Granular activated carbon enhances the anaerobic digestion of solid and liquid fractions of swine effluent at different mesophilic temperatures.

OBJECTIVES: This study evaluated the effect of particle size and dosage of granular activated carbon (GAC) on methane production from the anaerobic digestion of raw effluent (RE) of swine wastewater, and the solid (SF) and liquid (LF) fractions. The effect of temperature using the selected size and dosage of GAC was also evaluated. METHODS: 60 mL of swine wastewater were inoculated with anaerobic granular sludge and GAC at different dosages and particle size. The cultures were incubated at different temperatures at 130 rpm. The kinetic parameters from experimental data were obtained using the Gompertz model. RESULTS: The cultures with the LF and GAC (75-150 µm, 15 g/L) increased 1.87-fold the methane production compared to the control without GAC. The GAC at 75-150 µm showed lower lag phases and higher Rmax than the cultures with GAC at 590-600 µm. The cumulative methane production at 45 °C with the RE + GAC was 7.4-fold higher than the control. Moreover, methane production at 45 °C significantly increased with the cultures LF + GAC (6.0-fold) and SF + GAC (2.0-fold). The highest production of volatile fatty acids and ammonium was obtained at 45 °C regardless of the substrate and the addition of GAC contributed to a higher extent than the cultures lacking GAC. In most cases, the kinetic parameters at 30 °C and 37 °C were also higher with GAC. CONCLUSIONS: GAC contributed to improving the fermentative and methanogenesis stages during the anaerobic digestion of fractions, evidenced by an improvement in the kinetic parameters.

Azo dye biotransformation mediated by AQS immobilized on activated carbon cloth in the presence of microbial inhibitors.

In this work, anthraquinone-2-sulfonate (AQS) was covalently immobilized onto activated carbon cloth (ACC), to be used as redox mediator for the reductive decolorization of reactive red 2 (RR2) by an anaerobic consortium. The immobilization of AQS improved the capacity of ACC to transfer electrons, evidenced by an increment of 3.29-fold in the extent of RR2 decolorization in absence of inhibitors, compared to incubations lacking AQS. Experiments conducted in the presence of vancomycin, an inhibitor of acidogenic bacteria, and with 2-bromoethane sulfonic acid (BES), an inhibitor of methanogenic archaea, revealed that acidogenic bacteria are the main responsible for RR2 biotransformation mediated by immobilized AQS. Nonetheless, the results also suggest that some methanogens are able to maintain their capacity to use immobilized AQS as an electron acceptor to sustain the decolorization process, even in the presence of BES.

Biotransformation of 4-nitrophenol by co-immobilized Geobacter sulfurreducens and anthraquinone-2-sulfonate in barium alginate beads.

Geobacter sulfurreducens and anthraquinone-2-sulfonate (AQS) were used suspended and immobilized in barium alginate during the biotransformation of 4-nitrophenol (4-NP). The assays were conducted at different concentrations of 4-NP (50-400 mg/L) and AQS, either in suspended (0-400 µM) or immobilized form (0 or 760 µM), and under different pH values (5-9). G. sulfurreducens showed low capacity to reduce 4-NP in absence of AQS, especially at the highest concentrations of the contaminant. AQS improved the reduction rates from 0.0086 h-1, without AQS, to 0.149 h-1 at 400 µM AQS, which represent an increment of 17.3-fold. The co-immobilization of AQS and G. sulfurreducens in barium alginate beads (AQSi-Gi) increased the reduction rates up to 4.8- and 7.2-fold, compared to incubations with G. sulfurreducens in suspended and immobilized form, but in absence of AQS. AQSi-Gi provides to G. sulfurreducens a barrier against the possibly inhibiting effects of 4-NP.

Application of redox mediators in bioelectrochemical systems.

