Microplastics removal mechanisms in constructed wetlands and their impacts on nutrient (nitrogen, phosphorus and carbon) removal: A critical review.

Microplastics (MPs) are now prevalent in aquatic ecosystems, prompting the use of constructed wetlands (CWs) for remediation. However, the interaction between MPs and CWs, including removal efficiency, mechanisms, and impacts, remains a subject requiring significant investigation. This review investigates the removal of MPs in CWs and assesses their impact on the removal of carbon, nitrogen, and phosphorus. The analysis identifies crucial factors influencing the removal of MPs, with substrate particle size and CWs structure playing key roles. The review highlights substrate retention as the primary mechanism for MP removal. MPs hinder plant nitrogen uptake, microbial growth, community composition, and nitrogen-related enzymes, reducing nitrogen removal in CWs. For phosphorus and carbon removal, adverse effects of MPs on phosphorus elimination are observed, while their impact on carbon removal is minimal. Further research is needed to understand their influence fully. In summary, CWs are a promising option for treating MPs-contaminated wastewater, but the intricate relationship between MPs and CWs necessitates ongoing research to comprehend their dynamics and potential consequences.

Enhanced denitrification using high-molecular-weight poly(lactic acid) blended with lactide as an effective carbon source.

The utilization of wasted Poly(lactic acid) (PLA) as low-cost carbon sources in solid-phase denitrification is hindered by its low biodegradability, which can be attributed to its high molecular weight. This study presents a new approach by blending high-molecular-weight PLA with a small amount of ʟ-lactide (PLA/LAx) to treat nitrate-contaminated wastewater. The addition of ʟ-lactide enhanced the release of carbon from high-molecular-weight PLA. An impressive denitrification efficiency of 96.7% was achieved, accompanied by extremely low levels of accumulated NO2--N (0.1 mg/L) and NH4+-N (0.4 mg/L). The quantity of ʟ-lactide used significantly impacted the bacterial community structure. A high abundance of the phyla Bacteroidota and Chloroflexi associated with polymer degradation was observed. The most dominant denitrifier was the genus unclassified_f__Rhodocyclaceae belonged to the phylum Proteobacteria. This study demonstrates that blending PLA with just 5 wt% lactide can transform it into a highly effective solid-phase carbon source to eliminate nitrates.

The function and community structure of arbuscular mycorrhizal fungi in ecological floating beds used for remediation of Pb contaminated wastewater.

Arbuscular mycorrhizal fungi (AMF) have been demonstrated to be ubiquitous in aquatic ecosystems. However, their distributions and ecological functions are rarely studied. To date, a few studies have combined sewage treatment facilities with AMF to improve removal efficiency, but appropriate and highly tolerant AMF strains have not been explored, and the purification mechanisms remain unclear. In this study, three ecological floating-bed (EFB) installations inoculated with different AMF inocula (mine AMF inoculum, commercial AMF inoculum and non-AMF inoculated) were constructed to investigate their removal efficiency for Pb-contaminated wastewater. The AMF community structure shifts in the roots of Canna indica inhabiting EFBs during the three phases (pot culture phase, hydroponic phase and hydroponic phase with Pb stress) were tracked utilizing quantitative real-time polymerase chain reaction and Illumina sequencing techniques. Furthermore, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were used to detect the Pb location in mycorrhizal structures. The results showed that AMF could promote host plant growth and enhance the Pb removal efficiency of the EFBs. The higher the AMF abundance, the better the effect of the AMF on Pb purification by EFBs. Both flooding and Pb stress decreased the AMF diversity but did not significantly inhibit the abundance. The three inoculation treatments showed different community compositions with different dominant AMF taxa in different phases, and an uncultured Paraglomus species (Paraglomus sp. LC516188.1) was found to be the most dominant (99.65 %) AMF in the hydroponic phase with Pb stress. The TEM and EDS analysis results showed that the Paraglomus sp. could accumulate Pb in plant roots through their fungal structures (intercellular mycelium, intracellular mycelium, etc.), which alleviated the toxic effect of Pb on plant cells and limited Pb translocation. The new findings provide a theoretical basis for the application of AMF in plant-based bioremediation of wastewater and polluted waterbodies.

