Recent innovations in microplastics and nanoplastics removal by coagulation technique: Implementations, knowledge gaps and prospects

Revealing the removal behavior of five neglected microplastics in coagulation-ultrafiltration processes: Insights from experiments and predictive modeling

Typical water treatment processes are essential for mitigating the risk of microplastic contamination in drinking water. The integration of experiments and machine learning offers a promising avenue to elucidate microplastic removal behavior, yet relevant studies are scarce. To address this gap, this study combined experimental and artificial neural network (ANN) modeling to explore the removal behavior and mechanisms of five neglected microplastics in typical coagulation-ultrafiltration processes. Experimental results demonstrated that coagulation achieved an optimal removal rate of 37.0-56.0 % for the five microplastics, and subsequent ultrafiltration almost completely removed all residual microplastics. Five ANN models were constructed and optimized by adjusting activation functions and employing batch normalization, accurately predicting microplastic removal, with high R² values of 0.9972-0.9987. X-ray photoelectron spectroscopy elucidated the involvement of AlIV and AlVI species, hydrogen bonding, and π-π interaction in coagulation. Two-dimensional correlation spectroscopy explored the sequential formation of six chemical bonds (C-H, Al-O-Al, C-O, COO-, C=O, and -OH) and potential mechanisms. Moreover, theoretical calculations clarified the interfacial interactions between microplastics and ultrafiltration membrane, highlighting the roles of hydrophobic attraction and acid-base interaction. This study expands our understanding of microplastic removal in drinking water treatment, providing valuable mechanistic and modeling insights.

Effects of microplastics accumulation and antibiotics contamination in anaerobic membrane bioreactors for municipal wastewater treatment

Municipal wastewater treatment plants are the main collection points for plastics and antibiotics. Anaerobic membrane bioreactor (AnMBR) is one the most potential municipal wastewater treatment technologies. This study evaluated the impact of microplastic (aged polyvinyl chloride, aged PVC, 1.5 g/L), antibiotics (ciprofloxacin, CIP, 100 μg/L) and their interaction effect on AnMBR treatment performance and membrane fouling. Results showed that the inhibition of CIP on AnMBR organic removal and methane production was intensified, owing to the CIP adsorption on aged PVC. The enzyme activities of electron transport (ETS), adenosine triphosphate (ATP) and F420 were also significantly restrained by 47-52 % with combined exposure. The combined effects also significantly aggravated the membrane fouling of AnMBR, which shorted the membrane operational period by half due to more soluble microbial products (SMP) secretion. The microbial diversity analyses indicated that aged PVC and CIP addition can accumulate some main anaerobic fermentation bacteria but inhibit the archaea. The abundance of related enzyme in the acetoclastic and hydrogenotrophic methanogenesis decreased with the sole aged PVC and CIP addition and severely inhibited with their combine effect. The absolute abundance mcrA significantly reduced by 92 % with combined exposure, validating the negative impact on methanogenic activity. These findings provide valuable insight into the AnMBR implementation in complex wastewater treatment.

Making waves: Reevaluating iron dosing for carbon recovery in mainstream wastewater treatment system

Wastewater treatment plants face significant challenges in shifting from energy-intensive operations to carbon-neutral, energy-efficient systems. One promising strategy is the iron-enhanced primary treatment process (Fe-CEPT), which focuses on capturing organic carbon for energy recovery rather than biological oxidation. However, while Fe-CEPT has been implemented in wastewater treatment, its potential effects on downstream processes have often been overlooked. This viewpoint takes a comprehensive look at iron dosing for carbon recovery in mainstream wastewater treatment systems. Fe-CEPT has proven effective at capturing particulate organics and phosphorus. However, it is less successful in removing soluble organic carbon. Additionally, the high iron content in sludge, typically between 100 to 200 mg Fe/g SS, has been shown to severely inhibit methane production. This finding contrasts with earlier studies that suggested iron could enhance methane production. It was found the elevated iron levels bind around 20 % of the carbon in the sludge, limiting its bioavailability. These findings indicate that coupling Fe-CEPT with anaerobic digestion may not be an effective method for carbon recovery. A more promising approach that involves limiting iron dosing to less than 10 mg Fe/L in a high-rate activated sludge (HRAS) system is proposed. This strategy combines the benefits of iron dosing and HRAS system, offering a potential pathway to enhance carbon recovery, improve phosphorus management, and reduce the environmental impact of wastewater treatment processes.

