Separation of rare metals using silica gel modified with a surfactant micelle containing an extractant

• The amount of EHPNA impregnated in modified silica gel was 0.205 mmol/g-silica gel.
• The modified silica gel could be used as a stable adsorbent at a pH below 2.0.
• In was separated from Ga–In–Al mixture solution in the range of pH from 1.0 to 1.5.
• Separation of Ga–In–Al was achieved in a column packed with the modified silica gel.

Solvent extraction is widely used as a universal method to extract and separate metals in industry. However solvent extraction has some disadvantages, for example, the use of a large amount of organic solvent which could be an environmental hazard. Therefore a solid-phase extraction method is proposed in this research. First of all, a silica gel modified with a nonionic surfactant (TritonX-100) micelle including 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (EHPNA) as a metal extractant has been prepared. The amount of extractant contained in the modified silica gel was measured to evaluate the characteristics of the modified silica gel. Then the stability of the modified silica gel was examined. Next, the adsorption equilibrium and the adsorption rate of the metals (aluminum, gallium and indium) were measured to estimate the adsorption mechanism of the metals. Finally, the mutual separation of aluminum, gallium and indium using the modified silica gel packed in a column was investigated. The amount of EHPNA impregnated in the modified silica gel was 0.205 mmol/g-silica gel. The modified silica gel can be used as a stable adsorbent below pH 2.0 without leaking EHPNA. The results of adsorption equilibrium showed that the higher the pH, the higher was the extent of metal adsorbed. And among aluminum, gallium and indium, only indium was recovered with a three-metal mixture solution ranging pH 1.0 to pH 1.5. In contrast, as a result of examination of the separation of each metal using the modified silica gel packed in a column, the mutual separation of aluminum, gallium and indium was achieved by repeating the adsorption and desorption processes.