Treatment of Water Supplies Contaminated with Toxic Pollutants Using Tailored Soils
Natural clays have a large internal surface area that is potentially available for adsorption of hazardous organic compounds. The exchangeable cations in clays 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 organic pollutants. Therefore, the interlamellar area in clays is normally inaccessible to organic pollutants.
The natural cation in the clay may be exchanged with tetraalkylammonium (TAA+) ions. These quaternary amines are rigid tetrahedrons capable of displacing the natural exchangeable cations and associated interlamellar water. Clays tailored by this process have a predictable basal spacing, which is a function of the TAA+ molecule size. According to Theng et al. (1967), basal spacing in moist conditions vary from 1308 A for tertamethylammonium (TMA+) to 16.5 A for tetra-n-butyl ammonium). The interlamellar space created by TAA+ treated clays favors the selective removal of low molecular weight organic pollutants, which due to their steric properties can conform to the interlamellar space of the tailored clay. Other larger organic pollutants are prevented access into the clay by this selective straining mechanism.
Restoration of aquifers affected by petrochemicals may be accomplished by flushing the aquifer through natural soils that have been previously tailored with TAA+ molecules. The modified soils can be selectively tailored to preferentially adsorb targeted hazardous substances, while allowing innocuous materials to pass unaltered through the treated layers of soil (or be retained in underlying soil strata). This innovative process for the removal of BTX (benzene, toluene and xylenes) from ground waters contaminated by gasoline and other petrochemical spills is expected to be both economically and technically feasible.
The adsorptive properties of TAA+ tailored soils for the selective removal of hazardous organic pollutants found in waters contaminated by petrochemical spills was investigated. Effectiveness of the tailoring agents was evaluated by comparing adsorption isotherms of BTX on treated and untreated soils. Column studies were performed to estimate kinetic properties of the tailored and untailored natural soils. Modelling of pollutant transport in the column studies was used to obtain engineering design parameters.