The bioreduction of U(VI) and Pu(IV): Experimental and thermodynamic studies.

The experimental and thermodynamic bioreduction of U(VI)aq and Pu(IV)am was studied in order to more accurately predict their transport velocities in groundwater and assess the contamination risks to the associated environments. The results obtained in this study emphasize the impact of carbonate-calcium and humic acids at 7.1 and anoxic solutions on the rate and extent of U(VI)aq and Pu(IV)am bioreduction by Shewanella putrefaciens. We found that the bioreduction rate of U(VI)aq became slow in the presence of NaHCO3/CaCl2. The more negative standard redox potentials of the ternary complexes of U(VI)-Ca2+-CO32- accounted for the decreased rate of bioreduction, e.g., [Formula: see text]  = -0.6797 V ≪ [Formula: see text]  = 0.3862 V. The bioreduction of Pu(IV)am seemed feasible, while humic acids accepted the adequate extracellular electrons secreted by S. putrefaciens, and the redox potential of Eh(HAox/HAred) was lower than Eh(PuO2(am)/Pu3+), e.g., Eh(HAox/HAred) ≦ Eh(PuO2(am)/Pu3+) if humic acids accepted ≧ 7.952 × 10-7 mol of electrons. The standard redox potentials, Eho(PuO2(am)/Pu3+) = 0.9295 V ≫ [Formula: see text]  = -0.6797 V, cannot explain the reduction extent of Pu(IV)am (8.9%), which is notably smaller than that of U(VI)aq (74.9%). In fact, the redox potential of Pu(IV)am was distinctly negative under the experimental conditions of trace-level Pu(IV)am (∼2.8 × 10-9 mol/L Pu(IV) if Pu(IV)am was completely dissolved), e.g., Eh(PuO2(am)/Pu3+) = -0.1590 V (α(Pu3+) = 10-10 mol/L, pH = 7.1). Therefore, the chemical factor of Pu3+ activity, leading to a rapid drop in Eh(PuO2(am)/Pu3+) at trace-level Pu(IV)am, was responsible for the relatively small reduction extent of Pu(IV)am.

New insights into the role of calcium in the bioreduction of uranium(VI) under varying pH conditions.

The effect of calcium in the uranium-contaminated groundwater on U(VI)aq bioreduction remains uncertain. Some studies indicated that the presence of calcium may inhibit the bioreduction. However, our calculations show the negative standard molar Gibbs free energy of reduction. The bioreduction of the ternary uranyl-carbonate-calcium complexes seems thermodynamically favorable at specific pH. Sorption and reduction experiments were conducted to gain new insights of calcium into the bioreduction. The results show that the complexes were greatly reduced by Shewanella putrefaciens in the slightly acidic pH ~6.0 and alkaline pH ~7.9 solutions with the relatively high CaCl2 (1.0-6.0 mmol/L) although the reduction was difficult at the nearly neutral pH ~6.9. At pH ~6.9, the removal percentage of U(VI)aq decreased from 97.0% to 24.4% with increasing CaCl2 from 0 to 6.0 mmol/L, in contrast to the increasing percentage from 50.9% to 89.7% at pH ~7.9. The obvious removal of U(VI)aq was ascribed to the bioreduction instead of the biosorption, as evidenced by XPS, HRTEM and UV-vis spectra. The calculations such as [Formula: see text] and [Formula: see text] partially accounted for the reduction mechanisms. Accordingly, the U(VI)aq bioreduction is a promising method to remediate the groundwater even rich in calcium and carbonate.

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.

pH-dependent microbial reduction of uranium(VI) in carbonate-free solutions: UV-vis, XPS, TEM, and thermodynamic studies.

U(VI)aq bioreduction has an important effect on the fate and transport of uranium isotopes in groundwater at nuclear test sites. In this study, we focus on the pH-dependent bioreduction of U(VI)aq in carbonate-free solutions and give mechanistic insight into the removal kinetics of U(VI)aq. An enhancement in the removal of U(VI)aq with increasing pH was observed within 5 h, e.g., from 19.4% at pH 4.52 to 99.7% at pH 8.30. The removal of U(VI)aq at pH 4.52 was due to the biosorption of U(VI)aq onto the living cells of Shewanella putrefaciens, as evidenced by the almost constant UV-vis absorption intensity of U(VI)aq immediately after contact with S. putrefaciens. Instead, the removal observed at pH 5.97 to 8.30 resulted from the bioreduction of U(VI)aq. The end product of U(VI)aq bioreduction was analyzed using XPS and HRTEM and identified as nanosized UO2. An increasing trend in the biosorption of U(VI)aq onto heat-killed cells was also observed, e.g., ~ 80% at pH 8.38. Evidently, the U(VI)aq that sorbed onto the living cells at pH > 4.52 was further reduced to UO2, although biosorption made a large contribution to the initial removal of U(VI)aq. These results may reveal the removal mechanism, in which the U(VI)aq that was sorbed onto cells rather than the U(VI)aq complexed in solution was reduced. The decreases in the redox potentials of the main complex species of U(VI)aq (e.g., [Formula: see text] and [Formula: see text]) with increasing pH support the proposed removal mechanism.

