IJCRR - 5(3), February, 2013
Pages: 14-22
Date of Publication: 18-Feb-2013
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POLYCHLORINATED DIBENZO P DIOXINS AND FURANS- A REVIEW
Author: D. J. Mukesh Kumar, D. Deepika, B. Srinithya, P. T. Kalaichelvan
Category: General Sciences
Abstract:Polychlorinated dibenzo p dioxins and furans are considered as the highly toxic pollutants among the organic pollutants emitted from various industries as byproducts. They are almost found in all the section of the global ecosystem. They can persistent in the environment and with stand for a long period of time and considered to be accumulated in the food web. This review paper describes about the different sources of emission of this organic compound from the environment. Various remediation methods for the reduction of PCDD/PCDF from the environment were also discussed in this paper.
Keywords: Persistent organic pollutants, Dioxins, Toxic equivalent factor, Sources
Full Text:
INTRODUCTION
Persistent organic pollutants (POPs) are the most dangerous pollutants released by human activity into the environment. Persistent organic pollutants have been released as unwanted byproducts from industries for decades. In current days, these POPs reveals few unusual characteristics like Persistence iREMEDIATION/REDUCTION OF PCDD/PCDFn environment, Bio-accumulation, and can travel to a long distance. They also cause adverse health effects to both humans and animals since it is highly toxic and persistent in the environment (GEF, 2009) There are various types of persistent organic pollutants; they are short listed into 12 compounds which also include polychlorinated dibenzo-pdioxin and polychlorinated dibenzofurans (PCDD/PCDF). Some forms of carbon such as coal, charcoal, graphite, inactivated amorphous carbon can generate PCDD/PCDF. The generation of PCDD/PCDF from these carbon sources is due to the degeneration and deformation of graphite structures. (Physician Consensus Statement, March 1998) The polychlorinated dibenzo-p-dioxin (PCDD) has 75 derivatives and polychlorinated dibenzofurans (PCDF) have 135 derivatives which are commonly referred to ‘dioxins’ (Kutz et al., 1999). These 210 individual derivatives of PCDD and PCDF have been emitted from both thermal and industrial processes. Characteristics of Higher chlorinated PCDD/PCDF (Kutz et al., 1999).
- Less soluble in water
- Low volatile
- Absorb strongly to surfaces
MODE OF TRANSFER TO ENVIRONMENT
Terrestrial food chain
Air→plants/soil→cattle→milk/meat→man PCDD/PCDFs are less lipophillic, poor water soluble and hence bind to the granular particles such as soil and sediments. They might also accumulate in adipose tissues of organisms. They are accumulated on plant surfaces through deposition (wet/dry) of chemicals which adhere to the particles present in the atmosphere and also by diffusive transport which transfers gaseous chemicals in the air to the plant surfaces. Some amounts of PCDD/PCDFs rested on soil, move back to the environment by resuspending less chlorinated congeners (Kutz et al., 1999).
Release to air
Release of PCDD/PCDFs into air is either by stationary or diffused sources. Stationary source: This particularly involves industrial activities such as production and manufacturing processes. Diffused or dispersed source- which mostly relates to products containing PCDD/PCDFs. Some of the processes which release PCDD/PCDFs into air are:
1. Combustion processes
2. Metal processing operations
3. Drying and baking operations
4. Thermal processes
Even though the development of many technologies have tried to decrease and control the levels of emission of PCDD/PCDFs into air, it is more important to prevent the shifting of emission from one media to another (Environment and Social Development Organization, 2005) Release to water Dumping of wastes, applying pesticides directly, discharge of waste water runoff from contaminated sites result in release of PCDD/PCDFs into water (Environment and Social Development Organization, 2005) Release to land Direct application of contaminated products and deposition of PCDD/PCDFs through environmental process are the ways of releasing PCDD/PCDFs into land (Environment and Social Development Organization, 2005)
MODE OF ACTION
PCDDs and PCDFs are collectively called as dioxins which enter into human cell by using Aryl hydrocarbon (Ah) receptor which is generally present in the cell. The two main remarkable things about aryl hydrocarbon receptor are:
1. Even after 30 years of research, scientists do not know why it is present in the cell. Ah receptor’s normal ligand and its function are yet unknown.
