I recently had the opportunity to present a lecture on biodegradation to a master watershed class at the University of Arizona’s Yavapai extension office. According to the Master Watershed website, “[t]he Master Watershed Steward Program educates and trains citizens across the state of Arizona to serve as volunteers in the protection, restoration, monitoring, and conservation of their water and watersheds.” I was excited to present to this audience because they clearly have the passion and motivation to take action in their communities, and they are easily the most interactive and engaging group to which I have ever presented. I began my lecture with three questions:
- What is biodegradation?
- What are some examples of biodegradation?
- Is biodegradation a good thing?
The class provided a strong definition of biodegradation and successfully listed many examples, including wastewater treatment plants, composting, reduction of plant detritus in the environment, and landfilling. Most intriguing was their response to whether biodegradation is good or bad; the overwhelming response was that biodegradation was good. Since the demographics of this group included young and old, from a variety of careers and backgrounds, it is probably fair to say that their response is representative of the public’s perceptions. In reality, biodegradation is a complex process reliant upon a wide range of variables and – given a certain set of circumstances – biodegradation can certainly be a negative and potentially damaging process. We can briefly examine two examples where biodegradation did not give the positive results that people associate with the process.
When people think of massive chemical spills in the ocean, they often think of massively damaging petroleum spills. There are, however, other chemical spills that pose a risk too. Take the molasses spill from an underwater pipeline in September 2013 at the Honolulu Harbor in Hawaii. An estimated 233,000 gallons of molasses—assumed an innocuous material, because we enjoy ingesting it regularly as a society—was released into the harbor, where it sank and suffocated thousands of animals living on the ocean floor. Animals were initially suffocated by either the formation of a barrier between the animal and oxygen containing water or by clogging the gills. As the molasses spread, it began to dissolve into the bulk ocean water, where microbial organisms proliferated as they degraded the sugar rich solution. The increase in microbe populations led to the depletion of dissolved oxygen in the water (a process called eutrophication), which led to further suffocations. While not as obviously toxic as petroleum, the conditions offered by the molasses spill resulted in rapid biodegradation of the contaminant, and a high mortality rate of animals in and around the spill.
A second example of negative impacts is when a compound is not fully mineralized (broken down to carbon dioxide and water) during biodegradation, and secondary products, which can be more toxic than the parent compound, are produced. For instance, when the common ground water contaminants perchloroethylene (PCE), a common dry cleaning chemical, and trichloroethylene (TCE), a common degreaser, are degraded in anaerobic conditions, they are transformed to the more toxic daughter products cis-1,2-dichloroethene and vinyl chloride, where the former is a suspected carcinogen and the latter is a known carcinogen (1-3). This transformation due to biodegradation is of major concern for human and environmental health, especially when considering groundwater supplies.
Given the above examples, it is not my intention to argue against biodegradation as a consideration of contaminant design and/or control. On the contrary, I am a staunch advocate of using biodegradation as a mechanism to manage contaminants in industrial processes and the environment alike. I introduced these examples because it is important to impart the complex nature and potential impacts of biodegradation.
“Design for Degradation”, the tenth principle of green chemistry, is a laudable and important guide for chemists and researchers as they conceive and design green processes and chemicals. It is, however, a principle that must be thoroughly thought-out and tested before it can be used to label a process “green”, especially when the method of degradation is biodegradation. Ask yourself questions such as:
- Do reagents and the product chemical degrade completely into simple compounds (carbon dioxide, water, methane, ammonia, etc.) or are secondary compounds generated which have similar or higher toxicity?
- Is the product accessible in systems, or does the product partition into areas not accessible for degradation? That is, is the compound available for degradation?
- Is the compound only degradable in a very narrow set of conditions or by a limited number of organisms?
Furthermore, the twelve principles of green chemistry do not carry equal weight at all times. The degradation principle is an example of one which may vary. Consider the surfactant industry; by the very nature of their use, surfactants are widely and copiously introduced to soil and water systems. In this case, the degradation principle should be heavily weighted when assessing whether a compound is green or not. Contrastingly, when a material is produced in a research lab in volumes of only milligrams or grams, the degradation principle carries less weight.
The weight assigned to the degradation principle should be determined by the scale of production and distribution. In essence, chemicals produced in mass and widely distributed are at higher risk of large-scale spills and have a difficult to control fate; these chemicals should be degradable to be considered “green”. Chemicals produced for narrow applications and in small masses are less likely to enter environmental systems and are easier to control; they may still be considered “green” chemicals if not degradable, given they abide by the other principles. This being said, degradability is always desirable and encouraged for all compounds.
1. Doğan-Subaşı, E., et al. Quantitative and functional dynamics of Dehalococcoides spp. and its tceA and vcrA genes under TCE exposure. Biodegradation 2013, 1-12.
2. Jennings, L.K., et al. Bioaugmentation for Aerobic Degradation of CIS-1, 2-Dichloroethene, In Bioaugmentation for Groundwater Remediation, Anonymous ; Springer: 2013; pp. 199-217.
3. Mundle, S.O.C., et al. Monitoring Biodegradation of Ethene and Bioremediation of Chlorinated Ethenes at a Contaminated Site Using Compound-Specific Isotope Analysis (CSIA). Environ. Sci. Technol. 2012, 46 (3), 1731-1738.