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April-June 2006
Studies on the chemical mobility of fluorine in rocks
Xu, Luo, Feng, Tan
145
STUDIES ON THE CHEMICAL MOBILITY OF FLUORINE IN ROCKS
Lirong Xu,a Kunli Luo,b Fujian Feng,b Jian’an Tanb
Jinan, China
SUMMARY: Sequential extraction of various kinds of rocks reveals significant
differences in the chemical mobility of fluorine (F, as fluoride ion). In carbonate rocks
F is very labile, and the proportion of leachable F in them generally exceeds 50%.
Fluorine in Lower Cambrian “black rocks” is closely related to their different
metamorphic grades, which in black carbonaceous slate with higher metamorphosed
grade has mostly lower leachability than in black shale and black siliceous rocks. In
general, the percentage of leachable F is high in phosphorite rocks (between 40.84
and 99.96%) and low in phyllite rocks (12.87–14.92%). The leachable F in diabase is
directly proportional to its F content. There are also differences in chemical mobility
of F in stone coal of different ages. F in Silurian strata stone coal has higher
leachability than in stone coal of Cambrian formations.
Keywords: Chemical mobility of F; Fluorine in rocks; Sequential extraction of F.
INTRODUCTION
Fluorine (F, as fluoride ion) is widely distributed in rocks and is in constant
movement and cycles. In the human body it is derived mainly from water, food,
and air. When intake of F is excessive, biochemical and pathological changes in
the body lead to endemic dental and skeletal fluorosis. Rocks of different types are
the ultimate source of F in the environment. As might be expected, the impact of F
in rocks on the environment depends on not only its concentration but also its
modes of occurrence and chemical mobility.
In recent years especially, sequential chemical extraction experiment have been
applied to study modes of occurrence and chemical mobility of trace elements in
solid samples such as soil and industrial waste.1-5 For example, extraction
experiments have been conducted to study water leaching of typical soils in China
and their relation to fluorosis.6 However, such experiments do not appear to have
been applied to investigate the chemical mobility of F in rocks and coal. In this
paper, sequential chemical extraction was used to appraise how different types of
rocks and coal can contribute to the amount and impact of F in the environment.
MATERIALS AND METHODS
Thirty rock samples were collected from Paleozoic strata in the Daba Area south
of Shaanxi province, and 35 rock samples were obtained from corresponding
strata in the Yangtze Platform of Yunnan, Guizhou, and Hunan provinces (Figure).
To secure fresh and representative samples, all rocks were collected only on
stratigraphic sections. The following rock types are included in the samples:
carbonate rock, black shale, stone coal, carbonaceous slate, phyllite, diabase, and
phosphorite.
aFor
Correspondence: Dr Lirong Xu, Department of City Development, Jinan University,
250002, No.13 Shungeng Road, Jinan, China; E-mail: [email protected] bInstitute of
Geographical Sciences and Natural Resource Research, CAS, 100101, Beijing, China.
146
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Xu, Luo, Feng, Tan
146
Figure. Map showing locations of the sampling sites.
Distilled water, acetic acid, and nitric acid were used in sequence to extract F
from the rocks. The total concentration of F in rocks was determined by the
combustion-hydrolysis /fluoride-ion selective electrode (ISE) method according to
GB/T4633-1997.7,8 For quality control, standard reference materials (GBW11122
[coal, China], Chinese Standard Sample Study Center, Chinese Academy of
Measurement Sciences) were randomly analyzed with each batch of rock samples.
In all our F analyses the relative standard deviation was less than 10%, and the
detection limit was 10-8.
The sequential extraction procedure was carried out as follows:
Step 1: water-soluble F: One-gram rock samples crushed to 100 mesh were
placed in 50-mL centrifuge tubes, and 30 mL of distilled water was added.
Samples were stirred with glass rods. The tubes were stoppered and placed on a
shaker for 15 hr at 25±0.5°C. Solid residues were separated from the extract by
centrifugation (6000 rpm) and filtration of the supernatant liquid into polyethylene
tubes. These solutions were stored at 4°C prior to analysis.
After centrifugation, 20 mL of distilled water was added to the solid residues.
