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Influence of soil pH on the sorption of ionizable chemicals: Modeling advances
Author(s) -
Franco Antonio,
Fu Wenjing,
Trapp Stefan
Publication year - 2009
Publication title -
environmental toxicology and chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.1
H-Index - 171
eISSN - 1552-8618
pISSN - 0730-7268
DOI - 10.1897/08-178.1
Subject(s) - partition coefficient , chemistry , sorption , dissociation (chemistry) , colloid , dissolved organic carbon , ionic strength , fulvic acid , dissociation constant , environmental chemistry , acid dissociation constant , genetic algorithm , octanol , adsorption , aqueous solution , chromatography , organic chemistry , humic acid , fertilizer , biochemistry , receptor , evolutionary biology , biology
The soil–water distribution coefficient of ionizable chemicals ( K d ) depends on the soil acidity, mainly because the pH governs speciation. Using pH‐specific K d values normalized to organic carbon ( K OC ) from the literature, a method was developed to estimate the K OC of monovalent organic acids and bases. The regression considers pH‐dependent speciation and species‐specific partition coefficients, calculated from the dissociation constant (p K a ) and the octanol–water partition coefficient of the neutral molecule (log P n ). Probably because of the lower pH near the organic colloid–water interface, the optimal pH to model dissociation was lower than the bulk soil pH. The knowledge of the soil pH allows calculation of the fractions of neutral and ionic molecules in the system, thus improving the existing regression for acids. The same approach was not successful with bases, for which the impact of pH on the total sorption is contrasting. In fact, the shortcomings of the model assumptions affect the predictive power for acids and for bases differently. We evaluated accuracy and limitations of the regressions for their use in the environmental fate assessment of ionizable chemicals.