Co-removal of 2,4-dichlorophenol and nitrate using a palladized biofilm: Denitrification-promoted microbial mineralization following catalytic dechlorination

Insight into the microbial community of denitrification process using different solid carbon sources: Not only bacteria

There is a lack of understanding about the bacterial, fungal and archaeal communities' composition of solid-phase denitrification (SPD) systems. We investigated four SPD systems with different carbon sources by analyzing microbial gene sequences based on operational taxonomic unit (OTU) and amplicon sequence variant (ASV). The results showed that the corncob-polyvinyl alcohol sodium alginate-polycaprolactone (CPSP, 0.86±0.04 mg NO3--N/(g·day)) and corncob (0.85±0.06 mg NO3--N/(g·day)) had better denitrification efficiency than polycaprolactone (PCL, 0.29±0.11 mg NO3--N/(g·day)) and polyvinyl alcohol-sodium alginate (PVA-SA, 0.24±0.07 mg NO3--N/(g·day)). The bacterial, fungal and archaeal microbial composition was significantly different among carbon source types such as Proteobacteria in PCL (OTU: 83.72%, ASV: 82.49%) and Rozellomycota in PVA-SA (OTU: 71.99%, ASV: 81.30%). ASV methods can read more microbial units than that of OTU and exhibit higher alpha diversity and classify some species that had not been identified by OTU such as Nanoarchaeota phylum, unclassified_ f_ Xanthobacteraceae genus, etc., indicating ASV may be more conducive to understand SPD microbial communities. The co-occurring network showed some correlation between the bacteria fungi and archaea species, indicating different species may collaborate in SPD systems. Similar KEGG function prediction results were obtained in two bioinformatic methods generally and some fungi and archaea functions should not be ignored in SPD systems. These results may be beneficial for understanding microbial communities in SPD systems.

Biodegradation of nitrate and p-bromophenol using hydrogen-based membrane biofilm reactors in parallel

ABSTRACTP-bromophenol (4-BP) is a toxic halogenated phenolic organic compound. The conventional treatment processes for 4-BP elimination are costly and inefficient, with complete mineralization remaining a challenge for water treatment. To overcome these limitations, we investigated the treatment of 4-BP in a membrane biofilm reactor (MBfR) using hydrogen as an electron donor. The pathway of 4-BP degradation within the H2-MBfR was investigated through long-term operational experiments by considering the effect of nitrate and 4-BP concentrations, hydrogen partial pressure, static experiments, and microbial community diversity, which was studied using 16S rRNA. The results showed that H2-MBfR could quickly remove approximately 100% of 4-BP (up to 20 mg/L), with minimal intermediate product accumulation and 10 mg/L of nitrate continuously reduced. The microbial community structure showed that the presence of H2 created an anaerobic environment, and Thauera was the dominant functional genus involved in the degradation of 4-BP. The genes encoding related enzymes were further enhanced. This study provides an economically viable and environmentally friendly bioremediation technique for water bodies that contain 4-BP and nitrates.

Enhanced removal of 2,4-dichlorophenol by a novel biotic-abiotic hybrid system based on zeolitic imidazolate framework-8

Biotic-abiotic hybrid systems have recently emerged as a potential technique for stable and efficient removal of persistent contaminants due to coupling of microbial catabolic with abiotic adsorption/redox processes. In this study, Burkholderia vietnamensis C09V (B.V.C09V) was successfully integrated with a Zeolitic Imidazolate Framework-8 (ZIF-8) to construct a state-of-art biotic-abiotic system using polyvinyl alcohol/ sodium alginate (PVA/SA) as media. The biotic-abiotic system (PVA/SA-ZIF-8 @B.V.C09V) was able to remove 99.0 % of 2,4-DCP within 168 h, which was much higher than either PVA/SA, PVA/SA-ZIF-8 or PVA/SA@B.V.C09V (53.8 %, 72.6 % and 67.2 %, respectively). Electrochemical techniques demonstrated that the carrier effect of PVA/SA and the driving effect of ZIF-8 collectively accelerated electron transfer processes associated with enzymatic reactions. In addition, quantitative-PCR (Q-PCR) revealed that ZIF-8 stimulated B.V.C09V to up-regulate expression of tfdB, tfdC, catA, and catC genes (2.40-, 1.68-, 1.58-, and 1.23-fold, respectively), which encoded the metabolism of related enzymes. Furthermore, the effect of key physical, chemical, and biological properties of PVA/SA-ZIF-8 @B.V.C09V on 2,4-DCP removal were statistically investigated by Spearman correlation analysis to identify the key factors that promoted synergistic removal of 2,4-DCP. Overall, this study has created an innovative new strategy for the sustainable remediation of 2,4-DCP in aquatic environments.

