Supercritical fluid technology - an eco-friendly approach for resource recovery from e-waste and plastic waste: A review

Occurrence and ecotoxicological impacts of polybrominated diphenyl ethers (PBDEs) in electronic waste (e-waste) in Africa: Options for sustainable and eco-friendly management strategies

Polybrominated diphenyl ethers (PBDEs) are persistent contaminants used as flame retardants in electronic products. PBDEs are contaminants of concern due to leaching and recalcitrance conferred by the stable and hydrophobic bromide residues. The near absence of legislatures and conscious initiatives to tackle the challenges of PBDEs in Africa has allowed for the indiscriminate use and consequent environmental degradation. Presently, the incidence, ecotoxicity, and remediation of PBDEs in Africa are poorly elucidated. Here, we present a position on the level of contamination, ecotoxicity, and management strategies for PBDEs with regard to Africa. Our review shows that Africa is inundated with PBDEs from the proliferation of e-waste due to factors like the increasing growth in the IT sector worsened by the procurement of second-hand gadgets. An evaluation of the fate of PBDEs in the African environment reveals that the environment is adequately contaminated, although reported in only a few countries like Nigeria and Ghana. Ultrasound-assisted extraction, microwave-assisted extraction, and Soxhlet extraction coupled with specific chromatographic techniques are used in the detection and quantification of PBDEs. Enormous exposure pathways in humans were highlighted with health implications. In terms of the removal of PBDEs, we found a gap in efforts in this direction, as not much success has been reported in Africa. However, we outline eco-friendly methods used elsewhere, including microbial degradation, zerovalent iron, supercritical fluid, and reduce, reuse, recycle, and recovery methods. The need for Africa to make and implement legislatures against PBDEs holds the key to reduced effect on the continent.

An in-depth study of the oxidative liquefaction process for polymeric waste reduction and chemical production from wind turbine blades and personal protective equipment used in the medical field

An in-depth study of the oxidative liquefaction process has been provided to degrade the polymeric waste from personal protective equipment (PPEs) and wind turbine blades (WTBs). Thermogravimetric investigations demonstrate that WTBs have three prominent peaks throughout the degradation, whereas PPEs display solitary peak features. Experiments are carried out employing specific experimental design approaches, namely the Central Composite Face-Centered Plan (CCF) for WTBs and the Central Composition Design with Fractional Factorial Design for PPEs in a batch-type reactor at temperature ranges of 250-350 °C, pressures of 20-40 bar, residence times of 30-90 min, H2O2 concentrations of 15-45 %, and waste/liquid ratios of 5-25 % for WTBs. These values were 200-300 °C, 30 bar, 45 min, 30-60 % and 5-7 % for PPE. A detailed comparison has been provided in the context of total polymer degradation (TPD) for PPE and WTBs. Liquid products from both types of wastes after the oxidative liquefaction process are subjected to gas chromatography with flame ionization detection (GC-FID) to identify the existence of oxygenated chemical compounds (OCCs). For WTBs, TPD was 20-49 % and this value was 55-96 % for PPE while the OCC yield for WTBs (36.31 g/kg - 210.59 g/kg) and PPEs (39.93 g/kg - 212.66 g/kg) was also calculated. Detailed optimization of experimental plans was carried out by performing the analysis of variance (ANOVA) and optimization goals were maximum TPD and OCCs yields against the minimum energy consumption, though a considerable amount of complex polymer waste can be reduced and high concentrations of OCC can be achieved, which could be applied for commercial and environmental benefits.

