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What is BTEX?

BTEX refers to the group of compounds:  benzene, toluene, ethylbenzene, and total xylenes which are naturally occurring components of petroleum that end up largely in gasoline as a result of the refining process.  Refineries adjust the amounts of these compounds to meet vapor pressure and octane standards for gasoline.

Benzene is a volatile organic compound.  It is used in the production of synthetic materials and consumer products such as synthetic rubber, plastics, nylon, insecticides, paints, dyes, resins, and cosmetics.

Toluene occurs naturally as a component of many petroleum products.  Toluene is used as a solvent for paints, coatings, gums, oils, and resins.

Ethylbenzene is used mostly as a gasoline and aviation fuel additive.  It may also be present in consumer products such as paints, inks, plastics, and pesticides.

There are three forms of xylene: ortho-, meta-, and para-.  Ortho-xylene is the only naturally occurring form, the other two being man-made.  Xylenes are used in gasoline and as a solvent in printing, rubber, and leather industries. 

How does BTEX enter the environment?

The main concern of the Department of Human Services (OHD) is the improper storage and leakage of gasoline and BTEX chemicals from faulty and ill-maintained underground storage tanks.  With improper storage, these chemicals may easily leach into the groundwater and contaminate public and private water systems.  Other sources of BTEX contamination to groundwater are large bulk facilities, surface spills, and pipeline leaks. 

When released into the environment, BTEX components may become attached to soil and rock particles where they eventually find their way into groundwater.  Once in the groundwater, these compounds may persist longer than if they were exposed to air, therefore affecting water supplies for months or even years. 

  

What health effects are expected from exposure to BTEX?

Exposure to humans can occur by either ingestion (drinking water from contaminated wells), or by inhalation (exposure to BTEX contaminated water via showering or laundering).  Acute exposure to gasoline and its components Technical Bulletin - Health Effects Information

BTEX aka benzene, toluene, and xylenes has been associated with skin and sensory irritation, central nervous system depression, and effects on the respiratory system. 

Prolonged exposure to these compounds also affects these organs as well as the kidney, liver and blood systems.  According to the EPA, there is sufficient evidence from both human epidemiological and animal studies that benzene is a human carcinogen.  Workers exposed to high levels of benzene in occupational settings were found to have an increase in leukemia

What is Bioremediation?

Bioremediation is use of biological processes to degrade,
break down, change, and/or essentially remove
contaminants or impairments of quality from soil and water.
Bioremediation is a natural process which relies
on bacteria, fungi, and plants to alter contaminants
as these organisms carry out their normal life functions.
Metabolic processes of these organisms are capable
of using chemical contaminants as an energy source,
rendering the contaminants harmless or less
toxic products in most cases. 

Why do we need Bioremediation?

Many substances with toxic properties have been brought into the environment through human activity. These substances vary in level of toxicity and danger to human health. Many of these substances eventually come in contact with and are isolated by soil. Conventional methods to remove, reduce, or mitigate toxic substances introduced into soil or ground water via human activities and processes include pump and treat systems, soil vapour extraction, incineration, and containment. Utility of each of these conventional methods of treatment of contaminated soil and/or water suffers from recognizable drawbacks and may involve some level of risk.

The emerging science and technology of bioremediation offers an alternative method to detoxify contaminants. Bioremediation has been demonstrated and is being used as an effective means of mitigating:

• hydrocarbons
• halogenated organic solvents
• halogenated organic compounds
• non-chlorinated pesticides and herbicides
• nitrogen compounds
• metals (lead, mercury, chromium)
• radionuclides

How does Bioremediation work?

Bioremediation technology exploits various naturally occurring mitigation processes: natural attenuation, biostimulation, and bioaugmentation.

Bioremediation which occurs without human intervention other than monitoring is often called natural attenuation. This natural attenuation relies on natural conditions and behavior of soil microorganisms that are native to soil.

Biostimulation also utilizes native microbial populations to remediate contaminated soils. Biostimulation consists of adding nutrients and other substances to soil to speed up natural attenuation processes.
Bioaugmentation involves introduction of exogenic microorganisms (sourced from outside the soil environment) capable of detoxifying a particular contaminant, sometimes employing genetically altered microorganisms.

