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

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.

Prediction of heavy metal ion distribution and Pb and Zn ion concentrations in the tailing pond area

The pollution caused by tailings ponds has resulted in ecological damage, with soil contamination significantly impacting the daily lives of residents in the vicinity of mining areas and the future development of mining areas. This study assesses the transport status of heavy metal pollution in tailings areas and predicts its impact on future pollution levels. This study focused on lead-zinc tailing ponds, exploring the spatial and chemical distribution characteristics of heavy metals based on the distributions of Pb, Zn, As, Cu, Cr, Cd, Hg, and Ge ions. The concentrations of the major heavy metal ions Pb and Zn in tailings ponds were predicted via the exponential smoothing method. ① The total accumulation of Pb and Zn in the mine tailings ranges from 936.74~1212.61 mg/kg and 1611.85~2191.47 mg/kg, much greater than the total accumulation of the remaining six heavy metals. The total accumulation of associated heavy metal Cu was high, and the lowest total heavy metals were Hg and Ge at only 0.19 mg/kg and 1.05 mg/kg. ② The analyses of soil heavy metal chemical forms reveal that the heavy metals Pb and Zn had the highest exchangeable state content and state ratio and the strongest transport activity in the industrial plaza and village soils. Pb and Zn are the heavy metals with the greatest eco-environmental impacts in the mining area. ③ The predicted results show that the soil concentrations of the heavy metals Pb and Zn around the tailings area in 2026 are 1.335 and 1.191 times the predicted time starting values. The concentrations of the heavy metals Pb and Zn at the starting point of the forecast are already 3.34 and 3.02 times the upper limits of the environmental standard (according to environmental standards for gravelly grey calcium soils). These results have significant implications for heavy metal pollution risk management.

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.

Homology and heterogeneity of soil trace elements of coal power production bases in arid and semi-arid areas of Northwest China

Coal power activities could cause regional fluctuations of trace elements, but the distribution information of these trace elements in arid and semi-arid areas is insufficient. In this study, the soil trace elements (As, B, Be, Cd, Co, Cr, Cu, Fe, Ga, Ge, Mn, Mo, Ni, Pb, Sb, Sn, Sr, Ti, Tl, and Zn) of Ningdong Coal Power Production Base in China were monitored. Results showed that the concentrations of B, Tl, Mn, Pb, Cr, K, Cu, and Co exceeded background values. The maximum risk index reached 265.66, while the trace elements posed a cancer risk to children. Combining correlation analyses (CA), principal component analysis (PCA), and positive matrix factorization (PMF) techniques, it indicated that trace elements were mainly coming from coal combustion (34.15%), livestock farming (17.44%), traffic emissions (12.42%), and natural factors (35.99%). This study reveals the sources and potential ecological risks of soil trace elements in the Ningdong Coal and Power Production Base. It provides a scientific basis for developing targeted environmental management measures and reducing human health risks.

Iron ore tailings valorization through separate characterization and upgradation of different tailings streams of an Iranian iron ore processing plant

In iron ore processing plants, different tailing streams are usually transferred to the tailings thickener for partial dewatering and finally transferred to the tailings dam as a single stream. Therefore, the mixing of different tailings streams happens. This way can challenge the process of reprocessing the tailings in the tailings dam since the mixing of different tailings streams causes more complexity in the mineralogical composition as well as the chemical composition of the tailings in the tailings dam. To solve this problem, the idea of separate characterization and separate upgradation of different tailings streams of an iron ore processing plant was carried out and a comparison was made between the results of magnetic upgradation of each tailings streams with the total tailings (i.e., the tailings in the tailings dam, which is a mixture of different tailings streams of the plant). Hence, the different tailings streams of an iron ore processing plant were sampled and characterize for total iron, FeO content, particle size distribution, mineralogical composition by X-ray diffraction (XRD), magnetic behavior by Davis tube tests, and dry solid tonnage rate. The characterization results showed that the iron grade and dominant iron ore mineral vary from one stream to another tailings stream of the iron ore processing plant. For instance, the total iron content of different tailings streams varies in the range of 18.46 to 64.68% and the dominate iron ore mineral in the Cobber tailings was hematite, but in the other tailings streams it was magnetite. The magnetic upgradation of the Cobber and Rougher tailings and also the total tailings were performed separately by the wet magnetic separation at different magnetic field intensities of 2000, 3500, 5000, and 15,000 Gauss. A concentrate with the highest iron grade of 61.79% and yield of 52.15% was produced from magnetic upgradation of the Rougher tailings, but magnetic upgradation of the total tailings produced a concentrate with the iron grade of 37% and yield of 15.2%. A comparison between the magnetic upgradation of the total tailings and the Cobber and Rougher tailings revealed that the upgradation of Rougher tailings results in a concentrate with higher iron grade and yield than the total tailings.

