Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Bio-reduction of sulfide minerals to recover invisible gold
Hol, A. - \ 2011
Wageningen University. Promotor(en): Cees Buisman, co-promotor(en): Renata van der Weijden; J.P.A. de Weert. - S.l. : s.n. - ISBN 9789085859222 - 117
redoxreacties - pyriet - extractie - goud - redox reactions - pyrites - extraction - gold

Sulfide minerals, like pyrite and arsenopyrite, are of economical interest due to the presence of invisible gold locked inside these minerals. As fine grinding is often not sufficient to liberate the gold from these minerals, additional destruction techniques, based on chemical and biological oxidation processes, are required to access the gold via cyanidation. These techniques have proven to be successful to reach satisfactory gold recoveries, but operation costs are high and challenging waste streams (sulfuric acid) are produced. As an alternative to the oxidation methods in this thesis the bio-reduction of sulfide minerals was proposed and investigated. Bio-reduction, the use of hydrogen to convert mineral-sulfur to hydrogen sulfide, has as major advantage that the hydrogen sulfide can be recovered from the solution (to produce bio-sulfur) leaving a waste stream without diluted sulfuric acid. Furthermore, electrical energy will be saved. Theoretically, this more environmental friendly alternative should work, but no bio-reduction reaction was observed in practice. It appeared that sulfur/sulfate reducing bacteria were not able to use sulfur when fixed in the crystal lattice of pyrite and arsenopyriteat the selected conditions (pH 5, 35ºC). In order to make the mineral-sulfur bio-available for these bacteria the sulfur should first enter the solution. As alternative the combination between partial bio-oxidation and bio-reduction was therefore investigated. Partial bio-oxidation at pH 2 results in the formation of elemental sulfur, which can serve as a substrate at pH 5 for the sulfur/sulfate reducing bacteria. This combined method was found to be successful at 35°C, as the gold leachability of the used concentrate was increased from 6% to 39%. Optimization of this process is needed to reach gold recoveries >90% in 1 or 2 ox/red cycles, but when successful a new method (called the Paroxsul process) with a lower environmental impact, less costs, and application to a large number of minerals, is ready to be introduced to the precious metal industry.

