Tóth, Géza

English

Periodica Polytechnica (Budapest University of Technology and Economics)

DETERMINATION OF THE MERCURY CONTENT IN NATURAL WATERS BY ACTIVATION ANALYSIS By G. TOTH Department of Chemical Technology, Technical University Budapest (Received March 31, 1973) Presented by dr. 1. SZEBENYI In view of increasing ecological efforts, the knowledge and determination of trace contaminants in natural waters is steadily gaining importance. Among trace elements, particularly selenium, arsenic, antimony and mercury have been objects for the development of analytical methods all based on activation, but utilizing different enrichment methods, depending on the character of the sample to be analyzed. In the determination of trace elements in water by activation analysis, difficulties arise from the fact that usually only samples around some ml can be submitted to direct activation [l], and therefore concentration of the sample before irradiation is inevitable in most cases. This operation, however, involves the risk of contaminating the sample, particularly if the concentration of the trace element to be determined is of the p.p.b. order. It is therefore preferable to use a minimum amount of reagents for this purpose, and to avoid the use of reagents in which the element to be deter-mined is a typical contaminant. In the present paper, a method for the enrichment and determination by activation analysis of trace amounts of certain metals will be reported. The method is based on the spontaneous deposition of metal traces on pal-ladium black, joint activation of the palladium black -with the metal traces, and separation of the radioisotope to be used for the determination from palla-dium black by isotope exchange. The method has the advantage that before irradiation, the water sample is not submitted to contact ,tith any reagent except palladium black, and hence the risk of its being contaminated is reduced to a minimum. The procedure will be demonstrated on the example of mercury deter-mination in three water samples. 1* 4 G. T6TH Mercury deposition on palladium black The extent to which mercury present in the ionic form is deposited on palladium black preliminarily saturated "\vith hydrogen is shown in Fig. l. Plot 1 refers to a mercury nitrate concentration of 2 mg/l, plot 2 to 10 mg/l. The amount of palladium black was 100 mg in both experiments, the sample volume was 1000 ml. As demonstrated by plot 1, 100 mg palladium black is capable of binding 2 mg Hg quantitatively. This, in the case of a mercury concentration of 10 p.p.b. is equivalent to the mercury content in a 200 litre water sample. . % 100~~~~~~~~--o-~~--~ 90~~--.--~--.--~· 80 70 60 50 2 2 3 I; 5 6 7 8 9 10 x 100 ml Fig. 1. Extent of HgH deposition on 100 mg palladium black. Concentration of mercury nitrate in the solution: 1 : 2 mg/litre; 2 : 10 mg/litre The thermal stability ·f mercury deposited on palladium black Reckoning with the possibility that the temperature of the palladium black + trace element sample may reach 200 to 300 QC in the course of irra-diation, we studied the thermal stability of the mercury deposited on palla-dium black. The results are presented in Fig. 2, demo:nstrating that up to 400 QC, no measurable amounts of mercury are evolved, so that no mercury losses must be considered during activation. 1 00 I'<>---o--o--~_ % 80 60 40 20 200 300 400 500 600 700 800 900°C Fig. 2. Evaporation losses of mercury deposited on palladium black as a function of tem-perature (Y-axis: Residual mercury on palladium black, %) JIERCURY COSTE-,T LV _YATCRAL WATERS Removal of 203Hg and/or 197Hg from palladium black by heterogeneous isotope exchange 5 In the course of activation, in addition to the radioisotopes 2°~Hg and 197Hg from the mercury deposited on the palladium black, several palladium radioisotopes (lOSPd, l07Pd and I11Pd) as well as l11Ag will also be produced from the palladium black itself. Therefore, before measuring the gamma activity of the radioactive mercury isotopes, they must be separated from palladium black. For this purpose the irradiated sample was brought into con-tact with a mercury nitrate solution. As the result of the exchange reaction Pd - *Hg + H g2 + ~ Pd - Hg + *Hg2 (1) a fraction (depending on exchange time) of the radiomercury on the palladium black passed into the solution in the form of mercury(II) ions, while radio-active palladium isotopes and l11Ag remained in the solid phase. In our earlier work [2,3] we demonstrated that the kinetics of isotope exchange in systems consisting of metal monolayer on a noble metal and metal ion can he described by the relationship In (1 - F) (A) (B) , -'--'---'---'-~ . Rmax ·In t I" In (1 - F)'=l (A) . (B) , (2) where (1 - F) is the