Review articleDioxins sources and current remediation technologies — A review
Introduction
Dioxins are a class of structurally and chemically related polyhalogenated aromatic hydrocarbons that mainly includes polychlorinated dibenzo-p-dioxins (PCDDs or dioxins), dibenzofurans (PCDFs or furans) and the ‘dioxin-like’ biphenyls (PCBs). They constitute a group of persistent environmental chemicals and usually occur as a mixture of congeners. Their presence in the incinerator fly ash samples was firstly reported by Dutch and Swiss scientists in the year 1977 and 1978, respectively (Buser et al., 1978, Olie et al., 1977). However, dioxins had come to public attention in the year 1982 when an explosion at ICMESA factory in Seveso, Italy, deposited these chemicals over an area of 2.8 km2 (Wilson, 1982).
Only 7 of the 75 possible PCDD congeners, and 10 of the 135 possible PCDF congeners, those with chlorine substitution in the 2,3,7,8 positions, have dioxin-like toxicity. Likewise, there are 209 possible PCB congeners, only 12 of which have dioxin-like toxicity (USEPA, 1994a, USEPA, 1994b). These dioxin-like PCB congeners have four or more chlorine atoms and are sometimes referred to as coplanar PCBs, since their rings can rotate into the same plane. Physical and chemical properties of each congener vary according to the degree and position of chlorine substitution. Fig. 1 and Table S-1 depict the basic structural formula of PCDDs, PCDFs, and PCBs together with the numbering convention at the positions on benzene rings where chlorine or other halogen atoms can be substituted.
The largest release of these chemicals today is open burning of household waste, municipal waste, medical waste, landfill fires, and agricultural and forest fires (Dyke et al., 1997). Dioxin and furan compounds exhibit little potential for significant leaching or volatilization once sorbed to particulate matter. The available evidence indicates that PCDDs and PCDFs, particularly the tetra- and higher chlorinated congeners, are extremely stable compounds under most environmental conditions. The only environmentally significant transformation process for these congeners is believed to be photodegradation of nonsorbed species in the gaseous phase, at the soil–air or water–air interface (Tysklind et al., 1993).
PCDDs/PCDFs entering the atmosphere are removed either by photodegradation or by deposition. Burial in-place, resuspension back into the air, or erosion of soil to water bodies appears to be the predominant fate of PCDDs/PCDFs sorbed to soil. The ultimate environmental sink of PCDDs/PCDFs is believed to be aquatic sediments. Levels of PCDDs/PCDFs in fish and invertebrates have been found to be higher than those in the water column, suggesting bioaccumulation (Atkinson, 1991). Conversely, a little information exists on the environmental transport and fate of the 12 coplanar PCBs (Sakai et al., 2001).
The general population exposure to dioxins chemicals occurs as an exposure to a mixture of different congeners (Masuda et al., 1998). Clearly, however, many of the effects are mediated through an interaction with the aryl hydrocarbon receptor (AhR). Dioxins induces a broad spectrum of biological responses, including induction of gene expression for cytochrome P450, CYP1A1, and CYP1A2, disruption of normal hormone signaling pathways, reproductive and developmental defects. Briefly, it indicates that the inappropriate modulation of gene expression represents the initial steps in a series of biochemical, cellular and tissue changes that result in the toxicity observed (Mandal, 2005). The variation in toxicity amount the dioxins and furans and the effect at the AhR is 10,000 fold, with TCDD being the most potent. Fig. 2, depicts a schematic model of the action of dioxin in cell.
The toxicity of dioxins are expressed as toxic equivalent quantities (TEQs) where the most toxic congener TCDD is rated as 1.0 and the less toxic congeners as fractions of this. The toxicity of dioxins is mediated through the aryl hydrocarbon receptor; a toxic equivalency factor (TEF) is used, assuming that the effects are additive and act via a common mechanism to cause toxicity (Boening, 1998, Kerkvliet, 2002). The TEF system was initiated for dioxins and furans in 1998 by NATO/CCMS scheme, adopted internationally and termed International-TEFs (I-TEFs). Many of the other PCDDs and PCDFs and certain PCBs are less potent than TCDD but vary considerably in their respective concentrations. Each congener can be assigned a potency value relative to TCDD [TEF]. When a TEF is multiplied by the congener concentration level, a toxic equivalency (TEQ) value is obtained. In the early 1990s, WHO added TEFs for PCBs. The coplanar-polychlorinated biphenyls have less potency, but their concentrations are often much higher than concentrations of TCDD (Kang et al., 1997, Patterson, 1994), so their relative contribution to the total TEQ is potentially sizable. The 7 dioxin congeners, 10 furan congeners (all chlorinated in at least the 2,3,7,8 position) and the 12 PCBs which exhibit ‘dioxin-like activity’ were rated with TEFs (Giesy and Kannan, 1998) (seeTable S-2). Thus, the toxic contribution of the PCDDs and PCDFs and certain PCBs can then be compared. In 1998 and 2005 the WHO expert meeting derived consensus TEFs for both human and wildlife risk assessment (Van den Berg, 1998, Van den Berg, 2006).
