T H E E F F E C T OF V A R I O U S T Y P E S OF C E M E N T D U S T ON SULPHUR
DIOXIDE OXIDATION
IN T H E A I R
V. V A D J I C a n d M. G E N T I L I Z Z A
Institute for Medical Research and Occupational Health, University of Zagreb, Zagreb, Yugoslavia and R. H A L L E
JUCEMA, Zagreb, Yugoslavia
(Received 17 November, 1987) Abstract. The effect of various types of cement dust on the behaviour of sulphur dioxide in the air was investigated on model systems in different experimental conditions. Experiments were carried out with PC-15z-45s (Portland-blast furnace cement containing 15070 blast furnace slag), PC-25p-35s (Portland-pozzolan cement containing 25~ pozzolan) and EFD (electrofilter dust). EFD most effectively removed SO2 from the air stream. The next efficacious was PC-I 5z-45s, whereas PC-25p-35s was the least efficient. The efficacy of cement dusts for SO 2 removal from the air stream depended on their chemical and granulometric composition and in particular on the size of specific surface. The rate of reaction was also influenced by experimental conditions - - relative humidity, the length of contact, that is, the flow rate of gaseous mixture through the reactor, and the amount of cement dust. The experimental data show that in the contact between SO2 and cement dust catalytic oxidation of SO2 to sulphates takes place. Sulphates remain bound to the surface, from which they cannot be thermally desorbed, but can be released by extraction in the Soxhlet apparatus.
1. Introduction
A study of homogeneous and heterogeneous oxidations of sulphur dioxide to sulphates in ambient air (Urone and Schroeder, 1969; Urone et. al., 1968; Bufalini, 1971) is of major interest, because products of SO 2 oxidation are harmful to human health. Measurements of SO 2, total suspended particulates and sulphates as indicators of air pollution make part of a wide range of epidemiological investigations of acute and chronic effects on health (Waller, 1979) which have been carried out in USA by EPA (Environmental Protection Agency) as part of the CHESS project (Community Health and Environmental Surveillance System). It is not yet clear whether sulphates are better indicators of the presence of hazardous components in the air than SO2 itself nor, whether they are better indicators of the air pollution due to combustion than total suspended particulates measured in the samples collected from large volumes of air. The effect of sulphates on health probably varies in dependence on their physical and chemical form. In masses of polluted air sulphates are most often present in the form of ammonEnvironmental Monitoring and Assessment 11 (1988) 59-68. 9 1988 by Kluwer Academic Publishers.
60
V. VADJIC ET AL,
ium sulphate, which does not have the same effect as sulphuric acid. Presumably, in the atmosphere polluted with cement dust calcium sulphate is formed. Epidemiological studies conducted in real conditions in general population (Gomzi, 1983) provide main evidence of the association between air pollution and human health. Our laboratory investigations of the heterogeneous reactions of SO 2 with various types of soot demonstrated that SO2 in contact with soot was merely adsorbed on the surface, but remained chemically stable (Gentilizza and Vadji6, 1985) whereas in contact with various metal oxides SO2 underwent catalytic oxidation to sulphates (Vadjid and Gentilizza, 1985; Vadji6 and Gentilizza, 1986). The relationship between sulphates and SO2 in the ambient air polluted with cement dust was studied in the vicinity of a cement factory emitting SO2 and cement dust. Measurements were carried out in an industrial suburban area with several minor plants emitting SO2. Earlier data (Gentilizza and Vadjid, 1986) demonstrated that cement dust may influence SO2 oxidation into sulphates. Therefore, we continued to study the effect of various types of cement dust on oxidation of SO2 to sulphates at ambient temperature and in different laboratory conditions. Our experiments with pure mixtures containing known concentrations of all present reactants which were conducted in strictly controlled conditions, cannot correspond in full to the measurements in ambient air where other pollutants may interfere and meteorologicaI conditions are not completely under control.
