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Environmental persistence and transformation processes

Many abiotic and biotic processes that function in concert to eliminate (i.e., degrade) toxic chemicals exist in nature. In this way, many chemicals released into the environment pose minimal hazard simply because of their limited life span in the environment. Chemicals that have historically posed environmental hazards (e.g.: polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethane (DDT), 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)) resist degradative processes and thereby persist in the environment for very long periods of time. These chemicals are only slowly or not easily degraded (Table 1). Trace metals represent an extreme case of persistence because as elements, metals cannot be broken down in the environment. The continued disposal of persistent chemicals into the environment can result in their accumulation to environmental levels sufficient to pose toxicity. Such chemicals can continue to pose hazard long after their disposal into the environment has ceased. For example, one decade after the contamination of Lake Apopka (Florida) with some pesticides (e.g.: diclofol and DDT), the populations of alligators continued to experience severe reproductive impairment. Additionally, a significant contamination of Lake Ontario (North America) by the pesticide mirex occurred from the 1950s through the 1970s. Some studies done 20 years later shown that 80% of the mirex deposited into the lake persisted. Also, the estuarine sediments in the United Kingdom are contaminated with metals that date back to tin mining by the Romans.


Table 1 - Environmental half-life of some chemical contaminants.

Contaminant Half-Life Media
DDT 10 years Soil
TCDD 9 years Soil
Atrazine 25 months Water
Benzoperylene (polycyclic aromatic hydrocarbon (PAH)) 14 months Soil
Phenanthrene (PAH) 138 days Soil
Carbofuran 45 days Water

Source: Hodgson E. (2010).

Therefore, the potential environmental hazard associated with the use of a chemical is directly related to its persistence in the environment, which in turn depends on the rates of chemical transformation reactions. Transformation reactions can be divided into two classes:
  • Reversible reactions - involve continuous exchange among chemical states (ionization - e.g.: precipitation and dissolution -, complexation). These reactions alter the fate and toxicity of chemicals, but they do not irreversibly change the structure or properties of the chemical. Additionally, reversible reactions are usually abiotic, although biological processes can still exert great influence over them (e.g.: through a change in pH or production of complexing agents).
  • Irreversible reactions - permanently transform a parent chemical into a reaction product (photolysis, hydrolysis, and many redox reactions). Many redox reactions are reversible, however they are included in the irreversible reactions because many of these reactions influencing the fate of contaminants are irreversible on the temporal and spatial scales which are important to toxicity. These reactions alter the structure and properties of a chemical forever and can be abiotic or can be mediated directly by biota, particularly bacteria.

For more information on these reactions see Hodgson E. 2010 (pp. 561-563)


3.1 ABIOTIC DEGRADATION

There are a plethora of environmental forces that compromise the structural integrity of chemicals in the environment. Many prominent abiotic degradative processes occur due to the influences of light (photolysis) and water (hydrolysis). The effect of physical factors on degradation through photolysis and hydrolysis studies and the identification of the product formed can indicate the loss rate of the hazardous chemical or the possible formation of hazardous degradation products.


Photolysis - Light, primarily in the ultraviolet range, has the potential to break chemical bonds and therefore can contribute significantly to the degradation of some chemicals. Photolysis can take place wherever sufficient light energy exists. Hence, this process is most likely to occur in the atmosphere (in the gas phase and in aerosols and fog/cloud droplets), surface waters (in the dissolved phase or at the particle-water interface), and in the terrestrial environment (on plant and soil/mineral surfaces). The photolysis rate in surface waters depends on light intensity at the air-water interface, the transmittance through this interface, and the attenuation through the water column. Open ocean waters ("blue water") can transmit blue light to depths of 150 m, while highly eutrophic or turbid waters might absorb all light within 1 cm of the surface. Photolysis is dependent upon the intensity of the light but also the capacity of the pollutant molecules to absorb the light. For example, unsaturated aromatic compounds such as polycyclic aromatic hydrocarbons tend to be highly susceptible to photolysis due to their high capacity to absorb light energy. Some of these transformation products can be more toxic than the parent compound. Light energy can also facilitate the oxygenation of environmental contaminants via hydrolytic or oxidative processes. The photooxidation of the organophosphorus pesticide parathion is shown in fig. 2.

Figure 2 - The effect of sunlight (photooxidation) and precipitation (hydrolysis) on the degradation of parathion. Source: Hodgson E. (2010).


