Impact and application of electron shuttles on the redox (bio)transformation of contaminants : a review

During the last two decades, extensive research has explored the catalytic effects of different organic molecules with redox mediating properties on the anaerobic (bio)transformation of a wide variety of organic and inorganic compounds. The accumulated evidence points at a major role of electron shuttles in the redox conversion of several distinct contaminants, both by chemical and biological mechanisms. Many microorganisms are capable of reducing redox mediators linked to the anaerobic oxidation of organic and inorganic substrates. Electron shuttles can also be chemically reduced by electron donors commonly found in anaerobic environments (e.g. sulfide and ferrous iron). Reduced electron shuttles can transfer electrons to several distinct electron-withdrawing compounds, such as azo dyes, polyhalogenated compounds, nitroaromatics and oxidized metalloids, among others. Moreover, reduced molecules with redox properties can support the microbial reduction of electron acceptors, such as nitrate, arsenate and perchlorate. The aim of this review paper is to summarize the results of reductive (bio)transformation processes catalyzed by electron shuttles and to indicate which aspects should be further investigated to enhance the applicability of redox mediators on the (bio)transformation of contaminants.F.P. van der Zee thanks the Portuguese Fundaicao para a Ciencia e a Tecnologia for financial support (Grant SFRH/BPD/39086/2007). F. J. Cervantes greatly acknowledges a grant from Council of Science and Technology of Mexico (Grant SEP-CONACYT-C02-55045)

Fate of aniline and sulfanilic acid under denitrifying conditions

Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BDP/1874

Fate of aniline and sulfanilic acid in UASB bioreactors under denitrifying conditions

Two upflow anaerobic sludge blanket (UASB) reactors were operated to investigate the fate of aromatic amines under denitrifying conditions. The feed consisted of synthetic wastewater containing aniline and/or sulfanilic acid and a mixture of volatile fatty acids (VFA) as the primary electron donors. Reactor 1 (R1) contained a stoichiometric concentration of nitrate and Reactor 2 (R2) a stoichiometric nitrate and nitrite mixture as terminal electron acceptors. The R1 results demonstrated that aniline could be degraded under denitrifying conditions while sulfanilic acid remains. The presence of nitrite in the influent of R2, caused a chemical reaction that led to immediate disappearance of both aromatic amines and the formation of an intense yellow coloured solution. HPLC analysis of the influent solution, revealed the emergence of three product peaks: the major one at retention time (Rt) 14.3 min and two minor at Rt 17.2 and 21.5 min. In the effluent, the intensity of the peaks at Rt 14.3 and 17.2 min was very low and of that at Rt 21.5 min increased (~3-fold). Based on the mass spectrometry analysis, we propose the structures of some possible products, mainly azo compounds. Denitrification activity tests suggest that biomass needed to adapt to the new coloured compounds, but after a 3 days lag phase, activity is recovered and the final (N2 + N2O) is even higher than that of the control.Fundação para a Ciência e a Tecnologia (FCT

Lab-scale bioreactors for aromatic amines reduction under denitrification conditions

Under anaerobic conditions, azo dyes are readily decolourised as a result of the reductive transformation of the azo group leading to the formation of aromatic amines which are known to be even more toxic than the original dyes. A logical concept for the removal of azo dyes in biological wastewater treatment systems is based on the combination of anaerobic/aerobic treatment, for the degradation of also aromatic amines. A drawback of aerobic treatment is that many aromatic amines are prone to autoxidation. Nitrate/Nitrite are powerful electron acceptors as alternative to oxygen, avoiding the autoxidation. Our research consisted of operating two bioreactors with the objective to investigate the fate of aromatic amines under denitrifying conditions. The reactors were fed with synthetic wastewater contained aniline and/or sulfanilic acid and a mixture of volatile fatty acids as the primary electron donors. Reactor 1 (R1) contained a stoichiometric concentration of nitrate and Reactor 2 (R2) a mixture of nitrate and nitrite as terminal electron acceptors. The R1 results demonstrated that aniline could be degraded under denitrifying conditions while sulfanilic acid remains. The presence of nitrite in the effluent of R2, at low pH, caused a chemical reaction that led to immediate disappearance of both aromatic amines and the formation of an orange colour solution. HPLC analysis revealed the presence of phenol as a product of aniline. Other compounds were detected by LC_MS. The overall COD removal was always higher in R1 than in R2, suggesting toxicity of nitrite and/or the formed products. Whereas a replacement of amino-groups by hydroxyl-groups holds promise for biodegradability, the results indicate that the chemical reaction is more complex, resulting in the formation of compounds that were not mineralized during the course of the experiment

Activated carbon as a redox mediator: effect of AC surface chemistry and solution pH on dye reduction

