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 silica gel (CSG) for its ability to remove metal ions including Pb2+, Cu2+, Ni2+, Zn2+, Cd2+ and UO22+ from solution. The equilibration time was shorter for UO22+ (less than 1 h) than for the heavy metal ions (2–3 h) and adsorption by DSG took longer to reach equilibrium compared with CSG, probably due to the smaller mean diameter of its pores. The adsorption process showed first order kinetics for all the metals studied. The mass transfer coefficients and overall rate constants were determined for each of the metals. The adsorption rates of the metal ions, in order 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 characteristic for each metal. The adsorption of the metal ions obeyed the Langmuir isotherm and followed the preferential order UO22+>Pb2+>Cu2+>Ni2+>Zn2+>Cd2+. Similarity of the isotherms for the individual metals was observed 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 each metal was considerably reduced with an increasing concentration of the other, 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.