Technetium precipitation

In a recent study of the complexation of technetium with humic acid (HA) Sekine et al. [34,35] obtained interesting results which show competition between Tc,v-0(0H) i precipitate formation and Tcin-HA precipitate formation during a reduction process of pertechnetate with Sn2+. A weighable amount of... 

In closing, recovery of technetium from waste solution should be touched upon. Studies of the base hydrolysis of technetium P-diketone complexes revealed that all of the complexes studied decompose in an alkaline solution even at room temperature, until technetium is finally oxidized to pertechnetate. These phenomena are very important for the management of technetium in waste solutions. Since most metal ions precipitate in alkaline solution, only technetium and some amphoteric metal ions can be present in the filtrate [29]. A further favorable property of pertechnetate is its high distribution coefficient to anion exchangers. Consequently, it is possible to concentrate and separate technetium with anion exchangers from a large volume of waste solution this is especially effective using an alkaline solution [54],... 

Ballestra et al. [32] described a radiochemical measurement for determination of "technetium in rain, river, and seawater, which involved reduction to technetium (IV), followed by iron hydroxide precipitation and oxidation to the heptavalent state. Technetium (VII) was extracted with xylene and electrode-posited in sodium hydroxide solution. The radiochemical yield was determined by gamma counting on an anticoincidence shield GM-gas flow counter. The radiochemical yield of 50 to 150 litre water samples was 20-60%. 

Tetraphenylarsonium pertechnetate is precipitated in the presence of perchlorate as the carrier. The mixed salts are disolved in concentrated sulfuric acid and the solution is electrolyzed at platinum electrodes. The black deposit (TcOj) obtained is dissolved in perchloric acid, technetium heptoxide is distilled out of the solution... 

Procedure The irradiated molybdenum is dissolved in cone, sulfuric acid and technetium is distilled with the acid. The distillate obtained is diluted to 4 M H2SO4, heated to boiling and treated with bromic water. A platinum salt (1 mg of Pt/200 ml solution) is added to the solution as collector, and technetiiun is coprecipitated with the platinum sulfide. The precipitate is dissolved in NH OH/ HjOj mixture and the solution evaporated to dryness. The residue is dissolved in cone. HjSO or HCIO4 and technetium separated from platimun by distillation. The solution is diluted and the sulfide precipitated. 

Nitron-, thallium, cesium-, and silver pertechnetate are appreciably soluble in water and therefore less suitable for precipitation and separation of technetium. From aqueous ammonia solution, pertechnetate can be co-precipitated with MgMH PO ... 

Co-precipitation of Re S with platinum sulfide from cone, hydrochloric acid solutions of microamounts of technetium and rhenium is suitable for the separation of technetium from rhenium , since technetium is only slightly co-precipitat-ed under these conditions (Fig. 7). At concentrations of 9 M HCl and above, virtually no technetium is co-precipitated with platinum sulfide at 90 °C, whereas rhenium is removed quantitatively even up to 10 M HCl. The reduction of pertechnetate at high chloride concentration may be the reason for this different behavior, because complete co-precipitation of technetiiun from sulfuric acid solutions up to 12 M has been observed. However, the separation of weighable amounts of technetium from rhenium by precipitation with hydrogen sulfide in a medium of 9-10 M HCl is not quantitative, since several percent of technetiiun coprecipitate with rhenium and measurable amounts of rhenium remain in solu-tion . Multiple reprecipitation of Re S is therefore necessary. 

Fig. 7. Co-precipitation of microamounts of technetium and rhenium with platinum sulfide at 90 °C as a function of hydrochloric acid concentration ...

Satisfactory separation of pertechnetate from perrhenate can be achieved by reducing Tc (VII) with hydrochloric acid and co-precipitating technetium with Fe(OH)j Technetium can then be oxidized to the heptavalent state by oxidizing the precipitate in cone, nitric acid and precipitating Fe(OH)j with ammonia. 

Some precipitation methods have been applied to the separation of technetium from molybdenum when the former occurs as a radio-active daughter-product of the latter. The separation of technetium is performed by co-precipitation with tetraphenylarsonium perrhenate from an alkaline molybdate solution . In this way, also ruthenium remains in solution. Molybdate may be precipitated away from pertechnetate using 8-hydroxyquinoline " or a-benzoinoxim Pb or Ag" ions can also be used . Kuzina and Spitsyn have developed a method for concentrating technetium from ammoniacal molybdate solutions by co-precipitat-ing pertechnetate with the slightly soluble crystalline MgNH PO. ... 

