Removal of microcystin-LR and microcystin-RR by graphene oxide: adsorption and kinetic experiments

Nanostructured Magnetic Particles for Removing Cyanotoxins: Assessing Effectiveness and Toxicity In Vitro

The rise in cyanobacterial blooms due to eutrophication and climate change has increased cyanotoxin presence in water. Most current water treatment plants do not effectively remove these toxins, posing a potential risk to public health. This study introduces a water treatment approach using nanostructured beads containing magnetic nanoparticles (MNPs) for easy removal from liquid suspension, coated with different adsorbent materials to eliminate cyanotoxins. Thirteen particle types were produced using activated carbon, CMK-3 mesoporous carbon, graphene, chitosan, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidised cellulose nanofibers (TOCNF), esterified pectin, and calcined lignin as an adsorbent component. The particles' effectiveness for detoxification of microcystin-LR (MC-LR), cylindrospermopsin (CYN), and anatoxin-A (ATX-A) was assessed in an aqueous solution. Two particle compositions presented the best adsorption characteristics for the most common cyanotoxins. In the conditions tested, mesoporous carbon nanostructured particles, P1-CMK3, provide good removal of MC-LR and Merck-activated carbon nanostructured particles, P9-MAC, can remove ATX-A and CYN with high and fair efficacy, respectively. Additionally, in vitro toxicity of water treated with each particle type was evaluated in cultured cell lines, revealing no alteration of viability in human renal, neuronal, hepatic, and intestinal cells. Although further research is needed to fully characterise this new water treatment approach, it appears to be a safe, practical, and effective method for eliminating cyanotoxins from water.

Coupling physiochemical adsorption with biodegradation for enhanced removal of microcystin-LR in water

To innovate the design of water treatment technology for algal toxin removal, this research investigated the mechanisms of cyanotoxin microcystin-LR (MC-LR) removal by a coupled adsorption-biodegradation. Eight types of woody carbonaceous adsorbents with and without Sphingopyxis sp. m6, a MC-LR degrading bacterium, were tested for MC-LR removal in water. All adsorbents showed good adsorption capability, removing 40 % to almost 100 % of the MC-LR (4.5 mg/L) within 48 h in batch experiments. Adding Sphingopyxis sp. m6 continuously promoted MC-LR biological removal, and successfully broke the barrier of adsorption capacity of tested adsorbents, removing >90 % of the MC-LR in most of the coupled adsorption-biodegradation tests, especially for those adsorbents had low physiochemical adsorption capacity. Variance partitioning analysis indicated that mesopore was the dominant contributor to adsorption capacity of MC-LR in pure adsorption treatments, which acted synergistically with electrical conductivity, polarity and total functional groups on the absorbent. Pore structure was the key factor beneficial for the growth of Sphingopyxis sp. m6 (51% contribution) and subsequent MC-LR biological removal rate (80 % contribution). Overall, pinewood-based carbonaceous adsorbents (especially pinewood activated carbon) exhibited the highest adsorption capacity towards MC-LR and provided the most favorable conditions for biological removal of MC-LR, largely because of their high mesopore volume, total functional groups and electric conductivity. The research outcomes not only deepened the quantitative understanding of mechanisms for MC-LR removal by the coupled process, but also provided theoretical basis for future materials' selection and modification during the practical application of coupled process.

Influence of biochar on the removal of Microcystin-LR and Saxitoxin from aqueous solutions

The present study assessed the effective use of biochar for the adsorption of two potent HAB toxins namely, Microcystin-LR (MCLR) and Saxitoxin (STX) through a combination of dosage, kinetic, equilibrium, initial pH, and competitive adsorption experiments. The adsorption results suggest that biochar has excellent capabilities for removing MCLR and STX, with STX reporting higher adsorption capacities (622.53-3507.46 µg/g). STX removal required a minimal dosage of 0.02 g/L, while MCLR removal needed 0.4 g/L for > 90%. Similarly, a shorter contact time was required for STX removal compared to MCLR for > 90% of toxin removed from water. Initial pH study revealed that for MCLR acidic conditions favored higher uptake while STX favored basic conditions. Kinetic studies revealed that the Elovich model to be most suitable for both toxins, while STX also showed suitable fittings for Pseudo-First Order and Pseudo-Second Order in individual toxin systems. Similarly, for the Elovich model the most suited kinetic model for both toxins in presence of each other. Isotherm studies confirmed the Langmuir-Freundlich model as the best fit for both toxins. These results suggest adsorption mechanisms including pore filling, hydrogen bonding, π-π interactions, hydrophobic interactions, electrostatic attraction, and dispersive interactions.

Efficient removal of microcystin-LR from contaminated water using water-stable MIL-100(Fe) synthesized under HF-free conditions

Cyanobacterial algal hepatotoxins, called microcystins (MCs), are a global health concern, necessitating research on effective removal methods from contaminated water bodies. In this study, we synthesized non-fluorine MIL-100(Fe) using an environmentally friendly room-temperature method and utilized it as an adsorbent to effectively remove microcystin-LR (MC-LR), which is the most toxic MC congener. MIL-100(Fe) was thoroughly characterized, and its adsorption process was investigated under various conditions. Results revealed rapid MC-LR adsorption, achieving 93% removal in just 5 min, with the pseudo-second-order kinetic model indicating chemisorption as the primary mechanism. The Langmuir isotherm model demonstrated a monolayer sorption capacity of 232.6 µg g-1 at room temperature, showing favorable adsorption. Furthermore, the adsorption capacity increased from 183 µg g-1 at 20 °C to 311 µg g-1 at 40 °C, indicating an endothermic process. Thermodynamic parameters supported MC-LR adsorption's spontaneous and feasible nature onto MIL-100(Fe). This study highlights MIL-100(Fe) as a promising method for effectively removing harmful biological pollutants, such as MC-LR, from contaminated water bodies in an environmentally friendly manner.

Physico-chemical treatments for the removal of cyanotoxins from drinking water: Current challenges and future trends

Cyanobacteria are highly prevalent blue-green algae that grow in stagnant and nutrient-rich water bodies. Environmental conditions, such as eutrophication and human activities, increased the cyanobacterial blooms in freshwater resources worldwide. The excessive bloom formation has also resulted in an alarming surge of cyanobacterial toxins. Prolonged exposure to cyanotoxins is a potential threat to natural ecosystems, animal and human health by the spoilage of the quality of bathing and drinking water. Various molecular and analytical methods have been proposed to monitor their occurrence and understand their global distribution. Moreover, different physical, chemical, and biological approaches have been employed to control cyanobacterial blooms and their toxins to mitigate their occurrence. Numerous strategies have been engaged in drinking water treatment plants (DWTPs). However, the degree of treatment varies greatly and is primarily determined by the source, water properties, and operating parameters such as temperature, pH, and cyanotoxin variants and levels. A comprehensive compilation of methods, from traditional approaches to more advanced oxidation processes (AOPs), are presented for the removal of intracellular and extracellular cyanotoxins. This review discusses the effectiveness of various physicochemical operations and their limitations in a DWTP, for the removal of various cyanotoxins. These operations span from simple to advanced treatment levels with varying degrees of effectiveness and differing costs of implementation. Furthermore, mitigation measures applied in other toxin systems have been considered as alternative strategies.

