Nature’s Lessons: Chlorinated Organic Compounds
Introduction
We hear and read reports about industrially produced chlorinated organic
compounds. It is a little-known fact, however, that many chlorinated
organic compounds occur naturally and that many are indispensable to life
on Earth. Although a chemical, if toxic, is no less dangerous because it
is natural, it is important and useful to understand the natural
occurrences of chlorinated organic compounds for two reasons. First, their
presence in nature establishes a baseline concentration which cannot be
eliminated and must be recognized when considering environmental quality
issues. Second, if chlorinated organic compounds are produced naturally,
there is reason to believe that natural mechanisms exist to degrade such
compounds (Winterton, 2000).
Chlorine: Background Check
Chlorine is a naturally occurring chemical element produced originally,
along with the approximately 90 other elements. found on the Earth, in
nuclear reactions in stars. Each chemical element is a unique building
block of matter. Chlorine is the 19th most abundant element in the Earth
(Anders & Ebihara, 1982). An element’s chemical properties dictate its
“behavior” and distribution throughout the various environmental “spheres”
of the Earth. These are the: hydrosphere (water environment); atmosphere
(gaseous envelope surrounding our planet); geosphere (rocks and soils);
and biosphere (living and dead organisms).
Chlorine is found in trace amounts throughout the atmosphere and
biosphere, but concentrates heavily in the hydrosphere due to the high
solubility in water of some of its common salts. Sodium chloride, ordinary
table salt, is one such salt. One inadvertent taste of seawater at the
beach confirms the saltiness of seawater, a testament to its chloride
content. “Evaporite” salt deposits of the geosphere are formed when
seawater evaporates in isolated marine basins, forming massive deposits of
salt minerals. Chloride minerals from brines and salt deposits are sources
of chlorine for the chemical industry. Salt deposits are called “salt
domes” when, over the course of millions of years, they slowly migrate
into dome-like shapes, rising buoyantly due to their lower density in
comparison to the sediments around them. During this slow upward
migration, salt domes trap oil and natural gas in surrounding sediments
and rocks, forming economically important deposits of these energy
resources. Scientists believe that chlorine is also enriched in
inaccessible regions deep in the Earth.
Weathering of surface rocks and minerals over millions of years yields
great quantities of dissolved chlorine to rivers. Rivers drain the Earth’s
surface, transporting huge amounts of dissolved and particulate matter to
the oceans. By this process Earth’s oceans have become great chemical
“sinks” for chlorine. Indeed, one chloride “ion” (the term used to
describe dissolved salt components in water) may reside in the oceans for
100,000,000 years! While this chemical “fate” for chlorine has been
well-documented by Earth chemists, known as geochemists, for some time,
recent attention focuses on some of the lesser known, “supporting roles”
for chlorine in the natural world.
Chlorine is a co-factor in the production of oxygen during photosynthesis
(Hall & Rao, 1999; Kirk, 1991; Dismukes, 1986). Oxygen is an essential
component of respiration for humans and terrestrial animals. Researchers
have determined that chlorine plays a critical supporting role in complex
electrochemical reactions that take place in plants. (Electrochemical
reactions involve the precise transfer of electrons from one type of atom
to another.) These important reactions ultimately result in the chemical
cleaving of oxygen from water, H2O, making oxygen abundantly available in
the atmosphere. In fact, depletion of chloride in plants inhibits oxygen
production. Chlorine is also an important participant in complex natural
atmospheric chemical reactions. This element is introduced into the lower
atmosphere when wind stimulates turbulence over the oceans, forming fine
mists of seawater which give rise to chlorine and chlorine compound
vapors, with the help of solar energy (Keene, 1999).
Organochlorines in Nature
Scientists have documented over 2,000 naturally occurring
chlorine-containing organic compounds (G.W. Gribble, personal
communication, 1-5-01). Organic compounds are a large family of chemicals
composed of both geometrically simple and complex chains, rings and
branching molecules of mostly carbon and hydrogen. They include the
important chemicals of life. Organic chemicals, including chlorinated
organic compounds, are synthesized industrially also and used for the
manufacture of countless items of everyday use, such as plastics and
pharmaceuticals. The presence of chlorinated organic compounds in soils,
sediments, and water bodies is usually attributed to contamination
resulting from manufacturing and waste incineration. It is now clear,
however, that many of these compounds are produced naturally by organisms
and inorganic chemical processes, i.e., processes not involving organic
chemicals.
Nature began producing chlorinated organic chemicals millions of years
before the appearance of the first chemical manufacturing plant!
Scientists are just beginning to understand the natural production and
functions of these compounds. As analytical methods used to detect all
types of chemicals improve, chlorinated organic compounds are being found
throughout Earth’s environmental spheres.
For example, chloroform, a chlorinated organic solvent and by-product of
drinking water chlorination, has been discovered in uncontaminated
groundwater below a spruce forest. Researchers think chloroform is
produced naturally when soil microorganisms excrete certain
chloride-emitting enzymes. The chloride reacts with soil organic matter,
resulting in natural chloroform (Gron, 1999).
Chloroform Molecule
Chloroform Molecule Legend:
Sphere ColorAtom
GreyCarbon
GreenChlorine
YellowHydrogen
Chlorinated organic compounds in soils, once solely attributed to the
deposition of air pollutants transported through the atmosphere, are known
now to be natural soil constituents.
Soil bacterium
Pseudomonas aeurginosa
Interestingly, mushrooms represent a major source of chlorinated organic
compounds in soils. Some of these compounds have molecular structures
which are identical or very similar to industrial organochlorine
compounds, yet they are important ecological agents in the decomposition
of woody forest debris (Field, 1999).
Organisms of various types synthesize chlorinated organic compounds for
specific purposes, many of which await our discovery. The human body
employs chlorine to fight infection and kill bacteria, yeast, fungi, and
even tumor cells. Recently discovered natural chlorinated organic
compounds synthesized by organisms as diverse as seaweed, tree frogs,
marine sponges, mosses and algae show promise for potent medicinal use
against infections and cancer (Gribble, 1999).
Conclusions
Chlorine and chlorinated organic compounds are ubiquitous in nature,
filling numerous vital roles, and functioning in ways that researchers are
still deciphering. Organochlorine compounds have even been detected in a
type of meteorite whose composition is believed to be representative of
the non-gaseous, primordial dust of the Solar System, from which the Sun
and planets formed (Winterton, 2000). It is incorrect to presume an
industrial origin for all chlorinated organic compounds found in the
environment; we must recognize the existence of both naturally and
industrially generated chlorinated organic compounds. A greater
understanding of chlorine’s naturally occurring organic compounds will
enhance our ability to manage its industrially generated counterparts.