Effect of Ionic Liquid Additives on the Solubility Behavior and

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Effect of Ionic Liquid Additives on the Solubility Behavior and Morphology of Calcium Sulfate Dihydrate (Gypsum) in the Aqueous Sodium Chloride System and Physicochemical Solution Properties at 30 °C Jignesh Shukla, Mohit J. Mehta, and Arvind Kumar*

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Salt and Marine Chemicals Division, CSIR-Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, G. B. Marg, Bhavnagar 364002, India S Supporting Information *

ABSTRACT: We have investigated the solubility behavior of calcium sulfate dihydrate (gypsum, CaSO4·2H2O) in aqueous NaCl solutions upon the addition of ammoniumor imidazolium-based ionic liquids (ILs), viz., ethylammonium lactate (EAL), 1-ethyl-3methyl imidazolium hydrogen sulfate ([C2mim]HSO4), and 1-butyl-3-methyl imidazolium hydrogen sulfate ([C4mim]HSO4) at 30 °C. The addition of ammonium lactate ILs increased solubility significantly, ∼500%, whereas the addition of imidazolium hydrogen sulfate ILs decreased the solubility by ∼60%. The addition of ILs shifted the solubility maximum of CaSO4·2H2O toward either higher or lower concentration of NaCl in solution depending on the nature of IL additive. Solution properties such as density (ρ) and speed of sound (u) have been measured for the quaternary systems (CaSO4·2H2O + NaCl + ILs + H2O) at 30 °C. Solution isentropic compressibility (κS) has been derived from measurements of u and ρ. Physical and derived properties have been fitted with suitable polynomial equations. Crystal growth formation and surface morphology of CaSO4·2H2O crystals recrystallized from different aqueous IL solutions have also been examined. IL additives have shown good potential for calcium sulfate scale removal and as calcium sulfate precipitation agent.

1. INTRODUCTION Measurements of physical properties and interpretation of solution behavior of aqueous electrolyte systems are necessary for understanding complex ion−solvent or ion−ion interactions and phase behavior.1,2 Data generated from such studies are also important for the recovery/separation of pure salts from industrial waste streams. Gypsum (calcium sulfate dehydrate, CaSO4·2H2O) is an inorganic salt known for its widespread applications. The effectiveness and capability of gypsum as a functional building material, in mineral processing, and in contaminated soil recovery make gypsum an essential salt for commercial purposes.3−5 However, the use of calcium sulfate also has its disadvantages. Sparingly soluble gypsum is quite prone to precipitation in aqueous systems, and hence it is quite difficult to remove it from an aqueous system. Gypsum is one of the primary culprits of scale in reverse osmosis systems, seawater desalination, industrial water recovery, and hydrometallurgical operations.6−11 In such processes, the scaling can cause severe problems like impeding the process flow, fouling in reverse osmosis membranes, and hence decreased performance and increased costs. Apart from these problems, gypsum also precipitates in small amounts along with crystallization of common salt (NaCl) from natural brines, thus making it impure in terms of contamination of Ca2+and SO42− contents.12 The presence of such ionic impurities is detrimental for chlor-alkali production, where NaCl is used as basic raw material. Therefore, © XXXX American Chemical Society

the objective of the present study is primarily to alter the gypsum solubility in alkaline solutions by use of additives. A considerable amount of work has been done on gypsum dissolution behavior in aqueous or brine systems; in particular, the effect of electrolytes has been extensively investigated. The references of such studies have been systematically provided in our previous works.13,14 However, the effect of organic solvents/ additives on dissolution characteristics of gypsum in aqueous or brine systems is rarely studied,15−19 and ionic liquids (ILs) have not at all been investigated. ILs (ionic salts having a melting point ≤100 °C) are replacing almost every conventional organic solvents and additives in almost every process. ILs can provide highly conducting media, dissolve enzymes, act as a biphasic system for separation and a solvent for various reactions, be involved in the desulfurization process of fuels, and so on.20−25 Apart from this, from a solvation standpoint, ILs are generally considered to be “very polar” solvents, and the solubility of various inorganic salts has been investigated in ILs. It has been seen that both cations and anions in the IL affected solubility, but anions had more of an effect than cations. Overcoming the lattice energy of the solid matrix has been found to be the key to the solubility of inorganic salts in ILs.26−29 Received: January 29, 2018 Accepted: May 18, 2018

