Measurements of association and partition equilibria of n-alkylbenzenes with fulvic acid, humic acid, sodium dodecylbenzenesulfonate, n-dodecane, and 1-octanol in aqueous media by headspace gas chromatography

Natural organic matter (NOM) such as humic acids (HA), fulvic acids (FA), and humin in soils and sediments plays an important role in partitioning, binding, and transport of organics and metal ions in nature. We have developed an equilibrium headspace gas chromatographic (HSGC) technique and the theory to study the association and partitioning of n-alkylbenzenes (benzene, toluene, ethylbenzene, n-propylbenzene, and n-butylbenzene) with aqueous FA, HA, and sodium dodecylbenzenesulfonate (SDBS). The alkylbenzenes were chosen because they are the major constituents of gasoline and also toxic to human health and the environment. The precise measurement of n-alkylbenzenes in the vapor phase in equilibrium with the liquid phase allowed determination of solute - monomer surfactant (S), association constants (K11), solute- aggregate, Sn, association constant (Kn1), critical aggregation constants for FA, HA, and SDBS. It also allowed measurements of the mole fraction based partition coefficients (Kx), infinite dilution activity coefficients of solute inside the aggregate pseudo-phase (gammam), and the transfer free energies of alkyl chain -CH2- and benzene from aqueous to aggregate pseudo-phase. Theory shows that the parameters can be calculated without the concentration of the solute. All measurements were made at infinite dilution (<10-6 mole fraction) where solute-solute interactions are nonexistent. Almost no such studies could be found in the literature and, therefore, constitute a fundamental contribution in this field. This study shows that there is strong binding and partitioning of n-alkylbenzenes with FA and HA present as single molecule or as aggregates. We show that FA and HA forms aggregates containing hydrophobic pseudo-phase at 4- 8 gammaM concentrations. Both K11 and Kn1 values were of the order of at least 105 M-1 and 2-3 orders of magnitude higher than that of synthetic surfactants. The pseudo-phase partition coefficient, Kx values are also of the order of 107. The n-alkylbenzenes in FA pseudo-phase shows the lowest gamma m (0.0001) among the three systems studied and one of the lowest known values. Most importantly, the transfer free energy of partitioning of a -CH2 - group -155 cal/mol compared to that of benzene, -9722 cal/mol, indicates that the aggregate pseudo-phase is polarizable benzene-like and less n-alkane aliphatic-like. This is also true for HA and SDBS, but not for SDS. The high solubilization efficiencies of FA and HA for hydrocarbons was used to extract 30-60% of hydrocarbons from diesel contaminated sand. Also, FA was used to study the solubilization efficiency of aromatic hydrocarbons from gasoline contaminated water. Finally, the theory of association equilibria was successfully applied to liquid-liquid extraction of n-alkylbenzenes from water into n-dodecane and 1-octanol. This experiment allowed pre-extraction and post-extraction calculation of molecular association constant, K11, and critical phase ratio (cpr) for extraction, solute partition coefficient, Kx (organic/ water), and infinite dilution activity coefficient of solute in organic phase, gammas.