Chemical Geology 132 (1996) 91-102
Effects of organic acids on the dissolution of orthoclase at 80°C
and pH 6
R.E. Blake *, L.M. Walter
The University of Michigan, Department of Geological Sciences, Ann Arbor, M148109-1063, USA
Received 24 April 1995; accepted 13 December 1995
The dissolution of K-rich feldspar (orthoclase), quartz and AI(OH) 3 was investigated at 80°C and pH 6 in buffered
solutions of organic acids. Previous studies of the effects of organic acids (OA) on feldspar dissolution have typically been
conducted in acidic, low-ionic strength solutions often under conditions which preclude isolation of effects of pH from those
due to organic acids. Our experiments were conducted at constant pH, temperature, ionic strength, and buffer composition to
allow direct comparison of experiments with and without OA. The dissolution experiments were conducted under
closed-system conditiolas to: (1) determine the magnitude of mineral solubility enhancement by OA; (2) examine changes in
reaction stoichiometry as equilibrium is approached; and (3) investigate the effects of OA on secondary mineral
The carboxylic acid species, oxalate and citrate, significantly enhanced the dissolution of orthoclase at pH 6. The
concentrations of Si artd A1 in 10 mM oxalate and citrate solutions were nearly 3 times that in solutions of acetate buffer
without oxalate or citrate. Aluminum was below the limit of detection (< 0.007 mM) in the acetate buffer alone. Citrate
increased the release of Si and AI from orthoclase more than did oxalate at the same concentration. Equilibrium modeling
indicates that solutions with oxalate and citrate attained supersaturation with respect to gibbsite, kaolinite, and smectite, and
saturation with respect to quartz. Nevertheless, orthoclase dissolution remained congruent with respect to Si and AI release.
Separate experiments using pure quartz and AI(OH) 3 suggest that a mechanism other than formation of Si-organic
complexes may be involved in the OA-enhanced release of Si from orthoclase. The increase in dissolved silica was modest
in 10 mM oxalate and l0 mM citrate solutions reacted with quartz, and did not increase with increasing OA concentration.
The solubility of quart;, was similar in solutions of oxalate and citrate. In contrast, AI(OH) 3 dissolution was 50% greater in
citrate than in oxalate, similar to the behavior of orthoclase. Citrate may be more effective in dissolving orthoclase than
oxalate due to a stronger interaction between citrate and AI, rather than due to a synergistic effect of Al-citrate and Si-citrate
complexes. These resui[ts have implications for the mass transport of Si and A1 in diagenetic environments.
Organic acids (OA), specifically the carboxylic
acids, are of geochemical interest because they form
* Corresponding author.
soluble complexes with metals such as Pb and AI
(Giordano and Barnes, 1981; Fein, 1991a; Palmer
and Bell, 1994) thereby decreasing free-metal activ-
ity and enhancing metal solubility and mass trans-
port. Formation of organic-Al and organic-Si com-
plexes may significantly affect aluminosilicate diage-
nesis in the subsurface environment. In sandstone
0009-2541/96//$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved All rights reserved.
PH S0009-2541 (96)00044-7
R.E. Blake, L.M. Walter/Chemical Geology 132 (1996) 91-102
reservoirs, formation of secondary porosity, precipi-
tation of clays, and quartz cementation all may be
affected. In near-surface aquifers, permeability may
be enhanced or retarded by precipitation of
porosity-occluding secondary aluminosilicate miner-
als. Therefore, a thorough understanding of the con-
ditions under which organic-Al, -Si complexes are
most important is needed.
Specifically, determination of the magnitude of
mineral solubility enhancement and the effects of
fluid composition on the stability of organic-Al and
-Si complexes are required, especially under the
near-equilibrium conditions of many natural waters.
Although computational methods and predictive
models are available for estimating the stability of
metal-organic complexes and their role in enhancing
the solubility of aluminosilicate minerals (Harrison
and Thyne, 1992; Shock and Koretsky, 1995), the
experimental database from which such models are
derived is very limited. It does not include many of
the complexes of interest to studies of aluminosili-
cate reactions (i.e. divalent and trivalent OA ligands)
under conditions relevant to burial diagenesis (i.e.
