Flotation separation of waste plastics for recycling-A review

Recycling of Electrical Cables-Current Challenges and Future Prospects

Civilization and technical progress are not possible without energy. Dynamic economic growth translates into a systematic increase in demand for electricity. Ensuring the continuity and reliability of electricity supplies is one of the most important aspects of energy security in highly developed countries. Growing energy consumption results not only in the need to build new power plants but also in the need to expand and increase transmission capacity. Therefore, large quantities of electric cables are produced all over the world, and after some time, they largely become waste. Recycling of electric cables focuses on the recovery of metals, mainly copper and aluminum, while polymer insulation is often considered waste and ends up in landfills. Currently, more and more stringent regulations are being introduced, mainly environmental ones, which require maximizing the reduction in waste. This article provides a literature review on cable recycling, presenting the advantages and disadvantages of various recycling methods, including mechanical and material recycling. It has been found that currently, there are very large possibilities for recycling cables, and intensive scientific work is being carried out on their development, which is consistent with global climate policy.

Chemical recycling of polyolefins: a closed-loop cycle of waste to olefins

The unsuitable disposal of plastic wastes has caused serious environmental pollution, and finding a green manner to address this problem has aroused wide concern. Plastic wastes, especially polyolefin wastes, are rich in carbon and hydrogen, and chemical recycling shows distinct advantages in their conversion into olefins and realizes a closed-loop cycling of plastic wastes. Plastic wastes should be labeled before disposal. The necessity for, and methods of, pretreatment are introduced in this paper and the whole recycling process of polyolefin wastes is also summarized. As the core technology pyrolysis, including thermal, catalytic and solvolysis processes, is introduced in detail due to its potential for future development. We also briefly describe the feasible strategies of pyrolytic oil refining and life cycle assessment of the chemical recycling process. In addition, suggestions and perspectives concerning the industrial improvement of polyolefin chemical recycling are proposed.

Study on the impact of photoaging on the generation of very small microplastics (MPs) and nanoplastics (NPs) and the wettability of plastic surface

Photoaging is one of the important reasons for the sharp increase of waste plastics, especially microplastics (MPs), in the environment. Therefore, studying the photoaging of plastics is of great significance for controlling plastic pollution from the source. Nevertheless, there are few studies on plastic photoaging from the perspective of polymer structure. Besides, the capacity of different types of plastics to generate MPs with small particle size is relatively little studied. In view of this, we conducted a preliminary study on the capacity of different types of plastics to generate MPs using flow cytometry. We also studied the impact of photoaging on different types of plastics. The results showed that flow cytometry can be used to quantify very small MPs (1-50 μm) and nanoplastics (NPs) (< 1 μm). Furthermore, photoaging often accelerates the generation of MPs and roughens plastic surface. Besides, photoaging can introduce some oxygen-containing groups onto plastic surface, thereby changing the wettability of plastic surface. Moreover, benzene rings in polymer structures may inhibit the generation of MPs but may promote the transformation of the plastic surface from hydrophobic to hydrophilic during photoaging. Although the changes in properties of plastics caused by photoaging have adverse effects on the environment, some new processes and materials still can be developed based on photoaging of plastics. This work contributes to a better understanding of the photoaging of plastics from the perspective of polymer structure, which has certain positive significance for controlling plastic pollution from the source.

The Key to Solving Plastic Packaging Wastes: Design for Recycling and Recycling Technology

Confronted with serious environmental problems caused by the growing mountains of plastic packaging waste, the prevention and control of plastic waste has become a major concern for most countries. In addition to the recycling of plastic wastes, design for recycling can effectively prevent plastic packaging from turning into solid waste at the source. The reasons are that the design for recycling can extend the life cycle of plastic packaging and increase the recycling values of plastic waste; moreover, recycling technologies are helpful for improving the properties of recycled plastics and expanding the application market for recycled materials. This review systematically discussed the present theory, practice, strategies, and methods of design for recycling plastic packaging and extracted valuable advanced design ideas and successful cases. Furthermore, the development status of automatic sorting methods, mechanical recycling of individual and mixed plastic waste, as well as chemical recycling of thermoplastic and thermosetting plastic waste, were comprehensively summarized. The combination of the front-end design for recycling and the back-end recycling technologies can accelerate the transformation of the plastic packaging industry from an unsustainable model to an economic cycle model and then achieve the unity of economic, ecological, and social benefits.

Enabling the Polymer Circular Economy: Innovations in Photoluminescent Labeling of Plastic Waste for Enhanced Sorting

It is widely accepted that moving from a linear to circular economy for plastics will be beneficial to reduce plastic pollution in our environment and to prevent loss of material value. However, challenges within the sorting of plastic waste often lead to contaminated waste streams that can devalue recyclates and hinder reprocessing. Therefore, the improvement of the sorting of plastic waste can lead to dramatic improvements in recyclate quality and enable circularity for plastics. Here, we discuss current sorting methods for plastic waste and review labeling techniques to enable enhanced sorting of plastic recyclates. Photoluminescent-based labeling is discussed in detail, including UV-vis organic and inorganic photoluminescent markers, infrared up-conversion, and X-ray fluorescent markers. Methods of incorporating labels within packaging, such as extrusion, surface coatings, and incorporation within external labels are also discussed. Additionally, we highlight some practical models for implementing some of the sorting techniques and provide an outlook for this growing field of research.

