Summary of experimental conditions and main findings of studies on hydrodechlorination of PCBs by bimetallic systems. Bimetallic systems have several potential benefits. They have induced physicochemical properties, such as catalytic performance Kang et al. Magnetic metals in combination with noble metals form bimetallic systems which exhibit irreplaceable advantages in pollutions remediation, since noble metals [e.
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Additionally, two active metals of a bimetallic system can enhance catalytic activity thus treat PCBs and other chlorinated organic compounds in short times with no specialized laboratory equipment Sakrittichai, Bimetallic systems also have distinct drawbacks. Some magnetic metals such as cobalt Co , Ni, and noble metals such as Ag, Au, Pt, are too costly to be used in large commercial operations. Moreover, the reaction rate will significantly reduce, when the passivation of metals carry on during the remediation process Doong and Wu, More generally, the zero-valent metals involved in hydrodehalogenation of PCBs by bimetallic systems such as iron, zinc or magnesium can generate partially dehalogenated products that may be more toxic than the previous target compounds transformation of target compounds to phenol Grittini et al.
The outside ion of nZVI particles can react with water and oxygen to form an outer hydr oxide layer in aqueous environments Nurmi et al. As a result, this outer oxide layer allows electron transfer from the metal through the oxide conduction band or localized band. Furthermore, the outer oxide layer could serve as an adsorbent for PCBs. In , Schreier and Reinhard reported dehalogenation of alkyl halides RX by Fe powder in oxygen-free and buffered water, previously a method with difficult to predict outcomes. Since then, a three step mechanisms was proposed to explain the dehalogenation process of alkyl halides Matheson and Tratnyek, On the basis of the proposed mechanism, nZVI with a large surface area has been successfully used to achieve the dehalogenation process in water or PCB-contaminated soil in the presence of catalysts.
A study conducted by Gardner et al. Long et al. The nZVI can provide an appropriate pH 8—9 and reduce volatile fatty acid inhibition thereby stimulating the growth of microorganisms and by generating hydrogen gas via corrosion of nZVI, thereby enhance dehalogenation. Liu et al. They found that In contrast, Figure 2.
Proposed mechanisms for abiotic dechlorination of PCBs: Electron transfers from Fe 0 to monochlorobiphenyl Gomes et al. NZVI-remediation could offer advantages as compared to other in situ remediation technologies. For example, nZVI has been used to remediate PCBs as an advanced technology in the environment due to its catalytic properties and high degradation performance Ahmadi et al. A large surface area of nZVI particles can increase reactivity and, therefore, results in high speed contaminant degradation Nurmi et al.
Additionally, nZVI can be used in recalcitrant contaminated water such as deep contaminated ground water since nZVI can suspend in water for a longer time due to its small size Singh et al. However, the application of nZVI is still controversial. To date, the nZVI technologies in practice are mostly the application of chlorinated solvents in aquifers and most cases of nZVI applications are lab scale and have taken place in North America in comparison to the few field applications in Europe e.
As a consequence, the particle surface reactivity will be limited Lee and Jou, In addition, the nZVI particles will undesirably agglomerate and convert to large particles because the most reactive particles agglomerate more readily Bardos et al. The toxicological effect of nZVI particles are usually related to their nano-scale size. The nZVI used in remediation are typically 10— nm as aforementioned and the unique toxicity of nZVI has been only observed in nanoparticles smaller than 30 nm Bardos et al.
Some studies reported the toxicity of nZVI is specific to bacteria and biota such as collembolan srpingtails and lumbricina earthworms Bardos et al. In the early s, it was found that biofilm covered activated carbon particles can efficiently remove organic pollutants Dussert and Tramposch, Since then, the combination of bacterial biofilm and activated carbon has been widely used as a practiced treatment method for wastewater treatment, post-treatment, potable water purification, and organic contaminants removal Tammaro et al.
Kolb and Wilderer used an integrated activated carbon adsorption and biodegradation system to reduce the concentration of critical components e. Islam et al. Such successful attempts have led to the recognition of the combination of bacterial biofilm and activated carbon as a viable remediation technology for PCB-contaminated sediment as suggested by the USEPA. Some preliminary research of PCB-contaminated sediments treated with granular activated carbon GAC have been conducted.
A bench-scale study on bioaugmentation of PCBs in aqueous wastes conducted by Ghosh et al. Sediment treated with 3. These studies showed that the indigenous bacteria found in sediment were capable of PCB dehalogenation. However, microbial degradation of PCBs are limited in performance due to the relatively low abundance of PCB-dechlorinating microorganisms and low bioavailability of PCBs, especially for in-situ microbial degradation Kjellerup and Edwards, There was an insignificant decrease in the overall concentration of parent PCB congeners.
Tri-, tetra-, and penta-chlorinated congener concentrations were still relatively high, while mono- and di- chlorinated congeners were dominant in the presence of activated carbon Kjellerup et al.
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Therefore, a combination of adsorbent sequestration and bioaugmentation through biofilm covered activated carbon systems is proposed to enhance the biodegradation of low concentration PCBs in sediment. An adsorbent e. Such a combination of PCB-dechlorinating microbial communities in the form of biofilms on surfaces of activated carbon provide several advantages. A compact space between biofilms with large cell density and activated carbon surface could allow microorganisms to utilize PCBs as an electron acceptor thereby enabling subsequent degradation.