> 25°C, high ionic strength, complex fluid chem-
istry). Furthermore, predictions of mathematical
models require testing by experimental studies and
studies of natural systems at the appropriate condi-
Previous experimental studies of the effects of
organic acids on mineral dissolution have focused
primarily on reactions with feldspars (Huang and
Kiang, 1972; Mast and Drever, 1987; Bevan and
Savage, 1989; Welch and Ullman, 1993). Bennett et
al. (1988) and Bennett (1991) studied the effects of
several organic ligands on the dissolution rate and
solubility of quartz. Early studies often lacked suffi-
cient experimental control, especially of solution pH
(Huang and Keller, 1970; Huang and Kiang, 1972;
Surdam et al., 1984; Stoessell and Pittman, 1990).
The degree of protonation of an organic ligand,
which varies with pH, will likely control reactions at
mineral surfaces and the formation of surface com-
plexes (Benoit et al., 1993). Past experimental stud-
ies have also reported problems with contamination
from metal reaction vessels (Ti, Au), suspected inter-
ferences from pH buffers, degradation of reactant
species, and other analytical difficulties (Stoessell
and Pittman, 1990; Manning et al., 1992).
Formation of organic acid complexes with A1 and
Si has been suggested as the mechanism whereby
organic acids enhance feldspar and quartz dissolution
rate and solubility. Alternatively, others have sug-
gested that such OA complexes are insignificant and
that the primary role of organic acids is to buffer pH
in a region of pH-enhanced dissolution (Mast and
Drever, 1987; Bevan and Savage, 1989; Stoessell
and Pittman, 1990). The significance of Si-OA com-
plexes and the complexing ability of acetate also
remain uncertain (Franklin et al., 1994; Fein and
Hestrin, 1994). These uncertainties are largely due to
inadequate separation of pH-related effects on disso-
lution behavior from OA-promoted effects. Addition-
ally, variations among experimental conditions (pH,
mineral composition, T, buffer composition, OA
species and concentrations) make it difficult to inte-
grate results and synthesize a model for the role of
OA in aluminosilicate dissolution.
More recent studies of OA-feldspar and OA-
quartz interactions have better experimental control
and have focused on precise determination of disso-
lution kinetics. Typically these studies involved di-
lute, acidic solutions in flow-through reactors main-
tained far from equilibrium with respect to both the
dissolving mineral and to possible secondary miner-
als (Mast and Drever, 1987; Welch and Ullman,
1993; Franklin et al., 1994; Knauss and Copenhaver,
1995). These conditions are optimal for determining
dissolution kinetics, but do not permit assessment of
the effects of OA on overall mineral solubility as
well as on changes in dissolution stoichiometry as
equilibrium is approached. Also, the extremely low
pH values are not relevant to many natural fluids
which are buffered at near-neutral pH and near-equi-
librium conditions by surrounding rocks and soils
(Bevan and Savage, 1989; Nagy and Lasaga, 1992;
Oxburgh et al., 1994; Gautier et al., 1994).
A major goal of the present study was to isolate
the effects of OA on aluminosilicate mineral dissolu-
tion by conducting experiments under conditions of
constant solution composition, pH, temperature, and
water:rock ratio, and making direct comparisons be-
tween identical experiments with and without OA.
Also, experiments were conducted over a broad range
of OA concentrations and compositions to better
constrain controls on dissolution behavior. Experi-
ments investigated the cumulative effects of organic
R.E. Blake, LM. Walter/Chemical Geology 132 (1996) 91-102
acids on the dissolution of various feldspars, quartz,
and AI(OH) 3 at 50-80°C and near-neutral pH where
many aluminosilicate, s are highly insoluble and where
organic ligands may be most effective. Complete
analysis of the effects of OA on dissolution rates and
the effects of ionic strength (0-2 M NaC1) and
competing complexing cations on OA-enhanced re-
actions, will be presented in a future publication.
Here we present initial results of this larger study,
focusing on one feldspar composition, and compar-
ing the effects of two organic acid species, oxalate
and citrate, on the dlissolution of orthoclase, quartz,
and AI(OH) 3 as a function of OA concentration at
80°C and pH 6. Specifically, we report on the effects
of OA on the extent and stoichiometry of the ortho-
clase dissolution reaction, and compare the behavior
of Si vs. AI during aluminosilicate dissolution.
