Btex..

Benzene, Toluene, Ethyl Benzene and Xylene (BTEX) are the volatile components commonly associated with petroleum products. Benzene, toluene and xylenes are found naturally in petroleum products like crude oil, diesel fuel and gasoline. Ethylbenzene is a gasoline and aviation fuel additive. The term BTEX reflects that benzene, toluene, ethylbenzene and xylenes are often found together at contaminated sites. Because they are all highly toxic and soluble in water, they represent a significant hazard for humans.The main source of BTEX contamination is the leakage of gasoline from faulty and poorly maintained underground storage tanks. They are considered one of the major causes of environmental pollution because of widespread occurrences of leakage from underground petroleum storage tanks and spills at petroleum production wells, refineries, pipelines, and distribution terminals. Because of the high concentration of BTEX compounds in petroleum and the massive use of petroleum products as energy source, as solvents and in the production of other organic chemicals, their presence in water creates a hazard to public health and the environment. Contamination of groundwater with the BTEX compounds is difficult to remedy because these compounds are relatively soluble in water and can diffuse rapidly once introduced into an aquifer. Advanced detection methods of BTEX contamination are being developed nowadays. This paper describes one of the advanced methods namely Detection using microchip induced laser fluorescence (LIF). It is a very compact system and can be used to detect contamination in both soil and water. Thus even though one of the BTEX compounds (ethylbenzene) cannot be detected by this method, it is most commonly used.

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BTEX is the abbreviation used for four compounds found in petroleum products. The compounds are benzene, toluene, ethylbenzene and xylenes. These organic chemicals make up a significant percentage of petroleum products like crude oil, diesel, gasoline etc. Ethylbenzene is a gasoline and aviation fuel additive. They are also used extensively in manufacturing processes.

is used in the production of synthetic materials and

consumer products, such as synthetic rubber, plastics, nylon, insecticides and paints. is used as a solvent for paints, coatings, gums, oils and resins. may be present in consumer products such as paints, inks, plastics and pesticides. are used as a solvent in printing, rubber and leather industries.

The BTEX chemicals are present in a standard gasoline blend in approximately 18%(w/w), and the group is considered to be the largest one that is related to any health hazards. Naphthalenes make up only 1%(w/w) of gasoline. Benzene, which is recognized as the most toxic compound among BTEX, represents 11%, toluene represents 26%, ethylbenzene 11% and xylene 52% of the total BTEX fraction in gasoline.

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BTEX contamination of soil and groundwater can occur by the accidental spill of gasoline, diesel fuel and leakage from underground storage tanks in pumping stations. Once released to the environment, BTEX can volatilize, dissolve, attach to soil particles or degrade biologically. Volatilization occurs when chemicals evaporate, allowing them to move from a liquid into the air. Volatilization of the BTEX components of gasoline commonly occurs when you pump gasoline into your car, and is responsible for the characteristic odour. This phenomenon can also occur within the air pockets present in soils. BTEX can also dissolve into water, allowing it to move in the groundwater. Since BTEX can "stick" to soil particles, these chemicals move slower than the groundwater. BTEX can also dissolve into water, allowing it to move in the ground water. Because of their polarity and very soluble characteristics, BTEX will be able to enter the soil and groundwater systems and cause serious pollution problems. If oxygen is present in sufficient quantities, BTEX can also degrade biologically, though very slowly.

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Exposure to BTEX can occur by ingestion, inhalation or absorption through the skin. Inhalation of BTEX can occur while pumping gasoline or while showering or bathing with contaminated water. Absorption of these chemicals can occur by spilling gasoline onto one's skin or by bathing in contaminated water. Acute exposures to high levels of gasoline and its BTEX components have been associated with skin and sensory irritation, central nervous system depression and effects on the respiratory system. These levels are not likely to be achievable from drinking contaminated water, but are more likely from occupational exposures. Prolonged exposure to these compounds has similar effects, as well as the kidney, liver and blood systems. According to the U.S. Environmental Protection Agency (U.S. EPA), there is sufficient evidence from both human and animal studies to believe that benzene is a human carcinogen. Workers exposed to high levels of benzene in occupational settings were found to have an increase incidence in leukaemia.

