Linear and non-linear relationships between soil sorption and hydrophobicity

Area of Science:

• Environmental Chemistry
• Soil Science
• Physical Chemistry

Background:

• Understanding chemical sorption to soil is crucial for environmental risk assessment.
• The octanol/water partition coefficient (log P) is often used to predict soil sorption (Koc).
• The relationship between log P and log Koc is complex and varies with compound properties.

Purpose of the Study:

• To investigate the relationship between log Koc and log P across a wide range of compounds.
• To identify factors influencing soil sorption for hydrophobic, hydrophilic, and highly hydrophobic chemicals.
• To evaluate the suitability of the octanol/water system as a surrogate for soil sorption prediction.

Main Methods:

• Analysis of a large dataset correlating log Koc and log P.
• Application of linear solvation energy relationships to assess sorption mechanisms.
• Utilizing principal component analysis to compare different solvent/water partition systems.

Main Results:

• For moderately hydrophobic compounds (0.5 < log P < 7.5), Koc is linearly related to log P, dominated by organic carbon content.
• Hydrophilic compounds (log P < 0.5) exhibit higher observed Koc than predicted, due to significant hydrophilic contributions to sorption.
• Highly hydrophobic compounds (log P > 7.5) show decreased log Koc with increasing hydrophobicity due to low solubility limiting availability.
• Sorption potential increases with molecular size (dispersion interactions) and decreases with basicity (H-bonding with water).
• The octanol/water system is the closest surrogate but not ideal for predicting soil sorption of hydrophobic compounds.

Conclusions:

• Soil sorption is influenced by both hydrophobic and hydrophilic interactions, with varying dominance depending on compound properties.
• Log P is a useful but not universally accurate predictor of soil sorption (Koc).
• Molecular size and basicity are key factors affecting sorption potential beyond simple hydrophobicity.