Sorption of 2,4,6-Trichlorophenol in Model Humic Acid-Clay Systems XIAO-PING WANG, ²,‡ XIAO-QUAN SHAN,* ,² LEI LUO, ² SHU-ZHEN ZHANG, ² AND BEI WEN ² State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, and Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100085, China Humic acids and clays are important soil components that influence the sorption and desorption of organic contaminants; however, it is unclear how humic acids influence the sorption of organic contaminants onto clays and their subsequent desorption. Sorption and desorption of 2,4,6- trichlorophenol (2,4,6-TCP) by and from humic acid-modified K + - and Ca 2+ -montmorillonite and -illite were compared with unmodified clays using batch equilibration methods. Commercial humic acid and the humic acid extracted from forest soil were employed in this experiment. The adsorbed amount of 2,4,6-TCP by commercial humic acid was almost twice as large as that adsorbed by the extracted soil humic acid. More 2,4,6-TCP was sorbed onto K + - and Ca 2+ -illite than onto K + - and Ca 2+ - montmorillonite. K + clays were more effective in adsorbing 2,4,6-TCP than Ca 2+ clays. Sorption of 2,4,6-TCP on humic acid-modified Ca 2+ - and K + -montmorillonite and -illite increased as compared with unmodified clays. The sorption nonlinearity of 2,4,6-TCP on humic acid-modified Ca 2+ - and K + - illite increased remarkably as compared with the unmodified clays. The sorption nonlinearity of 2,4,6- TCP on humic acid-modified Ca 2+ - and K + -montmorillonite increased slightly in contrast to unmodified montmorillonites. By comparing sorption and desorption results, we observed hysteresis for all sorbents including humic acids, clays, and humic acid-modified clays. Sorption nonlinearity and hysteresis were dependent on the structure of humic acids. Higher aromaticity of humic acids resulted in greater sorption nonlinearity and desorption hysteresis. In addition, sorption capacity (K f ′) was positively correlated with the humic acid content of the sorbents. These results show that modification of humic acids on clays can not only increase the adsorption ability of clays but also affect the sorption nonlinearity of 2,4,6-TCP, and the desorption hysteresis was probably due to the structural characteristics of humic acids. KEYWORDS: Sorption and desorption; 2,4,6-trichlorophenol; humic acid-modified montmorillonite and illite INTRODUCTION In soil, organic matter and minerals are often associated such that it is unclear how the presence of the former influences the sorption and desorption properties of the latter. Sorption and desorption are important processes that influence the mobility, bioavailability, and toxicity of organic contaminants in the soil environment. Soil organic matter (SOM) content, surface characteristics, pH, soil texture, and clay minerals affect the retention of organic contaminants (1-3), with soil minerals and SOM being the major components influencing the sorption of organic contaminants including pesticides. Pesticide distribution patterns in soil are controlled by adsorption and partitioning. Pesticides in the liquid phase are readily available for plant uptake, and it is well-established that the uptake of nonpolar organic contaminants from aqueous solution is strongly correlated with soluble organic matter (or organic C) contents (4-8). Reliance on SOM-normalized sorption coefficients (K om , K oc ) to predict the transport of organic contaminants in soil implied that SOM is the sole sorption domain but ignored the potential contributions of soil mineral fractions. Mingelgrin and Chiou (9, 10) noted that soil behaves as a dual sorbent, in which SOM functions as a partitioning medium and mineral fractions as conventional adsorbents. However, for moderately and strongly polar functional group- containing compounds including many pesticides, multiple sorption mechanisms may be operative (11, 12). The relevant mechanisms include solute partitioning into SOM as well as * To whom correspondence should be addressed. Tel: +86-10-62923560. Fax: +86-10-62923563. E-mail: xiaoquan@mail.rcees.ac.cn. ² Research Center for Eco-Environmental Sciences. ‡ Institute of Tibetan Plateau Research. 3548 J. Agric. Food Chem. 2005, 53, 3548-3555 10.1021/jf048106r CCC: $30.25 © 2005 American Chemical Society Published on Web 04/02/2005