Interaction of natural and crosslinked chitosan membranes with Hg(II) ions

Fundamental investigation on adsorption and desorption of Hg(II) ions on chitosan membranes was performed. Batch experiments were used to study how these phenomena are affected by pH, concentration of Hg(II) ions, crosslinking agent (glutaraldehyde or epichlorohydrin) and desorbent solution. The adsorption depended on pH and speciation of mercury. Maximum capacity was reached near zero-charge-pH, the pHZPC. The amount of adsorbed Hg(II) ions increased when the adsorbent was crosslinked, mainly with glutaraldehyde. Since amino groups are usually unavailable in this situation, other groups play an important role in adsorption, such as imino bonds, unreacted aldehyde terminals and also unreacted hydroxyl groups from original chitosan. Hg(II) equilibrium concentrations were determined and the Langmuir model was fitted to the experimental data. The maximum adsorbed amounts, at pH 6.0, were 25.3, 30.3 and 75.5 mg/g, respectively, to natural, epichlorohydrin-crosslinked and glutaraldehyde-crosslinked chitosan membranes (wet weight). The pHZPC was in the range of 6–10 for the three different membranes. X-ray fluorescence analysis using synchrotron radiation demonstrated that natural chitosan presents more homogeneous adsorption than with crosslinked chitosan, within a 10 μm × 10 μm area. However, mappings of Hg in larger scale areas (1 mm × 1 mm), using energy dispersive X-ray spectroscopy, indicated a homogeneous distribution of Hg-adsorbent sites. The desorption process was carried out using NaCl (1 mol/L) and EDTA (10–4 mol/L) as eluent solutions. The best metal recovery was obtained when NaCl solution was used.