People are exposed primarily through foods that are contaminated with PCDDs and PCDFs as a result of the accumulation of these substances in the food chain and in high-fat foods, such as, dairy products, eggs, animal fats, and some fish. Further, the exposure also includes industrial accidents (Baccarelli et al., 2002) and several miscellaneous exposures (Yoshimura, 2003). The approximate estimation of human exposure pathways is shown in Fig. 3.
Several adverse health effects have been associated with dioxins, including soft tissue, sarcomas, lymphomas, skin lesions (chloracne), stomach cancer, biochemical liver-test abnormalities, elevated blood lipids, fatal injury, immune system and neurological effects (Mitrou et al., 2001). Moreover, carcinogenic, genetic, reproductive, and developmental effects have been observed in many animal studies although species differ dramatically in sensitivity to these chemicals (Cole et al., 2003, Huff et al., 1994). TCDD has the LD50 (lethal dose) of 0.04 mg/kg for rats. However, other dioxin isomers have LD50 values up to 100 mg/kg for rats (Kao et al., 2001).
A number of countries and organizations have studied various approaches to the health risk assessment of dioxins with regard to dioxin as carcinogenic promoters and have defined tolerable daily intake (TDI) based on No Observed Adverse Effect Level (NOEAL) derived from animal studies (EuropeanCommission, 1994, Steenland and Deddens, 2003). In assessing the risk of 2,3,7,8-TCDD the USEPA came up with a virtual safe dose of 6 fg/kg body weight per day. The two most recent health risk assessments, carried out by the Health Council of the Netherlands in 1996 and WHO in 1998, are based on developmental effects initiated during gestation and/or lactation. The international risk assessments of dioxins are summarized inTable S-3. These doses are based on the carcinogenicity of 2,3,7,8-TCDD and provides protection from toxic effects as well. The reactions of the various member states of the European Union to these risk evaluations have put an emission limit of 0.1 ng/m3 I-TEQ primarily waste incineration plants and tolerable daily intake of 1–4 pg I-TEQ/day/kg body.
Apart from the toxicity of dioxins and its presence in the environment, many scientists have shown the compound to be highly resistant to biodegradation. This resistance may be due to its very low water solubility and high octanol–water partition coefficients (Orazio et al., 1992). Thus, public health risk from environmental exposure to dioxins from contaminated sites can be significant. As a result, a clean-up of environmental dioxins contamination is an area requiring more attention.
Section snippets
Dioxins sources
Earlier human tissue samples show very low levels of dioxins than found today (Ligon et al., 1989). Studies of the sediments near industrial areas of the United States have shown that dioxins were very low until about 1920 (Alcock and Jones, 1996, Czuczwa et al., 1984). These studies show increases in dioxins concentrations from 1920s and continuing until about 1970. Some decline in concentrations has been observed this time. These findings can be explained by the corresponding trends of
Techniques of dioxin remediation, reduction and prevention
It was observed that dioxins enter into the environment mainly from the flue gas originated from incineration and combustion processes, formation of fly ash (originated from incineration and combustion processes) and dioxins contaminated soil occurred due to industrial and reservoir sources. Therefore, it was decided to highlight a comprehensive state-of-the-art study on the remediation, reduction and prevention of these components which are threatening the environment.
Future prospects and conclusions
Dioxins compounds are environmentally and biologically stable and, as a result, human exposure is chronic and wide spread. An exposure to such type of chemicals can damage the immune system, leading to increased susceptibility and it can disrupt the functions of several hormones. Major routes of dioxins entering into the atmosphere are incineration and combustion sources and therefore, more attention is required for the enhanced understanding of the precursor and de novo mechanisms of dioxins
Acknowledgements
The authors acknowledge Fundação Para a Ciência e a Tecnologia and FEDER (ref. SFRH/BPD/14848/2003) for the financial support.
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