Graphene as a rational interface for enhanced adsorption of microcystin-LR from water

Cyanotoxins such as microcystin-LR (MC-LR) represent a global environmental threat to ecosystems and drinking water supplies. The study investigated the direct use of graphene as a rational interface for removal of MC-LR via interactions with the aromatic ring of the ADDA1 chain of MC-LR and the sp2 hybridized carbon network of graphene. Intra-particle diffusion model fit indicated the high mesoporosity of graphene provided significant enhancements to both adsorption capacities and kinetics when benchmarked against microporous granular activated carbon (GAC). Graphene showed superior MC-LR adsorption capacity of 75.4 mg/g (Freundlich model) compared to 0.982 mg/g (Langmuir model) for GAC. Sorption kinetic studies showed graphene adsorbs 99% of MC-LR in 30 min, compared to zero removal for GAC after 24 hr using the same MC-LR concentration. Density functional theory (DFT), calculations showed that postulated π-based interactions align well with the NMR-based experimental work used to probe primary interactions between graphene and MC-LR adduct. This study proved that π-interactions between the aromatic ring on MC-LR and graphene sp2 orbitals are a dominant interaction. With rapid kinetics and adsorption capacities much higher than GAC, it is anticipated that graphene will offer a novel molecular approach for removal of toxins and emerging contaminants with aromatic systems.

Immobilization of Microcystin by the Hydrogel-Biochar Composite to Enhance Biodegradation during Drinking Water Treatment

Microcystin-LR (MC-LR), the most common algal toxin in freshwater, poses an escalating threat to safe drinking water. This study aims to develop an engineered biofiltration system for water treatment, employing a composite of poly(diallyldimethylammonium chloride)-biochar (PDDA-BC) as a filtration medium. The objective is to capture MC-LR selectively and quickly from water, enabling subsequent biodegradation of toxin by bacteria embedded on the composite. The results showed that PDDA-BC exhibited a high selectivity in adsorbing MC-LR, even in the presence of competing natural organic matter and anions. The adsorption kinetics of MC-LR was faster, and capacity was greater compared to traditional adsorbents, achieving a capture rate of 98% for MC-LR (200 μg/L) within minutes to tens of minutes. Notably, the efficient adsorption of MC-LR was also observed in natural lake waters, underscoring the substantial potential of PDDA-BC for immobilizing MC-LR during biofiltration. Density functional theory calculations revealed that the synergetic effects of electrostatic interaction and π-π stacking predominantly contribute to the adsorption selectivity of MC-LR. Furthermore, experimental results validated that the combination of PDDA-BC with MC-degrading bacteria offered a promising and effective approach to achieve a sustainable removal of MC-LR through an "adsorption-biodegradation" process.

A Theoretical Study of the Interactions between Persistent Organic Pollutants and Graphene Oxide

Persistent organic pollutants (POPs) have adverse effects on the human health and ecosystem functioning. Graphene oxide (GO) has been developed to remove trace levels of POPs from wastewater samples. However, many questions involved in these processes are still unresolved (e.g., the role of π-π interaction, the effect of GO on the degradation of POPs, and so on). Revealing the microscopic interactions between GO and POPs is of benefit to resolve these questions. In the present study, a quantum chemical calculation was used to calculate the molecular doping and adsorption energy between eight representative POPs and GO. The influences of GO on the thermodynamic parameters, such as the Gibbs free energy and the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gap, were also reported. We found the molecular doping is dependent on the species of POPs. The adsorption energy of the majority of POPs on GO is between 7 and 8 kJ/mol. Consequently, the GO may make degradation of POPs in wastewater more productive and lead to a change of kinetics of the degradation of POPs.

Graphene-Mediated removal of Microcystin-LR in chitosan/graphene composites for treatment of harmful algal blooms

Water quality can be severely impacted by algal blooms alone, yet cyanotoxins, such as microcystin (MC), are potent underlying hazards produced by various species of cyanobacteria. Currently there is a need for environmentally compatible and economically viable media to address large scale application for HAB impacted waters. This study evaluated the interactions of chitosan/graphene (CSG) composites with three different species of cyanobacteria: Anabaena sp, Synechocystis sp, and Microcystis aeruginosa for both removal of algal optical density and toxins. Although results suggest that CSG has an algae dependent removal of density with a range of 40-90% removal, graphene/CSG is highly effective at MC toxin removal, removing >94% of MC-LR produced by Microcystis aeruginosa. Characterization by SEM and XRD revealed that 750 m²/g surface area graphene, imparts graphene morphology and functionality into the chitosan matrix surface, potentially enabling π-π interactions between graphene and the aromatic ring of microcystin. This proposed π-π removal mechanism of microcystin via the CSG chitosan biopolymer substrate offers a promising sustainable and selective media suitable for deployable treatment of HAB impacted waters.

How UV radiation and pH alternation impact graphene oxide mediated environmental toxicant adsorption and resulting safety characteristics - A toxicology study beyond a classic carrier effect

Once released into water, the widely used graphene oxide (GO) is likely to adsorb classical environmental pollutants, exemplified by Microcystin-LR (MCLR) that is a representative double-bond rich liver-toxic endotoxin. While GO-mediated carrier effect is fairly predictable, the involvement of environmental factors like UV and pH may add additional level of sophistication as these factors may impact the adsorption capacity of GO to MCLR. Here, we firstly investigated the changes of GO structure under different UV-radiation durations and pH conditions with a view to establish the correlation in terms of MCLR adsorption onto GO. We demonstrated that GO reduction especially oxygen-containing groups reduction induced by UV- radiation caused the compromised adsorption MCLR capacity on GO. Besides, the higher pH decreased the non-biological MCLR adsorption to GO by reducing GO defect sites and increasing electrostatic repulsion. These abiotic discoveries were further investigated to compare the safety features of GO-MCLR complex. Under dark condition (pH = 7), we revealed the cytotoxicity of GO-MCLR to normal liver cells, which involved the ROS generation and cell ferroptosis caused by Fe²⁺ accumulation. Introduction of UV and pH alternation in environment impacted GO-mediated environmental toxicant adsorption and resulting safety characteristics, which reminded us environmental factors should not be ignored in the GO-mediated carrier effect.

