Surface-modified microbubbles (colloidal gas aphrons) for nanoparticle removal in a continuous bubble generation-flotation separation system

Micro and nanobubbles-assisted advanced oxidation processes for water decontamination: The importance of interface reactions

Micro and nanobubbles (MNBs), as an efficient and convenient method, have been widely used in water treatment. Composed of gas and water, MNBs avoid directly introducing potential secondary pollutants. Notably, MNBs exhibit significant advantages through interface reactions in assisting AOPs. They overcome barriers like low mass transfer coefficients and limited reactive sites, and shorten the distance between pollutants and oxidants, achieving higher pollutant removal efficiency. However, there is a lack of systematic summary and in-depth discussion on the fundamental mechanisms of MNBs-assisted AOPs. In this critical review, the characteristics of MNBs related to water treatment are outlined first. Subsequently, the recent applications, performance, and mechanisms of MNBs-assisted AOPs including ozone, plasma, photocatalytic, and Fenton oxidation are overviewed. We conclude that MNBs can improve pollutant removal mainly by enhancing the utilization of reactive oxygen species (ROS) generated by AOPs due to the effective interface reactions. Furthermore, we calculated the electrical energy per order of reaction (EE/O) parameter of different MNBs-assisted AOPs, suggesting that MNBs can reduce the total energy consumption in most of the tested cases. Finally, future research needs/opportunities are proposed. The fundamental insights in this review are anticipated to further facilitate an in-depth understanding of the mechanisms of MNBs-assisted AOPs and supply critical guidance on developing MNBs-based technologies for water treatment.

Enhanced flotation removal of polystyrene nanoplastics by chitosan modification: Performance and mechanism

Nanoplastics are difficult to remove from water using conventional flotation processes due to their stability and resistance to biodegradation. Here, polystyrene nanoplastics (PSNPs) were selected as the object of study. In addition, chitosan (CTS), an environmentally friendly natural cationic polymer, was selected to modify the air flotation process to improve the separation of PSNPs using air flotation. Adding chitosan effectively enhanced the removal of PSNPs using air flotation from 3.1 % to 96.7 %. The residual concentration decreased from 9.69 mg/L to 0.33 mg/L. Removal of PSNPs by CTS-modified air flotation was maintained at 92.8 % even when the air flotation time was significantly shortened. The zeta potential alterations demonstrated robust electrostatic attraction within the CTS-modified air flotation process. The contact angle measurements indicated that incorporating CTS could enhance the hydrophobic interaction between bubbles and PSNPs. PSNPs particles around 100 nm agglomerated to form floating flocs with a particle size of more than 4500 nm. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) images confirmed the presence of tight adhesion between PSNPs and CTS, indicating the presence of bridging adsorption during the process. The major PSNPs removal mechanisms included electrostatic attraction, enhancement of hydrophobicity, and bridging adsorption. Increasing the aeration volume could improve the removal rate, but this improvement was finite. Weakly acidic and low ionic strength conditions favored PSNPs removal. The CTS-modified air flotation process showed great potential for PSNPs removal from real water bodies.

A novel chitosan and polyferric sulfate composite coagulant for biogas slurry pretreatment by simultaneous flocculation and floatation: Performance and underlying mechanisms

Biogas slurry from anaerobic digestion is rich in nutrients but has not been fully utilized due to a high content of suspended solids (SS) causing clogging during agricultural irrigation. This study aimed to evaluate the performance of a novel chitosan and polyferric sulfate (CTS-PFS) composite coagulant for simultaneous flocculation and floatation to enhance SS removal while preserving nutrients in biogas slurry. Orthogonal method was used for experimental design to determine the optimal synthesis and operational conditions of CTS-PFS. Results show that CTS-PFS outperformed individual CTS and PFS coagulant in terms of SS removal and nutrient (nitrogen, phosphorus, and potassium) preservation. Compared to individual CTS and PFS coagulation, the combination of CTS and PFS at the mass ratio of 1:6 showed significantly higher performance by 41.5 % increase in SS removal and 5.2 % reduction in nutrient loss. The improved performance of CTS-PFS was attributed to its formation of polynuclear hydroxyl complexes with ferric oxide groups (e.g. Fe-OH, Fe-O-Fe, Fe-OH-Fe and COO-Fe) to strengthen charge neutralization and adsorption bridging. Data from this study further confirm that CTS-PFS enhanced the removal of small suspended particles and dissolved organic matter in the molecular weight range of 0.4-2.0 kDa and preserved ammonia and potassium better in biogas slurry. Bubbles were generated as hydrogen ions from coagulant hydrolysis interacted with bicarbonate and carbonate in biogas slurry for removing the produced flocs by floatation. Floc flotation was more effective in CTS-PFS coagulation due to the significant production of uniform bubbles, evidenced by the reduction in the viscosity of biogas slurry.

