Download the dissolution of asbestos fibres in water

Clay Minerals (1987) 22, 21-35
THE DISSOLUTION
OF A S B E S T O S F I B R E S I N W A T E R
JANET
R. G R O N O W
University of Cambridge, Department of Engineering, Trumpington Street, Cambridge CB2 1PZ
(Received 2 December 1985; revised 9 June 1986)
A B ST R A C T : The interaction of chrysotile and crocidolite with water has been investigated in
an attempt to identify the factors affecting the rate and the degree of dissolution of asbestos
fibres within groundwater systems at landfill sites. Dissolution experiments were used to
investigate rate laws and to obtain apparent activation energies for the dissolution of the two
minerals. The activation energies related to transport-controlled processes, but as the overall
dissolution occurred so slowly it was unlikely to be controlled by processes with such low
activation energies. Congruent dissolution of both minerals tended to increase with temperature
and time, suggesting that in the long-term environmental situation, congruent dissolution of
these two asbestos minerals would occur. However these experiments show that, as the reaction
was so slow, there is little likelihood of reduction of the asbestos pollution hazard by the complete
dissolution of fibres on prolonged contact with natural waters.
This investigation deals with the rate and degree of dissolution of asbestos fibres in water,
and is related to the possible pollution of water supplies by asbestos waste deposited in
landfill sites. Several authors (e.g. Harris & Grimshaw, 1971) have noted the high solubility
of chrysotile, particularly in solutions of low pH. This suggests that there is a possibility that,
on prolonged contact with water, asbestos fibres might disintegrate and be rendered
harmless. For this reason, the aim of the investigation was to look at both the mechanism and
the degree of dissolution of asbestos fibres in water and to relate the solubility of the fibres to
the environmental situation.
Most silicate minerals undergo incongruent dissolution at low temperatures. The rate of
dissolution may be controlled by: (i) reaction of the unaltered mineral at the mineral-solution
interface (Lagache, 1965); (ii) the diffusion rate of ions through a leached surface layer (Luce
et al., 1972); the rate of diffusion of ions through a precipitated surface layer (Wollast, 1967;
Helgeson, 1971).
Minerals that are included in the general term asbestos fall into two groups. These are the
serpentines, of which chrysotile (white asbestos) is the only fibrous form, and the amphibole
group which includes crocidolite (blue asbestos), amosite (brown asbestos), tremolite,
actinolite and anthophyllite. The slight misfit in the dimensions of the octahedral and
tetrahedral sheets of chrysotile causes the mineral to roll carpet-like into fibres, with the
brucite sheet on the outside; the average diameter of the chrysotile fibres is 300 A. The
fibrous varieties of the amphiboles are not cylindrical but are formed with the fibre axis
perpendicular to the crystallographic z-axis. Individual crocidolite fibre diameters are of the
order of 200 A or less.
Chrysotile may be represented by the structural formula
Mg6Si4Oa 0(OH)s
with some substitution of F C +, F e 3§ and Ca 2+ in octahedral sites. The formula
Na2_3(Fe3+,A13+)2 (Fe2+, Mg2+)3SisO22(OH)2
9 1987 The Mineralogical Society
Janet R. Gronow
22
TABLE1. Major element analysesofasbestossamples
studied.
Chrysotile
Crocidolite
MnO
CO 2
F
39.01
0.41
0.97
0.17
41.30
0.07
0-01
0"01
2'48
13"81
0'03
0.02
0.03
0.76
0.03
51-08
8.34
8.93
17.4l
4.06
1.30
6.14
0'06
0"12
2"80
0"37
0-01
0.16
Total
less O = F
99.11
0.01
100.78
Total
99.10
100.78
SiO2
A1203
FezO3
FeO
MgO
CaO
Na20
K20
H20H20+
Ti02
P2Os
represents crocidolite, where K + may be substituted for N a + at the ' A ' sites and Ca 2+ for N a +
in octahedral sites and there may be some substitution of Fe, A13+ and Mg 2+ by Ti 4+ and
Mn > .
