The mechanism for enhanced oxidation degradation of dioxin-like PCBs (PCB-77) in the atmosphere by the solvation effect
The reaction pathways of PCB-77 in the atmosphere with ·OH, O2, NOx, and 1O2 were inferred based on density functional theory calculations with the 6-31G* basis set. The structures the reactants, transition states, intermediates, and products were optimized. The energy barriers and reaction heats were obtained to determine the energetically favorable reaction pathways. To study the solvation effect, the energy barriers and reaction rates for PCB-77 with different polar and nonpolar solvents (cyclohexane, benzene, carbon tetrachloride, chloroform, acetone, dichloromethane, ethanol, methanol, acetonitrile, dimethylsulfoxide, and water) were calculated. The results showed that ·OH preferentially added to the C5 atom of PCB-77, which has no Cl atom substituent, to generate the intermediate IM5. This intermediate subsequently reacted with O2 via pathway A to generate IM5a, with an energy barrier of 7.27 kcal/mol and total reaction rate of 8.45 × 10−8 cm3/molecule s. Pathway B involved direct dehydrogenation of IM5 to produce the OH-PCBs intermediate IM5b, with an energy barrier of 28.49 kcal/mol and total reaction rate of 1.15 × 10−5 cm3/molecule s. The most likely degradation pathway of PCB-77 in the atmosphere is pathway A to produce IM5a. The solvation effect results showed that cyclohexane, carbon tetrachloride, and benzene could reduce the reaction energy barrier of pathway A. Among these solvents, the solvation effect of benzene was the largest, and could reduce the total reaction energy barrier by 25%. Cyclohexane, carbon tetrachloride, benzene, dichloromethane, acetone, and ethanol could increase the total reaction rate of pathway A. The increase in the reaction rate of pathway A with benzene was 8%. The effect of solvents on oxidative degradation of PCB-77 in the atmosphere is important.Graphical abstractThe reaction pathways of PCB-77 in the atmosphere with •OH, O2, NOx, and 1O2 were inferred based on density functional theory calculations with the 6-31G* basis set. Different polar and nonpolar solvents: cyclohexane, benzene, carbon tetrachloride, chloroform, acetone, dichloromethane, ethanol, methanol, acetonitrile, dimethylsulfoxide, and water were selected to study the solvation effect on the favorable reaction pathways. The investigated results showed what kind of pathway was most likely to occur and the solvent effect on the reaction pathway
<!>The pollution influence of PCBs<!>The industrial recycling options of PCBs<!>The atmospheric degradation research status of PCBs<!>Research content<!>Computational methods<!><!>Addition pathway of intermediate IM5b with singlet oxygen (1O2)<!>
<p>The reaction pathways of PCB-77 in the atmosphere with •OH, O2, NOx, and 1O2 were inferred based on density functional theory calculations with the 6-31G* basis set. Different polar and nonpolar solvents: cyclohexane, benzene, carbon tetrachloride, chloroform, acetone, dichloromethane, ethanol, methanol, acetonitrile, dimethylsulfoxide, and water were selected to study the solvation effect on the favorable reaction pathways. The investigated results showed what kind of pathway was most likely to occur and the solvent effect on the reaction pathway</p><!><p>Polychlorinated biphenyls (PCBs) have been widely used as flame retardants, dielectric and heat transfer fluids, and plasticizers in industrial and household products [1]. As typical chlorinated organic pollutants, large quantities of PCB byproducts are released into the environment causing serious pollution and adverse impacts on ecosystems [2, 3]. Because of their volatility, persistence, bioaccumulation, and high toxicity, they have been listed by the United Nations Environment Programme as one of 12 kinds of persistent organic pollutants that need to be controlled [4]. The physical and chemical properties of PCBs are very stable. Once PCBs come into the environment, they remain in the environment for a long time and are very difficult to degrade (half-life is about 40 years) [5]. PCBs are not degraded by hydrolysis or similar reactions in the environment at a marked rate, and only a small fraction of PCBs are converted by photolysis in the soil and by photolysis