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Volume 291, Number 5501, Issue of 5 Jan 2001, pp. 48-49.
Toward Sustainable Chemistry
Terry Collins*


Chemistry has an important role to play in achieving a sustainable
civilization on Earth. The present economy remains utterly dependent on a
massive inward flow of natural resources that includes vast amounts of
nonrenewables. This is followed by a reverse flow of economically spent
matter back to the ecosphere. Chemical sustainability problems are
determined largely by these economy-ecosphere material flows (see the
figure, below), which current chemistry education essentially ignores. It
has become an imperative* that chemists lead in developing the
technological dimension of a sustainable civilization.

When chemists teach their students about the compositions, outcomes,
mechanisms, controlling forces, and economic value of chemical processes,
the attendant dangers to human health and to the ecosphere must be
emphasized across all courses. In dedicated advanced courses, we must
challenge students to conceive of sustainable processes and orient them by
emphasizing through concept and example how safe processes can be developed
that are also profitable.

Green or sustainable chemistry can contribute to achieving sustainability
in three key areas. First, renewable energy technologies will be the
central pillar of a sustainable high-technology civilization. Chemists can
contribute to the development of the economically feasible conversion of
solar into chemical energy and the improvement of solar to electrical
energy conversion. Second, the reagents used by the chemical industry,
today mostly derived from oil, must increasingly be obtained from renewable
sources to reduce our dependence on fossilized carbon. This important area
is beginning to flourish, but is not the subject of this essay. Third,
polluting technologies must be replaced by benign alternatives. This field
is receiving considerable attention, but the dedicated research community
is small and is merely scratching the surface of an immense problem that I
will now sketch.

Many forces give rise to chemical pollution, but there is one overarching
scientific reason why chemical technology pollutes. Chemists developing new
processes strive principally to achieve reactions that only produce the
desired product. This selectivity is achieved by using relatively simple
reagent designs and employing almost the entire periodic table to attain
diverse reactivity. In contrast, nature accomplishes a huge range of
selective biochemical processes mostly with just a handful of
environmentally common elements. Selectivity is achieved through a reagent
design that is much more elaborate than the synthetic one. For example,
electric eels can store charge via concentration gradients of biochemically
common alkali metal ions across the membranes of electroplaque cells. In
contrast, most batteries used for storing charge require biochemically
foreign, toxic elements, such as lead and cadmium. Because of this
strategic difference, manmade technologies often distribute throughout the
environment pers
istent pollutants that are toxic because they contain elements that are
used sparingly or not at all in biochemistry.

Persistent bioaccumulative pollutants pose the greatest chemical threat to
sustainability. They can be grouped into two classes. Toxic elements are
the prototypical persistent pollutants; long-lived radioactive elements are
especially dangerous examples. New toxicities continue to be discovered for
biologically uncommon elements. The second class consists of
degradation-resistant molecules. Many characterized examples originate from
the chlorine industry and are also potently bioaccumulative. For example,
polychlorinated dibenzo-dioxins and -furans (PCDDs and PCDFs) are deadly,
persistent organic pollutants. They can form in the bleaching of wood pulp
with chlorine-based oxidants, the incineration of chlorine-containing
compounds and organic matter, and the recycling of metals. The United
Nations Environmental Program (UNEP) International Agreement on persistent
organic pollutants lists 12 "priority" pollutant compounds and classes of
compounds for global phaseout. All are organochlorines.

Imagine all of Earth's chemistry as a mail sorter's wall of letter slots in
a post office, with the network of compartments extending toward infinity
(see the figure, below). Each compartment represents a separate chemistry
so that, for example, thousands of compartments are associated with
stratospheric chemistry or with a human cell. An environmentally mobile
persistent pollutant can move from compartment to compartment, sampling a
large number and finding those compartments that it can perturb. Many
perturbations may be inconsequential, but others can cause unforeseen
catastrophes, such as the ozone hole or some of the manifestations of
endocrine disruption.$A4 Most compartments remain unidentified and even for
known compartments, the interactions of the pollutant with the
compartment's contents can usually not be foreseen, giving ample reason for
scientific humility when considering the safety of persistent mobile
compounds. We should heed the historical lesson that persistent pollutants
are capable of envi
ronmental mayhem, and treat them with extreme caution. In cases where the
use of a persistent pollutant is based on a compelling benefit, as with DDT
(dichlorodiphenyltrichloroethane) in malaria-infested regions, chemists
must face the challenge of finding safe alteratives.

Consider, for instance, the alarming reproductive damage that can be
inflicted by minute quantities of endocrine-disrupting chemicals (EDCs),
such as PCDDs, polychlorobiphenyls (PCBs), and the pesticides endosulfan
and atrazine. EDCs disrupt the body's natural control over the reproductive
system by mimicking or blocking the regulatory functions of the steroid
hormones or altering the amounts of hormones in the body. Uncertainty still
clouds our understanding of their full impact, but mass sterilization is
one limiting conceivable outcome of ignoring the demonstrated dangers of
EDCs. Our present knowledge strongly suggests that anthropogenic EDCs
should be identified and eliminated altogether.

Stringent regulations based on the precautionary principle and the
principle of "reversed onus" should be developed to guard against the
release of new environmentally mobile persistent compounds; a precise
definition of persistence also needs to be developed. This would provide a
regulatory foundation for weeding persistent bioaccumulative compounds out
of all technology, and highlight where research is needed to find safe
alternatives. Groundbreaking legislative proposals toward this goal are
about to be considered in the Swedish Parliament.

In their current formal training, all chemistry students will learn that
the chlorination of phenol proceeds by a mechanism known as electrophilic
aromatic substitution. But very few will learn of EDCs and their dangers or
come to know that prime examples of EDCs, namely PCDDs, are produced in
trace quantities whenever phenol is chlorinated. This hazardous omission
illustrates one important type of content that is simply missing from the
conventional curriculum.

Green chemistry can dramatically reduce environmental burdens of both
classes of persistent pollutants by moving the elemental balance of
technology closer to that of biochemistry. Significant reductions in the
dispersal of many persistent pollutants have already been achieved. By the
late 17th century, the use of lead oxide as a correcting agent for acidic
wine was banned on pain of death in Ulm in the duchy of Wurtemburg.|| More
recently, large reductions in lead pollution have been achieved in what are
recognizable examples of green chemistry, for instance, by replacement of
lead additives in paint with safe alternatives, by the development of
cleaner batteries, and by the as yet unfinished and sometimes flawed
progression away from tetraethyl lead toward safer combustion promoters in
fuels. PCDDs and PCDFs have been greatly reduced in the pulp and paper
industry by the replacement of chlorine with chlorine dioxide as the
principal bleaching agent.

folyt kov.

The author is at the Department of Chemistry, Carnegie Mellon University,
Pittsburgh, PA 15213, USA. E-mail: .