Recovery of lithium and cobalt from lithium cobalt oxide and lithium nickel manganese cobalt oxide batteries using supercritical water

This study investigates the eco-friendly extraction of metal oxides from LCO and NMC batteries using supercritical water. Experiments were conducted at 450 °C with a feed rate of 5 mL min-1 and varying battery/PVC ratios (0.0, 2.0, and 3.0). The products were analyzed by X-ray diffractometry (XRD), atomic absorption spectrometry (FAAS) and gas chromatography-mass spectrometry (GC-MS). Results show the presence of cobalt chloride (CoCl2) and lithium (Li) in the liquid products, achieving 100% cobalt recovery under all conditions. The gaseous products obtained hydrogen with molar compositions up to 78.3% and 82.7% for LCO:PVC and NMC:PVC batteries, respectively, after 60 min of reaction. These findings highlight the potential of this methodology for lithium-ion battery recycling.

Transforming pollution into solutions: A bibliometric analysis and sustainable strategies for reducing indoor microplastics while converting to value-added products

Microplastics (MPs), as emerging indoor contaminants, have garnered attention due to their ubiquity and unresolved implications for human health. These tiny particles have permeated indoor air and water, leading to inevitable human exposure. Preliminary evidence suggests MP exposure could be linked to respiratory, gastrointestinal, and potentially other health issues, yet the full scope of their effects remains unclear. To map the overall landscape of this research field, a bibliometric analysis based on research articles retrieved from the Web of Science database was conducted. The study synthesizes the current state of knowledge and spotlights the innovative mitigation strategies proposed to curb indoor MP pollution. These strategies involve minimizing the MP emission from source, advancements in filtration technology, aimed at reducing the MP exposure. Furthermore, this research sheds light on cutting-edge methods for converting MP waste into value-added products. These innovative approaches not only promise to alleviate environmental burdens but also contribute to a more sustainable and circular economy by transforming waste into resources such as biofuels, construction materials, and batteries. Despite these strides, this study acknowledges the ongoing challenges, including the need for more efficient removal technologies and a deeper understanding of MPs' health impacts. Looking forward, the study underscores the necessity for further research to fill these knowledge gaps, particularly in the areas of long-term health outcomes and the development of standardized, reliable methodologies for MP detection and quantification in indoor settings. This comprehensive approach paves the way for future exploration and the development of robust solutions to the complex issue of microplastic pollution.

Degradation of Waste Tetra Pak Packaging with Hydrothermal Treatment in Sub-/Supercritical Water

Tetra pak packaging is one of the most frequently used types of packaging in the food industry. The recycling of the tetra pak packaging waste presents a difficult task because of its multi-layered, multi-component structure. In this study, the degradation of tetra pak packaging in subcritical (SubCW) and supercritical (SCW) water was investigated. The experiments were carried out in one (SCW) or two stages (SubCW and SCW), whereby the influence of the reaction temperature and time on the yield and composition of the products obtained was investigated. The maximum oil phase yield achieved in a one-stage and a two-stage degradation process was 60.7% and 65.5%, respectively. The oil and gas phases were composed of different types of hydrocarbons. Higher temperature and longer time led to higher amounts of saturated aliphatic hydrocarbons in both the oil and gas phases. The aqueous phase contained sugars (glucose, fructose) and sugar derivatives (levulinic acid, glyceraldehyde, furfurals). Based on these results, the degradation pathway of waste tetra pak packaging in SubCW and SCW was proposed. The results of the study show that the degradation of waste tetra pak packaging with SubCW and SCW is a promising recycling process.

Physicochemical reactions in e-waste recycling

Electronic waste (e-waste) recycling is becoming a global concern owing to its immense quantity, hazardous character and the potential loss of valuable metals. The many processes involved in e-waste recycling stem from a mixture of physicochemical reactions, and understanding the principles of these reactions can lead to more efficient recycling methods. In this Review, we discuss the principles behind photochemistry, thermochemistry, mechanochemistry, electrochemistry and sonochemistry for metal recovery, polymer decomposition and pollutant elimination from e-waste. We also discuss how these processes induce or improve reaction rates, selectivity and controllability of e-waste recycling based on thermodynamics and kinetics, free radicals, chemical bond energy, electrical potential regulation and more. Lastly, key factors, limitations and suggestions for improvements of these physicochemical reactions for e-waste recycling are highlighted, wherein we also indicate possible research directions for the future.

