Characterizing the Reactivity of Metallic Iron for Water Treatment: H$_2$ Evolution in H$_2$SO$_4$ and Uranium Removal Efficiency

Abstract

Metallic iron (Fe$^0$) has been demonstrated as an excellent material for decentralized safe drinking water provision, wastewater treatment and environmental remediation. An open issue for all these applications is the rational material selection or quality assurance. Several methods for assessing Fe$^0$ quality have been presented, but all of them are limited to characterizing its initial reactivity. The present study investigates H$_2$ evolution in an acidic solution (pH 2.0) as an alternative method, while comparing achieved results to those of uranium removal in quiescent batch experiments at neutral pH values. The unique feature of the H$_2$ evolution experiment is that quantitative H$_2$ production ceased when the pH reached a value of 3.1. A total of twelve Fe$^0$ specimens were tested. The volume of molecular H$_2$ produced by 2.0 g of each Fe$^0$ specimen in 560 mL H$_2$SO$_4$ (0.01 M) was monitored for 24 h. Additionally, the extent of U(VI) (0.084 mM) removal from an aqueous solution (20.0 mL) by 0.1 g of Fe$^0$ was characterized. All U removal experiments were performed at room temperature (22 ± 2 °C) for 14 days. Results demonstrated the difficulty of comparing Fe$^0$ specimens from different sources and confirmed that the elemental composition of Fe$^0$ is not a stand-alone determining factor for reactivity. The time-dependent changes of H$_2$ evolution in H$_2$SO$_4$ confirmed that tests in the neutral pH range just address the initial reactivity of Fe$^0$ materials. In particular, materials initially reacting very fast would experience a decrease in reactivity in the long-term, and this aspect must be incorporated in designing novel materials and sustainable remediation systems. An idea is proposed that could enable the manufacturing of intrinsically long-term efficient Fe$^0$ materials for targeted operations as a function of the geochemistry.