I
leh University of California Riverside, 92502 and K. M. Chan California State University Long Beach, 90801
Sulfur Compounds Pollution, health effects, and biological function
Sulfur is one of the relatively abundant elements on earth, in the ocean and the atmosphere. The natural occurring sulfur is in the forms of metallic sulfide, hydrogen sulfide in oil, elemental sulfur, organic sulfides in coal (as calcium sulfate) or gypsum. In the atmosphere, the major sulfur compounds are hydrogen sulfide (HzS), sulfur dioxide (SOz), and sulfate ( S O P ) : they come from both natural environment and pollution emissions. Natural sulfur compound emissions are sulfate-containing aerosols from sea spray, and HzS mainly from the decomposition of organic matter in swamp areas, bogs and tidal flats. Areas of volcanic activity are minor sources of HzS. The H.S is later oxidized to SO?. The emission of SOz comes exclusively from pollution sources; about 70% is estimated to result from roal combustion, and 16% from the cumbustion of petroleum products, namely residual fuel oil (Table 1) ( I ) . During the past few years there has been an increasing geochemical evidence for the widespread dispersal of sulfur comwunds in the elobal environment (2). . . Among - all other air pollutants, sulfur compounds are noted to be relativelv reactive. Sulfur dioxide emitted to the environment has a rather short life of about 4-7 days, and is scavenged to form sulfate aerosols. In the polluted ground layer, S 0 4 Z - j S 0 ~ in aerosols fluctuates between 1:10 and 1:40 with SOz being the dominating component, whereas in the countryside, such ratio is only about 1:l to 1:2. Vertically above the inversion and haze layers, the sulfur dioxide concentration decreases rapidly. Such kind of horizontal and vertical distribution pattern studies indicate that sulfur dioxide from emissions are convected to a rather extended layer of the atmosphere. Of course, the ultimate fate of atmosnheric sulfur dioxide is the formation of sulfate-containing aerosols by the interactions with vegetations and the oxidations by molecular oxygen in the of transition metal catalysts or by oxidants, e.g., ozone, peroxyacetyl nitrate generated from photochemical reactions in the atmosphere (3). The rate of formation of sulfate is slow. A more rapid atmospheric sulfur dioxideammonia liquid phase reaction is currently being considered (4). Once sulfate-containing aerosols are formed, they will be scavenged from the at&osphere by precipitation.~arge size particles tend to settle down as dry fall-out. Removal of silfur dioxide through microbial or~chemicalreactions with soil particles seems to be also feasible. Laboratory experiments demonstrated that the removal of sulfur dioxide concentration can reach the rate u p to 100 ppm in 15 min. Through surface runoff and erosion, these sulfur compounds a r e ultimately transported to streams, and eventuallv to the oceans. The contribution from pollution is about 28% of the total amount of sulfur influx to the world's oceans, which is about 100 metric tonsjyear. Such an introduction of sulfur to the ocean through river discharges raises the sulfate/chloride ratio for river water in those highly industrialized continents .of Europe and North America and lowers the p H of water in lakes nearby any emission source of sulfur compounds. The long term geochemical effects of such dispersal of
Table 1.
Estimated Global Emissionsof Sulfur Compounds ( 1 )
Compound
source
emissions assulfur (tonslyear)
coal combustion petroleum refining petroleum combustion smeltingaperation industrial emissions marine emissions terrestrial emjssions marme emmlon
SO2
HvS ---
~~~~~
SO?
~~~
~~~
11 x 108
8 X 10'
3X
lo6
30 X 10" 70 X 108 44
x lo6
The estimates based on 1965world data.
~
~~
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Journal of Chemical Education
Table 2. Air Pollution Episodes (5)
attributed mortality
Date November 1950 Decemberl952 November 1953 January 1956 December 1957 December 1962 January 1963 November 1966
Poza Riea, Mexico
London, England New York, USA London, England London, England London, England New Yark, USA New York, USA
22 4000
220
1000 700-800 700
300 168
sulfur compounds in the global environment deserve more attention. Pollution and Health Effects
The increased use of sulfur-containing fuels and the general industrial expansion over recent years have led to substantial rises in the levels of atmospheric sulfur pollutants in many places throughout the world such as Britain and eastern United States. The episodes associated with this type of pollution, so called "London type smog," are listed in Table 2 (5). During four days in December, 1952, over the southeastern part of England, many people were affected by the smog and i t resulted in an excess over expected mortality of approximately 4000 deaths. When SOz is absorbed by leaf tissue of the plants, cells' convert it to either sulfate or sulfide. The affected areas first appear water-soaked, then become dry and papery. Different species of plants show great difference in their susceptibility to SOz. Plants with fleshy leaves, e.g., pine and citrus, show their resistance, whereas plants with thin leaves, e.g., barley, cotton, grapes and wheat are generally sensitive to SOz. When a guinea pig is exposed to SO? a t , a concentration near polluted air, the SOz causes swelling and increased exudation of fluid of the lungs, leading eventually to the complete obstruction of air passages. For humans, C0z causes respiratory disease, e.g., bronchostenosis. Sulfates in aerosol exert irritant effects as SOz. -. but lesser in degree (6). The oxidation of SO2 to sulfuric acid is promoted by catalysts like manganese and ferrous salts in-the polluted air. The conversion of SOz to HzSOl produces more harmful effects than ,302. The disaster in England 1952 was
probably due to the potential toxic properties of sulfuric acid. A number of studies show that animals have different susceptibility to sulfuric acid mist (6). Rabbits, rats, and mice are less sensitive than guinea pigs. Guinea pigs can tolerate about 2 mg/m3 of sulfuric acid mist for more than three months with onlv minor ~ulmonarychance. However, exposure at a coneentratiod higher than tGs, even for a short neriod, will cause the death of a guinea pig.