Key adsorbents and influencing factors in the adsorption of micro- and nanoplastics: A review

Microplastics and nanoplastics (MNPs) are emerging contaminants in drinking water sources that pose serious risks to human health and ecosystems. Several removal strategies, such as adsorption, exist but present challenges for their industrial scalability. This review provides a concise overview of MNP adsorption mechanisms and highlights the limited but critical exploration of column adsorption in the literature, emphasizing its importance for large-scale applications. Special attention is given to carbon-based materials due to their cost-effectiveness, environmental friendliness and sustainability. Other adsorbents (e.g., metal-organic frameworks, clays) are also discussed for their promising performance in realistic water matrixes. To predict and optimize the efficiency of adsorbents, leading simulation models are reviewed. Taken together, this work provides a comprehensive overview of the fundamental factors, such as adsorption mechanisms, adsorbent selection and experimental conditions, to optimize MNP adsorption. By highlighting the underexplored area of column-based processes, it provides valuable information to advance adsorption as a viable industrial-scale solution for MNP contamination.

Revealing the removal behavior of polystyrene nanoplastics and natural organic matter by AlTi-based coagulant from the perspective of functional groups

The interactions of nanoplastics (NPs) with natural organic matter (NOM) are influenced by their surface functional groups. In this study, the effects of representative functional groups on the interactions among polystyrene nanoplastics (PS-COOH and PS-NH2), hydrophilic low molecular weight (LMW) substances (salicylic acid (SA), phthalic acid (PA), and gluconic acid (GA)), and a novel AlTi-based coagulant were investigated. We found that PS-NH2 (83.02 % - 93.38 %) was easier to remove over a wider pH range than PS-COOH (6.94 % - 91.07 %). PS-COOH and PS-NH2 were both able to interact with SA (-OH, -COO-, and benzene ring) through hydrogen bonding, π-π conjugation, and n-π electron donor-acceptor interactions. However, the binding of PS-COOH/PS-NH2 with SA has no effect on the interaction strength between SA and PATC due to the preferential occupation of the coagulant binding sites by SA. The lower SA removal in the PS-COOH@SA system was attributed to its stronger electrostatic repulsion and hydrophilicity. PATC could form carboxylate outer and C-O inner complexes with SA and carboxylate inner complexes with PA. In this study, the analysis of the interaction mechanisms among metal-based coagulants, NPs, and LMW substances lays a theoretical foundation for further research and understanding of coagulation theory.

Synergistically piezocatalytic and Fenton-like activation of H2O2 by a ferroelectric Bi12(Bi0.5Fe0.5)O19.5 catalyst to boost degradation of polyethylene terephthalate microplastic (PET-MPs)

Pollution of microplastics (MPs) has been drastically threating human health, however, whose elimination from the environment by current approaches is inefficient due to their high molecular weight, stronghydrophobicity and stable covalent bonds. Herein, we report a novel and highly-efficient route to degrade MPs contaminants through synergistically piezocatalytic and Fenton-like activation of H2O2 by a ferroelectric Bi12(Bi0.5Fe0.5)O19.5 catalyst under ultrasound treatment. For 10 g/L polyethylene terephthalate microplastics (PET-MPs), the synergistic strategy reached a 28.9 % removal rate in 72 h, which is greatly enhanced in comparison to the individual piezocatalysis and Fenton (Fenton-like) activation. By optimizing the types of oxidants (H2O2, peroxymonosulfate and peroxydisulfate) and bismuth ferrite catalysts (non-piezoelectric Bi2Fe4O9 and piezoelectric BiFeO3/Bi12(Bi0.5Fe0.5)O19.5), it was revealed that H2O2 is the best oxidant, and the piezoelectric Bi12(Bi0.5Fe0.5)O19.5 with a high aspect-ratio morphology showed higher activity than the Bi2Fe4O9 and BiFeO3. The catalyst dosage and H2O2 concentration were further optimized, and the good durability of the catalyst was also demonstrated through multiple uses. Different characterization technologies demonstrated the occurrence of PET-MPs oxidation and fragmentation during the treatment process. The plausible mechanism of synergistically piezocatalytic and Fenton-like H2O2 activation was proposed based on measurements of band structure, piezoelectric property and reactive oxygen species generation. Finally, we detected the intermediates and determined a possible degradation route of PET-MPs. The toxicity assessment indicated that the produced intermediates have low toxicity and potential risks to the environment.

