (8.6012); 3,5-dinitrobenzoic acid (2.7314); 2-methyl-4,6-dinitrophenol (4.3115); hexanoic acid (4.852); 4-hydroxybenzoic acid (4.572); isocil (8.143); maleic hydrazide (5.6516), N-methylaniline (4.852), 3-methylaniline (4.712), 4-methylaniline (5.082); 3-methyl-4-bromoaniline (4.033); 1-naphthalenamine (3.922); N,N-dimethylaniline (5.072); 4-nitrobenzoic acid (3.432); pentachlorophenol (4.7417); phenylacetic acid (4.3018); phthalic acid (2.942); 4nitrophenol (7.152); pyridine (5.232); quinoline (4.902); picloram (∼2.3 1 9 ); pirimicarb (4.54 2 0 ); 2-(2,4,5trichlorophenoxy)propanoic acid (2.8417); 2,4,5-trichlorophenoxy acetic acid (2.85−3.4621); terbacil (9.017); 2,3,4,6tetrachlorophenol (5.2222); thiabendazole (4.7323); 2,4,6trichlorophenol (6.1524); 3,4,5-trichlorophenol (7.7324); 2,4,5trichlorophenol (7.0725); 3,5,6-trichloro-2-pyridinol (4.626); triclopyr (2.727); 3-trifluoromethyl-4-nitrophenol (6.0728); 3,4-dinitrobenzoic acid (2.443); dinoseb (4.6224); fenac (3.823); imazalil (6.5310); and sulfometuron methyl (3.810). Consequently, experimental KOC values for these compounds would be pH dependent, a key point that does not appear to have been considered by Phillips et al.1 Furthermore, Phillips et al.1 present a number of molecular models for terrestrial and aquatic humic acids and aquatic fulvic acids in the SMILES molecular language (see Table S1 in ref 1). The SMILES formats for these model humic and fulvic acids appear to be in the neutral forms of each compound. By
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definition, humic and fulvic acids are predominantly (if not nearly entirely) dissociated into their anionic forms in natural waters and moist soil systems. Basic amino moieties on some of these model humic and fulvic acids may also be protonated under environmentally relevant conditions. Consequently, it is unclear how these neutral form model humic and fulvic acid structures can be used to accurately model interactions with solutes (particularly ionizable solutes as listed above), given that in environmental systems these macromolecules would be ionized.
Sierra Rayne*
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Chemologica Research, P.O. Box 74, 318 Rose Street, Mortlach, Saskatchewan S0H 3E0, Canada
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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