Hazardous Wastes

In this section, we consider the nature of various types of hazardous wastes and discuss how individual samples of such wastes can be destroyed as an alternative to simply dumping them and thereby deferring the problem to a later date.

Currently there are more than 50,000 hazardous waste sites and perhaps 300,000 leaking underground storage tanks in the United States alone. The Superfund program of the U.S. EPA was created to remediate waste sites; its eventual cost is estimated to be $31 billion (see Box 16-1).

The Nature of Hazardous Wastes

A substance can be said to be a hazard if it poses a danger to the environment, especially to living things. Thus hazardous wastes are substances that have been discarded or designated as waste and that pose a danger. Most of the hazardous wastes with which we shall deal are commercial chemical substances or by-products from their manufacture; biological materials are not considered here.

In Chapter 10, we paid a, great deal of attention to substances that were toxic, i.e., they threaten the health of an organism when they enter its body. Other common types of hazardous materials include those that are

• ignitable and burn readily and easily;

• corrosive because their acid or base character allows them to easily corrode other materials;

• reactive in senses not covered by ignition or corrosion, i.e., by explosion; and

• radioactive.

Some waste materials are hazardous in more than a single category. The Management of Hazardous Wastes

There are four strategies in the management of hazardous waste. In order of decreasing preference, they are;

• Source reductions The deliberate minimization, through process planning, of hazardous waste generation in the first place. The green chemistry cases presented throughout this text provide many examples of this strategy.

• Recycling and reuse: The use in a different process as raw materials, whether by the same company or a different one, of hazardous wastes generated in a process.

• Treatment: The use of any physical, chemical, biological, or thermal process—including incineration—-that reduces or eliminates the hazard from the waste. Examples of such technology are discussed in subsequent sections.

• Disposal: Burial of the nonliquid waste in a properly designed landfill. In the past, liquid hazardous wastes were often injected into deep underground wells.

Landfills that arc specially designed to accommodate hazardous wastes have several characteristics in addition to those discussed for sanitary landfills. The locations of such landfills should be

• in an area with clay or silt soil, to provide an additional barrier to leachate dispersal, and

• away from groundwater sources.

Often the hazardous wastes in such landfills are grouped according to their physical and chemical characteristics so that incompatible materials are not placed near each other.

Toxic Substances

As mentioned, toxic wastes are those that can cause a deterioration in the health of humans or other organisms when they enter a living body. Their characteristics, as well as many examples, were discussed in Chapters 10-12 and 15 particularly and so will not be reviewed in detail here. Those of main concern are heavy metals, organochlorine pesticides, organic solvents, and PCBs.

As an example of the magnitude of the problem of toxic substance waste management, consider the PCBs that are still used in the capacitors in the ballast tubes of fluorescent light fixtures. Within the sealed capacitor container is a thick, gel-like liquid of concentrated PCB oil that is absorbed in several layers of paper. A typical capacitor contains about 20 g—about a tablespoon—of liquid PCB. Although each ballast does not contain much PCB, the number of these lighting fixtures in use in the developed world is huge. Thus the ultimate collection and disposal of PCBs from these sources will be a task requiring many years and many dollars. Disposal methods for toxic organic compounds arc (11 scussed later in this chapter.

Incineration of Toxic Waste

Incinerators that deal with hazardous waste are often more elaborate than those that burn municipal waste because it is important that the material be more completely destroyed and that emissions be more tightly controlled.

In some cases, the waste (e.g., PCBs) will not ignite on its own and must be added to an existing fire fueled by other wastes or by supplemental fuel such as natural gas or petroleum liquids. Modern facilities employ very hot flames, ensure that there is sufficient oxygen in the combustion zone, and keep the waste compounds in the combustion region long enough to ensure that their destruction and removal efficiency, DEE, is essentially complete, i.e., >99.9999%, called "six nines." The presence of carbon monoxide at a concentration greater than 100 ppm in the gaseous emissions is often used as an indicator of incomplete combustion.

About 3 million tonnes of hazardous wastes are burned annually in the-United States, although this is only 2% of the amount generated. Three-quarters of the hazardous waste is dealt with by aqueous treatment, and 12% is disposed of on land or injected into deep wells.