The U.S. EPA has established permissible levels for chemical contaminants in drinking water supplied by public water systems. These levels are called Maximum Contaminant Levels (MCLs). To derive these MCLs, the US EPA uses a number of conservative assumptions, thereby ensuring adequate protection of the public The MCL is set so that a lifetime exposure to the contaminant at the MCL concentration would result in no more than 1 to 100 (depending on the chemical) excess cases of cancer per million people exposed.

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The U.S. EPA recommends that exposure to BTEX be minimized. To avoid or reduce exposure to BTEX, people should use water supplies having concentrations of these compounds that are below the MCL or apply appropriate water treatment or filtration systems. If necessary, short-term reductions in exposure may be accomplished by using bottled water for food and beverage preparation and avoiding bathing or showering with the contaminated water. With in-home treatment processes, such as activated charcoal filtration, it is usually possible to remove sufficient BTEX from water to meet the MCL and thereby minimize health risks. If benzene is present above the MCL, treatment should be applied to all household water because of inhalation hazards.

Since the BTEX compounds are very toxic to humans and aquatic life, their sensitive and rapid determination is of critical importance. There are lots of established methods for determining BTEX contaminants in water namely liquid-liquid extraction, 6

solid phase extraction, gas chromatography, air stripping etc. But these methods exhibit high levels of sensitivity and selectivity. So they require well-trained personnel for its successful operation. If a small error occurs during sampling, the analytical result obtained using the best instrument will be inevitably wrong. Most existing methods for detecting BTEX are time-consuming, complicated and very expensive for routine screening. Also these methods require skill for its operation. There has been a lot of development in this area recently and many advanced techniques for the detection of BTEX contaminations have been developed. The use of lasers and optic fibres are some among them. Three advanced techniques of detection of BTEX contamination are: 1. Raman Dipstick method 2. Bioassay method 3. Detection using Microchip Induced Fluorescence Sensor

Raman dipstick method is the detection of BTEX contamination using long path length fibre optic Raman dipstick. Determination of BTEX components via optical remote sensing is attractive because eliminates many of the problems in other established methods. Samples are interrogated through the long-path length ‘dip-stick’. It is directly inserted into the liquid of interest or an extension hose is attached to the end of the ‘dipstick’, providing a low profile and more flexible means of sample interrogation. Fibreoptic spectroscopic techniques used for detection include visible absorption, infrared absorption, fluorescence and Raman spectroscopy. Of these techniques,

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Raman spectroscopy is particularly strong candidate for detecting BTEX analytes in water because it offers a high degree of selectivity and is compatible with aqueous matrices. Even though this method is very simple and cheaper, practically a lot of problems are there. Turbidity of the sample could block collection of Raman scattering from the sample. Also the presence of interfering compounds can lead to diminished sensitivity. If the interfering compounds are fluorescent it will mask Raman signals.

Detection of BTEX compounds using Toluene Dioygenese peroxide coupling reaction is called bioassay method. It is simple, sensitive, whole-cell-based bioassay system for detection of bio-available BTEX compounds based on a method developed for screening of oxygenese activity. This bioassay system requires no sophisticated instruments and exquisite techniques. The bioassay has long term storage stability so that it can be used for field monitoring of BTEX compounds and its tracking in contaminated water. The convenience of multiple sample-handling makes this whole cell assay an attractive candidate to be developed as a field diagnostic method for on-site BTEX contamination.

The Laser-Induced Fluorescence (LIF) method takes advantage of both time and wavelength information to investigate the contamination of BTX compounds in soil and water. This method is studied in detail in the next chapter.