Treatment and remediation of metal-contaminated water and groundwater in mining areas by biological sulfidogenic processes: A review.

Heavy metal pollution in the mining areas leads to serious environmental problems. The biological sulfidogenic process (BSP) mediated by sulfidogenic bacteria has been considered an attractive technology for the treatment and remediation of metal-contaminated water and groundwater. Notwithstanding, BSP driven by different sulfidogenic bacteria could affect the efficiency and cost-effectiveness of the treatment performance in practical applications, such as the microbial intolerance of pH and metal ions, the formation of toxic byproducts, and the consumption of organic electron donors. Sulfur-reducing bacteria (S0RB)-driven BSP has been demonstrated to be a promising alternative to the commonly used sulfate-reducing bacteria (SRB)-driven BSP for treating metal-contaminated wastewater and groundwater, due to the cost-saving in chemical addition, the high efficiency in sulfide production and metal removal efficiency. Although the S0RB-driven BSP has been developed and applied for decades, the present review works mainly focus on the developments in SRB-driven BSP for the treatment and remediation of metal-contaminated wastewater and groundwater. Accordingly, a comprehensive review for metal-contaminated wastewater treatment and groundwater remediation should be provided with the incorporation of the SRB- and S0RB-driven BSP. To identify the bottlenecks and to improve BSP performance, this paper reviews sulfidogenic bacteria presenting in metal-contaminated water and groundwater; highlight the critical factors for the metabolism of sulfidogenic bacteria during BSP; the ecological roles of sulfidogenic bacteria and the mechanisms of metal removal by sulfidogenic bacteria; and the application of the present sulfidogenic systems and their drawbacks. Accordingly, the research knowledge gaps, current process limitations, and future prospects were provided for improving the performance of BSP in the treatment and remediation of metal-contaminated wastewater and groundwater in mining areas.

Electrospun flexible core-sheath PAN/PU/ß-CD@Ag nanofiber membrane decorated with ZnO: enhance the practical ability of semiconductor photocatalyst.

It is necessary to effectively separate photocatalytic materials from water bodies and reuse catalysts for industrial wastewater treatment. Herein, a novel nanofiber membrane with enhanced light absorption and reusability of photocatalytic materials was prepared. The three-dimensional porous structure of the nanofibers helps the photocatalyst efficiently degrade pollutants. Specifically, a high-efficiency photocatalyst carrier with a nanofiber structure (PAN/PU/ß-CD@Ag nanofiber membrane) was prepared by electrospinning and a simple silver plating process, and then ZnO NPs were synthesized in situ on the nanofiber membrane during the hydrothermal process. Under visible-light irradiation, the ZnO-loaded PAN/PU/ß-CD@Ag nanofiber membranes exhibited excellent photocatalytic performance for the degradation of methylene blue (MB, 71.5%) and tetracycline hydrochloride (TCH, 70.5%). Additionally, a possible pathway of charge migration in this system was proposed. This design may provide a new idea for the preparation of visible-light photocatalytic nanofiber membranes and their treatments of wastewater containing dyes and hormones.

Reductive transformation of hexavalent chromium by ferrous ions in a frozen environment: Mechanism, kinetics, and environmental implications.