The two most common forms of toxic waste incinerators are the rotary kiln and the liquid injection types. The rotary kiln incinerator can accept wastes of all types, including inert solids such as soil and sludges. The wastes are fed into a long (>20 m) cylinder that is inclined at a slight angle (about 5° from the horizontal) away from the entrance end and slowly rotates so that unburned material is continually exposed to the oxidizing conditions of 650-1100°C; see Figure 16-16. Over a period of about an hour, the waste makes its way down the cylinder and is largely combusted. The hot exit gases from the kiln are sent to a secondary (nonrotating) combustion chamber equipped with a burner in which the temperature is 950-1200°C. The gas stays in the chamber for at least two seconds so that destruction of organic molecules is essentially complete. In some installations, liquid wastes can be fed directly into this chamber as the fuel. The gases exiting the secondary chamber are rapidly cooled to about 230°C by an evaporating water spray (in some cases with heat recovery), since they would otherwise destroy the air pollution equipment that they next enter.

FIGURE 16-16 Schematic diagram of the components of a rotary kiln incinerator, including air pollution equipment.

Rotary kiln incinerator

Rotary kiln incinerator

Liquid wastes and air

Water spray

Secondary combustion chamber

pollution control equipment v

Solid ash +

wastewater

Gases ' to air

Rotary kiln and other types of incinerators usually employ the same series of steps to purify the exhaust of particulates and of acid gases before it is released into the air, as do garbage incinerators, e.g., a gas scrubber and a bag-house filter.

Cement kilns are a special type of large rotary kiln used to prepare cement from limestone, sand, clay, and shale. Very high temperatures of 1700°C or more are generated in cement kilns in order to drive off the carbon dioxide from limestone, CaC03, in the formation of lime, CaO, In addition to hotter combustion temperatures (compared to incinerators), wastes in cement kilns are burned with the fuel right in the flame; the residence time of the material in the kiln is also longer. Liquid hazardous wastes are sometimes used as part of the fuel (up to 40%) for these units, the remainder being a fossil fuel, usually coal. Recently, techniques have been developed that allow kilns to handle sludges and solids. Cement kilns burn more hazardous waste (about a million tonnes a year) than do commercial incinerators in the United States, although even more waste is incinerated on-site by chemical industries.

In the liquid injection incinerator—a vertical or horizontal cylinder— pumpable liquid wastes are first dispersed into a fine mist of small droplets. The finer these droplets, the more complete their subsequent combustion, which occurs at about 1600°C with a waste residence time of a second or two. A fuel or some "rich" (easily combustible) waste is used to produce the high temperatures of the combustion. In contrast to the rotary kiln, only a single combustion chamber is used, although in some modern versions a secondary input of air is introduced to improve oxygen distribution and generate more complete combustion. The exit gases can be passed through a spray dryer to neutralize and remove acid gases, followed by a baghouse filter to remove particulates, before being released into the outside air.

The incineration of hazardous waste has garnered much attention from environmentalists and some of the general public because of the potential release of toxic substances—particularly from the stack into the ait— resulting from the operation of these units. Of special concern are the organic products of incomplete combustion, PICs, that have been found both in gases and adsorbed on particles emitted from incinerators. The PICs must be formed in the post-flame region because they could not survive the temperatures of the flame. Some of the most prevalent PICs are methane and benzene.

In their research to understand the production of these pollutants, scientists and engineers have discovered that reactions can occur downstream of the flame in "quench" zones and in pollution control devices, where temperatures fall below 600°C. Both gas-phase and surface-catalyzed processes apparently occur. For example, trace amounts of various dioxins and furans form at 200—400°C on fly ash and soot surfaces where the processes may be catalyzed by transition metal ions. A temperature of about 400°C is optimal for dioxin formation; helow this the formation reaction is slow and above it they are quickly decomposed. It has not been established whether the dioxins and furans result from the coupling on surfaces of precursor compounds such as chloroben-zenes and chlorophenols, or from the so-called de novo synthesis involving chlorine-free furan- or dioxin-like structures reacting with inorganic chlorides.

Concern has also been expressed about increased emissions that could occur when the incinerator is being closed down and during accidents or power failures, when lower temperatures would result for some time, since much greater quantities of dioxins and furans could presumably form under such conditions. Fugitive emissions, which include emissions from valves, minor ruptures, incidental Spills, CtC* BFG also a concern. The dust emitted from cement kiln incinerators has been found to contain toxic metals and some PICs. In fact, the health risk from toxic metal ion emissions (usually as oxides or chlorides) from the hazardous waste incinerators is found to exceed that from the toxic organics. As in the case of municipal incinerators, the solid residue from hazardous waste units can amount to one-third of the original waste volume and contains traces of toxic materials, as does the wastewater from the scrubber units.

