Nickel Adsorption by Biogenic Selenium Nanoparticles (Senps) : Characterization and Simulation of Non-Symmetric Breakthrough Curves in Continuous-Flow Packed-Bed Columns

Nanoparticles are promising novel adsorptive materials for heavy metals removal from water streams. In this work we present for the first time the adsorption of Nickel by biogenic selenium nanoparticles (SeNPs) both in freely suspended and immobilized form. Immobilization was carried out in calcium alginate matrix. The nature and morphology of the adsorbent were characterized using FTIR, SEM and rheological analysis. Kinetic and equilibrium data are obtained from batch experiments. Adsorption is fast and equilibrium is attained within one hour at Ni concentration between 10 to 100 mg/L Ni2+ at a sorbent to liquid ratio 7.5 g/100 mL. The pseudo-nth order kinetic model describes equally well the kinetic data as the pseudo-first and pseudo-second order model. Langmuir isotherm fits better the adsorption equilibrium with maximum loading Q0 of 29.411 mg Ni/g SeNPs and 0.6506 mg Ni/g alginate-SeNPs. Continuous flow experiments in packed beds with variable lengths and different flow rates have shown early elution points and non-symmetric sigmoidal profiles. This pattern is attributed to the low adsorption efficiency of alginate-SeNPs, the short column lengths and the non-uniform flow pattern. The Bohart-Adams equation fits adequately the main sigmoidal part of the breakthrough curves, while it fails to predict the early breakthrough data. The model accuracy is significantly improved by applying the advection-dispersion-reaction equation (ADR) under two novel concepts: (a) the rapid equilibrium model with the introduction of an efficiency coefficient accounting for all the kinetic and sorption limitations and (b) the split flow model accounting for any non-uniform flow conditions in the column. Repeated sorption-desorption experiments show that alginate-SeNPs retain 59% of the original adsorption capacity even after eight cycles