The solubilities of significant organic compounds in HLW tanks upernate solutions - FY 1997 progress report

The solubilities of seven sodium salts of organic acids that are thought to exist in high-level waste at the Hanford Site were measured in tank supernatant simulant solutions during FY 1997. This solubility information will be used to determine if these organic salts could exist in solid phases (saltcake or sludges) in the waste where they might react violently with the nitrate or nitrite salts present in the tanks. The solubility of sodium acetate was measured in simulated waste supernate solutions at 25C, 30C, 40C, and 50C that were both unsaturated and saturated with sodium nitrate. Solubilities of sodium glycolate, citrate, ethylenediaminetetraacetate (EDTA), nitrilotriacetate (NTA), formate, and oxalate were measured in simulated waste supernate solutions that were saturated with sodium nitrate. In addition, solubilities of sodium EDTA, citrate, glycolate, and NTA were measured in a complex waste matrix. The organic compounds were selected because they are expected to exist in relatively high concentrations in the tanks. The solubilities of sodium glycolate citrate, EDTA, NTA, and formate were high over the temperature and sodium hydroxide concentration ranges expected in the tanks. The solubility of sodium oxalate in solutions saturated with sodium nitrate were quite low. The presence of additional sodium in the waste simulant solutions that were saturated with sodium nitrate slightly lowered the solubilities of each of the organic salts. Solubilities were, however, high enough to prevent solid sodium salts of all the organic acids from precipitating from tank supernate solutions, except for sodium oxalate. The total organic carbon concentrations (TOC) of actual tank supernates are generally much lower than the TOC ranges for the simulated supernate solutions saturated (at the solubility limit) with the organic salts. This is true even if all the dissolved carbon in a given tank supernate is due to only one of these soluble compounds (an unlikely situation). Solubilities of all the organic salts, except for glycolate, decrease with increasing sodium hydroxide and sodium nitrate concentration because of the common ion effect of Na+. Sodium glycolate solubility increased with increasing hydroxide concentration. The complex waste solutions had sodium ion concentrations 3.4 to 7. 0 molar higher than unsaturated solutions. This caused a significant lowering of the solubilities of the organic sodium salts due to the common ion effect of sodium. Results of EDTA adsorption measurements show that EDTA or EDTA-metal complexes can be adsorbed onto hydrous metal oxides (that make up the sludge layers in the tanks) under conditions simulating the high-level waste tanks. The extent of adsorption is not large and depends on the concentration of hydroxide in the waste solutions. Higher hydroxide concentrations lower adsorption of EDTA. Adsorption also depends on the type of metal hydrous oxide present. Adsorption data for Fe(III), Cr(III), and NI(IV) hydrous oxides show that chromium(III) hydrous oxide adsorbs EDTA most effectively