Environmental Chemistry
D a v i d H. Klein Hope College Hollond, Michigan 49423
Some General and Analytical Aspects of Environmental Mercury Contamination
O n March 24, 1970, the Canadian government announced a ban on the sale and export of fish taken from thc Canadian waters of Lake St. Clair because these fish were contaminated wit,h mercury. This action focused the attention of thc public, both inside and outsidc thc scientific c~mmunit~y, on the mercury problem. It appears that mercury may be simply the first of a scries of metals which will be found to havc contaminated the environment. Hopefully the experience gained in handling the mercury problem will serve thc historical function of providing guidelines on how future problems can be met more gracefully. Accidental and intentional mercury poisonings have a long history, but beforc the 1950's there are no recorded cases of environrncntal mercury problems. Thcsc problems began in 1950 when a factory in Minamata, Japan, began discharging mercury-containing wastes into Minamata Bay. The bay is quite small and is surrounded by a population of about 10,000 who live in small fishing villages. The first case of Minamata disease, since diagnosed as methylmercury poisoning, occurred in 1953 and was followed by many other cases. The epidemic of poisonings did not come to thc attention of public health authorities until 1956 by which time there wcre about 80 cases. In latc 1958, the pathological symptoms of Minamata disease wcrc identified with those found in accidental alkyl mercury poisoning, and in 1959, the causative agent was shown to be methylmercury in the fish and shellfish of the bay. Governmental agencies and the factory management did not aid the Japanesc study group in identifying the causativc agent, or the source of the causative agent, and the resulting difficulties delayed corrective action so much that in 1964-65 a second outbreak of the disease occurred, this time among fish eaters living near Niigata, Japan. The Minamata poisonings included 121 cases with 46 deaths; the Niigata poisonings included 47 cases with 6 deaths. In 1966, the government began to require the monitoring of waste streams which might contain mercury. Sweden became aware of the mercury problem as a result of a marked die-off of seed eating birds and their predators. These animals, as well as many Swedish agricultural products, were found to contain elevated levels of mercury, which was attributed to the rather large scale use of alkylmercurial seed dressings in Swedish agriculture. The use of these compounds was severely curtailed in 1965, and the mercury levels in the biota of terrestrial food chains has since decreased. However, it was also observed that mercury contamination in aquatic food chains was greater than could reasonably be attributed to run-off of agricultural mercurials, and that this contamination did not
decrease with the prohibition of thcse chemicals. Thus, all dischargers of mercury into Swedish waters were implicated. In 1967, Jensen and Jernelijv reported that inorganic mercury could be convertcd, in the environment, into methylmercury, thus completing the definition of our present mercury problem (1) Mercury, discharged into the waters in any form, can be methylated (2) . . Methvlmercurv accumulates in Lhe fish and other biota of that waterwaf (3)Animals, including man, eating those fish in sufficient quantity, will suffer permanent damage Excellent reviews of the history and development of the present situation have appeared in several popular magazines (1-3). At least in broad outline, this much of the mercury problem is clcar, and the details of thesc aspects of the problem are being further cxplored in a number of laboratories. Thcrc are other aspects, however, which are much less clear, and which need a good deal more attention before they are clarificd. Mcrcury is a naturally occurring element, and so presumably is to be found in any material analyzed, provided the analytical method is sufficiently sensitive. It is a ma& ter of some significance to define thosc lcvels of mcrcury which arc "natural," i.e., which would be observed in the absence of any society-related inputs, since life has presumably evolved to tolerate these lcvels. The oldest mercury analyses which appear to be reliable, however, are those of the German chemist Stock in the 19301s, and Gcrmany in the 1930's could hardly be considered a natural environment in the sense defined above. Therefore, the natural levels in some segment of the environment are usually taken to be the lowest levels observed in that scgment. Thcsc levels might better be termed "background" levels. The term "normal" has also been used, but 'hormal" has a connotation of acceptability which is not always warranted-earthquakcs, plague, and war all are normal, but not acceptable. A knowledge of background Estimates o f Background Levels o f Mercury i n Samples Not Known to b e Contaminated. Compiled from Various Sources
