Zhi-Hua Yuan

Interaction of silver nanoparticles with pure nitrifying bacteria

In this study, Nitrosomonas europaea ATCC 19718 was exposed to silver nanoparticles (AgNPs) of different particle size (7±3 and 40±14nm) and different coatings (polyvinyl alcohol and adenosine triphosphate disodium). For all different AgNPs used in the study, large aggregates were gradually formed after addition of AgNPs into the media containing N. europaea. The scanning electron microscopy and energy dispersive X-ray spectroscopy of the microstructures suggested that bacterial cells and electrolytes had significant effects on AgNP aggregation. Size- and coating-dependent inhibition of ammonia oxidation by AgNPs was observed, and our analysis suggested that the inhibition was not only due to the released dissolved silver, but also the dispersity of AgNPs in the culture media. Electron microscopy images showed AgNPs could cause the damage of cell wall of N. europaea and make the nucleoids disintegrated and condensed next to cell membrane. Surface-enhanced Raman scattering signals also implied the damage of cell membrane caused by AgNPs. Further protein expression analysis revealed that AgNPs would inhibit important protein functions, including biosynthesis, gene expression, energy production and nitrification to further cause toxicity to N. europaea. Our findings explain the susceptibility of N. europaea to inhibition by AgNPs and the possible interaction between each other. Future research is needed to characterize these effects in more complex cultures and media such as activated sludge and wastewater.

In Situ Study of the Antibacterial Activity and Mechanism of Action of Silver Nanoparticles by Surface-Enhanced Raman Spectroscopy

Silver nanoparticles (Ag NPs) are extensively used as an antibacterial additive in commercial products and their release has caused environmental risk. However, conventional methods for the toxicity detection of Ag NPs are very time consuming and the mechanisms of action are not clear. We developed a new, in situ, rapid, and sensitive fingerprinting approach, using surface-enhanced Raman spectroscopy (SERS), to study the antibacterial activity and mechanism of Ag NPs of 80 and 18 nm (Ag80 and Ag18, respectively), by using the strong electromagnetic enhancement generated by Ag NPs. Sensitive spectra changes representing various biomolecules in bacteria were observed with increasing concentrations of Ag NPs. They not only allowed SERS to monitor the antibacterial activity of Ag NPs of different sizes in different water media but also to study the antibacterial mechanism at the molecular level. Ag18 were found to be more toxic than Ag80 in water, but their toxicity declined to a similar level in the PBS medium. The antibacterial mechanism was proposed on the basis of a careful identification of the chemical origins by comparing the SERS spectra with model compounds. The dramatic change in protein, hypoxanthine, adenosine, and guanosine bands suggested that Ag NPs have a significant impact on the protein and metabolic processes of purine. Finally, by adding nontoxic and SERS active Au NPs, SERS was successfully utilized to study the action mode of the NPs unable to produce an observable SERS signal. This work opens a window for the future extensive SERS studies of the antibacterial mechanism of a great variety of non-SERS-active NPs.

Removal of silver nanoparticles by coagulation processes

Commercial use of silver nanoparticles (AgNPs) will lead to a potential route for human exposure via potable water. Coagulation followed by sedimentation, as a conventional technique in the drinking water treatment facilities, may become an important barrier to prevent human from AgNP exposures. This study investigated the removal of AgNP suspensions by four regular coagulants. In the aluminum sulfate and ferric chloride coagulation systems, the water parameters slightly affected the AgNP removal. However, in the poly aluminum chloride and polyferric sulfate coagulation systems, the optimal removal efficiencies were achieved at pH 7.5, while higher or lower of pH could reduce the AgNP removal. Besides, the increasing natural organic matter (NOM) would reduce the AgNP removal, while Ca(2+) and suspended solids concentrations would also affect the AgNP removal. In addition, results from the transmission electron microscopy and X-ray diffraction showed AgNPs or silver-containing nanoparticles were adsorbed onto the flocs. Finally, natural water samples were used to validate AgNP removal by coagulation. This study suggests that in the case of release of AgNPs into the source water, the traditional water treatment process, coagulation/sedimentation, can remove AgNPs and minimize the silver ion concentration under the well-optimized conditions.

Reaction of silver nanoparticles in the disinfection process

This study investigated the dissolution, aggregation, and reaction kinetics of silver nanoparticles (AgNPs) with the three types of water disinfectants (ultraviolet, sodium hypochlorite, and ozone) under the different conditions of pH, ionic strength, or humic acid (HA). The physicochemical changes of AgNPs were measured by using UV-Vis spectroscopy, transmission electron microscopy, and inductively coupled plasma optical emission spectrometer. The results showed that when AgNPs contacted the disinfectants, oxidative dissolution was the primary reaction. In addition, the reaction kinetics studies revealed that the reaction rate of AgNPs with disinfectants was significantly influenced by different disinfectants along with different pH and the presence of sodium nitrate and HA. Our research demonstrated the potential effect of disinfectants on AgNPs, which will improve our understanding of the fate of AgNPs in the disinfection processes in the water and wastewater treatment plant.

