Production and prediction of hydroxyl radicals in distinct redox-fluctuation zones of the Yellow River Estuary.

Hydroxyl radicals (·OH) produced in subsurface sediments play an important role in biogeochemical cycles. One of the major sources of·OH in sediments is associated with reduced compounds (e.g., iron and organic matter) oxygenation. Moreover, the properties of iron forms and dissolved organic matter (DOM) components varied significantly across redox-fluctuation zones of estuaries. However, the influence of these variations on mechanisms of·OH production in estuaries remains unexplored. Herein, sediments from riparian zones, wetlands, and rice fields in the Yellow River Estuary were collected to systematically explore the diverse mechanisms of·OH generation. Rhythmic continuous·OH production (82-730 µmol/kg) occurred throughout the estuary, demonstrating notable spatial heterogeneity. The amorphous iron form and humic-like DOM components were the key contributors to·OH accumulation in estuary wetlands and freshwater restoration wetlands, respectively. The crystalline iron form and protein-like DOM components influenced the capabilities of iron reduction and continuous·OH production. Moreover, the orthogonal partial least squares models outperformed various multivariate models in screening crucial factors and predicting the spatiotemporal production of·OH. This study provides novel insights into varied mechanisms of·OH generation within distinct redox-fluctuation zones in estuaries and further elucidates elemental behavior and contaminant fate in estuarine environments. ENVIRONMENTAL IMPLICATION: Given that estuaries serve as sinks for anthropogenic pollutants, various organic pollutants (e.g., emerging contaminants such as antibiotics) have been widely detected in estuarine environments. The production of·OH in sediments has been proven to affect the fate of contaminants. Therefore, the varied mechanisms of·OH in estuarine environments, dominated by diverse iron forms and DOM components, were explored in this study. MLR and OPLS models exhibited good performance in screening crucial factors and predicting·OH production. Our work highlights that in estuarine subsurface environments, the presence of·OH potentially leads to a natural degradation of pollutants.

Unraveling the neglected role of elemental sulfur in chromate removal by sulfidated microscale zero-valent iron.

Elemental sulfur (S0), as an oxidation product of low-valent sulfur, is widely believed to inhibit the reactivity of sulfidated zero-valent iron (S-ZVI). However, this study found that the Cr(VI) removal and recyclability of S-ZVI with S0 as the dominant sulfur species were superior to those FeS or iron polysulfides (FeSx, x > 1) dominated ones. The more S0 directly mixed with ZVI, the better Cr(VI) removal obtained. This was ascribed to the formation of micro-galvanic cells, the semiconductor properties of cyclo-octasulfur S0 with sulfur atom substituted by Fe2+, and the in situ generations of highly reactive iron monosulfide (FeSaq) or polysulfides precursors (FeSx,aq). The Cr(VI) sequestration of FeSx,aq was 1.2-2 times that of FeSaq, and the reaction rate of amorphous iron sulfides (FexSy) in the removal of Cr(VI) by S-ZVI was 8- and 66-fold faster than that of crystalline FexSy and micron ZVI, respectively. The interaction of S0 with ZVI required direct contact and needed to overcome the spatial barrier caused by FexSy formation. These findings reveal the role of S0 in Cr(VI) removal by S-ZVI and guide the future development of in situ sulfidation technologies to utilize the highly reactive FexSy precursors for field remediation.

Enhanced reductive degradation of chloramphenicol by sulfidated microscale zero-valent iron: Sulfur-induced mechanism, competitive kinetics, and new transformation pathway.

Crystalline iron sulfide (FeSx, i.e., FeS or FeS2) minerals as sulfur sources were used to prepare the mechanochemically sulfidated microscale zero-valent iron ((FeSx+ZVI)bm). Metastable FeS and FeS2 precursors were generated via aqueous coprecipitation and applied to fabricate FeSx@ZVI samples. (FeSx+ZVI)bm and FeSx@ZVI exhibited better chloramphenicol (CAP) degradation than ZVI due to the increase in specific surface areas, the decrease of electrochemical impedance, the formation of galvanic cells, and sulfur-induced pitting and local acidity. (FeSx+ZVI)bm had better CAP removal capacity than FeSx@ZVI under different S/Fe molar ratios, initial pH, and oxygen conditions. At the same time, FeSx@ZVI showed better electron utilization under oxic conditions, related to their Fe0 and sulfur spatial distribution. Nitro reduction and dechlorination of CAP by (FeSx+ZVI)bm produced nitroso, azoxy, amine, and monodechlorination products, while dechlorination was not involved in the degradation process of CAP by FeSx@ZVI. A new transformation pathway of nitroso-CAP to amine-CAP mediated by azoxy products is proposed via coupling a chain decay multispecies model and DFT calculations. The larger competitive reaction rates among O2, CAP, and its degradation products was determined by their lower LUMO energy. The contribution of direct electron transfer to nitro reduction was greater than that of atomic hydrogen, but the opposite was true for dechlorination. FeSx@ZVI had a larger DET contribution than (FeSx+ZVI)bm, and FeS2 promoted the DET contribution better than FeS. Toxicity assessment indicated that the rapid transformation of nitroso and azoxy products was crucial for eliminating the biotoxicity of CAP.

Elemental sulfur generated in situ from Fe(III) and sulfide promotes sulfidation of microscale zero-valent iron for superior Cr(VI) removal.

