Understanding the nanoscale adhesion forces between the fungal pathogen Candida albicans and antimicrobial zinc-based layered double hydroxides using single-cell and single-particle force spectroscopy.

Antifungal resistance has become a very serious concern, and Candida albicans is considered one of the most opportunistic fungal pathogens responsible for several human infections. In this context, the use of new antifungal agents such as zinc-based layered double hydroxides to fight such fungal pathogens is considered one possible means to help limit the problem of antifungal resistance. In this study, we show that ZnAl LDH nanoparticles exhibit remarkable antifungal properties against C. albicans and cause serious cell wall damage, as revealed by growth tests and atomic force microscopy (AFM) imaging. To further link the antifungal activity of ZnAl LDHs to their adhesive behaviors toward C. albicans cells, AFM-based single-cell spectroscopy and single-particle force spectroscopy were used to probe the nanoscale adhesive interactions. The force spectroscopy analysis revealed that antimicrobial ZnAl LDHs exhibit specific surface interactions with C. albicans cells, demonstrating remarkable force magnitudes and adhesion frequencies in comparison with non-antifungal negative controls, e.g., Al-coated substrates and MgAl LDHs, which showed limited interactions with C. albicans cells. Force signatures suggest that such adhesive interactions may be attributed to the presence of agglutinin-like sequence (Als) adhesive proteins at the cell wall surface of C. albicans cells. Our findings propose the presence of a strong correlation between the antifungal effect provided by ZnAl LDHs and their nanoscale adhesive interactions with C. albicans cells at both the single-cell and single-particle levels. Therefore, ZnAl LDHs could interact with C. albicans fungal pathogens by specific adhesive interactions through which they adhere to fungal cells, leading to their damage and subsequent growth inhibition.

Efficiency of saturated vertical-flow filters planted with Panicum maximum reeds for passive wastewater treatment.

A vertical-flow unit containing four filters filled with shale was used to study the removal of phosphorous, nitrogen and organic matter of an urban residual wastewater during a period of 90 days. The influence of both the shale granulometry and the plant density of Panicum Maximum were studied. The decrease of the shale granulometry led to a significant improvement of all the measured parameters, while the presence of plants did only influence the phosphate retention with a lower extent. By comparing the results to previous studies, we hypothesised that the effect of the root system of Panicum maximum would be different depending on the size and the depth of the reactors. For practical application, adjusting the material granulometry was proposed to be the most important parameter for improving the filtration efficiency. Concomitantly, adjusting the plant density helps to control the clogging percentage of the filters.

Synthesis and characterization of a new monometallic layered double hydroxide using manganese.

This article reports for the first time the synthesis of an LDH using only manganese as the divalent and trivalent metallic ion. Analysis of the pH, redox potential, and chemical composition during the oxidation of a manganese basic salt using persulfate indicates the oxidation of 1/3 of the initial MnII ions, in agreement with the paramagnetic structure and XPS analysis. Infrared, Raman spectra and thermogravimetric analysis results were similar to the ones obtained with Fe-LDH also known as green rust. X-Ray diffractograms and Rietveld refinement were used to determine the structure of this solid. Thermodynamic considerations predict that this solid could reduce nitrate into gaseous nitrogen without further reduction to ammonium or ammonia unlike what is observed for Fe-LDH.

Understanding the role of surface interactions in the antibacterial activity of layered double hydroxide nanoparticles by atomic force microscopy.

