Filtration methods, nano

A holy grail for DON (and DOM in general) remains a rapid and portable method to quantitatively isolate and desalt a large dissolved sample (Bronk, 2002). Such a method would not only allow a wealth of diverse techniques to be brought to bear on the largest unknown fraction of DON, but would allow direct isotopic measurements and compound-specific mass balances. Examples of some approaches currently being explored to improve DON recovery on both small and large scales include homemade ion-retardation resins (Bronk, unpublished data), electrodialysis (Vetter et al, 2007), and use of nano-filtration membranes coupled to standard ultrafiltration approaches (McCarthy et al unpubhshed data). 

Biocatalysts are not always immobilized on membranes in bioreactors, though. As enzymes are macromolecules and often differ greatly in size from reactants they can be separated by size exclusion membrane filtration with ultra- or nano-filtration. This is used on an industrial scale in one type of enzyme membrane reactor for the production of enantiopure amino acids by kinetic racemic resolution of chemically derived racemic amino acids. The most prominent example is the production of L-methionine on a scale of 400 t/y (Liese et al, 2006). The advantage of this method over immobilization of the catalyst is that the enzymes are not altered in activity or selectivity as they remain solubilized. This principle can be applied to all macromolecular catalysts which can be separated from the other reactants by means of filtration. So far, only enzymes have been used to a significant extent. 

Separation/desalination/purification using membranes is essentially a mechanical process and usually no heating is needed but sometime operates using pressure, thus consiunes less energy. The main division of this method are (i) Electro-Dialysis Reversal (EDR), (ii) Reverse Osmosis (RO), (iii) Nano-Filtration (NF) and (iv) Membrane Distillation. 

The rare earth group can be effectively separated from many other elements which form volatile chlorides by thermal methods because the rare earth chlorides are not volatile. Fusion of the rare earth nitrates with NaNOs at temperatures under 300 C results in the selective formation of Ce02- The melt is simply dissolved in water and the Ce02 separated by filtration. These and other thermal methods are summarized by Aladjem (1970), pp. 138-139. 

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