Further study on particle size, stability, and complexation of silver nanoparticles under the composite effect of bovine serum protein and humic acid

Silver nanoparticles (AgNPs) are widely used due to their unique antibacterial properties and excellent photoelectric properties. Wastewater treatment plants form a pool of AgNPs due to the social cycle of wastewater. During biological treatment processes, the particle size and stability of AgNPs change. We studied the particle size changes and stability of silver nanoparticles in the presence of bovine serum albumin (BSA) and humic acid (HA). The experimental results indicated that silver nanoparticles can complex with the functional groups in BSA. For AgNP-BSA composites, as the BSA concentration increases, the size of the silver nanoparticles first decreases and then increases. AgNPs can combine with the amide, amino, and carboxyl groups in HA. As the concentration of HA increases, the particle size and large particle size distribution of AgNPs increase. This increasing trend is more obvious when the HA concentration is lower than 20 mg L-1. When HA and BSA exist at the same time, HA will occupy the adsorption sites of BSA on the surface of AgNPs, and the AgNP-HA complex will dominate the system. This study aims to provide key operational control strategies for the process operation of wastewater treatment plants containing AgNPs and theoretical support for promoting water environment improvement and economic development such as tourism.

Performance of constructed wetlands with different water level for treating graphene oxide wastewater: Characteristics of plants and microorganisms

Constructed wetlands (CWs) have been expected advantages in emerging pollutant removal, but with less known on their characteristic when treating wastewater containing graphene oxide (GO). In present study, we investigated characteristics of Iris pseudacorus, microorganisms, and pollutant removal in CWs with 60 cm and 37 cm water level (termed HCW and LCW). Plants in LCW had higher chlorophyll content and lower activities of antioxidant enzyme (superoxide dismutase, catalase, peroxidase) as well as malondialdehyde content. Substrate enzyme activities were affected by time and CW type. LCW increased only dehydrogenase activities, while HCW increased catalase, urease, neutral phosphatase, and arylsulfatase activities. Sequencing analysis revealed that microbial community showed higher richness and diversity in LCW, but this dissimilarity could be eased by time-effect. Proteobacteria (25.62-60.36%) and Actinobacteria (13.86-56.20%) were stable dominant phyla in CWs. Ratio of Proteobacteria/Acidobacteria indicated that trophic status of plant rhizosphere zone was lower in LCW. Nitrospirae were enriched to 0.16-0.68% and 0.75-1.42% in HCW and LCW. The enrichment of phyla Proteobacteria and Firmicutes in HCW was attributed to class Gammaproteobacteria and genus Enterococcus. GO transformation showed some reductions in CWs, which could be affected by water depth and substrate depth. Overall, HCW achieved nitrogen and phosphorus removal for 48.78-62.99% and 95.01%, which decreased by 8.41% and 7.31% in LCW. COD removal was less affected reaching 93%. This study could provide some new evidence for CWs to treat wastewater containing GO.

Investigation on the Toxicity of Nanoparticle Mixture in Rainbow Trout Juveniles

The environmental impacts of nanoparticle mixtures in the aquatic environment is not well understood. The purpose of this study examined the sub-lethal toxicity of low concentrations (ug/L range) of selected nanoparticles alone and in mixtures in juvenile trout. Fish were exposed to to individual and two environmentally relevant mixtures of silver (nAg), copper oxide (nCuO) and cerium oxide (nCeO) nanoparticles for 96 h at 15 °C. After the exposure period, fish were depurated overnight and tissue levels in Ag, Ce, Cu and Zn were determined along with a suite of effects biomarkers such as oxidative stress/inflammation, denatured protein tagging (ubiquitin), DNA strand breaks (genotoxicity) and acetylcholinesterase (AChE) activity. The data showed that these nanoparticles behaved as suspended matter but were nevertheless bioavailable for fish with bioconcentration factors of 6, 8 and 2 for nAg, nCeO and nCuO respectively. Only nCuO alone increased malonaldehyde (lipid peroxidation) contents but all nanoparticles increased DNA damage, protein-ubiquitin labeling, and decreased AChE activity. Globally, the toxicity of nCeO and nCuO was generally stronger than nAg, and antagonist effects were found in the mixtures. The interactions involved in these antagonisms are not well understood but do not involve the liberation of free ions and labile zinc in tissues. In conclusion, the bioavailability and toxicity of these nanoparticles are influenced by mixtures of nanoparticles, which is likely to occur in contaminated environments.