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One drawback of biofilm covered activated carbon system is that some slow growing PCB dechlorinating microorganisms might be bloom-growing thus affected by other easily-degradable organic compounds or pollutants Abromaitis et al. For example, some researchers pointed out that the concentration of persistent organic compounds e. Soil and aquatic environments e. More advanced alternatives to unsustainable PCB remediation techniques have been sought. Electrokinetic remediation can be conducted in-situ. This generally includes an external, direct current source, and an anode and cathode immersed in the electrolytic solution.
The organic pollutants driven by ionic migration and electrophoresis will migrate to the electrodes when the direct current is applied. Other technologies e. Fan et al. Electrodialytic remediation combines electrodialysis with the electrokinetic movement of ions to remove heavy metals Ottosen et al. Two surfactants saponin and Tween 80 were involved in this study to enhance PCB desorption and removal from polluted soil.
Chun et al. The effectiveness of nZVI based reductive dehalogenation described in this paper are quite specific. One major concern in nZVI based reductive dehalogenation processes is the reduction of surface reactivity due to iron oxide formation on the surface of the nanoparticles.
As aforementioned, combining two or several different remediation technologies as a potentially viable alternative for PCBs remediation can overcome the deficiencies associated with individual treatment methods Naddeo et al. Furthermore, optimization of PCBs remediation efficiency can be achieved within appropriate costs. In recent years, the effectiveness of nZVI has been proven in the treatment of a wide variety of contaminants such as chlorinated methanes, brominated methanes, trihalomethanes, chlorinated ethenes, chlorinated benzenes, and other polychlorinated hydrocarbons Nowack, Regarding nZVI particles coupled with electrokinetic remediation discussed above, some studies indicated that nZVI particles in combination with a second metal e.
Metal-covered nZVI particles can effectively reduce the activation energy of the pollutants and increase the reaction rate of dechlorinaton reactions Liou et al. Additionally, metal-covered nZVI particles can rapidly achieve dehalogenation through increasing the particle surface area and surface activity Clark et al. Zhuang et al. Le et al. After nano-bio treatment, the toxicity of the residual PCBs in terms of toxic equivalent values decreased from Four criteria including the cost, removal efficiency, time duration, and environmental burdens were provided to determine a comprehensive framework for PCB remediation strategies and the combinations of these technologies.
More importantly, most of these technologies does not generate toxic byproducts except the supercritical water oxidation and bimetallic systems. The generated byproducts from these technologies are the major drawbacks that could be released to the environment with negative impacts. However, these technologies usually require excessive costs. Therefore, any efforts relative to the implementation of these technologies to the contaminated filed sites or large commercial operations will encounter with challenges. They usually cannot immediately apply to the PCB sources due to requiring a long remediation and monitoring time as compared to the technologies mentioned above.
Even though phytoremediation and microbial degradation have a limited remediation efficiency with a long-term remediation time, the major advantage is the relatively low implementation costs. Therefore, they could be applied to an in-situ contamination site as a full-scale application. Activated carbon and the biofilm covered activated carbon obtained the highest scores according to this table.
A low cost and relatively high remediation efficiency allow them to be applied to either in-situ or ex-situ PCBs remediation.
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A compact space between biofilms and activated carbon surface could allow bacteria to efficiently attack PCB molecular as an electron acceptor thereby. However, it should be noted that some studies on PCBs remediation by a combination of anaerobic biofilm and activated carbon involved in this paper were conducted under a strictly experimental condition e.
A successful treatment of PCBs not only depends on its remediation efficiency, it is also need to consider the cost, time duration, and environmental burdens. On the basis of the discussion of this paper, therefore, the future vision of PCBs remediation could be a comprehensive treatment based on the biofilm covered activated carbon particles as a microbial inoculum delivery system.
PCBs are very toxic chemical compounds. Thus, a comprehensive assessment of the environmental impacts of PCB destruction is an important criterion for selection of PCB remediation technologies.
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The comprehensive assessment of PCB formation and destruction are only available for the incineration process Buekens and Huang, ; Weber, For instance, the destruction efficiency of a wastewater treatment facility could be difficult to evaluate due to low concentration levels in the effluent streams. This would require continuous PCB sampling and long-term data monitoring and collection such as time series analysis on the basis of PCB emission. Therefore, the assessment of the PCB destruction technologies should include two essential parts: 1 assessment of destruction efficiencies for in-situ and ex-situ PCB remediation technologies and the environmental impacts or risks associated with the implementation and operation of the respective technology; 2 continuous PCB sampling and long-term data monitoring and collection in order to assess the reliability of the proposed PCB remediation technology.
This paper reviewed the treatment strategies for PCBs remediation. Therefore, they are not appropriate for an in-situ contamination site as a full-scale application. Activated carbon and the biofilm covered activated carbon approaches obtained the highest scores according to this table as compared to the technologies mentioned above. PCBs are complex chemicals, so knowledge of their chemical and physical properties is important to better understand their transport and fate thereby for selecting appropriate remediation approaches.
The possibility of PCB remediation by using multiple technologies discussed in this paper need more data and pilot scale experiments in order to evaluate the effectiveness. The future vision of PCB remediation could be a comprehensive treatment solution. Because successful treatment of PCBs not only depends on the appreciated selection of the most effective remediation technology, it is also needs to consider the public acceptance and environmental and human health impacts of the remediation technology, neither of which have been achieved.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Abramowicz, D.
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Abromaitis, V. Biodegradation of persistent organics can overcome adsorption—desorption hysteresis in biological activated carbon systems. Chemosphere , — doi: Acar, Y. Principles of electrokinetic remediation Environ. CrossRef Full Text. Agarwal, S.