Xiaohua Zhou

The kinetic stability of colloid-associated plutonium: Settling characteristics and species transformation

The colloid-associated plutonium-239, as the dominant species of Pu in natural environment, was formed through sorption of Pu onto in situ colloids. In case of chemical perturbations present in pore water, the fate and transport of Pu would be therefore impacted by changes in sorption affinity of Pu for the colloid surfaces. The present study reveals that colloidal (239)Pu exhibited the kinetic stability in two respects. First, in situ colloids isolated from the vadose zone sediments at Lop Nor, when in contact with solutions of high ion concentrations or low pH, were significantly aggregated and then exhibited fast settling. Kinetics settling characteristics were described by the parameters, including settling index and characteristic time. Second, Pu dissociation from colloid surfaces occurred immediately after the introduction of Na(+). However, the dissolved species was still unstable and had the potential for re-association with the fraction of colloids that had not settled out from the suspensions due to small size and then remained in suspension. This implies that Pu sorption sites on initial colloids were changed to the sites of suspended colloids.

Colloid-associated plutonium transport in the vadose zone sediments at Lop Nor

A framework to describe the characteristics of pore water in unsaturated media was established in order to study transport of colloid-associated (239)Pu (i.e., colloidal Pu) through the vadose sediments. Effluent concentrations and recoveries of Pu were found to decrease with increasing ionic strength. However, they would remain approximately constant at a critical value of 0.0289 M (Na(+)) though ionic strengths were further increased. Fast deposition rate coefficient (k(fast)) was thus experimentally determined. To our knowledge, this relationship between the mobility of colloidal Pu and the critical ionic strength was the first time observed. On the other hand, significant detachment of colloidal Pu once retained in the sediments was not observed during the subsequent chemical and physical perturbations. But slow release and transport could persist as long as flow continued. The threshold infiltration intensity (0.166 cm/min) revealed a nonmonotonic dependence of the cumulative amount of detached colloidal Pu on the intensity.

Insights into transport velocity of colloid-associated plutonium relative to tritium in porous media

Although faster transport velocities of colloid-associated actinides, bacteria, and virus than nonreactive solutes have been observed in laboratory and field experiments, some questions still need to be answered. To accurately determine the relative velocity (UPu/UT) of 239Pu and tritium representative of the bulk water, a conceptual model of electrostatic interactions coupled with the parabolic water velocity profile in pore channels is developed. Based on the expression for UPu/UT derived from this model, we study the effects of water flow rates and ionic strengths on the UPu/UT. Also, the velocity relationship between Pu, tritium and Sr2+ is explored. The results show that UPu/UT increased fairly linearly with decreasing water flow rates; UPu/UT declined approximately exponentially with increasing Na+ concentrations; the charge properties of colloid-associated Pu (negative), tritium (neutral) and Sr2+ (positive) had a close association with their transport velocities as UPu : UT : USr2+ = 1.41 : 1 : 0.579.

Plutonium partitioning in three-phase systems with water, colloidal particles, and granites: New insights into distribution coefficients

The traditional sorption experiments commonly treated the colloid-associated species of low-solubility contaminants as immobile species resulted from the centrifugation or ultrafiltration, and then solid/liquid distribution coefficients (Ks/d) were determined. This may lead to significantly underestimated mobility of the actinides in subsurface environments. Accordingly, we defined a new distribution coefficient (Ks/d+c) to more adequately describe the mobile characteristics of colloidal species. The results show that under alkaline aqueous conditions the traditional Ks/d was 2-3 orders of magnitude larger than the Ks/d+c involving the colloidal species of (239)Pu. The colloid/liquid distribution coefficients Kc/d≫0 (∼10(6)mL/g) revealed strong competition of the colloidal granite particles with the granite grains for Pu. The distribution percentages of Pu in the three-phase systems, depending on various conditions such as particle concentrations, Na(+) concentrations, pH and time, were determined. Moreover, we developed the thermodynamic and kinetic complexation models to explore the interaction of Pu with the particle surfaces.