The transformation between hexavalent chromium (Cr6+) and trivalent chromium (Cr3+) has a significant impact on ecosystems, as Cr6+ has higher levels of toxicity than Cr3+. In this regard, a variety of Cr6+ reduction processes occurring in natural environments have been studied extensively. In this work, we investigate the reductive transformation of Cr6+ by ferrous ions (Fe2+) in ice at -20 °C, and compare the same process in water at 25 °C. The Fe2+-mediated reduction of Cr6+ occurred much faster in ice than it did in water. The accelerated reduction of Cr6+ in ice is primarily ascribed to the accumulation of Cr6+, Fe2+, and protons in the grain boundaries formed during freezing, which constitutes favorable conditions for redox reactions between Cr6+ and Fe2+. This freeze concentration phenomenon was verified using UV-visible spectroscopy with o-cresolsulfonephthalein (as a pH indicator) and confocal Raman spectroscopy. The reductive transformation of Cr6+ (20 µM) by Fe2+ in ice proceeded rapidly under various Fe2+ concentrations (20-140 µM), pH values (2.0-5.0), and freezing temperatures (-10 to -30 °C) with a constant molar ratio of oxidized Fe2+ to reduced Cr6+ (3:1). This result implies that the proposed mechanism (i.e., the redox reaction between Cr6+ and Fe2+ in ice) can significantly contribute to the natural conversion of Cr6+ in cold regions. The Fe2+-mediated Cr6+ reduction kinetics in frozen Cr6+-contaminated wastewater was similar to that in frozen Cr6+ solution. This indicates that the variety of substrates typically present in electroplating wastewater have a negligible effect on the redox reaction between Cr6+ and Fe2+ in ice; it also proposes that the Fe2+/freezing process can be used for the treatment of Cr6+-contaminated wastewater.

The nitrogen removal characterization of a cold-adapted bacterium: Bacillus simplex H-b.

The removal efficacy of biological nitrogen removal process is inhibited by low temperatures. Herein, a psychrotrophic bacterium strain, Bacillus simplex H-b, was isolated and identified with the potential to conduct heterotrophic nitrification and aerobic denitrification in the temperature range from 5 to 37 °C. At 10 °C, the removal efficiencies of initial nitrate-N (63 mg/L), nitrite-N (10 mg/L) and ammonium-N (60 mg/L) were 67.29%, 78.69% and 82.16%, with the maximum removal rate of 0.56, 0.18 and 0.74 mg/L/h, respectively. Additionally, both the accumulation level of ATP (adenosine triphosphate) and the formation of extracellular polymeric substances was found to increase with the decrease of temperature from 37 °C to 10 °C, indicating strain H-b might resist low temperature stress through its cellular extreme environment resistant mechanism and further suggesting the newly isolated strain could serve as a promising candidate for nitrogen contaminated wastewater treatment, especially under low-temperature condition.

Development of highly efficient bundle-like hydroxyapatite towards abatement of aqueous U(VI) ions: Mechanism and economic assessment.

The exploration of emergency materials with ultra-fast adsorption rate and great adsorption capability of released U(VI) ions is essentially urgent. The present work successfully fabricated bundle-like hydroxyapatite (B-HAP) microstructures which composed of numerous nanorods by employing a facile and green method. The B-HAP was applied to treat the U(VI) containing wastewater. The abatement of U(VI) by B-HAP was very rapid and the saturated adsorption capacity was superior; over 96.7 % of U(VI) was abated within 5 min, and the maximum adsorption capacity was as high as to 1305 mg/g, signifying the feasibility and effectiveness of this B-HAP in the treatment of uranium-contaminated wastewater due to nuclear accidents. It is worthy to note that other ions in solution exhibited relatively low interference on its performance, indicating that B-HAP has great application potential to capture U(VI) from radioactive-contaminated wastewater as well. The U(VI) removal mechanism by B-HAP was confirmed with results from XRD, FT-IR and XPS. Chernikovite [H2(UO2)2(PO4)2·8H2O] was newly formed after U(VI) abatement by B-HAP. Economic assessment suggested B-HAP and its application on U(VI) abatement were cost-effective. With characteristics of high adsorption rate, large capacity, and strong antijamming ability, B-HAP has great application potential as an emergency treatment material for nuclear accidents.

Effects of levofloxacin exposure on sequencing batch reactor (SBR) behavior and microbial community changes.