2. Experimental The material and methods were as described in a previous paper (Vadji6 and Gentilizza, 1985, Vadji6 and Gentilizza, 1986). A mixture of SO2 and air was prepared following a procedure for the preparation of calibration mixtures of gases and vapours. A system of three bottles connected in series with the same initial concentration of the gas mixture in each was used. The rate of dilution followed an exponential equation which served to calculate a theoretical curve of dilution (Setterling, 1953). The mixture of SO2 and air first flowed into the reactor with cement dust at a known and controlled flow rate, and was then passed through the washing bottle containing the absorbing solution in which the SO2 fraction that had not reacted with cement dust was retained. The amount of SO2 lost in contact with cement dust was calculated as the difference between the theoretical quantity of SO2 (calculated from the curve of dilution) and the amount absorbed in the absorbing solution after having passed through the reactor with cement dust. Control samples of SO2 were collected in the same manner, but with the reactor empty. The volume of the individual samples was 6 dm 3. Sulphur dioxide was sampled alternatively, in sodium tetrachloromercurate (TCM) solution with analysis using the West and Gaeke spectrophotometric method (West and Gaeke, 1956) and in hydrogen peroxide solution with analysis by the
THE EFFECT OF VARIOUS TYPES
61
sulphate method (OECD, 1964; Fritz and Yamamura, 1955). The TCM method is specific for SO2, whereas the sulphate method besides measuring SO2 also r~.easures SO3 and sulphates. The higher values obtained by the sulphate method suggest a partial conversion of SO 2 to SO 3 in the gaseous phase. Sulphur dioxide desorption from cement dust samples was carried out using improvized equipment. The reactor with cement dust (after reaction with SO2 had taken place) was placed into a temperature-controlled heater. Gas carrier was nitrogen. Desorbed SO 2 was sampled into a washing bottle containing 50 cm 3 TCM and analyzed using the TCM method. The extraction of sulphates from cement dust was accomplished with redistilled water in a Soxhlet apparatus. The extract contained water-soluble sulphates. It was submitted to cation separation in an ion exchange column and sulphates were determined using the sulphate method (Gentilizza and Vadji6, 1980). 3. Results and discussion Table I shows the results of chemical analysis and Table II those of granulometric distribution for all the three types of cement dust. The highest percentage of small particles in EFD points to the largest specific surface of 5113 cm 2 g - 1. The specific surface of PC- 15z-45s was 4149 cm 2 g - 1 and that of PC-25p-35s was 3500 cm 2 g - l Table III shows decreased values o f the mass concentrations of SO2 (w =P/o x 100) after it passed through various types and amounts of cement dust in different experimental conditions and at ambient temperature. Our laboratory investigations were directed first to measuring a decrease in the mass concentration of SO2 after contact with various types of cement dust. In TABLE I Chemical analysis of various types of cement dust % Components
Humidity Loss of ignation Non-soluble SiO 2 A1203 Fe203 CaO MgO SO 2 Na20 K20
EFP
PC-15z-45s
PC-25p-35s
0.36 35.27 -11.84 3.72 1.40 43.33 1.88 1.06 0.17 0.58
0.09 -0.89 20.88 5.64 3.34 62.59 2.47 1.98 0.34 0.67
0.83 1.38 12.82 17.72 5.70 3.31 54.00 2.04 1.49 0.32 0.43
EDF - electrofilter dust PC-15z-45s - Portland-blast furnance cement containing 15% blast furnance slag PC-25p-35s - Portland-pozzolan cement containing 25~ pozzolan
62
v. VADJI(~ ET AL. T A B L E II Granulometric distribution of various types of cement dust %
Particle size (#g)
EFD
PC-15z-45s
PC-25p-35s
0- 5 5- 10 10- 15 15- 20 20- 25 25- 30 30- 35 35- 40 40- 45 45- 50 50- 55 55- 60 60- 90 90-120 120-150 200
17.0 20.3 14.2 7.6 6.0 5.6 5.1 6.0 1.3 1.9 1.4 0.9 7.1 3.1 1.9 0.6
9.0 14.0 13.8 18.5 14.5 8.5 5.5 5.0 3.0 1.5 0.7 0.2 3.6 2.2 0.0 0.0
10.7 12.3 10.9 10.1 8.8 7.7 6.8 6.1 5.6 5.0 3.8 2.9 6.3 2.3 0.5 0.2
Specific surface cm 2 g - ~
5113
4149
3500
EFD - electrofilter dust PC-15z-45s - Portland-blast furnance cement containing 15% blast furnance slag PC-25p-35s - Portland-pozzolan cement containing 25% pozzolan T A B L E III Percent of SO: decrease after passage through various types of cement dust at different experimental conditions Cement type
quantity mg
Range of SO2 mass concentration mg d m - 3
EFD PC-15z-45s PC-25p-35s PC-15z-45s PC-15z-45s PC-15z-45s
10 10 10 10 10 50
49.4-7.6 45.0-6.9 47.0-7.1 47.2-7.2 37.5-5.8 50.4-7.8
w PH N s*
-----
0.1 0.1 0.1 0.1 0.2 0.1
Air quality s*
N
Percent of SO2 removal
20 20 20 20 20 20
57.9 52.5 50.4 28.8 42.3 82.4
76% RH
+ + + + + +
arithmetic mean relative humidity number o f experimental data air priviously dried over silica-gel
experiments humidity percent
Flow rate of gaseous mixture dm 3 m i n - l
we used
and
10 m g o f E F D ,
PC-15z-45s
a flow rate of 0.1 dm 3 min -I
removal
of SO2 in contact
and PC-25p-35s
(Figure
with the cement
1). T h e
a t 76~
mean
dusts under
values
relative of the
our experimental
THE
EFFECT
OF
VARIOUS
63
TYPES
6FD 75.
9 A9
"o ol =,
r~
@A
9
f01.
S-
~>~
o
TCM
CONTROL
9
TCM
CEMENT
"
H202 CONTROL
&
H202 CEMENT
--
10 20
PC
-'}5
30 40 S0 60
Z
-
70
THEORETICAL
CURVE
80 90 100 "r10 120
/.5 S
PC - 25 p - 35 S
7-
5
ea
Aea~ -o
101
0 u~
S-
m;-
A
o~
$-
@A
D ' 10
+ 20
' 30
' AO
+ 50
c"-~ ~ 60 70
~ 90
, 90
AIR
~ ]00
"-I. 'T 1~0 '/20
FLOW
-r
m
+
i
+
i
:,,o 3a ,m so
VO L U M E
,
+
,
r
,
+
,_
m, ++'o ~o ~o loo ++o ~:'o
tdm 3)
F i g . 1, E f f e c t o f d i f f e r e n t t y p e s o f c e m e n t d u s t o n S O 2 d e c r e a s e (10 r a g c e m e n t d u s t , R H 7 6 % , r a t e 0. l d m 3 r a i n - ').
