Life Sciences Research on Effect of PCBs in Water Systems

Introduction
Polychlorinated biphenyls (PCBs) are among the most toxic persistent organic pollutants (POPs). POPs degrade very slowly and have the tendency to accumulate in organisms and be magnified up the food web and thus persist for a long time in the environment and cause serious ecological damage. Its massive production and use for almost a century since its commercialization in 1929 has left about 1.7 billion tons or more, circulating in the environment, some of which are still in use. (ATSDR, 2000 and 2014)
PCBs are very much inert and chemically stable. PCBs became very attractive for industrial applications for their longetivity. It is resistant to hydrolysis, oxidation and temperature changes. Its non-flammability and dielectric properties also contributed to its widespread use. It had been incorporated in various products as a coolant, dielectric and insulating components in electrical equipment such as capacitors, transformers, motors, magnets, liquid-filled electrical cables and circuit breakers. It has also been added in inks, construction materials, paints, sealants, adhesives, textiles, plasticizers, hydraulic fluids, lubricating oils, and pesticides. Having been spread with the advent of power supply, and being found in all electrical appliances and modern architecture, PCBs have made its mark in modern living. (Füll, 2001; ATSDR, 2000 and 2014)
PCBs come in the form of mixtures of up to 209 congeners. Though most of PCB congeners are not that toxic, a number of PCBs are very similar in structure to the most toxic chemicals known – be polychlorinated dibenzo-p-dioxin (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCB materials also often contain impurities of dioxins and furans. The process of manufacturing PCBs entails side production of these toxic chemicals during pyrolysis. When the material is heated at temperatures of ____, polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are produced. This could also happen during incineration for waste disposal and during fire incidents. (ATSDR, 2000 and 2014)
Mono-ortho-substituted and non-ortho PCBs have phenyl rings that can rotate and adopt a coplanar configuration similar to PCDDs and PCDFs. Despite their resistance to chemical reactions, being similar in shape and polarity to certain biological ligands makes it possible for them bind to certain bodily enzymes having the AL-receptor and disrupt molecular cell signaling. Being non-biodegradable, these are hence retained by the enzyme once bound, thus the enzyme becomes perpetually turned on or off depending on the function of the ligand. This leads to chronic dysfunctions and complications. (ATSDR, 2000 and 2014)
PCB manufacturing had been banned in the USA since 1979 because of its environmental and health toxicity. Monsanto, the originator and major producer of the most commercialized PCB, Aroclor, stopped its production in 1977. They were responsible for about 99% of PCB production of the United States and that accounts for about half of the world supply of PCBs. (ATSDR, 2000 and 2014)
At the peak of PCBs popularity, massive losses in wildlife, agriculture and food industry happened worldwide due to intake of PCBs from contaminated water and feeds and grazing on PCB contaminated land. Chemical waste effluents had been improperly discharged into water systems. PCB contaminated meat, fish, diary, eggs, and other foods, consequently poisoned humans as well. PCBs could also be toxic via inhalation and skin contact, especially for those working in the electrical and electronics industry which is the biggest and fastest growing in this technological age. (ATSDR, 2000 and 2014)
Production was believed to have ceased in the mid-1980s in the US. However, it was only in 2001 that a worldwide agreement had been signed in the Stockholm Convention on Persistent Organic Pollutants (POPs) and it took effect only in 2004. The provisions of the convention included elimination of intentional production and use, minimizing unintentional by-production, and promoting environmentally sound waste disposal of PCBs among other POPs. However, it allows use of closed application PCB containing equipment up to year 2025, provided that it does not leak. The USA, Italy, Israel, and Malaysia did not support the convention but had their own regulations. Some countries such as China continued production of PCBs. (Füll, 2001)
Environmental Emissions
Though production has greatly reduced, PCBs still continue to enter the environment as by-product from thermal processes in chemical manufacturing plants. These industries are not regulated or required to check for PCB levels. Industrial effluents contain significantly higher concentrations of PCBs than agricultural effluents.
PCBs could form through any chemical reaction containing organic carbon and chlorine. PCBs could also come from coal mining, burning of fuels, organic wastes incineration, various recycling operations, and building fires and demolitions. Liquid and gaseous fuels containing or contaminated with PCBs contribute to PCB emissions, along with dioxins and furans. Incomplete combustion of highly chlorinated PCBs followed by slow cooling of the flue gas also increases the likelihood of emitting dioxins.
As a significant amount of PCBs are still in use, disposal and weathering of PCB containing materials and equipment is still a major source of PCB emissions to the environment. Volatilization and leaching from landfills could be avoided by stabilizing and enclosing PCBs in plastic containers for proper disposal. However, leakage and evaporation from improperly disposed wastes and mismanaged dump sites could occur. Incineration of wastes could also contribute significant amounts when combustion is not complete. (Füll, 2001)
Where PCB regulations are poorly implemented, or not applied, PCBs may still be disposed into water systems. However, with improving awareness and policies, PCBs now primarily enter the water system through wet deposition of air particulates than direct dumping into waters. Aside from reemissions from contaminated water and soil, dusts from old establishments made from PCB containing construction materials PCBs contributes to air-borne PCBs. These are carried by winds around the world and settles into distant lands and water systems.
PCBs are semi-volatile and have low vapor pressure thus limiting reemission from water and soil. However, since production declined, much of PCB sources are airborne, PCBs are those that are emitted into the air through fuel combustion, building demolitions, fires, and weathering, wastes incineration and partitioning and volatilization from dumpsites. Aerosol and gaseous PCBs could go back to earth surface through wet and dry deposition.
PCBs that are deposited in water attach to suspended sediments. Release of PCBs into the water from re-suspended bottom sediments is possible depending on water temperature and pH and the concentrations, surface area and polarities of various other dissolved sediments which may repel or attract and hold the PCBs better.
However, ultimately, in the long run, the sediments would sink into solid matter at the bottom, whether directly or when organisms where they are attached or that ingest them eventually die. The bottom sediments and soil environment is the ultimate sink for PCBs because the hydrophobic nature and chemical inertness of PCBs that keeps them there once bound. Besides, PCB disposal are mostly confined on land and leaching is unlikely. PCBs bind strongly to non-polar particles. Thus, depth and breadth of penetration is limited unless the bottom sediments are poor in organic matter content and are loose and dredged in water that would allow more mobility. Nevertheless, normally, PCBs would not leach deep into the bedrock water supply, unless directly intentionally disposed therein.
PCB deposits in soil and sediments, however, pose ecological risks, as these serve as habitat and food to various organisms at the bottom of the food web. Even minute amounts in the environment could be concentrated up to a million fold in an organism as it accumulates across the food web and magnifies up the food chains. Primary producers, bottom dwellers and filter feeders have the highest exposure of PCBs from the soil environment. Plankton and plankton feeders that thrive on the surface where air-borne PCBs are deposited, are also at high risk. Because of hydrophobicity, PCBs are often sobred in suspended sediments, especially in microplastics, which are easily mistaken as food. However, concentrations could be higher in organisms on top of...