Waste acrylonitrile butadiene styrene (ABS) incorporated with polyvinylpyrrolidone (PVP) for potential water filtration membrane

Acrylonitrile butadiene styrene (ABS) is one of the most common fused-filament feedstocks for 3D printing. The rapid growth of the 3D printing industry has resulted in huge demand for ABS filaments; however, it generates a large amount of waste. This study developed a novel method using waste ABS to fabricate electrospun nanofiber membranes (ENMs) for water filtration. Polyvinylpyrrolidone (PVP) was employed to modify the properties of waste ABS, and the effect of PVP addition in the range of 0-5 wt% was investigated. The results showed that adding PVP increased the viscosity and surface tension but decreased the conductivity of the precursor solution. After electrospinning, PVP could reduce the number of beads, increase the porosity and fiber diameter, and improve the wettability of the fabricated fibers. Moreover, the bilayer of ABS ENMs achieved a high flux value between 2951 and 48 041 L m^-2 h^-1 and a high rejection rate of 99%. Our study demonstrates a sustainable strategy to convert waste plastics to inexpensive materials for wastewater treatment membranes.

Enhancement of leaching copper by organic agents from waste printed circuit boards in a sulfuric acid solution

Leaching copper from waste printed circuit boards (WPCBs) by hydrometallurgy has always been a hot research topic. At atmospheric pressure, hydrogen peroxide (H₂O₂) was used as an oxidant to study the leaching behavior of copper from WPCBs in sulfuric acid (H₂SO₄) solution with ethylene glycol (EG). To elucidate the leaching mechanism of copper from WPCBs, the effect of various parameters on the leaching performance with or without EG was investigated. The results showed that the copper leaching process from WPCBs in the presence of EG was found to conform to the ash diffusion-controlled shrinking core model according to the kinetic curve and a activation energy of 18.38 kJ/mol. Moreover, the presence of EG strengthened the stability of H₂O_2, improved dispersity and increased electrical activity of WPCBs, which enhanced the leaching of copper from WPCBs in the high leaching temperature (>323.15 K). As a result, apart from the fact that the optimal leaching concentration of H₂O₂ was reduced by the addition of EG, the improved copper leaching efficiency from WPCBs was achieved by the addition of EG, as demonstrated by a maximum copper leaching efficiency of 98.01% and a maximum loss rate of 29.68%. Besides, the mineralogical and morphological properties of leaching residue validated the leaching results. Based on this, our findings confirmed the enhanced leaching performance of copper from WPCBs by EG, which benefited for the efficient recovery of copper from WPCBs.

Application of persulfate-based oxidation processes to address diverse sustainability challenges: A critical review

Over the past years, persulfate (PS) is widely applied due to their high versatility and efficacy in decontamination and sterilization. While treatment of organic chemicals, remediation of soil and groundwater, sludge treatment, disinfection on pathogen microorganisms have been covered by most published reviews, there are no comprehensive and specific reviews on its application to address diverse sustainability challenges, including solid waste treatment, resources recovery and regeneration of ecomaterials. PS applications mainly rely on direct oxidation by PS itself or the reactive sulfate radical (SO₄^•-) or hydroxyl radical (^•OH) from the activation of peroxodisulfate (PDS, S₂O₈^2-) or peroxymonosulfate (PMS, HSO₅^-) in SO₄^•--based advanced oxidation processes (SO₄^•--AOPs). From a broader perspective of environmental cleanup and sustainability, this review summarizes the various applications of PS except pollutant decontamination and elaborates the possible reaction mechanisms. Additionally, the differences between PS treatment and conventional technologies are highlighted. Challenges, research needs and future prospect are thus discussed to promote the development of the applications of PS-based oxidation processes in niche environmental fields. In all, this review is a call to pay more attention to the possibilities of PS application in practical resource reutilization and environmental protection except widely reported pollutant degradation.

Separation of plastic wastes using froth flotation - An overview

Despite various initiatives and efforts, plastic solid waste (PSW) has become a major global problem due to decades of relentless use of plastics. Since non-biodegradable plastics can persist in the environment for hundreds of years, threatening animal and human life, discarding them into the environment is not a viable option. Plastic recycling is a critical research area that requires urgent attention since less than 10% of the seven billion tons of globally generated plastic waste has been recycled so far. With recent technological developments, it is now possible to recycle many types of PSW using a variety of methods. This review provides an overview of the froth flotation technology that is currently being researched for PSW recycling. Fundamental working principles, the current state of the development, and limitations of this technique are reviewed. It is suggested that froth flotation with continuous development has tremendous potential to result in a more efficient and environmentally friendly approach to PSW recycling.

