7 Aspects of chemical weathering
7.3 Leaching experiments
7.3.3 Leached fractions
The concentrations (g/kg) of the individual leached fractions from the Schlaitdorf sandstone (fine and coarse grain sizes) in the different leachants (demineralized water, pH 7, pH 8, and saturated gypsum solution) are shown in figure 7.9; and figure A7.1 as well as in table A7.3 in the appendix. The leachabilities of the individual leached fractions (% of wt. %) from the Schlaitdorf sandstone (fine and coarse grain sizes) in the different leachants (demineralized water, pH 7, pH 8, and saturated gypsum solution) are shown in table A7.4 in the appendix.
Due to a measuring error, data for the third one-week elution period (Ch1Wo3) in pH 7 is missing.
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leached fractions from SS fine in aq. demin (g/kg)
leachabiliy from SS fine in aq. demin (% of wt.
%)
leached fractions from SS fine in pH 7 (g/kg)
0,00
leached fractions from SS fine in pH 8 (g/kg)
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leachabiliy from SS fine in oversat. gypsum sol.
(% of wt. %)
Total of leachabiliy from SS fine (% of wt. %)
Figure 7.9 Concentrations (g/kg) and leachabilities (% of wt. %) of the leached elements from the fine sample fraction from the Schlaitdorf sandstone (please note the different scales).
a b
Comparing the concentrations of the leached fractions from the fine sample fractions from the Schlaitdorf sandstone in the different leachants, calcium is the most leached fraction except in pH 8, in which sodium shows the highest concentrations. (Fig. 7.9 a,c,e,g,I).
Comparing the leachabilities of the different fractions in the various leachants, sodium and potassium show high leachabilities in demineralized water, pH 7 and pH 8 (Fig. 7.9 b, d, f). In the saturated gypsum solution the calcium fraction shows a significantly higher leachability (Fig. 7.9 h; Tab. A7.4 in the appendix). In the saturated gypsum solution highest concentrations of leached fractions are detectable (Fig. 7.9 g). In the other three leachants, the concentration of the total leached fractions differs insignificantly (Fig. 7.9 a, c, e). In general, the calcium fraction shows highest concentrations as well the highest leachability from the Schlaitdorf sandstone (Fig. 7.9 i and j).
The high sodium concentration in pH 8 as well as the high calcium concentrations in the saturated gypsum solution might indicate cation exchange processes. In the buffers of pH 3.4, pH 7 and pH 8.4 borax (Na2B4O7·10 H2O) is contained (see buffer preparation in the appendix Tab. A7.1). Here the sodium ions, and in the gypsum solution the calcium ions, of the salts might be replaced by cations leached from the Schlaitdorf sandstone. This assumption is underlined by the higher yield of calcium ions in the four week elution period (Ch2Wo4) in comparison with the first one-week elution period (Ch1Wo1) (Fig. 7.9 g). Cation exchange is a time dependent process. Thus, over longer leaching periods more ions can be exchanged.
However, the high yields of calcium associated as a total with high leachability indicate the major impact of the dissolution of the carbonate cement in the Schlaitdorf sandstone.
As expected, the concentrations of the leached fractions are much lower in the coarse sample fractions from Schlaitdorf sandstone compared with the fine sample fractions due to the smaller reactive surfaces of the former (Tab. 7.8a). Noticeable are the relatively high sodium concentrations as well as the leachabilities in demineralized water, pH 7 and pH 8 (Fig. A7.1 a, c, e in the appendix). Higher concentrations as well as higher leachabilities would have been expected for the calcium and also for the magnesium fraction. These two derive from the dolomite cement of the stone, which is relatively soluble in the fine as well as in the coarse sample fractions. High element yields in the coarse sample fractions are assumed to come from smaller grain sizes in the natural stone. The high yields of sodium might be explained by a cation exchange process with borax. This readily soluble salt forms other salts with magnesium and calcium (boracite and colemanite) which show lower solubility compared with borax. Thus, magnesium as well as calcium ions from the dolomite cement, are “caught” by the BO3
anion and sodium is released. Noticeable as well is the high concentration of calcium in the saturated gypsum solution. A possible explanation could
leached concentrations leachabilities
Table 7.8 (a) Average concentrations (g/kg) and total concentrations (g/kg) of leached elements from the fine and coarse sample fractions from the different stones in demineralized water, pH 3.4, pH 7 and pH 8.4; (b) average leachabilities (% of wt. %) and total leachabilities (% of wt. %) of leached elements from the fine and coarse sample fractions from the different stones in demineralized water, pH 3.4, pH 7 and pH 8.4
a b
Drachenfels trachyte
The concentrations (g/kg) of the individual leached fractions from the Drachenfels trachyte (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) and the respective totals are shown in figure 7.10; and figure A7.2 and table A7.5 in the appendix. The leachabilities of the individual leached fractions (% of wt. %) from the Drachenfels trachyte (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) are shown in table A7.6 in the appendix.
