Effects of hydrodynamic chromatography on colloid-facilitated migration of radionuclides in the fractured rock

Research on safety early warning of uranium tailings dam based on abnormal radioactive indexes of water leachate

The radioactive index value of the leachate of the uranium tailings dam is affected by the internal damage of the dam. Therefore, a way of using the deviation of the radioactive index concentration in the leachate to warn the instability of the dam is innovatively proposed in this paper. Firstly, the SSA-BP algorithm is used to predict and analyze the five groups of parameters U, Ra, ∑ α, ∑ β and Rn. Then, the deviation between the actual value and the predicted value is computed. Finally, an early warning is given based on the entropy weight extension decision-making model. The model is verified by the leachate environment monitoring data of a uranium tailings dam in southern China from 2016 to 2020, which shows that the model can effectively caution of the instability of the uranium tailings dam and provides a reference for the subsequent decommissioning management.

Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs+, Sr2+, and Eu3

Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a ⁶⁰Co source. The non-irradiated and irradiated HA molecules were fractionated by ultrafiltration into four categories: > 100, 50-100, 10-50, and < 10 kDa. Total organic carbon measurements and potentiometric titration analysis suggested that (1) some gamma-irradiated HA molecules were degraded into smaller molecules and (2) radiolytic degradation caused phenolic -OH became the predominant functional group in the small molecular-weight fractions of HA. The effect of absorbed dose of gamma rays on the distributions of Cs⁺, Sr²⁺, and Eu³⁺ ions in the molecular-weight fractions of the metal-HA systems was examined to discuss the complexation affinity. The metal ions were distributed in the smaller molecular-weight fractions at different doses, which corresponded to the degradation of HA molecules. For a predetermined absorbed dose, Cs⁺ ions did not change the molecular-weight distribution of the total organic carbon content of the degraded HA molecules. Conversely, the Sr²⁺ and Eu³⁺ ions redistributed organic carbon toward the larger molecular-weight fractions.

Colloid Transport in a Single Fracture–Matrix System: Gravity Effects, Influence of Colloid Size and Density

A numerical model was developed to investigate the influence of gravitational force on the transport of colloids in a single horizontal fracture–matrix system. Along with major transport phenomena, prominence was given to study the mass flux at the fracture–matrix interface, and colloid penetration within the rock matrix. Results suggest that the gravitational force significantly alters and controls the velocity of colloids in the fracture. Further, it was shown that the colloid density and size play a vital part in determining the extent that gravity may influence the transport of colloids in both fracture and rock matrix. The mass flux transfer across the fracture–matrix interface is predominantly dependent on the colloidal size. As large as 80% reduction in penetration of colloids in the rock matrix was observed when the size of the colloid was increased from 50–600 nm. Similarly, the farther the density of colloid from that of the fluid in the fracture (water), then the higher the mitigation of colloids in the fracture and the rock matrix. Finally, a non-dimensional parameter “Rock Saturation Factor” has been presented in the present study, which can offer a straightforward approach for evaluating the extent of penetration of colloids within the rock matrix.

Continuum-based models and concepts for the transport of nanoparticles in saturated porous media: A state-of-the-science review

