Metal cations enhanced the bonding of Sb(V) on adsorbent surfaces: An effective strategy for adsorbent engineering

Abstract

The adsorption capacity of the adsorbent for pentavalent antimony (Sb(V)) is limited by its strong electronegativity and high solubility, and the mechanism of how the presence of coexisting cations in the wastewater can modulate this adsorption process is still understudied. In this study, the effect of coexisting metal cations on Sb(V) bonding on the adsorbent surface was determined using magnetic carbon nanotube adsorbents (FeOx@CNTs) as an example. The experimental design and theoretical calculations demonstrated that the metal cations could enhance the removal of Sb(V) by the adsorbent (4-fold enhancement of Sb(V) adsorption in the presence of Cd(II) coexistence). The addition of Cd(II) increased the FeOx@CNTs surface potential and enhanced the adsorption performance. The complexation of hydroxyl functional groups with iron oxides and the co-precipitation of FeOx/Cd(II) with Sb (V) were the primary factors to enhance the removal of Sb(V). Density functional theory (DFT) calculations demonstrated that the Sb(V) adsorption energy (Ead) on FeOx was −4.28 eV, while the introduction of Cd(II) increased the adsorption energy to −5.09 eV, facilitating an easier and more stable adsorption process. Analysis of the partial density of states (PDOS) and d-band center theory argued the Sb(V) chemisorption on the FeOx surface and its enhanced electron transport capacity (the presence of Cd(II) brings the d-band center of FeOx closer to the Fermi energy level from −0.786 to −0.739 eV). In the Sb(III) and Sb(V) synergistic adsorption system, the addition of Cd(II) not only did not inhibit the adsorption performance of Sb(III), but also enhanced the Sb(V) uptake on FeOx@CNTs. This study lays a foundation for exploring the mechanism of co-existing metal cation adsorption in real antimony (Sb)-containing wastewater, and also provides a new idea and an effective strategy for adsorbent engineering.