Efficient removal of nanoplastics by iron-modified biochar: Understanding the removal mechanisms

Tiny plastic particles, particularly nanoplastics, are becoming major threats to aquatic and biotic life owing to their unique physico-chemical characteristics. Thus, in the present work, biochar (BC) was fabricated using "Ulva prolifera green tide" as a biowaste raw material by slow pyrolysis technique to examine its potential in removing nanoplastics from the environment. The findings depicted that nanoplastics removal efficiency by BC was V-shaped with initial pH increased from 2 to 11, and the main removal mechanism changed from adsorption to heterogeneous aggregation between nanoplastics, biochar colloids, and leached substances from BC. When the solution pH crossed the pHpzc of BC (2.3), the aggregation kinetics were well-fitted by the logistic model and displayed as an S-shaped curve with a lag period. Characterization results indicated that biochar colloids were the key enabler with a critical concentration of 72.01 mg L-1 at neutral pH. Keeping in mind the removal mechanisms and contribution of biochar colloids, iron-modified biochar (Fe-BC) was produced to enhance the overall removal efficiency. The Fe-BC demonstrated a two-phase removal process of pre-adsorption and post-aggregation, successfully realized to minimize lag time and enhance aggregation performance. The theoretical removal capacity of Fe-BC against nanoplastics could reach up to 1626.3 mg g-1, which was three-fold higher than that of BC. Further, the Fe-BC was suggested to be recycled and reused at least three times by ultrasound, followed by co-pyrolysis for green and efficient degradation of nanoplastics. Overall, the findings offer a promising approach for removing and recycling nanoplastics in the environment.

Unraveling the synergistic mechanisms of coagulation combined with oxidation for the treatment of sewer overflow: The interaction between iron species and NaClO

During wet weather, sewer overflow pollution can pose a serious threat to surface water. In order to reduce the impact of overflow discharge on receiving waters, ferric chloride (Fe(Ⅲ))/potassium ferrate (Fe(Ⅵ))/polyacrylamide (PAM) coagulation (Fe(Ⅲ)/Fe(Ⅵ)/PAM) combined with sodium hypochlorite (NaClO) oxidation was proposed. Different combinations were constructed, including pre-oxidation coagulation (NaClO-Fe(Ⅲ)/Fe(Ⅵ)/PAM), pre-coagulation oxidation (Fe(Ⅲ)/Fe(Ⅵ)/PAM-NaClO), and synchronous coagulation oxidation (NaClO+Fe(Ⅲ)/Fe(Ⅵ)/PAM). The combined processes achieved efficient removal of conventional contaminants, and the produced byproducts were controlled, especially in the NaClO-Fe(Ⅲ)/Fe(Ⅵ)/PAM. The obvious discrepancy in the sulfamethoxazole (SMX) removal was observed in different processes. NaClO affected the distribution of hydrolyzed iron species, and the proportion of active iron in the NaClO-Fe(Ⅲ)/Fe(Ⅵ)/PAM significantly increased. More complexation sites were generated in the NaClO-Fe(Ⅲ)/Fe(Ⅵ)/PAM, which can complex with the coagulant and then effectively transfer to the flocs. The composition of the flocs further confirmed the differences in coagulation characteristics. The generated·OH played a crucial role in SMX removal in the NaClO+Fe(Ⅲ)/Fe(Ⅵ)/PAM, and ClO·was responsible for partial removal of ammonia nitrogen (NH4+-N). The contribution of high-valent iron species was confirmed, and the introduction of NaClO promoted the generation of iron species. This study may provide an ideal for overflow treatment to improve the urban water environment.

Octopus-inspired flocculant for oily wastewater decontamination: Hydrophilic-hydrophobic convertibility and auto-separation characters

Unilateral hydrophobic flocculant and unsatisfactory floc separation constrained the efficacious purification of oil-containing wastewater. Illumined by the hunting behavior of mimic octopus, a biomimetic flocculant (CNSDA) with temperature-sensitive chains (color pouch) and hollow silica cores (mantle) was manufactured to derive hydrophilic-hydrophobic convertibility and auto-separation capabilities. Physical-chemical information of CNSDA was elucidated through characterization analysis. The flocculation behaviors of temperature-sensitive chains and hollow silica cores were evaluated by flocculation experiments. Results indicated that the configuration of CNSDA molecular chains varied from extension to constriction and revealed hydrophobicity as the temperature crossed 29.6 ℃. Compared with 20 ℃, the flocculation efficiencies rocketed at 40 ℃ by CNSDA, and excess flocculants were adsorbed by as-formed flocs through nonpolar interactions (the residual was low to 2.27 % at 160 mg/L). Concomitantly, the contracted molecular chains were contributed to generating dense flocs with low moisture content that flocked into large ones and expedited the solid-liquid separation process (60 % shorter than cationic polyacrylamide) with the auxiliary of low-density cores. The hydrophobic adsorption mechanism actuated by temperature-sensitive character was the decisive factor for high-efficiency flocculation. This study can provide meaningful references for the conception and exploitation of oily wastewater disposal agents.

