Automated analysis of microplastics based on vibrational spectroscopy: are we measuring the same metrics?

The traditional manual analysis of microplastics has been criticized for its labor-intensive, inaccurate identification of small microplastics, and the lack of uniformity. There are already three automated analysis strategies for microplastics based on vibrational spectroscopy: laser direct infrared (LDIR)-based particle analysis, Raman-based particle analysis, and focal plane array-Fourier transform infrared (FPA-FTIR) imaging. We compared their performances in terms of quantification, detection limit, size measurement, and material identification accuracy and speed by analyzing the same standard and environmental samples. LDIR-based particle analysis provides the fastest analysis speed, but potentially questionable material identification and quantification results. The number of particles smaller than 60 µm recognized by LDIR-based particle analysis is much less than that recognized by Raman-based particle analysis. Misidentification could occur due to the narrow tuning range from 1800 to 975 cm-1 and dispersive artifact distortion of infrared spectra collected in reflection mode. Raman-based particle analysis has a submicrometer detection limit but should be cautiously used in the automated analysis of microplastics in environmental samples because of the strong fluorescence interference. FPA-FTIR imaging provides relatively reliable quantification and material identification for microplastics in environmental samples greater than 20 µm but might provide an imprecise description of the particle shapes. Optical photothermal infrared (O-PTIR) spectroscopy can detect submicron-sized environmental microplastics (0.5-5 µm) intermingled with a substantial amount of biological matrix; the resulting spectra are searchable in infrared databases without the influence of fluorescence interference, but the process would need to be fully automated.

Geochemical markers of the Anthropocene: Perspectives from temporal trends in pollutants.

Determining stratigraphic markers of the Anthropocene is important for demarcating Global Stratotype Section and Point (GSSP). Heavy metals and persistent organic pollutants (POPs) are candidate geochemical markers of the Anthropocene, but no study has comprehensively evaluated temporal trends in these pollutants in sediment cores globally. 454 data points for 8 heavy metals and 8 POPs were compiled to reconstruct their temporal trends and evaluate their global consistency. The heavy metals did not increase rapidly in the 20th century, and their temporal trends were locally but not globally consistent, which are not suitable geochemical markers of the Anthropocene. POPs rapidly increased beginning in the mid-20th century but have declined in the past decade, and these data are more consistent globally. The time of the peak concentration and period of rapid increase for polychlorinated biphenyls (PCBs) occur near the boundary of the Anthropocene and are consistent globally. Forty-five percent of the studies evaluated used only 210Pb chronology for dating, which creates definite uncertainty in the analysis. In GSSP candidate sections, PCBs could be considered candidate markers of the Anthropocene.

Raman spectra and surface changes of microplastics weathered under natural environments.

Raman spectroscopy can be used to effectively analyze submicron- to microsized microplastics, but Raman spectra of weathered microplastics commonly show deviations from those of unweathered microplastics and are often affected by fluorescence. However, studies of weathering-induced surface changes in microplastics have been limited to laboratory simulations. To systematically study Raman spectra and surface changes of microplastics weathered under natural environments, we collected microplastics from sediments around waste plastics processing and recycling industries in Laizhou City, Shandong Province, East China. Raman spectra of weathered microplastics differ greatly from standard spectra of unweathered plastic material. Peaks in the Raman spectra of weathered microplastics are weakened and even invisible. A preliminary Raman database of weathered microplastics (RDWP) including 124 Raman spectra of weathered microplastics was built to accurately identify microplastics in natural environments, and it is open to all users. FTIR spectroscopy revealed the presence of oxygen-containing functional groups and CC bonds related to oxidation and chain scission. SEM showed that weathered microplastics had rough surfaces and that PP was more easily fractured than PE. Complementary C and O elemental maps suggested that the O/C ratio is a potential indicator of oxidation degree. EDS revealed titanium on PET and PVC surfaces, which is related to titanium dioxide typically used as a light-blocking aid. Our data document that Raman spectroscopy has great potential in the identification of naturally weathered microplastics and that combined spectral and elemental analyses can be useful in deciphering the degradation processes of microplastics under natural conditions. CAPSULE: Raman spectra of weathered microplastics differ greatly from standard spectra. A Raman database of weathered microplastics is established. Surface changes of weathered microplastics were systematically studied.

The rapid increases in microplastics in urban lake sediments.

Microplastics have received widespread attention as an emerging global pollutant. However, the research on the abundance and characteristics of microplastics entering the environment throughout history has been limited. Meanwhile, the determination of the start of the Anthropocene is important because humans have become a vital force affecting the environment and Earth surface processes. It is unclear whether the plastic can be used as an artefact to indicate the start of the Anthropocene. In this study, combined with 137Cs, 210Pb, and spherical carbonaceous particles (SCP) high-resolution chronology, a microplastics-time curve was established by using the sedimentary record from an urban lake in Wuhan city. The microplastic abundance increased from 741 items·kg-1 to 7707 items·kg-1 over the past 60 years. The microplastics were mainly fibres and composed of polyester and rayon polymers, which indicated that the microplastics most likely originated from textiles. The surfaces of the older microplastics were rough and weathered with many absorbed elements. Microplastics are similar to fossils belonging to the Anthropocene, and may be used as an indicator. The comparison of microplastic-time curves in different records on a global scale will be necessary in the future.