Remediation of spent oxalic acid nuclear decontamination solutions using ozone
Creators: name: Ketusky, Edward
Publisher: name: Lancaster University
Year: 2018

description: The Savannah River Site (SRS) has forty-three remaining very large
      underground tanks containing significant quantities of nuclear waste
      generated primarily from cold-war radiochemical separations. All of the
      tanks eventually must be closed. As part of decommissioning/closing the
      nuclear waste tanks, even residual quantities of the waste must be
      removed. Although most sludge can be removed mechanically, chemically
      cleaning (i.e. decontamination) with oxalic acid is used to aid in the
      removal of residual quantities.
      
      
      Although oxalic acid works for cleaning the tanks, its downstream impacts
      are considered detrimental. To better understand and quantify the impacts,
      detailed models were developed to account for different potential
      processing strategies for handling the spent oxalic acid nuclear
      decontamination slurries. Although the results vary, the models show that
      regardless of the oxalate handling strategies: 1) significant washing to
      decrease sodium concentration/solids concentration in vitrification feed
      will be required, and 2) the creation of copious future additions of feed
      for salt processing will be unavoidable.
      
      
      Using a Theory of Inventive Problem Solving (TRIZ) approach, a modified
      form of the Chemical Oxidation Reduction Decontamination (CORD)
      ultraviolet (UV) light was identified as being used with an analogous but
      already resolved problem that could be adapted to SRS HLW tank cleaning. A
      novel preliminary process called Enhanced Chemical Cleaning (ECC) was
      envisioned. 
      
       
      
      As part of maturing the technology, the literature review identified three
      possible oxalate decomposition mechanisms associated with ECC. They are:  
      
      1)A heterogeneous non-Advanced Oxidation Process (AOP) mechanism where the
      target organic adsorbs onto the surface of a solid, often particulate,
      metal oxide at a so-called active site, followed by ozone attack on the
      sorbed organic; 
      
      
      2)A homogeneous non-AOP mechanism that operates under low pH acidic
      conditions and which involves complexation of the catalysing metal ion
      with the oxalate followed by ozone attack on the complex; and, 
      
      
      3)A homogeneous AOP mechanism that operates at a high basic pH and which
      involves metal ions catalysing the formation of hydroxyl radicals from
      ozone, with the said hydroxyl radicals then driving the oxalate
      decomposition.
      
      
      Process testing was conducted using slurries made from simulants designed
      to be chemically similar to real High-Level Waste (HLW) sludge types.
      Testing using slurries made from real HLW sludge was also performed, but
      because of safety limitations associated with handling HLW, only a much
      smaller scale test apparatus could be used. With the much smaller scale
      test apparatus, the purpose of the real HLW based testing was
      confirmatory. Each of the simulant decomposition test slurry was created
      using an Fe-rich or an Al/Mn-rich sludge simulant using either 1 wt% or
      2.5 wt% oxalic acid. The real HLW based slurries were formed using 2 wt%
      oxalic acid.
      
      
      As part of the main postulate of this effort, both the simulant
      decomposition test slurries and real HLW based slurries demonstrate that
      UV light increased the decomposition rate. Even without UV, by adding only
      ozone, the oxalate decomposition was completed on an industrially relevant
      timescale. 
      
      Also using simulant based testing, the overall oxalate decomposition
      exhibited four distinct stages related to the metal catalysts: 
      
      
      Stage One – At short ozonation times, ozone decomposes Fe oxalates and
      solubilise Fe from ozone action on the metal oxide constituents of the
      sludge.
      
      
      Stage Two – At intermediate ozonation times, as a result of the loss of
      the solution capacity to complex (and so solubilise) Fe, Mn, and Ni ions
      due to O<sub>3</sub> driven oxalate decomposition, as well as
      the pH increase that accompanies that decomposition, Fe begins to
      precipitate. Oxalate decomposition is still primarily catalysed by Fe ions
      during this stage. 
      
      Stage Three – At intermediate ozonation times, Fe precipitation is near
      complete, and oxalate decomposition is now driven by ozone and Mn
      catalysis – Mn playing a major role in determining the final time to
      process endpoint of 1.1 × 10<sup>-3</sup> M oxalate in
      solution. 
      
      Stage Four – At long ozonation times, the process endpoint with Mn
      precipitation now near completion with Ni being the dominant metal ion in
      solution.
      
      Constructed plots compare the pH and remaining oxalate concentration, both
      as a function of time, suggesting some relationship. Regression analysis
      of the negative log of the oxalate concentration shows the R2 values are
      all greater than 0.80, confirming correlation. Thus, pH can be used as a
      field measure for confirming when oxalate decomposition is complete. 
      
      
      As a principal hypothesis of this effort, using simulant based testing,
      both the scavenging effects of “all-ready present” nitrite (a soluble
      component of the sludge simulants) and oxalate mineralisation-derived
      carbonate are advantageously used in lieu of introducing hydroxyl radical
      probe compounds to the process. Specifically, differing nitrite
      concentrations between slurries showing no impact on the decomposition
      rates, and the build-up of carbonate not inhibiting the decomposition
      process strongly suggest that the decomposition is not the result of
      radicals. Instead, the oxalate decomposition is likely the result of a
      direct reaction of ozone with metal complexed oxalate (i.e. mechanism 2
      discussed above).