Dehua Xia

Graphene and g-C3N4 nanosheets cowrapped elemental α-sulfur as a novel metal-free heterojunction photocatalyst for bacterial inactivation under visible-light.

A new class of metal-free heterojunction photocatalysts was prepared by wrapping reduced graphene oxide (RGO) and g-C3N4 (CN) sheets on crystals of cyclooctasulfur (α-S8). Two distinctive structures were fabricated by wrapping RGO and CN sheets in different orders. The first was RGO sheets sandwiched in heterojunction of CN sheets and α-S8 (i.e., CNRGOS8), while the second structure was the other way around (i.e., RGOCNS8). Both structures exhibited antibacterial activity under visible-light irradiation. CNRGOS8 showed stronger bacterial inactivation than RGOCNS8 in aerobic conditions. However, RGOCNS8 was more active than CNRGOS8 under anaerobic condition. A possible mechanism was proposed to explain the differences between photocatalytic oxidative inactivation and reductive inactivation. As a proof-of-concept, this work could offer new inroads into exploration and utilization of graphene sheets and g-C3N4 sheets cowrapped nanocomposites for environmental applications.

Mesoporous zinc ferrite: Synthesis, characterization, and photocatalytic activity with H2O2/visible light

Mesoporous ZnFe(2)O(4) (meso-ZnFe(2)O(4)) was synthesized by a hydrothermal process in which cetyltrimethylammonium bromide (CTAB) participates in the reaction to produce nanocrystals. Synthesized ZnFe(2)O(4) was characterized by energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, scanning electronic microscopy (SEM), transmission electron microscopy (TEM), and diffuse reflectance spectra (DRS). The meso-ZnFe(2)O(4) was resulted from the agglomeration of nanoparticles with size of 5-10nm. The photocatalytic activity of ZnFe(2)O(4) under visible light (λ>400 nm) was evaluated by the degradation of Acid Orange II (AOII) at different sintering temperatures, the amount of ZnFe(2)O(4), and the concentration of H(2)O(2). The photocatalytic degradation of AOII was almost complete within 2h in H(2)O(2)/visible light system. The high efficiency for AOII degradation was attributed to the strong absorption of ZnFe(2)O(4) in visible-light region and the generation of reactive OH by H(2)O(2) in the system. The involvement of OH in oxidizing AOII was examined by determining the photocurrent of ZnFe(2)O(4), [OH], and degradation rates using different scavengers. Organic compounds as intermediates of the degradation process were identified by LC/MS. Moreover, ZnFe(2)O(4) retained their degradation efficiencies for a series of repetitive batch runs, indicating the true photocatalytic process.

A Recyclable Mineral Catalyst for Visible-Light-Driven Photocatalytic Inactivation of Bacteria: Natural Magnetic Sphalerite

Motivated by recent studies that well-documented mineral photocatalyst for bacterial inactivation, a novel natural magnetic sphalerite (NMS) in lead-zinc deposit was first discovered and evaluated for its visible-light-driven (VLD) photocatalytic bactericidal properties. Superior to the reference natural sphalerite (NS), vibrating sampling magnetometeric (VSM) analysis revealed the ferromagnetic property of NMS, indicating its potential for easy separation after use. Under the irradiation of fluorescence tubes, NMS could inactivate 7 log10 Gram-negative Escherichia coli K-12 without any regrowth and metal ions leached out from NMS show no toxicity to cells. The cell destruction process starting from cell wall to intracellular components was verified by TEM. Some products from damaged cells such as aldehydes, ketones and carboxylic acids were identified by FTIR with a decrease of cell wall functional groups. The relative amounts of potassium ion leakage from damaged cells gradually increased from initial 0 to approximately constant concentration of 1000 ppb with increasing reaction time. Superoxide radical (•O2(-)) rather than hydroxyl radical (•OH) was proposed to be the primary reactive oxidative species (ROSs) responsible for E. coli inactivation by use of probes and electron spin resonance (ESR). H2O2 determined by fluorescence method is greatly involved in bacterial inactivation in both nonpartition and partition system. Multiple cycle runs revealed excellent stability of recycled NMS without any significant loss of activity. This study provides a promising natural magnetic photocatalyst for large-scale bacterial inactivation, as NMS is abundant, easily recycled and possessed an excellent VLD bacterial inactivation ability.