Assessing the effect of SiO2 and TiO2 nanoparticles on granule stability and microbial community shift in aerobic granular sludge process

The widely used SiO₂ and TiO₂ nanoparticles (NPs) can accumulate in industrial wastewaters, thereby posing challenge to biological wastewater treatment processes. In this work, the performance of aerobic granular sludge (AGS) reactors fed with wastewater containing 50 mg L^-1 SiO₂ and TiO₂ NPs were investigated. The results showed that the granules could resist the NPs in wastewater (no disintegration of granules was observed). SiO₂ NPs had a negative effect on the settleability of granules, with the SVI₃₀ increased by 64.5% and protein secretion decreased by 29.9%. To the contrary, the settleability of granules was improved in the presence of TiO₂ NPs due to the increase of the protein secretion. Possibly because of the compact and layered structure of granules, in the presence of both types of NPs, no obvious reduction of the overall removal efficiency of organics was found, and nanoparticle-resistant strains were enriched. The overall nitrification and denitrification efficiencies were hardly affected by SiO₂ NPs while significantly inhibited by TiO₂ NPs. Some functional genera, such as Hyphomicrobium and Acidovorax, showed growth inhibition with TiO₂ NPs, which might be responsible for the reduction of nitrification and denitrification efficiencies.

Effects of Heavy Metals on the Performance and Mechanism of Anaerobic Ammonium Oxidation for Treating Wastewater

Persistence of ammonium nitrogen and heavy metals in wastewater still remains a challenge, and many wastewater treatment plants face the challenge of removing nitrogen under heavy metal stresses. There is no preferred method for the biological treatment of wastewater containing nitrogen and heavy metals with the possible exception of the anaerobic ammonium oxidation (anammox), since it has shown promise for removing nitrogen under heavy metal stresses. This article reviews the recent research results of the nitrogen-removal performance and mechanism by the anammox process under heavy metal stresses, mainly discussing the enhancing and inhibition effects of heavy metals on the performance of the Anammox reactor. The influencing mechanism of heavy metals on the microbial community and extracellular polymeric substances is also presented, and examples are given for explanation. The main problems of the present research are pointed out, and it is proposed that unifying the metal ion concentrations of inhibiting or promoting anammox activity is necessary for the development and industrial application of the anammox process. The information of this review can offer a great possibility for achieving desired nitrogen removal in wastewater treatment under heavy metal stresses and with significant energy savings.

A review on the generation, discharge, distribution, environmental behavior, and toxicity (especially to microbial aggregates) of nano-TiO2 in sewage and surface-water and related research prospects