Colloid-associated plutonium aged at room temperature: evaluating its transport velocity in saturated coarse-grained granites

The fate and transport of colloidal contaminants in natural media are complicated by physicochemical properties of the contaminants and heterogeneous characteristics of the media. Size and charge exclusion are two key microscopic mechanisms dominating macroscopic transport velocities. Faster velocities of colloid-associated actinides than that of (3)H2O were consistently indicated in many studies. However, dissociation/dissolution of these sorbed actinides (e.g., Pu and Np), caused by their redox reactions on mineral surfaces, possibly occurred under certain chemical conditions. How this dissolution is related to transport velocities remains unanswered. In this study, aging of the colloid-associated Pu (pseudo-colloid) at room temperature and transport through the saturated coarse-grained granites were performed to study whether Pu could exhibit slower velocity than that of (3)H2O (UPu/UT <1). The results show that oxidative dissolution of Pu(IV) associated with the surfaces of colloidal granite particles took place during the aging period. The relative velocity of UPu/UT declined from 1.06 (unaged) to 0.745 (135 d) over time. Size exclusion limited to the uncharged nano-sized particles could not explain such observed UPu/UT <1. Therefore, the decline in UPu/UT was ascribed to the presence of electrostatic attraction between the negatively charged wall of granite pore channels and the Pu(V)O2(+), as evidenced by increasing Pu(V)O2(+) concentrations in the suspensions aged in sealed vessels. As a result of this attraction, Pu(V)O2(+) was excluded from the domain closer to the centerline of pore channels. This reveals that charge exclusion played a more important role in dominating UPu than the size exclusion under the specific conditions, where oxidative dissolution of colloid-associated Pu(IV) was observed in the aged suspensions.

Effects of humic acid concentration on the microbially-mediated reductive solubilization of Pu(IV) polymers

The role of humic acid concentration in the microbially-mediated reductive solubilization of Pu(IV) polymers remains unclear until now. The effects of humic concentration (0-150.5mg/L) on the rate and extent of reduction of polymeric Pu(IV) were studied under anaerobic and pH 7.2 conditions. The results show that Shewanella putrefaciens, secreting flavins as endogenous electron shuttles, cannot notably stimulate the reduction of polymeric Pu(IV). In the presence of humic acids, the reduction rate of polymeric Pu(IV) increased with increasing humic concentrations (0-15.0mg/L): e.g., a 102-fold increase from 4.1×10-15 (HA=0) to 4.2×10-13mol Pu(III)aq/h (HA=15.0mg/L). The bioreduced humic acids by S. putrefaciens facilitated the extracellular electron transfer to Pu(IV) polymers and thus the reduction of polymeric Pu(IV) to Pu(III)aq became thermodynamically favorable. However, the reduction rate did not increase but decrease with increasing humic concentrations from 15.0 to 150.5mg/L. Humic coatings formed on the polymer surfaces at relatively high humic concentrations limited the electron transfer to the polymers and thus decreased the reduction rate. The finding of the dynamic role of humic acids in the bioreductive solubilization may be helpful in evaluating Pu mobility in the geosphere.

The dynamic role of pH in microbial reduction of uranium(VI) in the presence of bicarbonate

The negative effect of carbonate on the rate and extent of bioreduction of aqueous U(VI) has been commonly reported. The solution pH is a key chemical factor controlling U(VI)aq species and the Gibbs free energy of reaction. Therefore, it is interesting to study whether the negative effect can be diminished under specific pH conditions. Experiments were conducted using Shewanella putrefaciens under anaerobic conditions with varying pH values (4-9) and bicarbonate concentrations ( [Formula: see text] , 0-50 mmol/L). The results showed a clear correlation between the pH-bioreduction edges of U(VI)aq and the [Formula: see text] . The specific pH at which the maximum bioreduction occurred (pHmbr) shifted from slightly basic to acidic pH (∼7.5-∼6.0) as the [Formula: see text] increased (2-50 mmol/L). At [Formula: see text]  = 0, however, no pHmbr was observed in terms of increasing bioreduction with pH (∼100%, pH > 7). In the presence of [Formula: see text] , significant bioreduction was observed at pHmbr (∼100% at 2-30 mmol/L [Formula: see text] , 93.7% at 50 mmol/L [Formula: see text] ), which is in contrast to the previously reported infeasibility of bioreduction at high [Formula: see text] . The pH-bioreduction edges were almost comparable to the pH-biosorption edges of U(VI)aq on heat-killed cells, revealing that biosorption is favorable for bioreduction. The end product of U(VI)aq bioreduction was characterized as insoluble nanobiogenic uraninite by HRTEM. The redox potentials of the master complex species of U(VI)aq, such as [Formula: see text] , [Formula: see text] , and [Formula: see text] , were calculated to obtain insights into the thermodynamic reduction mechanism. The observed dynamic role of pH in bioreduction suggests the potential for bioremediation of uranium-contaminated groundwater containing high carbonate concentrations.