Process and systematic study of gold recovery from flexible printed circuit boards (FPCBs) using a choline chloride-ethylene glycol system

The traditional hydrometallurgy technology has been widely used to recover precious metals from electronic waste. However, such aqueous recycling systems often employ toxic/harsh chemicals, which may cause serious environmental problems. Herein, an efficient and environment-friendly method using a deep eutectic solvent (DES) mixed system of choline chloride-ethylene glycol-CuCl2·2H2O is developed for gold (Au) recovery from flexible printed circuit boards (FPCBs). The Au leaching and precipitation efficiency can reach approximately 100 % and 95.3 %, respectively, under optimized conditions. Kinetic results show that the Au leaching process follows a nucleation model, which is controlled by chemical surface reactions with an apparent activation energy of 80.29 kJ/mol. The present recycling system has a much higher selectivity for Au than for other base metals; the two-step recovery rate of Au can reach over 95 %, whereas those of copper and nickel are < 2 %. Hydrogen nuclear magnetic resonance spectroscopy (HNMR) and density functional theory (DFT) analyses confirm the formation of intermolecular hydrogen bonds in the DES mixed system, which increase the system melting and boiling points and facilitate the Au leaching process. The Au leaching system can be reused for several times, with the leaching efficiency remaining > 97 % after five cycles. Moreover, ethylene glycol (EG) and choline chloride (ChCl) act as aprotic solvents as well as coordinate with metals, decreasing the redox potential to shift the equilibrium to the leaching side. Overall, this research provides a theoretical and a practical basis for the recovery of metals from FPCBs.

Novel method for recovering valuable metals from Sn ash: Vacuum carbothermal reduction-directional condensation

Sn ash recycling is an industry with positive development prospects, as it provides better-protected resources, promotes sustainable development, and lays a solid foundation for future development. In this study, an innovative vacuum carbothermal reduction-directional condensation process was developed. The thermodynamic analysis results indicated that the initial reaction pressure and temperature for the carbothermal reduction of the system was 1-10 Pa and 998-1063 K, respectively. The saturation vapor pressure, separation coefficient, and condensation temperature of Sn, Pb, and Zn in the reduced products differed significantly, and their separation could be achieved by controlling the volatilization and condensation temperatures. A single-factor experiment investigated the effects of carbon ratio, temperature, and time on the reduction efficiency, direct yield, and recovery rate. The optimal experimental conditions were the ratio of MeO to C of 4:1, temperature of 1373 K, and time of 120 min. Sn, Pb, and Zn products were obtained at different positions. This process shortens the traditional process, reduces the reduction cost of Sn, and enables the implementation of the process, making it environmentally friendly.

E-waste plastic liquefaction using supercritical Toluene: Evaluation of reaction parameters on liquid products

Solvothermal liquefaction (STL) is a thermochemical conversion technique that employs solvents other than water to transform waste plastics into valuable compounds. The objective of this study was to explore the potential use of supercritical toluene, a nonpolar solvent, for the depolymerization of four electrical waste (e-waste) thermoplastics, namely polyamide (PA), polycarbonate (PC), polyoxymethylene (POM), and polyether ether ketone (PEEK), into liquid products. Depolymerization experiments were carried out in batch reactors at three reaction temperatures (325, 350, and 375 °C), and three residence times (1, 3, and 6 h). The findings revealed that increasing STL temperature and extending the reaction time enhances the depolymerization of e-waste thermoplastics. The highest STL conversation (100 %) was observed for POM, and the lowest STL conversation (32.23 %) was observed for PEEK. Additionally, the ultimate analysis showed that the liquid product obtained from STL at 375 °C and 6 h exhibited higher heating values (HHV) within the range of 31.43 to 35.31 MJ/kg. Thermogravimetric analysis (TGA) demonstrated that the boiling point distributions of liquid products are highly dependent on thermoplastic type. Finally, the reaction mechanisms of STL for PA, PC, POM, and PEEK were proposed based on gas chromatography-mass spectrometry (GCMS) analysis.

