Single and binary heavy metal adsorption using alginic acid structure: Experimental and density functional theory investigations

The adsorption of Cd²⁺ and Co²⁺ using alginic acid (AA) was performed by density functional theory (DFT) calculations. To validate the theoretical findings, Cd²⁺ and Co²⁺ were extracted from aqueous solutions using brown algae, where AA constituted the major component. The study encompassed both experimental and theoretical aspects, addressing single and binary systems. Various parameters, including adsorption energy, thermodynamics, quantum chemical, natural bond orbital, quantum theory of atoms, electron-localization-function, non-covalent interaction, UV–vis, and restrained electrostatic potential (RESP), were calculated using a DFT-based method to assess the adsorption of heavy metals (Cd²⁺ and Co²⁺) onto AA in both single and binary systems. The DFT calculations confirmed higher Co²⁺ adsorption onto AA compared to Cd²⁺. Moreover, the determination of the charge distribution on the adsorbent atoms, accomplished through the RESP method, revealed that oxygen atoms within AA allocate more negative charge for Co²⁺ adsorption than Cd²⁺, signifying a stronger affinity for Co²⁺ than Cd²⁺. The computed adsorption energies were −25.40 and −9.32 kcal/mol for the Co²⁺-AA and Cd²⁺-AA complexes, respectively. AA, the predominant compound in brown algae, has been recognized for its significant capability in adsorbing heavy metals. To further verify the accuracy of the DFT calculations, batch adsorption processes of Co²⁺ and Cd²⁺ were conducted using brown algae. Isothermal and kinetic studies demonstrated higher Co²⁺ adsorption (single = 102.10 mg/g and binary = 85.20 mg/g) on brown algae than Cd²⁺ (single = 60.01 mg/g and binary = 41.12 mg/g). The close agreement between the DFT-based calculations and experimental data underscored the remarkable potential of AA in removing heavy metals. Furthermore, the RESP method exhibited excellent predictive ability for heavy metal adsorption in single and binary systems.