**7. What can be done to control or remove POPs in the environment?**

The Stockholm Convention on POPs clearly provides suggestive solutions by restricting the production and use of POPs in the environment. The Convention requires that production and use of POPs be stopped and their stockpiles destroyed so that the resulting products are no longer POP. Article 6 of the Convention addresses the identification and management of POP waste. The Convention requires that such wastes be managed in a safe, efficient, and environmentally sound manner and that the disposal be done such that the POP content be destroyed or irreversibly transformed so it is no longer a POP.

Stockpiles of POPs are well documented worldwide, which include obsolete pesticides, PCBs discarded from use (PCB oils and liquids), and heavily contaminated soil (e.g., soil surrounding landfills and deep wells containing POPs). To eliminate these stockpiles of POPs from the environment, two basic POP destruction technologies are suggested, namely, combustion technology and non-combustion technology.

The combustion technologies (hazardous waste incinerators, rotary kilns, furnaces, boilers, IR incinerators, etc.) are usually believed to be the most economically appropriate way for concentrated POP waste treatment. This is why in industrialized nations POP wastes are routinely burnt in incinerators, and for most countries combustion technologies still remain the most economically acceptable way to treat POP waste on the macroscale. Modern waste incinerators are designed to satisfy the required emissions levels.

Over the last 15 years, a number of non-combustion technologies have been demonstrated to effectively treat POP wastes in countries such as Canada, the USA, Australia, and Japan. Some methods, particularly for PCBs, are outlined by the UNEP. However, even developed noncombustion technologies can hardly be competitive with incineration. Other non-combustion technologies lack in research or technical development, and most of them have a scarce commercial operation history.

Current studies aimed at minimizing POPs in the environment are investigating their behavior in photocatalytic oxidation reactions. POPs that are found in humans and in aquatic environments the most are the main subjects of these experiments. Aromatic and aliphatic degradation products have been identified in these reactions. Photochemical degradation is negligible compared to photocatalytic degradation. A method of removal of POPs from marine environments that has been explored is adsorption. It occurs when an absorbable solute comes into contact with a solid with a porous surface structure. Current efforts are more focused on banning the use and production of POPs worldwide rather than the removal of POPs [20, 22–24].

## **8. Conclusion**

accumulation of less-chlorinated forms in sediments. More intensive studies on microbial dehalogenation of PCDDs/PCDFs in the environment started appearing in the past decade. Microbial dehalogenation of PCDDs/PCDFs takes place by removal mainly at positions 1, 4, 6, and/or 9 and led to much more toxic congeners, including 2,3,7,8-TCDD, in some cases. The maximum yield of cell protein coincided with the production of less-chlorinated DD isomers, where no methanogenic activity was detected. Experiments with sediment microcosms spiked with the much less toxic congener 1,2,3,4-TCDD revealed that reductive dehalogenation occurred at the lateral positions with 1,2,4-TrCDD as the main intermediate, leading to 2-MCDD as the final end product. The available information indicates that MCDDs/MCDFs

**Scheme 8.** Possible degradation pathways of reductive dechlorination of 1,2,3,4-TCDD (A) and 1,2,3,7,8-PeCDD (B) by

*Dehalococcoides* sp. strain CBDB1.

are not dehalogenated further [14, 19–21] (**Scheme 8**).

**Scheme 7.** Degradation of PCBs in the environment.

26 Persistent Organic Pollutants

POPs pose one of the most challenging problems in environmental science and technology. Their fate, transport, and biodegradation in the environment occur via complex networks, involving complicated interactions with other contaminants and with slow biodegradation and various physiological, chemical, and biological processes. Those processes can be used and modified in order to diminish their environmental concentration. There are more promising results of such activities performed by various researchers worldwide. Nevertheless, the still existing challenge is to develop a bioremediation strategy that involves and integrates different types of solutions, on the scale of the whole ecosystem, in order to optimize the effectiveness of pollutant removal from the environment. For POP stockpiles, the combustion technologies remain by far the most effective and economic method of removing them from the environment.

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