In Situ Synthesis of Silver Nanoparticles on Cellulose Fibers Using D-Glucuronic Acid and Its Antibacterial Application

The development of ecofriendly procedures to avoid the use of toxic chemicals for the synthesis of stable silver nanoparticles (AgNPs) is highly desired. In the present study, we reported an eco-friendly and green technique for in situ fabrication of AgNPs on bleached hardwood pulp fibers (bhpFibers) using D-glucuronic acid as the only reducing agent. Different amounts of D-glucuronic acid were introduced and its effect on the size and distribution of AgNPs on the bhpFibers was discussed. The morphology and structures of bhpFibers@AgNPs were proved by electron microscope-dispersive spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). Then, a series of bhpFibers@AgNPs with different AgNPs loadings were also prepared by adjusting the concentration of the AgNO₃ solution. After a papermaking process via vacuum filtration, the prepared papers displayed an outstanding antibacterial performance against Escherichia coli (gram -negative) and Staphylococcus aureus (gram-positive). It is foreseeable that the bhpFibers@AgNPs have a promising application in the field of biomedical.

Kinetics and mechanism of the advanced oxidation process of Cr(III) to Cr(VI) by SO4 −˙ free radicals in slightly acidic simulated atmospheric water

In a previous work, we showed that the oxidation of Cr(III) to Cr(VI) by OH˙ present in the atmospheric water droplets has the potential to threaten the people’s health since non-toxic species is transformed into environmental carcinogens. The same oxidation might be initiated by the SO4 −˙ free radicals. Here, we shed some light on the detailed mechanisms of this oxidation reaction occurring in ambient atmosphere. Steady state irradiation and pulse radiolysis technique were used to generate SO4 −˙. The advanced oxidation process mechanism was investigated at pH 4 and 6 selected as typical values of cloud water acidity. Our findings showed that the oxidation is pseudo-first order with respect to Cr(III) and is pH dependent. In the suggested reaction mechanism, the electron transfer proceeds via an inner sphere mechanism, with formation of the [Cr(III)–SO4 −˙] precursor adduct, followed by an electron transfer inside the adduct, from Cr(III) to SO4 −˙, to form Cr(IV): Cr(III) + SO 4 − · ⇌ [ C r ( III ) – SO 4 − · ] → Cr(IV) + SO 4 2 − . $${\rm{Cr(III)}} + {\rm{S}}{{\rm{O}}_4}^{ - \cdot}[Cr({\rm{III}})-{\rm{S}}{{\rm{O}}_4}^{ -\cdot }] \to {\rm{Cr(IV)}} + {\rm{S}}{{\rm{O}}_4}^{2 - }.$$ At pH 4, the equilibrium constant and the rate constant are 7.52 × 104 M−1 and 2.47 × 104 s−1, respectively. At pH 6 these values become 1.90 × 105 M−1 and 1.41 × 104 s−1, respectively.

Quinic acid and hypervalent chromium: a spectroscopic and kinetic study

The redox reaction between an excess of quinic acid (QA) and Cr^VI involves the formation of intermediates, namely, Cr^IV and Cr^V species, which in turn react with the organic substrates. As observed with other substrates that have already been studied, Cr^IV does not accumulate during this reaction because of the rate of the reaction. Its rate of disappearance is several times higher than that of the reaction of Cr^VI or Cr^V with QA. Kinetic studies indicate that the redox reaction proceeds via a combined mechanism that involves the pathways Cr^VI → Cr^IV → Cr^II and Cr^VI → Cr^IV → Cr^III, which is supported by the observation of superoxo-Cr^III (CrO₂²⁺) ions, free radicals, and oxo-Cr^V species as intermediates and the detection of Cr^VI ester species. The present study reports the complete rate laws for the QA/chromium redox reaction.

The redox reaction between an excess of quinic acid (QA) and Cr^VI involves the formation of intermediates, namely, Cr^IV and Cr^V species, which in turn react with the organic substrates.

