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Influence of pH and Temperature on Basaluminite Dissolution Rates Patricia Acero, and Karen A. Hudson-Edwards ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00128 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 20, 2018
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ACS Earth and Space Chemistry
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Influence of pH and Temperature on Basaluminite Dissolution
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Rates
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Patricia Acero1 and Karen A. Hudson-Edwards1,2*
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don WC1E 7HX, UK.
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Exeter, Penryn, Cornwall, TR10 9FE, UK. *Corresponding author Tel: +44-(0)1326-259-489;
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Email:
[email protected] Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet St., Lon-
Now at Environment & Sustainability Institute and Camborne School of Mines, University of
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Resubmitted to:
ACS Earth and Space Chemistry
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Date of resubmission:
28 December 2017
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Keywords:
basaluminite; aluminum; dissolution; rate; incongruent; activation
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energy.
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TOC Art:
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Abstract
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The processes, rates, and controlling factors of basaluminite (Al4(SO4)(OH)10·4H2O) dissolution
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were assessed using batch dissolution experiments in both H2SO4 and HCl at pHs of 2.4, 2.9-3.1,
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3.5-3.6 and 4.0-4.1, and temperatures of c. 279, 293, 303 and 312 K. Basaluminite dissolution is
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incongruent over most of the studied pH range, giving generally a lower Al/S ratio in solution
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than in the pristine basaluminite sample. The lower Al/S ratio may be at least partially explained
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by the preferential release of sulfate compared to Al from the dissolving basaluminite. The disso-
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lution rates range between 10–7.6 and 10–9.1 mol·m−2·s−1. At 291-293K, the slowest rates were
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observed at pH 4.1 in H2SO4 solutions, while at pH 3.0, the slowest rates were observed at 279 K
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in HCl solutions. Decreases in pH and increases in temperature increase dissolution rates. The
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influence of pH and temperature on the basaluminite dissolution rate, expressed as Al release,
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can be described by the following expression: .±. ± ⁄ = 10 . ±.
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Where rateAl is the basaluminite dissolution rate, based on the rate of Al release from dissolving
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basaluminite (in mol·m−2·s−1); aH+ is the activity of hydrogen ions in solution; R is the Universal
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gas constant (in kJ·mol−1·K−1) and T is temperature (in K). In light of the calculated value for the
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activation energy (78±3 kJ·mol−1), basaluminite dissolution appears to be surface-controlled. The
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reaction for basaluminite dissolution under the experimental conditions is proposed to be
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Al4(SO4)(OH)10·4H2O + 10 H+→ 4 Al3+ + SO42- + 14 H2O.
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ACS Earth and Space Chemistry
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INTRODUCTION
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Basaluminite (Al4(SO4)(OH)10·4H2O) is one of the most common aluminum hydroxysulfates
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associated with acid mine drainage and acid sulfate soils1-5 and is one of the main minerals
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thought to control the solubility of Al in acid sulfate waters1. Basaluminite is a nano-sized to
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microcrystalline variety of felsöbányaite6,7 and was discredited by the International Mineralogi-
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cal Association in 2006. However, we retain the term ‘basaluminite’ in this manuscript, as others
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have8, because of its prevalence in the scientific literature and in thermodynamic databases.
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There has been some controversy about the mechanisms and products of basaluminite dissolu-
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tion1,2,9 . Adams and Rawajfih1 proposed that basaluminite dissolved incongruently, producing Al
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hydroxide as a secondary product:
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Al4(OH)10SO4 + 2H2O ↔ 3Al(OH)3 + Al3+ + OH- + SO42-
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Nordstrom3 refuted this proposed equation because of the preferential formation of basaluminite
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from low pH waters, and because the aluminum ion cannot be independent of the solid phases
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present at chemical equilibrium9. Nordstrom3 proposed the following alternative reaction for
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incongruent basaluminite dissolution:
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Al4(OH)10SO4 + 2H2O ↔ 4Al(OH)3 + 2H+ + SO42-
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Excessive concentrations of aluminum can lead to toxicity in plants10, animals11, particularly
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fish12, and humans11,13. To protect environmental health and to better predict the controls on Al
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cycling in the surface environment, there is a need to understand the mechanisms, rates and
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products of the dissolution of Al-bearing minerals such as basaluminite. Previous studies have
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determined the dissolution kinetics of the Al oxyhydroxysulfate alunite14,15, but those for
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basaluminite remain unknown. To help bridge this knowledge gap, the kinetics of basaluminite
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dissolution under conditions similar to those commonly found in low-temperature aquatic acid
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mine drainage environments are assessed in this work. With this aim, batch dissolution experi-
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ments in both H2SO4 and HCl at pH values between 2.5 and 4, at temperatures between 279 and
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312 K were carried out using pure synthetic basaluminite as a starting material. The evolution of
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dissolved concentrations and reacting solids during the experiments were monitored and inter-
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preted, together with geochemical modelling and mineralogical analyses. Rate expressions in-
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cluding the influence of pH and temperature were obtained, and possible dissolution reactions
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are discussed. Therefore, our study is the first to derive dissolution rate data for basaluminite that
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can be used to predict its environmental behavior.