Comparing the concentrations of the leached fractions from the fine sample fractions from the Drachenfels trachyte, a continuous decrease can be observed for consecutive elution periods (Ch1Wo1 to Ch1Wo3) (Fig. 7.10 a, c, e, g, i). Sodium and potassium fractions show relatively regular values of around 0.1 to 0.2 g/kg in all the leachants (Fig .7.10 a, c, e, g, i).
Calcium shows higher concentrations in the pH 3.4 leachant (Fig. 7.10 c). The highest concentrations of leached fractions are given in the pH 3.4 leachant (Fig. 7.10 c). The total concentration of leached fractions is about double compared to the other leachants. The highest yields in concentration in total are given by sodium and potassium (Fig. 7.10 k). The highest leachability in total as the percentage of leached fractions relative to the element concentration in the host rock gives magnesium (Fig. 7.10 l). In the demineralized water sodium shows the highest leachability (Fig. 7.10 b). In the other leachants (pH 3.4, pH 7, and pH 8), calcium shows the highest leachability (Fig. 7.10 d, f, h). In the saturated gypsum solution a decrease of calcium concentration is detected (Fig. 7.10 i). The highest leachability in the saturated gypsum solution shows magnesium (Fig. 7.10 j). The highest concentrations of leached fractions are detectable in the pH 3.4 leachant with calcium being the main supplier (Fig. 7.10 c). In the coarse sample fractions from the Drachenfels trachyte, the concentrations of leached fractions and leachabilities are much lower compared to the fine sample fractions (Fig. A.7.2 in the appendix). The overall relative tendencies are comparable, except for slightly higher concentrations of the calcium fraction in the demineralized water, pH 3.4 and pH 8 for the longer elution period of four weeks (Ch2Wo4) (Fig. A.7.2 a, c, g in the appendix).
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Leached fractions from DT fine in aq. demin (g/kg)
Leachability from DT fine in aq. demin (% of wt. %)
Leached fractions from DT fine in pH 7 (g/kg)
Leached fractions from DT fine in pH 8 (g/kg)
Total of leachability from DT fine (% of wt.
%)
Leached fractions from DT fine in pH 3.4 (g/kg) elements from the fine sample fraction from the Drachenfels trachyte (please note the different scales).
The data shows a more or less steady release of sodium and potassium, with very regular values in all the leachants in terms of concentration and leachability. This might be attributed to the corrosion of the high plagioclase and sanidine content in the cryptocrystalline matrix of the host rock. As for acidic impact, higher calcium yields are found in terms of concentration as well as in terms of leachability. This indicates a higher acid sensitivity of the minerals. The calcium content may derive from the weathering of the anorthite content in the plagioclases, from the minor content of apatite and from the accessory calcite. The higher leachabilities – but low concentrations – in the demineralized water, pH 7 and pH 8 in the fine sample fraction, as well as the slightly higher concentrations in the coarse sample fraction over longer elution periods may indicate calcite content. This mineral shows higher solubility than the silicate minerals; thus, higher leachability is detected. Since calcite is ascertained in the Drachenfels trachyte only in minor concentrations, only small quantities of leached calcium are achieved. The high magnesium leachability and the reduced calcium concentration in the saturated gypsum solution may indicate a cation exchange process from the clay mineral content in the Drachenfels trachyte.
