Dissolution Stoichiometry

RESULTS AND INTERPRETATION OF EXPERIMENTAL WORK

4.1 WEATHERING BY DISSOLUTION

4.1.1. Surface Coordination of Inorganic Ions

4.1.2.1. Dissolution Stoichiometry

(2.4) together, taking the initial surface to be 10 percent sil ica (90 percent magnesium hydroxide), and given a pHzpc of 8.9, a better fit to the data is obtained using PKal s = 8.0, PK a2 s = 10.0, pKas s = ^7.5

and C = 4.0 C/V cm2• These values were arrived at by iteration and are used in the interpretations of dissolution rate data that follow.

Lines with the same slope as these best-fit

ines for PKal s

and PKa2 s are also plotted on Figure 4.4. These 1 ines are not directly

comparable to the plotted points, as the points are based on an

initial magnesium hydroxide surface and the 1 ines are based on a mixed magnesium hydroxide-sil ica initial surface. The relation derived in prel iminary experiments for brucite, which suggests a capacitance of 3.1 C/V cm 2 and a lower PK

a2 s (9.6), is also shown. The latter experiments were IIfast

^ll

titrations and are described in Appendix IV.

The calcul ated rel ation of pH vs based on the above constants is shown on Figure 4.3. The model describes observations best near the pHZpC and deviates most at high pH. The fit could be made better by using different capacitances above and below the pHzpc ' which would add one more fitting parameter to the model. As is apparent from the data of others, the charge-potential relations on metal-oxide

amphoteric surfaces may be quite different above and below the pHzpc (Hohl

Stumm, 1976; Kummert

Stumm, 1980; Sigg

Stumm, 1981).

4.i.2. Dissolution of Chrysotile

;,"1 :0.05

~ O. i ^<.)::

~ ::>

U '-"

20 .0 60 80 100 12C 20 40 60 80 10e 120

C' . 4 0.15

(, ."1 7.0 Mg c pH 7.0 Si

N _E " " "1 7.3 NaCI

..

e x pH 7.5 NaCI

..:: 0.3 ^.!. p"1 S.C ..., _~ pH 8.0

~"1 b.~ ... ₊ pH 8.5

..

'"

^" " "1 9.0 ^~^0.1 ⁰ pH 9.0

f' .:- ,,"1 e co pH 9.5

-

'!'

c L ' - 0

~ l'. I U

~ 0

0 '.' ²⁰ ⁴⁰ ⁶⁰ ⁶⁸ ^18C ¹²⁰

:,4 0.15

0 ,,"1 7.)~ Mg c 1'"1 7.93 Si

N " ^p"1 ^{8. 15} NaHC03 ...

" ^{p'i 8.15} ^NaHCO;,

E e

~ .3 ^t:. ;:>11 15.27

NaNOs

..., _t:. _{pH 8.27}

NaN03

" ^;:"1 ^'3,^{I C}

,

p"1 9. I:)

" ^,)"1 ^'3,20 ^~^O.ⁱ ^~ ^pl1 ^9.20

€' e co pl1 9.35

6

^.~I.- ^'!'₀

:O.C5

~ o. ^~,_c:

.,

L: 20 40 60 60 ,,,,...

'"oJ 120 20 120

0.4 0.15

Cl .,'"1 8 "

.

" C.Ci"j Mg ⁰ pl1 8.C C.CiN Si

N " ^i''"I ^8.8 ^C.ⁱ^"j ^NGzS04 ... _x _pl1 _8.C _C._J_~

NGzS°4

e e

t:. p"1 8.5 C.Ci'" '" ^to ^{pH 8.5} ^O.OiN

... C'. 3

..

1='"1 8.5 O. i "j

,

+ p'i 8.5 O. IN

,

.;) O. i

f; e

-

^{6(>' -}

^..

₀

:0.C5

~ (t. I ^<.)

., c: _~

20 40 60 6r, ," lac 120 0

20 40 60 80 10C 120

Ti~e. h 0 u r 5 T I ." e • hours

Fig. 4.5. Extent of magnesium (Mg) and s il

ca (5i) release from chrysotil e in various electrolytes; KN03 and NaCl were 0.1 M,

NaHC03/NaN03 was at. fi xed' PC02 = 350 ppm; 10 giL chrysotile, N2 atm. ,

25 C.

studied are shown on Figure 4.6. The more rapid magnesium release during the first few hours of each constant-pH experiment has two possible expl anations. First,

could be due to preferential dissolution of fine-grained impurities and dissolution at surface discontinuities. These discontinuities resul t in part from the grinding done during sample preparation causing dislocations in the crystal structure; these dislocations yield "higher-energy" surface sites, more accessible to proton attack. Discontinuities may also be present in the parent mater ial, before sampl e preparation. For

feldspars and pyroxenes, it has been shown that a steady-state

dissolution rate occurs essentially from the outset of dissolution in pre-weathered (washed, HF-etched) material. That is, removal of higher-energy surface sites in a prior step yields a more homogeneous starting material. Using freshly-ground material, feldspar and

pyroxene dissolution rates are initially high and decl ine to steady-state after about fiv"e and 20 days respectively (Holdren

Berner, 1979; Schott et al., 1981). The same effect has been observed on

-A1

2 0

3 (Stumm et al., 1983).