64
V. VADJIt~ E T AL.
'/6 %
SILICA
RH
5
GEL
5-
"" ?E9~
10I.
0
5
9
10'
9
5 O 9
TCM CONTROL TCM CEMENT H202 CONTROL
9
. 10
.
. 20
.
30
&0-
. 50
.
60.
. 70
.
. 80
AIR
. 90 .
100 110 120
FLOW
H202 CEMENT THEORETICAL CURVE
,b 2'0 ~, ~ ~'o ~
VOLUME
;o
' 60
9'o loo
1'10
1~o
{din 3 )
Fig. 2. Effect of relative humidity on SO 2 decrease (10 mg PC-15z-45s, rate 0.1 d m 3 min-~).
conditions indicated (Table III) that EFD having the largest specific surface removed SO2 from the air most efficiently - 5 7 . 9 . For PC-15z-45s, with smaller specific surface, the percentage of removed SO 2 was lower - 52.5 and for PC-25p-35s it was 50.4 because of the smallest specific surface. In further investigations we used PC-15Z-45S, the cement dust of medium efficacy. Firstly, we investigated the effect of relative humidity on the decrease in the mass concentration of SO2 after passing it through 10 mg of cement at a flow rate of 0.1 dm 3 min-1 (Figure 2). The figure (left-hand side) illustrates the experiment in which the influence of 76~ relative humidity was very strong compared to the experiment where the air was dried over silica-gel before entering the reaction system containing the mixture of SO2 and air (right-hand side of the figure). Other conditions were identical in both experiments. The mean value of the mass fraction of removed SO2 at a relative humidity of 76% was 52.5, but it was as low as 28.8 when the air had been dried over silica-gel (Table III). These data are in accordance with those found in the literature (Judeikis et al., 1978; Li et al., 1968; Haury et al., 1978) and emphasize the importance of relative humidity in heterogeneous reactions taking place between SO2 and suspended particulates. Figure 3 illustrates the effect of the flow rate of the SO2 and air gaseous mixture of the decrease in the mass concentration of SO2 in contact with 10 mg of PC-15z45s, at a relative humidity of about 76~ Owing to a longer contact between gaseous
65
THE EFFECT OF VARIOUS TYPES
RATE 0,1 dm 3 rain 1
RATE
0,2
-1
dm 3 rain
5
~E "r
lo1.
101
9 9
C)
5 O m
O
%
9
TCM CONTROL TC M CEMENT H202 CONTROL
A 9
10 20 3
S0
60 70 80
,
90 100 110 120
AIR
FLOW
9 9
H202 CEMENT THEORETICAL CURVE
,
10 20
VOLUME
,
,
,
30 40 50
,
,
,
J
60 70 80
,
f
J
90 100 110 120
( tim 3}
Fig. 3. Effect of flow rate on SO 2 decrease (10 mg PC-15z-45s, RH 76070).
50 mg
10 mg 7-
S
"o
~.,0 ~
9
9
101.
o~ (/)
% s
1o 2'0 ;o ~
io ~'o 7o 8~ ,'o ,~o ,;o 1;,o AIR
FLOW
Fig. 4. Effect of various amounts of PC-15z-45s
O 9 A
TCM CONTROL TCM CEMENT H202 CONTROL
9
H202 CEMENT THEORETICAL CURVE
1
0
VOLUME on S0 2
9
0 9
100 110 I 0
( d m 3)
decrease (RH 7607o, rate 0.1 dm 3 min-~).
66
V. VADJI(~ ET AL.
S O 2 and cement dust a lower flow rate of the gaseous mixture (0.1 dm 3 min -1)
enhanced the decrease in the SO 2 mass concentration to a greater extent than a higher flow rate (0.2 dm 3 min-1). The mean values of the mass fraction of removed SO 2 under the above conditions were 52.5 at a rate of 0.1 dm 3 m i n - 1 and 42.3 at a rate of 0.2 dm 3 m i n - 1 (Table III). The effect of different amount of PC-15z-45s on the decrease in the mass concentration of SO2 was studied at the optimum flow rate (0.1 dm 3 m i n - l) and at a relative humidity of about 76%. Samples of 10, 50, and 100 mg of cement dust were used in experiments. Figure 4 shows the results of SO 2 removal from the air stream after contact with l0 and 50 mg of cement dust. The percentage of the mass fraction of removed SO 2 after contact with 10 mg of cement dust was 52.5. Greater quantities of cement dust enhanced the effect so that with 50 mg the percentage was 82.4 (Table III) and with 100 mg of cement all the SO2 was removed from the air stream. According to our previous study of SO2 oxidation to sulphates (Vadji6 and Gentilizza, 1985; Vadji6 and Gentilizza, 1986) 50 mg of MnO 2 completely removed SO 2 from the air stream by catalysis. Partial oxidation of SO2 to gaseous SOa (10-20%) also took place and the rest of SO 2 was oxidized to sulphates. In the gaseous phase SO3 was determined using alternatively the TCM and sulphate methods. The presence of SO 3 could not be detected by the T C M method, which is specific for SO2, whereas using the sulphate method, which in addition to SO a also detects SO 3, 10-20% of SO 3 was measured. Although SO2 removal from the air stream was complete with 100 mg of cement dust, the data obtained by the TCM and sulphate methods did not show any difference. To determine whether under our experimental conditions SO 2 is merely adsorbed onto the cement dust surface or is also converted to sulphates, the samples of cement dust which had been in contact with SO2 were submitted to thermal desorption at 250 ~ At this temperature desorption did not take place and it is assumed that SO 2 was not only adsorbed on, but was also chemically bound to the surface of cement dust. Cement dust samples and the bound SO 2 were extracted in the Soxhlet apparatus. All of the SO 2 bound onto cement dust surface was extracted and detected as sulphate. To compare the results with adsorbed SO2 the sulphates detected in the sample following extraction in the Soxhlet apparatus were calculated and shown as SO 2 (Figure 5). 4. Conclusions Our experimental data show that S O 2 conversion to sulphate depends on the chemical composition of cement dust, especially on its granulometric composition. The size of the specific surface o f cement dust has a positive effect on SO2 conversion, because it is assumed that SO2 is first adsorbed onto the surface and then, by catalytic oxidation, converted to sulphates. Sulphates remain bound onto cement
THE EFFECT OF VARIOUSTYPES
1
67
ill i !10 IO!ASOREI D S !l 1
2
3
,~
: J li I I I Jl i
2
4
SAMPLES
Fig. 5. Sulphate extraction from PC-15z-45s in Soxhlet extractor.
