Turning Plastic Waste into Carbon Capture Materials: A Promising Route toward Sustainable CO₂ Adsorption

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Carbon capture technologies are becoming increasingly important as industries seek practical solutions to reduce greenhouse gas emissions while maintaining economic viability. Among the many available approaches, adsorption using porous solid materials has attracted considerable attention because of its relatively low energy requirement, operational flexibility, and scalability. At the same time, plastic waste—particularly polyethylene terephthalate (PET)—continues to accumulate worldwide, creating another significant environmental challenge. Integrating these two issues into a single engineering solution offers an attractive pathway toward circular economy implementation.

The study reviewed in this article investigates whether activated carbon produced from PET plastic waste can function as an effective adsorbent for high-pressure carbon dioxide capture. Beyond evaluating adsorption performance, the research also develops reliable thermodynamic data that can support future adsorption system design. This work demonstrates how waste valorization, materials engineering, and carbon capture technology can converge to address both climate change mitigation and plastic waste management through a scientifically robust and experimentally validated approach.

Bibliographic Information

Item Information
Article Title High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications
Authors Awaludin Martin, Erman Taer, Nasruddin, and Nur Khotimah
Journal Teknomekanik
Volume & Issue Volume 8, Issue 1
Publication Year 2025
Pages 52–66
DOI https://doi.org/10.24036/teknomekanik.v8i1.36172
Publisher Universitas Negeri Padang
License Creative Commons Attribution 4.0 International (CC BY 4.0)

1. Research Background

  • Carbon dioxide emissions remain a major engineering challenge. Fossil fuel combustion continues to dominate global energy production, making industrial carbon emissions one of the principal contributors to climate change. Engineering solutions that can capture carbon before atmospheric release are therefore receiving increasing scientific and industrial attention.
  • Adsorption has emerged as one of the most promising carbon capture technologies. Compared with several conventional separation methods, adsorption systems can offer relatively lower energy consumption, operational simplicity, and flexible integration into industrial processes. Their performance, however, depends heavily on the quality of the adsorbent material.
  • Activated carbon remains one of the most widely used adsorbents. Owing to its high surface area, extensive pore structure, chemical stability, and relatively low production cost, activated carbon has become an attractive material for gas separation and carbon dioxide adsorption applications across multiple engineering sectors.
  • Plastic waste presents an opportunity rather than merely an environmental burden. Polyethylene terephthalate (PET), commonly used in beverage bottles and packaging, accumulates rapidly in municipal waste streams. Converting this abundant waste into high-value activated carbon supports both waste reduction and resource recovery within a circular economy framework.
  • Engineering sustainable adsorbents requires balancing performance and environmental responsibility. Many commercially available adsorbents deliver excellent adsorption performance but rely on relatively expensive raw materials or energy-intensive manufacturing processes. Waste-derived adsorbents offer an alternative that may simultaneously reduce production costs and environmental impacts.
  • High-pressure adsorption behavior remains insufficiently characterized for PET-derived activated carbon. Although previous studies have demonstrated that activated carbon can adsorb carbon dioxide effectively, relatively limited information is available regarding the adsorption performance of PET-derived activated carbon under elevated pressures combined with different operating temperatures.
  • Reliable thermodynamic models are essential for engineering design. Experimental adsorption data alone are insufficient for designing industrial adsorption systems. Engineers require mathematical models capable of accurately describing adsorption equilibrium and adsorption heat, allowing future simulations, reactor design, and process optimization.
  • This study bridges materials engineering and carbon capture technology. Rather than focusing solely on producing activated carbon from recycled PET, the researchers comprehensively evaluate adsorption behavior under high-pressure isothermal conditions while validating adsorption models that can support future carbon capture system development.
  • The novelty lies in combining waste valorization with advanced adsorption characterization. The research integrates sustainable material production, experimental adsorption testing, thermodynamic modeling, and adsorption heat analysis into a single engineering investigation. This holistic approach provides knowledge that extends beyond material synthesis toward practical implementation in carbon capture technologies.