Source identification and migration fate of metal(loid)s in soil and groundwater from an abandoned Pb/Zn mine

Understanding the spatial distribution, source identification, and migration fate of toxic metals is crucial for managing the potential risks associated with metal(loid)s in abandoned Pb/Zn mines. This study provides a comprehensive analysis of the heterogeneous characteristics, contamination sources, and migration fate of metal(loid)s in both mine soil and groundwater. The results reveal that the abandoned mine soil is primarily contaminated with As and Pb, whereas groundwater in the mining and smelting area is mainly contaminated with Pb. The concentrations of As and Pb in the soil reached a maximum of 37.5 mg/kg and 289 mg/kg, respectively, significantly exceeding the local background values of 13.6 mg/kg for As and 29 mg/kg for Pb. The sources of soil contamination were attributed to historical smelting activities (31.4 %) for As, Cd, Hg, and Sb, while Pb and Mn were primarily derived from the ore-deposited belt (21.5 %). Machine learning predictions indicate that the migration of As in the soil can extend up to six meters or more, predominantly influenced by the presence of grit and silt. As a significant source of groundwater contamination, both soil As and Cd can infiltrate the groundwater through convection or diffusion processes. In conclusion, it is imperative to address the long-term release of heterogeneous metal ores in the soil of abandoned mine sites, as this can severely deteriorate the quality of both soil and groundwater.

Study on diversity, nitrogen-fixing capacity, and heavy metal tolerance of culturable Pongamia pinnata rhizobia in the vanadium-titanium magnetite tailings

Introduction

The diversity, nitrogen-fixing capacity and heavy metal tolerance of culturable rhizobia in symbiotic relationship with Pongamia pinnata surviving in vanadium (V) - titanium (Ti) magnetite (VTM) tailings is still unknown, and the rhizobia isolates from the extreme barren VTM tailings contaminated with a variety of metals would provide available rhizobia resources for bioremediation.

Methods

P. pinnata plants were cultivated in pots containing the VTM tailings until root nodules formed, and then culturable rhizobia were isolated from root nodules. The diversity, nitrogen-fixing capacity and heavy metal tolerance of rhizobia were performed.

Results

Among 57 rhizobia isolated from these nodules, only twenty strains showed different levels of tolerance to copper (Cu), nickel (Ni), manganese (Mn) and zinc (Zn), especially strains PP1 and PP76 showing high tolerance against these four heavy metals. Based on the phylogenetic analysis of 16S rRNA and four house-keeping genes (atpD, recA, rpoB, glnII), twelve isolates were identified as Bradyrhizobium pachyrhizi, four as Ochrobactrum anthropic, three as Rhizobium selenitireducens and one as Rhizobium pisi. Some rhizobia isolates showed a high nitrogen-fixing capacity and promoted P. pinnata growth by increasing nitrogen content by 10%-145% in aboveground plant part and 13%-79% in the root. R. pachyrhizi PP1 showed the strongest capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals, which provided effective rhizobia strains for bioremediation of VTM tailings or other contaminated soils. This study demonstrated that there are at least three genera of culturable rhizobia in symbiosis with P. pinnata in VTM tailings.

Discussion

Abundant culturable rhizobia with the capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals survived in VTM tailings, indicating more valuable functional microbes could be isolated from extreme soil environments such as VTM tailings.

