Optimization of Surface Treatment Using Sodium Hypochlorite Facilitates Coseparation of ABS and PC from WEEE Plastics by Flotation

A hydrophilic/hydrophobic switch on polymer surface triggered by calcite towards separation of hazardous PVC from plastic mixtures

Hydrophilic modification of polymer surfaces is crucial for the emerging flotation separation of plastic waste towards resources recycling. In this study, we investigated a novel hydrophilic regulation induced by calcite to modify the surface wettability of PVC, ABS, PS, PC, and PET. The interactions between calcite and plastic molecules contributed to the selective formation of hydrophilic calcite shells on plastic surfaces. Due to the strong polarity of C-Cl bonds on PVC surfaces, calcium ions tended to act as bridges between calcite particles and PVC, and a non-covalent interaction leaded to strong physical adsorption of calcite on PVC surfaces. However, other plastics adsorbed fewer calcite than PVCs owing to weaker interactions. Like a hydrophilic/hydrophobic switch, ultrasonic cleaning erased hydrophilic calcite shells to restore the hydrophobicity of hydrophilic plastics without destroying the chemical structure of polymers. The recovery and purity of PVC were 100 % and 96.3 %, under the conditions of calcite dosage 1.5 g L-1, coating time 15 min, coating temperature 50 °C, pH 10, and rotational speed 900 r min-1. Its attractive applicability and reversibility were further demonstrated by the flotation separation of actual plastic mixtures and the eversible floatability of sinking plastics.

Synthesis of Polyphosphate Flame Retardant Bisphenol AP Bis(Diphenyl Phosphate) and Its Application in Polycarbonate/Acrylonitrile-Butadiene-Styrene

The flame retardant bisphenol AP bis(diphenyl phosphate) (BAPDP) is synthesized from triphenyl phosphate and bisphenol AP via transesterification, producing a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) with a high flame retardancy and thermal stability. In this study, flame-retardant PC/ABS blends with various BAPDP contents are prepared, and their flame retardancy is studied using the limit oxygen index, vertical combustion, thermogravimetric analysis, and cone calorimeter testing. With a BAPDP content of 20 wt%, the product exhibits a limiting oxygen index of 25.4% and achieves the UL-94 V-0 grade, with a thermal deformation temperature of 72.6 °C. BAPDP improves the flame retardancy of the PC/ABS blends and exhibits fewer adverse effects on the thermal deformation temperature than other commercial flame retardants at the same concentration.

Development of a Zr-Based Metal-Organic Framework (UiO-66) for a Cooperative Flame Retardant in the PC/ABS

Polycarbonate/acrylonitrile butadiene styrene (PC/ABS) blends are widely used as engineering plastic alloys; however, they have a low fire safety level. To improve the flame-retardant property of PC/ABS, a zirconium-based metal-organic framework material (UiO-66) was synthesized with zirconium chloride and terephthalic acid and used as a flame-retardant cooperative agent. Its flame-retardant performance and mode of action in the PC/ABS blends were carefully investigated. The results showed that UiO-66 had good thermal stability and delayed the pyrolysis of the materials, thus significantly enhancing the efficiency of intumescent flame retardants. By compounding 7.0 wt% hexaphenyloxy-cyclotri-phosphazene (HPCTP) with 3.0 wt% UiO-66, the PC/ABS blends reached a limiting oxygen index value of 27.0% and V0 rating in the UL-94 test, showing significantly improved resistance to combustion dripping. In addition, UiO-66 enhanced the smoke and heat suppression characteristics of the intumescent flame-retardant materials. Finally, the flame-retardant mode of action in the blends was indicative of UiO-66 having a cooperative effect on the flame-retardant performance of PC/ABS/HPCTP materials. This work provides good ideas for further development of the flame-retardant ABS/PC.

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.

Recovery of non-metallic useable materials from e-waste

Tremendous amounts of electric and electronic wastes (e-waste) are generated daily, and their indiscriminate disposal may cause serious environmental pollution. The recovery of non-metallic materials from e-waste is a strategy to not only reduce the volume of e-waste but also avoid pollutant emissions produced by indiscriminate disposal of e-waste. Pyrolysis, sub/supercritical water treatment, chemical dissolution, and physical treatment (e.g., ball milling, flotation, and electrostatic separation) are available methods to recover useable non-metallic materials (e.g., resins, fibers, and various kinds of polymers) from e-waste. The e-waste-derived materials can be used to manufacture a large variety of industrial and consumer products. In this regard, this work attempts to compile relevant knowledge on the technologies that derive utilizable materials from different classes of e-waste. Moreover, this work highlights the potential of the e-waste-derived materials for various applications. Current challenges and perspectives on e-waste upcycling to useable materials are also discussed.

