Effects of treated industrial wastewaters and temperatures on growth and enzymatic activities of duckweed (Lemna minor L.)

Alginate and chitosan surface coating reduces the phytotoxicity of CeO2 nanoparticles to duckweed (Lemna minor L.)

Little is known about the effect of surface coatings on the fate and toxicity of CeO2 nanoparticles (NPs) to aquatic plants. In this study, we modified nCeO2 with chitosan (Cs) and alginate (Al) to obtain positively charged nCeO2@Cs and negatively charged nCeO2@Al, respectively, and exposed them to a representative aquatic plant, duckweed (Lemna minor L.). Uncoated nCeO2 could significantly inhibit the growth of duckweed, induce oxidative damage and lead to cell death, whereas nCeO2@Cs and nCeO2@Al exhibited lower toxicity to duckweed. ICP-MS analysis revealed that the Ce content in duckweed from the nCeO2 group was 1.74 and 2.85 times higher than that in the nCeO2@Cs and nCeO2@Al groups, respectively. Microscopic observations indicated that the positively charged nCeO2@Cs was more readily adsorbed on the root surface of duckweed than the negatively charged nCeO2@Al. The results of XANES and LCF demonstrated that a certain percentage of Ce(Ⅳ) was reduced to Ce(Ⅲ) after the interaction of the three NPs with duckweed, but the degree of biotransformation differed among the treatments. Specifically, the absolute contents of Ce(III) produced of nCeO2@Cs and nCeO2@Al through biotransformation were reduced by 55.5% and 83.5%, respectively, compared with that of the nCeO2 group, which might be the key factor for the diminished phytotoxicity of the coated nCeO2 to the duckweed. These findings were valuable for understanding the toxicity of metal-based NPs to aquatic plants and for the synthesis of environmentally friendly nanomaterials.

Domestic wastewater treatment by Pistia stratiotes in constructed wetland

The objective of the study was to evaluate the performance of Pistia stratiotes for treatment of domestic wastewater in a free surface water flow constructed wetland. The objective of the study was to evaluate contaminants removal efficiency of the constructed wetland vegetated with P. stratiotes in treatment of domestic wastewater against Hydraulic retention time (HRT) of 10, 20 and 30 days was investigated. This asks for newer and efficient low-cost nature-based water treatment system which along with cost takes into consideration the sustainability of the ecosystem. Five constructed wetland setups improved the wastewater quality and purify it significantly by reducing the TDS by 83%, TSS by 82%, BOD by 82%, COD by 81%, Chloride by 80%, Sulfate by 77%, NH3 by 84% and Total Oil and Grease by 74%. There was an increase in pH of about 11.9%. Color and odor of wastewater was also improved significantly and effectively. It was observed that 30 days' HRT was optimum for the treatment of domestic wastewater. The final effluent was found to be suitable as per national environmental quality standards and recycled for watering plants and crop irrigation but not for drinking purposes. The treatment in constructed wetland system was found to be economical, as the cost of construction only was involved and operational and maintenance cost very minimal. Even this research was conducted on the sole purpose of commuting the efficiency of pollutant removal in short span time.

Alleviation of aqueous nitrogen loss from paddy fields by growth and decomposition of duckweed (Lemna minor L.) after fertilization

Runoff loss of nitrogen from paddy fields has received increasing attention in recent years. Duckweed is an aquatic plant frequently found in paddy fields. In this study, the effects of duckweed (Lemna minor L.) in floodwater on aqueous nitrogen losses from paddy fields were systematically investigated. Results demonstrated that the growth of duckweed decreased total nitrogen concentrations in floodwater and nitrogen runoff loss from paddy fields by 16.7%-18.3% and 11.2%-13.6%, respectively. Moreover, compared with NO₃^-, NH₄⁺ was preferentially removed by duckweed. ¹⁵N isotope tracer experiments revealed that the growth and decomposition of duckweed acted as a "buffer" against the nitrogen variation in floodwater after fertilization. During the growth of duckweed, leaves were found to be the principal organ to assimilate NH₄⁺ and release NO₃^- by using non-invasive micro-test technology. Duckweed degradation increased the content of hydrophobic acids and marine humic-like substances in floodwater, which promoted the migration of nitrogen from floodwater to soil. Redundancy analysis and structural equation models further illustrated that pH and temperature variation in floodwater caused by duckweed played a greater role in aqueous nitrogen loss reduction than the nitrogen accumulation in duckweed. This study suggested that the growth of duckweed in paddy fields was an effective supplementary method for controlling aqueous nitrogen loss during agricultural production.

