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 deter mination of concentration and size distribution of colloidal matter wa s tested in a surface water and in a deep groundwater. Well-defined co lloids (sizes predominately in the range 50-500 nm) of alpha-Fe2O3, ga mma-Al(OH)3, SiO2, kaolinite, illite and a humic acid in aqueous solut ions were used as references for calibration of the PCS-instrument (si gnal vs. concentration). The intensity of the scattered light was depe ndent on the composition of the colloid. The concentration ranges, whe re 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 h umic acid, respectively (in homogeneous systems). The colloidal popula tions in the surface waters (inlet and outlet of a lake) had a size di stribution in the range 100-500 nm. The measured count rates were arou nd 330 kcps and 30 kcps at the inlet and outlet, respectively, which c orrespond to totally 2.8 mg/l and 0.9 mg/l of colloidal matter accordi ng to gravimetric determinations (collected on clogged 0.015 mum filte rs). Comparisons of concentrations estimated from measurements of refe rence colloids with PCS and gravimetric determinations of the colloida l 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 grou ndwater was below the detection limit for the PCS-equipment, which cor responds 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 PC S for non-disturbing size characterisation and concentration measureme nts of colloids in aquatic systems have been demonstrated, as well as some limitations of the method.