Second, the initial release could reflect rapid dissolution of a portion of a uniform outer brucite layer. The initial rate is

approximately 1000-fold faster than the constant magnesium release rate that occurs after the first day. Sil ica dissolution occurs at a constant rate essentially from the outset. This initial rate for magnesium release from chrysotile is of the same order of magnitude as

that for bruc ite d issol ution (Figure 4.7). The gradual decl ine in

rate could in part reflect a decrease in the total number of surface

:0.C5

~ 0.1 ....

.,

~ ~

u ....,

0 20 40 60 60 100 120 ⁰ 20 40 60 6(; l:JG

."

^.^,,-

(I 4 0.15

d ; '1 ⁷.S MO ⁰ pH 7.5 Si

'" " ^,,'1 ^6.C ^CA ¹⁰ ...

" ^pH ^8.0 ^{CA 10}

E E

-.: 0. ³ .:. p'1 &.5 '-

'"

^A ^pl-l ^8.5

.. 0 0.1

;,

^(',

..

"

_:0.C5

-: (I. I ., '" ^~₀

[I 0

I.: 2C 40 50 60 loe :2:

C.4 0.15

0 ; '1 ⁷.5 MO ⁰ ^pH 7.5 Si

'" E ^x ,'" ^6.C ^OX ^...E

•

^pH ^8.0 ^OX

.:: (" 3 ^.:. ^p'1 6 _{• .,J}<-

'"

^A ^pH ^8.5

...

,

0 0.1

E E

..

,.

-

⁰

;0.C5

,

o· i

'"

^~0

,-' u

(! 0

I,; 2C 40 120 20

0.4 O. i 5 _T _I

0 ~'1 7.5 MO 0 p'1 7.5 ^Si

... _E _• _pl-l _8.C _OX ₁₀

..

_E x pl-l 8.0 OX 10

'-' .:. pl-l 8.5 '-' .:. pH 8.5

'- 0.:3 _'-

0 .., O. I -

E e:

'6°.

..

: 0. e5

-

~ 0.1 ^u

.., .., ~

u u

°

²⁰ ¹⁰⁰ ¹²⁰ ⁰ ²⁰ ⁴⁰ ⁶⁰ ⁸⁰ ^loe ¹²⁰

T I ." ~ • hour'S

Fig. 4.6. Extent of magnesium (Mg) and s

ⁱ¹

i ca

(Si)

release from

chrysotil e in the presence of

and

mmollL catechol (CA) and

oxalate (OX);

0.1 M

NaCl,

giL chrysotil e,

atm. ,

C.

2.0~---'----~----'---'---~----~---r----~---.

N E

~ 6 '0 I.

ID E

E

1.2

:::J

'iii

~ co

~ 0.8

0.4

Locli 7.5

~LOdi

^8.5

OL-__

____

- L ____ ~ ____ ~ __ ~ ____ ~ ____ ~ ____ ~ __ ~

o w

^~ ^~ ⁰⁰ ¹⁰⁰

Time, hours

Fig. 4.7. Extent of magnesium release from brucite at constant

pH; 0.1 M

NaCl,

²⁰

mg/L (Gabb) and

⁸⁰

mg/L (Lad;),

atm.,

²⁵

c;

pH values 7.5, 8.0

and

^8.5.

sites initially removed. Intercepts range from 0.05 to 0.35x10- 9 mol/cm2, or approximately 5-15 percent of the total number of surface sites.

Al though the actual composition of the outer surface (fraction 5 m vs. 5s) was not known at the outset, the magnitude of the surface charge observed and the small anount of Mg2+ dissolved during sampl e preparation (section 3.1.1) suggest that 5m

5s initially. Thus the apparent small shift in surface composition could not account for a 1000-fold decl ine in dissolution rate. The decl ine must then be due to removal of higher-energy sites created during sample grinding. The f ac t that s

ic a rel ease is 1 inear from the outset supports thi s

conclusion.

Chowdhury and Kitchener (1975) observed preferential release of magnesium from chrysotile at broken fiber ends or places where fiber

bundles were danaged. This direct, al though qual itative, observation further suggests that the more rapid initial dissolution is due to surface discontinuities.