Adsorbents Used for Microcystin Removal from Water Sources: Current Knowledge and Future Prospects

The increasing occurrence of toxic cyanobacteria in water sources, driven by climate change and eutrophication, is of great concern worldwide today. Cyanobacterial blooms can negatively affect water bodies and generate harmful secondary metabolites, namely microcystins (MCs), which significantly impair water quality. Various adsorbents used for MC removal from water sources were assessed in this investigation. Activated carbon constitutes the most widely used adsorbent for treating contaminated waters due to its high affinity for adsorbing MCs. Alternative adsorbents have also been proposed and reported to provide higher efficiency, but the studies carried out so far in this regard are still insufficient. The mechanisms implicated in MC adsorption upon different adsorbents should be further detailed for a better optimization of the adsorption process. Certainly, adsorbent characteristics, water pH and temperature are the main factors influencing the adsorption of MCs. In this context, optimization studies must be performed considering the effectiveness, economic aspects associated with each adsorbent. This review provides guidelines for more practical field applications of the adsorption in the treatment of waters actually contaminated with MCs.

Safety assessment of graphene oxide and microcystin-LR complex: a toxicological scenario beyond physical mixture

Background

Nanomaterials have been widely used in electrochemistry, sensors, medicine among others applications, causing its inevitable environmental exposure. A raising question is the “carrier” effect due to unique surface properties of nanomaterials, which may collectively impact the bioavailability, toxicokinetic, distribution and biological effects of classic toxicants. Noteworthy, this aspect of information remains largely unexplored.

Methods

Here, we deliberately selected two entities to mimic this scenario. One is graphene oxide (GO), which is made in ton quantity with huge surface-area that provides hydrophilicity and π–π interaction to certain chemicals of unique structures. The other is Microcystin-LR (MCLR), a representative double-bond rich liver-toxic endotoxin widely distributed in aquatic-system. Firstly, the adsorption of GO and MCLR after meeting under environmental conditions was explored, and then we focused on the toxicological effect and related mechanism of GO-MCLR complex on human skin cutin forming cells (HaCaT cells) and normal liver cells (L02 cells).

Results

Abiotically, our study demonstrated that GO could effectively adsorb MCLR through hydrogen bonding and π–π interaction, the oxidation degree of GO-MCLR decreased significantly and surface defect level raised. Compared to GO or MCLR, GO-MCLR was found to induce more remarkable apoptosis and ferroptosis in both HaCaT and L02 cells. The underlying mechanism was that GO-MCLR induced stronger intracellular reactive oxygen species (ROS) and mtROS generation, followed by Fe²⁺ accumulation, mitochondrial dysfunction and cytoskeletal damage.

Conclusions

These results suggest that the GO-MCLR complex formed by GO adsorption of MCLR may exhibit more toxic effects than the single material, which demonstrates the necessity for assessing nano-toxicant complexity. Our discovery may serve as a new toxicological paradigm in which nanomaterial mediated surface adsorption effects could impact the degree of cytotoxicity and toxicological mechanisms of classic toxins.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00466-x.

Rapid removal of trace haloacetic acids from drinking water by a continuous adsorption process using graphene oxide

Significant health risks are caused by trace levels of haloacetic acids (HAAs) in drinking water. We used graphene oxide (GO), a high-performance absorbent, to remove monochloroacetic acid (MCAA), dichloroacetic acid (DCAA) and trichloroacetic acid (TCAA). 31.6%, 27.1% and 30.2% of MCAA, DCAA and TCAA in drinking water could be rapidly removed within 2 min by the interaction of intermolecular hydrogen bonds between GO and HAAs. On the other hand, as a type of weak interaction, intermolecular hydrogen bonds are easy to fracture, which leads to the recovery of GO. The removal efficiency of MCAA, DCAA and TCAA monotonously decreased with increasing pH from 3 to 11. Temperature was not an important influence on the removal efficiency of HAAs, and only affected the interaction of intermolecular hydrogen bonds between GO and HAAs. A continuous adsorption process was used for further improving the removal efficiency of HAAs, and the concentration of total HAAs decreased from 436 to 52.5 μg L^-1 after five adsorption processes. The total contact time was just 2.25 min, which was faster than other reported adsorbents, and total HAAs could be decreased by 88%. The innovative process in this study provides an effective method for application of GO to rapidly remove HAAs in drinking water.

Okara and okara modified and functionalized with iron oxide nanoparticles for the removal of Microcystis aeruginosa and cyanotoxin

Eutrophicating compounds promote the growth of cyanobacteria, which has the potential of releasing toxic compounds. Alternative raw materials, such as residues, have been used in efficient adsorption systems in water treatment. The aim of the present study was to apply the residue Okara in its original form and modified by hydrolysis with immobilization of magnetic nanoparticles as an adsorbent. For the removal, the cyanobacteria Microcystis aeruginosa was chosen, as well as its secondary metabolites, L-amino acids leucine and arginine (MC-LR microcystin), from aqueous solutions. The adsorbents presented a negative surface charge, and the x-ray diffraction (DRX) outcomes successfully demonstrated the immobilization of iron oxide nanoparticles on the adsorbents. The adsorbent with the best result was the Okara hydrolyzed and functionalized with iron oxide, which showed a 47% (qe = 804.166 cel/g) and 85% (qe = 116.94 µg/L) removal for the cyanobacteria cells and chlorophyll-a, respectively. The kinetics study demonstrated a pseudo-first-order adsorption with maximal adsorption in 480 minutes, removing 761 µg/L of chlorophyll-a. In this trial, a low organic material removal has occurred, with a removal rate of 5% (qe = 0.024 mg/g) in the analysis of compounds in absorbance by ultraviolet light (UV) monitored by optical density determination in 254 nm (OD254). Nevertheless, the reaction system with the presence of organic material removed 53,28% of the MC-LR toxin, with adsorption capacities of 2.84 µg/L in a preliminary trial conducted for two hours, arising as a potential and alternative adsorbent with a capacity of removing cyanobacteria and cyanotoxin cells simultaneously.

