Behaviors of CrO42- and Cr3+ during Removal Based on Ettringite Synthesis and Ca(OH)2-Al2(SO4)3 Addition

The sulfate and Al3+ in ettringite (Ca6Al2(SO4)(3)(OH)(12)center dot 26H(2)O) can substitute for other oxyanions such as chromate ions (CrO42-) and cations such as Cr3+, respectively. Ettringite synthesis in the presence of CrO42- or Cr3+ was attempted by adding solid Ca(OH)(2) to an Al-2(SO4)(3) solution to examine the behavior of these chromium species during uptake by ettringite. By supplying chromate ions and a sufficient amount of OH to a Ca(OH)(2)-Al-2(SO4)(3) solution, CrO42--substituted ettringite was obtained; however, the formation of SO42- ettringite was dominant. By suspending ettringite in K2CrO4, CrO42- ettringite was also obtained via the ion-exchange mechanism. In the case of the OH-deficient solution, in which weakly acidic CrO3 was employed instead of K2CrO4, a mixture of amorphous Al(OH)(3) and gypsum was precipitated, and no ettringite was precipitated. On the other hand, Ca(OH)(2)-Al-2(SO4)(3)-CrCl3 solutions, which contained Cr3+, produced precipitable mixtures of gypsum, Ca(OH)(2), Al(OH)(3), and Cr(OH)(3). The co-precipitation of Cr(OH)(3) with other hydroxides such as Al(OH)(3) was more favored than the formation of Cr3+ ettringite. Simultaneous removal of CrO42- and Cr3+ by Ca(OH)(2)-Al-2(SO4)(3) addition was demonstrated. Sedimentation properties such as the terminal velocity of settling down and particle size of the obtained precipitates were estimated.