Utilization of milk protein as an environmental material: Accumulation of metal ions using a protein-inorganic hybrid material

Abstract

Casein is a well-known phosphoprotein that possesses many metal ion-binding sites in its molecular structure; thus, a casein-containing hybrid material can be used for accumulating metal ions from an aqueous solution. A casein-inorganic hybrid material was prepared by mixing casein and a silane coupling reagent, bis3-(trimethoxysilyl)propylamine. This casein-inorganic hybrid material was stable in an aqueous solution owing to the electrostatic interactions between the phosphate group in casein and the amino group in the silane coupling reagent. When the casein-inorganic hybrid material was incubated in an aqueous Cu(II) ion-containing solution, the hybrid material turned blue owing to the accumulation of the Cu(II) ions. Therefore, we demonstrated the accumulation of various metal ions using the casein-inorganic hybrid material from aqueous solutions. The hybrid materials could selectively accumulate heavy and light metal ions and did not interact with the rare-earth metal ions. In addition, the metal ion-accumulated casein-inorganic hybrid materials could be recycled by washing with an aqueous EDTA solution or hydrochloric acid.