Thermally enhanced adsorption and persulfate oxidation-driven regeneration on FeCl3-activated biochar for removal of microcystin-LR in water

In this study, a cyclic process of adsorption and persulfate (PS) oxidation-driven regeneration using FeCl₃-activated biochar (FA-BC) was suggested as a novel remediation process to remove microcystin-LR (MC-LR) from water. For enhancing overall treatment efficiency and cost effectiveness, the impacts of temperature on adsorption and PS oxidation-driven regeneration were investigated. The increase of temperature resulted in the increase of MC-LR adsorption rate on FA-BC due to the enhanced MC-LR diffusivity in water. Moreover, the MC-LR oxidation and PS reaction rates during the PS regeneration on FA-BC were remarkably improved by factors of 3.4 and 3.5 with increasing temperature from 20 °C to 50 °C. Both diffusion and desorption of MC-LR from FA-BC were thought to be the key factors for controlling the MC-LR oxidation rate during the PS regeneration of MC-LR. In addition, the decrease of pH (from 10 to 4) and increase of PS concentration (from 100 to 400 mg/L) enhanced the regeneration efficiency for MC-LR-spent FA-BC. The four cycles of adsorption-PS regeneration (200 mg/L PS, pH 6, and 50 °C) resulted in 92.81% regeneration efficiency in DI water and 82.89% in lake water. However, the four cycles of adsorption-PS regeneration led to the reduction of surface area (from 835 to 413 m²/g), oxidation of carbon surface and slight reduction of Fe⁰ on FA-BC. In overall, the cyclic adsorption-PS regeneration at higher temperature could provide practical reuse of FA-BC for cost-effective treatment of aqueous MC-LR.

Biochar from constructed wetland biomass waste: A review of its potential and challenges

Constructed wetland is considered a promising approach for water remediation due to its high efficiency, low operation costs, and ecological benefits, but the large amounts of wetland plant biomass need to be properly harvested and utilized. Recently, wetland plant derived biochar has drawn extensive attention owing to its application potential. This paper provides an updated review on the production and characteristics of wetland plant derived biochar, and its utilization in soil improvement, carbon sequestration, environmental remediation, and energy production. In comparison to hydrothermal carbonization and gasification, pyrolysis is a more common technique to convert wetland plant to biochar. Characteristics of wetland plant biochars varied with plant species, growth environment of plant, and preparation conditions. Wetland plant biochar could be a qualified soil amendment owing to its abundant nutrients. Notably, wetland plant biochar exhibited considerable sorption capacity for various inorganic and organic contaminants. However, the potentially toxic substances (e.g. heavy metal and polycyclic aromatic hydrocarbons) retained in wetland plant biochar should be noticed before large-scale application. To overcome the drawbacks from the scattered distribution, limited productivity, and seasonal operation of constructed wetlands, the economic feasibility of wetland plant biochar production system could be improved via using mobile pyrolysis unit, utilizing local waste heat, and exploiting all the byproducts. Future challenges in the production and application of wetland plant derived biochar include the continuous supply of feedstock and proper handling of potentially hazardous components in the biochar.

Phototransformation of Graphene Oxide on the Removal of Sulfamethazine in a Water Environment

Graphene oxide (GO) is widely used in various fields and has raised concerns regarding its potential environmental fate and effect. However, there are few studies on its influence on coexisting pollutants. In this study, the phototransformation of GO and coexisting sulfamethazine (SMZ) under UV irradiation was investigated, with a focus on the role of reactive oxygen species. The results demonstrated that GO promoted the degradation of SMZ under UV irradiation. The higher the concentration of GO, the higher the degradation rate of SMZ, and the faster the first-order reaction rate. Two main radicals, ∙OH and ¹O₂, both contributed greatly in terms of regulating the removal of SMZ. Cl⁻, SO₄²⁻, and pH mainly promoted SMZ degradation by increasing the generation of ∙OH, while humic acid inhibited SMZ degradation due to the reduction of ∙OH. Moreover, after UV illumination, the GO suspension changed from light yellow to dark brown with increasing absorbance at a wavelength of 225 nm. Raman spectra revealed that the I_D/I_G ratio slightly decreased, indicating that some of the functional groups on the surface of GO were removed under low-intensity UV illumination. This study revealed that GO plays important roles in the photochemical transformation of environmental pollutants, which is helpful for understanding the environmental behaviors and risks of nanoparticles in aquatic environments.

Selective removal of common cyanotoxins: a review

The development of cyanobacterial blooms can have adverse effects on water bodies and may produce cyanotoxins. Several physical and chemical methods have been applied to remove cyanotoxins, but they have been significantly challenged due to extensive energy footprint and over-used chemicals, which limits practical application on a large scale. Selective removal has been regarded as the most promising approach recently for the elimination of prevalent and major bloom-forming cyanotoxins (e.g., microcystins and cylindrospermopsin) as natural organic matters and radical scavengers are ineluctably present in real scenarios. This paper reviews current advancements in research on selective oxidation and adsorption of cyanotoxins. Its goal is to provide comprehensive information on the treatment mechanism and the process feasibility involved in the cyanotoxin removal from real-world waters. Moreover, perspectives of cyanotoxin control and in situ selective elimination approaches are also reviewed. It is expected that the information gathered and discussed in this review can provide a useful and novel reference and direction for future pilot-scale applications.

Adsorption and regeneration on iron-activated biochar for removal of microcystin-LR

Novel iron activated biochars (FA-BCs) were prepared via simultaneous pyrolysis and activation of FeCl₃-pretreated bermudagrass (BG) for removing microcystin-LR (MC-LR) in aqueous solution. Compared to the raw BC (without activation), the surface area and adsorption capacity of FA-BC at iron impregnation ratio of 2 (2 g FeCl₃/g BG) were enhanced from 86 m²/g and 0.76 mg/g to 835 m²/g and 9.00 mg/g. Moreover, FA-BC possessed various iron oxides at its surface which provided the catalytic capacity for regeneration of MC-LR spent FA-BC and magnetic separation after the MC-LR adsorption. Possible mechanisms for the MC-LR adsorption onto FA-BC would include electrostatic attraction, π⁺-π, hydrogen bond, and hydrophobic interactions. The detailed adsorption studies indicated mainly chemisorption and intra-particle diffusion limitation would participate in the adsorption process. The thermal regeneration at 300 °C kept high regeneration efficiency (99-100%) for the MC-LR spent FA-BC during four cycles of adsorption-regeneration. In addition, the high regeneration efficiency (close to 100%) was also achieved by persulfate oxidation-driven regeneration. FA-BC also exhibited high adsorption capacity for the MC-LR from the real lake water to meet the MC-LR concentration below 1 μg/L as a safe guideline suggested by WHO.

