Dynamics of radionuclide electromigration in intact granite: An kinetic adsorption-advection-dispersion model and its application.

Granite, as the natural barrier for the disposal of high-level radioactive waste, plays an important role in ensuring environmental and public safety. The safety assessment of the repository depends on the reliable migration parameters of radionuclides in granite. In this study, we developed a kinetic adsorption-advection-dispersion model based on first-order adsorption kinetics. It introduces a first-order adsorption rate coefficient to describe the kinetics of adsorption process and accounts for other crucial mechanisms affecting the migration of radionuclide ions, namely, the electromigration, electroosmosis, and dispersion. This model is then applied to interpret the experimental results of electromigration of tracer ions in intact granite. The results show that for the weakly adsorbed radionuclides studied, iodide and selenite, the effective diffusion coefficients and formation factors calculated by this model are in constant with those derived from the classical advection-dispersion model based on linear adsorption equilibrium. By contrast, for the moderately or strongly adsorbed tracer ions studied, including cobalt, cesium, and strontium, the migration parameters calculated by this model exhibit significantly less uncertainty than those obtained from the advection-dispersion model simulations. The advection-dispersion model based on the first order adsorption kinetics introduces the first order adsorption rate coefficient, and considers the influence of electromigration, electroosmosis and dispersion mechanism, which helps to explain the migration mechanism of nuclide ions in intact granite more accurately.

Study of natural attenuation after acid in situ leaching of uranium mines using isotope fractionation and geochemical data.

Acid in situ leaching (AISL) is a subsurface mining approach suitable for low-grade ores which does not generate tailings, and has been adopted widely in uranium mining. However, this technique causes an extremely high concentration of contaminants at post-mining sites and in the surroundings soon after the mining ceases. As a potential AISL remediation strategy, natural attenuation has not been studied in detail. To address this problem, groundwater collected from 26 wells located within, adjacent, upgradient, and downgradient of a post-mining site were chosen to analyze the fate of U(VI), SO42-, δ34S, and δ238U, to reveal the main mechanisms governing the migration and attenuation of the dominant contaminants and the spatio-temporal evolutions of contaminants in the confined aquifer of the post-mining site. The δ238U values vary from -0.07 ‰ to 0.09 ‰ in the post-mining site and from -1.43 ‰ to 0.03 ‰ around the post-mining site. The δ34S values were found to vary from 3.3 ‰ to 6.2 ‰ in the post-mining site and from 6.0 ‰ to 11.0 ‰ around the post-mining site. Detailed analysis suggests that there are large differences between the range of isotopic composition variation and the range of pollutants concentration distribution, and the estimated Rayleigh isotope fractionation factor is 0.9994-0.9997 for uranium and 1.0032-1.0061 for sulfur. The isotope ratio of uranium and sulfur can be used to deduce the migration history of the contaminants and the irreversibility of the natural attenuation process in the anoxic confined aquifer. Combining the isotopic fractionation data for U and S with the concentrations of uranium and sulfate improved the accuracy of understanding of reducing conditions along the flow path. The study also indicated that as long as the geological conditions are favorable for redox reactions, natural attenuation could be used as a cost-effective remediation scheme.

Assessment of the antimicrobial effects of sodium hypochlorite on Enterococcus faecalis by D2O-labeled single-cell Raman micro-spectroscopy.