A technique for the determination of Tc amounts as little as 4 x 10 g by neutron activation analysis has been described by Foti et al. . Tc in triply distilled water is irradiated in a thermal neutron flux of 5 x 10 neutrons per cm and per second to produce °°Tc. Other radionuclides are removed by co-precipi-tation with Fe(OH)j. Then, °°Tc is co-precipitated twice with tetraphenylarsonium perrhenate which can be removed by sublimation. The chemical purification of °°Tc requires 40-45 s and the technetium yield is about 53%. 

Pertechnetate forms a blue complex and perrhenate a brownish-yellow complex with K4[Fe(CN) ] in presence of bismuth amalgam. This permits the spectrophotometric determination of both elements in the same solution . The adsorption maxima of the technetium and rhenium complexes are at 680 and 420 nm, respectively. The molar extinction coefficients are 10,800 for technetium and 4,000 for rhenium. Metals forming color or precipitates with K4[Fe(CN) ] must first be removed. 

The use of a special microtechnique has permitted the precipitation, weighing, and determination of about 2 /xg of technetium with a standard deviation of 0.08 /xg . The precipitate is filtered, washed with ice-cold water, dried at 110 °C and weighed as (CgH5) AsTcO. Permanganate, perchlorate, periodate, iodide, fluoride, bromide, thiocyanate anions and mercury, bismuth, lead, silver, tin and vanadyl cations as well as nitrate concentrations above 0.5 M interfere with the determination. 

Technetium can also be precipitated and weighed as nitron pertechnetate CjqHj N TcO which is precipitated at 80 °C from a weak sulfuric acid or acetic acid solution with an excess of a solution of 5% nitron in 3% acetic acid. The precipitate is washed with cold water, dried at 100 °C and weighed. Nitrate, perchlorate, permanganate, periodate, chloride, bromide, and iodide ions disturb the determination. 

Thiourea complexes of technetium(III) have been shown to be excellent precursors for the synthesis of a large number of Tc and Tc complexes. The [Tc(tu)6] cation is readily formed in a mixture of pertechnetate, tu and 12 M HCl and precipitates as chloride. Due to the different redox potentials of the homologous elements technetium and rhenium, the reducing capacity of HCl is not sufficient to apply the same method for rhenium and tin chloride is added as reducing... 

Tin and americium were so extensively sorbed under all conditions that isotherm data could not be obtained. These elements are not significantly mobile in the Mabton Interbed aquifer. Values of Freundlich constants for technetium, radium, uranium, neptunium, and plutonium are given in Table IV. The Freundlich equation did not fit the selenium sorption data very well probably because of slow sorption kinetics or precipitation. Precipitation was also observed for technetium at 23°C for concentrations above 10 7M. This is about the same solubility observed for technetium in the sandstone isotherm measurements. Linear isotherms were observed only in the case of radium sorption. In general, sorption on the Mabton Interbed was greater than on the Rattlesnake Ridge sandstone. This is probably due to the greater clay content of the Mabton standard. 

Table VH shows that hydrazine is the only important variable for technetium sorption on each of the geologic solids. Hydrazine causes technetium to be removed from solution either by sorption or by precipitation of the reduced technetium species. Hydrazine is a powerful reducing agent and should reduce TcO/ to technetium(IV) according to standard half-cell potentials. No Tc02 was observed however, since the technetium passed through...

E. O. Fischer was the first to prepare arene complexes of technetium. Arene ligands in these compound are typically six-electron donors. The nature of the Tc-arene bond is probably very similar to the one of the Tc-Cp bond. The [Tc(arene)2] complexes (arene = benzene or hexamethylbenzene) are prepared by the reaction of TcCLt with the appropriate arene in the presence of AICI3/AI. In a typical reaction, TcCLt, AICI3, and aluminum were heated with benzene in a sealed tube to 135 °C for two days. The product is precipitated as a salt using the hexafluorophosphate ion. A variety of other 18-electron compounds can be prepared from this material, particularly through chemical reduction. 

Under reducing conditions, Tc(IV) is the dominant oxidation state because of biotic and abiotic reduction processes. Technetium(IV) is commonly considered to be essentially immobile, because it readily precipitates as low-solubility hydrous oxides and forms strong surface complexes on iron and aluminum oxides and clays. 

In addition, technetium may be fixed by bacteri-aUy mediated reduction and precipitation. Several types of Fe(III)- and sulfate-reducing bacteria have been shown to reduce technetium, either directly (enzymatically) or indirectly through reaction with microbially produced Fe(II), native sulfur, or sulfide (Lyalikova and Khizhnyak, 1996 Lloyd and Macaskie, 1996 Lloyd et al, 2002). 

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