dust surface and cannot be thermally desorbed at 250 ~ but can be extracted in the Soxhlet apparatus and detected as sulphate. E F D was most efficacious in removing SO~ from the air stream. PC-15z-45s was less so, and PC-25p-35s was the least effective. The amount of cement dust has a marked effect on the investigated reaction. With 100 mg of cement dust SO2 conversion to sulphate is complete. High relative humidity and low flow rate of the gaseous mixture, that is, the length o f contact between reactants, also have a positive effect on SO2 conversion. It can be assumed that in the ambient air at a constant concentration o f SO 2 and cement dust in emissions a possibility of SO 2 conversion to sulphate is greater at a lower wind rate because of longer contact o f reactants in the fume cloud. It can also be hypothesized that if a fume cloud emitted from a cement factory were carried by the wind stream towards some other larger SO 2 emitter, heterogenous catalytic oxidation of SO 2 to sulphates on cement dust particles might take place.
References Bufalini, M.: 1971, 'Oxidation of Sulfur Dioxide in Polluted Atmosphere, A Review', Environ. Sci. TechnoL 5, 685-700. Fritz, J. S. and Yamamura, S. S.: 1955, 'Rapid Microtitration of Sulfate', Anal. Chem. 27, 1461-1464. Gentilizza, M. and Vadji6, V.: 1985, 'The Effect of Various Types of Soot on the Behaviour of Sulphur Dioxide in the Air Investigated on Model Systems', Sci. Total Environ. 41, 45-53. Gentilizza, M. and Vadji6, V.: 1986, 'The Relationship Between the Mass Concentrations of Sulphate and Sulphur Dioxide in Air Polluted with Cement Dust', Sci. Total Environ. 48, 231-237.
68
V. VADJI(~ ET A.L.
Gentilizza, M. and Vadji6, V.: 1980, 'Evaluation of the Method for the Determination of Sulphate Concentrations in Airborne Particles", Arh. Hig. Rada ToksikoL 31, 219-226. Gomzi, M.: 1983, 'Health Effects of Sulfur Compounds and Other Pollutants', Za~st. Atmos. 3, 101-106 (in Croatian). Haury, G., Jordan, S., and Hofmann, C.: 1978, 'Experimental Investigation of the Aerosol Catalyzed Oxidation of SO 2 Under Atmospheric Conditions', Atmos. Environ. 12, 281-287. Judeikis, H. S., Stewart, T. B., and Wren, A. G.: 1978, 'Laboratory Studies of Heterogeneous Reactions of SO2', Atmos. Environ. 12, 1633-1641. Li, K., Rothfus, R. R., and Adey, A. H.: 1968, 'Effect of Macroscopic Properties of Manganese Oxides on Adsorption of Sulfur Dioxide', Environ. Sci. Technol. 2, 619-621. Methods of Measuring Air Pollution OECD, Paris 1964. Setterlind, A. N.: 1953, 'Preparation of Known Concentrations of Gases and Vapors in Air', Am. Ind. Hyg. Assoc. Q. 14, 113-120. Urone, P. and Schroeder, V. H.: 1969, 'SO 2 in the Atmosphere: A Wealth of Monitoring Data, but a few Reaction Studies', Environ. Sci. Technol. 3, 436-445. Urone, P., Lutsep, H., Noyes, C. M., and Parcher, J. F.: 1968, 'Static Studies of Sulfur Dioxide Reactions in Air', Environ. Sci. Technol. 2, 611-618. Vadji6, V. and Gentilizza, M.: 1985, 'The Effect of MnO 2 and some Manganese Salts on the Behaviour of Sulphur Dioxide in the Air Investigated on Model Systems', Sci. Total Environ. 44, 245-251. Vadji6, V. and Gentilizza, M.: 1986, 'The Effect of Metal Oxides on the Behaviour of Sulphur Dioxide in the Air Investigated on Model Systems', Staub-Reinhalt. Luft 46, 125-127. Waller, R. E.: 1979, 'The Effect of Sulphur Dioxide and Related Urban Air Pollutants on Health', Za~st Atmos. 16, 25-28 (in Croatian). West, P. V. and Gaeke, G. C.: 1956, 'Fixation of Sulfur Dioxide as Disulfitomercurate (II) and Subsequent Colorimetric Estimation', Anal Chem. 28, 1816-1819.
