On-site monitoring and numerical simulation on groundwater flow and pollution plume evolution in a hexavalent-chromium contaminated site.

Accurately ascertaining spatiotemporal distribution of pollution plume is critical for evaluating the effectiveness of remediation technologies and environmental risks associated with contaminated sites. This study concentrated on a typical Cr(VI) contaminated smelter being currently remediated using pump-and-treat (PAT) technology. Long-term on-site monitoring data revealed that two highly polluted regions with Cr(VI) concentrations of 162.9 mg/L and 234.5 mg/L existed within the contaminated site, corresponding to previous chromium slag yard and sewage treatment plant, respectively. The PAT technology showed significant removal performance in these highly polluted areas (>160 mg/L) after six months of pumping, ultimately achieving complete removal of the pollutants in these high-pollution areas. Numerical simulation results showed that although the current remediation scheme significantly reduced the Cr(VI) pollution degree, it did not effectively prevent the incursion of the pollution plume into the downstream residential area after 20 years. Additionally, an improved measure involving supplementary pumping wells was proposed, and its remediation effects were quantitatively evaluated. Results indicated that the environmental pollution risk of groundwater downstream could be effectively mitigated by adding pumping wells, resulting in a reduction of the pollution area by 20 % in the case of adding an internal well and 41 % with the addition of external wells after 20 years. The findings obtained in this study will provide an important reference and theoretical guidance for the reliability analysis and design improvement of the PAT remediation project.

Effects of ionic strength and bentonite ratio on the migration of Cr(VI) in clayey soil-bentonite engineered barrier.

Soil-bentonite (S-B) barriers have been widely used for heavy metal pollution containment. This study conducted batch adsorption tests and diffusion-through tests to evaluate how ionic strength and bentonite ratio influence the migration of Cr(VI) in natural clay-bentonite mixtures. The test results indicated that the adsorption of Cr(VI) exhibited an obvious anion adsorption effect, the pH of the soil mixture increased with the addition of bentonite, resulting in a decrease in the positive surface charge. This change led to a decrease in Cr(VI) adsorption capacity, from 775.19 mg/kg for pure clay to 378 mg/kg for mixture samples with excessive bentonite. Furthermore, as the ionic strength increases from 0 to 0.1 M, the Cr(VI) adsorption capacity increases slightly due to the weakening of electrostatic repulsion on the clay particle surface, but the effective diffusion coefficient (De) increases by 21.97%. The compression of the diffusion double layer (DDL) under high ionic strength conditions enlarges the diffusion path and enhances the migration of Cr(VI) through the pore flow paths. Moreover, hydrated bentonite effectively fills the interaggregate pores of natural clay, thus creating narrower and more tortuous flow paths. However, excessive bentonite increases the pH value and pore volume, resulting in changes to the soil microstructure and disrupting the continuous skeleton of natural clay, which is unfavorable for Cr(VI) containment. Based on the study of the Cr(VI) contaminated site, a bentonite ratio of 2:10 is recommended for optimal natural performance of the natural clay-bentonite barrier.

Leaching behavior of copper tailings solidified/stabilized using hydantoin epoxy resin and red clay.

Tailings produced by mining engineering and metal smelting industries have become a major challenge to the ecological environment and human health. Environmental compatibility, mechanical stability, and economic feasibility have restricted the treatment and reuse of tailings. A novel solidification/stabilization technology using hydantoin epoxy resin (HER) and red clay for copper tailing treatment was developed, and the leaching behaviors of solidified/stabilized copper tailings were investigated in this paper. The leaching characteristics were analyzed by toxicity characteristic leaching procedure (TCLP) leaching tests. Besides, the influence of red clay content and acid rain on the permeability characteristics and leaching characteristics were investigated based on flexible-wall column tests and microstructure tests. The results showed that the copper tailings solidification/stabilization technology with HER and red clay had excellent performances in toxicity stabilization. The leaching concentration of Cu in TCLP tests and flexible wall column tests remained within the limit specified by the Chinese national standard, and the concentration of Cu decreased significantly with the increase of the red clay content. Moreover, acid rain leaching changed the mineral composition and microstructure of solidified tailings, and the porosity of the samples increased with the dissolution of soluble minerals. Additionally, the hydraulic conductivities decreased slightly with the increase in the pH value of acid rain, and the solidified sample with 5% red clay had the lowest hydraulic conductivity.