Application potential of Vaccinium ashei R. for cadmium migration retention in the mining area soil

Despite numerous reports on phytoremediation of heavy metals contaminated soil, there are few reports on plant retention of heavy metals in the mining area slope. This study was the first of its kind to explore the cadmium (Cd) retention capacity of the blueberry (Vaccinium ashei Reade). Firstly, we investigated the stress response of blueberry to different soil Cd concentrations (1, 5, 10, 15, 20 mg/kg) to assess its potential for phytoremediation by pot experiments. The results showed that the blueberry biomass exposed to 10 and 15 mg/kg Cd was significantly increased compared with the control (1 mg/kg Cd); the blueberry crown increased by 0.40% and 0.34% in 10 and 15 mg/kg Cd-contaminated soil, respectively, compared with control; the blueberry heigh did not even change significantly in each treatment group; the total chlorophyll content, peroxidase and catalase activity of blueberry were enhanced in 5-20 mg/kg Cd treatments. Furthermore, the Cd contents of blueberry in the root, stem and leaf increased significantly as the Cd concentration of soil increased. We found that more Cd accumulated in blueberry root: the bioaccumulation concentration factor was root > stem > leaf for all groups; the residual-Cd (Cd speciation) in soil increased by 3.83%-411.11% in blueberry-planted versus unplanted groups; blueberry improved the Cd-contaminated soil micro-ecological environment including soil organic matter, available K and P, as well as microbial communities. Then, to investigate the effect of blueberry cultivation on Cd migration, we developed a bioretention model and revealed that soil Cd transport along the model slope was significantly weakened by blueberry cultivation, especially at the bottom of the model. In a word, this research suggests a promising method for the phytoremediation of Cd-contaminated soil and the reduction of Cd migration in mining areas.

Environmental risk of tailings pond leachate pollution: Traceable strategy for leakage channel and influence range of leachate

Identifying the leakage channel and the influencing range is essential for controlling the environmental risks of leachate from the tailings pond. The investigation of leachate pollution in tailings pond has the defect of focusing only on the scope of tailings pond in recent studies. This study innovatively built a comprehensive investigation and accurate verification system for leachate leakage of tailings pond integrated with the aeromagnetic survey, ground penetrating radar, hydrochemistry and isotope coupling methods. Geophysical exploration found that among the four fault zones, and the F1 was the channel for leachate to recharge the groundwater 2.53 km away from the tailings pond. The fissures inside the tailings pond were connected with the natural fissures outside, forming a leachate migration channel. The hydrochemistry and isotope characteristics showed that the groundwater far away from the tailings pond were polluted by arsenic containing leachate, which verified the geophysical exploration results. The significant correlation between arsenic and SO2-4 concentration indicated that arsenic in leachate originated from the oxidation release of sulfide minerals (i.e., arsenopyrite). This study sheds light on the comprehensive investigation of leachate leakage in the tailings pond. This development method also provides guidance for environmental risk identification of other 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). K_d, Q_max, and ɛ_acc (accessible porosity) of Cr(VI) increase through increasing ionic strength, while Zn(II) shows the opposite trend; D_e (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.

Immobilizing lead and copper in aqueous solution using microbial- and enzyme-induced carbonate precipitation

Inappropriate irrigation could trigger migration of heavy metals into surrounding environments, causing their accumulation and a serious threat to human central nervous system. Traditional site remediation technologies are criticized because they are time-consuming and featured with high risk of secondary pollution. In the past few years, the microbial-induced carbonate precipitation (MICP) is considered as an alternative to traditional technologies due to its easy maneuverability. The enzyme-induced carbonate precipitate (EICP) has attracted attention because bacterial cultivation is not required prior to catalyzing urea hydrolysis. This study compared the performance of lead (Pb) and copper (Cu) remediation using MICP and EICP respectively. The effect of the degree of urea hydrolysis, mass and species of carbonate precipitation, and chemical and thermodynamic properties of carbonates on the remediation efficiency was investigated. Results indicated that ammonium ion (NH4+) concentration reduced with the increase in lead ion (Pb2+) or copper ion (Cu2+) concentration, and for a given Pb2+ or Cu2+ concentration, it was much higher under MICP than EICP. Further, the remediation efficiency against Cu2+ is approximately zero, which is way below that against Pb2+ (approximately 100%). The Cu2+ toxicity denatured and even inactivated the urease, reducing the degree of urea hydrolysis and the remediation efficiency. Moreover, the reduction in the remediation efficiency against Pb2+ and Cu2+ appeared to be due to the precipitations of cotunnite and atacamite respectively. Their chemical and thermodynamic properties were not as good as calcite, cerussite, phosgenite, and malachite. The findings shed light on the underlying mechanism affecting the remediation efficiency against Pb2+ and Cu2+.

