Sustainable bioreduction of toxic levels of chromate in a denitrifying granular sludge reactor

Bioreduction of chromate in a syngas-based membrane biofilm reactor

This study leveraged synthesis gas (syngas), a renewable resource attainable through the gasification of biowaste, to achieve efficient chromate removal from water. To enhance syngas transfer efficiency, a membrane biofilm reactor (MBfR) was employed. Long-term reactor operation showed a stable and high-level chromate removal efficiency > 95%, yielding harmless Cr(III) precipitates, as visualised by scanning electron microscopy and energy dispersive X-ray analysis. Corresponding to the short hydraulic retention time of 0.25 days, a high chromate removal rate of 80 µmol/L/d was attained. In addition to chromate reduction, in situ production of volatile fatty acids (VFAs) by gas fermentation was observed. Three sets of in situ batch tests and two groups of ex situ batch tests jointly unravelled the mechanisms, showing that biological chromate reduction was primarily driven by VFAs produced from in situ syngas fermentation, whereas hydrogen originally present in the syngas played a minor role. 16 S rRNA gene amplicon sequencing has confirmed the enrichment of syngas-fermenting bacteria (such as Sporomusa), who performed in situ gas fermentation leading to the synthesis of VFAs, and organics-utilising bacteria (such as Aquitalea), who utilised VFAs to drive chromate reduction. These findings, combined with batch assays, elucidate the pathways orchestrating synergistic interactions between fermentative microbial cohorts and chromate-reducing microorganisms. The findings facilitate the development of cost-effective strategies for groundwater and drinking water remediation and present an alternative application scenario for syngas.

Growth-dependent cr(VI) reduction by Alteromonas sp. ORB2 under haloalkaline conditions: toxicity, removal mechanism and effect of heavy metals

Bacterial reduction of hexavalent chromium (VI) to chromium (III) is a sustainable bioremediation approach. However, the Cr(VI) containing wastewaters are often characterized with complex conditions such as high salt, alkaline pH and heavy metals which severely impact the growth and Cr(VI) reduction potential of microorganisms. This study investigated Cr(VI) reduction under complex haloalkaline conditions by an Alteromonas sp. ORB2 isolated from aerobic granular sludge cultivated from the seawater-microbiome. Optimum growth of Alteromonas sp. ORB2 was observed under haloalkaline conditions at 3.5-9.5% NaCl and pH 7-11. The bacterial growth in normal culture conditions (3.5% NaCl; pH 7.6) was not inhibited by 100 mg/l Cr(VI)/ As(V)/ Pb(II), 50 mg/l Cu(II) or 5 mg/l Cd(II). Near complete reduction of 100 mg/l Cr(VI) was achieved within 24 h at 3.5-7.5% NaCl and pH 8-11. Cr(VI) reduction by Alteromonas sp. ORB2 was not inhibited by 100 mg/L As(V), 100 mg/L Pb(II), 50 mg/L Cu(II) or 5 mg/L Cd(II). The bacterial cells grew in the medium with 100 mg/l Cr(VI) contained lower esterase activity and higher reactive oxygen species levels indicating toxicity and oxidative stress. In-spite of toxicity, the cells grew and reduced 100 mg/l Cr(VI) completely within 24 h. Cr(VI) removal from the medium was driven by bacterial reduction to Cr(III) which remained in the complex medium. Cr(VI) reduction was strongly linked to aerobic growth of Alteromonas sp. The Cr(VI) reductase activity of cytosolic protein fraction was pronounced by supplementing with NADPH in vitro assays. This study demonstrated a growth-dependent aerobic Cr(VI) reduction by Alteromonas sp. ORB2 under complex haloalkaline conditions akin to wastewaters.

Phosphomolybdic acid enhancing hexavalent chromium bio-reduction in long-term operation: Optimal dosage and mechanism analysis

The bio-reduction of Cr(VI) is regarded as a feasible and safe strategy to treat Cr pollution. The optimal concentration of phosphomolybdic acid (PMo12) for Cr(VI) reduction and the catalytic mechanism of electron behavior (electron production, electron transport and electron consumption) were revealed in denitrifying biofilm systems. The results showed that 0.1 mM PMo12 could achieve 92.5 % removal efficiency of 90 mg/L Cr(VI), which was 47.7 % higher than that of PMo12-free system, and improve the extracellular fixation capacity of Cr(III). The activity of peroxidase (POD) was significantly promoted by PMo12 to repair oxidative stress damage caused by Cr(VI) reduction. Additionally, analysis of electron behavior demonstrated that PMo12 could enhance key indicators of electron production, transport and consumption. This led to rapid activation of the electron pathway inhibited by Cr(VI), enabling simultaneous efficient nitrogen removal and Cr(VI) reduction in the biofilm system. This discovery will provide an efficient technique for Cr-containing wastewater treatment.

