FILTER EFFICIENCY OF 3 SAPROLITES FOR NATURAL CLAY AND IRON-OXIDE COLLOIDS

The mobility of natural soil colloids during saturated flow through un disturbed saprolite columns was studied. The colloids were isolated fr om a surface soil and consisted of clay and iron oxide particles (<0.2 mu m) partially coated with adsorbed natural organic matter. Seventy- five intact saprolite columns (7.7 cm length, 6.6 cm diameter) were co llected at three field sites, and each column was characterized with r espect to chemical and physical properties. Dilute colloidal suspensio ns were passed through the saprolite columns, and the effluents were a nalyzed for colloidal Fe, Al, and Si. The results demonstrate that col loids can be rather mobile in some saprolites, whereas other saprolite s are efficient filters for colloidal particles. The elemental ratios between colloidal Fe, Al, and Si remained constant, indicating that th ere was no preferential filtration of certain mineral phases. The coll oid filter efficiency tended to decrease-with increasing pore water ve locity, increasing macroporosity, decreasing iron oxide content, and i ncreasing base saturation of the saprolites. The results correlate wel l with simple filtration theories.