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Most organic molecules when excited with ultra rays re emit less energetic optical radiation. This emitted radiation is known as fluorescence and is characterized by its intensity as a function of both time and wavelength. Since this information is linked to the physical characteristics of an individual molecular species, it provides a powerful means to perform chemical analyses. By the observation of wavelength and time we can detect, identify and quantify the chemical species within an aqueous solution. The Laser-Induced Fluorescence (LIF) takes advantage of both time and wavelength information to investigate the contamination of BTX compounds in soil and water. The device provides excitation using a passively Q-switched microlaser pumped by fibre-coupled near-infrared diode laser and generates short pulses of 266nm radiation at a repetition rate near 10 kHz. The microchip laser focusing optics and collection system are very compact and the entire assembly can be placed in a monitoring well or contained within the shaft of a cone penetrometer. Thus the UV radiation necessary to excite fluorescence in environmental pollutants such as gasoline is generated at the point of contamination while the infrared diode pump laser remains above the ground. This configuration takes advantage of the excellent transmission of infrared energy through fibre optics cable and minimizes the ultraviolet attenuation.

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This is a case study done by Bloch. J, Germaine J.T, Hemond H.F and Johnson.B.Sinfield.

The experimental apparatus used to evaluate the performance of the LIF probe includes spectroscopic hardware, a test cell and a data acquisition system.

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The microchip laser is pumped by a 1 W continuous wave 808nm diode laser through a fibre optic cable. The microchip laser is mounted in the probe using a threaded fitting that contains a sapphire window and an UV silica focusing optic. This laser is passively Q-switched and provides 200ps pulses of 1064nm light at a repetition rate of approximately 10 kHz. After two stages of frequency doubling and subsequent filtering to remove 1064 and 532nm lights, a fourth harmonic is generated in the UV at 266nm which is focused just outside the probe’s sapphire window. Molecular fluorescence excited by the UV microchip laser is imaged through the same window onto the tip of a 550µm diameter silica return fibre. The output of the fibre is focused on the entrance slit of a 1/8m scanning monochromator. A fast photomultiplier tube (PMT) is used to detect light intensity at the exit slit of the monochromator. A trigger signal is generated using an UV silica beam splitter mounted within the monochromator to direct a small fraction of the light entering the spectrometer on to a second PMT. The PMTs are operated at approximately 800 V.

The test cell employed in the laboratory experiments is used to simulate immersion of the probe in a liquid. The cell consists of a rectangular stainless-steel block clamp that is placed around the probe. The test sample is placed in a cylindrical hole in the clamp located directly above the laser output window of the probe. The hole contains approximately 1.5cm3 of sample solution, although only a small fraction of this volume is actually interrogated by the laser. Sample loss around the probe/clamp interface is prevented using a Teflon gasket. Volatilization is prevented by a stainless steel cap fitted with a fluorocarbon rubber o-ring.

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A 1.5 GHz digital storage oscilloscope is used as a fast analog-to-digital converter to acquire fluorescence signals at a sampling rate of 5 giga-samples per second for a period of 50ns referenced to the trigger; 500 traces are typically averaged for each measurement. The PMT output signal is measured across a 50Ω load. A personal computer is used to control the monochromator grating and the oscilloscope.

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Test solutions were prepared by dilution of stock solutions, which comprised distilled demineralised water maintained in equilibrium with reagent-grade compounds. During each test, a volume of the desired sample was sealed in the test cell with zero headspace. This procedure minimized loss by leakage or volatilization throughout the duration of a LIF test. Fresh solutions were used for each test. In addition, blanks of distilled demineralised water were measured whenever test solutions were changed, to demonstrate that no residual contamination remained in the test cell.

A series of tests were performed to determine the sensor’s sensitivity to BTX compounds and its time-response. Each test involved recording the time-dependent fluorescence spectrum (from 275 to 350nm) of one of the BTX compounds at a particular concentration in water. The spectra from each test were analyzed to determine: 1. The total fluorescence signal gathered from the test medium 2. The fluorescence lifetime of the compound in solution 3. The wavelength of the peak fluorescence emission 4. The peak fluorescence intensity The total fluorescence signal was determined by integrating over time and wavelength using trapezoidal integration; results are presented in arbitrary units. Absolute signal amplitudes were typically on the order of 101mV at the oscilloscope input for peak concentration levels. A plot of the fluorescence signal, integrated in time, versus emission wavelength is referred to as an emission wavelength spectrum.

