Superfund Research Program
Advances Made in Developing Organoclays for Use in Hazardous Waste Remediation
Recently, a great deal of interest has been generated by organically-modified clays (organoclays) and their potential applications in the realm of hazardous materials remediation strategies. Organoclays are clay-based sorbent systems that are easily produced by replacing the naturally occurring inorganic cations present on the clay surface with a particular organic cation. This transforms the natural clay into one that adsorbs organic contaminants from water and other media. Native clays, the basis for organoclays, are abundant and readily available; therefore organoclays are relatively inexpensive to produce. Together, these characteristics provide the great potential of organically-modified clays for use in decontaminating water and abating chemical contaminants.
Investigators within the Superfund Basic Research Program have made significant contributions to this promising area of technology by investigating organoclays for their application in the remediation of organic chemical contaminants. Researchers at both Texas A&M and Michigan State Universities are utilizing cation-exchange technology to develop chemically modified clays that remove organic contaminants from water. Scientists at Texas A&M have recently produced a clay that not only readily adsorbs the toxic environmental pollutant pentachlorophenol (PCP), but also retains the adsorbed PCP for extended periods of time. Results of recent experiments at Texas A&M indicate that the modified clay possesses both high affinity and large capacity for adsorption of PCP. Leachate containing concentrations of PCP as high as 10 mg/L were reduced to non-detectable levels with the use of this organoclay.
To more fully extend the breadth of this clay's application, a novel method for adsorbent-adsorbate contact has been developed at Texas A&M, in which the organoclay is fixed to sand or another porous matrix. The porous nature of this medium results in a large increase in movement of PCP through the organoclay's matrix and thus enhances the removal of this organic contaminant from water. The porous sorbent also provides the basis for using the organoclay in more efficient fixed-bed sorbent systems. In carrying out water remediation activities on hazardous waste sites, costly charcoal-based fixed-bed systems are often used. This new organoclay fixed-bed system could provide a less expensive and highly effective alternative to charcoal in decontaminating water that contains PCP. Ultimately this recent achievement in clay-based remediation is significant in demonstrating that clay-based fixed-bed sorbent systems are now possible for the removal of PCP from water.
In addition to utilizing the abundance of native clays for developing water decontamination systems, the natural presence of clays in soils and subsurface environments offers a unique possibility for developing clay-based in situ remediation technologies. Researchers at Michigan State University are currently developing a very promising technology known as in situ sorptive zone development. Their vision for this technology is based on the idea that clays present in subsoils and aquifers can potentially be converted to highly sorptive organoclays via subsurface injections of cationic surfactants. The creation of such subsurface zones could be used to intercept and immobilize advancing organic contaminant plumes, thereby protecting down-gradient water sources. Furthermore, this potential technology offers the possibility of concentrating pollutants in a defined zone that can be managed to enhance remediation efforts. For example, the contaminants that have been trapped in a sorptive zone could be treated by bioremediation.
In making this vision a reality, scientists have been investigating various cationic surfactants for their comparative affinity with native inorganic cations and for their ability to remove dissolved organic contaminants from water. Cationic surfactants are soap-like materials that are commonly found in commercial household products, such as detergents. Such substances are being combined with clays and soils in a laboratory setting to create novel, highly sorptive organoclay materials.
The Michigan State scientists have determined that the large organic cation, hexadecyltrimethylammonium (HDTMA), has a much higher affinity than native inorganic cations for exchange sites on clays. As with pure clays, HDTMA was also demonstrated to have a high affinity for soil cation exchange sites in soils and subsoils containing clays. The HDTMA modified soils exhibited excellent sorptive capabilities for removing neutral organic contaminants (NOC) from water. Enhanced NOC uptake from water occurred up to 350 times higher than in unmodified soil, and the same sorptive mechanisms were operative as in pure HDTMA-clay. These laboratory studies are laying the foundation for conducting field trials of the proposed technology.
Additionally, the scientists have investigated the hydraulic conductivity of the HDTMA modified clays and soils. An evaluation of how in situ treatment of soil may affect the flow of materials through the sorptive zone has been conducted. Experiments revealed that HDTMA modified clays and soils maintain their natural conductivity of water and other solvents. These studies are significant in that they are suggesting that the creation of subsurface sorptive zones for capturing and abating NOCs are possible.
In a time when cost minimization is an important consideration for hazardous waste remediation efforts, organoclays provide an inexpensive, yet highly effective alternative to some conventional remediation technologies. The new organoclay materials developed by both groups of researchers offer the possibility of improved methods of decontaminating polluted water and subsurface environments.
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