1. Sand Mine Particulate Analysis
Callie Fischer, Julie Zhang, Joe Oster, Dr. Patricia Cleary| Chemistry Department
Abstract
Calibration
Investigation of the Interferences
Concern has arisen about levels of silica in ambient particles near frac sand mines. In order to assess these levels of silica, we have adopted an XRD analysis to test real air samples. Calibrations were constructed with silica standards containing 10μg - 500μg respirable silica on filter media with detection limits of μg. Real air samples were collected at a frac sand site using a personal Cascade Impactor. Filter substrates were pre-weighed and post-weighed to determine the total dry mass of particles sampled and preliminary XRD results show as much as 16% of the mass can be attributed to crystalline silica in the samples. Problems, interferences and intercomparison with the SEM results will be discussed.
Calibration curve was created using 10μg, 20μg, 50μg, 100μg, 250μg and 500μg silica standard filters, with detection limit of 28μg.
One of the current focuses of our work is the inconsistency between XRD and SEM quantifications of silica content. We are investigating possible sources of interference that could be influencing the observed results.
This scan was obtained from a real air sample. The red peaks correspond to the silver material of which the filter is composed. The shorter, blue peaks correspond to silicon dioxide (in the form of quartz) present in the air sample. Green peaks were identified to be calcite.
Field Sampling Strategy
Teflon and polycarbonate filters were placed inside the personal Cascade Impactors to collect samples for 24 hours near a high-dust producing area in an active sand mine in Northwestern Wisconsin.
Method Development
Results
Sample Collection & Preparation
Sampling Date: 07/25/2015
This scan is a zoom-in graph, showing the major silica peak of this air sample. Net area of this peak was calculated to estimate the mass of quartz using the calibration curve.
Sample Collection
Pre-weigh filters
Place filters in the Cascade Impactors
Send out the Cascade Impactors to the field
Sample 24 hours on site
Post-weigh filters
Sample Preparation
Ash filters in Muffle Furnace
Break up particles in Ultrasonic Bath
Filter suspension
Heat fix to glass slide
Mount on XRD
Cascade Impactor Stages
Peak Net Area
Quartz Estimate Mass (𝝁g)
Particle Mass Deposited on Filters (𝝁g)*
Quartz% on Filters
A (2.5-10μm)
0.3736 63
510
12%
B ( μm)
0.1007 28
320 9%
C ( μm)
24 (Below DL)
130 -
D ( μm)
18 (Below DL)
140
Blank
15 (Below DL) 60
Comparison with SEM spectrum
Sampling Date: 09/30/2015
Cascade Impactor Stages
Peak Net Area
Quartz Estimate Mass (𝝁g)
Particle Mass Deposited on Filters (𝝁g)
Quartz% on Filters
A (2.5-10μm)
0.1951 39
250
16%
B ( μm)
19 (Below DL) 70 -
C ( μm)
D ( μm)
14 (Below DL) 50
Blank
Most particles deposited on the filters were not pure substances. It is difficult to identify compositions using XRD, since peaks could overlap.
Therefore, SEM technique was used and showed that many of the particles on filters contain large amounts of Mg, Ca and AL. Silica level is relatively low.
Sampling Date: 01/15/2016
Cascade Impactor Stages
Peak Net Area
Quartz Estimate Mass (𝝁g)
Particle Mass Deposited on the Filters (𝝁g)
Quartz% on Filters
A (2.5-10μm)
16 (Below DL)
1330 -
B ( μm)
15 (Below DL)
560
C ( μm)
330
D ( μm)
14 (Below DL)
Blank
X-Ray Diffraction
Comparison with other minerals
X-rays are diffracted generating specific spectra corresponding to particular crystalline lattice structures. These spectra are measured by a detector and converted to a signal resulting in particular peaks on a scan. Peak area under the curve was calculated to determine the Quartz mass in our research.
So far we have identified one mineral in particular, muscovite, as a possible source of interference due to the close proximity between the signals for the major peaks of muscovite and quartz.
*Total particle mass was measured from a Teflon filter on a parallel sampler.
This is a photo of the X-ray diffractometer used in our research. It is composed of three parts: (1) a cathode ray tube which conducts X-rays and projects them in a beam onto the sample, (2) a sample holder, and (3) an X-ray detector which receives the diffracted X-Rays.
Tables above are three sets of data we collected from the field since summer So far, the Quartz% of particles deposited on filter show as much as 16% of the mass can be attributed to crystalline silica. However, most of results are below our current detection limit. Therefore, the estimated Quartz mass on filters show zero attribution to crystalline silica.
Acknowledgements
We would like to acknowledge and thank NCUR, LTS printing services, Anthony Wagner and the Materials Science Center, as well as the Office of Research and Sponsored Programs and Dr. Matthew Evans for everything they do in providing the opportunity to participate in these research experiences.