Accelerated natural attenuation
It is clear as we enter the new millennium that we can manipulate the atom and the electron at will. Carbon, a basic element that was once used for nothing more sophisticated than the pigment of choice for cave paintings, can now be transformed into soccer ball-shaped molecular cages called "buckyballs," with a myriad of micro-packaging applications. Modern culture is governed by managed electron flow generating everything from e-commerce to the cell phone. On Earth, illnesses are understood and cured at the molecular level. In outer space, we go to Mars looking for water and life,and probe the further galactic depths with orbiting telescopes. So, a call to return to simple solutions for complex problems appears to be misplaced and counterintuitive. To rally around more moderate strategies becomes a challenge.
It is becoming apparent, however, that we are paying an environmental price for our progress. Expecting technology to reliably cover its own tracks is unrealistic. When our aquifers first became unhealthy, restoration efforts began with available and palpable mechanical solutions. The approach was direct but heavy-handed just pump out the water and clean it up. However, this strategy was somewhat misguided or, at a minimum, over-applied. The industry had a hammer and everything looked like a nail. Eventually most mechanical systems were seen as a long-term, uneconomical and incomplete response to the realities of environmental restoration. Then, in an almost reactionary turn of events, someone said, "The emperor has no clothes."
The first paradigm shift
Contaminated groundwater plumes were suddenly became viewed as self-limiting, and regulators began to loosen the reins as the lexicon and mathematics of "exposure pathways" and "acceptable risk" took hold. Eventually, the healing forces of nature were marshaled into service to complete the conversion efforts. We now have monitored natural attenuation, whose advocates counsel that, under the right conditions, we may be able to just watch the problem go away. In reality, monitored natural attenuation does nothing to proactively improve the environment; it only establishes a platform to understand, quantify and accept the risk that is already present. Unfortunately, this paradigm has the aura of a "do-nothing" technology and, though not without merit, is constantly forced to defend itself against these charges.
Though we now have a full spectrum of remediation options, we have only skirted the problem. On the one hand, we know that we can over-remediate and, on the other hand, we are counseled to eliminate heroic efforts. The question is, can we find a middle ground? Is there a strategy which incorporates the useful elements of monitored natural attenuation, recognizing that nature has a solution, and combining it with some level of proactive response to avoid both the stigma of inaction and the continuing risk posed by untreated contamination? Can we propose a level of action that is catalytic in nature so that a little bit of effort can rapidly accelerate the rate of natural processes and have a major impact on the problem? In other words, can a moderate strategy, between the extremes, be a serious contributing factor to solving our groundwater problems? And, finally, is this plausible in a world where we have grown accustomed to controlling atoms and electrons at will?
The paradox is that, in spite of our technological sophistication, we are still humbled by the fact that aquifer remediation is a macroscopic, multi-variable problem. There are exceptions, but, in general, groundwater contamination problems are hard to characterize, understand, monitor and solve. Brute force is not usually the answer - at least not at an affordable price. We seem to be on the right track by giving nature a chance to work. The new paradigm being advanced here is that we should offer nature a helping hand. In many circumstances, this is sensible and proactive action closes sites more quickly, efficiently and economically.
A natural solution
A concise phrase that defines a new paradigm in groundwater cleanup is "accelerated natural attenuation." Through this terminology we establish that nature offers a solution natural attenuation, while invoking the promise that there is a way to accelerate the process. Accelerating natural attenuation makes the process readily applicable to environmental problem solving. Unassisted natural remediation processes operate on a time scale that may be unsatisfactory to many who are responsible for our environmental health and safety, or who are concerned with property transfer issues.
To understand how we can intercede in and manage the natural attenuation process we must first dissect its components in order to identify those that can be engineered. Attenuation means, "to reduce in force, value, amount, or degree." However, this definition alone does not reveal the several subsets of the natural attenuation process. In physical attenuation, we spotlight forces such as dilution and volatilization that are self-evident in their mode of action. Chemical attenuation references the less dominant to absent geochemical reactions that are extremely site specific, and that may even be detrimental in terms of the overall chemical risk factors in the system. Managing physical and chemical natural attenuation mechanisms takes on large proportions, invoking the cumbersome and expensive mechanical systems and the wealth of time it sometimes takes to generate results. Into this background enters the promise of affordably managing biological natural attenuation.
Dance of the electrons
Clearly, environmental contamination can be neutralized by microbes, or more accurately, by their metabolic activities. When we examine bioremedial metabolism closely, it is basically a dance of electrons taking place in the biochemical sphere of the cell. Beyond that, the process is about the energy that is transferred in this dynamic, allowing the entropic forces of the universe, which would unravel the complex high energy state of the living cell, to be countered. Simply put, contaminant molecules can enter into the electron flow and energy transfer processes of the cell and be destroyed, ironically while keeping the system going, by giving life to the processing organism. So, aside from any romantic notions, what is critical relative to the physical and chemical attenuation processes is that the efficiency of the biological attenuation processes, the "electron dance," can be enhanced by reasonable and affordable means.
