Quantifying the contribution of different sorption mechanisms for 2,4-dichlorophenoxyacetic acid sorption by several variable-charge soils

Seunghun Hyun, Linda S. Lee

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60 Citations (Scopus)

Abstract

Previous research with phenolic, carboxylic, and urea type organic acids demonstrated that hydrophilic sorption was primarily due to anion exchange, which was linearly correlated to chemical acidity (pKa) and the soil anion exchange capacity. However, for dichlorophenoxyacetic acid (2,4-D), sorption by a kaolinitic soil was much higher than expected relative to all other organic acid-soil data. The enhanced sorption was hypothesized to involve calcium bridging of 2,4-D to hydrophilic domains. In this study, the mechanisms contributing to 2,4-D sorption by variable-charged soils were probed and quantified by measuring sorption from CaCl2, KCI, CaSO4, KH2PO4, and Ca(H2-PO4)2 solutions. Linear sorption coefficients estimated for 2,4-D sorption from the different electrolytes decreased as follows: CaCl2 > KCl > CaSO4 > Ca(H2PO4)2 ≅ KH 2PO4. Differences in 2,4-D sorption from CaCl2 and phosphate solutions were attributed to sorption by hydrophilic domains, which ranged between 46 and 94% across soils. Differences in 2,4-D sorption from CaCl2 and KCl were attributed specifically to Ca-bridging between 2,4-D's carboxyl group and the silanol edge on kaolinite and quartz and ranged from negligible to 40% depending on the soil mineral type. Differences in sorption from CaCl2 and CaSO4 was attributed to anion exchange, which ranged from 16 to 91%, followed the trends with pKa developed previously for other organic acids, and correlated well to the soil anion to cation exchange capacity ratio (AEC/CEC). The sum of anion exchange and Ca-bridging contributions agreed well with the sorption estimated to be from hydrophilic domains. All other sorption was attributed to hydrophobic processes, which correlated well to a linear free-energy relationship between pH-dependent organic carbon-normalized sorption coefficients and pH-dependent octanol-water partition coefficients developed for several other organic acids.

Original languageEnglish
Pages (from-to)2522-2528
Number of pages7
JournalEnvironmental Science and Technology
Volume39
Issue number8
DOIs
Publication statusPublished - 2005 Apr 15
Externally publishedYes

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2,4-Dichlorophenoxyacetic Acid
2,4 dichlorophenoxyacetic acid
Sorption
Soil
sorption
Soils
Anions
Acids
soil
Organic acids
Ion exchange
Negative ions
organic acid
ion exchange
Kaolin
Octanols
Quartz
cation exchange capacity
Electrolytes
Minerals

ASJC Scopus subject areas

  • Environmental Science(all)
  • Environmental Chemistry
  • Environmental Engineering

Cite this

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title = "Quantifying the contribution of different sorption mechanisms for 2,4-dichlorophenoxyacetic acid sorption by several variable-charge soils",
abstract = "Previous research with phenolic, carboxylic, and urea type organic acids demonstrated that hydrophilic sorption was primarily due to anion exchange, which was linearly correlated to chemical acidity (pKa) and the soil anion exchange capacity. However, for dichlorophenoxyacetic acid (2,4-D), sorption by a kaolinitic soil was much higher than expected relative to all other organic acid-soil data. The enhanced sorption was hypothesized to involve calcium bridging of 2,4-D to hydrophilic domains. In this study, the mechanisms contributing to 2,4-D sorption by variable-charged soils were probed and quantified by measuring sorption from CaCl2, KCI, CaSO4, KH2PO4, and Ca(H2-PO4)2 solutions. Linear sorption coefficients estimated for 2,4-D sorption from the different electrolytes decreased as follows: CaCl2 > KCl > CaSO4 > Ca(H2PO4)2 ≅ KH 2PO4. Differences in 2,4-D sorption from CaCl2 and phosphate solutions were attributed to sorption by hydrophilic domains, which ranged between 46 and 94{\%} across soils. Differences in 2,4-D sorption from CaCl2 and KCl were attributed specifically to Ca-bridging between 2,4-D's carboxyl group and the silanol edge on kaolinite and quartz and ranged from negligible to 40{\%} depending on the soil mineral type. Differences in sorption from CaCl2 and CaSO4 was attributed to anion exchange, which ranged from 16 to 91{\%}, followed the trends with pKa developed previously for other organic acids, and correlated well to the soil anion to cation exchange capacity ratio (AEC/CEC). The sum of anion exchange and Ca-bridging contributions agreed well with the sorption estimated to be from hydrophilic domains. All other sorption was attributed to hydrophobic processes, which correlated well to a linear free-energy relationship between pH-dependent organic carbon-normalized sorption coefficients and pH-dependent octanol-water partition coefficients developed for several other organic acids.",
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