2. Materials and methods
2.1. Preparation of 'solids
Potassium-rich feldspar, orthoclase (from India),
and quartz (Hot Springs, Arkansas) were obtained
from Ward's Natural Science Establishment. The
orthoclase was first coarse-crushed to a size conve-
nient for picking out visible impurities under a
binocular microscope. This separated material was
ground using an ag~Lte mortar and pestle and sieved
to obtain the 38-106 Ixm size fraction. In order to
remove fine-grained material produced in grinding,
grains were wet-sieved and ultrasonically cleaned in
ethanol until the s~apematant remained clear (ap-
proximately 20 rinses). Cleaned feldspar was then
dried at 80°C. Magnetic grains and particles were
removed using a Fr~aaz isodynamic magnetic separa-
tor. Scanning electron microscopic (SEM) examina-
tion showed that clean, unreacted feldspar grain sur-
faces were smooth aad free of ultrafine particles. The
surface area of the material was estimated to be 0.3
m2/g from BET surface area measurements of
feldspars of similar composition and size compiled
by Blum (1994). The initial chemical composition of
the feldspar was determined by forming a
feldspar/lithium me, taborate flux which was subse-
quently dissolved in HC1 and analyzed for major
elements by ICP-AES. Based on the compositional
Chemical analysis of orthoclase feldspar used in the dissolution
Element or oxide (wt%)
analysis (Table 1), the mineral formula was deter-
mined to be: (K0.7Na0.3)All.0Si3.008, with Si/AI =
Clear quartz crystal points were crushed in a
shatter box, and the 38-125 ~m size fraction was
wet-sieved using distilled deionized water then ultra-
sonically cleaned in ethanol. The quartz grains were
treated further by etching in dilute HF, then rinsed in
0.1 N NaOH followed by deionized water, and
finally, dried and re-sieved (Iler, 1979 and references
therein). Reagent grade AI(OH) 3 powder (J.T. Baker
Company) was treated by ultrasonic cleaning in
ethanol to remove ultrafine material, then dried at
80°C. The grain size of cleaned AI(OH) 3 was 38-106
Following the dissolution experiments, reacted
materials were recovered by filtration onto 0.45-txm
nylon filters then dried and re-examined under SEM
for physical evidence of dissolution and the presence
of secondary mineral precipitates.
2.2. Experimental solutions
Experimental solutions were prepared from
reagent grade chemicals and distilled deionized wa-
ter. Solutions of 0.5, 3, and 10 mM oxalate and
citrate were prepared by adding appropriate amounts
of lithium oxalate and lithium citrate to a pH 6 stock
buffer solution comprising lithium acetate (0.7 M)
and acetic acid (0.04 M). This buffer solution pro-
vided a constant ionic medium of relatively high
ionic strength. It has been reported that acetate does
not form significant complexes with Si (Bennett et
R.E. Blake, L.M. Walter/Chemical Geology 132 (1996) 91-102
al., 1988) or A1 (Fein, 1991a; Benezeth et al., 1994)
under the conditions of our experiments and acetate
is usually the dominant organic acid species found in
natural formation waters (Carothers and Kharaka,
1978; Lundegard, 1985; Fisher, 1987; MacGowan
and Surdam, 1990).
The lithium salts of acetate, oxalate, and citrate
were used to minimize effects of alkali cations on
quartz dissolution. Quartz dissolution rates are in-
creased by low concentrations of electrolytes at
near-neutral pH in the order: NaC1 = KC1 > LiC1
(Dove and Crerar, 1990), while feldspar dissolution
rates are decreased in NaC1 solutions (0.01-0.1 M)
at pH 3 (Stillings and Brantley, 1995). Microbial
growth inhibitors were not added to solutions to
prevent degradation of the organic compounds. Anal-
yses of oxalate and citrate concentrations of solu-
tions kept at room temperature in polyethylene bot-
tles for over one year showed no change in concen-
tration or other evidence of microbial growth. Even
solutions kept at 80°C for several months showed no
change in concentration of oxalate or citrate or signs
of degradation to CO 2 or smaller OA fragments such
as formate. Crossey (1991) estimated the half-life of
oxalate species at 80°C to range from 2,500 to
28,000 years at pH 5 and 7, respectively. Thermal
decomposition proceeds more rapidly, however, at
temperatures of 150°C (Manning et al., 1992).
2.3. Experimental methods
Orthoclase dissolution experiments: Solution compositions and
final Si and AI concentrations, 80°C and pH 6
Mineral OA OA conc.
buffer alone (a) 0.14 < 0.007
Acetate buffer: 0.7 M lithium acetate/0.04 M acetic acid.
batch conditions in a constant-temperature rotating
water bath which provided continuous, gentle agita-
tion. The batch method was employed to allow
observations of the effects of organic acids on
feldspar dissolution as equilibrium with respect to
the dissolving mineral and to secondary mineral
phases was approached.
