Surface charge development at the Barite-Water interface in NaCl media, from 15 to 50˚C
Heather Williams and Moira K. Ridley
Department of Geosciences, Texas Tech University
RESULTS
Background corrected, Ionic Strength = 0.06 m
0.0
Excess / Deficit Micromol H+ / m2
T=15 C
T=25 C
T=35 C
T=50 C
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.2
-0.3
-0.4
-0.5
T=15 C
T=25 C
T=35 C
T=50 C
-0.6
-0.7
3
-0.7
4
5
6
7
8
9
3
4
5
6
7
8
9
10
11
The proton induced surface charge
of barite shows a strong temperature
dependence, as the surface becomes
progressively more positive (deficit
of H+ in solution) with increasing
temperature.
pH
The titration curves are shallow from
pH 3 to 8 then steepen significantly.
The barite surface is positively charged
over the entire experimental pH range,
and at all ionic strengths and
temperatures studied. The results
suggest that the zero net proton
charge (pHznpc) value is at high pH,
likely above a pH of 9.
STEM/TEM (upper) and SEM (lower)
images of the barite used in the
titrations. The particles are euhedral
and approximate 50 nm in diameter.
Barium
Oxygen
Sulfur
o
Varying Ionic Strength at T = 25 C
-0.1
Barite surface charge
is strongly dependent
on ionic strength.
Surface charge
decreases as ionic
strength decreases.
I=0.03 m
I=0.03 m
I=0.06 m
I=0.1 m
-0.2
-0.3
-0.4
-0.5
4
5
6
7
8
9
pH
Orthorhombic crystal structure of barite
+
Background titrations at Ionic Strength = 0.06 m
Delta = Measured mH - Calculated mH
XRD Analysis
+
1000
Barite is commonly used in
drilling fluids for oil and gas
exploration.
Intesnity (cps)
800
The effective radii of industrial
pipes are reduced by barite; thus,
decreasing necessary fluid flow.
10
pH
Excess / Deficit Micromol H+ / m2
Barite is found in a variety of geologic environments,
including marine deposits from evaporation and biogenic
processes, as well as hydrothermally mineralized veins.
Barite is also of industrial importance. For example, it is
found as scale in pipes which causes a decrease in the
effective radii of pipes and limits fluid flow. Barite is an
additive to drilling fluids used for oil and gas exploration.
Consequently, a number of studies have been performed
analyzing crystal growth and the dissolution behavior of
barite. Similarly, numerous studies have been performed to
evaluate the reactive processes occurring between the
surface of calcite and aqueous solutions. Barite has a crystal
structure similar to calcite; additionally, the solubility of
barite has been shown to increase when calcite or gypsum
are present. Therefore, studies to evaluate the surface
reactivity of barite in aqueous solution may assist in
elucidating calcite–water interactions.
In this study, potentiometric titrations were performed
to understand the surface charging behavior of barite in
NaCl media at a variety of ionic strengths (0.03, 0.06, and
0.1 m) and temperatures (15, 25, 35, 50, and 65 ˚C). All
experiments were performed in NaCl media. Sodium
chloride media was selected as it most closely matches the
natural environmental conditions were barite is found.
Excess / Deficit Micromol H+ / m2
INTRODUCTION
Varying Temperatures at Ionic Strength = 0.03 m
600
400
200
0.00010
T=15C
T=25C
T=35C
T=50C
0.00005
0.00000
-0.00010
4
5
6
7
8
9
10
11
pH
0
CHARACTERIZATION
The commercial barite sample used in this study was characterized
extensively. Characterization included SEM and TEM imaging, XRD
analysis, and BET surface area measurements.
40
60
80
2-theta (deg)
Results from XRD analysis of the barite sample.
XRD confirmed that the sample was crystalline
synthetic barite.
At each experimental condition base and acid
calibrations solutions, and a base titrant were
prepared. The experimental solutions were
prepared from concentrate HCl, NaOH, and NaCl
stock solution, and 18 MΩ water. The
potentiometric titrations were performed using a
single titration cell. The titration procedure
included a two point calibration of the electrode,
first at base then acid pH. The synthetic barite
powder was added to the acid calibration solution,
allowed to equilibrate then the solid–solution
mixture was titrated with the base titrant. For
each titration, a constant mass of ~ 0.65 g of barite
was used. The maximum variation in the mass of
solid added was 5 %. In addition, background
titrations were performed at each experimental
condition. These titrations were conducted in the
same manner as the barite titration, but omitted
the solid (i.e., only the prepared solutions were
used). The background titrations accounted for
any pH-dependent solution effects.
Ionic Strength (m)
0.03, 0.06, 0.1
Temperature (⁰C)
15, 25, 35, 50, 65
-0.00005
3
20
10
MATERIALS & METHODS
Background titrations show pH-dependent solution
effects. The titrations were fitted to polynomial
functions and used to correct the concentration of
calculated H+ in the solid-solution titrations.
Experimental Conditions: at each ionic strength,
titrations were performed at all 5 temperatures.
ACKNOWLEDGEMENTS
Thanks to Dr. A. Stack for providing the barite samples,
and Ms. J. Riedel for help with sample characterization.