Section Five
SORPTION PROCESSES
14    USE OF TAILORED CLAYS FOR SELECTIVE ADSORPTION
OF HAZARDOUS POLLUTANTS
Fernando Cadena, Associate Professor
Civil Engineering Department
New Mexico State University
Las Cruces, New Mexico 88003
Shannon W. Jeffers, Engineer
U.S. Army Corps of Engineers
Omaha, Nebraska 68114
INTRODUCTION
Activated carbon is a common adsorbent used in the removal of hazardous pollutants from aqueous solutions. Unfortunately, activated carbon adsorption is a non-selective process. Common innocuous organics, which are typically present at much higher concentrations than the more troublesome
hazardous pollutants, may interfere with the treatment processes. Consequently, significant capacity
to remove hazardous waste by activated carbon adsorption is frequently reduced by competition with
non-hazardous molecules. In contrast, the technology presented herein offers a selective treatment
alternative with potential technical and economic advantages.
The large surfaces found in most natural clays account for the excellent capacity of these minerals
to adsorb certain types of pollutants. In particular, the smectite group which includes bentonite and
montmorillonite clays, possesses outstanding adsorption characteristics for certain hazardous organic
compounds.
Smectites are naturally occurring clays found with other minerals in common geological formations
of bentonite, beidellite, and nontronite.' The effective size of smectite clays is typically below 2.0 ^m.
The capacity of these clays to adsorb high molecular weight pollutants is a well understood phenomenon. Factors that favor this natural adsorption capacity include the relatively low solubility of such
pollutants, and the large area of the adsorbent (up to 800 m2/g).
Molecular adsorption on smectites may involve chemical as well as physical effects. Even neutral
molecules, such as hydrocarbons, have been reported to exhibit weak interactions with the net
negative charge of smectites and the positive charge of associated cations. However, these interactions
are much weaker than ion-ion or ion-dipole interactions found in certain priority pollutants such as
phenols and aromatic amines.
Organic compounds with molecular weights below 150 amu are weakly or not adsorbed by pure
smectites (unless they carry a charge and can enter into ion exchange reactions). The interaction forces
under these conditions are the same in magnitude as hydration forces, so that competition reactions
with water molecules for the surface dominate.2 Although a certain amount of anion exchange may
occur on the edges of smectite, most exchange adsorption will be cationic, with positively charged
organic molecules displacing inorganic cations.' Thus, negatively charged compounds are not readily
adsorbed on the negative charges of smectite.2 However, soluble compounds with molecular weights
above 150 amu, whether or not charged, are typically adsorbed by smectites3.
The mineral composition of bentonite, a common member of the smectite family, is quite variable
depending on the source. In general, sodium montmorillonite represents the main component in these
clays (up to 80% by weight). Other inert minerals and clays comprise the remaining portion of the
clay. Typical cation exchange capacity (CEC) of bentonitic clays is approximately 80 meq/100 g.4.5
This CEC value was assumed to be valid in the experimental work presented in this publication.
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