Characterization of metal ion interactions with chitosan by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) is employed to study chemical interactions between three metal ions — Cu(II), Mo(VI), and Cr(VI) — and chitosan, a natural biopolymer extracted from crab shells. Three forms of chitosan are used — flakes, beads, and modified beads obtained by glutaraldehyde cross-linking. XPS provides identification of the sorption sites involved in the accumulation of metals, as well as the forms of species sorbed on the biopolymer. It is found that sorption occurs on amine functional groups for all the three metals. With copper, the sorption step is not followed by reduction of the metal. More complex phenomena are involved in molybdate removal. A partial reduction (about 20–25% of the total molybdenum content) occurs with chitosan beads and flakes. The distribution of reduced Mo(V) on the surface of the sorbent differs from that in the bulk of the sorbent for raw chitosan beads, while the glutaraldehyde cross-linking allows uniform distribution of reduced Mo(V) throughout the sorbent. The difference between these two forms of chitosan can be related to a complementary photoreduction step occurring on the surface of the biopolymer. For chromium, a similar trend with molybdenum is followed but to a greater extent; with cross-linked sorbents all chromate previously sorbed is reduced to Cr(III), while with raw chitosan beads Cr(VI) reduction does not exceed 60%.