A

DOI: 10.1021/acs.jced.8b00093 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Compounds with Their CAS Registry Number, Source, Mass Fraction Purity, Purification Method, and Analysis

a

chemical name

CAS registry no.

source

mass fraction puritya

purification

analysis method

calcium sulfate dihydrate sodium chloride ethylamine lactic acid 1-ethyl-3-methyl imidazolium hydrogen sulfate 1-butyl-3-methyl imidazolium hydrogen sulfate ethylammonium lactate

10101-41-4 7647-14-5 75-04-7 50-21-5 412009-61-1 262297-13-2 NA

HPLC India HPLC India TCI Japan TCI Japan Sigma-Aldrich India Sigma-Aldrich India synthesized

0.98 0.999 0.35 0.85 0.95 >0.95 0.95

none none none none none none none

none none none none none none NMR

Purity as stated by the supplier.

Table 2. Solubility of CaSO4·2H2O in Aqueous−NaCl Solutions in the Presence of ILs, Solution Density, ρ, and Speed of Sound, u, at 30°C at Pressure P = 0.1 MPaa NaCl (mol·kg−1) 0 0.7608 1.7218 2.8431 3.8842 4.9253 5.4860 0 0.5606 1.6418 2.6028 3.6439 4.6050 5.0855 0 0.6006 1.6818 2.6428 3.6039 4.6050 5.0054 0 0.0520 1.6418 2.8030 3.7641 4.9253 5.3258 0 0.4805 1.4415 2.4827 3.4437 4.3647 4.8453

CaSO4 (mol·kg−1) 5% EAL 0.0550 0.0667 0.0771 0.0831 0.0815 0.0738 0.0687 10% EAL 0.0914 0.1010 0.1128 0.1176 0.1147 0.1078 0.1007 10% [C2mim]HSO4 0.0089 0.0104 0.0126 0.0132 0.0124 0.0140 0.0090 5% [C4mim]HSO4 0.0116 0.0148 0.0187 0.0198 0.0191 0.0159 0.0141 15% [C4mim]HSO4 0.0062 0.0083 0.0103 0.0106 0.0096 0.0076 0.0063

Ρ (g·cm3)

u (m·s−1)

NaCl (mol·kg−1)

1.0050 1.0287 1.0595 1.1004 1.1368 1.1710 1.1898

1535.7 1572.2 1621.7 1671.6 1721.1 1772.3 1796.4

0 0.6807 1.7619 2.6829 3.7641 4.8453 5.3258

1.0267 1.0461 1.0806 1.1141 1.1496 1.1818 1.1975

1594.6 1619.4 1668.8 1714.0 1759.1 1800.7 1821.0

0 0.6407 1.7218 3.0833 3.8442 4.8853 5.2857

1.0267 1.0437 1.0807 1.1152 1.1498 1.1829 1.1982

1545.1 1573.3 1628.7 1680.8 1731.2 1775.5 1795.2

0 1.0411 1.5617 2.5227 3.2835 4.4048 4.8534

1.0090 1.0276 1.0672 1.1051 1.1368 1.1773 1.1919

1535.1 1561.8 1619.8 1679.2 1722.1 1776.0 1797.6

0 0.5606 1.5617 2.6293 3.6840 4.6050 5.1256

1.0334 1.0504 1.0837 1.1163 1.1503 1.1821 1.1977

1574.3 1599.9 1649.4 1696.5 1741.8 1785.4 1810.6

CaSO4 (mol·kg−1) 7.5% EAL 0.0759 0.0850 0.0938 0.0970 0.0949 0.0886 0.0843 5% [C2mim]HSO4 0.0116 0.0418 0.0175 0.0190 0.0180 0.0162 0.0147 15% [C2mim]HSO4 0.0069 0.0083 0.0089 0.0093 0.0085 0.0071 0.0059 10% [C4mim]HSO4 0.0096 0.0121 0.0147 0.0153 0.0142 0.0119 0.0096

Ρ (g·cm−3)

u (m·s−1)