Biogenic Palladium Improved Perchlorate Reduction during Nitrate Co-Reduction by Diverting Electron Flow in a Hydrogenotrophic Biofilm

Microbial reduction of perchlorate (ClO4-) is emerging as a cost-effective strategy for groundwater remediation. However, the effectiveness of perchlorate reduction can be suppressed by the common co-contamination of nitrate (NO3-). We propose a means to overcome the limitation of ClO4- reduction: depositing palladium nanoparticles (Pd0NPs) within the matrix of a hydrogenotrophic biofilm. Two H2-based membrane biofilm reactors (MBfRs) were operated in parallel in long-term continuous and batch modes: one system had only a biofilm (bio-MBfR), while the other incorporated biogenic Pd0NPs in the biofilm matrix (bioPd-MBfR). For long-term co-reduction, bioPd-MBfR had a distinct advantage of oxyanion reduction fluxes, and it particularly alleviated the competitive advantage of NO3- reduction over ClO4- reduction. Batch tests also demonstrated that bioPd-MBfR gave more rapid reduction rates for ClO4- and ClO3- compared to those of bio-MBfR. Both biofilm communities were dominated by bacteria known to be perchlorate and nitrate reducers. Functional-gene abundances reflecting the intracellular electron flow from H2 to NADH to the reductases were supplanted by extracellular electron flow with the addition of Pd0NPs.

How halogenated aromatic compounds affect the electron supply and consumption in glucose supported denitrification?

Halogenated aromatic compounds possess bidirectional effects on denitrifying bio-electron behavior, providing electrons and potentially interfering with electron consumption. This study selected the typical 4-chlorophenol (4-CP, 0-100 mg/L) to explore its impact mechanism on glucose-supported denitrification. When COD(glucose)/COD(4-CP)=28.70-3.59, glucose metabolism remained the dominant electron supply process, although its removal efficiency decreased to 73.84-49.66 %. When COD(glucose)/COD(4-CP)=2.39-1.43, 4-CP changed microbial carbon metabolism priority by inhibiting the abundance of glucose metabolizing enzymes, gradually replacing glucose as the dominant electron donor. Moreover, 5-100 mg/L 4-CP reduced adenosine triphosphate (ATP) by 15.52-24.67 % and increased reactive oxygen species (ROS) by 31.13-63.47 %, causing severe lipid peroxidation, thus inhibiting the utilization efficiency of glucose. Activated by glucose, 4-CP dechlorination had stronger electron consumption ability than NO2--N reduction (NO3--N > 4-CP > NO2--N), combined with the decreased nirS and nirK genes abundance, resulting in NO2--N accumulation. Compared with the blank group (0 mg/L 4-CP), 5-40 mg/L and 60-100 mg/L 4-CP reduced the secretion of cytochrome c and flavin adenine dinucleotides (FAD), respectively, further decreasing the electron transfer activity of denitrification system. Micropruina, a genus that participated in denitrification based on glucose, was gradually replaced by Candidatus_Microthrix, a genus that possessed 4-CP degradation and denitrification functions after introducing 60-100 mg/L 4-CP.

