Estimation of Humic-Based Sorption Enthalpies from Nonlinear Isotherm Temperature Dependence: Theoretical Development and Application to Substituted Phenylureas
- F. C. Spurlock *
Thermodynamic assessments of organic chemical-soil interactions are important for understanding various aspects of sorption phenomena, including isotherm nonlinearity. Expressions were derived here for calculating organic carbon based Freundlich nonlinear sorbed phase partial molar enthalpies (ΔH̄1) and entropies (ΔS̄1) from isotherm temperature dependence. It was shown that, in general, any changes in nonlinearity with temperature must be evaluated when estimating enthalpy effects from isotherm temperature dependence. The results were applied to nonlinear soil sorption data of three substituted phenylurea herbicides between 281 K and 297 K. The ΔH̄1 estimated at 289 K were found to range down to approximately −50 kJ mol−1, and were associated with negative ΔS̄1. Differential heats of sorption, which also include solution phase interactions, were ≈ 10 to 15 kJ mol−1 more exothermic than ΔH̄1. The ΔH̄1 were significantly less exothermic as sorbed phase concentrations increased. For diuron (1,1-dimethyl-3-(3,4-dichlorophenyl) urea), a compound that displays comparable solubility and octanol-water partition coefficient as naphthalene, ΔH̄1 at higher concentrations were similar to values estimated for naphthalene (approximately −4 to + 10 kJ mol−1, calculated here from literature data), a nonpolar compound without polar functional groups. The data suggest that specific interactions between the phenylureas and soil organic matter moieties are dominant at low sorbed phase concentrations, but become less important relative to nonspecific London-van der Waals forces as sorbed phase concentration increases. The urea isotherms are therefore nonlinear, with nonlinearity fundamentally related to the nature of substituted phenylurea sorbed phase interactions.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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