By-products of the Thermal Treatment of Hazardous Waste: Formation and Health Effects

Abstract

Destruction of toxic chemicals by thermal treatment can be a highly effective method for remediation of sites contaminated with hazardous substances. Of the 977 Superfund source control treatment projects in the United States from 1982 to 2005, 16% used incineration or other thermal treatments (the proportion is similar for 126 projects in the period 2002–2005).¹ However, as with other technologies, if thermal treatments are not matched correctly with the site or are improperly operated, harmful by-products can form, requiring further treatment.

Origin and Formation of By-products

The emission of toxic by-products from combustion/thermal treatment devices is a complex subject,^2–5 and the uncertainty in emissions has led to substituting moderate temperature, thermal treatment techniques for generally more efficient combustion technologies. Toxic combustion by-products include two broad categories of organic pollutants that are defined under the U.S. Resource Conservation and Recovery Act (RCRA): residual, undestroyed emissions of so-called Principal Organic Hazardous Constituents (POHCs) contained in the feed-stock and Products of Incomplete Combustion (PICs) formed during the thermal treatment.

Today, thermal treatment of Superfund wastes includes more than incineration. The term “incineration” is normally used for processes, in which wastes are fed to a combustor that utilizes an open flame to induce at least part of the waste destruction. Traditional incineration has been partially replaced by thermal processes, in which the waste is not fed directly into the flame (e.g., thermal destruction), and thermal desorption, in which toxic chemicals are first desorbed from the medium, absorbed onto a solid or semi-solid matrix, transported off-site, and then treated further, usually by incineration. However, the chemistry of the pollutant formation is very similar in flame processes and nonflame processes; many of the pollutants are common to all types of thermal treatment, although their exact form, gas-solid distributions, and concentrations may vary from process to process. The nature of combustion by-products is determined by the chemicals that are treated and the conditions under which they react. Although incinerators, catalytic oxidizers, thermal desorbers, and accidental fires are quite different from an engineering perspective, the underlying chemistry forming pollutants is closely related.

During the incineration process, few, if any, organic components of the feed-stock survive direct contact with the flame, and other than soot, only minimal organic by-products are formed.¹ The vast majority of the observed pollutants in the effluent must therefore originate from chemistry occurring outside the flame. Most of the pollutants are probably formed in the high-temperature, post-flame zone or at even lower temperatures further downstream in quenched gases, or as a result of surface-mediated reactions.

The Chemical Reaction Zone Model (see Figure 1) details the types of reactions occurring within a given zone and their impact on emissions.² Zones 2 and 3 reactions, which form ultrafine particulate of soot and fly-ash, also transform metals into catalytically active forms and catalyze the formation of new toxic by-products in Zone 5. Once formed in Zone 5, the temperatures are too low to result in their destruction and the pollutants are emitted into the atmosphere. Waste incinerators and accidental fires contain all five of the combustion zones, thermal destruction devices contain only Zones 3, 4, and 5. Thermal desorbers consist of low-temperature components of Zone 3, as well as Zone 4 and 5. Catalytic oxidizers contain Zones 4 and 5 only. Zones 3 through 5 may well be responsible for the vast majority of toxic combustion by-products.

Figure 1. The Chemical Reaction Zone Model.

Notes: Zone 1, the pre-flame fuel zone, is characterized by a wide range of temperatures (near ambient to 1200 °C), residence times on the order of 0.1 sec, and low excess air conditions. Zone 2, the high-temperature flame zone, is characterized by temperatures of 1000–1800 °C at which essentially every organic compound will undergo complete conversion to its most thermodynamically stable end-products (i.e., carbon dioxide, water, hydrochloric acid, nitric oxide, and, under local pyrolysis conditions, soot). Zone 3, the post-flame thermal zone, is a chemistry-rich zone, where various types of radical-molecule reactions occur. It is characterized by temperatures of ~ 600–1100 °C, residence times of a few seconds, and both oxygen- rich as well as oxygen-depleted regions. Zone 4, the gas quench cool zone, exists downstream of the flame and post-flame zones and can be characterized by either gradual or rapid quenching of the gas temperature. Zone 5, the surface catalysis cool zone, is fundamentally different from the other four zones in that one must now consider the effects of surfaces at temperatures between 200 and 600 °C.