Enhanced adsorption of phenolic compounds using biomass-derived high surface area activated carbon: Isotherms, kinetics and thermodynamics

The progress of industrial and agricultural pursuits, along with the release of inadequately treated effluents especially phenolic pollutant, has amplified the pollution load on environment. These organic compounds pose considerable challenges in both drinking water and wastewater systems, given their toxicity, demanding high oxygen and limited biodegradability. Thus, developing an eco-friendly, low-cost and highly efficient adsorbent to treat the organic pollutants has become an important task. The present investigation highlights development of a novel adsorbent (CFPAC) by activation of Cassia fistula pod shell for the purpose of removing phenol and 2,4-dichlorophnenol (2,4-DCP). The significant operational factors (dosage, pH, concentration, temperature, speed) were also investigated. The factors such as pH = 2 and T = 20°C were found to be significant at 1.6 g/L and 0.6 g/L dosage for phenol and 2,4-DCP respectively. Batch experiments were further conducted to study isotherms, kinetic and thermodynamics studies for the removal of phenol and 2,4-DCP. The activated carbon was characterised as mesoporous (specific surface area 1146 m2/g, pore volume = 0.8628 cc/g), amorphous and pHPZC = 6.4. At optimum conditions, the maximum sorption capacity for phenol and 2,4-DCP were 183.79 mg/g and 374.4 mg/g respectively. The adsorption isotherm was better conformed to Redlich Peterson isotherm (phenol) and Langmuir isotherm (2,4-DCP). The kinetic study obeyed pseudo-second-order type behaviour for both the pollutants with R2 > 0.999. The thermodynamic studies and the value of isosteric heat of adsorption for both the pollutants suggested that the adsorption reaction was dominated by physical adsorption (ΔHx < 80 kJ/mol). Further, the whole process was feasible, exothermic and spontaneous in nature. The overall studies suggested that the activated carbon synthesised from Cassia fistula pods can be a promising adsorbent for phenolic compounds.

Reduction and precipitation of chromium(VI) using a palladized membrane biofilm reactor

H2-driven reduction of hexavalent chromium (Cr(VI)) using precious-metal catalysts is promising, but its implementation in water treatment has been restricted by poor H2-transfer efficiency and high catalyst loss. We investigated the reduction of Cr(VI) through hydrogenation catalyzed by elemental-palladium nanoparticles (PdNPs) generated in-situ within biofilm of a membrane biofilm reactor (MBfR), creating a Pd-MBfR. Experiments were conducted using a Pd-MBfR and a non-Pd MBfR. The Pd-MBfR achieved Cr(VI) (1000 μg L-1) reduction of >99 % and reduced the concentration of total Cr to below 50 μg L-1, much lower than the total Cr concentration in the non-Pd MBfR effluent (290 μg L-1). The Pd-MBfR also had a lower concentration of dissolved organic compounds compared to the non-Pd MBfR, which minimized the formation of soluble organo-Cr(III) complexes and promoted precipitation of Cr(OH)3. Solid-state characterizations documented deposition of Cr(OH)3 as the product of Cr(VI) reduction in the Pd-MBfR. Metagenomic analyses revealed that the addition and reduction of Cr(VI) had minimal impact on the microbial community (dominated by Dechloromonas) and functional genes in the biofilm of the Pd-MBfR, since the PdNP-catalyzed reduction process was rapid. This study documented efficient Cr(VI) reduction and precipitation of Cr(OH)3 by the Pd-MBfR technology.

Effective boosting of halogenated α, β-unsaturated C4-dicarbonyl electrocatalytic hydrodehalogenation by 1 T'-MoS2/Ti3C2T2 (T = O, OH, F) heterojunctions: A theoretical study

Halogenated α, β-unsaturated C4-dicarbonyl (X-BDA), a novel family of high-toxicity ring cleavage products, is produced during the disinfection of phenolic compounds. The technique of electrocatalytic hydrodehalogenation (ECH) is efficient in rupturing carbon-halogen bonds and generating useful chemicals. This study used first principles to examine the ECH reaction mechanism of X-BDA and the subsequent hydrogenation reaction of the toxic derivative BDA over the 1 T'-MoS2/Ti3C2T2 (T = O, OH, F) catalysts. The catalytic activity of Ti3C2T2 (T = O, OH, F) catalysts decreases gradually with -OH, -F, -O functional group. The loading of 1 T'-MoS2 onto the Ti3C2T2 surface improves the stability and selectivity of Ti3C2T2. In particular, 1 T'-MoS2/Ti3C2(OH)2 is most conducive to the ECH reaction of X-BDA via a direct-indirect continuous reduction process. It exhibits excellent removal capability towards Cl-BDA, with decreasing reactivity in the order of the Cl-, Br-, and I-BDA. The material offers a solution to the challenging dechlorination issue. The dehalogenated product BDA can be hydrogenated to produce 1,4-butanedial, 1,4-butanediol, and 1,4-butenediol. Three valuable chemicals can be obtained by exerting an applied potential of - 0.65 V. This work suggests that the formation of heterojunction catalyst may lead to new strategies to improve ECH for environmental remediation applications.

