In Situ Liquid Cell Observations of Asbestos Fiber Diffusion in Water

Our plain-English summary of In Situ Liquid Cell Observations of Asbestos Fiber Diffusion in Water, by L. Wu, C. Ortiz, Y. Xu, et al (Environmental Science & Technology, 2015).

The study

• Authors: L. Wu, C. Ortiz, Y. Xu, J. Willenbring, D. Jerolmack
• Journal: Environmental Science & Technology
• Published: 2015;49(22):13340-9
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

We present real-time observations of the diffusion of individual asbestos fibers in water. We first scaled up a technique for fluorescent tagging and imaging of chrysotile asbestos fibers and prepared samples with a distribution of fiber lengths ranging from 1 to 20 μm. Experiments were then conducted by placing a 20, 100, or 150 ppm solution of these fibers in a liquid cell mounted on a spinning-disk confocal microscope. Using automated elliptical-particle detection methods, we determined the translation and rotation and two-dimensional (2D) trajectories of thousands of diffusing chrysotile fibers. We find that fiber diffusion is size-dependent and in reasonable agreement with theoretical predictions for the Brownian motion of rods. This agreement is remarkable given that experiments involved non-idealized particles at environmentally relevant concentrations in a confined cell, in which particle-particle and particle-wall interactions might be expected to cause deviations from theory. Experiments also confirmed that highly elongated chrysotile fibers exhibit anisotropic diffusion at short time scales, a predicted effect that may have consequences for aggregate formation and transport of asbestos in confined spaces. The examined fibers vary greatly in their lengths and were prepared from natural chrysotile. Our findings thus indicate that the diffusion rates of a wide range of natural colloidal particles can be predicted from theory, so long as the particle aspect ratio is properly taken into account. This is an important first step for understanding aggregate formation and transport of non-spherical contaminant particles, in the environment and in vivo.

In plain English

This laboratory experiment filmed individual chrysotile asbestos fibers moving randomly in water (Brownian motion, or diffusion). It studied fibers in water, not people or animals. The fibers came from a chrysotile ore block from the El Dorado Mine, Salt River, Arizona. They were tagged with a fluorescent protein and prepared at lengths of 1 to 20 μm. Suspensions at 20, 100 and 150 ppm were sealed in a thin liquid cell and recorded with a confocal microscope, and software tracked thousands of fibers. Measured movement rates differed from the predictions of existing theory for rod-shaped particles by a factor of 2. Rotation rates differed by a larger range, a factor of 4. Longer fibers turned much more slowly, as the theory predicts. Over a time scale of 0.1 seconds, the longest fibers had a diffusion coefficient along their length about 6 times their diffusion coefficient across it. The authors reported that diffusion generally slowed as concentration rose. They concluded that the theory predicts the fibers' motion to first order. They noted that tracking only two dimensions, and fibers drifting out of focus, introduced errors.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experiment: fluorescent-tagged chrysotile fibers filmed in water in a sealed liquid cell under a spinning-disk confocal microscope, with automated tracking, and results compared with theory for Brownian motion of rods
• Who was studied: Thousands of chrysotile asbestos fibers, 1 to 20 μm long, prepared from a raw chrysotile ore block (purity 90%) from El Dorado Mine, Salt River, Arizona; experiments at the University of Pennsylvania
• Exposure: Chrysotile fiber suspensions in water at 20, 100 and 150 ppm, fibers 1 to 20 μm long, tagged with a DksA protein fluorescent probe
• Main results:
  • For translational (side-to-side) movement of the fibers, measured diffusion rates differed from theoretical predictions by a factor of 2, which the authors described as small
  • Translational diffusion depended on fiber length, but the effect was mild
  • Rotational diffusion fell steeply with fiber length (an inverse cubic dependence), as theory predicts, and the experiments confirmed this trend
  • For rotation, the difference between measured and theoretical rates had a larger range than for translation, a factor of 4
  • For fibers with the smallest aspect ratio class (length-to-width about 1 to 3), the measured rotational diffusion coefficient was 2.3 × 10−1 ± 3.3 × 10−2 rad2/s, against a theoretical prediction of 1.3 × 10−1
  • For fibers with the largest aspect ratio class (about 9 to 11), the measured rotational diffusion coefficient was 9.0 × 10−4 ± 3.6 × 10−4 rad2/s, against a theoretical prediction of 2.6 × 10−3
  • The longest fibers (aspect ratio of order 10) showed anisotropic diffusion: at a time scale of 0.1 s, the ratio of the diffusion coefficient along the fiber axis to the diffusion coefficient across it was about 6
  • Short, low aspect ratio fibers moved equally in all directions over the whole observation period
  • For fibers with an aspect ratio of 10, the time for movement to become equal in all directions was of order 10 s, consistent with observations
  • Increasing concentration from 20 to 150 ppm appeared to produce a modest change in diffusion rate, with diffusion generally decreasing as concentration increased
• The authors' conclusion: The authors concluded that existing theories of single-particle Brownian motion of rods predict, to first order, the movement and rotation of the chrysotile fibers. They suggested the theory may also describe diffusion of asbestos and other fibers in lung fluids or porous solids.
• Limitations the authors state:
  • Fibers moved in three dimensions but were tracked in two, so measured diffusion rates differ from true values by a small but unknown amount
  • Fibers could move in and out of the focal plane, introducing errors in position measurements
  • Rotation measurements were more error-prone than translation measurements
  • Imperfections in the fibers may have caused uneven fluorescent coating, leading to errors in identifying and characterizing fibers
  • Quantifying the concentration effect would need a much wider range of concentrations, which was hard to achieve because optics degrade at higher concentrations
  • The liquid cell experiment is far simpler than the natural environment

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile.

Medical terms in this study, explained in our Lexicon: Chrysotile, Fiber Length, Aspect Ratio (Length-to-diameter ratio).

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