known inhumans to be caused by positive or negative sulfur balance. The positive sulfur balance implies that sulfur intake exceeds sulfur excretion. For example, hypermethioninemia is characterized by an accumulation of methionine in the plasma and tissues, and cystathioninuria results from deficient activity of (3-cystathionase. It is very clear that sulfur compounds are essential to the biosphere, and many life processes will be upset by the imbalance of the sulfur cycle.
Biological Function
One essential feature in the movement of sulfur through the biological phase of its cycle is the incorporation of cysteine and methionine into polypeptide chain during protein synthesis, and the formation of disulfide bonds between cysteine residues in polypeptide chains to impart essential spatial configurations to the protein. The presence of methionine, cysteine, and disulfide bonds in the protein is critical to the functioning of these proteins as enzymes and as structural units in all living cells (7). In addition to the above proteins, there are a number of lower molecular weight organic sulfur compounds necessary for the metabolic processes of living cells. S-adenosylmethionine, lipoic acid, coenzyme A, glutathione, thiamine, and biotin are such examples. These sulfur compounds are essential for ,transmethylation, acetylation, COz transfer and the maintenance of proper redox potentials in the cell. As the metabolism of sulfur compounds in the animal body progresses, these compounds are eventually oxidized and excreted as sulfate. The excreted sulfate may recycle (the biological cycle) via the actions of bacteria and plants. Within the plant, the sulfate ion (S01'-) enters into a series of reactions involving stepwise reduction to sulfides and the incorporation of the sulfur into cysteine. Plants also synthesize methionine from cysteine through cystathionine as an intermediate (see below). The ability to perform the reduction of sulfate and the synthesis of sulfur amino acids is an important marker of the boundary between the plants and the animals. Plants perform this series of reactions but animals do not. The animals eat nrotein sources to obtain the sulfur amino = - ~ ~ from - ~ - nlant ~ acids. Inorganic sulfur in the diet does not appear to contribute substantially to sulfur nutrition. The dietary needs for sulfur amino acids can be met by methionine as shown in Table 3 (8). The metbionine requirements vary with age and also sex. Nevertheless, several diseases have been ~
Table 3. Daily Methionine Requirement ( 8 )
Subiect infants 11-yr old boys young men voune women
daily requirement (rnelka)
PALP = pyridorel phosphate, a cofaotor
Conclusion
From both chemical and biochemical points of view, sulfur and its compounds are essential to many facets of life processes. However, human activities have continuously emitted an alarming quantity of sulfur to the environment. In many cases a poisonous sulfuric acid-containing smog was formed causing high mortality. Our present efforts should concentrate on reducing the atmospheric sulfur pollutants. New legislative measures to impose a tax on air-polluting sulfur emissions u p to 15 cents/lb, to be effective in 1976, has been proposed by the President. Our basic chemical education in sulfur and its compounds should aim at low-sulfur content energy and a natural balance of sulfur in our ecosphere. Literature Cited (11 Robinson, E., end Robbin., a. C., Stanford Ekearch Institute Project PR-6755, Web. 1968). (2) Georgii, H. W., J. Geaphya. RPJ.,75.2365 (19701. 131 Leh. F.,"Chemistry,"inpreu. (41 Abelea, F. B., Ciark, L. E., Fornee, L. E.. and Leather, G. R.. Science, 173, 914 119701. 15) De Villien, A. J.. "Man and His Envimnmcnt," (Editor Ward. M. A ) , Pergamon Press. I910. Vol 1. p 163. (6l Audus M.. Arch Enulmn. Hwlth 23,459 (1971). (71 Roy, A. B., and Tmdinger. P. T.. "The Biachemistry of Inorganic Compound. of Sulfur,"Csmbridgest thoUnivenify Press, 1970. ( 8 ) Swendseid. M. E., and Wong, M.. "Sulfur in Nutrition," (Editors: Muth, 0. H.. andGldfiold. J. E.I,TheAviPublishingCo.I~~., 1970. p.212.
Volume 50, Number4, April 1973
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