Developing ion electroadsorption stereoselectivity, by pore size adjustment with chemical vapor deposition onto active carbon fiber electrodes.: Case of Ca2+/Na+ separation in water capacitive desalination

By using carbon chemical vapor deposition (CVD) onto active porous carbon fibers, we were able to reduce the pore size of the active carbon to obtain selective carbon molecular sieves (CMS) of variable properties. The pore size of these carbons was characterized by the adsorption of molecules of different dimensions (molecular probe adsorption) from the gas phase. By manipulating the several experimental parameters of the CVD process, the pore size of the CMS could be finely tuned to fall between sizes of alkaline earth cations (Ca2+; Mg2+, 6-7 angstrom in diameter), and the much smaller size of hydrated monovalent cations (4 angstrom in diameter). This was proven by voltammetric measurements of the pore-tailored carbon electrodes in the corresponding salt solutions, which also provided a measure of the differential capacity of the electrical double layer (EDL) vs the potential (e.g., at potentials negative to the potential of the zero charge, the EDL capacity was very small for CaCl2 and MgCl2 solutions, while it was high for NaCl solutions). The CVD process applied herein did not significantly reduce the specific surface area and the pore volume of the carbons. This implies that the carbon deposits are only superficial and that the pore blocking effect takes place only at the pore mouth. Hence, the adsorption processes related to these carbons involve surface barrier mechanisms. The importance of the selectivity thus obtained relates to a possible use of these carbon electrodes in electrochemical water desalination processes. By using these carbons as electrodes in capacitive water deionization processes, it is possible to selectively remove Na+ cations, leaving Call and Mg2+ ions (which are important nutrients) in the treated water.