Recent Advances in the Decontamination and Upgrading of Waste Plastic Pyrolysis Products: An Overview

Extensive research on the production of energy and valuable materials from plastic waste using pyrolysis has been widely conducted during recent years. Succeeding in demonstrating the sustainability of this technology economically and technologically at an industrial scale is a great challenge. In most cases, crude pyrolysis products cannot be used directly for several reasons, including the presence of contaminants. This is confirmed by recent studies, using advanced characterization techniques such as two-dimensional gas chromatography. Thus, to overcome these limitations, post-treatment methods, such as dechlorination, distillation, catalytic upgrading and hydroprocessing, are required. Moreover, the integration of pyrolysis units into conventional refineries is only possible if the waste plastic is pre-treated, which involves sorting, washing and dehalogenation. The different studies examined in this review showed that the distillation of plastic pyrolysis oil allows the control of the carbon distribution of different fractions. The hydroprocessing of pyrolytic oil gives promising results in terms of reducing contaminants, such as chlorine, by one order of magnitude. Recent developments in plastic waste and pyrolysis product characterization methods are also reported in this review. The application of pyrolysis for energy generation or added-value material production determines the economic sustainability of the process.

Process development options for electronic waste fractionation to achieve maximum material value recovery

Revised legislation and bans on imports of waste electrical and electronic equipment (WEEE) into many Asian countries for treatment are driving the need for more efficient WEEE fractionation in Europe by expanding the capacity of treatment plants and improving the percentage recovery of materials of economic value. Data from a key stakeholder survey and consultation are combined with the results of a detailed literature survey to provide weighted matrix input into multi-criteria decision analysis calculations to carry out the following tasks: (a) assess the relative importance of 12 process options against the 6 industry-derived in-process economic potential criteria, that is, increase in product quality, increase in recycling rate, increase in process capacity, decrease in labour costs, decrease in energy costs and decrease in disposal costs; and (b) rank 25 key technologies that have been selected as being the most likely to benefit the efficient sorting of WEEE. The results indicate that the first stage in the development of any total system to achieve maximum economic recovery of materials from WEEE has to be the selection and application of appropriate fractionation process technologies to concentrate valuable components such as critical metals into the smallest possible fractions to achieve their recovery while minimising the disposal costs of low-value products. The stakeholder-based study has determined the priority for viable technical process developments for efficient WEEE fractionation and highlighted the economic and technical improvements that have to be made in the treatment of WEEE.

Separation of virgin plastic polymers and post-consumer mixed plastic waste by sinking-flotation technique

The main objective of this research is to separate virgin polymers (PA, PC, PP, HDPE; PS, and ABS) and post-consumer plastic waste from municipal solid waste (MSW) using the sinking-flotation technique. Separation was carried out on a pilot scale in an 800-l useful volume container with 160 rpm agitation for one hour. Tap water, ethanol solutions, and sodium chloride at different concentrations were used as densification medium. Virgin polymers were separated into two groups: low-density (HDPE and PP) and high-density polymers groups (PS, ABS, PA, and PC). Polymers whose density was less than that of the medium solution floated to the surface, while those whose density was greater than those of the medium solution sank to the bottom. The experimental results showed that complete separation of HDPE from PP achieved 23% ethanol v/v, whereas high-density polymers separated up to 40% w/v sodium chloride. Polymer recovery ranged from 70 to 99.70%. In post-consumer recycled plastic waste, fractions of 29.6% polyolefins, 37.54% PS, 11% ABS, 8% PA, 12% PC PET, and PVC were obtained. Finally, cast plates were made of the post-consumer waste to properly identify the polymer type present in the separated fractions.

Flotation separation of poly (ethylene terephthalate and vinyl chloride) mixtures based on clean corona modification: Optimization using response surface methodology

Postconsumer polyethylene terephthalate (PET) has potential applications in many areas of manufacturing, but contamination by hazardous polyvinyl chloride (PVC) in common waste streams can reduce its recyclable value. Separating collected PET-PVC mixtures before recycling remains very challenging because of the similar physicochemical properties of PET and PVC. Herein, we describe a novel flotation process with corona modification pretreatment to facilitate the separation of PET-PVC mixtures. Through water contact angle, surface free energy, X-ray photoelectron and FT-IR characterization, we found that polar hydroxyl groups can be more easily introduced on the PVC surface than on the PET surface induced by corona modification. This selective wetting can suppress the floatability of PVC, leading to the separation of PET as floating product. A reliable mechanism including two different hydrogen-abstraction pathways was established. Response surface methodology consisting of Plackett-Burman and Box-Behnken designs was adopted for optimization of the combined process, and control parameters were solved based on high-quality prediction models, with fitting from significant variables and interactions. For physical or chemical circulation strategies with PET purity prioritization, the validated purity of the product reached 96.05% at a 626 W corona power, 5.42 m/min passing speed, 24.78 mg/L frother concentration and 286 L/h air flow rate. For the energy recuperation strategy with PET recovery prioritization, the factual recovery reached 98.08% under a 601 W corona power, 6.04 m/min passing speed, 27.55 mg/L frother concentration and 184 L/h air flow rate. The current work provides technological insights into the cleaner disposal of waste plastics.

