2,4,6-trichlorophenol (TCP) photobiodegradation and its effect on community structure

Integration of microbial electrochemical systems and photocatalysis for sustainable treatment of organic recalcitrant wastewaters: Main mechanisms, recent advances, and present prospects

In recent years, microbial electrochemical systems (MESs) have demonstrated to be an environmentally friendly technology for wastewater treatment and simultaneous production of value-added products or energy. However, practical applications of MESs for the treatment of recalcitrant wastewater are limited by their low power output and slow rates of pollutant biodegradation. As a novel technology, hybrid MESs integrating biodegradation and photocatalysis have shown great potential to accelerate the degradation of bio-recalcitrant pollutants and increase the system output. In this review, we summarize recent advances of photo-assisted MESs for enhanced removal of recalcitrant pollutants, and present further discussion about the synergistic effect of biodegradation and photocatalysis. In addition, we analyse in detail different set-up configurations, discuss mechanisms of photo-enhanced extracellular electron transfer, and briefly present ongoing research cases. Finally, we highlight the current limitations and corresponding research gaps, and propose insights for future research.

Synergistic degradation for o-chlorophenol and enhancement of power generation by a coupled photocatalytic-microbial fuel cell system

A hierarchically photocatalytic microbial fuel cell system (PMFC) coupled with TiO₂ photoanode and bioanode was established to enhance the power generation based on single-chamber MFC. Compared with the conventional anaerobic mode, oxygen in the solution could be utilized by the photoanode of PMFC to improve the removal of o-chlorophenol (2-CP). The maximum power densities were increasing from 261 (MFC) to 301 mW/m² (PMFC). The removal efficiency of 2-CP (5 mg/L) in PMFC was 76.20% and higher than that in MFC (19.33%) and by photocatalysis (49.23%). The electron-hole separation efficiencies were decreasing with the increasing of dissolved oxygen, causing a low efficiency of photocatalysis, due to the reduction of the current density of the systems. The abundance of Geobacter sp., PHOS-HE36 fam., and Pseudomonas sp. was increased with illumination, contributing to improve the electricity production and 2-CP degradation. The only detective intermediate of 1,2-dichlorobenzene in PMFC indicated that the microbes could regulate the degradation pathway of 2-CP in the coupling system. These findings provided an feasible method for the effective degradation of refractory organic compounds and simultaneous energy recovery by combining photocatalysis and microbial power generation.

Degradation of low-concentration perfluorooctanoic acid via a microbial-based synergistic method: assessment of the feasibility and functional microorganisms

Perfluorooctanoic acid (PFOA) is persistent in the environment. The activities of microorganisms alone are insufficient for the decomposition of PFOA, but microorganisms can contribute positively to the degradation of PFOA in synergistic systems. Herein, a synergistic system combining photocatalytic decay with microbial degradation of the transformation products was applied to degrade 500.0 μg L^-1 PFOA. The microorganisms increased the total removed percentage by 30.7% to a final percentage of 79.7 ± 9.4% in comparison with the photocatalytic method alone. Moreover, an additional 44.2% of removed total organic carbon and additional defluorination percentage of 24.5% were obtained after the synergistic tests. The 16S RNA sequencing analysis indicated that Stenotrophomonas, Bacillus, Pseudomonas, and Brevundimonas were highly enriched in the functional microbial community, which was simultaneously shaped by photocatalysis and substances. This study found it would be feasible to use a synergistic method containing photocatalysis and a microbial community for the degradation of low-concentrations of PFOA, and the results provided a reference to modified the removal efficiency of the synergistic system by looking insight into the relationship between the functional microbial community and PFOA.

Intimate coupling of photocatalysis and biodegradation for wastewater treatment: Mechanisms, recent advances and environmental applications

Due to the increase of emerging contaminants in water, how to use new treatment technology to make up for the defects of traditional wastewater treatment method has become one of the research hotspots at present. Intimate coupling of photocatalysis and biodegradation (ICPB) as a novel wastewater treatment method, which combines the advantages of biological treatment and photocatalytic reactions, has shown a great potential as a low-cost, environmental friendly and sustainable treatment technology. The system mainly consists of photocatalytic materials, porous carriers and biofilm. The key principle of ICPB is to transform bio-recalcitrant pollutants into biodegradable products by photocatalysis on the surface of porous carriers. The biodegradable products were mineralized simultaneously through the biofilm inside the carriers. Because of the protection of the carriers, the microorganism can remain active even under the UV-light, the mechanical force of water flow or the attack of free radicals. ICPB breaks the traditional concept that photocatalytic reaction and biodegradation must be separated in different reactors, improves the purification capacity of sewage and saves the cost. This review summarizes the recent advances of ICPB photocatalysts, carriers and biofilm being applied, and focuses on the mechanisms and reactor configurations which is particularly novel. Furthermore, the possible ongoing researches on ICPB are also put forward. This review will provide a valuable insight into the design and application of ICPB in environment and energy field.