Understanding the mechanisms of the interactions between zinc-based layered double hydroxides (LDHs) and bacterial surfaces is of great importance to improve the efficiency of these antibiotic-free antibacterial agents. In fact, the role of surface interactions in the antibacterial activity of zinc-based LDH nanoparticles compared to that of dissolution and generation of reactive oxygen species (ROS) is still not well documented. In this study, we show that ZnAl LDH nanoparticles exhibit a strong antibacterial effect against Staphylococcus aureus by inducing serious cell wall damages as revealed by the antibacterial activity tests and atomic force microscopy (AFM) imaging, respectively. The comparison of the antibacterial properties of ZnAl LDH nanoparticles and micron-sized ZnAl LDHs also demonstrated that the antibacterial activity of Zn-based LDHs goes beyond the simple dissolution into Zn2+ antibacterial ions. Furthermore, we developed an original approach to functionalize AFM tips with LDH films in order to probe their interactions with living S. aureus cells by means of AFM-based force spectroscopy (FS). The force spectroscopy analysis revealed that antibacterial ZnAl LDH nanoparticles show specific recognition of S. aureus cells with high adhesion frequency and remarkable force magnitudes. This finding provides a first insight into the antibacterial mechanism of Zn-based LDHs through direct surface interactions by which they are able to recognize and adhere to bacterial surfaces, thus damaging them and leading to subsequent growth inhibition.

Direct contact, dissolution and generation of reactive oxygen species: How to optimize the antibacterial effects of layered double hydroxides.

Infections by pathogenic bacteria have been threatening several fields as food industries, agriculture, textile industries and healthcare products. Layered double hydroxides materials (LDHs), also called anionic clays, could be utilized as efficient antibacterial materials due to their several interesting properties such as ease of synthesis, tunable chemical composition, biocompatibility and anion exchange capacity. Pristine LDHs as well as LDH-composites including antibacterial molecules and nanoparticles loaded-LDHs were proven to serve as efficient antibacterial agents against various Gram-positive and Gram-negative bacterial strains. The achieved antibacterial effect was explained by the following mechanisms: (1) Direct contact between the materials and bacterial cells driven by electrostatic interactions between positively charged layers and negatively charged cell membranes, (2) Dissolution and gradual release over time of metallic ions or antibacterial molecules, (3) Generation of reactive oxygen species.

Birnessite: A New Oxidant for Green Rust Formation.

Iron and manganese are ubiquitous in the natural environment. FeII-FeIII layered double hydroxide, commonly called green rust (GR), and MnIII-MnIV birnessite (Bir) are also well known to be reactive solid compounds. Therefore, studying the chemical interactions between Fe and Mn species could contribute to understanding the interactions between their respective biogeochemical cycles. Moreover, ferromanganese solid compounds are potentially interesting materials for water treatment. Here, a {Fe(OH)2, FeIIaq} mixture was oxidized by Bir in sulphated aqueous media in the presence or absence of dissolved O2. In oxic conditions for an initial FeII/OH- ratio of 0.6, a single GR phase was obtained in a first step; the oxidation kinetics being faster than without Bir. In a second step, GR was oxidised into various final products, mainly in a spinel structure. A partial substitution of Fe by Mn species was suspected in both GR and the spinel. In anoxic condition, GR was also observed but other by-products were concomitantly formed. All the oxidation products were characterized by XRD, XPS, and Mössbauer spectroscopy. Hence, oxidation of FeII species by Bir can be considered as a new chemical pathway for producing ferromanganese spinels. Furthermore, these results suggest that Bir may participate in the formation of GR minerals.

Stability of magnetic LDH composites used for phosphate recovery.

Layered double hydroxides (LDH) and their magnetic composites have been intensively investigated as recyclable high-capacity phosphate sorbents but with little attention to their stability as function of pH and phosphate concentration. The stability of a Fe3O4@SiO2-Mg3Fe LDH P sorbent as function of pH (5-11) and orthophosphate (Pi) concentration (1-300 mg P/L) was investigated. The composite has high adsorption capacity (approx. 80 mg P/g) at pH 5 but with fast dissolution of the LDH component resulting in formation of ferrihydrite as evidenced by Mössbauer spectroscopy. At pH 7 more than 60% of the LDH dissolves within 60 min, while at alkaline pH, the LDH is more stable but with less than 40% adsorption capacity as compared to pH 5. The high Pi sorption at acid to neutral pH is attributed to Pi bonding to the residual ferrihydrite. Under alkaline conditions Pi is sorbed to LDH at low Pi concentration while magnesium phosphates form at higher Pi concentration evidenced by solid-state 31P MAS NMR, powder X-ray diffraction and chemical analyses. Sorption as function of pH and Pi concentration has been fitted by a Rational 2D function allowing for estimation of Pi sorption and precipitation. In conclusion, the instability of the LDH component limits its application in wastewater treatment from acid to alkaline pH. Future use of magnetic LDH composites requires substantial stabilisation of the LDH component.