2. Every species above invertebrates has the Ah receptor.
TOXIC EQUIVALENT FACTOR (TEF)
Toxic equivalent factor (TEF) was developed to rank the toxicity level of individual Dioxins. Toxicity equivalent factor helps to represent increasing toxicities of complex PCDD/PCDF mixtures and are expressed as a single Toxic equivalent quantity value (Kutz et al., 1999). Polychlorinated Biphenyls toxicity equivalents are similar to 2, 3, 7, and 8,-TCDD and concentration of PCDD/PCDFs congeners are expressed through TEFs which helps us to know risk levels. It is used to evaluate human health risks posed by PCDD/PCDF mixtures by using TCDD (Tetra Chloro dibenzo p dioxin) as an index chemical (Risk Assessment Forum., 2010). To apply TEF method, dose-response function must be known since toxicity of index chemical gives scaling factor for each Dioxin like compounds (DLC) and this when multiplied with environmental exposure concentration gives individual PCDD and PCDF dose equal to index chemical dose. TEF values are determined by correlating the ratio of molar dose of Tetrachloro dibenzo-p-dioxin to produce 50% effect to that of chemical which is to be tested to produce 50% effect. The TEC (TCDD Equivalent Concentration) can be known by multiplying the TEF value of an individual dioxin to the concentration of that derivative in the tissue sample (Michael et al., 1996). Then the total TEC can be obtained by summing up all the TEC values of each congener. TEQ can be calculated by using the following equation (Otto Hutzinger et al., 2000) TEQ = ((PCDDi × TEFi) n ) + ((PCDFi × TEFi) n )
EXPOSURE AND HEALTH RISK ASSESSMENT
Exposure of PCDD/PCDFs into the environment affects the people severely. They were exposed to these compounds by inhaling them from air, eating plants and animals which are accumulated by high concentration of PCDD/PCDFs. These compounds alter the endocrine functions which end up infertility disorders, pregnancy problems. These are due to the strong resemblance/correspondence of the dioxin structure to the sex and steroid hormones. It also causes inflection in thyroid and testosterone levels in plasma, attenuating neurological effects and reduced glucose tolerance (WHO, 1996) Human carcinogenesis was predicted by study which was done based on human exposures by gathering an occupational group who differ in the order of magnitude of accumulation of PCDDs in tissues more than background levels (McGregor et al., 1998). A study explains the quantitative information about dose-response relationship. The dose-response relationship is considered to be important (McGregor et al., 1998). From the data collected in 1997, based on the results of single cohort studies done on small number of workers exposed to contaminated herbicides or on the basis of animal carcinogenicity, the results were not consistent and no assessment of PCDD/PCDFs were made (McGregor et al., 1998). In humans, dioxins carried to newborn can be determined by plasma concentrations where it stays in blood and onto the new born (Farland., 2003). As drug-receptor complex results in biological response similarly plasma concentrations can result in human exposure data whereas for animals Physiologically-Based Pharmacokinetic models are used to provide reasonable values (Farland., 2003). The exposure of PCDD/PCDF to humans occurs through intake of animal as a major source of food. Several countries have characterized Tolerable Daily Intake (TDI) of PCDD/PCDF depending on No Observed Adverse Effect Level (NOEAL) acquired from animal tissues (Kulkharni et al., 2007). A Tolerable Daily Intake for TCDD at 10pg/kg was established by WHO in 1990, based on TCDD induced liver cancer in rats. WHO highlighted that if the average intake over a long periods is not exceeded, then the TDI expresses a Tolerable Daily Intake for lifetime exposure and that irregular excursions above TDI would have no health effects. The range of TDI is the equal to the typical/average daily intake by all passages of emission to PCDD/PCDFs estimated as TEQs. WHO suggested that TDI is not applicable to breastfed newborns as the approach of TDI is in accordance with the dose absorbed throughout the lifespan (WHO, 1996). In addition to this, another consultation reported that even though for many concerns, humans might be less sensitive than animals, there are still some ambiguities remain regarding animal to human susceptivities. Moreover there are many dissimilarities occur in half lives for destruction of different TEQ compound mixtures. A combined uncertainty factor of 10 was proposed in order to account for all such ambiguities. By applying this uncertainty factor of 10, a TDI (Tolerable Daily Intake) range was developed (Otto Hutzinger et al., 2000).