Flotation separation of hazardous polyvinyl chloride towards source control of microplastics based on selective hydrophilization of plasticizer-doping surfaces

As the single largest chlorine source of plastics, hazardous polyvinyl chloride (PVC) has become an increasing environmental concern with the rapid microplastics accumulation. An advanced separation method is advocated to purify waste PVC plastics, optimize physical recycling, and protect aquatic and terrestrial environment safety. In this study, we proposed a novel scheme for the flotation separation of PVC plastics with diverse plasticizer contents (PVCs) via regulating hydrophilicity based on a selective ferric deposition. Rigid PVCs were prone to loading ferric ions and generating hydrophilic shells than flexible PVCs. Plasticizers can diffuse freely through the interior and surface of PVC plastics. Abundant plasticizers thereby overlaid the surface of flexible PVC and shielded PVC matrix from ferric ions. By regulating the ferric concentration, the wettability of PVCs was adjusted to separate rigid and flexible PVCs by froth flotation. Waste PVCs could also be separated from each other through the compound process of ferric deposition and flotation, further confirming its feasibility and stability. Thus far, this study supplies distinctive insights into the wettability regulation of plasticizer-doping PVC surfaces, contributes a pioneering hydrophilization method to PVCs separation and recycling, and mitigates hazardous PVC microplastics by source control.

Rational design of AIE-based fluorescent probes for hypochlorite detection in real water samples and live cell imaging

As one kind of important disinfectant and reactive oxygen species (ROS), hypochlorite (ClO^-), plays vital roles in both water treatment and cell homeostasis. In this work, by decorating a series of groups with different electron donating and withdrawing properties on tetraphenylethene (TPE), four aggregation-induced emission (AIE)-based fluorescent probes containing C˭C double bonds as the potential reaction sites named Probe A, B, C and D were constructed, and their sensing performance for ClO^- was systematically studied. The results showed that the substituents can not only effectively tune the photophysical properties of the probes, but also make a significant impact on their sensing performance for ClO^-. Combined with the theoretical calculation results, it can be inferred that the reactivity of the probes for ClO^- can be greatly enhanced with the increase of electron cloud density on the C˭C double bonds by the introduction of strong electron-donating group (EDG) and electron-withdrawing group (EWG) adjacent to the double bonds. Finally, the best performing Probe D was selected and then successfully applied to ClO^- detection in real water samples and live cell imaging.

Insights into Adsorption of Humic Substances on Graphitic Carbon Nitride

Graphitic carbon nitride (CN) has been widely used in environmental pollution remediation. However, the adsorption of organic compounds on CNs, which has practical significance for the environmental application of CNs, is poorly understood. For the first time, this study systematically investigated the adsorption behaviors and mechanisms of humic substances (HSs), i.e., humic acid (HA) and fulvic acid (FA), on CNs derived from four typical precursors. Intriguingly, CN derived from urea (CN-U) showed a great capacity for HS adsorption due to its porous structure and large surface area, with maximum adsorption amounts of 73.24 and 51.62 mgC/g for HA and FA, respectively. The formation, influencing factors, and relative contributions of multiple interactions to HS adsorption on CNs were thoroughly elucidated. HS adsorption on CNs was mainly mediated by electrostatic interactions, π-π interactions, and H-bonding. The dominance of electrostatic interactions resulted in HS adsorption being highly dependent on pH and ionic strength. HS components with high aromaticity and high molecular weight were preferentially adsorbed due to π-π interactions. These multiple interactions were largely affected by amino groups and tri-s-triazine units of CNs, as well as the moieties of aromatic rings and oxygen-containing groups of HSs.

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.

Surface Reactions in Selective Modification: The Prerequisite for Plastic Flotation

Improper disposal of waste plastic has caused much environmental pollution, but plastic recycling can reduce the amount of new and residual waste plastic in the environment through source control. Plastic flotation can separate waste plastics with similar physical and chemical properties, which suggests its promising application in plastic recycling. With the help of the different hydrophilicities waste plastic can be separated by flotation, and hydrophilization can be accomplished by surface modifications. However, no systematic studies addressing these surface reactions have been published yet, and such modifications are a prerequisite for plastic flotation. In this critical review, we not only summarize the various modification mechanisms, including physical regulation, surface oxidation, surface degradation, dechlorination, and coating, but also have reasonably added additional information for some reactions covering surface reconstruction, plastic degradation, polymer stability, wastewater treatment, soil remediation, and chemical recycling of plastic. An entirely novel concept, the "plastic gene", is also proposed to elaborate on some contradictory results. Plastic flotation with clear surface reactions may promote plastic recycling and thereby control waste plastic at the source, save energy, and reduce microplastics. We also predict challenges for clean, efficient, and practical surface modifications and plastic flotation.