Montemerlo trachyte
The concentrations (g/kg) of the individual leached fractions from the Montemerlo trachyte (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) and the respective totals are shown in figure 7.11; and figure A7.3 as well as inn table A7.7 in the appendix. The leachabilities of the individual leached fractions (% of wt. %) from the Montemerlo trachyte (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) are shown in table A7.8 in the appendix.
Comparing the concentrations of the leached fractions from the fine sample fractions from the Montemerlo trachyte, a continuous decrease can be observed for consecutive elution periods (Ch1Wo1 to Ch1Wo3) (Fig. 7.11 a, c, e, g, i). The sodium fraction shows the highest values, except for in the saturated gypsum solution (Fig. 7.11 i). The calcium concentration is pronounced in the pH 3.4 leachant (Fig. 7.11 c). The highest concentrations of leached fractions are given in the pH 3.4 leachant (Fig. 7.11 c). The Magnesium concentration in pH 3.4 is noticeable, as it is in the saturated gypsum solution (Fig. 7.11 c and i).
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Leached fractions from MT fine in aq. demin (g/kg)
Leachability from MT fine in aq. demin (% of wt.
%)
Leached fractions from MT fine in pH 3.4 (g/kg)
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Leached fractions from MT fine in pH 7 (g/kg)
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Leached fractions from MT fine in pH 8 (g/kg)
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Total leachability from MT fine (% of wt. %)
Figure 7.11 Concentrations (g/kg) and leachabilities (% of wt. %) of the leached
The highest yields in concentration in total are given by sodium and magnesium (Fig. 7.11 k).
The leachability for magnesium is always relatively high (Fig. 7.11 d, f, h, j); thus, the total for the magnesium is significant (Fig. 7.11 l). In the saturated gypsum solution, a decrease of calcium concentration is detected as being associated with a strong increase in magnesium concentration (Fig. 7.11 i). The highest leachability in the saturated gypsum solution shows magnesium (Fig. 7.11 j).
Comparing the different total yields of leached fractions, the highest values are detected for the pH 3.4 leachant with all four elements being the major supplier (Fig. 7.11 c).
In the coarse sample fractions from the Montemerlo trachyte, the concentrations of leached fractions and leachabilities are much lower compared with the fine sample fractions (Fig.
A7.3 in the appendix). Noticeable are the high calcium yields in the demineralized water and pH 3.4, as well as in pH 8 (Fig. A7.3 a, c, g in the appendix).
Compared with the Drachenfels trachyte, the leached concentrations have a higher variance in the Montemerlo trachyte. The potassium concentration is significantly lower in the Montemerlo trachyte. A part of the leached sodium, potassium and calcium fractions might derive from the feldspar corrosion. The relatively high calcium yields especially in the coarse sample fraction of the Montemerlo trachyte can be attributed to the calcite content in the host rock. The potassium and the pronounced magnesium concentrations and leachabilities, which are significant in the pH 3.4 leachant from the fine grained sample as well as in the demineralized water, pH 3.4 and pH 8 leachants from the coarse grained sample, indicate a biotite and a certain clay mineral weathering. The high magnesium concentrations associated with a strong decrease in concentration of calcium in the saturated gypsum solution may indicate cation exchange processes from the clay minerals contained in the Montemerlo trachyte.
Obernkirchen and Bozanov sandstone
The concentrations (g/kg) of the individual leached fractions from the Obernkirchen sandstone (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) and the respective totals are shown in figure 7.12; and figure A7.4 as well as inn table A7.9 in the appendix. The leachabilities of the individual leached fractions (% of wt. %) from the Obernkirchen sandstone (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) are shown in table A7.10 in the appendix.
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Leached fractions from OS fine in aq. demin (g/kg)
Leachability from OS fine in aq. demin (% of wt %)
Leached fractions from OS fine in pH 3.4 (g/kg)
Leached fractions from OS fine in pH 7 (g/kg)
Leached fractions from OS fine in pH 8 (g/kg)
Total leachability from OS fine (% of wt %)
Figure 7.12 Concentrations (g/kg) and leachabilities (% of wt. %) of the leached
The concentrations (g/kg) of the individual leached fractions from the Bozanov sandstone (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) and the respective totals are shown in figure 7.13; and figure A7.5 as well as inn table A7.11 in the appendix. The leachabilities of the individual leached fractions (% of wt. %) from the Obernkirchen sandstone (fine and coarse grain sizes) in the different leachants (demineralized water, pH 3.4, pH 7, pH 8, and saturated gypsum solution) are shown in table A7.12 in the appendix.
