Sulfide-modified Zero-valent Iron Research Articles

Encapsulation/capture of S-nZVI particles by PANa-PAM hydrogel limits their leakage and improves Cr(VI) removal

Sulfide-modified zero-valent iron (S-nZVI) has been shown to be an effective material for removal of various contaminants from wastewater. However, the release of this powder material into the environment brings some hidden troubles to its application. Therefore, poly(sodium acrylate)-poly(acrylamide) (PANa-PAM) hydrogel was used in this study to capture S-nZVI particles within their frameworks, and encapsulate them from being released into the environment. The structure and composition characterization of the composites (S-nZVI@H) showed that the S-nZVI particles were tightly encapsulated by the cross-linked network frameworks of the PANa-PAM hydrogel. The content of Fe in S-nZVI@H was 123.77 mg/g, and only 6.34% were leached out after being used three times in a row. The maximum removal capacity of Cr(VI) by S-nZVI@H was 9.24 mg/g. Moreover, S-nZVI@H showed excellent reusability and stability. The proposed mechanism of Cr(VI) removal by S-nZVI@H can be described as uptake-reduction-codeposition. The synergistic effect of PANa-PAM hydrogel and S-nZVI particles realized stabilization and solidification of Cr(VI). These results demonstrate that the encapsulation of S-nZVI particles by PANa-PAM hydrogel not only limited their leakage to the environment, but also facilitated the removal of Cr(VI).

Effects of magnetic field on selenite removal by sulfidated zero valent iron under aerobic conditions

A novel strategy of sulfide modified zero valent iron (S-ZVI) coupled with the magnetic field (MF) is developed for selenite (Se(IV)) removal. The original ZVI particle size (30 μm), S/Fe ratio (1:80), solution pH (5), S-ZVI loading (0.75 g L−1), and MF intensity (20 mT) can exhibit the optimal enhancement effects of MF on Se(IV) removal by S-ZVI. Common corrosion promoters (Cl−, PO43−, SO42−, Mg2+, and Ca2+) and inhibitors (NO3−, SiO32−, and CO32−) show positive and negative effects on Se(IV) removal by S-ZVI, respectively. But MF can alleviate promoting or inhibiting effects of coexisting ions on Se(IV) removal by S-ZVI, and well preserve the reactivity of S-ZVI from background ions in water. Furthermore, MF can also enhance the reactivity of S-ZVI towards Se(IV) during consecutive experiments, the promotion factor (the ratio of kobs with MF to kobs without MF) increased from 2.57 to 5.83 with the increase of cycles. MF can not only improve the reactivity of ZVI covered with iron oxide or iron hydroxide but also effectively enhance the ability of ZVI covered with iron sulfide. S-ZVI exhibited good stability and recyclability in the presence of MF. XANES analysis of selenium species reveals that the reductive product of Se(IV) with or without MF is primarily Se(0), and Se(IV) removal by S-ZVI can be ascribed to adsorption and reduction. This work indicates that MF may widen the application of S-ZVI for pollutants removal in environmental remediation.

Sulfamethazine removal by peracetic acid activation with sulfide-modified zero-valent iron: Efficiency, the role of sulfur species, and mechanisms

Sulfide-modified zero-valent iron (S-Fe0) has been regarded as a promising method to enhance the catalytic activity of Fe0. Therefore, peracetic acid (PAA) firstly activated by S-Fe0 was conducted to remove sulfamethazine (SMT) in this study. Only 13.1% SMT could be removed within 60 min in Fe0/PAA system while 86.5% SMT could be removed within 60 min in S-Fe0/PAA system at condition of [PAA] = 100 μΜ, [S-Fe0] = 20 mg/L, [SMT] = 5 mg/L, pH = 4.0. The effect of S/Fe ratio, S-Fe0 dosage, PAA concentration and initial pH on SMT removal were investigated. The characterization of S-Fe0 such as SEM, XPS, contact angle and Tafel confirmed that the formation of sulfur species on Fe0 surface could enhance the catalytic performance of Fe0. S2− played the major role and SO32- played the minor role in accelerating the conversion of Fe3+ to Fe2+. •OH and R-O• exhibited 67.1% and 32.5% contribution for SMT removal in S-Fe0/PAA system, respectively. The possible degradation pathway of SMT was also proposed. Cl− and HCO3– had a negligible adverse effect on SMT removal in S-Fe0/PAA system indicating the good adaptability of inorganic ions. In addition, S-Fe0/PAA system had the good recycle ability of S-Fe0 and showed a better performance in removing SMT in the natural fresh water. Therefore, this study notably showed the potential applicability of S-Fe0/PAA system in degrading SMT in natural fresh water.

