Uptake of oxo-anions by cements through solid-solution formation: experimental evidence and modelling

Abstract

Summary Uptake experiments were carried out with selenate and chromate on fresh and leached Portland and high-alumina cements; and in addition with selenate and selenite on synthetic ettringite. In all experiments with cements, exceptionally high uptake could be observed under conditions where significant amounts of secondary ettringite were formed. Experimental data obtained for pure ettringite corroborated the important role of this mineral phase. However, uptake kinetics show opposite trends in these two systems, which can be viewed as end-members of the same process: Where a fast precipitation of secondary ettringite occurred, initial uptake was high, with Kd values in the range of ≍1-5 m3/kg for Se(VI), but decreased with time. Uptake by pre-formed (primary) ettringite initially gave lower Kd values (≍0.1 m3/kg), which increased with time. After prolonged equilibration times, the two systems started to approach each other. Concurrent measurements of sulphate concentrations allowed to extract a mean partition coefficient for the solid-solution formation of selenite and selenate with ettringite. Based on this simple solid-solution model, a pragmatic quantitative relation was developed that permits to estimate Kd for Se(VI) on whole cement as a function of the concentrations of sulphate in the cement and in the solution. A test against experimental data shows reasonably good agreement between measurements and calculations. This relation can also be directly applied to estimate Kd values of chromate (and perrhenate) by different cements, indicating the same uptake mechanism. The approach may be less well suited for Se(IV), whose uptake on cement appears to be related mainly to minerals other than ettringite.