The tubes were stoppered and the solids were again extracted by shaking for 15
min. The supernatant liquid was discarded, taking care that no residues were lost.
These residues were separated from the extract by centrifugation (6000 rpm) and
used for the next extraction.
Step 2: F bound to carbonates: Thirty milliliters of 20% (V/V) acetic acid/water
solutions were added in residues from Step 1 in the centrifuge tube. Samples were
stirred with glass rods. Reagent blanks were kept for measurements. The tubes
were stoppered, and the solids were extracted by shaking for 15 hr at 25±0.5°C.
Solid residues were separated from the extract by centrifugation (6000 rpm), and
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April-June 2006
Studies on the chemical mobility of fluorine in rocks
Xu, Luo, Feng, Tan
147
supernatant liquid was filtered into polyethylene tubes. As before, these solutions
were stored at 4°C prior to analysis.
The residues were handled in the same manner as in Step 1 and used for the next
extraction.
Step 3: F bound to sulfide compounds: Thirty milliliters of 15% (V/V) nitric
acid/water solutions were added to the residues from Step 2 in the centrifuge
tubes. Samples were stirred with glass rods and digested in a covered vessel at
room temperature for 1 hr and then for an additional hour at 85°C on a water bath.
The tubes were stoppered and the solids were extracted by shaking for 15 hr at the
temperature of 25±0.5°C. Solid residues were separated from the extract by
centrifugation (6000 rpm), and the supernatant liquid was filtered into a
polyethylene tubes. Again, these solutions were stored at 4°C prior to analysis. In
this process, HNO3 solutions at 85°C on a water bath can dissolve F bound to
sulfide such as pyrite, marcasite, galena, etc.9,10
The residues were handled by the same method as in Step 1 and used for the next
extraction.
Step 4: F in residues: the concentration of F in residues from Step 3 was
determined by the same method as the original samples. Reagent blanks were kept
for measurements in each extraction step.
RESULTS AND DISCUSSION
In this sequential extraction study, the sum of all the sequential extractions of F
is assumed to be the mobile fluorine under natural leaching conditions over long
periods of time. F retained in the residue from the last extraction step is assumed to
be inert under natural leaching conditions.11 The lower the percentage of F
retained in the residue, the more mobile was F in it and the greater its expected
environmental impact on soil, water, and human health.
Table 1 shows the chemical mobility of F in carbonate rocks calculated from the
combined leachate concentrations obtained by the sequential extractions.
Carbonate rocks in this study consist mainly of dolomite, siliceous dolomite, and
limestone.
Table 1. Chemical mobility of fluorine in carbonate rocks
Sampling
Wa
Acb
HNO3c Leachable F Total F in rock Leachable Fd
site
(mg/kg) (mg/kg)
(mg/kg)
(mg/kg)
samples
percentage
(%)
(mg/kg)
Dolomite
Daba Area
5.34
151.1
112.4
268.8
269.8
99.63
Dolomite
Daba Area
5.40
110.2
61.14
176.7
177.7
99.44
Limestone
Daba Area
2.94
160.9
159.7
323.5
496.6
65.14
Siliceous dolomite Daba Area
3.63
106.6
134.0
244.2
411.2
59.39
Dolomite
Hunan
10.83
576.4
10.08
597.3
790.2
75.59
Dolomite
Guizhou
6.96
212.5
60.19
279.5
354.7
78.80
Limestone
Guizhou
14.22
514.5
54.35
583.1
1012
57.62
a
W = F in distilled water.
bAc = F in acetic acid/water solutions.
cHNO = F in nitric acid/water solutions.
3
d
Leachable percentage = percentage all leachable F as fraction of total F.
Rock type
Carbonate rocks are widely distributed worldwide. The results show that F in
such rocks is very mobile with the leachable percentage generally more than 50%;
in some of them it is extracted almost entirely. Carbonate rocks are composed of
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148
carbonate minerals such as calcite and dolomite, which dissolve in acids. As a
result, F bound to carbonates was extracted during their dissolution. However, the
concentration of F in carbonates in the Daba Area is comparatively low, and the
leachable quantity of F in them is also lower.