Concentration Sample
Volume
(PP~)
49, Number I, Jonuory 1972
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Environmental Chemistry
levels is indispensable in diagnosing possible mercury contamination. Most of the available information is summarized in a thorough, relatively recent report (4). However, specific information on the mercury content of particular segments of the ecology in particular regions of the world, is often lacking. The author's est.imates of some background levels are presented in the table. Once the background levels in an area are established, it is possible to assess the extent to which the area has been altered, either by natural processes related to the underlying geology or by the activities of man. Intent,ional or uncontrolled discharges have been bringing inventoried mercury into the environment a t a rate of about 4000 tons per year (6). Large, but a t present unknown, amounts of uninventoried mercury are also discharged, because of the common occurrence of mercury as a t,race element in most raw materials: Since billions of tons of coal and crude oil derivatives are burned annually, it is of some significance to be able to define the mercury content of various coals and crudes. Current estimates are that about 3000 tons of mercury enter the environment annually through combust,ion of fossil fuels, but these estimates can easily be in error by a factor of 2 to 5. Much more of the geochemistry of mercury which is relevant to the present problem remains to be explored. For example, the reported concentrations of mercury in seawater range from 0.03 ppb to about 2 ppb, with numerous reports in between. Are the differences due to different methods of analysis, or are they real? If real, arc they due to differences in natural inputs of mercury, or to differences in manmade input,s? Such questions await resolution. The few available reports indicate that the mercury content of rainwater varies from undetectable t,o a few tenths of a ppb, with an average of about 0.2 ppb. If 0.2 ppb is a true world average, then about 80,000 tons of mercury are transported annually between the at,mosphere and the earth. This 80,000 tons, if real, dwarfs any known society-related source of mercury discharge to the at,mosphere, and so the value for mercury in rainwater, on which this figure is based, demands extensive study. Many undergraduate institutions are now capable, or can easily become capable, of carrying out significant studies directed toward answering such questions as out,lined above. Many of these questions can be recast into regional t,erms, and mill fall naturally into smallar research problems of a scale well suited to ur?dergraduate research students. I n addition to their obvious relevance, such research problems have intrinsic scientific validity in unraveling thc geochemistry of mercury. Il'urther, such problems are excellent teaching tools, in that they require precise analytical techniques on sometimes unwilling samples, and they demand a careful consideration of sample selection and data treatment, topics often neglected in ~ndergraduat~e courses. The mercury problem could have been handled much more smoothly, had there been in existence a larger body of data on its geocliemistry and distribution. Undergraduak research students can contribute to the building up of such a body of data, for mercury as well as for the other heavy metals. 8
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Journal of Chemical Education
Mercury Analysis
The reduction-aeration-flameless atomic absorption method for mercury analysis is relatively simple and quite sensitive, and is applicable to a wide variet.y of samples after appropriate pre-treatment. The sample is digested to convert all the mercury to Hg(II), which is then reduced to Hg(0) by addition of Sn(I1). The Hg(0) is then voltatilized by aeration of the test solution, and the air stream is directed into the sample compartment of a general or special purpose atomic absorption spectrophotometer. Absorbance at 253.7 nm is proportional to the mercury concentration in the air stream, which in turn is hopefully proportional to thc total mercury concentration in the sample. Alkylmercurials are determined gas chromatographically, using an electron capture detector, following a series of extractions, baclc-extractions, and back-baclcextractions, which will not be discussed here. Determination of Total Mercury