Comparing the concentrations of the leached fractions from the fine sample fractions from the Obernkirchen and Bozanov sandstones, a continuous decrease can be observed for consecutive elution period (Ch1Wo1 to Ch1Wo3). Obernkirchen and Bozanov sandstone show comparable concentrations of leached fractions from their fine grained samples, with potassium showing the highest yields (Fig. 7.12 and 7.13). In the saturated gypsum solution the concentrations of magnesium and potassium are higher from the Bozanov sandstone than from the Obernkirchen sandstone. This is associated with a significant decrease in calcium, indicating cation exchange processes.
The leachabilities of the various elements from the two sandstones differ: in the Obernkirchen sandstone, potassium shows significantly higher leachability. Although the concentrations are very small, this is easily understandable since the leachability reflects the percentage of leached fraction relative to the initial element content in the host rock. The leached potassium fraction may originate from accessory muscovite components. From the Bozanov sandstone, similar concentrations of potassium are leached, but the leachability is less. This indicates that the Obernkirchen sandstone is a pure arenite sandstone with minor amounts of other minerals, whereas for the Bozanov sandstone rock fragments and feldspar components as well as a certain clay mineral content are reported (Koch 2006; Přikryl et al.
2010).
Comparing the fine sample fraction from Obernkirchen sandstone with the coarse sample fraction, leached concentrations are about three times higher in the fine grained sample (Tab. 7.8a). Looking at the total concentration of leached elements from the Bozanov sandstone the difference seems to be within the measurement accuracy (Tab. 7.8a). If the single leachants are regarded, it is obvious that the yield from the fine sample fraction is much higher than that from the coarse sample fraction. The strong depletion of sodium in pH 8 (Tab. A7.5g in the appendix) changes the value of the total. .
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Leached fractions from BS fine in aq. demin (g/kg)
Leachability from BS fine in aq. demin (% of wt
%)
Leached fractions from BS fine in pH 3.4 (g/kg)
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Leached fractions from BS fine in pH 7 (g/kg)
-0,10
Leached fractions from BS fine in pH 8 (g/kg)
-3,50
Leached fractions from BS fine in oversat.
gypsum sol. (g/kg)
Total leachability from BS fine in aq. demin (%
of wt %)
7.3.4 Discussion
Comparing the pH values from the first to the third one-week leaching periods of charge 1 (Fig. 7.7), a decrease is obvious. This is reflected by the total concentration of leached elements from the various stones (see Fig. 7.10–7.13 diagram c).
The concentrations of the leached elements differ from leachant to leachant, but a tendency for a primary leached fraction can be observed for each stone: from the Schlaitdorf sandstone this is calcium; from the Drachenfels trachyte, potassium; from the Montemerlo trachyte, the highest yields are given by sodium and magnesium; from the Obernkirchen sandstone, the highest concentrations of potassium are detected; from the Bozanov sandstone, potassium and magnesium; from Stenzelberg latite, the highest yields are found for sodium; and from Londorf basalt lava, this is magnesium and sodium. The highest yields are generally found in the pH 3.4 leachant.
The negative values found for the calcium fraction in the saturated gypsum solution seems to be confusing at first sight. They can be explained by cation exchange processes. In rocks containing clay minerals, the cation exchange should preferably take place with the clay mineral content of the various host rocks; e.g., Drachenfels and Montemerlo trachyte as well as Bozanov sandstone. Thus, they may function as a sort of indicator for the clay mineral content. In the Stenzelberg latite and in the Londorf basalt lava, where strong decreases in calcium and significantly higher concentrations of magnesium are found (Tab. A7.13–A716 in the appendix), this might involve tracing back to the hornblende and the olivine weathering.
This points to how the cation exchange process with the calcium ions from gypsum cannot be ascribed to a specific mineral. In saturated gypsum solution, an only slightly increased solubility of K-feldspar is detected in comparison with demineralized water (Snethlage 1984).
Therefore, a minor impact is deduced on the feldspar dissolution by gypsum within the pore space of sandstones.