Injection of NH3(gas) is under consideration as a novel remediation technique for uranium (U) at the Hanford Site in Washington state. The goal of NH3(gas) injection is to raise the pH to dissolve natural silicate and aluminosilicate minerals from sediments. Then, co-precipitation processes are expected to immobilize U as NH3 dissipates and the aqueous phase returns to natural pH and becomes saturated. Our previous work showed that incongruent dissolution is occurring in batch experiments which suggests formation of secondary precipitates. However, there is a need to identify mineralogy alterations which may control the fate of U following treatment. The objective of this study was to investigate solid phase transformations due to precipitation of secondary minerals following treatment of pure minerals with NH3(gas). Duplicate suspensions of minerals were aged for one month in the presence of NH3(gas) with background electrolyte solutions emulating natural conditions. Then, half of the samples were aerated to return to a natural, neutral pH. Following treatment with NH3(gas) or treatment with NH3(gas) followed by aeration, all samples were washed with pH-adjusted ultrapure water. Then, minerals were characterized via X-ray diffraction (XRD), Fourier-Transform IR (FTIR) spectroscopy, and scanning electron microscopy (SEM). Solid phase characterization results show that mineralogy is changing based on changes in (1) peak intensities and ratios as measured in FTIR, (2) particle surface area as analyzed in BET, and (3) altered physical morphology based on SEM-EDS imaging. For example, the Si-O stretching bands (700-1200 cm-1) observed in FTIR are decreased in NH3-treated samples versus those treated with NH3 and aerated back to neutral pH likely due to enhanced dissolution of aluminosilicates. Further, the increase in Si-O bond peak intensity for aerated samples suggests that precipitation of secondary minerals is occurring.

Potential impacts to local mineralogy from remediation with ammonia gas

256th National Meeting (2018)

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