Hormetic activation of nano-sized rare earth element terbium on growth, PSII photochemistry, antioxidant status and phytohormone regulation in Lemnaminor

Soils contaminated with rare earth elements (REEs) can damage agriculture by causing physiological disorders in plants which are evaluated as the main connection of the human food chain. A biphasic dose response with excitatory responses to low concentrations and inhibitory/harmful responses to high concentrations has been defined as hormesis. However, not much is clear about the ecological effects and potential risks of REEs to plants. For this purpose, here we showed the impacts of different concentrations of nano terbium (Tb) applications (5-10-25-50-100-250-500 mg L^-1) on the accumulation of endogeneous certain ions and hormones, chlorophyll fluoresence, photochemical reaction capacity and antioxidant activity in duckweed (Lemna minor). Tb concentrations less than 100 mg L^-1 increased the contents of nitrogen (N), phosphate (P), potassium (K⁺), calcium (Ca²⁺), magnesium (Mg²⁺), manganese (Mn²⁺) and iron (Fe²⁺). Chlorophyll fluorescence (F_v/F_m and F_v/F_o) was suppressed under 250-500 mg L^-1 Tb. In addition, Tb toxicity affected the trapped energy adversely by the active reaction center of photosystem II (PSII) and led to accumulation of inactive reaction centers, thus lowering the detected level of electron transport from photosystem II (PSII) to photosystem I (PSI). On the other hand, 5-100 mg L^-1 Tb enhanced the activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), NADPH oxidase (NOX), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR) and glutathione S-transferase (GST). Tb (5-50 mg L^-1) supported the maintenance of cellular redox status by promoting antioxidant pathways involved in the ascorbate-glutathione (AsA-GSH) cycle. In addition to the antioxidant system, the contents of some hormones such as indole-3-acetic acid (IAA), gibberellic acid (GA), cytokinin (CK) and salicylic acid (SA) were also induced in the presence of 5-100 mg L^-1 Tb. In addition, the levels of hydrogen peroxide (H₂O₂) and lipid peroxidation (TBARS) were controlled through ascorbate (AsA) regeneration and effective hormonal modulation in L. minor. However, this induction in the antioxidant system and phytohormone contents could not be resumed after applications higher than 250 mg L^-1 Tb. TBARS and H₂O₂, which indicate the level of lipid peroxidation, increased. The results in this study showed that Tb at appropriate concentrations has great potential to confer tolerance of duckweed by supporting the antioxidant system, protecting the biochemical reactions of photosystems and improving hormonal regulation.

Rare earth element scandium mitigates the chromium toxicity in Lemna minor by regulating photosynthetic performance, hormonal balance and antioxidant machinery

Chromium (Cr) toxicity is a serious problem that threatens the health of living organisms and especially agricultural production. The presence of excess Cr leads to biomass loss by causing the imbalance of biochemical metabolism and inhibiting photosynthetic activity. A new critical approach to cope with Cr toxicity is the use of the rare earth elements (REEs) as an antioxidant defence system enhancer in plants. However, the effect of scandium (Sc), which is one of the REEs, is not clear enough in Lemna minor exposed to Cr toxicity. For this purpose, the photosynthetic and biochemical effects of scandium (50 μM and 200 μM Sc) treatments were investigated in Lemna minor under Cr stress (100 μM, 200 μM and 500 μM Cr). Parameters related to photosynthesis (F_v/F_m, F_v/F_o) were suppressed under Cr stress. Stress altered antioxidant enzymes activities and hormone contents. Sc applications against stress increased the activities of superoxide dismutase (SOD), NADPH oxidase (NOX), ascorbate peroxidase (APX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), and glutathione S-transferase (GST). In addition to the antioxidant system, the contents of indole-3-acetic acid (IAA), abscisic acid (ABA) and jasmonic acid (JA) were also rearranged. However, in all treatment groups, with the provision of ascorbate (AsA) regeneration and effective hormone signaling, reactive oxygen species (ROS) retention which result in high hydrogen peroxide (H₂O₂) content and lipid peroxidation (TBARS) were effectively removed. Sc promoted the maintenance of cellular redox state by regulating antioxidant pathways included in the AsA-GSH cycle. Our results showed that Sc has great potential to confer tolerance to duckweed by reducing Cr induced oxidative damage, protecting the biochemical reactions of photosynthesis, and improving hormone signaling.

