Understanding the activation mechanism of Ca2+ ion in sodium oleate flotation of spodumene: A new perspective

Adsorption of glycine at the anatase TiO2/water interface: Effects of Ca2+ ions

Adsorption reactions of amino acids (AAs) on TiO₂ nanoparticles (NPs) play an important role in the available nutrients in soils and sediments. The pH effects on glycine adsorption have been studied, but little is known about its coadsorption with Ca²⁺ at the molecular level. Combined attenuated total reflectance Fourier transform infrared (ATR-FTIR) flow-cell measurements and density functional theory (DFT) calculations were used to determine the surface complex and corresponding dynamic adsorption/desorption processes. The structures of glycine adsorbed onto TiO₂ were closely associated with its dissolved species in the solution phase. The presence of Ca²⁺ exerted different influences on glycine adsorption within pH 4-11, thus affecting its migration rate in soils and sediments. The mononuclear bidentate complex at pH 4-7, involving the COO^- moiety of zwitterionic glycine, remained unchanged in the absence and presence of Ca²⁺. At pH 11, the mononuclear bidentate complex with deprotonated NH₂ can be removed from the TiO₂ surface upon coadsorption with Ca²⁺. The bonding strength of glycine on TiO₂ was much weaker than that of the Ca-bridged ternary surface complexation. Glycine adsorption was inhibited at pH 4 but was enhanced at pH 7 and 11.

Surface change of microplastics in aquatic environment and the removal by froth flotation assisted with cationic and anionic surfactants

Microplastics (MPs) are increasingly released into the environment due to the widespread usage and improper management of plastics. Considerable research efforts have been devoted to the remediation of MPs. Froth flotation has been demonstrated as an effective method to remove MPs in water and sediment. However, there is a lack of knowledge on the regulation of the hydrophobicity/hydrophilicity of MPs surfaces. We found that exposure to the natural environment resulted in the increased hydrophilicity of MPs. The flotation efficiencies of polyvinyl chloride (PVC), polypropylene (PP), polystyrene (PS), and polyethylene glycol terephthalate (PET) MPs decreased to zero after six months of natural incubation in rivers. According to various characterizations, the hydrophilization mechanism is mainly correlated with surface oxidation and the deposition of clay minerals. Inspired by surface wettability conversion, we applied surfactants (collectors) to enhance MPs hydrophobicity and flotation efficiency. Anionic sodium oleate (NaOL) and cationic dodecyl trimethyl ammonium chloride (DTAC) were used to regulate surface hydrophobicity. The effects of collector concentration, pH, conditioning time, and metal ions on MPs flotation were thoroughly elucidated. Characterizations and adsorption experiments were performed to describe the heterogeneous adsorption of surfactants on MPs surfaces. The interaction between surfactants and MPs was explained through density functional theory (DFT) simulations. The dispersion energy between hydrophobic hydrocarbon chains attracts collectors on the MPs surface, and the collector molecules wrap and laminate to MPs surfaces. Flotation using NaOL exhibited a higher removal efficiency, and NaOL was environmentally friendly. Subsequently, we investigated the activation of Ca²⁺, Fe³⁺, and Al³⁺ to further improve the collecting efficiency of NaOL. Under the optimized conditions, MPs in natural rivers could be removed by froth flotation. This study shows the great promise of froth flotation for the application of MPs removal.

Insight into the microplastics release from disposable face mask: Simulated environment and removal strategy

The fight against the COVID-19 epidemic significantly raises the global demand for personal protective equipment, especially disposable face masks (DFMs). The discarded DFMs may become a potential source of microplastics (MPs), which has attracted much attention. In this work, we identified the detailed source of MPs released from DFMs with laser direct infrared spectroscopy. Polypropylene (PP) and polyurethane (PU) accounted for 24.5% and 57.1% of released MPs, respectively. The melt-blown fabric was a dominant MPs source, however, previous studies underestimated the contribution of mask rope. The captured polyethylene terephthalate (PET), polyamide (PA), polyethylene (PE), and polystyrene (PS) in airborne only shared 18.4% of released MPs. To deepen the understanding of MPs release from medical mask into the aquatic environment, we investigated the effects of environmental factors on MPs release. Based on regression analysis, the effects of temperature, incubation time, and wearing time significantly affect the release of MPs. Besides, acidity, alkalinity, sodium chloride, and humic acid also contributed to the MPs release through corroding, swelling, or repulsion of fibers. Based on the exposure of medical mask to simulated environments, the number of released MPs followed the order: seawater > simulated gut-fluid > freshwater > pure water. Considering the risk of MPs released from DFMs to the environment, we innovatively established a novel flotation removal system combined with cocoamidopropyl betaine, achieving 86% removal efficiency of MPs in water. This work shed the light on the MPs release from DFMs and proposed a removal strategy for the control of MPs pollution.