A comprehensive review on chromium (Cr) contamination and Cr(VI)-resistant extremophiles in diverse extreme environments

Chromium (Cr) compounds are usually toxins and exist abundantly in two different forms, Cr(VI) and Cr(III), in nature. Their contamination in any environment is a major problem. Many extreme environments including cold climate, warm climate, acidic environment, basic/alkaline environment, hypersaline environment, radiation, drought, high pressure, and anaerobic conditions have accumulated elevated Cr contamination. These harsh physicochemical conditions associated with Cr(VI) contamination damage biological systems in various ways. However, several unique microorganisms belonging to phylogenetically distant taxa (bacteria, fungi, and microalgae) owing to different and very distinct physiological characteristics can withstand extremities of Cr(VI) in different physicochemical environments. These challenging situations offer great potential and extended proficiencies in extremophiles for environmental and biotechnological applications. On these issues, the present review draws attention to Cr(VI) contamination from diverse extreme environmental regions. The study gives a detailed account on the ecology and biogeography of Cr(VI)-resistant microorganisms in inhospitable environments, and their use for detoxifying Cr(VI) and other applications. The study also focuses on physiological, multi-omics, and genetic engineering approaches of Cr(VI)-resistant extremophiles.

Response of immobilized denitrifying bacterial consortium to tetracycline exposure

Tetracycline (TC) as one of the most widely used antibiotics commonly exists in aquaculture tail water and piggery wastewater, causing risks to human. However, the response of immobilized anaerobic denitrifying bacterial consortium to TC exposure lacks systematic research. In this study, the denitrification performance and the compositional shift of extracellular polymeric substances (EPS) and microbial community under TC stress were investigated. The inhabitation effect of TC on nitrate reduction of the immobilized bacterial consortium became evident at high concentrations (50 mg/L and 100 mg/L). Nitrite reduction was more sensitively inhibited than nitrate reduction. The inhabitation effect was mainly due to the fact that TC damaged cell membranes and subsequently effect the intracellular enzymes activities relating to denitrification (NAR and NIR activities). About 50% of TC can be removed by the immobilized bacterial consortium under all tested TC concentrations. Three-dimensional excitation-emission matrix (3D-EEM) results implied that the tryptophan like substances of EPS were obviously quenched with increasing TC concentration. EPS played an important role in TC removal. The denitification performance of the immobilized bacterial consortium under TC stress was attributed to the genera Paraccoccus, Pseudoxanthomonas, Diaphorobacter and Pseudomonas. Initial TC concentration obviously affected the microbial communities. This study may facilitate the management of aquaculture tail water and piggery wastewater contaminated with nitrate and antibiotics.

Bio-capture of Cr(VI) in a denitrification system: Electron competition, long-term performance, and microbial community evolution

Chromium is widely applied in industries as an important metal resource, but the discharge of Cr(VI) containing wastewater leads to the loss of chromium resources. This study proposed a bio-capture process of chromium in a denitrification system. The bio-capture potentiality was explored by investigating the electron competition between Cr(VI) and nitrogen compounds reduction, the long-term bio-capture performance, and the microbial community evolution. In the competition utilization of electron donors, both NO₃^--N and NO₂^--N took precedence over Cr(VI), and NO₂^--N reduction was proved to be the rate-limiting step. Under the optimum conditions of 20 mg/L NO₃^--N and 6 h HRT, 99.95% of 30 mg/L Cr(VI) could be reduced, and 220980 μg Cr/g MLSS was captured by the biofilm, which was fixed in intercellular as Cr(III). Microbiological analysis confirmed that the bio-reduction of Cr(VI) and NO₃^--N was mediated by synergistic interactions of a series of dominant bacteria, including genera Acidovorax, Thermomonas, and Microbacterium, which contained both the denitrification genes (narG, narZ, nxrA, and nirK) and chromate reduction genes (chrA and chrR). This study proved the feasibility of chromium bio-capture in denitrification systems and provided a new perspective for the Cr(VI) pollution treatment.

Application of metagenomics to biological wastewater treatment

Biological wastewater treatment is a process in which the microbial metabolism of complex communities transforms pollutants into low- or non-toxic products. Due to the absence of an in-depth understanding of the diversity and complexity of microbial communities, it is very likely to ignore the potential mechanisms of microbial community in wastewater treatment. Metagenomics is a technology based on molecular biology, in which massive gene sequences are obtained from environmental samples and analyzed by bioinformatics to determine the composition and function of a microbial community. Metagenomics can identify the state of microbes in their native environments more effectively than traditional molecular methods. This review summarizes the application of metagenomics to assess microbial communities in biological wastewater treatment, such as the biological removal of phosphorus and nitrogen by bacteria, the study of antibiotic resistance genes (ARGs), and the reduction of heavy metals by microbial communities, with an emphasis on the contribution of microbial diversity and metabolic diversity. Technical bottlenecks in the application of metagenomics to biological wastewater treatment are elucidated, and future research directions for metagenomics are proposed, among which the application of multi-omics will be an important research method for future biological wastewater treatment.

