Luis H. Alvarez

Immobilized redox mediator on metal-oxides nanoparticles and its catalytic effect in a reductive decolorization process.

Different metal-oxides nanoparticles (MONP) including α-Al(2)O(3), ZnO and Al(OH)(3), were utilized as adsorbents to immobilize anthraquinone-2,6-disulfonate (AQDS). Immobilized AQDS was subsequently tested as a solid-phase redox mediator (RMs) for the reductive decolorization of the azo dye, reactive red 2 (RR2), by anaerobic sludge. The highest adsorption capacity of AQDS was achieved on Al(OH)(3) nanoparticles, which was ∼0.16 mmol g(-1) at pH 4. Immobilized AQDS increased up to 7.5-fold the rate of decolorization of RR2 by anaerobic sludge as compared with sludge incubations lacking AQDS. Sterile controls including immobilized AQDS did not show significant (<3.5%) RR2 decolorization, suggesting that physical-chemical processes (e.g. adsorption or chemical reduction) were not responsible for the enhanced decolorization achieved. Immobilization of AQDS on MONP was very stable under the applied experimental conditions and spectrophotometric screening did not detect any detachment of AQDS during the reductive decolorization of RR2, confirming that immobilized AQDS served as an effective RMs. The present study constitutes the first demonstration that immobilized quinones on MONP can serve as effective RMs in the reductive decolorization of an azo dye. The immobilizing technique developed could be applied in anaerobic wastewater treatment systems to accelerate the redox biotransformation of recalcitrant pollutants.

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.

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.

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.

Decolorization and biogas production by an anaerobic consortium: effect of different azo dyes and quinoid redox mediators

The inhibitory effect of azo dyes and quinoid compounds on an anaerobic consortium was evaluated during a decolorization process and biogas production. In addition, the impact of quinoid compounds such as lawsone (LAW) and anthraquinone-2,6-disulfonate (AQDS) on the rate of decolorization of Direct Blue 71 (DB71) was assessed. The anaerobic consortium was not completely inhibited under all tested dye concentrations (0.1-2 mmol l(-1)), evidenced by an active decolorization process and biogas production. The presence of quinoid compounds at different concentrations (4, 8, and 12 mmol l(-1)) also inhibited biogas production compared to the control incubated without the quinoid compounds. In summary, the anaerobic consortium was affected to a greater extent by increasing the quantity of azo dyes or quinoid compounds. Nevertheless, at a lower concentration (1 mmol l(-1)) of quinoid compounds, the anaerobic consortium effectively decolorized 2 mmol l(-1) of DB71, increasing up to 5.2- and 20.4-fold the rate of decolorization with AQDS and LAW, respectively, compared to the control lacking quinoid compounds.

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.

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 S2− L−1), with a fixed concentration of p‐cresol (45 mg C L−1). 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 S2− L−1. 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 S2− L−1; whereas at 100 mg S2− L−1 removal efficiencies were 77% and 59% for p‐cresol and nitrate, respectively. On the other hand, sulfide (20 mg L−1) 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.

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.

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.