Comparison of chromatography and desiccant silica gels for the adsorption of metal ions - I. adsorption and kinetics

In this study desiccant silica gel (DSG was compared with chromatography si lica gel (CSG) for its ability to remove metal ions including Pb2+, Cu2+, N i2+, Zn2+, Cd2+ and UO22+ from solution. The equilibration lime was shorter for UO22+ (less than 1 h) than for the h eavy metal ions (2-3 h) and adsorption by DSG look longer to reach equilibr ium compared with CSG, probably due to the smaller mean diameter of its por es. The adsorption process showed first order kinetics for all the metals s tudied. The mass transfer coefficients and overall rate constants were dete rmined for each of the metals. The adsorption rates of the metal ions, in o rder of decreasing magnitude, were UO22+ > Pb2+ >- Cu2+ > Zn2+ >Ni2+ > Cd2 for both DSG and CSG. Metal uptake was found to increase rapidly within a two pH unit range chara cteristic for each metal. The adsorption of the metal ions obeyed the Langm uir isotherm and followed the preferential order UO22+ > Pb2+ > Cu2+ > Ni2 >Zn2+ >Cd2+. Similarity of the isotherms for the individual metals was obs erved for both DSG and CSG, indicating a comparable removal capacity of DSG and its possible use as an alternative to CSG in adsorption applications. Adsorption from lead(II)-uranium(VI) solutions showed that the uptake of ea ch metal was considerably reduced with an increasing concentration of the o ther, the adsorption of lead(II) being more strongly influenced by uranium( VI) than vice versa due to the higher affinity of silica gel for the latter . (C) 1999 Elsevier Science Ltd. All rights reserved.