Visible-light-driven photocatalytic bacterial inactivation and the mechanism of zinc oxysulfide under LED light irradiation

Zinc oxysulfide (ZnO0.6S0.4) nanoparticles, prepared via a coprecipitation–calcination method, were used as an effective visible-light-driven (VLD) photocatalyst for the inactivation of a typical Gram-negative bacterium, Escherichia coli K-12 for the first time. An energy-saving white light emitting diode (LED) lamp was employed as the visible light (VL) source. Compared to the only UV-responsive pure ZnO and ZnS, the light active region of ZnO0.6S0.4 was expanded as far as 550 nm in the VL region. Significantly, the obtained ZnO0.6S0.4 nanoparticles showed considerable VLD photocatalytic bacterial inactivation activity under white LED irradiation. The mechanism of inactivation was investigated in-depth. Photogenerated holes (h+) and hydrogen peroxide (H2O2) were predominantly responsible for the bacterial inactivation. Moreover, H2O2 was evidenced to be derived only from electrons in the conduction band of ZnO0.6S0.4 in the present photocatalytic system. The integrated damage from the direct oxidation effect of the h+ and continuous accumulation of H2O2 resulted in a high bacterial inactivation efficiency of ZnO0.6S0.4 nanoparticles under visible white LED lamp irradiation. The destruction process of bacterial cells by the ZnO0.6S0.4 photocatalyst was also monitored. This was shown to begin with an attack of the cell membrane and then end in the release of intracellular components.

Photocatalytic nanomaterials for solar-driven bacterial inactivation: recent progress and challenges

Nanostructured photocatalysts have attracted ever-growing research attention in the application of solar energy for water disinfection. Over the past few decades, various photocatalytic nanomaterials have shown superior bacterial inactivation activity to their bulk counterparts, due to their enhanced interfacial charge separation and large surface area providing more active sites. This review presents an overview of the current research activities that focus on the development of nanostructured photocatalysts for water disinfection, including 0D, 1D and 2D (low-dimensional) nanostructures. The synthesized methods, characterization and photocatalytic bacterial inactivation performances are systematically summarized and discussed. In particular, a new conceptual direction to develop naturally occurring materials is highlighted, especially for accelerating practical industrial application. Moreover, the photocatalytic bacterial inactivation process and mechanisms based on the role of reactive species are briefly reviewed from a tutorial point of view. Finally, future research opportunities and challenges associated with the development of highly efficient and cost-effective nano-photocatalysts for water disinfection using inexhaustible solar energy are also pointed out.

Role of in Situ Resultant H2O2 in the Visible-Light-Driven Photocatalytic Inactivation of E. coli Using Natural Sphalerite: A Genetic Study

This study investigated how a natural sphalerite (NS) photocatalyst, under visible light irradiation, supports photocatalytic bacterial inactivation. This was done by comparing parent E. coli BW25113, and its two isogenic single-gene knock-out mutants, E. coli JW0797-1 (dps(-) mutant) and JW1721-1 (katE(-) mutant), where both dps and KatE genes are likely related to H2O2 production. NS could inactivate approximately 5-, 7- and 7-log of E. coli BW25113, JW0797-1, and JW1721-1 within 6 h irradiation, respectively. The two isogenic mutants were more susceptible to photocatalysis than the parental strain because of their lack of a defense system against H2O2 oxidative stress. The ability of in situ resultant H2O2 to serve as a defense against photocatalytic inactivation was also confirmed using scavenging experiments and partition system experiments. Studying catalase activity further revealed that in situ H2O2 played an important role in these inactivation processes. The destruction of bacterial cells from the cell envelope to the intracellular components was also observed using field emission-scanning electron microscopy. Moreover, FT-IR was used to monitor bacterial cell decomposition, key functional group evolution, and bacterial cell structures. This is the first study to investigate the photocatalytic inactivation mechanism of E. coli using single-gene deletion mutants under visible light irradiation.

A novel wet-scrubbing process using Fe(VI) for simultaneous removal of SO2 and NO

The objective of this work was to develop a novel wet-scrubbing process using Fe(VI) for the simultaneous removal of gaseous NO and SO(2). The oxidation of SO(2) and NO with Fe(VI) was studied in aqueous solution at alkaline pH (9.0-11.0). A stoichiometric molar ratio for NO and SO(2) oxidation with Fe(VI) was determined to be nearly 3.0. Sulfate and nitrate was identified as final products by ion chromatography from the reaction at pH 9.0-11.0. The feasibility of simultaneous removal of multiple gas pollutants with the continuous feeding of ferrate in lab-scale was investigated from the view of industrial application. It was found that the removal efficiency of NO and SO(2) was enhanced with the increase of Fe(VI) concentration, more than 90% NO removal efficiency and 100% SO(2) removal efficiency were achieved by wet-scrubbing process using Fe(VI) at room temperature and ambient atmosphere. The results demonstrate that Fe(VI) could be an effective wet-scrubbing agent for the simultaneous removal of NO and SO(2).