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A plot of fluorescence signal versus time, at any individual emission wavelength, is termed as an intensity-time trace. The collection of all of the information available from a fluorescence test, in terms of both the time and wavelength, can be presented in a threedimensional plot referred to as a wavelength-time-intensity (WTI) profile. The peak fluorescence signal is defined as the highest intensity observed at any wavelength. Further, the total fluorescence signal is the volume under the WTI profile. Each compound has different fluorescence lifetime. Over the majority of the concentration ranges, each compound is having a linear relationship among the peak signal, the total fluorescence signal and the concentration of a compound in solution. The slopes of the lines relating compound concentration to either peak signal (the highest intensity observed at any wavelength) or total fluorescence signal (volume under the WTI profile) differ among the three compounds.

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The fluorescence-to-concentration relationship, essentially a calibration factor, varies among compounds for two reasons. First, the UV absorptivity varies for different compounds. Under equivalent conditions, a solution of benzene, for example, absorbs an 15

order of magnitude less at 266 nm than does an equimolar concentration of xylene. Second, the quantum yield (ratio of the number of emitted photons to absorbed photons) also differs among compounds. The quantum yield of benzene, for example, is less than one-third of that for o-xylene.

Detection of the presence of a contaminant relative to a baseline or background is the least challenging operation for a sensor. To achieve this goal the sensor need only reliably indicate the presence of a signal in excess of the "uncontaminated" background. The criterion for the detection of a compound was defined to be a minimum signal level of approximately three times the standard deviation of the background. Assuming that the random noise component of the background, which cannot be effectively subtracted from the fluorescence signature, can be characterized by a normal distribution, the three times standard deviation criterion should include approximately 99.9% of the noise. Thus any signal above this threshold could reasonably be interpreted as the product of fluorescence. The background associated with this LIF system was evaluated from ten full-spectrum scans of water blanks. The ten background scans has to be carried out on different days over a period of 3 months. The standard deviation of the background signal can be found out. The corresponding detection limits for benzene, toluene and o-xylene can be calculated. These values were obtained by dividing the three times standard deviation minimum required signal level by the slope of the plot relating peak fluorescence signal to aqueous concentration for each of the BTX compounds.

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Even though we can detect the presence of various BTEX compounds in water, the LIF can detect only Toluene and Xylene effectively. Benzene is notably more challenging top detect while Ethyl benzene cannot be detect at all. The LIF sensor can accurately measure fluorescence lifetimes as short as 2.5 ns. Also there is a clear straightline relationship between contaminant concentration and observed fluorescence intensity that can be used to calibrate the LIF sensor for quantitative analyses over environmentally relevant concentration ranges. It is recognized that the performance of the LIF sensor will degrade in natural environments involving contaminant mixtures and non aqueous media. These experiments also highlight that the capabilities of the LIF sensor are directly linked to the performance characteristics of the data acquisition system and, more importantly, illustrate the need to distinguish among a contaminant sensor’s ability to detect, to identify and to quantify chemical compounds. Each of these capabilities requires data of a different nature and quality, and success in achieving any of these levels of measurement can enable valuable practical applications.

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1. It is a very compact collection system. So it can be placed in a monitoring well or within a cone penetrometer. 2. LIF can be used for the detection of contamination both in water as well as in soil. 3. The intensity of fluorescence is a function of wavelength and time, which is linked to the physical characteristics of an individual molecular species, provides a powerful means to detect the contaminants. 4. It has the ability to detect the presence of a compound in solution or recognize a change in state, relative to background conditions. So it helps in finding leaks in landfill systems or indicates the presence of harmful agents in water. 5. Since it is possible to detect, identify and quantify the contamination, it is easy to select the type and extent of treatment to be given.

1. It is very difficult to detect the presence of Benzene in water. Also Ethylbenzene cannot be detected at all. 2. The entire system is costly as it has sophisticated instruments.

