3
of surfactants (24, 25), resulting in considerable cost sav-
ings as well.
SURFACTANT SELECTION
Many different chemical dust suppressants have been used,
including salts, asphalt emulsions, vegetable oils, molas-
ses, synthetic polymers, mulches, and lignin products (26).
Many of these are certainly non-toxic and will do no harm
to the environment or to workers. However, as Piechota
et al. (26) describes, it is important to have a full analy-
sis of any material as some may be toxic waste products
from other industries. Health issues for workers may arise,
and soil could become contaminated. A Safety Data Sheet
should be requested for any material considered as a dust
suppressant additive/surfactant.
The next consideration is the improvement in wettabil-
ity of the dust particles with the addition of the surfactants.
According to a comprehensive review by Zhao et al. (27),
these tests can be classified as static or dynamic tests. Static
tests include contact angle measurements, liquid surface
tension tests (lower surface tension should correlate to bet-
ter wetting), capillary rise tests, liquid penetration tests, and
the simplest or most common sink test or Walker test (28).
Dynamic tests can include wind tunnel or other field tests
(27). All these tests measure the potential to wet the surface
of a dust particle. Xu et al. (29) provides a good description
and review of these and other static tests methods.
In the Walker test as illustrated in Figure 2, an air-dried
sample of the dust is placed on top of the liquid in a small
beaker or graduated cylinder, and the time to completely
wet and settle the dust is measured. Tests can be conducted
easily with different surfactants and at different surfactant
concentrations. Quicker settling times give better wettabil-
ity. Note that the water used in these experiments must be
the water used on site as the contained ions will affect the
ability for the surfactant to reduce the water’s surface ten-
sion. This test is thought to simulate the particle capture
mechanism (28) and might be used to set initial concentra-
tions for field tests of any surfactant.
Foaming agents need to be evaluated for wettability as
well as for their ability to foam in water from the site.
Going forward, wettability might not be the most
important criterion for a good surfactant. As research pro-
gresses into the surface chemistry of dust and the health
effects of ROS and, specifically, silanols on respirable crys-
talline silica, it will be important to evaluate whether these
surfactants might increase the ROS content making the
dust more toxic or reduce the ROS content making the
dust less toxic. One such project is in progress at Penn State
with Arnold as the Principal Investigator (30). Results with
several potential surfactants are given in Table 3 for quartz
collected from coals representing different coal ranks. The
hydroxyl content (10–9 mol.L–1) on the surface of particles
pulverized to 10 microns to represent respirable dust
sizes was measured using a spectrofluorometer for the bare
surface in distilled water at pH 7 and for nine different
reagents at two concentrations each. As indicated, the OH
content generally decreased in the presence of the surfac-
tants, with the addition of some of the surfactants causing
a very large drop in OH content. It can also be noted, as
stated previously, that all the quartz samples did not exhibit
the same behavior in the presence of the various surfactants,
further indicating that testing must be conducted for each
individual sample. Also, note that all surfactants did not
reduce the OH content to the same extent. The presence
of other contaminants with the quartz may have played a
role and the interaction of the chemicals with the surfaces
certainly affected the results.
Figure 1. The zeta potential of quartz from various sources (20, 21, 22)
of surfactants (24, 25), resulting in considerable cost sav-
ings as well.
SURFACTANT SELECTION
Many different chemical dust suppressants have been used,
including salts, asphalt emulsions, vegetable oils, molas-
ses, synthetic polymers, mulches, and lignin products (26).
Many of these are certainly non-toxic and will do no harm
to the environment or to workers. However, as Piechota
et al. (26) describes, it is important to have a full analy-
sis of any material as some may be toxic waste products
from other industries. Health issues for workers may arise,
and soil could become contaminated. A Safety Data Sheet
should be requested for any material considered as a dust
suppressant additive/surfactant.
The next consideration is the improvement in wettabil-
ity of the dust particles with the addition of the surfactants.
According to a comprehensive review by Zhao et al. (27),
these tests can be classified as static or dynamic tests. Static
tests include contact angle measurements, liquid surface
tension tests (lower surface tension should correlate to bet-
ter wetting), capillary rise tests, liquid penetration tests, and
the simplest or most common sink test or Walker test (28).
Dynamic tests can include wind tunnel or other field tests
(27). All these tests measure the potential to wet the surface
of a dust particle. Xu et al. (29) provides a good description
and review of these and other static tests methods.
In the Walker test as illustrated in Figure 2, an air-dried
sample of the dust is placed on top of the liquid in a small
beaker or graduated cylinder, and the time to completely
wet and settle the dust is measured. Tests can be conducted
easily with different surfactants and at different surfactant
concentrations. Quicker settling times give better wettabil-
ity. Note that the water used in these experiments must be
the water used on site as the contained ions will affect the
ability for the surfactant to reduce the water’s surface ten-
sion. This test is thought to simulate the particle capture
mechanism (28) and might be used to set initial concentra-
tions for field tests of any surfactant.
Foaming agents need to be evaluated for wettability as
well as for their ability to foam in water from the site.
Going forward, wettability might not be the most
important criterion for a good surfactant. As research pro-
gresses into the surface chemistry of dust and the health
effects of ROS and, specifically, silanols on respirable crys-
talline silica, it will be important to evaluate whether these
surfactants might increase the ROS content making the
dust more toxic or reduce the ROS content making the
dust less toxic. One such project is in progress at Penn State
with Arnold as the Principal Investigator (30). Results with
several potential surfactants are given in Table 3 for quartz
collected from coals representing different coal ranks. The
hydroxyl content (10–9 mol.L–1) on the surface of particles
pulverized to 10 microns to represent respirable dust
sizes was measured using a spectrofluorometer for the bare
surface in distilled water at pH 7 and for nine different
reagents at two concentrations each. As indicated, the OH
content generally decreased in the presence of the surfac-
tants, with the addition of some of the surfactants causing
a very large drop in OH content. It can also be noted, as
stated previously, that all the quartz samples did not exhibit
the same behavior in the presence of the various surfactants,
further indicating that testing must be conducted for each
individual sample. Also, note that all surfactants did not
reduce the OH content to the same extent. The presence
of other contaminants with the quartz may have played a
role and the interaction of the chemicals with the surfaces
certainly affected the results.
Figure 1. The zeta potential of quartz from various sources (20, 21, 22)