Synergic degradation of 2,4,6-trichlorophenol in microbial fuel cells with intimately coupled photocatalytic-electrogenic anode

A microbial fuel cell system with intimately coupled photocatalytic-electrogenic anode (photocatalytic-MFC) was proposed for the synergetic degradation of 2,4,6-trichlorophenol (2,4,6-TCP) which has a structure of three chlorine groups connecting to a phenol ring and is well recognized as a recalcitrant pollutant for its high toxicity, bioaccumulation and persistence. The photocatalytic-electrogenic anode was prepared by coating mpg-C₃N₄ on a carbon felt anode, followed by inoculating with municipal sewage and acclimating with 2,4,6-TCP at gradient concentrations. Improved TCP degradation was achieved, showing 79.3% of TCP removal in 10 h with an original concentration of 200 mg L^-1, which was higher than that obtained with the unilluminated MFC (66.0%) and the photocatalytic-only process (56.1%). The coupled photocatalytic-electrogenic process demonstrated different degradation pathways compared with the photocatalytic-only process, with one open-chain compound (2-chloro-4-keto-2-hexenedioic acid, 2-CMA) detected in the photocatalytic-MFC system. Microbial community analysis revealed that Pseudomonas, instead of Geobacter observed in the unilluminated MFC bioanode, dominated in the photocatalytic-electrogenic anode MFC biofilm, which might be responsible for enhanced current generation in the coupled system. In addition, biofilm rich with Rhodococcus on air-cathode was also responsible for the enhanced TCP removal. This research provides an efficient strategy for the treatment of wastewater with recalcitrant contaminants by intimate-coupling of the photocatalytic and the electrogenic processes.

Roles of an easily biodegradable co-substrate in enhancing tetracycline treatment in an intimately coupled photocatalytic-biological reactor

Intimately coupled photocatalysis and biodegradation (ICPB) was realized in a macroporous carrier in which a photocatalyst was present on the outer surface, while a biofilm accumulated inside the carrier. In ICPB, photocatalysis products are rapidly biodegraded by a protected biofilm, leading to mineralization of the refractory organics, such as antibiotics. However, mineralization in ICPB could be compromised if the photocatalysis products remain refractory or are inhibitory. To address this, we attempted to increase metabolic activity by providing a readily biodegradable co-substrate (acetate) that could act as a source of energy and electrons to improve biotransformation and mineralization of the refractory antibiotic tetracycline (TCH). When we added acetate during ICPB of TCH, TCH removal increased by ∼5%, mineralization increased by ∼20%, and almost all photocatalysis products disappeared. Acetate addition also led to an increase in active biomass, an increase in the biomass's respiratory activity, and evolution of the microbial community to having more members able to biodegrade photocatalysis and biotransformation intermediates. Thus, providing an easily biodegradable co-substrate was an effective means for enhancing TCH removal and mineralization with the ICPB technology.

Rapid restoration of methanogenesis in an acidified UASB reactor treating 2,4,6-trichlorophenol (TCP)

Anaerobic bioreactors are often used for removal of xenobiotic and highly toxic pollutants from wastewater. Most of the time, the pollutant is so toxic that the stability of the reactor becomes compromised. It is well known that methanogens are one of the most sensitive organisms in the anaerobic consortia and hence the stability of the reactors is highly dependant on methanogenesis. Unfortunately few studies have focused on recovering the methanogenic activity once it has been inhibited by highly toxic pollutants. Here we establish a quick recovery strategy for neutralization of an acidified UASB reactor after failure by intoxication with an excess of TCP in the influent. Once the reactor returned to pH values compatible with methanogenesis, biogas production was re-started after one day and the system was re-acclimated to TCP. Successful removal of TCP from synthetic wastewater was shown for concentrations up to 70mg/L after restoration.

Support vector regression model of wastewater bioreactor performance using microbial community diversity indices: effect of stress and bioaugmentation

The relationship between microbial community structure and function has been examined in detail in natural and engineered environments, but little work has been done on using microbial community information to predict function. We processed microbial community and operational data from controlled experiments with bench-scale bioreactor systems to predict reactor process performance. Four membrane-operated sequencing batch reactors treating synthetic wastewater were operated in two experiments to test the effects of (i) the toxic compound 3-chloroaniline (3-CA) and (ii) bioaugmentation targeting 3-CA degradation, on the sludge microbial community in the reactors. In the first experiment, two reactors were treated with 3-CA and two reactors were operated as controls without 3-CA input. In the second experiment, all four reactors were additionally bioaugmented with a Pseudomonas putida strain carrying a plasmid with a portion of the pathway for 3-CA degradation. Molecular data were generated from terminal restriction fragment length polymorphism (T-RFLP) analysis targeting the 16S rRNA and amoA genes from the sludge community. The electropherograms resulting from these T-RFs were used to calculate diversity indices - community richness, dynamics and evenness - for the domain Bacteria as well as for ammonia-oxidizing bacteria in each reactor over time. These diversity indices were then used to train and test a support vector regression (SVR) model to predict reactor performance based on input microbial community indices and operational data. Considering the diversity indices over time and across replicate reactors as discrete values, it was found that, although bioaugmentation with a bacterial strain harboring a subset of genes involved in the degradation of 3-CA did not bring about 3-CA degradation, it significantly affected the community as measured through all three diversity indices in both the general bacterial community and the ammonia-oxidizer community (α = 0.5). The impact of bioaugmentation was also seen qualitatively in the variation of community richness and evenness over time in each reactor, with overall community richness falling in the case of bioaugmented reactors subjected to 3-CA and community evenness remaining lower and more stable in the bioaugmented reactors as opposed to the unbioaugmented reactors. Using diversity indices, 3-CA input, bioaugmentation and time as input variables, the SVR model successfully predicted reactor performance in terms of the removal of broad-range contaminants like COD, ammonia and nitrate as well as specific contaminants like 3-CA. This work was the first to demonstrate that (i) bioaugmentation, even when unsuccessful, can produce a change in community structure and (ii) microbial community information can be used to reliably predict process performance. However, T-RFLP may not result in the most accurate representation of the microbial community itself, and a much more powerful prediction tool can potentially be developed using more sophisticated molecular methods.