Sorption of copper by chemically modified aspen wood fibers

Sorption of copper (Cu2+) by untreated and treated (bleaching and hydrolysis) aspen wood fibers, cellulose and lignin was examined to understand the Cu2+ sorption behavior by these natural sorbents. All sorbents were characterized by solid-state 13C NMR and FTIR. Bleaching broke up aromatic structures and increased hydrophilicity of the fibers, whereas hydrolysis decreased carbohydrate content, producing a more hydrophobic structure. Copper sorption was a function of pH; the percentage of Cu2+ sorption steadily increased from pH 1.5 to 4.5 with a maximum sorption amount at around pH 5.5 for all the materials. All isotherms fitted well to the Langmuir equation. Bleached sample (BL) had a highest sorption capacity, followed by untreated (UTR), cellulose (CEL), and hydrolyzed (HHY), while lignin (LIG) had little Cu2+ sorption under the studied conditions. The results suggested that carboxyl (–COOH) and hydroxyl (–CHOH) in carbohydrates are mainly responsible for Cu2+ sorption, and that ion exchange may be a main sorption mechanism for the studied sorbents. Additionally, the sorption capacity for Cu2+ on all sorbents decreased with the increase of the initial concentrations of Ca2+, Na+ or Al3+. Copper sorption decreased rapidly at low initial concentrations of Ca2+, Na+ or Al3+. However, the decline of Cu2+ sorption slowed down when initial Na+ and Ca2+ concentration was higher than 0.05 M or initial Al3+ concentration was greater than 0.005 M, indicating that specific adsorption may be taking place. Therefore, the majority of sorbed Cu2+ to aspen wood fibers could be through ion exchange (especially, for UTR, BL and CEL), while a faction of sorbed Cu2+ via inner-sphere complex (or specific adsorption).