Polyolefins and Polyethylene Terephthalate Package Wastes: Recycling and Use in Composites

Plastics are versatile materials used in a variety of sectors that have seen a rapid increase in their global production. Millions of tonnes of plastic wastes are generated each year, which puts pressure on plastic waste management methods to prevent their accumulation within the environment. Recycling is an attractive disposal method and aids the initiative of a circular plastic economy, but recycling still has challenges to overcome. This review starts with an overview of the current European recycling strategies for solid plastic waste and the challenges faced. Emphasis lies on the recycling of polyolefins (POs) and polyethylene terephthalate (PET) which are found in plastic packaging, as packaging contributes a signification proportion to solid plastic wastes. Both sections, the recycling of POs and PET, discuss the sources of wastes, chemical and mechanical recycling, effects of recycling on the material properties, strategies to improve the performance of recycled POs and PET, and finally the applications of recycled POs and PET. The review concludes with a discussion of the future potential and opportunities of recycled POs and PET.

Sono-oxidation treatment of hazardous ABS/PC surface for its selective separation from ESR styrene plastics

This study reports the selective hydrophilization of the ABS/PC blend surface using the peroxide-sonochemical system and then its selective separation by froth flotation technique from other ABS-based plastics (ABS, ABS/PMMA) and PS/HIPS in electronic shredder residue (ESR). FT-IR and XPS measurements confirm that the hydrophilic moiety development on the ABS/PC surface led to increasing the wettability of ABS/PC and then decreased its floatability. The confocal scanning results also support the enhancement of microscale roughness of the treated ABS/PC surface. The enhanced surface roughness is attributed to the oxidative process which degrades hydrophobic moieties and promotes hydrophilic functional groups on the ABS/PC surface using commercial oxidant peroxide and ultrasound. This study also investigated removal of Br-containing compounds on the ABS/PC surface. The optimum conditions for selectively ABS/PC separation are peroxide concentration 2%, power cycle 70%, treatment time 5 min, temperature 50 °C, floating agent concentration 0.4 mg/L, flotation time 2 min, and airflow rate 0.5 L/min. ABS/PC was selectively separated from ESR styrene plastics with high recovery and purity of 98.9% and 99.8%, respectively. Hence, the developed novel surface treatments having removal of hazardous Br chemicals and none-formation of secondary pollutants should be applied for upgrading plastic recycling quality.

Enhanced flotation efficiency of metal from waste printed circuit boards modified by alkaline immersion

Efficient recycling of waste printed circuit boards by flotation has become a research focus. In this study, waste printed circuit boards were treated by alkaline immersion to enhance the flotation efficiency. Firstly, the SEM-EDS analysis of the crushed products shown that metal and nonmetal were completely liberated in the -0.25 mm fraction. When the printed circuit boards were modified by alkaline immersion, the recovery of metal increased from 64.34% to 72.35%. Further, the mixture of metal and nonmetal at the edge of nonmetal was discovered by EPMA. This was the cause of metal loss during the flotation process. Secondly, by adjusting the alkaline immersion time and pH value, a good flotation effect was achieved at 40 min alkaline immersion time and the pH = 11. Meanwhile, the XPS analysis of nonmetal found that the intensity of the OH peak was significantly enhanced, while the intensity of the O peak was evidently decreased. The change of the resin molecular structure indicated that the O linked to the benzene ring was broken under the action of alkaline immersion, resulting a free bond was generated on the benzene ring. This made the free OH adsorb to the free bond. This conduct promoted the dispersion of nonmetal in the slurry due to the increased nonmetal surface energy and metal hydrophilicity. Thus, this study provides a new route to improve the flotation efficiency of waste printed circuit boards.

Hydrophilic modification of polycarbonate surface with surface alkoxylation pretreatment for efficient separation of polycarbonate and polystyrene by froth flotation

Waste polystyrene (PS) and polycarbonate (PC) are crucial components arising from mixtures of plastic products, whose recycling is significantly limited by separation efficiency. In this work, to assist the flotation separation of PC and PS, we proposed a novel modification technology of surface alkoxylation pretreatment (SAP) where PC surface reacted with glycerol and urea. The SAP could selectively transform the hydrophobic PC into hydrophilic plastic, while the PS remained its hydrophobic surface owing to the exclusion from SAP process. Benefiting from the hydrophilic PC, the separation efficiency of PS and PC could reach the maximum of 99.34% under optimum conditions (urea dosage of 5 g, pretreatment temperature of 130 °C, pretreatment time of 10 min, flotation time of 2.5 min, frother concentration of 16.5 mg/L, and airflow rate of 7.2 mL/min). The mechanism of SAP was systematically analyzed by wettability, surface morphology, molecular weight, and chemical reactions. Compared with PS plastic, the pretreated PC presented better wettability, rougher surface, and significantly reducing molecular weight. The improvement of PC hydrophilicity can be attributed to the cleavage of ester bonds on backbone chains and the introduction of hydrophilic hydroxyl groups. The effective SAP process proves that chemical recycling of waste plastic can provide a novel strategy for surface modification and flotation separation of PS and PC.

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.

