IntroductionAliphatic and aromatic compounds are the two major categories thatcontain hydrocarbons. The first group is also titled as non-aromatic, as Figure1 presents.
Benzene is the key molecule that forms aromatic hydrocarbons thoughtheir appearances are not similar in some cases.Figure 1 – Classification of hydrocarbons 1Polycyclic aromatic hydrocarbons (PAHs) are considered as one of themost stable forms of hydrocarbons that are largely concentrated in contaminatedsoils. Natural and anthropogenic sources are both significant and need to be assessed2.
PAHs molecular weight is the factor that determines their origin’s levelin earth. Obviously, higher molecular weight causes them to be found in deeperdepths. The contaminants are irregularly distributed throughout levels andlocations. As a prominent threat to humans health, PAHs have been thoroughlystudied and their mutagenic, teratogenic, and carcinogenic features have beenidentified 3-6. The pollutants are also categorized as a subset ofhydrophobic organic contaminants (HOCs). In most cases, two or three benzenerings are the skeleton of these contaminants.
The widespread nonpolarpollutants are detected in petrochemical products such as tar, oil and coal.Incomplete combustion is also another significant source while organicmaterials are involved 7.Those PAHs that have highly involved the researchers are categorized byEnvironmental Protection Agency (EPA) as 16 EPA priorities: acenaphthene,acenaphthylene, anthracene, benz(a)anthracene, naphthalene, benzo(a)pyrene,benzo(k)fluoranthene, benzo(b)fluoranthene, benzo(ghi)perylene, chrysene,dibenz(a,h)anthracene, fluoranthene, fluorene, indeno(1,2,3-cd)pyrene,phenanthrene, and pyrene. The EPA priority list has been considered assubcategory of a more comprehensive record of 126 major pollutants. Inaddition, 5 of them are recorded as 25 threatening compounds which areidentified to cause extremely perilous situation for the environment atsuperfund sites 8, 9.The experimental data shows that the remediation of PAHs contaminatedsoil is practically demanding and the issue is because of the pollutants’ highoctanol-water partition coefficient and insignificant water solubility 10.Clayey soil’s impermeability is considered as a natural obstacle thatexacerbates ground water pollution as it maintains PAHs.
Low permeability,however, hinders contamination removal 11. The low solubility of PAHs inwater, render its removal to be costly and time-consuming 12-14.Although chemical and physical features highly vary in PAHs, some commonfeatures are specified in researches. Insignificant volatility, low watersolubility, and lipophilicity should be mentioned as the most important ones. Onthe other hand, octanol-water partition coefficient (Kow) is another keycharacteristic that has been studied and specified as “high” forPAHs. Contaminant’s fate is determined by the latter index.
That means high Kowof PAHs signals their high solubility in lipid. Moreover, molecular weight is anotherindicator that shows higher volatility, higher solubility in water and greaterlipophilicity 15. Therefore, it can be concluded that removal of those PAHspossessing lower molecular weight is more manageable. Furthermore, PAHs’removal process becomes more strenuous as number of benzene rings increases16. The pre-mentioned characteristics are effective on decision making abouttreatment process. Biological methods are more efficient for the contaminationcases with PAHs having two or three rings are on the table 17, 18. These PAHsare recognized as light PAHs.
Another classification is based on contaminants’hydrophobicity in which those PAHs that have a Log octanol-water coefficientgreater than 5 are categorized as hydrophobic 19.There are several techniques that are propounded for remediation ofsites contaminated with PAHs. There are some pros and cons that have beenexpressed for each procedure. To begin with, a digest of the method’s procedureis provided and then shortcomings and superiorities are discussed. Efficacy,rates of contaminants’ removal and effective factors are also included.Combining techniques in order to improve the results is another strategy thathas been assessed in case studies.The treatment techniques that are investigated through the article areas: soil washing, oxidation, electrokinetic treatment, and phytoremediation.
OriginsBased on their sources of emission, PAHs are classified into threegroups of phytogenic (natural), petrogenic and pyrogenic. Their origin has akey role in recognizing the pollutants’ type and migration circumstances 20.They are transferred to the environment by adsorption, dissolution, dispersion,evaporation, and other processes 21-24.As petrogenics, products derived from crude oil, such as lubricants andfuels are another sources that usually leak from the tanks and consequentlypollute groundwater 25. The main PAH compounds in a petroleum source are asthe EPA 16 priority PAHs and the petroleum-specific alkylated (PAH1-PAH4) homologuesof selected PAHs. Namely: alkylated naphthalene, phenanthrene,dibenzothiophene, fluorene, and chrysene series. These components are alsocalled as “the alkylated five” or “five target” 26.
Lack of oxygen and high temperatureprovides the condition in which pyrogenic PAHs are produced by fuelscombustion. Cracking, pyrolysis and incomplete combustion are the mostprominent sources 27. The products are highly available in aquaticenvironments 28. During the last decades, one of the largest PAHs origins,which generates 5% of PAHs in urban areas, is power plants 29, 30. Contaminantsfrom aluminum smelt, agricultural and industrial runoff have also their largecontribution in PAH-generation activities. Obviously, the mentioned items arealso recognized as sources of petrogenic and natural PAHs.