Spatiotemporal distribution and pollution control of pollutants in a Cr(VI)-contaminated site located in Southern China.

Soil and groundwater Cr(VI) pollution resulting from improper disposal and accidental spills is a critical problem worldwide. In this study, a comprehensive study was conducted to assess the hydrogeological conditions of a contaminated site, obtain spatiotemporal distribution and trend forecasts of pollutant Cr(VI), and determine the feasibility of applying clayey engineered barriers for pollution control. The results showed that the hydraulic conductivity (K) of the clayey barrier (1.56E-5 m/d) is several orders of magnitude lower than that of the stratum beneath the contaminated site, with K values ranging from 0.0014 to 4.76 m/d. Cr(VI) exhibits high mobility and a much higher concentration in the vadose zone, with maximum values of 6100 mg/kg in topsoil and 2090 mg/L in the perched aquifer. The simulation results indicated that the groundwater in the vicinity of the contaminated site, as well as downstream of the Lianshui River, is seriously threatened by Cr(VI). Notably, the pollution plume could occur downstream of the Lianshui River after 8 years. The retention efficiency of clayey engineered barriers will decrease over time, at 61.6% after 8 years and 33% after 20 years. This work contributes to an in-depth understanding of Cr(VI) migration at contaminated sites.

Laboratory investigation on the retention performance of a soil-bentonite mixture used as an engineered barrier: insight into the effects of ionic strength and associated heavy metal ions.

Soil-bentonite (S-B) materials are promising backfill materials for use as engineered barriers in heavy metal-contaminated sites. The effects of contaminant exposure on the retention performance of the S-B barrier remain unrevealed. In this study, based on the pollution status of an abandoned ferroalloy factory located in southern China, the retention performance of the S-B mixture toward Cr(VI) and Zn(II) was studied through adsorption and diffusion experiments sequentially; the separate effect of ionic strength (binary solution) and the combined effect of ionic strength and associated heavy metal ion (ternary solution) were discussed. In NaCl-Cr(VI)/Zn(II) binary solutions, the adsorption of Zn(II) onto the S-B mixture is larger than that of Cr(VI). Kd, Qmax, and ɛacc (accessible porosity) of Cr(VI) increase through increasing ionic strength, while Zn(II) shows the opposite trend; De (effective diffusion coefficient) values for both Cr(VI) and Zn(II) increased with increasing ionic strength and follow a sequence of Cr(VI) > Zn(II), indicating a better retention performance of the S-B mixture to Zn(II). For a given ionic strength, the adsorption of Zn(II) was larger than that of Cr(VI), which can be attributed to the retention specificity of the S-B mixture to anion and cation. In Cr(VI)-Zn(II)-NaCl ternary solutions, the adsorptions of Cr(VI) and Zn(II) are enhanced in varying degrees when compared with their binary solution, which probably could be attributed to the ion bridge role of Cr(VI)/Zn(II) to connect each other that relatively increased the adsorption capacity of S-B material. This work will contribute to an in-depth understanding of the retention performance of the S-B mixture in complicated chemical environments and facilitate the selection of future remediation strategies.

Contaminant migration and the retention behavior of a laterite-bentonite mixture engineered barrier in a landfill.

Groundwater pollution has become increasingly severe in recent years, particularly owing to leachate leakage in landfills. In this study, the migration of Cu2+ in a landfill and the retention behavior of a compacted laterite-bentonite engineered barrier system toward the contaminant were analyzed by a numerical simulation based on laboratory and field test results. The results show that the hydraulic conductivity of the laterite-bentonite mixture decreased with an increase in the bentonite ratio: The hydraulic conductivities of the laterite-bentonite mixture were 4.718 × 10-7, 2.103 × 10-7, 7.899 × 10-8, 3.918 × 10-8, and 1.614 × 10-8 cm/s when the bentonite ratios were 0, 2%, 5%, 10%, and 20%, respectively. The hydraulic conductivity of laterite and of the mixture with a bentonite ratio of 2% decreased gradually under infiltration of deionized water and CuSO4 solutions with concentrations of 0.01 and 0.1 mol/L. This could be attributed to the increased degree of flocculation of laterite with the increase in the solution concentration. The results of the numerical simulation indicate that the migration range of Cu2+ after 3650 days was approximately 1500 m. The retention efficiency of a 0.5 m engineered barrier for Cu2+ was 67%. However, the retention efficiency exceeded 83% when the engineered barrier thickness was increased to 1.0 m. The results of the laboratory tests and numerical simulation demonstrate that a compacted laterite-bentonite engineered barrier system has a good retention effect on Cu2+. These observations may provide effective concepts for the prevention and control of groundwater pollution in landfills.

