Regenerative Properties of Spent Tailored Soils
Natural clays have a large internal surface area potentially available for adsorption of hazardous organic compounds. The exchangeable cations in clay occupy the interlamellar spaces that separate the clay platelets. Water molecules form a hydrated shell around the exchangeable cations due to the hydrophilic nature of these ions. The presence of such water molecules hinders adsorption of hydrophobic pollutants. Therefore, the interlamellar area in clays is normally inaccessible to organic pollutants.
Adsorption of organic pollutants with low molecular weight onto natural soils may be greatly enhanced by chemical modification, that is, tailoring, of the clay fraction of the soil. Tailoring agents include tetraalkylamine compounds which can effectively replace the natural exchangeable cation in the clays. Prior studies show that the adsorption capacity of soils for benzene and toluene are greatly increased by tailoring with tetramethylammonium (TMA+) cations.
Economic feasibility of using this process hinges on the potential to regenerate the spent soils. Regeneration by volatilization of the adsorbate was investigated for three different tailored soils: Wyoming bentonite, Glendale clay-loam, and a clay-loam soil collected in the Los Angeles area. A 48 percent decrease from the original value of K (the linear adsorption constant) was observed for bentonite after six regeneration cycles. The Los Angeles soil showed no significant degradation in its adsorption capacity for benzene after six regeneration cycles. The Glendale soil decreased its X values 62 percent from the original value after the same number of regeneration cycles.
The adsorption capacity of tailored bentonite for toluene was reduced by 97 percent after six regeneration cycles. This decaying effect does not necessarily represent a decrease in equilibrium adsorption capacity. Rather, the kinetics of adsorption may play a significant role as the adsorbent becomes more granular after repeated regeneration. Adsorption studies with o-xylene showed minimal adsorption for both virgin and regenerated tailored bentonite. For this reason, no regeneration studies were performed with o-xylene.