Chitosan metal-crosslinked beads applied for n-alkylmonoamines removal from aqueous solutions – a thermodynamic study

• Chitosan beads were successfully synthesized with divalent cations.
• Well-formed bead structures containing cations act as acceptor electron sites.
• n-Alkylmonoamine/bead interactions are favorably sorbed at the solid/liquid interface.
• The thermodynamic data were favorably obtained from calorimetric titrations.
• Crosslinked metal–chitosan beads can remove amine-like substances from an ecosystem.

Chitosan has ability in coordinating divalent cations when immersed in crosslinked beads, after dripping: (i) chitosan gel into a copper solution, (ii) nickel chitosan gel into sodium hydroxide and (iii) chitosan/cobalt gel into sodium tripolyphosphate. The amounts of (1.82; 1.27 and 0.44) mmol · g−1 for copper, nickel and cobalt cations in these well-formed structures were determined, to give nitrogen/metal ratios of 3.52; 2.09 and 8.51, indicating the least effectiveness for cobalt in the coordination. Copper cation is well-adjusted in the coordination model through free amino and hydroxyl electron pairs, while amino and acetamino groups for nickel and cobalt were used. The chitosan–hydrogen bond breaking in bead formation caused decreases in crystallinity to yield amorphous structures for cobalt and nickel. The water mass fraction released during heating depends on the hydration of the cations, with the highest value of 0.20 for cobalt. The quantitative aspects of the interaction among cations on beads and basic n-alkylmonoamines determined via sorption batch methodology adjusted to the Langmuir isothermal model, with maximum sorption quantities to saturate nickel of (2.50; 2.38; 2.03; 1.79) mmol · g−1 and copper of (2.59; 2.29; 2.28; 1.92) mmol · g−1 for ethyl- propyl-, butyl- and pentylamines, respectively. The interaction energies quantitatively determined from isothermal titration calorimetry (ITC) at the solid/liquid interface resulted in exothermic enthalpic values. These negative enthalpy values combined to the negative Gibbs free energies from the application of the Langmuir isothermal model to the ITC data gave positive entropic values. The entropy changes varied from (48 to 64) J · mol−1 · K−1 for nickel and between (18 and 31) J · mol−1 · K−1 for copper beads. These thermodynamic data strongly suggest that these easily synthesized crosslinked metal beads may be useful and practical designed agents to apply in waste effluent treatments for pollutant removal.

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