This article reviews the nano-effects and applications of different crystalline nano‑titanium dioxide (nano-TiO₂), identifies their discharge, distribution, behavior, and toxicity to aquatic organisms (focusing on microbial aggregates) in sewage and surface-water, summarizes related toxicity mechanisms, and critically proposes future perspectives. The results show that: 1) based on crystal type, application boundaries of nano-TiO₂ have become clear, extending from traditional manufacturing to high-tech fields; 2) concentration of nano-TiO₂ in water is as high as hundreds of thousands of μg/L (sewage) or several to dozens of μg/L (surface-water) due to direct application or indirect release; 3) water environmental behaviors of nano-TiO₂ are mainly controlled by hydration conditions and particle characteristics; 4) aquatic toxicities of nano-TiO₂ are closely related to their water environmental behavior, in which crystal type and tested species (such as single species and microbial aggregates) also play the key role. Going forward, the exploration of the toxicity mechanism will surely become a hot topic in the aquatic-toxicology of nano-TiO₂, because most of the research so far has focused on the responses of biological indicators (such as metabolism and damage), while little is known about the stress imprint caused by the crystal structures of nano-TiO₂ in water environments. Additionally, the aging of nano-TiO₂ in a water environment should be heeded to because the continuously changing surface structure is bound to have a significant impact on its behavior and toxicity. Moreover, for microbial aggregates, comprehensive response analysis should be conducted in terms of the functional activity, surface features, composition structure, internal microenvironment, cellular and molecular level changes, etc., to find the key point of the interaction between nano-TiO₂ and microbial aggregates, and to take mitigation or beneficial measures to deal with the aquatic-toxicity of nano-TiO₂. In short, this article contributes by 1) reviewing the research status of nano-TiO₂ in all aspects: application and discharge, distribution and behavior, and its aquatic toxicity; 2) suggesting the response mechanism of microbial aggregates and putting forward the toxigenic mechanism of nanomaterial structure; 3) pointing out the future research direction of nano-TiO₂ in water environment.

Unveiling the distinctive role of titanium dioxide nanoparticles in aerobic sludge digestion

Aerobic digestion is considered to be a common process for the stabilization of waste activated sludge (WAS) in the small-sized wastewater treatment systems, while the broad application of titanium dioxide nanoparticles (TiO₂ NPs) results in their unavoidable existence in WAS aerobic digestion, with its role in aerobic sludge digestion being never documented. This study set up a series of aerobic sludge digesters to evaluate the previously unknown role of TiO₂ NPs on the performance of the digesters. The volatile solids (VS) degradation percentage increased from 21.9 ± 0.6% to 26.9 ± 0.1% - 30.0 ± 0.3% with the different contents of TiO₂ NPs (0, 1, 20 and 50 mg/L). Similarly, the total inorganic nitrogen production increased from 23.1 ± 0.3 to 31.0 ± 0.1 mg N/g VS with the rising TiO₂ NPs concentrations from 0 to 50 mg/L. The microbial analysis suggested that TiO₂ NPs contributed to the accumulation of specific microbes correlated with the degradation of organic substances and the conversion of nitrogen compounds. Model-based analysis showed the higher biodegradability and hydrolysis rate of sludge with TiO₂ NPs. Further mechanistic studies indicated that the enhancement of WAS solubilization and the degradation of recalcitrant substances (e.g., humic acid and cellulose) contributed to the better performance of experimental aerobic digesters, which was confirmed by the fourier transformation infrared spectroscopy (FTIR) indicating the converting of these materials into biodegradable substrates for digestion with TiO₂ NPs. It could be inferred from this investigation that aerobic digestion rather than anaerobic digestion would be a more suitable treatment method for sludge containing TiO₂ NPs.

Recent advancements of spinel ferrite based binary nanocomposite photocatalysts in wastewater treatment

A lot of studies on spinel ferrites (MFe₂O₄, M = divalent metal ion) and their binary nanocomposites as photocatalysts in the decontamination of wastewater have been performed, because MFe₂O₄ nanoparticles are relatively stable, biocompatible and low-cost efficient photocatalyst. The separation of MFe₂O₄ photocatalyst is easy owing to its excellent magnetic behavior. With this background, the recent developments on photocatalytic performances of MFe₂O₄ based binary nanocomposites were comprehensively reviewed. Especially, a focus on MFe₂O₄/metal oxides, MFe₂O₄/carbon based materials, MFe₂O₄/polymers, MFe₂O₄/metal nanoparticles and MFe₂O₄/other compounds for the photocatalytic degradation of dyes, emerging contaminants and inorganic pollutants has been thoroughly given. The advantages of MFe₂O₄ based nanocomposites as photocatalysts were also discussed. In addition, the possible pathway of active free radical generation by these photocatalysts under visible and ultraviolet irradiation has been explained. A comparison of photocatalytic activities of MFe₂O₄ based binary nanocomposites with recent reports has been carried out. This review concludes that MFe₂O₄ based binary nanocomposites have potential capacity in water purification technology. Nevertheless, their practical utilization in water treatment plants still needs to be further studied.