Selection of Covalent Organic Framework Pore Functionalities for Differential Adsorption of Microcystin Toxin Analogues

Microcystins (MCs), produced by Microcystis sp, are the most commonly detected cyanotoxins in freshwater, and due to their toxicity, worldwide distribution, and persistence in water, an improvement in the monitoring programs for their early detection and removal from water is necessary. To this end, we investigate the performance of three covalent organic frameworks (COFs), TpBD-(CF₃)₂, TpBD-(NO₂)₂, and TpBD-(NH₂)₂, for the adsorption of the most common and/or toxic MC derivatives, MC-LR, MC-RR, MC-LA, and MC-YR, from water. While MC-LR and MC-YR can be efficiently adsorbed using all three COF derivatives, high adsorption efficiencies were found for the most lipophilic toxin, MC-LA, with TpBD-(NH₂)₂, and the most hydrophilic one, MC-RR, with TpBD-(NO₂). Theoretical calculations revealed that MC-LA and MC-RR have a tendency to be located mainly on the COF surface, interacting through hydrogen bonds with the amino and nitro functional groups of TpBD-(NH₂)₂ and TpBD-(NO₂)₂, respectively. TpBD-(NO₂)₂ outperforms the adsorbent materials reported for the capture of MC-RR, resulting in an increase in the maximum adsorption capacity by one order of magnitude. TpBD-(NH₂)₂ is reported as the first efficient adsorbent material for the capture of MC-LA. Large differences in desorption efficiencies were observed for the MCs with different COFs, highlighting the importance of COF-adsorbate interactions in the material recovery. Herein we show that efficient capture of these toxins from water can be achieved through the proper selection of the COF material. More importantly, this study demonstrates that by careful choice of COF functionalities, specific compounds can be targeted or excluded from a group of analogues, providing insight into the design of more efficient and selective adsorbent materials.

Application of a 2D-QSAR with a sine normalization method for the biodegradation of fluoroquinolones to poison cyanobacteria

Cyanobacteria are photosynthetic autotrophic aquatic prokaryotes. One of the methods for controlling cyanobacterial blooms is to destroy the phycobiliproteins required for photosynthesis. In this study, to improve the biodegradation of the fluoroquinolones through inhibit cyanobacteria, the molecular docking scores of 32 fluoroquinolones (FQs) with four categories of phycobiliproteins from cyanobacteria were calculated after sine normalization to characterize the binding ability between them. A two-dimensional quantitative structure-activity relationship (2D-QSAR) model was constructed based on the comprehensive scores. Danofloxacin (DAN) with the highest comprehensive score was chosen for molecular modification. When docking with four categories of phycobiliproteins from cyanobacteria, the docking values of DAN-11 and DAN-16 were increased up to 35.75%. Moreover, their functional characteristics and environmentally friendly predictive values were improved. When the DAN-11 and DAN-16 molecules docked with the other cyanobacterial phycobiliproteins, indicating that the designed DAN derivatives had general applicability to poison cyanobacteria, the weak interaction forces might increase the binding ability between the DAN derivatives and the receptor phycobiliprotein compared with the target molecule.

Prediction of varying microcystins during non-thermal plasma oxidation of harvested microalgal biomass

By capturing intracellular microcystins (MCs) release from microalgal cell destruction and extracellular MCs oxidation, this study suggests a mathematical model explaining the simultaneous removal of microalgae and their toxins (MC-LR, -RR, and -YR) in non-thermal plasma (NTP) application. Although the suggested model was built based on simplified kinetic assumptions, it can reasonably predict the behavior of extracellular MCs in a harvested/concentrated slurry of microalgae taken from a blooming site. After 24 h of NTP treatment, the experimental reduction of extracellular MCs was recorded up to ∼77 %. Regressions based on the experimental data reveal the degradation rate (8.60 d^-1) and release rate (0.37 d^-1) of MCs, which provides the essential physicochemical information about intracellular MCs release by microalgal cell destruction. Simulation results help to develop safe and useful control over the simultaneous treatment of harvested microalgal biomass and toxins. This study further demonstrates that the suggested model contributes to predicting the variation of MCs in mass management of microalgal biomass for sustainable utilization.

The Use of Biochar and Pyrolysed Materials to Improve Water Quality through Microcystin Sorption Separation

Harmful algal blooms have increased globally with warming of aquatic environments and increased eutrophication. Proliferation of cyanobacteria (blue-green algae) and the subsequent flux of toxic extracellular microcystins present threats to public and ecosystem health and challenges for remediation and management. Although methods exist, there is currently a need for more environmentally friendly and economically and technologically feasible sorbents. Biochar has been proposed in this regard because of its high porosity, chemical stability, and notable sorption efficiency for removing of cyanotoxins. In light of worsening cyanobacterial blooms and recent research advances, this review provides a timely assessment of microcystin removal strategies focusing on the most pertinent chemical and physical sorbent properties responsible for effective removal of various pollutants from wastewater, liquid wastes, and aqueous solutions. The pyrolysis process is then evaluated for the first time as a method for sorbent production for microcystin removal, considering the suitability and sorption efficiencies of pyrolysed materials and biochar. Inefficiencies and high costs of conventional methods can be avoided through the use of pyrolysis. The significant potential of biochar for microcystin removal is determined by feedstock type, pyrolysis conditions, and the physiochemical properties produced. This review informs future research and development of pyrolysed materials for the treatment of microcystin contaminated aquatic environments.

Recent Advancements in the Removal of Cyanotoxins from Water Using Conventional and Modified Adsorbents—A Contemporary Review

The prevalence of cyanobacteria is increasing in freshwaters due to climate change, eutrophication, and their ability to adapt and thrive in changing environmental conditions. In response to various environmental pressures, they produce toxins known as cyanotoxins, which impair water quality significantly. Prolonged human exposure to cyanotoxins, such as microcystins, cylindrospermopsin, saxitoxins, and anatoxin through drinking water can cause severe health effects. Conventional water treatment processes are not effective in removing these cyanotoxins in water and advanced water treatment processes are often used instead. Among the advanced water treatment methods, adsorption is advantageous compared to other methods because of its affordability and design simplicity for cyanotoxins removal. This article provides a current review of recent developments in cyanotoxin removal using both conventional and modified adsorbents. Given the different cyanotoxins removal capacities and cost of conventional and modified adsorbents, a future outlook, as well as suggestions are provided to achieve optimal cyanotoxin removal through adsorption.

The effects of co-exposure of graphene oxide and copper under different pH conditions in Manila clam Ruditapes philippinarum

Carbon nanomaterials (CNM), such as graphene oxide (GO), have been the focus of study in several areas of science mostly due to their physical-chemical properties. However, data concerning the potential toxic effects of these CNM in bivalves are still scarce. When present in the aquatic systems, the combination with other contaminants, as well as pH environmental variations, can influence the behavior of these nanomaterials and, consequently, their toxicity. Thus, the main goal of this study was to evaluate the effect of exposure of clam Ruditapes philippinarum to GO when acting alone and in the combination with copper (Cu), under two pH levels (control 7.8 and 7.3). A 28-day exposure was performed and metabolism and oxidative stress-related parameters were evaluated. The effects caused by GO and Cu exposures, either isolated or co-exposed, showed a direct and dependent relationship with the pH in which the organisms were exposed. In clams maintained at control pH (7.8), Cu and GO + Cu treatments showed lower lipid peroxidation (LPO) and lower electron transport system (ETS) activity, respectively. In clams maintained at low pH, glutathione-S-transferases (GSTs) activities were increased in Cu and Cu + GO treatments, whereas reduced glutathione (GSH) levels were increased in Cu treatment and ETS activity was higher in GO + Cu. Thus, it can be observed that clams responses to Cu and GO were strongly modulated by pH in terms of their defense system and energy production, although this does not result into higher LPO levels.