EXPERIMENTAL
METHODS
Initial characterization of samples
The sample of chrysotile used was from Thetford, Canada, and was provided by Cape
Asbestos Fibres Ltd. The crocidolite sample was from Cape Province, South Africa, and was
provided by Turner Asbestos Fibres.
Major element analyses of the samples are given in Table 1.
Cell dimensions o f a = 5.31 ~,, b = 9.24 A, c = 14.64 A and fl = 92.63 ~ for chrysotile and
a = 9.65/~, b = 18.80 A, c = 9"65 • and fl = 103.8 ~ for crocidolite were found from X-ray
powder photographs taken with a Gandolfi camera using Cu-Kct radiation. These agree
closely with published data and were obtained using the computer program Cellpar (Prewitt,
1974) which provides a least-squares refinement of assumed cell parameters based on X-ray
powder data.
Sample pretreatment
Samples of the two minerals were hand-picked, milled for 5-15 min and then sieved. The
sieved products ( - 125 + 105/~m) were washed several times with Aristar grade acetone.
Under the transmission electron microscope, they consisted of well-opened bundles of fibres,
110 pm long.
Dissolution o f asbestos fibres in water
23
There is no doubt that the pre-treatment of a solid sample has a large bearing on
subsequent reaction kinetics and can produce changes in reaction rates of more than one
order of magnitude (Boldyrev et al., 1979). Ball milling, apart from opening up the bundles of
asbestos fibres and so increasing the surface area, may cause structural changes (Uehara,
1975) and it almost certainly increases the frequency of lattice defects at the surface of the
sample.
The changes in particle size and concomitant changes in surface area may affect the
reaction in different ways.
(1) A change in available surface area may modify the relative influence of nucleation
and interfacial reaction.
(2) Surface stresses and surface energy may be changed. This is likely to alter surface
reactivity. An increase in the number of defects at the surface is likely to increase the
number of potential reaction sites.
(3) A variation in surface area may affect diffusion, which may change the rate of
reaction if transport mechanisms have any part to play in the rate-controlling process.
The preparation of mineral surfaces has been the subject of much controversy. There has
been a suggestion that the fresh mineral surface produced by etching with an HF/HzSO4
solution may be fluorinated by this technique under certain circumstances (Perry et al., 1983;
Berner et al., 1985). Ultrasonics were found to be unsatisfactory in this case because the
resulting suspensions of the asbestos fibres contained flocs from which it was impossible to
separate the fines. However, it is possible to assess the lattice damage produced in chrysotile
on milling, by monitoring the intensity of various peaks in the IR spectrum of the mineral, as
certain peaks have been shown to be sensitive to the effects of grinding (Harris, 1971). The
spectrum obtained from the 125-105 #m fraction showed that there was no significant
disruption of the silicate lattice during sample preparation.
The specific surface area of chrysotile prepared as described above was found to be 45.2 m 2
g-1 ; for crocidolite, the measured surface area was 24.7 m 2 g-1.
DISSOLUTION
EXPERIMENTS
Water used for the dissolutions had a resistivity of at least 18 megohm-cm and was found to
have no element present above the ten parts per billion level. Reaction was carried out in one
litre Nalgene Teflon FEP bottles under CO2-free nitrogen. Through ports in each lid, three
Tygon tubes were passed; a fourth port was used for pH measurement and was closed when
not in use. One tube was for sampling and the other two were for nitrogen inlet and outlet.
Two reaction jars containing 1 1 of water plus a Teflon-coated magnetic follower were
placed in a plastic water-bath mounted on two magnetic stirrers. The temperature of the
water-bath was kept constant, the solutions were stirred continuously and the pH was
monitored using a glass-calomel combination electrode.