2. Research Objectives

  • Evaluate the carbon dioxide adsorption performance of activated carbon synthesized from polyethylene terephthalate (PET) plastic waste under high-pressure isothermal conditions.
  • Investigate how adsorption temperature influences carbon dioxide uptake across multiple operating pressures.
  • Determine the maximum adsorption capacity of PET-derived activated carbon for potential carbon capture applications.
  • Develop adsorption equilibrium models capable of accurately representing experimental adsorption behavior using the Toth isotherm model.
  • Estimate the heat of adsorption using thermodynamic approaches, including the Clausius–Clapeyron equation and the Chakraborty–Saha–Koyama (CSK) model.
  • Assess the suitability of PET-derived activated carbon as a sustainable microporous adsorbent for future adsorption-based carbon capture systems.
  • Generate reliable experimental and thermodynamic data that can support numerical modeling, adsorption system optimization, and future engineering design.

3. Why This Research Matters

  • Supports climate change mitigation. Developing efficient adsorbents capable of capturing carbon dioxide directly contributes to reducing industrial greenhouse gas emissions and supports global decarbonization initiatives.
  • Transforms plastic waste into engineering resources. Instead of treating PET waste solely as an environmental problem, the study demonstrates its potential as a valuable raw material for advanced functional materials with environmental applications.
  • Promotes circular economy implementation. Converting post-consumer plastic into high-performance activated carbon illustrates how waste streams can be reintegrated into productive industrial value chains rather than disposed of in landfills.
  • Provides valuable engineering data. High-pressure adsorption measurements, equilibrium modeling, and adsorption heat estimation supply important design parameters for future adsorption columns, carbon capture reactors, and process simulations.
  • Improves sustainable materials development. The study highlights how waste-derived porous materials can compete with conventional adsorbents while simultaneously reducing dependence on virgin carbon sources.
  • Supports industrial carbon capture technologies. The findings may assist engineers working in power generation, petrochemical processing, natural gas purification, and other sectors requiring efficient gas separation technologies.
  • Strengthens interdisciplinary engineering research. This work integrates mechanical engineering, materials science, thermodynamics, environmental engineering, and chemical process engineering into a unified investigation with both scientific and practical significance.
  • Creates a foundation for future digital engineering. The experimentally validated adsorption models generated in this research can serve as reliable inputs for computational simulations, process optimization, and digital twins used in next-generation carbon capture systems.

4. Research Methodology

  • Research Type

    This study employed an experimental engineering approach combined with thermodynamic modeling to evaluate the carbon dioxide adsorption performance of activated carbon synthesized from polyethylene terephthalate (PET) plastic waste. The research integrates materials engineering, adsorption science, and numerical model validation to characterize the suitability of waste-derived activated carbon for carbon capture applications.

  • Raw Material

    The adsorbent precursor consisted of discarded polyethylene terephthalate (PET) plastic. After material sorting and preparation, the plastic waste was converted into activated carbon through controlled thermal processing, enabling the development of a microporous carbon structure suitable for gas adsorption.

  • Activated Carbon Preparation

    The PET material first underwent carbonization under a nitrogen atmosphere to decompose the polymer into carbon-rich material while preventing oxidation. The resulting carbon was subsequently subjected to physical activation using carbon dioxide at elevated temperature to enlarge the pore network and increase the available adsorption surface area.

    The effectiveness of the activation process was evaluated through iodine adsorption measurements and pore characterization reported by the researchers, confirming that the synthesized activated carbon possessed a well-developed microporous structure appropriate for adsorption applications.

  • Experimental Adsorption System

    Carbon dioxide adsorption experiments were conducted using a high-pressure volumetric adsorption apparatus consisting of interconnected charging and measuring chambers. Pressure and temperature were continuously monitored while adsorption equilibrium was established under carefully controlled operating conditions.

    The experimental system incorporated pressure transmitters, thermocouples, temperature-controlled water circulation, computerized data acquisition, and LabVIEW-based monitoring to ensure accurate measurement of adsorption behavior throughout each experiment.

  • Experimental Conditions

    Adsorption performance was evaluated under three isothermal operating temperatures:

    • 27°C
    • 35°C
    • 45°C
    Pressure was gradually increased from approximately 100 kPa to around 3500 kPa to investigate the influence of pressure on adsorption capacity over a broad operational range representative of high-pressure carbon capture systems.