Figure 2 presentsNational Priorities List (NPL) sites with PAHs contamination. Pyrogenic PAHs are generated by the combustion of fuels and biomasses athigh temperature when there is not sufficient oxygen. Incomplete combustion,pyrolysis, cracking and destructive distillation are among the main sources27. These materials are often solid and are highly found in aquaticenvironments 28. During the last decades, one of the largest sources of PAHs, whichgenerates 5% of PAHs in metropolises, is power plants 29, 30. Other sourcesinclude pollutants from aluminum smelt, industrial and agricultural runoff. Ofcourse, these are also considered as sources of petrogenic and natural PAHs.Figure 2 presents National Priorities List (NPL) sites with PAHs contamination.
Figure 2 – Distribution of NPL sites with PAHscontamination 31Chemical plants are also industry equipment that are considered asPAH-producers 32. Anthropogenic emissions of PAHs are occurred duringprocesses such as incomplete combustion and pyrolysis 33. Revindra et alintroduced a classification that includes domestic, mobile, industrial,agricultural, and natural sources of PAHs 34.Volcanic activities, wildfires, and oil seepages, are another examplesof PAHs sources.
Those PAHs considered as natural PAHs are mainly producedduring chemical conversions of organic materials in plants, planktons,microorganisms, fungi, and sediments etc 35. Some bacteria and plants areable to produce Benzoapyrene. Perylene in tropical termite nests is anotherexample of natural PAHs 36, 37.PAHs such as naphthalene (N0), benzobfluoranthene (BbF), andphenanthrene (P0), may also be produced by termite activities and vascular landplants 38, 39. Generally, PAHs are generated by more than one source and manyPAHs such as perylene may possibly be produced by either natural oranthropogenic sources.Health threatToxicity, mutagenicity, and carcinogenicity of most PAHs have beenconfirmed 40. They have high solubility in lipid and it has facilitated theirabsorption in mammals’ body 41.
The contaminants presence in food chain hasmade their swallow probable and the issue has endangered animal and human life41. Naphtalene is found to be a PAH that adheres to kidney, liver, and lungcells 42. Researches have confirmed their destructive effects on skin andeyesight of involved individuals 41.
Researches on indeno(1,2,3-c,d)pyrene,dibenz(a,h)anthracene, benzo(k)fluranthene, benzo(b)fluoranthene,benzo(a)anthracene, benzo(a)pyrene, have approved their carciogenicity 43,44. As a more abundant PAH, pyrene and its derivatives are reported to berather mutagenic 41. Having 4 rings, the contaminant’s elimination from theenvironment has been depicted as strenuous 45. Although pyrene’scarcinogenicity has not been showed, the pollutant is included in EPA listbased on its destructive effects on human health (it transforms into toxicbenzopyrene) 46. EPA has developed standard methods for analysis of PAHs in environmentas summarized in Table 1.Table 1 – EPA Standard methods developed to determinePAHsTreatment techniquesPAHs are able to affect every aspect of the environment including air,water, and soil. As an advantage, bioremediation does not leave any secondpollutant after treatment process and consequently it can be employed in alarge scale. However, these pollutants can be isolated, separated and removedby other physical and chemical techniques.
These methods of contaminantselimination, known as conventional methods 47, are discussed below.Soil washingSoil washing or solvent extraction has been acutely investigated byrecent researches 48. The process can be conducted either in-situ or ex-situ.The approach has been recognized as a procedure that does not cause significantdisturbance in ecology of the site 49. Obviously, method’s efficiency is highlydependent to parameters such as molecular weight that determines solubility ofPAHs in washing agent 50.Solutions used as surfactant need to be recycled in order to improveprocess’s efficiency and lower overall costs. On the other hand, in an effort toreplace toxic and costly agents, researchers have developed novel agents forcontaminants’ extraction 51. For instance, biosurfactants were successfullyused and improved removal process is reported in those studies that have usedthese agents 52, 53, 54.
Some eco-friendly materials such as vegetable oilhave also presented acceptable results in soil treatment processes 55.Wen examined solution’s pH effect on PAHs removal rate and for loamysoils; accordingly, the positive effect of alkaline condition on removal rateof washing process was confirmed. It was also concluded that saline environmenthas a destructive effect 54.
Soil washing is one of those techniques which are highly capable to be creativelycombined with other methods in order to improve treatment efficiency. Song etal integrated the method with ultrasonic technique and reported highly improvedresults. The result revealed that the initial concentration of phenanthrene,sonication time and washing flow rate had significant effect on soilcontamination removal, as illustrated in Figure 3 56.Figure 3 –The effects of flow rate and sonication timein combined method of treatment 56As table 2 presents, PAHs’ removal rate of 90% was reported in a studythat combined biological remediation and ozonation with washing technique 57.In most cases, washing residue is considered as a hazardous materialthat can threaten environment and human health. Therefore, independentprocedures need to be designed for treatment of washing effluent 58, 59. In ahigh efficient soil washing experiment carried out in 2012, coagulation was theseparate technique chosen for treatment of the effluent 60.
