Abstract
Nanoscale zero-valent iron (nZVI) has attracted considerable attention for its potential to sequestrate and immobilize heavy metals such as Cr(VI) from an aqueous solution. However, nZVI can be easily oxidized and agglomerate, which strongly affects the removal efficiency. In this study, graphene-based nZVI (nZVI/rGO) composites coupled with ultrasonic (US) pretreatment were studied to solve the above problems and conduct the experiments of Cr(VI) removal from an aqueous solution. SEM-EDS, BET, XRD, and XPS were performed to analyze the morphology and structures of the composites. The findings showed that the removal efficiency of Cr(VI) in 30 min was increased from 45.84% on nZVI to 78.01% on nZVI/rGO and the removal process performed coupled with ultrasonic pretreatment could greatly shorten the reaction time to 15 min. Influencing factors such as the initial pH, temperature, initial Cr(VI) concentration, and co-existing anions were studied. The results showed that the initial pH was a principal factor. The presence of HPO42−, NO3−, and Cl− had a strong inhibitory effect on this process, while the presence of SO42− promoted the reactivity of nZVI/rGO. Combined with the above results, the process of Cr(VI) removal in US-nZVI/rGO system consisted of two phases: (1) The initial stage is dominated by solution reaction. Cr(VI) was reduced in the solution by Fe2+ caused by ultrasonic cavitation. (2) In the following processes, adsorption, reduction, and coprecipitation coexisted. The addition of rGO enhanced electron transportability weakened the influence of passivation layers and improved the dispersion of nZVI particles. Ultrasonic cavitation caused pores and corrosion at the passivation layers and fresh Fe0 core was exposed, which improved the reactivity of the composites.
Highlights
With the rapid development of industrialization, urbanization, and agriculture activities, chromium (Cr) has played a great role both in industrial and agricultural production [1,2]
The results showed that Nanoscale zero-valent iron (nZVI) presented a much lower removal efficiency than nZVI/reduced graphene oxide (rGO). nZVI/rGO obtained the highest removal efficiency of 78%
In order to study the mechanism of Cr(VI) removal from an aqueous solution by the reaction, which illustrates that chromium was adsorbed onto nZVI/rGO
Summary
With the rapid development of industrialization, urbanization, and agriculture activities, chromium (Cr) has played a great role both in industrial and agricultural production [1,2]. Of all the known methods, sequestration by nanoscale zero-valent iron (nZVI) has been considered as an economic, efficient, and implemented process [15], owing to its large specific surface area, high surface reactivity, and low cost [16,17,18]. As an alternative, reduced graphene oxide (rGO), which could to a certain extent resemble pristine graphene’s properties [31,32] and be obtained by reduction of GO [33], has become a preferred support material for absorbent research. It is necessary to develop an activation method without removing the iron oxide shell [25]. Studies have shown that ultrasonic cavitation has the ability of cleaning and activating the passivation layer on the surface of nZVI particles. Characterizations such as SEM-EDS, BET, XRD, and XPS were performed to investigate the difference in structure between nZVI and nZVI/rGO, and to uncover the mechanism of Cr(VI) removal in the US-nZVI/rGO system
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