(449by) Adsorption of Water Contaminants in Continuous Flow Systems Using Carbon Cryogels with a Microhoneycomb Structure

A microhoneycomb monolith, which is a miniature version of a honeycomb-shaped monolith, has fairly straight macro channels, a few tens of μm in diameter which are formed by thin channel walls, a few μm in thickness. This monolith shows a minimized hydraulic resistance because of its straight channels and the lengths of the diffusion paths within its walls are extremely short¹. Such a monolith is suitable for high-throughput separation of diluted water contaminants, e.g. phenolic compounds and various dyes, because a low hydraulic resistance and fast response due to short diffusion path lengths are compatible. This cannot be achieved in a column packed with particles.

In this work, to demonstrate the potential of such microhoneycomb monoliths for the separation of typical contaminants, phenol and methylene blue adsorption were conducted in batch systems and continuous flow systems using carbon cryogels with a microhoneycomb structure (carbon microhoneycombs, CMHs). The obtained monolithic CMHs had fairly straight channels, 25-45 μm in diameter, and the thickness of the walls which form the channels was 5-10 μm. The pressure drop which occurred when water was passed through the CMHs was 90-370 times lower than that of a column packed with particles having similar diffusion path lengths. This indicates that the CMHs can process fluids without causing a severe hydraulic resistance when compared columns packed with particles. From nitrogen adsorption/desorption experiments, it was found that the CMHs have a hierarchical micro-meso porous structure giving BET surface areas in the range of 513-1070 m²Â·g^-1. The phenol and methylene blue adsorption isotherms of the CMHs are typical Langmuir-type which indicates monolayer adsorption. The monolayer uptake increases with the increase in carbonization temperature of the CMHs which represents that hydrophobic surfaces are preferable for the adsorption of phenol and methylene blue. The obtained phenol and methylene blue breakthrough curves show typical sigmoidal-shapes and the calculated dynamic capacities are equal to the values calculated from the isotherms. In addition, the length of unused bed, which represents the length of the zone in which adsorption takes place, is only a few mm. These results indicate that the adsorbates can quickly gain access to the adsorption sites in the CMHsâ?? channel walls even in flow systems. The simulated breakthrough curves of a column packed with particles which shows the same total pressure drop and has the same material properties are a few-times broader when compared with those of the CMHs measured under the same flow conditions. This result indicates that the CMHs can effectively separate contaminants in flow systems when compared with typical columns packed with particles. The results indicate that CMHs have a high potential to be used for effective separation of water contaminants such as phenol and methylene blue.

[1] Mukai, S. R.; Nishihara, H.; Tamon, H., Chem. Commun. 2004, (7), 874-875.