Supercritical fluid-based method for selective extraction and analysis of indole alkaloids from Uncaria rhynchophylla

The development of an approach based on simultaneous supercritical fluid extraction-sample cleanup, followed by supercritical fluid chromatography/tandem mass spectrometry (SFE-SFC-MS/MS) was as a tool for the extraction, separation and characterization of indole alkaloids of Uncaria rhynchophylla. A two-step SFE method was designed. A mixture of the U. rhynchophylla sample and an adsorbent named C18SCX with the ratio of 1:1 (w/w) was placed into an extraction cell. The extraction temperature was 40 °C and the pressure was 25 Mpa. In the first step, 10 % EtOH as the co-solvent was used to extract for 60 min, which was considered as a cleanup process to remove non-alkaloid components. In the second step, 0.1 % DEA was added to 10 % EtOH and it extracted for 60 min to obtain the desired extract. By introducing an additional adsorbent, the specificity of SFE towards alkaloids was greatly improved. An SFC-MS/MS method was then utilized for analysis of the SFE extract. Using 2-EP as stationary phase with the gradient elution of 0-10 min, 5-25 % EtOH (+0.05 % DEA) in CO2, column temperature 40 °C, and back pressure 13.8 Mpa, 10 peaks were separated within 8 min. Further MS/MS analysis confirmed that nine of the 10 peaks in the SFE extract were indole alkaloids. This study developed a supercritical fluid-based method specifically towards extraction and analysis of alkaloids, which is helpful to the study of alkaline compounds in complex samples.

A feasible approach for the treatment of waste computer casing plastic using subcritical to supercritical acetone: Statistical modelling and optimization

Electronic waste (e-waste) usage has increased tremendously with the rapid evolution of technologies. The accumulated e-waste has now emerged as one of the crucial concerns regarding environmental pollution and human health. Recycling e-waste is commonly focused on metal recovery; nevertheless, a significant fraction of plastics (20-30%) are in e-waste. There is an indispensable need to focus on e-waste plastic recycling in an effective way, which has been mostly overlooked to date. An environmentally safe and efficient study is conducted using subcritical to supercritical acetone (SCA) to degrade the real waste computer casing plastics (WCCP) in the central composite design (CCD) of response surface methodology (RSM) to achieve the maximum oil yield of the product. The experiment parameters were varied in the temperature span of 150-300 °C, residence time between 30 and 120 min, solid/liquid ratio between 0.02 and 0.05 (g/ml), and NaOH amount from 0 to 0.5 g. Adding NaOH into the acetone helps to achieve efficient degradation and debromination efficiency. The study emphasized the attributes of oils and solid products recovered from the SCA-treated WCCP. The characterization of feed and formed products is performed with different characterization techniques such as TGA, CHNS, ICP-MS, FTIR, GC-MS, Bomb calorimeter, XRF, and FESEM. The highest oil yield achieved is 87.89% from the SCA process at 300 °C, in 120min, 0.05 S/L ratio, and 0.5 g of NaOH. GC-MS results disclose that the liquid product (oil) comprises single- and duplicate-ringed aromatic and oxygen-containing compounds. Isophorone is the significant component of the liquid product obtained. Furthermore, SCA's possible polymer degradation mechanistic route, bromine distribution, economic feasibility, and environmental aspect were also explored. This present work represents an environmentally friendly and promising approach for recycling the plastic fraction of e-waste and recovering valuable chemicals from WCCP.

