Effect of Acid Functional Groups on Organic Acids Sorption by Variable Charge Soils from Methanol/water Mixture

The existing sorption models of organic acids to variable charged soils in water-cosolvent mixture are unpredictable as the functional groups of organic acids. The relative contribution of anion exchange to organic acid sorption by four variable charged soils was assessed a function of the acid functional groups of the organic acids in methanol/water. Benzoic acid and 2,4-D from carboxyl functional group and 2,4-DCP and 2,4,6-TCP from hydroxyl functional group were chosen as organic acid and two variable charged soils, Andisols and Oxisols, from Jeju island of Korea and HI USA were used. Organic acids sorption by soils was measured as a function of apparent pH and methanol volume fraction. Organic acid sorption by those soils through anion exchange appeared to be increasing along with chemical acidity of the organic acid (decreasing pKa). Sorption coefficient of neutral and anionic organic acids decreased log-linearly with increasing methanol volume fraction (f_c). The slope of neutral species (α_n, σ_n) was approximately two times greater than the one of ionized species (α_i, σ_i). For 2,4-DCP and 2,4,6-TCP, the cosolvent model with speciation adequately described as the log-linear decrement in sorption as increasing f_c in the range of 0 and 0.8. The sorption of Benzoic acid and 2,4-D increased with increasing f_c when f_c was greater than 0.2. This difference was highly-likely due to the complexation reactions of carboxyl functional group and soil surface in water-cosolvent. Further works are required to explain the interaction between functional group of organic acids and the soil surface and to develop techniques for direct probing of the complex on the surface for deriving adequate sorption model.