Physico-Chemical Properties of Inorganic NPs Influence the Absorption Rate of Aquatic Mosses Reducing Cytotoxicity on Intestinal Epithelial Barrier Model

Noble metals nanoparticles (NPs) and metal oxide NPs are widely used in different fields of application and commercial products, exposing living organisms to their potential adverse effects. Recent evidences suggest their presence in the aquifers water and consequently in drinking water. In this work, we have carefully synthesized four types of NPs, namely, silver and gold NPs (Ag NPs and Au NPs) and silica and titanium dioxide NPs (SiO₂ NPs and TiO₂ NPs) having a similar size and negatively charged surfaces. The synthesis of Ag NPs and Au NPs was carried out by colloidal route using silver nitrate (AgNO₃) and tetrachloroauric (III) acid (HAuCl₄) while SiO₂ NPs and TiO₂ NPs were achieved by ternary microemulsion and sol-gel routes, respectively. Once the characterization of NPs was carried out in order to assess their physico-chemical properties, their impact on living cells was studied. We used the human colorectal adenocarcinoma cells (Caco-2), known as the best representative intestinal epithelial barrier model to understand the effects triggered by NPs through ingestion. Then, we moved to explore how water contamination caused by NPs can be lowered by the ability of three species of aquatic moss, namely, Leptodictyum riparium, Vesicularia ferriei, and Taxiphyllum barbieri, to absorb them. The experiments were conducted using two concentrations of NPs (100 μM and 500 Μm as metal content) and two time points (24 h and 48 h), showing a capture rate dependent on the moss species and NPs type. Then, the selected moss species, able to actively capture NPs, appear as a powerful tool capable to purify water from nanostructured materials, and then, to reduce the toxicity associated to the ingestion of contaminated drinking water.

Effects of aging and transformation of anatase and rutile TiO2 nanoparticles on biological phosphorus removal in sequencing batch reactors and related toxic mechanisms

The effect of nanomaterials aging, namely the transformation of comprehensive characteristics after experiencing real or complex environmental behaviors, on their ecotoxicology is still lacking. Moreover, the mechanisms by which NPs influence biological phosphorus (P) removal during sewage treatment require further elucidation. Therefore, we used both pristine and aged anatase (TiO₂-A) and rutile (TiO₂-R) NPs to investigate the mechanisms by which NPs affect P removal in a SBR. At 0.1 mg/L, the four types of NPs (pristine and aged) had no significant effect on sludge purification after acute (72-h) exposure under simulated sunlight. However, at 50 mg/L-regardless of the crystalline phase of the NPs-SOP and COD removal efficiency dropped steeply to approximately 42.2-82.4 % (p < 0.05) and 69.8-83.3 % (p < 0.05), respectively, especially in the pristine TiO₂-NPs groups because of decrease of richness and diversity of genus level of PAOs and enzyme activity of both PPK and PPX, and the sluggish transformation of PHA and glycogen. Aging reduced the ability of NPs toxicity. The toxicity mechanisms of TiO₂-NPs included lipid peroxidation and contact damage, or leakage from bacterial cytoplasmic membrane, which are closely related to photooxidation capacity and aqueous solution stability-i.e., nanoscale effects-and the impacts of aging or inclusion.