Degradation of 1,2-dichloroethane by microbial communities from river sediment at various redox conditions
Zaan, B.M. van der; Weert, J.P.A. de; Rijnaarts, H. ; Vos, W.M. de; Smidt, H. ; Gerritse, J. - \ 2009
Water Research 43 (2009)13. - ISSN 0043-1354 - p. 3207 - 3216.
rivieren - sediment - biodegradatie - redoxreacties - oxidatie - micro-organismen - anaërobe omstandigheden - elbe - rivers - sediment - biodegradation - redox reactions - oxidation - microorganisms - anaerobic conditions - river elbe - halogenated aliphatic-compounds - dichloroeliminans strain dca1 - reductive dechlorination - methanogenic bacteria - contaminated aquifer - chlorinated ethenes - pure cultures - sequence data - dehalogenation - groundwater
Insight into the pathways of biodegradation and external factors controlling their activity is essential in adequate environmental risk assessment of chlorinated aliphatic hydrocarbon pollution. This study focuses on biodegradation of 1,2-dichloroethane (1,2-DCA) in microcosms containing sediment sourced from the European rivers Ebro, Elbe and Danube. Biodegradation was studied under different redox conditions. Reductive dechlorination of 1,2-DCA was observed with Ebro and Danube sediment with chloroethane, or ethene, respectively, as the major dechlorination products. Different reductively dehalogenating micro-organisms (Dehalococcoides spp., Dehalobacter spp., Desulfitobacterium spp. and Sulfurospirillum spp.) were detected by 16S ribosomal RNA gene-targeted PCR and sequence analyses of 16S rRNA gene clone libraries showed that only 2-5 bacterial orders were represented in the microcosms. With Ebro and Danube sediment, indications for anaerobic oxidation of 1,2-DCA were obtained under denitrifying or iron-reducing conditions. No biodegradation of 1,2-DCA was observed in microcosms with Ebro sediment under the different tested redox conditions. This research shows that 1,2-DCA biodegradation capacity was present in different river sediments, but not in the water phase of the river systems and that biodegradation potential with associated microbial communities in river sediments varies with the geochemical properties of the sediments
Sulfate reduction under acidic conditions for selective model recovery
Bijmans, M.F.M. - \ 2008
Wageningen University. Promotor(en): Cees Buisman, co-promotor(en): Piet Lens. - [S.l.] : S.n. - ISBN 9789085049258 - 156
sulfaat - redoxreacties - zuurgraad - metalen - terugwinning - afvalwaterbehandeling - bioreactoren - nikkel - ijzer - sulfaatreductie - sulfate - redox reactions - acidity - metals - recovery - waste water treatment - bioreactors - nickel - iron - sulfate reduction
Dit proefschrift heeft als doel om processen te ontwikkelen voor selectieve metaal herwinning uit afvalwater and processtromen die meerdere metalen bevatten, door gebruik te maken van sulfaat reductie onder zure omstandigheden
New insights in cellular biochemistry owing to the use of fluorescence microscopy
Borst, J.W. - \ 2006
Wageningen University. Promotor(en): Ton Visser. - [S.l. ] : S.n. - ISBN 908504474X - 132
fluorescentiemicroscopie - redoxreacties - lipidenperoxidatie - arabidopsis - fluorescence microscopy - redox reactions - lipid peroxidation - arabidopsis
Belasting van het oppervlaktewater in het landelijk gebied door uitspoeling van zware metalen; modelberekeningen t.b.v. emissieregistratie 2006 en invloed van redoxcondities
Bonten, L.T.C. ; Brus, D.J. - \ 2006
Wageningen : Alterra (Alterra-rapport 1340) - 46
zware metalen - oppervlaktewater - uitspoelen - waterverontreiniging - modellen - redoxreacties - bodemchemie - emissie - soil chemistry - heavy metals - surface water - leaching - water pollution - models - redox reactions - emission
Ten behoeve van de emissieregistratie zijn modelberekeningen uitgevoerd om een inschatting te maken van de uitspoeling van zware metalen uit bodems in het landelijk gebied naar het oppervlaktewater. Het modelinstrumentarium voor de berekening van deze uitspoeling is sterk verbeterd door een nieuwe schematisatie van zware metaalgehaltes in de bodem en een verbeterde hydrologie voor natuurgebieden. Daarnaast is de mogelijk invloed van de redoxeffecten geschat. Voor zand- en kleigronden lijken redoxeffecten nauwelijks een rol te spelen. De berekeningsresultaten met het verbeterde model komen beter overeen met metingen dan eerdere berekeningen. Landelijke vrachten van Zn, Cd en Ni zijn veel lager dan eerdere berekeningen
Chemically enhanced biological NOx removal from flue gases : nitric oxide and ferric EDTA reduction in BioDeNox reactors
Maas, P.M.F. van der - \ 2005
Wageningen University. Promotor(en): Gatze Lettinga, co-promotor(en): Piet Lens. - Wageningen : - ISBN 9085043166 - 224
stikstofoxiden - edta - redoxreacties - luchtverontreiniging - rookgassen - nitrogen oxides - edta - redox reactions - air pollution - flue gases