Nitrate reduction by mixed iron(II-III) hydroxycarbonate green rust in the presence of phosphate anions: the key parameters influencing the ammonium selectivity.

The reduction of nitrate anions by a mixed Fe(II)-Fe(III) carbonated green rust (GR) in aqueous medium is studied as a function of the initial pH and the initial concentrations of iron, phosphate and nitrate. The influence of these parameters on the fraction of nitrate removed and the production of ammonium is investigated by the help of statistical experimental designs. The goal is to determine experimental conditions that maximize the fraction of NO3(-) removed and concomitantly minimize the production of NH4(+). Increasing the phosphate concentration relatively to the initial Fe(II) concentration inhibits the reduction of nitrate probably due to a surface saturation of the lateral sites of the GR crystals. The kinetics of the reaction is greatly enhanced by increasing the initial pH at 10.5, however it leads to a global increase of the NH4(+) production. A partial saturation of the surface sites by phosphate leads to a global decrease of selectivity of the reaction towards ammonium. The evolution of the ratio of the NH4(+) concentration to the Fe(II) concentration confirms that the NO3(-) species are only partially transformed into ammonium. Interestingly at an initial pH of 7.5, the selectivity of the reaction towards NH4(+) is often lower than ∼30%. The reduction of nitrate by carbonated GR differs from the behavior of other GRs incorporating Cl(-), F(-) and SO4(2-) anions that fully transform nitrate into ammonium. Finally, if GR is intended to be used during a passive water denitrification process, complementary dephosphatation and ammonium treatments should be considered.

Abiotic process for Fe(II) oxidation and green rust mineralization driven by a heterotrophic nitrate reducing bacteria (Klebsiella mobilis).

Green rusts (GRs) are mixed Fe(II)-Fe(III) hydroxides with a high reactivity toward organic and inorganic pollutants. GRs can be produced from ferric reducing or ferrous oxidizing bacterial activities. In this study, we investigated the capability of Klebsiella mobilis to produce iron minerals in the presence of nitrate and ferrous iron. This bacterium is well-known to reduce nitrate using an organic carbon source as electron donor but is unable to enzymatically oxidize Fe(II) species. During incubation, GR formation occurred as a secondary iron mineral precipitating on cell surfaces, resulting from Fe(II) oxidation by nitrite produced via bacterial respiration of nitrate. For the first time, we demonstrate GR formation by indirect microbial oxidation of Fe(II) (i.e., a combination of biotic/abiotic processes). These results therefore suggest that nitrate-reducing bacteria can potentially contribute to the formation of GR in natural environments. In addition, the chemical reduction of nitrite to ammonium by GR is observed, which gradually turns the GR into the end-product goethite. The nitrogen mass-balance clearly demonstrates that the total amount of ammonium produced corresponds to the quantity of bioreduced nitrate. These findings demonstrate how the activity of nitrate-reducing bacteria in ferrous environments may provide a direct link between the biogeochemical cycles of nitrogen and iron.

Hydrolysis of mixed Ni(2+)-Fe3+ and Mg(2+)-Fe3+ solutions and mechanism of formation of layered double hydroxides.