In the realm of bioremedial metabolism, we can look at environmental contaminants in simple terms as either "electron donors" or "electron acceptors." To understand this, we must first know how we can engineer the process by reasonable and affordable means. For this, only three things have to be understood. The first is that electron donors and acceptors act in pairs. The second is that a contaminant molecule may be the electron acceptor or the electron donor and that it doesn't matter which one as long as it is destroyed in the process. The third relates to the fact that whichever role a contaminant molecule will play, pitcher or catcher so to speak, we can engineer into the system the missing part of the baseball battery. The engineering component is usually nothing more than the simple injection of the appropriate electron donating or electron accepting gas, liquid or solid into the contaminated aquifer a reasonable economic proposition by most standards.
When molecules act as electron acceptors in aerobic metabolism, they are playing an important role in the electron and energy flow process. For example, in aerobic metabolism, a cell derives energy from a fuel. In this case, the fuel is a molecule that has energy bound up in its structure. These molecules are also referred to as substrates, conveying the image that they are being acted upon in some way so that the energy can be released and redirected. When we consume the various arrays of carbohydrates, fats and other molecules in our diet, we are using them as substrates for energy; to a microorganism, petroleum hydrocarbons can serve this function as well. This differential condition in metabolism between organisms basically centers on "enzymatic competence." Microbes simply are capable, using their unique enzymes, of dismantling molecules that we cannot.
The action takes place in cell structures called mitochondria that are essentially cellular engines. The fuel, like ordinary sugar or the contaminant benzene, goes into this structure, and energy is produced. The specific dynamics of this process are that the mitochondria manage a flow of electrons that cascade "downhill." As the electrons take the fall downhill, energy is liberated, captured by other molecules and then shunted off to do work in other parts of the cell. The electrons are generated when the substrate is enzymatically dismantled. The downhill journey they follow is called an electron transport chain. This is analogous to the action of a turbine that captures the energy of water falling through a dam. The electron's fall has to have an endpoint, and this endpoint is oxygen. In other words, oxygen sits at the bottom of the electron transport chain and catches the "spent" electrons. It is from this process that we derive the concept of oxygen as an electron acceptor (and, it
should be noted, that the fuel molecule is the electron donor).
Providing subsurface oxygen sources
From an engineering perspective, the simple objective is to provide oxygen to the subsurface. One of the common methods of doing this has been air sparging, which is simply the act of blowing or sparging either air or pure oxygen into the subsurface. Since we are concerned here with contaminated groundwater, we reference this basic air injection process as it is applied to the saturated aquifer and not the vadose zone. Venting the vadose zone, when required, is a related technology and only pertinent in the prevention of future sources of groundwater contamination in the absence of excavation or other remedies.
So, in summary, we are discharging an oxygen as a gas, either in pure or diluted form, into the aquifer so that it can receive electrons from the degrading organic contaminants and maintain the continuity of the "electron dance." Unfortunately, aside from mechanical systems costs and ongoing operations and maintenance issues, the sparging process can create preferential flow paths in the subsurface leading to the formation of untreated zones.
Another option is to provide oxygen via liquids. The most efficient way to do that is through the implementation of hydrogen peroxide. Hydrogen peroxide is brought to the site in concentrated form and diluted for injection. The method, while still used, has several drawbacks with respect to safety, efficiency and longevity. Also, as with air sparging, it is often mechanically and operationally intensive and, once again, we find ourselves forcing something into the aquifer in high volume bringing on a secondary series of problems in plume management. Still, we are moving in the right direction because, while hydrogen peroxide additions may be flawed, use of peroxygens in general have merit at least when they are in solid form.
Of the solid peroxygens that are commercially viable and compatible with biological systems, the choices are restricted to calcium peroxide and magnesium peroxide. The advantage in using these compounds is that they can be injected infrequently as a slurry into the aquifer. However, even these solid peroxygens have a limited longevity, typically one or two months. So, remediation experts, looking for a long-lasting solid oxygen source that can avoid the need for multiple re-injections, have been using a special slow-release oxygen compound called ORC. This technology, which has been applied more than 5,000 times worldwide, is a patented formulation of magnesium peroxide intercalated with a food-grade phosphate ion. The term intercalation defines the fact that the phosphates permeate the magnesium peroxide crystal. This phenomenon partially inhibits the transmission of water into the structure - the catalytic event for oxygen release from magnesium peroxide. It is analogous to a method used in microel
ectronics where a nonconducting material is introduced into a conducting material to alter its transmissive properties. Therefore, controlling the hydration of the crystal effects a "timed" oxygen release. Under varied conditions of demand in the aquifer, intercalation can allow for product longevity of about six months with excursions to a year or more.
