Article pubs.acs.org/jced
Effects of Na, Ca, Mg, and Al Chloride Salts on Dissolution and Phase Stability of Calcium Sulfate Dihydrate in Aqueous Solutions at 278.15 K to 308.15 K Jian Sun,† Lisheng Wang,*,† and Guomin Yu†,‡ †
School of Chemical Engineering and Environment, Beijing Institute of Technology, Beijing 100081, China School of Biochemical and Chemical Engineering, Nanyang Institute of Technology, Nanyang 473000, China
‡
ABSTRACT: Dissolution trend and solubility data of calcium sulfate dihydrate (CaSO4·2H2O gypsum) in aqueous Na, Ca, Mg, and Al chloride salt solutions have been studied at 278.15 K to 308.15 K. The concentration ranges were up to 4.5 mol·L−1 for NaCl, 3.0 mol·L−1 for MgCl2, 2.0 mol·L−1 for CaCl2, and 1.6 mol·L−1 for AlCl3, respectively. Measurement results show that except for CaCl2, the other three salts allow the solubility of gypsum to rise a lot first and then descend smoothly after attaining the maximum. Because of common ions effect, the solubility of CaSO4·2H2O drops sharply in CaCl2 solution at low concentration and continues to fall almost linearly as concentration exceeds 0.2 mol•kg−1. Impacts of Na, Mg, Al chloride salts on solubility of CaSO4·2H2O are explained on the basis of dissolution and thermodynamic equilibrium. The solubility of CaSO4·2H2O in AlCl3 solution is highest compared with that in NaCl and MgCl2 solution because two sulfur-bearing chemical complex Al(SO4)2−(aq) and AlSO4+(aq) are expected to be formed in solution. Besides, a number of H+ ions produced in AlCl3 solution further promote the dissolution of CaSO4·2H2O. In addition, it is demonstrated that CaSO4·2H2O is fairly stable in the above chloride salts solution of high concentration (4.5 mol·L−1 for NaCl, 3.0 mol·L−1 for MgCl2, 2.0 mol·L−1 for CaCl2 and 1.6 mol·L−1 for AlCl3) for 2 weeks at 308.15 K by the evidence that no new peaks emerged in X-ray power diffraction results.
1. INTRODUCTION Calcium sulfate occurs in nature in form of three different minerals distinguished by the degree of hydration: gypsum(CaSO4·2H2O DH), hemihydrate(CaSO4·0.5H2O HH), and anhydrite (CaSO4 AH). Generally speaking, the use of calcium sulfate results in many problems, but calcium sulfate also has applications in various fields. In China, wastewater containing high levels of inorganic salt from petroleum extraction and refinery systems in discharging or treatment processes frequently lead to soil salinization due to the incomplete environmental mechanism; sparingly soluble gypsum that is prone to precipitate is one of a number of significant pollutants.1 Besides, gypsum scaling often contaminates industrial systems of reverse osmosis.2,3 Beneficially, the abilities and potency of calcium sulfate dihydrate are of importance in the production of functional building material, contaminated soil recovery, and mineral processing.4−6 It is obvious that none of these fields is independent of dissolution behaviors of gypsum. Therefore, determining the maximum extent to which calcium sulfate dihydrate is soluble in pure water or other solutions is meaningful environmentally and industrially. A number of articles since 19017 have reported the solubility data of gypsum in pure water or other electrolyte solutions, but these data, found scattered as literature data of solubility of gypsum in Na, Cl, and Mg chloride salt solutions at ordinary temperatures, were mainly measured before 1990 by EDTA potentiometric titration for calcium analysis.8−16 Besides, few investigators made efforts to © XXXX American Chemical Society
measure or predict with the help of a chemical model the solubility of gypsum in AlCl3 solutions.17,18 In this case, we intend to reexamine the solubility data of gypsum in NaCl, CaCl2, and MgCl2 and report accurate solubility data in AlCl3 at 278.15 K to 308.15 K with a microwave plasma−atomic emission spectrometer. We also attempted to give theoretical interpretations on the basis of dissolution and thermodynamic equilibrium for the distinct influence of the above-mentioned chloride salts on the solubility of gypsum. In addition, it is well-known that DH can transform to AH at elevated temperatures in solution. At low temperature the stable one is DH; at high temperature it is AH; HH sometimes is an intermediate type in transformation process. It is reported in the review of Freyer that solubility curves of calcium sulfate dihydrate and anhydrite in water intersected at a NaCl content of approximately 4 mol·kg−1 at 298 K,19 which suggests at such temperature above this concentration anhydrite presents the stable solid. The other aim of this paper was to examine if gypsum changes to a more indissolvable anhydrite in concentrated chloride solutions at ordinary temperatures, which is of importance to understand the salinization process of soil caused by gypsum (anhydrite) in brine solution undergoing long time solar heating. Received: January 2, 2015 Accepted: July 15, 2015
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DOI: 10.1021/acs.jced.5b00005 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
2. EXPERIMENTAL SECTION 2.1. Chemicals and Equipment. In this research, CaSO4· 2H2O (99.0 wt %, Beijing Chemical Works, China) and various chloride salts including NaCl, CaCl2·2H2O, MgCl2·6H2O, and AlCl3·6H2O (99.5 wt %, Beijing Chemical Works, China) were analytical grade and used as starting materials without further purification. Deionized water (conductivity