The hydrolytic behavior of mixed metallic solutions containing Ni(2+)-Fe(3+) and Mg(2+)-Fe(3+) has been studied with respect to the relative proportion of the divalent and trivalent cations in solution as well as the quantity of NaOH added. The combination of X-ray diffraction and vibrational spectroscopy provides a deep insight into both the nature of the phases and the structure of the formed LDH. The relative abundance of each phase is determined by using a mass balance diagram and is in good agreement with the solid characterization. We showed that the slow hydrolysis of mixed metallic solutions involved first the precipitation of Fe(3+) to form an akaganeite phase, and then the formation of a precursor on the iron oxyhydroxide surface, which transforms into LDH by diffusion of Fe(III) species from the akaganeite phase to the precursor. Interestingly, whatever the iron content in solution, the same fraction of Fe(III) is incorporated into the LDH phase which is correlated to the nature of the formed precursor. For Ni(2+)-Fe(3+) solution, the precursor is an α-Ni hydroxide, which formed a LDH phase with a very low iron content (x(layer) = 0.1), but a high charge density provided by structural hydroxyl default. This result unambiguously demonstrated that the LDH phase is formed from the precursor structure. For Mg(2+)-Fe(3+) solution, the precursor is structurally equivalent to a ß-Mg(OH)2 phase, leading to a LDH with a higher x(layer) value of ~0.2. In both cases, at the end of the titration experiments, a mixture of different phases was systematically observed. Hydrothermal treatment allows the recovery of a pure LDH phase exclusively for the Ni(2+)-Fe(3+) solution.

Fenton-like oxidation and mineralization of phenol using synthetic Fe(II)-Fe(III) green rusts.

BACKGROUND, AIM, AND SCOPE: In literature, the environmental applications of green rust (GR) have mainly been pointed out through the reduction of inorganic contaminants and the reductive dechlorination of chlorinated organics. However, reactions involving GR for the oxidation and mineralization of organic pollutants remain very scantly described. In this study, the ability of three synthetic Fe(II)-Fe(III) green rusts, GR(CO (3)(2-)), GR(SO(4)(2-)), and GR(Cl(-)), to promote Fenton-like reaction was examined by employing phenol as a model pollutant. Unlike the traditional Fenton's reagent (dissolved Fe(II) + H(2)O(2)), where the pH values have to be lowered to less than 4, the proposed reaction can effectively oxidize the organic molecules at neutral pH and could avoid the initial acidification which may be costly and destructive for the in situ remediation of contaminated groundwater and soils. The green rust reactivity towards the oxidative transformation of phenol was thoroughly evaluated by performing a large kinetic study, chemical analyses, and spectroscopic investigations. MATERIALS AND METHODS: The kinetics of phenol removal was studied at three initial phenol concentrations for three green rusts under similar conditions (pH = 7.1; 1 g L(-1) of GR; 30 mM H(2)O(2)) and reaction rates were calculated based on mass and surface area. The oxidation rate constants are compared with that of magnetite, a well-known mixed iron (II, III) oxide. The mineralization of phenol was investigated at various H(2)O(2) doses and GR concentrations. In order to describe the phenol transformation in GR/H(2)O(2) system, several investigations were performed including HPLC and ion exclusion chromatography analysis, TOC, dissolved iron, and H(2)O(2) concentration measurements. Finally, X-ray powder diffraction and Raman spectroscopy were used to identify the oxidation products of GRs. RESULTS AND DISCUSSION: In GR/H(2)O(2) system, the kinetics of phenol removal at neutral pH was very fast and independent of the initial phenol concentration. No aromatic intermediates were detected and final by-products are mainly of short chain organic acids (oxalic acid and formic acid). Green rusts exhibit different reactivity toward Fenton-like oxidation of phenol. Both on mass and surface area basis, the reactivity of Fe(II)-Fe(III) species toward the oxidation of phenol was highest for GR(Cl(-)), little less for GR(SO(4)(2-)) or GR(CO(3)(2-)), and even less for magnetite (Fe(3)O(4)). Phenol degradation pseudo-first order rate constants (k(surf)) values were found to be: 13 x 10(-4), 3.3 x 10(-4), 3.5 x 10(-4), and 0.4 x 10(-4) L m(-2) s(-1) for GR(Cl(-)), GR(SO(4)(2-)), GR(CO(3)(2-)), and Fe(3)O(4), respectively. The mineralization yield of phenol as well as the decomposition rate of H(2)O(2) was higher for GR(Cl(-)) than for GR(SO(4)(2-)) or GR(CO(3)(2-)), mainly due to the higher Fe(II) content of GR(Cl(-)). Both X-ray diffraction analysis and Raman spectroscopy showed that the oxidation of GR with H(2)O(2) may lead to feroxyhyte (delta-FeOOH), with possible formation of poorly crystallized goethite (alpha-FeOOH), depending on GR type. CONCLUSIONS: This original work shows that the heterogeneous Fenton-like reaction using GR/H(2)O(2) is very effective toward degradation and mineralization of pollutants. In summary, this study has demonstrated that the green rust-promoted oxidation reaction could contribute to the transformation of water contaminants in the presence of H(2)O(2.) RECOMMENDATIONS AND PERSPECTIVES: These results could serve as the basis for the understanding of the transformation of organic pollutants in iron-rich soils in the presence of chemical oxidant (H(2)O(2)) or for the development of wastewater treatment process. However, some experimental parameters should be optimized for a high-scale application. Further work needs to be done for the reactive transport and transformation of organic compounds in a green rust-packed column. The reusability of GR in mineral-catalyzed reaction should be also investigated.

