Sorption Experiments: Diuron (>99% purity) was purchased from Sigma–Aldrich (Sydney, Australia). High performance liquid chromatography (HPLC) grade solvents and calcium chloride were obtained from Merck Pty Limited (Victoria, Australia). A stock solution of 1000 mg L−1 diuron was prepared in methanol. Sorption experiments were carried out using a batch equilibration technique at ambient laboratory temperature (25 ± 3 °C), as reported previously ( Liyanage et al., 2006). For single point measurements on whole soil, an aliquot of soil (5 g) was equilibrated with 10 mL diuron solution (1.0 mg L−1) by end-over-end shaking for 16 h, based on a preliminary kinetics experiment ( Liyanage et al., 2006). For HF-treated soils only 50 mg of the sorbent was equilibrated with 5 mL diuron solution (1.0 mg L−1). At the end of the equilibration period, suspensions were centrifuged at 3000 × g for 20 min and 1 mL aliquots of the supernatant were filtered through 0.45 μm Teflon filters and analysed. Blanks without diuron and without sorbent were analysed and appropriate corrections were applied. Sorbed concentrations were calculated from the difference between the initial solution concentration and equilibrium solution concentration.