More specifically, groundwater represents 98% of the available fresh water on earth. The contamination of groundwater has increased significantly due to industrial developments over the last century. To illustrate how the modern human has had an impact on the environment, let us look at some numbers: In the United States alone, yearly 100 million tons of hazardous waste is generated and 4 million tons of toxic chemicals are released into streams; in addition, 1.2 million tons of toxic waste are emptied into landfills and 1.5 million tons are injected into deep wells for disposal. Again in the United States, of the 2 million underground storage tanks in gas stations, 450,000 are leaking gasoline and petroleum products to the subsurface. If these numbers do not impress you, remediation costs for contaminated sites in Europe are expected to exceed $1.5 trillion in the near future. These are only some of the impacts of heavy industrialization of which we are aware. The contaminants that are released into the environment severely threaten drinking water, agricultural, and surface waters.
The pollution process of the subsurface environment is ironically simple. Leaks from various contamination sources seep into the groundwater from where they travel and infiltrate the soil with which they come in contact. Contaminated soils and sediments slowly release these pollutants, which over long period of time have become a continuous source. Of course, all these contaminants can potentially cause cancer or have detrimental effects on the ecosystem or on human health. To give an example of how these pollutants easily spread around the globe, in a recent study of dairy products that were collected from countries all around the world, the same type of organic contaminant (PCB:
Polychlorinated Biphenyls) was found in those products that, for the most part, originated in the USA, although the usage and production of this contaminant has been banned since 1976. How have these pollutants managed to persist in the environment and travel all the way from USA to the rest of the world, even as far as Australia? There are several ways this can be done; first the released contaminant volatilizes into the air and travels, through atmospheric depositions of large quantities onto the grass that is eaten by the cow. Another way is that a fish swimming in contaminated fresh water is caught and becomes food at a dairy farm. This research exemplifies how the pollution of the environment can affect us, regardless of where we live on this planet.
It seems like a very gloomy picture though, if nobody is going to take any action against the contamination or clean up the toxic chemicals from vulnerable targets like drinking water sources or terrestrial lands. There are, of course, many precautions that have been undertaken to clean up the environment. This article is about one of the interesting ways in which we utilize microorganisms to clean up contaminated groundwater or drinking water.
It sounds a little strange for engineers to be dealing with bacteria that naturally exist and utilize them in cleaning up the environment. But this is what they are doing, using a technique called bioremediation, wherein natural microbes become stewards for destroying the pollutants in the environment. The simple technical description of bioremediation is the intentional use of the biodegradation process to eliminate environmental pollutants that have been intentionally or inadvertently released. The biodegradation used here is the microbial transformation process of toxic chemicals into nontoxic forms and sometimes mineralization into inorganic elements like carbon, oxygen and hydrogen.
This transformation, which is essentially a process of destruction, first requires the presence of microorganisms. These microorganisms almost always exist in nature, unless there are harsh conditions that prevent microbial growth. The microorganisms use inorganic or organic contaminants as their nutritional and growth source in the life cycle. For example, a commonly encountered organic contaminant, benzene, is composed of six carbon atoms and six hydrogen atoms. During the degradation process, the microorganisms synthesize enzymes that stimulate the breaking down of benzene into carbon dioxide and water or sometimes the simple elements of carbon and oxygen. These enzymes ease the reactions that produce cellular energy and the building blocks for the synthesis of new cells. In essence, the contaminant serves as a nutrient – food – so that the microorganism can continue its life.
The electron acceptors
During the process in which the microorganism feeds on the contaminant (the degradation process), the key issue is the electron acceptors, the complementary part of the chemical reaction that occurs during the biodegradation process. During the breaking down of the large organic molecules into small elements, excess electrons are released into the environment and therefore an electron acceptor is required to maintain the chemical equilibrium and continue the reaction mechanism. This necessity for oxygen or iron dioxide resembles the need for oxygen in our liver to break down the complex molecules that occur during energy production and new cell generation. The process is as simple as this: we breathe oxygen to live and so do microorganisms. Oxygen molecules act as convenient electron dumps for bacteria that usually lie near the soil surface. Depending on the electron acceptor types, degradation reactions are categorized as aerobic (using oxygen) or anaerobic (using nitrate, manganese, iron and sulfate as electron acceptors). Humans can only inhale oxygen, but most insects can utilize other molecules, like iron oxide or sulfur as well.
