The role of the oxygen functional groups in adsorption of copper (II) on carbon surface

The effect of carbon surface oxidation on the adsorption of Cu(II) ions from aqueous solution was studied in order to explain the role of the oxygen functional groups in the binding of copper ions. Pristine carbonaceous adsorbent was oxidized to a various extent of oxygen uptake (Fenton-like oxidation < persulphate in H₂SO₄ < H₂O₂ in HNO₃). Equilibrium adsorption tests were performed in acetate buffer at pH ≈ 5. The results show that the adsorption capacity of pristine adsorbent is expectable low (~0.1 mmol g^-1). The oxidized samples adsorb Cu(II) at a considerably higher level of ~1.4 mmol g^-1 despite the degree of surface oxidation. Analysis of the surface groups (FTIR, TPD) and surface charge (zeta potential) of used adsorbents and their Cu(II) saturated counterpart lead to the finding that Cu(II) ions are mostly bonded by complexation with the dissociated carboxylic groups (partly formed by anhydrides hydrolysis) probably in the form of Cu(Ac)⁺ formed in the acetate buffer. The extent of dissociation is given by equilibrium pH during the adsorption and does not depend on the total amount of the surface groups. Thus, the content of active sites and consequently adsorption capacity is independent on the degree of oxidation when pH is kept constant. The results indicate that even moderate oxidation treatment of carbonaceous materials can produce highly effective adsorbents for Cu(II) immobilization.