Azo dyes have a wide application in food, pharmaceutical, textile, leather, cosmetics and paper industries. These are the largest and most versatile classes of dyes used, but are recalcitrant to biodegradation and many are carcinogenic or cytotoxic. Their removal is a major concern when treating dye-containing wastewaters. Under anaerobic conditions, they are non-specifically reduced, a fortuitous but often slow process. Acceleration can be achieved by using electron-shuttling compounds that speed up the reaction, acting as redox mediators. Activated carbon (AC) has been shown as a feasible redox mediator and adsorbent material. In this study, the surface chemistry of a commercial AC (AC0) was selectively modified, without changing significantly its textural properties, by means of chemical oxidation with HNO3 (ACHNO3) (mild acidic surface properties) and thermal treatments under H2 (ACH2) or N2 (ACN2) flow (basic surface properties). Oxidation with 5% O2 (ACO2) ends not only in surface chemistry changes (acidic properties), but also in the textural properties. The effect of modified AC on anaerobic chemical dye reduction was assayed with sulphide as a reducing agent at different pH values: 5, 7 and 9. Four dyes from different classes were tested: Acid Orange 7, Reactive Red 2, Mordant Yellow 10 and Direct Blue 71. Batch experiments with low amounts of AC (0.1 g.L-1) demonstrated an increase of the first-order reduction rate constants (~ 9-fold) for all the dyes tested as compared with assays without AC. The reduction of AO7 and MY10 was highly dependent on the pH, with optimum rates at pH 5 and 7, respectively. Higher rates of RR2 and DB71 reduction were obtained at pH 5. In general, an increase of the rates with increasing the pHpzc was observed, following the trend ACHNO3 < ACO2 < AC0 < ACN2 < ACH2. Comparing the rates of single dyes, MY10 was reduced at the highest rate (12 ± 1 d-1) and RR2 at the lowest (1.3 ± 0.1 d-1)

Thermal modification of activated carbon surface chemistry improves its capacity as redox mediator for azo dye reduction

The surface chemistry of a commercial AC (AC0) was selectively modified, without changing significantly its textural properties, by chemical oxidation with HNO3 (ACHNO3 ) and O2 (ACO2 ), and thermal treatments under H2 (ACH2) or N2 (ACN2 ) flow. The effect of modified AC on anaerobic chemical dye reduction was assayed with sulphide at different pH values 5, 7 and 9. Four dyes were tested: Acid Orange 7, Reactive Red 2, Mordant Yellow 10 and Direct Blue 71. Batch experiments with low amounts of AC (0.1 g L−1) demonstrated an increase of the first-order reduction rate constants, up to 9-fold, as compared with assays without AC. Optimum rates were obtained at pH 5 except for MY10, higher at pH 7. In general, rates increased with increasing the pH of point zero charge (pHpzc), following the trend ACHNO3 < ACO2 < AC0 < ACN2 < ACH2 . The highest reduction rate was obtained for MY10 with ACH2 at pH 7, which corresponded to the double, as compared with non-modified AC. In a biological system using granular biomass, ACH2 also duplicated and increase 4.5-fold the decolourisation rates of MY10 and RR2, respectively. In this last experiment, reaction rate was independent of AC concentration in the tested range 0.1–0.6 g L−1.This work was supported by the PTDC/AMB/69335/2006 project grants. L Pereira holds a Pos-Doc fellowship (SFRH/BPD/20744/2004) and R. Pereira holds a fellowship (SFRH/BPD/39086/2007) from Fundacao para a Ciencia e Tecnologia. F.J. Cervantes greatly acknowledges a grant from Council of Science and Technology of Mexico (Grant SEP-CONACYT-C02-55045)

Warp or lag? The ionized and neutral hydrogen gas in the edge-on dwarf galaxy UGC 1281

The properties of gas in the halos of galaxies constrain global models of the interstellar medium. Kinematical information is of particular interest since it is a clue to the origin of the gas. Until now mostly massive galaxies have been investigated for their halo properties. Here we report on deep HI and H{\alpha} observations of the edge-on dwarf galaxy UGC 1281 in order to determine the existence of extra-planar gas and the kinematics of this galaxy. This is the first time a dwarf galaxy is investigated for its gaseous halo characteristics. We have obtained H{\alpha} integral field spectroscopy using PPAK at Calar Alto and deep HI observations with the WSRT of this edge-on dwarf galaxy. These observations are compared to 3D models in order to determine the distribution of HI in the galaxy. We find that UGC 1281 has H{\alpha} emission up to 25"(655 pc) in projection above the plane and in general a low H{\alpha} flux. Compared to other dwarf galaxies UGC 1281 is a normal dwarf galaxy with a slowly rising rotation curve that flattens off at 60 km/s and a central depression in its HI distribution. Its HI extends 70" (1.8 kpc) in projection from the plane. This gas can be explained by either a warp partially in the line-of-sight warp or a purely edge-on warp with rotational velocities that decline with a vertical gradient of 10.6 \pm 3.7 km/s/kpc. The line-of-sight warp model is the preferred model as it is conceptually simpler. In either model the warp starts well within the optical radius.Comment: Accepted for publication in MNRAS. 16 pages, 14 figure

Biodegradability of reduced azo dyes : exploring the second stage of sequential anaerobic-aerobic treatment of azo-dye-containing wastewater

Sequential anaerobic-aerobic treatment is the most logical biological strategy for the removal of azo dyes from wastewater. In the anaerobic stage, azo dyes, which generally resist aerobic biodegradation, are fortuitously reduced under anaerobic conditions, yielding colorless aromatic amines. In the aerobic stage, aromatic amines, which generally do not undergo further transformation under anaerobic conditions, are prone to oxidative (bio)transformation under aerobic conditions. Since research on aromatic amine biodegradation has been usually conducted with relatively stable, not easily autoxidizing aromatic amines, representing only a part of the aromatic amines from azo dyes, still much is unknown about the aerobic fate of azo dye cleavage products. In this screening study we investigated the biodegradability of seven chemically reduced azo dyes and two aromatic amines. The compounds were incubated with dyestuff-adapted sludge in the presence of either oxygen, perchlorate, nitrate, Fe(III) or Mn(IV) as the electron acceptors. CO2-production, O2- consumption and changes in HPLC-chromatograms and UV/VIS-spectra were monitored. Preliminary results indicate that biodegradation had been limited to relatively simple benzene-based aromatic amines