Synergistic effects of bismuth coupling on the reactivity and reusability of zerovalent iron nanoparticles for the removal of cadmium from aqueous solution

Removal of cadmium (Cd²⁺), a highly toxic heavy metal, from aqueous solutions was investigated using nano zerovalent iron (Fe⁰). Cadmium was efficiently removed by Fe⁰, although reactivity and reusability of Fe⁰ was significantly promoted by coupling with bismuth (Bi). At a reaction time of 20 min, 85% and 96% Cd²⁺ was removed by Fe⁰ and Bi/Fe⁰, respectively, at first cycle using [Cd²⁺]₀ = 10 mg/L and [Fe⁰]₀ = [Bi/Fe⁰]₀ = 1.0 g/L. However, Cd²⁺ removal efficiency was reduced to 12% and 80% at sixth cycle by Fe⁰ and Bi/Fe⁰, respectively. The X-Ray diffraction and energy dispersive X-Ray spectroscopy analysis proved successful formation of Fe⁰ by the chemical reduction method and also confirmed coupling of Bi with Fe⁰ to form bimetallic Bi/Fe⁰. The oxidation of Fe⁰ and Bi/Fe⁰ yielded electron that played significant role in the conversion of toxic Cd²⁺ into non-toxic Cd⁰. The reactivity of electron with Cd²⁺ was calculated to be 4.3 × 10⁹ M^-1 s^-1. The pH of solution showed pronounced effects on the reactivity of both Fe⁰ and Bi/Fe⁰. Removal of Cd²⁺ by both Fe⁰ and Bi/Fe⁰ followed pseudo-first-order kinetic model. The conversion of Cd²⁺ into non-toxic Cd⁰ proved Fe⁰ and Bi/Fe⁰ to be highly efficient and rewarding in detoxification of Cd²⁺ and other toxic metals in aqueous environments.