**6. Degradation pathways of POPs in the environment**

Despite the fact POPs are resistant to most of the degradation processes in the environment, some molecular alterations are possible in the environment that does not necessarily lead to simpler and less toxic compounds. Some of POP metabolites are equally complex and even more toxic than the parent molecules. Most of the degradation processes of POPs in the environment are assisted by microorganisms. Nevertheless, the half-lives of biodegradation processes of POPs are significantly long, thus accounting for their persistency in the environment. Some representative examples of POP degradations are as presented below.

### **6.1. Degradation of heptachlor**

Heptachlor is known to undergo oxidative dechlorinating in the soil to form 1-hydroxychlordene and by the influence of soil microorganisms to form heptachlor epoxide with a half-life of up to 112 days [15] as shown in **Scheme 1**.

#### **6.2. Degradation of DDT**

*p,p'*-DDT can be broken down in the soil by *Enterobacter aerogenes* microorganisms in the presence of UV light and/or iron catalyst to reduced dehydrochlorinated compounds, DDE, and DDD as well as oxidized derivatives which ultimately form *p,p'*-dichlorobenzophenone (**Scheme 2**). *o,p'*-DDT degrades in the same way as *p,p'*-DDT [16].

#### **6.3. Degradation of endosulfan**

The technical grade of endosulfan consists of two isomers, alpha and beta in the ratio of 70:30. In the environment, endosulfan is photolytically degraded to yield endosulfan sulfate in soil and endosulfan diol in aquatic environment. Endosulfan sulfate is equally toxic as the parent molecule (**Scheme 3**) [16].

#### **6.4. Degradation of hexachlorobenzene**

In anaerobic condition, biodegradation of HCB in an arable soil takes place with several dechlorination steps, indicating the following main HCB transformation pathways: HCB→pentachlorobenzene (QCB) → 1,2,3,5-tetrachlorobenzene (TeCB) → 1,3,5-TCB → 1,3-dichlorobenzene(DCB), with 1,3,5-TCB as the main intermediate dechlorination product [17] (**Scheme 4**).

**6.5. Degradation of aldrin/dieldrin**

**Scheme 2.** Degradation of *p,p'*-DDT in the environment.

**Scheme 1.** Degradation pathways of heptachlor in the soil.


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diol by two enzyme systems is present in excised roots. The enzymatic oxidation of aldrin in plants is known to be more species specific than the oxidation by microorganisms [10]

The NADPH2

(**Scheme 5**).

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**Scheme 1.** Degradation pathways of heptachlor in the soil.

The slow decomposition of PCDDs/PCDFs and PCBs in the environment and the hazards they pose for living organisms make them large-scale environmental degraders, especially because their toxicity can be further enhanced by their ability to accumulate in the soil and sediments and their bioaccumulation and biomagnification within aquatic and land food

Despite the fact POPs are resistant to most of the degradation processes in the environment, some molecular alterations are possible in the environment that does not necessarily lead to simpler and less toxic compounds. Some of POP metabolites are equally complex and even more toxic than the parent molecules. Most of the degradation processes of POPs in the environment are assisted by microorganisms. Nevertheless, the half-lives of biodegradation processes of POPs are significantly long, thus accounting for their persistency in the environ-

Heptachlor is known to undergo oxidative dechlorinating in the soil to form 1-hydroxychlordene and by the influence of soil microorganisms to form heptachlor epoxide with a half-life

*p,p'*-DDT can be broken down in the soil by *Enterobacter aerogenes* microorganisms in the presence of UV light and/or iron catalyst to reduced dehydrochlorinated compounds, DDE, and DDD as well as oxidized derivatives which ultimately form *p,p'*-dichlorobenzophenone

The technical grade of endosulfan consists of two isomers, alpha and beta in the ratio of 70:30. In the environment, endosulfan is photolytically degraded to yield endosulfan sulfate in soil and endosulfan diol in aquatic environment. Endosulfan sulfate is equally toxic as the parent

In anaerobic condition, biodegradation of HCB in an arable soil takes place with several dechlorination steps, indicating the following main HCB transformation pathways: HCB→pentachlorobenzene (QCB) → 1,2,3,5-tetrachlorobenzene (TeCB) → 1,3,5-TCB → 1,3-dichlorobenzene(DCB),

with 1,3,5-TCB as the main intermediate dechlorination product [17] (**Scheme 4**).

ment. Some representative examples of POP degradations are as presented below.

