Hydrophilic and Hydrophobic Sorption of Organic Acids by Variable-Charge Soils:  Effect of Chemical Acidity and Acidic Functional Group

Abstract

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(H2PO4)2, and KCl). 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 pKa 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 KCl and CaCl2 solutions. For predicting contributions from hydrophobic processes, a log-log linear relationship between pH-dependent octanol-water (Kow(pH)) and organic carbon-normalized sorption coefficients (Koc(pH)) appeared adequate.

Pharmaceuticals in soils of lower income countries: Physico-chemical fate and risks from wastewater irrigation

“…The more acid soils found in parts of Asia, noted above, are relatively rich in positively charged Al and Fe sesquioxides (FAO, 2014) which may provide sorption sites for anionic (acidic) APIs (Hyun and Lee, 2004;White, 2013). Soil pH will also influence the pH-dependent charge on the organic matter, clay minerals and metal sesquioxide components of the soil, which in turn might be expected to influence API sorption (Hyun and Lee, 2004).…”

“…Soil pH will also influence the pH-dependent charge on the organic matter, clay minerals and metal sesquioxide components of the soil, which in turn might be expected to influence API sorption (Hyun and Lee, 2004). …”

“…In relatively acidic environments however, clays may exhibit an anion exchange capacity (AEC) because of protonation of the surface hydroxyl groups. The AEC:CEC ratio will reflect the difference between the soil pore water pH and the pH of the net zero charge of the particular clay type (Hyun and Lee, 2004). Thus, ionic sorption of an API to the clay will be a function of the AEC:CEC ratio, soil solution pH and API pK a .…”

Describing chlorophenol sorption on variable-charge soil using the triple-layer model

“…In the normal soil pH range, most chlorophenols will totally or partially ionize, affecting their solubility, adsorption, transport, and bioavailability [1]. Many investigators have noted that pH plays an important role in the adsorption of compounds with acidic functional groups on activated carbon and soil because the neutral and ionic forms display very different adsorption properties [2][3][4][5][6][7]. However, chlorophenol sorption in soil has been generally described assuming hydrophobic partitioning of the neutral species to soil organic matter [6,8].…”

The Dechlorination of Pentachlorophenol by Zerovalent Iron in Presence of Carboxylic Acids

“…PCP and its impurities not only give rise to accumulation and biological amplification, but also cause immunological and endocrine disorders and infertility problems in human. The degradation and transformation of PCP have been paid great attention because its adverse effects on environment and human being might continue for a long time (Hyun and Lee 2004).…”