Chapter IV, Section C, Item 1: Contaminants: biological,
radionuclide, and chemical
When most think of an environmental issue, they conjure pollution,
most commonly chemical contamination. But what does it mean for
something to be contaminated? Anything in excess, including water
itself, can be unhealthy or even toxic at some concentration or
amount. Whether the contaminating agent is chemical, biological, or
radionuclide, health and environmental standards are set as
concentration limits in air, soil, and water. For air pollution, the
EPA has established National Ambient Air Quality Standards (NAAQS)
for 6 widespread, principal pollutants, as well as National Emission
Standards for Hazardous Air Pollutants (NESHAPS) to regulate
hazardous chemical and radionuclide emissions from industry, while
OSHA sets exposure limits in the workplace. For water pollution, the
EPA has set Maximum Contaminant Levels (MCL’s) for a host of
microbial, radionuclide, and chemical contaminants, expressed as
maximum allowable concentrations in drinking water. For soil
pollution, the EPA has guidelines called Preliminary Remedial Goals
(PRG’s) that, though not legally enforceable limits, become part of
legally enforceable abatement orders. These orders are largely to
prevent eventual contamination of both water and air, which do have
legally enforceable limits, or human exposure, which is regulated by
health laws. Soil contamination is a principal media of exposure to
lead and other heavy metals, making it a primary concern in
environmental clean-ups for property transfers. Other than setting
MCL limits on microbes in water, and providing sanitary guidelines
and indoor air quality recommendations, the EPA generally leaves
disease and other biological contamination issues to health
departments. The Food and Drug Administration (FDA) and Center for
Disease Control (CDC) set legal guidelines to limit exposure to
chemicals and microbes in food and the environment.
Biological contamination is actually the most water and food
resource limiting. Though toxic chemical problems are more common in
industrial societies, contributing to more heightened awareness with
the public, the media-sensational stories of death within days of
exposure to contamination in food or water are usually examples of
biological contamination. Bacterial outbreaks of e coli, typhoid, or
cholera can even occur in ground water wells, as was the case
recently in Canada. Bacteria and viruses are not the only form of
biological contamination. Ask any hiker. In many natural, outdoor
settings, the surface water is often of exceptional water quality,
and could be used untreated, if not for water-borne parasites such
as giardia or crypto sporidium. Unlike bacteria, they, like many
other protozoan parasites, are resistant to even doubling boiling.
The hiker carries a manual reverse osmosis unit, a water filtration
pump with filters, a miniature version of a reverse osmosis
treatment plants that utilities have to use if they draw from
surface water sources. Disease is more commonly known to spread from
airborne or solid surface contact, underscoring that biological
contamination can involve all media of exposure. However, water is
actually our most common disease vector. Eighty percent (80%) of the
world’s diseases are caused by poor water supply.
Radionuclide contamination is another form not typically conjured in
the context of pollution. Environmentally the nuclear issue is more
commonly perceived as a waste disposal issue due to the longevity of
hazardous radioactive wastes, the radiation itself sourced from the
elements, not the compounds. The source of the contamination or
waste is a radioactive isotope of an element. Radioactive releases
from power plants typically involve radioactive isotopes of iodine,
strontium, and cesium, which can be borne in air, water, or soil,
and are readily absorbed into human tissues. The actual hazard from
either nuclear waste or fallout is the radiation emitted from the
isotopic source: alpha, beta, or gamma radiation. With radioactive
decay, the source isotope transmutes into other radioactive and
non-radioactive isotopes. Eventually over time, usually a
considerable amount of time, the contamination may decay into a
non-hazardous stable isotope. But unlike most toxic chemical
compounds that degrade into more neutral forms, radioactive isotopes
are environmentally persistent, a legacy shared with several
non-radioactive elements as well, such as heavy metals, hazardous
from chemical toxicity.
Chemical contamination is the most prevalent environmentally, due to
the commercial handling of a very long laundry list of chemicals.
The relative environmental liability of each on that list is more
readily realized if categorized by source and type. A short list of
categories incorporates all but the most unique contaminants, and
includes: 1) solvents, usually chlorinated solvents, from industry,
transportation, and dry cleaning; 2) petroleum hydrocarbons from
industry and transportation; 3) polynuclear aromatic hydrocarbons
(PAHs) and poly-chlorinated biphenols (PCBs) from industry and power
transmission; 4) heavy metals from mining, smelting, and power
generation; 5) agricultural and industrial salts; 6) agricultural
herbicides and pesticides; and 7) potentially all of the above from
waste landfills. These categories do not include nutrients, acids,
caustics, and oxidizers, which usually affect water quality more
generally, and are herein discussed in separate environmental
classifications other than contaminants.
The energy debits to the complete energy economy from all
contaminant problems are related to the energies associated with
treatment. Whether the contaminated media is air, water, or soil,
these treatments are generally of four types: 1) removal, usually by
filtration, distillation, or ion exchange; 2) dilution; 3)
destruction by dissolution in a solvent, including acids, caustics,
and water; 4) destruction by oxidation, either by combustion or
respiration (or hydrolysis / acid); and 5) destruction by reduction
caused by an electron donor. The first two options do not destroy
the contaminant. Instead it changes its media, or gets dispersed to
an “acceptable” level in more of the existing media. New
environmental problems are thus exchanged for the old, usually a
waste disposal problem dealing with the contaminant’s new media, or
a degradation of a larger volume of the existing media to attain an
“acceptable” level of contamination. Though destruction of toxic
compounds by the last three options is thus preferred, and is often
even energetically favorable, the only remedial alternatives for
elemental contaminants are removal or dilution.