During bioremediation, microbes use chemical contaminants in the soil as an energy source and, through oxidation-reduction reactions, metabolize the target contaminant into useable energy for microbes. By-products (metabolites) released back into the environment are typically in a less toxic form than the primary contaminants.

For example, petroleum hydrocarbons can be degraded by microorganisms in the presence of oxygen through aerobic respiration. The hydrocarbon loses electrons and is oxidized while oxygen gains electrons and is reduced. The result is formation of carbon dioxide and water.

Three primary ingredients for bioremediation are:

1) Presence of a contaminant

2) An electron acceptor

3) Presence of microorganisms that are capable of degrading the specific contaminant.

Generally, a contaminant is degraded easily if it is a naturally occurring compound in the environment, or chemically similar to a naturally occurring compound, because microorganisms capable of its biodegradation are more likely to grow.

Factors affecting Bioremediation

Microorganisms have limits of acceptance for particular environmental conditions, as well as optimal conditions for peak performance. Factors that affect efficiency of microbial biodegradation are nutrient availability, moisture content, pH, and temperature of the soil matrix.

-Nutrient availability

Inorganic nutrients such as nitrogen and phosphorus are necessary for microbial activity and cell growth. It has been shown that “treating petroleum-contaminated soil with nitrogen can increase cell growth rate, decrease the microbial lag phase, help to maintain microbial populations at high activity levels, and increase the rate of hydrocarbon degradation”. However, it has also been shown that excessive amounts of nitrogen in soil cause microbial inhibition.

-Moisture content

All soil microorganisms require moisture for cell growth and function. Availability of water affects circulation
of water and soluble nutrients into and out of microorganism cells. However, excess moisture, such as in saturated soil, is undesirable because it reduces the amount of available oxygen for aerobic respiration.

-pH

pH of the soil is important because survival of most microbial species are limited to a certain pH range. In addition, soil pH can affect availability of nutrients.

-Temperature

Temperature influences rate of biodegradation by controlling rate of enzymatic reactions within microorganisms. Generally, “speed of enzymatic reactions in the cell approximately doubles for each 10^oC rise in temperature” (Nester et al., 2001). There is an upper limit to the temperature that microorganisms can withstand. Most bacteria found in soil, including many bacteria that degrade petroleum hydrocarbons, are mesophiles which have an optimum temperature ranging from 25 degree C to 45 degree C .Thermophilic bacteria (those which survive and thrive at relatively high temperatures) which are normally found in hot springs and compost loads exist indigenously in cool soil environments and can be activated to degrade hydrocarbons with an increase in temperature to 60 degree C. 

Some contaminants can stick to soil particles and microorganisms are unable to use them for biodegradation. Therefore, under these circumstances, bioavailability of contaminants does not solely depend on the characteristics of the contaminant but also on soil type.

Advantages and disadvantages of Bioremediation

Advantages
1) Bioremediation is a natural process.
2) It is cost effective.
3) Toxic chemicals are destroyed or removed from environment and not just merely separated.
4) Low capital expenditure.
5) Less energy is required as compared to other technologies
6) Less manual supervision.


Disadvantages
1) The process of bioremediaiton is slow. Time required is in day to months.
2) Heavy metals are not removed.
3) For insitu bioremediation site must have soil with high permeability.
4) It does not remove all quantities of contaminants.
5) Substantial gaps exist in the understanding of microbial ecology, physiology and genetic expression and site expression and site engineering. A stronger scientific base is required for rational designing of process and success.

Bioremediation for BTEX

There are an estimated 1 to 2 million underground storage tanks containing gasoline in the United States. Of this number it is estimated that 100,000 to 400,000 are leaking either into the soil or directly into the groundwater. In addition to leaking underground storage tanks, leakage from the subterranean portion of tanks at fuel storage facilities, such as the tank shown below, are contributing to the volume of petroleum hydrocarbons contaminating the subsurface environment.

