Characterization of respirable coal mine dust recovered from fibrous polyvinyl chloride filters by scanning electron microscopy

While respirable dust monitoring has long been done on the basis of mass concentration, it is increasingly recognized that specific dust characteristics may play an important role in health outcomes. Scanning electron microscopy with energy-dispersive X-ray (SEM-EDX) can be used for detailed particle analysis, but the quality of results can be dependent on the sample substrate; ideally, particles should be deposited on a smooth, uniform substrate like polycarbonate (PC). While direct air sampling onto PC filters is possible, this is not the standard approach in many situations. For example, in coal mines, polyvinyl chloride (PVC) filters are often used. They enable conventional mass-based analysis — of the total respirable dust, and the content of crystalline silica in this fraction — but are fibrous and thus not ideal for SEM-EDX or other microscopy. To address this issue, the current work tested a simple method to recover respirable dust particles from PVC filters and redeposit them to PC. Results generally indicated that recovered particles are representative of the original dust sample in terms of mineralogy distributions. Where this was not the case, the issue appeared to be with high particle loading density during SEM-EDX work rather than related to the recovery/redeposition method itself. This bodes well for applications where a single PVC filter sample could be used for nondestructive analysis to determine respirable dust mass and/or respirable silica mass, followed by preparation for SEM-EDX analysis to evaluate particle characteristics. The recovery and redeposition method might also be adapted for other applications, such as to enable the analysis of respirable dust samples collected by the continuous personal dust monitor (CPDM). Background Particle characteristics such as size and mineralogy can control lung deposition and response [1,2]. Understanding such characteristics in the context of respirable coal mine dust has become increasingly of interest due to the resurgence of occupational lung disease among coal miners in the United States and elsewhere [3,4]. While respirable coal mine dust monitoring is generally based on measurements of mass using physical filter samples, particle-level analysis is possible. SEM-EDX can be used to simultaneously acquire particle size and shape data (from the SEM images) along with chemistry (from the EDX spectra), and the chemistry can be interpreted to infer mineralogy [4]. Figure 1: Mineralogy distributions for laboratory-generated sample pairs (D = direct filter, R = recovered filter). Pairs for which the Freeman Halton test indicated disagreement (P value < 0.05)are outlined in red. One important condition for SEM-EDX analysis is that the particles are deposited on a suitable substrate to enable particle identification (from the background) and limited interference during the collection of EDX data. Track-etched PC filters are commonly used for this work because their smooth surface, low impurity and consistently sized pores are ideal for both sample collection and analysis [5]. Although dust samples can be collected on PC filters for direct SEM-EDX analysis, this is typically not the case in mines. Rather, respirable mine dust sampling is conventionally done using PVC filters. PVC filters are compatible with standard analytical methods to determine mass concentration of respirable dust and respirable crystalline silica (the basis of regulatory standards), but their fibrous nature is not ideal for SEM-EDX work. Notably, in U.S. coal mines, use of the CPDM has replaced some of the conventional respirable dust sampling since 2016 [4]. The physical sample collected by the CPDM is used to track mass concentration in real time, but it is presently not used for follow-up analysis — such as of crystalline silica or particle characteristics — due the nature of the filter substrate (fibrous borosilicate glass fiber).To enable the SEM-EDX analysis of respirable dust particles collected on fibrous filters, the current work tested a simple method for recovering dust from PVC and redepositing it on PC. Materials and methods Experiments were conducted using pairs of respirable dust samples. Each pair consisted of one sample on PC and one sample on PVC, which were collected side by side to create duplicates. A total of 10 sample pairs were generated in the laboratory using high-purity source materials — coal, rock dust, kaolinite and silica — that were chosen to represent the major particle types observed in real respirable coal mine dust samples [6]. Additionally, 44 pairs of field samples were used, which had been collected in different locations on prior projects. All sample collection was done using standard equipment: a 10-mm nylon cyclone and air pump operated at 2.0 L/m to pull respirable particles onto PVC or PC filters in closed styrene cassettes. Figure 2: Difference in the number percentage of particles in major mineralogy classes versus the difference in particle loading density (PLD) for (a) 10 laboratory-generated sample pairs and (b) 44 field sample pairs (D = direct filter, R = recovered filter).Pairs for which the Freeman Halton test indicated disagreement (P value < 0.05)are outlined in red. For SEM-EDX analysis, each original PC filter was analyzed directly, and the dust on the original PVC filter was recovered by sonication in isopropyl alcohol and redeposited onto a clean PC filter by vacuum, then analyzed. The SEM-EDX analysis was done using an automated routine, which has been described in detail [6]. Briefly, the routine moves across the filter to identify approximately 500 particles (about 1 to 10 µm in size), collecting data on each particle’s size and elemental abundances. The elemental data are then used to classify the particle into one of eight mineralogy classes: carbonaceous (C), mixed-carbonaceous (MC), aluminosilicates (AS), silica (S), other silicates (SL-O), carbonates (CB), heavy minerals (M) or other (O). Using the SEM-EDX-derived data, mineralogy distributions (number percent in each class) and D50 size (µm) were determined per filter, as was the particle loading density (PLD, number of particles per µm2 analyzed). Finally, results were compared between the “direct” and “recovered” filter in each pair. Results With respect to mineralogy distributions, there was typically good agreement between the direct and recovered filters for the laboratory-generated sample pairs (Fig. 1). Based on the Freeman-Halton exact test of independence, at 95 percent confidence, filters in just three of these 10 pairs were observed to disagree. This indicates the method used to recover particles from the PVC filters and redeposit them on PC filters generally yields a representative sample. Interestingly, the same statistical test indicated that filters in 25 of the 44 field sample pairs disagreed. However, careful analysis of these pairs with respect to their PLD values offers a possible explanation: on highly loaded filters, proximal particles can interfere with one another and skew apparent mineralogy distributions. Figure 2 clearly shows this effect, wherein greater differences in PLD between paired direct and recovered filters are correlated with greater differences in their apparent mineralogy distributions. Most notably, in any given pair, the higher-PLD filter appears to have a greater abundance of AS particles and a lower abundance of C and MC particles (interpreted as coal dust) than its lower-PLD counterpart. This suggests that on high-PLD filters, AS particles (classified by sufficient presence of aluminum and silicon content) are interfering with classification of coal dust particles (classified by the relative absence of elements other than carbon and oxygen). Another factor underlying the apparent disagreement in mineralogy distributions for some field sample pairs could be agglomeration of particles; agglomeration is more likely on the directly analyzed filters, which coincidentally also exhibited higher PLD in this study. Overall, the D50 particle sizes for direct filters were observed to be slightly larger than for the recovered filters, which is consistent with deagglomeration during the recovery/redeposition process. Conclusions A simple method for the recovery of respirable dust particles from fibrous PVC filters and redeposition onto a smooth PC substrate has been demonstrated. In general, the method appears to yield a representative sample that can be used for particle characterization by SEM-EDX. For this application, care must be taken to limit particle loading density, especially if an objective is to classify particles by their chemistry or inferred mineralogy. Moreover, it must be acknowledged that the recovery method likely deagglomerates particles; this might simplify classification but also prevent observation of native particulates. That said, neither loading density nor deagglomeration is expected to affect bulk particle analysis. Thus, the recovery method demonstrated here might also be applied to support analysis by other techniques. Selected references World Health Organization (1984) Evaluation of exposure to airborne particles in the work environment. World Health Organization, Geneva. Oberdörster G, Oberdörster E, Oberdörster J (2005) Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles. Environ Health Perspect 113(7) 823–839 Blackley DJ, Crum JB, Halldin CN, Storey E, Laney AS (2016) Resurgence of progressive massive fibrosis in coal miners—Eastern Kentucky. Morb Mortal Wkly Rep 65(49):1385–1389 NASEM (2018) Monitoring and sampling approaches to assess underground coal mine dust exposures. The National Academies, Washington Willis RD, Conner TL (2002) Guidelines for the application of SEM/EDX analytical techniques to particulate matter samples. US Environmental Protection Agency, Washington Sarver E, Keles Ç, Afrouz SG (2021) Particle size and mineralogy distributions in respirable dust samples from 25 US underground coal mines. Int J Coal Geol 247:103851

Characterization of respirable coal mine dust recovered from fibrous polyvinyl chloride filters by scanning electron microscopy

Authored by: A. Greth, S. Afrouz, C. Keles and E. Sarver

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