PCDD/PCDFs SOURCES
Incineration sources
Incineration is the waste treatment process that involves the burning of the organic substances which are present in the waste materials. They are been treated at high temperature which transfers the waste in Ash or flue gas. The incinerator units are categorized into
? Municipal Waste Combustors (MWC),
- Medical Waste Incinerators (MWI),
- Hazardous Waste Incinerators (HWI),
- Boilers and Industrial Furnaces (BIF),
- Cement kilns (CK),
- Biomass Combustors (BC).
i. Municipal waste incinerators
Municipal incineration is the most expensive technique. They involve skilled working and maintenance of the incineration, this technique is commonly being adopted by the developed companies The flue gases of Municipal solid waste incinerators are responsible for the emission of PCDD/PCDFs (Kim et al., 2001). The presence of dioxins and their precursors in municipal solid waste were observed to be around 50ng I-TEQ/kg (Abad et al., 2002). Some municipal waste incinerators include a. Mass Burn Refractory-Wall, b. Mass Burn Water Wall, c. Refused Derived Fuel, d. Modular Starved Air, e. Modular Excess Air f. Rotary Water Wall (Thomas et al., 1995)
ii. Hospital waste incinerators
Hospital wastes like used syringes, needles, test tubes, bandages, cell cultures, plastics, human anatomic remains, wastes contaminated with viruses, bacteria, fungi etc were not disposed in a proper way and hence the incineration (combustion) of high chlorinated wastes emit PCDD/PCDFs (Stanmore et al., 2000). Hospital waste incinerators are smaller and less organized than municipal waste incinerators and so they may be less functioned than municipal waste incinerators (Thomas et al., 1995)
iii. Sewage sludge incinerators
Sewage sludge incinerators are used mostly to burn dewatered sewage sludge which are also a main source of emitting PCDD/PCDFs (Fullana et al., 2004) Due to improper land filling and recycling, restriction to sea disposal has led to the usage of incinerators which emit PCDD/PCDFs into the environment (Kulkharni et al., 2007).
iv. Hazardous waste incinerators
The toxic compounds released from chemical processes are referred as hazardous wastes. These compounds can be carcinogenic, mutagenic, explosive, inflammable, oxidizing, corrosive and highly toxic based on their kinds. The incineration for such hazardous wastes is termed as hazardous waste incinerators. Such incineration process is also a source of emission of PCDD/PCDFs (Karademir et al., 2004). Food chain designing is used to figure out the emission of hazardous wastes to plants and animal tissues (Aykan Karademir., 2003)
Thermal and combustion processes
High temperature and combustion processes are considered to be the major source of emitting PCDD/PCDFs into the environment. Thermal processes are applicable to municipal, medical and chemical waste incineration. Iron, steel, nickel and magnesium are also produced in high temperature combustion processes. The main sources of PCDD/PCDFs to the atmosphere are flue gases released from thermal processes (Grzegorz Wielgosi?ski., 2010).
i. Thermal degradation of commercial products
Thermal processecing of commercial products are also responsible for the emission of PCDD/PCDFs. Combustion of PCBs produces polychlorinated dibenzo furans and burning of PVCs also lead to congeners of PCDD/PCDFs (Rajagopalan et al., 2004).
ii. Coal and wood burning
In non industrial processes, several studies have shown that the PCDD/PCDFs are present in the emission as well as in ash/soot from wood fires. While comparing coal fired utilities to wood burning, the emission of PCDD/PCDFs is very less, even though they are larger in size, and they also affect very large areas (Kulkharni et al., 2007).