The rate of mineral dissolution depends on pH (Luce et al., 1972), and it was necessary to
use buffer solutions as the pH of the contact suspensions changed continuously as the
hydrolysis of the mineral progressed. Buffers were used with reluctance because of concern
about the participation of components of the buffer in the rate-limiting step. Buffers with
negligible metal-buffer binding capacities were used (Good et al., 1966). 0.05 M NN-bis(2hydroxyethyl)-2-aminoethane sulphonic acid (BES) was used for solutions at pH 7, 0.05 M
Tris (hydroxmethyl) aminomethane (TRIS) was used for pH 9, and 0.05 M Tris
(hydroxmethyl) aminomethane citrate (monobasic TRIS citrate) was used for pH 4.
24
Janet R. Gronow
500 mg of fibres were added to the reaction vessel and 10 ml aliquots of the suspension
removed after 15 min and at set time intervals thereafter. Each aliquot was immediately
filtered through a 0.1 #m Millipore filter and acidified with Ultra-grade HC104 before
analysis by atomic absorption spectroscopy for Mg and Si and, for crocidolite, also for Fe.
The filters were dried and kept for X-ray photoelectron spectroscopy (XPS).
In the absence of COz, the amount of ferrous iron in solution is controlled by the solubility
of Fe(OH)2, which is ~ 104 at p H 7, so dissolutions were carried out under CO2-free nitrogen.
In the p H range 6-8, the amount of ferric iron in solution is limited by the low solubility of
Fe(OH)3 (4 x 10-10-5 x 10 -6 p p m Fe) (Mason, 1966) but the influence of other dissolved
constituents, or the formation of complex ions, can cause considerable deviation from
theoretical solubilities.
The possibility of hydrous ferric oxide formation during the crocidolite dissolution
experiments was relevant for four reasons. (1) It is possible that reactions involving iron could
have played a part in the rate-controlling process of dissolution (Siever & Woodford, 1979).
(2) As some colloidal iron could have been removed during filtration, it was decided not to use
iron to monitor the reaction. (3) It is known that colloidal iron adsorbs such ions as Si. F o r this
reason a dialysis bag, as used by W i l d m a n et al. (1968), was suspended in the crocidolite
reaction solutions. The solution inside this was analysed without filtration at the end of each
experiment. Generally there was no significant trend in the differences between the Mg and
Si concentrations in the final filtered solution and in the dialysis bag, indicating little
adsorption of Mg or Si onto any colloids formed after dissolution from the mineral surface. (4)
Iron colloids could have interfered with XPS of the residue on the filters.
OBSERVATIONS
Both chrysotile and crocidolite were found to dissolve incongruently and, in all cases, by
1024 h some sort of equilibrium was approached. Table 2 shows the theoretical number of
TABLE2. Calculated proportion of one surface layer removed after dissolution
Theoretical
p.p.m./1 layer
pH
Temp
(~
Actual p.p.m, in solution
Mg
Si
after 170 h
Mg
Si
Chrysotile
7
44
7
6
7
25
9
25
4
25
10.90
10.90
10.78
10.90
10.82
8.04
8.04
7.96
8.05
7.99
9-73
3.50
9.49
4.40
95.72
2.84
1.23
1.84
1.24
40.34
Crocidolite
7
44
7
6
7
25
9
25
4
25
0.85
0.85
0.86
0.85
0.86
8.27
8.26
8.30
8.21
8.29
1.20
0.50
0.83
0.36
1.54
4.60
2.82
1.36
2.66
4.02
Mg
Si
after 1024 h
10.18
1.22
Calculated proportion of
one layer
Mg
Si
after 170 h
Mg
Si
after 1024 h
2.88
0.89
0.32
0.88
0.40
8.84
0.35
0-15
0.23
0-15
5.05
0.94
0.36
3.15
1.41
0.59
0.96
0.42
1.80
0.56
0.03
0.16
0.32
0.48
1.42
0.37
25
Dissolution o f asbestos fibres in water
surface layers removed after each experiment, which gives an indication of the degree of
dissolution which had occurred, but it is not assumed that reaction actually took place
uniformly over the surface of the mineral fibres.
