Numerical simulation of Cd(II) removal by ostrich bone ash supported nanoscale zero-valent iron in a fixed-bed column system: Utilization of unsteady advection-dispersion-adsorption equation

A rigorous model, based on the numerical simulation of one-dimensional, unsteady of advection-dispersion-adsorption equation (UADAE) solved by the implicit finite difference method (IFDM), has been employed to simulate the dynamic adsorption of Cd(II) ions on ostrich bone ash (OBA)/nanoscale zerovalent iron particles (nZVI) composite (OBA/nZVI) in a fixed-bed column system. The accuracy of the numerical model was assessed by employing three criteria: mean residual error (MRE), normalized root mean square error (NRMSE) and coefficient of determination (R-2). The numerical model tracked the observed values very closely (R-2 = 0.9698). It was found that the numerical model had the excellent performance to simulate breakthrough curves (BTC) (NRMSE = 7.66%) and it tended to overestimate the BTC very slightly (MRE = 0.0029). The sensitivity analysis indicated that the axial dispersion coefficient (D) had more significant impact on the column dynamic simulation. Therefore, it is possible to describe the adsorption process for wastewater treatment systems in a more cost-effective and easier way with the suggested model application. The results obtained from SEM and XRD analyses confirmed that the OBA/nZVI composite preserved its capabilities even after one year of storage under room conditions. The results also obtained from XPS technique confirmed that the removal mechanism of Cd(II) ions, which have the standard reduction potential very close to that of Fe, is the sorption/surface complex formation without reduction of Cd(II) to Cd degrees by nZVI.