Application and development of foam extraction technology in wastewater treatment: A review

With the advancement of technology, wastewater treatment has become a significant challenge limiting the clean and sustainable development of chemical and metallurgical industries. Foam extraction, based on interfacial separation and mineral flotation, has garnered considerable attention as a wastewater treatment technology due to its unique physicochemical properties. Although considerable excellent accomplishments were reported, there still lacks a comprehensive summary of process features and contaminant removal mechanisms via foam extraction. According to the latest research progresses, the principles and characteristics of foam extraction technology, the classification and application of flotation reagents are systematically summarized in this work. Then comprehensively commented on the application fields and prospects of iterative flotation technology such as ion flotation, adsorption flotation and floating-extraction. The shortcomings and limitations of the current foam extraction technologies were discussed, and the feasible process intensification techniques were highlighted. This review aims to enchance the understanding of the foam extraction mechanism, and provides guidance for the selection appropriate reagents and foam extraction technologies in wastewater treatment.

A comparative study of removal efficiency of organic contaminant in landfill leachate-contaminated groundwater under micro-nano-bubble and common bubble aeration

Landfill leachate-contaminated groundwater is widespread all over the world. In order to study the organic contaminant removal efficiency of landfill leachate-contaminated groundwater under oxygen micro-nano-bubble (MNB) aeration, a series of lab-scale experiments of oxygen MNB aeration as well as common bubble (CB) aeration were conducted. Firstly, the difference in mass transfer, microbial activity enhancement, and contaminant removal efficiency between MNB and CB aeration was estimated. Then, the composition variations of dissolved organic matter (DOM) in groundwater treated by MNB or CB aeration were characterized by ultraviolet-visible (UV-VIS) absorption spectrum and fluorescence excitation-emission matrix (EEM). The test results showed that the oxygen utilization efficiency and volumetric oxygen transfer coefficient of MNB aeration were 10 and 50 times that of oxygen CB aeration, respectively. On the 30th day after MNB aeration, the dehydrogenase activity (DHA) of groundwater increased by 101.25%. Compared with CB aeration, the chemical oxygen demand (COD), 5-day biochemical oxygen demand (BOD₅), and ammonia nitrogen removal efficiency under MNB aeration increased by 29.72%, 13.43%, and 138.59%, respectively. With the biodegradation effect of MNB aeration, a large number of protein-like and soluble microbial by-product substances were degraded, and humic and fulvic acid-like substances were degraded to a certain level. Oxygen MNB aeration played a chemical oxidation effect while enhancing the biodegradation of groundwater, and it was an energy-efficient landfill leachate-contaminated groundwater treatment method.

Separation of emulsified crude oil from produced water by gas flotation: A review

The development and production of oil and gas fields would eventually result in a considerable amount of oily generated water, posing serious risks to humans and the environment. Nowadays, the oil concentration in the drainage stream of the produced water is strictly regulated, and many countries have established strict emission standards. As an indispensable oily wastewater treatment technology, flotation technology has attracted much attention because of its maturity, economy, practicality, and relative efficiency. Firstly, this paper summarizes and compares flotation techniques, such as dissolved gas flotation, induced gas flotation, electroflotation, and compact flotation units widely used in produced water treatment offshore in recent years. Considering the complexity of the mechanism of oil removal by air flotation, the mechanism of the oil droplet-bubble interaction is further discussed. The effects of flocculant, PH, and salinity on the oil droplet-bubble interaction in the flotation process were summarized from the perspective of the microscopic colloidal interface, which has a specific guiding role in improving the oil removal efficiency in the gas flotation process. Finally, the research status of produced water treatment by air flotation is summarized, and the feasible research direction is put forward.

Continuous air purification by aqueous interface filtration and absorption

The adverse impact of particulate air pollution on human health^1,2 has prompted the development of purification systems that filter particulates out of air^3-5. To maintain performance, the filter units must inevitably be replaced at some point, which requires maintenance, involves costs and generates solid waste^6,7. Here we show that an ion-doped conjugated polymer-coated matrix infiltrated with a selected functional liquid enables efficient, continuous and maintenance-free air purification. As the air to be purified moves through the system in the form of bubbles, the functional fluid provides interfaces for filtration and for removal of particulate matter and pollutant molecules from air. Theoretical modelling and experimental results demonstrate that the system exhibits high efficiency and robustness: its one-time air purification efficiency can reach 99.6%, and its dust-holding capacity can reach 950 g m^-2. The system is durable and resistant to fouling and corrosion, and the liquid acting as filter can be reused and adjusted to also enable removal of bacteria or odours. We anticipate that our purification approach will be useful for the development of specialist air purifiers that might prove useful in a settings such as hospitals, factories and mines.