Concerns about incineration have spurred the development of other technologies for disposing of hazardous waste. In molten salt combustion, wastes are heated to about 900°C and destroyed by being mixed with molten sodium carbonate. The spent carbonate salt contains NaCl, NaOH, and various metals from the combusted waste, which can be recovered so that the Na2C03 can be reused. No acidic gas is evolved, since it reacts within the salt. In fluidized-bed incinerators, a solid material such as limestone, sand, or alumina is suspended in air (fluidized) by means of a jet of air, and the wastes are combusted in the fluid at about 900°C. A secondary combustion chamber completes the oxidation of the exhaust gases. Plasma incinerators can achieve temperatures of 10,000°C by passing a strong electrical current through an inert gas such as argon. The plasma consists of a mixture of electrons and positive ions, including nuclei, and can successfully decompose compounds, producing much lower emissions than traditional incinerators. In such a thermal or hot plasma, all the particles travel at high speeds and are thermally hot. In a variant that is used to treat municipal solid waste, plasma is first created in air, which is then used to heat a mixture of waste, coke, and limestone to 1500°C or more in a second, oxygen-starved chamber. The inorganic compounds are converted into a slag that is innocuous enough to be used as a construction material. The organic compounds are broken down into syngas, the combination of carbon monoxide and hydrogen discussed in Chapter 8, which is then used as a fuel.

Supercritical Fluids

The use of supercritical fluids is another modern alternative to incineration.

The supercritical state of matter is produced when gases or liquids are

0.006

0.006

1 Critical point v

Supercritical fluid

1 Liquid /

Solid

Vapor

Temperature (°C)

subjected to very high pressures and, in some cases, to elevated temperatures. At pressures and temperatures at or beyond the critical point, separate gaseous and liquid phases of a substance no longer exist. Under these conditions, only the supercritical state, with properties that lie between those of a gas and those of a liquid, exists. For example, for water, the critical pressure is 218 atm (22.1 megapas-cals) and the critical temperature is 374°C, as illustrated in the phase diagram in Figure 16-17. Depending on exactly how much pressure is applied, the physical properties of the supercritical fluid vary between those of a gas (at relatively lower pressures) and those of a liquid (at higher pressures); the variation of properties with changes in pressure or temperature is particularly acute near the critical point. Thus the density of supercritical water can vary over a considerable range, depending upon how much pressure (beyond 218 atm) is applied. Other substances that readily form useful supercritical fluids are carbon dioxide (see Chapters 6 and 7; it is used for many extractions in the food industry, including the decaffeination of coffee beans), xenon, and argon. See Table 16-4 for their critical temperatures and pressures.

One rapidly developing innovative technology for destruction of organic wastes and hazardous materials such as phenols is supercritical water oxida^ tion (SCWO). Initially, the organic wastes to be destroyed are either dissolved in aqueous solution or suspended in water. The liquid is then subjected to very high pressure and a temperature in the 400-600°C range so that the water lies beyond its critical conditions and so is a supercritical fluid. The solubility characteristics of supercritical water differ markedly from those of normal liquid water: Most organic substances become much more soluble, and

0.01 100 374

Temperature (°C)

FIGURE 16-17 Phase diagram for water (not to scale). Notice the region for the supercritical state (shaded light green), which exists at temperatures and pressures beyond the critical point.

TABLE 16-4 1

Supercritical Fluid Characteristics

Substance

Critical Temperature (°C) Critical Pressure (atm)

Water

374.1

217.7

Carbon dioxide

31.3

72.9

Argon

150.9

48.0

Xenon

16.6

58.4

many ionic substances become much less soluble. Similarly, and also because very high pressures are applied, 02 is much more soluble in supercritical than in liquid water.

At the elevated temperatures associated with supercritical water, the dissolved organic materials are readily oxidized by the ample amounts of 02 that are pumped into and dissolve in the fluid. Hydrogen peroxide may be added to generate hydroxyl radicals, which initiate even faster oxidation. Because materials diffuse much more rapidly in the supercritical state than in liquids, the reaction is generally complete within seconds or minutes. One practical problem with the SCWO method is that insoluble inorganic salts that are formed in the reactions can corrode the high-pressure equipment used in the process and so shorten its lifetime; this problem can be solved by designing the reactor so that there are no zones where salts can build up.

The advantages of the SCWO technology include the rapidity of the destruction reactions and the lack of the gaseous NOx by-products that are characteristic of gas-phase combustion. The required pressure and temperature conditions are readily accessible with available high-pressure equipment. However, some intermediate products of oxidation—mainly organic acids and alcohols, and perhaps also some dioxiris and furans—are formed with the SCWO method, which raises concerns about the toxicity of the effluent from the process. In the variant of this technology in which a catalyst is used, the percentage conversion to fully oxidized products is increased and the amount of intermediates that persist is.decreased.