Reactions were carried out in 30-ml high-density
polyethylene bottles cleaned in 10% nitric acid and
rinsed with distilled deionized water. A 0.5-g aliquot
of solid and 30 ml of experimental solution were
added to each reaction vessel to yield a solution-to-
solid mass ratio of 60. Each bottle represented a
single time step and was extracted only once during
All experiments were conducted at 80°C in solu-
tions buffered at pH 6. The pH of 6 was chosen to
coincide with the region of minimum pH dependence
of feldspar and quartz dissolution rates (Knauss and
Wolery, 1988; Hellmann, 1994; Blum, 1994 and
references therein), and of minimum aluminum solu-
bility (Fein, 1991b).
Experimental conditions are summarized in Ta-
bles 2 and 3. The effects of oxalate and citrate on the
dissolution of orthoclase were investigated as a func-
tion of organic acid concentration. Additional experi-
ments were conducted using "pure" end-member A1
and Si phases to decouple the release of Si from that
of AI. Quartz and AI(OH) 3 were reacted separately
with solutions of 10 mM citrate and 10 mM oxalate,
under the same experimental conditions used in the
feldspar experiments. Experiments were run under
Quartz and AI(OH) 3 experiments: solution compositions and final
Si and AI concentrations, pH 6
OA OA conc.
AI(OH) 3 80
a Acetate buffer: 0.7 M lithium acetate/0.04 M acetic acid.
R.E. Blake, L.M. Walter/Chemical Geology 132 (1996) 91-102
the course of the experiments. Extracted fluids were
immediately filtered through 0.45-1xm nylon filters
and diluted to prevent precipitation upon cooling.
Solutions collected for A1 analysis were also acidi-
fied to pH < 2 with nitric acid.
2.4. Analytical methods
Experimental solutions were analyzed by induc-
tively coupled plasma atomic emission spectropho-
tometry, ICP-AES, (Leeman Plasmaspec II). The
analytical precision was + 2% for Si and + 3% for
AI. Organic acid anion concentrations were deter-
mined by ion chromatography (Dionex 4000i series
ion chromatograph using an AS10 column and con-
ductivity detection). The pH of experimental solu-
tions was measured at 80°C, before and after reac-
tion, using an Orion pH meter and a Ross combina-
tion electrode, calibrated at the reaction temperature.
Although maintenance of effective buffering capac-
ity has been a problem in some studies using more
dilute pH buffers (Mast and Drever, 1987; Knauss
and Copenhaver, 1995), our results showed no sig-
nificant changes ir~L pH and, therefore, effective
buffering of the se,lutions. The SEM analyses of
solids were performed using a Hitachi Model S-570
scanning electron microscope fitted with backscatter
and secondary electron detectors, and a Kevex Quan-
tum energy-dispersive X-ray analytical system.
2.5. Aqueous speciafion modeling
Aqueous speciation and the saturation state of
experimental solutiQns with respect to various min-
eral phases were modeled using the SOLMINEQ.88
equilibrium modeling program (Kharaka et al., 1988).
The SOLMINEQ.88 database includes equilibrium con-
stants for acetic and oxalic acids and stability con-
stants for several A1 complexes formed with their
anions. Additional stability constants for Li com-
plexes with acetate and oxalate, and for all of the
relevant citrate species (citric acid, Li-citrate, A1-
citrate) were obtained from Smith and Martell (1976,
1989)) and added to SOLMrNEQ.88 in order to model
our experimental systems. Stability constants for po-
tential Si-organic complexes have not been estab-
lished and therefore could not be added to the
3. Results and discussion
3.1. General remarks
The effectiveness of OA in promoting mineral
dissolution and increasing the capacity of solutions
to carry dissolved Si and A1 was determined by
comparing the total Si and AI concentrations in
solutions with and without OA. The general shape of
plots of Si and A1 concentration vs. time (Fig. 1 and
Fig. 2) were similar for all experiments and followed
a trend common for batch dissolution experiments
(Bevan and Savage, 1989; Bennett, 1991). Here, an
initial rapid increase in concentration with time is
followed by an asymptotic approach to constant con-
centration as metastable equilibrium is approached.
0.(~ ~ . . . .