1.0154 1.0373 1.0711 1.1049 1.1419 1.1805 1.1955

1564.6 1593.6 1643.6 1691.4 1745.7 1788.5 1808.7

1.0115 1.0332 1.0679 1.1152 1.1451 1.1806 1.1936

1526.7 1558.8 1616.6 1685.8 1725.6 1774.9 1796.4

1.0406 1.0738 1.0909 1.1240 1.1500 1.1875 1.2037

1564.4 1620.9 1649.5 1702.0 1738.5 1795.5 1820.4

1.0205 1.0402 1.0750 1.1119 1.1444 1.1778 1.1957

1555.2 1581.0 1633.0 1686.8 1733.8 1777.7 1805.2

a Standard uncertainty u: U(T) = 0.01 °C, U(P) = 10 kPa, combined expanded uncertainties Uc(x) = 2 × 10−4, Uc: Uc(ρ) = 5 × 10−4 g·cm−3, Uc(u) = 0.5 m·s−1, Ur(w) = 0.05 for IL content in solution (level of confidence = 0.95, k = 2).Water is used as a solvent and IL with varying mass fraction is used as additive.

sulfates, respectively, for investigations of the dissolution behavior of gypsum in aqueous NaCl solutions up to very high concentrations. Studies have been performed as an aid toward

Therefore, keeping very good physicochemical characteristics of ILs, herein we have used two different types of ILs, that is, alkylammonium lactates and alkyl imidazolium hydrogen B



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Table 3. Parameters Ai and Standard Deviations σ of Equation 1 for the Systems CaSO4·2H2O−NaCl−Water−ILs at 30°C ionic liquids

A0

5% EAL 7.5% EAL 10% EAL 5% [C2mim]HSO4 10% [C2mim]HSO4 15% [C2mim]HSO4 5% [C4mim]HSO4 10% [C4mim]HSO4 15% [C4mim]HSO4

0.0549 0.0762 0.0916 0.0118 0.0088 0.0068 0.0118 0.0097 0.0065


1.0034 1.0137 1.0266 1.0103 1.0243 1.0395 1.0097 1.0209 1.0338

5% EAL 10% EAL 15% EAL 5% [C2mim]HSO4 10% [C2mim]HSO4 15% [C2mim]HSO4 5% [C4mim]HSO4 10% [C4mim]HSO4 15% [C4mim]HSO4

1536.87 1564.05 1595.31 1527.37 1544.99 1566.82 1537.50 1555.32 1576.64


421.75 403.19 382.98 424.14 408.21 392.70 420.77 405.53 390.35

A1 Solubility (mol·kg−1) 0.0178 0.0141 0.0187 0.0047 0.0033 0.0020 0.0056 0.0044 0.0035 Density, ρ (g·cm−3) 0.0340 0.0341 0.0336 0.0346 0.0346 0.0335 0.0340 0.0339 0.00338 Speed of Sound, u (m·s−1) 47.56 46.59 44.69 51.04 50.44 52.19 48.97 48.61 48.16 Isentropic Compressibility, κS 1012 (Pa−1) −39.12 −36.58 −33.63 −41.73 −36.23 −39.58 −44.82 −40.69 −40.49

A2

σ

−0.0028 −0.0023 −0.0031 −7.89 × 10−4 −6.65 × 10−4 −4.55 × 10−4 −9.91 × 10−4 −8.80 × 10−4 −7.46 × 10−4

2.46 × 10−4 3.95 × 10−4 3.73 × 10−4 2.05 × 10−4 1.19 × 10−4 1.13 × 10−4 2.01 × 10−4 1.30 × 10−4 2.35 × 10−4 0.0009 0.0010 0.0004 0.0010 0.0010 0.0006 0.0006 0.0006 0.0002 1.12 2.48 0.90 1.07 1.88 1.48 1.79 0.91 1.30

1.77 1.64 1.54 2.11 2.34 2.53 4.02 3.29 4.16

0.65 1.81 0.43 0.55 1.06 0.63 0.60 0.74 0.83

using FE-SEM and by recording images on OptiMax Synthesis Workstation.