Microcystin Incidence in the Drinking Water of Mozambique: Challenges for Public Health Protection

Microcystins (MCs) are cyanotoxins produced mainly by freshwater cyanobacteria, which constitute a threat to public health due to their negative effects on humans, such as gastroenteritis and related diseases, including death. In Mozambique, where only 50% of the people have access to safe drinking water, this hepatotoxin is not monitored, and consequently, the population may be exposed to MCs. The few studies done in Maputo and Gaza provinces indicated the occurrence of MC-LR, -YR, and -RR at a concentration ranging from 6.83 to 7.78 µg·L-1, which are very high, around 7 times above than the maximum limit (1 µg·L-1) recommended by WHO. The potential MCs-producing in the studied sites are mainly Microcystis species. These data from Mozambique and from surrounding countries (South Africa, Lesotho, Botswana, Malawi, Zambia, and Tanzania) evidence the need to implement an operational monitoring program of MCs in order to reduce or avoid the possible cases of intoxications since the drinking water quality control tests recommended by the Ministry of Health do not include an MC test. To date, no data of water poisoning episodes recorded were associated with MCs presence in the water. However, this might be underestimated due to a lack of monitoring facilities and/or a lack of public health staff trained for recognizing symptoms of MCs intoxication since the presence of high MCs concentration was reported in Maputo and Gaza provinces.

Purification Behavior of Zn(II) in Water by Magnesium Hydroxyapatite: Surface Complexation, and Dissolution-Precipitation

As an innovative and economical material, hydroxyapatite does little harm to the environment. In this study, a magnesium hydroxyapatite (Mg-HAP) adsorbent was prepared by doping magnesium. Magnesium doping can increase the hydroxyl groups on the surface of Mg-HAP to form more adsorption sites and improve the removal effect of the heavy metal Zn(II) in water. This study was implemented to survey the effect of different sorption elements, including the liquor initial pH, initial concentration, dose of adsorbents, and other factors, on the adsorption effect. The outcomes show that the sorption effect was best at the time that the liquor was weakly acidic (pH = 6); At a pH of 6, the temperature of 25 °C when the optimal dosage of adsorbent is 0.25 g, the maximum adsorption amount is 62.11 mg/g. Through data fitting, the adsorption process can be accurately described as a pseudo-second-order dynamics model and the Langmuir isotherm equation. According to the thermodynamic analysis, the sorption of zinc ions by Mg-HAP belongs to the process of spontaneous endothermic and entropy increase, and the increase of temperature was conducive to adsorption. Material characterization and analysis indicate that surface complexation and dissolution-precipitation was the main mechanism for adsorption of Zn(II).

Waterborne microcystin-LR exposure induced chronic inflammatory response via MyD88-dependent toll-like receptor signaling pathway in male zebrafish

Waterborne microcystin-LR (MC-LR) released by cyanobacterial blooms in eutrophic water bodies have caused serious risk to aquatic animal and human health. In the present study, we for the first time conducted a comprehensive in vivo investigation on chronic inflammatory responses and its molecular pathways of different environmental relevant levels of MC-LR (0, 0.4, 2 and 10 μg/L) in male zebrafish (Danio rerio). The results showed that chronic MC-LR exposure caused splenic inflammatory changes including the formation of melano-macrophage centers, remarkable elevation of serum tumor necrosis factor alpha (TNFα) and interleukin 1 beta (IL1β) levels as well as significant upregulated expression of MyD88-dependent toll-like receptor (TLR/MyD88) signaling pathway genes (tlr4a, myd88, erk2, p38a, il1β and tnfα). The immunohistochemical and western blot results further validated that higher MC-LR concentrations tended to enhance the MyD88 signal. Moreover, significant decreases of serum C3 levels along with splenic c3b expression in the 10 μg/L exposure group proved that chronic MC-LR exposure could ultimately decrease the innate immunity of fish. Our findings revealed that chronic exposure of MC-LR could cause chronic inflammation through TLR/MyD88 signaling pathway and subsequently induce immune disorders in male zebrafish, which also urge us to pay more attention on the potential immunotoxicity of long-term exposure to low concentration of MC-LR.

Nanopolystyrene at predicted environmental concentration enhances microcystin-LR toxicity by inducing intestinal damage in Caenorhabditis elegans

We employed nematode Caenorhabditis elegans to determine the combinational effect between nanopolystyrene at predicted environmental concentration and microcystin-LR (MC-LR). Prolonged exposure to nanopolystyrene (1 μg/L) increased MC-LR (0.1 μg/L) toxicity in reducing brood size and locomotion behavior and in inducing oxidative stress. Moreover, the adsorption of MC-LR by nanopolystyrene particles played an important role in inducing the enhancement in MC-LR toxicity by nanopolystyrene particles. Additionally, only exposure to resuspension of nanopolystyrene (1 μg/L) caused the increased intestinal permeability in MC-LR (0.1 μg/L) exposed nematodes. Our data indicates the potential of nanopolystyrene at predicted environmental concentration in enhancing MC-LR toxicity on environmental organisms.

A review on exfoliation, characterization, environmental and energy applications of graphene and graphene-based composites

Because of an atom-thick two-dimensional structure with sp² hybridization, large specific area, high thermal conductivity, superior electron mobility, and chemical stability, graphene (GN) has developed substantial interest among researchers, exponentially accelerating GN based research. GN and its derivatives are the potentially attractive materials to develop composites for energy and environmental applications. This review covered a general overview on physical and chemical properties of GN and based composite materials, briefly summarizing exfoliation methodologies and characterization techniques in the first section. The environmental applications of GN and GN composites in detection of gases, bacteria as well as in the removal of organic and inorganic pollutants were comprehensively addressed in the second section. Third section focused on recent progress associated with the applications of GN and its composites in solar energy conversion, electrochemical energy devices, storage and production of hydrogen. Finally, conclusive remarks emphasizing unresolved problems and major future challenges were covered in the last section. In addition, the prospects and further development of GN and GN composites in energy, environment and bioscience were discussed.

