A thorough understanding of the adsorption of Ni (II), Cd (II) and Zn (II) on goethite using experiments and molecular dynamics simulation

The study evaluates the efficiencies of the removal of three heavy metals, such as Ni(Il), Cd(H) and Zn(II), from water using goethite as the adsorbent. Batch adsorption experiments have been carried out to study the effect of parameters like time, dosage, temperature, pH and metal ion concentration. The Ni(II) ion shows the least adsorption efficiency (78.1%) compared to Cd (II) (89.1%) and Zn (II) (85%) at a temperature of 90 degrees C for an initial metal concentration of 10 ppm and a goethite dosage of 10 g/L at a pH of 5-6. As per the Langmuir isotherm model, all the metal ions follow monolayer adsorption while the Freundlich model suggests that Cd(II) and Zn(II) have a higher probability of forming a multilayer on the mineral surface in comparison to Ni(Il). The mean free energy values, as well as the enthalpy differences, reveal that all the metal ions follow physisorption although the adsorption of Cd(II) and Zn(II) shows some characteristics of chemisorption, which is also indicated by the Fourier Transform Infrared spectra. The concentration profiles of the metal ions and salt molecules, as determined using the molecular dynamics simulations, indicate that Ni (H), in comparison to the other two ions, is placed farther from the goethite (0 1 0) surface. Both the experimental findings and the computational calculations, agree on the trend of adsorption strength, i.e. Cd (II) > Zn (II) > Ni (II).