iii. Cement kilns
Another major source of PCDD/PCDFs is cement kilns, which vary widely in their emission patterns and quantities. The variations may be because of many factors, which include kiln’s model, performing conditions, fuels and unrefined materials fed into it. Hazardous wastes like PCDD/PCDFs are produced during the combustion of varied mix of fuels including petroleum coke, coal, refinery distillation ends and other supporting fuels in kilns. Industrial sources i. Metal treatment and processing industries Copper smelting and electric arc furnaces in steel processing, which are also a high temperature processes emit PCDD/PCDFs (Rajagopalan et al., 2004). ii. Paper and pulp industries During pulp bleaching, the phenol present in the pulp reacts with chlorine and chlorinated compounds which results in emission of PCDD/PCDFs under high pressure and temperature conditions (Rajagopalan et al., 2004) iii. Photochemical processes In photochemical dechlorination, the higher chlorinated PCDD/PCDFs are converted to lower chlorinated congeners. For instance, hepta, hexa, penta, and tetra congeners are produced from the photochemical dechlorination of octa chlorinated dibenzo dioxins and octa chlorinated dibenzo furans (Rajagopalan et al., 2004) Reservoir sources The accumulation of PCDD/PCDFs in soils, sediments, organic matter, landfills sites and vegetation is mainly due to their persistent and hydrophobic nature (Kjeller Lo et al., 1995) PCDD/PCDF reservoirs where they are already present either as products or in the environment. They are not intentionally present but are from other sources. The main feature of reservoir sources is that they have the capability of releasing PCDD/PCDFs into the environment again. As a by-product they can be released into three mediaair, water, land as product and waste (Environment and Social Development Organization, 2005)
REMEDIATION/REDUCTION OF PCDD/PCDF
Remediation As PCDD/PCDFs are carcinogenic, understanding the pathway of their contamination is of more concern. Mostly the toxic PCDD/PCDFs are accumulated in soils and sediments which have their source from reservoirs. The estimated amount of soil polluted with PCDD/PCDFs was 500,000 tons, which needs remediation (Johnson., 2008). i. ex situ remediation technique Ex situ is the offsite convertion, in this the remediation technique are not directly located in the particular contaminated product. Ex situ thermal process is used to transport the contaminants from soil to vapor phase. It is achieved by three steps: Soil conditioning, thermal treatment, exhaust gas purification. In soil conditioning, the soil is broken into smaller granular particles and they are sieved in preparation for thermal treatment, which increases the heat supply to the soil and evaporation of pollutants occurs and finally the contaminants are transferred to the gas phase (Koning et al., 2000). The methods which come under ex situ remediation are: composting, Land farming, biopilling and bioreactor processing. This process degrades petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAH), benzene, toluene, ethylbenzene, xylenes (BTEX), phenol compounds, PCDD/PCDFs etc (Koning et al., 2000). ii. In situ remediation techniques This technique is applicable only for specific type of soils, which include high permeable and low organic content homogenous soils. It is suitable for removing pollutants that can be deprived in lower temperature. In this process a steam air is introduced into the soil at 60-100?C. The steam-air mixture must be controlled in this temperature range, since pollutants must not be transferred to ground water. After the injection of steam air mixture into the soil, the volatile and non volatile contaminants move from soil to vapor phase.
Treatment of fly ash
PCDD/PCDFs are formed generally during combustion of organic matter in the presence of chlorine, where flyash is the residue and results in major environmental pollution. (Nam et al., 2008) found that PCDD/PCDFs in flyash can be degraded by Sphingomonas wittichii strain RW1. From a study in 2005, it was identified that the 75.5% of PCDD and 83.8% of 2,3,7,8-TCDD were eliminated from flyash by degradation and adsorption on viable and non viable cell biomass (Nam et al., 2008) From a study of 2008, a biocatalyst was introduced to degrade PCDD/PCDFs, through a combination of mix of 4 bacterial and 5 fungal strains. 68.7% of PCDD and 66.8% of 2, 3, 7, 8- TCDD were degraded by the biocatalyst. It was confirmed that, an extracellular non specific enzyme named oxidases could degrade highly stable PCDDs and lignin in fly ash (Nam et al., 2008). Sorption / desorption of PCDD/PCDFs Cyclodextrin are sugar molecules which are bound together in a ring. They are produced from starch in the means of enzymatic conditions. These are widely used in chemical industries and environmental engineering. Cyclodextrin (CD) was used for degrading PCDD/PCDFs, which are present in soil and water. There are five classes of cyclodextrins commonly. They are, (1) α-cyclodextrin (ACD), (2) β-cyclodextrin (BCD), (3) hydroxypropyl-α-cyclodextrin (HPACD), (4) hydroxypropyl-β-cyclodextrin (HPBCD), and (5) hydroxypropyl-γ-cyclodextrin (HPGCD). Among these, the efficient CDs in removing the contaminants are hydroxypropyl-β-cyclodextrin (HPBCD), α-cyclodextrin (ACD), β-cyclodextrin (BCD) (Cathum et al., 2010). CDs are first made to trap (bound) to PCDD/PCDFs in soil and water. The initial concentration of unbound PCDD/PCDFs was monitored. After a certain period of days, CDs would remove PCDD/PCDFs present in soil and water. And the highest efficiency of removing all congeners of PCDD/PCDFs is 96%, which is achieved by hydroxypropyl-β-cyclodextrin (HPBCD). Following to that, α-cyclodextrin (ACD) has an efficiency of 45% and β- cyclodextrin (BCD) has an efficiency of 50% in removing PCDD/PCDFs. The CDs were selected based on the diameter of the molecular void and functional groups present in it (Cathum et al., 2010). Thermal desorption Many PCDD/PCDF contaminated sites are treated by thermal desorption method. It is a separation process, in which heat is used to separate hydrocarbons from contaminated soils. Equipment is designed for thermal desorption called thermal desorber. It supplies enough heat to the contaminant soil, due to which the constituents are evaporated and separated from the soil (Kulkharni et al., 2007).
