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Team:Oxford/P&P environmental impact
From 2014.igem.org
Chlorinated solvents are organic solvents containing chlorine atoms in their molecular structure. They have a huge range of uses by individuals, professionals, and industry.
The chlorinated solvent DCM has been used in industry for over 80 years. Its unique combination of properties - low boiling point, high solvency power, relative inertness, low toxicity and non flammability - has led to its wide variety of applications. It is the most widely-used of the chlorinated solvents, particularly for pharmaceutical production, and is also used as an extraction medium/process solvent (Eurochlor). For these reasons, our team has chosen to use DCM as the case study chemical for which we will develop a bioremedication mechanism eventually applicable to all chlorinated solvents.
The worldwide production of DCM alone is estimated at around 570,000 tonnes, of which approximately 270,000 tonnes are thought to be produced in western Europe. Figures for chlorinated solvents generally are far higher but difficult to estimate with any accuracy due to the wide range of applications, which sometimes involve the use of only a small amount of solvent which individually be discounted but cumulatively these small scale uses are significant.
Eurochlor, the EU body responsible for the European Chlorinated Solvents Association) analyses the uses and impact of chlorinated solvents in three categories:
Chlorinated solvents have too many industrial applications to list. Amongst the most essential are food production, cleaning, the textile industry, manufacturing, foam blowing, fire extinguishers, and as an extraction solvent and functional fluid.
These solvents are widely used in laboratories, as well as extensive use in dry cleaning, film cleaning and copying, aerosols, adhesives, and packaging.
Prominent uses include aerosols (despite controversies, chlorinated solvents remain present in a wide range of aerosol products including hairspray and deodorant), glue and other home decorating products such as paint and paint stripper. Chlorinated solvents are also common in home-use pest control sprays and are found in various washing and cleaning products.
Current disposal methods for chlorinated solvents are inadequate on every level. Because individual users may use chlorinated solvents infrequently or in very small amounts, they make not think it worthwhile, may not have the means, or may lack the knowledge/understanding of chlorinated solvents to ensure waste is disposed of properly. Cumulatively, these small incidents amount to a large volume of chlorinated solvents which are simply poured down the drain or otherwise dangerously disposed of, leading to grave and often long-lasting environmental damage.
For worse is the impact of the much greater volumes of chlorinated solvents used professionally and in industry. Although disposal is regulated and there are high penalties for failure to deal properly chlorinated waste, contamination remains an issue. Meanwhile, even that waste which is disposed of in accordance with procedure causes environmental harm; currently chlorinated solvents which can no longer be recycled are simply incinerated, released damaging compounds into the atmosphere.
Click the images below to discover more about the damage caused by chlorinated solvent waste to the atmosphere, biosphere, ground and surface water, and to the marine environment.
The Environmental Protection Agency has expressed fears that even short lived halocarbons may have a significant detrimental effect on the global atmosphere, as well as concerns about our relative lack of understanding of the environmental effects of these compounds2. Several chlorinated solvents are listed by the U. S. Environmental Protection Agency (U.S. EPA) as a hazardous air pollutant (HAP) under the U.S. Clean Air Act. However, various environmental NGOs and organisations maintain that current regulation of chlorinated solvent disposal is inadequate - for example, chlorinated solvents are not regulated under the Montreal Protocol despite evidence that they may contribute to ozone depletion.
TCE and PBRC have both been linked to photochemical smog. Both these chemicals are used extensively in the textiles industry and current disposal is inadequate. Photochemical smog is a unique form of air pollution, caused by reactions between sunlight and pollutants. The products of these reactions are generally 'secondary' pollutants such as hydrocarbons or ozone (which in the lower atmosphere is not desirable as it causes irritation to the respiratory tract).
Photochemical smog is known to cause respiratory problems in humans and animals. Because the chemicals can travel on the wind, the problem can potentially affect all areas although it tends to be most serious in large cities.
Dichloromethane has a Global Warming Potential (GWP) ten times greater than that of carbon dioxide, whilst trichloromethane has a GWP 30 times greater. At the time of writing, the GWPs of tetrachloroethene and trichloroethene are not known, but are expected to be comparable to those for DCM and TCM.
In the lower atmosphere degradation of chlorinated solvents is initiated by a reaction with the hydroxyl radical, and forms a variety of products including hydrochloric acid, formic acid, and phosgene (the colourless gas infamous for its use as a chemical weapon during World War One). These compounds dissolve in clouds and rain water, and are ultimately deposited from the atmosphere in acid the form of rain and snow.
Further, trichloroacetic acid (TCA) can be formed as a minor product in the atmospheric de-gradation of some chlorinated solvents. Studies have shown that TCA is broadly distributed in precipitation, surface water and soil on a global scale. Since the observed levels in soil in some areas have been found to exceed the accepted 'safe' levels (2.4 μg/kg for terrestrial organisms) the European Commission instructed producers of the relevant solvents to carry out extensive studies of the origin and fate of environmental TCA. Although the results of these studies suggest that TCA levels in soils could not be explained by precipitation alone, the European Union Risk Assessment nevertheless concluded that “it is considered unlikely that depo-sition of TCA from the atmosphere will by itself lead to levels of TCA in soil that pose a risk for ter-restrial organisms”.