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MATERIALS AND METHODS
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Analytical, mineralogical and other techniques. Elemental analyses (for Al and S) for all solu-
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tions obtained in this study were obtained via Inductively Coupled Plasma Optical Emission
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Spectrometry (ICP-OES) on a Varian 720-ES (axial configuration) using a simultaneous solid-
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state detector (CCD). Calibration with sets of five standards was performed and laboratory
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standards were also analyzed after every 10 samples and any drift in the measurements (general-
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ly less than 4%) was corrected accordingly. The quantification limits for Al and S were deter-
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mined to be 3.7 × 10-6 and 3.1 × 10-6 mol L-1, respectively. Sulfur concentrations were trans-
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formed to dissolved sulfate, which is the main stable species under the experimental conditions.
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XRD spectra were acquired using a PANalytical XPert Pro diffractometer with Co Kα1
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radiation and an ‘X’Celerator’ position-sensitive detector with the X-ray tube operated at 40 kV
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and 30 mA. Data were collected over the 2θ range from 5° to 110°, with a collection time of 13
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h.
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ACS Earth and Space Chemistry
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The surface area of the synthetic material was determined to be 55.6±0.2 m2·g-1 using the
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BET method16 in a Beckman Coulter SA3100 surface area analyzer using 5-point N2 adsorption
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isotherms.
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Variations of pH and temperature during the dissolution experiments were monitored by
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regular measurements using a Thermo Scientific Orion Star A121 pH meter with automated
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temperature correction. The pH meter was calibrated and checked using certified pH 2, 4 and 7
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buffer solutions.
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Synthesis and characterisation of basaluminite. Basaluminite was synthesized by titration
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using a modification of the method of Adams and Rawajfih1. For the synthesis, a 0.02 M
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Al2(SO4)3 solution was titrated dropwise with 1 M NaOH while vigorously stirring to pH 4.2.
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The solution was decanted and filtered, thoroughly washed with deionized water and then fil-
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tered again and dried at room temperature for five days.
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The purity of the obtained synthetic mineral phase was confirmed by XRD spectra, which
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showed a typical basaluminite diffraction pattern without any other peaks or significant back-
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ground from other phases (Figure S1).
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The composition of the synthetic precipitate was confirmed by ICP-OES after digestion
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with concentrated nitric acid. The formula of the resulting precipitate, based on the aluminum
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and sulfur proportions from the analyses, is Al3.98(SO4)(OH)10·nH2O. This corresponds to an
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almost perfectly stoichiometric basaluminite.
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Dissolution experiments. The effect of different pH values and temperatures on basaluminite
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dissolution kinetics was assessed by means of batch stirred experiments in H2SO4 solutions at pH
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values between 2.4 and 4.1 and at four different temperatures (around 279, 293, 303 and 312 K;
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see Table 1). This range of conditions is intended to represent the most usual conditions under
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basaluminite dissolution may take place in natural systems. Although higher pH values were also
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explored in preliminary experiments, they led to oversaturated solutions with respect to basalu-
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minite within a few minutes and, therefore, they have not been included in this study. The exper-
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iments at the two lowest temperatures were carried out in a controlled temperature room and the
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rest were performed using a magnetic stirred heating plate. The effect of HCl solutions on the
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dissolution kinetics was also explored between 293 K and 295 K and under the same pH range
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(2.4 to 4.0) as for the H2SO4 experiments. All the reported conditions were addressed at least by
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triplicate (and in most cases by quadruplicate) experiments to ensure the reproducibility of the
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obtained results.
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For each dissolution experiment, approximately 50 mg was quartered and split from of
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the synthetic basaluminite sample and placed in a beaker containing 200 mL of the target solu-
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tion. All the solutions were prepared using deionized water (