Competitive pseudo-ELISA based on molecularly imprinted nanoparticles for microcystin-LR detection in water

Microcystins (MCs) are dangerous cyanotoxins for the public health, and microcystin-LR (MC-LR) is one of most toxic, dangerous, and frequently found in water bodies. Typically, the detection of MCs is carried out by means of competitive ELISAs which, however, need special precautions for handling and storage, due to the stability of the antibodies used in this test. Molecularly imprinted nanoparticles (nanoMIPs) represents more robust and cost-effective alternative to antibodies. In this work, we developed a competitive pseudo-ELISA based on nanoMIPs (which are used in place of natural antibodies), for the detection of microcystin-LR (MC-LR). This pseudo-ELISA showed a linear response towards MC-LR, showing high affinity and low cross-reactivity against another analogue toxin (microcystin-YR). The analytical recovery of MC-LR in the analysis of water samples by the proposed pseudo-ELISA was 96 %–130 % and the limit of detection was 2.64 × 10−4 nM. The obtained results suggest that this competitive pseudo-ELISA could have high potential in the detection of toxins, due to its rapid, sensitive and accurate detection of toxin in water samples.

Graphene oxide as a tool for antibiotic-resistant gene removal: a review

Environmental pollutants, including antibiotics (ATBs), have become an increasingly common health hazard in the last several decades. Overdose and abuse of ATBs led to the emergence of antibiotic-resistant genes (ARGs), which represent a serious health threat. Moreover, water bodies and reservoirs are places where a wide range of bacterial species with ARGs originate, owing to the strong selective pressure from presence of ATB residues. In this regard, graphene oxide (GO) has been utilised in several fields including remediation of the environment. In this review, we present a brief overview of resistant genes of frequently used ATBs, their occurrence in the environment and their behaviour. Further, we discussed the factors influencing the binding of nucleic acids and the response of ARGs to GO, including the presence of salts in the water environment or water pH, because of intrinsic properties of GO of not only binding to nucleic acids but also catalysing their decomposition. This would be helpful in designing new types of water treatment facilities.

Graphene based adsorbents for remediation of noxious pollutants from wastewater

The contamination of water resources with noxious pollutants is a serious issue. Many aquatic systems are contaminated with different toxic inorganic and organic species; coming to wastewater from various anthropogenic sources such as industries, agriculture, mining, and domestic households. Keeping in view of this, wastewater treatment appears to the main environmental challenge. Adsorption is one of the most efficient techniques for removing all most all types of pollutants i.e. inorganics and organics. Nowadays, graphene and its composite materials are gaining importance as nano adsorbents. Graphene; a two-dimensional nanomaterial having single-atom graphite layer; has attracted a great interest in many application areas (including wastewater treatment) due to its unique physico-chemical properties. The present paper is focused on the remediation of noxious wastes from wastewater using graphene based materials as adsorbents, and it contains all the details on materials - i.e., from their synthesis to application in the field of wastewater treatment (removal of hazardous contaminants of different chemical nature - heavy and rare-earth metal ions, and organic compounds - from wastewater effluents. The efficiency of the adsorption and desorption of these substances is considered. Certainly, this article will be useful for nano environmentalist to design future experiments for water treatment.

Photocatalytic degradation of Microcystin-LR by visible light active and magnetic, ZnFe2O4-Ag/rGO nanocomposite and toxicity assessment of the intermediates

In this work, we aimed to study photocatalytic degradation of Microcystin-LR (MC-LR), a cyanotoxin known to cause acute as well as chronic toxicity and even mortality. The nanocomposite (NC) based on zinc ferrite (ZnFe₂O₄) was modified with graphene oxide (GO) and Ag nanoparticles (NPs) to enhance its photocatalytic properties under visible light. The so-formed ZnFe₂O₄-Ag/rGO NC exhibited superior performance in visible light allowing complete degradation of MC-LR within 120 min of treatment with pseudo rate constant, k = 0.0515 min^-1, several times greater than other photocatalysts, TiO₂ (k = 0.0009 min^-1), ZnFe₂O₄ (k = 0.0021 min^-1), ZnFe₂O₄-Ag (k = 0.0046 min^-1) and ZnFe₂O₄/rGO (k = 0.007 min^-1) respectively. The total organic carbon analysis revealed that only 22% of MC-LR was mineralized on 120 min of treatment time indicating presence of different intermediate by-products. The intermediates formed during photocatalytic treatment were identified using liquid chromatography-mass spectrometry (LCMS) based on which probable degradation pathways were proposed. The attack from OH radicals formed during the photocatalytic process resulted to hydroxylation and subsequent cleavage of diene bond. The toxicity assessment with Daphnia magna revealed that the degradation process has alleviated toxicity of the MC-LR and no toxic intermediates were formed during the treatment which is very important from eco-toxicological view point. Therefore, ZnFe₂O₄-Ag/rGO has a good potential in the field of environmental applications as visible light active and magnetic photocatalyst with enhanced performance.

Removal of Microcystin-LR from spiked natural and synthetic waters by anion exchange

Cyanobacterial blooms are becoming a serious challenge across the globe due to changing climate and rainfall patterns as a consequence of human activities. In the present study, the fundamental interactions involved during the removal of Microcystin-LR (MCLR), one of the most commonly occurring cyanobacterial toxins, were investigated by employing strongly basic anion exchange (IX) resins. Several factors including the stoichiometric coefficients, competitive fractions and solute affinities were determined under various concentrations of inorganic ions and natural organic matter. The results indicated that suphates were the most competitive fractions with high affinity (α (affinity coefficient) values ~ 9) followed by nitrates (α ~ 4.7) and NOM fractions (α ~ 4.5, p < 0.05). The Equivalent Background Concentration Mode (EBC), that arises from the Ideal Adsorption Solution Theory (IAST), indicated a competitive fraction of ~2 μeq/L NOM, which approximates to <10% of the initial NOM concentrations, indicating a small fraction of the NOM resulting in the competitive effect. Further, studies with natural surface waters indicated that the MCLR uptake could be modeled using the IAST-EBC model and the IX resin could simultaneously removal of >90% of NOM, inorganic ions and MCLR at resin dosages of 3.6 meq/L or higher.