TEM of fibres which had undergone 1024 h dissolution produced no evidence of changes at
the surface of the minerals and there was no reduction in the intensity of diffraction
patterns from bundles of parallel fibres. Both observations are to be expected if reaction did
in fact occur at the limited rate indicated by Table 2.
The resolution of the scanning electron microscope available was insufficient to obtain an
indication of the nature of the surface reaction. SEM and X-ray microanalysis of chrysotile on
which the lichen Lecanora atra had been growing and actively secreting oxalic acid (Wilson et
al., 1981) has shown evidence of an X-ray amorphous silica gel layer up to 100 pm thick on the
surface of fibre bundles. But work by Holdren & Berner (1979) and Berner et al. (1980) on
feldspars, amphiboles and pyroxenes, and by Fung & Sanipelli (1982) on microcline, resulted
in scanning electron micrographs of mineral surfaces with etched patterns, which suggested
that the chemical reaction at the interface, and not diffusion, was the rate-limiting process.
The Mg: Si ratios obtained from XPS of the filters from chrysotile and crocidolite
dissolutions at pH 4 are given in Table 3, together with the Fe:Si ratio for crocidolite.
Analysis depths were ~22 A for Mgls, 56 A for Si2p and 42 A for Fe.
According to Luce et al. (1972), cases where transport is limited by diffusion of the reacting
species through a leached mineral surface layer lead to so-called parabolic kinetics, where the
quantity of species per unit area of mineral surface found in solution is proportional to the
square root of the time in solution. The chrysotile analysis in Table 3 indicates the presence of
an Mg-depleted layer, which suggests that dissolution at pH 4 may be diffusion controlled.
Fig. 1 demonstrates that this was the only chrysotile dissolution to show parabolic kinetics,
but then, as can be seen from Table 2, it was the only chrysotile dissolution in which more
than one theoretical surface layer was removed and therefore was the only dissolution in
which a leached layer could have been achieved.
The fibres from the low-pH chrysotile dissolution were investigated in more detail by XPS
to see if there was any shift in the position of silicon peaks which might indicate a difference
in the structure of the silicate framework in the leached layer, but none was obvious.
TABLB3. XPS of fibres from low-pH dissolutions
Contact solution
(mole dm-3 x 10 -4)
Time (h)
Mg :Si
Chrysotile
0.25
25
79
170
1.02
0.55
0.34
0.13
Crocidolite
0.25
2
25
50
0.07
0.06
0.06
0.07
Fe :Si
0.30
0.31
0.30
0.33
Mg
Si
4.5
19.9
31'1
39'3
0.4
5-4
10.1
10.3
0"20
0.32
0'50
0'56
0.14
0.22
0"43
1.05
26
Janet R. Gronow
35
30
G"
I
o 25
x
r
20
pHi,
~15
10
pH7
_-
~/__
6oCJ
25% pH9
0
2
4
6
8
10
12
1~
{ hour) 1/2
FIG. 1. Chrysotile dissolution: Mg concentration in contact solution to indicate 'parabolic
kinetics'.
However, this is reasonable as the mineral has a layer structure and incongruent dissolution
may not lead to a significant change in the structure of the tetrahedral silicate sheet.
If the pattern of incongruent dissolution suggested by the concentrations of Mg and Si in
the crocidolite contact solution (Table 3) was followed, a leached layer approximately one
unit-cell thick over the mineral surface after 50 h would be expected, producing an Mg: Si ratio
of 0.03. However no such layer was evident from the XPS analysis. This suggests that reaction
occurred at discrete sites rather than in a uniform layer. The Fe: Si ratios produced by XPS of
leached crocidolite were no higher than expected from the atomic ratios of the unleached
mineral and do not indicate an Fe-rich layer on the surface of the crocidolite fibres. In fact the
analysis showed no surface layer on any sort, neither leached nor precipitated, indicating that
the rate-limiting step of the dissolution process at 50 h could not be diffusion through a
surface layer.