of sunlight and ultraviolet light. Contaminated water and soil are also difficult to recover and have high residual in the environment. Under the stress of PCBs, the whole community structure of soil bacteria will change, which leads to the change of soil ecological environment quality [6]. Because of its lipophilic and hydrophobic characteristics, its long-term accumulation and the high bioaccumulation of some isoforms and congeners, PCBs have strong accumulation in the organism and are gradually enriched through the food chain. If the water contains 0.01 μg/L PCBs, its accumulation in the fish can reach 20 × 104 times of the concentration of water, and which in the body of birds and animals who eat fish will be higher. Some big fish in the sea and fierce birds in the air, such as sharks, seals and raptors, the concentration in their body can be 107–108 times higher than the surrounding environment [7]. The characteristics of the PCBs enable them to be migrated remotely and can be dispersed globally through the pathway of volatiles—atmospheric transport—sedimentation. From Antarctic penguins to the Arctic seals, they have been detected PCBs in their bodies. PCBs pollution has become a global problem. Related toxicological studies have shown that PCBs have an induced effect on lesions, or even concretization, of skin, liver, gastrointestinal system, nervous system, reproductive system, and immune system. Some homologues affect the reproduction of mammals and birds, and are potentially carcinogenic to human health. They are prone to accumulate in adipose tissue, causing brain, skin and visceral diseases and affect the nervous, reproductive and immune systems [8].</p><!><p>The PCBs are the platform upon which microelectronic components such as semiconductor chips and capacitors are mounted. PCBs provide the electrical interconnections between components and are found in virtually all electrical and electronic equipment. Every year, 20–50 million tons of waste electrical and electronic equipment (WEEE) are generated worldwide, which could bring large amount of waste PCBs. Recycling of waste PCBs is an important subject not only from the treatment for waste but also from the recovery of precious metal, The typical metals in PCBs consist of copper, iron, tin, nickel, lead, zinc, silver, gold, and palladium [9]. Due to its complex composition, PCBs recycling requires a multidisciplinary approach intended to separate fibers, metals and plastic fractions and reduce environmental pollution. Recycling process for waste PCBs includes three processes which is pretreatment, physical recycling, and chemical recycling. PCBs recycling generally start from the pretreatment stage, which include disassembly of the reusable and toxic parts and then PCBs are treated using physical recycling or chemical recycling process [10]. In general, the recycling methods of waste PCBs can be summarized as physical recycling methods and chemical recycling methods. Physical processing for the separating the metal fraction and non-metal fraction from waste PCBs includes shape separation, magnetic separation, electric conductivity-based separation, density-based separation and corona electrostatic separation. The chemical recycling methods include pyrolysis, gasification and combustion. Metal fraction can be treated by pyrometallurgical, hydrometallurgical or biotechnological process [11].</p><p>Industrial recovery of PCBs is indeed an environmentally friendly technology and 85% of the PCBs wastes produced by industry are to be recycled [12]. Furthermore, Large amount of the PCBs wastes were untreated wastes and exposed to wind and rain, easy to evaporate into the atmosphere or deposited into the soil, which can pose a serious threat to local water, air environment and human health, and lead to PCBs are not easy to be recycled.</p><!