Influence of (calcium-)uranyl-carbonate complexation on U(VI) sorption on Ca- and Na-bentonites.
The influence of uranyl-carbonate and calcium-uranyl-carbonate complexations on the kinetics of U(VI) (approximately 3.4 x 10(-3) mol L(-1)) sorption from NaNO3 and Ca(NO3)2 solutions on Na- and Ca-bentonites at circumneutral ambient conditions was investigated. Complexation of U(VI) in Ca2UO2(CO3)3(aq) aqueous species, dominating the U(VI) speciation in Ca(NO3)2 solution, reduces its adsorption on bentonite by a factor of 2-3 in comparison with that in (UO2)2CO3(OH)3- species, dominating in NaNO3 solution, within the studied period of time (21 days). As a result of the dissolution of accessory calcite, Ca2UO2(CO3)3(aq) can be formed in the initially Ca-free solution in contact with either Na- or Ca-bentonite. U(VI) adsorption on Na-bentonite is a factor of approximately 2 higher than that on Ca-bentonite for solutions with the Ca2UO2(CO3)3(aq) complex dominating aqueous U(VI) speciation. This favors use of Na-bentonite over that of Ca-bentonite in final disposal of radioactive waste. Furthermore, the observed strong correlation between U(VI) adsorption and Mg release as a result of montmorillonite dissolution indicates in agreement with previous findings that under the applied conditions U(VI) is adsorbed on the edge surface of montmorillonite, which is a major mineral phase of the studied clays.
Meleshyn
A
A
Center for Radiation Protection and Radioecology, Leibniz Universität Hannover, Herrenhäuser Strasse 2, 30419 Hannover, Germany. meleshyn@zsr.uni-hannover.de
Azeroual
M
M
Reeck
T
T
Houben
G
G
Riebe
B
B
Bunnenberg
C
C
D000327:Adsorption; D000538:Aluminum Silicates; D001546:Bentonite / Q000032:analysis / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D017610:Calcium Compounds / Q000032:analysis; D002254:Carbonates / Q000737:chemistry*; D000077215:Clay; D019015:Geologic Sediments / Q000032:analysis*; D007700:Kinetics; D008274:Magnesium / Q000737:chemistry; D008670:Metals / Q000032:analysis; D009566:Nitrates / Q000032:analysis; D012964:Sodium / Q000737:chemistry*; D014501:Uranium / Q000737:chemistry*
D000538:Aluminum Silicates; D017610:Calcium Compounds; D002254:Carbonates; D008670:Metals; D009566:Nitrates; D001546:Bentonite; D014501:Uranium; C031618:sodium nitrate; D012964:Sodium; D008274:Magnesium; C059948:calcium nitrate; D002118:Calcium; D000077215:Clay