0 10(3 300 400 500 600
l - , • , - , • , . , . , • ,
700 0 100 200 300 400 500 600 700
•  •
• buffer alone
time (hrs) time (hrs)
Fig. 1. Orthoclase dissolution experiments at 80°C and pH 6, Si release vs. time in (a) 0.5-10 mM oxalate, and(b) 0.5-10 mM citrate,
R.E. Blake, L.M. Walter/Chemical Geology 132 (1996) 91-102
0.5 and 0
• A A
AI < detection
. , .m,
~  buffer alone
100 200 300 400 500 600 700
Fig. 2. Orthoclase dissolution experiments at 80°C and pH 6. AI release vs. time in (a) 0.5-10 mM oxalate, and (b) 0.5-10 mM citrate.
Note that AI is below detection (0.007 mM) in 0.5 mM oxalate and in the 0.7 M acetate buffer alone.
Mineral solubility is defined rigorously as the
total amount of mineral that has dissolved at equilib-
rium (e.g., Nordstrom and Munoz, 1994). The total
amounts of Si and A1 released from dissolving ortho-
clase, quartz and AI(OH) 3, are taken here as an
indicator of the amount of dissolution or relative
"solubility" of the solids in different solutions. Al-
though it could not be precisely determined whether
solutions were near equilibrium with the dissolving
solid at the end of our experiments, the solution
concentrations of Si and A1 began to level off and no
longer changed significantly with time. This repre-
sents approach to a state of constant or steady Si and
AI release reflecting metastable equilibrium with
some aluminosilicate phase.
3.2. Si- and Al-release during orthoclase dissolution
The total concentrations of both Si and A1 were
up to 2.5 times greater in reaction mixtures contain-
ing oxalate than in the acetate buffer without oxalate
(Fig. 1; Table 2). In general, Si and A1 concentra-
tions increased with increasing oxalate concentration,
consistent with results of previous studies; however,
solutions of 0.5 mM oxalate did not significantly
enhance orthoclase dissolution relative to the acetate
buffer alone. Aluminum was below detection (<
0.007 mM) in the 0.5 mM oxalate solution. This
result suggests that a threshold concentration of ox-
alate (> 0.5 mM) is required to enhance orthoclase
dissolution under these conditions. This constraint
may explain why Mast and Drever (1987) reported
that oxalate at concentrations up to 1.0 mM did not
affect the dissolution rate of oligoclase at 22°C.
Similarly, Bennett et al. (1988) only observed en-
hanced quartz dissolution in oxalate solutions ex-
ceeding concentrations of ~ 2 m M.
Aluminum was also below detection in the acetate
buffer (0.7 M acetate) alone. Huang and Longo
(1992) observed a decrease in initial rates of release
of A1 and Si from K-feldspar with increasing acetate
concentration and suggested that acetate may adsorb
onto the feldspar surface and thus retard dissolution;
however, changes in acetate concentration were also
accompanied by large changes in pH (from pH 4 to
pH 7) in their experiments making it difficult to
isolate the effects of acetate from pH. Bennett et al.
(1988) also suggested that acetate may adsorb onto
quartz surfaces based on the decrease in quartz disso-
lution rates with increasing acetate concentration at
pH 7. Manning et al. (1991) observed a decrease in
quartz solubility with increasing acetate concentra-
tion at pH 6 and 150°C, 500 bar. The importance of
a retarding/adsorption effect of acetate cannot be
similarly evaluated by the present study as only one
acetate buffer concentration (0.7 M) was used. As
shown in Fig. 1, however, Si release from orthoclase
increased steadily in the acetate buffer solution
showing that the orthoclase was continuously dis-
solving, but that the A1 was not being mobilized into
All solutions containing citrate (0.5-10 m M) in-
R.E. Blake, L.M. Walter~Chemical Geology 132 (1996) 91-102
creased the release of Si and A1 from orthoclase
(Figs. lb and 2b) relative to the acetate buffer alone.
Citrate enhanced orthoclase dissolution more than
oxalate at the same concentration. Solutions of 0.5
mM citrate increased Si and A1 release by the same
amount as solutions of 3 m M oxalate (Figs. 1 and
2). One explanation for more enhanced orthoclase
solubility in the presence of citrate may be due to the
speciation of citrate at pH 6. Carboxylic acid specia-
tion, structure, and pH relationships are shown in
Fig. 3. Acid dissociation constants for citric acid
indicate that at pH 6, citrate, although present domi-
nantly as the doubly deprotonated bi-citrate anion (2
-CO0- groups), is also present as the fully deproto-
nated species with 3 -CO0-
react with the mineral surface or dissolved species.
