Influence of matrix on external mass transfer resistance in immobilized urease membranes

The influence of seven kinds of chemical modification of acrylonitrile copolymer membrane over both: transport characteristics of modified membranes and the diffusion-limitations of substrate in immobilized urease membranes were studied. It was found that the modification improved the hydrophilicity of the membranes and urea diffusion coefficient. The kinetic parameters Vmax and Km of the immobilized urease were also determined. Besides this, a good correlation between diffusion coefficient of urea through the modified membranes and Km was observed. For all the membranes studied, diffusion-limitations of substrate inside of the porous membrane were not found because there was not immobilized urease in membrane pores, but there was external membrane mass transfer resistance. To establish the external mass transfer resistance, the values of mass transfer coefficient (KL) and Nernst diffusion layer were determined. In the present experiments, a good correlation between hydrophilicity and the thickness of the boundary layer on membrane surface and KL, respectively, was observed. This results proved the influence of the modified matrix on mass transfer coefficient. All the immobilized systems studied were found to have weaker external mass transfer resistance and the values of the effectiveness factor calculated were higher than 0.95. A common feature of all immobilized systems was their quite high effectiveness factor, which clearly showed that the modified acrylonitrile copolymer membranes studied were suitable carriers for immobilization of the enzyme urease.