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 organic matter(SOM). Competitive sorption is one indication that such interactions may take place. Competitive sorption was tested between atrazine (AT) and other co-solutes in water suspensions of natural and model sorbents. The co-solutes included several s-triazine analogs, a substituted 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), rubbery polymers (polyethylene, cellulose, chitin), a glassy polymer [poly(2,6-diphenyl-p-phenylene oxide)], and a mesoporous silica gel. The rubbery polymers afforded linear single-solute isotherms and no competition, both consistent with ideal (Henry's law) partition sorption. The other sorbents, including the glassy polymer, gave nonlinear single-solute isotherms and significant competition between AT and its analogs 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 materials, the results indicate that, like glassy polymers, SOM is a dual-mode sorbent. Sorption occurs by a partition mechanism and a hole-filling mechanism. The holes are conceptualized as specific sites inside the matrix where complexation follows the Langmuir isotherm and where a degree of specificity is exhibited. In the mineral and peat Soils, from one-third to one-half of AT sorption occurs in the hole domain. Combined with previous data, it appears that dual-mode sorption in SOM is applicable to polar and nonpolar compounds alike. For silica, the results suggest that the spatial sorption domain of TCE is distinct (possibly further from the surface) than that of the s-triazines.