Application of froth flotation in the separation of polyvinyl chloride and polycarbonate for recycling of waste plastic based on a novel surface modification

The complicated stream of waste plastic impedes the recycling of polyvinyl chloride (PVC) and polycarbonate (PC), which can be settled by flotation separation. We proposed a novel chlorine dioxide (ClO₂) pretreatment to assist the separation of PVC and PC by froth flotation, and clarified possible surface reactions of hydrophilic PC by contact angles, scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The hydrolysis and further rearrangement of carbonic esters (O(CO)O) may be deemed as the main reason for hydrophilic PC, introducing oxygenated functional groups, such as hydroxyl groups (COH), carboxyl groups (COOH), and tiny acyl chloride (ClCO), on PC surfaces. The robustness of this process was proved by efficient flotation separation of PVC and PC under various conditions of size fractions, frother concentration, mass ratio, and flotation time. The optimal pretreatment conditions for flotation separation of PVC and PC are temperature of 70 °C, ClO₂ concentration of 0.5 g/L, and treatment time of 70 min. The optimal recovery and purity of PC in sunken plastic can stably maintain 97% and 99%, respectively. Compared with waste plastic, raw PC embraces a high floatability after ClO₂ pretreatment, revealing that ageing is conducive to surface modification.

Optimizing green ferrate (VI) modification towards flotation separation of waste polyvinylchloride and acrylonitrile-butadiene-styrene mixtures

The recycling of waste plastics is of considerable significance with environmental and economic benefits, while available separation approaches have been considered as a major bottleneck for its widespread application. Thus, we proposed a simple method, flotation along with surface modification, to separate waste acrylonitrile-butadienestyrene and polyvinylchloride mixtures. Single-factor experiment was conducted to determine the critical parameters in surface modification. Surface response methodology using Box-Behnken Design was performed to optimize separation performance. The quadratic models were generated to predict the floatability of acrylonitrile-butadienestyrene and the difference between the floatability of polyvinylchloride and acrylonitrile-butadienestyrene. The model was also utilized to determine optimized conditions by desirability approaches. The optimized conditions were: concentration = 0.18 M, temperature = 75.00 °C, treatment time = 11.50 min along with stirring rate = 200 rpm. The efficient separation of acrylonitrile-butadienestyrene and polyvinylchloride was achieved, yielding recovery of 98.40% and purity of 98.43%. The experimental responses well agreed with predicted values, demonstrating the accuracy of the prediction model. The formed hydrophilic groups, coated iron oxide, and signs of corrosion were confirmed as the major mechanism for the selective surface hydrophilization of acrylonitrile-butadienestyrene. Consequently, this method is feasible for separation of waste acrylonitrile-butadienestyrene and polyvinylchloride mixtures, and can be expected to promote the sustainable recycling of waste plastics.

Combination of sodium hypochlorite pretreatment and flotation towards separation of polycarbonate from waste plastic mixtures

This study developed a novel method, surface pretreatment using sodium hypochlorite along with flotation, to facilitate separation of waste polycarbonate from plastic mixtures for recycling. Surface pretreatment was observed that has an obviously negative effect on the floating ratio of polycarbonate and the floating ratio of poly-methyl-methacrylate, polystyrene, and polyvinylchloride was not affected in flotation, and this difference in floating ratio can be expected to separate polycarbonate from plastic mixtures. The optimum conditions obtained included sodium hypochlorite concentration of 0.05 M, pretreatment temperature of 70.0 °C, pretreatment time of 60.0 min, frother dosage of 10.8 mg/L, and flotation time of 4.0 min. Under optimum conditions, polycarbonate was separated effectively from multiple plastic mixtures, and the purity and recovery were 99.8% and 100.0%, respectively. The major mechanism of surface pretreatment was ascertained by the aid of Fourier transform infrared, scanning electron microscope, energy dispersive spectrometer, and X-ray photoelectron spectroscopy, and the hydrophilic groups, pitting, and protuberances introduced on polycarbonate surface caused the reduced floating ratio of polycarbonate. Accordingly, this method can be expected to improve the recycling quality of waste plastics, and provides technological insights in the environmentally friendly disposal of waste plastics.