The emission of nitrogen oxides (NOx) to the atmosphere is a major environmental problem. To abate NOx emissions from industrial flue gases, to date, mainly chemical processes like selective catalytic reduction (SCR) are applied. All these processes require high temperatures (>300 °C) and expensive catalysts. Therefore, biological NOx removal techniques using denitrification may represent promising alternatives for the conventional SCR techniques. However, water based biofiltration requires relatively long scrubber/bioreactor retention times, i.e. big reactor volumes, due to the slow mass transfer of NO from the gas into the liquid phase. BioDeNOx is in principle a welcome alternative for conventional NOx removal techniques like SCR and water-based biofiltration, since it does not need high temperatures and catalysts, while scrubber retention times can be very short (<10 seconds) due to the chemically enhanced NO absorption. In this thesis, the BioDeNOx concept was investigated with special attention to the bioreactor key conversions: NO and Fe(III)EDTA- reduction. This study showed stable NO removal from the gas phase with efficiencies up to 80 %. It was found that the NO removal from the gas phase is primary determined by NO absorption kinetics. Therefore, a high Fe(II)EDTA2- concentration is required, i.e. the FeEDTA absorption liquor should be in the reduced state. However, a totally reduced system should be avoided, since this will induce sulfide accumulation. The latter process is unwanted, since already low sulfide concentrations showed an incomplete NO reduction due to inhibition of N2O reduction to N2. To achieve satisfying NO removal from the gas and to avoid sulfdogenic conditions, the redox potential of BioDeNOx reactors should be steered between -180 and -200 mV versus Ag/AgCl (pH 7.2±0.2). An ethanol dosing system that is controlled by the redox potential signal was shown to be a proper manner to do so
Reductive decolourisation of azo dyes by mesophilic and thermophilic methanogenic consortia
Cervantes, F.J. ; Santos, A.B. dos; Madrid, M.P. de; Stams, A.J.M. ; Lier, J.B. van - \ 2005
Water Science and Technology 52 (2005)1-2. - ISSN 0273-1223 - p. 351 - 356.
slib - ontkleuring - azoverbindingen - kleurstoffen (dyes) - anaërobe behandeling - redoxreacties - microbiële afbraak - afvalwaterbehandeling - sludges - decolorization - azo compounds - dyes - anaerobic treatment - redox reactions - microbial degradation - waste water treatment - anaerobic sludge - redox mediators - quinone-respiration - anthraquinone dyes - granular sludge - transformation - toxicity - bioreactors - inhibition - reactor
The contribution of acidogenic bacteria and methanogenic archaea on the reductive decolourisation of azo dyes was assessed in anaerobic granular sludge. Acidogenic bacteria appeared to play an important role in the decolourising processes when glucose was provided as an electron donor; whereas methanogenic archaea showed a minor role when this substrate was supplemented in excess. In the presence of the methanogenic substrates acetate, methanol, hydrogen and formate, methane production became important only after colour was totally removed from the batch assays. This retardation in methane production may be due to either a toxic effect imposed by the azo dyes or to the competitive behaviour of azo dyes to the methanogenic consortia for the available reducing equivalents. Keywords Azo dyes; decolourisation; methanogenic consortium; redox mediator; inhibitors
Effect of different redox mediators during thermophilic azo dye reduction by anaerobic granular sludge and comparative study between mesophilic (30C) and thermophilic (55C) treatments for decolourisation of textile wastewaters
Bezerra Dos Santos, A. ; Bisschops, I.A.E. ; Cervantes, F.J. ; Lier, J.B. van - \ 2004
Chemosphere 55 (2004)9. - ISSN 0045-6535 - p. 1149 - 1157.
afvalwater - degradatie - consortia - toxiciteit - waterverontreiniging - ontkleuring - anaërobe behandeling - afvalwaterbehandeling - redoxreacties - azoverbindingen - kleurstoffen (dyes) - slib - waste water - degradation - consortia - toxicity - water pollution - decolorization - anaerobic treatment - waste water treatment - redox reactions - azo compounds - dyes - sludges - waste-water - reactor - carbon - bn6
The impact of different redox mediators on colour removal of azo dye model compounds and textile wastewater by thermophilic anaerobic granular sludge (55 C) was investigated in batch assays. Additionally, a comparative study between mesophilic (30 C) and thermophilic (55 C) colour removal was performed with textile wastewater, either in the presence or absence of a redox mediator
The impact of different redox mediators on colour removal of azo dye model compounds and textile wastewater by thermophilic anaerobic granular sludge (55 degreesC was investigated in batch assays. Additionally, a comparative study between mesophilic (30 degreesC and thermophilic (55 degreesC colour removal was performed with textile wastewater, either in the presence or absence of a redox mediator. The present work clearly evidences the advantage of colour removal at 55 degreesC compared with 30 degreesC when dealing with azo coloured wastewaters. The impact of the redox mediators anthraquinone-2,6-disulfonate (AQDS), anthraquinone-2-sulfonate (AQS) and riboflavin was evident with all dyes, increasing decolourisation rates up to 8-fold compared with the mediator-free incubations. The generation of the hydroquinone form AH(2)QDS, i.e. the reduced form of AQDS, was extremely accelerated at 55 degreesC compared with 30 degreesC. Furthermore, no lag-phase was observed at 55 degreesC. Based on the present results we postulate that the production/transfer of reducing equivalents was the process rate-limiting step, which was accelerated by the temperature increase. It is conclusively stated that 55 degreesC is a more effective temperature for azo dye reduction than 30 degreesC which on the one hand can be attributed to the faster production/transfer of reducing equivalents, but also to the decrease in activation energy requirements. (C) 2004 Elsevier Ltd. All rights reserved.
The contribution of biotic and abiotic processes during azo dye reduction in anaerobic sludge
Zee, F.P. van der; Bisschops, I.A.E. ; Blanchard, V.G. ; Bouwman, R.H.M. ; Lettinga, G. ; Field, J.A. - \ 2003