DISCUSSION
The dissolution of a solid may be divided into the following processes (Bircumshaw &
Riddiford, 1952): (i) diffusion ofthe reacting species towards the surface; (ii) adsorption onto
the surface; (iii) reaction at the surface; (iv) desorption of products; (v) diffusion of products
away from the surface. Processes (i) and (v) are transport phenomena, (ii), (iii) and (v) are
Dissolution o f asbestos f i b r e s in w a t e r
27
surface reactions. Generally (ii) and (v) are too fast to be rate-determining processes, so the
rate-limiting step is most~ikely to be either (i), (iii) or (v).
Heterogeneous reactions may be divided into three types:
(1) Transport-controlled processes, where the rate of chemical reaction at the reaction
interface is very much faster than the rate of transport of the reactant to, or the product
from, the surface.
(2) Chemically controlled processes, where the rate of the chemical reaction is much
slower than either of the transport processes.
(3) Intermediate situations, where both processes have similar rates.
The general equation describing the type (3) reaction above, involving the dissolution from
a surface of area S into a fluid of volume V, is:
Vdc
J = -S d-T = k t ( C ' -
C) = kc(C ~ -
C') n
(1)
where J is the mass flux of a species C into the bulk solution (mole m -2 h-l), C' is the surface
concentration, C is the bulk fluid concentration and k t is the mass transport coefficient for
species C. C~ is the equilibrium concentration at the surface and kc is the reaction rate
constant, the dimensions of which depend on the value of n.
Reaction types (1) and (2) are limiting cases of this: in type (1) reactions the chemical
reaction rate is much faster than the transport rate and, because J remains finite, (C's - C')
must be small, i.e. C's --- C'. So equation (1) becomes:
Vdc
J-
Sdt
-
kt(C~ -
C)
(2)
Similarly, in type 2 reactions, (C' - C) will be small, i.e. C' - C and equation (1) becomes:
Vdc
J-
S dt -
k~(C; -
Cy
(3)
This implies that a reaction order other than unity indicates that the dissolution is controlled
by the chemical reaction at the surface, but this is not necessarily so in a multi-component
system.
CHRYSOTILE
DISSOLUTION
The rate of a dissolution may be measured as a change in concentration of a species as a
function of time. The rate law for dissolution may be given by:
J = k[C e
-
-
C]a[n+] b
(4)
where k is the reaction rate parameter, C is the concentration of the measured species in the
bulk solution, Ce is the steady-state Concentration to which the solution tends, and a and b are
the reaction orders with respect to species C and hydrogen ion concentrations respectively.
Taking logs of equation (4) and keeping p H constant gives:
log J = log k' + a log [Ce - C]
(5)
where k' = k[H§ b.
A plot of log J vs log [Ce - C] has a slope of a, the reaction order with respect to C. For Mg,
values of J were obtained from the slope of the plot of Mg concentration vs time at constant
pH (Fig. 2).
28
Janet R. Gronow
44.6~
35
magnesium
30
~"25'
sample surface area 22.5m t
E20 i
"U
15
silicon
10.
Z,L.6~
25.1~
6.0~C
5 84
0
20
z.O
60
80
100
120
140 (h)
FIG. 2. Chrysotile dissolution at pH 7: Mg and Si concentrations in contact solution.
It is difficult to determine the point at which a mineral reaction nears equilibrium, since
small changes in concentration over long periods of time are difficult to detect. Plummer &
Mackenzie (1974) described a method of estimating mineral solubility from the rate at which
equilibrium is approached during dissolution. They found that, provided the surface
reactions did not change with time and that the time to half saturation was greater than 10 h,
extrapolation to infinite time of a plot of species concentration vs (time) --~ produced a good
estimate of the species equilibrium concentration.
It was not possible to use this method to estimate the Mg equilibrium concentration for
chrysotile dissolution, as the time to half saturation was < 10 h. However, it can be seen from
Table 2 that in all chrysotile dissolutions at pH 7 less than one theoretical surface layer was
removed and, as is indicated later, the activation energy for Si dissolution appears to be
greater than that for the dissolution of Mg. This may justify taking the amount of Mg in one
surface layer as the Mg equilibrium value to which the solution tends, assuming that the
slower release of Si from the tetrahedral sheet limits further Mg dissolution in sheets beneath
this. Applying this method, values of log J vs log [Mge - Mg] were obtained and plotted in
Fig. 3, to give a reaction order of 4.1 _+ 1.0 with respect to Mg. The linear least-squares
analysis used by Latham & Burgess (1977) was employed in all these calculations to find the
best straight-line fit for the data obtained.
