The two-step methodology is environmentally friendly and advantageous to implement
Scientists from the Ural Federal University and the Ural Branch of the Russian Academy of Sciences have proposed a new way to eliminate polychlorinated biphenyls. This is a toxic industrial waste that accumulates in the adipose tissues of humans and animals and pollutes the environment. The new two-step approach is environmentally safe and can be implemented on the basis of existing disposal facilities. A description of the methodology and experimental results have been published in the journal Chemical Papers. The development was supported by the Russian Foundation for Basic Research (project № 18-29-24126).
Polychlorinated Biphenyls (PCBs) are chloroaromatic compounds of Hazard Class 2 (Highly Hazardous). These compounds can accumulate in the human body, causing cancer, allergies, and severe poisoning, including death. It is estimated that about 10 thousand transformers and 500 thousand capacitors were used in Russian power systems, where PCBs were used as dielectric.
The most effective way of destroying PCBs is incineration, but for successful elimination of substances it is necessary to follow the rules: high turbulence, presence of the substance in the reaction zone for two or three seconds and the temperature in the combustion zone above 2000 °С. High temperature is necessary to avoid formation of dioxins and furans - cumulative poisons formed from PCBs at high, but insufficient for complete combustion, temperatures. Creation of furnaces with all the requirements requires separate design measures and high-tech capacities, that is why the Ural scientists have developed a two-stage method, facilitating the process of PCBs elimination. The technology is advantageous in that it does not require construction of special plants with furnaces capable of maintaining the temperature of at least 2000 °С. The new approach will require lower temperatures that can be maintained in existing furnaces at waste disposal plants.
"The first step is chemical functionalization - replacing the chlorine atoms in the original PCB structures with the hydroxyl group OH. It is chlorine that is dangerous in combustion, making it important to minimize its amount in the composition of the substance. Unfortunately, it is impossible to replace all chlorine atoms by traditional chemical methods, but it is possible to replace a few atoms with more neutral chemical groups. Then the combustion of functionalized PCBs will produce only water and compounds that are safe for health," says Tatiana Gorbunova, coauthor of the study and leading researcher of the Laboratory of Organofluorine Compounds of Postovskii Institute of Organic Synthesis of the Ural Branch of the Russian Academy of Sciences.
The decisive step in the destruction scheme is thermal destruction. After functionalization, the combustion of new polychlorinated biphenyl derivatives is more efficient and at lower temperatures. Scientists have found that a temperature of 600-700°C is sufficient to incinerate chemically processed PCBs. The end products of thermal destruction are mineral substances: water, carbon dioxide, hydrochloric acid, which do not pose direct danger to humans. Dioxins and furans are not formed in the final products.
The results of our experiments show that hydroxy derivatives of polychlorinated biphenyls can be incinerated at any waste incineration plant. For example, in 2015 in Yaroslavl there was launched a plant for the disposal of hazardous waste, the main technology there is represented by just thermal methods. But in the furnaces of the plant only solid waste contaminated with PCBs is destroyed: oiled consumables and contaminated soil. Liquid technical mixtures are forbidden by the legislation, because of danger of formation of cumulative poisons. Therefore, we propose to solve this problem by a comprehensive approach and neutralize the original hazardous substances. Cooperation of scientific potential of our group and capacities of waste incineration plants may become the optimal solution of the problem of liquid stockpiles of PCBs," explains Tatiana Kulikova, senior researcher of the Research Laboratory of Advanced Functional Inorganic Materials of UrFU.
Scientists note that the proposed disposal scheme requires further industrial environmental expertise.
Polychlorinated biphenyls are a group of organic compounds including 1-10 chlorine atoms bonded to any carbon atom of a biphenyl (organic compound, aromatic hydrocarbon). PCBs have high thermal and electrical insulation characteristics, thermal and fire resistance, good solubility in fats, oils and organic solvents. This has contributed to their wide use as dielectrics in transformers and condensers, hydraulic fluids, coolants and coolants, lubricating oils, components of paints, varnishes, adhesives, etc.
PCBs are one of the most widespread anthropogenic pollutants and are classified as persistent organic pollutants (POPs) - a class of highly hazardous chemical agents posing a serious threat to human health and the environment. POPs are characterized by resistance to degradation, acute and chronic toxicity, and long-range transboundary transport by air, water, or with migratory species.
In 2001, after a series of international negotiations within the framework of the Intergovernmental Committee under the UN Environment Program, the Stockholm Convention was adopted. It provides for the reduction or elimination of emissions from the intentional production and use of persistent organic pollutants, as well as the reduction or elimination of emissions from their unintentional production. The Russian Federation signed the Stockholm Convention in 2002 and ratified it in 2011 (Federal Act of 27 June 2011 No. 164-FZ). According to the Stockholm Convention on Persistent Organic Pollutants POPs, by 2028 all stockpiles of PCBs on their territories have to be eliminated.