DIOXIDE OXIDATION
IN T H E A I R
V. V A D J I C a n d M. G E N T I L I Z Z A
Institute for Medical Research and Occupational Health, University of Zagreb, Zagreb, Yugoslavia and R. H A L L E
JUCEMA, Zagreb, Yugoslavia
(Received 17 November, 1987) Abstract. The effect of various types of cement dust on the behaviour of sulphur dioxide in the air was investigated on model systems in different experimental conditions. Experiments were carried out with PC-15z-45s (Portland-blast furnace cement containing 15070 blast furnace slag), PC-25p-35s (Portland-pozzolan cement containing 25~ pozzolan) and EFD (electrofilter dust). EFD most effectively removed SO2 from the air stream. The next efficacious was PC-I 5z-45s, whereas PC-25p-35s was the least efficient. The efficacy of cement dusts for SO 2 removal from the air stream depended on their chemical and granulometric composition and in particular on the size of specific surface. The rate of reaction was also influenced by experimental conditions - - relative humidity, the length of contact, that is, the flow rate of gaseous mixture through the reactor, and the amount of cement dust. The experimental data show that in the contact between SO2 and cement dust catalytic oxidation of SO2 to sulphates takes place. Sulphates remain bound to the surface, from which they cannot be thermally desorbed, but can be released by extraction in the Soxhlet apparatus.
1. Introduction
A study of homogeneous and heterogeneous oxidations of sulphur dioxide to sulphates in ambient air (Urone and Schroeder, 1969; Urone et. al., 1968; Bufalini, 1971) is of major interest, because products of SO 2 oxidation are harmful to human health. Measurements of SO 2, total suspended particulates and sulphates as indicators of air pollution make part of a wide range of epidemiological investigations of acute and chronic effects on health (Waller, 1979) which have been carried out in USA by EPA (Environmental Protection Agency) as part of the CHESS project (Community Health and Environmental Surveillance System). It is not yet clear whether sulphates are better indicators of the presence of hazardous components in the air than SO2 itself nor, whether they are better indicators of the air pollution due to combustion than total suspended particulates measured in the samples collected from large volumes of air. The effect of sulphates on health probably varies in dependence on their physical and chemical form. In masses of polluted air sulphates are most often present in the form of ammonEnvironmental Monitoring and Assessment 11 (1988) 59-68. 9 1988 by Kluwer Academic Publishers.
60
V. VADJIC ET AL,
ium sulphate, which does not have the same effect as sulphuric acid. Presumably, in the atmosphere polluted with cement dust calcium sulphate is formed. Epidemiological studies conducted in real conditions in general population (Gomzi, 1983) provide main evidence of the association between air pollution and human health. Our laboratory investigations of the heterogeneous reactions of SO 2 with various types of soot demonstrated that SO2 in contact with soot was merely adsorbed on the surface, but remained chemically stable (Gentilizza and Vadji6, 1985) whereas in contact with various metal oxides SO2 underwent catalytic oxidation to sulphates (Vadjid and Gentilizza, 1985; Vadji6 and Gentilizza, 1986). The relationship between sulphates and SO2 in the ambient air polluted with cement dust was studied in the vicinity of a cement factory emitting SO2 and cement dust. Measurements were carried out in an industrial suburban area with several minor plants emitting SO2. Earlier data (Gentilizza and Vadjid, 1986) demonstrated that cement dust may influence SO2 oxidation into sulphates. Therefore, we continued to study the effect of various types of cement dust on oxidation of SO2 to sulphates at ambient temperature and in different laboratory conditions. Our experiments with pure mixtures containing known concentrations of all present reactants which were conducted in strictly controlled conditions, cannot correspond in full to the measurements in ambient air where other pollutants may interfere and meteorologicaI conditions are not completely under control.
