PAHs

PAHs (Polycyclic Aromatic Hydrocarbons)

Pollutant Information

Chemical Formula:	Various (C₁₀H₈ to C₂₄H₁₄+)
Pollutant Family:	Organic priority pollutants
CAS Numbers:	91-20-3 (naphthalene), 50-32-8 (benzo[a]pyrene), others
Molecular Weight:	128-300+ g/mol (varies by compound)
Solubility:	Low water solubility, decreases with ring number
Persistence:	Highly persistent, bioaccumulative
Regulatory Status:	EPA priority pollutants, many carcinogenic

Overview

Polycyclic aromatic hydrocarbons (PAHs) are a group of over 100 different organic compounds composed of two or more fused benzene rings in various structural configurations. These ubiquitous environmental pollutants are formed primarily through incomplete combustion of organic materials, making them widespread contaminants from both natural and anthropogenic sources. PAHs are classified as priority pollutants due to their toxic, mutagenic, and carcinogenic properties, with 16 compounds identified by the EPA as particularly concerning. They are categorized as light PAHs (2-4 rings) and heavy PAHs (5+ rings), with the heavier compounds being more stable, persistent, and toxic in the environment.

Characteristics

Physical Properties

• State at room temperature: Solid (except naphthalene which can be crystalline)
• Color/Appearance: Colorless to pale yellow, becoming darker with increasing molecular weight
• Odor: Light PAHs have characteristic aromatic odors; heavy PAHs are often odorless
• Melting point: Increases with molecular weight (80°C naphthalene to 300°C+ for heavy PAHs)
• Boiling point: 218°C (naphthalene) to 500°C+ (heavy PAHs)
• Density: 0.9-1.4 g/cm³, generally denser than water

Chemical Properties

• pH effects: Stable across pH range; bioavailability can be affected by soil pH
• Oxidation states: Stable aromatic structures; can form reactive metabolites
• Reactivity: Resistant to chemical degradation; can form DNA adducts
• Stability: Highly stable due to aromatic ring structure
• Bioavailability: Decreases with increasing molecular weight and soil aging

Environmental Behavior

• Mobility in soil: Low mobility; strongly adsorb to organic matter and clay
• Water solubility: Very low and decreases with increasing ring number
• Volatility: Light PAHs can volatilize; heavy PAHs remain in solid phase
• Adsorption: Strong sorption to soil organic matter, sediments, and particulates
• Half-life: Weeks to years depending on environmental conditions and molecular weight

Sources & Contamination Pathways

Industrial Sources

• Coal tar and petroleum processing facilities
• Coke production and steel manufacturing
• Aluminum smelting operations
• Creosote wood preservation plants
• Gas manufacturing plants (former manufactured gas plants)
• Asphalt production and road construction

Transportation Sources

• Vehicle exhaust emissions (especially diesel)
• Tire wear particles and road runoff
• Fuel spills and leakage
• Airport runway de-icing operations
• Maritime vessel emissions and fuel spills

Urban/Residential Sources

• Residential heating (wood, coal, oil combustion)
• Barbecuing and meat cooking
• Cigarette smoke and tobacco products
• Urban stormwater runoff
• Contaminated building materials (coal tar-based sealants)
• Waste incineration and landfills

Natural Sources

• Forest fires and natural combustion
• Volcanic emissions
• Diagenesis of organic matter in sediments
• Biosynthesis by bacteria, fungi, and plants (limited amounts)

Environmental & Health Impacts

Effects on Land/Soil

• Soil chemistry: Alters soil organic matter dynamics, affects nutrient cycling
• Soil biology: Toxic to soil microorganisms, reduces microbial diversity at high concentrations
• Plant uptake: Limited direct uptake, but can affect plant growth and development
• Groundwater contamination: Light PAHs can migrate to groundwater, especially from coal tar sites
• Ecosystem disruption: Bioaccumulation in soil fauna, food web contamination

Effects on Water Systems

• Aquatic toxicity: Acute and chronic toxicity to fish, invertebrates, and algae
• Bioaccumulation: Accumulation in fatty tissues of aquatic organisms
• Water quality: Taste and odor problems, aesthetic degradation
• Sediment contamination: Long-term reservoir of PAHs in aquatic systems

Human Health Effects

Acute Exposure

• Skin and eye irritation from direct contact
• Respiratory irritation from inhalation
• Nausea and vomiting from high-dose ingestion

Chronic Exposure

• Multiple cancers (lung, skin, bladder, gastrointestinal)
• Respiratory problems and reduced lung function
• Immune system suppression
• Developmental and reproductive effects
• Genotoxic effects and DNA damage
• Cardiovascular disease

Vulnerable Populations

• Children: Higher exposure rates, developing organ systems more susceptible
• Pregnant women: Risk of birth defects and developmental disorders
• Occupational workers: Coke oven workers, roofers, road construction workers
• Urban populations: Higher exposure from traffic and industrial emissions
• Subsistence communities: Exposure through contaminated traditional foods