The United States Environmental Protection Agency (EPA) estimates that roughly 11 million gallons of gasoline per year are lost due to leaking underground storage tanks. Gasoline as well as other fuels contain BTEX which are hazardous compounds regulated by the EPA. These BTEX compounds may comprise greater than 60% of the mass that goes into solution when gasoline is introduced to water.Due to their relatively high solubility, BTEX compounds are the hydrocarbons most frequently reported as groundwater contaminants. Considering that gasoline leaks from underground storage tanks are a major source of groundwater contamination and that almost 50% of the drinking water supply in the United States comes from groundwater wells, there is high potential for drinking water contamination resulting from leaking underground storage tanks.

Once groundwater contamination has occurred, there are numerous techniques available to either contain the pollutant or treat the aquifer. These techniques can range from ex-situ technologies such as excavation and subsequent treatment of aquifer material or pump and treat methods, to physical containment via slurry walls and other impermeable structures, to in situ (a.k.a. intrinsic) remediation via biological and/or chemical transformation of hazardous materials into either less toxic or non-toxic compounds. Of these various techniques, in situ biologically-based remediation has the potential to provide an efficient and cost effective remediation procedure while minimizing site disturbance.

1) Intrinsic bioremediation- The microorganisms which are used for biodegradation are tested for the natural capability to bring about biodegradation. So the inherent metabolic ability of the microorganisms to degrade certain pollutants is the intrinsic bioremediation. The ability of surface bacteria to degrade a given mixture of pollutants in ground water is dependent on the type and concentration of compounds, electron acceptor and the duration of bacteria exposed to contamination. Therefore, the ability of indigenous bacteria degrading contaminants can be determined I laboratory by using the techniques of plate count and microcosm studies. The conditions of site that favour intrinsic bioremediation are ground water flow throughout the year carbonate minerals to buffer acidity produced during biodegradation, supply of electron acceptors and nutrients for microbial growth and absence of toxic compounds.

2) Engineered in situ bioremediation- When the bioremediation process is engineered to increase the metabolic degradation efficiency (of pollutants) it is called engineered in situ bioremediation. This is done by supplying sufficient amount of nutrients and oxygen supply, adding electron acceptors and maintaining optimal temperature and pH. This is done to overcome the slow and limited bioremediation capability of microorganisms.

Advantages and disadvantages of in situ Bioremediation

Advantages of in situ bioremediation
a) The method ensures minimal exposure to public or site personnels.
b) There is limited or minimal disruption to the site of bioremediation.
c) Due to these factors it is cost effective.
d) The simultaneous treatment of contaminated soil and water is possible.

Disadvantages of in situ bioremediation
a) The sites are directly exposed to environmental factors like temperature, oxygen supply etc.
b) The seasonal variation of microbial activity exists.
c) Problematic application of treatment additives like nutrients, surfactants, oxygen etc.
d) It is a very tedious and time consuming process.

Requirements for BTEX Biodegradation

• Presence of appropriate microbial population--generally, microorganisms that are able to degrade petroleum products are ubiquitous in the subsurface environment
• Energy source and a carbon source--organic carbon is utilized both as an energy source by releasing electrons during transformation and it is also used by the cell for maintenance and growth
• Electron acceptor--the electrons released by the carbon transformation must be taken up by some other chemical
• Nutrients--for bacterial growth to occur certain nutrients are needed (e.g., nitrogen, phosphorous, calcium, potassium, magnesium, iron, etc.)
• Appropriate environmental conditions--microbial activity is dependent upon many environmental conditions such as temperature, pH, salinity, pressure, concentration of pollutants, and presence of inhibitors.

Electron Acceptors and Electron Tower Theory

In situ bioremediation can be divided into natural and enhanced methods. As noted in the previous section, naturally occurring bioremediation occurs when a sufficient energy source, carbon source, electron acceptor concentration, and nutrient concentration are available to a native biological population. The rate of naturally occurring bioremediation of BTEX compounds is often limited by either the concentration of an appropriate electron acceptor or a nutrient needed during the transformation of the BTEX into a non-toxic compound. Enhanced in situ bioremediation attempts to stimulate biodegradation by adding either the limiting electron acceptor or the appropriate nutrients to the subsurface environment until the (hydro)carbon substrate becomes the limiting factor in reaction kinetics.

