LABORATORY DETERMINATION OF DISSOLUTION RATES OF MAMMOTH MOUNTAIN SOILS
4.3 Results and Discussion
4.3.7 A Decrease in Specific Surface Area Observed
with Ksp = 1.95 x 10-3 (Stumm and Morgan, 1996). Typical concentration ranges in the pH-stat reactors were 1500-2000 µM Al and 700-1000 µM Si. At the low pH (2.78) of these experiments, none of these batch systems approached saturation with respect to kaolinite, gibbsite or amorphous SiO2. Values of the log of the saturation quotient are around -3 for kaolinite, around -2 for gibbsite, and around –1 for amorphous SiO2. As will be discussed in Chapter 5, when similar experiments were conducted at higher pH values of 3.5 and 4.0, the solutions did reach saturation with respect to kaolinite and gibbsite (the solubility of amorphous SiO2 is pH-independent in the acidic pH range).
Table 4.4 Effect of decreases in specific surface area (A) on normalized dissolution rates dissolution rates (µeqH+ /m2 hr) Solid materialA (m2 /g)% Decreasew/ initial Aw/ final ARfin/Rinit initial finalin ARinitRfin High-CO217.363.7349.30.080.162.0 High-CO217.364.3840.40.110.191.7 High-CO217.363.0858.20.120.282.4 Control 12.130.4280.00.271.345.0 Control 12.130.8858.70.260.642.4 Control 12.130.5574.00.281.083.8 High-CO221.430.3575.20.180.744.0 High-CO221.430.3972.90.180.653.7 Control 20.980.5444.90.210.381.8 Control 20.980.4752.40.150.312.1 High-CO233.751.3165.00.100.282.9 High-CO233.751.2865.90.090.272.9 Control 32.881.3951.70.140.292.1 Control 32.881.0065.20.120.342.9 Obsidian0.180.0949.80.130.282.2 Table 4.4 Effect of decreases in specific surface area (A) on normalized dissolution rates dissolution rates (µeqH+ /m2 hr) Solid materialA (m2 /g)% Decreasew/ initial Aw/ final ARfin/Rinit initial finalin ARinitRfin High-CO217.363.7349.30.080.162.0 High-CO217.364.3840.40.110.191.7 High-CO217.363.0858.20.120.282.4 Control 12.130.4280.00.271.345.0 Control 12.130.8858.70.260.642.4 Control 12.130.5574.00.281.083.8 High-CO221.430.3575.20.180.744.0 High-CO221.430.3972.90.180.653.7 Control 20.980.5444.90.210.381.8 Control 20.980.4752.40.150.312.1 High-CO233.751.3165.00.100.282.9 High-CO233.751.2865.90.090.272.9 Control 32.881.3951.70.140.292.1 Control 32.881.0065.20.120.342.9 Obsidian0.180.0949.80.130.282.2
al., 1998). In the latter case, specific surface area decreased by 61-94%, a range similar to that of the decreases observed in these soil dissolution experiments.
The loss of mass during the > 300 hours of dissolution determined by comparing the measured weights of the pre- and post-dissolution material was on the order of 1-4%.
This is consistent with estimates of the amount of material dissolved during the experiments based on summing the concentrations of solutes released (0.5-2 %). This order of mass loss can be reconciled with the decrease in specific surface area if
variations in specific surface area among different mineral phases and different particle sizes are considered. The highly soluble ultra fine particles may have been a major contributor to the initially measured specific surface area, but a minor contributor to the overall weight of the sample.
The significant decrease in specific surface area of these soils invited a more detailed analysis of when during the < 300 hours of dissolution this change was occurring. Dissolution experiments were conducted over varying periods of time with both a soil sample (control site 1) and the obsidian sample to assess the temporal
component of the observed decrease in specific surface area (Fig. 4.7 a & b). In both the soil and the obsidian sample, the specific surface area reduction appears to be primarily occurring in the initial 100 hours or so, the same time period as the initial nonlinear portion of the H+ consumption and Si release data. This suggests that the decrease in specific surface area is associated with the dissolution of fine particles and other highly soluble phases that occurs in the initial 100 hours.
(a)
0 0.5 1 1.5 2 2.5
0 100 200 300 400 500
length of experiment (hours) Specific surface area ( m2 / g )
(b)
0 0.05 0.1 0.15 0.2 0.25
0 100 200 300 400 500
length of experiment (hours) Specific surface area (m2 / g)
Figure 4.7 - Comparison of final specific soil surface area measurements after dissolution experiments of varying duration. t = 0 represents the initial soil surface area. Series of experiments with (a) soil from control site 1, (b) the obsidian sample. These experiments demonstrate that the decrease in surface area is associated primarily with the initial, rapid phase of dissolution.
The dissolution rates reported in the previous sections are derived from the linear portion of the dissolution data which occurs after the initial dissolution phase and,
apparently, after the decrease in specific surface area. A more accurate calculation of the dissolution rates, therefore, would use the post-dissolution values rather than the initial values for specific surface area. When the dissolution rates are recalculated using the post-dissolution values, the rates increase by a factor of 1.7-5 (Table 4.4). This re-
calculation does not, however, change the pattern of relative rates shown in Fig 4.5;
control site 1 still exhibits the highest dissolution rate.
To my knowledge, only two studies have reported dissolution rates that
incorporate observed changes in specific surface area. In a study of albite dissolution, the final specific urface area was used to calculate dissolution rates (Burch et al., 1993). In a study of feldspar dissolution, a linear increase in specific surface area from the initial to final value with time was assumed and the specific surface area estimated for the time of sampling was used to calculate the dissolution rate (Stillings and Brantley, 1995). The observed changes in specific surface area observed in these experiments and their demonstrated effect on the calculated weathering rates provide reason to question the informal convention of normalizing all dissolution rates to initial surface area. The erroneous normalization of rates to initial values of specific surface area is a possible explanation for the well-documented poor reproducibility of rates determined in different laboratories by different researchers. In the attempt to compare the dissolution rate of obsidian determined in this study to rates of other minerals present in the soil in Table 4.3, the rate calculated using the final specific surface area is almost twice that of the rate determined by the initial value. To improve researchers’ confidence in reported rates, the specific surface area of the solid material should always be measured before and after dissolution experiments, allowing for consideration of how to best calculate and report dissolution rates in each case.