Biodegradation Mechanism and Technology of Polycyclic Aromatic Hydrocarbons

DIAO+Shuo　WANG+Hong-qi　ZHENG+Yi-nan　HUA+Fei

【Abstract】Polycyclic aromatic hydrocarbons are a class of potentially hazardous chemicals of environmental and health concern.PAHs are one of the most prevalent groups of contaminants found in soil.Biodegradation of complex hydrocarbon usually requires the cooperation of more than single specie.This paper reviews the existing screening methods of PAH-degrading bacteria.It studied the mechanism and technical applications of the co-metabolism in PAHs.Author gives the suggestions and prospects in Biodegradable trend of PHAs.

【Key words】PHAs；Biodegradable；Co-metabolism

1 Introduction

Polycyclic aromatic hydrocarbons （PAHs） are a group of persistent organic compounds composed of two or more fused aromatic rings with low water solubility.PAHs comprise a large and heterogeneous group of organic contaminants which are formed and emitted as a result of the incomplete combustion of organic material（Muller，H.，Breure，A.M.，Rulkens，W.H.，2004）.Anthropogenic contamination sources，such as road traffic and combustion of fossil fuels predominate，but there are also natural sources，e.g.volcanic eruptions and forest fires（Lee，P.K.，Ong，S.K.， Golchin，J.，Nelson，G.L.，2001；hodadoust，A.P.，Bagchi，R.，Suidan，M.T.Brenner R.C.，Sellers，N.G.，2000）.

Due to their toxic，mutagenic and carcinogenic properties（Menzie， 1992），16 PAHs have been listed as priority pollutants by US Environmental Protection Agency（EPA）.The 16 regulated PAH comprise both low and high molecular weight species，and seven of them are designated as known human carcinogens.All of the carcinogenic PAH （cPAH） are high molecular weight compounds（Amellal，N.，Portal，J.M.， Berthelin J.，2001；.Michael，D.，Aitken，Shu-Hwa Chen， Chikoma Kazunga， Randall，B.，Marx，2004）

Due to the stable nature of PAHs，it is very slow to be degraded by natural conditions.Recently，bioremediation，which is an economic and efficient alternative method to other remediation processes such as chemical or physical ones，has been developed as a environment clean-up technique（O.Potin，E.Veignie，C.Rafin，2004；B.Antizar-Ladislao，J.Lopez-Real，A.J.Beck，2004）.It has many advantages in economic，safe and residue.

It has three aspects main research in application of bioremediation：screening of degradation strains，degradation mechanism research，and application technology.

2 Screening of Degradation Strains

Currently，the microbes，which have the ability to degrade hydrocarbons，have been found in more than 70 genera and 200 species.The vast majority of them are bacterium and fungus.

Early research started in the microbial degradation of PAH naphthalene.Many microorganisms，which can degrade naphthalene，have been found.But according to the study，the ability of degrading PAHs with different bacteria is different.Such as the Pseudomonas have a wide range and various ability in degradation of PAHs.

We need to filter and determine the superiority strain of PAHs degradation in the process of PAHs biodegradation，then applied in the repair of PAHs pollution.

The main screening process is：

1）Enrichment and separation of strains.Using a certain kind of PAHs（such as naphthalene， etc.），as the sole carbon and energy sources in a dark room within the inorganic salt medium culture avoid photolysis. Usually，we use HPLC method for determination of degradation efficiency.

2）Species identification was done using 16SrDNA classification by morphological observation，hydrogen peroxide reaction， gram staining and oxidation reaction.

3 Degradation Mechanism and Application Technology

3.1 Metabolic Mechanism

In general，the numbers of the benzene ring and the rate of？degradation will be lower and lower with the increase of polycyclic aromatic hydrocarbons.Therefore，PAHs with low molecular weight can be degraded rapidly in environment.They consist briefly in environment. PAHs with high molecular weight are completely opposite.There are two main types of microbial metabolism：1）PHAs as sole carbon source and energy2）Co-metabolism with other organic matter.In the two methods， co-metabolism plays a leading role in decomposition and mineralization of PAHs refractory pollutants.

