Occurrence and dynamics of microplastics and emerging concern microparticles in coastal sediments: Impact of stormwater upgrade and port-associated facilities

Urban runoff is a significant source of microplastic pollution in aquatic environments, especially in coastal areas. Despite urban stormwater runoff being considered a major pathway of anthropogenic particles there's no studies about the impact of stormwater upgrades on microparticle transport. Moreover, due to the influence of anthropogenic activities, including maritime traffic and maintenance, on coastal environments, it is crucial to identify plastic debris from both inland and in-shore sources. This study evaluates characteristics, abundance, and distribution of microplastics in subtidal sediments from the southwestern Atlantic region, influenced by a recently upgraded stormwater outfall and port facilities. Herein, we have analyzed temporal trends, including seasonal dynamics and their relation with the pre- and post-upgrade of the stormwater outfall. Three main types of anthropogenic microparticles were observed: common plastic (MPs), paints (Pps), and tire wear particles (TWPs). Microparticle groups varied in morphology, color, and size distribution, including uncommon microparticle debris. Analysis by FTIR and Raman spectroscopies allows the identification of polyethylene, polypropylene, polyethylene terephthalate, polyvinyl chloride, polystyrene, polyamide, and polyacrylonitrile polymers for MPs mainly. Pigments such as Naphthol AS, phthalocyanine, and quinacridone have been identified in Pps. SEM-EDS and FTIR analysis of collected TWPs revealed similar trace metals constituents and infrared signals to those observed in tire road samples. Spatial and temporal abundances of microparticle groups were significantly different (p < 0.05), mainly related to the distance from the stormwater outfall and seasons. TWPs showed the most significant seasonal increment rate among pre and post-upgrade periods. Furthermore, the upgrades made to the stormwater system have been observed to intensify the transport and increase the presence of specific anthropogenic microparticles in subtidal sediments. Based on these findings, the occurrence of TWPs emerges as a reliable urban runoff indicator to differentiate ex and in-situ sources in multipolluted coastal environments.

Study of a stream in Argentina with a high concentration of microplastics: Preliminary analysis of the methodology

This study presents for the first time results about the microplastic concentration and their origin in a stream of the Pampas region in Argentina, receiving wastewater of an intermediate city. The most appropriate procedure to analyze and quantify the microplastics in the studied system is the use of an oxidative digestion process using a 30% H₂O₂ solution to eliminate the organic matter in the samples. A high quantity of MPs, on the order of millions of microplastics per m³ of water, was estimated in the Langueyú stream. 56% of the microplastics correspond to fibers with diameters between 10 and 15 μm and lengths less than 500 μm, while 44% are fragments with sizes of tens of micrometers. Raman microspectroscopy was used to identify the type of fibers. The characteristics of the microplastic fibers released in a wash load test are comparable with those observed in the Langueyú stream, in particular, the average sizes and the distribution of the diameters of the MPFs are similar. The processes in the sewage treatment plant, prior to their discharge in the stream, would affect the color of the fibers and their length.

Optimizing graphene production in ultrasonic devices

In this work, we investigate the efficiency of graphene production from graphite by ultrasonic-induced exfoliation under different operational conditions. An ultrasonic bath and an ultrasonic horn tip were used, to show how the graphene production efficiency depends on the acoustic intensity and cavitation mechanism using graphite crystal with different initial sizes and characteristic lengths. Operational variables such as geometry, location and type of vessel containing the liquid phase, were also explored. All of them have effect on the cavitation process and directly influence the amount of graphene obtained. Remarkably, we found that the initial size of the graphite crystals has a marked impact in exfoliation process. Based on the above arguments we propose a combined protocol with a specific sequence to maximize the amount of graphene obtained.

A study of polymer chain diffusion by surface enhanced Raman: effects of plasmonic substrate topology

We report on a new methodology to track chain interdiffusion between polymer slabs based on Raman enhanced by plasmonic substrates. Diffusion is studied in a deuterated-polystyrene/polystyrene (dPS/PS) polymer pair, designed to provide a well-characterized diffusion behavior. The bilayer, 160 nm thick in total, is supported on a plasmonic substrate that provides local amplification of Raman signals in sample regions of close proximity to it. Gold-based substrates with structures of inverted pyramids, spherical nanoparticles and tipped pillars were investigated. Interdiffusion between dPS and PS is promoted upon annealing and followed in situ by dynamic spectral acquisition. A simple model that describes the coupling between the sampled region arising from the plasmonic effect and the diffusion process is employed to interpret spectral evolution data. It is shown that a highly regular topology and surface continuity are key features of the plasmonic substrate in order to provide reliable results. With pyramidal substrates, the most suitable substrates for this application, data are consistent with diffusion coefficients in the range 10-13-10-15 cm2 s-1 and dimensions of sampled regions below 40 nm. The strategy provides a reliable labeling-free technique to investigate polymer interdiffusion on the nanoscale.

