Impact of the redox mediator sodium anthraquinone-2,6-disulphonate (AQDS) on the reductive decolourisation of the azo dye Reactive Red 2 (RR2) in one- and two-stage anaerobic systems

Color removal in acidogenic reactor followed by aerobic granular sludge reactor: Operational and microbiological aspects

This work investigated the operational and microbiological aspects of the decolorization of the azo dye Reactive Black 5 in acidogenic reactors followed by aerobic granular sludge (AGS) reactors, evaluating the effect of the acidogenic hydraulic retention time (HRT) (3, 2, and 1 h), effluent recirculation in the AGS reactor (50 mL min-1), dye concentration (50 and 100 mg L-1), and the redox mediator sodium anthraquinone-2-disulfonate (AQS) (50 μM). The acidogenic reactors were mainly responsible for the dye decolorization, with AQS significantly improving its efficiency and enabling the use of a shorter HRT (2 h). The recirculation effect was not so evident, probably masked by the adaptation of the acidogenic microbiota. Increasing the dye concentration did not affect the total decolorization, but reduced nitrogen removal in the AGS reactors. Furthermore, the dye and its byproducts may have negatively affected the long-term AGS stability. While the acidogenic microbiota maintained its diversity, the AGS tended to become more specialist. However, in both, some abundant genera that may have acted in reducing the dye were found, such as Clostridium_sensu_stricto_1 and Raoutella in the acidogenic sludge and Dechloromonas and Defluviicoccus in the AGS.

Accelerating denitrification and mitigating nitrite accumulation by multiple electron transfer pathways between Shewanella oneidensis MR-1 and denitrifying microbial community

The bio-denitrification was usually retarded by the unbalance of electron generation and consumption. In this study, mixing S. oneidensis MR-1 with denitrifying microbial community increased the nitrogen removal rate by 74.74 % via the interspecies electron transfer (IET), and reduced the accumulated nitrite from 9.90 ± 0.81 to 0.02 ± 0.03 mg/L. Enhanced denitrification still appeared but relatively decreased, when S. oneidensis MR-1 was separated by a dialysis bag (MW < 3000), indicating mediated interspecies electron transfer (MIET) counted in IET. The results of electron transfer activity and sludge conductivity suggested DIET and MIET jointly transfer electrons from MR-1 to electroactive denitrifying bacteria (EDB), improving denitrifying reductase activities. Electron distribution among denitrifying reductases was found to be associated with the IET rate. Microbial insights showed the total abundance of EDB was increased, and denitrifying genes were correspondingly enriched. Pseudomonas was found to cooperate with exoelectrogens in a complicated microbial community.

Effects of the antibiotics trimethoprim (TMP) and sulfamethoxazole (SMX) on granulation, microbiology, and performance of aerobic granular sludge systems

This work assessed the effect of the antibiotics trimethoprim (TMP) and sulfamethoxazole (SMX) on the granulation process, microbiology, and organic matter and nutrient removal of an aerobic granular sludge (AGS) system. In addition, after the maturation stage, the impact of the redox mediator anthraquinone-2,6-disulfonate (AQDS) (25 μM) on the biotransformation of the antibiotics was evaluated. The reactor R1 was maintained as a control, and the reactor R2 was supplemented with TMP and SMX (200 μg L^-1). The ability to remove C, N, and P was similar between the reactors. However, the structural integrity of the AGS was impaired by the antibiotics. Low TMP (∼30%) and SMX (∼60%) removals were achieved when compared to anaerobic or floccular biomass aerobic systems. However, when the system was supplemented with AQDS, an increase in the removal of TMP (∼75%) and SMX (∼95%) was observed, possibly due to the catalytic action of the redox mediator on cometabolic processes. Regarding the microbial groups, whereas Proteobacteria and Bacterioidetes increased, Planctomycetes decreased in both reactors. However, TMP and SMX presence seemed to inhibit or favor some genera during the formation of the granules, possibly due to their bactericidal action.

Efficient Biodegradation of Azo Dyes Catalyzed by the Anthraquinone-2-sulfonate and Reduced Graphene Oxide Nanocomposite

An anthraquinone-2-sulfonate and reduced graphene oxide nanocomposite (AQS@rGO) was prepared and the improvement on the biotic reduction of a pollutant, i.e., azo dye, was demonstrated. Electron paramagnetic resonance signal of the semi-quinone radical in the well-dispersed solid AQS@rGO solution was detected. Although the as-prepared AQS@rGO has a negligible adsorption capacity toward methyl orange (MO) dye, the decolorization efficiencies in both flask experiments and sequencing operation reactors in the presence of AQS@rGO were increased by more than 1.5 times as compared to that with graphene oxide, and an efficient and continuable catalytic effect on the decolorization of azo dyes in seven operation periods was maintained. The catalytic effect on reduction was caused by the formation of a space-charge layer, which facilitates the efficient e- transfer from the conductive rGO sheets to the C=O of the AQS molecule. The results suggested that the AQS@rGO may act as an efficient insoluble redox mediator, which is important for the pollution control by accelerating the extracellular electron transfer.

