Retention of Vanadium(V), Molybdenum(VI) and Tungsten(VI) by Kaolin

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Abstract
The ability of kaolin (approximately H2Al2Si2O8 · H2O), a well known clay mineral constituent, to retain V(V) andMo(VI) species from 10−5 and 10−4 M solutions, and W(VI) species from 10−4 M solutions was investigated in the pH range 2–7.5 using a solid/liquid ratio 1:100 (w:v). The equilibration time in the adsorption studies was 72 h, and the shaking time in the desorption studies was 17 h. 0.02 M KCl was used as ionic medium and desorbing liquid. V(V), Mo(VI) and W(VI) were all retained by kaolin to some extent, and the amounts of the elements which could be washed away from kaolin with 0.02 M KCl were very small. The pH dependence of the adsorption was considerable: when added alone, all three elements were adsorbed strongly at about pH 4, but above pH 6.5 only V was retained in noticeable amounts. When V, Mo and W were added simultaneously, the adsorption curves showed that adsorption of Mo(VI) predominates at pH 4, whereas W(VI) and V(V) predominate at pH 5–6 and pH > 6.5, respectively.