Redox mediators (RM) are natural or artificial compounds used by microorganisms as electron acceptors and electron donors during electron transfer. Evidence collected in the last years indicates that the application of RM in bioelectrochemical systems (BES) enhanced the electron transfer from microorganisms to anodes and from cathodes to microorganisms. This review summarizes the results of using soluble or immobilized RM in BES to produce electricity and for the treatment of contaminants from wastewater effluents. In addition, future research focused on biohydrogen production, recovery or removal metals, and the use of humic substances (HS) extracted from natural environment is proposed.

Mechanism of anaerobic bio-reduction of azo dye assisted with lawsone-immobilized activated carbon.

Lawsone redox (LQ) mediator was covalently bound to granular activated carbon (GAC) by Fischer esterification. A high LQ adsorption capacity on GAC was achieved (∼230 mg/g), and desorption studies showed strong chemical stability. Furthermore, kinetic experiments with solid-phase redox mediator (RM) and their controls (soluble RM, GAC and anaerobic sludge) were tested for decolorization of congo red dye at initial concentration of 175 mg/L. Benzidine, a by-product of complete congo red reduction, was also measured by HPLC analysis along the kinetic experiments. The highest percentage of decolorization after 24 h of incubation was achieved in cultures with soluble (77%) and immobilized (70%) LQ. In contrast, low decolorization efficiency was reached in anaerobic bio-reduction assays with unmodified GAC (47%) and anaerobic sludge (28%) after 24 h. Removal of congo red by adsorption onto LQ-GAC was negligible. The rate of benzidine production was slower than decolorization rate, suggesting that one azo bond of congo red is selectively broke and followed by a slower breaking of the second azo bond, consequently, appearance of benzidine in solution. These issues could be attributed to the steric rearrangement and the inhibitory effects of the produced aromatic amines in the biotransformation process.

Quinone-functionalized activated carbon improves the reduction of congo red coupled to the removal of p-cresol in a UASB reactor.

In this research was immobilized anthraquinone-2-sulfonate (AQS) on granular activated carbon (GAC) to evaluate its capacity to reduce congo red (CR) in batch reactor and continuous UASB reactors. The removal of p-cresol coupled to the reduction of CR was also evaluated. Results show that the immobilization of AQS on GAC (GAC-AQS) achieved 0.469mmol/g, improving 2.85-times the electron-transferring capacity compared to unmodified GAC. In batch, incubations with GAC-AQS achieved a rate of decolorization of 2.64-fold higher than the observed with GAC. Decolorization efficiencies in UASB reactor with GAC-AQS were 83.9, 82, and 79.9% for periods I, II, and III; these values were 14.9-22.8% higher than the obtained by reactor with unmodified GAC using glucose as energy source. In the fourth period, glucose and p-cresol were simultaneously fed, increasing the decolorization efficiency to 87% for GAC-AQS and 72% for GAC. Finally, reactors efficiency decreased when p-cresol was the only energy source, but systems gradually recovered the decolorization efficiency up to 84% (GAC-AQS) and 71% (GAC) after 250 d. This study demonstrates the longest and efficient continuous UASB reactor operation for the reduction of electron-accepting contaminant in presence of quinone-functionalized GAC, but also using a recalcitrant pollutant as electron donor.

Influence of redox mediators and salinity level on the (bio)transformation of Direct Blue 71: kinetics aspects.

The rate-limiting step of azo dye decolorization was elucidated by exploring the microbial reduction of a model quinone and the chemical decolorization by previously reduced quinone at different salinity conditions (2-8%). Microbial experiments were performed in batch with a marine consortium. The decolorization of Direct Blue 71 (DB71) by the marine consortium at 2% salinity, mediated with anthraquinone-2,6-disulfonate (AQDS), showed the highest rate of decolorization as compared with those obtained with riboflavin, and two samples of humic acids. Moreover, the incubations at different salinity conditions (0-8%) performed with AQDS showed that the highest rate of decolorization of DB71 by the marine consortium occurred at 2% and 4% salinity. In addition, the highest microbial reduction rate of AQDS occurred in incubations at 0%, 2%, and 4% of salinity. The chemical reduction of DB71 by reduced AQDS occurred in two stages and proceeded faster at 4% and 6% salinity. The results indicate that the rate-limiting step during azo decolorization was the microbial reduction of AQDS.

