Role of Humic Acid in Enhancing Dissolved Air Flotation for the Removal of TiO2 Nanoparticles

A two-stage dissolved air flotation saturator configuration for significant microbubble improvement

The presence of suspended contaminants in water and wastewater, such as algae, colloids, fats and oil, necessitates the use of systems such as dissolved air flotation (DAF) for their removal. In the current study, a novel setup has been proposed for bubble enhancement. An industrial scale (pilot) DAF system was tested at saturator pressures of 3-7 atm and flow rates of 5-20 L/min in three different configurations, namely, empty, packed, and the innovative two-stage (TS) configuration. In the TS system, after the nucleation of micro bubbles, the water is returned to the saturator to undergo pressurization for a second time before it is passed through the nozzle once more and is released. The results show that the highest volume of released air as well as the smallest microbubbles are seen in the TS configuration, followed by packed mode, with the empty configuration showing the least favourable results. Moreover, the bubbles produced at the lowest residence time and pressure (3 atm) with the novel setup are better than the bubbles produced by the standard configuration, even with pressures as high as 7 atm. Thus, the novel TS setup can allow for significantly lower energy requirements and lower capital costs. For real-world application of the TS system, the feed for the saturator could be extracted from within or near the contact zone, i.e. where the bubbles are released in the DAF tank.

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.

Removal of flocculated TiO2 nanoparticles by settling or dissolved air flotation

Engineered nanoparticles of TiO₂ (TiO₂-NPs) are used in the industry for a great number of applications. After their usage, the particles end up in aquatic environments, contaminating supply waters and watercourses. Bench-scale studies report removal of TiO₂-NPs (450 nm, the mean volumetric diameter) by flocculation followed by settling or by dissolved air flotation (4 bar saturation pressure and 30% recycling ratio). Floc formation was conducted after heterocoagulation with iron hydroxide (30-40 mg L^-1Fe³⁺⁾ and gelatinized corn starch (10-20 mgL^-1) as flocculant, at pH 7. Particle size distribution and zeta potential, removal efficiencies as a function of time and microphotography of flocs were analyzed. Mechanisms involve ferric hydroxide precipitation, heterocoagulation with the nanoparticles and flocculation of the loaded carrier precipitates with gelatinized starch. Best results showed removals between 95-100% of TiO₂-NPs, either by settling or flotation after 5 min. Clear treated waters with low turbidity < 3 nephelometric turbidity units (NTU) and TiO₂-NPs concentrations <1 mg L^-1 were obtained. A practical advantage in DAF was the higher solids content (1.9% w/w) of the sludge, when compared to settling (0.7% w/w). This would facilitate the sludge dewatering and disposal, but DAF has the disadvantage of the poor efficiency at high concentration of the nanoparticles of titanium oxide (>100 mg L^-1). Conversely, the removal by settling of the flocs increased at high dosages. It is believed that both processes are sustainable in terms of reagents and the removal efficiencies of TiO₂ nanoparticles from water.

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.

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 pH dependent surface charging and points of zero charge. VIII. Update

A critical review of the points of zero charge (PZC) obtained by potentiometric titration and of isoelectric points (IEP) obtained by electrokinetic measurements. The results from the recent literature are presented with experimental details (temperature, method, type of apparatus, etc.), and they are compared with the zero points of similar materials reported in older publications. Most studies of PZC and IEP reported in the recent papers were carried out for metal oxides and hydroxides, especially alumina, iron oxides, and titania, and the results are consistent with the PZC and IEP of similar materials reported in older literature, and summarized in previous reviews by the same author. Relatively few studies were carried out with less common materials, and IEP of (nominally) VO₂ and BN have been reported for the 1st time.

Comparison of Bubble Size Distributions Inferred from Acoustic, Optical Visualisation, and Laser Diffraction

Bubble measurement has been widely discussed in the literature and comparison studies have been widely performed to validate the results obtained for various forms of bubble size inferences. This paper explores three methods used to obtain a bubble size distribution—optical detection, laser diffraction and acoustic inferences—for a bubble cloud. Each of these methods has advantages and disadvantages due to their intrinsic inference methodology or design flaws due to lack of specificity in measurement. It is clearly demonstrated that seeing bubbles and hearing them are substantially and quantitatively different. The main hypothesis being tested is that for a bubble cloud, acoustic methods are able to detect smaller bubbles compared to the other techniques, as acoustic measurements depend on an intrinsic bubble property, whereas photonics and optical methods are unable to “see” a smaller bubble that is behind a larger bubble. Acoustic methods provide a real-time size distribution for a bubble cloud, whereas for other techniques, appropriate adjustments or compromises must be made in order to arrive at robust data. Acoustic bubble spectrometry consistently records smaller bubbles that were not detected by the other techniques. The difference is largest for acoustic methods and optical methods, with size differences ranging from 5–79% in average bubble size. Differences in size between laser diffraction and optical methods ranged from 5–68%. The differences between laser diffraction and acoustic methods are less, and range between 0% (i.e., in agreement) up to 49%. There is a wider difference observed between the optical method, laser diffraction and acoustic methods whilst good agreement between laser diffraction and acoustic methods. The significant disagreement between laser diffraction and acoustic method (35% and 49%) demonstrates the hypothesis, as there is a higher proportion of smaller bubbles in these measurements (i.e., the smaller bubbles ‘hide’ during measurement via laser diffraction). This study, which shows that acoustic bubble spectrometry is able to detect smaller bubbles than laser diffraction and optical techniques. This is supported by heat and mass transfer studies that show enhanced performance due to increased interfacial area of microbubbles, compared to fine bubbles.