RETRACTED ARTICLE: Experimental study and numerical prediction of HTO and 36Cl- diffusion in radioactive waste at Téguline Clay.

Statement of Retraction: Experimental study and numerical prediction of HTO and 36Cl- diffusion in radioactive waste at Téguline ClayWe, the Editor and Publisher of Environmental Technology, have retracted the following article:Yun-yi Zhang, Yong He, Bing-bing Li & Ke-neng Zhang, Experimental study and numerical prediction of HTO and 36Cl- diffusion in radioactive waste at Téguline Clay, Environmental Technology, Latest Articles, 10.1080/09593330.2021.1939433Following publication, errors have been identified in the experimental work and numerical simulation as follows: 1) The experimental results presented in this paper are too limited to effectively support the numerical analysis.2) Although XRD, MIP, SEM and other microstructural tests were conducted and presented in this paper, the results of these microstructural tests have not been used in the numerical analysis, leading to incorrect results shown in Figure 11.3) There is a serious discrepancy between the simulated results and the real conditions, due to an incorrect selection of diffusion parameters and initial concentration (200 mg/L and 500 mg/L) as well as an incorrect simulation model and boundary condition (Figure 4).4) The simulation software (Geo-studio + MATLAB) appears to be unsuitable for the simulation work in this paper due to physical, mechanical and adsorption properties of the sample not being considered in the simulation.These errors were identified and acknowledged by the authors. The errors are too substantial to be corrected in a corrigendum and will alter the conclusions of the paper. There are no concerns regarding research or publishing conduct, and the authors plan to revisit the relevant mature results of their research and may resubmit the revised paper for consideration by the journal.We have been informed in our decision-making by our policy on publishing ethics and integrity and the COPE guidelines on retractions.The retracted article will remain online to maintain the scholarly record, but it will be digitally watermarked on each page as "Retracted".

Experimental and numerical study on heavy metal contaminant migration and retention behavior of engineered barrier in tailings pond.

Heavy metal pollution is a serious environmental problem globally, particularly in mines and tailings ponds. In this study, based on laboratory and field tests, the migration of heavy metal contaminants in a tailings pond and the retention behavior of a compacted bentonite engineered barrier system on the heavy metal contaminants were analyzed by a numerical simulation. The results demonstrate that the hydraulic conductivity of compacted bentonite is lower than that of the tailings from the laboratory tests. The hydraulic conductivity of the tailings sand decreased with an increase in the dry density and increased with an increase in the concentration of the chemical solution, which could be attributed to the large amounts of fine-grained soil contained in the tailings, according to the grain size distribution test. The hydraulic conductivity of the tailings from the engineering geological survey was between 2.0 × 10-6 and 9.0 × 10-5 m/s, and followed the order: tail coarse sand ＞ tail silty sand ＞ tail medium sand ＞ tail fine silt. The numerical simulation of the seepage could satisfactorily describe the actual working condition of the tailings dam. With the groundwater seepage, the migration range of the heavy metal contaminant in the researched tailings pond reached a maximum of 45 m for 5 years. The retention efficiencies of the 0.2 m engineered barrier against the heavy metal contaminant for 15 and 30 years were 45.4% and 57.2%, respectively. Moreover, the retention efficiency would exceed 87% when the engineered barrier thickness is increased to 0.5 m. The results of model validation show that the calculated results are in good agreement with the measured ones. These findings can provide effective ideas for the prevention and control of environmental pollution in mines and tailings ponds.