The Fate of Anthropogenic Nanoparticles, nTiO2 and nCeO2, in Waste Water Treatment

Wastewater treatment is one of the main end-of-life scenarios, as well as a possible reentry point into the environment, for anthropogenic nanoparticles (NP). These can be released from consumer products such as sunscreen or antibacterial clothing, from health-related applications or from manufacturing processes such as the use of polishing materials (nCeO2) or paints (nTiO2). The use of NP has dramatically increased over recent years and initial studies have examined the possibility of toxic or environmentally hazardous effects of these particles, as well as their behavior when released. This study focuses on the fate of nTiO2 and nCeO2 during the wastewater treatment process using lab scale wastewater treatment systems to simulate the NP mass flow in the wastewater treatment process. The feasibility of single particle mass spectroscopy (sp-ICP-MS) was tested to determine the NP load. The results show that nTiO2 and nCeO2 are adsorbed to at least 90 percent of the sludge. Furthermore, the results indicate that there are processes during the passage of the treatment system that lead to a modification of the NP shape in the effluent, as NP are observed to be partially smaller in effluent than in the added solution. This observation was made particularly for nCeO2 and might be due to dissolution processes or sedimentation of larger particles during the passage of the treatment system.

Differential responses of encoding-amoA nitrifiers and nir denitrifiers in activated sludge to anatase and rutile TiO2 nanoparticles: What is active functional guild in rate limiting step of nitrogen cycle?

The long-terms effects of different crystal-composition TiO₂ nanoparticles (NPs) on nitrogen-cycle-related functional guilds in activated sludge remain unclear, especially under natural light irradiation. Accordingly, activated sludge was exposed to anatase TiO₂-NPs (TiO₂-A) and rutile TiO₂-NPs (TiO₂-R) for up to 45 days. With markedly (p < 0.05) reducing nitrification-/denitrification-enzymatic-activities and abundances of ammonia-oxidizing-microorganisms (AOMs) and nitrite-reducing-bacteria (NRB), TiO₂-NPs triggered bacteria and archaea UPGMA clustering and a deep modification of N-cycling functional diversity guided by crystal structure. in situ¹³C-DNA-SIP confirmed ammonia-oxidizing-bacteria (AOB) (Nitrosomonas and Nitrosospira) in original sludge as main active AOMs with 75.4 times more abundance than ammonia-oxidizing-archaea (AOA), while AOA within Nitrosopumilus and Nitrososphaera genera were the main active AOMs and tended to aggregate inside sludge after 10-mg/L TiO₂-NPs exposure. Encoding-nirK NRB were more sensitive, while encoding-nirS Zoogloea with a total share of 4.97% to 14.93%, etc. were the main active NRB. AOB was more sensitive to TiO₂-A, while TiO₂-R showed the stronger toxicity to AOA and NRB resulting from differences in water environmental behaviors and crystal characteristics of two TiO₂-NPs. This work expands understanding of the ecological risks of titanium-dioxide-crystal-NPs in aquatic environment and may help devise better methods to alleviate environmental stress caused by NPs at wastewater treatment plants.

Metal oxide nanoparticles resonate to ammonium removal through influencing Mg2+ absorption by Pseudomonas putida Y-9

Most metal oxide nanoparticles (NPs) can impact ammonium removal, but the underlying mechanism remains unclear. In this study, high doses of NiO, CuO, ZnO and TiO₂ (>1 mg/L) inhibited the ammonium removal performance of Pseudomonas putida Y-9. Interestingly, low levels of CuO NPs (0.1, 0.5 mg/L) and NiO NPs (0.1 mg/L) enhanced ammonium removal efficiency. Moreover, a decrease in Mg²⁺ levels was significantly positively correlated with ammonium removal efficiency, while negatively correlated with the Ti²⁺, Zn²⁺, and Cu²⁺ release of NPs. Further research on effect of NPs and their corresponding cations on ammonium removal revealed that four NPs affected Mg²⁺ absorption in Y-9 via different routes, thus impacting NH₄⁺ removal efficiency, i.e., the effect of NiO NPs was caused by itself, TiO₂ NPs' impact was solely due to the release Ti⁴⁺, while the influence of CuO NPs and ZnO NPs was based on both the particles and released ions.