**6. Degradation pathways of POPs in the environment**

(**Scheme 2**). *o,p'*-DDT degrades in the same way as *p,p'*-DDT [16].

chains (**Figure 3**) [14].

22 Persistent Organic Pollutants

**6.1. Degradation of heptachlor**

**6.2. Degradation of DDT**

**6.3. Degradation of endosulfan**

**6.4. Degradation of hexachlorobenzene**

molecule (**Scheme 3**) [16].

of up to 112 days [15] as shown in **Scheme 1**.

**Scheme 2.** Degradation of *p,p'*-DDT in the environment.

#### **6.5. Degradation of aldrin/dieldrin**

The NADPH2 -dependent enzymatic oxidation of aldrin to dieldrin and aldrin to aldrindiol by two enzyme systems is present in excised roots. The enzymatic oxidation of aldrin in plants is known to be more species specific than the oxidation by microorganisms [10] (**Scheme 5**).

**Scheme 3.** Degradation of endosulfan in the environment.

undergo the biphenyl degradation pathway (BP pathway) to be degraded to accessible carbon

(BphA, B, C, D, E, F, G) to convert biphenyl to TCA cycle intermediates (pyruvate and Acyl-CoA) and benzoate. However, there are few microorganisms that can dechlorinate substrate under natural conditions. Even with selective media, the accumulation of PCB-dechlorinating microorganisms is still slow, which is one reason for the slow degradation rate. As a result, PCBs usually go through a co-metabolism pathway that involves different microorganism

It has been well reported that several species of strictly or facultatively anaerobic bacteria are capable of de-halogenating chlorinated aliphatic and aromatic compounds. Some of these dehalogenation processes have been shown to couple to ATP synthesis via a chemiosmotic mechanism. The reductive dehalogenation linked to energy conservation is called "halorespiration" or "dehalorespiration." For example, a sulfate-reducing bacterium, *Desulfomonile tiedjei* strain DCB-1, has been shown to conserve energy for growth from reductive dehalogenation of 3-chlorobenzoate by an uncharacterized chemiosmotic process. Bacterial dehalorespiration with tri- or tetrachlorinated benzene as a terminal electron acceptor is also known to take place. Gibbs free energy of formation of various PCDDs/PCDFs and redox potentials for PCDD/PCDF substrate/product couples indicate that the reductive dehalogenation of PCDDs/PCDFs is an exergonic reaction, and this implies that microorganisms acquire energy

via anaerobic electron transport with PCDDs/PCDFs as terminal electron acceptors.

Despite its physiological and ecological importance, the biological reductive dehalogenation of PCDDs/PCDFs as well as of PCBs has been studied much less than the aerobic biodegradation of dioxin-related compounds. This biological reaction has so far been studied mostly in sediments, sludge, and soils containing anaerobic microbial consortia. Earlier work related to this subject showed changes in PCDD-/PCDF-congener distribution patterns and the resultant

in the aerobic environment. BP pathway is a pathway that utilizes series of enzymes

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or CO2

species [7, 14, 17, 25] (**Scheme 7**).

**Scheme 6.** Degradation of lindane in the environment.

**6.8. Degradation of dioxins by reductive dehalogenation**

**Scheme 4.** Degradation of hexachlorobenzene.

**Scheme 5.** Degradation of aldrin in the environment.

#### **6.6. Degradation of lindane**

Dehydrohalogenation of lindane to γ-hexachlorocyclohexane takes place in moist soil and is attributed to the soil microorganisms such as *Bacillus coli* and *Clostridium sporogenes*. Also these bacteria produced trace amount of benzene and monochlorobenzene from lindane (**Scheme 6**) [18].