Reductive transformation and mineralization of an azo dye by hydroxysulphate green rust preceding oxidation using H(2)O(2) at neutral pH.

In this study, the reactivity of hydroxysulphate green rust (GR(SO(4)(2-))) toward reductive transformation, oxidative degradation and mineralization of organic compounds was evaluated using Methyl Red (MR) as model pollutant. The GR(SO(4)(2-)) was synthesized by co-precipitation method and characterized by X-ray diffraction (XRD), Mössbauer spectroscopy and Fourier Transform Infrared (FTIR) analyses. Reductive decolourization of MR solution occurred in the presence of GR(SO(4)(2-)), while no total organic carbon (TOC) decay was observed during the equilibration time. Significant TOC removal (87%) was noted when H(2)O(2) was added to the GR(SO(4)(2-))/MR mixture after the preliminary reduction step. UV-Vis analysis, dissolved iron and H(2)O(2) concentration measurement, and batch sorption test showed that the heterogeneous Fenton-like reaction is the main mechanism by which the pollutant was mineralized. Increasing of H(2)O(2)/Fe(II) ratio did not affect significantly the mineralization rate of MR. However, slight decolourization of MR and absence of TOC abatement were noted when both MR and H(2)O(2) were simultaneously mixed with the GR(SO(4)(2-)). XRD analysis, Mössbauer spectroscopy and FTIR spectroscopy revealed that the oxidation end-products of GR(SO(4)(2-)) were mainly a poorly crystallized goethite when GR was oxidized after equilibrating with MR in solution. However, a badly crystallized iron oxide was formed when GR was immediately oxidized. In all cases, the interlayer anion (SO(4)(2-)) was ejected from GR structure to aqueous solution. These results suggest that the GR(SO(4)(2-))/H(2)O(2) system could be used to promote the reduction/oxidation reaction of organic pollutants.

In situ redox flexibility of FeII-III Oxyhydroxycarbonate green rust and fougerite.

Bacterial activity is commonly thought to be directly responsible for denitrification in soils and groundwater. However, nitrate reduction in low organic sediments occurs abiotically by FeII ions within the fougerite mineral (IMA 2003-057), giving the bluish-green color of gleysols. Fougerite, the mineral counterpart of FeII-III oxyhydroxycarbonate, FeII6(1-x)FeIII6xO12H2(7-3x)CO3, provides a unique in situ redox flexibility, which can adapt x = {[FeIII]/[Fetotal]} between 1/3 and 2/3 as shown using Mössbauer spectroscopy. Chemical potential and Eh-pH diagrams for this system were determined from electrode potential monitored during deprotonation of hydroxycarbonate FeII4FeIII2(OH)12CO3 to assess the possibility of reducing pollutants in the field. Bioreduction of ferric oxyhydroxides in anoxic groundwater yields dissolved FeII, whereas HCO3- anions produced from organic matter are incorporated into fougerite layered double oxyhydroxide structure. Thus, fougerite is the solid-state redox mediator acting as electron shuttle that helps bacterial activity for reducing nitrate by coupling dissimilatory FeIII reduction and oxidation of FeII with reduction of NO3-. It is proposed that this system could be used in the remediation of soils and nitrified waters.