Sorption of acidic organic solute onto kaolinitic soils from methanol-water mixtures

Abstract

The fate of the acidic organic solute from the soil-water-solvent system is not well-understood. In this study, the effect of the acidic functional group of organic solute in the sorption from cosolvent system was evaluated. The sorption of naphthalene (NAP) and 1-naphthoic acid (1-NAPA) by three kaolinitic soils and two model sorbents (kaolinite and humic acid) were measured as functions of the methanol volume fractions (f _c ≤ 0.4) and ionic compositions (CaCl₂ and KCl). The solubility of 1-NAPA was also measured in various ionic compositions. The sorption data were interpreted using the cosolvency-induced sorption model. The K _m values (= the linear sorption coefficient) of NAP with kaolinitic soil for both ionic compositions was log linearly decreased with f _c. However, the K _m values of 1-NAPA with both ionic compositions remained relatively constant over the f _c range. For the model sorbent, the K _m values of 1-NAPA with kaolinite for the KCl system and with humic acid for both ionic compositions decreased with f _c, while the sorption of 1-NAPA with kaolinite for the CaCl₂ system was increased with f _c. From the solubility data of 1-NAPA with f _c, no significant difference was observed with the different ionic compositions, indicating an insignificant change in the aqueous activity of the liquid phase. In conclusion, the enhanced 1-NAPA sorption, greater than that predicted from the cosolvency-induced model, was due to an untraceable interaction between the carboxylate and hydrophilic soil domain in the methanol-water system. Therefore, in order to accurately predict the environmental fate of acidic pesticides and organic solutes, an effort to quantitatively incorporate the enhanced hydrophilic sorption into the current cosolvency-induced sorption model is required.

Keywords:

• 1-Napthoic acid, cosolvency, sorption, kaolinitic soil

Acknowledgments

This work was supported, in part, by the Basic Science Research Program through the National Research Foundation of Korea (NFR), funded by the Ministry of Education, Science and Technology (2010-0008507) and, in part, by a Korea University Grant.

Notes

^a NAP = Naphthalene; 1-NAPA = 1-Naphthoic acid. Data from Schwarzenbach et al.^{[ 15 ]} and Burgos and Pisutpaisal.^{[ 24 ]}

^b Molecular Mass (g mol⁻¹).

^c Acid dissociation constant.

^d Aqueous solubility (mol L⁻¹).

^e Octanol-water partitioning constant. For 1-NAPA, log K_ow of neutral species is reported.

^a 1:10 (g mL⁻¹)

^b Organic content (%)

^c Oxalate-extractable Fe or Al (g kg⁻¹)

^d Citrate-Bicarbonate-Dithionite extractable Fe or Al (g kg⁻¹)

^e Exchangeable cations measured by the ammonium acetate method (cmol kg⁻¹)

^f Anion exchange capacity (cmol₍₊₎ kg⁻¹) or cation exchange (cmol_(-) kg⁻¹) at the pH value reported

^g Zero point of charge

^h The fraction of hydrophilic sorption of 1-NAPA

^a NAP = Naphthalene, 1-NAPA = 1-Naphthoic acid. ^b Volume fraction of methanol in mixture solution.

^c Apparent equilibrium isotherm pH at which the sorption experiment was constructed.

^d K _m values measured from methanol-10 mM CaCl₂ mixture. ^e K _m values measured from the methanol-10 mM KCl mixture.

^f Standard deviation of K _m value.