Effects of OA on feldspar dissolution have been
reported to increase with increasing number of car-
boxyl functional groups (-COOH) (Huang and
groups available to
at pH 6
mono -- pk = 4.8
H3C H3c4"C~0 -
Acetic acid Acetate anion
p,, =1., % ,,o
OH I pk= = 4.6 -- o/C - C"O -
H-- C- H,/~'-I Pkl = 3.1
NO_ C._~OVH [ "
p, = 4.8
Pk3=6. 5 HO--~--
H- C- Hc~o -
Fig. 3. Schematic structu:ral formulas, speciation, and acid dissoci-
ation constants (pK=) for selected carboxlyic acids, pK= values
are at 75°C for acetic and oxalic acids (Kharaka et al., 1988) and
50°C for citric acid (Weast, 1984).
CItrnte (mM~ Ovadlile (mM~
o.o, ~ ~ "=:';2z
0.00 ...... , • " . = . - . . . . . , .....
0.3 0.4 0.5
Fig. 4; Stoichiometry of Si and A1 release from orthoclase. Dashed
vertical line represents quartz saturation in the acetate buffer
Kiang, 1972; Surdam et al., 1984; Reed and Hajash,
1992). The effect of protonation on the capacity of
organic acids to promote feldspar and quartz dissolu-
tion cannot be firmly established due to differences
in the pH among different studies. However, Benoit
et al. (1993) showed that protonation of ligands
strongly affects their adsorption onto Al-oxides. More
work on the role of protonation in OA-promoted
aluminosilicate mineral dissolution is required.
3.3. Stoichiometry of orthoclase dissolution
Several studies have reported preferential release
of A1 from feldspar in the presence of OA (Welch
and Ullman, 1993; Franklin et al., 1994). In contrast,
we observed that Si and A1 were released stoichio-
metrically in oxalate and citrate solutions (Fig. 4).
Dissolution was incongruent in the acetate buffer
alone and in 0.5 m M oxalate. Solids recovered from
experiments with the acetate buffer alone had a
visibly "clumpy" aggregate texture viewed with the
unaided eye and secondary aluminosilicate precipita-
tion was substantiated by SEM examination and
EDS analysis (Fig. 5).
Equilibrium modeling of experimental solutions
approaching steady-state Si and A1 concentration
indicated that supersaturation with respect to several
aluminosilicates including gibbsite, kaolinite, and
smectite and saturation with respect to quartz, was
reached in solutions with oxalate (3-10 mM) and
citrate (0.5-10 mM). Model calculations did not
include Si-organic complexes which may account for
R.E. Blake, L.M. Walter//Chemical Geology 132 (1996) 91-102
Fig. 5. SEM photomicrograph of a typical reacted orthoclase grain
from experiments with 0.7 M acetate buffer without oxalate or
citrate. Precipitation and overgrowth of a secondary aluminosili-
cate phase (arrows) was confirmed by EDS analysis.
the apparent supersaturation of these solutions with
Si-bearing minerals. However, even with considera-
tion of Al-oxalate and Al-citrate species, which were
included in the modeling, solutions containing ox-
alate and citrate remained supersaturated with re-
spect to the A1 minerals, gibbsite and boehmite. This
apparent inconsistency may reflect our limited ability
to model organic systems due to the paucity of
available thermochemical data for metal-organic
species at relevant conditions.
Oxalate and citrate at concentrations as low as 3
and 0.5 mM, respectively, have the capacity to
significantly enhance the solubility of orthoclase in a
region of minimum A1 solubility, presumably via
formation of metal-organic complexes, and may fa-
cilitate Si and AI transport by inhibiting the precipi-
tation of secondary clay minerals. The mobility and
total mass transport of Si and AI could be increased
quite significantly in fluids containing these organic
acid species under open system conditions.
3.4. Experiments with pure quartz and AI(OH) 3
Results presented thus far show similar behavior
of Si and A1 during organic acid-enhanced dissolu-
tion of feldspar. The release of both Si and AI are
enhanced by oxalate and citrate, and the effect gener-
ally increases with increasing OA concentration. It
has been postulated that organic acid anions form
complexes with both AI and Si during feldspar disso-
lution (Surdam et al., 1984; Bennett et al., 1988).
