Aerobic removal of iodinated contrast medium by nano-sized zero-valent iron: A combination of oxidation and reduction

Nanoscale zero-valent iron reverses resistance of Pseudomonas aeruginosa to chloramphenicol

Zero-valent iron (ZVI) has been extensively studied for its capacity to remove various contaminants in the environments. However, whether ZVI affects bacterial resistance to antibiotics has not been fully explored. Herein, it was unexpected that, compared with microscale ZVI (mZVI), nanoscale ZVI (nZVI) facilitated the susceptibility of Pseudomonas aeruginosa (P. aeruginosa) to chloramphenicol (CAP), with a decrease in the minimal inhibitory concentration (MIC) of about 60 %, demonstrating a nanosize-specific effect. nZVI enhanced CAP accumulation in P. aeruginosa via inhibitory effect on efflux pumps activated by MexT, thus conferring the susceptibility of P. aeruginosa to CAP. Circular dichroism spectroscopy revealed that the structure of MexT was changed during the evolution. More importantly, molecular dynamic simulations uncovered that, once the structure of MexT changed, it would be more likely to interact with nZVI, resulting in more serious changes in its secondary structure, which was consistent with the increasing susceptibility of P. aeruginosa to CAP. Collectively, this study elucidated the size-specific effect and the underlying mechanism of ZVI on the bacterial evolution of susceptibility toward antibiotics, highlighting the potentials of nZVI-based technologies on the prevention of bacterial resistance to antibiotics, one of the most important issue for globally public health.

Ferrous ion enhanced Fenton-like degradation of emerging contaminants by sulfidated nanosized zero-valent iron with pH insensitivity

In this study, the performance and mechanism of the integrated sulfidated nanosized zero-valent iron and ferrous ions (S-nZVI/Fe2+) system for oxygen activation to remove emerging contaminants (ECs) were comprehensively explored. The S-nZVI/Fe2+ system exhibited a 2.4-8.2 times of increase in the pseudo-first order kinetic rate constant for the oxidative degradation of various ECs compared to the S-nZVI system under aerobic conditions, whereas negligible removal was observed in both nZVI and nZVI/Fe2+ systems. Moreover, remarkable EC mineralization efficiency and benign detoxification capacity were also demonstrated in the S-nZVI/Fe2+ system. We revealed that dosing Fe2+ promoted the corrosion of S-nZVI by maintaining an acidic solution pH, which was conducive to O2 activation by dissolved Fe2+ and surface-absorbed Fe(II) to produce •OH. Furthermore, the generation of H* was enhanced for the further reduction of Fe(III) and H2O2 to Fe(II) and •O2-, resulting in the improvement of consecutive single-electron O2 activation for •OH production. Additionally, bisphenol A (BPA) degradation by S-nZVI/Fe2+ was positively correlated with the S-nZVI dosage, with an optimum S/Fe molar ratio of 0.15. The Fenton-like degradation process by S-nZVI/Fe2+ was pH-insensitive, indicating its robust performance over a wide pH range. This study provides valuable insights for the practical implementation of nZVI-based technology in achieving high-efficiency removal of ECs from water.

Removal of Iodine-Containing X-ray Contrast Media from Environment: The Challenge of a Total Mineralization

Iodinated X-ray contrast media (ICM) as emerging micropollutants have attracted considerable attention in recent years due to their high detected concentration in water systems. It results in environmental issues partly due to the formation of toxic by-products during the disinfection process in water treatment. Consequently, various approaches have been investigated by researchers in order to achieve ICM total mineralization. This review discusses the different methods that have been used to degrade them, with special attention to the mineralization yield and to the nature of formed by-products. The problem of pollution by ICM is discussed in the first part dedicated to the presence of ICM in the environment and its consequences. In the second part, the processes for ICM treatment including biological treatment, advanced oxidation/reductive processes, and coupled processes are reviewed in detail. The main results and mechanisms involved in each approach are described, and by-products identified during the different treatments are listed. Moreover, based on their efficiency and their cost-effectiveness, the prospects and process developments of ICM treatment are discussed.