Electron donors are similar to electron acceptors; they just address the other side of the equation. As in the previous discussions, we see that in this aspect of chemistry the reactions are paired. Consequently, we need an electron acceptor to receive the electrons that are donated. In the previous example, certain kinds of organic molecules (petroleum hydrocarbons) serve as electron donors, and oxygen is the electron acceptor. In this case, the electron acceptor can actually be the contaminant itself most commonly a higher-order chlorinated hydrocarbon such as perchloroethylene (PCE); the electron donor is an organic material that decomposes to release electrons.
A closer look at the last statement regarding decomposing organic materials presents an interesting picture. What is needed in an electron donor is an organic material that can be fermented by microorganisms to produce hydrogen gas. It is this hydrogen gas that is the actual source of electrons that drives the destruction of chlorinated hydrocarbons by a process called reductive dechlorination. Some of the best organic materials that can be used for this application are simple molecules such as small chain organic acids and alcohols. Complex natural products such as molasses are sometimes used, but they need to be broken down further before they can be effective.
As in the previous example involving a controlled release oxygen source, there is also a controlled release hydrogen source. One of these, which has recently become available is HRC, a benign polylactate ester specially formulated for the slow release of lactic acid (an organic acid) upon hydration. When a compound like HRC is injected into an aquifer, the water can hydrate the ester linkage and separate the polylactic acid complex from the glycerol backbone. The release of the polylactic acid complex is a moderately slow process, depending on the size and nature of the microbial population in the vicinity of the application.
Polylactic acid release is the first part of a multi-step slow-release hydrogen mechanism. When the polylactate complex is released, it takes even more time to be turned into individual lactic acid molecules. Then, when there is finally some free lactic acid, it can be fermented by indigenous anaerobic microbes. In the fermentation reactions, the lactic acid is converted to several other organic acids and produces hydrogen along the way. The resulting hydrogen can then be used by reductive dehalogenating microorganisms, which are capable of dechlorinating the contaminants. Depending on site conditions, the application of slow-release hydrogen compounds can facilitate anaerobic remediation for up to several years, depending on how it is formulated.
In summary, since oxygen is an electron acceptor and hydrogen is an electron donor, we can characterize the technology, which involves their formulation as slow release compounds, as "time-released electron donors and electron acceptors." This strategy is extremely sensible and very responsive to the needs of the marketplace. The key is providing the optimum level of oxygen or hydrogen on a constant "low flow" basis.
Advantages of time release donors and acceptors
The various limitations of mechanical solutions for in situ
remediation, such as air sparging or continuous injection, are well documented. The focal point should not be the limitations of these systems, but rather the merits of what Dr. Herb Ward of Rice University calls "engineered bioattenuation." Engineered bioattenuation has merits that are continually accruing with its ongoing acceptance and implementation. Given that this is a viable option for the accelerated natural attenuation of contaminated sites, the following are the some advantages of using slow-release compounds to achieve this goal and set new directions for aquifer restoration.
Low capital, design and O&M costs.
Since slow-release compounds are part of a passive, in situ
approach, substantial design, capital and operations/maintenance (O&M) costs are avoided. Actively engineered systems are expensive, time-consuming and often burdened with extensive and costly design considerations. Sometimes the design costs alone of mechanical systems will approach or exceed the costs of a slow-release substrate mediated biological treatment.
Minimal site disturbance. In situ injection treatments overcome the requirement for aboveground equipment after initial operations, thereby allowing remediation without disrupting normal business or commercial activities. Applying slow-releasing substrates to the subsurface is fast and easy. After application, there are no aboveground indications that the materials have been applied, because they work silently below ground. The disadvantages of visible and active systems are self-evident, with respect to aesthetics, safety and theft.
Applicability at difficult-to-manage sites.
Slow-release compounds are ideal for sites where geological or physical conditions make active systems inappropriate. Particularly in clay soils, where pumping is difficult and sparging promotes channeling, the slow release of diffusable materials has advantages.
Limited disturbance of the contaminant plume. Any mechanical action in the aquifer has the potential to distort the dynamics of a contaminant plume usually not to the benefit of the project. Sparging is of particular concern in this regard. Injections of slow release materials are minimally invasive and limit site disturbance.
Constant and persistent source of electron acceptor/donor. Slow release electron donors and electron acceptors will remain where emplaced and generate diffusable active agents slowly over time. Particularly in the case of chlorinated hydrocarbons, since plumes are difficult to locate, a continuous source of diffusable materials increases the effectiveness of contact, containment and remediation.
In summary, the new paradigm in aquifer restoration can be described as "the use of controlled release electron acceptors and electron donors to accelerate natural bioattenuation". Engineering these systems is a low cost, passive and effective way to respond to the massive array of groundwater challenges we face today and will face in the future.
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This article appeared in the Feb. 2000 issue of EP, Vol. 11, No. 2, p. 14.
Photo courtesy John A. Karachewski
This article originally appeared in the 02/01/2000 issue of Environmental Protection.