Consistency of Surface Charge and Dissolution Results

Time, hours

4.2.2.2. Consistency of Surface Charge and Dissolution Results

One check on the consistency of the dissolution, surface-charge and mobil ity data is to compare the extent of charge

reversal due to dissolution, predicted from the surface-chemical

model, with the extent of charge reversal observed in the mobil

ity

experiments. From Figure 4.8, the magnesium and s11 ica components

dissolve (pH 8) at 1.90x10- 16 and O.73x10- 16 mol/cm2·s respectively.

+4

(J)

E +2

...

_u

E ...

:t. >

..

⁰

>.

...c

-

0

-2

~

-4

f

₁ ⁶

I

^{6 6}

0

0 0

1 1

6 AHMED

0 SMITH 8 TRIVE 01

!

MEAN AND RANGE OF CURRENT DATA

3

4 5

6 7 8 9 10

pH

Fig. 4.22. Initial mobility of chrysotile; results of Ahmed (1981) and Smith & Trivedi (1974) shown for reference; current data are taken from mobility vs.

time results of Appendix III.

Assuming that these rates hold as the surface becomes more

silica-like, the fraction of silica on the surface of a chrysotile fiber is directly proportional to time. Using the surface-equilibrium constants derived in section 4.1.1, the surface charge can then be computed as a function of time (Figure 4.23). This calculation suggests that the surface charge at pH

8

should be zero after about ten weeks, which is about five-fold longer than the two-week observed time for mobility reversal (Figure 4.21).

This lack of close agreement between the two sets of data reflects the oversimplified nature of the surface-chemical model used to

describe the compound, heterogeneous chrysotile surface. The charge-potential relation and surface acidity constants were

detennined for a 1 imited set of conditions and are not expected to fit data taken under greatly different experimental conditions. For

example, the surface-acidity constants were detennined during five-day experiments, duri ng which time the surface was made up of a 1 arge fraction of >Mg-OH sites and a smaller fraction of >Si-OH sites.

Adsorption of protons at the >Mg-OH sites was responsible for the positive surface charge below pH 8.9 and the charge-potential relation was dete nni ned 1 a rgely by the natu re of the more abu ndant >Mg-OH sites

rather than by the >Si-OH sites. The same charge-potential relation would not necessarily apply to a surface with a greater fracti on of

>Si-OH sites. Further, the acidity constant for the >Si-OH sites, K s

(2.3),

was assumed, based on previous studies on a silica surface

as

with no >Mg-OH sites present. Because of the initially small relative abundance of >Si-OH sites on chrysotile, varying the value of Kass over an order of magnitude had 1 ittle effect on the model fit to the

C\I

E

( )

"'- E o

0) I

+0.2

o

o ⁵⁰

Days

100 150

pHS

o ^-0.2 o ..

-0.4

o ^{0.2 0.4 0.6 0.8 1.0}

Fraction Silica

Fig. 4.23. Predicted chrysotile surface charge during dissolution; fraction of surface that is silica vs.

magnesium increases with time; time scale based on

surface being 10 percent silica at the outset.

surface charge results in the five-day experiments. The magnitude of Kass has a greater effect on the calculated charge for a surface with a greater fraction of >Si-OH sites, corresponding to the larger times

on Fig u re 4.23.

4.2.2.3. Effect of Model Organic Anions

The addition of organic anions makes the surface charge and mobility of chrysotile negative in two ways -- by

adsorption of negatively charged species to the surface and by

enhanci ng release of magnesium from the surface. Results of a ten-day mobil ity and adsorpti on experiment are shown on Figure 4.24. The effect of adsorbed catechol in givi ng chrysotile a negative mobil ity is apparent duri ng the fi rst 24 hours after addi ng the organic.

During that time, only about 15 percent of the catechol is adsorbed, which corresponds to 40 percent of the maximum adsorption density.

The UV-absorbance peaks for catechol showed a progressive

broadening in this experiment (Figure 4.25), like that noted above in the dissolution experiments (Figure 4.19). Along with this

broadening, or apparent oxidative polymerization, is a decrease in the magnitude of the negatlve mobility. This is consistent with

oXldation of adsorbed organics to form unCharged, pol.Y.1T1eric species.