The essentials of the method in general use are described by Hatch and Ott (67,as a mcthod for determination of mercury in metals, rocks, and soils. Approximately one-gram samples are dissolved in sulfuric or nitric-sulfuric acid. Any organic material present is oxidized with hydrogen peroxide and potassium permanganate. A sodium ehloride-hydroxylamine solution is added, to reduce any excess oxidizing agent while keeping the mercury as Hg(I1). Finally a 10% solut,ion of stannous sulfate is added, the solution is quickly mixed, and placed into the gas train. Hatch and Ott use an air flow of 2 l/min, provided by a mechanical pump, and a closed syst,em. Air passes from the pump, through the test solution, through a magnesium perchlorate drying agent, through a quartz-windowed absorption cell, and back to the pump. Absorption values reach a stable maximum value after about three minutes. Uthe et al. (7) recommend a procedure for the analysis of fish tissue, which requires only a part.ia1 digedion of the sample. Samples of 0.1-0.5 g are treated a t 60°C witah sulfuric acid and potassium permanganate, in closed vessels to minimize volatility losses of mercury. The samples are then reduced with hydroxylamine and stannous chloride, and aerated. The Dow Chemical Co. has published a general method, again based on reduction-aeration, with adaptations to brines, sludges, air, water, etc. (8). Samples are oxidized either with permanganate or with aqua regia. I n the aerat,ion step, a great,er air flow rate is used, and the system is open. The recorder trace from the spect.rophotometer gives a peak, since the mercury is not recirculated, but sensitivity is somewhat improved, as the relat,ive volumes of the absorption cell and the rest of the gas train are such that most of the mercury can be brought into the cell a t once. A different set of rduction conditions has been proposed by Magos and Cernik (9),who note that Sn(I1) in strongly basic solution can reduce mercury bound to sulfur. They have used their method to estimate mercury in undigested samples of blood and urine. To the sample is added a 1% solution of cysteine in 1 N nitric acid; the cysteine binds any free Hg(1I) to
Environmental Chemistry
prevent its premature reduction. Solid stannous chloride is then added, and the reduction reaction is triggered by the addition of excess 30% sodium hydroxide. In our laboratories, this method has been found to work well for natural waters, but not for sewage influents and effluents, as the reagents cannot be made sufficientlyconcentrated. Samples of air or other gases may be tested by passing the test gas stream through an acidic permanganate solution, which retains the mercury. The solution may then be reduced and aerated by the standard procedure. Alternatively, the gas stream may be passed through a wad of goldwire, or a gold covered glass frit, which amalgamates the mercury, and from which the mercury can be removed by heating. The sensitivity of the methods depends strongly on the ratio between cell volume and the dead volume in the gas train. Ideally, the dead volume should he smaller than the cell volume, since then the total sample can he in the light beam at once. This ideal is difficult to attain, but certainly can be approached. All of these methods, and many others, work very well after the operator has gained somc experience, and the apparatus has been broken in. Typically, a new operator, or different apparatus, will produce erratic results for some time. We observe very high blanks for the first few runs each day, which presumably result from mercury condensing inside the air train or glassware, but after a few runs acceptably small blanks (a few nanograms) arc obtained. Similarly, changing the tubing or a reaction vessel may lead to high or low results for a short time, due to contamination by mercury, or adsorption on the walls. Such effects, which tend to be the rule in these analyses, demand the frequent running of blanks, standards, and spiked samples. Preparation of standard mercury solutions presents some difficulties, since the working standards should be quite dilute. We use a standard of 50 ng/ml, prepared by dilution of 1000 ng/ml stock solution. Both of these solutions tend to be unstable, losing mercury by adsorption on the container wall. They should be prepared fresh, rather frequently, a t least during the initial stages of the analytical program. We have found that most of the mercury is lost within a few days from a 50 ng/ml standard solution stored in a new volumetric flask, but if we use a flask aged by contact with the stock solution, the loss is negligible. Conventional reagent grade chemicals are quite satisfactory. Even NaOH and HCI, which we suspected might contain traces of mercury, have caused no difficulty. One lot of stannous chloride, out of four which we have tested, was found to be rather heavily contaminated, so it is necessary to check each new batch of reagents. The mercury in samples collected for analysis may be lost by evaporation or adsorption, so analysis should begin soon after the samples are collected. This is particularly important for water analysis, but is somewhat less critical when analyzing biological materials and some highly organic sediments, in which the mercury is bound sufficiently to resist evaporation. Because of the volatility of mercury, most samples should be measured out for analysis on an "as received" basis, rather than first oven dried. Moisture content should be determined on a separate aliquot, and this
