Comparison of chromatography and desiccant silica gels for the adsorption of metal ions. II. Fixed-bed study

The adsorption of uranium(VI) and lead(II) on fixed beds of chromatography silica gel (CSG) and desiccant silica gel (DSG) was investigated at initial pH 5.15 and an initial concentration of 0.5 and 0.1 mM for uranium(VI) and lead(II), respectively, and exhibited a constant pattern of breakthrough curves at various bed heights and flow rates. The experimental data gave a good fit to the bed depth-service time (BDST) model at 50% breakthrough with the bed capacity close to the value predicted from batch study results. The adsorption performance of the silica gel columns could be well described by the Clark model at the ratios of concentration of effluent to influent (C/C0) up to 0.3 for uranium(VI) and 0.5 for lead(II). Above these levels, the model was approximately valid for uranium(VI), while a large deviation occurred with lead(II) adsorption. Application of the Wolborska model to the data at C/C0 ratios lower than 0.5 enabled the determination of the kinetic coefficients for mass transfer in these systems. Furthermore, it was found that uranium(VI) could be effectively separated from lead(II) by using a column packed either with CSG or DSG at pH 3.50. Complete removal of uranium(VI) adsorbed to a packed bed of silica gel was obtained by employing 4.5 M HNO3 as eluant. Six and seven bed volumes of 4.5 M HNO3 were required for the elution of uranium(VI) from CSG and DSG, respectively. For lead(II), complete desorption from the column required 3.0 and 3.5 bed volumes of 1.5 M HNO3 solution for CSG and DSG, respectively. For both uranium(VI) and lead(II) it was found that the DSG-packed column exhibited better performance than CSG in terms of service time and volume of solution treated, and gave similar value of the adsorbent usage rate, suggesting that DSG can be used instead of CSG in column applications.