Detection and remediation of pollutants to maintain ecosustainability employing nanotechnology: A review

Environmental deterioration due to anthropogenic activities is a threat to sustainable, clean and green environment. Accumulation of hazardous chemicals pollutes soil, water and air and thus significantly affects all the ecosystems. This article highlight the challenges associated with various conventional techniques such as filtration, absorption, flocculation, coagulation, chromatographic and mass spectroscopic techniques. Environmental nanotechnology has provided an innovative frontier to combat the aforesaid issues of sustainable environment by reducing the non-requisite use of raw materials, electricity, excessive use of agrochemicals and release of industrial effluents into water bodies. Various nanotechnology based approaches including surface enhance scattering, surface plasmon resonance; and distinct types of nanoparticles like silver, silicon oxide and zinc oxide have contributed significantly in detection of environmental pollutants. Biosensing technology has also gained significant attention for detection and remediation of pollutants. Furthermore, nanoparticles of gold, ferric oxide and manganese oxide have been used for the on-site remediation of antibiotics, organic dyes, pesticides, and heavy metals. Recently, green nanomaterials have been given more attention to address toxicity issues of chemically synthesized nanomaterials. Hence, nanotechnology has provided a platform with tremendous applications to have sustainable environment for present as well as future generations. This review article will help to understand the fundamentals for achieving the goals of sustainable development, and healthy environment.

Highly efficient nano-Fe/Cu bimetal-loaded mesoporous silica Fe/Cu-MCM-41 for the removal of Cr(VI): Kinetics, mechanism and performance

Zero valent iron (Fe⁰) can reduce Cr(VI) in water, where Fe⁰ and Fe(Ⅱ) are possible electron donors, but passivation and aggregation easily occur to Fe⁰. To improve the performance of Fe⁰, a new hybridization strategy of Fe/Cu bimetal and silica-based mesoporous molecular sieve MCM-41 for the removal of Cr(VI) from water has been proposed. The results show that the two-dimensional mesoporous structure of MCM-41 can provide skeleton support for Fe⁰, improve the mass transfer rate, and overcome the aggregation bottleneck of Fe⁰. The Cr(VI) removal rate reached 98.98% (pH = 2) after 40 min. The analytical results revealed Cr(VI) removal process: Cr(VI) adsorbed onto Fe/Cu-MCM-41 by electrostatic attraction and other molecular inter-atomic forces. The second metal, Cu, can inhibit the passivation of Fe⁰ and promote Fe(Ⅱ)through the formation of Fe/Cu battery, thereby promoting the electron transfer. The resulting Cr(Ⅲ) is precipitated as FeCr₂O₄ and Cr_xFe_1-x(OH)₃.

The synergistic effect of attapulgite in the highly enhanced photoreduction of Cr(VI) by oxalic acid in aqueous solution

Attapulgite (ATP), a widely existed clay in nature, was firstly and successfully applied to enhance the photoreduction of highly toxic Cr(VI) by oxalic acid (Ox). In ATP + Ox + UV system, batch effects (Ox concentration, initial Cr(VI) concentration, ATP dosage, and reusability of ATP) were investigated. By studying the impact of the initial pH in the solution, the change of pH and Fe species concentration as well as Ox concentration during the reaction, the free radical scavenging test, and the role of ATP, the mechanism of Cr(VI) removal by ATP + Ox + UV system was revealed. The methyl orange (MO) removal of ATP + Ox + UV system was also inspected. The results indicated that ATP showed the obvious enhancement in efficient photoreduction of Cr(VI) by Ox in water. The Fe and Si components in ATP played an important role in Cr(VI) removal by ATP + Ox + UV system: most of Cr(VI) was reduced by Fe(II) and CO₂^•‒ produced by the Fe(III)-Ox complex from the dissolved Fe component in ATP under UV irradiation. Some of Cr(VI) was reduced by e^- and CO₂^•‒ from the oxidation of Ox by h⁺ generated by the photocatalyzed SiO₂ in ATP. Furthermore, ATP + Ox + UV system also showed excellent MO removal performance, indicating the great potential in practical applications.

Mechanistic role of nitrate anion in TCE dechlorination by ball milled ZVI and sulfidated ZVI: Experimental investigation and theoretical analysis

