Electrostatic Gatekeeper Ion Rectification and Electrochemical Fouling Recovery of Carbon Nanotube Membranes

By reducing diameter of carbon nanotubes (<1.5nm) and increasing functional chemistry density at we are able to mimic protein channel electrostatic paddles functionality and demonstrate ion exclusion of small ions.  Prior studies with larger multiwalled CNTs could have only modest rectification of large Ru(bipyr)32+ cations [1].  Recent results with charged entrances showed significant electroosmotic pumping at low ionic concentrations (<~20mMol) [2].  In this work, highly charge and sterically bulky dye molecules resulted in significant ionic rectification factors ( 4-7) for small ions (KCl, LiCl, ZnSO4 and BaCl2) in high ionic strength (0.1M) aqueous solutions. Mass transport studies of electrically neutral molecule, caffeine through these CNTs indicate that an enhanced charge repulsion instead of steric hindrance mechanism dominates the ionic rectification phenomenon. Electrochemical Impedance Spectroscopy (EIS) measurements are also employed to investigate the underlying mechanism of the ionic rectification.

The fouling behavior of carbon nanotube (CNT) membranes was investigated for large protein biomolecules and a wide variety of small molecules. The CNT membranes are largely fouling resistant, even to untreated river water, due to size exclusion and inert graphitic core that supports fast fluid flow. However it was found that bovine serum albumin (BSA) and naphthalene significantly foul membranes due to solution coagulation and π-π stacking respectively. Small SWCNTs (<1.5 nm i.d.) are not fouled with BSA when precipitation is prevented, showing that size exclusion at CNT tips can prevent fouling. Electrochemical oxidation, bubble generation and ionic pumping are shown to recover membrane performance. Electrochemical oxidation at greater than 1.4 V is seen to oxidize CNTs as well as bio-foulant, but H2 bubble generation at -2 V lifts foulants without damage to membrane allowing for repeated cycles.  Ionic pumping using large cations is seen to remove small molecule foulants adsorbed to CNT core. The relatively narrow class of foulants and three complementary methods of membrane defouling make the CNT membrane platform a potentially robust system for a wide variety of chemical separations and environmental water treatments.  The described electrochemical recovery method offers a protein mimetic method of active pore regeneration.