Adsorption behavior of tungsten (VI) onto activated carbon.

Abstract

The adsorption behavior of tungsten(VI) onto activated carbon was studied by using a column method. The result was compared with those of molybdenum(VI), chromium(VI) and other ions. The adsorption of tungsten(VI) shows remarkable pH-dependency, like that of molybdenum(VI) and chromium(VI). The adsorption-pH curves shift toward the higher pH in the following order: HClO4 <HNO3 < H2SO4< HCl. As much as 0.54 g of tungsten(VI) was adsorbed onto 5.0 g of activated carbon at pH 2.0, where the pH value was adjusted by using HNO3. The amount of tungsten(VI) adsorbed was one half as large as that of molybdenum(VI) and 100 times larger than those of other metal ions such as chromium(VI) and copper(II) under the optimum conditions. Tungsten(VI) may be adsorbed in the forms of polymerized species. The adsorption of tungsten(VI) onto activated carbon was remarkably affected by the addition of 0.2 mol dm-3 NaX( X = NO3-, ClO4-, Cl- and SO42-) solution. The amounts of tungsten (VI) adsorbed increased in the following order: NaClO4 <NaNO3 <Na2SO4 <NaCl at pH 2.0. The salt effect on the adsorption of tungsten(VI) may be ascribed to the effect of the anion on the equilibrium constant of the formation of the polymerized species and/or on the formation rate of the polymerized species. Ten percent of all the tungsten(VI) adsorbed was eluted by passing a 100 ml portion of 5.0 mol dm-3 HNO3 through the column packed with 5.0 g of activated carbon loaded with 0.41 g of tungsten(VI). The electron microscope photographs of activated carbon and activated carbon loaded with tungsten(VI) are also reported.