the assessment of the potential, drowning out precipitation using ILs as a separation technique. We have also reported the solution properties such as density (ρ) and speed of sound (u) for the quaternary systems (CaSO4·2H2O + NaCl + ILs + H2O) at 30 °C and estimated the solution isentropic compressibility (κS) for the data. The data will be useful to test or develop the models of the solubility of mineral salts in hydrate inhibitors/water/salt solutions. The precipitation of CaSO4·2H2O from aqueous solutions normally results in the formation of small needle-like crystals with a monoclinic, prismatic structure having water molecules between the calcium and sulfate ions in the unit cell.30 However, it has been possible to change the morphology of CaSO4·2H2O using various kinds of organic or inorganic additives.31−34 Herein we have observed the changes in the surface morphology and studied the real-time crystallization kinetics (crystal growth and particle size distribution) of CaSO4·2H2O precipitated from aqueous solutions containing imidazolium hydrogen sulfate ILs

2. EXPERIMENTAL SECTION Materials. All chemicals used in the study are mentioned in Table 1. Synthesis of ILs. Ammonium lactate IL EAL was synthesized by neutralization of lactic acid with a respective base as per literature procedure.35,36 In brief, ethylamine was taken into a round-bottomed flask placed in ice bath. Lactic acid was added dropwise while under constant stirring. After complete addition, the reaction mixture was allowed to stir at room temperature for 24 h. Resulting ILs were dried in a rotary evaporator and stored in a vacuum desiccator having phosphorus pentoxide as a desiccant. NMR spectra of the synthesized IL (Figure S1, Supporting Information) compared with literature35,36 indicate the formation of ILs with good purity. Preparation of Solutions. All solutions were prepared by weight using analytical balance with a precision of ±0.0001 C



Article

Figure 1. Solubility behavior of CaSO4·2H2O at various concentration of IL aqueous solution with varying NaCl. ILs are (A) ethylammonium lactate, (B) 1-ethyl-3-methyl-imidazolium hydrogen sulfate, and (C) 1-butyl-3-methyl-imidazolium hydrogen sulfate (A: ◆, miliQ; ■, 5%; ●, 7.5%; ▲, 15% and C,D: ◆, miliQ; ■, 5%; ●, 10%; ▲, 15%). Lines are polynomial fit to the experimental data.

Figure 2. Density (ρ) of CaSO4·2H2O + NaCl + H2O + ILs solutions. ILs are (A) ethylammonium lactate, (B) 1-ethyl-3-methylimidazolium hydrogen sulfate, and (C) 1-butyl-3-methylimidazolium hydrogen sulfate. Lines are linear fit to the experimental data.

D



Article

Figure 3. Speed of sound (u) in CaSO4·2H2O + NaCl + H2O + ILs systems. ILs are (A) ethylammonium lactate, (B) 1-ethyl-3-methylimidazolium hydrogen sulfate, and (C) 1-butyl-3-methylimidazolium hydrogen sulfate. Lines are linear fit to the experimental data.

Figure 4. Isentropic compressibility of CaSO4·2H2O + NaCl + H2O + ILs solutions. ILs are (A) ethylammonium lactate, (B) 1-ethyl-3methylimidazolium hydrogen sulfate, and (C) 1-butyl-3-methylimidazolium hydrogen sulfate. Lines are linear fit to the experimental data.

(Shimadzu ATX) in Millipore-grade water. Stock solutions of the different amounts of ILs + saturated NaCl were prepared by adding NaCl to an aqueous solution of ILs. A range of solutions

was then prepared by diluting this stock solution with aqueous IL solution and adding an excessive amount of CaSO4·2H2O. All of the prepared solutions were stirred overnight in a shaker at 35 E



Article

Figure 5. SEM micrographs of (A,B) native CaSO4, (C−F) crystals grown in water, (G−I) precipitated using [C2mim]HSO4, and (J−L) precipitated using [C4mim]HSO4.

rpm and 30 °C. The solutions were centrifuged at 3500 rpm, 10 min (REMI R-8M), and the supernatant was used as a final solution for all analyses. No liquid−liquid demixing was observed in any of the solvent−NaCl−water systems, even at very high NaCl concentration at 30 °C. Liquid samples were withdrawn periodically and analyzed for different ions, as described in our previous work.37 In brief, the Ca2+ and Cl− concentrations were determined volumetrically using standard EDTA and AgNO3 solutions. Repeated analytical experiments showed an error of

Effect of Ionic Liquid Additives on the Solubility Behavior and

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