Researchers in the Louisiana State University Superfund Research Program have determined that many pollutants are formed by interactions with transition metals associated with environmental particulate matter. A previously unknown class of pollutants, Environmentally Persistent Free Radicals (EPFRs), is formed when a molecule chemisorbs on a metal and electron transfer occurs to reduce the metal and form EPFRs (see Figure 2). Using laboratory surrogate particles, studies have shown that the radical is stabilized by the surface and the reaction rate with oxygen is reduced to the point that the radical has a half-life of several days. These EPFRs are similar to semiquinone radicals contained in cigarette smoke, which are known to be biologically active. EPFRs were detected in various combustion-generated soots. They were initially thought to only form in the post-flame zone of combustors between 150 and 600 °C. A sampling of Baton Rouge air revealed airborne fine particulate matter (PM_2.5) consistently contained 10¹⁷–10¹⁸ radicals/g of EPFRs with long lifetimes comparable to those observed in laboratory studies (see Figure 3).

Figure 2. The formation of EPFRs.

Notes: Mechanisms of EPFRs from chemisorption of 2-monochlorophenol by elimination of HCl or H₂O and transfer of an electron from the adsorbate to the transition metal. The reduced form of the metal is produced (e.g. Cu(I) from Cu(II) or Fe(II) from Fe(III), and the EPFR).

Figure 3. Electron paramagnetic resonance spectra of EPFRs in Baton Rouge PM_2.5 sampled on August 19, 2009.

Notes: The EPFRs exhibited a slow decay over 54 days after sampling, with a half-life of 21 days.

Health Effects

Atmospheric ultrafine and fine particles are largely formed by combustion sources as primary particulate emissions or as secondary particles formed by atmospheric chemical reactions of combustion emissions of sulfur and nitrogen oxides.⁶ Ultrafine particles are not efficiently captured by air pollution control devices and are transported over long distances. Recent epidemiological studies have documented strong associations between air pollution, particularly PM, and acute health effects.^7–12 An increased body burden for virtually every metal and organic chemical contained in incinerator particulate emissions, as well as the majority of known carcinogens and other chronically toxic chemicals, has been observed downwind of incinerators.^11,12 Ultrafine particles are capable of penetrating deep into the lungs and then into the circulatory system, ¹³ and may translocate to other organs. Since ultrafine and fine PM have high surface-to-volume ratios, it is reasonable to assume that they are principal carriers of biologically active species and also may lead to greater exposure to those species. Organics, metals, and particles may act as a single, integrated system initiating oxidative stress in exposed individuals eventually leading to adverse health effects.

Our research indicates that inhalation of EPFRs initiates inflammatory responses in the lung (see Figure 4) that are indicative of chronic lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD) in humans. The presence of emphysematous-type lesions in the lungs of EPFR-exposed rodents further suggest that EPFR exposure may be responsible for the COPD-phenotype observed in nonsmokers, an area of unquestionable global importance for which data are scarce. Instillation of EPFRs into the lungs also produces oxidative stress and inflammation in the heart, suggesting that acute exposure to EPFRs may increase cardiac vulnerability to ischemic injury.

Figure 4. Light micrographs of rat lungs following exposure to air (A) or EPFRs (B).

Notes: Black arrows denote significant inflammation surrounding the bronchioles; line denotes increase smooth muscle mass surrounding the bronchioles; and white arrow denotes emphysematous-type lesions of the alveolar spaces.

Management Implications and Future Research Needs

Because of the poor public perception of incineration, thermal treatment has been partially substituted for treatment where incineration would be a good choice. However, it is unlikely low-temperature thermal treatment has lower emissions of by-products than incineration. For example, less carryover of contaminated soils and higher gas temperatures can be achieved by reducing the airflow in a thermal treatment device. However, reduced airflow can result in oxygen-starved pyrolysis pockets, which produce more pollutants. Even in a high-temperature incinerator, increased combustion efficiency can reduce pollutant formation, but some pollutants are formed after the combustion zone in the post-flame cool zone. Thus, efficient combustors operating at high temperatures and oxidizing conditions can still have cool zones, where the time at temperatures between 200 and 600 °C are long and many pollutants are formed.

Furthermore, many of the pollutants formed in the cool zone are associated with ultrafine particles, and these particles increase the dosage of the pollutants by acting as a carrier for inhalation exposure. Even installing particulate control devices can be counterproductive, because they do not efficiently collect ultrafine particles and can extend the cool zone. Changes in combustion conditions can also determine whether pollutants are chemisorbed or physisorbed to particles. It is not currently known which particles will be more persistent in the environment or more bioavailable. These complex questions can only be answered through an interdisciplinary approach in which these issues and more are considered in a holistic approach. Understanding the health impacts of combustion-generated pollutants will allow a focus of attention and resources on the most toxic pollutants so that improved, health-effect engineered, thermal treatment technologies can be developed.