HAZARDOUS WASTES IN THE ATMOSPHERE

21.15 HAZARDOUS WASTES IN THE ATMOSPHERE

Hazardous-waste chemicals can enter the atmosphere by evaporation from hazardous-waste sites, by wind erosion, or by direct release. Hazardous-waste chem- icals usually are not evolved in large enough quantities to produce secondary air pollutants. (Secondary air pollutants are formed by chemical processes in the atmosphere. Examples are sulfuric acid formed from emissions of sulfur oxides and oxidizing photochemical smog formed under sunny conditions from nitrogen oxides and hydrocarbons.) Therefore, species from hazardous-waste sources are usually of most concern in the atmosphere as primary pollutants emitted in localized areas at a Hazardous-waste chemicals can enter the atmosphere by evaporation from hazardous-waste sites, by wind erosion, or by direct release. Hazardous-waste chem- icals usually are not evolved in large enough quantities to produce secondary air pollutants. (Secondary air pollutants are formed by chemical processes in the atmosphere. Examples are sulfuric acid formed from emissions of sulfur oxides and oxidizing photochemical smog formed under sunny conditions from nitrogen oxides and hydrocarbons.) Therefore, species from hazardous-waste sources are usually of most concern in the atmosphere as primary pollutants emitted in localized areas at a

(21.15.1) Primary air pollutants such as these are almost always of concern only adjacent to

H 2 SO 4 + 2NaCN → Na 2 SO 4 + 2HCN(g)

the site or to workers involved in site remediation. One such substance that has been responsible for fatal poisonings at hazardous-waste sites, usually tanks that are

undergoing cleanup or demolition, is highly toxic hydrogen sulfide gas, H 2 S. An important characteristic of a hazardous-waste material that enters the atmosphere is its pollution potential. This refers to the degree of environmental threat posed by the substance acting as a primary pollutant, or to its potential to cause harm from secondary pollutants.

Another characteristic of a hazardous-waste material that determines its threat to the atmosphere is its residence time, which can be expressed by an estimated atmospheric half-life, τ 1/2 . Among the factors that go into estimating atmospheric half-lives are water solubilities, rainfall levels, and atmospheric mixing rates.

Hazardous-waste compounds in the atmosphere that have significant water solubilities are commonly removed from the atmosphere by dissolution in water. The water may be in the form of very small cloud or fog particles or it may be present as rain droplets.

Some hazardous-waste species in the atmosphere are removed by adsorption onto aerosol particles . Typically, the adsorption process is rapid so that the lifetime of the species is that of the aerosol particles (typically a few days). Adsorption onto solid particles is the most common removal mechanism for highly nonvolatile constituents such as benzo[a]pyrene.

Dry deposition is the name given to the process by which hazardous-waste species are removed from the atmosphere by impingement onto soil, water, or plants on the earth’s surface. These rates are dependent upon the type of substance, the nature of the surface that they contact, and weather conditions.

A significant number of hazardous-waste substances leave the atmosphere much more rapidly than predicted by dissolution, adsorption onto particles, and dry deposition, meaning that chemical processes must be involved. The most important of these are photochemical reactions, commonly involving hydroxyl radical, HO • . Other reactive atmospheric species that may act to remove hazardous-waste

compounds are ozone (O 3 ), atomic oxygen (O), peroxyl radicals (HOO • ), alkyl- peroxyl radicals (ROO • ), and NO 3 . Although its concentration in the troposphere is relatively low, HO • is so reactive that it tends to predominate in the chemical processes that remove hazardous-waste species from air. Hydroxyl radical undergoes abstraction reactions that remove H atoms from organic compounds,

(9.15.2) and may react with those containing unsaturated bonds by addition as illustrated by

R-H + HO • → R • + H 2 O

the following reaction:

C C + HO .

H C C OH

The free radical products are very reactive. They react further to form oxygenated species, such as aldehydes, ketones, and dehalogenated organics, eventually leading to the formation of particles or water-soluble materials that are readily scavenged from the atmosphere.

Direct photodissociation of hazardous-waste compounds in the atmosphere may occur by the action of the shorter wavelength light that reaches to the troposphere and is absorbed by a molecule with a light-absorbing group called a chromophore:

R–X + h ν → R • + X • (9.15.4) Among the factors involved in assessing the effectiveness of direct absorption of

light to remove species from the atmosphere are light intensity, quantum yields (chemical reactions per quantum absorbed), and atmospheric mixing. The requirement of a suitable chromophore limits direct photolysis as a removal mechanism for most compounds other than conjugated alkenes, carbonyl com- pounds, some halides, and some nitrogen compounds, particularly nitro compounds, all of which commonly occur in hazardous wastes.