Effect of hydrodynamic dispersion on phosphatase reactions in a soil column

Columns of artificially-prepared soil crumbs were perfused with solutions of p-nitrophenyl phosphate and rates of hydrolysis of this substrate as catalyzed by soil phosphatase were measured. Hydrolysis proceeded by a zero-order or by a first-order reaction at high and low concentrations of substrate, respectively. Steady-state concentrations of substrate and products from the columns can be described essentially in terms of plug flow kinetics with but minor corrections for hydrodynamic dispersion. Values of the zero and first-order rate constants corrected for dispersion are 3 and 9 per cent greater, respectively, than those assuming plug flow kinetics. The apparent value of the first-order rate constant is about 70 percent higher than the true value because of electrostatic repulsion between the negatively-charged substrate and the soil particles.