Comment on "Observation of mutual diffusion of macromolecules in PS/PMMA binary films by confocal Raman microscopy" by C. Hu, X. Chen, J. Chen, W. Zhang and M. Q. Zhang, Soft Matter, 2012, 8, 4780

A paper by Hu et al. (Soft Matter, 2012, 8, 4780) reports on the use of confocal Raman microscopy to resolve mutual diffusion between polystyrene (PS) and poly(methyl methacrylate) (PMMA). In-depth optical sectioning is employed to measure the diffusive broadening of the originally planar PS-PMMA interface, from which tracer and mutual diffusion coefficients and values for the PS-PMMA thermodynamic interaction parameter are extracted. Here, a reinterpretation of Hu's data that leads to a completely different scenario is presented, as apparent diffusive broadening can be mostly attributed to optical distortions inherent to the probe methodology. It also explains the lack of consistency of kinetic and thermodynamic parameters obtained by the authors from their diffusion analysis in comparison with earlier published data on this system. Overall, it highlights the importance of carrying out appropriate data analysis when confocal Raman microscopy is applied in dry depth-profiling investigations.

Subtle Structural Aspects of Propylene-Based Copolymers as Revealed by Raman Spectroscopy

Raman spectroscopy is used to elucidate fine details of the rather complex microstructure of ethylene-propylene copolymers (EPCs). This paper is focused on a series of commercial EPCs (Versify by Dow) with well-characterized ethylene content. Particular emphasis is given on the analysis of crystal type and content and their relation with EPC chain microstructure. Information provided by Raman is compared with that obtained by differential scanning calorimetry (DSC), a well-established technique widely used in the polymer field. Temperature-resolved Raman experiments are also carried out to interpret more precisely the complex melting patterns observed in the DSC traces. These experiments reveal with more detail the crystal chemical composition and melting temperature ranges of EPC samples, key features to design processing conditions that guarantee optimum lifetime and recyclability of overmolded parts.

Depth-profiling by confocal Raman microscopy (CRM): data correction by numerical techniques

The data obtained in confocal Raman microscopy (CRM) depth profiling experiments with dry optics are subjected to significant distortions, including an artificial compression of the depth scale, due to the combined influence of diffraction, refraction, and instrumental effects that operate on the measurement. This work explores the use of (1) regularized deconvolution and (2) the application of simple rescaling of the depth scale as methodologies to obtain an improved, more precise, confocal response. The deconvolution scheme is based on a simple predictive model for depth resolution and the use of regularization techniques to minimize the dramatic oscillations in the recovered response typical of problem inversion. That scheme is first evaluated using computer simulations on situations that reproduce smooth and sharp sample transitions between two materials and finally it is applied to correct genuine experimental data, obtained in this case from a sharp transition (planar interface) between two polymeric materials. It is shown that the methodology recovers very well most of the lost profile features in all the analyzed situations. The use of simple rescaling appears to be only useful for correcting smooth transitions, particularly those extended over distances larger than those spanned by the operative depth resolution, which limits the strategy to the study of profiles near the sample surface. However, through computer simulations, it is shown that the use of water immersion objectives may help to reduce optical distortions and to expand the application window of this simple methodology, which could be useful, for instance, to safely monitor Fickean sorption/desorption of penetrants in polymer films/coatings in a nearly noninvasive way.

Depth profiling by confocal Raman microspectroscopy: semi-empirical modeling of the Raman response

It has been well documented that the use of dry optics in depth profiling by confocal Raman microspectroscopy significantly distorts the laser focal volume, thus negatively affecting the spatial resolution of the measurements. In that case, the resulting in-depth confocal profile is an outcome of several contributions: the broadening of the laser spot due to instrumental factors and diffraction, the spreading of the illuminated region due to refraction of the laser beam at the sample surface, and the influence of the confocal aperture in the collection path of the laser beam. Everall and Batchelder et al. developed simple models that describe the effect of the last two factors, i.e., laser refraction and the diameter of the pinhole aperture, on the confocal profile. In this work, we compare these theoretical predictions with experimental data obtained on a series of well-defined planar interfaces, generated by contact between thin polyethylene (PE) films (35, 53, 75, and 105 microm thickness) and a much thicker poly(methyl methacrylate) (PMMA) piece. We included two refinements in the above-mentioned models: the broadening of the laser spot due to instrumental factors and diffraction and a correction for the overestimation in the decay rate of collection efficiency predicted by Batchelder et al. These refinements were included through a semiempirical approach, consisting of independently measuring the Raman step-response in the absence of refraction by using a silicon wafer and the actual intensity decay of a thick and transparent polymer film. With these improvements, the model reliably reproduces fine features of the confocal profiles for both PE films and PMMA substrates. The results of this work show that these simple models can not only be used to assist data interpretation, but can also be used to quantitatively predict in-depth confocal profiles in experiments carried out with dry optics.

Interphase evolution in polymer films by confocal Raman microspectroscopy

Liquid-glassy polymer diffusion is an important topic in polymer physics, with several mechanistic aspects that still remain unclear. Here we describe the use of confocal Raman microspectroscopy (CRM) to study directly several features of interphase evolution in a system of this type. The interphase studied was generated by contact between liquid polystyrene (PS) and glassy polyphenylene oxide (PPO). Interphase evolution on thin films made from these polymers was followed by depth profiling in combination with immersion optics. We also applied regularized deconvolution to improve the spatial resolution of the measurements. With the help of these techniques, we examined interphase PPO concentration profiles and kinetics of interphase evolution in the range 120-180 degrees C, well below the glass transition temperature of the PPO-based films (185 degrees C). Overall, the experiment captures the most important features needed to discern the mechanistic factors that control this process. In this sense, confocal Raman microspectroscopy emerges as one of the best experimental techniques for the study of diffusion kinetics in this type of system.