Alternatively, it has been suggested that AI is com-
plexed by OA which disrupts the tetrahedral struc-
ture, resulting in passive, concomitant release of Si
(Huang and Keller, 1970). Mast and Drever (1987)
concluded that detachment of silicon rather than
aluminum was the rate-limiting step in the dissolu-
tion of oligoclase. Experimental results for "pure"
end-member A1 and Si phases, under the same condi-
tions used in the feldspar experiments, help resolve
The solubility of quartz was increased in solutions
of 10 mM oxalate and 10 mM citrate relative to the
acetate buffer alone (Fig. 6). In contrast to ortho-
clase, the effect of citrate on quartz was almost
identical to that for oxalate. Additional experiments
with quartz at 70°C in solutions of 2-10 mM citrate
and 10-20 mM oxalate also showed no significant
increase in the effect on quartz solubility with in-
creasing organic acid concentration (Fig. 6b). These
observations contrast with the results of Bennett et
al. (1988) who investigated the effects of oxalate and
citrate on quartz dissolution at 25°C and pH 7, and
found a greater effect for citrate relative to oxalate at
the same concentration,
The apparent discrepancy with our results may be
reconciled by a speciation effect, as was suggested to
explain the greater effect of citrate on orthoclase
dissolution vs. oxalate. At pH 7, citric acid (a tricar-
boxylic acid) is present dominantly as the fully
deprotonated species (pK 3 ~ 6.5). At pH 6, the
percentages of fully deprotonated oxalic acid (a di-
carboxylic acid; pK 2 ~ 4.2) and doubly deproto-
nated citric acid (pK 2 ~ 4.8) would be closer to
equal which may explain their similar effects on
quartz solubility at this pH (Fig. 3). At pH 7, citrate
has 3 -COO- groups available to participate in
solution and surface complexation reactions, explain-
ing the greater effect of citrate on quartz solubility at
pH 7. Reports from previous studies of decreasing
effects of organic acids with decreasing solution pH
are consistent with this interpretation (Wogelius and
Walther, 1991; Welch and Ullman, 1993). In addi-
R.E. Blake, L.M. Walter//Chemical Geology 132 (1996) 91-102
• buffer alone
0.4 __ EquiL WRT Qualtz
0.2 ~ •
100 2 200
500 " 600
• buffer alo!3e
-- Equn. WIlT Quartz
Fig. 6. Quartz dissolution in oxalate (10-20 mM) and citrate
(2-10 mM) at pH 6 and (a) 80°C and (b) 70°C. Quartz solubility
increased in solutions with oxalate and citrate relative to the
acetate buffer alone, but did not increase significantly with in-
creasing oxalate and citrate concentration.
tion to the effect of pH on the availability of depro-
tonated -COOH groups, Bennett et al. (1988) in-
voked the possible influence of pH on protonation of
-OH groups at quartz surfaces to explain a decrease
in quartz dissolutio~L rates between pH 7 and 5. The
similar behavior of oxalate and citrate at pH 6 (this
study) and dissimilar behavior at pH 7 suggests
strong control by the protonation state of the organic
Bennett et al. (1988) also presented spectroscopic
evidence for the existence of Si-citrate and Si-
oxalate complexes .'at pH 7. The spectroscopic data
have been recently questioned by Knauss and
Copenhaver (1995), and Fein and Hestrin (1994)
questioned the existence of a Si-oxalate complex.
Fein and Hestrin (1994) concluded that aqueous
Si-oxalate complexation was of negligible impor-
tance at pH 4.7-5.1 based on silica dissolution ex-
periments in 0.7-71 mM oxalate at 80°C. Franklin
et al. (1994) also did not observe enhanced solubility
of hydrothermal quartz in solutions of 0.07 m acetate
(pH 4.7) and 0.07 m acetate + 0.005 m oxalate (pH
4.4) relative to distilled water, at 100°C and 347 bar
pressure. Differences among results of these studies
may be due to differences in pH, as well as due to
differences in concentration of organic acids.
During dissolution of pure AI(OH) 3, A1 concen-
trations increased dramatically in solutions of 10
mM oxalate and 10 mM citrate (Fig. 7). The effect
for citrate was greater than for oxalate, similar to
results for orthoclase, and in contrast to those for
quartz. The concentration of A1 in the 10 mM citrate
experiment reached 0.62 m M by 24 days, whereas
the 10 mM oxalate solution had an A1 concentration
of only 0.41 mM at this point in time (Table 2). The
A1 concentration decreased in the acetate buffer alone
from a modest value of 0.02 m M to below detection
(< 0.007 mM) within the first few days of reaction,
suggesting precipitation of a secondary A1 phase.
