Organic Acid Sorption by Variable Charge Soils in Methanol-Water System : The Effects of Functional Group and Aqueous Ionic Composition.

Sorption of hydrophobic organic compounds (HOC) by soils can be successfully described by log linear cosolvency model. However, application of similar model to sorption of ionizable hydrophobic organic compounds (HIOC) by variable charge soils is limited due to further consideration of the additional effect of functional group, ionic speciation of solute, and ionic composition of solution. In this study, naphthalene (NAP) and 1-naphthoic acid (1-NAPA) sorption by 3 Korean andisols (SD, DH, and EG) were obtained from methanol-5mM CaCl₂ mixture and methanol-10mM KCl mixture. Lower cosolvent volume fraction (f_c = 0, 0.1, 0.2, 0.3, 0.4, 0.5) was employed to minimize the effect of cosolvent-induced modification on soil sorption domain. Equilibrium liquid pH was adjusted to near the natural soil pH by adding diluted HCl or NaOH and >92% of 1-NAPA was anionic in liquid phase under the given experimental condition. Log K_m of NAP from both ionic composition was log linearly decreased with increasing f_c with ασ values being 4~11% greater in methanol-CaCl₂ system relative to methanol-KCl system. Greater values for methanol-5mM CaCl₂ mixture can be attributed to larger decrease of activity coefficient of NAP. However, the sorption of 1-NAPA remains relatively constant under same condition (i. e., no apparent correlation between log K_m and f_c for 1-NAPA). And no measurable differences of sorption behavior of 1-NAPA were observed between in methanol-CaCl₂ system and methanol-KCl system. The results mean that the contribution of ionic charge effects to sorption enhancement of 1- NAPA with increasing f_c is negligible. So the other mechanisms are needed to invoke to explain sorption enhancement of 1-NAPA. To derive adequate conceptual models, cosolvency power (σ) by solubility experiments with methanol and soil extract solutions, and the relative fraction of hydrophilic sorption domain (f_Hphilic) to overall sorption for 1-NAPA were investigated. And the sorption of 1-NAPA by kaolinite (dominant clay mineral of DH soil) will be also measured.