Long-Term Continuous Test of H2-Induced Denitrification Catalyzed by Palladium Nanoparticles in a Biofilm Matrix

Pd0 catalysis and microbially catalyzed reduction of nitrate (NO3--N) were combined as a strategy to increase the kinetics of NO3- reduction and control selectivity to N2 gas versus ammonium (NH4+). Two H2-based membrane biofilm reactors (MBfRs) were tested in continuous mode: one with a biofilm alone (H2-MBfR) and the other with biogenic Pd0 nanoparticles (Pd0NPs) deposited in the biofilm (Pd-H2-MBfR). Solid-state characterizations of Pd0NPs in Pd-H2-MBfR documented that the Pd0NPs were uniformly located along the outer surfaces of the bacteria in the biofilm. Pd-H2-MBfR had a higher rate of NO3- reduction compared to H2-MBfR, especially when the influent NO3- concentration was high (28 mg-N/L versus 14 mg-N/L). Pd-H2-MBfR enriched denitrifiers of Dechloromonas, Azospira, Pseudomonas, and Stenotrophomonas in the microbial community and also increased abundances of genes affiliated with NO3--N reductases, which reflected that the denitrifying bacteria could channel their respiratory electron flow to NO3- reduction to NO2-. N2 selectivity in Pd-H2-MBfR was regulated by the H2/NO3- flux ratio: 100% selectivity to N2 was achieved when the ratio was less than 1.3 e- equiv of H2/e- equiv N, while the selectivity toward NH4+ occurred with larger H2/NO3- flux ratios. Thus, the results with Pd-H2-MBfR revealed two advantages of it over the H2-MBfR: faster kinetics for NO3- removal and controllable selectivity toward N2 versus NH4+. By being able to regulate the H2/NO3- flux ratio, Pd-H2-MBfR has significant implications for improving the efficiency and effectiveness of the NO3- reduction processes, ultimately leading to more environmentally benign wastewater treatment.

Efficient removal of Cl-DBPs by direct-indirect continuous hydrodechlorination reduction reaction on Ti3C2X2 surface: A theoretical calculation

Degradation of Chlorine-containing disinfection by-products(Cl-DBPs) on surface by electrocatalytic hydrodechlorination(EHDC) is considered a promising advanced water treatment method. Cl-DBPs have ecological toxicity and health risks so that it is urgent to degrade DBPs. We designed and verified the degradation performance of the EHDC of 18 kinds of DBPs (TAAs, TANs, TALs, TNMs, TAcAms, THMs) with different substituents led by the Ti₃C₂X₂(X = O/OH) system by the first-principles. On the surface of Ti₃C₂(OH)₂, DBPs react with atomic hydrogen (*H) by a direct-indirect continuous reduction mechanism to eliminate the Cl atom in turn. Dissociative adsorption of DBPs on the surface of Ti₃C₂(OH)₂ simultaneously realizes the first electron transfer step and forms H vacancy, which makes its electrocatalytic activity superior to that of Ti₃C₂O₂. Removing the six types of DBPs only needs to add -0.1 V of applied potential. In addition, we investigated the impact of substituents and chlorination degree on the reactivity of DBPs removal. The strong electron-withdrawing group is more conducive to the dechlorination reaction. Dehalogenation is much favorable in thermodynamics as the increase in chlorination degree. This study provides important insights and efficient catalysts for the degradation of DBPs and shows the potential of MXenes in eliminating chloride in water.