Detection & Testing

Sampling Methods

• Soil sampling: Composite sampling, attention to historical contamination patterns
• Water sampling: Both dissolved and particulate phases, use amber glass containers
• Sediment sampling: Core sampling to assess historical deposition
• Air sampling: High-volume samplers with quartz filters and polyurethane foam
• Biological sampling: Fat-containing tissues for bioaccumulation studies

Analytical Methods

• GC-MS (Gas Chromatography-Mass Spectrometry) - Gold standard for PAH analysis
• HPLC-Fluorescence - Sensitive detection for fluorescent PAHs
• EPA Methods 8270, 8310 for soil and water analysis
• Field screening kits - Semi-quantitative immunoassay methods

Regulatory Limits

• EPA drinking water standards: Various limits (0.2-200 μg/L depending on compound)
• EPA soil screening levels: 0.1-61 mg/kg (residential use)
• OSHA workplace exposure: 0.2 mg/m³ (coal tar pitch volatiles)
• International guidelines: WHO, EU standards for water, soil, and air

Bioremediation Organisms

Bacteria

• Pseudomonas spp. - Extensive PAH degradation pathways, naphthalene and phenanthrene specialists
• Rhodococcus spp. - High molecular weight PAH degradation, including pyrene and chrysene
• Burkholderia cepacia - Efficient benzo[a]pyrene degradation from activated sludge
• Sphingomonas spp. - Anthracene and phenanthrene degradation
• Mycobacterium spp. - Fluoranthene and pyrene metabolism
• Bacillus spp. - Consortium member for multi-ring PAH degradation

Fungi

• Scopulariopsis brevicaulis - 77% total PAH removal, especially effective on benzo[a]pyrene
• Coniothyrium spp. - 26.5% total PAH degradation, specializes in high molecular weight PAHs
• Fusarium spp. - 27.5% PAH removal, effective on multiple ring compounds
• Cladosporium spp. - Marine-derived strain for anthracene and other PAHs
• Aspergillus spp. - Widely studied for PAH biotransformation
• White-rot fungi (Phanerochaete chrysosporium) - Lignin peroxidase system for PAH oxidation

Plants (Phytoremediation)

• Alfalfa (Medicago sativa) - Rhizosphere enhancement, effective in intercropping systems
• Tall fescue (Festuca arundinacea) - Superior PAH removal, especially heavy PAHs
• Willow (Salix spp.) - Deep root system, effective for soil and groundwater treatment
• Hybrid poplar (Populus spp.) - Fast growth, large-scale phytoremediation applications
• Switchgrass (Panicum virgatum) - Native grass, 70% TPH reduction in greenhouse studies
• Eastern gamagrass (Tripsacum dactyloides) - Effective rhizosphere enhancement

Other Organisms

• Microalgae (Oscillatoria spp.) - Naphthalene biotransformation in aquatic systems
• Halophilic archaea (Natrialba spp.) - PAH degradation in high-salinity environments
• Mixed microbial consortia - Synergistic degradation effects, prevents toxic intermediate accumulation
• Earthworm - Enhance PAH bioavailability and microbial activity in soil

Bioremediation Strategies

Rhizosphere-Enhanced Bioremediation

• Plant-microbial partnerships: Root exudates stimulate PAH-degrading bacteria
• Intercropping systems: Alfalfa-tall fescue combinations for enhanced removal
• Mycorrhizal inoculation: Arbuscular mycorrhizal fungi enhance plant performance
• Bioaugmentation: Introduction of specific PAH-degrading bacterial strains

Fungal Bioremediation (Mycoremediation)

• Indigenous fungal treatment: Site-specific fungi isolated from contaminated soil
• Composting with fungi: Mushroom compost for enhanced degradation
• White-rot fungi applications: Ligninolytic enzyme systems for PAH oxidation
• Biofilm reactors: Fungal biofilms for water treatment applications

Large-Scale Phytoremediation

• Hybrid poplar plantations: 15,000+ trees for former industrial sites
• Constructed wetlands: Sedge and grass systems for contaminated sediments
• Phytostabilization: Immobilization of PAHs to prevent migration
• Harvesting strategies: Multiple harvest cycles to maximize contaminant removal

Alternative Treatment Methods

Physical Treatment

• Soil washing: Surfactant-enhanced extraction of PAHs from soil
• Thermal treatment: Incineration, thermal desorption for heavily contaminated soil
• Air sparging: Removal of volatile PAHs from groundwater
• Activated carbon: Adsorption treatment for water and air emissions

Chemical Treatment

• Chemical oxidation: Fenton's reagent, permanganate, ozone treatment
• Surfactant washing: Enhanced solubilization and extraction
• Photocatalytic degradation: UV/TiO₂ systems for water treatment
• Electrochemical treatment: Electrokinetic remediation