2. Experimental The material and methods were as described in a previous paper (Vadji6 and Gentilizza, 1985, Vadji6 and Gentilizza, 1986). A mixture of SO2 and air was prepared following a procedure for the preparation of calibration mixtures of gases and vapours. A system of three bottles connected in series with the same initial concentration of the gas mixture in each was used. The rate of dilution followed an exponential equation which served to calculate a theoretical curve of dilution (Setterling, 1953). The mixture of SO2 and air first flowed into the reactor with cement dust at a known and controlled flow rate, and was then passed through the washing bottle containing the absorbing solution in which the SO2 fraction that had not reacted with cement dust was retained. The amount of SO2 lost in contact with cement dust was calculated as the difference between the theoretical quantity of SO2 (calculated from the curve of dilution) and the amount absorbed in the absorbing solution after having passed through the reactor with cement dust. Control samples of SO2 were collected in the same manner, but with the reactor empty. The volume of the individual samples was 6 dm 3. Sulphur dioxide was sampled alternatively, in sodium tetrachloromercurate (TCM) solution with analysis using the West and Gaeke spectrophotometric method (West and Gaeke, 1956) and in hydrogen peroxide solution with analysis by the
THE EFFECT OF VARIOUS TYPES
61
sulphate method (OECD, 1964; Fritz and Yamamura, 1955). The TCM method is specific for SO2, whereas the sulphate method besides measuring SO2 also r~.easures SO3 and sulphates. The higher values obtained by the sulphate method suggest a partial conversion of SO 2 to SO 3 in the gaseous phase. Sulphur dioxide desorption from cement dust samples was carried out using improvized equipment. The reactor with cement dust (after reaction with SO2 had taken place) was placed into a temperature-controlled heater. Gas carrier was nitrogen. Desorbed SO 2 was sampled into a washing bottle containing 50 cm 3 TCM and analyzed using the TCM method. The extraction of sulphates from cement dust was accomplished with redistilled water in a Soxhlet apparatus. The extract contained water-soluble sulphates. It was submitted to cation separation in an ion exchange column and sulphates were determined using the sulphate method (Gentilizza and Vadji6, 1980). 3. Results and discussion Table I shows the results of chemical analysis and Table II those of granulometric distribution for all the three types of cement dust. The highest percentage of small particles in EFD points to the largest specific surface of 5113 cm 2 g - 1. The specific surface of PC- 15z-45s was 4149 cm 2 g - 1 and that of PC-25p-35s was 3500 cm 2 g - l Table III shows decreased values o f the mass concentrations of SO2 (w =P/o x 100) after it passed through various types and amounts of cement dust in different experimental conditions and at ambient temperature. Our laboratory investigations were directed first to measuring a decrease in the mass concentration of SO2 after contact with various types of cement dust. In TABLE I Chemical analysis of various types of cement dust % Components
Humidity Loss of ignation Non-soluble SiO 2 A1203 Fe203 CaO MgO SO 2 Na20 K20
EFP
PC-15z-45s
PC-25p-35s
0.36 35.27 -11.84 3.72 1.40 43.33 1.88 1.06 0.17 0.58
0.09 -0.89 20.88 5.64 3.34 62.59 2.47 1.98 0.34 0.67
0.83 1.38 12.82 17.72 5.70 3.31 54.00 2.04 1.49 0.32 0.43
EDF - electrofilter dust PC-15z-45s - Portland-blast furnance cement containing 15% blast furnance slag PC-25p-35s - Portland-pozzolan cement containing 25~ pozzolan
62
v. VADJI(~ ET AL. T A B L E II Granulometric distribution of various types of cement dust %
Particle size (#g)
EFD
PC-15z-45s
PC-25p-35s
0- 5 5- 10 10- 15 15- 20 20- 25 25- 30 30- 35 35- 40 40- 45 45- 50 50- 55 55- 60 60- 90 90-120 120-150 200
17.0 20.3 14.2 7.6 6.0 5.6 5.1 6.0 1.3 1.9 1.4 0.9 7.1 3.1 1.9 0.6
9.0 14.0 13.8 18.5 14.5 8.5 5.5 5.0 3.0 1.5 0.7 0.2 3.6 2.2 0.0 0.0
10.7 12.3 10.9 10.1 8.8 7.7 6.8 6.1 5.6 5.0 3.8 2.9 6.3 2.3 0.5 0.2
Specific surface cm 2 g - ~
5113
4149
3500
EFD - electrofilter dust PC-15z-45s - Portland-blast furnance cement containing 15% blast furnance slag PC-25p-35s - Portland-pozzolan cement containing 25% pozzolan T A B L E III Percent of SO: decrease after passage through various types of cement dust at different experimental conditions Cement type
quantity mg
Range of SO2 mass concentration mg d m - 3
EFD PC-15z-45s PC-25p-35s PC-15z-45s PC-15z-45s PC-15z-45s
10 10 10 10 10 50
49.4-7.6 45.0-6.9 47.0-7.1 47.2-7.2 37.5-5.8 50.4-7.8
w PH N s*
-----
0.1 0.1 0.1 0.1 0.2 0.1
Air quality s*
N
Percent of SO2 removal
20 20 20 20 20 20
57.9 52.5 50.4 28.8 42.3 82.4
76% RH
+ + + + + +
arithmetic mean relative humidity number o f experimental data air priviously dried over silica-gel
experiments humidity percent
Flow rate of gaseous mixture dm 3 m i n - l
we used
and
10 m g o f E F D ,
PC-15z-45s
a flow rate of 0.1 dm 3 min -I
removal
of SO2 in contact
and PC-25p-35s
(Figure
with the cement
1). T h e
a t 76~
mean
dusts under
values
relative of the
our experimental
THE
EFFECT
OF
VARIOUS
63
TYPES
6FD 75.
9 A9
"o ol =,
r~
@A
9
f01.
S-
~>~
o
TCM
CONTROL
9
TCM
CEMENT
"
H202 CONTROL
&
H202 CEMENT
--
10 20
PC
-'}5
30 40 S0 60
Z
-
70
THEORETICAL
CURVE
80 90 100 "r10 120
/.5 S
PC - 25 p - 35 S
7-
5
ea
Aea~ -o
101
0 u~
S-
m;-
A
o~
$-
@A
D ' 10
+ 20
' 30
' AO
+ 50
c"-~ ~ 60 70
~ 90
, 90
AIR
~ ]00
"-I. 'T 1~0 '/20
FLOW
-r
m
+
i
+
i
:,,o 3a ,m so
VO L U M E
,
+
,
r
,
+
,_
m, ++'o ~o ~o loo ++o ~:'o
tdm 3)
F i g . 1, E f f e c t o f d i f f e r e n t t y p e s o f c e m e n t d u s t o n S O 2 d e c r e a s e (10 r a g c e m e n t d u s t , R H 7 6 % , r a t e 0. l d m 3 r a i n - ').
64
V. VADJIt~ E T AL.
'/6 %
SILICA
RH
5
GEL
5-
"" ?E9~
10I.
