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Environ Sci Technol 47, 2013, 6711 6712 .pdf

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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


© 2013 American Chemical Society

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

Sierra Rayne*

Chemologica Research, P.O. Box 74, 318 Rose Street,
Mortlach, Saskatchewan S0H 3E0, Canada


Corresponding Author

*E-mail: sierra.rayne@live.co.uk.

The authors declare no competing financial interest.


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dx.doi.org/10.1021/es401135s | Environ. Sci. Technol. 2013, 47, 6711−6712

Environmental Science & Technology


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