Efficient anaerobic treatment of synthetic textile wastewater in a UASB reactor with granular sludge enriched with humic acids supported on alumina nanoparticles.

A novel technique to co-immobilize humus-reducing microorganisms and humic substances (HS), supported on γ-Al2O3 nanoparticles (NP), by a granulation process in an upflow anaerobic sludge bed (UASB) reactor is reported in the present work. Larger granules (predominantly between 1 and 1.7 mm) were produced using NP coated with HS compared to those obtained with uncoated NP (mostly between 0.25 and 0.5 mm). The HS-enriched granular biomass was then tested for its capacity to achieve the reductive decolorization of the recalcitrant azo dye, reactive red 2 (RR2), in the same UASB reactor operated with a hydraulic residence time of 12 h and with glucose as electron donor. HS-enriched granules achieved higher decolorization and COD removal efficiencies, as compared to the control reactor operated in the absence of HS, in long term operation and applying high concentrations of RR2 (40-400 mg/L). This co-immobilizing technique could be attractive for its application in UASB reactors for the reductive biotransformation of several contaminants, such as nitroaromatics, poly-halogenated compounds, metalloids, among others.

Biodegradation of p-cresol and sulfide removal by a marine-denitrifying consortium.

The simultaneous removal of sulfide and p-cresol was carried out by using a marine-denitrifying consortium collected in the coastal zone of Sonora, Mexico. Different experimental conditions were used to evaluate the capacity of the consortium to simultaneously eliminate nitrate, sulfide, and p-cresol. For instance, the first set of assays was conducted at different sulfide concentrations (20, 50, and 100 mg S(2À) L(À1) ), with a fixed concentration of p-cresol (45 mg C L(À1) ). The second set of assays was developed at different concentrations of p-cresol (45, 75, and 100 mg C L(-1) ), in the presence of 20 mg S(2À) L(À1) . In all cases, the concentration of nitrate was stoichiometrically added for the complete oxidization of the substrates. The results showed removal efficiencies up to 92% for p-cresol and nitrate at 20 and 50 mg S(2À) L(À1) ; whereas at 100 mg S(2À) L(À1) removal efficiencies were 77% and 59% for p-cresol and nitrate, respectively. On the other hand, sulfide (20 mg L(À1) ) was completely removed under different concentrations of p-cresol tested, with a partial accumulation of nitrite according to the increment of p-cresol concentration. The results obtained indicate that the marine consortium was able to simultaneously remove the pollutants studied.

Humus-reducing microorganisms and their valuable contribution in environmental processes.

Humus constitutes a very abundant class of organic compounds that are chemically heterogeneous and widely distributed in terrestrial and aquatic environments. Evidence accumulated during the last decades indicating that humic substances play relevant roles on the transport, fate, and redox conversion of organic and inorganic compounds both in chemically and microbially driven reactions. The present review underlines the contribution of humus-reducing microorganisms in relevant environmental processes such as biodegradation of recalcitrant pollutants and mitigation of greenhouse gases emission in anoxic ecosystems, redox conversion of industrial contaminants in anaerobic wastewater treatment systems, and on the microbial production of nanocatalysts and alternative energy sources.

Simultaneous biodegradation of phenol and carbon tetrachloride mediated by humic acids.

The capacity of an anaerobic sediment to achieve the simultaneous biodegradation of phenol and carbon tetrachloride (CT) was evaluated, using humic acids (HA) as redox mediator. The presence of HA in sediment incubations increased the rate of biodegradation of phenol and the rate of dehalogenation (2.5-fold) of CT compared to controls lacking HA. Further experiments revealed that the electron-accepting capacity of HA derived from different organic-rich environments was not associated with their reducing capacity to achieve CT dechlorination. The collected kinetic data suggest that the reduction of CT by reduced HA was the rate-limiting step during the simultaneous biodegradation of phenol and CT. To our knowledge, the present study constitutes the first demonstration of the simultaneous biodegradation of two priority pollutants mediated by HA.