Redox and complexation chemistry of the CrVI/CrV-D-glucaric acid system

When an excess of uronic acid over Cr(VI) is used, the oxidation of D-glucaric acid (Glucar) by Cr(VI) yields D-arabinaric acid, CO2 and Cr(III)-Glucar complex as final redox products. The redox reaction involves the formation of intermediate Cr(IV) and Cr(V) species. The reaction rate increases with [H(+)] and [substrate]. The experimental results indicated that Cr(IV) and Cr(V) are very reactive intermediates since their disappearance rates are much faster than Cr(VI). Cr(IV) and Cr(V) intermediates are involved in fast steps and do not accumulate in the redox reaction of the mixture Cr(VI)-Glucar. Kinetic studies show that the redox reaction between Glucar and Cr(VI) proceeds through a mechanism combining one- and two-electron pathways: Cr(VI) → Cr(IV) → Cr(II) and Cr(VI) → Cr(IV) → Cr(III). After the redox reaction, results show a slow hydrolysis of the Cr(III)-Glucar complex into [Cr(OH2)6](3+). The proposed mechanism is supported by the observation of free radicals, CrO2(2+) (superoxo-Cr(III) ion) and oxo-Cr(V)-Glucar species as reaction intermediates. The continuous-wave electron paramagnetic resonance, CW-EPR, spectra show that five-coordinate oxo-Cr(V) bischelates are formed at pH ≤ 4 with the aldaric acid bound to oxo-Cr(V) through the carboxylate and the α-OH group. A different oxo-Cr(V) species with Glucar was detected at pH 6.0. The high g(iso) value for the last species suggests a mixed coordination species, a five-coordinated oxo-Cr(V) bischelate with one molecule of Glucar acting as a bi-dentate ligand, using the 2-hydroxycarboxylate group, and a second molecule of Glucar with any vic-diolate sites. At pH 7.5 only a very weak EPR signal was observed, which may point to instability of these complexes. This behaviour contrasts with oxo-Cr(V)-uronic species, and must thus be related to the Glucar acyclic structure. In vitro, our studies on the chemistry of oxo-Cr(V)-Glucar complexes can provide information on the nature of the species that are likely to be stabilized in vivo.

Reduction of hypervalent chromium in acidic media by alginic acid

Selective oxidation of carboxylate groups present in alginic acid by Cr(VI) affords CO2, oxidized alginic acid, and Cr(III) as final products. The redox reaction afforded first-order kinetics in [alginic acid], [Cr(VI)], and [H(+)], at fixed ionic strength and temperature. Kinetic studies showed that the redox reaction proceeds through a mechanism which combines Cr(VI)→Cr(IV)→Cr(II) and Cr(VI)→Cr(IV)→Cr(III) pathways. The mechanism was supported by the observation of free radicals, CrO2(2+) and Cr(V) as reaction intermediates. The reduction of Cr(IV) and Cr(V) by alginic acid was independently studied and it was found to occur more than 10(3) times faster than alginic acid/Cr(VI) reaction, in acid media. At pH 1-3, oxo-chromate(V)-alginic acid species remain in solution during several hours at 15°C. The results showed that this abundant structural polysaccharide present on brown seaweeds is able to reduce Cr(VI/V/IV) or stabilize high-valent chromium depending on pH value.

A laboratory study of the oxidation of non toxic Cr(III) to toxic Cr(VI) by OH(•) free radicals in simulated atmospheric water droplets conditions: potential environmental impact

In atmospheric waters, oxidation of Cr(III) to Cr(VI) by OH(•) free radicals is a major environmental hazard since non-toxic species is transformed into toxic one. It is important to obtain some details concerning this oxidation reaction. In this study we simulated this oxidation by steady state radiolysis using (60)Co radioactive source and pulse radiolysis technique using a 2.5MeV van de Graaff electron accelerator and investigated its kinetics in the pH range 1 to 9. Our findings showed that the reaction was highly pH dependant with a maximum yield at pH 4. The electron transfer proceeds via an inner sphere mechanism with (i) formation of the [OH(•)-Cr(III)] adduct with an equilibrium constant of 2.34×10(4)mol(-1)dm(-3) then (ii) followed by an electron transfer from Cr(III) to OH(•) within the adduct with a rate constant of 2.51×10(4)s(-1). The implication of this oxidation to atmospheric chromium contamination is discussed.

Detection and structural characterization of oxo-chromium(V)-sugar complexes by electron paramagnetic resonance

This article describes the detection and characterization of oxo-Cr(V)-saccharide coordination compounds, produced during chromic oxidation of carbohydrates by Cr(VI) and Cr(V), using electron paramagnetic resonance (EPR) spectroscopy. After an introduction into the main importance of chromium (bio)chemistry, and more specifically the oxo-chromium(V)-sugar complexes, a general overview is given of the current state-of-the-art EPR techniques. The next step reviews which types of EPR spectroscopy are currently applied to oxo-Cr(V) complexes, and what information about these systems can be gained from such experiments. The advantages and pitfalls of the different approaches are discussed, and it is shown that the potential of high-field and pulsed EPR techniques is as yet still largely unexploited in the field of oxo-Cr(V) complexes. Subsequently, the discussion focuses on the analysis of oxo-Cr(V) complexes of different types of sugars and the implications of the results in terms of understanding chromium (bio)chemistry.