Dissolutlon Process

2.2. DISSOLUTION MODEL

2.2.1. Dissolutlon Process

Dissolution is a multi-step process lnvolving both

redistribution of canponents in space by diffusion and chemical

reactions at the solid-liquid interface that facilitate flux of

canponents across the interface. Figure 2.4 shows the steps involved in dissolution at the surface of a solid suspended in a moving fluid.

Either diffusion (steps a,b,h or

i)

or chemical reaction (steps c-g) may be rate 1 imi t i ng.

For a reaction-limited process, steps c-g recognize that dissol ution occurs preferentially at sites of higher energy,

conslstent with the accepted terrace-ledge-kink (TLK) mcx:lel of growing or subliming solid surfaces (Boudart,

1975).

For example, it is

suggested that cation release fran an oxide may proceed similarly to vaporization of metal in vacuum or anodic dissolution of metal

(Valverde

&

Wagner,

1976):

z+ . z+ z+ z+

Me (klnk)

~

Me (step)

~

Me (ads)

~

Me (aq)

Electron-transfer, ion-exchange or canp1exation reactions may alter the rate of cation transfer fran kink sites to bulk solution by sl owi ng or speedi ng a surface-chemical reacti on or changi ng the rate of diffusion. For example, Stumm et a1. (1983) suggest that

protonation of surface >MOH groups and ligand exchange -- formation of suitable inner-sphere canp1exes of the form >ML -- weaken remaining meta1-0xlde bonds and enhance detachment of the >M-L or >M-OH

2 group.

Alternately, specifically-adsorbing solutes may block access to surface sites and prevent proton or 1 igand attack.

The rate of a su rface reacti on may be 1 imited by both the total surface area and the number of reactive sites per unit area that are available for reaction (Wadsworth,

1975).

In laboratory experiments, the initial concentration of kink sites, or sites of higher energy, may be different fran the steady-state number present at later stages

of dissolution. Size reduction by grinding the parent material

(0)

Transport

Por1icle

(b) Bdy.-layer diffusion

(c) Surface diffusion

(d) Complexation

(e) Activated cpx.formation

(1)

Detachment

(g)

Surface diffusion

(h)

Bdy.-layer

"

^\

~ ,

I / I /

( i )

Transport

O~O 00 00

diffusion

(j)

Solution complexation

Fig. 2.4. Dissolution process.

creates fresh surfaces; the distribution of energy levels of surface sites will depend on the relative number of cracks, tips and

dislocations formed and on the particle-size distribution. These surface properties are determi ned by the i ntensHy and durati on of grinding. The dissolution rate for freshly-ground material typically decreases over several hours or days to a steady-state val ue, whereas pre-weathered material exhibits steady-state dissolution from the outset (Holdren & Berner, 1979; Schott et al., 1981; Stumm et al., 1983).

Where dissolution is limited by surface reaction, large

well-developed etch pits are observed to form at pOints of excess energy such as surface dislocations. For transport-controlled

dissolution, attack is more rapid and more uniformly distributed over the surface (Berner, 1981).

Dissolution may be diffusion limited, either in bulk solution (Rickard & Sj~berg, 1983; Zutic & Stumm, 1982), or through a surface precipitate or leached layer that forms when some solid components are

released into solution at a faster rate than are others (Wadsworth, 1975; Luce et al., 1972). A surface leached layer could result from

+ +

selective exchange of cations for H (or H30 ); a surface precipitate could result from reformation of surface material as selective ions are removed, or from precipitation of less-soluble, amorphous material as a well-ordered crystal dissolves. In addition to the sequence of Figure 2.4, two additional steps would occur: b.2) diffusion of

reactive species through th~ surface layer and b.3) diffusion of dissolved species away, through the surface layer.

Past studies of chrysotile dissolution have found that magnesium is released into solution in excess of the 3:2 ratio in pure chrysotile.

This was observed for dissolution of a significant fraction of the solid at acidic pH (Morgan et al., 1973) and during at least the initial one to three days of dissolution near netural pH (Luce, 1969;

Hostetler & Christ, 1968). Selective removal of the outer brucite (Mg(OH)2) layer could account for the greater magnesium release at short times. Continued release of magnesium at a higher rate could result in build-up of a silica surface layer. A surface gel or

leached layer may form on chrysotile under natural-water conditions in

t 1 t t . . F' 1 t 1 H Mg2+ b

a eas wo Sl tuat1 ons. 1 rst, at eM to nelJ ra p, may e undersaturated and removed to solution while released silica

precipitates as an amorphous surface layer due to oversaturation.