The energy of treatment by removal with filtration or distillation
was previously discussed in terms of the desalination of water, part
of the assessment of the energy value of water. Though desalination
involves distillation from, or filtration of, various salts, these
processes are available for many other contaminants. The principles
are the same: either heat energy is used to separate the water from
the contaminant by a phase change, or work energy is used to
pressurize water through a membrane. With ion exchange, an undesired
contaminant is replaced with a desired one, usually substituting one
for the other in a crystal or resin. A familiar example is home
water softening, where calcium in the water is replaced by sodium
from a salt source. Sometimes “undesirable” versus “desirable” is a
matter of perspective: calcium is removed because it is hard to
clean off of fixtures, but the added sodium in the water can cause
hypertension, while the chloride is often released in the
environment from septic systems. Along with road salt, the chloride
adds to a growing problem, the increased salination of groundwater.
For all three contaminant removal processes, as in the example of
desalination, a geometric increase in the amount of energy is
required for an increase in water purity.
The energy demands and associated costs of contaminant removal can
quickly become so prohibitive that “going the other way,” diluting
the contaminant rather than concentrating it into a residuum, is the
only feasible option. As the old saying goes, “dilution is the
solution for pollution.” Though it may reflect an old philosophy,
dilution is still considered part of the solution, not only in waste
disposal, but also in meeting water supply standards. A salient
example became political football recently: the lowering of the MCL
for arsenic in drinking water from 50 ppb to 10 ppb. The lowering of
any MCL results in increased costs to water utilities to meet the
stricter standard. Treatment to the new standard is nearly
impossible without some measure of “blending,” a type of dilution
where the contaminant concentration is reduced by blending
uncontaminated water from another source into the contaminated
source. Blending sacrifices a little water quality from one source
for the sake of a greater water supply, and is feasible where the
water supply is sufficient to provide the uncontaminated mixing
component. With the arsenic example in arid regions, however,
dilution is not an option, and the cost of treatment by removal
methods is costly to water utilities. Dilution of contaminants into
the environment generally is still a method of waste disposal,
especially into the oceans. The question is how much the environment
can absorb, and at what cost.
Dissolution can be a method of contaminant destruction, but at the
very least, a byproduct of the contaminant remains in a solvent, and
if the byproduct or the combined solution is also hazardous, another
waste treatment requirement arises. The most common solvent is
water, but many different solvents exist for different substances.
Trichloroethylene, a very environmentally persistent chlorinated
solvent, was used to dissolve oils and greases from metal parts. In
this case the solvent became a major contaminator of ground water
supplies. Sometimes the solvent may destroy the contaminant, but
more often, the contaminant exists in solution and may precipitate
from solution. In this way, the dissolution becomes merely a means
of dilution and not destruction of the contaminant. Some substances
that are not soluble in water may be soluble in another substance
such as fat. Fat soluble contaminants that are also insoluble in
water can become very troublesome, accumulating in animal tissue
without a way to be rid from the body. The concentrations of these
bioaccumulators actually increase up the food chain, which as why
such examples as mercury and PCB may occur at high concentrations in
fish, even though the concentrations may be relatively low in the
water.
Oxidation is the most common way to destroy contaminants.
Oxidation-reduction is a specific way to describe a class of
reactions in chemistry, but in fact, all reactions can be expressed
as the oxidation of one species, losing electrons, followed by the
reduction of another species, gaining them. Energy is used to break
existing chemical bonds between elements–ignition–then electrons are
re-arranged and energy is re-released to form new bonds in new
compounds. The released energy may be more or less than the energy
used to get it all started, but ignition always requires some
initial energy investment. In the process, the reducing agent is
oxidized and the oxidizing agent is reduced, but given the confusion
that can arise, it is easier conceptually to just track the
electrons, referring to one reactant as an electron donor and the
other an electron acceptor. In near-surface environments including
shallow groundwater, the most common electron acceptor is oxygen. If
the contaminant is a willing electron donor, oxidation will occur,
usually in the form of either combustion or respiration. In a waste
incinerator, we add the ignition energy needed to get combustion
started. In the environment, most reactions get going with the help
of microbes, i.e. they are catalyzed by microbes that use the
exchange of electrons in respiration to draw energy. Though
oxidation reactions are usually energetically favorable, the
reactions often need to be stimulated by providing nutrients to the
microbes and increasing dissolved oxygen levels. These result in
treatment costs.
Reduction is required to get rid of some of our most stubborn
contaminants, including oxidized metals, other inorganic compounds,
acids, excess nutrients, and chlorinated solvents. Unlike oxidation
reactions, these reduction reactions are not energetically
favorable, and require reducing conditions, an environment devoid of
oxygen uncommon near the Earth’s surface. The contaminants require
an electron donor to breakdown, usually in the form of reduced
metals, other inorganic compounds, bases, organic matter, petroleum
hydrocarbons, and hydrogen gas. An electron acceptor pecking order
for those electrons also exists in the environment, electrons
usually going first to ubiquitous oxygen, then nitrate, iron, and
sulphate before reducing chlorinated solvents and organic matter to
methane.
But sometimes an electron donor can wreak havoc. A particularly
notorious electron donor, reduced sulfur, is unfortunately
ubiquitous in most of our fossil fuels, particularly in coal. Sulfur
also occurs as metal sulfides in exposed bedrock, and as sulfide
metal ore deposits. Sulfide has the unfortunate knack of creating
acid when oxidized.