Quinoline Sorption on Kaolinite–Humic Acid Complexes

pH and compound ionization. In contrast to the case for neutral PACs, an equilibrium partition model, whereby Adsorption of quinoline (pKa 5 4.92) and background electrolyte NHCs sorb in direct proportion to fOC, is not supported (LiCl) onto specimen kaolinite was measured as a function of surfaceby the available data. Retention of cationic NHCs may bound humic acid (HA) concentration ( fOC 5 0–0.5%), pH (3–10), and ionic strength (1–10 mM ). Complexation of HA on the kaolinite be strongly dependent on the nature and density of surface (4.5 mg C kg21) reduced the point of zero net charge (pznc) anionic surface functional groups associated with soil for kaolinite by more than one pH unit and resulted in a significant clays and native organic matter, but prior studies have increase in negative surface charge. Maximum sorption of quinoline not included measurements of soil surface composition occurred near its pKa for all sorbents. Below the pKa, quinoline sorpand its variation with solution chemistry. Competition tion increases with increasing pH and decreasing proton competition. with background electrolyte for cation sorption sites Above the pKa, sorption is reduced in parallel with (but offset to a may also be a factor. As a result, soil selectivity for higher pH from) the ionized fraction. Competition with Li for surface cationic NHCs vs. inorganic cations will probably affect sites is apparent from diminished quinoline adsorption with increasing contaminant fate. ionic strength, but sorption of the ionized form of quinoline is always The role of mineral-bound humic substances in NHC favored and kaolinite exhibits selectivity for cationic quinoline over Li (Kexc 5 65 at pH 5). However, increasing fOC diminishes quinoline sorption is unknown. Bound humic materials may influsorption and selectivity (Kexc 5 32 at pH 5) and increases sorption ence mineral sorbent affinity by altering the type and reversibility relative to uncoated kaolinite. Humic acid alone exhibits charge of surface functional groups (Davis, 1982; lower selectivity for quinoline (Kexc 5 4 at pH 5). The results indicate Kretzschmar et al., 1997) or by altering the relative that mineral-sorbed humic substances can diminish retention of catpredominance of hydrophobic sorption sites (Schlautionic quinoline despite an increase in overall cation-exchange capacity. man and Morgan, 1993). Whereas the former effect may be significant for the ionized compound, the latter is more likely to influence retention of the neutral species. R in soil of nonpolar polycyclic aromatic Prior work has indicated that cation exchange of ionized compounds (PACs) results primarily from interacquinoline is important in its adsorption to subsurface tion with solid-phase humic matter. Sorption has often material at acid pH (Zachara et al., 1986; 1987; Ainsbeen correlated with the mass fraction of soil organic worth et al., 1987). Hence, if sorption of humic subC ( fOC) and sorbate hydrophobicity [e.g., the octanolstances to kaolinite increases the net negative charge of water partition coefficient, KOW] (Means et al., 1980; the surface, affinity for adsorption of cationic NHCs Hassett et al., 1981; Chiou, 1989; Wershaw, 1991; may be likewise affected. Further work is needed to Schwarzenbach et al., 1993). Most mineral surfaces identify the relative importance of mineral vs. organic weakly adsorb low quantities of nonpolar PACs soil constituents to NHC sorption as a function of solu(Schwarzenbach and Westall, 1981; Karickhoff, 1984; tion chemistry. The objective of our study was to examZhang et al., 1990), and sorption increases with the ine the effects of mineral-bound humic substances on amount of mineral-bound organic matter (Murphy et the sorption of the NHC quinoline (C9H7N, pKa 5 4.94) al., 1994). For nonpolar compounds, variation in the to kaolinite across the pH range encountered in natural sorbed quantity of nonionic PAC with pH and ionic soils and water. strength has been attributed to conformational variability of humic substances (Schlautman and Morgan, 1993). MATERIALS AND METHODS A broad group of environmental contaminants Preparation of Electrolyte-Saturated Kaolinite termed ionizable PACs includes the N heterocyclic compounds (NHCs), whose weakly basic N heteroatom is All solutions were prepared using distilled water that was protonated to cationic form in the acidic pH range of passed through a MilliQ UV-plus water purification system. Poorly crystallized Georgia kaolinite (KGa-2) was acquired natural waters (Southworth and Keller, 1984; Zachara from the Source Clay Minerals Repository and 250 g were et al., 1987). The few studies that have been conducted added slowly to 1 L of MilliQ H2O while stirring. The kaolinite to date indicate that NHC (e.g., quinoline) sorption on was dispersed for size fractionation by adjusting suspension soils (Zachara et al., 1986, 1987) and humic substances pH to 9.5 with dropwise addition of LiOH. The suspension (Nielsen et al., 1997) is affected significantly by solution was size fractionated by centrifugation, and particles with equivalent settling diameters .2 mm were discarded. The suspension was flocculated by addition of concentrated LiCl soluJon Chorover and Mary Kay Amistadi, Soil Science Program, Dep. of Agronomy, Pennsylvania State Univ., University Park, PA 16802; William D. Burgos, Dep. of Civil and Environmental Engineering; Abbreviations: BET, Brunauer–Emmett–Teller; CPMAS-NMR, Pennsylvania State Univ., University Park, PA 16802; Patrick G. cross-polarization magic angle spinning nuclear magnetic resonance Hatcher, Dep. of Chemistry, Ohio State Univ., Columbus, OH 43210. spectroscopy; DOC, dissolved organic C; EGME, ethylene glycol moContribution from the Dep. of Agronomy, Pennsylvania State Univ. noethyl ether; fOC, mass fraction of soil organic C; HA, humic acid; Received 13 April 1999. *Corresponding author ([email protected]). NHC, N heterocyclic compound; PAC, polycyclic aromatic compound; pznc, point of zero net charge; TOC, total organic C. Published in Soil Sci. Soc. Am. J. 63:850–857 (1999).