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

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The sulfate and Al3+ in ettringite (Ca6Al2(SO4)3(OH)12 · 26H2O) 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 Al2(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–Al2(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–Al2(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–Al2(SO4)3 addition was demonstrated. Sedimentation properties such as the terminal velocity of settling down and particle size of the obtained precipitates were estimated.