Water Research 37 (2003)13. - ISSN 0043-1354 - p. 3098 - 3109.
rioolafvalwater - anaërobe behandeling - azoverbindingen - kleurstoffen (dyes) - redoxreacties - afvalwaterbehandeling - sewage effluent - anaerobic treatment - azo compounds - dyes - redox reactions - waste water treatment - redox mediators - decolorization - bacteria - system
Azo dye reduction results from a combination of biotic and abiotic processes during the anaerobic treatment of dye containing effluents. Biotic processes are due to enzymatic reactions whereas the chemical reaction is due to sulfide. In this research, the relative impact of the different azo dye reduction mechanisms was determined by investigating the reduction of Acid Orange 7 (AO7) and Reactive Red 2 (RR2) under different conditions. Reduction rates of two azo dyes were compared in batch assays over a range of sulphide concentrations in the presence of living or inactivated anaerobic granular sludge
Azo dye reduction results from a combination of biotic and abiotic processes during the anaerobic treatment of dye containing effluents. Biotic processes are due to enzymatic reactions whereas the chemical reaction is due to sulfide. In this research, the relative impact of the different azo dye reduction mechanisms was determined by investigating the reduction of Acid Orange 7 (AO7) and Reactive Red 2 (RR2) under different conditions. Reduction rates of two azo dyes were compared in batch assays over a range of sulphide concentrations in the presence of living or inactivated anaerobic granular sludge. Biological dye reduction followed zero order kinetics and chemical dye reduction followed second-order rate kinetics as a function of sulfide and dye concentration. Chemical reduction of the dyes was greatly stimulated in the presence of autoclaved sludge; whereas chemical dye reduction was not affected by living or gamma-irradiated-sludge. Presumably redox-mediating enzyme cofactors released by cell lysis contributed to the stimulatory effect. This hypothesis was confirmed in assays evaluating the chemical reduction of AO7 utilizing riboflavin, representative of the heat stable redox-mediating moieties of common occurring flavin enzyme cofactors. Sulfate influenced dye reduction in accordance to biogenic sulfide formation from sulfate reduction. In assays lacking sulfur compounds, dye reduction only readily occurred in the presence of living granular sludge, demonstrating the importance of enzymatic mechanisms. Both chemical and biological mechanisms of dye reduction were greatly stimulated by the addition of the redox-mediating compound, anthraquinone-disulfonate. Based on an analysis of the kinetics and demonstration in lab-scale upward-flow anaerobic sludge bed reactors, the relative importance of chemical dye reduction mechanisms in high rate anaerobic bioreactors was shown to be small due to the high biomass levels in the reactors. (C) 2003 Elsevier Science Ltd. All rights reserved.
Activated Carbon as an Electron Acceptor and Redox Mediator during the Anaerobic Biotransformation of Azo Dyes
Zee, F.P. van der; Bisschops, I.A.E. ; Lettinga, G. ; Field, J.A. - \ 2003
Environmental Science and Technology 37 (2003)2. - ISSN 0013-936X - p. 402 - 408.
slib - koolstof - redoxreacties - verontreinigende stoffen - azoverbindingen - kleurstoffen (dyes) - anaërobe behandeling - oxidatie - afvalwaterbehandeling - sludges - carbon - redox reactions - pollutants - azo compounds - dyes - anaerobic treatment - oxidation - waste water treatment - sp strain bn6 - oxidative dehydrogenation - catalytic-oxidation - hydrogen-sulfide - fluidized-bed - reduction - sludge - decolorization - degradation - quinones
The role of AC as redox mediator in accelerating the reductive transformation of pollutants as well as a terminal electron acceptor in the biological oxidation of an organic substrate is described. This study explores the use of AC as an immobilized redox mediator for the reduction of a recalcitrant azo dye in laboratory-scale anaerobic bioreactors, using volatile fatty acids as electron donor
Activated carbon (AC) has a long history of applications in environmental technology as an adsorbent of pollutants for the purification of drinking waters and wastewaters. Here we describe novel role of AC as redox mediator in accelerating the reductive transformation of pollutants as well as a terminal electron acceptor in the biological oxidation of an organic substrate. This study explores the use of AC as an immobilized redox mediator for the reduction of a recalcitrant azo dye (hydrolyzed Reactive Red 2) in laboratory-scale anaerobic bioreactors, using volatile fatty acids as electron donor. The incorporation of AC in the sludge bed greatly improved dye removal and formation of aniline, a dye reduction product. These results indicate that AC acts as a redox mediator. In supporting batch experiments, bacteria were shown to oxidize acetate at the expense of reducing AC. Furthermore, AC greatly accelerated the chemical reduction of an azo dye by sulfide. The results taken as a whole clearly suggest that AC accepts electrons from the microbial oxidation of organic acids and transfers the electrons to azo dyes, accelerating their reduction. A possible role of quinone surface groups in the catalysis is discussed.
Anaerobic azo dye reduction
Zee, F.P. van der - \ 2002
Wageningen University. Promotor(en): G. Lettinga; J.A. Field. - S.l. : S.n. - ISBN 9789058086105 - 142
anaërobe behandeling - azoverbindingen - kleurstoffen (dyes) - redoxreacties - antrachinonen - anaerobic treatment - azo compounds - dyes - redox reactions - anthraquinones

