APPLICABILITY OF PHOTON-CORRELATION SPECTROSCOPY FOR MEASUREMENT OF CONCENTRATION AND SIZE DISTRIBUTION OF COLLOIDS IN NATURAL-WATERS

The suitability of photon correlation spectroscopy (PCS) for the determination of concentration and size distribution of colloidal matter was tested in a surface water and in a deep groundwater. Well-defined colloids (sizes predominately in the range 50-500 nm) of alpha-Fe2O3, gamma-Al(OH)3, SiO2, kaolinite, illite and a humic acid in aqueous solutions were used as references for calibration of the PCS-instrument (signal vs. concentration). The intensity of the scattered light was dependent on the composition of the colloid. The concentration ranges, where a quantitative determination of the colloid could be achieved, were 0.03-2 mg/l, 0.1-2 mg/l, 0.1-7 mg/l, 0.5-10 mg/l, 0.5-50 mg/l and 0.5-75 mg/l for alpha-Fe2O3, gamma-Al(OH)3, SiO2, kaolinite, illite and humic acid, respectively (in homogeneous systems). The colloidal populations in the surface waters (inlet and outlet of a lake) had a size distribution in the range 100-500 nm. The measured count rates were around 330 kcps and 30 kcps at the inlet and outlet, respectively, which correspond to totally 2.8 mg/l and 0.9 mg/l of colloidal matter according to gravimetric determinations (collected on clogged 0.015 mum filters). Comparisons of concentrations estimated from measurements of reference colloids with PCS and gravimetric determinations of the colloidal matter showed that a combination of the two methods would be needed. The measurements of a deep groundwater were performed on-line and in situ with a PCS technique. The content of colloidal matter in the groundwater was below the detection limit for the PCS-equipment, which corresponds to a concentration not above 0.5 mg/l and probably below 0.1 mg/l in the present system. The feasibility and advantages of using PCS for non-disturbing size characterisation and concentration measurements of colloids in aquatic systems have been demonstrated, as well as some limitations of the method.