CHARACTERISTICS OF MEMBRANE FILTERS IN RELATION TO ALUMINUM STUDIES IN SOIL SOLUTIONS AND NATURAL-WATERS

Potential vitiative effects of Millipore 0.45-, 0.22-, 0.05- and 0.025-mu-m filter elements on aqueous samples were investigated. All the filter elements evaluated exhibited a cation exchange capacity (CEC); in the case of 0.025-mu-m filter elements, the CEC increased with pH. Filtration of 20-mu-M AlCl3/1.65 mM CaCl2 solutions (I almost-equal-to 0.005 M) to 0.45, 0.22 and 0.05-mu-m resulted in no significant change in solution Al concentration. Filtration through 0.025-mu-m filter elements significantly reduced the total Al present in solutions adjusted to pH 4.5 and 5.0; the Al concentration of solution adjusted to pH 4.0 was not altered. The removal of Al from pH 4.5 and pH 5.0 solutions was attributed to the retention of Al hydroxide micro-particulates formed during pH adjustment of the AlCl3/CaCl2 solution. Adsorption of Al by the filter element was not significant. Soil solutions were analysed without filtering and after filtration to 0.45, 0.22, 0.05 and 0.025-mu-m. Filtration had no effect on the concentration of the major anions (Cl-, SO4(2-), NO3-) or cations (Ca++, Mg++, Na+, K+). However, filtration to 0.05 and 0.025-mu-m resulted in significant reductions in the concentration of Al, Fe, Si and organic C in some samples. In soil solutions for which filtration resulted in the removal of large amounts of particulate material, the solution appeared less coloured after filtration and exhibited lower absorbance in the visible spectrum (400-600 nm). However, the absence of colour did not preclude the presence of colloidal particles.