Efficient degradation of 2,4-dichlorophenol in water by sequential electrocatalytic reduction and oxidation with a Pd-MWCNTs/Ni-foam electrode

Our previous study indicated excellent dechlorination efficiency and phenol conversion rate in the electrocatalytic reduction of 2,4-dichlorophenol (2,4-DCP) with a Pd-MWCNTs/Ni-foam electrode; it is deserved to investigate whether this electrode can efficiently degrade phenol in electro-Fenton oxidation (EFO) process and realize the effective mineralization of 2,4-DCP in aqueous solution. In this work, the sequential electrocatalytic reduction and oxidation of 2,4-DCP were studied after examining phenol degradation in the EFO process. The results showed that the removal efficiency of 0.31 mM phenol could reach 96.76% after 90-min degradation with the rate constant of 0.0367 min^-1, and hydroxy radicals (·OH) were the main active species in the EFO process. In the sequential electrocatalytic reduction and oxidation processes, the removal efficiencies of 2,4-DCP, phenol, and total organic carbon (TOC) reached 99.72%, 97.07%, and 61.45%, respectively. The possible degradation mechanism of 2,4-DCP was proposed through monitoring the reaction products, and the stability and reusability of the electrode were also examined. This study suggested that 2,4-DCP in wastewater can be effectively mineralized to realize its efficient degradation through the sequential electrocatalytic reduction and oxidation.

Biological reduction and hydrodechlorination of chlorinated nitroaromatic antibiotic chloramphenicol under H2-transfer membrane biofilm reactor

Chlorinated nitroaromatic antibiotic chloramphenicol (CAP) is a persistent pollutant that is widely present in environments. A H₂ transfer membrane biofilm reactor (H₂-MBfR) and short-term batch tests were setup to investigate the co-removal of CAP and NO₃^-. Results showed that the presence of CAP (<10 mg L^-1) has no effect on the denitrification process while 100% removal efficiency of CAP can be obtained when nitrate was absent. Nitroaromatic reduction and completely dechlorination were successfully realized when CAP was removed. The CAP transformation product p-aminobenzoic acid (PABA) was detected and batch tests revealed that the hydroxy carboxylation was far faster than nitroaromatic reduction when p-nitrobenzyl alcohol (PNBOH) was conversed to p-aminobenzoic acid (PABA). The path way of CAP degradation was proposed based on the intermediate's analysis. Microbial community analysis indicated that Pleomorphomonadaceae accounts for the dechlorination of CAP.

Coremoval of Energetics and Oxyanions via the In Situ Coupling of Catalytic and Enzymatic Destructions: A Solution to Ammunition Wastewater Treatment

Ammunition wastewater contains toxic nitrated explosives like RDX and oxyanions like nitrate and perchlorate. Its treatment is challenged by low efficiency due to contaminant recalcitrance and high cost due to multiple processes needed for separately removing different contaminant types. This paper reports a H₂-based low-energy strategy featuring the treatment of explosives via catalytic denitration followed by microbial mineralization coupled with oxyanion reduction. After a nitrate- and perchlorate-reducing biofilm incapable of RDX biodegradation was coated with palladium nanoparticles (Pd⁰NPs), RDX was rapidly denitrated with a specific catalytic activity of 8.7 g_cat^-1 min^-1, while biological reductions of nitrate and perchlorate remained efficient. In the subsequent 30-day continuous test, >99% of RDX, nitrate, and perchlorate were coremoved, and their effluent concentrations were below their respective regulation levels. Detected intermediates and shallow metagenome analysis suggest that the intermediates after Pd-catalytic denitration of RDX ultimately were enzymatically utilized by the nitrate- and perchlorate-reducing bacteria as additional electron donor sources.

Enhanced removal of diclofenac via coupling Pd catalytic and microbial processes in a H2-based membrane biofilm reactor: Performance, mechanism and biofilm microbial ecology

Diclofenac (DCF) is a most widely used anti-inflammatory drug, which has attracted worldwide attention given its low biodegradability and ecological damage, especially toxic effects on mammals including humans. In this study, a H₂-based membrane biofilm reactor (H₂-MBfR) was constructed with well-dispersed Pd nanoparticles generated in situ. The Pd-MBfR was applied for catalytic reductive dechlorination of DCF. In batch tests, DCF concentration had significantly effect on the rate and extent DCF removal, and NO₃^- had negative impact on DCF reductive dechlorination. Over 67% removal of 0.5 mg/L DCF and 99% removal of 10 mg/L NO₃^--N were achieved in 90 min, and the highest removal of 97% was obtained at 0.5 mg/L DCF in the absence of NO₃^-. Over 78 days of continuous operation, the highest steady-state removal flux of DCF was 0.0097 g/m²/d. LC-MS analysis indicated that the major product was 2-anilinephenylacetic acid (APA). Dechlorination was the main removal process of DCF mainly owing to the catalytic reduction by PdNPs, microbial reduction, and the synergistic reduction of microbial and PdNPs catalysis using direct delivery of H₂. Moreover, DCF reductive Dechlorination shifted the microbial community in the biofilms and Sporomusa was responsible for DCF degradation. In summary, this work expands a remarkable feasibility of sustainable catalytic removal of DCF.