Assessment of the fate of silver nanoparticles in the A2O-MBR system

In this study, we employed a bench scale A(2)O-MBR (anaerobic-anoxic-oxic membrane bioreactor) system to systematically investigate the behavior and distribution of silver nanoparticles (AgNPs) in the activated sludge. The results showed that AgNPs would aggregate and form Ag-sulfur complexes in the activated sludge, and the dissolved silver only reached 13.6 μg/L when AgNPs of 5mg/L was spiked into the A(2)O-MBR. The long-term mass balance analysis showed that most of the silver contents were accumulated in the bioreactor and wasted excess sludge. Only a small fraction (less than 0.5%) of silver could get across the hollow fiber membranes with 0.1 μm nominal pore size in the effluent. In addition, the comparison between total AgNP concentration in aerobic sludge supernatant and effluent suggested that the membrane modules played a role in controlling the discharge of AgNPs into the effluent, especially under a higher influent concentration of AgNPs. Our results also showed that the adsorbed AgNPs or silver complexes in activated sludge still could release dissolved silver at the ambient pH. Thus, since activated sludge could be a sink for AgNPs, the risks of AgNPs in wasted excess sludge during utilization and disposal should be further studied.

Enhanced adsorption of arsenite from aqueous solution by an iron-doped electrospun chitosan nanofiber mat: Preparation, characterization and performance

A novel iron-doped chitosan electrospun nanofiber mat (Fe@CTS ENM) was synthesized, which was proven to be effective for the removal of arsenite (As(III)) from water at neutral pH condition. The physiochemical properties and adsorption mechanism were explored by SEM-EDS and X-ray photoelectron spectroscopy (XPS). Batch adsorption experiments were carried out to evaluate the As(III) adsorption performance of the Fe@CTS ENM with various process parameters, such as adsorbent dose, solution pH, initial As(III) concentration, contact time, ionic strength, coexisting anions, and natural organic matter. The experimental results indicated that the maximum adsorption capacity was up to 36.1 mg g-1. Especially, when the adsorbent dosage was higher than 0.3 g L-1, the As(III) concentration was reduced from 100 µg L-1 to less than 10 µg L-1, which indicated the Fe@CTS ENM could effectively remove trace As(III) from water over a wide pH range (from 3.3 to 7.5). Kinetics study demonstrated that the adsorption equilibrium was achieved within 2.0 h, corresponding to a fast uptake of As(III). The presence of common co-ions and humic acid had little effect on the As(III) adsorption. XPS analysis suggested that the FeO, COH, COC and CN groups on the adsorbent surface play dominant roles in the adsorption of As(III). Adsorption-desorption regeneration test further demonstrated that no appreciable loss in the adsorption capacities was observed, which confirmed that the Fe@CTS ENM maintained a desirable life cycle that was free of complex synthesis processes, expensive and toxic materials, qualifying it as an efficient and low-cost As(III) adsorbent.

Preparation, characterization and performance of an electrospun carbon nanofiber mat applied in hexavalent chromium removal from aqueous solution

Hexavalent chromium, Cr(VI), a highly toxic oxyanion known as a carcinogen and mutagen, is an issue of concern due to its adverse impact on human health. Therefore, development of effective technologies and/or materials for Cr(VI) removal from water has been of great interest for researchers. In this study, an electrospun carbon nanofiber (CNF) mat was prepared via electrospinning polyacrylonitrile (PAN), followed by thermal pre-oxidation and carbonization. Scanning electron microscopy (SEM) observation showed that the fiber diameter of the CNF with carbonization temperature of 950°C (CNF950) was about 266 nm. Potentiometric titration analysis demonstrated that the point of zero charge pH (pHpzc) of CNF950 was around 7.93. CNF950 demonstrated high adsorption capacity and fast adsorption kinetics for Cr(VI) at pH < 3. Langmuir isotherm calculations showed that the maximum adsorption capacity of Cr(VI) on CNF950 was 118.8 mg/g at pH 2. The adsorption isotherm of Cr(VI) on CNF950 was well described by the Redlich-Peterson model, revealing that Cr(VI) adsorption was the result of a combination of monolayer and multilayer adsorption, depending on the initial Cr(VI) concentration. Solution pH greatly affected Cr(VI) adsorption onto CNF950 due to the electrostatic interaction, and the adsorption capacity was relatively high when pH was below 3. X-ray photoelectron spectroscopy (XPS) analysis revealed that the removal of Cr(VI) might be the result of a combination of redox reaction and electrostatic adsorption. The adsorption-saturated CNF950 could be regenerated by NaOH solution. This study extends the potential applicability of electrospun CNF mats for Cr(VI)-contaminated water purification.