Combined effects of temperature and nutrients on the toxicity of cadmium in duckweed (Lemna aequinoctialis)

Global anthropogenic changes are altering the temperature and nutrients of the ecosystem, which might also affect the extent of cadmium (Cd) toxicity in organisms. This study aimed to investigate the combined effects of temperature and nutrient availability (here, nitrogen [N] and phosphorus [P]) on Cd toxicity in duckweed (Lemna aequinoctialis). The growth parameters, nutrient uptake, and Cd tolerance of plantlets reached their highest values for duckweed grown in medium with 28 mg/L N and 2.4 mg/L P (N:P = 11.67) at 25 °C under 1 mg/L CdCl₂ exposure. Raising the temperature (from 18 °C to 25 °C) and levels of N and P (from 0.01 N/P to 2 N/P) enhanced photosynthetic capacity and nutrient uptake, thus promoting plant growth and diluting the toxic effects of Cd. Although Cd uptake increased with increasing temperature, duckweed with relatively high biomass exhibited a lower accumulation of the toxic metal because their growth rate exceeded Cd uptake rate. Increasing N and P supply also enhanced the tolerance of duckweed to Cd by limiting Cd bioavailability. Our study therefore suggests the importance of combined effects from temperature and nutrients for Cd toxicity and provides novel insights for a comprehensive analysis of Cd toxicity associated with the environmental factors of a particular ecosystem.

Impact of aquatic microplastics and nanoplastics pollution on ecological systems and sustainable remediation strategies of biodegradation and photodegradation

The extreme degree of microplastics contamination and its negative impact on ecosystems has become a serious and emerging global concern. Microplastics are mainly generated from products that are used primarily in our everyday lives and are also generated from the fragmentation of larger plastic wastes. It easily penetrates the food chain and, when ingested by aquatic animals or humans, can pose serious health problems. Recently, several technologies have been developed to control the unrestricted spread of microplastics and possibly eradicate them; however, still under investigation. In this review, we have illustrated the types of microplastics, their harmful effect on living things, and the progress to degrade them to protect the environment and life on earth. Several promising and eco-friendly technologies including microbial and enzymatic approaches are enticing to eliminate the microplastics. Also, the photodegradation of microplastics contaminations appeals as a more fascinating approach. The metal oxide-assisted photodegradation of microplastics has also been taken into account. This work presented an impact on the comprehensive research for the effective degradation of different microplastic compositions as well as emerging green approaches for a sustainable environment and a healthier life.

Antioxidative Responses of Duckweed (Lemna minor L.) to Phenol and Rhizosphere-Associated Bacterial Strain Hafnia paralvei C32-106/3

Duckweed (L. minor) is a cosmopolitan aquatic plant of simplified morphology and rapid vegetative reproduction. In this study, an H. paralvei bacterial strain and its influence on the antioxidative response of the duckweeds to phenol, a recalcitrant environmental pollutant, were investigated. Sterile duckweed cultures were inoculated with H. paralvei in vitro and cultivated in the presence or absence of phenol (500 mg L⁻¹), in order to investigate bacterial effects on plant oxidative stress during 5 days. Total soluble proteins, guaiacol peroxidase expression, concentration of hydrogen peroxide and malondialdehyde as well as the total ascorbic acid of the plants were monitored. Moreover, bacterial production of indole-3-acetic acid (IAA) was measured in order to investigate H. paralvei’s influence on plant growth. In general, the addition of phenol elevated all biochemical parameters in L. minor except AsA and total soluble proteins. Phenol as well as bacteria influenced the expression of guaiacol peroxidase. Different isoforms were associated with phenol compared to isoforms expressed in phenol-free medium. Considering that duckweeds showed increased antioxidative parameters in the presence of phenol, it can be assumed that the measured parameters might be involved in the plant’s defense system. H. paralvei is an IAA producer and its presence in the rhizosphere of duckweeds decreased the oxidative stress of the plants, which can be taken as evidence that this bacterial strain acts protectively on the plants during phenol exposure.