Azo dyes, aromatic moieties linked together by azo (-N=N-) chromophores, represent the largest class of dyes used in textile-processing and other industries. The release of these compounds into the environment is undesirable, not only because of their colour, but also because many azo dyes and their breakdown products are toxic and/or mutagenic to life. To remove azo dyes from wastewater, a biological treatment strategy based on anaerobic reduction of the azo dyes, followed by aerobic transformation of the formed aromatic amines, holds promise. However, the first stage of the process, anaerobic azo dye reduction, proceeds relatively slow. Therefore, this thesis research aimed at optimising anaerobic azo dye reduction, by studying the reaction mechanism and by consequently applying the obtained insights.

In this thesis it is shown that non-adapted anaerobic granular sludge has the capacity to non-specifically reduce azo dyes. As there was no correlation between a dye's reduction rate and its molecular characteristics (i.e. its size and its number of sulphonate groups and other polar substituents), it is unlikely that the mechanism of azo dye reduction involves cell wall penetration. Moreover, the presence of bacteria is not a prerequisite: azo dyes can also be reduced by sulphide in a purely chemical reaction. As dye containing wastewater usually contains sulphate and other sulphur species that will be biologically reduced to sulphide during treatment in anaerobic bioreactors, azo dye reduction will be a combination of biotic and abiotic processes. However, it was demonstrated that under normal conditions in high-rate anaerobic bioreactors (high sludge content, moderate sulphide levels), chemical azo dye reduction by sulphide hardly contributes to the overall reaction. Anaerobic azo dye reduction is therefore mainly a biological process, either a direct enzymatically catalysed reaction involving non-specific enzymes or a reaction with enzymatically reduced electron carriers. Azo dye reduction by sludge that had not earlier been exposed to dyes was found to relate to the oxidation of endogenous substrate and, especially, to the oxidation of hydrogen when present in bulk concentrations. Enrichment was required for the utilisation of electrons from volatile fatty acids for dye reduction.