#### **6.7. Degradation of PCBs**

Various microorganisms are involved in a two-stage process of degradation of PCBs, which happens in aerobic and anaerobic environments. Degrading PCBs is similar to the degradation of biphenyl. However, the chlorines on PCBs prevent them from being utilized as a substrate of biphenyl degradation. Due to high chemical stability, PCBs cannot be used as energy sources. However, due to the chlorination, PCBs can be used as electron acceptors in anaerobic respiration to store energy, which is also the first stage of the degradation pathway, reductive dechlorination. Once the PCBs are dechlorinated to a certain degree, usually lower than five chlorines presenting in the structure and one aromatic ring has no chlorine, they can Degradation Pathways of Persistent Organic Pollutants (POPs) in the Environment http://dx.doi.org/10.5772/intechopen.79645 25

**Scheme 6.** Degradation of lindane in the environment.

**6.6. Degradation of lindane**

**Scheme 5.** Degradation of aldrin in the environment.

**Scheme 4.** Degradation of hexachlorobenzene.

**Scheme 3.** Degradation of endosulfan in the environment.

24 Persistent Organic Pollutants

**6.7. Degradation of PCBs**

(**Scheme 6**) [18].

Dehydrohalogenation of lindane to γ-hexachlorocyclohexane takes place in moist soil and is attributed to the soil microorganisms such as *Bacillus coli* and *Clostridium sporogenes*. Also these bacteria produced trace amount of benzene and monochlorobenzene from lindane

Various microorganisms are involved in a two-stage process of degradation of PCBs, which happens in aerobic and anaerobic environments. Degrading PCBs is similar to the degradation of biphenyl. However, the chlorines on PCBs prevent them from being utilized as a substrate of biphenyl degradation. Due to high chemical stability, PCBs cannot be used as energy sources. However, due to the chlorination, PCBs can be used as electron acceptors in anaerobic respiration to store energy, which is also the first stage of the degradation pathway, reductive dechlorination. Once the PCBs are dechlorinated to a certain degree, usually lower than five chlorines presenting in the structure and one aromatic ring has no chlorine, they can undergo the biphenyl degradation pathway (BP pathway) to be degraded to accessible carbon or CO2 in the aerobic environment. BP pathway is a pathway that utilizes series of enzymes (BphA, B, C, D, E, F, G) to convert biphenyl to TCA cycle intermediates (pyruvate and Acyl-CoA) and benzoate. However, there are few microorganisms that can dechlorinate substrate under natural conditions. Even with selective media, the accumulation of PCB-dechlorinating microorganisms is still slow, which is one reason for the slow degradation rate. As a result, PCBs usually go through a co-metabolism pathway that involves different microorganism species [7, 14, 17, 25] (**Scheme 7**).

#### **6.8. Degradation of dioxins by reductive dehalogenation**

It has been well reported that several species of strictly or facultatively anaerobic bacteria are capable of de-halogenating chlorinated aliphatic and aromatic compounds. Some of these dehalogenation processes have been shown to couple to ATP synthesis via a chemiosmotic mechanism. The reductive dehalogenation linked to energy conservation is called "halorespiration" or "dehalorespiration." For example, a sulfate-reducing bacterium, *Desulfomonile tiedjei* strain DCB-1, has been shown to conserve energy for growth from reductive dehalogenation of 3-chlorobenzoate by an uncharacterized chemiosmotic process. Bacterial dehalorespiration with tri- or tetrachlorinated benzene as a terminal electron acceptor is also known to take place. Gibbs free energy of formation of various PCDDs/PCDFs and redox potentials for PCDD/PCDF substrate/product couples indicate that the reductive dehalogenation of PCDDs/PCDFs is an exergonic reaction, and this implies that microorganisms acquire energy via anaerobic electron transport with PCDDs/PCDFs as terminal electron acceptors.

Despite its physiological and ecological importance, the biological reductive dehalogenation of PCDDs/PCDFs as well as of PCBs has been studied much less than the aerobic biodegradation of dioxin-related compounds. This biological reaction has so far been studied mostly in sediments, sludge, and soils containing anaerobic microbial consortia. Earlier work related to this subject showed changes in PCDD-/PCDF-congener distribution patterns and the resultant 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 are not dehalogenated further [14, 19–21] (**Scheme 8**).

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

destroyed or irreversibly transformed so it is no longer a POP.

incinerators are designed to satisfy the required emissions levels.

technology and non-combustion technology.

mercial operation history.

of POPs [20, 22–24].

**8. Conclusion**

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

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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

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

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 com-

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

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,

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

**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.