Iron oxide minerals promote simultaneous bio-reduction of Cr(VI) and nitrate: Implications for understanding natural attenuation

Nitrate and Cr(VI) coexist in aquifers, posing a potential threat to ecological environment and public health. Iron oxide minerals (hematite and magnetite) exist ubiquitously in groundwater, which are hot spots for biogeochemical transformation. However, there is still a knowledge gap anout the effect of iron oxide minerals on bioreduction of nitrate and Cr(VI). Here we observed that iron oxide minerals can significantly improve the ability of microorganisms to simultaneously reduce nitrate and Cr(VI), the reduction rates of nitrate and Cr(VI) increased by 7.3 and 8.5 times, respectively. The addition of minerals reinforced biofilm formation and shaped microbial communities with a new dominant strain of Azoarcus. The expression levels of functional genes were also upregulated, including napA, narG, nfsA, yieF, POD, and CAT. Furthermore, nitrate and chromate reductases' activities increased by 11 and 5 folds, respectively. These results demonstrated that iron oxide minerals participated in the bio-transformation of nitrate and Cr(VI) co-contamination, alleviating oxidative stress, shaping the microbial community, and ultimately accelerating bio-transformation. These findings offer a window into the biological transformation of co-contamination in the presence of iron oxide minerals, and insights to reveal strategies for microbial detoxification and to develop promising approaches for dealing with complex pollution conditions.

Evaluating the nitrogen-contaminated groundwater treatment by a denitrifying granular sludge bioreactor: effect of organic matter loading

A sequential bed granular bioreactor was adapted to treat nitrate-polluted synthetic groundwater under anaerobic conditions and agitation with denitrification gas, achieving very efficient performance in total nitrogen removal at influent organic carbon concentrations of 1 g L^-1 (80-90%) and 0.5 g L^-1 (70-80%) sodium acetate, but concentrations below 0.5 g L^-1 caused accumulation of nitrite and nitrate and led to system failure (30-40% removal). Biomass size and settling velocity were higher above 0.5 g L^-1 sodium acetate. Trichosporonaceae dominated the fungal populations at all times, while a dominance of terrestrial group Thaumarchaeota and Acidovorax at 1 and 0.5 g L^-1 passed to a domination of Methanobrevibacter and an unclassified Comamonadaceae clone for NaAc lower than 0.5 g L^-1. The results obtained pointed out that the denitrifying granular sludge technology is a feasible solution for the treatment of nitrogen-contaminated groundwater, and that influent organic matter plays an important role on the conformation of microbial communities within it and, therefore, on the overall efficiency of the system.

Enrichment and characterization of an effective hexavalent chromium-reducing microbial community YEM001

Chromium (Cr) is one of the most widely used heavy metals in industrial processes, resulting in water and soil pollution that seriously threaten environmental safety. In this paper, we have directionally enriched a Cr(VI)-reducing bacterial community YEM001 from no-Cr(VI) polluted pond sedimental sludge by selectively growing it in Cr(VI)-containing media. This community could effectively reduce Cr(VI) in laboratory rich media containing different concentrations of Cr(VI), such as 61% reduction at 435 mg/L Cr(VI), 85% reduction at 355 mg/L Cr(VI), and complete reduction at 269 mg/L Cr(VI) in 93.5 h. It was also able to completely reduce 100 mg/L and 300 mg/L Cr(VI) in landfill leachate and natural sludge in 48 h, respectively. Optimal pH for Cr(VI) reduction of the YEM001 is between 7 and 8 and the best efficiency for Cr(VI) reduction occurs at 30 °C. Metagenomic data demonstrated that the YEM001 community was composed of multiple bacteria, including well-known Cr(VI)-reducing bacteria and non-Cr(VI)-reducing bacteria. Delftia, Comamonas, Alicycliphilus, Acidovorax, Bacillus, and Clostridioides account for 83% of total community abundance. The stability of the composition of the YEM001 community and its Cr(VI)-reducing activity allows for its application in bioremediation of environmental Cr(VI) pollution.