The method of Plummer & Mackenzie (1974) was used to estimate the Si equilibrium
concentration in the chrysotile solutions, as time to half saturation exceeded 10 h; this gave a
reaction order of 3.5 _+ 0.9 with respect to Si (Fig. 3).
Dissolution of asbestos .fibres in water
magnesium
29
silicon
reaction order 3 , 8 /
/
7
/
reoction order 3.3
reaction order 4.1
i
5
3.5
36
3.7
3.8
3.9
- Iog[C
e
4.0
4.3
4.4
4.5
46
4.7
4.8
- C]
FIG. 3. Chrysotile dissolution at pH 7: reaction orders with respect to Mg and Si.
The effect of p H on chrysotile dissolution can be seen in Fig. 4. Measurement of log J at
different p H values provides a value for b, the reaction order with respect to pH. This was
obtained by least-squares analysis of values of log J a t 25~ at different times for both Mg and
Si. For Mg, the rate of the reaction was found to be approximately proportional to [H § 1/3 and
95
pH &
H 40 Si
f
o 80
X
"(3
-6 65
50
/
sample surface area 22.5m2
/
351 /
.. . . . . . . _ . . . . . . .
pH72Mg
20 / ' "
"~--'-~-0
20
40
pH90 Mg
60
FIG. 4. Chrysotile dissolution at 25~
80
pH. 7.2 Si
pH 9.0
100 120 1/,0 hours
concentration of Mg and Si in contact solution.
30
J a n e t R. Gronow
the dissolution with respect to Si approximately proportional to [H+]2/3. There was a trend in
the value of reaction order with time (0.29-0.38 for Mg and 0.05-0.64 for Si) which indicates
that log J should strictly have been measured at constant distance from equilibrium, but this
was not possible with the data available. That none of the reaction order values obtained were
unity suggests, but in no way proves, that the dissolution was surface-reaction-rate-, rather
than transport-controlled.
Taking logs of the Arrhenius equation gives
log k' = log A - E A / 2 . 3 0 3 R T
(6)
where k' is the reaction rate constant given in equation (5) and EA is the apparent activation
energy for the reaction. The reaction rate was measured when Mg concentrations were at a
constant distance from equilibrium, and values of log J vs 1/T(K) were used to obtain an
apparent activation energy for the dissolution of Mg of ~ 27 kJ mole -1. Analysis of the initial
reaction rate gave the much lower value of 9 kJ mole -1 . These values lie on either side of the
22.9 kJ mole -1 obtained by Choi & Smith (1971), which they consider to be indicative of
diffusion-controlled processes, since diffusion of ions through liquids have activation
energies of the order of 20 kJ mole -1.
Analysis of the initial reaction rate gave a value of 32 kJ mole- ~for the apparent activation
energy of Si removal. Judging by the interval estimates calculated with 90~ probability for
the slopes of the linear least-squares analyses, the error on these values could have been as
much as 50~. Since the dissolution of the first layer of Mg is not dependent on the removal of
Si, it is feasible to obtain different activation energies for Mg and Si dissolution initially.
However, after the removal of the freely available Mg on the surface, any other Mg reaction
may be limited by one of two factors; the exposure of fresh Mg, which will depend on the
mechanism with the higher activation energy which limits Si dissolution, or by diffusion of
Mg ions through the silicate layer.