exchange fraction, i.e. the radiomercury fraction not exchanged till the moment t, (A) and (B) mercury concentrations on the surface of the solid phase and in the solution, resp., and Rmax the exchange rate. 100 rxloo 10 10 100 min Fig. 3. Time dependence of exchange fraction. CJ::: CJ 50 pg mercury on 100 mg palla-dium black; 00 10 pg mercury on 100 mg palladium black; ••• 1 pg mercury on 100 mg palladium black 6 G. TDTH Under the given experimental conditions, namely when the surface con-centration of mercury (A) is much lower than mercury concentration in the solution (B), Eq. (2) assumes a simpler form: In (1 - F) --:- -kmax . (B) . In t + In (1 - F)t=l (3) i.e. the time dependence of the exchange fraction is invariable "With (A). In other words, under othenvise identical conditions, identical fractions of the mercury will pass into the solution by isotope exchange from palladium blacks containing different amounts of mercury. Some experimental data confirming this statement are presented in Fig. 3. Determination of the mercury content in water samples We analyzed water samples taken from the Budapest section of the Danube, from the river Tisza and from the lake Velence. Sample volumes as listed in Table 1 were admitted to a small chromatograph column filled "\\ith 100 mg palladium black previously saturated with hydrogen. The flow rate of the water samples was adjusted to .--....,200 ml/hr. Hydrogen was continuously led into the water in the column above the palladium black, to avoid an escape of the hydrogen sorbed by the palladium. The water samples were passed through the column without previous filtration, to avoid adsorption of mercury present in a non-ionic form on the filter, since this - presumably significant effect could falsify remlts. The mercury standard series was prepared by admitting mercury nitrate solutions containing 1, 10 and 50 Vg Hg, resp., to columns containing 100 mg palladium black each, previously saturated with hydrogen. As demonstrated by Fig. 1, the mercury quantities applied for standards are quantitatively precipitated on the platinum black under the given experimental conditions. The palladium blacks with the mercury contents of the water samples and with the mercury standards, resp., were actiyated in reactor with a neutron flux of '""'-' 1017 m -2S -1 for 3 hours. After activation the samples were added to 5 ml each of a O.llVI mercury nitrate solution, agitated for 30 minutes and separated by filtration. An aliquot of the solutions was used for activity measurement carried out with a NaI(TI) scintillation detector coupled with a 25fi-channel amplitude analyzer. For the evaluation of the gamma spectra the 77 ke V photopeaks of 197Hg was used. The results are summarized in Table 1. MERCURY CONTENT IN iYATURAL WATERS Table 1 Mercury concentrations found in natural waters Origin of water sample River Danube River Tisza Lake Velence Tested volume 3000 ml 2900 ml 2450 ml Discussion Hg found, p.p.b. 1.0 1.7 <0.2 <0.1 7 The described method is little intended for routine tests, but primarily for soh-ing tasks where - o, .. ing to the extremely low mercury content of the samples - very large volumes have to be concentrated before activation. The described procedure allows such concentration from samples having vol-umes of several 10 litres. It appears that the procedure is successfully applicable for concentrating all trace elements which are spontaneously deposited on palladium black, viz. copper, selenium and tellurium. \Ve intend to develop analytical procedures for the determination of these elements too. The author wishes to express his gratitude to Professor Gabor FOLDL.\.K, Director of the Isotope Institute of the Hungarian Academy of Sciences, for enabling the experimen-tal work. Summary A method was developed for determining mercury traces in natural waters. The pro-cedure is based on the spontaneous deposition of mercury nn palladium black, activation of the palladium black together with the mercury deposited on it in a nuclear reactor, and separation of the radioactive mercury isotopes from palladium black and other isotopes formed in activation by means of isotope exchange. References 1. BECKl'\ELL, D. E.-MARSH, R. H.-ALLIE. E. JR.: J. Anal. Chern., 43, 1230 (1971). 2. TOTH, G.: Radiochim. Acta, 17, 12 (1972). 3. MILLER. J.-TOTH. G.: Magy. Kern. Foly .. 78, 265 (1972). Dr. Geza TOTH H-1521 Budapest 

DETERMINATION OF THE MERCURY CONTENT IN NATURAL WATERS BY ACTIVATION ANALYSIS

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DETERMINATION OF THE MERCURY CONTENT IN NATURAL WATERS BY ACTIVATION ANALYSIS

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Periodica Polytechnica (Budapest University of Technology and Economics)