"_i_eHanford Ferrocyanide Task Team is addressing issues involving ferrocyanide precipitates in single-shell waste storage tanks (SSTs), in particular the storage of waste in a safe manner. This Task Team, composed of researchers from Westinghouse Hanford Company (WHC), Pacific Northwest Laboratory (PNL), and outside consultants, was formed in response to the need for an updated analysis of safety questions about the Hanford ferrocyanide tanks. " The Ferrocyanide Safety Project at PNL is part of the Waste 'Tank Safety Program led by WHC. The overall purpose of the WHC program, sponsored by the U.S. Department of Energy's Tank. Farm Project Office, is to 1) maintain the ferroeyanide tanks with minimal risk of an accident, 2) select one or more strategies to assure safe storage, 3) close out the unreviewed safety question (USQ), and 4) identify ultimate disposal options to be used when waste is removed from the tanks. This annual report gives the resu!t_ of the work conducted by PNL in FY 1992 on Subtask 3.4, Aging Studies, which is part of Task 3, Chemical Nature of Ferrocyanide in Wastes. Subtask 3.4 deals with the aging behavior and solubilization of ferrocyanide tank waste sludges in a basic aqueous environment. Investigated were the effects of pH variation, ionic strength, salts present in SSTs, and gamma radiation on solubilization of vendor-prepared Na_NiFe(CN)6. Vendor-prepared Na_NiFe(CN)6 dissolves in aqueous base to give primarily insoluble Ni(OH)2 and soluble Na4Fe(CN)6. Precipitation of Ni(OH)2 apparently drives the ferrocyanide dissolution. From the gamma pit experiments, the in_,_uble solids contained more than one iron cyanide species, yet to be identified. The rate of dissolution of Na2NiFe(CN)6 in aqueous base increases with increasing pH. At pH 14, 95% dissolution is observed after 0.5 h. Addition of 1 M Na + ions in the form of Na2SO4 suppresses dissolution at pH 13, presumably because of a common ion effect. However, 1 M Na + in the form of SST simulant salts (sodium salts of phosphate, carbonate, nitrate, nitrite, sulfate, and hydroxide) resulted in an enhancement of the rate of solubilization most likely arising from buffering of the solution by phosphate. Even when the solution is not stirred, dissolution is relatively rapid. Approximately 40% of the Na_NiFe(CN)6 dissolves in 24 h in an unstirred solution containing SST salts. Gamma radiation does not appear to greatly affect the dissolution reaction. Similar rates were observed in unstirred irradiated and control solutions. A more complex mixture of iron cyanides in the insoluble fraction of the gamma radiation experiments is obtained, suggesting the possibility that an iron cyanide species re-precipitates from solution. Further work is needed to determine the identity of this species. Solubilization work will continue next fiscal year. In addition, cyanide and ferrocyanide hydrolysis studies will be conducted.

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"_i_eHanford Ferrocyanide Task Team is addressing issues involving ferrocyanide precipitates in single-shell waste storage tanks (SSTs), in particular the storage of waste in a safe manner. This Task Team, composed of researchers from Westinghouse Hanford Company (WHC), Pacific Northwest Laboratory (PNL), and outside consultants, was formed in response to the need for an updated analysis of safety questions about the Hanford ferrocyanide tanks. " The Ferrocyanide Safety Project at PNL is part of the Waste 'Tank Safety Program led by WHC. The overall purpose of the WHC program, sponsored by the U.S. Department of Energy's Tank. Farm Project Office, is to 1) maintain the ferroeyanide tanks with minimal risk of an accident, 2) select one or more strategies to assure safe storage, 3) close out the unreviewed safety question (USQ), and 4) identify ultimate disposal options to be used when waste is removed from the tanks. This annual report gives the resu!t_ of the work conducted by PNL in FY 1992 on Subtask 3.4, Aging Studies, which is part of Task 3, Chemical Nature of Ferrocyanide in Wastes. Subtask 3.4 deals with the aging behavior and solubilization of ferrocyanide tank waste sludges in a basic aqueous environment. Investigated were the effects of pH variation, ionic strength, salts present in SSTs, and gamma radiation on solubilization of vendor-prepared Na_NiFe(CN)6. Vendor-prepared Na_NiFe(CN)6 dissolves in aqueous base to give primarily insoluble Ni(OH)2 and soluble Na4Fe(CN)6. Precipitation of Ni(OH)2 apparently drives the ferrocyanide dissolution. From the gamma pit experiments, the in_,_uble solids contained more than one iron cyanide species, yet to be identified. The rate of dissolution of Na2NiFe(CN)6 in aqueous base increases with increasing pH. At pH 14, 95% dissolution is observed after 0.5 h. Addition of 1 M Na + ions in the form of Na2SO4 suppresses dissolution at pH 13, presumably because of a common ion effect. However, 1 M Na + in the form of SST simulant salts (sodium salts of phosphate, carbonate, nitrate, nitrite, sulfate, and hydroxide) resulted in an enhancement of the rate of solubilization most likely arising from buffering of the solution by phosphate. Even when the solution is not stirred, dissolution is relatively rapid. Approximately 40% of the Na_NiFe(CN)6 dissolves in 24 h in an unstirred solution containing SST salts. Gamma radiation does not appear to greatly affect the dissolution reaction. Similar rates were observed in unstirred irradiated and control solutions. A more complex mixture of iron cyanides in the insoluble fraction of the gamma radiation experiments is obtained, suggesting the possibility that an iron cyanide species re-precipitates from solution. Further work is needed to determine the identity of this species. Solubilization work will continue next fiscal year. In addition, cyanide and ferrocyanide hydrolysis studies will be conducted.