ARCANUM | DESIGN: Nanofluidic bulk active ion pump using porous pyrolytic carbon

ORIGINALITY

Original, to the best of my knowledge, as of March 2011.  Idea date approximately January 2007.  May share some similarities to ideas explored in research into implantable artificial dialyzers.


SOURCES OF INSPIRATION

Dialysis; animal kidneys; mass spectrometry; cyclotrons


EXPOSITION

The removal of ions from solution touches on many disparate areas of modern life, such as medicine, engineering, public health, process chemistry, and farming, to name but a few.  An efficient means of selectively transporting ions against a concentration gradient would almost certainly lead to cheaper and cleaner water supplies, more efficient industrial processes, less pollution, and possibly even medical advances such as fully-functional implantable artificial kidneys.

Removing ions from aqueous solution has traditionally been the province of bulk techniques, such as reverse osmosis[1], ion-exchange[2], distillation[3], and electrodialysis[4].  These share the common feature of being energetically and/or economically expensive, as well as relatively nonselective.  Ion-exchange resins have the added problem that the resin beads must be periodically recharged, during which time they cannot be used for ion removal (duty cycle <100%), and that they must replace the captured ion with one or more ions totaling the same charge, meaning no net decrease in dissolved solid concentration.

In animals, the job of maintaining appropriate ion concentrations in the bloodstream is largely accomplished by the kidneys.  In order to efficiently maintain control of blood ion concentrations, very efficient and selective nanoscale ion-transport systems have evolved to "pump" ions in the proper direction, usually against a concentration gradient.  A major problem is that most of these biological ion-pumps are powered by transport of some other substance along its concentration gradient (ex:  the sodium-glucose antiporter pump SGLT1[5]), by co-transport of some other substance along the same concentration gradient (ex:  the Na-K-Cl symporter system in the kidney[6]), or by active powering by ATP.  The first case leads to a "Hobbes' choice" where the increasing concentration of some other potentially undesirable contaminant in the diluate must be tolerated in order to remove another, less-desirable contaminant; the second leads to a case where one must remove other, potentially-desirable ions from the diluate in order to facilitate removal of undesirable ions; the final case leads to the requirement of multiple entire biochemical pathways to produce the chemical power necessary to power the transport system.

In gaseous or plasma phase, however, ions are easy to separate individually, owing to their unique interactions with magnetic and electric fields.  Every elemental ion has a different charge and mass; using these unique physical characteristics, ions can be separated from one another easily, as in a cyclotron or mass spectrometer.  However, this is of little use in industrial processes, as converting substances to gas or plasma is extremely expensive energetically and economically, and the flow rate is minimal at best; it is of even less use in biological systems, for the obvious reasons.


DESCRIPTION

The core of this idea is the observation of the electrical anisotropy and biocompatibility of pyrolytic carbon sheets[7].  Pyrolytic carbon is a form of graphite, consisting of parallel sheets of graphene, with some impurities.  Graphene's planar structure allows electron transport within sheets at conductivities potentially higher than silver[8], while cross-sheet electron transport is significantly less efficient.  (I shall refer to each sheet of graphene henceforth as a "sheet", and of the collection of parallel sheets as a whole as a "sheaf".)


This would seem to lead to the possibility of providing different amounts of charge to each sheet, with relatively limited current leakage between sheets.  If the sheaf were constructed or machined such that a small cylindrical "tunnel" oriented perpendicular to the sheet existed, the tunnel would have a spatially-inhomogeneous electric field inside it, owing to the relative difference in charge between individual sheets.  If charge could be pumped in and out of these sheets in a controlled fashion, the time-varying electric field in the tunnel would create a time-varying force on a charged particle inside it.  Since different ions would have different mass-to-charge ratios, this would seem to offer the possibility of selectively accelerating particular ions in aqueous solution, using electrical power to "pump" them from one side of the sheaf to another by carefully applying particular charge pulses to various sheets within the sheaf at particular intervals.


Careful control of the charge applied to each sheet should permit "gating" of particular batches of ions once within the tunnel; ions of mass lower than that desired should move faster through the tunnel, where they can be "reflected" by appropriately-charged sheets, and ions of mass higher than that desired should move much more slowly, thereby enabling "cutoff" of transport at a certain separation from the "desired" group of ions.  This should lead to relatively selective transport of ions through the sheaf, presumably as a function of the total thickness of the sheaf.


If such a device is feasible, the applications are obvious.  Pyrolytic carbon is a highly-biocompatible material; such a device could serve as an artificial glomerulus, enabling the construction of an electrically-powered implantable dialyzer that should function adequately as a replacement for those suffering from kidney disease.  (In some ways, such a device might even be an improvement over the natural model, since it would be immune to various poisons and toxins that can damage biological kidneys.)  Pyrolytic carbon is also resistant to most forms of chemical attack, thereby enabling its use in industrial processes.  It would also enable selective recovery of ions in wastewater and possibly even from the active process, potentially improving the efficiency and reducing pollution from those processes.  It may, if efficient enough, be useful as a means of desalinating drinking water.




BENEFITS

1. Outlined above.  Obviously, this is highly speculative.

PROBLEMS

1. Obviously, there's no guarantee that you can make pyrolytic carbon, or any other substance, do this.  The addition of transverse holes to the sheaf may be impossible, or may lead to changes in the electronic structure of the sheet that lead to increased inefficiency, difficulty of production, or both.
2. The envelope-level description above does not take into account solvation effects.  Ions in aqueous solution are surrounded by configurations of water molecules, significantly altering their behavior. These solvation effects may well preclude the efficient transport of ions in this manner.
3. The ions may interact with the pyrolytic carbon in some fashion, changing its electrical characteristics over time.  The electrical characteristics of pyrolytic carbon are highly dependent on the level of impurities embedded in it; electronic "pumping" may abstract impurities from the solution, and may even propagate them within the structure.
4. The tunnels within the sheaf may become filled, irreversibly, with ions that come out of solution.  If an ion is able to abstract an electron, or donate an electron, to the sheaf itself, then it will lose its charge and will no longer be susceptible to forces imposed by the electric field.  (It may be possible to re-ionize the substance by applying an electric field of the appropriate magnitude, though.)
5. Artificially segregating ions in solution may lead to situations where insoluble salts are produced by accident, either in the solution or within the sheaf.
6. It will be significantly less efficient at lower ion concentrations.
7. It may require some form of mechanical mixing or turbulence to homogenize ion distribution within the solution.

CALCULATIONS

To be added.



DRAWINGS


To be added.



REFERENCES

1. http://en.wikipedia.org/wiki/Reverse_osmosis
2. http://en.wikipedia.org/wiki/Ion_exchange
3. http://en.wikipedia.org/wiki/Distillation
4. http://en.wikipedia.org/wiki/Electrodialysis 
5. http://en.wikipedia.org/wiki/SGLT1
6. http://en.wikipedia.org/wiki/Na-K-2Cl_symporter
7. http://www.espimetals.com/index.php/online-catalog/353-carbon-c
8. http://en.wikipedia.org/wiki/Graphene#Electronic_transport
9. Applied Surface and Colloid Chemistry, R.M. Pashley and M.E. Karaman, Wiley, 2004, ISBN 0-470-86883-X