Hydrophobic and Hydrophilic Sorption of Organic Acids On Humic Acid From Methanol/Water System.

             Organic acids (i.e., pesticide, herbicide, antibiotics) from different sources can result in soil contamination problems. In waste disposal/spill sites, the soil solution may consist of a mixture of water and water miscible various organic solvents (i.e., cosolvent). However, the existing sorption models of organic acids to soils in cosolvent/water mixture are unpredictable for organic acids carrying carboxylic acid group; sorption of benzoic acid by variable charge soils increased log-linearly with increasing methanol volume fraction. In this study, sorption mechanism was primarily observed for an understanding of interaction between organic acid and organic matter domain. Sorption of benzoic acid carrying carboxyl functional group and 2,4,6-trichlorophenol (2,4,6-TCP) carrying hydroxyl functional group by humic acid was measured as a function of apparent pH (pH^app) and methanol volume fractions (f_c = 0.0, 0.2, 0.4, 0.6, and 0.8). The sorption coefficient (K_m) of benzoic acid and 2,4,6-TCP by humic acid were decreased with the addition of methanol. The K_m value of benzoic acid was lower than that of 2,4,6-TCP in methanol/water mixtures. The difference of K_m value determined for benzoic acid and 2,4,6-TCP was related to both the polarity of the solute and solvent. Sorption data of both neutral (pH^app < 3) and anionic species (pH^app > 8) of 2,4,6-TCP was decreased log-linearly with increasing f_c . In case of benzoic acid, sorption of neutral (pH^app < 3) and anionic species (pH^app > 8) also decreased log-linearly with increasing f_c. However, the slope of the relationship was less for anionic species. It is likely that the anionic species of the organic acids have a much lower K_m than the respective neutral species and hydrophobic sorption domain have higher contents than hydrophilic sorption domain. The sorption of both benzoic acid and 2,4,6-TCP on humic acid can be sufficiently explained by hydrophobic interaction between neutral species and hydrophobic domain as a function of f_c.