Insights into the Use of Phytoremediation Processes for the Removal of Organic Micropollutants from Water and Wastewater; A Review

Greater awareness of micropollutants present in water and wastewater motivates the search for effective methods of their neutralization. Although their concentration in waters is measured in micro- and nanograms per liter, even at those levels, they may cause serious health consequences for different organisms, including harmful effects on the functioning of the endocrine system of vertebrates. Traditional methods of wastewater treatment, especially biological methods used in municipal wastewater treatment plants, are not sufficiently effective in removing these compounds, which results in their presence in natural waters. The growing interest in phytoremediation using constructed wetlands as a method of wastewater treatment or polishing indicates a need for the evaluation of this process in the context of micropollutant removal. Therefore, the present work presents a systematic review of the effectiveness in the removal of micropollutants from polluted waters by processes based on plant used. The article also analyzes issues related to the impact of micropollutants on the physiological processes of plants as well as changes in general indicators of pollution caused by contact of wastewater with plants. Additionally, it is also the first review of the literature that focuses strictly on the removal of micropollutants through the use of constructed wetlands.

Microalgal and duckweed based constructed wetlands for swine wastewater treatment: A review

Constructed wetlands for swine wastewater treatment have been one of the most exciting research topics. Usually hydrophytes based constructed wetlands could not adapt well to high concentration of ammonia nitrogen in swine wastewater, while microalgal and duckweed based constructed wetlands are promising for the nutrient removal. In this critical review, the important roles of microalgae and duckweeds played in wastewater treatment in constructed wetlands were first summarized. Performances including biomass growth, nutrient removal capacities and mechanisms of microalgal and duckweed based constructed wetlands were reviewed for swine wastewater treatment. Challenges for the applications of constructed wetlands including microalgal and duckweed based ones were discussed which includes a better understanding and utilization of synergistic effects among microalgae and duckweeds, difficulty and costs in harvesting biomass, applications in various field conditions including low temperatures, and selections of various types of microalgal and duckweed species. Future research needs were also proposed accordingly.

Application of Novel C-TiO2-CFA/PAN Photocatalytic Membranes in the Removal of Textile Dyes in Wastewater

The existence of dye effluent in environmental water bodies is becoming a growing concern to environmentalists and civilians due to negative health effects. In this study, a novel poly(acrylonitrile)-membrane-supported carbon-doped titanium dioxide–coal fly ash nanocomposite (C-TiO2-CFA/PAN) was prepared and evaluated in the removal of textiles dyes (methyl orange and golden yellow) in water. The C-TiO2-CFA nanocomposite was prepared via sol-gel synthesis and immobilized on PAN membrane prepared via phase inversion technique. The photocatalyst was characterized by FTIR, XRD, BET surface area analysis, SEM, EDX, and DRS. FTIR analysis confirmed the existence of the expected functional groups, and XRD revealed that the C-TiO2 was predominantly in the anatase phase, which exhibited the highest photocatalytic activity. The optimum C-TiO2-CFA photocatalyst load on the PAN membrane was 2% w/w, and it achieved degradation efficiencies of 99.86% and 99.20% for MO and GY dyes, respectively, at pH 3.5, using a dye concentration of 10 ppm, under sunlight irradiation, in 300 min. The novel 2% C-TiO2-CFA/PAN photocatalytic membrane proved to be very effective in the removal of textile dyes’ water. Three reusability cycles were carried out, and no significant changes were observed in the photocatalytic efficiencies. Immobilization on PAN membrane allowed easy recovery and reuse of the photocatalyst.

Phytoremediation potential of Azolla filiculoides for sodium dodecyl benzene sulfonate (SDBS) surfactant considering some physiological responses, effects of operational parameters and biodegradation of surfactant

In this study, phytoremediation potential of the Azolla filiculoides Lam. was examined for sodium dodecyl benzene sulfonate (SDBS) anionic surfactant. Furthermore, the effect of surfactant treatment on some physiological characteristics of Azolla was studied. The surfactant bioremoval efficiency was studied under variable conditions including treatment time, initial surfactant concentration, Azolla fresh weight, temperature, and pH. Results showed that surfactant removal efficiency of A. filiculoides was significantly enhanced with increasing of temperature, initial surfactant concentration, and amount of Azolla. SDBS led to a reduction in growth rate and total chlorophyll content, but effect index of Azolla increased by higher concentrations of surfactant. In contrast, antioxidant enzymes activities including polyphenol oxidase, ascorbate peroxidase, catalase, and peroxidase, as well as nonenzymatic antioxidants such as total carotenoids and anthocyanin contents significantly increased probably due to the ability of plant to overcome oxidative stress induced by SDBS. An increase in antioxidant activity based on 2, 2-diphenyl-1-picrylhydrazil (DPPH) confirmed this fact. An increase in the amount of hydrogen peroxide and reduction in membrane stability index indicated the induction of oxidative stress. As a result of SDBS biodegradation, 6 homologs of sulfophenyl carboxylates (SPCs) including C₂ to C₇-SPC and benzenesulfonate ring were identified by liquid chromatography-mass spectroscopy (LC-MS) analysis.