OBJECTIVES: To systematically evaluate the feasibility of D2O-labeled single-cell Raman micro-spectroscopy in drug resistance research and test the susceptibility of Enterococcus faecalis (E. faecalis)to sodium hypochlorite. METHODS: 1) The growth of E. faecalis in different doses of D2O and the regularity of D2O intake were evaluated through absorbance measurement and D2O-labeled single-cell Raman micro-spectroscopy to examine the universality of D2O-labeled single-cell Raman micro-spectroscopy in bacterial resistance research. 2) Broth dilution method and absorbance measurement were performed to determine the minimum inhibitory concentration (MIC) of NaClO against E. faecalis and the MIC based on metabolic activity (MIC-MA) in vitro via D2O-labeled single-cell raman micro-spectroscopy. RESULTS: 1) The growth of E. faecalis was not significantly inhibited by ≤40% D2O in the medium. E. faecalis could actively metabolize D2O and exhibit a C-D ratio in specific areas of Raman micro-spectroscopy results. The C-D ratio of E. faecalis at the stationary phase was positively correlated with D2O concentration. 2) The MIC and MIC-MA of NaClO against E. faecalis were 0.45 and 0.9 g·L-1, respectively. The concentration of MIC-MA was twice that of MIC. CONCLUSIONS: D2O-labeled single-cell Raman micro-spectroscopy is important in screening antimicrobial agents and evaluating the efficacy of antimicrobial agents. It is suitable for evaluating the effect of drugs on bacterial metabolic activities. NaClO showed an effective antimicrobial activity against E. faecalis. E. faecalis ceased propagation yet remained highly metabolically active when it was exposed to NaClO at the MIC level. The metabolic activity of most cells was inhibited only when they were exposed to NaClO at the MIC-MA level.

Geographic Variation Did Not Affect the Predictive Power of Salivary Microbiota for Caries in Children With Mixed Dentition.

Dental caries is one of the most prevalent chronic oral diseases, affecting approximately half of children worldwide. The microbial composition of dental caries may depend on age, oral health, diet, and geography, yet the effect of geography on these microbiomes is largely underexplored. Here, we profiled and compared saliva microbiota from 130 individuals aged 6 to 8 years old, representing both healthy children (H group) and children with caries-affected (C group) from two geographical regions of China: a northern city (Qingdao group) and a southern city (Guangzhou group). First, the saliva microbiota exhibited profound differences in diversity and composition between the C and H groups. The caries microbiota featured a lower alpha diversity and more variable community structure than the healthy microbiota. Furthermore, the relative abundance of several genera (e.g., Lactobacillus, Gemella, Cryptobacterium and Mitsuokella) was significantly higher in the C group than in the H group (p<0.05). Next, geography dominated over disease status in shaping salivary microbiota, and a wide array of salivary bacteria was highly predictive of the individuals' city of origin. Finally, we built a universal diagnostic model based on 14 bacterial species, which can diagnose caries with 87% (AUC=86.00%) and 85% (AUC=91.02%) accuracy within each city and 83% accuracy across cities (AUC=92.17%). Although the detection rate of Streptococcus mutans in populations is not very high, it could be regarded as a single biomarker to diagnose caries with decent accuracy. These findings demonstrated that despite the large effect size of geography, a universal model based on salivary microbiota has the potential to diagnose caries across the Chinese child population.

Salivary biochemical indices related to early childhood caries.

OBJECTIVES: This study aimed to compare the salivary biochemical indices between caries-free individuals and those with early childhood caries (ECC), and construct a saliva-based caries diagnostic model by analyzing the correlation between salivary biochemical indices and caries severity. METHODS: A total of 120 children aged 4-6 years were selected and divided into two groups: individuals with ECC (C group, n=60) and healthy children (H group, n=60). Salivary samples were collected to compare the pH, total protein, and ion concentrations between the two groups. The correlation between the salivary biochemical indices and caries severity was examined, and an ECC diagnostic model was established. RESULTS: The NO3- concentration significantly decreased in the C group, whereas the Cl-, Br-, NH4+, and Mg2+ concentrations significantly increased in the C group (P<0.05). In addition, the salivary caries severity had a significantly negative correlation with the NO3- concentration but had a positive correlation with Br-, Cl-, and NH4+ concentrations (P<0.05). The ECC diagnostic model based on salivary biochemical indices could yield satisfactory results in terms of distinguishing the C and H groups with over 85% accuracy. CONCLUSIONS: Salivary biochemical indices can contribute to the diagnosis and risk assessment of ECC.

The predictive power of saliva electrolytes exceeds that of saliva microbiomes in diagnosing early childhood caries.