Fenton micro-reactor on a bubble: A novel microbubble-triggered simultaneous capture and catalytic oxidation strategy for recalcitrant organic pollutant removal

A novel catalyst-functionalized microbubble system was developed to trigger both of the Fenton reaction and the flotation separation on the gas-liquid interface of bubbles for efficiently removing the recalcitrant organic pollutants from waters. The Fe(II)-functionalized colloidal microbubbles (FCMBs) were featured as large specific surface area, great bubble density and high ·OH activation capacity. Approximately 98.2% and 93.1% of the triphenylmethane and aromatic azo pollutants were removed within 0.5 min, respectively. Particularly, at the lowest Fe(II) dose of 0.2 mmol/L, the FCMB-triggered Fenton still achieved 7.4-20.6% higher removal than the traditional Fenton method at 0.5 min. In addition to the Fenton oxidative degradation mechanism, the FCMBs themselves were able to capture and remove 20.1-36.8% of pollutants from water. Thus, FCMBs served as micro-reactors in terms of: (i) the target molecules and intermediates were adhered and separated by FCMBs; and (ii) the FCMBs enhanced the mass transfer of catalyst and exposed sufficient active sites on the bubble surface for catalytic oxidation reaction. Compared with the traditional Fenton, the present method showed the robust tolerance of pH (4.0-9.5) and salinity (up to 40‰) at decreased Fe(II) doses, and the bio-toxicity of intermediates was obviously lower. The FCMB-triggered pollutant capture and catalytic oxidation technology exhibited a great potency in engineering implementation given the flexible bubble construction, the integration and simplification of treatment unit, as well as the decreased chemical doses.

Insights into capture-inactivation/oxidation of antibiotic resistance bacteria and cell-free antibiotic resistance genes from waters using flexibly-functionalized microbubbles

The spread of antibiotic resistance in the aquatic environment severely threatens the public health and ecological security. This study investigated simultaneously capturing and inactivating/oxidizing the antibiotic resistant bacteria (ARB) and cell-free antibiotic resistance genes (ARGs) in waters by flexibly-functionalized microbubbles. The microbubbles were obtained by surface-modifying the bubbles with coagulant (named as coagulative colloidal gas aphrons, CCGAs) and further encapsulating ozone in the gas core (named as coagulative colloidal ozone aphrons, CCOAs). CCGAs removed 92.4-97.5% of the sulfamethoxazole-resistant bacteria in the presence of dissolved organic matter (DOM), and the log reduction of cell-free ARGs (particularly, those encoded in plasmid) reached 1.86-3.30. The ozone release from CCOAs led to efficient in-situ oxidation: 91.2% of ARB were membrane-damaged and inactivated. In the municipal wastewater matrix, the removal of ARB increased whilst that of cell-free ARGs decreased by CCGAs with the DOM content increasing. The ozone encapsulation into CCGAs reinforced the bubble performance. The predominant capture mechanism should be electrostatic attraction between bubbles and ARB (or cell-free ARGs), and DOM enhanced the sweeping and bridging effect. The functionalized microbubble technology can be a promising and effective barrier for ARB and cell-free ARGs with shortened retention time, lessened chemical doses and simplified treatment unit.

Study on the cell-collector-bubble interfacial interactions during microalgae harvesting using foam flotation

Foam flotation is an economical and efficient technology for microalgae harvesting. However, the mechanism of cell-collector-bubble interfacial interactions remains to be elucidated. There are two distinct hypotheses regarding the mechanism of microalgae foam flotation. In this study, the cationic surfactant N-cetyl-N-N-N-trimethylammonium bromide (CTAB), which acts as a partition between Chlorella sorokiniana cells and bubbles, is quantified and the zeta potential response of cells and bubbles after adsorption of CTAB is calculated to reveal the interfacial mechanism of the cells-collector-bubble interfacial interactions. The results indicated that more than 90% of CTAB was preferentially adsorbed on the bubbles, which reversed the surface charge of bubbles from negative (-20 mV) to positive (6.1 mV). However, only 0%-3% CTAB was observed on the microalgae cells, suggesting its limited influence on the negatively charged microalgae cells (from -22.3 to -18.6 mV). During microalgae foam flotation, the nonpolar tails of CTAB were first inserted into the bubble through hydrophobic interactions, leaving the positively charged polar heads outside; further, the CTAB-covered positively charged bubbles captured the negatively charged cells by electrostatic attraction. A feasible mechanism was proposed to understand the interfacial interaction of the microalgae cell-CTAB-bubble. By understanding the mechanism of foam flotation, efficient and cost-effective collectors and devices for microalgae harvesting using foam flotation can be developed.

