Experimental Determination of Gibbsite Solubility in Concentrated Mixtures of NaOH + NaNO₃ and NaOH + NaNO₂ at Elevated Temperatures—Applications to Hanford Tank Waste

At the Hanford Site, there are approximately 56 million gallons (MGal) of hazardous radioactive waste (Page et al., 2018). Of the total 56 MGal waste, about 21 MGal are liquid supernatant, along with approximately 23 MGal being soluble saltcake, and approximately 12 MGal are insoluble sludge (Colburn and Peterson, 2020). Aluminum (Al) is the second most prevalent component in the sludge phase, after sodium (Na) (Colburn and Peterson, 2020; Westesen and Peterson, 2022; Westesen et al., 2023). The Hanford supernatants are characterized with high pH, high concentrations of sodium, nitrate, nitrite, and aluminate [Al(OH)₄^–]. Therefore, a fundamental understanding of aluminum chemistry under the conditions relevant to Hanford Tank Waste provides the key knowledge basis for development of a scientifically sound and effective pretreatment approach. Such an approach will be advantageous both for aluminum dissolution and removal from HLW sludge, and for prevention of precipitation of aluminum phase(s) from supernatants.<br/><br/>In this work, we report our experimental results regarding the solubility of gibbsite in mixtures of NaOH + NaNO₃ and NaOH + NaNO₂ at elevated temperatures starting from 60^oC. In our experiments, equilibrium is primarily approached from the direction of undersaturation. NaOH concentrations range from 1.0 mol:kg^–1 to 3.0 mol:kg^–1 in the mixtures. The concentrations of both NaNO₃ and NaNO₂ are in a range from 1.0 mol:kg^–1 to 5.0 mol:kg^–1 in the mixtures. In the supernatant liquid from Tank 241-AN-105, the nitrate and nitrite concentrations are 4.08 and 4.58 mol:kg^–1 (McCoskey et al., 2015), respectively. Therefore, the nitrate and nitrite concentrations in our experimental matrices cover their respetive concentrations present in the supernatant liquid. Based on our previously evaluated Pitzer parameters for Na⁺—Al(OH)₄^– (Xiong, 2014), our current experimental data will be used to calculate the Pitzer interaction parameters for NO₃^–—Al(OH)₄^– and NO₂^–—Al(OH)₄^–.<br/><br/>Acknowledgements: Sandia National Laboratories is a multi-mission laboratory operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. SAND2024-07900A. This research is funded by a Hanford Tank Waste R & D Award from the U.S. Department of Energy.<br/><br/>References<br/><br/>Colburn, H.A. and Peterson, R.A., 2021. A history of Hanford tank waste, implications for waste treatment, and disposal. <i>Environmental Progress & Sustainable Energy</i>, <i>40</i>, p.e13567.<br/><br/>McCoskey, J.K., Cooke, G.A., Herting, D.L., 2015. Chemical Equilibrium of Aluminate in Hanford Tank Waste Originating from Tanks 241-AN-105 and 241-AP-108. Washington River Protection Solutions, LAB-RPT-14-00011 R0.<br/><br/>Page, J.S., Reynolds, J.G., Ely, T.M. and Cooke, G.A., 2018. Development of a carbonate crust on alkaline nuclear waste sludge at the Hanford site. <i>Journal of hazardous materials</i>, <i>342</i>, pp.375-382.<br/><br/>Westesen, A. and Peterson, R., 2022. Speciation of aluminum phases at the Hanford Site. <i>Environmental Progress & Sustainable Energy</i>, <i>41</i>, p.e13789.<br/><br/>Westesen, A., Wells, B. and Peterson, R., 2023. Identifying challenge sludges to process at the Hanford site. <i>Particulate Science and Technology</i>, <i>41</i>, pp.453-459.<br/><br/>Xiong, Y., 2014. A Pitzer model for the Na–Al(OH)₄–Cl–OH system and solubility of boehmite (AlOOH) to high ionic strength and to 250°C. <i>Chemical Geology</i>, <i>373</i>, pp.37-49.