The sorption of selenite
and selenate to ettringite (3
CaO·Al
2O
3·3CaSO
4·32H
2O), "monosulfate" (3CaO·Al
2O
3·CaSO
4·12H
2O),
and cal
cium sili
cate hydrate (C-S-H) wasinvestigated in order to underst
and Se immobilization by
cement in hazardous wastes. Sorption kineti
cs were fastwith equilibrium between the minerals
and Se spe
cies rea
chedwithin 1 d. Selenite is suggested to sorb by surfa
cerea
ctions,
and for ettringite, a sorption maximum of 0.03mol kg
-1 was determined. Distribution ratios (
Rd) for selenitewere 0.18, 0.38,
and 0.21 m
3 kg
-1 for ettringite, monosulfate,
and C-S-H, respe
ctively. At high selenite additions,CaSeO
3 was pre
cipitated with a solubility produ
ct of
Kso= 10
-7.27 (
I = 0, 25
![](/images/entities/deg.gif)
C). Selenate sorbed only weakly toettringite (
Rd = 0.03),
and no signifi
cant sorption toC-S-H was found. In
contrast, sorption to monosulfatewas strong (
Rd = 2.06). With in
creasing selenate addition,XRD analyses revealed
changes in the interlayer distan
ceof monosulfate, in parallel with an in
crease of the ettringitefra
ction. Substitution of sulfate is suggested to be therelevant pro
cess. This indi
cates that selenate is sorbedmore effi
ciently by monosulfate-ri
ch
cement, while the
cement
composition is of minor importan
ce for selenite sorption.