Two-stage anaerobic process benefits removal for azo dye orange II with starch as primary co-substrate

Two-stage anaerobic system (S1: R1 (acidogenic phase) + R2 (methanogenic phase)) and the one-stage control (S0) were established to investigate the effect of phase separation on the removal of an azo dye orange II, i.e., Acid Orange 7 (AO7), with starch as the primary co-substrate. Although final AO7 removal from two systems showed no statistical differences, the first-order rate constants for AO7 removal (k_AO7-) and sulfanilic acid (SA) formation (k_SA) were higher in S1. Kinetic analysis showed that k_AO7- and k_SA in S1 were 2.7-fold and 1.7-fold of those in S0, respectively, indicating the benefit of phase separation to the AO7 reduction. However, this benefit only appeared in the period with influent AO7 concentrations higher than 2.14 mM. Otherwise, this advantage would be hidden due to the longer HRT (5 d) and sufficient electron donor (1.0 g starch L^-1). Within S1, R1 only contributed about 10% of the entire AO7 removal, and k_AO7- in R1 (0.172 h^-1) was much lower than in R2 (0.503 h^-1). The methanogenic phase rather than acidogenic phase was the main contribution to AO7 removal, because the influent of R2 had more available electron donors and suitable pH condition (pH 6.5-7.0) for the bio-reduction process.

Catalytic Oxidation Process for the Degradation of Synthetic Dyes: An Overview

Dyes are used in various industries as coloring agents. The discharge of dyes, specifically synthetic dyes, in wastewater represents a serious environmental problem and causes public health concerns. The implementation of regulations for wastewater discharge has forced research towards either the development of new processes or the improvement of available techniques to attain efficient degradation of dyes. Catalytic oxidation is one of the advanced oxidation processes (AOPs), based on the active radicals produced during the reaction in the presence of a catalyst. This paper reviews the problems of dyes and hydroxyl radical-based oxidation processes, including Fenton's process, non-iron metal catalysts, and the application of thin metal catalyst-coated tubular reactors in detail. In addition, the sulfate radical-based catalytic oxidation technique has also been described. This study also includes the effects of various operating parameters such as pH, temperature, the concentration of the oxidant, the initial concentration of dyes, and reaction time on the catalytic decomposition of dyes. Moreover, this paper analyzes the recent studies on catalytic oxidation processes. From the present study, it can be concluded that catalytic oxidation processes are very active and environmentally friendly methods for dye removal.

Highly efficient nitrate removal in a heterotrophic denitrification system amended with redox-active biochar: A molecular and electrochemical mechanism

Biochar is widely used in water treatment because of its porous structure, however, the effects of biochars on denitrification remain unclear. Here, we combined molecular biological and electrochemical techniques to investigate effects of biochars (formed at 300 °C, 500 °C and 800 °C) on denitrification. Results showed that biochar at 300 °C increased total nitrogen removal by 415% and decreased N₂O accumulation by 78%. Mechanistic research demonstrated that it achieved the highest electron transfer efficiency and denitrifying enzyme activities. Further study evidenced that biochar at 300 °C increased the abundance of denitrifiers such as Pseudomonas. Correlation analysis indicated that nitrate reductase and nitrite reductase activities were the key factors influenced by biochar during denitrification. Overall, this study suggested that biochar at 300 °C could act as the bio-engineer of electron shuttle and the stimulator of denitrification, achieving high rate nitrogen removal and significant reduction of N₂O accumulation from high-strength wastewater.

Enhanced bioelectricity generation and azo dye treatment in a reversible photo-bioelectrochemical cell by using novel anthraquinone-2,6-disulfonate (AQDS)/MnOx-doped polypyrrole film electrodes

A novel anthraquinone-2,6-disulfonate/MnO_x-doped polypyrrole film (AQDS/Mn/PPy) electrode was prepared by one-step electropolymerization method and was used to improve performance of a reversible photo-bioelectrochemical cell (RPBEC). The RPBEC was operated in polarity reversion depended on dark/light reaction of alga Chlorella vulgaris by which sequential decolorization of azo dye and mineralization of decolorization products coupled with bioelectricity generation can be achieved. The results showed that formation of uniform AQDS/Mn/PPy film significantly enhanced electroactive surface area and electrocatalytic activity of carbon electrode. The RPBEC with AQDS/Mn/PPy electrodes demonstrated 77% increases in maximum power and 73% increases in Congo red decolorization rate before polarity reversion, and 198% increases in maximum power and 138% increases in decolorization products mineralization rate after polarity reversion, respectively, compared to the RPBEC with bare electrode. This was resulted from simultaneous dynamics improvement in half-reaction rate of anode and photo-biocathode due to enhanced electron transfer and algal-bacterial biofilm formation.

