Harm and countermeasures of dioxins and their analogs

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Abstract: It introduces the toxicity, properties, hazards, and formation conditions of dioxins and their analogs. Since dioxins mainly originate from the incineration of chlorine-containing waste, the formation of dioxins and their analogs can be suppressed by controlling the waste incineration process; this article introduces the principles and procedures of several treatment methods. Keywords: dioxins ； Toxicity of dioxin analogs ； Dioxins generated during waste incineration are abbreviated as PCDD/Fs. Halogenated aromatic compounds with dioxin-like activity are collectively referred to as dioxin-like compounds, including polychlorinated biphenyls (PCBs), chlorinated diphenyl ethers and chlorinated naphthalenes, brominated compounds (PBDD/Fs and PBBs), as well as other mixed halogenated compounds. There are two ways humans can be exposed to dioxins, namely through the environment and food. The WHO believes that dioxins are difficult to eliminate once they enter the body and can cause cancer; in 1997 it declared TCDD to be the most toxic dioxin, the world’s top carcinogen, with just one drop being enough to kill 1,000 people. The formation of dioxins requires the following conditions: ① Compounds containing benzene rings (benzene, phenols, etc.) ； ②Compounds containing chlorine (hydrogen chloride, chlorine gas, etc.) ； ③Reaction catalysts (iron, copper, etc.) ； ④The reaction temperature ranges from 300 to 600°C. Dioxins have high melting and boiling points; they are solids at room temperature, insoluble in water, but soluble in carbon tetrachloride. PCDD/Fs are highly stable in the environment, exhibit slow biodegradation, and remain stable at low temperatures; they generally do not degrade until heated to 800°C. However, temperatures above 1000°C are required to destroy them in large quantities, and they can be reformed once cooled down again. 1 　Suppression techniques: Dioxins are not natural substances; they are formed during the combustion of chlorinated hydrocarbons. In 1900, humans invented a method for electrolyzing salt to produce sodium and chlorine; free chlorine was later widely used in the production of pesticides, solvents, plastics, etc. Since then, dioxins have begun to accumulate in the environment. According to statistics, over 95% of dioxins originate from the burning of waste. The fly ash generated from the incineration of municipal solid waste contains PCDD/Fs; the concentration of 2,3,7,8-TCDD is 0.1–7.5 ng/m3, while the concentration of 1,2,3,7,8-P5CDD is 3–10 times higher than that. The concentration of 2,3,7,8-TCDF is 0.1–50 ng/m3. It may be higher in waste incineration ash containing polyvinyl chloride. Based on the mechanism of dioxin formation, during waste incineration, chlorine is oxidized to hydrogen chloride or chlorine gas; coupled with the large amount of dust present in the exhaust gases, dioxins are easily generated within a certain range of incineration temperatures. Figure 1 shows the proportion of dioxins generated at various stages during a representative waste incineration test. It can be seen that dioxins may be produced during the heating to start combustion, the cooling to stop combustion, as well as during normal operation. Rapid heating and cooling, along with maintaining the normal operating temperature as high as 800°C, can **reduce the amount of dioxins generated during these three stages. In addition, it is necessary to ensure complete and stable combustion of the waste; sufficient residence time allows unburned gases to mix thoroughly with air. An appropriate oxygen concentration must be maintained, along with slow gas flow, to facilitate automatic combustion control. Heat is recovered during the gas cooling process to rapidly cool the combustion gases, prevent an excess of fly ash carriers, avoid fly ash accumulation, and remove chlorine. The exhaust gases should also be cooled through a dust collection process, and denovo synthesis inhibitors should be added. According to available data, in Japan, for waste incinerators built after 1995—whether they are fully continuous, semi-continuous, or batch types—appropriate control measures have led to a significant reduction in the concentrations of dioxins in the equipment, incineration ash, and fly ash. 2 Treatment of dioxins: Appropriate measures can be taken during waste incineration to deal with dioxins. ①Capture technologies: include electric furnace dust collectors and bag filters, as well as activated carbon adsorption. ②Decomposition techniques: incineration, thermal decomposition, photodecomposition, chemical decomposition, ozone decomposition, supercritical water decomposition, biological decomposition, catalytic oxidative decomposition, etc. 　Pyrolysis for the treatment of municipal and industrial waste as well as sludge requires higher technical standards and stricter operational control conditions compared to incineration; as a result, the equipment and treatment costs are also higher. However, pyrolysis offers significant advantages over incineration: it not only recovers heat but also generates gas. Additionally, since pyrolysis operates under anaerobic conditions, there is less waste gas and dust generated, which helps to reduce the amount of dioxins released into the atmosphere. This method is widely used in countries such as Japan. Corona discharge in an electrostatic precipitator can decompose dioxins by breaking the chemical bonds within their molecules, thereby completely removing the pollutants. The catalytic oxidation decomposition method is a purification technique that uses a catalyst to lower the ignition temperature of pollutants in exhaust gases, thereby enabling their oxidation and decomposition at lower temperatures (see Figure 2). With the addition of a catalyst, dioxins can be decomposed at 250–350°C, which reduces energy consumption and the requirements for the material of the reactor. A Japanese company has developed a catalyst that can rapidly degrade dioxins, which is mainly used in municipal waste incinerators and industrial waste burners. It enables the rapid degradation of dioxins in gases (to 99%), and this catalyst and device are effective when installed at any location in the exhaust system. In a bag filter, dust-containing gases can be filtered through the dust layer, thereby reducing the concentration of dioxins in the fly ash resulting from waste incineration; the process is shown in Figure 3. The principle and apparatus of supercritical water oxidation are shown in Figure 4. 3 　Conclusion Given the hazards of dioxins and their analogs, as well as the increasing amount of municipal waste being incinerated in our country, research on dioxins and their analogs and efforts to prevent and control them should be given greater attention and strengthened.