Typical electron acceptors utilized by microorganisms are oxygen, nitrate, iron (III), sulfate, and carbon dioxide. When oxygen is utilized as the electron acceptor, microbial respiration is termed aerobic. When other electron acceptors are utilized, it is termed anaerobic. Depending on the mode of respiration, microbes can be classified into three categories: (1) aerobic; (2) anaerobic; and (3) facultative Aerobes thrive only in oxygenated environments using dissolved oxygen as an electron acceptor. Strict anaerobes grow only under highly reduced conditions, where oxygen is effectively absent. Strict anaerobes use electron acceptors such as sulfate or carbon dioxide. Many microorganisms are able to adapt to both aerobic and anaerobic conditions, but are typically more active in the presence of oxygen .These organisms are termed facultative, and most microbes utilizing nitrate as an electron acceptor tend to be facultative.The following figure adapted from Jorgensen (1989) illustrates the sequence and products of electron acceptor utilization for oxidation of organic carbon.

The electron tower, as depicted above, relates the amount of energy a given microbial population can gain from electron acceptor to the electron acceptors position on the 'tower'. Microbes tend to oxidize organic substrates by the using the electron acceptor that provides the most energy. Note that oxygen, which is at the top of the electron 'tower', provides microbes with more free energy (via oxygen reduction) than any other electron acceptor. Carbon dioxide, which is used as the electron acceptor by methanogenic bacteria, yields the least energy of all the electron acceptors, and is therefore located at the bottom of the 'tower'. Thus, the electron tower provide above schematically depicts the order of electron acceptor utilization based on the free energy a given microbial population can gain from reduction of a given electron acceptor.

Field evidence also seems to suggest that natural in situ bioremediation may employ different electron acceptors at various locations throughout a given site. Lyngkilde et al. (1991) report an electron acceptor utilization order determined by measuring field concentrations for each electron acceptor. The trends observed at this observed at this site, as schematically depicted in the figure below, indicate that respiratory conditions of the plume vary from highly reactive aerobic conditions, through anoxic nitrate and iron reduction, to highly reduced sulfate and methanogenic conditions. Note that the order of electron acceptor respiration agrees well with the utilization order predicted by the electron tower theory.

Figure adapted from Lyngkilde et al., 1991. 

Source: Brauner (1995)

Evidence of Biological Degradation of BTEX Compounds

Early research into in situ bioremediation identified only aerobic degradation of petroleum hydrocarbon groundwater contaminants. Further research, however, indicated that petroleum hydrocarbons, such as benzene, toluene, ethylbenzene, and xylenes (the BTEX compounds), are some of the most aerobically biodegradable found in the subsurface environment . Petroleum aromatic compounds have been shown to degrade by cleavage of the aromatic carbon ring as shown here for benzene:

Although natural or artificial recharge may stimulate aerobic biodegradation by reintroducing oxygen to anaerobic regions, the low solubility of oxygen and the rapid reaction rates typical of aerobic environments can severely limit the aerobic biodegradation of petroleum hydrocarbons

Recent research has recognized the importance of anaerobic degradation of aromatic hydrocarbons, as shown for the BTEX compounds in Table 1 below. Research has shown degradation of aromatic hydrocarbons using nitrate, iron (III), and sulfate as electron acceptors (see Baker and Herson, 1994 for summary). Methanogenic contaminant reduction (i.e., using carbon dioxide as an electron acceptor to form methane gas) has also been demonstrated, but at significantly slower rates than the other degradation processes.The most recent discovery of iron (III) reduction is evidenced by reduced contaminant concentrations and increased levels of aqueous phase iron (II) under anaerobic conditions. Additional research shows that iron (III) is preferentially used over sulphate, but also shows that iron (III) utilization is inhibited by the presence of oxygen and nitrate. For the BTEX compounds in particular, research has shown toluene, ethylbenzene, and some of the xylenes to degrade anaerobically, with toluene being the most anaerobically degradable. Anaerobic degradation of ethylbenzene and xylene appear to be most significant when they are cometabolized with toluene. Evidence of anaerobic degradation of benzene, though, has been inconclusive.

Table 1. Research supporting biodegradation of BTEX compounds. (Modified from Bedient et al., 1994).

