Highly efficient removal and separation of Sr2+, Co2+, In3+ and Ni2+ in single and mixed ion systems on low-cost Zeolite, and its stability analysis: DFT and experimental investigations

In view of the increasing demand for clean water, the aim of this work was to elucidate, by molecular Monte Carlo (MC) simulations and experimental investigations, the capture of Sr2+, In3+, Ni2+ and Co2+ in single and mixed system ions using low cost-effective zeolites based on natural kaolin. Recycled zeolites were characterized by NMR-MAS, FT-IR, XRD, SEM, EDX-mapping and TGA/DSC analysis. XRD analysis after adsorption showed that the peak intensity decreased due to the presence of adsorbed ions on the zeolite surface. NMR-MAS revealed the chemical shift of 29Si and 27Al after adsorption due to the presence of Co and Ni in the samples, which tend to affect the chemical environment of the orbital spin. The maximum capacities (Qm,exp) obtained were ∼780, ∼600, ∼1000 and ∼1300 mg/g for Sr2+, In3+, Ni2+ and Co2+, respectively. The Langmuir isotherm provided the most accurate non-linear fit to the experimental data, with an R² > 0.993, indicating that the metal ions undergo homogeneous monolayer adsorption on the adsorbent surface. The Freundlich isotherm showed the presence of multilayer adsorption and heterogeneous adsorption energy. The removal of ions was favored by ion exchange and chemical reaction on monolayers with energetic heterogeneity at the zeolite surface. Pseudo-first order non-linear kinetic models were favorable with (R2 ≥ 0.99) indicating the presence of chemisorption. MC modelling showed that ion capture was favorable in neutral media, with individual adsorption energies (dEads/dNi) of -0.03, -0.05, -5.8 and -13.8 kcal/mol for Co, Ni, Sr and In, respectively, in a mixed ion system at 298 K. The affinity of zeolite to adsorbed Co(II) on Ni(II), Al(III), Bi(III), Ca(II), In(III), Sr(II), K(I), Cd(II), Cr(III) and Pb(II) was observed by the highest values of the distribution coefficient (Kd) and low separation factor (α). The selectivity order was Co > Ni > In > Ca > Sr > Cd > Mn > Pb > K > Al > Cr > Bi. Surface and river water influenced the adsorption capacity compared to distilled and tap water. Zeolite exhibited high stability during the removal of Sr, In, Ni and Co in both single and mixed ions systems. Removal rates were found to be in the range of ∼84 % to ∼94 % and ∼55 % to ∼80 %, respectively, after the fifth cycle.