Study on the removal of hazardous Congo red from aqueous solutions by chelation flocculation and precipitation flotation process

Dyes are intensively used in textile and dyeing industries, and substantial volumes of organic wastewater with residual dye require treatment before discharges to public waterways. Flotation separation is an efficient and widely used method for the treatment of massive organic dye wastewaters. The key scientific problems for dye flotation separation lie in the mineralization transformation of dissolved dye to tangible flocs. In this work, a high-efficiency removal of hazardous azo dye Congo red (CR) from simulated wastewaters via metal ions chelation flocculation followed by flotation separation was proposed. It's demonstrated that CR can be chelated by the trivalent metal ions, including Al(III), Fe(III), and its mixture to form hydrophobic flocs, and then the flocs were efficiently removed via flotation in a microbubble column. The effects of chelation flocculation and flotation separation conditions on the removal efficiencies of CR, COD, and chromaticity from CR simulated wastewaters were optimized. Chelation effect of CR by trivalent metal ions was in this order: Al(III)+Fe(III)>Fe(III)>Al(III). The chelation mechanism suggested that CR molecules gradually changed from hydrazones to electronegative azo with the increase of pH to 6-7, and electrostatic attraction between the Al₃(OH)₄⁵⁺ or Fe(OH)₂⁺ with the CR was favorable for the chelation reaction, in which the metal ions chelated with N atoms on naphthalene ring and amino groups of CR. Over 99% CR was removed under the optimal chelation and flotation conditions: chelation by composite Al(III)/Fe(III) with a concentration of 25 mg/L at pH of 7 for 25min; followed by flotation with SDS concentration of 20 mg/L and air flow rate of 50 mL/min for 20min. Under this condition, the COD and chromaticity removal efficiency were over 96% and 98%, respectively, and the turbidity was lower than 0.1 NTU, meeting the water discharge requirement. Eventually, resourceful utilization of flotation sludge via calcination was conducted to prepare Al-Fe spinel refractory material.

Novel treatment of Microcystis aeruginosa using chitosan-modified nanobubbles

In this study, we treated harmful Microcystis aeruginosa cyanobacteria using chitosan-modified nanobubbles. The chitosan-modified nanobubbles (255 ± 19 nm) presented a positive zeta potential (15.36 ± 1.17 mV) and generated significantly (p < 0.05) more hydroxyl radicals than the negatively charged nanobubbles (-20.68 ± 1.11 mV). Therefore, the interaction between the positively charged chitosan-modified nanobubbles and negatively charged M. aeruginosa (-34.81 ± 1.79 mV) was favored. The chitosan-modified nanobubble treatment (2.20 × 10⁸ particles mL^-1) inactivated 73.16% ± 2.23% of M. aeruginosa (2.00 × 10⁶ cells mL^-1) for 24 h without causing significant cell lysis (≤0.25%) and completely inhibited the acute toxicity of M. aeruginosa toward Daphnia magna. The inactivation was correlated (r² = 0.97) with the formation of reactive oxygen species (ROS) in M. aeruginosa. These findings indicated that the hydroxyl radicals generated by the chitosan-modified nanobubbles disrupted cell membrane integrity and enhanced oxidative stress (ROS formation), thereby inactivating M. aeruginosa. Moreover, the penetration of the chitosan-modified nanobubbles and cell alterations in M. aeruginosa were visually confirmed. Our results suggested that the chitosan-modified nanobubble treatment is an eco-friendly method for controlling harmful algae. However, further studies are required for expanding its practical applications.

Influence of particle size on the aggregation behavior of nanoparticles: Role of structural hydration layer

More and more attention has been paid to the aggregation behavior of nanoparticles, but little research has been done on the effect of particle size. Therefore, this study systematically evaluated the aggregation behavior of nano-silica particles with diameter 130-480 nm at different initial particle concentration, pH, ionic strength, and ionic valence of electrolytes. The modified Smoluchowski theory failed to describe the aggregation kinetics for nano-silica particles with diameters less than 190 nm. Besides, ionic strength, cation species and pH all affected fast aggregation rate coefficients of 130 nm nanoparticles. Through incorporating structural hydration force into the modified Smoluchowski theory, it is found that the reason for all the anomalous aggregation behavior was the different structural hydration layer thickness of nanoparticles with various sizes. The thickness decreased with increasing of particle size, and remained basically unchanged for particles larger than 190 nm. Only when the distance at primary minimum was twice the thickness of structural hydration layer, the structural hydration force dominated, leading to the higher stability of nanoparticles. This study clearly clarified the unique aggregation mechanism of nanoparticles with smaller size, which provided reference for predicting transport and fate of nanoparticles and could help facilitate the evaluation of their environment risks.

