The application of adsorption to contaminated groundwater remediation is not only an important subject, but one we could expand upon into several volumes unto itself. At best, all we can do is try to provide a concise overview in this volume.
When we discuss this subject, we cannot separate groundwater treatment from subsurface soil treatment, as the two often go hand in hand. Furthermore, carbon adsorption is not the only technology applicable to subsurface (soil and water) remediation, but it has indeed been successfully applied over the last twenty or so years to countless site remediation activities. Te selection of the proper remediation technology depends largely upon local site conditions such as the hydrogeology, properties of the groundwater, the nature of the contaminants, soil properties, and a range of other parameters and properties. The standard jargon among remediation specialists that focus only on the groundwater remediation aspect, refer to the treatment technology as pump and treat.
Applications have traditionally focused on the removal of man-made contaminants. These include members of the BETX (benzene-ethylene-toluene-xylene) family of components associated with gasoline, and industrial solvents, of which the most notorious ones are the chlorinated hydrocarbons. BETX constituents entered the groundwater from a combination of activities practiced by industry over the years. Perhaps the most common activity in the U.S. whereby groundwater became contaminated by gasoline constituents is from the old practice of storing and disepnsing of this products from single-wall, bare steel underground tanks. Having worked for many years with contractors in site remmediations of auto service stations and industrial facilities, one begins to appreciate how poor this old technology was, and that better than 90 % of these vessels leaked after say 15 years of service (see Figure 16 as an example). It wasn't until the Underground Storage tank regulations (Subtitle C) of RCRA (Resource Conservation and Recovery Act) that the magnitude of this problem was really understood, and groundwater contamination issues began to be addressed on an aggressive basis. Substandard underground storage tank systems led to discharges of gasoline from pitting and corrosion of the vessel walls and in transfer lines and dispensers, and from the overfilling of these vessels. In many cases these were not catastrophic leaks, but rather slow steady losses that cumulatively saturated the soil with contaminants slowly permeating down to the groundwater.
Other industrial solvents have entered into the groundwater simply by poor industry practices, and in some cases, downright negligence and disregard for public safety. This may seam like a harsh statement, but we must recognize that environmental legislation as we know it today, did not exist 30 some odd years ago. As such there was no real driving force for industry to protect the environment, other than say their own sense of public safety. Since pollution controls cost money, from a business standpoint, it makes little sense to invest in technologies aimed at cleaning up pollution, and three or more decades ago, the concepts of pollution prevention were simply not within the mainstream of industry thinking. Although a gross over-generalization, certainly there certainly were companies that simply dumped spent solvents directly on bare ground, and in some situations, were well aware of the fact that these discharges could potentially reach groundwater sources that supplied drinking water to the public. In other situations, companies acted out of pure ignorance. Up until and throughout the 1950s, and even later, many industry people believed that because many of the solvents discussed below evaporate quickly, a simply means of disposal was to spread waste or spent solvents directly onto the ground or in unlined lagoons.
Today we have laws that protect our water resources, along with widespread public awareness of the health effects of toxic pollutants. At the same time there are rapidly expanding demands on the potable water supplies by the ever growing population of the United States. Consequently, research into the behavior of chemicals in groundwater and in the human body greatly expanded in the public and private sectors of many developed countries. Many industrial areas have discovered contaminated water supplies, which should not at all be surprising, given the large scale of the petrochemical industries, and their rapid and spectacular growth over the last century in the developed world. Government agencies responded by establishing Maximum Contaminant Level Goals (MCLGs) and maximum contaminant levels (MCLs) for many toxic chemical compounds. The MCLGs are EPA's non-enforceable levels, based solely on possible health risks and exposure. Based on the MCLGs, EPA has established enforceable MCLs, which are set as close to the MCLGs as possible, considering the ability of public water systems to detect and remove contaminants using suitable treatment technologies. Today, once a chemical regulated under current federal, state, or local law is identified in soil or water above the MCL, facility operators or property owners are required to initiate assessment and remediation of the contamination within a specified amount of time. If a responsible party cannot be identified or does not have the financial means to clean up the contamination, the government (namely the taxpayers) pays for the required cleanup. Unfortunately, the cost of cleaning up discharges of certain chemicals can greatly exceed the value of the contaminated property. Although the remediation of contaminated soil is a simple process, usually involving the excavation and disposal of the impacted media, or in-situ treatment (thermal methods or vapor extraction), if the contaminant has reached the groundwater (the source of potable water used by most municipal water suppliers), the risk to the public welfare, remedial cost, and amount of time required to remove the contaminants can increase substantially.
The most commonly used remediation technique for the recovery of organic contaminants from ground water has been pump- and-treat, which recovers contaminants dissolved in the aqueous phase. In this regard, the application of carbon adsorption has found extensive, but not exclusive use. Vacuum extraction (also called soil venting) has also become popular for removal of volatile organic contaminants from the unsaturated zone in the gaseous phase. Both of these techniques can, in the initial remediation phase, rapidly recover contaminants at concentrations approximately equal to the solubility limit (pump-and-treat), or the maximum gas phase concentration of the contaminant (vacuum extraction). The maximum gas phase concentration will depend on whether the contaminant is present as a free phase or as a solute in the aqueous phase. During this initial phase, large amounts of the contaminants may be removed. The second phase of the remediation, however, is characterized by rapidly declining contaminant concentration in the effluent as the rate of mass transfer into the flowing phase controls the rate of removal. The third phase of the remediation is characterized by a tailing in the effluent of low contaminant concentrations. However, low effluent concentrations may not be a reliable indication of low contaminant levels remaining in the subsurface. Diffusion of contaminants from less-permeable areas into the regions where flow is occurring or the slow desorption of contaminants from the soil surface may control contaminant removal during this phase, and termination of the extraction process before these processes are complete may lead to significant rebounding of the ground water and/or soil air concentrations. This means that the rate-limiting properties of the systems are different in each of the three phases of the remediation: in the first phase, the solubility of the contaminant in the aqueous phase (pump-and-treat) or its maximum gas phase concentration (vacuum extraction); in the second phase, it is the mass transfer step, i.e., dissolution into the aqueous phase (pump-and-treat), or vaporization (vacuum extraction); and, during the third phase, it is diffusion from low permeability areas or the desorption rate.
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