Investigation into the effects of temperature and stirring rate on the solid-phase extraction of diuron from water using a C18 extraction disk

A novel experimental method for determining the equilibrium constant, Keq, and the uptake rate constant, kup, for the solid-phase extraction (SPE) of diuron from water using a C18 Empore extraction disk is reported. Log Keq and log kup are determined at 7.0, 11.0, 18.0 and 23.0°C and for stirring rates of 100, 200 and 400 rpm. From a Van ‘t Hoff plot of log Keq versus T−1 the enthalpy of sorption, ΔH0, is shown to be negative which indicates that the thermodynamic process of uptake is exothermic. The rate of stirring has no effect on log Keq over the temperature range 7.0–23.0°C. The enthalpy of activation, ΔH0, calculated from Arrhenius plots of log kup versus T−1 at 100, 200 and 400 rpm show that the kinetic process of uptake is endothermic. At 100 rpm the rate of uptake is limited by the aqueous diffusion of diuron. At 200 rpm or greater the aqueous diffusion layer around the disk is sufficiently small to prevent diffusion from being a limiting factor. The method described in this paper is limited to the analysis of analytes that contain a significant UV chromophore and are relatively soluble in water, but it can also be used to investigate pH and salinity effects on the SPE of diuron from water.