Moreover, in order to have a successful clean up, scientists need to satisfy chemical and nutritional requirements and this is challenging for engineers. As these electron acceptors are not always readily available, engineers supplement the electron acceptors in the contaminated environment by methods like pumping air into the ground. Sometimes the microorganisms that are necessary to degrade the potential pollutant do not exist and the engineers must first inject the bacteria so that they can consume the pollutants as food.
One of the microorganisms most frequently studied for its degradation potential for organic and inorganic contaminants is Geobacter metallireducens, or the geobacter. Since it was first discovered, more than 20 years ago, researchers at the University of Massachusetts have been studying this incredible creature; however, they admit that there are many things that they still do not know about it. The geobacter was the first organism found to oxidize organic compounds to carbon dioxide using iron oxides as the electron acceptor. In other words, the geobacter gains its energy by using iron oxides (a rust-like mineral) in the same way that humans use oxygen. The main nutrient for the geobacter can be organic or inorganic pollutants for, and it breathes iron oxide in the way the human inhales oxygen. The geobacter can consume soil and groundwater contaminants like benzene and the gasoline additive MBTE, even in an oxygen-free environment. The geobacter, which has been found almost everywhere, even living in the dental spit-sinks, also flourishes in uranium-contaminated sites, converting soluble radioactive material to a material that is insoluble in groundwater, therefore making it easier to isolate for cleaning up. At present the geobacter is being put to work in actual clean up projects. As our understanding of the functioning of the species has improved, it has become possible to use this information to modify environmental conditions in order to accelerate the rate of contaminant degradation.
More interestingly, researchers have discovered that the geobacter spits out unwanted electrons into the circuit while consuming contaminants for energy. The geobacter exhales electricity through 20 to 30 hair-like structures, just 3 to 5 nanometers in diameter, to its surroundings. Although there is hardly enough microbe-produced electricity generated to solve the world's energy problems, a fuel cell measuring a cubic meter would generate 2 kilowatts, and some engineers are talking about powering sewage treatment plants with a type of geobacter that harvests off the sewage itself. Just to give an idea about the direction of future research, researchers are now working on a selected gene of the geobacter. The gene that limits electricity production will be modified so that electricity production can be boosted during the degradation process. Given that, it would not be surprising if there were technology that created energy while cleaning contaminated soil or groundwater.
Clean water is a basic need for every human being, and it is our moral obligation to work as stewards for the environment; the first thing we must do is to stop contaminating the planet. However, we are faced with resources that have been previously contaminated. As one result of an increasing sense of responsibility toward nature, we are at a point where we can use natural microorganisms or plants as clean-up tools. Although mankind harshly contaminates the environment while creating an industrial and technological world, it is quite ironic that we still rely on the marvels of such divinely ordained solutions to sustain life.
Martin Alexander, Biodegradation and Bioremediation, 199, Academic Press, San Diego CA USA
Pedro J. Alvarez, Walter A. Illman, Bioremediation and Natural Attenuation: Process Fundamentals and Mathematical Models, 2005, Wiley and Sons, NY, USA
www.geobacter.org Geobacter project, University of Massachusetts, Amherst Environmental Biotechnology Center
Jana Weiss, Olaf Papke, and Ake Bergman, A Worldwide Survey of Polychlorinated Dibenzo-p-dioxins, Dibenzofurans, and Related Contaminants in Butter, AMBIO: A Journal of the Human Environment Volume 34, Issue 8 (December 2005), pp. 589–597