Examination of the reduction of twenty chemically distinct azo dyes by anaerobic granular sludge revealed a large variation in the reaction rates. Especially reactive azo dyes with triazyl reactive groups were slowly reduced. For these common occurring reactive dyes, long contact times may be necessary to reach a satisfying extent of decolourisation. Consequently, they pose a serious problem for applying high-rate anaerobic treatment as the first stage in the biological degradation of azo dyes. However, this problem can be overcome by using redox mediators, compounds that speed up the reaction rate by shuttling electrons from the biological oxidation of primary electron donors or from bulk electron donors to the electron-accepting azo dyes.

It was observed that one of the constituent aromatic amines of the azo dye Acid Orange 7 had an autocatalytic effect on the dye's reduction, probably by acting as a redox mediator. Other compounds, e.g. the artificial redox mediator anthraquinone-2,6-disulphonate (AQDS), a compound that is known to catalyse the reductive transfer of several pollutants, and the commonly occurring flavin enzyme cofactor riboflavin, were found to be extremely powerful catalysts, capable of raising the pseudo first-order reaction rate constants by orders of magnitude. Moreover, a large stimulatory effect was found for autoclaved sludge, presumably due to the release of internal electron carriers, e.g. enzyme cofactors like riboflavin, during autoclaving.

AQDS was successfully applied to improve the continuous reduction of Reactive Red 2 (a reactive azo dye with a triazyl reactive group) in a lab-scale anaerobic bioreactor that was operated under moderate hydraulic loading conditions. Without AQDS, the reactor's dye removal efficiency was very low, which gave rise to severe dye toxicity towards the biological activity. Addition of catalytic concentrations of AQDS to the reactor influent caused an immediate increase of the dye removal efficiency and recovery of the methane production. Eventually, almost complete RR2 colour removal could be reached.

Though effective AQDS dosage levels are low, continuous dosing has disadvantages with respect to the costs and the discharge of this biologically recalcitrant compound. Therefore, the feasibility of activated carbon (AC), which is known to contain quinone groups at its surface, to act alternatively as an insoluble/immobilised redox mediator was explored. Incorporation of AC in the sludge of lab-scale anaerobic bioreactors that treated Reactive Red 2 in synthetic wastewater containing volatile fatty acid as primary electron donor resulted in enhanced continuous dye reduction as compared to the control reactors without AC. The effect of AC was in large excess of its dye adsorption capacity. In addition, it was shown that bacteria could utilise AC as terminal electron acceptor in the oxidation of acetate. Moreover, AC catalysis of chemical azo dye reduction by sulphide was demonstrated. These results clearly suggest that AC accepts electrons from the microbial oxidation of organic acids and transfers the electrons to azo dyes, thereby accelerating their biological reduction.

The research presented in this thesis makes clear that the reduction of azo dyes can be optimised by utilising redox mediators, i.e. either by continuous dosing of soluble quinones or by incorporation of AC in the sludge blanket. The potential of using redox mediators is probably not limited to enhancing azo dye reduction but may be extrapolated to other non-specific reductive (bio)transformations, e.g. reduction of halogenated or nitroaromatic compounds. The potential of using redox mediators is furthermore probably not limited to wastewater treatment but may also apply to bioremediation of soils polluted with e.g. polychlorinated solvents or nitroaromatic pesticides.

Quinones as electron acceptors and redox mediators for the anaerobic biotransformation of priority pollutants
Cervantes-Carrillo, F.J. - \ 2002
Wageningen University. Promotor(en): G. Lettinga; J.A. Field. - S.l. : S.n. - ISBN 9789058085672 - 162
anaërobe behandeling - anaërobe afbraak - verontreinigende stoffen - chinonen - humuszuren - redoxreacties - anaerobic treatment - anaerobic digestion - pollutants - quinones - humic acids - redox reactions
Humus is the most abundant organic fraction in the biosphere. It is composed of a complex structure in which recalcitrant polymers prevail with a residence time lasting decades or even centuries. Despite the recalcitrance of humic substances, they have recently been recognized to play an important role on the anaerobic conversion of organic matter by serving as an electron acceptor for microbial respiration. Quinone moieties are the responsible electron-accepting groups accounting for the microbial reduction of humus. Quinones and humus not only serve as terminal electron acceptors for microbial respiration, but they also function as redox mediators during the transfer of electrons in microbial and chemical reactions. In this dissertation the impact of humus and quinone analogues on the anaerobic biotransformation of ecologically important substrates, as well as priority pollutants, was evaluated.