Early childhood caries (ECC) is one of the most prevalent chronic diseases affecting children worldwide, and thus its etiology, diagnosis, and prognosis are of particular clinical significance. This study aims to test the ability of salivary microbiome and electrolytes in diagnosing ECC, and their interplays within the same population. We here simultaneously profiled salivary microbiome and biochemical components of 331 children (166 caries-free (H group) and 165 caries-active children (C group)) aged 4-6 years. We identified both salivary microbial and biochemical dysbiosis associated with ECC. Remarkably, K+, Cl-, NH4 +, Na+, SO4 2-, Ca2+, Mg2+, and Br- were enriched while pH and NO3 - were depleted in ECC. Moreover, the dmft index (ECC severity) positively correlated with Cl-, NH4 +, Ca2+, Mg2+, Br-, while negatively with pH and NO3 -. Furthermore, machine-learning classification models were constructed based on these biomarkers from saliva microbiota, or electrolytes (and pH). Unexpectedly, the electrolyte-based classifier (AUROC = 0.94) outperformed microbiome-based (AUROC = 0.70) one and the composite-based one (with both microbial and biochemical data; AUC = 0.89) in predicting ECC. Collectively, these findings indicate ECC-associated alterations and interplays in the oral microbiota, electrolytes and pH, underscoring the necessity of developing diagnostic models with predictors from salivary electrolytes.

Quantitative Analysis of Salivary Oral Bacteria Associated with Severe Early Childhood Caries and Construction of Caries Assessment Model.

To construct a saliva-based caries risk assessment model, saliva samples from 176 severe early childhood caries (S-ECC) children and 178 healthy (H) children were screened by real-time PCR-based quantification of the selected species, including Streptococcus mutans, Prevotella pallens, Prevotella denticola and Lactobacillus fermentum. Host factors including caries status, dmft indices, age, gender, and geographic origin were assessed in their influence on abundance of the targeted species, which revealed host caries status as the dominant factor, followed by dmft indices (both P < 0.01). Moreover, levels of S. mutans and P. denticola in the S-ECC group were significantly higher than those in the healthy group (P < 0.001 for S. mutans and P < 0.01 for P. denticola). Interestingly, the co-occurrence network of these targeted species in the S-ECC group differed from that from the healthy group. Finally, based on the combined change pattern of S. mutans and P. pallens, we constructed an S-ECC diagnosis model with an accuracy of 72%. This saliva-based caries diagnosis model is of potential value for circumstances where sampling dental plague is difficult.

Low concentration of Fe(II) to enhance the precipitation of U(VI) under neutral oxygen-rich conditions.

Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16-20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52-56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27-29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O2 and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H+ concentration ([H+]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O2 has potential applications in the separation and recycling of aqueous uranium.

Distribution of radon and risk assessment of its radiation dose in groundwater drinking for village people nearby the W-polymetallic metallogenic district at Dongpo in southern Hunan province, China.

Radon in the household water (especially groundwater) which is an important source of indoor radon, has become a potential health hazard to residents. In this study, radon concentrations in groundwater sampled from five villages near Dongpo W-polymetallic metallogenic region were measured using RAD-7 detector with RAD H2O accessory, and the effect of regional geology and mineralization on radon concentration in groundwater was studied. In addition, we also estimated the radiation doses received by people via ingestion of radon in water and inhalation of the radon from the indoor air while using water. The results show that the radon concentration in groundwater samples varies from 1.29 Bq L-1 to 31.31 Bq L-1 with 10.47 Bq L-1 on average, and about 31.3% of the groundwater samples analyzed have a higher radon concentration than the maximum contaminant level of 11.1 Bq L-1 recommended by United States Environmental Protection Agency (USEPA). The relatively high radon level in groundwater can be attributed to a relatively high uranium background produced by the magmatic activity and magmatic-hydrothermal system. The values of annual effective dose (AEDing) due to ingestion of radon in groundwater range from 0.002 mSv y-1 to 0.055 mSv y-1, 0.005 mSv y-1 to 0.11 mSv y-1 and 0.008 mSv y-1 to 0.188 mSv y-1 for adult, child and infant respectively. The values of annual effective dose due to the inhalation of radon released from water are 63.6, 15.4 and 3.8 times of those through the ingestion of radon in groundwater by the adults, children and infants, respectively. In addition, the values of estimated total annual effective doses are 0.020-0.480 mSv y-1, 0.017-0.406 mSv y-1 and 0.020-0.484 mSv y-1 for adult, child and infant, respectively. These values are much lower than the reference dose level of 1 mSv y-1 recommended by World Health Organization (WHO) and United Nations Scientific Committee on the Effect of Atomic Radiation (UNSCEAR).