Phenol Removal Capacity of the Common Duckweed (Lemna minor L.) and Six Phenol-Resistant Bacterial Strains From Its Rhizosphere: In Vitro Evaluation at High Phenol Concentrations

The main topic of this study is the bioremediation potential of the common duckweed, Lemna minor L., and selected rhizospheric bacterial strains in removing phenol from aqueous environments at extremely high initial phenol concentrations. To that end, fluorescence microscopy, MIC tests, biofilm formation, the phenol removal test (4-AAP method), the Salkowski essay, and studies of multiplication rates of sterile and inoculated duckweed in MS medium with phenol (200, 500, 750, and 1000 mg L⁻¹) were conducted. Out of seven bacterial strains, six were identified as epiphytes or endophytes that efficiently removed phenol. The phenol removal experiment showed that the bacteria/duckweed system was more efficient during the first 24 h compared to the sterile duckweed control group. At the end of this experiment, almost 90% of the initial phenol concentration was removed by both groups, respectively. The bacteria stimulated the duckweed multiplication even at a high bacterial population density (>10⁵ CFU mL⁻¹) over a prolonged period of time (14 days). All bacterial strains were sensitive to all the applied antibiotics and formed biofilms in vitro. The dual bacteria/duckweed system, especially the one containing strain 43-Hafnia paralvei C32-106/3, Accession No. MF526939, had a number of characteristics that are advantageous in bioremediation, such as high phenol removal efficiency, biofilm formation, safety (antibiotic sensitivity), and stimulation of duckweed multiplication.

Phytoremediation processes of domestic and textile effluents: evaluation of the efficacy and toxicological effects in Lemna minor and Daphnia magna

Phytoremediation has been proposed as a potential biotechnological strategy to remediate effluents before their release into the environment. The use of common aquatic plant species, such as macrophytes (e.g., Lemna spp.) as a cleanup solution has been proposed decades ago. However, the effectiveness of such processes must be assessed by analyzing the toxicity of resulting effluents, for the monitoring of wastewater quality. To attain this purpose, this work intended to quantify the efficacy of a Lemna-based wastewater phytoremediation process, by analyzing toxicological effects of domestic and textile effluents. The toxic effects were measured in Lemna minor (same organisms used in the phytoremediation process, by quantifying toxicological endpoints such as root length, pigment content, and catalase activity) and by quantifying individual parameters of Daphnia magna (immobilization, reproduction, and behavior analysis). Phytoremediation process resulted in a decrease of chemical oxygen demand in both effluents and in an increase in root length of exposed plants. Moreover, textile effluent decreased pigments content and increased catalase activity, while domestic effluent increased the anthocyanin content of exposed plants. D. magna acute tests allowed calculating a EC₅₀ and Toxic Units interval of 53.82-66.89%/1.85-1.49, respectively, to raw textile effluent; however, it was not possible to calculate these parameters for raw and treated domestic effluent (RDE and TDE). Therefore, in general, the acute toxicity of effluent toward D. magna was null for RDE, and mild for the treated textile effluent (TTE), probably due to the effect of phytoremediation. Exposure to textile effluents (raw and treated) increased the total number of neonates of D. magna and, in general, both textile effluents decreased D. magna distance swim. Moreover, although both effluents were capable of causing morphological and physiological/biochemical alterations in L. minor plants, organisms of this species were able to survive in the presence of both effluents and to remediate them.

Identification of textile wastewater in water bodies by fluorescence excitation emission matrix-parallel factor analysis and high-performance size exclusion chromatography

Identifying the causes of water body pollution is critical because of the serious water contamination in developing countries. The textile industry is a major contributor to severe water pollution due to its high discharge of wastewater with high concentrations of organic and inorganic pollutants. In this study, fluorescence excitation emission matrix-parallel factor (EEM-PARAFAC) analysis was applied to characterize textile industry wastewater and trace its presence in water bodies. The EEM spectra of textile wastewater samples collected from 12 wastewater treatment plants (WWTPs) revealed two characteristic peaks: Peak T1 (tryptophan-like region) and Peak B (tyrosine-like region). Two protein-like components (C1 and C2) were identified in the textile wastewater by PARAFAC analysis. The components identified from different textile WWTPs were considered identical (similarity >0.95). C1 and C2 were not sensitive to changes in pH, ionic strength, or low humic acid concentration (TOC < 4 mg/L). Therefore, C1 combined with C2 was proposed as a source-specific indicator of textile wastewater, which was further demonstrated by conducting high-performance size exclusion chromatography analysis. These results suggested that EEM-PARAFAC analysis is a reliable means of identifying textile wastewater pollution in water bodies and may also enable the identification of other industrial wastewater.