A novel bulk density-based recognition method for kitchen and dry waste: A case study in Beijing, China

Identification technology of household kitchen and dry solid waste has played a major part in improving the accuracy of residents' separation by intelligent outdoor trashcan, which is an effective integral solid waste management tool for growing household solid waste (HSW). Our study aims to present a novel and simple recognition method for kitchen and dry waste based on bulk density. In three communities in Beijing, 270 bagged waste samples were collected, and their moisture content, separation accuracy, and bulk density, characterized. Then a bulk density index was developed to straightforwardly express residents' waste source separation accuracy by linear regression analysis above physical properties. In the 3 Beijing communities, we demonstrated a clear distinction in the bulk density index, for dry, mixed, and kitchen waste of <115, 115-211, >211 kg/m³, respectively. Our results provide a theoretical basis for the establishment of an intelligent waste supervision system, which is of great significance for waste management in developing countries like China.

Characterization of post-consumer plastic film waste from mixed MSW in Spain: A key point for the successful implementation of sustainable plastic waste management strategies

The purpose of this paper is to provide a full characterization of post-consumer plastic film recovered from mixed municipal solid waste (MSW) treatment plants in Spain. Currently, this type of plastic waste is not recycled due to technical or economic barriers and is still sent to landfill. Different types of municipal plastic waste (MPW) from manual and automated sorting were studied: i) colour plastic film recovered by ballistic separators and then manual sorting in different seasons; ii) colour plastic film recovered by automated sorting (air suction); and iii) white plastic film from primary manual sorting process. The samples were characterized by different techniques, including the ultimate and proximate analysis, Higher Heating Value (HHV) and Lower Heating Value (LHV), metal content, Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry (DTG), Fourier Transform Infrared (FT-IR) analysis and Differential Scanning Calorimetry (DSC). The results were compared to those obtained for pretreated colour and white plastic film waste and contrasted with industrial recycled film granules of polyethylene (as a reference material for packaging film). Additionally, pretreated plastic film samples were also characterized by analyzing viscosity, Pressure-Volume-Temperature (PVT) diagram, specific heat capacity and halogen and sulphur contents. Characterization data from this study will contribute to identify and develop potential recycling alternatives for a more sustainable municipal plastic waste management, which is recognized as a priority in the European Circular Economy Action Plan to use resources in a more sustainable way.

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.

MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: III. Anticipating impacts of ageing on identification

Ageing of polymers entails important structural changes and degrades their functional properties, particularly their aspect. Since sorting is a primordial step in recycling to achieve acceptable mechanical properties, the use of promising technologies such as MIR-HSI (Mid-Infrared Hyperspectral Imagery), which could overcome black plastics sorting issue, has to take into account the influence of ageing on identification. As ageing strongly impacts spectra, it can create confusion between materials, especially in an automatized scheme. Based on laboratory FTIR-ATR (Fourier-Transform Infrared Attenuated Total Reflection), this work investigates spectral evolutions of natural and accelerated photodegradation of Waste of Electric and Electrical Equipment plastics (WEEE) as PE, PP, HIPS, ABS and PC to help identifying a polymer despite its ageing degree. Oxidation marks were described and retrieved within a stock of about one hundred of real waste samples, then differentiated from other sources of spectral alteration as formulation. Laboratory ageing data were found to be consistent and often more extreme than real waste samples values. Generally, styrenics showed stronger spectral alteration than polyolefins despite their respective aspects. No significant spectral alteration of PC was obtained here or observed in the waste stock. As an important oxidation marker, the carbonyl peak was also found to often enable fast identification through its wavenumber. If well taken in account, ageing should not induce confusion with other polymers, even formulated, as characteristic signals are different. Finally, the different industrial sub-ranges within MIR are not affected at the same degree, possibly influencing a technological choice for industrial sorting.

Analyzing the intention of the households to drop off mobile phones to the collection boxes: empirical study in Malaysia

Purpose

To achieve proper waste management, the disposal of electronic waste (e-waste) is one suitable method. Most developing countries, including Malaysia, are facing lack of e-waste recycling facilities and low household participation. Using a survey method using a questionnaire, this study aims to examine the intention of Malaysian households to drop-off their mobile phones to the nearest collection boxes (n= 600).

Design/methodology/approach

This study expanded the theory of planned behavior by adding environmental awareness and knowledge. In addition, the cost of disposal and the convenience of the available disposal infrastructure were measured as two parts of the perceived behavioral control.

Findings

The results of this study show that environmental knowledge and awareness have a significant impact on attitudes toward recycling intention of the households. In addition, it was also found that the attitude and cost of disposal infrastructure is positively related to household intention.

Originality/value

These results show that if e-waste collection boxes are provided to the nearest community and e-waste management information is distributed, this will increase household participation in e-waste management.

