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    Investigating hydrodynamic cavitation as an efficient means for removal of per- and polyfluoroalkyl substances from solution
    (Elsevier BV, 2024-11-11) Kabiri S; Jafarian M; Navarro DA; Whitby CP; McLaughlin MJ
    With nearly five decades of per- and polyfluoroalkyl substances (PFASs) being associated with firefighting and industrial activities, these compounds inevitably accumulate in both ground and surface water. PFAS contamination in water has emerged as a significant environmental and public health concern, particularly perfluorooctanesulfonic acid (PFOS), which is often found in higher concentrations compared to other PFAS and has more pronounced adverse health effects. Addressing PFAS contamination requires treating large volumes of water, making technologies that rapidly separate and concentrate PFASs highly favoured. The strong surface activity of PFAS, such as PFOS, enables them to generate colloidal gas aphrons (CGAs) during high shear mixing of their aqueous solutions, where PFASs can be separated and collected as foam. This study aims to evaluate the effectiveness of high shear mixing in separating PFOS from solution, leveraging its accumulation at air–water interfaces. High shear-assisted PFOS separation was tested by varying parameters like rotational speed (4000 to 10,000 rpm), mixing time (30 s to 30 min), and the effect of electrolytes. Results showed greater PFOS separation in the presence of electrolytes, particularly monovalent cations like Na+, compared to divalent cations such as Ca2+, due to the creation of more stable CGAs with smaller sizes. At a mixing rate of 6000 rpm, 85 % of PFOS was removed in 30 s from a highly contaminated PFOS solution (10 mg/L), with over 95 % separation after 5 mixing cycles. While high-shear mixing was efficient in PFOS separation from highly contaminated solutions it was less efficient for low-level contaminated solutions (less than 1 mg/L). These results suggest that hydrodynamic cavitation induced by high-shear mixing seems promising for enhancing the separation of PFOS from heavily contaminated solutions. This technique could serve as a standalone method or be integrated with other PFAS removal technologies to enhance the overall efficiency of PFAS removal from polluted water sources.
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    Particle coating using foams and bubbles : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Bioprocess Engineering at Massey University, Palmerston North, New Zealand
    (Massey University, 2017) Singh, Shakti
    This thesis investigates powder coating using foams or bubbles. The work initially started on foams. Wettability studies first showed that foams can be used to coat powders. Research then focussed on the fundamental unit of foams, the bubble. An experimental apparatus was designed and built to perform particle-bubble impact studies in air. Bubble solutions comprised of water, hydroxypropyl methylcellulose (HPMC) and sodium dodecyl sulphate (SDS). Four distinct physical behaviours occur when a particle impacts a bubble: (i) particle capture, (ii) particle slide-off, (iii) bubble burst and (iv) bubble self-healing. The rate processes that occur during particle-bubble impact are; (i), surface area creation by bubble film stretching; (ii), delivery of surface active molecules to the newly created surface; and (iii), stress dissipation as the film is stretched. The ability of the solutions to do (ii) and (iii) are highly complex relying on the thermodynamic equilibrium of the solutions and the local perturbations in the near surface region. Therefore, establishing quantitative boundaries of behaviour is a difficult exercise. It is proposed that, for solutions above the cac or cmc, (critical aggregate concentration, critical micelle concentration) where self-healing occurs, the rate of (ii) > rate of (i) and the rate of (iii) > rate of (i). For solutions below the cac, where bursting occurs, the opposite is true, the rate of (ii) < rate of (i) and the rate of (iii) < rate of (i). Intermediate behaviours such as slide-off of capture are within the range of self-healing behaviours, but where the energy of the particle is insufficient to penetrate the bubble. These behaviours are explained by complexation theory. For SDS concentration ≥ cac and cmc, small aggregates of SDS and HPMC locally supply surfactant to the surface of the stretching bubble film. This maintains low surface tension stress and self-healing results. For SDS concentrations < cac, self-healing occurs because the complexation is a HPMC-SDS sea containing SDS islands. The HPMC-SDS sea structure is sufficiently interlinked to simply stretch with the film, while the SDS islands de-aggregate quickly in the near surface region to supply the newly created surface with surfactant. Here the supply rate is faster than the stretching and so the new surface area is populated with SDS molecules. In contrast bursting occurs when the complexation is HPMC-SDS islands in a SDS sea. Here, the rapid film extension is so fast that the islands of HPMC-SDS become isolated and the film loses structural homogeneity. Furthermore, the rate of new surface creation is too fast for diffusion of SDS molecules from the bulk ‘sea’ to the newly created surface. This results in both an inhomogeneous structure and local increases in surface tension, causing both stress concentration in the film and the Marangoni effect. Extensional viscosity measurements, conducted in collaboration with Monash University, Australia, produced three behaviours as solutions were thinned: bead-on-string, blob and long-lived filaments. Solutions which produced long lived filaments here correspond to those that self-healed during particle impact (when the impact velocity was sufficient). It is proposed that this long-lived filament behaviour is due to the SDS concentration being > cmc, where the SDS micelles act like ‘ball-bearings’ between the extending HPMC chains. Coatings were characterised by SEM and gravimetric measurement. Cross-sectional imaging of the soft particle that penetrated self-healing bubbles were found to have a continuous coating layer around the particle. Surface topography of bubble coated particles were compared with classical droplet coated single particles from the literature. Bubble coated particles were found to be smoother than the droplet coated particle. The knowledge gained here was used to suggest how an industrial-scale particle coater using bubbles may be designed.