Understanding variability in algal solid-liquid separation process outcomes by manipulating extracellular protein-carbohydrate interactions

Coagulation-flocculation followed by sedimentation or dissolved air flotation (DAF) are processes routinely used for separating microalgae from water; however, during algae separation then can exhibit inconsistent separation, high coagulant demand, and high operating cost. To circumvent these problems, previous studies reported the development of a novel DAF process in which bubbles were modified instead of particles. While this process was shown to be sustainable and inexpensive, the problem of inconsistent algal separation across species remained. Recent research has suggested that this could be due to the varying concentration and character of algal-derived proteins and carbohydrates within the extracellular organic matter (EOM) and their associated interactions. This hypothesis is tested in the current study using the novel modified-bubble DAF process, which has been highly susceptible to EOM protein and carbohydrate concentrations and character. Biomolecular additives (commercially available proteins and carbohydrates, and algal-extracted proteins) of widely differing molecular weight (MW) and charge were dosed in varying proportions into samples containing either Chlorella vulgaris CS-42/7, Microcystis aeruginosa CS-564/01, or Microcystis aeruginosa CS-555/1 after removing the intrinsic EOM. These cell-rich suspensions were then subject to flotation using cationic bubbles modified with poly(diallyldimethylammonium chloride) (PDADMAC). When additives were dosed independently, separation increased from <5% to up to 62%. The maximum separation was obtained when the dose was double the respective biopolymer concentration measured in the intrinsic EOM for the equivalent species, and, in the case of protein additives, when MW and charge were >50 kDa, and >0.5 meq·g^-1, respectively, irrespective of the species tested. When evaluating steric- and charge-based protein-carbohydrate interactions on cell separation by simultaneously dosing high MW and high charge protein- and carbohydrate-additives, enhanced separation of up to 79% was achieved. It is suggested that enhanced cell separation is achieved due to proteins and carbohydrates bridging with cells and forming protein-carbohydrate-cell suprastructures in the presence of a flocculant, e.g. PDADMAC, and this only occurs when the intrinsic EOM comprises proteins and carbohydrates that have high MW (>25 kDa) and charge (>0.2 meq·g^-1), and interactions with each other and with the cell surface.

Removal of micron-scale microplastic particles from different waters with efficient tool of surface-functionalized microbubbles

Microplastic (MP) contamination in water has garnered significantly global concerns. The MP removal particularly challenges when the particle size decreases to several microns and other contaminants co-exist. This study used the coagulative colloidal gas aphrons (CCGAs) to simultaneously remove the micron-scale MP particles (~5 µm in diameter) and dissolved organic matter (DOM). Carboxyl-modified poly-(methyl methacrylate) (PMMA) and unsurface-coated polystyrene (PS) were chosen as target MPs. Over 94% of PS particles and almost 100% of color were simultaneously removed with lower CCGA consumption than the scenarios with either contaminant in water. The PMMA removal was not as high as the PS removal since the HA polyanions could compete with the negatively-charged PMMA for CCGAs. High salinity reduced the removal of HA by changing its interfacial behaviors without impacting the MP separation. In river water or influent of wastewater treatment plant, the MP particles were almost completely eliminated whereas the DOM (tyrosine-like or tryptophan-like) was partially removed. The fluorescence quenching titration revealed that CCGAs preferably captured the free DOM and the DOM-coated MP particles through complexation interaction. The study denoted that the CCGA system could be a robust tool for efficiently and synergistically removing micron-scale MPs and DOM from different water matrixes.

The role of the pulp-froth interface on particle detachment and selectivity

The region between the pulp and the froth also known as pulp/froth interface in mineral flotation processes separates the pulp from the froth. Various researchers suggest particle detachment occurs around this region significantly affecting mineral recovery and grade. One of the causes pointed out is sudden deceleration of bubble-particle aggregate upon collision with the interface while another theory suggests detachment to be caused by bubble coalescence. A possible cause of divergence in views may be in the different methods of investigation employed. Though, more than several researchers indicate that detachment occurs, it is not conclusive whether kinetic energy changes or bubble coalesce or a combination of the two is responsible for detachment if any. Thus, this review examines and presents work that has been done on the role of the pulp-froth interface on particle detachment and selectivity. The review also considers the behaviour of a bubble with various interface as found in literature with a view of inferring the dominant cause of detachment at the interface.