Source: Brauner (1995)

Anaerobic biodegradation rates for aromatic hydrocarbons are typically an order of magnitude or more less than aerobic rates. However, anaerobic biodegradation may still significantly influence substrate reduction due to longer reaction times. These longer reaction rates can be attributed to the inefficiency of anaerobes relative to aerobes and the inhibitory effects of alternate electron acceptors present in the subsurface, especially oxygen. The most important factor in determining if a contaminant plume can be successfully remediated may be the identification of the terminal electron accepting process.

Casestudy 1

Examples of Bioremediation Description 18: U.S. Army Contracting Command Overseas Remediation.

An overseas U.S. Army Military Base. This base has been active since the Korean War. A Lieutenant with the Army Corps of Engineers contacted Alabaster Corp. about a bioremediation project he was conducting. For many years large volumes of military equipment had been utilized at the subject location. The location had many UST or underground fuel storage tanks as well as AST or above ground fuel storage tanks. The facility operates and maintains a large amount of equipment ranging from typical tractors, cranes, bulldozers, and other earth moving apparatus to heavy military items such as tanks and other combat designed vehicles. The concrete slab throughout most of the very large subject area was heavy and thick to withstand the tremendous weight of the equipment. Large amounts of hydrocarbons had accumulated around and underneath various large cemented areas as well as within soil or non-cemented areas.

Various Army Engineering experiments with bioremediation had showed only typical to minimal results. The current options being considered were destroying the concrete slab, excavating the contaminated earth and re-filling this with clean fill dirt before relaying the new concrete slab. This included several huge area around a large military base. The officers in charge were making a last attempt to keep this project within a budget. Many details must be left confidential.

Pollution: Hydrocarbons Accumulated From Years of Operation.

Large volumes of hydrocarbon contamination. Total TPH ranged from minimal detection levels to as high as well over 100,000 mg/kg. The concrete itself had a saturated amount of hydrocarbons accumulated within. The drinking water supply was effected. Ground water contamination had detectable amounts of BTEX components and other GRO or gasoline range organics. Many details must be left confidential.

Solution: Alabaster Corp. Bioremediation Products. Slab Injection, etc.

A very large volume of Alabaster Corp. Bioremediation products were supplied to the U.S. Army Contracting Command for this overseas base. These included products like our BCC#1 Concentrate (Sold as CS2 or Super Concentrate) and microbial blend AB with Booster.

Case Studies in Bioremediation Results: The Project was Successful.

The Army engineers involved were well acquainted with the process of bioremediation and utilizing the products. They were more than typically equipped and knowledgeable about various techniques involved with applying the products. They had several flexible and creative solutions to contracting dilemmas which would have made this project difficult and more expensive for most. Many details must be left confidential. However, we were verbally assured that they remediated “the substances” they intended. Over the next few months they tripled the original order.

Casestudy 2

Background


Gasoline  SPH has been historically present in three monitor wells for a number of years.  Three  mobile  dual-phase extraction events conducted by another provider fail

Hydrogeology

Groundwater is present in  silty clay at depths ranging from approximately  3 to  7 feet below grade

Treatment Methodology
EcoVac Services was contacted to implement SURFAC®and ISCO-EFR® at this site to remove SPH and reduce BTEX concentrations below the site’s CALs.  EcoVac Services’  patented  SURFAC®and ISCOEFR® processes are the combination of surfactant and oxidant injection, respectively, with dual phase/multi-phase extraction.   The processes described herein  are  patentprotected and represent the intellectual property of EcoVac Services, Inced to remove SPH at the site.




SURFAC® and ISCO-EFR® Implementation
A single SURFAC® application was implemented at the site in September and October 2007 (four field days)  to remove SPH from three monitor wells. ISCO-EFR® was implemented in July 2008 and October 2009  (a total of four field days)  to reduce BTEX concentrations in four monitor wells to below CALs




Results and Conclusions
A single  SURFAC® application successfully removed SPH from this site (four field days). ISCO-EFR® was implemented using Activated Sodium Persulfate which  reduced BTEX concentrations to well below the site’s CALs (four field days).  A table showing the reduction in BTEX  concentrations achieved by ISCO-EFR® is shown below

References

Bioremediation: Hope / Hype for Environmental Cleanup

For those who would like to get a better understand of Bioremediation, I would recommand you to watch the video. It's really helpful.