Herein, we compared the effect of different extra iron and sulfur precursors on the sulfidation efficiency, physicochemical properties, and reactivity of post-sulfidated microscale zero-valent iron (S-ZVI). S0@ZVI was synthesized from in situ S0 generated via reaction of Fe(III) with S2-, which resulted in 23-fold higher Cr(VI) removal compared with S0com/ZVI synthesized using commercial S0. The direct formation of FeSx film via reaction between S0 and ZVI played a crucial role in enhancing the removal of Cr(VI) by S0@ZVI, with 16- and 12-fold faster rates compared with FeS@ZVI and FeS2@ZVI prepared via precipitated reaction of Fe(II) with S2- and sulfur mixtures, respectively. The incorporated sulfur, sulfidation sequence, and sulfidation time determined the performance of S0@ZVI. A combination of batch experiments and kinetic models was used to determine the chemical composition of reduced Cr(VI) products. S0@ZVI immobilized Cr(VI) as Fe0.5Cr0.5(OH)3 via surface heterogeneous reactions, and partial Cr(VI) was homogeneously reduced to soluble Cr(acetate)3 or Fe0.75Cr0.25(OH)3(aq) by dissolved Fe(II). The insights gained from this study will facilitate the fabrication of highly reactive S-ZVI and elucidate the mechanism of Cr(VI) removal.

Morphology and structure of in situ FeS affect Cr(VI) removal by sulfidated microscale zero-valent iron with short-term ultrasonication.

The properties of sulfidated zero-valent iron (S-ZVI) are considered to be determined by the entire structure of Fe0 and FexSy as a whole, but few studies focus on the influence of the morphology and structure of the external FexSy layer on the performance of S-ZVI. In this study, after the sulfidation of microscale ZVI in acetate (HAc-NaAc) and 2-(N-morpholino) ethanesulfonic acid (MES) buffer solution, the S-mZVIHAc-NaAc surface presented the in situ growth of the FeS nanosheet, while the S-mZVIMES surface was dominated by agglomerated FeS sub-micron particles. Under short-term ultrasonication, S-mZVIHAc-NaAc was superior to removing Cr(VI) than S-mZVIMES, and the clearance of the passivation layer by ultrasound maximized the conductivity of the FeS nanosheet to strengthen the sulfidation contribution. However, agglomerated FeS particles were easily separated from S-mZVIMES by ultrasonication, resulting in the suppression of its sulfidation contribution. The removal of Cr(VI) by S-ZVI increased linearly with FeS content, and the chemical combination of FeS with ZVI had more significant synergy than their physical mixture. The FeS nanosheet with excellent conductivity and large vertical space benefited the generation of dissolved and surface-associated Fe(II) as electron donors and structural Fe(II) as the electron shuttle. Understanding the relationship between FeS structure and S-ZVI performance will pave a way for optimizing the synthesis of S-ZVI.

Humic acid addition sequence and concentration affect sulfur incorporation, electron transfer, and reactivity of sulfidated nanoscale zero-valent iron.

Nanoscale zero-valent iron (nZVI) is extensively used in field remediation and can be sulfidated in situ with sulfide or sulfate-reducing bacteria to enhance its performance. Humic acid (HA) widely exists in nature, but its influence on both the sulfidation process of nZVI and the reactivity of sulfidated nZVI (S-nZVI) has been rarely reported. Herein, we first synthesized S-nZVI by one-pot (S1-nZVI) and two-step (S2-nZVI) approaches with adding HA before (pre-added) or after (post-added) FexSy generation, respectively. Then, we evaluated their reactivity on Cr(VI) removal and analyzed the effects of HA on sulfidation regarding electron transfer resistance, sulfur incorporation, and structure characterization. Pre-added HA inhibited the Cr(VI) removal by S1-nZVI more seriously than by S2-nZVI and nZVI, and stronger inhibition was observed at higher HA concentrations. The inhibitory effect can be attributed mainly to the adsorbed HA increasing the impedance of the material and the free HA impeding the generation and deposition of FexSy. Different from the inhibition of pre-added HA at all studied HA concentrations, the Cr(VI) removal by both S1-nZVI and S2-nZVI with post-added HA was enhanced at specific HA concentrations. The reason for this phenomenon was that the dispersion and specific surface area of S-nZVI were improved, thereby offsetting the inhibition from both impedance increase and sulfur loss. This work suggests that the presence of HA can affect the sulfidation process and the property of S-nZVI, which is conducive to evaluating the performance of S-nZVI produced both by injection and in situ in the subsurface contaminant remediation.

Warrior's armor: Study on the aging of sulfidated micro-sized zero valent iron in air and its subsequent reactivity for chloramphenicol degradation in different acid systems.

In the practical application process, the reactivity and performance of ZVI-based materials when being placed in the air for a few days, weeks or months was worth studying. Most studies on the aging of ZVI were carried out in solution, only considering the reactivity of ZVI in aqueous solution. In this work, we investigated the degradation of chloramphenicol (CAP) in sulfuric acid (SA) and citric acid (CA) systems by sulfidated micro-sized zero-valent iron (S-mZVI) in air with different aging days. The results showed that with the increase of aging days in the air, the degradation effect of S-mZVI on CAP in different acid systems showed a similar trend (first increasing and then decreasing), the removal effect of S-mZVI on CAP reached the best within the aging time of 5-9 days. The degradation path of CAP could be divided into oxidation path and reduction path. The XPS and XRD characterization results of the materials on different aging days indicated that the characteristic peak of Fe3O4 was detected on the surface of the materials with the increase of aging days, which may be the reason for changing degradation efficiencies of CAP by S-mZVI for different aging days. In addition, in different systems of SA and CA, the degradation curves of CAP differed. This might be caused by two reasons: (1) CA could adsorb on S-mZVI while SA could not; (2) The initial pH of the CA system played a more significant effect on CAP degradation compared to that of the SA system.