The apparent activation energies obtained relate only to reactions at pH 7. The XPS results
suggest that, at a lower pH, Mg dissolution may be controlled by diffusion through a leached
layer, and the activation energy for this process is likely to be considerably higher (of the
order of 100 kJ mole- i). The dissolution of Si at low pH was initially slower than that of Mg,
which suggests that Si dissolution may be limited by the surface reaction of the tetrahedral
silicate sheet during this stage. However the overall reaction rate at pH 4 was markedly
higher than at pH 7, which suggests that the activation energy at pH 4 might be lower than at
pH 7, unless the pre-exponential factor A in equation (6) was solely responsible for the
difference. So it is possible that the overall reaction at pH 7 is controlled by a process with a
considerably higher activation energy than the values obtained here.
CROCIDOLITE
DISSOLUTION
Values of the reaction rate for crocidolite dissolution with respect to both Mg and Si were
found from Fig. 5. A value of 1.8 + 0.4 was obtained for the reaction order with respect to
Mg. The method of Plummer & Mackenzie (1974) was used to estimate both the Mg and Si
equilibrium values and gave a reaction order of 2.1 +0.4 with respect to Si (Fig. 6).
The dissolution profiles of crocidolite with respect to Mg at 25~ and different pH values
can be seen in Fig. 7. The reaction order with respect to pH was found to be 0.08. The trend in
reaction order with time (0.06-0.1) again indicates that the reaction rate should strictly have
been measured at constant distance from equilibrium.
Dissolution of asbestos fibres in water
31
I
/
t
' Z./-,.6~ Si
J
6
X
7'
t
o?
I
I
66i
-6
Z.Z,.6~
~5~
/
Mg
_ - ~ - - 25.1~ Si
~
251~
Mg
6.0~
Mg
/
6.0~ Si
........
0
20
40
60
80
100
120
hours
FIG. 5. Crocidolite dissolution at pH 7 : concentration of Mg and Si in contact solution. Sample
surface area 12.5 m E .
ro=20
ro=1.8
~8
ro
0
I
=
ro =1.8
212
6.0~
Si
6.0% Mg
/*Z,.6~ Mg
/,Z,.6~ C Si
38
/,.0
L,.2
/,/,
4.6
4.8
58
-Iog[C e - C]
5.2
5.4
5.6
FIG. 6. Crocidolite dissolution at pH 7: reaction order (ro) with respect to Mg and Si.
Fig. 7 shows that the response of Si dissolution rate to pH is non-linear, suggesting that
there may be a different dissolution mechanism at different values of pH. This non-linear
dependence on pH was also noted by Schott et al. (1981) when investigating the dissolution of
Si from enstatite and diopside.
An apparent activation energy for the removal of Mg from crocidolite of 28 kJ mole-~ was
obtained from values of log J at pH 7 at different temperatures and at equal distances from
32
Janet R. Gronow
pH 4.0 Si
8
pH 8.9 Si .- -"
~-7
I
O
.- -"
pH l,,O
Mg
x6
O')
'E
-0 5
-5
4
"
1
"/t
r/
"
p H_7,2 Mg
,r
/z
/
'
pH 8,9 Mg
f
0
20
L0
60
80
hours
100
120
140
FIG. 7. Crocidolite dissolution at 25~ : concentration of Mg and Si in contact solution. Sample
surface area 12.5 m 2.
equilibrium. A value of 17 kJ mole -1 was obtained from the initial reaction rates. These
values are low enough to indicate that the removal of Mg from the mineral is transport
controlled. This suggests that the loss of the other metal ions in the ' I beam sandwich' may
also be transport-controlled, provided that the oxidation of Fe(II) from the MI site does not
play a part in the dissolution mechanism and that the differences in the Madelung site
energies are not significant.
For Si, values of log J for two different temperatures at the same distance from equilibrium
gave an apparent activation energy of 27 kJ mole -1 . Analysis of the initial reaction rates of Si
dissolution at different temperatures produced an apparent activation energy of 32 kJ
mole- 1.
These values refer to dissolution at pH 7 only and were of the same order of magnitude as
those obtained by Choi et al. (1974), but were only approximately one half of those obtained
by Schott et al. (1981) for the dissolution of non-fibrous iron-free amphiboles.
