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TECHNICAL REPORTS

Organic Compounds in the Environment

Sorption of Aromatic Compounds to Clay Mineral and Model Humic Substance–Clay Complex: Effects of Solute Structure and Exchangeable Cation

State Key Lab. of Pollution Control and Resource Reuse, and School of the Environment, Nanjing Univ., Jiangsu 210093, P.R. China

^* Corresponding author (zhud{at}nju.edu.cn).

Received for publication May 2, 2007. Clay minerals and humic substance (HS)–clay complexes are widely distributed in soil environments. Improved predictions on the uptake of organic pollutants by soil require a better understanding of fundamental mechanisms that control the relative contribution from organic and inorganic constituents. Five selected aromatic compounds varying in electronic structure, including nonpolar phenanthrene (PHEN), 1,2,4,5-tetrachlorobenzene (TeCB), polar 1,3-dinitrobenzene (DNB), 2,6-dichlorobenzonitrile (dichlobenil [DNL]), and 1-naphthalenyl methylcarbamate (carbaryl [CBL]), were sorbed separately from aqueous solution to Na⁺–, K⁺–, Cs⁺–, and Ca²⁺–saturated montmorillonites with and without the presence of dissolved HS at pH about 6. Upon normalizing for hydrophobic effects by solute aqueous solubility, the overall trend of sorptive affinity to HS-free K⁺–clay is DNB >> DNL, CBL > PHEN, TeCB, indicating preferential adsorption of the polar solutes. With the presence of HS, sorption of PHEN, TeCB, and CBL increases by several times compared with the pure clay, attributed to HS-facilitated hydrophobic partition (PHEN and TeCB) or H-bonding (CBL). The enhanced sorption of PHEN by HS is cation dependent, where Cs⁺ shows the strongest facilitative effect. Coadsorption of HS does not affect sorption of DNB and DNL to clays except that of DNB to Ca²⁺–clay because cation–dipole interactions between the polar group (NO₂ or CN) of solute and weakly hydrated exchangeable cations dominate the overall sorption.

Abbreviations: CBL, carbaryl • DNB, 1,3-dinitrobenzene • DNL, 2,6-dichlorobenzonitrile • GC, gas chromatography • HOC, hydrophobic organic compounds • HS, humic substance • K_d, solid-to-solution distribution coefficient • K_OC, organic carbon-based distribution coefficient • K_OW, solute n-octanol–water partition coefficient • NAC, nitroaromatic compounds • OC, organic carbon • PAH, polycyclic aromatic hydrocarbon • PHEN, phenanthrene • SOM, soil organic matter • S_W, water solubility • TeCB, 1,2,4,5-tetrachlorobenzene • XRD, X-ray diffraction