Bio degradation
Anaerobic reductive dechlorination (ARD) and aerobic dioxygenation are the two methods which have been studied for microbial degradation of PCDD/PCDFs (Field, Chang., 2008). In anaerobic reductive dechlorination, the hydrogen molecules replace the chlorine molecules (Mohn., 2008). In biodegradation process the reduction of toxic compounds are done by means of micro organisms. The micro organisms reduce Persistent Organic Pollutants (POPs) especially dioxin and dioxin like compounds by oxidation and cleavage of aromatic rings in presence of oxygen (Chang., 2008). The genes involved in anaerobic dechlorination and aerobic angular dioxygenation called as dioxin detoxification genes (DDGs) are accountable for reducing toxicity of PCDD/PCDFs. Pseudomonas and Sphingomonas (RW1) are efficient strains for oxidizing PCDD/PCDFs (Chang ., 2008). Lower chlorinated congeners are reduced under aerobic conditions and higher chlorinated congeners are reduced under anaerobic conditions. Intermediate products are formed during aerobic and anaerobic processes are less equal or more toxic than the original pollutants (Kao et al., 2000).
Destruction of PCDD/PCDFs by Carbon Nanotubes
Carbon NanoTubes can be seen as smooth and flat graphite sheets that have been deformed, so that carbon atoms twist in a helical manner to form a tiny tube (Baughman et.al., 1999). They are of two types namely single walled carbon nanotubes (SWCNT) and multi walled carbon nanotubes (MWCNT). The characteristics of CNTs are high surface area, interstitial space with a cluster of nanotubes, electron mobility, electrical conductivity, (Dai et al., 2002) mechanical properties , (Saridara et al., 2005) like chemical and thermal stabilities. Thus Carbon NanoTubes are used to carry catalysts that help in destruction of dioxins (Lina Zhou et al., 2010). As dioxins have high melting and boiling point, the estimation of adsorption isotherms is difficult. A technique was developed, which depends on Temperature Programmed Desorption (TPD) to study adsorption isotherm of dioxins (Yang et al., 1999). From a study, it was identified that doped graphene and doped nanotubes are efficient in disruption of dioxins (Kang ., 2005). Vanadia – tungsta – titania catalysts are more effective than noble metal catalysts to reduce the concentration of dioxins below 0.1ng TEQ/Nm3 (Lina Zhou et al., 2010). The adsorption and absorption of carbon nanotubes only transfer the contaminants from gas phase to solid or liquid phase, but the catalysts alone cause the elimination of dioxin totally (Yoshikawa et al., 2004).
FUTURE ASPECTS AND CONCLUSION
As PCDD/PCDFs are highly toxic and more persistent, understanding about the source of contamination is essential. PCDD/PCDFs are emitted from various sources like industrial sources, reservoir sources, combustion sources and incineration sources. These POPs affect human health adversely; especially it causes infertility, endocrine disruption, modulation of sex and thyroid hormones. To summarize, biodegradation is most effective reduction method for converting toxic PCDD/PCDFs into non toxic and harmless compounds. In future, the destruction of gas phase PCDD/PCDFs contaminants by adsorption and catalytic destruction using Carbon Nano Tubes can be a promising method to reduce the PCDD/PCDF contamination.
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