In-situ formation and immobilization of biogenic nanopalladium into anaerobic granular sludge enhances azo dyes degradation

Azo dyes are toxic and recalcitrant wastewater pollutants. An innovative technology based on biogenic nanopalladium (Bio-Pd) supported anaerobic granular sludge (AGS) was developed for azo dyes reduction. In-situ formation of Bio-Pd in the AGS was observed by Scanning Electron Microscopy coupled with Energy Dispersive Spectrometer (SEM-EDS). The Pd associated AGS (Pd-AGS) showed enhanced decolorization rates to the three azo dyes of Congo Red, Evans Blue and Orange II, with the degradation kinetic constants increased by 2.3-10 fold compared to the control AGS in the presence of electron donor formate. Impacts of different electron donors on Orange II decolorization were further investigated. Results showed that formic acid, formate, acetate, glucose, ethanol and lactate could serve as electron and hydrogen donors to stimulate Orange II decolorization by the Pd-AGS, and their activities followed the order: formic acid > formate > ethanol > glucose > lactate > acetate. Most of the Bio-Pd was bound with microbes in the AGS with a small fraction in the extracellular polymer substances (EPS). Transmission Electronic Microscopy analysis revealed that the Bio-Pd formed in the periplasmic space, cytoplasm and on the cell walls of bacteria. This study provides a new concept for azo dye reduction, which couples sludge microbial degradation ability with Bio-Pd catalytic ability via in-situ formation and immobilization of Bio-Pd into AGS, and offers an alternative for the current azo dye treatment technology.

Catalytic performance of functionalized polyurethane foam on the reductive decolorization of Reactive Red K-2G in up-flow anaerobic reactor under saline conditions

Soluble anthraquinone compounds including anthraquinone-2-sulfonate (AQS) and anthraquinone-2,6-disulfonate can accelerate anaerobic decolorization of azo dyes. To realize the application of these compounds, the catalytic performance and stability of AQS-modified polyurethane foam (AQS-PUF) for Reactive Red K-2G decolorization were investigated in an up-flow anaerobic bioreactor under saline conditions. The results showed that the optimal influent pH value and hydraulic retention time were 7 and 10 h, respectively, in a continuous-flow bioreactor amended with AQS-PUF (R1). Under the above conditions, R1 (93.8 % color removal) displayed better decolorization performance than the bioreactor amended with PUF (R2, 64 % color removal) in 10 days, when influent K-2G concentration was 50 mg/L. Moreover, compared with R2, R1 could more effectively cope with 50-400 mg/L K-2G and exhibited better stability with over 85 % color removal efficiency within 75 days. Further bacterial community analysis using polymerase chain reaction-denaturing gradient gel electrophoresis showed that AQS-reducing bacteria played an important role in accelerating K-2G decolorization in R1. Extracellular polymeric substances analysis found that biofilm formed on AQS-PUF had very limited negative effects on K-2G decolorization. The catalytic performance of used AQS-PUF only decreased less than 9 % in batch experiments. These findings indicate that AQS-PUF has potential application for the treatment of azo dye-containing wastewater.

Study the catalyzing mechanism of dissolved redox mediators on bio-denitrification by metabolic inhibitors

The effect of dissolved redox mediators on denitrification by Paracoccus versutus sp. GW1 were studied with α-AQS, AQS, 1,5-AQDS, AQDS and 2,7-AQDS. AQDS presented the best catalytic effect which increased denitrification rate by 1.5-fold with 0.17 mmol L(-1). Denitrification rate was found to be pseudo-zero order with 0-0.32 mmol L(-1) AQDS (k=19.874C AQDS+10.301 (R(2)=0.9984)). The accelerating mechanism for respiratory chain was also explored by using metabolic inhibitors. Addition of AQDS eased the inhibitions of rotenone, dicumarol, sodium azide on respiratory chain, and enhanced the nitrite accumulation during bio-denitrification process. AQDS exhibited slight catalytic effect on nitrite denitrification. These results allowed us to deduce the accelerating site of AQDS on the denitrification respiratory chain. Increased nitrate denitrification by AQDS might mainly attribute to the accelerated electrons transfer from NADH to nitrate reductase via complex I and ubiquinone/ubiqunol. This study provided further understanding of the mechanism of redox mediators on denitrification.