Impact of hydraulic retention time on swine wastewater treatment by aerobic granular sludge sequencing batch reactor

In this study, the effect of hydraulic retention time (HRT) on the performance of aerobic granular sludge (AGS) treating swine wastewater (SW) in sequencing batch reactor (SBR) was investigated. The HRT was set at 4.8, 6, 8, 12, and 16 h and performed in five parallel aerobic granular sludge sequence batch reactors (AGSBRs), respectively. The results showed that sedimentation performance and biomass concentration were improved by decreasing the HRT from 16 to 8 h. However, when further decreasing HRT from 8 to 4.8 h, the AGS performance became worse. The AGS process with HRT of 8 h exhibited high pollutant removal efficiency, and an abundant microbial community and a stable microbial community structure were observed. High-throughput 16S rRNA analysis revealed that there were significant differences between the microorganisms in AGS samples with HRT of 8 h at the phylum level and that the dominant microbes changed as the process proceeded. At the genus level, the species and relative abundance of microorganisms gradually evolved for AGS stability in all cases.

The role of natural Fe(II)-bearing minerals in chemoautotrophic chromium (VI) bio-reduction in groundwater

To date, comparatively little is known about the role of natural Fe(II)-bearing minerals in bioremediation of chromium (VI) contaminated aquifers subject to chemoautotrophic conditions. This work employed four kinds of Fe(II)-bearing minerals (pyrite, mackinawite, wustite, and magnetite) as inorganic electron donors to support Cr(VI) bio-reduction. In batch experiments, mackinawite (FeS) performed best, with Cr(VI) removal efficiency of 98.1 ± 1.21 % in 96 h. Continuous column experiments lasting 180 d implied that groundwater chemistry and hydrodynamics influenced the Cr(VI) removal process. A breakthrough study suggested that biotic and abiotic contributions to Cr(VI) reduction were 76.0 ± 1.12 % and 24.1 ± 1.43 %, respectively. Cr(VI) was reduced to insoluble Cr(III), whereas Fe(II) and S(-II) in mackinawite were finally oxidized to Fe(III) and sulfate. Mackinawite evolved progressively into pyrrhotite. High-throughput 16S rRNA gene sequencing indicated that mackinawite-driven Cr(VI) reduction was mediated through synergistic interactions of microbial consortia; i.e. autotrophs as Acidovorax synthesized volatile fatty acids as metabolic intermediates, which were consumed by Cr(VI) reducers as Geobacter. Genes encoding enzymes for S oxidation (soxB) and Cr(VI) reduction (chrA, yieF) were upregulated. Cytochrome c participating in Fe(II) oxidation increased significantly. This work advances the development of sustainable techniques for Cr(VI) polluted groundwater remediation.

Microbial Chromate Reduction Coupled to Anaerobic Oxidation of Elemental Sulfur or Zerovalent Iron

Chromate (Cr(VI)), as one of ubiquitous contaminants in groundwater, has posed a major threat to public health and ecological environment. Although various electron donors (e.g., organic carbon, hydrogen, and methane) have been proposed to drive chromate removal from contaminated water, little is known for microbial chromate reduction coupled to elemental sulfur (S(0)) or zerovalent iron (Fe(0)) oxidation. This study demonstrated chromate could be biologically reduced by using S(0) or Fe(0) as inorganic electron donor. After 60-day cultivation, the sludge achieved a high Cr(VI) removal efficiency of 92.9 ± 1.1% and 98.1 ± 1.2% in two independent systems with S(0) or Fe(0) as the sole electron donor, respectively. The deposited Cr(III) was identified as the main reduction product based on X-ray photoelectron spectroscopy. High-throughput 16S rRNA gene sequencing indicated that Cr(VI) reduction coupled to S(0) or Fe(0) oxidation was mediated synergically by a microbial consortia. In such the consortia, S(0)- or Fe(0)-oxidizing bacteria (e.g., Thiobacillus or Ferrovibrio) could generate volatile fatty acids as metabolites, which were further utilized by chromate-reducing bacteria (e.g., Geobacter or Desulfovibrio) to reduce chromate. Our findings advance our understanding on microbial chromate reduction supported by solid electron donors and also offer a promising process for groundwater remediation.

Analysis of uranium removal capacity of anaerobic granular sludge bacterial communities under different initial pH conditions

The bacterial community of an anaerobic granular sludge associated with uranium depletion was investigated following its exposure to uranium under different initial pH conditions (pH 4.5, 5.5, and 6.5). The highest uranium removal efficiency (98.1%) was obtained for the sample with an initial pH of 6.5, which also supported the highest bacterial community richness and diversity. Venn diagrams visualized the decrease in the number of genera present in both the inoculum and the uranium-exposed biomass as the initial pH decreased from 6.5 to 4.5. Compared with the inoculum, a significant increase in the abundances of the phyla Chloroflexi and Proteobacteria was observed following uranium exposure. At initial pH conditions of 6.5 to 4.5, the proportions of the taxa Anaerolineaceae, Chryseobacterium, Acinetobacter, Pseudomonas, and Sulfurovum increased significantly, likely contributing to the observed uranium removal. Uranium exposure induced a greater level of dynamic diversification of bacterial abundances than did the initial pH difference.