Second, the chemical affinity for dissolution may be sufficient to enable forming a new lower-entropy surface phase by rearrangement of canponents remaining after the more-soluble Mg2+ is removed, forming a surface leached layer. Recent expeMmental evidence on feldspars, amphiboles and pyroxenes suggests that these do not occur to an extent sufficient to control reaction rate. Rather, chemical reaction at the mineral-water interface controls dissolution (Aagaard & Helgeson, 1982; Berner & Schott, 1982; Berner & Holdren, 1979; Berner, 1978).

Evidence on serpentines is inconclusive (Thanassin et al., 1977).

Berner (1978) observed that for selected minerals there is a reasonably good correlation between the solubility of a mineral and

the rate-control 1 ing mechanism by which it dissolves. Minerals with sol ub il it ies on the order of 10-3 mol/L or lower (pH 8 in water) tend to be 1 imited by surface reaction and those more soluble are 1 imited by transport. Surface-reaction 1 imited dissolution is generally slower than dissolution 1 imited by transport in solution. This correlation would be true if precipitation, the reverse of

dissolution, involved a rate-l imiting chemical step that was common to several different minerals and that occurred at approximately the sane rate in different minerals. For a reversible reaction

k

_{diss -}

-

Keqkprec' where kdiss and kprec are the rate constants for dissolution and precipitation respectively and Keq is the equil ibrium solubil ity product. If kprec is of similar magnitude for different minerals, then kdiss is proportional to Keq. If the rate-l imiting step in precipitation involves loss of one or more water molecules, analogous to solution complexation (Morel, 1983), then it is unlikely that kprec for a variety of minerals could be approximately equal. Water

exchange rates for different cations vary by several orders of magnitude. Concepts of common rate-l imiting steps or quantitative

relations between Keq and kdiss on kprec have not been incorporated into theories of nucleation and crystal growth.

Based on the correlation between solubility and dissolution

.

mechanism, brucite dissolution should be transport 1 imited and sil ica dissolution,. reaction-rate 1 imited. This suggests that chrysotile should be either reaction 1 imited or a mix of reaction and transport 1 im ited.

Experimental results on chrysotile dissolution in strong acid can be interpreted as being either transport or reaction-rate 1 imited.

(assuming

50

m2/g) the average dissolution rate is on the order of

10-13 _10- 12

mol/cm

²

·s. The amount of magnesium released in these experiments was proportional to the square root of time, which is consistent with a transport-control

1

ed reaction (Luce et al.,

1972).

Electron micrographs of the fresh and magnesium-leached material showed

1

ittle change in fiber morphology, suggesting that both

reactants (protons) and products (magnesium ions) diffused through an outer, porous silica

1

ayer that becane progress ively thicker as

dissolution proceeded (Morgan et al.,

1973).

It is not known

if

the s

i1

ic a remained as a

1

eached 1 ayer due to slow d issol ut ion or

dissolved and reprecipitated from a saturated solution. Chowdhury

(1975)

observed release rates for magnesium from chrysotile on the order of

10-15

mOl/cm

²

s at pH

7

and

37

C, which is considerably lower than the rate in strong acid. (Surface area of the material was not reported in his experiments, so a v~ue ^of

⁵⁰

^{m 2} /g was assumed.)

The calcul ated diffusion coefficient from the data of Morgan et al.

(1973),

estimated following Luce et al.

(1972),

is on the orde:- of

10-16

cm

²

/s. This is near the

10-17

cm

²

/s estimated for magnesium leaching from chrysotile in

0.1

Noxal ic acid (Thomassin et al.,

1977) •

Ionic diffusion coefficients in sil icate minerals are in general

not known, and may range from

10-10

to

10- 30

cm

²

/s (Petrovic,

1976)

versus the 10- 5 cm 2 /s for ion ic d iffus ion in water (Sherwood et al .,

1975).

Values for a surface-leached layer will depend on the

structure of the pores, or diffusion channels, through the sol ide Petrovic suggests that diffusion through an amorphous aluminum hydroxide precipitate in feldspars should proceed with diffusion coefficients comparable to those for diffusion through

comparably-sized porous sol ids, on the order of

10-

9

_{_10-}

6 cm2/s. That is, dissolution that imp1 ies diffusion rates much smaller than these is too slow to be diffusion controlled.

These order-of-magnitude comparisons fail to suggest whether chrysotile dissolution in strong acid is reaction-rate or diffusion 1 imited. The diffusion 1 imitation could be in a surface-leached layer rather than a precipitate coating. Observations do, however, provide an upper bound on the expected dissolution rate in low pH media such as stomach ac ids --

10-12 _10-13

mo1/cm

²

·s, which corresponds to complete dissolution of a single fiber within a few days.

2.2.3. Reaction Kinetics