Technetium sulfides

An exception is technetium sulfide (TcjSy), known as " Tc-sulfur colloid (Stern et al. 1966). Scavenging molecules like phosphinimine (R3P = N-SiMe3) have been reported to incorporate TcO, producing organic molecules containing Tc(VII) (Katti et al. 1993 Singh et al. 1995). 

Chemical separation of technetium in soils is not easy, but it is fairly well-known that under aerobic conditions pertechnetate Tc(YII) is readily transferred to plants while under anaerobic conditions insoluble TcCh (or its hydrate) is not transferred to them. Even under aerobic conditions, however, the transfer rate decreases with time [28], indicating that soluble pertechnetate changes to insoluble forms by the action of microorganisms which produce a local anaerobic condition around themselves [29,30]. Insoluble technetium species may be TcOz, sulfide or complexes of organic material such as humic acid. 

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. 

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 ...

FIGURE 21.2 Primary mineral sources of metals. The s-block metals occur as chlorides, silicates, and carbonates. The d- and p-block metals are found as oxides and sulfides, except for the group 3B metals, which occur as phosphates, and the platinum-group metals and gold, which occur in uncombined form. There is no mineral source of technetium (Tc in group 7B), a radioactive element that is made in nuclear reactors. 

As mentioned before, subsequent phosphate treatment does not affect the stable sulfide, and TCLP results show excellent stabihzation of Cr in any oxidation state. Alternatively, a small amount of reductant in the waste will convert chromate into lower oxidation states. Such methods, however, are not preferred, because the reductant may also affect the solubility of other hazardous compounds. The exception is technetium-containing radioactive waste, in which chromate is also a contaminant. As we shall see in Chapter 17, a reductant is essential for stabihzation of technetium, and that will also help in stabilization of chromium. 

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). 

Fission products that are compatible with the uraninite crystal stmcture—the REE, yttrium, neodymium, and zirconium—were largely retained in the uraninite core, the reactor clays, minor phosphate phases, and uranium and zirconium silicate phases (Gauthier-Lafaye et ai, 1996). Lighter REE—lanthanum, cerium, and praseodymium—were partially lost from the reactor. Einally, molybdenum, technetium, mthe-nium, rhodium, and other metallic elements were retained in the metal/metal oxide inclusions and arsenide/sulfide inclusions in the core, and in the reactor clays (Hidaka et ai, 1993 Jensen and Ewing, 2001). 

BockW. D., BruhlH., Trapp T., and Winkler A. (1989) Sorption properties of natural sulfides with respect to technetium. Int. Symp. Sci. Basis Waste Manage. VII, 973—977. 

Redox-type waste contains considerable mercury, which must be removed. Advantage is taken of the presence of mercury to use it as a carrier for the ruthenium and technetium when this group is precipitated as the sulfides. This involves fairly corrosive chemical solutions, but they can be handled in equipment fabricated with special grades of stainless steel. The filtrate contains only the alkaline and rare earths which are then precipitated as carbonates, the same as the Purex-type procedure. The waste from this step is treated separately. 

FIGURE 20.1 Metals and their best-known minerals. Lithium is faund in spadumene (LiAISi20, and beryllium in beryl (see Table 20.1). The rest of the alkaline earth metals are faund in minerals that are carbanates and sulfates. The minerals for Sc, Y, and La are the phasphates. Same metals have more than one type of important mineral. For example, in addition to the sulfide, iron is found as the oxides hematite (Fe202) and magnetite (Fe O ) and aluminum, in addition to the oxide, is found in beryl (Be2Al2Si Ots). Technetium (Tcj is a synthetic element. 

Chemical name Technetium " Tc colloidal rhenium sulfide injection Ph. Eur.) Technetium Tc 99m sulfur colloid injection USP) " Tc-(Re)-sulfide colloid " Tc-sulfur colloid Listed trade names HepatoCis (TCK-1) (1) Sulfotec Sorin (2) ... 

Tc-(Re)-sulfide colloid is formed at acidic pH in the boiling water bath. The reaction is based on the formation of colloidal sulfur and technetium heptasulfide (TC2S7) at acidic pH. Rhenium is used as a carrier (Larson and Nelp 1966 Patton et al. 1966). Ten milligrams of sodium thiosulfate pentahydrate generate 2 mg of colloidal sulfur (Stern et al. 1966). The added Tc eluate should not be less than 5 ml, the Tc activity not less than 370 MBq (10 mCi). 

The most exposed organs are the liver, spleen, and red marrow. Calculations of the absorbed radiation dose resulting from liver and spleen scintigraphy are based on technetium-labeled colloids (International Commission on Radiological Protection 1987). The effective dose equivalent is 0.014 mSv/MBq. The effective whole-body dose in adults (70 kg) resulting from an intravenous injection of 185 MBq of Tc-(Re)-sulfide colloid is 2.6 mSv. 

Patton, DP, Garcia, EN, Webber, MM (1966) Simplified preparation of technetium-99m-sulfide colloid for liver scaiming. Am J Roentgenol Radium Ther Nucl Med 97 880 Ponto JA, Swanson DP, Freitas JE (1987) CUnical manifestations of radio-pharmaceutical formulation problems. In Hladik WB III, Saha GB, Study KT (eds) Essentials of nuclear medicine science. Williams Wilkins, Baltimore, pp 268-289 Schuind F, Schoutens A, Verhas M, Verschaeren A (1984) Uptake of colloids by bone is dependent on bone blood flow. Eiu" J Nucl Med 9 461-463... 

To prepare the compounds RbioTe68i4, CsioTe68i4, RbioRe68i4, and CsioReeSn, a mixture of the alkali carbonate and rhenium or technetium in 5 1 molar ratio was in each case converted at 800 °C in a stream of hydrogen doped with sulfur. The duration of these reactions was between 10 and 16 h. The preparation of the technetium compoimds was performed in a radiochemical laboratory of the For-schungszentrum Julich. The Tc isotope used decomposes by emission. The compounds were obtained as black, lustrous crystals embedded in the partially solidified melt (Fig. 2). All four sulfides are unstable in air. 

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