Plant growth promotion and alleviation of salinity stress in Capsicum annuum L. by Bacillus isolated from saline soil in Xinjiang

To maintain the growth and development of pepper in saline condition, candidates of plant growth promoting rhizobacteria (PGPR) were isolated, and detected to plant growth promoting (PGP) potential under salt stress was investigated. Thirteen bacterial strains with 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, WU-1-13, were isolated from saline soil in Xinjiang, China. The isolates were shown to belong to the genera Bacillus by partial sequencing analysis of their respective 16 S rRNA genes. Seven isolates had the ability to solubilize phosphate. Moreover, the amount of solubilized phosphate was significantly high (P < 0.05), which ranged from 157.33 μg/mL to 922.41 μg/mL. All tested bacterial strains were shown to produce a large amount of ACC deaminase and NH₃. Furthermore, nine strains were detected for siderophore production. On the aspect of extracellular enzyme, all bacterial isolates produced lipase, amylase and cellulose, whereas only a minority produced chitinase (15.4%) and 10 isolates produced β-glucanase or protease. In growth room experiments, the results showed that the strain WU-5 exhibited better growth promotion of pepper seedlings in terms of fresh weight (75.60%), dry weight (86.68%), shoot length (12.12%) and root length (146.52%) over the control under saline stress followed by WU-13. Furthermore, seedlings accumulated high amounts of proline induced by the different PGPR inoculation treatments to alleviate the negative effects of salt stress. Further growth-promoting assays under different salt stress were set up to confirm that the fresh and dry weight, shoot and root length of pepper plants inoculated by three strains all were significantly higher than non-inoculated control under different saline stress. In summary, the results demonstrated that WU-9, which induced high levels of proline production and antioxidant enzyme activities, and three strains (WU-5, WU-9 and WU-13) can be of great value in maintaining the growth and development of seedlings on saline lands.

Effect of cerium on growth and antioxidant metabolism of Lemna minor L

An increasing input rate of rare earth elements in the environment is expected because of the intense extraction of such elements form their ores to face human technological needs. In this study Lemna minor L. plants were grown under laboratory conditions and treated with increasing concentrations of cerium (Ce) ions to investigate the effects on plant growth and antioxidant systems. The growth increased in plants treated with lower Ce concentrations and reduced in plants treated with higher concentrations, compared to control plants. In plants treated with higher Ce concentrations lower levels of chlorophyll and carotenoid and the appearance of chlorotic symptoms were also detected. Increased levels of hydrogen peroxide, antioxidant metabolites and antioxidant activity confirmed that higher Ce concentrations are toxic to L. minor. Ce concentration in plant tissues was also determined and detectable levels were found only in plants grown on Ce-supplemented media. The use of duckweed plants as a tool for biomonitoring of Ce in freshwater is discussed.

The Role of Nanobubbles in the Precipitation and Recovery of Organic-Phosphine-Containing Beneficiation Wastewater

Dissolved air flotation (DAF) is broadly applied in wastewater treatment, especially for the recovery of organic pollution with low concentration. However, the mechanism of interaction between nanoscale gas bubbles and nanoparticles in the process of DAF remains unclear. Here, we investigated the role of nanobubbles in the precipitation of styryl phosphoric acid (SPA)-Pb particles and recovering organic phosphine containined in beneficiation wastewater by UV-vis (ultraviolet-visible) spectra, microflotation tests, nanoparticle tracking analysis, dynamic light scattering, and atomic force microscopy measurements. As suggested from the results, nanobubbles can inhibit the crystallization of SPA-Pb precipitation, which makes the sediment flotation recovery below 20%. After the precipitation crystallization is completed, nanobubbles can flocculate precipitated particles, which can promote the flotation recovery of precipitated particles to 90%. On the basis of the results, we proposed a model to explain the different roles of nanobubbles in the process of precipitation and flotation of SPA-Pb particles. This study will be helpful to understand the interaction between nanobubbles and nanoparticles in the application of flotation.