Isotopic Organic and Inorganic Mercury Exchange in River Water Akira Kudo*, Hirokaisu Akagi, D. C. Mortimer, and Donald R. Miller Division of Biological Sciences, National Research Council of Canada, Ottawa, Ont., Canada K 1A OR6 1
Isotopic exchanges between organic and inorganic mercury were observed for Ottawa River water as well as for polar and nonpolar organic solvents. The concentration of mercury investigated ranges from 0.1 to 100 ppm, and the exchanges were observed for 24 h under vigorous mechanical mixing at a temperature of 21 "C. No or very little (less than 0.52%) isotopic exchange was observed in the Ottawa River water. There was no difference between the two exchange directions, i.e., 203HgC12 CH3HgC1 and CH3203HgC1 HgC12. The additional conditions of adding salts, acid, or alkali, changing temperature to 40 'C, and prolonging the mixing period did not change the results.
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Materials and Methods
A combined technique of radioactive tracers and thin-layer chromatography (TLC) was applied to determine, quantitatively, organic and inorganic forms of mercury (13).Mercuric chloride and methylmercuric chloride, both labeled with radioactive mercury-203, were used to observe isotopic mercury exchange from both sides, Le., 203HgC12 CH3HgCl and CH3203HgC1 HgC12. A dithizone-benzene extraction method was applied for this experiment. The recovery of mercury for the determination was up to 91%. Dithizone-benzene extract was then applied to the TLC (Polygram, cel400) with a methylcellosolve-water solvent system. Rf values for mercuric and methylmercuric
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+
.
1
1
'
'
1
'
022% 024%
[CH~H~CJ]-[~~JJZ]
:
850ppb
1094%
'
I
I
009%
y;
000% j
~~~~1
170ppb 0040,~Elhonol C&OH
ooosb
I 042%
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Isotopic mercury exchanges between organic and inorganic forms have been observed for nearly 25 years (1-5). Two schools (Ingold et al. vs. Abraham et al.) are debating the detailed mechanisms of the exchange of mercuric and alkyl mercuric compounds for high mercury concentration ranges (over 10 000 ppm) in polar organic solvents (6-10). However, no direct observation of isotopic mercury exchange has been reported for aquatic environmental conditions for the low concentration range of mercury (under 100 ppb) in which the most environmental mercury concentrations in water fall (11, 12).This reports a recent observation that there is no or very little isotopic mercury exchange in low concentration for the Ottawa River water as well as for polar and nonpolar organic solvents.
1
Acetone
(CH,),CO
O05%[CH;H4CLf--[ H g C J i 1062yo 170ppb
: 850ppb
026%
Methylcellmolve
0
Benzene C S HS
075% 1
I
0
20
IO
1
, IO
0
Figure 2. Isotopic exchange rates between methylmercuric and mercuric chloride in various solutions (after 24 h)
forms, respectively, were 0.36 ana 0.66. All experiments were conducted a t 21 OC and for triplicate samples. Radioactive methylmercuric chloride (New England Nuclear) had a radiological impurity as mercuric chloride (2.18 f 0.10%). This was caused by degradation of radioactive methylmercury. With an increase in time, the proportion of the impurity increased. For example, after 6 months in the dark, the radioactive methylmercuric chloride solution contained 20% of radioactive impurity as mercuric chloride. However, at this time, radioactivity of the solution decreased to 7.033% of the original value (half-life 47 days). Therefore, this amount of the impurity was subtracted from experimental results. Two sets of experiments were carried out to observe isotopic mercury (mercuric and methylmercuric) exchange: changing mercury concentration in the Ottawa River water (0.1, 1, 10, and 100 ppm for each form), and changing solvents (water, methanol, ethanol, acetone, methylcellosolve, benzene, and chloroform) and fixing mercury concentration at a 1 (0.170 ppm, radioactive) to 5 (0.850 ppm, stable) ratio. To promote the isotopic exchange, vigorous mechanical mixing was applied to the samples for 24 h. Results and Discussion
Isotopic Exchange Rote(%) Figure 1. Isotopic mercury exchanges for various concentrations in Ottawa River water (after 24 h , concentration ratio was 1 to 1 for both directions)
No or very little (less than 0.52%) isotopic exchange was observed in the Ottawa River water (Figure 1).There was no difference between the two exchange directions, i.e., 203HgC12 CH3HgCl and CH3203HgC1 HgC12. Furthermore, there was no difference in exchange rates for various mercury concentrations ranging from 0.1 to 100 ppm. Additional experiments were also carried out under different conditions: mercury concentrations 1-10,l-0.1 ratio (radioactive); adding salt (2 N NaCl), acid (2 N HCl), or alkali (2 N NaOH); changing temperature to 40 OC; and prolonging the mixing period up to 4 days. Similar results (no or very little isotopic exchange) were obtained. Organic solvents did not change the low isotopic exchange rates after 24 h of mixing (Figure 2). This was true for both exchange directions, 203HgC12 CH3HgCl and CH3203HgC1 HgC12. To increase isotopic exchange rates, mercury concentration of the stable form was five times (0.850 ppm) higher than that of the radioactive form (0.170 pprn). Slightly higher exchange rates were observed in chloroform for both direc-
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Volume 11, Number 9, September 1977
907