Coagulation performance and mechanism of different novel covalently bonded organic silicon-aluminum/iron composite coagulant for As(V) removal from water: The role of hydrolysate species and the effect of coexisting microplastics

Arsenate [As(V)] pollution is a challenge for water treatment, and the effect of coexisting microplastics (MPs) on As(V) removal is still not clear. In this study, series novel covalently bonded organic silicon-aluminum/iron composite coagulants (CSA/F) with different Al/Fe molar ratios were prepared for enhancing As(V) removal. The effect mechanism of MPs (PS MPs and PS-COOH MPs) on As(V) removal by using CSAF coagulation was analyzed. CSAF and CSF showed significantly better As(V) removal performance than other coagulants under the same conditions, especially CSF, more than 90 % As(V) removal was achieved at dosage of 20 mg/L and pH of 4.0-8.0. Interestingly, the introduction of silane coupling agent and the increase of Fe content in CSA/F changed the Al/Fe species distribution. Charge neutralization dominant in As(V) removal by using CSA, whereas adsorption and net sweeping contributed to As(V) coagulation by using CSAF and CSF with higher iron proportion at neutral pH. 3 µm MPs were removed by net sweeping of amorphous Al/Fe hydroxides, while 26 µm MPs were charge-neutralized or surface adsorbed by coagulant hydrolysates. The aliphatic C-H and -COOH functional groups of MPs were the main sites of hydrogen bonding adsorption with the hydroxyl groups of coagulant hydrolysates. This study is conducive to mitigating the environmental toxicity of arsenic and provides new insights into the interaction mechanism between composite pollutants and coagulants in waters.

High-efficiency microplastic removal in water treatment based on short flow control of hydrocyclone: Mechanism and performance

Microplastics have been identified as a potentially emerging threat to water environment and human health. Therefore, there is a pressing demand for effective strategies to remove microplastics from water. Hydrocyclone offers a rapid separation and low energy consumption alternative but require reduction of microparticle entrainment by short flow, which limits the effectiveness for small density differentials and ultralow concentrations separation. We proposed an enhanced mini-hydrocyclone with overflow microchannels (0.72 mm width) based on the active control of short flow in hydrocyclone for microplastic removal from water. The overflow microchannels effectively redirect the particles that would typically be entrained by the short flow, leading to higher separation efficiency. Simulation results show overflow microchannels effectively reduced short flow to 0.7 %, a reduction of up to 94 % compared to conventional hydrocyclones. The hydrocyclone with overflow microchannel demonstrated a removal efficiency exceeding 98 % for 8 μm plastic microbeads at ultralow concentrations (10 ppm), which is a 33.7 % improvement over conventional hydrocyclone. Compared with other methods (e.g., filtration, adsorption, coagulation) for microplastic removal, this work achieves rapid separation capability and long period operation, highlighting hydrocyclone as a promising approach for microplastic removal in industry-scale water treatment.

Uncovering the performance and intrinsic mechanism of different hydrolyzed AlTi species in polystyrene nanoplastics coagulation

Hydrolyzed AlTi species are essential metal-based coagulants in a coagulation process to remove nanoplastics (NPs). Understanding the molecular interactions between hydrolyzed AlTi species and NPs is key to promoting coagulation efficiency. In this study, the coagulation performance and intrinsic mechanism of different AlTi species (including monomeric AlTi and polymeric AlTi species-Al13Ti13) for NPs removal were systematically investigated. We found that the polymeric AlTi species exhibited higher turbidity removal (95.0 %) and lower residual Al content (20.67 μg/L) at a low dosage over monomeric AlTi species. Al13 and Al13Ti13 formed by in situ hydrolysis were the dominant species to destabilize and aggregate NPs at pH 6. Main coagulation mechanisms were dominated by charge neutralization, complexation between the aliphatic CH of NPs and Al/Ti-OH, and cation-π interaction between polycations and the aromatic structure of NPs. The preformed Al13Ti13 showed multiple positive charge binding sites assisting its easy adsorption on NPs by electrostatic attraction, and then formed microscale aggregates through charge neutralization or intermolecular interaction. The preformed Al13Ti13 demonstrated a high stability and coagulation performance with respect to pH changes in raw water, whereas the promotion of μ-OH bridges dissociation by OH- and the presence of electrostatic repulsion significantly decreased the NPs removal by monomeric AlTi at high pH. This study provides valuable theoretical insights into the interaction between NPs and various hydrolyzed AlTi species.