Impact of adding metal nanoparticles on anaerobic digestion performance - A review

Anaerobic digestion is the most widely adopted biological waste treatment processes with renewable energy production. The effects of adding metal nanoparticles (NPs) on improving digestion performance are well noted. This paper reviewed the traditional view on the cytotoxicity of NPs to living organisms and the contemporary view of mechanisms for enhancement in anaerobic digestion performance in the presence of metal NPs. The complicated interactions acquire further studies for comprehending the physical and chemical interactions of metal NPs to the constituent compounds and to the living cells, and the involvement of mechanisms such as direct interspecies electron transfer for better design and control of the "NP strategy" for anaerobic digestion performance enhancement.

Simultaneous removal of nitrogen and phosphorous by heterotrophic nitrification-aerobic denitrification of a metal resistant bacterium Pseudomonas putida strain NP5

A novel strain NP5 with efficient heterotrophic nitrification, aerobic denitrification and phosphorus accumulation ability was isolated and identified as Pseudomonas putida strain NP5. The removed ammonium and phosphate were mainly converted into intracellular components by assimilation, and negligible nitrification intermediates and N₂O were accumulated during heterotrophic nitrification. In addition, the optimal conditions for nutrient removal were: succinate as carbon source, C/N 10, P/N 0.2, temperature 30 °C, salinity 0% and shaking speed 160 rpm. Besides, strain NP5 possessed an exceptional heavy metal and nanoparticles resistance. Cr⁶⁺ was found to be the most toxic among the tested metals, and it could be removed simultaneously. Moreover, an obvious phosphorus release was observed under anaerobic condition, and repeated exposure to the anaerobic/aerobic conditions could significantly improve the nutrient removal. Furthermore, the successful expression of key enzymes for nitrogen and phosphorous removal provided additional evidence for possibility of simultaneous nitrification, denitrification and phosphorus removal.

TiO2 nanoparticles in irrigation water mitigate impacts of aged Ag nanoparticles on soil microorganisms, Arabidopsis thaliana plants, and Eisenia fetida earthworms

Treated wastewater is reclaimed to irrigate crops in a growing number of arid and semi-arid areas. In order to study the impacts of metallic nanoparticles (NPs) present in treated wastewater on soil ecosystems, a soil micro-ecosystem containing Arabidopsis thaliana plants, soil microorganisms, and Eisenia fetida earthworms was developed. The soil was irrigated with deionized water containing environmentally relevant concentrations of 70 µg/L of TiO₂ NPs; or 20 µg/L of an Ag mixture, which included 90% (w/w) Ag₂S NPs, 7.5% (w/w) Ag⁰ NPs, and 2.5% (w/w) Ag⁺ to represent speciation of aged Ag NPs in treated wastewater; or a combination of the TiO₂ NPs and the Ag mixture to reflect the frequent presence of both types of materials in treated wastewater. It was found that TiO₂ NPs alone were not toxic to the soil micro-ecosystem. Irrigation water containing 20 µg/L of the Ag mixture significantly reduced the soil microbial biomass, and inhibited the growth of plants and earthworms; however, a combination of 70 µg/L of TiO₂ and 20 µg/L of Ag did not show toxic impact on organism growth compared to the Control of deionized water irrigation. Taken together, these results indicate the importance of investigating the effects of different nanomaterials in combination as they are introduced to the environment-with environmentally relevant concentrations and speciation-instead of only selecting a single NP type or residual ion. Moreover, the results of this study support the safe application of reclaimed water from wastewater treatment plants for use in agricultural lands in regard to limited concentrations of aged NPs (i.e., TiO₂ and Ag) if present in combination.