Table 2 shows the chemical mobility of F in “black rocks” determined from the
combined leachate F concentration obtained by the sequential extractions.
The lower Cambrian “black rock” formations are widely distributed in South
China, which have high concentration of organic carbon (5–10% in general,
sometimes close to 20%). The lower Cambrian black rock formations are
composed of black shale, black siliceous rock, and black carbonaceous siliceous
rock, along with stone coal, phosphorite, V-ore, and Mo-Ni rich layers.12
Rock type
Black shale
Table 2. Chemical mobility of fluorine in “black rock” formations
Acb
HNO3c Leachable F Total F in rock
Sampling site
Wa
(mg/kg) (mg/kg) (mg/kg)
(mg/kg)
samples
(mg/kg)
Daba Area
24.75
30.17
33.98
88.90
973.7
Guizhou
13.32
40.11
177.9
231.3
3063
Guizhou
18.66
20.67
40.64
79.97
537.1
Hunan
21.12
149.5
159.1
329.7
2055
Hunan
14.28
87.24
325.3
426.8
1224
Daba Area
Black siliceous Guizhou
rock
Guizhou
28.41
13.26
27.30
82.08
33.96
55.26
100.61
394.2
20.66
211.1
441.4
103.2
Daba Area
0.11
0.74
1.89
2.74
Black
2.82
11.49
55.25
69.56
carbonaceous Daba Area
slate
Guizhou
5.01
28.86
18.77
52.64
a
W = F in distilled water.
b
Ac = F in acetic acid/water solutions.
cHNO = F in nitric acid/water solutions.
3
d
Leachable percentage = percentage all leachable F as fraction of total F.
Leachable Fd
percentage
(%)
9.13
7.55
14.89
16.04
34.87
897.5
1511
622.2
23.52
29.21
16.59
370.0
963.5
1162
0.74
7.22
4.53
The results in Table 2 show that there are great differences in the mobility of F in
black rocks in terms of their different rock types and metamorphic grades. The
leachable percentage of F in black shale, black siliceous rock, and black
carbonaceous slate is mostly 0.74–34.87%, and the majority of F was retained in
the residues from all the sequential extraction. There was a negative correlation
between the mobility of F in rocks and their metamorphic grades. For example,
black carbonaceous slate has a higher metamorphic grade than black shale and
black siliceous rock, and the results in Table 2 show that F in the former has lower
leachability than the in latter. These minerals were reassembled after the black
carbonaceous slate had been metamorphosed, and some F may have entered into
the stabilized mineral crystal lattice, which is therefore relatively inert under
surface conditions.
Table 3 shows the chemical mobility the F in stone coal in Daba Area
determined from sequential extractions of such coal in that locale. Stone coal
differs from ordinary coal in that it is of ancient formation from early plant life,
whereas ordinary coal is of more recent formation from higher plants. Generally
speaking, stone coal has higher ash content and lower heating ability than ordinary
coal. In China, stone coal has limited reserves, being distributed mainly in the
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Xu, Luo, Feng, Tan
149
south, especially in mountainous areas, where it is widely used as a local
household fuel.
Table 3. Chemical mobility of fluorine in stone coal of Daba Area
Geologic Sampling site Wa (mg/ Acb
HNO3c Leachable F Total F in Leachable Fd
period
kg)
(mg/kg) (mg/kg)
rock samples percentage
(mg/kg)
(%)
(mg/kg)
Stone coal Cambrian Hanwang,
17.1
79.2
296.9
393.2
1378
28.53
Ziyang County
Stone coal Cambrian Tiefo, Ziyang
4.38
3.94
97.8
106.1
349.8
30.33
County
Stone coal Silurian
Haoping,
3.22
4.04
414.2
421.5
874.5
48.20
Ziyang County
Stone coal Silurian
Haoping,
24.62
120.1
208.3
353.0
757.9
46.58
Ziyang County
Stone coal Silurian
Haoping,
10.80
122.1
117.4
250.3
309.4
80.90
cinder
Ziyang County
a W = F in distilled water.
b
Ac = F in acetic acid/water solutions.
cHNO = F in nitric acid/water solutions.