  • Sample Preparation

    Prior to adsorption measurements, the activated carbon samples were vacuum degassed to eliminate residual moisture and previously adsorbed gases. Void-volume calibration was subsequently performed before carbon dioxide was introduced into the adsorption chamber. Each experiment was repeated three times to improve measurement reliability and reproducibility.

  • Adsorption Modeling

    Experimental adsorption data were analyzed using the Toth adsorption isotherm model. This model was selected because it is capable of describing adsorption on heterogeneous porous surfaces while maintaining good predictive capability across both low- and high-pressure operating regions.

  • Thermodynamic Analysis

    Beyond equilibrium adsorption analysis, the researchers estimated adsorption heat using two complementary thermodynamic approaches:

    • Clausius–Clapeyron equation
    • Chakraborty–Saha–Koyama (CSK) model
    These analyses provide additional understanding of the energetic characteristics governing the adsorption process and support future adsorption system design.

  • Supporting Software and Data Processing

    Experimental measurements were processed using LabVIEW for data acquisition, while carbon dioxide density calculations utilized REFPROP thermophysical property software. Statistical indicators, including regression coefficients and standard errors, were employed to evaluate model accuracy and agreement between experimental observations and mathematical predictions.

  • Validation Strategy

    Validation focused on comparing experimental adsorption measurements with predictions generated by adsorption isotherm models. Model performance was assessed through regression analysis, goodness-of-fit statistics, and consistency across multiple operating temperatures, ensuring that the selected mathematical models accurately represented the observed adsorption behavior.

5. Key Findings

PET-Derived Activated Carbon Demonstrated Strong CO₂ Adsorption Performance

The study confirms that activated carbon synthesized from polyethylene terephthalate waste possesses considerable potential as a carbon dioxide adsorbent. Under high-pressure conditions, adsorption capacity increased steadily with increasing operating pressure, demonstrating that the developed microporous material can effectively capture carbon dioxide across a wide pressure range.

The highest adsorption capacity reported in the study reached approximately 0.213 kg of CO₂ per kilogram of adsorbent at 27°C under the maximum investigated pressure. This result indicates that PET waste can be transformed into a functional engineering material capable of supporting adsorption-based carbon capture technologies while simultaneously reducing plastic waste.

Adsorption Capacity Was Strongly Influenced by Temperature

One of the clearest experimental observations is the inverse relationship between adsorption temperature and carbon dioxide uptake. Lower operating temperatures consistently produced greater adsorption capacities, whereas adsorption performance gradually decreased as temperature increased from 27°C to 45°C.

This behavior agrees with the thermodynamic characteristics of physical adsorption, where lower temperatures enhance intermolecular attraction between carbon dioxide molecules and the adsorbent surface. The experimental trends therefore reinforce existing adsorption theory while providing quantitative performance data for PET-derived activated carbon.

Pressure Became the Primary Driver of Adsorption Capacity

Across all investigated temperatures, increasing pressure resulted in progressively higher carbon dioxide adsorption. The adsorption curves exhibited consistent upward trends, indicating that elevated pressure increases the amount of carbon dioxide available for adsorption inside the microporous structure.

This finding is particularly important for industrial carbon capture applications because many post-combustion and gas purification systems operate under elevated pressures. The results demonstrate that PET-derived activated carbon maintains stable adsorption behavior within pressure conditions relevant to engineering practice.

The Toth Isotherm Successfully Represented Experimental Behavior

A major contribution of the study lies in validating the Toth isotherm as an accurate mathematical representation of the adsorption equilibrium. The fitted model reproduced the experimental adsorption data with excellent agreement across all investigated temperatures.

The reported coefficient of determination exceeded 99%, indicating that the Toth model captures the adsorption characteristics of PET-derived activated carbon with high reliability. Such predictive capability is essential for process simulation, adsorption column design, and optimization studies that rely on mathematical adsorption models rather than direct experimentation.

Thermodynamic Analysis Confirmed Favorable Adsorption Characteristics

The calculated adsorption heat obtained from both the Clausius–Clapeyron equation and the Chakraborty–Saha–Koyama model demonstrates consistent thermodynamic behavior. The agreement between these analytical approaches strengthens confidence in the reliability of the experimental measurements.

