Hydrophilic and hydrophobic sorption of organic acids by variable-charge soils: Effect of chemical acidity and acidic functional group

Seunghun Hyun, Linda S. Lee

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53 Citations (Scopus)


Sorption of organic acids by variable-charge soil occurs through both hydrophilic and hydrophobic Sorption. In this study, the effect of chemical acidity and the type of acidic functional group on the relative contribution of hydrophilic and hydrophobic processes to sorption by a gibbsite-dominated and a kaolinite-dominated variable-charge soils was quantified by measuring sorption isotherms from different electrolytes (CaCl2, Ca(H2PO 4)2, and KCI). The A1 soil is dominated by gibbsite whereas the DRC soil is primarily kaolinite. The organic acids investigated include five chlorinated phenols (pentachlorophenol, 2,3,4,6-tetrachlorophenol,2,4,6- trichlorophenol, 2,4,5-trichlorophenol, and 2,4-dichlorophenol) with pK a values ranging from 4.69 to 7.85 and two acidic herbicides (2,4-D (pKa = 2.8) and prosulfuron (pKa = 3.76)) that contain carboxyl and urea functional groups, respectively. Anion exchange of chlorinated phenols and prosulfuron on both variable-charge soils as well as 2,4-D sorption on the A1 soil was linearly correlated to chemical acidity. The effective positive surface charge [AEC/(AEC + CEC)] and the anionic fraction of the organic acid in solution, which are both pH-dependent, were sufficient to estimate the contribution of anion exchange to organic acid sorption except for 2,4-D sorption by DRC soil. The latter was much greater than would be predicted from the pKa of 2,4-D. Calcium bridging between silanol edge group and 2,4-D was hypothesized and corroborated by differences in sorption measured from KCI and CaCl2 solutions. For predicting contributions from hydrophobic processes, a log-log linear relationship between pH-dependent octanol-water (KowpH) and organic carbon-normalized sorption coefficients (KocpH) appeared adequate.

Original languageEnglish
Pages (from-to)5413-5419
Number of pages7
JournalEnvironmental Science and Technology
Issue number20
Publication statusPublished - 2004 Oct 15
Externally publishedYes

ASJC Scopus subject areas

  • General Chemistry
  • Environmental Chemistry


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