Comparative studies of cobalt sorption and desorption on bentonite, alumina and silica: effect of pH and fulvic acid

The fate and transport of toxic metal ions and radionuclides in the environment are generally controlled by sorption reactions. The extent of sorption of divalent metal cations is controlled by a number of factors including the presence of other cosorbing or complexing ions. In this work, the effects of pH and bentonite Co(II) sorption sites occupied by fulvic acid (FA), γ-Al2O3 and silica concentrations were studied using batch experiments. The general consensus that humic substances (HSs) tend to enhance metal ion sorption at low pH and to reduce metal ion sorption at high pH was shown to be true. The addition order of fulvic acid (FA) and Co(II) to sorbent suspension was also studied, and the results indicated the sorption of Co(II) on bentonite, γ-Al2O3 and silica was not influenced by the addition order. Sorption-desorption hysteresis of Co(II) on bentonite indicated that the sorption was irreversible and the sorbed Co(II) on bentonite was difficult to be desorbed. The results of scanning electron microscopy (SEM) indicated that the presence of FA made bentonite particles easily aggregate. Sorption mechanisms were discussed.