Toxicity of Three Crystalline TiO2 Nanoparticles in Activated Sludge: Bacterial Cell Death Modes Differentially Weaken Sludge Dewaterability

The eco-toxicities of different crystalline phases of TiO₂-NPs are controversial, and the effects and mechanisms on activated sludge are unclear. Therefore, we assessed the acute-toxicities (8-h exposure) of P25, anatase, and rutile TiO₂-NPs in activated sludge using flow cytometry under simulated sunlight (hereafter-sun) and evaluated the relationship between sludge dewatering and bacterial cell death modes using Pearson's correlation coefficients ( r). Additionally, the response of the microbial community structure was examined by high throughput sequencing. Bacterial survival and death were observed by confocal laser scanning microscopy. Toxicity indicators (e.g., lactate dehydrogenase (LDH) and reactive oxygen species (ROS)) were determined. Overall, TiO₂-NPs toxicity was concentration-dependent and crystalline-phase-dependent. The responses of bacterial communities to crystalline phases were more obvious than that of dosage. P25-sun and anatase-sun caused necrosis-like cell death via strong photo-oxidation confirmed by 131%/123% (1 mg/L) and 301%/254% (50 mg/L) LDH released by the control, while rutile-sun induced apoptosis-like death via intracellular ROS production increased to 165% (1 mg/L) and 420% (50 mg/L) of the control. P25 and anatase NPs had higher protein and polysaccharide affinities, while rutile NPs exhibited stronger attachment onto phospholipids. TiO₂-NPs-sun reduced activated sludge dewaterability. Specific resistance to filtration (SRF) showed the strongest positive correlation with tightly bound extracellular polymeric substances (EPS) and total soluble microbial byproducts ( r = 0.974, p < 0.01) and was closely related to EPS content and composition, especially the increased bound water (BW) content and sludge protein concentrations. High Pearson correlation coefficients were observed between early apoptotic cells and BW content ( r = 0.952, p < 0.01) resulting from massive polysaccharides and between necrotic (including late apoptotic) cells and SRF ( r = 0.959, p < 0.01) resulting from high protein and EPS concentrations. Thus, in response to TiO₂-NPs, bacterial cell death modes differentially weakened sludge dewatering.

The revolution of performance, sludge characteristics and microbial community of anammox biogranules under long-term NiO NPs exposure

Given the increasing applications of NiO nanoparticles (NPs) in battery products, the potential effects of NiO NPs on anaerobic ammonium oxidation (anammox) systems were studied for the first time. The results showed that the anammox system performance obviously differed under the stresses of different NiO NPs concentrations. After the withdrawal of NiO NPs, the nitrogen removal performance of the anammox reactor returned to nearly that of the initial phase within 35 days. Compared with 0 mg L^-1 NiO NPs, the specific anammox activity first increased and then decreased to the minimum value of 116.8 ± 13.8 mg TN g^-1 VSS d^-1 at 60 mg L^-1 NiO NPs. The variations in the heme c contents and extracellular polymeric substance amounts were similar to the variations in the specific anammox activity throughout the whole experiment. Additionally, the relative abundance of the dominant bacteria (Candidatus kuenenia) increased from 20.44% at 60 mg L^-1 NiO NPs to 23.14% at the end of the last phase. Thus, the potential effects of NiO NPs on anammox systems should be a cause for great concern.

Evaluating the effects of metal oxide nanoparticles (TiO2, Al2O3, SiO2 and CeO2) on anammox process: Performance, microflora and sludge properties

The increasing use of engineered metal oxide nanoparticles (MONPs) in consumer products raises great concerns about their environmental impacts, but their potential impacts on anaerobic ammonium oxidation (anammox) process in wastewater treatment remain unclear. In this study, the presence of MONPs (1, 50, 200 mg L^-1) exhibited no visible effects on the nitrogen removal performance of anammox reactors, but high levels (200 mg L^-1) of SiO₂NPs, Al₂O₃NPs and CeO₂NPs had a distinct effect on shaping the anammox community. Long-term exposure of MONPs caused different responses in the relative abundance of Ca. Kuenenia, the level of functional gene HzsA and the activities of three key enzymes involved in anammox metabolism, but no significant inhibition effects on specific anammox activity were detected. Overall, the effects of MONPs on anammox community structure and sludge properties depended on their types and levels and followed the order SiO₂ > CeO₂ > Al₂O₃ > TiO₂.