Solubility of thorium oxides and hydroxides

The X-ray amorphous Th(IV) precipitates, called either amorphous hydroxides Th(OH)4(am) or hydrous oxides Th02-xH20(am) or Th02(am, hyd) as in other NEA-TDB reviews, are not well-defined compounds cf. [2003GUI/FAN]). The water content and the particle or crystallite size depend on the preparation method, pretreatment, alteration and temperature [1984GRE/LIE], [2000RAI/MOO], 

In addition to systematic discrepancies in solubility up to 3-4 orders of magnitude, the different data sets are widely scattered. This is at least partly due to experimental problems with the phase separation procedures, e.g., insufficient removal of colloids, and in particular at the low thorium concentrations at pH 6, sorption effects on the filter material. 

As none of the reported equilibrium constants for the formation of the neutral species Th(OH)4(aq) (or Th ,(OH)4 ,(aq)) which is predominant in neutral to alkaline solutions, is free of ambiguities, the reported solubility data in that pH range are used to evaluate an operational value of logic A°i can be used for geochemical modelling. 

Studies based on titration until precipitation / colloid formation  

The pH-independent solubility of Th(OH)4(am) or Th02(am, hyd) in neutral to alkaline solutions is usually ascribed to the reaction  

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Experimental studies on the hydrolysis of the Th" ion, its complexes with strong inorganic hgands, and the solubility of thorium oxides or hydroxides ate usually performed with low concentrations of thorium in perchlorate, chloride, and nitrate media. There is no evidence for complex formation between Th" and CIO4 however, chloride and nitrate form weak Th(lV) complexes as discussed in Sections Vlll.2.2.1 and X. 1.3.3, respectively. For the evaluation of equihbrium constants at zero ionic strength from data in chloride and nitrate media we have therefore the general problem to decide if the activity of Th", , should be calculated using a... 

We have shown that phytic acid readily hydrolyzes to produce phosphate with a projected lifetime of 100-150 years in the absence of microbiological effects, that actinide-phytate compounds are insoluble, and that europium and uranyl phytates are converted to phosphates within a month at 85 °C. Thorium solubility, on the other hand, is controlled by hydroxide or oxide species. Furthermore, the solubilities of radiotracer europium and uranyl are reduced by phosphate dosing of a simulated groundwater solution, even in the presence of citric acid. In the same systems, neptunium(V) solubility is only affected by 0.01 M phosphate at pH greater than 7. The results of these tracer-scale immobilization experiments indicate that phosphate mineral formation from representative deposits is under thermodynamic control. 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

Heating the ore with sulfuric acid converts neodymium to its water soluble sulfate. The product mixture is treated with excess water to separate neodymium as soluble sulfate from the water-insoluble sulfates of other metals, as well as from other residues. If monazite is the starting material, thorium is separated from neodymium and other soluble rare earth sulfates by treating the solution with sodium pyrophosphate. This precipitates thorium pyrophosphate. Alternatively, thorium may be selectively precipitated as thorium hydroxide by partially neutralizing the solution with caustic soda at pH 3 to 4. The solution then is treated with ammonium oxalate to precipitate rare earth metals as their insoluble oxalates. The rare earth oxalates obtained are decomposed to oxides by calcining in the presence of air. Composition of individual oxides in such rare earth oxide mixture may vary with the source of ore and may contain neodymium oxide, as much as 18%. 

The possibility of dissolving the mixed hydroxide in HNOs and obtaining direct extraction of thorium (and uranium) from the nitrate solution has been studied [155,156], but does not seem to be too promising, possibly due to the partial oxidation of tripositive cerium to the tetrapositive state. Kraitzer [157] was able to separate thorium from the mixed hydroxide cake by extracting the cake with sodium carbonate buffer at pH 9.5—10. Thorium was found to form a soluble carbonate complex and a recovery of better than 99% of thorium was claimed after only four extractions. 

Thorium has the oxidation state of (IV) in all of its important compounds. Its oxide, ThCL. and its hydroxide are entirely basic. The nature of the 10ns present in a number of solutions of the soluble compounds is not known with certainty. Complex ions involving sulfate are suggested by the increased solubility of the sulfate in solutions of the acid sulfates. Similarly, other complex ions are suggested by the solubility of the carbonate in excess alkali carbonate and of the oxalate in ammonium oxalate. Such ready complex ion formation is consistent with the high positive charge of the thorium-flV) ion. 

The chemistry of actinide ions is generally determined by their oxidation states. The trivalent, tetravalent and hexavalent oxidation states are strongly complexed by numerous naturally occurring ligands (carbonates, humates, hydroxide) and man-made complexants (like EDTA), moderately complexed by sulfate and fluoride, and weakly complexed by chloride (7). Under environmental conditions, most uncomplexed metal ions are sorbed on surfaces (2), but the formation of soluble complexes can impede this process. With the exception of thorium, which exists exclusively in the tetravalent oxidation state under relevant conditions, the dominant solution phase species for the early actinides are the pentavalent and hexavalent oxidation states. The transplutonium actinides exist only in the trivalent state under environmentally relevant conditions. 

NEC/ALT] in oversaturated solution. In both cases (solubility studies from underand oversaturation) solid-liquid equilibrium is followed by measuring the thorium concentration and pH as a function of time. In equilibrium with Th oxide or hydroxide the total thorium concentration is given by ... 

Figure VII-14 shows in addition that the solubility data of thorium hydroxide or hydrous oxide can be divided into two pH regions. At pH < 6, the thorium concentration shows a steep decrease with increasing pH while at pH 6-14 the thorium concentration remains at a constant level. In the following sections the published data are discussed and evaluated as follows ...

Gadolinium is produced from both its ores, monazite and bastnasite. After the initial steps of crushing and beneficiation, rare earths in the form of oxides are attacked by sulfuric or hydrochloric acid. Insoluble rare earth oxides are converted into soluble sulfates or chlorides. When produced from monazite sand, the mixture of sand and sulfuric acid is initially heated at 150°C in cast iron vessels. Exothermic reaction sustains the temperature at about 200 to 250°C. The reaction mixture is cooled and treated with cold water to dissolve rare earth sulfates. The solution is then treated with sodium pyrophosphate to precipitate thorium. Cerium is removed next. Treatment with caustic soda solution fohowed by air drying converts the metal to cerium(lV) hydroxide. Treatment with hydrochloric or nitric acid sol-... 

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