Constrain in solving Langmuir–Hinshelwood kinetic expression for the photocatalytic degradation of Auramine O aqueous solutions by ZnO catalyst

Langmuir–Hinshelwood (L–H) and first-order expressions are the most widely used expressions to explain the kinetics of heterogeneous catalytic system. Previously lot of researchers approximated the L–H kinetic expression to first-order expression just to easily obtain the parameters involved in the L–H expression. However, it is inappropriate to alter a kinetic model, which is having definite assumptions behind it without any validation. This study reports the constrain in approximating the L–H kinetic expression to first-order kinetic expression using the experimental kinetic data of photocatalytic degradation of Auramine O aqueous solution in the presence of ZnO catalyst. Further, the present investigation showed that it is not appropriate to approximate the Langmuir–Hinshelwood kinetics to first-order kinetics. In addition, a second-order kinetic model is proposed and is found to well represent the experimental data of Auramine O degradation by ZnO catalyst for the range of initial dye concentration studied.