By quantifying adsorption heat, the researchers provide valuable engineering parameters that extend beyond equilibrium measurements alone. These thermodynamic properties are essential for designing regeneration strategies, estimating process energy requirements, and developing numerical simulations for adsorption-based carbon capture systems.

The Research Demonstrates a Practical Circular Economy Strategy

Beyond its scientific findings, the study illustrates a practical engineering pathway for transforming an environmental pollutant into a value-added functional material. Rather than viewing PET solely as municipal waste, the research demonstrates how discarded plastic can become a useful precursor for advanced carbon capture materials.

This dual environmental benefit—reducing plastic waste while simultaneously mitigating carbon dioxide emissions—represents one of the most compelling aspects of the study. It highlights how sustainable materials engineering can integrate waste valorization with climate change mitigation technologies, creating broader environmental value than either objective could achieve independently.

6. Scientific Contribution

  • Introduces a sustainable adsorbent derived from plastic waste. The study demonstrates that polyethylene terephthalate (PET) waste can be transformed into a functional microporous activated carbon capable of capturing carbon dioxide under high-pressure conditions, expanding the range of sustainable precursor materials available for adsorption technologies.
  • Provides experimentally validated high-pressure adsorption data. Reliable adsorption measurements obtained under multiple temperatures and pressures enrich the experimental database required for adsorption engineering, particularly for waste-derived activated carbon materials.
  • Validates the applicability of the Toth adsorption isotherm. By demonstrating excellent agreement between experimental observations and model predictions, the research confirms that the Toth model is well suited for describing adsorption equilibrium on heterogeneous PET-derived activated carbon.
  • Expands thermodynamic understanding of PET-based adsorbents. The estimation of adsorption heat using both the Clausius–Clapeyron equation and the Chakraborty–Saha–Koyama model provides valuable information regarding adsorption energetics that extends beyond conventional equilibrium studies.
  • Integrates materials engineering with environmental engineering. Rather than treating material synthesis and adsorption evaluation as separate investigations, the study combines adsorbent fabrication, experimental characterization, equilibrium modeling, and thermodynamic analysis within a single engineering framework.
  • Supports future computational modeling. The experimentally validated adsorption parameters generated in this research can serve as reliable input data for numerical simulations, adsorption column design, digital process optimization, and future carbon capture system development.

7. Industrial Implications

  • Carbon Capture and Storage (CCS). The adsorption characteristics demonstrated by PET-derived activated carbon indicate potential application in adsorption-based carbon capture units for power plants, cement production, petrochemical industries, and other carbon-intensive sectors.
  • Waste-to-Value Manufacturing. The research illustrates how discarded PET plastics can become valuable engineering materials rather than environmental liabilities, supporting industrial recycling initiatives and circular manufacturing strategies.
  • Adsorbent Manufacturing. Manufacturers of activated carbon may consider recycled PET as an alternative feedstock capable of producing microporous materials with competitive adsorption performance while reducing dependence on conventional raw materials.
  • Environmental Engineering. Engineers responsible for industrial emission control can utilize the reported adsorption and thermodynamic data when evaluating alternative adsorbent materials for gas separation processes.
  • Process Design and Simulation. The validated adsorption isotherm parameters provide valuable inputs for process simulators, adsorption column sizing, regeneration analysis, and optimization of pressure swing adsorption (PSA) or related adsorption technologies.
  • Quality Assurance. The rigorous experimental methodology—including repeated measurements, regression analysis, and thermodynamic validation—offers a reference for evaluating adsorption performance in future industrial material qualification studies.
  • Industry 4.0 and Digital Engineering. As manufacturing increasingly relies on predictive simulation and digital process optimization, experimentally validated adsorption models such as those presented in this study become essential for digital twins and intelligent process control.
  • Sustainable Manufacturing. The work supports industrial sustainability by simultaneously addressing plastic waste utilization, carbon emission reduction, and resource efficiency through an integrated engineering approach.