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.

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.

Coagulation behavior of kaolin-anionic surfactant simulative wastewater by polyaluminum chloride-polymer dual coagulants

In this study, polyaluminum chloride (PAC) and cationic polymers were used to treat kaolin suspension in the presence of sodium dodecyl benzene sulfonate (SDBS). Effects of PAC dosage, pH, and rotation rate on the coagulation efficiency and floc properties were studied. And the interaction of chemicals and kaolin-SDBS was discussed. Results showed that dual coagulants could decrease the influence of SDBS on the turbidity removal compared with PAC. PAC + polyacrylamide dual coagulant showed superior performance, and the maximal removal ratios of turbidity and dissolved organic carbon were 98.5 and 42.2%, respectively. Optimal coagulation performance was achieved at pH 5-7, where charge neutralization of Al hydrolysates and bridging of polyacrylamide were the primary mechanisms. And flocs with compact structure and small size were formed. Flocs coagulated by PAC were prone to be broken at the pH of raw water after introducing high rotation rate. After dosing polyacrylamide, floc size was enhanced under alkaline condition. Meanwhile, flocs showed stronger recoverability and an open structure because the regeneration mechanism was mainly the bridging effect of polyacrylamide.

BS12-assisted flotation for the intensification of SNPs separation from CMP wastewater using a novel flotation column

In view of the extremely small size, high stable dispersion and intricate colloidal nature of silica nanoparticles (SNPs) in chemical mechanical polishing (CMP) wastewater, they might not only have hazards for environment and human health, but also cause low separation efficiency by classical water-treatment processes. Thus, it would be an important challenge to develop an efficient flotation technology for the separation SNPs. For this propose, this paper firstly presented the interaction between SNPs and dodecyl dimethyl betaine (ambient-friendly surfactant). Secondly, a novel flotation column was developed for strengthening interfacial adsorption by micro-bubbles and enhancing foam drainage by internal of regular-decagonal hollow frustum (RHF). One vital finding was that the mixture of micro-bubbles and macro-bubbles was conducive to improving the flotation performance. Under the suitable operating conditions, the enrichment ratio (E) and recovery percentage (R) of SNPs could reach 30.4±1.5 and 90.8±4.5%, respectively. The great E and R were obtained simultaneously, revealing a good participation of RHF in the flotation. Without a doubt, owing to the low chemical reagent addition and the high flotation performance, it was clear that our flotation has huge implications for the separation of nanoparticles from their wastewaters.

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

The treatment of nanoparticle (NP) polluted aqueous suspensions by flotation can be problematic due to the low probability of collision between particles and bubbles. To overcome this limitation, the present work focuses on developing an enhanced flotation technique using the surface-functionalized microbubbles - colloidal gas aphrons (CGAs). The CGA generator was adapted to be air flow rate controlled based on the classical Sebba system; thus it could be well adopted in a continuous flotation process. Cetyl trimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) were employed for CGA creation. Positively surface-charged CTAB-CGAs (∼44.1 μm in size) and negatively surface-charged SDS-CGAs (∼42.1 μm in size) were produced at the optimum stirring speed of 8000 rpm. The half-life of CGAs varied from 100 s to 340 s under the tested conditions, which was largely sufficient for transferring CGAs from bubble generator to flotation cell. The air flow led to less stable CTAB-CGAs but apparently enhanced the stability of SDS-CGAs at higher air flow rates. In the presence of air flow, the drainage behavior was not much related to the type of surfactants. The continuous CGA-flotation trials highlighted the effective separation of silica nanoparticles - the removal efficiencies of different types of SiO₂ NPs could reach approximately 90%-99%; however, at equivalent surfactant concentrations, no greater than 58% of NPs were removed when surfactants and bubbles were separately added into the flotation cell. The SiO₂ NPs with small size were removed more efficiently by the CGA-flotation process. For the flotation with CTAB-CGAs, the neutral and basic initial SNP suspension was recommended, whereas the SDS-CGAs remained high flotation efficiency over all investigated pH. The good performance of CGA-flotation might be interpreted: most of the surfactant molecules well covered/coated on the surfaces of stable CGAs and thus fully contacted with NPs, resulting in the efficient utilization of surfactants.