data used to correct the sample weight to a dry weight basis. Fish analyses are usually reported on a wet weight basis. This is somewhat unfortunate, as the water content of fish is both high and variable, while the mercury is primarily bound to the solids. Thus fish tissue containing, say 5 mg Hg/g of dry protein will be reported, on a fresh weight basis, as 0.5 ppm if the fresh tissue is 90% water, and as 1.0 ppm if the fresh tissue is 80% water. Instrumentation
The basic instrument requirements are quite simplea narrow-hand source of 253.7 nm radiation, a sample cell, a detector, and some readout device. Any commercial or home-made spectrophotometer which permits ready access to the lamp compartment, and has a large cell area, can be used. The light source may he a mercury hollow cathode lamp, available from AA suppliers, or a low pressure discharge, such as the PenRay (Ultra Violet Products, San Gabriel, Calif.). The latter is stable, inexpensive, intense, and can be mounted into most spectrophotometers with little difficulty. Requirements of the absorption cell are minimal-it nceds quartz windows, should be as large as but not much larger than, the light beam, and should be as long as convenient, so as to maximize the per cent of thc mercury which will be in the beam. Conventional UV detectors are fine. Gas for aeration may be obtained either from a pump or a cylinder. The flow rate should be from about 2 up to 6 or 7 l/sec, and is not critical so long as it is held constant, as the flow rate determines the peak shape. Several commercial instruments designed specifically for mercury analysis are available. Two of these, the Coleman and the Thermotron, can be obtaincd for about 51000. Both have self-contained pumps, so that the sample bubbling bottle and drying tube are the only external pieces of apparatus required. The Thermotron instrument has been in use in our lsboratory for sevcral months, and has been used for analyses ranging from Lake Michigan water, a t below 0.05 pph, through sewage sludges a t 0.5 ppm, to fish a t 2 ppm. Both instruments are sufficiently compact and rugged to perform well in the field. According to a recent publication (10) Dr. Bruce RiIcDuffie set up a mercury lab on a pier a t Hampton Bays, Long Island, to monitor mercury levels in the swordfish brought in by contestants in the Shinnecock Swordfish Tournament. Not to be outdone, we have mounted the Thermotron instrument, with battery pack, in a 13-ft Boston Whaler, to do on-site analysis of water samples of our local waters. Conclusions
The mercury problem is more than a midwestern lake full of poisoned fish. There is much good chemistry, especially geochemistry, biochemistry, and analytical chemistry, yet to be done on mercury and on other toxic metals. This work can he done, and donc well, by individuals or small groups, working with inexpensive equipment, on readily available samples. It is the hope of the author that this paper will encourage other chemists in undergraduate institutions to channel their energies and the energies of their students into this field, where the needs are great and the time is short. Volume 49, Number 1 , January 7 972
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Environmental Chemistry Literature Cited (1) M o m ~ o n sP., . *xo MONTAOUE. K., Sot.Rcu., 54,50 (February 6.1971). (2) HOLMEB, J.. Eapdre. 75. 135 (May. 1971). (3) Novma. S.. Enuironment. 11, 2 (1969); L 6 m o ~ n .G.. ibid.. p. 10. G R A NN ~.. ibid., ? 18. (4) "Mercury in the Environment." Profesaianal Pspe.713. US. Geologiosl ~ U W ~ Y1970. , (5) KLEIN. D. H., AND G O ~ ~ D B B R EO . D.. . Emiron. Sci. Tcchnal., 4, 765 (1970).
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lournol o f Chemical Education
(6) HATOX,W. R., (7)
AND
Ow, W.
L..A n d . Chem., 40,2085 (1968).
Bd. Con.. J' F"27, AnMamoNa' 805 (1970).F' A' "'
M' "'
J'
RBs'
(8) Axon. "Determination of Mercury by Atomia Absorption Speotrophotometry." Dow Cbemioal Co., Midland, Michigan. April. 1970.
,.,AND CERNIE,A. A_,Brit. J . Ind. Mad., 26, 144 (1969). (9) M ~ o o sL , H., spans IIGUS., 35,46 (JUIY 12, 19'11). (10) B o ~ ER.