The Reference Dose (an estimate of the maximum level of continuous exposure to the human population which is unlikely to pose any significant risk of detrimental effects (excluding the risk of cancer of the course of a lifetime)) for DCM is 0.06 milligrams per kilogram of body weight per day. Worryingly, DCM has been found in some urban air and at some hazardous waste sites at average concentrations of 11 ppb of air, and the average daily intake of methylene chloride from outdoor air in three U.S. cities may reach 309 micrograms per day, suggesting it is entirely possible that intake may exceeds the reference dose in individual cases. The most frequent and dangerous exposure to chlorinated solvents generally occur in workplaces where the chemical is present. Workers are at risk of breathing in chlorinated solvents or accidently coming into skin contact with chemicals. Previous studies have shown concentrations of up to 1,000 ppm of DCM in air (note that 1 part per million is 1,000 times more than 1 part per billion) have been detected in general work areas, and even higher concentrations of up to 1,400 ppm have been detected in samples in the breathing zone of some workers. Such exposure levels far exceed the current recommended federal limits; The National Institute for Occupational Safety and Health (NIOSH) estimated that 1 million workers may be exposed to dangerous levels of dichloromethane, and for chlorinated solvents generally the figure is much higher.
DCM and other chlorinated solvents can have a devastating impact on human health. Case studies of DCM poisoning during paint stripping operations have shown that overexposure can be fatal to humans. Acute inhalation exposure can cause short term damage to the central nervous system including detriment to visual, auditory, and psychomotor functions, and irritation to the nose and throat. The major effects of chronic inhalation of DCM are also effects on the nervous system, including headaches, nausea, memory loss, and possibly dizziness. There is currently a lack of research indicating whether there may be developmental or reproductive effects in humans, although animal studies have previously shown that if DCM passes through the placental barrier there is a high risk of skeletal variations and/or lower fetal body weight. DCM is also considered to be a probable human carcinogen. Although research in this area is incomplete, animal studies have shown a sharp increase in liver and lung cancer and in mammary gland tumors following exposure to DCM. The US Environmental Protection Agency has concluded that, by a weight of evidence evaluation, 'dichloromethane is [and should be treated as] carcinogenic by a mutagenic mode of action'.
Tests have shown that acute exposure to DCM causes moderate acute toxicity from oral/inhalation exposure in many animals. Chronic exposure can lead to problems with the liver, kidneys, nervous and cardiovascular systems of a variety of animals. There is also a risk, to humans as well as to animals, that DCM will be broken down by the body form carbon monoxide, which can cause respiratory problems and can ultimately be fatal.
The No Observed Effect Concentration (NOEC) for the most sensitive species of plants was 46 μg/m. In some areas levels may exceed this, due to contamination of soil and groundwater. There is currently a worrying lack of understanding and research into the effects of chlorinated solvents on plant life.
TCE and TeCE are amongst the common contaminants and are particularly tricky to deal with due to the fact that their biodegradation pathways start off with reductive dechlorination to vinyl chloride, which in an anaerobic environment works fine. But then the process often gets stuck at vinyl chloride as that is typically oxidised in groundwater. With VC being far more carcinogenic than TCE and TeCE this is a problem.
TCE is probably the prevalent groundwater contaminant these days. In a public health statement, the Agency for Toxic Substances and Disease Registry (ASTDR) admitted that we do not know precisely how long chlorinated solvents may remain in the soil. What we do know, however, is that chlorinated solvents are a 'big deal' in groundwater - in fact, they are the most frequently detected groundwater contaminant in the USA. ASTDR also concedes that there is a possibility of contamination of drinking water by chlorinated solvents including dichloromethane1.
Chlorinated solvent pollution also affect surface water - although these chemicals tend to volatilise, and are extensively diluted in big rivers, the environmental and drinking water quality standards are very low in comparison to their solubility. The figures are not trivial; according to the US Agency for Toxic Substances, averages of 68 ppb of methylene chloride in surface water and 98 ppb methylene chloride in groundwater have been found at some hazardous waste sites1 .
Conventional water treatment techniques (coagulation, sedimentation, filtration and chlorination) have been found to have a little or no effect in reducing concentrations of DCM in drinking water. Due to the volatile organic nature of DCM, there are two existing treatment technologies that public water systems can use: air stripping and granular activated carbon (GAC) adsorption The U.S. EPA recommends packed tower aeration (PTA) as a best available technology (BAT) for DCM removal in drinking water below the U.S. EPA Maximum Contaminant Level of 5 µg/L. However it should be noted that the selection of an appropriate treatment process for a specific water supply will depend on the characteristics of the raw water supply and the operational condition of the specific treatment method.
Chlorinated solvents are generally highly volatile and only sparingly soluble in water. Even if traces of solvents are briefly present in aqueous waste streams, they volatilise from rivers and lakes with a half-life of about a month or less, unless they are trapped in groundwater. Nevertheless, presence of chlorinated solvents is a concern due to its potential impact on marine life...
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