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Barium sulfate crystallization from synthetic seawater Matthew Boon, and Franca Jones Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00729 • Publication Date (Web): 08 Jul 2016 Downloaded from http://pubs.acs.org on July 11, 2016
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Barium sulfate crystallization from synthetic seawater Matthew Boon, Franca Jones* Department of Chemistry, Nanochemistry Research Insitute, Curtin University GPO Box U1987 WA 6845 *Corresponding author, email :
[email protected] phone: (618) 9266 7677 fax: (618) 9266 2300
RECEIVED DATE (to be automatically inserted) Running Head Title: Barium sulfate crystallization in simulated seawater
ABSTRACT: Barium sulfate was crystallized in a synthetic seawater mixture that was chosen to better reflect ocean conditions. The synthetic seawater contained monovalent ions, magnesium, strontium and calcium as well as bicarbonate and boric acid. The natural pH of the synthetic seawater is 8.1 and this seawater was used to determine the impact on morphology, nucleation rates and incorporation of foreign ions. It was found that dendritic and diamond-shaped particles are both formed. The main parameters influencing the formation of dendritic particles were the saturation index and the ion ratio but there was also a significant synergistic effect with the other ions present. The diamond-shaped particles formed later at lower saturation index. The nucleation rate in synthetic seawater was found to be higher than expected based on an ion ratio basis. This is most probably due to the fact that the divalent ions induce a higher nucleation rate by lowering the surface free energy. Strontium was found to be the dominant ion substituting for barium with some calcium substitution also occurring. Finally, the presence of silicate appeared to form dendritic particles with a larger aspect ratio and more impurities being present but
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little other impacts. The lack of change in the homogenous nucleation rate supports a heterogenous nucleation hypothesis.
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Barium sulfate crystallization from simulated seawater Matthew Boon, Franca Jones* Department of Chemistry, Nanochemistry Research Insitute, Curtin University GPO Box U1987 WA 6845 *Corresponding author, email :
[email protected] phone: (618) 9266 7677 fax: (618) 9266 2300
RECEIVED DATE (to be automatically inserted) Running Head Title: Barium sulfate crystallization in simulated seawater
ABSTRACT: Barium sulfate was crystallized in a synthetic seawater mixture that was chosen to better reflect ocean conditions. The synthetic seawater contained monovalent ions, magnesium, strontium and calcium as well as bicarbonate and boric acid. The natural pH of the synthetic seawater is 8.1 and this seawater was used to determine the impact on morphology, nucleation rates and incorporation of foreign ions. It was found that dendritic and diamond-shaped particles are both formed. The main parameters influencing the formation of dendritic particles were the saturation index and the ion ratio but there was also a significant synergistic effect with the other ions present. The diamond-shaped particles formed later at lower saturation index. The nucleation rate in synthetic seawater was found to be higher than expected based on an ion ratio basis. This is most probably due to the fact that the divalent ions induce a higher nucleation rate by lowering the surface free energy. Strontium was found to be the dominant ion substituting for barium with some calcium substitution also occurring. Finally, the presence of silicate appeared to form dendritic particles with a larger aspect ratio and more impurities being present but
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little other impacts. The lack of change in the homogenous nucleation rate supports a heterogenous nucleation hypothesis.
KEYWORDS: barium sulfate, silicate, seawater, incorporation, strontium, calcium. BRIEFS: Synthetic seawater influences barium sulfate morphology, nucleation rate and impurity incorporation.
Introduction Barium sulfate, an inorganic, sparingly soluble sulfate salt is not a common solid to be found in seawater but it is found as a biomineral in some organisms.1, 2 For example, in green alga3 it has been found that a sulfate ‘trap’ leads to the formation of the mixed Ba/Sr sulfate. There is also interest in barite (the mineral name of barium sulfate) crystallization in seawater because of the ‘barite paradox’ whereby it appears to form in undersaturated seawater.4,
5
This paradox has been linked to both the
occurrence of strontium and silicate in seawater.5-7 The presence of barite in sediments is often taken as an indicator of significant biological productivity (i.e. as an ocean productivity proxy) as is strontium sulfate.8,
9
Thus it is important that the chemistry of these proxies is is well understood. Calcium
carbonate and barium sulfate are also both minerals of interest from a scaling perspective.10-13 Barium sulfate is most important to off-shore oil production, where its lower solubility means it is the most likely to crystallize when aquifer water mixes with seawater.14, 15 Thus, understanding the crystallization process of barium sulfate in seawater is of benefit both from a fundamental and practical aspect.
While silica and silicates are one of the most common minerals found on Earth, making up >90% of the Earth’s crust, they can be difficult to investigate since the chemistry of silicates can involve inorganic polymerisation of the silicate species, depending on the pH and composition of the solution.16 There are, however, a few studies available on crystallization in the presence of silicate. The Pina group ACS Paragon Plus Environment
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investigated calcium carbonate17 and strontium sulfate18,
19
crystallization where they found that
polysilicic acids have a different effect to the monosilicic acids. Another study is that of Lakshminarayanan, on calcite, which showed that the silicate ion was incorporated into the structure.20 Thus, the impact of silicate is very sensitive to pH and the mono/polymeric form. The study involving strontium sulfate showed increasing silicate concentration led to the formation of the hemihydrate and finally an amorphous solid.18,
19
Work by the group of Garcia-Ruiz21 has shown that barium
carbonate/silicate structures having curved surfaces are formed from purely inorganic sources for which they coined the term ‘biomorphs’. From this work it was shown that silicate at high pH is required to form such structures. Barium sulfate has also been investigated in the presence of silicate22. Our previous work on barium sulfate crystallization showed that at pH 10 fibrous structures are formed, however, what is lacking, is similar work in seawater. This is because, as previously stated, silica has been implicated as being an important component in causing the barite paradox5-7.