Hydrolysis - Water, often in combination with light energy or heat, can break chemical bonds. Hydrolytic reactions commonly result in the insertion of an oxygen atom into the molecule with the commensurate loss of some component of the molecule. Ester bonds, such as those found in organophosphate pesticides (e.g.: parathion; fig. 1), are highly susceptible to hydrolysis, which dramatically lowers the environmental half-lives of these chemicals. Hydrolytic rates of chemicals are influenced by the temperature and pH of the aqueous media. Rates of hydrolysis increase with increasing temperature and with extremes in pH. Therefore, pH and temperature influence the fate and the effects of a contaminant.

Hydrolysis is usually associated with surface waters but also takes place in the atmosphere (fogs and clouds), in groundwater, at the particle-water interface of soils and sediments, and in living organisms.


3.2 BIOTIC DEGRADATION

Many environmental contaminants are susceptible to abiotic degradative processes, however such processes often occur at low rates. On the other hand, the environmental degradation of chemical contaminants can occur at greatly accelerated rates through the action of microorganisms. Microorganisms (bacteria, archaea, and fungi) most frequently degrade organic and inorganic compounds by using them as electron acceptors, electron donors, or as sources of nutrients such as sulphur or nitrogen. For example, many agricultural chemicals are susceptible to fungal or bacterial degradation, being frequently broken down to products that can enter the carbon, nitrogen, and oxygen cycles. These biotic degradative processes are enzyme mediated and typically occur at rates that far exceed abiotic degradation, as referred above. Moreover, the biotic degradative processes can lead to complete mineralization of chemicals to water, carbon dioxide, and basic inorganic constituents.

Biotic degradation includes those processes associated with abiotic degradation (e.g.: hydrolysis and oxidation) and other processes such as the scission of ringed structures (ring cleavage), the removal of chlorine atoms (dehalogenation), and the removal of carbon chains (dealkylation). In some cases, the products of degradation or metabolites can be more harmful to the environment than the original parent compound.

Bioremediation - The process by which microorganisms are used to facilitate the removal of environmental contaminants. To better understand the bioremediation process, see the following videos:

https://www.youtube.com/watch?v=PgCMbqI71rI

https://www.youtube.com/watch?v=Srem6sjemPg

https://www.youtube.com/watch?v=rCYxsbAXXvg


Biodegradation or biological degradation is known as a biotic process where the organic chemicals can be decomposed primarily by microorganisms. During biodegradation, microorganisms feed off the chemical structure, breaking it down and using it in order to grow. The microorganisms are the primary converters of complex organic chemicals into inorganic substances, although soil and sediment invertebrates also play a major role in the biodegradation process. In many instances, higher organisms are able to metabolize compounds, but they generally play a less significant role in environmental systems. Moreover, the biodegradation process represents a significant loss mechanism in soil and sediments and can take place in virtually any environmental situation, aerobic or anaerobic. Under aerobic conditions, the microorganisms use oxygen as they break down the organic material. Unfortunately, in doing so, there is a risk that a considerable proportion of the dissolved oxygen present in the aqueous environment will be consumed to such an extent that many other aquatic organisms that depend on this gas may not survive.

Note: Only organic compounds can be biodegraded and the majority of these will eventually break down in the environment, though the actual rate of degradation varies as some chemicals will degrade more rapidly than others. Therefore, the inorganic chemicals (e.g.: sodium silicate) cannot be biodegraded, but they may be degraded by other means.


See the following Video:


3.3 NONDEGRADATIVE ELIMINATION PROCESSES

Many processes that contribute to the regional elimination of a contaminant by altering its distribution are operative in the environment:
  • Contaminants with sufficiently high vapour pressure can evaporate from contaminated compartments (aquatic or terrestrial) and can be transferred through the atmosphere to new locations. Such processes of global distillation are considered largely responsible for the worldwide distribution of relatively volatile organochlorine pesticides such as hexachlorobenzene and lindane.
  • Entrainment by wind and upper atmospheric currents of contaminant particles or dust onto which the contaminants are sorbed also contributes to contaminant redistribution.
  • Sorption of contaminants to suspended solids in an aquatic environment with commensurate sedimentation can result in the removal of the contaminants from the water column and their redistribution into bottom sediments. Sediment sorption of contaminants greatly reduces bioavailability since the propensity of a lipophilic chemical to partition from sediments to organisms is significantly less than its propensity to partition from water to organisms. Often the amount of organic carbon in sediments is associated to the bioavailability of contaminants. Sulphur also can affect metal bioavailability in sediments due to its high affinity to many metals. Therefore, more highly water-soluble contaminants can be removed and redistributed through runoff and soil percolation. For example, the herbicide atrazine is ubiquitous in surface waters due to its extensive use and has the propensity to migrate into groundwater because of its relatively high water solubility and low predilection to sorb to soil particles.