Dissimilatory and Cytoplasmic Antimonate Reductions in a Hydrogen-Based Membrane Biofilm Reactor

A hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated to investigate the bioreduction of antimonate [Sb(V)] in terms of Sb(V) removal, the fate of Sb, and the pathways of reduction metabolism. The MBfR achieved up to 80% Sb(V) removal and an Sb(V) removal flux of 0.55 g/m²·day. Sb(V) was reduced to Sb(III), which mainly formed Sb₂O₃ precipitates in the biofilm matrix, although some Sb(III) was retained intracellularly. High Sb(V) loading caused stress that deteriorated performance that was not recovered when the high Sb(V) loading was removed. The biofilm community consisted of DSbRB (dissimilatory Sb-reduction bacteria), SbRB (Sb-resistant bacteria), and DIRB (dissimilatory iron-reducing bacteria). Dissimilatory antimonate reduction, mediated by the respiratory arsenate reductase ArrAB, was the main reduction route, but respiratory reduction coexisted with cytoplasmic Sb(V)-reduction mediated by arsenate reductase ArsC. Increasing Sb(V) loading caused stress that led to increases in the expression of arsC gene and intracellular accumulation of Sb(III). By illuminating the roles of the dissimilatory and cytoplasmic Sb(V) reduction mechanism in the biofilms of the H₂-MBfR, this study reveals that the Sb(V) loading should be controlled to avoid stress that deteriorates Sb(V) reduction.

Long-term effects of Cu(II) on denitrification in hydrogen-based membrane biofilm reactor: Performance, extracellular polymeric substances and microbial communities

Divalent copper (Cu(II)) frequently coexists with nitrate (NO₃^-) in industrial wastewater and the effect of Cu(II) on the autotrophic denitrification system using H₂ as the electron donor remains unknown. In this study, the hydrogen-based membrane biofilm reactor (H₂-MBfR) was operated continuously over 150 days to explore the effect of Cu(II) on the performance of autotrophic denitrification system and understand the key roles of EPS and microbial community. More than 95% of 20 mg-N/L NO₃^- was removed at 1-5 mg/L Cu(II), and the removal rate of NO₃^--N was stabilized to 82% at 10 mg/L Cu(II) after a short period, while NH₄⁺ and NO₂^- in effluent were hardly detected, indicated that high concentration of Cu(II) did not permanently inhibit the denitrification performance in H₂-MBfR. Colorimetric determination showed that Cu(II) stimulated the secretion of EPS, in which the protein (PN) content was much higher than polysaccharide (PS). The PN/PS ratios increased from 0.93 to 1.99, and the PN was more sensitive to copper invasion. The results of three-dimensional excitation-emission matrix illustrated that tryptophan was the main component of EPS chelating Cu(II) to reduce toxicity. The results of Fourier-transform infrared demonstrated that hydroxyl, carboxyl, and protein amide groups bound and reduced Cu(II). Furthermore, Cu(II) was effectively removed (>80%), and the results of distribution and morphology analysis of Cu(II) show that the electron-dense deposits of monovalent copper (Cu(I)) were found in EPS and biofilms and the reduction of Cu(II) to Cu(I) was an obvious self-defense reaction of biofilm to copper stress. The microbial richness and diversity decreased with the long-term exposure to Cu(II), while the relative abundance of denitrifiers Azospira and Dechloromonas increased. This study provides a scientific basis for the optimal design of treatment system for removal of nitrate and recovery of heavy metals simultaneously.