Consortia obtained from many different environments including sandy, organic rich, and contaminated sediments, as well as anaerobic and aerobic sludges, showed the capacity for oxidizing a wide variety of ecologically significant substrates, such as lactate and acetate, when the humic model compound, anthraquinone-2,6-disulfonate (AQDS), was provided as a final electron acceptor. AQDS-reducing microorganisms out-competed methanogens for most of the substrates supplied indicating that quinone reduction is a widespread physiological process, which may contribute to important carbon cycling process in many different environments. Quinone and humus reduction was also found in pure cultures of different microorganisms, such as Desulfitobacterium spp. and Methanospirillum hungatei, indicating that the ubiquity of quinone reduction may be due to the wide diversity of microorganisms with the capacity for reducing humic substances. The results also illustrate that phylogenetically distinct microorganisms can channel electrons from anaerobic substrate oxidation via quinone reduction towards the reduction of metal oxides. Quinone respiring microorganisms could also be enriched and immobilized in the microbial community of an anaerobic granular sludge of a upflow anaerobic sludge blanket (UASB) reactor. The feasibility to immobilize quinone-reducing microorganisms can be applied to accelerate the conversion of xenobiotics susceptible to reductive biotransformations such as azo dyes and polychlorinated compounds in continuous bioreactors.

The long-term goal of this research was to explore the capacity of humus respiring consortia for oxidizing priority pollutants through the reduction of humic substances. Anaerobic granular sludge originated from different wastewater treatment plants were shown to oxidize phenol and p -cresol coupled to the reduction of AQDS. Both phenolic contaminants were converted to methane in the absence of the humic analogue, but addition of AQDS as an alternative electron acceptor diverted the flow of electrons from methanogenesis towards quinone reduction. Priority pollutants, which were not degraded under methanogenic conditions, could also be mineralized by humus-respiring consortia when humic substances were provided as an electron acceptor. Enriched sediments from different origins readily mineralized uniformly labeled [ 13C]toluene to 13CO 2 when humic acids or AQDS were provided as terminal electron acceptors. Negligible recovery of 13CO 2 occurred in the absence of humic substances. Additionally, the electrons in the toluene mineralized were recovered stoichiometrically as reduced humus or AH 2 QDS (reduced form of AQDS).

Humic substances were also shown to accelerate the transfer of reducing equivalents required for the anaerobic conversion of different pollutants containing electron-withdrawing groups. AQDS supplemented at sub-stoichiometric levels in granular sludge incubations enhanced the rate of conversion of carbon tetrachloride (CT) leading to an increased production of inorganic chloride. Negligible dechlorination occurred in sterile controls with autoclaved sludge and considerably less dechlorination was achieved in active controls lacking AQDS. A humus respiring enrichment culture, composed primarily of a Geobacter sp., derived from the same granular sludge was also shown to dechlorinate CT, yielding similar products as the AQDS-supplemented sludge consortium. Addition of catalytic levels of AQDS to a UASB reactor continuously treating the azo dye, acid orange 7 (AO7), also enhanced the biotransformation of this pollutant to the corresponding aromatic amines. High efficiency (>90 %) of decolorization of AO7 occurred even at a hydraulic residence time of 2 hours with a molar ratio of AQDS/AO7 as low as 1/100, whereas 70 % of color removal occurred in the absence of AQDS under the same hydraulic conditions.

The evidences provided in this study indicate that humic substances may play an important role on the stabilization of organic matter, as well as on the intrinsic bioremediation of contaminated environments, by serving as a terminal electron acceptor. The application of humic substances for achieving the bioremediation of contaminated aquifers can be considered. Humus and quinones can also be applied in anaerobic reactors to enhance the conversion of priority pollutants containing electron-withdrawing groups.

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