Aqueous speciation and pH effect on the sorption behavior of uranium by montmorillonite

Abstract

Clay minerals occur widely in nature and play a very important role in agriculture, mineral recovery and chemical manufacturing. Among the many properties which affect clay behaviour, water binding and ion exchanging appear to be the most important. The study of the cation exchange capacity of soils is of great theoretical and practical importance since the CEC determines in many ways the behavior of nutrients, chemical amendments, and many toxic compounds entering the soils. Sorption interactions with montmorillonite and other clay minerals in soils are potantially important mechanisms for attenuating the mobility of heavy metal cations through the subsurface environment. In this work the cation exchange capacity (CEC) of montmorillonite from west Anatolia, and sorptions interactions with montmorillonite for attenuating the mobility of uranium were studied. The CEC value was found to be 77 meq/100 g montmorillonite. The relative importance of test parameters e.g., contact time, particle size, pH and U(+6) aqueous speciation was determined. The results show that sorption on montmorillonite is a funtion of pH depending strongly on the aqueous U(+6) species. It reaches a maximum at near neutral pH(pH~7). At low and at high pH solutions the sorption values of uranium are poor. These sorption values were attributed to the formation of aqueous U(+6) carbonate complexes in alkaline conditions and the ionexchange process between UO2 +2 species and interlayer cations on montmorillonite in acidic solutions.

Reversible surface-sorption-induced electron-transfer oxidation of Fe(II) at reactive sites on a synthetic clay mineral

“…Fletcher and Sposito, 1989;Charlet et al, 1993;Zachara et al, 1993;Zachara and Smith, 1994;Baeyens and Bradbury, 1997;Bradbury and Baeyens, 1997, 2002Akcay, 1998;Turner et al, 1998). However, the influence of other background ionic media such as Fe(II)-containing water is poorly documented.…”

Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

“…5−7 U(VI), usually in the form of a soluble uranyl cation (UO 2 2+ ), is predominant under oxidizing conditions. 8,9 Under reducing conditions, U(VI) can be reduced to U(IV) (uranous ion, U 4+ ), which occurs as sparingly soluble uraninite (UO 2 ), amorphous U(IV)-phosphate phases, monomeric U(IV) adsorbed to metal oxide surfaces, or persistent colloids. 6,10−19 The decrease in solubility of U(VI) upon reduction to U(IV) has driven significant research efforts to focus on the microbial reduction and immobilization of U(VI) as a promising technology to remediate uranium contamination in soil and groundwater systems.…”

Microbial reduction of chlorite and uranium followed by air oxidation

“…In aerobic groundwater, U(VI) carbonate complexes are often the predominant uranium species. These anionic or neutral U species tend to sorb weakly to solid phases and, therefore, can be relatively mobile in the environment (Akcay, 1998;Arnold et al, 1998). Under anoxic conditions U(VI) can be reduced to sparingly soluble U(IV) minerals and precipitated from groundwater (Lovley and Phillips, 1992).…”