The effect of laterite density on radon diffusion behavior.

Radon generated in porous media such as soils and rocks migrates into indoor and outdoor air mainly by diffusion, possessing significant hazards to human health. In order to reduce these hazards of radon, it is of great importance to study the diffusion behavior of radon. In this study, we systematically measured the radon diffusion coefficient of laterite with the density ranging from 0.917gcm-3 to 2.238gcm-3, and studied the effect of laterite density on the radon diffusion. The results show that the radon diffusion coefficient of the laterite generally decreases with the increasing laterite density. In addition, three possible relationships between the radon diffusion coefficient and the laterite density are found out as follows: (1) the linear correlation with a slope of -4.48 × 10-6 for laterite with density ranging from 0.917 to 1.095gcm-3, (2) the exponential correlation for laterite with density from 1.095 to 1.63gcm-3, (3) linear correlation with a slope of -3.1 × 10-7 for laterite with density from 1.63 to 2.238gcm-3. The complex relationship between the radon diffusion coefficient and density is caused by the change of porosity and tortuosity of the laterite. Therefore, we suggest that a suitable density should be adopted while using the laterite to effectively cover uranium tailings or economically produce building materials that can curb the radon exhalation.

Fractal kinetic characteristics of hard-rock uranium leaching with sulfuric acid.

In order to study the fractal dynamic properties of uranium leach mining and discuss the influence of ore crushing on the dynamics of leach mining, uranium mine ore rocks in southern China were selected as the research object and an acid leaching experiment was performed on the ore samples with different fractal dimensions of 1.1, 1.4, 1.7, 2.0, 2.3 and 2.6. In the column leaching experiment, a PVC pipe with an inner diameter of 112 mm and a height of 1500 mm was used. The uranium content was determined by using titanium trioxide that was placed into a 0.1 mg ml-1 standard uranium solution, and a sampling rate of once daily with a 5 ml volume of leaching solution was adopted after 8 h drenching time. The results show that the flow rate of the leaching solution depends on the distribution of the ore's particle size, that is, a larger fractal dimension results in a smaller flow rate. The concentration of the uranium leaching solution reaches a maximum value which subsequently decreases with time on the third day of the experiment, and it seems that the changes in the uranium concentration tend to be stable at around 15 days. Moreover, the concentration seems to increase with the increasing fractal dimension, and the fractal dimension of the ore particle size has a significant impact on the leaching kinetics. When the fractal dimension is between 1.1 and 2.6, the uranium dissolution rate, K, increases with the increasing fractal dimension. The kinetic reaction of the uranium leaching is a liquid-solid one, which is controlled by chemical reactions in the earlier phase. While the middle reaction phase is mainly chemical-diffusion reaction coupling, and the latter part of the reaction is controlled by diffusion. As the fractal dimension increases, the liquid-solid reaction controlled by diffusion appears at earlier phases. When the fractal dimension is greater than 2.0, the time of entering the diffusion control phase only changed little with the increasing of the fractal dimension. At last, a fractal dimension of 2.0 is suggested for the acid leaching of uranium ore crushing.

Fractal Theory and Field Cover Experiments: Implications for the Fractal Characteristics and Radon Diffusion Behavior of Soils and Rocks.

Radon diffusion and transport through different media is a complex process affected by many factors. In this study, the fractal theories and field covering experiments were used to study the fractal characteristics of particle size distribution (PSD) of six kinds of geotechnical materials (e.g., waste rock, sand, laterite, kaolin, mixture of sand and laterite, and mixture of waste rock and laterite) and their effects on radon diffusion. In addition, the radon diffusion coefficient and diffusion length were calculated. Moreover, new formulas for estimating diffusion coefficient and diffusion length functional of fractal dimension d of PSD were proposed. These results demonstrate the following points: (1) the fractal dimension d of the PSD can be used to characterize the property of soils and rocks in the studies of radon diffusion behavior; (2) the diffusion coefficient and diffusion length decrease with increasing fractal dimension of PSD; and (3) the effectiveness of final covers in reducing radon exhalation of uranium tailings impoundments can be evaluated on the basis of the fractal dimension of PSD of materials.