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tions. In general, the exchange rates are higher for CH3203HgC1 HgClP. However, the higher exchange rate observed in this direction might suggest decomposition of methylmercuric chloride to mercuric chloride (not isotopic exchange) during experimental processes. Clarkson (14) also observed no or a negligible isotopic exchange rate between metallic mercury and organomercurials covalently bonded to a carbon atom. Elemental mercury vapor and organomercury in tissue were placed in a sealed box during 45 h. In this case, the mercury concentration in tissue was 10 000 ppm. For a mercury concentration of 20 000 ppm, Hughes et al. (6)observed the exchange rate of 67.6% during 8 h. However, only one direction (HgBr2 CH3HgBr) was observed, and the exchange reaction was conducted in ethanol a t 100.2 “C, which is far from natural aquatic conditions. Of course, radioactive methylmercuric chloride (CH3203HgC1) has been commercially produced through application of the isotopic exchange phenomena. For example, the exchange technique [203Hg(N03)2 CH3HgClI is used under very limited specific conditions by the New England Nuclear Corp. Isotopic exchange reactions between methylmercuric and mercuric chloride only were observed in this study. No other forms of mercury compounds were investigated. Furthermore, this study was conducted in very limited environmental conditions. Therefore, at this stage the results must be treated considering these limiting factors. If this result (no or very little isotopic exchange) is applicable for low mercury concentrations (at most 100 ppm) and various aquatic environmental conditions, it is possible that the mercury transport and transfer mechanism in natural environments can be investigated using radioactive mercury tracers for each individual mercury compound. For example,
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the methylation rate from inorganic mercury or demethylation rate in aquatic systems can be determined quantitatively. Acknowledgment
We thank E. Javorsky and M. Y. Pinard for their assistance. Literature Cited
XL (2), 96 (1951). (2) Nefedov, V. D., Sinotova, E. N., “Collected Works on Radiochemistry”, p 113, Leningrad Univ. Press, 1955. (3) Nefedov, V. D., Sinotova, E. N., 2. Fiz. Khim., 30,2356 (1956). (4) Reutov, 0. A., Izv. Akad. Nauk SSSR, Otd. Khim. Nauk, 684 (1958). (5) Reutov, 0. A., Angew. Chem., 72,198 (1960). (6) Hughes, E. D., Ingold, C. K., Thorpe, F. G., Volger, H. C., J. Chem. Soc., 1961, p 1133. (7) Charman, H. B., Hughes, E. D., Ingold, C. K., Volger, H. C., ibid., p 1142. (8) Hughes, E. D., Volger, H. C., ibid., p 2359. (9) Abraham, M. H., Dcdd, D., Johnson, M. D., Lewis, E. S., O’Ferrall More, R. A., J . Chem. SOC.( B ) , 1971, p 762. (10) Abraham, M. H., Grellier, P. L., Hogarth, M. J., J . Chem. SOC. Perkin Trans. 2, 1974, p 1613. (11) Kudo, A,, Townsend, D. R., Miller, D. R., Prog. Water Technol., 9,923 (1977). (12) Miller, D. R., Ed., Ottawa River Project (#3), National Research Council of Canada, Ottawa, Ont., Canada, 1976. (13) Takeshita, R., Akagi, H., Fujita, M., Sakagami, Y., J . Chromatogr., 51,283 (1970). (14) Clarkson, T. W., “Chemical Fallout, Current Research on Persistent Pecticides”, M. W. Miller and G. G. Berg, Eds., p 274, Charles C. Thomas, Publ., Illinois. (1) Cross, J. M., Pinajian, J. J., J . Am. Pharm. Assoc.,
Received for review December 21, 1976. Accepted April 26, 1977. Research curried out us part of the Ottawa River Project, a joint study of the National Research Council Laboratories (NRCL)and the University of Ottawa.
Physical Model of Marine PhytoplanktonChlorination at Coastal Power Plants Joel C. Goldman* and John A. Davidson Woods Hole Oceanographic Institution, Woods Hole, Mass. 02543
w A physical model of the impact of coastal power plant chlorination on marine phytoplankton was developed. Several steady state biomass levels of the test alga Phaeodactylurn tricornuturn were established in continuous cultures by varying the concentration of phosphorus in the growth medium. The algae were then exposed to combinations of chlorine and temperature stress for 1 h, and deviations from the prestress steady state were observed over long periods of time. Three types of response occurred that were a function of the applied chlorinehiomass ratio: no effect, effect followed by recovery, and complete washout. It is suggested from the results that, given the typical organic loads present in coastal waters, excessive levels of chlorine may be in current use, and that the effects on entrained phytoplankton probably have little bearing on the ultimate effect on the standing crop in the receiving water. The most important considerations were the hydrologic, biological, and chemical qualities of the receiving waters themselves.
Based on recent reports there is a general consensus of opinion that chlorination practice at coastal power plants for fouling control leads to serious impairment of phytoplankton productivity in entrained waters ( I ) . This conclusion, for the most part, has been based on two types of studies: comparative 908
Environmental Science 8 Technology
field estimates of photosynthetic activity (measured by the classical 14C incubation technique) at the intake and discharge stations of power plants (2-5),and by laboratory studies using similar techniques for observing immediate and short-term responses of natural and artificially cultured phytoplankton populations after chlorine exposure ( I , 5 ) . There are a number of reasons why the above techniques at best only provide a partial picture of the impact of chlorine on phytoplankton populations in receiving waters: entrained phytoplankton represent only a fraction of the actual population residing in the receiving water; phytoplankton generation times are rapid enough so that even though there may be serious inhibition of photosynthesis and actual destruction of a large percentage of entrained phytoplankton, those surviving organisms are capable of recovery and subsequent restocking of the standing crop when returned to the receiving water; in many plants chlorination is practiced during only a small fraction of the day; any persistent effects of the toxicant in marine receiving waters are minimal-both free chlorine and chloramine compounds, or their bromine counterparts which are formed in seawater (6),are quickly degraded through the combined oxidation of organic matter and the inorganic decomposition of the halites to highly oxidized forms, not recoverable by the standard amperometric technique (5, 7,8);and the formation and presence of persistent organochlorine compounds in trace quantities, although not
