A DFT study on Ca-Alginate interactions with divalent transition metals

Abstract

Alginate is a naturally occurring linear polysaccharide that is extracted from brown algae. This is generally produced as water-soluble Na alginate which in turn is transformed into water-insoluble Ca-Alginate beads. These beads are potential adsorbents for the removal of heavy metal ions from water through ion exchange with calcium ions present in the Ca-Alginate matrix. To investigate the feasibility of ion exchange of various heavy metal ions, Density Functional Theory (DFT) calculations were carried out using the Gaussian-16 simulation package. The heavy transition metal ions with + 2 valence in the 4th, 5th, and 6th periods of d-block were considered for the study as most of the heavy metals (e.g., Cu, Cr, Ni, Co, Mn, etc.) are present in this block and significantly contribute to water pollution. The widely accepted egg-box molecular model representation of alginate structures is used for computation purposes. For computation purposes, one monomer attached from top and bottom to the metal center is considered. The computations were carried out in both vapor and aqueous solvent phases and the trends of binding energy were found to be invariant. The binding energies of the alginate complexes formed with Ni, Pd, and Pt metal ions of the 4th, 5th, and 6th series were the highest amongst all the metals considered in this work.