Humic acids facilitated microbial reduction of polymeric Pu(IV) under anaerobic conditions.

Flavins and humic substances have been extensively studied with emphasis on their ability to transfer extracellular electrons to insoluble metal oxides. Nevertheless, whether the low-solubility Pu(IV) polymers are microbially reduced to aqueous Pu(III) remains uncertain. Experiments were conducted under anaerobic and slightly alkaline conditions to study the difference between humic acids and flavins to transport extracellular electrons to Pu(IV) polymers. Our study demonstrates that Shewanella putrefaciens was unable to directly reduce polymeric Pu(IV) with a notably low reduction rate (3.4×10-12mol/L Pu(III)aq within 144h). The relatively high redox potential of flavins reveals the thermodynamically unfavorable reduction: Eh(PuO2(am)/Pu3+)

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.

Plutonium partitioning in water-granite and water-α-FeOOH systems: from a viewpoint of a three-phase system.

Traditional sorption experiments commonly treat the colloidal species of low-solubility contaminants as immobile species when separated by centrifugation or ultrafiltration. This study shows that, from a viewpoint of a three-phase system, the mobile Pu species, especially the colloidal species, play an important role in Pu partitioning in water-granite and water-α-FeOOH systems. A new distribution coefficient term Ks/(d+c) was defined to take the mobile colloidal species into consideration, and it differs to the traditional distribution coefficient Ks/d by orders of magnitude in the water-granite and water-α-FeOOH systems. This term, Ks/(d+c), can quantitatively describe Pu partitioning in the suspension, in particular the fraction of mobile species that dominate Pu migration in the environment. The effects of ionic strength (I) and pH on the Pu partitioning in water-granite and water-α-FeOOH systems are well interpreted with respect to the zeta potential change of granite grains, α-FeOOH colloid particles and polymeric Pu. It is concluded that the presence of the α-FeOOH colloid with a low concentration (<10 mg L(-1)) is favorable for the stability of colloidal Pu and leads to large proportion of mobile Pu, especially colloid-associated Pu, which will migrate much faster than dissolved Pu in groundwater.

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.

Goethite colloid enhanced Pu transport through a single saturated fracture in granite.

α-FeOOH, a stable iron oxide in nature, can strongly absorb the low-solubility plutonium (Pu) in aquifers. However, whether Pu transports though a single saturated fracture can be enhanced in the presence of α-FeOOH colloids remains unknown. Experimental studies were carried out to evaluate Pu mobilization at different water flow velocity, as affected by goethite colloids with various concentrations. Goethite nanorods were used to prepare (α-FeOOH)-associated Pu suspensions with α-FeOOH concentration of (0-150) mgL(-1). The work experimentally evidenced that α-FeOOH colloid does enhance transport of Pu through fractured granites. The fraction of mobile (239)Pu (RPu, m=41.5%) associated with the α-FeOOH of an extremely low colloid concentration (0.2mgL(-1)) is much larger than that in absence of α-FeOOH (RPu, m=6.98%). However, plutonium mobility began to decrease when α-FeOOH concentration was increased to 1.0mgL(-1). On the other hand, the fraction of mobile Pu increased gradually with the water flow velocity. Based on the experimental data, the mechanisms underlying the (α-FeOOH)-associated plutonium transport are comprehensively discussed in view of its dynamic deposition onto the granite surfaces, which is decided mainly by the relative interaction between the colloid particle and the immobile surface. This interaction is a balance of electrostatic force (may be repulsive or attractive), the van der Walls force, and the shear stress of flow.

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, granite grains, and different colloids.

Low-solubility contaminants with high affinity for colloid surfaces may form colloid-associated species. The mobile characteristics of this species are, however, ignored by the traditional sorption/distribution experiments in which colloidal species contributed to the immobile fraction of the contaminants retained on the solids as a result of centrifugation or ultrafiltration procedures. The mobility of the contaminants in subsurface environments might be underestimated accordingly. Our results show that colloidal species of (239)Pu in three-phase systems remained the highest percentages in comparison to both the dissolved species and the immobile species retained on the granite grains (solid phase), although the relative fraction of these three species depended on the colloid types. The real solid/liquid distribution coefficients (K s/d) experimentally determined were generally smaller than the traditional K s/d (i.e., the K s+c/d in this study) by ~1,000 mL/g for the three-phase systems with the mineral colloids (granite particle, soil colloid, or kaolinite colloid). For the humic acid system, the traditional K s/d was 140 mL/g, whereas the real K s/d was approximately zero. The deviations from the real solid/liquid K s/d were caused by the artificially increased immobile fraction of Pu. One has to be cautious in using K s/d-based transport models to predict the fate and transport of Pu in the environment.

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 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.

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.