Microcystin-LR Enrichment from Freshwater by a Recombinant Plant-derived Antibody Using Sol-Gel-Glass Immunoextraction

Eutrophication of water bodies can promote cyanobacterial (blue-green algae) blooms, which has become a source of increasing concern for both recreational and drinking water use. Many bacterial species can produce toxins that pose threats to wildlife, domestic animals and humans. Microcystin-leucine-arginine (MC-LR) is the most frequent and most toxic microcystin congener. For the first time, lab-scale investigations were performed to test the application of a recombinant plant-derived anti-MC-LR antibody immobilized on an immunoaffinity support material to selectively extract the toxin from spiked freshwater samples. As a comparison, its hybridoma-derived counterpart (murine monoclonal antibody) was evaluated. The antibody-doped material was prepared via an optimized sol-gel process; its stability and binding efficiency of MC-LR in spiked freshwater samples were thoroughly tested using the ELISA and orthogonal LC-MS methods. For removal, two column-based procedures with sequential or continuous cyclic sample addition and a suspension mode (moving adsorbent) were tested. Noteworthy the results obtained with a crude antibody fraction were fully compatible with the highly purified preparation. This study paves the way for further investigation being focused on novel applications of plant-derived anti-MC-LR antibodies in bioremediation to selectively deplete the toxin from freshwater: a green and promising technology without secondary pollution.

Simulating graphene oxide nanomaterial phototransformation and transport in surface water

The production of graphene-family nanomaterials (GFNs) has increased appreciably in recent years. Graphene oxide (GO) has been found to be the most toxic nanomaterial among GFNs and, to our knowledge, no studies have been conducted to model its fate and transport in the environment. Lab studies show that GO undergoes phototransformation in surface waters under sunlight radiation resulting in formation of photoreduced GO (rGO). In this study, the recently updated Water Quality Analysis Simulation Program (WASP8) is used to simulate time-dependent environmental exposure concentrations of GO and its major phototransformation product, rGO, for Brier Creek, GA, USA at two flow scenarios under a constant loading of GO to the river for a period of 20 years. Analysis shows that the degree of phototransformation is closely associated with river flow condition: up to of 40% of GO undergoes phototransformation at low flow condition, whereas only 2.5% of GO phototransformation occurs at mean flow condition. River flow and heteroaggregation exhibit a 'competing' effect in determining the formation of rGO heteroagglomerates. Mass fraction analysis indicates that the vast majority of rGO heteroagglomerates settle to the sediment layers due to the settling of suspended solids. Simulation of natural recovery after removal of the GO source suggests that free GO and rGO are the immediate contaminants of concern in the studied surface water system, while rGO heteroaggregated with suspended solids can have a long-term ecological impact on both the water column and sediments.

Conjugated microporous polymer nanosheets and nanotubes as novel absorbents for microcystin-LR: insights from theoretical investigations

Our calculations demonstrated that CMP nanosheet and nanotube materials can be developed as novel adsorbents for harmful microcystin-LR adsorption/removal.

Environmental risks of ZnO nanoparticle exposure on Microcystis aeruginosa: Toxic effects and environmental feedback

The vast majority of studies measure the toxic effect of organisms exposed to nanoparticles (NPs) while there is still a lack of knowledge about the influence of NPs on the aquatic environment. It is unknown whether or not the interaction between NPs and algae will result in the variation of algal organic matter (AOM) and stimulate the production of more algal toxins. In this study, zinc oxide nanoparticles (nano-ZnO) as a typical representative of metal oxide NPs were used to evaluate the toxic effects and environmental feedback of Microcystis aeruginosa. Reactive oxygen species (ROS) and malondialdehyde (MDA) were measured to explain the toxicity mechanism. Changes of AOM, including the production of toxins, the molecular weight distribution and the excitation-emission matrices of algal solution were also studied as environmental feedback indicators after nano-ZnO destroyed the algae. As the nano-ZnO exceeded the comparable critical concentration (1.0 mg/L), the algae were destroyed and intracellular organic matters were released into the aquatic environment, which stimulated the generation of microcystin-LR (MC-LR). However, it is worth noting that the concentration of nano-ZnO would need to be high (at mg/L range) to stimulate more MC-LR production. These findings are expected to be beneficial in interpreting the toxicity and risks of the releasing of NPs through the feedback between algae and the aquatic environment.

Adsorption of Trace Estrogens in Ultrapure and Wastewater Treatment Plant Effluent by Magnetic Graphene Oxide

In the current study, graphene oxide, Fe3+, and Fe2+ were used for the synthesis of magnetic graphene oxide (MGO) by an in situ chemical coprecipitation method. Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction were used to characterize the well-prepared MGO. The prepared MGO was used as an adsorbent to remove five typical estrogens (estrone (E1), 17β-estradiol (E2), 17α-ethinylestradiol (17α-E2), estriol (E3), and synthetic estrogen (EE2)) at the ppb level from spiked ultrapure water and wastewater treatment plant effluent. The results indicated that the MGO can efficiently remove estrogens from both spiked ultrapure water and wastewater treatment plant effluent in 30 min at wide pH ranges from 3 to 11. The temperature could significantly affect removal performance. A removal efficiency of more than 90% was obtained at 35 °C in just 5 min, but at least 60 min was needed to get the same removal efficiency at 5 °C. In addition, an average of almost 80% of the estrogens can still be removed after 5 cycles of MGO regeneration but less than 40% can be reached after 10 cycles. These results indicate that MGO has potential for practical applications to remove lower levels of estrogens from real water matrixes and merits further evaluation.

Surface modified TiO2 floating photocatalyst with PDDA for efficient adsorption and photocatalytic inactivation of Microcystis aeruginosa

Microcystis aeruginosa, as the most common cyanobacteria, often grows uncontrollably in eutrophic lakes with the accumulation of microcystin-LR (MC-LR) in water, which heavily pollutes water and hence imposes tremendous threat to aquatic animals and human beings. To remediate the harmful algae polluted water, here we synthesize a series of poly dimethyl diallyl ammonium chloride (PDDA) modified TiO₂ floating photocatalysts, PDDA@NPT-EGC, and apply them as a visible light driven multifunctional material. The fabricated PDDA@NPT-EGC composites have a worm-like structure with PDDA particles distributed on their surfaces, and the concentration of PDDA can affect the agglomerative condition and distribution of PDDA particles and the photoelectric properties of catalysts. Among these catalysts, the PDDA@NPT-EGC with 0.2 wt% PDDA (0.2PDDA@NPT-EGC) shows the highest adsorption and photocatalytic activity. Compared with the NPT-EGC, the dark adsorption efficiency for the 0.2PDDA@NPT-EGC after 3 h increases from 70.4% to 88.9%, and the total removal efficiency after visible light irradiation for 2 h increases from 77.8% to 92.6%. In addition, the 0.2PDDA@NPT-EGC exhibits a removal efficiency of 96.55% for photocatalytic degradation of MC-LR after irradiation for 3 h. The Adda side chain of MC-LR molecule is found to degradate gradually in the photocatalytic degradation process, indicative of the elimination of biotoxicity for MC-LR molecule in the reaction. We demonstrate that the 0.2PDDA@NPT-EGC is remarkably competitive in both algae inactivation and MC-LR removal, which shall hold substantial promise in remediation of algae pollution in eutrophic waters.