HYDROLYSIS OF UREA BY GELATIN-IMMOBILIZED UREASE - SEPARATION OF KINETIC AND DIFFUSION PHENOMENA IN A MODEL IMMOBILIZED-ENZYME REACTOR SYSTEM

Experiments and appropriate mathematical models are presented to elucidate and separate the effects of mass transfer and immobilization on the apparent kinetics of urea hydrolysis by urease immobilized within a crosslinked gelatin film. Diffusion of urea through the gelatin matrix appears to exert the major influence on the observed kinetics. Diffusion coefficients are measured, and a model for the effectiveness factor is presented, accounting for this aspect of mass transfer control. A secondary, but significant, influence on apparent kinetics arises because the reaction products lead to an increased pH level which, because of diffusion resistance, remains high within the gelatin matrix. For pH levels in the 6.7-9.0 range, the activity of urease is a strongly decreasing function of pH. An approximate model accounting for ionic equilibrium allows this pH-diffusion effect to be introduced so as to lead to predictions of the apparent kinetics that are compared with experimental observations. The immobilization procedure apparently leads to some loss of activity due to an interaction of the gelatin crosslinking reaction with the enzyme itself.