Adsorption properties of oxidized carboxymethyl starch and cross-linked carboxymethyl starch for calcium ion

The adsorption properties of oxidized carboxymethyl starch (OCMS) and cross-linked carboxymethyl starch (CCMS) with carboxymethyl groups for calcium ion in aqueous solution were investigated. Removal efficiency of calcium ion from aqueous solution was found to be dependent on initial concentration of calcium ion, the adsorbent dose and temperature. Ionization of carboxymethyl groups in OCMS and CCMS was necessary for the effective calcium removal. When the pH of the solution was adjusted below 4.0, OCMS and CCMS were no longer an effective calcium-sequestering agent. Regardless of starch type, the adsorption capacities of two anionic starches reached a state of equilibrium within 30 min for starch dispersion in calcium salt solution. Adsorption of calcium ion followed modified Freundlich kinetics with the adsorption rate for adsorption on OCMS being higher followed by that on CCMS. The adsorption rate constant was found to decrease with the increase in the temperature. Adsorption isotherm models were developed wherein the best fit was obtained in the Langmuir model for OCMS and Freundlich isotherm for CCMS, respectively. Thermodynamic study indicated that the adsorption process of calcium ion on two anionic starches was exothermic, and the enthalpy change (ΔHθ), the entropy change (ΔSθ), and free energy change (ΔGθ) of the adsorption process were calculated with adsorption isotherm data and basic thermodynamic relations. It was assessed that adsorption occurred by strong electrostatic interactions with the negative adsorption enthalpy.