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Abstract
A study was carried out on the transport of neodymium from nitric acid solution, using a flat sheet contained liquid membrane technique, in order to investigate the feasibility of treating low-level radioactive wastewaters using hollow fiber membrane contactors. The solvent consisted of n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) as the active carrier in n-dodecane. The effect of various modifiers was explored and a mixture of tributyl phosphate (TBP) and isotridecanol was identified as the most effective modifier, minimizing acid co-extraction and preventing third phase formation. The effects of various parameters on Nd transport were examined. These included the effect of acidity, Nd, and sodium nitrate concentrations in the feed and the hydrophilicity of the membranes at the feed and strip interfaces.
The extraction of Nd from nitric acid to a solvent mixture of CMPO, TBP, and isotridecanol was found to be mainly governed by membrane diffusion. At the feed interface, it was found that a hydrophobic membrane offers higher resistance to mass transfer than a hydrophilic membrane, due to a combination of low distribution coefficient and low diffusivity of the Nd–CMPO complex. Resistance at the strip interface was also lower with a hydrophilic membrane. The rate of acid transport from feed to strip solution was higher with a hydrophilic membrane at the feed interface, and this was found to reduce Nd recovery in the strip solution. These results highlight the importance of the effect of membrane hydrophilicity on the overall mass transfer rates.