Lability-specific enrichment of typical engineered metal (oxide) nanoparticles by surface-functionalized microbubbles from waters

Enrichment of metallic engineered nanoparticles (MENPs) from environmental waters is a prerequisite for their removal, reliable analyses, and environmental process interpretations. This work investigated the enrichment of typical MENPs with different degrees of lability using surface-functionalized microbbubles. During the process, the transformation/dissolution characteristics of MENPs were considered, and the impact of surfactant or coagulant dose, pH of MENP suspensions, and water matrix was systematically investigated. Results show that the colloidal gas aphrons (CGAs) were capable of enriching over 90.0% of ionic Ag(I) which ended up as AgBr and Ag₂CO₃ in floats when the pH of suspension was 6.0. The polyaluminum chloride-modified CGAs with positive surface charges were good at capturing the particulate ZnO-NPs (~84.8%) but failed to collect the ionic species. It should be noted that the total MENP enrichment efficiency closely related to the content proportions of different species. In the river water, both of the dissolved natural organic matter (fulvic acids) and the electrolytes might influence the enrichment process by affecting the species transformation of Ag-NPs and ZnO-NPs. For the stable TiO₂-NPs, 97.1% of the nanoparticles were captured by CGAs. FAs apparently reinforced the enrichment performance since the molecules acted as bridge and facilitated the attachment between TiO₂-NP and CGAs. This work contributes to establishing the robust microbubble-induced enrichment method considering the characteristics of MENP contaminants.

What occurs in colloidal gas aphron-induced separation of titanium dioxide nanoparticles? Particle fate analysis by tracking technologies

As an important method of enriching, separating and removing nanoparticles, colloidal gas aphrons (CGAs) need to be investigated for the fate and interfacial behaviors of particles during the process. It is beneficial to sufficiently interpreting the process performance and mechanisms. This study employed complementary tracking technologies to analyze the extensively-used engineered nanoparticles - TiO₂ nanoparticles (TiO₂-NPs) in effluent and floats of CGA process. Results denote that, at the optimum SDS relative dosage of 0.78 mg/mg TiO₂, the particle number concentration was largely reduced by 2-4 orders of magnitude based on nanoparticle tracking analysis (NTA) whilst approximately 84.0% of TiO_2-NPs were separated according to inductively coupled plasma-mass spectrometry (ICP-MS). NTA shows the change of overall particle dispersion status in the water phase while ICP-MS provides the Ti-related separation effect. Particularly, the particle size variation for the scenario of overdosing CGAs was clearly observed by NTA. Micro-Raman, dynamic laser scattering and small angle laser light scattering exhibited advantages in obtaining the configuration and morphology of flocs. The large flocs with open structure were apt to form and be favorably separated at the appropriate CGA dosage. However, overdosing CGAs weakened the capture capacity of bubbles and gave rise to small and dense aggregates. This work, for the first time, shows the change of nanoparticles in water and solid phases using the important and novel nanoparticle collection method - CGA technology. It also provides a reference to other flotation-related technologies for studying the nanoparticle fate and the process performance.

The progressive steps for TPH stripping and the decomposition of oil refinery sludge using microbubble ozonation

Total petroleum hydrocarbons (TPH) in activated petroleum waste sludge (PWS) hindered the disintegration of sludge, and microbubble ozonation (MB-O₃) was explored to separate the TPH and solids particle, enhance the decomposition of PWS, and improve the efficiency of ozonation. The maximum solubilization of PWS reached to approximately 41.9% at an ozone dose of 5.40 gO₃/gTS, two times higher than the control one. The ozone mass transfer coefficient of k_La increased from 0.1101 min^-1 to 0.2293 min^-1 in MB-O₃, resulting in the formation of a higher concentration of 1.29 μg/L hydroxyl radicals. The medium diameter sharply declined from 38.6 μm to 17.5 μm, and more porous surface of sludge flocs was observed, indicating that MB-O₃ destroyed the water-oil-gel structure and contributed to the stripping of TPH. The soluble chemical oxygen demand was released by 390% with respect to initial value (from 764 to 3740 mg/L) and acetic acid was the predominant component with yield of 590 ± 7.1 mg/L, which could be served as an additional carbon source. This study provides an efficient approach to achieve sludge disposal and simultaneous enhance the stripping of total petroleum hydrocarbons from oil refinery sludge.