0
5
9
10'
9
5 O 9
TCM CONTROL TCM CEMENT H202 CONTROL
9
. 10
.
. 20
.
30
&0-
. 50
.
60.
. 70
.
. 80
AIR
. 90 .
100 110 120
FLOW
H202 CEMENT THEORETICAL CURVE
,b 2'0 ~, ~ ~'o ~
VOLUME
;o
' 60
9'o loo
1'10
1~o
{din 3 )
Fig. 2. Effect of relative humidity on SO 2 decrease (10 mg PC-15z-45s, rate 0.1 d m 3 min-~).
conditions indicated (Table III) that EFD having the largest specific surface removed SO2 from the air most efficiently - 5 7 . 9 . For PC-15z-45s, with smaller specific surface, the percentage of removed SO 2 was lower - 52.5 and for PC-25p-35s it was 50.4 because of the smallest specific surface. In further investigations we used PC-15Z-45S, the cement dust of medium efficacy. Firstly, we investigated the effect of relative humidity on the decrease in the mass concentration of SO2 after passing it through 10 mg of cement at a flow rate of 0.1 dm 3 min-1 (Figure 2). The figure (left-hand side) illustrates the experiment in which the influence of 76~ relative humidity was very strong compared to the experiment where the air was dried over silica-gel before entering the reaction system containing the mixture of SO2 and air (right-hand side of the figure). Other conditions were identical in both experiments. The mean value of the mass fraction of removed SO2 at a relative humidity of 76% was 52.5, but it was as low as 28.8 when the air had been dried over silica-gel (Table III). These data are in accordance with those found in the literature (Judeikis et al., 1978; Li et al., 1968; Haury et al., 1978) and emphasize the importance of relative humidity in heterogeneous reactions taking place between SO2 and suspended particulates. Figure 3 illustrates the effect of the flow rate of the SO2 and air gaseous mixture of the decrease in the mass concentration of SO2 in contact with 10 mg of PC-15z45s, at a relative humidity of about 76~ Owing to a longer contact between gaseous
65
THE EFFECT OF VARIOUS TYPES
RATE 0,1 dm 3 rain 1
RATE
0,2
-1
dm 3 rain
5
~E "r
lo1.
101
9 9
C)
5 O m
O
%
9
TCM CONTROL TC M CEMENT H202 CONTROL
A 9
10 20 3
S0
60 70 80
,
90 100 110 120
AIR
FLOW
9 9
H202 CEMENT THEORETICAL CURVE
,
10 20
VOLUME
,
,
,
30 40 50
,
,
,
J
60 70 80
,
f
J
90 100 110 120
( tim 3}
Fig. 3. Effect of flow rate on SO 2 decrease (10 mg PC-15z-45s, RH 76070).
50 mg
10 mg 7-
S
"o
~.,0 ~
9
9
101.
o~ (/)
% s
1o 2'0 ;o ~
io ~'o 7o 8~ ,'o ,~o ,;o 1;,o AIR
FLOW
Fig. 4. Effect of various amounts of PC-15z-45s
O 9 A
TCM CONTROL TCM CEMENT H202 CONTROL
9
H202 CEMENT THEORETICAL CURVE
1
0
VOLUME on S0 2
9
0 9
100 110 I 0
( d m 3)
decrease (RH 7607o, rate 0.1 dm 3 min-~).
66
V. VADJI(~ ET AL.
S O 2 and cement dust a lower flow rate of the gaseous mixture (0.1 dm 3 min -1)
enhanced the decrease in the SO 2 mass concentration to a greater extent than a higher flow rate (0.2 dm 3 min-1). The mean values of the mass fraction of removed SO 2 under the above conditions were 52.5 at a rate of 0.1 dm 3 m i n - 1 and 42.3 at a rate of 0.2 dm 3 m i n - 1 (Table III). The effect of different amount of PC-15z-45s on the decrease in the mass concentration of SO2 was studied at the optimum flow rate (0.1 dm 3 m i n - l) and at a relative humidity of about 76%. Samples of 10, 50, and 100 mg of cement dust were used in experiments. Figure 4 shows the results of SO 2 removal from the air stream after contact with l0 and 50 mg of cement dust. The percentage of the mass fraction of removed SO 2 after contact with 10 mg of cement dust was 52.5. Greater quantities of cement dust enhanced the effect so that with 50 mg the percentage was 82.4 (Table III) and with 100 mg of cement all the SO2 was removed from the air stream. According to our previous study of SO2 oxidation to sulphates (Vadji6 and Gentilizza, 1985; Vadji6 and Gentilizza, 1986) 50 mg of MnO 2 completely removed SO 2 from the air stream by catalysis. Partial oxidation of SO2 to gaseous SOa (10-20%) also took place and the rest of SO 2 was oxidized to sulphates. In the gaseous phase SO3 was determined using alternatively the TCM and sulphate methods. The presence of SO 3 could not be detected by the T C M method, which is specific for SO2, whereas using the sulphate method, which in addition to SO a also detects SO 3, 10-20% of SO 3 was measured. Although SO2 removal from the air stream was complete with 100 mg of cement dust, the data obtained by the TCM and sulphate methods did not show any difference. To determine whether under our experimental conditions SO 2 is merely adsorbed onto the cement dust surface or is also converted to sulphates, the samples of cement dust which had been in contact with SO2 were submitted to thermal desorption at 250 ~ At this temperature desorption did not take place and it is assumed that SO 2 was not only adsorbed on, but was also chemically bound to the surface of cement dust. Cement dust samples and the bound SO 2 were extracted in the Soxhlet apparatus. All of the SO 2 bound onto cement dust surface was extracted and detected as sulphate. To compare the results with adsorbed SO2 the sulphates detected in the sample following extraction in the Soxhlet apparatus were calculated and shown as SO 2 (Figure 5). 4. Conclusions Our experimental data show that S O 2 conversion to sulphate depends on the chemical composition of cement dust, especially on its granulometric composition. The size of the specific surface o f cement dust has a positive effect on SO2 conversion, because it is assumed that SO2 is first adsorbed onto the surface and then, by catalytic oxidation, converted to sulphates. Sulphates remain bound onto cement
THE EFFECT OF VARIOUSTYPES
1
67
ill i !10 IO!ASOREI D S !l 1
2
3
,~
: J li I I I Jl i
2
4
SAMPLES
Fig. 5. Sulphate extraction from PC-15z-45s in Soxhlet extractor.