3.2 The metabolic mechanism of polycyclic aromatic hydrocarbons as the only carbon source and nitrogen source

The mechanism of this metabolic pathway：Oxygen is added on the phenyl ring to form C-0 with the induction of polycyclic aromatic hydrocarbons and the catalysis of monooxygenases or dioxygenase，then reducing the number of benzene rings with C-C rupture by hydrogenation and dehydration.Wherein，the fungus produce monooxygenase，and the bacteria produce dioxygenase.

The intermediate product is not the same in different ways，it depends on the position of hydroxyl group.The product structure may be symmetrical，positive or others. Catechol is the most common intermediate. There are two kinds of metabolic pathways in catechol，meta-position and ortho-position.

The degradation will produce 2-hydroxy adipic acid semialdehyde， 2-keto-4-pentenoic acid， pyruvic acid，acetaldehyde or cis，cis - muconic acid，adipic ketone，succinic acid，etc.All of these are synthesized in cellular proteins，and the final product is water and carbon dioxide（Sims R C，Overcash M R.，1983）.

3.3 Co-metabolism mechanism of PAHs

Co-metabolism is using an easily degradable material as nutrients to support microbial growth and reproduction，while degrading another substance.However，the degradation and transformation of the second material can not make the microbes obtaining energy， carbon or any other nutrients.

The disconnect of benzene in PAHs is to rely on the role of dioxygenase（Figure 1）.The degree of PAHs degradation is subject to the activity of dioxygenase.

Fig.1 Microbial oxidation way of polycyclic aromatic hydrocarbons under the action of oxygenase

Since metabolic enzymes of PAHs can be induced，we can improve enzyme activity and enhanced degradation by adding a matrix analogue. Various factors（for instance，low toxicity，low price，easy to get，not used by other non-PAHs degrading bacteria，can improve the content and activity of microbial dioxygenase） need to be considered in matrix analogues （inducer）selection（Leadbetter E R，Foster J W.，1959）.

3.4 Application Technology

In the method of application of PAHs degrading bacteria，some investigators directly add degrading bacteria to PAHs in the environment for microbiological repair；and some researchers use immobilized bacteria.

Immobilized materials are generally inorganic carrier and an organic carrier.The methods of immobilized microorganisms are mainly adsorption， covalent bonding， cross-linking and non-immobilized carrier.It can greatly enhance the microorganism concentration participate in the reaction，increased microbial resistance to environmental impact.

Chinese researchers have already successfully achieved high efficiency biodegradation of PAHs.Baoli Cai used Pseudomonas.ND24 inoculated into sterilized soil containing 0.2g / dL in naphthalene，the removal of naphthalene was 98.2% after 14d. Xin Wang used PVA- boric acid compound carrier embedded zoogloea bacteria，the rate of phenanthrene and pyrene degradation got 84.89% after 186h.Furthermore，PAHs degrading bacteria can also be used for biological remediation of contaminated water.

4 Suggestion and Expectation

As more and more microorganisms are found has the ability to degrade PAHs，PAHs degradation efficiency will be greatly improved. Many strains of microorganisms have the ability to degrade PAHs，but these strains can not degrade PAHs macromolecules

We need to build efficient new strains by genetic recombination or other new breeding techniques to find new degradation to expand the scope of microbial degradation of contaminants.Construction of high-performance genetic engineering bacteria for bioremediation of environmental pollution and degrading recalcitrant pollutants is an important research topic in the field of environmental engineering in the future.

Acknowledgments

This paper is sponsored by the National Natural Science Foundation of China（NO.41372232） and the National High Technology Research and Development Program of China（863 Program）（NO.2013AA06A205）.

Compliance with Ethical Standards

Funding：This study was funded by the National Natural Science Foundation of China（NO.41372232）and the National High Technology Research and Development Program of China（863 Program）（NO. 2013AA06A205）

Conflict of Interest：Shuo Diao declares that she has no conflict of interest.

Hongqi Wang declares that he has no conflict of interest.

Yinan Zheng declares that he has no conflict of interest.

Fei Hua declares that she has no conflict of interest.

Ethical approval： This article does not contain any studies with human participants or animals performed by any of the authors.

Informed consent： Informed consent was obtained from all individual participants included in the study.

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