Environmental applications of nanoparticles (NP) increasingly result in widespread NP distribution within porous media where they are subject to various concurrent transport mechanisms including irreversible deposition, attachment/detachment (equilibrium or kinetic), agglomeration, physical straining, site-blocking, ripening, and size exclusion. Fundamental research in NP transport is typically conducted at small scale, and theoretical mechanistic modeling of particle transport in porous media faces challenges when considering the simultaneous effects of transport mechanisms. Continuum modeling approaches, in contrast, are scalable across various scales ranging from column experiments to aquifer. They have also been able to successfully describe the simultaneous occurrence of various transport mechanisms of NP in porous media such as blocking/straining or agglomeration/deposition/detachment. However, the diversity of model equations developed by different authors and the lack of effective approaches for their validation present obstacles to the successful robust application of these models for describing or predicting NP transport phenomena. This review aims to describe consistently all the important NP transport mechanisms along with their representative mathematical continuum models as found in the current scientific literature. Detailed characterizations of each transport phenomenon in regards to their manifestation in the column experiment outcomes, i.e., breakthrough curve (BTC) and residual concentration profile (RCP), are presented to facilitate future interpretations of BTCs and RCPs. The review highlights two NP transport mechanisms, agglomeration and size exclusion, which are potentially of great importance in controlling the fate and transport of NP in the subsurface media yet have been widely neglected in many existing modeling studies. A critical limitation of the continuum modeling approach is the number of parameters used upon application to larger scales and when a series of transport mechanisms are involved. We investigate the use of simplifying assumptions, such as the equilibrium assumption, in modeling the attachment/detachment mechanisms within a continuum modelling framework. While acknowledging criticisms about the use of this assumption for NP deposition on a mechanistic (process) basis, we found that its use as a description of dynamic deposition behavior in a continuum model yields broadly similar results to those arising from a kinetic model. Furthermore, we show that in two dimensional (2-D) continuum models the modeling efficiency based on the Akaike information criterion (AIC) is enhanced for equilibrium vs kinetic with no significant reduction in model performance. This is because fewer parameters are needed for the equilibrium model compared to the kinetic model. Two major transport regimes are identified in the transport of NP within porous media. The first regime is characterized by higher particle-surface attachment affinity than particle-particle attachment affinity, and operative transport mechanisms of physicochemical filtration, blocking, and physical retention. The second regime is characterized by the domination of particle-particle attachment tendency over particle-surface affinity. In this regime although physicochemical filtration as well as straining may still be operative, ripening is predominant together with agglomeration and further subsequent retention. In both regimes careful assessment of NP fate and transport is necessary since certain combinations of concurrent transport phenomena leading to large migration distances are possible in either case.

Strontium migration in a crystalline medium: effects of the presence of bentonite colloids

The effects of bentonite colloids on strontium migration in fractured crystalline medium were investigated. We analyzed first the transport behaviour of bentonite colloids alone at different flow rates; then we compared the transport behaviour of strontium as solute and of strontium previously adsorbed onto stable bentonite colloids at a water velocity of approximately 7.1·10(-6)m/s-224m/yr. Experiments with bentonite colloids alone showed that - at the lowest water flow rate used in our experiments (7.1·10(-6)m/s) - approximately 70% of the initially injected colloids were retained in the fracture. Nevertheless, the mobile colloidal fraction, moved through the fracture without retardation, at any flow rate. Bentonite colloids deposited over the fracture surface were identified during post-mortem analyses. The breakthrough curve of strontium as a solute, presented a retardation factor, R(f)~6, in agreement with its sorption onto the granite fracture surface. The breakthrough curve of strontium in the presence of bentonite colloids was much more complex, suggesting additional contributions of colloids to strontium transport. A very small fraction of strontium adsorbed on mobile colloids moved un-retarded (R(f)=1) and this fraction was much lower than the expected, considering the quantity of strontium initially adsorbed onto colloids (90%). This behaviour suggests the hypothesis of strontium sorption reversibility from colloids. On the other hand, bentonite colloids retained within the granite fracture played a major role, contributing to a slower strontium transport in comparison with strontium as a solute. This was shown by a clear peak in the breakthrough curve corresponding to a retardation factor of approximately 20.

Field study of the migration of 90Sr in a weak loess aquifer

The migration of 90Sr in a weak loess aquifer was investigated in-situ in an Underground Research Facility. Quartz containing 90Sr, 3H and other nuclides was introduced into the aquifer with a stainless steel pipe. The local water flow field in the aquifer was monitored using a 3-dimensional sampling system. The contaminated area of the aquifer was marked out wholly with the help of a pre-installed stainless steel frame and cut into small pieces (samples) 3-dimensionally, and analyzed. The result of the -3year experiment indicated that 90Sr had normal concentration distributions in Y and Z directions. The migration rate of the mass center for 90Sr in X direction was calculated as 3.5×10-5cm/d, the distribution coefficient for 90Sr in the aquifer was calculated as 7.4×102ml/g and the diffusion coefficient for 90Sr in the aquifer in X direction was calculated as 1.3×10-11m2/s.