In the wet air oxidation process, temperatures (typically 120-320°C) and pressures lower than those required to achieve supercritical conditions for water are used to efficiently oxidize aqueous wastes (often catalytically). The oxidation is efficient because the amount of oxygen that dissolves at the enhanced pressures favors the reaction. The process is generally much slower than in supercritical water, requiring about an hour. The method is cheaper to operate, however, than the relatively expensive SCWO technology.

Supercritical carbon dioxide has been used to extract organic contaminants such as the gasoline additive MTBE from polluted water. After extraction, the pressure can be lowered, at which point the carbon dioxide becomes a gas, leaving behind the liquid contaminants to be incinerated or otherwise oxidized. Similarly, supercritical fluids could be used to extract contaminants such as PCBs and DDT from soils and sediments.

Nonoxidative Processes

All the processes just described employ oxidation as the means of destroying the hazardous wastes. However, a closed-loop chemical reduction process has been devised that has no uncontrolled emissions, using a reducing rather than an oxidizing atmosphere to destroy hazardous wastes. One advantage to the absence of oxygen is that there is no opportunity for the incidental formation of dioxins and furans. The reducing atmosphere is achieved by using hydrogen gas at about 850°C as the substance with which the preheated mist of wastes reacts. The carbon in the wastes is converted to methane (and some transiently to other hydrocarbons such as benzene, which are subsequently hydrogenated to produce additional methane). The oxygen, nitrogen, sulfur, and chlorine are converted into their hydrides. The process is actually enhanced by the presence of water, which under these reaction conditions can act as a reducing agent and form additional hydrogen by the water-gas shift reaction with methane (see Chapter 8). PAH formation, which is characteristic of other processes at high temperatures in the absence of air, is suppressed by maintaining the hydrogen level at more than 50%. The gas output is cooled and scrubbed to remove particulates. The hydrocarbon output from the process is subsequently burned to provide heat for the system; thus there are no direct emissions to the atmosphere.

PROBLEM 16-7

Construct and balance chemical equations for the destruction of the PCB molecule with the formula C^HgC^ (a) by combustion with oxygen to yield C02, HzO, and HC1 and (b) by hydrogenation to yield methane and HC1.

Chemical dechlorination methods for the treatment of chlorine-containing organic wastes, especially PCBs from transformers, have been developed and used in various parts of the world, though they are rather expensive to operate. The basic idea is to substitute a hydrogen atom or some other nonhalogen group for the covalently bound chlorine atoms on the molecules, thereby detoxifying them. The mostly dechlorinated wastes can then be incinerated or disposed of in landfills. The commonly used reagent for this purpose is MOR, the alkali metal (M = sodium or potassium) salt of a polymeric alcohol. In the reaction, an —OR group replaces each of the chlorines, which depart as the salt MCI:

+ 5 MOR
+ 5 MCI

The process is carried out at temperatures above 120°C in the presence of potassium hydroxide, KOH, and occurs most efficiently for highly chlorinated PCBs. An alternative dechlorination method is the reaction of the PCBs with dispersions of metallic sodium to give sodium chloride and a polymer containing many biphenyl units joined together.

Review Questions

1. Define the term solid waste and name its five largest categories for developed countries,

2. Describe the components and steps in the creation of a sanitary landfill.

3. Describe the three stages of waste decomposition that occur in a sanitary landfill, including the products of each stage. Are all stages equal in production or consumption of acidity?

4. Define the term leachate, explain how this substance arises, and list several of its common components. How can leachate be controlled and how is it treated?

5. Explain the difference between the two common types of MSW incinerators.

6. What is the difference between bottom ash and fly ash in an incinerator? Describe some of the air pollution control devices found on incinerators.

7. What is meant by the four Rs in waste management?

8. Why can the recycling of metals often be justified by economics alone?

9. Describe the processes by which paper and rubber tires can be recycled.

10. What are the common types of consumer packaging plastics that can be recycled? What four ways are used to recycle plastics?

11. What are some of the arguments for and against the recycling of plastics?

12. What is meant by a life cycle assessment and what are the two main uses for LCAs?

13. Describe the main inorganic constituents of soil. How do clay, sand, and silt particles differ in size?

14. What are the names and origins of the principal organic constituents of soil? Are both types of acids soluble in base? In acid?

15. What is meant by a soil's cation-exchange capacity 1 What are its common units?

16. What is meant by a soil's reserve acidity 1 How does it arise?

17. Describe several methods, including chemical equations, by which soils that are too acid or too alkaline can be treated.