Assessing the impact of alumina nanoparticles in an anaerobic consortium: methanogenic and humus reducing activity.

The impact of γ-Al(2)O(3) nanoparticles (NP) on specific methanogenic activity (SMA) and humus reducing activity (HRA) in an anaerobic consortium was evaluated. SMA in sludge incubations without γ-Al(2)O(3) was always higher compared with those performed in the presence of 100 g/L of γ-Al(2)O(3). Nevertheless, the SMA in incubations with γ-Al(2)O(3) was not completely inhibited, indicating that some methanogenic microorganisms were physiologically active even in the presence of γ-Al(2)O(3) NP during the incubation period (~400 h). SMA and HRA of the anaerobic consortium were also conducted in the presence of γ-Al(2)O(3) NP coated with humic acids (HA). Microbial HA reduction occurred 3.7-fold faster using HA immobilized on γ-Al(2)O(3) NP (HA(Imm)), compared with the control with suspended HA (HA(Sus)). Furthermore, immobilized HA decreased the toxicological effects of γ-Al(2)O(3) NP on methanogenesis. Scanning electron microscopy (SEM) images revealed cell membrane damage in those sludge incubations exposed to uncoated γ-Al(2)O(3) NP. In contrast, cell damage was not observed in incubations with HA-coated γ-Al(2)O(3) NP. Methanogenesis out-competed microbial humus reduction regardless if HA was HA(Imm) or HA(Sus). The present study provides a clear demonstration that HA immobilized in γ-Al(2)O(3) NP are effective terminal electron acceptor for microbial respiration and suggests that HA could mitigate the toxicological effects of metal oxide NP on anaerobic microorganisms.

Immobilized humic substances on an anion exchange resin and their role on the redox biotransformation of contaminants.

A novel technique to immobilize humic substances (HS) on an anion exchange resin is presented. Immobilized HS were demonstrated as an effective solid-phase redox mediator (RM) during the reductive biotransformation of carbon tetrachloride (CT) and the azo model compound, Reactive Red 2 (RR2). Immobilized HS increased ∼4-fold the extent of CT reduction to chloroform by a humus-reducing consortium in comparison to incubations lacking HS. Immobilized HS also increased 2-fold the second-order rate constant of decolorization of RR2 as compared with sludge incubations lacking HS. To our knowledge, the present study constitutes the first demonstration of immobilized HS serving as an effective solid-phase RM during the reductive biotransformation of priority contaminants. The immobilizing technique developed could be appropriate for enhancing the redox biotransformation of recalcitrant pollutants in anaerobic wastewater treatment systems.

Voltage noise influences action potential duration in cardiac myocytes.

Stochastic gating of ion channels introduces noise to membrane currents in cardiac muscle cells (myocytes). Since membrane currents drive membrane potential, noise thereby influences action potential duration (APD) in myocytes. To assess the influence of noise on APD, membrane potential is in this study formulated as a stochastic process known as a diffusion process, which describes both the current-voltage relationship and voltage noise. In this framework, the response of APD voltage noise and the dependence of response on the shape of the current-voltage relationship can be characterized analytically. We find that in response to an increase in noise level, action potential in a canine ventricular myocytes is typically prolonged and that distribution of APDs becomes more skewed towards long APDs, which may lead to an increased frequency of early after-depolarization formation. This is a novel mechanism by which voltage noise may influence APD. The results are in good agreement with those obtained from more biophysically-detailed mathematical models, and increased voltage noise (due to gating noise) may partially underlie an increased incidence of early after-depolarizations in heart failure.