Impact of salinity on colloidal ozone aphrons in removing phenanthrene from sediments

Polycyclic aromatic hydrocarbons (PAHs) tend to adsorb and accumulate on sediments owing to their hydrophobicity and persistence. Salinity is the predominant factor determining the PAH partition between aqueous and solid phases in freshwater, estuaries and seawater. This study focuses on the impact of salinity on the phenanthrene (PHE) removal from sediments using an in situ and targeted remediation technology - colloidal ozone aphrons (COAs). The ozone-encapsulated colloidal aphrons exhibited increasing air holdup but decreasing stability with the salinity increasing from 0.5‰ to 35‰. The hydrophobic attraction between Tween-20-coated bubbles and the hydrophobic solid surface weakened at high salinities. The presence of inorganic ions in the aqueous phase could lead to the salting-out of nonionic compounds (PHE, Tween-20 and even ozone), hindering detaching and degrading PHE from the solid phase. Anyhow, COAs achieved high efficiencies of washing (88.0-90.2%) and oxidative degradation (74.0-76.5%) particularly for the hydrophobic sediments with highly concentrated PHE (200.4 μg/kg) over the investigated salinities. The flushing effect imposed by the bubble flow played an important role, which was not greatly influenced by salinity. Although the dissolved natural organic matter competed with PHE for COAs and led to low PHE removal, the efficiency was improved by successive COA addition.

Comparison of coagulative colloidal microbubbles with monomeric and polymeric inorganic coagulants for tertiary treatment of distillery wastewater

The flotation using coagulative colloidal gas aphrons (CCGAs) is of great potential in effectively removing the recalcitrant dissolved organic matter (DOM) and colorants from the bio-chemically treated cassava distillery wastewater. As bubble modifier, the monomeric and polymeric inorganic coagulants need to be studied considering their distinct influence on the surfactant/coagulant complex, the properties of colloidal aphrons as well as the process performance and mechanisms. Such studies help to create robust CCGAs with high flotation potential. In this work, the commonly-used monomeric and polymeric Al(III)- and Fe(III)-coagulants were combined with the cationic surfactant - cetyl trimethylammonium bromide (CTAB) to generate CCGAs. The CCGAs functionalized with Al(III)-coagulants (both monomeric and polymeric ones) were featured as small bubble size, strong stability and high air content. Particularly, the monomeric Al(III)-coagulant (AlCl₃ in this work) resulted in low surface tension and high foamability when being mixed with CTAB in the bubble generation solution. Those CCGAs achieved high removal efficiencies of DOM and colorants at low coagulant concentrations. The molecular weight of DOM in effluent was well controlled below 1 kDa by CCGAs. For the flocs obtained from CCGA-flotation, the characteristic Raman band of DOM and colorants showed the layer-by-layer variation of Raman intensity which decreased from the outer layer to the center. In contrast with the conventional coagulation-flotation, the reduction of coagulant dosage by CCGAs was 67% (AlCl₃), 25% (polyaluminum chloride), 60% (Fe₂(SO₄)₃) and 40% (polyferric sulfate). The sludge production could then be largely reduced, and meanwhile, the retention time was shortened by 9.5 min.

Efficient elimination and re-growth inhibition of harmful bloom-forming cyanobacteria using surface-functionalized microbubbles

The elimination of cyanobacteria is frequently required for treating and controlling the waters with harmful algal blooms. In this study, an improved flotation technology was developed using colloidal gas aphrons (CGAs) surface-modified with the inorganic coagulant of polyaluminum chloride (PACl); the Microcystis aeruginosa (M. aeruginosa) cells were efficiently removed and their re-growth was effectively inhibited. The so-created coagulative CGAs (CCGAs) exhibited the attractive characteristics of both CGAs and PACl for the cell removal. The experimental results clearly showed that 94.2-99.2% of cells were removed within 3 min at the optimum dosage of cetyltrimethyl ammonium bromide (CTAB) and PACl at three different initial cell densities (OD₆₈₀ = 0.05, 0.26 and 0.76); and the re-growth of M. aeruginosa did not occur in 10 days. The flocs derived from the CCGA-flotation were of smaller size and looser configuration in contrast with those obtained from coagulation-flotation. The CCGAs were robust in charge neutralization, cell capture, cell attack and destruction. Even at low CTAB dosages, those bubbles could provide large surface area for capturing the M. aeruginosa cells in both unicellular and colonial form compared with the unmodified CTAB-CGAs. The CCGAs reduced 59.5-87.9% of CTAB dosage with the assistance of PACl and the required flotation retention time was largely shortened in comparison with the sedimentation and flotation-based treatment options. This would lead to low treatment cost and sludge production. The present work provides a novel insight into the development of flotation technologies for treating and controlling dense harmful algal blooms.

