A distributed reactivity model for sorption by soils and sediments .5. The influence of near-surface characteristics in mineral domains

Phenanthrene sorption by eight particulate mineral solids was examined. The polar surfaces of these inorganic solids comprised three general types with respect to particle structure; namely, (i) the external surfaces of non-porous alpha-Al2O3, quartz, kaolinite, and amorphous SiO2, (ii) the internal pore surfaces of three mesoporous silica gels, and (iii) the internal interlayer surfaces of a swelling bentonite clay. Our experimental data reveal that (i) equilibrium is attained rapidly for all sorbents studied; (ii) the isotherms for all sorbents other than the bentonite are linear; and (iii) the external surfaces have surface-area-based sorption coefficients that are comparable to those of the interlayer surfaces of bentonite at high residual aqueous-phase concentrations, but an order of magnitude greater than those for the internal pore surfaces. The differences in sorption behavior exhibited by the three types of surfaces appear to result from differences in their relative accessibility to the sorbate, as affected by near-surface particle or pore geometry and by the preferential sorption of water molecules. The results suggest that the slow rates of sorption of hydrophobic organic contaminants often observed for soils and sediments is likely attributable to processes other than solute diffusion to ''sorption sites'' on internal mineral surfaces.