3
dLeachable percentage = percentage all leachable F as fraction of total F.
Rock type
The results in Table 3 show that there are some differences in the chemical
mobility of F in stone coal of different ages. The leachable percentage of F in
stone coal is mostly 28.53–48.20%, and F in Silurian stone coal (mean 47.39%)
has higher leachability than Lower Cambrian stone coal (mean 29.43%) in the
Daba Area. The leachable quantity of F in stone coal is relatively high probably
because of its high F concentration. It is noteworthy that the F concentration in
stone coal cinder after combustion in Haoping accounts for nearly 41% of the total
amount of F in the stone coal. Thus half the F in stone coal can be expected to be
released into the air during burning. In fact, endemic fluorosis from combustion of
stone coal is very serious in the Daba Area of Haoping. Since stone coal cannot be
fully burned because of the low operating temperature of domestic stoves, a
considerable part of F is retained in the cinder after combustion. Table 3 shows
that F in Haoping stone coal cinder has very mobile, since the leachable
percentage of F is almost 81%. Consequently, the release of F from these coal
cinder deposits on their environmental water and soil should not be negligible.
Table 4 shows the result of the mobility of F in siliceous rock, phosphorite,
diabase and phyllite.
The results in Table 4 show that the leachable percentage of F in phyllite is low,
only about 13 to 15%. Generally speaking, the leachable F in diabase is in direct
proportion to its F concentration, i.e., the higher the F concentration in original
diabase is, the more F is leachable.
In addition, the leachable percentage of F in phosphorite rocks ranges from
about 41% to nearly 100%. Thick phosphorite layers are frequently present in
black rocks of South China, in which the F concentration is mostly more than 104
mg/kg.13,14 Table 4 shows that F is mobile in phosphorite. Although phosphorite
layers in Lower Cambrian black rock series are not present widely in the world,
the pollution from F in them must be recognized and considered when they are
utilized as a resource because of their high and mobile F content.
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Xu, Luo, Feng, Tan
150
Table 4. Chemical mobility of fluorine in other rock types
Wa
(mg/kg)
Acb
(mg/kg)
HNO3c
(mg/kg)
Phosphorite
Daba Area
Daba Area
Guizhou
Guizhou
Yunan
Yunan
Yunan
Yunan
33.03
36.06
213.9
92.13
77.43
46.89
73.53
9.04
42.33
62.28
443.5
247.6
491.4
571.2
340.5
76.06
432.8
769.8
16254
9294
9433
18882
18099
13704
Diabase
Daba Area
Daba Area
Daba Area
Daba Area
1.35
2.88
5.97
1.53
11.16
33.07
105.8
21.39
34.83
35.95
459.9
268.7
Rock type
Sampling site
Leachable F Total F in rock Leachable Fa
(mg/kg)
samples
percentage
(%)
(mg/kg)
508.2
1244
40.85
868.1
1770
49.05
16911
16930
99.89
9634
9648
99.85
10002
10900
91.76
19500
19520
99.90
18513
18520
99.96
13790
14510
95.04
47.34
71.90
571.7
291.6
Daba Area
18.37
121.5
1.60
141.5
Daba Area
11.7
116.6
27.80
156.1
a W = F in distilled water.
bAc = F in acetic acid/water solutions.
c
HNO3 = F in nitric acid/water solutions.
dLeachable percentage = percentage all leachable F as fraction of total F.
Phyllite
405.7
421.2
906.7
523.5
11.67
17.07
63.05
55.70
1100
1046
12.86
14.92
ACKNOWLEDGMENTS
The authors express their heartfelt thanks to Lianhua Xiang, Mineral Bureau of
Ziyiang County, Shaanxi, for his help in field-work. The Chinese National Key
Project (Grant No. 40171006), the Project Sponsored by the Scientific Research
Foundation for Doctor of Jinan University (Grant No. B0425) and the Knowledge
Innovation Foundation of Institute of Geographical Sciences and Natural
Resource, Chinese Academy of Sciences (Grant No. SJ10G-A01-03) supported
this work.
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