The adaptation mechanisms of bacterial community for nitrogen removal performance exposed to fluctuated levels of levofloxacin (LVX) during wastewater treatment in SBRs were investigated. Although LVX is completely synthetic, the results of minimum inhibitory concentration (MIC, 32 mg-LVX/L) and minimum bactericidal concentration (MBC, 512 mg-LVX/L) of the sampled sludge showed that the LVX resistance/tolerance for bacterial growth has already existed in the actual wastewater treatment plants (WWTPs). The key bacteria, i.e. Nitrosomonas sp. (ammonia-oxidizing bacteria), Nitrospira sp. (nitrite-oxidizing bacteria) and Thauera sp. (the predominant denitrifiers), decreased with LVX exposure, and the recovery of biological process in the reactor was disturbed due to LVX exposure. However, after stopping exposure their population was quickly increased and thus the performance was recovered. The results of the non-metric multidimensional scaling and microbial community by sequencing showed the LVX concentration was a crucial factor to the change of bacterial communities and controlled the quantitative evolution of the communities in our systems. This effect was more pronounced as the LVX concentration was higher. The results suggested the removal of residual antibiotics to accomplish under no effect concentration before biological treatment is important to suppress emerging and increasing of the antibiotic resistant bacteria in WWTPs.

Insight into Cr6+ reduction efficiency of Rhodococcus erythropolis isolated from coalmine waste water.

A microbial treatment of Cr6+ contaminated wastewater with a chromium reducing bacteria isolated from coal mine area was investigated. In a series of batch study metal removal was executed under different parametric conditions which include pH (2-7), temperature (20-50 °C), initial Cr6+ concentration (1-100 mg/L), substrate utilization and its overall effect on biomass generation. Impact of oxygen availability was checked at different agitation speed and its role on the remedial process. Liquid phase reduction of Cr6+ was measured in terms of substrate reduction and total biomass yield. The bacterium species isolated was able to tolerate Cr6+ over a wide range from 1 to 100 mg/L before it reached minimum inhibition concentration. Apart from Cr6+, the bacterial isolate showed tolerance towards Fe, As, Cu, Ag, Zn, Mn, Mg and Pb. Removal mechanism adopted by the bacterium recommended that it employed accumulation of Cr6+ as Cr3+ both within and outside the cell. Classical Monod equation was used to determine the biokinetics of the bacterial isolate along with the interference of metal ion concentration and substrate utilization. Cr6+ removal was found prominent even in bimetallic solutions. The bacterial isolate was confirmed to be Rhodococcus erythopolis by 16s rRNA molecular characterization. Thus the bacterial isolate obtained from the coal mine area proved to be a potential agent for microbial remediation of Cr6+ laden waste water.

Acute toxicity reduction and toxicity identification in pigment-contaminated wastewater during anaerobic-anoxic-oxic (A/A/O) treatment process.

In China, a considerable part of industrial wastewater effluents are discharged into the municipal wastewater treatment plants (WWTPs) after pretreatment in their own wastewater treatment plants. Even though the industrial effluents meet the professional emission standards, many micro-pollutants still remained, and they could be resistant in the municipal WWTPs with conventional activated sludge process. Pigment wastewater was chosen in this study, and the acute toxicity reduction and identification of the pigment-contaminated wastewater treated by the conventional anaerobic-anoxic-oxic (A/A/O) process were evaluated. Results indicated that the raw pigment-contaminated wastewater was acutely toxic to Photobacterium phosphoreum (P. phosphoreum), Daphnia magna (D. magna) and Danio rerio (D. rerio). The acute toxicity was decreased in some degree after A/A/O treatment, but the final effluent still exhibited acute toxicity to D. magna and D. rerio with the toxic units (TU) of 1.1 and 2.0, respectively. Chemical analyses showed the presence of various refractory and toxic nitrogen-containing polycyclic and heterocyclic compounds in the pigment-contaminated wastewater. Toxicity identification by combining chemical analyses and correlation analysis showed that N-containing refractory organic toxicants were the main toxicity source for the pigment-contaminated wastewater, and several toxicants showed significant correlation with P. phosphoreum and D. magna. This study indicated that the A/A/O process was not efficient for pigment-contaminated wastewater treatment, and it was irradiative for technology improvement in the WWTPs receiving pretreated industrial wastewater effluents.