Simple and effective chitosan based films for the removal of Hg from waters: Equilibrium, kinetic and ionic competition

• The removal of Hg from water by chitosan films was investigated at realistic levels.
• The removal of μM levels of Hg by 50 mg L−1 of chitosan films range between 80% and 90%.
• The removal efficiency of Hg was negatively affected in natural waters.
• The sorption capacity of chitosan film was improved after the grafting reaction.
• An economical and friendly method was developed to remove Hg(II) from aqueous media.

The efficiency of chitosan cross-linked with genipin (Chg) and cross-linked with genipin and grafted with caffeic acid (Chg+caf) to remove Hg(II) from waters was investigated. An optimal dose of 50 mg L−1 for both chitosan films was selected based on the equilibrium removal percentage and on the contact time required to attain the equilibrium. The sorption extent was dependent on the initial Hg(II) concentration (CHg,0CHg,0), with removal efficiencies varying between 79% and 82% for CHg,0=0.05CHg,0=0.05 mg L−1 and between 89% and 94% for CHg,0=0.50CHg,0=0.50 mg L−1. Under ion competition, the Chg and Chg+caf films showed to be effective and selective for mercury in multimetallic solutions containing also cadmium and lead. In the case of natural river and seawaters, the mercury speciation played an important role in the overall sorption process, reducing the percentage removal of Hg. In terms of modeling, the kinetic data were well described by pseudo-first and pseudo-second order models, depending on the experimental conditions. The diffusion models suggested that the entire sorption process of Hg(II) by both Chg and Chg+caf films was essentially controlled by pore diffusion. The equilibrium data were well described by the Sips isotherm, and the estimated capacity was 2.2 and 4.0 mg g−1 for Chg and Chg+caf films, respectively. In the whole, the results showed that the sorption efficiency was improved by grafting caffeic acid to the polymeric chains of chitosan cross-linked with genipin.