8. Research Limitations

  • The investigation focuses on laboratory-scale adsorption experiments. Performance under continuous industrial operating conditions remains to be demonstrated through pilot-scale or commercial-scale validation.
  • Only pure carbon dioxide adsorption was investigated. Industrial flue gases generally contain multiple components, including nitrogen, oxygen, water vapor, sulfur oxides, and nitrogen oxides that may influence adsorption behavior.
  • The adsorption experiments were conducted under three controlled temperatures. Broader operating conditions could provide a more comprehensive understanding of adsorption performance.
  • Although adsorption equilibrium was thoroughly characterized, long-term cyclic adsorption–desorption stability was not extensively evaluated within this study.
  • The activated carbon originated exclusively from PET waste. Comparative evaluation with additional plastic waste streams could further clarify the influence of precursor composition on adsorption performance.
  • The study emphasizes equilibrium adsorption and thermodynamic analysis rather than detailed adsorption kinetics or mass transfer behavior.
  • Economic feasibility, life-cycle assessment, and large-scale production costs were outside the scope of the investigation and therefore require separate evaluation before industrial implementation.

9. Future Research Opportunities

  1. Investigate adsorption performance using realistic industrial flue gas mixtures containing multiple gaseous species.
  2. Evaluate adsorption–desorption cycling stability to determine long-term adsorbent durability and regeneration efficiency.
  3. Optimize carbonization and activation parameters to maximize micropore development while minimizing production energy consumption.
  4. Compare PET-derived activated carbon with activated carbon synthesized from other plastic wastes, agricultural biomass, and industrial by-products.
  5. Develop kinetic adsorption models that complement the equilibrium analysis presented in this study.
  6. Perform computational fluid dynamics (CFD) and process simulations using the experimentally validated adsorption parameters.
  7. Conduct techno-economic assessment and life-cycle assessment to evaluate industrial feasibility and environmental benefits.
  8. Investigate surface modification or chemical functionalization techniques to further enhance carbon dioxide selectivity and adsorption capacity.
  9. Integrate PET-derived activated carbon into pressure swing adsorption (PSA) or temperature swing adsorption (TSA) systems for pilot-scale validation.
  10. Explore machine learning and artificial intelligence approaches for predicting adsorption behavior using experimental datasets generated from waste-derived adsorbents.

10. Potential for Public Policy Citation (Overton)

This article demonstrates moderate-to-high potential for citation in public policy documents because it addresses two strategic environmental priorities simultaneously: carbon emission reduction and plastic waste utilization. Although the research is laboratory-based, its findings directly support engineering solutions aligned with sustainable development and circular economy implementation.

Government agencies responsible for climate mitigation, waste management, and industrial innovation could reference this work when preparing technical reports or long-term environmental strategies. The study is particularly relevant to policies encouraging waste valorization, low-carbon manufacturing, and sustainable material development.

Potential policy applications include:

  • National Carbon Capture, Utilization and Storage (CCUS) roadmaps.
  • Plastic waste recycling and circular economy strategies.
  • Industrial decarbonization programs.
  • Sustainable manufacturing and green industry initiatives.
  • Research and innovation funding priorities for advanced materials.
  • Technical guidance related to adsorption-based carbon capture technologies.
  • University–industry collaboration programs focused on waste-derived engineering materials.

Nevertheless, because the study evaluates material performance under controlled laboratory conditions, additional pilot-scale demonstrations and industrial validation would strengthen its suitability for inclusion in engineering standards, regulatory guidelines, and large-scale implementation policies.

11. Who Should Read This Paper?

  • Researchers in Carbon Capture and Climate Engineering
    Scientists investigating adsorption technologies, carbon capture systems, sustainable materials, and greenhouse gas mitigation will find valuable experimental data and validated thermodynamic models that can support future investigations.
  • Materials Scientists
    Researchers working on porous materials, activated carbon, waste valorization, and advanced functional materials can use this study as a reference for developing environmentally sustainable adsorbents from recycled resources.
  • Mechanical and Chemical Engineers
    Engineers involved in adsorption system design, gas separation processes, thermal systems, and industrial process optimization will benefit from the experimentally validated adsorption characteristics and equilibrium modeling presented in this work.
  • Environmental Engineers
    Professionals developing technologies for emission reduction, waste management, and environmental remediation can gain practical insights into integrating plastic recycling with carbon capture technologies.
  • Graduate Students
    Master's and doctoral students studying adsorption science, thermodynamics, sustainable manufacturing, or environmental engineering can use this article as a valuable example of experimental methodology and scientific reporting.
  • Industrial Practitioners
    Engineers and technical specialists working in carbon capture, activated carbon manufacturing, gas purification, petrochemical processing, and sustainable manufacturing may find useful engineering data applicable to industrial process development.
  • Policy Makers and Sustainability Experts
    Government agencies, environmental organizations, and policy analysts interested in circular economy implementation, plastic waste utilization, and industrial decarbonization may consider the study as scientific evidence supporting future innovation strategies.
  • Educators
    University lecturers teaching materials engineering, environmental engineering, thermodynamics, or sustainable manufacturing can use this article as an excellent case study illustrating interdisciplinary engineering research.

