COMPETITIVE SORPTION BETWEEN ATRAZINE AND OTHER ORGANIC-COMPOUNDS IN SOILS AND MODEL SORBENTS

This is part of a larger study that addresses the question of whether site-specific sorption of organic compounds takes place in soil organi c matter(SOM). Competitive sorption is one indication that such intera ctions may take place. Competitive sorption was tested between atrazin e (AT) and other co-solutes in water suspensions of natural and model sorbents. The co-solutes included several s-triazine analogs, a substi tuted benzene analog (5-chloro-1,3-dimethoxybenzene), and a dissimilar compound, trichloroethene (TCE). The sorbents included a mineral soil (3% OM), a peat soil (93% OM), soil humic acid particles (99% OM), ru bbery polymers (polyethylene, cellulose, chitin), a glassy polymer [po ly(2,6-diphenyl-p-phenylene oxide)], and a mesoporous silica gel. The rubbery polymers afforded linear single-solute isotherms and no compet ition, both consistent with ideal (Henry's law) partition sorption. Th e other sorbents, including the glassy polymer, gave nonlinear single- solute isotherms and significant competition between AT and its analog s and weak or no competition between AT and TCE. A thermodynamic model , ideal adsorbed solution theory (IAST), was incapable of consistently simulating competition or lack thereof. For the SOM-containing materi als, the results indicate that, like glassy polymers, SOM is a dual-mo de sorbent. Sorption occurs by a partition mechanism and a hole-fillin g mechanism. The holes are conceptualized as specific sites inside the matrix where complexation follows the Langmuir isotherm and where a d egree of specificity is exhibited. In the mineral and peat Soils, from one-third to one-half of AT sorption occurs in the hole domain. Combi ned with previous data, it appears that dual-mode sorption in SOM is a pplicable to polar and nonpolar compounds alike. For silica, the resul ts suggest that the spatial sorption domain of TCE is distinct (possib ly further from the surface) than that of the s-triazines.