ADSORPTION OF Ca(II), Pb(II) AND Ag(I) ON SULFONATO-SILICA HYBRID PREPARED FROM RICE HULL ASH - Sulastri, Siti; Student of Doctorate Program, Department of Chemistry, Faculty of Mathematics and Natural Sciences, Universitas Gadjah Mada, Yogyakarta (Universitas Gadjah Mada, 2011)