Disposal Methods

• Secure landfill: Containment of heavily contaminated materials
• Incineration: High-temperature destruction in specialized facilities
• Stabilization/solidification: Cement or polymer-based immobilization
• Beneficial reuse: Treated materials for construction applications (with restrictions)

Case Studies

Intercropping Phytoremediation Field Trial - Agricultural Site

• Location: PAH-contaminated agricultural field
• Contamination source: Historical industrial activities and atmospheric deposition
• Treatment method: Alfalfa and tall fescue intercropping system
• Organisms used: Medicago sativa (alfalfa) + Festuca arundinacea (tall fescue)
• Duration: 7-month growing season field study
• Results: 30.5% total PAH removal (intercropping) vs 19.9% (monoculture) vs -0.6% (unplanted)
• Innovation: Demonstrated synergistic effects of plant combinations
• Source: Journal of Soils and Sediments, 2012

Hybrid Poplar Phytoremediation - Former Oil Tank Farm, Iowa

• Location: Former petroleum storage facility, Iowa, USA
• Contamination source: Petroleum product storage and spills
• Treatment method: Large-scale hybrid poplar tree plantation
• Organisms used: Populus deltoids X Populus nigra (15,000+ trees)
• Duration: Multi-year study from 1999-2009
• Results: Significant reduction in soil and groundwater contamination
• Scale: 54,000 square meters (14 acres) treated
• Source: Water Environment Research, 2009

Indigenous Fungal Remediation - Former Gasworks Site

• Location: Former gasworks facility with aged PAH contamination
• Contamination source: Historical coal gas production operations
• Treatment method: Native soil fungi isolated and applied to contaminated soil
• Organisms used: 21 filamentous fungi isolates, including Scopulariopsis brevicaulis
• Duration: 28-day laboratory and field applications
• Results: Up to 77% total PAH removal, 89% phenanthrene and 75% benzo[a]pyrene removal
• Innovation: Site-specific fungi more effective than introduced species
• Source: International Biodeterioration & Biodegradation, 2004

Prevention Strategies

Source Reduction

• Improved combustion efficiency in industrial processes
• Cleaner transportation fuels and emission control technologies
• Alternative wood preservation methods (avoiding creosote)
• Better waste management and incineration practices
• Green energy alternatives to reduce fossil fuel combustion

Best Management Practices

• Stormwater management to reduce PAH transport from urban areas
• Proper handling and disposal of coal tar-based products
• Regular monitoring at former industrial sites
• Protective equipment for workers in high-exposure industries
• Public education about PAH sources and reduction strategies

Groups & Projects Working on This Pollutant

• University of Iowa - Civil & Environmental Engineering - Jerald Schnoor Lab - Phytoremediation research, hybrid poplar studies
• Purdue University - Agronomy Department - Plant-soil interactions, rhizosphere enhancement for PAH degradation
• Environmental Protection Agency - Superfund Research Program - Large-scale remediation technology development
• Pacific Northwest National Laboratory - Environmental Microbiology - Microbial consortium development for PAH bioremediation
• University of Reading - Soil Research Centre - Fungal bioremediation, mycoremediation technology development
• Institut National de la Recherche Agronomique (INRA) - France - Phytoremediation following bioremediation treatment studies

Resources

Scientific Literature

• Ghosal et al. (2016). "Current State of Knowledge in Microbial Degradation of Polycyclic Aromatic Hydrocarbons: A Review." Frontiers in Microbiology
• Cerniglia (1992). "Biodegradation of polycyclic aromatic hydrocarbons." Biodegradation 3:351-368
• Haritash & Kaushik (2009). "Biodegradation aspects of PAHs: a review." Journal of Hazardous Materials

Government Resources

• EPA Polycyclic Aromatic Hydrocarbons (PAHs) Information
• ATSDR Toxicological Profile for Polycyclic Aromatic Hydrocarbons
• NIOSH Criteria for PAH Occupational Exposure
• EPA CLU-IN Phytoremediation Technology Overview

Testing & Analysis

• Environmental laboratories - GC-MS analysis for 16 priority PAHs
• Field test kits - EnviroLogix PAH soil test, Millipore immunoassays
• Academic research facilities - Advanced analytical methods and method development
• Consulting firms - Site assessment and remediation design services

Related Pollutants

• BTEX (Benzene, Toluene, Ethylbenzene, Xylenes) - Often co-occur at petroleum sites
• Total Petroleum Hydrocarbons (TPH) - Broader category including PAHs
• Phenols - Degradation products and co-contaminants at coal tar sites
• Heavy metals - Co-contamination at industrial sites, especially copper and zinc
• Dioxins and furans - Similar sources from combustion processes
• PCBs (Polychlorinated Biphenyls) - Historical industrial co-contaminants

Last updated: June 24, 2025
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