The objective of the study was to elucidate the binding of He(2+), methyl mercury, and phenyl mercury on chitosan and barbital-immobilized chitosan by potentiometric titration and XPS and FTIR spectroscopy. Adsorption experiments were conducted at room temperature (30 degrees C) to optimize the percentage removal of different forms of mercury, and the data were fitted to Langmuir and Freundlich adsorption isotherms. Speciation of inorganic, methyl, and phenyl mercury was achieved by sequentially eluting inorganic mercury using 0.1 N perchloric acid followed by methyl mercury using 0.015 N NaCl and finally phenyl mercury using 0.05 N EDTA. Potentiometric titration data and a plea spectrum approach showed the presence of self-assembled amine/protonated amine, amine, and hydroxyl/amine functional groups. The total number of functional groups was found to be 158.05 mol center dot g(-1) and 68.6 mol center dot g(-1) for barbital-immobilized chitosan and chitosan, respectively, suggesting that a larger number of binding sites are available in barbital-immobilized chitosan for bonding to mercury. FTIR, pH, and XPS studies show that the possible functional groups in the metal binding include amino functional groups in the case of chitosan and amide groups in the case of barbital-immobilized chitosan. The peak areas of XPS Hg 4f spectra show greater adsorption for Hg using barbital-immobilized chitosan as adsorbent compared to chitosan.