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This treatment is used after groundwater has been pumped out of the aquifer. The contaminated water is passed through the organoclay and carbon unit where the organics are adsorbed and collected. This is accomplished through the adsorption of the chemical substance onto a carbon matrix. The effectiveness of this process is related to the quality of the organoclay and the properties of the contaminants. This process can be extremely effective when using a high quality organoclay such as Aqua Technologies ET1 Activated Clay. Because of the surrounding groundwater petroleum high interest in area. The petroleum subsurface requires the and techniques. has been a lot within this how to set up processes is

health concerns soil and contamination from products, there is a researching this continual release of products into the environment need for monitoring remediation Even though there of research going on area the problem of actual remediation still very complex

Direct push groundwater circulation wells (DP-GCW) are a promising technology for remediation of groundwater contaminated with dissolved hydrocarbons and chlorinated solvents. In these wells, groundwater is withdrawn from the formation at the bottom of the well, aerated and vapor stripped and injected back into the formation at or above the water table. Previous field studies have shown that: (a) GCWs can circulate significant volumes of groundwater; and (b) GCWs can effectively remove volatile compounds and add oxygen In this work, we describe the development and field-testing of a system of DP-GCWs for remediation of volatile organics such as benzene, toluene, ethylbenzene, and toluene (BTEX). The GCWs were constructed with No. 20 slotted well screen (2.4 cm ID) and natural sand pack extending from 1.5 to 8.2 m below grade. Air is introduced ~ 7.5 m below grade via 0.6 cm tubing. Approximately 15% of the vertical length of the air supply tubing is wrapped in tangled mesh polypropylene geonet drainage fabric to provide surface area for biological growth and precipitation of oxidized iron.

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These materials were selected to allow rapid installation of the GCWs using 3.8 cm direct push Geoprobe® rods, greatly reducing well installation costs. Laboratory testing of these sparged wells and computational fluid dynamics (CFD) modeling showed that these wells, although they used only about 1 L/min of air, could circulate about 1 L/min of water through the surrounding aquifer. This flow was sufficient to capture all of a flowing contaminant if the wells are sufficiently closely together, about 1 meter on center depending on the air flow rate supplied, in a line across the plume. The CFD work showed the details of this ability to capture, and also showed that unforeseen heterogeneities in the aquifer such as a gradient of permeability or a thin impermeable layer (such as a clay layer) did not prevent the system from working largely as intended. The system was tested in a petroleum contaminated aquifer near Rocky Point, NC. The contaminant plume there is approximately 10 m deep, 50 m wide and contains up to 4 mg/L total BTEX and 75 mg/L dissolved iron. An extensive pilot test was first performed to estimate the zone of influence for a single well. At this site an air injection rate of 1.2 L/min resulted in a water flow rate of 1 to 2 L/min based on bromide dilution tests in the GCW. The GCW increased the dissolved oxygen concentration in the discharge water to between 6 and 8 mg/L and reduced contaminant concentrations to less than 20 μg/L total BTEX. Monitoring results from a 73 day pilot test were then used to define the zone of influence for a single DP-GCW and to design a full scale barrier system

IN SITU ISCO involves the delivery of chemical oxidants directly to the subsurface contamination source zones and downgradient groundwater contamination plumes. This is commonly achieved by either temporary injection points or permanent injection wells (see Exhibit 2). Upon direct contact with organic contaminants, a chemical oxidation reaction occurs, which mineralizes the contaminant compound and produces non-toxic end products such as carbon dioxide (CO2), water, and in the case of chlorinated solvents (e.g., trichloroethene), inorganic chloride salts (Interstate Technology Regulatory Council [ITRC] 2005). The contaminants susceptible to chemical oxidation include total petroleum hydrocarbons (TPH)

BTEX contamination is a threat to the mankind as well as to animals and plants. Prolonged exposure to the compounds even in small quantities is highly fatal. The reason why the BTEX entering our soil and groundwater system, are considered such a serious 20

problem is that they all have some acute and long term toxic effects. Benzene is carcinogenic to humans. So the detection of these compounds is of utmost importance. There are a lot of advanced methods of detection of BTEX contamination emerging nowadays. One of the advanced techniques, detection using laser induced fluorescence (LIF) is studied in this paper. LIF is a very compact system. This method detects contaminants relative to a baseline or background. This method of detection is quick compared to the other methods which are time consuming. One of the BTEX compounds, ethylbenzene cannot be detected using this method. Since it is possible to detect, identify and quantify the BTX contamination, it is easy to select the type and extent of treatment to be given. Though this method is a bit costly, it provides a powerful, accurate and reliable means to detect the contaminants in both water and soil.

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