THERMODYNAMIC CONSIDERATIONS IN THE SORPTION OF ORGANIC CONTAMINANTS BY SOILS AND SEDIMENTS .1. THE ISOSTERIC HEAT APPROACH AND ITS APPLICATION TO MODEL INORGANIC SORBENTS

Isosteric heats of sorption of phenanthrene from aqueous solution were determined for a hydrophobic sorbent (graphite) and for four hydrophi lic sorbents (alpha-Al2O3 and three mesoporous silica gels). The Claus ius-Clapeyron equation was used in conjunction with temperature-correc ted aqueous-phase solute activity coefficients to compute isosteric he ats from measured temperature-dependent Freundlich isotherm parameters . The results reveal that sorption of phenanthrene by graphite is exot hermic, whereas its sorption by each of the other four model sorbents is primarily endothermic. This is consistent with the expected occurre nce of distinctly different molecular interactions of solute and solve nt molecules at hydrophobic and hydrophilic surfaces. Phenanthrene com petes favorably with water for sorption on hydrophobic surfaces, but c annot compete effectively with water for sorption on hydrophilic surfa ces; in the latter cases, the low level of sorption that does occur is driven by entropy gain by water molecules in bulk phase. This report on enthalpy relationships and molecular-level interpretation of observ ed sorption behavior for rigid solid sorbents is the first in a series of papers on the subject. Subsequent papers utilize the experimental approach and mechanistic information developed here to explore operati ve sorption mechanisms in the more complex realms of physically expand able and chemically more heterogeneous soil/sediment organic matrices.