CONCLUSIONS
There is little doubt that the activation energies obtained here relate to transport-controlled
processes. However, it is suggested that the overall reaction proceeds too slowly to be
controlled by processes with such low activation energies, when one considers that at neutral
and high pH less than the equivalent of one surface layer had been removed after one and a
half months reaction time.
Dissolution of asbestos fibres in water
33
On comparing these results with both the methods and results of other workers (e.g. Choi et
al., 1974; Grandstaff, 1977; Schott et al., 1981 ; Chou & Wollast, 1984) it appears that it is the
pre-treatment of the minerals which is the critical factor. As the calculations in these
experiments were the result of the first sixty hours reaction, it may be the dissolution of
ultrafine particles produced by comminution which was measured. This is probably also the
reason for the discrepancies in this work in the apparent activation energies of Mg
dissolution calculated using the initial reaction rates and at equal distance from equilibrium.
It has also been suggested (Chou & Wollast, 1984) that the presence of fine-grained material
and highly strained areas is the reason for the observed 'parabolic dissolution kinetics'
proposed by Luce et al. (1972).
Heterogeneous reactions involve the formation of a reaction border, resulting from an
inefficiency in transport processes, or from chemical reactivity. The latter may involve two
processes, that of nucleation and the reaction at the interface, and all three factors are
affected by surface conditions. The course of a reaction depends on the way in which
potential reaction sites are distributed. Nucleation is only possible when some sort of surface
reaction has proceeded to a sufficient extent, for enough nucleus-forming species to be
released. The rate of nucleation varies differently with respect to the rate of the interfacial
process and may not always be rate-limiting.
Although the actual surface area may affect the degree to which transport processes control
the overall process, the results suggest that the surface of the minerals is in a condition in
which the surface energy of sufficient potential sites is high enough that the chemical reaction
is no longer the rate-limiting process, but that as the number or energy of available new sites
decreases so the progress of the chemical reaction may become rate-limiting.
XPS results indicate that the dissolution of chrysotile may occur through an Mg-depleted
surface layer, but no such layer formed on the crocidolite surface. However, for both
chrysotile and crocidolite there is a tendency towards congruent dissolution with increasing
temperature and increasing time. This suggests that, in the long term, there will be a slow
congruent dissolution of both types of asbestos in water. The dissolution of crocidolite may be
slowed further, under suitable conditions, by the precipitation of ferric oxides on the surface.
Seiver & Woodford (1979) found that oxidation of Fe in mafic minerals did not occur within
the existing structure and therefore did not contribute to the dissolution process during
oxidation, but could reduce it by the formation of an unreactive coating.
Chrysotile dissolution is more rapid at low pH than at high pH but also closer to
congruency, presumably since the initially faster removal of Mg ions exposes a large area of
the tetrahedral silicate sheet to hydrolytic attack, so it is unlikely that a silica relic, devoid of
metal cations, would remain in suspension. Even if this was the case, research indicates that
it is the shape of the fibre and not its chemical composition which is responsible for the
carcinogenic properties of asbestos, so the hazard would not be reduced by the loss of Mg
from the structure.
In the environmental context the system is likely to be an open one, so approaching
equilibrium may not always be a factor in the dissolution rate. All asbestos deposited in
landfills is likely to have undergone a manufacturing process. However, the pretreatment of
the mineral is unlikely to have much effect on the long-term dissolution rate; the conditions in
which the fibres are retained will be of most importance. For example, pH will effect
chrysotile dissolution most dramatically and Eh is likely to have more control over the
dissolution of Fe-rich crocidolite. The present work shows that only in extreme conditions of
low pH will there be any chance of the complete disintegration of the minerals within a
groundwater system.
34
Janet R. Gronow
ACKNOWLEDGEMENTS
This work was carried out under the direction of the Thames Water Authority as part of a Natural
Environment Research Council CASE Studentship. I would like to thank my supervisor, Dr C. V. Jeans for his
support and advice during that time and Dr A. A. Hodgson for his helpful review of the paper.
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