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Compost-assisted Bioremediation of Polycyclic Aromatic Hydrocarbons

Introduction

Polycyclic aromatic hydrocarbons (PAHs) reach the environment from incomplete combustion of organic substances, and heavy industries and transport, which use fossil fuel, are the main anthropogenic sources for contamination by PAHs in terrestrial and aquatic environments (Ghosal et al. 2016). These PAH organic compounds consist of carbon (C) and hydrogen (H) atoms with two or more fused benzene rings. Although a large number of different PAH compounds exist in soil and aquatic environments, only around 16 of these compounds are considered as priority pollutants (Table 1) (Keith 2015, Lukic et al. 2017b). The specific characteristics of PAH compounds are their high hydrophobicity and low water solubility. These characteristics impact then active adsorption to various soil components, thereby affecting their bioavailability and subsequent degradation. PAH contamination can cause various health hazards to humans and other living organisms. For example, some of the PAH compounds, including benzo-anthraceue, benzo-fluorautlieue, benzo-pyreue, chrysene, dibenzo-anthracene, and indeno-pyrene, are considered as potential human carcinogens. Additionally, chrysene and benzo-pyreue may cause genetic disorders and impair fertility. Similarly, naphthalene, benzo-authracene, and benzo-pyrene have been shown to cause teratogenicity or embryo toxicity in animals. Bioremediation of PAH contaminated soil is an attractive technology because of lower capital investments, a limited interruption of contaminated site activity, and a green-based environmentally friendly approach compared to other chemically-based remediation treatment technologies (Table 2). Composting of organic contaminants promotes the bioremediation of these compounds.

Sources of PAHs

PAHs are distributed extensively in the environment, and a range of PAH compounds have been identified in waste materials, landfill leachates, soils, sediments, groundwater, and in the atmosphere

Table 1. Structure and physico-chemical properties of major priority polycyclic aromatic hydrocarbons (PAH) pollutants

(modified from Keith 2015, Lukic et al. 2017).

No.

PAH compound

Melting point (°C)

'og K0

Water solubility at 25°C (pgL-1)

Vapour pressure (Pa at 25°C)

1

Naphthalene (C10H8)

SI

3,00-4.00

3.17 x 10J

10.9

2

Acenaphthylene (C12H8)

95

3.70

/

5.96 x l0-‘

3

Acenaphthene (C12H10)

96.2

3.92-5.07

3.93 x Ю!

5.96 x 10-‘

4

Fluoreue (C13H10)

115-116

4.18

1.98 x 10s

8.86 x 10-=

5

Anthracene (C14H10)

218

4.46-4.76

73

2.0 x 10-1

6

Phenanthrene

100.5

4.45

1.29 x Ю!

1.8 x 10-2

7

Fluoranthene (C14H10)

108.8

4.90

260

2.54 x l0-‘

8

Pyrene (C16H10)

150.4

4.90

135

8,86 x 10-4

9

Benzo[a]anthraceue (C1SH20)

160.7

5.61-5.70

14

7.3 x 10-6

10

Chrysene (C1SH20)

253.8

5.61

2

5.7 x 10-7

11

Benzo[b]fluoranthene (C20H12)

168.3

6.57

1.211 (20°C)

/

12

Benzo[k]fluoranthene (C20H12)

215.7

6.84

0.76

/

13

Benzo[a]pyrene (C20H12)

178.1

6.04

3.8

8.4 x 10-7

14

Dibenzo[a,h]anthracene (C2,H14)

266.6

5,80-6.50

0.5 (27°C)

3.7 x 10-10

15

Indeno[l,2,3-cd]pyrene (C22H12)

163

7.66

62

/

16

Benzo[g,h,i]perylene (C22H12)

278.3

7.23

0.26

6 x 10‘8

(Li et al. 2019). Both natural processes and anthropogenic activities contribute to PAH input to the environment. Natural processes such as bush fires and volcanic eruptions contribute to PAH input to the environment. The major anthropogenic activities contributing to PAH input to the environment include partial combustion of fossil fuels in transport, disposal of petroleum hydrocarbon products, and waste incineration. Most of the industries using fossil fuels in their production system, such as petroleum refining and coal gasification, generate PAHs. The concentration of PAHs in contaminated media including soils and sediments depends mainly on the anthropogenic sources of contamination (Soroji et al. 2007).