Separation of hazardous polyvinyl chloride from waste plastics by flotation assisted with surface modification of ammonium persulfate: Process and mechanism

Plastic separation becomes an effective method to improve the plastic recycling by concentrating a single component from complex plastic mixtures. Based on advanced oxidation process, surface modification assisted by ammonium persulfate ((NH₄)₂S₂O₈) was applied to selectively wet plastic surface, achieving the separation of hazardous polyvinyl chloride (PVC) from acrylonitrile butadiene styrene (ABS), polystyrene (PS), and polycarbonate (PC) in forth flotation. The mechanisms were investigated through contact angle, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), as well as scanning electron microscope (SEM). The floatability of PS, PC, and ABS reduces owing to the introduction of carbonyl (O = CO), hydroxyl (-OH), and amide (O = C-NH₂) on plastic surfaces, which is the result of the oxidation by sulfate radical (SO₄^∙-) and the hydrolysis of nitrile group (CN) and butadiene (CC). Then, available reaction equations of ABS, PS, and PC were established to supplement the mechanisms of surface modification. The optimal conditions for flotation separation of PVC are (NH₄)₂S₂O₈ concentration 0.2 M, temperature 70 °C, pretreatment time 30 min, pH 10, flotation time 4 min, terpineol dosage 20 mg/L, and particle size 3-4 mm. The recovery and purity of PVC reach 100 % and 99.7 ± 0.2 % respectively, favoring the reuse of separated waste plastic.

Stochastic induction time of attachment due to the formation of transient holes in the intervening water films between air bubbles and solid surfaces

HYPOTHESIS

Bubble attachment to hydrophobic solid surfaces is influenced by the liquid film instability. Inclusion of transiently formed holes within the film rather than the so-called hydrophobic force in the theory is expected to better describe and explain film rupture and triple contact line formation in the bubble-surface attachment process. The significance of surface hydrophobicity and hole formation renders the stochastic nature of the induction time of attachment.

EXPERIMENTS

A combination of high-speed video microscopy and theoretical analysis was applied to investigate the induction time of attachment and critical film thickness of air bubbles rising freely perpendicularly to silica surfaces of different hydrophobicities.

FINDINGS

Film rupture occurred statistically for shorter induction times and thicker films on the more hydrophobic surface, rejecting the conjecture of hydrophobic force. Computed results of the critical base radius of the transient holes causing film rupture were merged together nicely, independently of surface hydrophobicity. The paper sheds light on the significance of hydrophobicity on the attachment process by means of a novel and easily implemented methodology, without relying on the debatable hydrophobic force.

Classification of Black Plastics Waste Using Fluorescence Imaging and Machine Learning

This work contributes to the recycling of technical black plastic particles, for example from the automotive or electronics industries. These plastics cannot yet be sorted with sufficient purity (up to 99.9%), which often makes economical recycling impossible. As a solution to this problem, imaging fluorescence spectroscopy with additional illumination in the near infrared spectral range in combination with classification by machine learning or deep learning classification algorithms is here investigated. The algorithms used are linear discriminant analysis (LDA), k-nearest neighbour classification (kNN), support vector machines (SVM), ensemble models with decision trees (ENSEMBLE), and convolutional neural networks (CNNs). The CNNs in particular attempt to increase overall classification accuracy by taking into account the shape of the plastic particles. In addition, the automatic optimization of the hyperparameters of the classification algorithms by the random search algorithm was investigated. The aim was to increase the accuracy of the classification models. About 400 particles each of 14 plastics from 12 plastic classes were examined. An attempt was made to train an overall model for the classification of all 12 plastics. The CNNs achieved the highest overall classification accuracy with 93.5%. Another attempt was made to classify 41 mixtures of industrially relevant plastics with a maximum of three plastic classes per mixture. The same average classification accuracy of 99.0% was achieved for the ENSEMBLE, SVM, and CNN algorithms. The target overall classification accuracy of 99.9% was achieved for 18 of the 41 compounds. The results show that the method presented is a promising approach for sorting black technical plastic waste.

MIR spectral characterization of plastic to enable discrimination in an industrial recycling context: II. Specific case of polyolefins

Sorting at industrial scale is required to perform mechanical recycling of plastics in order to obtain properties that could be competitive with virgin polymers. As a matter of fact, the most part of the various types of plastic waste are not miscible and even compatible. Mid-Infrared (MIR) HyperSpectral Imagery (HSI) is viewed as one of the solutions to the problem of black plastic sorting. Many Waste of Electrical and Electronic Equipment (WEEE) plastics are black. Nowadays, these materials are difficult to sort at an industrial scale because the main used pigment to produce this color, carbon black, masks the Near-Infrared (NIR) spectra of polymers, the currently most used technology for acute sorting in industrial conditions. In this study, laboratory Fourier-Transform Infrared (FTIR) in Attenuated Total Reflection mode (ATR) has been used as a theoretical toolbox based on physical chemistry to help building an automated HSI discrimination despite its limited conditions, especially shorter wavelengths ranges. Weaker resolution and very short acquisition times are other HSI limitations. Helping fast and exhaustive laboratory characterizations of polymeric waste stocks is the other goal of this study. This study focusses on polyolefins as they represent the second biggest fraction of WEEE plastics (WEEP) after styrenics and since little quantities mixed to styrenics during mechanical recycling can lead to important decrease in mechanical properties. Twelve references were thus evaluated and compared between each other and with real waste samples to highlight spectral elements, which can enable differentiation. Charts compiling the signals of discussed polymers were built aiming to the same objective.