A kinetic model for 2,4-dichlorophenol adsorption and hydrodechlorination over a palladized biofilm

Adsorption and catalytic hydrodechlorination (HDC) of aqueous 2,4-DCP by palladium nanoparticles (Pd0NPs) associated with a biofilm (i.e., a Pd0-biofilm) was investigated in terms of the removal efficiency of 2,4-DCP, dechlorinated product selectivity, and reduction kinetics. Experiments were executed with Pd0-biofilm and with abiotic Pd0NPs-film alone. The 2,4-DCP-adsorption capacity of Pd0-biofilm was 2- to 5-fold greater than that of abiotic Pd0NPs-film, and the adsorption accelerated dechlorination by Pd0-biofilm, including selectivity to phenol instead of mono-chlorophenols. A mechanistic kinetic model was developed to represent the sequential adsorption and reduction processes. Modeling results represented well the removal of 2,4-DCP and quantified that Pd0-biofilm had a strong affinity for adsorbing 2,4-DCP. The strong adsorption increased the volume-averaged concentration of 2,4-DCP concentration inside the Pd0-biofilm, compared to the concentration in the bulk liquid. This increase in the local concentration of 2,4-DCP led to a 2- to 4-fold increase in the reduction rate of 2,4-DCP in Pd0-biofilm, compared to abiotic Pd0NPs-film. Thus, coupling Pd0NPs with the biofilm promoted 2,4-DCP removal and full dechlorination despite its low concentration in bulk water.

How phenol stresses anammox for the treatment of ammonia-rich wastewater: Phenomena, microbial community evolution and molecular modeling

Phenol is a biotoxic organic compound and found in large quantities in ammonia-rich wastewater discharged from coking and petrochemical industries. In this work, phenol was fed to the system of anaerobic ammonia oxidation (anammox), and the possible inhibitory mechanism was speculated using the characterization of granular sludge, analysis of microbial community and molecular docking simulations. The results showed that phenol (0-300 mg/L) did not significantly inhibit anammox. However, phenol did activate denitrification, which increased the nitrogen removal rate (NRR) by 0.94 kg N/(m³·d). Moreover, when phenol concentration reached t400 mg/L, the NRR was inhibited by 70%, while the extracellular polymeric substance (EPS) of granular sludge was reduced. Phenol resulted in the reduction of Candidatus_Kuenenia and promoted the proliferation of phenol-degrading denitrifying bacteria, Azoarcus and Thauera. Molecular docking indicated that phenol, 2-nitrophenol and 4-nitrophenol could bind the nitrite reductase (NirS), which prevented the first step of the anammox reaction.

Perturbation of clopyralid on bio-denitrification and nitrite accumulation: Long-term performance and biological mechanism

The contaminant of herbicide clopyralid (3,6-dichloro-2- pyridine-carboxylic acid, CLP) poses a potential threat to the ecological system. However, there is a general lack of research devoted to the perturbation of CLP to the bio-denitrification process, and its biological response mechanism remains unclear. Herein, long-term exposure to CLP was systematically investigated to explore its influences on denitrification performance and dynamic microbial responses. Results showed that low-concentration of CLP (<15 mg/L) caused severe nitrite accumulation initially, while higher concentrations (35-60 mg/L) of CLP had no further effect after long-term acclimation. The mechanistic study demonstrated that CLP reduced nitrite reductase (NIR) activity and inhibited metabolic activity (carbon metabolism and nitrogen metabolism) by causing oxidative stress and membrane damage, resulting in nitrite accumulation. However, after more than 80 days of acclimation, almost no nitrite accumulation was found at 60 mg/L CLP. It was proposed that the secretion of extracellular polymeric substances (EPS) increased from 75.03 mg/g VSS at 15 mg/L CLP to 109.97 mg/g VSS at 60 mg/L CLP, which strengthened the protection of microbial cells and improved NIR activity and metabolic activities. Additionally, the biodiversity and richness of the microbial community experienced a U-shaped process. The relative abundance of denitrification- and carbon metabolism-associated microorganisms decreased initially and then recovered with the enrichment of microorganisms related to the secretion of EPS and N-acyl-homoserine lactones (AHLs). These microorganisms protected microbe from toxic substances and regulated their interactions among inter- and intra-species. This study revealed the biological response mechanism of denitrification after successive exposure to CLP and provided proper guidance for analyzing and treating herbicide-containing wastewater.