Sulfadiazine removal by peroxymonosulfate activation with sulfide-modified microscale zero-valent iron: Major radicals, the role of sulfur species, and particle size effect

Sulfide-modified zero-valent iron (S-Fe0) is regarded as a promising method to enhance the catalytic activity of Fe0 for peroxymonosulfate (PMS) activation. However, the roles of sulfidation and the application of the sulfidation treatment method are worth to further investigation. In our study, the effects of the S/Fe ratio, Fe0 dosage, and initial pH on sulfadiazine (SDZ) removal were investigated. The characterization of S-Fe0 with SEM, XPS, contact angle and Tafel analysis confirmed that the formation of sulfur species on the Fe0 surface could enhance the catalytic performance of Fe0. S2− played the major role and SO32- played the minor role in accelerating the conversion of Fe3+ to Fe2+. EPR tests, radical quenching and quantitative determination experiments identified •OH as playing the major role and SO4•− also playing an important role in SDZ removal in S-Fe0/PMS system. Sulfidation produced no notable change in the role of •OH and SO4•−. A possible degradation pathway of SDZ was proposed. Effect of sulfidation on various sizes of Fe0 was also studied which demonstrated that the smaller sizes of Fe0 (< 8 µm) were more effective in the sulfidation method treatment. S-Fe0/PMS system also showed a good performance in removing antibiotics in natural fresh water.

Enhanced removal of sulfadiazine by sulfidated ZVI activated persulfate process: Performance, mechanisms and degradation pathways

Sulfidation treatment has been considered as an effective method to improve the activity of zero valent iron (ZVI) for reductive removal of contaminants. However, the role and mechanism of sulfide-modified zero valent iron (S-ZVI) in advanced oxidation reaction were worth further exploration. In this study, influences of different reaction conditions, including initial pH, iron dosage, persulfate (PDS) dosage and inorganic ions on sulfadiazine (SDZ) removal by mechanical ball-milling sulfidated microscale zero valent iron (S-ZVIbm)/PDS process and mechanical ball-milling microscale zero valent iron (ZVIbm)/PDS were comparatively investigated. By contrastive analyses, S-ZVIbm exhibited more excellent performance on activation of PDS. Meanwhile, SO42−, Cl−, and NO3− showed different effects on SDZ degradation by ZVIbm/PDS and S-ZVIbm/PDS. The potential mechanisms that S-ZVIbm exceeding ZVIbm in activating PDS were clarified from two aspects, which were better iron dissolution and iron circulation caused by iron sulfides generated during sulfidation treatment. Besides, chemical quenching experiments and electron paramagnetic resonance (EPR) studies identified that both SO4·- and OH were generated in ZVIbm/PDS and S-ZVIbm/PDS, and SO4·- was the dominant one. Two different degradation pathways of SDZ in the S-ZVIbm/PDS process were proposed based on the experimental intermediates identification and density functional theory (DFT) calculation. According to the results obtained in this study, the S-ZVIbm/PDS process was recommended as an effective method to degrade contaminants.

Sulfide-modified zerovalent iron for enhanced antimonite sequestration: Characterization, performance, and reaction mechanisms

Zerovalent iron (ZVI) is commonly used for water treatment under aerobic conditions such as sequestration of metals. Sulfide-modified ZVI (S-ZVI) is attracting increasing attention for its easy preparation and high reactivity with environmental pollutants. The processes responsible for contaminant removal can be a complex mixture of redox, sorption, and coprecipitation processes. In this paper, ZVI and S-ZVI were used to sequester antimonite (Sb(III)). The rates of Sb(III) sequestration were determined in open, well-mixed, batch reactors. The effects of various experimental variables were investigated, including pH, iron dose, initial concentrations of Sb(III), aging time of the ZVI and S-ZVI, addition of Fe2+, mixing rate, etc. The results showed that S-ZVI can significantly enhance the Sb(III) sequestration, and under basic conditions in this study, the kobs (0.018 min−1) obtained in the S-ZVI system was approximately 15 times higher than the 0.0012 min−1 obtained in the ZVI system. Solid phase characterizations were conducted to assess the influence of sulfidation on the morphology and surface geochemistry of ZVI. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS) confirmed the presence of sulfur. X-ray photoelectron spectroscopy (XPS) indicated the oxidation of Sb(III) to Sb(V) and adsorption and coprecipitation onto the iron oxides is the mainly sequestration process. The FeS layer on ZVI is more conductive than oxides and therefore accelerates electron transfer. In addition sulfidation promotes the corrosion of iron and the formation of ferrous iron, which further enhances the ferric iron oxide formation, thereby favoring adsorption and oxidation of Sb(III).