12. Final Thoughts

This study presents a carefully designed experimental investigation demonstrating that polyethylene terephthalate (PET) plastic waste can be transformed into a functional microporous activated carbon suitable for carbon dioxide adsorption under high-pressure conditions. Rather than addressing plastic pollution and greenhouse gas emissions independently, the research successfully combines both environmental challenges into a single engineering solution based on sustainable materials development.

One of the principal strengths of the study lies in its comprehensive approach. The authors move beyond material synthesis by incorporating systematic adsorption experiments, equilibrium modeling, thermodynamic analysis, and quantitative validation. The excellent agreement between experimental observations and the Toth isotherm model increases confidence in the reported results and provides valuable engineering parameters for future adsorption system design and numerical simulation.

From an engineering perspective, the work offers practical significance because it provides experimentally validated data that may support future development of adsorption-based carbon capture technologies. Although additional pilot-scale investigations and long-term performance evaluations remain necessary before industrial implementation, the research establishes an important scientific foundation for future advances in sustainable adsorbent development.

Overall, this article represents a meaningful contribution to sustainable materials engineering, carbon capture research, and circular economy innovation. By demonstrating how waste-derived materials can achieve competitive adsorption performance while simultaneously addressing environmental pollution, the study illustrates the growing role of interdisciplinary engineering research in supporting future low-carbon technologies.

Suggested Citation

Teknomekanik (UNP) Style

Martin, A., Taer, E., Nasruddin, & Khotimah, N. High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications. Teknomekanik, 8(1), 52–66. https://doi.org/10.24036/teknomekanik.v8i1.36172

APA (7th Edition)

Martin, A., Taer, E., Nasruddin, & Khotimah, N. (2025). High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications. Teknomekanik, 8(1), 52–66. https://doi.org/10.24036/teknomekanik.v8i1.36172

IEEE Style

A. Martin, E. Taer, Nasruddin, and N. Khotimah, "High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications," Teknomekanik, vol. 8, no. 1, pp. 52–66, 2025, doi:10.24036/teknomekanik.v8i1.36172.

Harvard Style

Martin, A., Taer, E., Nasruddin & Khotimah, N. 2025, 'High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications', Teknomekanik, vol. 8, no. 1, pp. 52–66. https://doi.org/10.24036/teknomekanik.v8i1.36172

Vancouver Style

Martin A, Taer E, Nasruddin, Khotimah N. High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications. Teknomekanik. 2025;8(1):52–66. doi:10.24036/teknomekanik.v8i1.36172.

Chicago (Author–Date)

Martin, Awaludin, Erman Taer, Nasruddin, and Nur Khotimah. 2025. "High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications." Teknomekanik 8 (1): 52–66. https://doi.org/10.24036/teknomekanik.v8i1.36172.

MLA (9th Edition)

Martin, Awaludin, et al. "High-pressure adsorption isothermal on a novel microporous material from polyethylene terephthalate plastic waste in carbon dioxide capture applications." Teknomekanik, vol. 8, no. 1, 2025, pp. 52–66. https://doi.org/10.24036/teknomekanik.v8i1.36172.

Editorial Note

Editorial Note: This blog post is an independent scholarly review intended for educational and scientific communication purposes. It summarizes and discusses the published article in the author's own words while providing full attribution to the original publication, consistent with the principles of the Creative Commons Attribution 4.0 International (CC BY 4.0) license.

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