Table 2 – Employing combined techniques for treatmentof PAHs polluted soilsOxidation technique has been confirmed to have effective results onheavy PAHs which are more challenging to be separated. The technique can becarries out in two methods: conventional and advanced. Conventional processincludes hydrogen peroxide, ozone, and permanganate 61, 62. In AdvancedOxidation Process (AOP), two or morereagents are employed to form more highly reactive radicals 63. Among variousoxidants for soil treatment, ozone (gas) and fenton (liquid) are the mostconventional and studied ones 62, 64. The technique is defined in threemodes, namely, in-situ, ex-situ and off-situ. The last mode is considered toachieve high results and that is because of the easy-to-control environment itprovides off the site, where the soil is transferred. Ex-situ mode processesthe extracted soil in site area.
In-situ mode directly injects treatmentmaterials to the soil 64. Oxidation can employ ozone or fenton reagent. Thelatter one is the most studied techniques and is similarly carried out in threemodes 65-68.
In-situ Fenton oxidation process and pollutants transportmechanisms are summarized in Figure 4.Figure 4 – In-situ Fenton oxidation process andtransport mechanisms of the pollutants; (A) Hazardous waste site contaminatedwith PAHs. Injection well plus two monitoring wells are located in the vicinityof the critical area; (B) H2O2 reacts after injection.
Contaminants conversionvia oxidation occurs with a high pace. ;( C) Groundwater displaces because ofhigh pneumatic pressure of O2;(D) After the oxidation process, contamination ofthe studied area is curbed. The results are assessed by comparing data frommonitoring wells 69.Hydroxyl radicals are product of an iron salt and hydrogen peroxide.Degradation of organic matter is carried out by these transient radicals in twoforms.
It can be readily added to the organic material or cause the organicmaterial to lose their hydrogen 70. Fenton oxidation can be integrated withother techniques such as bioremediation to improve PAHs removal rate 71, 72. Remediation of soil by ozone is executed within wells that areconstructed vertical or horizontal. Ozone is injected in aqueous or gaseousform 64, 70. Gan concluded that light PAHs, ozone is more efficient 70.Another technique for oxidationof organic pollutants is via pressurized hot water. There is a specifiedcritical point (Tc= 374 °C, Pc= 221 bar) in which physical and chemicalcharacteristics of water are not easy to be identified 73.
Table 3presents the findings of different researches on PAH treatment in soil.Table 3 – Studies of Oxidation technique on PAH treatment in soil.ElectrokineticIn many studies, researchers have to deal with obstacles which hinderpollutants’ removal. One of these factors is low permeability of soil thathighly lowers methods’ removal efficiency. Electrokinetic treatment was initiallydesigned to treat inorganic pollutants. During recent decades, electro osmosishas been resulted in positive outcomes specifically for low permeable soils75,76. Similar to previous methods of remediation, this approach can alsobeen integrated with oxidation or bioremediation to ensure trusted outcomes77.
Meanwhile, more research is needed to evaluate all aspects of the process78. Electroremediation is recognized as a in-situ approach that forcespollutants’ movement. Water electrolysis equations are as: At the anode: 2H2O ? O2(g) + 4H+ + 4e?At the cathode: 4H2O + 4e? ? 2H2(g) + 4OH? As illustrated in equations, two solutions, acidic and alkali are the placewhere electrical current moves components toward electrodes. Electromigrationand electro- osmosis are two forms of migration. In electromigration, ions movetoward opposite charged electrodes, while electro-osmosis, is the movement ofliquid across the soil.Figure 5 illustrates mechanisms of migration. Figure 5 – Migration details in electroremediation technique Therefore, for neutral organic materials, electro-osmosis is the processthat drives them toward electrods. The next stage is cleaning electrode polesfrom pollutants.
Soluble and Porous material environment facilitates substances’ movementand improves removal efficiency. In clayey media, agents such as co-solvants, and cyclodextrins can beemployed to lower the destructive role of low permeability 81. Ana T. Lima etal conducted experiments on electro-osmosis was deployed to eliminate PAH fromclayey soil by using 0.01 M NaCl, solely and also combined with surfactantTween 80. The surfactant is placed in anode plate vicinity. It was aimed todrive surfactant through the media and improve its contact with PAH. Usingsurfactants, PAHs movement with the liquid is facilitated 78.
They concluded,having lower molar mass and consequent higher solubility, surfactants highlyfacilitate naphthalene and acenaphtylene migration. It was also found that timeis not a main factor influencing process efficacy 78. Table 4 – Studies of electrokinetic remediation of soils contaminated byPAH