Surface treatment by the Fe(III)/sulfite system for flotation separation of hazardous chlorinated plastics from the mixed waste plastics

A novel advanced oxidation process by a combination of Fe(III) and sulfite for surface treatment of waste plastic mixtures is proposed. The Fe(III)/sulfite system has been found to enhance hydrophilicity of the mixed waste plastics, including acrylonitrile butadiene styrene (ABS), polystyrene (PS) and polycarbonate (PC), while it has little effect on hazardous polyvinyl chloride (PVC), thus promoting separation of PVC from the mixed waste plastics by flotation. Radical scavenging experiments indicate that sulfate radicals are the main reactive species. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) results imply the formation of CO or CO groups on the treated plastics surface except for PVC and a plausible mechanism for oxidizing plastics with sulfate radicals is proposed. PVC with 100.00% recovery and 99.84% purity is achieved under optimum surface treatment conditions of sodium sulfite concentration 10 mM, ferric sulfate concentration 0.4 mM, pH 6.0, temperature 25 °C and treatment time 15 min. Consequently, surface treatment by the Fe(III)/sulfite system is an effective technology for separating hazardous PVC from the mixed waste plastics by flotation.

Fuel oil generated from the cogon grass-derived Al-Si (Imperata cylindrica (L.) Beauv) catalysed pyrolysis of waste plastics

This research investigated pyrolysis as a potential method to manage plastic waste in Sichang Island, Thailand. Pyrolysis was chosen to convert waste plastic into fuel oil using Al-Si catalysts derived from cogon grass. The study consisted of three stages. The first stage determined the composition of the waste plastics found in Sichang Island. High-density polyethylene (48%) comprised the highest proportion of the waste plastics, followed by low-density polyethylene (22%), polyethylene terephthalate (13%), polypropylene (10%), and polystyrene (7%). In the second stage, the Al-Si catalysts were prepared from cogon grass (Imperata cylindrica (L.) Beauv) by treating it with acid and calcination. The optimum conditions to extract silica from cogon grass through acid treatment were heating at 700 °C for 2 h, which yielded 97.7% of amorphous silica with a surface area of 172 m²/g and a pore volume of 0.43 cc/g. This amorphous silica was combined with an aluminum precursor to form Al-Si catalysts with 20-80 wt% of Al-Si. The results showed that the surface area of the catalyst increased with increasing aluminum content. The optimum ratio was 60 wt% of Al-Si with a surface area of 200 m²/g. In the final stage, the catalytic properties of the previously prepared Al-Si catalysts in the pyrolysis of waste plastics were evaluated. The catalyst enhanced the plastic cracking process and the oil yield while decreasing the reaction time. The optimum ratio of 60% Al-Si to 10% waste plastic provided the maximum oil yield of 93.11% and the minimum reaction time of 20 min. The results showed that catalytic cracking with 60% Al-Si contributed to a high quantity of oil yield, similar to using a commercial Al-Si catalyst. The results of this research will be applied as an alternative method of recycling plastic for sustainable waste management in Sichang Island.

Dissolution of adhesive resins present in plastic waste to recover polyolefin by sink-float separation processes

This study investigated the dissolution of adhesive resins present in polyolefin films that cause plastic materials to adhere to each other. The process of dissolution was made by the use of ethyl acetate and followed by separation through the sink-float process. The objective was to separate and characterize polyolefin films from plastic solid waste derived from recycled post-consumer paper. Through these procedures, 6% polyethylene of high-density (HDPE), 14% polyethylene of low-density (LDPE) and 39% polypropylene (PP) were separated and recovered from plastic waste. Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analyzes (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were conducted to determine the chemical, thermal and mechanical properties of the recovered polymers and to establish a comparison with standard commercial polymers. It demonstrated that recovered material kept their chemical, thermal, and mechanical properties. This process indicates possible economic viability considering the demand, the market value of the PP, and the required investment to be implemented in the recycling process that could be amortized in a short period of time. Moreover, the organic solvent used in the dissolution process can be easily recovered by distillation.

Separation of polyvinylchloride and acrylonitrile-butadiene-styrene combining advanced oxidation by S2O82-/Fe2+ system and flotation

A combining technology of advanced oxidation by S₂O₈^2-/Fe²⁺ system and flotation was proposed for separating polyvinyl chloride (PVC) and acrylonitrile butadiene styrene (ABS). In this research, sodium persulfate was activated by heating and ferrous ions. The separation efficiency of PVC/ABS oxidized by S₂O₈^2-/Fe²⁺ was higher than that by sodium persulfate. The mechanism of this process was investigated through contact angle, Fourier transform infrared spectroscopy (FT-IR) inductively coupled plasma (ICP), nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS). The floatability of ABS reduced owing to the introduction of oxygen-containing functional groups such as carbonyl (OCO) and hydroxyl (OH), which was a result of oxidation by sulfate radicals (SO₄^·-). The optimal conditions for separating PVC and ABS were: Na₂S₂O₈ concentration 0.1 M, molar ratio (S₂O₈^2-/Fe²⁺) 200, treatment time 10 min, flotation time 4 min, frother concentration 14.7 mg L^-1 and airflow rate 6.8 mL min^-1. Novel kinetics of pretreatment time and flotation were proposed and researched in this work. The max rate constant of PVC/ABS flotation was 0.64 min^-1. In addition, the pretreatment solution can be reused for three times with superior performance. The recovery and purity of PVC reached 100% and 99.7%, respectively. According to reasonable evaluation, the combination of S₂O₈^2-/Fe²⁺ advanced oxidation and flotation is a practical and efficient technology for separating PVC and ABS.

