DESORPTION OF HALOGENATED ORGANICS FROM MODEL SOLIDS, SEDIMENTS, AND SOIL UNDER UNSATURATED CONDITIONS .1. ISOTHERMS

Desorption isotherms spanning 4-5 orders of magnitude in vapor concentration were measured for chloroform, trichloroethylene, and tetrachloroethylene under unsaturated conditions at 100% relative humidity. The mechanisms affecting isotherm shape were investigated using model solids, aquifer materials, and soil spanning a range in physical properties. Uptake from the vapor phase was examined in terms of four sorption mechanisms: (1) mineral surface adsorption, (2) partitioning into natural organic matter, (3) partitioning into surface-bound water, and (4) adsorption in micropores. Evidence is presented that a heretofore overlooked mechanism-adsorption in micropores-contributes significantly to sorbate uptake. and contributes to isotherm nonlinearity on solids with low natural organic matter contents. Micropores are those pores less than several adsorbate diameters in width and are implicated as showing enhanced adsorption as compared to pores of larger dimension. Isotherm shape on solids with low natural organic matter appears to be dominated by intraaggregate microporosity.