Abstract
Kinetic experiments using cement pastes for Cr(VI) removal revealed that cement hydration products can uptake Cr(VI) considerably in the high pH region. The removal mechanism was investigated by a series of batch equilibrium tests with a range of Cr(VI) doses. The overall behavior of Cr(VI) removal was adequately described by a Langmuir isotherm, indicating a finite capacity of cement pastes in removing chromate oxyanions. The Cr(VI) removal behavior in the batch equilibrium tests was conceptually modeled as precipitation of Cr(VI)-bearing ettringite in lower Cr(VI) dose conditions, followed by anionic exchange with interlayer sulfate ions of ettringite at doses above a certain level. This model was well supported by the experimental results and the SEM images that exhibited development of the more acicular crystals of ettringite with the higher dose of chromate ions. The Cr(VI) removed by cement pastes was completely released upon dissolution of ettringite phases at pH 10.5 in the Cr(VI) release experiments.
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