Offensive waste valorisation in the UK: Assessment of the potentials for absorbent hygiene product (AHP) recycling

Offensive human waste refers to non-hazardous waste that contains body fluids from non-infectious humans, comprised of post-consumer Absorbent Hygiene Products (AHPs), swabs, dressings, bedding, gloves, and other materials. While this waste category requires more stringent handling, storage and disposal measures than general waste, its non-hazardous nature suggests that there are opportunities for waste valorisation. An inventory of 200 offensive human waste bags collected from various healthcare institutions in South-Eastern England show that about 76% of the waste is comprised of AHPs, most of which are adult incontinence pads and child nappies. Mixed plastics comprised of predominantly HDPE represent 9% of the waste. To evaluate the potentials for offensive human waste valorisation, small-scale separation tests involving artificially-soiled nappies and associated mixed plastic packaging wastes have been performed. Findings suggest that about 50% of the total superabsorbent polymer is recoverable from fluff pulp fractions, recoveries of which are unaffected by the presence of ionic species typically present in human waste. On the other hand, recovery of mixed plastic packaging is more challenging. Overall, however, findings suggest that viable AHP recycling is possible if recyclate materials are targeted towards non-food related markets outlets such as the construction and land remediation sectors.

Sustainable hydrophilization to separate hazardous chlorine PVC from plastic wastes using H2O2/ultrasonic irrigation

Polyvinyl chloride (PVC) products comprise a large portion of plastic wastes and cause severe environmental burdens in thermal recycling such as toxic release and disposal difficulties. Selective separation methods for PVC containing hazardous chlorine are required for the development of suitable disposal or material recycling processes. However, separating PVC selectively from municipal plastic waste mixtures is difficult due to their similar hydrophobic surface and appearance densities. This study presents a one-step, selective separation technique for PVC using H₂O₂ solution under ultrasonic irrigation to promote the selective development of hydrophilicity only on the PVC surface. The combined treatment helped to decrease air bubbles attached on the PVC surface because of increased wettability, which allowed the treated PVC to settle on the bottom of the flotation reactor. However, the remaining plastic wastes were easily floated off because they maintained their hydrophobicity. The combined treatment with a low concentration of 3% H₂O₂ and ultrasonic irrigation for 30 min afforded 100% purity and recovery of the PVC separated from the municipal plastic waste mixture. This proposed treatment is therefore a promising and inexpensive way to improve plastic recycling quality through selective PVC separation by the selective development of hydrophilicity on its surface.

Adsorption behavior and selectivity mechanism of flotation reagents applied in ternary plastic mixtures

Plastic flotation attracts increasing attention in the process of recycling and will bring potential application in industry after theoretical perfection. For a separated ternary system of polyethylene terephthalate (PET), polyvinyl chloride (PVC) and polyethylene (PE), adsorption behavior and selectivity mechanism of flotation reagents were investigated by multi-characterization tests and batch equilibrium adsorption method. Quantitative adsorption results indicate that frother polyethylene glycol (PEG) only acts on gas and liquid phases in the flotation system with negligible adsorption capacity onto solid phase. For depressant sodium lignosulphonate (SL), the pseudo-first-order and Langmuir isotherm models are suitable for corresponding kinetic and equilibrium data of PET or PVC. Thermodynamic parameters further indicate that the adsorption of SL is a spontaneous and endothermic process, which neither belongs to the pure physisorption nor to the pure chemisorption. Adsorption models of SL were established based on hydrogen bond, with three clear bonding types (OH…π*, OH…O, and OH…Cl). Selectivity mechanism can be attributed to the selective hydrogen bond acceptors and donors, which are provided by specific plastic and depressant, respectively. In the light of these theoretical fundings, new targeted reagents or pre-treatments are expected to be developed towards more complex flotation system.

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

Waste acrylonitrile butadiene styrene (ABS) and polycarbonate (PC) as dominant components in waste electrical and electronic equipment (WEEE) plastics show significant potential for recycling, which is severely restricted by efficient separation method. We proposed a novel surface treatment method using sodium hypochlorite for facilitating coseparation of ABS and PC from WEEE plastics by flotation for recycling. Optimization of surface treatment process was performed with response surface methodology using Box-Behnken design. A quadratic model was generated for predicting the floating rate of ABS and PC, and it was also used to optimize the coseparation performance. The optimum conditions were determined and included concentration of 0.05 M, temperature of 69.5 °C, contact time of 56.5 min, and stirring rate of 200 rpm. Under optimum conditions, the coseparation of ABS and PC was effectively achieved; the recovery and the purity of ABS and PC reached 97.4% and 100.0%, respectively. The formation of oxygen-bearing groups and morphological changes were confirmed as major mechanism to induce the surface hydrophilization of ABS and PC. Consequently, this method is feasible for selective coseparation of ABS and PC from WEEE plastics, and it provides technological insights in the sustainable deposal of WEEE plastics.