Mechanistic role of NO₃^- in trichloroethylene (TCE) dechlorination by ball milled, micro-scale sulfidated and unsulfidated ZVI (e.g., S-mZVI^bm and mZVI^bm) was explored through experiments and density functional theory (DFT) calculations. Sulfidation inhibited NO₃^- reduction by mZVI^bm as S weakened its interaction with NO₃^-. mZVI^bm reduced NO₃^- within 2 h. This just resulted in a short-term electron competition during the dechlorination process by mZVI^bm and hardly affected its sluggish dechlorination kinetics (complete TCE dechlorination in 11 d). On the contrary, NO₃^- suppressed TCE dechlorination by S-mZVI^bm. This was attributed to that inhibited NO₃^- reduction by S-mZVI^bm (40 % reduction in 6 h) induced continuous electron competition with TCE during the time span of its dechlorination by S-mZVI^bm. NO₃^- reduction was also observed to facilitate formation/crystallization of Fe₃O₄ on both ZVI particles, promoting dechlorination by mZVI^bm after 4 d while not taking effect to the S-mZVI^bm/TCE system, as its dechlorination time was too short for the surface of S-mZVI^bm to transform. This observation has important implication on groundwater remediation by ZVI or sulfidated ZVI PRBs under a scenario of upgradient anthropogenic release of NO₃^-.

Biological treatment of non-biodegradable azo-dye enhanced by zero-valent iron (ZVI) pre-treatment

Zero-valent iron (ZVI) pre-treatment in sequential strategy for removal of non-biodegradable azo-dye Orange II by activated-sludge was quantitatively examined. The decolorization and TOC (total organic carbon) removal of Orange II by ZVI pre-treatment were examined in the ranges of pH from 3 to 11 and ZVI dosage from 500 to 2000 mgL^-1. While the decolorization was enhanced with decreasing pH and the optimal pH for decolorization was found at pH 3, the TOC removal rate at pH 3 remained at 22.2% and the maximum TOC removal rate of 78.2% was obtained at pH 4. The decolorization and TOC removal of Orange II were monotonously increased with increasing ZVI dosage. To quantify the ZVI pre-treatment, the contributions of redox degradation, complexation/precipitation and adsorption to TOC removal by ZVI were defined. Novel kinetic models for the ZVI pre-treatment and activated-sludge post-treatment were developed. The proposed kinetic models satisfactorily predicted the transitional behaviors of the ZVI pre-treatment and activated-sludge post-treatment and the contributions of redox degradation, complexation/precipitation and adsorption to TOC removal by the ZVI pre-treatment. The complete removal of non-biodegradable azo-dye Orange II of 300 mgL^-1 was accomplished by 78.2% removal after 360 min ZVI pre-treatment with the ZVI dosage of 1000 mgL^-1 at pH 4 and subsequently 21.8% removal after 480 min activated-sludge post-treatment. The ZVI pre-treatment integrated with activated-sludge post-treatment was proved to be an effective strategy for treating non-biodegradable pollutants.

Highly efficient remediation of groundwater co-contaminated with Cr(VI) and nitrate by using nano-Fe/Pd bimetal-loaded zeolite: Process product and interaction mechanism

Hexavalent chromium and nitrate co-contaminated groundwater remediation are attracting extensive attention worldwide. However, the transformation pathways of chromium and nitrate and the interplay mechanism between them remain unclear. In this work, zeolite-supported nanoscale zero-valent iron/palladium (Z-Fe/Pd) was synthesized and used for the first time to simultaneously remediate Cr(VI) and nitrate. Transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses confirmed that nanoscale zero-valent iron/palladium was successfully loaded onto zeolite and it exhibited good dispersibility and oxidation resistance. Results of batch experiments showed that the Cr(VI) and nitrate removal efficiencies decreased from 95.5% to 91.5% to 45% and 73%, respectively, with the initial solution pH increasing from 3.0 to 8.0. The removal rates and efficiencies of Cr(VI) and nitrate under anoxic conditions were higher than those under open atmosphere because the dissolved oxygen diminished the electron selectivity toward the target pollutants. Moreover, the presence of Cr(VI) inhibited nitrate reduction by forming Fe(III)-Cr(III) hydroxide to impede electron transfer. Cr(VI) removal was promoted by nitrate, within limits, by balancing the consumption and generation rate of Fe₃O₄, which enhanced electron migration from the Fe(0) core to the external surface. The removal capacities of Cr(VI) and nitrate reached 121 and 95.5 mg g^-1, respectively, which were superior to the removal capacities of similar materials. Results of product identification, XRD, and XPS analyses of spent Z-Fe/Pd indicated that the reduction of Cr(VI) was accompanied by adsorption and co-precipitation, whereas the reduction of nitrate was catalyzed by the synergism of Fe(0) and Pd(0). An alternative to the simultaneous remediation of Cr(VI) and nitrate from groundwater under anoxic conditions is provided.