Some limited literature is available on the impact of the different inorganic ions on barium sulfate crystallization.
14, 23-29
Many of these are from the perspective of off-shore oil production where a
‘seawater’ is mixed with an ‘aquifer’ water. These studies are mainly interested in the amount or rate of scale formation or use limited ions for their “seawater” equivalent 14, 24-28; of these, two were found to be most relevant23, 29. These results show that the morphology of the formed barium sulfate is significantly altered in the presence of seawater. However, even literature on how different inorganic ions incorporate or impact on barium sulfate is sparse. Ions that have been investigated to date are: a)
the alkali cations and halide anions30-32, which have been shown to promote barite crystallization
b)
the alkali earth metal cations such as calcium and strontium33, 34 and lead35, which can substitute for barium ions in the solid
c)
zinc and lanthanum36, which both inhibit the growth of barite while lanthanum ions can substitute ~3% into the barite lattice.
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d)
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carbonate37, this study showed that the growth rate is reduced in the presence of carbonate and that the 2D island shape becomes elliptical. Eventually, witherite (BaCO3) may form.
Unfortunately, all of these were investigated separately and in pure systems. There is little to no information on what the combination of ions does to the structure of barite other than its morphological changes23, 29.
Seawater is itself a complex mixture of inorganic and organic species that changes depending on the region, depth and time of year. In this work, we have used a synthetic seawater recipe based on that by Berges38. In addition, we have chosen barium sulfate because solids such as calcium carbonate have issues of polymorph formation. Three common crystalline polymorphs are known for calcium carbonate (vaterite, aragonite, calcite) at room temperature and pressure while only barite is known for barium sulfate. Finally, as already alluded to, barium sulfate crystallization in seawater is relevant for a variety of reasons ranging from off-shore oil production to ocean productivity proxies. These considerations make barium sulfate crystallization a suitable candidate as a model system and we have used it as such to gain some fundamental insights into crystallization generally.39-41
The work discussed herein investigates, as a starting point, only the inorganic ions in seawater. This combination of ions is the most complex investigated in the literature to date and it is important to determine whether the crystallization behavior of barium sulfate is modified from that previously observed. In addition, the impact of silicate ions on barium sulfate crystallization in the synthetic seawater (SSW) will be investigated as it has been suggested to play a role in the barite paradox. The impact on the morphology, nucleation rate, crystallinity and structure (through XRD) and molecular simulation data are presented and discussed with respect to crystallization phenomena and to possible impacts on ocean productivity proxies.
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Materials and Methods All materials were analytical grade reagents used as received. Ultrapure water (resistivity > 18 MΩ cm at 22 °C) was used in the preparation of all solutions.
Synthetic Seawater (SSW) Solutions The SSW solutions were made up according to Berges38 whereby two salt solutions (salt solution 1 and 2 listed in Table 1 below) are prepared separately and then mixed together to form the SSW. This is done to avoid crystallization of calcium carbonate prior to start of the crystallization reaction. Table 1 lists the ions in the SSW and their final concentrations. The natural pH for the mixed system was found to be 8.1 . Ion groupings were also investigated whereby only the monovalent, divalent ions, or bicarbonate + borate were investigated. When this occurred the concentrations were equivalent to that in the SSW listed in Table 1. In these instances two separate solutions were not necessary and barium chloride was added to the prepared solution containing the desired sulfate concentration to commence crystallization. Once prepared, it is important to know which species are expected to form from the SSW. The supersaturation was calculated using the PHREEQC program42 (using the phreeqc database). Each of the ions and their concentrations were inputted and the activities calculated. The saturation index is then derived from the following: S.I. = log [aiajak…/Ksp]
Eqn 1.
where the ai, aj etc. refer to the activities of the ions (and is called the Ion Activity Product) and the Ksp refers to the solubility product of the solid containing species i, j, k etc..
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Table 1. Synthetic Seawater Stock Solutions and final concentrations. Values in parentheses are the concentrations used to obtain an SI of 2.5 Stock solution
Final concentration
g L-1 21.19 0.599 0.0863 0.0028 0.0230 0.174
mM 363 8.04 0.725 0.0657 0.372 2.07
ppm 21,213 599 86.2 2.76 23.2 173
3.55
Varied (0.25)
0-3551 (35.5)
Salt solution 2 MgCl2.6H2O CaCl2.2H2O SrCl2.6H2O
9.592 1.344 0.0218
41.2 9.14 0.082
8,375 3,307 79.5
Na2SiO3.9H2O BaCl2
1000 20.8236
Varied Varied (0.06)
0-813 0-5205 (12.5)
Salt solution 1 NaCl KCl KBr NaF H3BO3 NaHCO3 Na2SO4
Table 2 lists the species where the saturation index calculated by PHREEQC was greater than 0. For those species whose saturation index is
