UK Dust Network – 1 st Workshop Claire Horwell 24th May 2007
description
Transcript of UK Dust Network – 1 st Workshop Claire Horwell 24th May 2007
UK Dust Network – 1st WorkshopClaire Horwell24th May 2007
Cutting edge techniques for the analysis of volcanic ash and other natural particles
Objectives:
• To use mineralogy and geochemistry to make rapid assessments of the potential health hazard of volcanic ash (and other natural dusts).
• To understand WHY a mineral or dust may trigger a pathogenic respiratory response.
Questions to be answered:
• Is the dust small enough to enter the lungs?
• What is the composition of the dust?
• Is the surface of the dust reactive?
• Are individual particles ‘pure’?
• Other ideas?
Grain size
• Is the dust small enough to enter the lungs?
• Grain size analysis techniques:
– Laser diffraction• 70 analyses from around the world
– SEM with image analysis
– Sieving• > 63 m only
• New predictive technique
Malvern Mastersizer 2000
Horwell, 2007, accepted for publication
Composition of heterogeneous dusts
SEM image of volcanic ash
• Volcanic ash is often composed of tens of minerals.
• Some are considered toxic e.g. crystalline silica.
• Analytical techniques:– SEM-EDX gives individual particle
compositions but not polymorphs.
– XRD-PSD gives quantity of minerals in a bulk sample. High res. so no overlap between plagioclase and cristobalite.
– Raman-SEM allows polymorphic determination of individual crystals/particles.
XRD spectrum for Soufriere Hills dome-collaspe ash
0
100
200
300
400
500
600
700
800
900
1000
10 30 50 70 90Degrees 2 theta
Co
un
ts
Quartz STD
Cristobalite STD
Monterrat ash separate
Reactivity of surfaces• Electron Spin Resonance detects
free and surface radicals.
• Radicals formed by breaking bonds during fragmentation
• Radicals are highly reactive, damaging DNA, proteins, lipids etc.
• Likely to be one of several triggering mechanisms for chronic lung disease.
Production of silica surface radicals
Horwell et al. Environmental Research, 2003
• Soufrière Hills dome-collapse ash shows no generation of silica radicals (peaks expected at point A).
• Distinctive curve and peak (at point B) shows interaction of iron.
• Crystalline silica alone (Talvitie residue) has less iron but no significant generation of silica radicals.
Production of hydroxyl radicals:Fenton Reaction: Fe2+ + H2O2 Fe3+ + OH- + HO
Horwell et al. Environmental Research, 2003
Fe2+ release vs. hydroxyl radical production at 30 mins
0
2
4
6
8
10
12
0 1 10 100 1000
Fe2+ release (after 7 days, mol/m2)
Hyd
roxy
l ra
dic
al g
ener
atio
n
(30
min
s,
mo
l/m
2)
MON SHI
MER ELR
SAK MSH
M03 FUE
LAN TUN
PIN PAC
ETN CER2
V1872 V1904
V1906 V1871
Minusil 1906A
Production of hydroxyl radicals
Basaltic
Andesitic/Dacitic
Tephritic/Phonolitic
Minusil 5 Quartz standardHorwell, Fenoglio & Fubini, in review
Purity of crystalline silica• One could say that if the ash is
respirable and contains x-silica then it is a potential health hazard.
• BUT toxicological and epidemiological evidence appears to suggest that volcanic silica isn’t very toxic.
• We can use mineralogy to determine WHY x-silica is/ is not toxic.
• The problem: Difficult to analyse differences in composition at the nano-scale.
• Timeliness: New technology for high resolution micro-analysis e.g. TEM-EDX, FIB thinning etc.
How pure is volcanic x-silica?
SEM-EDX spectrum of cristobalite
1 mm
• Crystalline silica in volcanic ash may be modified by more-inert components. E.g. it is known that Al ameliorates toxicity.
• Evidence: SEM-EDX work indicates that silica particles are impure.
• The silica particles may be modified by:
– occlusion by glass
– intergrowth with glass or plagioclase
– substitution of Si from atomic structure by Al & Na.
Early results – dome rock
Cristobalite in dome rock vugh
1 mm
– In dome rock we see euhedral and platey crystals which have grown in cracks and vesicles by vapour-phase deposition.
– Raman-SEM confirms these are cristobalite.
Horwell, Williamson & Le Blond, in prep.
0.266
0.267
0.268
0.269
0.27
0.271
0.272
0.273
0.274
100 200 300 400 500 600 700 800 900 1000
0
0.2
0.4
0.6
0.8
1
1.2
MVO287 prismatic cristobalite 50x obj
MVO287 cristob
Reference cristobalite
Raman spectra from cristobalite in dome rock
Early results – dome rock
1 mm
Electron microprobe shows that the cristobalite is compositionally distinct from volcanic quartz, containing impurities of Al and Na.
Horwell, Williamson & Le Blond (in prep.)
Cristobalite structure
0
0.5
1
1.5
2
2.5
3
94 95 96 97 98 99 100 101
SiO2 (Wt. %)
Al 2
O3
(W
t. %
)
MVO945vp
MVO945qz
MVO819pl
MVO332vp
MVO1236pl
MVO1236vp
MVO1236qz
MVO1406vp
MontR1vp
MontR1qz
MVO617
MVO287
quartz
euhedral cris.
platey cris.
Case Study 1 – Volcanic ashEarly results
1 mm
SEM-EDX Elemental maps
In thin section, cracked appearance.
Devitrification of glass also produces crystalline silica.
Blue = Si
Pink = K
Green = Al
Case Study 1 – Volcanic ashEarly results
1 mm
SEM-EDX Elemental maps
In thin section, cracked appearance.
Devitrification of glass also produces crystalline silica.
Would not fragment as microlites.
Blue = Si
Pink = K
Green = Al
?
Where do we go from here?
1 mm
• Could alpha and beta forms of cristobalite and quartz have different toxicities?
• Extremely unusual to preserve beta variety but impurities potentially make it possible.
• Integration of work with toxicology.
• Application of techniques to other natural dusts e.g. coal and desert.
A useful tool for predicting the respirable fraction:
y = 0.5256x - 0.505R2 = 0.9754
0
2
4
6
8
10
12
14
16
18
20
0 5 10 15 20 25 30 35
< 10 m (cum. vol. %)
< 4
m
(cu
m. v
ol.
%)
y = 0.1877x - 1.9179R2 = 0.8608
-5
0
5
10
15
20
0 20 40 60 80 100
< 63 m (cum. vol. %)
< 4
m (
cum
. vo
l. %
)Data collected by Malvern Mastersizer
2000 laser diffractometer.
n = 65 samples from volcanoes worldwide.