Removal Kinetics of Chromium by Nano-Magnetite in Different Environments of Groundwater

Chromium (Cr) is a toxic contaminant and ubiquitously present in the environment. This study was conducted to investigate the removal of Cr by nano-magnetite ($\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$) in different biogeochemical environments of groundwater. Size of $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ used was in the range of 50–100 nm, and it contained Fe (58.88%) and O (31.13%). Removal rates of total Cr (${\mathrm{Cr}}_{\mathrm{T}}$) by $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ were significantly improved ($0.40–0.84{\min }^{-1}$) as the concentrations of $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ were increased ($0.25–3.75\mathrm{g}/\mathrm{L}$). However, removal rates of ${\mathrm{Cr}}_{\mathrm{T}}$ by $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ were decreased ($1.52–0.66{\min }^{-1}$) as the concentrations of ${\mathrm{Cr}}_{\mathrm{T}}$ increased ($0.5–1.5\mathrm{mg}/\mathrm{L}$). In addition, the removal rate of ${\mathrm{Cr}}_{\mathrm{T}}$ by $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ was significantly increased ($0.56–1.63{\min }^{-1}$) as the pH increased (pH 5.5–pH 9). Removal rates of ${\mathrm{Cr}}_{\mathrm{T}}$ by $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ were significantly decreased ($0.93–0.28{\min }^{-1}$) as the concentrations of humic acid (HA) increased ($0.25–3.75\mathrm{g}/\mathrm{L}$). This study revealed that the reactive surface of $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ plays an important role to enhance maximum removal of ${\mathrm{Cr}}_{\mathrm{T}}$. The surface charge of $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ with or without ${\mathrm{Cr}}_{\mathrm{T}}$ is directly related to the reactive surface area of $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$, which has not been well reported in the literature. Chemisorption reaction mechanisms may occur during the removal kinetics of ${\mathrm{Cr}}_{\mathrm{T}}$ by $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$. Experimental findings from this study proved that $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$ has the capability to remove ${\mathrm{Cr}}_{\mathrm{T}}$ and provided fundamental knowledge on the potential reaction mechanisms of ${\mathrm{Cr}}_{\mathrm{T}}$ removal by $\mathrm{nano}\text{-}{\mathrm{Fe}}_3{\mathrm{O}}_4$.