The interactions between mixed waste from discarded surgical masks and face shields during the degradation in supercritical water

The widespread use of surgical masks made of polyolefin and face shields made of polyester during pandemics contributes significantly to plastic pollution. An eco-friendly approach to process plastic waste is using supercritical water, but the reaction of mixed polyolefin and polyester in this solvent is not well understood, which hinders practical applications. This study aimed to investigate the reaction of waste surgical masks (SM) and face shields (FS) mixed in supercritical water. Results showed that the optimal treatment conditions were 400 °C and 60 min, achieving a liquid oil yield of 823.03 mg·g-1 with 25 wt% FS. The interaction between polypropylene (PP), polyethylene terephthalate (PET), and iron (Fe) in SM and FS mainly determined the production of liquid oil products such as olefins and benzoic acid. The methyl-branched structure of PP enhanced PET hydrolysis, resulting in higher production of terephthalic acid (TPA). The degradation of PP was facilitated by the acidic environment created by TPA and benzoic acid in the reaction. Moreover, the hydrolysis of PET produced carboxylic acid, which coordinated with Fe3+ to form Fe-H that catalyzed the polymerization of small olefins, contributing to higher selectivity for C9 olefins. Therefore, this study provides valuable insights into the degradation mechanism of mixed PPE waste in supercritical water and guidance for industrial treatment.

Valorization of e-waste via supercritical water technology: An approach for obsolete mobile phones

The improper handling of electronic waste has not only severe environmental impacts but also results in the loss of high economic potential. To address this issue, the use of supercritical water (ScW) technology for the eco-friendly processing of waste printed circuit boards (WPCBs) obtained from obsolete mobile phones has been explored in this study. The WPCBs were characterized via MP-AES, WDXRF, TG/DTA, CHNS elemental analysis, SEM and XRD. A L9 Taguchi orthogonal array design was employed to evaluate the impact of four independent variables on the organic degradation rate (ODR) of the system. After optimization, an ODR of 98.4% was achieved at a temperature of 600 °C, a reaction time of 50 min, a flowrate of 7 mL min-1, and the absence of an oxidizing agent. The removal of the organic content from the WPCBs resulted in an increase in the metal concentration, with up to 92.6% of the metal content being efficiently recovered. During the ScW process, the decomposition by-products were continuously removed from the reactor system through the liquid or gaseous outputs. The liquid fraction, which was composed of phenol derivatives, was treated using the same experimental apparatus, achieving a total organic carbon reduction of 99.2% at 600 °C using H2O2 as the oxidizing agent. The gaseous fraction was found to contain hydrogen, methane, CO2, and CO as the major components. Finally, the addition of co-solvents, namely ethanol and glycerol, enhanced the production of combustible gases during the ScW processing of WPCBs.

A sustainable route for the recovery of metals from waste printed circuit boards using methanesulfonic acid

The rapid increase in electronic waste (e-waste) generation and its unsustainable management pose a threat to the environment and human well-being. However, various valuable metals are present in e-waste, which makes it a potential secondary source to recover metals. Therefore, in the present study, efforts were made to recover valuable metals (Cu, Zn, and Ni) from waste printed circuit boards (WPCB) of computers using methanesulfonic acid (MSA). MSA is contemplated as a biodegradable green solvent and has a high solubility for various metals. The effect of various process parameters (MSA concentration, H₂O₂ concentration, stirring speed, liquid to solid ratio, time, and temperature) was investigated on metal extraction to optimize the process. At the optimized process conditions, 100% extraction of Cu and Zn was achieved, while Ni extraction was around 90%. The kinetic study for metal extraction was performed using a shrinking core model and findings showed that MSA-aided metal extraction is a diffusion-controlled process. Activation energies were found to be 9.35, 10.89, and 18.86 kJ/mol for Cu, Zn, and Ni extraction, respectively. Furthermore, the individual recovery of Cu and Zn was achieved using the combination of cementation and electrowinning, which resulted in 99.9% purity of Cu and Zn. The current study proposes a sustainable solution for the selective recovery of Cu and Zn from WPCB.