On the average, the ratio of mol ar magnesium to sil ica rel ease rates is about 2.0 over the pH range 7-9, versus the 1.5 molar ratio

in the sol id. This suggests that the outer surface, initially

magnesium hydroxide, becomes more sil ica-l ike with aging time in

water. In

con~rast,

dissolution behavior reflecting magnesium, iron

and sil ica molar ratios in the sol id was observed for other sil icate

minerals, including 01 ivine (Grandstaff, 1980), faya1 ite and bronzite (Schott

Berner, 1983) and diopside and trenolite (Schott et a1., 1981) Dissolution of three other layer si1 icates -- antigorite, talc and ph10gophite -- over several weeks also showed magnesium being released at a faster rate than silica (Lin

Clemency, 1981).

The shift from a magnesium hydroxide to sil ica surface should resul t in a gradual decrease in the rate of magnes ium rel ease from chrysotile and an increase in the rate of sil ica release. Eventually

the dissolution rates for the two components should converge to a steady-state ratio of 3:2 for Mg:Si and the surface composition should remain constant. This was not observed in any of the five-day

dissolution experiments.

Dalam dokumen In Partial Fulfillment of the Requirements f or the Deg ree of (Halaman 108-114)

RESULTS AND INTERPRETATION OF EXPERIMENTAL WORK

4.1 WEATHERING BY DISSOLUTION

4.1.1. Surface Coordination of Inorganic Ions

4.1.2.1. Dissolution Stoichiometry

(2.4) together, taking the initial surface to be 10 percent sil ica (90 percent magnesium hydroxide), and given a pHzpc of 8.9, a better fit to the data is obtained using PKal s = 8.0, PK a2 s = 10.0, pKas s = 7.5

and C = 4.0 C/V cm2• These values were arrived at by iteration and are used in the interpretations of dissolution rate data that follow.

Lines with the same slope as these best-fit

ines for PKal s

and PKa2 s are also plotted on Figure 4.4. These 1 ines are not directly

comparable to the plotted points, as the points are based on an

initial magnesium hydroxide surface and the 1 ines are based on a mixed magnesium hydroxide-sil ica initial surface. The relation derived in prel iminary experiments for brucite, which suggests a capacitance of 3.1 C/V cm 2 and a lower PK

a2 s (9.6), is also shown. The latter experiments were IIfast

titrations and are described in Appendix IV.

amphoteric surfaces may be quite different above and below the pHzpc (Hohl

Stumm, 1976; Kummert

Stumm, 1980; Sigg

Stumm, 1981).

4.i.2. Dissolution of Chrysotile

..

..

'"

-

,

6

.,

.

..

,

,

-

..

Fig. 4.5. Extent of magnesium (Mg) and s il

ca (5i) release from chrysotil e in various electrolytes; KN03 and NaCl were 0.1 M,

NaHC03/NaN03 was at. fi xed' PC02 = 350 ppm; 10 giL chrysotile, N2 atm. ,

25 C.

studied are shown on Figure 4.6. The more rapid magnesium release during the first few hours of each constant-pH experiment has two possible expl anations. First,

feldspars and pyroxenes, it has been shown that a steady-state

dissolution rate occurs essentially from the outset of dissolution in pre-weathered (washed, HF-etched) material. That is, removal of higher-energy surface sites in a prior step yields a more homogeneous starting material. Using freshly-ground material, feldspar and

pyroxene dissolution rates are initially high and decl ine to steady-state after about fiv"e and 20 days respectively (Holdren

Berner, 1979; Schott et al., 1981). The same effect has been observed on

2 0

3 (Stumm et al., 1983).

Second, the initial release could reflect rapid dissolution of a portion of a uniform outer brucite layer. The initial rate is

approximately 1000-fold faster than the constant magnesium release rate that occurs after the first day. Sil ica dissolution occurs at a constant rate essentially from the outset. This initial rate for magnesium release from chrysotile is of the same order of magnitude as

that for bruc ite d issol ution (Figure 4.7). The gradual decl ine in

rate could in part reflect a decrease in the total number of surface

.,

."

'"

;,

..

"

•

'"

,

..

,.

-

,

'"

..

'6°.

..

-

°

Fig. 4.6. Extent of magnesium (Mg) and s

i ca

release from

chrysotil e in the presence of

and

mmollL catechol (CA) and

oxalate (OX);

NaCl,

giL chrysotil e,

atm. ,

C.

E

~LOdi

OL-__

(2.4) together, taking the initial surface to be 10 percent sil ica (90 percent magnesium hydroxide), and given a pHzpc of 8.9, a better fit to the data is obtained using PKal s = 8.0, PK a2 s = 10.0, pKas s = ^7.5

^..