Ozone-encapsulated colloidal gas aphrons for in situ and targeting remediation of phenanthrene-contaminated sediment-aquifer

The hydrophobic polycyclic aromatic hydrocarbons (PAHs) are apt to adhere tightly to the sediments in aquifer and thus pose great threats to the aquatic environment of groundwater and surface water as well as human health. The present study constructed functionalized microbubbles, named colloidal ozone aphrons (COAs), by dissolving ozone-contained air into the nonionic surfactant (Tween-20) solution at the pressure of 300 kPa for the in situ remediation of phenanthrene (PHE)-contaminated sediments. The COA system aimed at improving the PHE elimination in terms of (i) enhancing the migration and transportation ability of the bubble system in the contaminated aquifer matrix, (ii) accurately desorbing the target hydrophobic contaminants from sediments, and (iii) reinforcing the in situ oxidation degradation immediately after or simultaneously when the PAHs are desorbed into the aqueous phase. Experimental results demonstrated that the COAs exhibited similar characteristics as the classical colloidal gas aphrons (CGAs), including the high stability (half-life time > 200 s), typical morphology and average bubble size (114-162 μm); higher air hold-up of COAs was achieved (i. e. > 20%) compared with the air-microbubbles (1-2%) obtained under the same generation conditions. Although the encapsulated ozone could oxidize the surfactant-layers, the properties and behaviors of COAs were not greatly affected. The surfactant multi-layers endowed the COAs with strong hydrophobic attraction with PHE, great migration capacity and enlarged oxidation area in the sediment matrix. Approximately 96.9% of PHE was removed from the sediments and 84.9% of the overall PHE was oxidized at the high ozone concentration of 0.6 mg/L when the initial PHE concentration was 240.0 μg/kg. The COA-involved remediation technology provided the insight of combining the processes of washing and oxidizing through adopting the particularly conceived microbubbles. The in situ and selective removal of hydrophobic organic contaminants from sediments in aquifer was well achieved in this study.

Polyaluminum chloride-functionalized colloidal gas aphrons for flotation separation of nanoparticles from water

The present work used the coagulative colloidal gas aphron (CCGA)-involved flotation as a robust technology to efficiently remove the typical engineered nanoparticles - silica nanoparticles (SNPs) from water. The inorganic polymer coagulant - polyaluminum chloride (PACl) was used to surface-functionalize the zwitterionic surfactant (C15B)-based CGAs. Results denote that the physicochemical conditions of PACl/C15B mixed solution markedly influenced the flotation behaviors by changing the properties of CCGAs. The C15B molecules showed different dissociated states and interaction behaviors with Al species with the variation of pH. The addition of salt into the PACl/C15B mixed solution decreased the foamability of solution, and the bubbles collapsed before they could efficiently capture SNPs in their rising trajectory. The optimum SNP removal (87.2%) was obtained when the pH and the additional ionic strength of PACl/C15B mixed solution were ∼4.7 and ≤ 1.0 g NaCl/L, individually, and the pH of SNP suspension was ∼9.4. Importantly, modifying PACl on microbubbles took greater advantages than directly using it as coagulant in terms of SNP removal and PACl utlization. The CCGAs were robust since their colloidal attraction and collision efficiency with SNPs were simultaneously enhanced. The PACl was more efficiently utilized during flotation whilst the regular chemical-dosing unit was omitted.

Using Fe(III)-coagulant-modified colloidal gas aphrons to remove bio-recalcitrant dissolved organic matter and colorants from cassava distillery wastewater

Efficient removal of bio-recalcitrant dissolved organic matter (DOM) and colorants is essential for discharging or reusing the distillery wastewater. The present work adopted a novel microbubble system - Fe(III)-coagulant-modified colloidal gas aphrons (CGAs) in flotation as tertiary treatment of the bio-chemically treated cassava distillery wastewater. Approximately 93% of bio-recalcitrant color and around 79% of dissolved organic carbon (DOC) were removed at the initial pH of 9.0 and 7.1, individually. The modified CGAs exhibited strong ability of complexation and electrostatic attraction of the polyanions of DOM and colorants. But the 1-10 kDa DOM was found to be resistant to the CGA capture. Compared with directly dosing coagulant, the Fe(III)-coagulant-modified CGAs consumed ∼47% and ∼21% less coagulant to achieve the optimum decoloration efficiency and DOC removal, respectively. In the flotation with Fe(III)-coagulant-modified CGAs, the coagulant-dosing system could be omitted while the coagulant utilization was improved.