The dissolution behavior of AI(OH) 3 in oxalate and
citrate solutions was similar to that of the A1 release
during dissolution of orthoclase.
Differences in the effect of oxalate and citrate on
the dissolution of quartz and AI(OH) 3 at pH 6
suggest that the greater effect of citrate observed
Citrate (raM} Oxalate ~rnM'J
• 10 o
~,~ 0.4 Q
0.C~ • m
Fig. 7. A1 (OH) 3 dissolution in 10 mM oxalate and citrate at 80°C
and pH 6. A1 solubility was greatly enhanced in citrate and oxalate
solutions relative to the acetate buffer alone, and was significantly
greater in citrate than in oxalate solutions. In the acetate buffer
alone, A1 concentrations decreased very early in the experiments
to levels below detection ( < 0.007 mM).
100 R.E. Blake, LM. Walter/Chemical Geology 132 (1996) 91-102
during orthoclase dissolution, is due to a stronger
interaction of citrate with A1. This interpretation is
also supported by our observations of an even greater
effect of citrate relative to oxalate in experiments
with more Al-rich plagioclase feldspars (labradorite,
Si/AI = 1.4) (Blake and Walter, 1993). These exper-
iments also show that the release of Si from quartz is
enhanced by oxalate and citrate but, possibly by a
mechanism other than the formation of a Si-OA
complex. Note, however, that the Si/AI ratios for
feldspar dissolution in citrate solutions reflect con-
gruent dissolution (Fig. 4) with no preferential re-
lease of A1 in the presence of citrate. This result
suggests that the concomitant increase in Si release
may be a "passive" result of disruption of the
mineral framework driven by formation of an A1-
4. Summary and conclusions
Results of experiments investigating the effects of
organic acids on the dissolution of orthoclase at 80°C
an pH 6 indicate that the carboxylic acid species,
oxalate and citrate, can significantly enhance ortho-
clase solubility in a region of minimum pH-promo-
ted feldspar dissolution and of minimum A1 solubil-
ity. The major results of this study are summarized
(1) Release of both Si and AI from orthoclase
increased in the presence of oxalate and citrate.
Concentrations of Si and A1 increased by almost a
factor of 3 in solutions of 10 mM citrate and l0 mM
oxalate, relative to buffered solutions without oxalate
or citrate. Citrate had a greater effect on orthoclase
dissolution than did oxalate at the same concentra-
tion. The concentration of A1 was always below the
limit of detection in acetate buffer solutions (0.7 M
acetate) without oxalate or citrate.
(2) Oxalate and citrate at concentrations as low as
3 and 0.5 m M, respectively, appear to have pre-
vented the precipitation of secondary aluminosili-
cates over the duration of our experiments (up to 24
(3) In contrast to many previous studies, Si and
Al were released in stoichiometric proportion during
orthoclase dissolution in the presence of citrate (0.5-
10 mM) and oxalate (3-10 mM), indicating no
preferential release of A1 and, hence, congruent dis-
(4) Dissolution experiments with pure quartz and
with AI(OH) 3 conducted under the same conditions
as orthoclase experiments, demonstrated that both Si
and A1 solubility are enhanced by oxalate and citrate,
but possibly by different mechanisms, and that cit-
rate interacts more strongly with A1 than with Si
during the dissolution of orthoclase.
Our results suggest that the presence of oxalate
and citrate (or other organic species with similar
carboxyl functional groups), in aquifer fluids may
significantly enhance the dissolution of K-rich
feldspar (and quartz) as well as the mobility and
mass transport of Si and A1. The subsequent precipi-
tation of Si triggered by changes in the stability of
organic complexes as a result of changing fluid
chemistry or due to thermal degradation of organic
ligands, may provide an alternative mechanism to
explain the volumes of silica cement observed in
sandstone formations without requiring large vol-
umes of fluid saturated with respect to quartz.
Whether the net effect of organic acids is to enhance
the dissolution of aluminosilicate minerals and create
secondary reservoir porosity, or to create barriers to
groundwater and contaminant flow via the precipita-
tion of A1 and Si from supersaturated organic acid
solutions, it is important to determine the conditions
under which organic acids can be most effective and
to understand the mechanisms of OA-promoted dis-
Financial support was provided by British
Petroleum Research Center and by the University of
Michigan Rackham Merit Fellowship. Max Coleman
and Ted Huston are thanked for their advice and
support over the course of this study.
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