18. Describe the processes by which soil in arid areas becomes salty and alkaline.

19. What do the terms sediments and pore water mean?

20. By what three ways are heavy metals bound to sediments?

21. How can mercury stored in sediments be solubilized and enter the food chain?

22. Describe how mine tailings are usually stored and how this represents a potential environmental problem.

23. How can sediments contaminated by heavy meta Is be remediated so they can be used on agricultural fields?

24. Describe two ways by which contaminated sediments can be treated without removing the sediments themselves.

25. List the three categories of technologies commonly used to remediate contaminated soils. Give examples of each.

26. List three conditions that must be fulfilled if bioremediation of soil is to be successful.

27. Describe the two ways in which PCBs in sediments are bioremediated.

28. Define phytoremediation and list the three mechanisms by which it can operate. What are hyperaccumulators ?

29. Define the term hazardous waste. What are the five common types?

30. List, in order of decreasing desirability, the four strategies used in the management of hazardous waste.

31. Name and describe the three common types of incinerators used to destroy hazardous waste. What does DRE stand for and how is it defined?

32. Define the term PiCs and describe how they are formed.

33. Explain the advantages and disadvantages of supercritical water oxidation for the destruction of hazardous wastes.

34. How is the chemical reduction process carried out? What advantages does it have over oxidation methods?

Green Chemistry Questions

See the discussion of focus areas and the principles of green chemistry in the Introduction before attempting these questions.

1. What are the environmental advantages of using polyaspartate as an antiscalant/dispersant versus polyacrylate?

2. The synthesis of polyaspartate as developed by Donlar won a Presidential Green Chemistry Challenge Award.

(a) Into which of the three focus areas for these awards does this award best fit?

(b) List at least three of the twelve principles of green chemistry that are addressed by the chemistry developed by Donlar Corporation.

3. The development of recyclable carpeting by Shaw Industries won a Presidential Green Chemistry Challenge Award.

(a) Into which of the three focus areas for these awards does this award best fit?

(b) List three of the twelve principles of green chemistry that are addressed by the chemistry developed by Shaw Industries.

4. What are the environmental advantages of using polyolefin-backed nylon fiber carpeting in place of PVC-backed carpeting?

Additional Problems

1. Consider a small city of 300,000 people in the northern United States, southern Canada, or central Europe, and suppose that its residents on average produce 2 kg/day of MSW, about one-quarter of which will decompose anaerobically to release methane and carbon dioxide evenly over a 10-year period. Calculate how many homes could be heated by burning the methane from the landfill, given that residential requirements are about 108 kj/year in that climate zone. Consult Chapter 7 for data on methane combustion energetics.

2. By reference to the general solubility rules for sulfides found in introductory chemistry textbooks, deduce which metals would not have their sediment availabilities determined by AVS. Which of these metals would occur in the form of insoluble carbonates instead and thus be biologically unavailable in marine environments?

3. The acid volatile sulfide concentration in a sediment is found to be 10 /xmol/g. The majority of the sulfide is present in the form of insoluble FeS, since the Fe2+ concentration in the sediment is 450 jig/g. What mass of mercury in the form of Hg2+ can be tied up by the remaining sulfide in 1 tonne of such sediment? Assume that a negligible amount of iron is not tied up as FeS,

4. For the PCB congener 2,4,4',5-tetrachloro-biphenyl, deduce (a) the chlorine substitution pattern on the benzoic acid that results from its aerobic degradation and (b) the various PCB congeners with one or two chlorines that could result from its anaerobic degradation,

5. Generate lists of the aspects of production, distribution, and disposal that you would employ in a life cycle assessment to decide which is the more environmentally friendly container for beer: glass bottles or aluminum cans. Do you think the conclusions of such an analysis would depend significantly on the extent of recycling of the containers?

6. Waste having a high cellulose content, such as paper, wood scraps, and corn husks, can be converted to ethanol for use as a fuel. One way this can be done is to first perform an acid hydrolysis of the cellulose to convert it into glucose. This is then followed by fermentation of the glucose to produce ethanol. Write a balanced equation for the fermentation of glucose, C6H1206, into ethanol, C2H5OH. Given that the pyranose monomer unit in cellulose has the formula C6Hj0O5 (i.e., glucose — H20), determine the volume of ethanol that could be produced in this way from 1.00 tonne of hardwood scraps (46% cellulose), assuming 100% conversion in both steps. Ethanol's density is 0.789 g/mL.

7. One way to reduce the lifetime of plastic wastes is to make them photodegradable, so that some of the bonds in the polymers will break on absorption of light. When designing a photodegradable plastic, what range of sunlight wavelengths would it be best for the plastic to absorb? What is the main limitation of the breakdown of these plastics? If you were to design a photodegradable plastic that would break down on absorption of 300-nm light, what would be the maximum bond energy of the bonds to be cleaved?

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