That the decrease in absorbance at A= 256 nm is associated with

catechol removal from sol utl on was confi rmed by the TOe analyses shown on Figure 4.24. Other mechanisms for reduction of the magnitude of the mobility, such as shifting the plane of shear due to polymer adsorption (Kavanagh et al.. 1975), charge neutralization by countenons. or changes in the double-layer parameters seem less likely.

~ o

e

CJ) I

QO.2

~

o

~e E~

::1..> -I

-

:li

~ o -2

o

200 ²⁴⁰

Add Catechol

l

-3~ __ ~ __ ~ __ L -_ _ L -_ _ L -_ _ L-~~~~~ _ _ ~~~ _ _ ~

o

40 80 120 160- ^{200 240}

Time, hours

Fig. 4.24. Chrysotile surface charge and mobility resulting

from catechol adsorption; pH 8, 0.01 M NaCl, 25 C, N2- C02

(350 ppm) atm., solids 10.9 mg/L (mobility) and 1.02 g/L

(adsorption); 1.9 x 10-9 mol/cm2 catechol added.

std 1.0 - - - -

.. ..

120 hrs - - - -

236 hrs - - - - std 0.5 - - -

...

- _---- --

-

^....

^_---'

240

nm

\:\

,

_,

\

,

, , _, ,

....

...

~~-

Fig. 4.25. UV scans of catechol in the presence of chrysotile; experimental results on Fig. 4.15.

o

were withdrawn from the stock suspension whose mobility is shown on Figure 4.21, catechol or oxalic added, and the effect noted. Parallel adsorption experiments were run. All mobility curves showed

qualitatively the same behavior, as did those for preliminary

experiments done with phthalic acid (Appendix III). Within 24 hours after adding the organic, a new, lower steady-state mobility value was

reached. Figure 4.26 summarizes these latter steady-state values for the range of conditions studied. If charge reversal was due only to anion adsorption at the maximum densities suggested by Figures 4.15 and 4.18 there should be little charge enhancement beyond an organic to surface molar ratio of one. The presence of an enhancement at higher ratios suggests either multi-layer adsorption, increased magnesium removal like that observed on Figure 4.13, or both.

The effect of different initial mooilities, prior to organics addition, was small; this was evidenced by the similar post-addition mobilities for pH 8, equimolar catechol and surface, for initial mobilities of fran +1 toO (Figures 4.26 and Appendix III). Oxalate

has 1 ess of an effect on chrysotil e mobil ity than does catechol, consistent with the above-noted lower adsorption of oxalate on the surface. The mixed effect of catechol and oxalate on dissolution of the magnesium-hydroxide layer was noted on F,igure 4.11 and 4.13.

However the lesser adsorption of oxalate is counter to the stronger magnesium-oxalate solution complexation equilibria noted in Table 2.2

±,-2 ..

:.c _o

:E

.0.00015

0.015

90

Fig. 4.26. Mobility of chrysotile in the presence

of various catechol concentrations (mg C/cm2),

where 0.015 mg C/cm2

=

200 10-9 mol catechol/cm2

or 100-fold excess organic relative to surface

site dens ity.

anion adsorption and enhanced magnesium hydroxide dissolution can also be seen by compar ing the Alox and chrysotil e mobi1

it

ies of Figures 4.26-4.28. The pH iep of Alox in the absence of organic matter is seen from Figure 4.28 to be about 9.4. which is near the pHzpc of 8.9 for chrysotile. From 10- 3 to 10-2 mg C/cm2 (catechol) causes a small charge reversal of Alox at pH 8 (Figure 4.27) due to adsorption. The same concentrations cause a greater charge reversal on chrysotile.

Concentrations on the order of 10-4 mg C/cm2 at pH 8 cause a charge reversal on chrysotile but not on Alox. From Figure 4.14 it is seen that catechol adsorbs more strongly onto Alox than onto chrysotile.

The greater magnitude mobility on chrysotile in the presence of

catechol (Figure 4.27) is then due 1 argely to enhanced dissolution

rather than greater adsorption.