Interactions of PAH

PAHs undergo a number of reactions including adsorption, volatilization, photolysis, and redox reactions, although microbial transformation is the major natural attenuation process of PAH- contaminated sites. Low molecular weight (LMW) PAH compounds, consisting of 2 or 3 rings, have been reported to cause acute toxicity but are not carcinogenic, while high molecular weight (HMW) PAHs, consisting of 4 to 7 rings, are relatively less toxic but may be carcinogenic, mutagenic, or teratogenic (Bauer et al. 2018, Keith 2015, Ghosal et al. 2016). The most important physical and chemical properties of the 16 priority PAH pollutants are reported in Table 1 (Lukic et al. 2016). These properties control the interactions of PAH with soil components and subsequent bioavailability and biodegradation processes. For example, water solubility of PAHs is likely to decrease with increasing number of fused benzene rings, indicating that HMWPAHs are more slowly mobilised from solid substrates and dissolved into water than LMW PAHs, and, therefore, are less subjected to biodegradation (Yamada et al. 2003). Similarly, partitioning based on octanol and water (K0„.) is generally used to predict the affinity of an organic pollutant to be retained onto organic substrates. Higher Kow of an organic compound indicates its lower biodegr adability and higher potential for its bioaccumulation (Jonker and Vanderheijden 2007). In soils, the partition value of PAHs based on soil organic matter (Koc) indicates the extent of sorption and mobility of these compounds, and

Table 2. Remediation technologies for contaminated soil and sediments (Modified from Lukic et al. 2017).

Remediation treatment

Major contaminants

Primary environmental media

Remediation process

Bioremediatiou

PAH, TPH, Pesticides

Soil and sediments

Microorganism mineralize the contaminants to a less toxic, environmentally acceptable foim.

Phytoremediation

Heavy metals, PAH, TPH, Pesticides

Soil, sedunents and groundwater

Higher plants are used to extract, accumulate, sequester, and detoxify contaminants.

Natural attenuation (NA), Monitored natural attenuation (MNA), Enginerred monitored natural attenuation (EMNA)

PAH, TPH, Pesticides

Soil, sedunents and groundwater

Native microorganisms are stimulated (i.e., biostunulation) to facilitate the degradation of mostly organic contaminants

Chemical

PAH, PFAS, TPH, Pesticides

Soil, sedunents and groundwater

Chemical reactions involving oxidising, reducing agents destroy, decompose, or neutralize contaminants.

Thermal

PAH, PFAS, TPH, Pesticides

Soil and sedunents

Heat is employed to destroy contaminants through incineration, gasification, and pyrolysis.

Physical

Heavy metals, PFAS, PAH, TPH, Pesticides

Soil, sedunents and groundwater

Contaminated soil is removed to a landfill site or contained at the contaminated site.

Chemical umnobihzation

Heavy metals, PAH, PFAS, TPH, Pesticides

Soil, sedunents and groundwater

Chemical amendments are used to immobilize, thereby reducing the bioavailability of contammants.

Permeable reactive barrier

Heavy metals, PAH, TPH, Pesticides

Groundwater

Applied mainly to ‘filter’ contaminants in aquatic media such as surface and groundwater sources.

Solidification/vitnfication

Heavy metals, PAH, TPH, Pesticides

Soil and sedunents

Solidification refers to the encapsulation of contammants within a monolithic solid of high structural integrity. Vitrification involves tlie use of high temperatures using plasma to fuse contaminated material.

Integrated remediation techniques

Heavy metals, PAH, PFAS, TPH, Pesticides

Soil, sedunents and groundwater

Multiple remediation technologies can be applied to the degradation of contammants.

PAH = polycyclic aromatic hydrocarbons; PFAS = Poly- andperfluoroalkyl substances; TPH = total petroleum hydrocarbon.

the higher the Koc the stronger the partition onto soil organic matter rather than mobilization in the aqueous phase (Zhang et al. 2009, Appert-Collin et al. 1999). Furthermore, the electrochemical stability, resistance toward biodegradation, persistency in environmental media, and carcinogenic index of PAHs increase with increasing number of aromatic rings and hydropliobicity, while volatility of PAHs is likely to decrease with increasing molecular weight (Kanaly and Hariyama 2000, Ghosal et al. 2016).

Most of the PAHs are hydrophobic, leading to a high adsorption onto to soil organic matter and consequent persistency in soils and sediments (Zhang et al. 2017). The major pathways of PAH exposure include ingestion, inhalation, and dermal contact, thereby impacting human and animal health. The bioavailability of PAHs in environmental substrates, such as soils and sediments, is influenced by both the extent of adsorption and also the contact period between substrates and contaminants. The residence period in soils (i.e., ageing) allows the diffusion of contaminants into soil micropores, leading to then ready incorporation into stable phases and decreasing the mobility and bioavailability of contaminants. Bioaccumulation refers to the tendency of PAHs to accumulate in the tissue of organisms resulting from the exposure to a contaminated medium, or by ingestion of contaminated food sources. The bioaccumulation factor (BAF), which refers to the ratio of contaminant concentration in an organism to that in the ambient environmental substrates such as soil and sediment, is usually used to predict the potential for uptake and accumulation and subsequent monitoring of contaminant hazard to human and ecosystem health (Chapman et al. 1996, Feijtel et al. 1997, Sample et al. 1999).

 
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