Innovative solutions for the removal of emerging microplastics from water by utilizing advanced techniques

Microplastic pollution is one of the most pressing global environmental problems due to its harmful effects on living organisms and ecosystems. To address this issue, researchers have explored several techniques to successfully eliminate microplastics from water sources. Chemical coagulation, electrocoagulation, magnetic extraction, adsorption, photocatalytic degradation, and biodegradation are some of the recognized techniques used for the removal of microplastics from water. In addition, membrane-based techniques encompass processes propelled by pressure or potential, along with sophisticated membrane technologies like the dynamic membrane and the membrane bioreactor. Recently, researchers have been developing advanced membranes composed of metal-organic frameworks, MXene, zeolites, carbon nanomaterials, metals, and metal oxides to remove microplastics. This paper aims to analyze the effectiveness, advantages, and drawbacks of each method to provide insights into their application for reducing microplastic pollution.

Differences of microplastics and nanoplastics in urban waters: Environmental behaviors, hazards, and removal

Microplastics (MPs) and nanoplastics (NPs) are ubiquitous in the aquatic environment and have caused widespread concerns globally due to their potential hazards to humans. Especially, NPs have smaller sizes and higher penetrability, and therefore can penetrate the human barrier more easily and may pose potentially higher risks than MPs. Currently, most reviews have overlooked the differences between MPs and NPs and conflated them in the discussions. This review compared the differences in physicochemical properties and environmental behaviors of MPs and NPs. Commonly used techniques for removing MPs and NPs currently employed by wastewater treatment plants and drinking water treatment plants were summarized, and their weaknesses were analyzed. We further comprehensively reviewed the latest technological advances (e.g., emerging coagulants, new filters, novel membrane materials, photocatalysis, Fenton, ozone, and persulfate oxidation) for the separation and degradation of MPs and NPs. Microplastics are more easily removed than NPs through separation processes, while NPs are more easily degraded than MPs through advanced oxidation processes. The operational parameters, efficiency, and potential governing mechanisms of various technologies as well as their advantages and disadvantages were also analyzed in detail. Appropriate technology should be selected based on environmental conditions and plastic size and type. Finally, current challenges and prospects in the detection, toxicity assessment, and removal of MPs and NPs were proposed. This review intends to clarify the differences between MPs and NPs and provide guidance for removing MPs and NPs from urban water systems.

Recent advances in microplastic removal from drinking water by coagulation: Removal mechanisms and influencing factors

Microplastics (MPs) are emerging contaminants that are widely detected in drinking water and pose a potential risk to humans. Therefore, the MP removal from drinking water is a critical challenge. Recent studies have shown that MPs can be removed by coagulation. However, the coagulation removal of MPs from drinking water remains inadequately understood. Herein, the efficiency, mechanisms, and influencing factors of coagulation for removing MPs from drinking water are critically reviewed. First, the efficiency of MP removal by coagulation in drinking water treatment plants (DWTPs) and laboratories was comprehensively summarized, which indicated that coagulation plays an important role in MP removal from drinking water. The difference in removal effectiveness between the DWTPs and laboratory was mainly due to variations in treatment conditions and limitations of the detection techniques. Several dominant coagulation mechanisms for removing MPs and their research methods are thoroughly discussed. Charge neutralization is more relevant for small-sized MPs, whereas large-sized MPs are more dependent on adsorption bridging and sweeping. Furthermore, the factors influencing the efficiency of MP removal were jointly analyzed using meta-analysis and a random forest model. The meta-analysis was used to quantify the individual effects of each factor on coagulation removal efficiency by performing subgroup analysis. The random forest model quantified the relative importance of the influencing factors on removal efficiency, the results of which were ordered as follows: MPs shape > Coagulant type > Coagulant dosage > MPs concentration > MPs size > MPs type > pH. Finally, knowledge gaps and potential future directions are proposed. This review assists in the understanding of the coagulation removal of MPs, and provides novel insight into the challenges posed by MPs in drinking water.