Dual Roles of Capsular Extracellular Polymeric Substances in Photocatalytic Inactivation of Escherichia coli: Comparison of E. coli BW25113 and Isogenic Mutants

ABSTRACT The dual roles of capsular extracellular polymeric substances (EPS) in the photocatalytic inactivation of bacteria were demonstrated in a TiO2-UVA system, by comparing wild-type Escherichia coli strain BW25113 and isogenic mutants with upregulated and downregulated production of capsular EPS. In a partition system in which direct contact between bacterial cells and TiO2 particles was inhibited, an increase in the amount of EPS was associated with increased bacterial resistance to photocatalytic inactivation. In contrast, when bacterial cells were in direct contact with TiO2 particles, an increase in the amount of capsular EPS decreased cell viability during photocatalytic treatment. Taken together, these results suggest that although capsular EPS can protect bacterial cells by consuming photogenerated reactive species, it also facilitates photocatalytic inactivation of bacteria by promoting the adhesion of TiO2 particles to the cell surface. Fluorescence microscopy and scanning electron microscopy analyses further confirmed that high capsular EPS density led to more TiO2 particles attaching to cells and forming bacterium-TiO2 aggregates. Calculations of interaction energy, represented by extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) potential, suggested that the presence of capsular EPS enhances the attachment of TiO2 particles to bacterial cells via acid-base interactions. Consideration of these mechanisms is critical for understanding bacterium-nanoparticle interactions and the photocatalytic inactivation of bacteria.

Natural magnetic pyrrhotite as a high-Efficient persulfate activator for micropollutants degradation: Radicals identification and toxicity evaluation

This study discusses the SO4- based process mediated by natural magnetic pyrrhotite (NP) for the degradation of refractory organic micropollutants. Complete degradation of 20μM phenol in distilled water (DW) was obtained within 20min using NP/PS (persulfate) system. Electron paramagnetic resonance spectra indicated aerobic and acidic conditions favored the generation of both SO4- and OH species, but only OH signal was survived at alkaline condition. The leaked Fe2+ and Fe(II) of NP collectively work to activate PS and generate surface and bulk SO4- and OH simultaneously. The identified intermediates indicate the transformation of benzene ring is the key step for phenol degradation by NP/PS system. Moreover, phenol degradation was greatly inhibited in surface water (SW, 29%) and wastewater (WW, 1%), but 25.9% and 17.5% of TOC removal were obtained in the SW and WW during NP/PS treatment, respectively. Additionally, the acute toxicity tests for NP/PS process exhibited a fluctuating trend depending on the water matrix, while the genotoxic activity analysis indicated that the treated phenol solutions were not genotoxic but cytotoxic. Overall, this study provides a solution to abate refractory organic micropollutants in water systems.

One-step synthesis of silicon carbide foams supported hierarchical porous sludge-derived activated carbon as efficient odor gas adsorbent

Hierarchical porous sludge-derived activated carbon coated on macroporous silicon carbide (SiC) foams substrate has been facilely fabricated via a simple one-step strategy by utilizing sludge as carbon source, and jointly using zinc chloride and hexadecanol as pore forming agents. The sludge-derived carbon has been confirmed to be hierarchical macro-meso-microporous structure based on detailed characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectra and nitrogen adsorption-desorption measurement. The adsorption tests showed that the hierarchical porous sludge-derived activated carbon fabricated by one-step pore-forming (zinc chloride and hexadecanol microemulsion mixture) possesses excellent adsorption capacity (259.9mgg-1, breakthrough time reach 90min and saturation end-time up to 140min) of methyl mercaptan (CH3SH). The excellent adsorption performance can be attributed to the macroporous SiC foam skeleton and the mesopores channel formed by nonionic surfactant hexadecanol micelles, as well as the micropores activated by ZnCl2 as odor capture sites. The proposed pore-forming strategy paves an avenue for the sludge disposal and even the development of bio-derived materials.