Summary of - Development of an inter-confirmatory plastic characterization system using spectroscopic techniques for waste management

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Article

Abstract

Study Background: The article presented titled “Development of an inter-confirmatory plastic characterization system using spectroscopic techniques for waste management” by Dr. Adarsh U.K., Dr. E. Bhoje Gowd, Dr. Aseefhali B., Dr. Kartha V.B., Santhosh C., and Dr. Unnikrishnan V.K. is a report on development of a multimodal spectroscopy system by combining laser induced breakdown spectroscopy (LIBS) and Raman spectroscopy in a single optical system. The performance of the developed compact and cost-effective system was initially assessed using pure plastic samples such as Nylon 1, 1, PP, PMMA, PC and PLA. Then the performance of the system in characterizing post-consumer plastic samples were evaluated by using PE, PP, and PET samples collected from household waste materials. The performance evaluation of the system shows the importance of combining multiple techniques together in a system in overcoming the limitations of individual spectroscopic technique and the developed systems shows the potential to be used as a plastic sorting sensor.

Research Goals and Hypothesis: The objective of the research is to design and develop a multimodal spectroscopy system to characterize postconsumer plastic waste based on the polymer type for the purpose of plastic waste sorting for recycling applications. The major emphasis is to develop a methodology to sort plastic solid waste (PSW) by utilizing the elemental and molecular properties of the plastic classes. Since each of the polymer type has different elemental and molecular signatures, LIBS and Raman spectroscopy can give characteristic signals that can be used for sorting plastics. The application of multimodal spectroscopy can help to overcome the limitations of individual techniques.

Methodological Approach:

  1. The optical system is designed to operate using a single source – single detector configuration to carry out both LIBS and Raman spectroscopy of plastics. A nanosecond (6 ns) pulsed laser operating at 532 nm and 10 Hz is used as the source and the combination of a Czerny-Turner (CT) spectrograph and non-gated CCD is used for the detection of LIBS-Raman signals.
  2. The optical system is shared in a cage system assembly for LIBS and Raman measurements that makes the proposed system compact and cost-effective compared to the conventional approach with designs having multiple optical systems for the multimodal applications.
  3. A set of laboratory-made pure plastic samples (PP, PMMA, PC, PLA, and Nylon-11) were used to evaluate and standardise the performance of the system.
  4. The optimization of parameters for the system is performed by considering the laser energy and signal exposure time needed by the system. For LIBS measurements, a laser energy of 7 mJ is applied, considering the ablation threshold and detector saturation limit observed. The signals are recorded in single-shot mode with 10 ms exposure time where a single laser pulse from the laser is used for the excitation of the plastic sample for the measurement.
  5. For Raman spectroscopy, a laser energy of 3 mJ is utilized with an exposure time of 10 ms (single-shot measurement same as LIBS). This allowed the system to capture both LIBS and Raman signals with a total exposure time of 30 ms.
  6. The evaluation of performance of the system was done by using conventional laboratory based techniques like XRD and ATR-FTIR.
  7. A micro-controller based translational stage was used to simulate the constant movement of plastic samples across the laser beam to match circumstances in a real industry settings.

Results and Discoveries:

  1. Based on a detailed analysis of classification performance (evaluated by using Principal Component Analysis (PCA)) of spectral data in different emission regions, the spectral region from 400 nm to 520 nm gives the best classification based on LIBS data.
  2. The system could record LIBS and Raman signals from the plastic samples which are under constant movement across the laser spot with a speed of 4 mm/sec.
  3. The LIBS analysis of different colour types of post-consumer plastics under the categories PET, PP, and HDPE showed good classification in PCA score analysis based on the variations in emission lines from C2, N, and O.
  4. The Raman spectroscopy analysis on the set of post-consumer plastics showed good classification based on characteristic Raman bands. However, some of the plastic samples (e.g. dark coloured HDPE) gave strong fluorescence background that forced to remove the spectral data from those samples from the analytical set. This shows the importance of hyphenated systems in identification-sorting applications.
  5. The concept of laser ablation surface cleaning is utilized to remove the surface contaminants from the plastic sample surface for a meaningful and error-free analytical results.
  6. The Raman spectroscopy analysis of different colour types of PP showed that it is difficult to gather Raman scattering information from dark coloured samples due to negligibly small scattering and chances of absorption.
  7. The comparison of analytical results obtained with the developed LIBS-Raman system with conventional systems like XRD, ATR-FTIR, and NIR reflectance showed the feasibility of the proposed system in characterizing plastics.

Citation to the base paper:

Adarsh U.K, E. Bhoje Gowd, Aseefhali Bankapur, V.B. Kartha, Santhosh Chidangil, Unnikrishnan V.K, (2022), Development of an Inter-Confirmatory Plastic Characterization System Using Spectroscopic Techniques for Waste Management, Waste Management, 150, 339-351, https://doi.org/10.1016/j.wasman.2022.07.025

Publication Date: 2022

Recommended Citation: Adarsh U.K, E. Bhoje Gowd, Aseefhali Bankapur, V.B. Kartha, Santhosh Chidangil, Unnikrishnan V.K, Development of an Inter-Confirmatory Plastic Characterization System Using Spectroscopic Techniques for Waste Management

Publication Date

2022

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