A DISTRIBUTED REACTIVITY MODEL FOR SORPTION BY SOILS AND SEDIMENTS .2. MULTICOMPONENT SYSTEMS AND COMPETITIVE EFFECTS

The uptake of individual hydrophobic organic compounds (HOCs) from aqueous solution by soils which exhibit heterogeneous reactivity is shown to be reduced in the presence of other HOCs. The observed behavior is consistent with expectations for competitive surface adsorption phenomena, being most marked at low solution-phase concentrations and coinciding with single-solute isotherm nonlinearity. Nonlinear sorption and competitive effects appear to relate to components or fractions of soil having different reactivity than that commonly attributed to soil organic matter. In particular, HOC uptake by the organic components of soils deriving from certain sedimentary rocks appears to occur by surface reactions rather than by partitioning to organic matrices. Ideal adsorbed solution theory (IAST) is used to predict mixed-solute competitive effects from single-solute sorption isotherms. The results are examined within the context of the distributed reactivity model (DRM), wherein overall sorption behavior by heterogeneous solids is explicitly attributed to different mechanisms. Competitive reductions in the sorption of HOCs in systems dominated by nonlinearly sorbing soil components and characterized by a two-compartment DRM are found to be modeled reasonably well with IAST.