Mineralogical investigation of settled and airborne dust collected from Cluj County, Romania

Abstract

Solid contaminants get into the air as dust, varying widely both in composition and size distribution.
Dusts are classified into two groups on the basis of grain size: airborne dust (< 10 fim) and settled dust
(> 10 urn) (Fig. 1). Dust can be of natural or anthropogeneous origin. Natural dust originate from soil
erosion, forest fires, biogenic materials, sea salts above the sea surface, and volcanic eruptions.
Anthropogeneous dust arise mainly from industrial activity and transportation.
Phase (mineral) composition and size distribution of dust (Fig. 2) make it possible to establish the
effect that dust have on the environment and health.
In Romania, county-level environmental agencies collect settled dust samples monthly. The collected
samples are then weighted. Settled dust samples were collected and studied from 18 localities (Fig. 3) in
the period March-July 2003 by the Environmental Protection Agency of Cluj County. Our main aim was
to determine the phase (mineral) composition of these dust samples.
The collected samples were separated on the basis of grain size fractions >400 цт , 400-250 u.m,
250-125 u,m, 125-63 urn, 63-32 p.m, 32-1 u.rn and <1 p.m. (The first six fractions were obtained by wet
sieving, while the last one by settling.) Grain size distributions were then considered. Fractions above
63 u.m were examined using studied by light microscopy. Routine powder X-ray measurements have
been carried out on the samples, while the respective characteristics most typical grains were studied
with scanning electron microscopy (SEM equipped with EDX).
Minerals of the settled dust originate predominantly from soil erosion, accompanied by industrial
contaminants (Fig. 4, Tables 1-2). There is a strong correlation between grain size and composition: the
proportion of mineral constituents increases with decreasing grain size.
Among the identified constituents, the following are of natural origin: quartz, calcite (Fig. 5C),
feldspar, dolomite, mica, chlorite, anhydrite (?), goethite, kaolinite-group minerals, amphibole group,
ruffle (Fig. 5B), kyanite, asbestos (Figs 5А and 9), ilmenite (Fig. 5D), halite and the biogenic materials like
insects and plant fragments. Of anthropogenic origin are the silicon-carbide, corundum, cristobalite,
tridymite, spinel-structure compounds (?) and plastic materials. On the X-ray powder diffraction
patterns micas and kaolinite-group minerals occur mainly in the lower grain size fractions (63-1 um,
63-32 um). Cristobalite was present in large amounts in some of the samples (Figs. 6 and 7). This material
may eventually get enriched in the respirable fraction, too, (Figs. 6B and 7), carrying an increased health
risk. Tridymite is subordinate relative to cristobalite, and occurs only in samples that contain siïiconcarbide as well.
In salt-rich samples the salt may well originate from a surface-exposed salt dome. However,
contamination from the winter salt treatment of the roads cannot be excluded either. In the settlement of Turda, large amounts of calcite originate from local outcrops of limestone. However, the nearby cement
factory, which uses limestone as the base material for cement production, should also be taken into
consideration.
The composition of asbestos is transitional between that of serpentine and amphibole (Fig. 9).