Surface Modulation and Chromium Complexation: All-in-One Solution for the Cr(VI) Sequestration with Bifunctional Molecules

Abstract

The sulfidation of zero valent iron (ZVI) to an Fe@FeS x (S-ZVI) composite has been intensively explored in the ZVI field. Yet, further benefits from the FeS x coating layer are seldom realized, especially those effectively using its intrinsic physical and chemical properties for elaborate design. Here, we demonstrate that in a traditional Cr(VI) sequestration reaction, the FeS x layer displays a great utility in immobilizing molecules containing hydroxyl groups (−OH) and hence, attracting Cr(VI) complexes chelated with carboxyl organics (RCOOH). Such intermolecular attraction readily promotes the diffusion of the Cr(VI) complexes to the S-ZVI surface, affording a higher reaction rate for the Cr(VI) sequestration process. In addition, the above mechanism was used to guide a rational selection of molecules incorporating both hydroxyl and carboxyl functional groups with a proper ratio and thereby, a significantly improved reaction efficiency was achieved. Furthermore, the FeS x phase was revealed to be consumed in the reaction, acting as a supplementary reductant. This work is the first to unveil the relationship between molecules with specific functionalization and the FeS x phase, providing a general rule in choosing appropriate reaction media for Cr(VI) sequestration and related reactions.

Recent Advances in Sulfidated Zerovalent Iron for Contaminant Transformation

“…For instance, typical mZVI particles used in sulfidation studies has been characterized as roughly spherical and irregular, − unlike the spherical and smooth nZVI particles. During the two-step method, mZVI is first pretreated with a buffer − or acid washed with HCl , in order to breakdown the oxide shell prior to sulfidation. For instance, Xu et al mixed 1 g of mZVI with a deoxygenated solution of acetic acid (HAc) and sodium acetate (NaAc) (0.2 M, pH 6) for 30 min before adding 1 M of Na 2 S and equilibrating for 12 h to precipitate Fe 2+ as FeS.…”

“…The shell was comprised predominantly of iron sulfides (FeS or FeS 2 ) and iron oxides (Fe 2+ -O). Subsequent studies adopted or modified the method by Xu et al, making it the most common approach for the synthesis of S-mZVI two‑step , and Na 2 S the preferred sulfidation agent, though other sulfur sources are gaining popularity (e.g., S 0 and Na 2 S 2 O 3 ). ,,,− ,− …”

Chemical Bond Bridging across Two Domains: Generation of Fe(II) and In Situ Formation of FeSx on Zerovalent Iron

“…XPS analysis on the surfactant FeS x has been carried out with the purpose of quantifying the contents and chemical states for the elements. It has shown that ZVI is covered by the iron (hydr)­oxide layer of FeOOH, Fe 2 O 3 , and Fe 3 O 4 , and the inner Fe 0 can hardly be detected because the X-ray beam can only penetrate several nanometers of the sample depth (Figure D). , Compared with ZVI, the Fe 2p 3/2 XPS spectra of SZVI (inherent) , SZVI (generated) , and SZVI (external) clearly show the peak of FeS x at ∼707 eV. Because the binding energy of Fe in FeS x is close to that of Fe 0 , the analysis of reduced S species (S 2– and S 2 2– ) in S 2p XPS spectra is used to rule out the possibility of peak 707 eV being Fe 0 .…”

“…It has shown that ZVI is covered by the iron (hydr)oxide layer of FeOOH, Fe 2 O 3 , and Fe 3 O 4 , and the inner Fe 0 can hardly be detected because the X-ray beam can only penetrate several nanometers of the sample depth (Figure 1D). 29,44 Compared with ZVI, the Fe 2p 3/2 XPS spectra of SZVI (inherent) , SZVI (generated) , and SZVI (external) clearly show the peak of FeS x at ∼707 eV. Because the binding energy of Fe in FeS x is close to that of Fe 0 , the analysis of reduced S species (S 2− and S 2 ) peaks in the S 2p XPS spectra have a positive linear correlation with the area of peak ∼707 eV in Fe 2p XPS spectra (Figure S5 and Tables S2−S3), indicating that the peak at ∼707 eV can be ascribed to FeS x .…”

Simply closing the reactor improves the electron efficiency of zerovalent iron toward various metal(loid)s removal

“…Over the past two decades, zerovalent iron (ZVI), as an efficient, inexpensive, and environmental‐friendly reagent to remove various contaminants, has gained considerable attention in the field of groundwater remediation and wastewater treatment (Gillham & Ohannesin, 1994; Guan et al, 2015; Matheson & Tratnyek, 1994). Researches on the sequestration of metal(loid)s by ZVI have been motivated extensively, due to the reducing potential of ZVI and the adsorption capacity of the iron (hydr)oxides generated from ZVI corrosion (Chen et al, 2019; Gao et al, 2020; Lin et al, 2020; Liu et al, 2009; Su et al, 2014). Plenty of studies have proved that ZVI corrosion is a necessity for metal(loid)s sequestration by ZVI (Fan, Li, et al, 2019; Guan et al, 2015; Sun et al, 2016), and the removal of metal(loid)s by ZVI is hard to be achieved when ZVI corrosion is strongly inhibited (i.e., passivation) (Fan, Li, et al, 2019; Qin et al, 2017).…”