dust surface and cannot be thermally desorbed at 250 ~ but can be extracted in the Soxhlet apparatus and detected as sulphate. E F D was most efficacious in removing SO~ from the air stream. PC-15z-45s was less so, and PC-25p-35s was the least effective. The amount of cement dust has a marked effect on the investigated reaction. With 100 mg of cement dust SO2 conversion to sulphate is complete. High relative humidity and low flow rate of the gaseous mixture, that is, the length o f contact between reactants, also have a positive effect on SO2 conversion. It can be assumed that in the ambient air at a constant concentration o f SO 2 and cement dust in emissions a possibility of SO 2 conversion to sulphate is greater at a lower wind rate because of longer contact o f reactants in the fume cloud. It can also be hypothesized that if a fume cloud emitted from a cement factory were carried by the wind stream towards some other larger SO 2 emitter, heterogenous catalytic oxidation of SO 2 to sulphates on cement dust particles might take place.
References Bufalini, M.: 1971, 'Oxidation of Sulfur Dioxide in Polluted Atmosphere, A Review', Environ. Sci. TechnoL 5, 685-700. Fritz, J. S. and Yamamura, S. S.: 1955, 'Rapid Microtitration of Sulfate', Anal. Chem. 27, 1461-1464. Gentilizza, M. and Vadji6, V.: 1985, 'The Effect of Various Types of Soot on the Behaviour of Sulphur Dioxide in the Air Investigated on Model Systems', Sci. Total Environ. 41, 45-53. Gentilizza, M. and Vadji6, V.: 1986, 'The Relationship Between the Mass Concentrations of Sulphate and Sulphur Dioxide in Air Polluted with Cement Dust', Sci. Total Environ. 48, 231-237.
68
V. VADJI(~ ET A.L.
Gentilizza, M. and Vadji6, V.: 1980, 'Evaluation of the Method for the Determination of Sulphate Concentrations in Airborne Particles", Arh. Hig. Rada ToksikoL 31, 219-226. Gomzi, M.: 1983, 'Health Effects of Sulfur Compounds and Other Pollutants', Za~st. Atmos. 3, 101-106 (in Croatian). Haury, G., Jordan, S., and Hofmann, C.: 1978, 'Experimental Investigation of the Aerosol Catalyzed Oxidation of SO 2 Under Atmospheric Conditions', Atmos. Environ. 12, 281-287. Judeikis, H. S., Stewart, T. B., and Wren, A. G.: 1978, 'Laboratory Studies of Heterogeneous Reactions of SO2', Atmos. Environ. 12, 1633-1641. Li, K., Rothfus, R. R., and Adey, A. H.: 1968, 'Effect of Macroscopic Properties of Manganese Oxides on Adsorption of Sulfur Dioxide', Environ. Sci. Technol. 2, 619-621. Methods of Measuring Air Pollution OECD, Paris 1964. Setterlind, A. N.: 1953, 'Preparation of Known Concentrations of Gases and Vapors in Air', Am. Ind. Hyg. Assoc. Q. 14, 113-120. Urone, P. and Schroeder, V. H.: 1969, 'SO 2 in the Atmosphere: A Wealth of Monitoring Data, but a few Reaction Studies', Environ. Sci. Technol. 3, 436-445. Urone, P., Lutsep, H., Noyes, C. M., and Parcher, J. F.: 1968, 'Static Studies of Sulfur Dioxide Reactions in Air', Environ. Sci. Technol. 2, 611-618. Vadji6, V. and Gentilizza, M.: 1985, 'The Effect of MnO 2 and some Manganese Salts on the Behaviour of Sulphur Dioxide in the Air Investigated on Model Systems', Sci. Total Environ. 44, 245-251. Vadji6, V. and Gentilizza, M.: 1986, 'The Effect of Metal Oxides on the Behaviour of Sulphur Dioxide in the Air Investigated on Model Systems', Staub-Reinhalt. Luft 46, 125-127. Waller, R. E.: 1979, 'The Effect of Sulphur Dioxide and Related Urban Air Pollutants on Health', Za~st Atmos. 16, 25-28 (in Croatian). West, P. V. and Gaeke, G. C.: 1956, 'Fixation of Sulfur Dioxide as Disulfitomercurate (II) and Subsequent Colorimetric Estimation', Anal Chem. 28, 1816-1819.