><p>Pathways for transformation and removal of PCBs in the environment are a hot topic. Because PCBs degrade very slowly, they are now ubiquitous in air, water, soil, sediment, and biota. During transportation, PCBs can be removed and transformed through photolysis, wet and dry deposition, and chemical reactions with ·OH, NO3, Cl radicals, and O3. The radical ·OH is considered to be the main initiator for the removal of volatile organic compounds in the atmosphere [13–15]. The reaction with ·OH is considered the dominant removal pathway of PCBs from the atmosphere [16]. Rate constants for the gas phase reactions of 14 PCB congeners with ·OH have been measured from 323 to 363 K, the experimental data suggested that the more highly chlorinated PCBs would become progressively less reactive with OH radicals, which would increase their lifetimes compared to lower chlorinated PCBs [17]. Using the MPWB1K functional, Sun et al. [18] investigated the degradation process of PCB-47 with oxygen and nitrogen oxides, and found that the main degradation product of PCB-47 was glyoxal, the reaction rate with ·OH was 1.27 × 10−12 cm3/molecule s, and the half-life was 9.1 days. Lee et al. [19] studied the mechanisms for the formation of polychlorinated dibenzodioxin–OH adducts using density functional theory (DFT), and found that carbon atoms connected to oxygen atoms were the main sites of ·OH addition. Altarawneh et al. studied the atmospheric degradation of polychlorinated dibenzofurans initiated by ·OH addition and found a reaction rate of 2.70 × 10−11 cm3/molecule s. They suggested that the ·OH adduct immediately reacted with O2 to generate polychlorinated dibenzofuran–OH–O2 adducts [20]. Because of the reactivity of ·OH, most studies on the reactions of persistent organic pollutants with free radicals or reactive molecules in the atmosphere have focused on ·OH [21]. Because of the complexity of environmental conditions, it is difficult to use computational and theoretical chemistry to accurately evaluate environmental effects, and particularly the effect of solvents. More complete reaction mechanisms for oxidative degradation induced by ·OH, and studies of the effects of different solvents on the reaction mechanisms are required.</p><!><p>Quantitative structure activity relationship studies have shown that PCBs with meta- and para-chlorine substituents are the most similar to dioxins among the PCBs. Toxicological studies have shown that PCBs that are not coplanar or ortho-substituted, such as the 14 dioxin like PCBs (congener numbers 77, 81, 105, 114, 118, 123, 126, 156, 157, 169, 170, 180, and 189) have high reactivity and toxicity [22]. In the present study, PCB-77 (3,3′,4,4′-Tetrachlorobiphenyl), which has two meta- and two para-chlorine substituents, was selected for study by quantum chemistry calculations using DFT. The results were used to predict the degradation reactions of PCBs with the common oxidants ·OH, O2, NOx, and 1O2. The intermediates and transition states in the reactions were predicted, and the rate constants were calculated by transition state theory (TST). The effects of different solvents on the reaction pathways were studied based on the solvation effect.</p><!><p>The Gaussian 09 program [23] was used to perform all calculations. The degradation reactions of PCB-77 were optimized for all reactants, transition states, intermediates, and products. The optimal structure was obtained using DFT at the B3LYP/6-31G*level. This method has yielded satisfying results in previous research [24, 25]. The vibrational frequencies were calculated at the same level, and showed that the transition state has only one imaginary frequency, and the intermediate for each transition state has no imaginary frequency. The intrinsic reaction coordinate was calculated to identify the connections between reactants, transition states, and products. To obtain more accurate Gibbs free energies, the basis set of 6-311G(2df, p) was used with the same method to calculate the single point energies based on the optimal material configuration, taking into consideration the zero point energy correction. The reaction rates at 298 K and standard atmospheric pressure were calculated using transition state theory (TST), taking into consideration the tunneling effect, with KiSTheIP software [26]. The polarizable continuum model (PCM) was used to optimize the reaction parameters of the intermediates and transition states of PCB-77 degradation, and analyze the effect of solvation. PCM is widely used in studies of the solvation effect as the classical quantum chemical calculation method [27]. Many studies show that the results of PCM can be directly compared with the outcome of the experimental measurements,and the results are in good agreement with the experimental values [28–30]. To eliminate systematic error between the theoretical and experimental values, the calculations were multiplied by a calibration factor of 0.960 [31].