Solubility of Ca-6[Al(OH)(6)](2)(CrO4)(3) center dot 26H(2)O, the chromate analog of ettringite; 5-75 degrees C

Ca-6[Al(OH)(6)](2)(CrO4)(3) . 26H(2)O, the chromate analog of the sulfate mineral ettringite, was synthesized and characterized by X-ray diffraction, Fourier transform infra-red spectroscopy, thermogravimetric analyses, energy dispersive X-ray spectrometry, and bulk chemical analyses. The solubility of the synthesized solid was measured in a series of dissolution and precipitation experiments conducted at 5-75 degrees C and at initial pH values between 10.5 and 12.5. The ion activity product (IAP) for the reaction Ca-6[Al(OH)(6)](2)(CrO4)(3) . 26H(2)O reversible arrow 6Ca(2+) + 2Al(OH)(4)(-) + 3CrO(4)(2-) + 4OH(-) + 26H(2)O varies with pH unless a CaCrO4(aq), complex is included in the speciation model. The log K for the formation of this complex by the reaction Ca2+ + CrO42- = CaCrO4(aq) was obtained by minimizing the variance in the IAP for Ca-6[Al(OH)(6)](2)(CrO4)(3) . 26H(2)O. There is no significant trend in the formation constant with temperature and the average log K is 2.77 +/- 0.16 over the temperature range 5-75 degrees C. The log solubility product (log K-SP) of Ca-6[Al(OH)(6)](2) (CrO4)(3) . 26H(2)O at 25 degrees C is -41.46 +/- 0.30. The temperature dependence of the log K-SP is log K-SP =A-B/T + D log(T) where A = 498.94 +/- 48.99, B = 27,499 +/- 2257, and D = -181.11 +/- 16.74. The values of Delta G(r,298)(0) and Delta H-r,298(0) for the dissolution reaction are 236.6 +/- 3.9 and 77.5 +/- 2.4 kJ mol(-1). the values of Delta C-P,r,298(0), and Delta S-r,298(0) are -1506 +/- 140 and -534 +/- 83 J mol(-1) K-1. Using these values published standard state partial molal quantities for constituent ions, Delta G(f,298)(0) = -15,131 +/- 19 kJ mol(-1), Delta H-f,298(0) = -17,330 +/- 8.6 kJ mol(-1), Delta S-298(0) = 2.19 +/- 0.10 kJ mol(-1) K-1, and Delta C-Pf,298(0) = 2.12 +/- 0.53d kJ mol(-1) K-1, were calculated. (C) 2000 Elsevier Science Ltd. All rights reserved.