</p><!><p>Optimized structure of PCB-77</p><p>The reaction pathway of PCB-77 with ·OH (a for the addition reaction rath, b for the H extraction reactionpath) [∆E (kcal/mol): energy barrier, ∆H (kcal/mol): reaction heats]</p><p>The optimization structure of transition state of PCB-77 with ·OH</p><p>Two stage reaction pathway of intermediate IM5 (a), reaction pathway of intermediate IM5a with NO (b), and the transition state structure of IM5a with O2/NO (c). [∆E (kcal/mol): energy barrier, ∆H (kcal/mol): reaction heats]</p><p>Addition reaction pathway of intermediate IM8 with ·OH, O2, NO2 (a) and the transition state structure of IM8a with NO (b). [∆E (kcal/mol): energy barrier, ∆H (kcal/mol): reaction heats]</p><!><p>Reactions of IM5 and IM8 in the atmosphere can generate IM5b, which is hydroxylated PCBs. OH-PCBs have been widely detected in the abiotic environment in air, rain, and snow. These compounds have potential estrogenic activity, thyroid effects in vitro and in vivo, and are toxic [37, 38]. 1O2 is an important reactive oxygen species. The electron configuration of its molecular orbital is different from ground state oxygen. 1O2 is a form of excited state molecular oxygen, and can easily react with unsaturated compounds because of its high activity. Studies have found that chlorophenol and 1O2 reactions are the main contributors to decomposition [39]. We deduced an addition reaction pathway of IM5b with 1O2. Generally, the following four principal types of oxygen addition reactions that produce aromatic and unsaturated compounds are recognized [40]: (1) 1,3-addition to a double bond connected to a hydrogen-carrying group to form allylic hydroperoxides; (2) 1,4-cycloaddition to a system containing at least two conjugated double bonds to form 1,4-peroxides; (3) [p2 + p2] 1,2-cycloaddition to an isolated double bond to form 1,2-peroxides; and (4) 1,4-addition to phenols and naphthols to form hydroperoxide ketones.</p><!><p>Twelve reaction pathways of IM5b with 1O2</p><p>Gibbs free energy change (∆G) of twelve reaction pathways unit: Kcal/mol</p><p>Reaction energy barrier of IM5b and 1O2 in twelve possible reaction pathways unit: Kcal/mol</p><p>Open loop reaction pathway of intermediate a2 (a) and the transition state structure (b). [∆E (kcal/mol): energy barrier, ∆H (kcal/mol): reaction heats]</p><p>The energy barrier of the reaction path A</p><p>Degradation reaction energy barrier for path A under different solvents unit: Kcal/mol</p><p>The degradation rates (k, cm3/molecule s) for pathway A at 298 K and standard atmospheric pressure, and the Arrhenius equation at 238–357 K (gas phase)</p><p>Degradation Rates (k, cm3/molecule s) for Pathway A in Different Solvents at 298 K and Standard Atmospheric Pressure</p><p>PCB-77 can react with ·OH in ·OH addition and H abstraction reactions in the atmosphere, and ·OH addition is dominant. The ·OH is added to the carbon atoms without substituents. After the addition reaction with ·OH, the reaction can diverge into pathway A or B. The energy barrier of pathway A is lower than that of pathway B, and it is more likely to occur. Most of the pathways in the reaction will produce OH-PCBs, and the OH-PCBs can easily react with singlet oxygen (1O2). This reaction occurs via one of the three types (a, c, and d) of the four principal types of oxygen addition reactions to aromatic and unsaturated compounds.</p><p>In the atmosphere, the reaction rates of each step of pathway A increase with increasing temperature, and the final step of the whole reaction process is conversion of IM5a2 to IM5a4.</p><p>The reaction energy barrier and reaction rate of optimal pathway A are affected by changes in the solvent. Among the investigated solvents, cyclohexane, carbon tetrachloride, and benzene can reduce the reaction energy barrier, and cyclohexane, carbon tetrachloride, benzene, dichloromethane, acetone, and ethanol can improve the reaction rate. The largest reduction in the energy barrier and increase in the reaction rate were obtained with benzene.</p>