Process, influencing factors, and simulation of the lateral transport of heavy metals in surface runoff in a mining area driven by rainfall: A review

The lateral transport of heavy metals can expand the scope of original contamination, and an accurate prediction of heavy metal migration is necessary to control heavy metal transport. However, previous studies have mainly focused on the migration of soil pollutants in the runoff-soil-groundwater system, whereas research on the lateral migration of heavy metals in surface soil driven by rainfall is relatively scarce. Therefore, in this study we analyzed the horizontal migration of water-soluble heavy metals with surface runoff and non-water-soluble heavy metals with sediment particles, investigated the main factors affecting the processes of runoff and sediment transport and the main factors affecting the mobility of heavy metals in soils, summarized the existing methods for the simulation of heavy metal transportation. The construction of a lateral migration model based on the migration mechanism of soil heavy metals, the hydrological model, and the application of the lateral migration model should be the focus of future research. This study provides a theoretical basis for establishing a model of the lateral migration of soil heavy metals and is of great significance for the prevention and control of the risks related to the lateral migration of soil heavy metals.

Effect of seepage conditions on the microstructural evolution of loess across north-west China

Loess features metastable microstructure and is deemed susceptible to chemical contaminant permeation. However, studies on the loess permeability evolution under water and chemical environments are remarkably limited. In this study, the response of the loess to the water and sodium sulfate seepages was analyzed using the temporal relationship of cations concentration, X-ray diffraction and fluorescence (XRD and XRF), mercury intrusion porosimetry (MIP), and scanning electron microscope (SEM) tests. The permeability evolution characteristics were identified, and its underlying mechanisms were revealed from aspects of the diffuse double layer (DDL) theory and physiochemical actions. The discharge of Mg²⁺ and precipitation of calcium carbonate, referred also to as the dedolomitization, degraded the macro permeability when subjected to the water seepage test. The salt-induced swelling, induced by the intrusion of Na⁺ into the DDL, caused an increase in the micropore fraction under the sodium sulfate seepage test, thereby increasing the macro permeability.

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The k evolution of the loess under the water and Na₂SO₄ seepages is investigated

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The dedolomitization takes part in the k degradation under the water seepage

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The Na⁺ intrusion into the double layer enhances the k under the Na₂SO₄ seepage

Insights into the spatiotemporal differences in tailings seepage pollution by assessing the diversity and metabolic functions of the soil microbial community

The formation of tailings ponds depends on the long-term accumulation of tailing and high terrain. Its seepage pollution characteristics may have gradient variations on spatiotemporal scales. Used three nearby metal tailings ponds with different service times, we aimed to reveal seepage pollution trends on spatiotemporal scales and the response of soil microbial community. The results showed that the degree of seepage pollution was negatively correlated with the distance from the tailings pond on the spatial scale, while the seepage pollution showed higher levels in tailings ponds with longer service times on the temporal scale (RI = 248.04-2109.85). The pollution effect of seepage persisted after the tailings pond was discontinued (RI = 226.72). Soil microbial diversity increased with spatial scale expansion. The proportion of Actinomyces gradually increased and Proteobacteria decreased. Cr (r = 0.21) and Fe (r = 0.22) contributed more to the microbial community changes. Functional predictions showed that pathways related to signal transduction and energy metabolism were more abundant in the tailings pond. In contaminated areas, the proportion of nitrate respiration and cellulolysis functional communities had decreased, and some potentially pathogenic human taxa had accumulated. These results emphasized that there was pollution accumulation on temporal scale and pollution dispersion on spatial scale around tailings ponds, and the response of the microbial community further illustrated these trends.

Study on Preparation of Polymer-Modified Bentonite and Sand Mixtures Based on Osmotic Pressure Principle

Polymer-modified bentonite and sand mixtures (PMBS) are widely used in the engineering field due to their low cost and low permeability. In this study, different ionic types of polyacrylamides were used to modify bentonite to improve its swelling properties and impermeability. The physicochemical properties of polymer-modified bentonite were characterized by X-ray diffraction, particle size distribution, IR spectroscopy, SEM, and free swell index (FSI) to further demonstrate the successful organic modification of bentonite. To investigate the impermeability mechanism of PMBS from the perspective of osmotic pressure, the colloidal osmotic pressure of bentonite and hydraulic conductivity were compared. The results showed that anionic polyacrylamide (APAM) had the most obvious improvement on the swelling properties of bentonite, and 3% APAM increased the FSI of bentonite from 15 mL/2 g to 41 mL/2 g. With the increase in polymer dosage, the colloidal osmotic pressure of bentonite increased and the hydraulic conductivity of PMBS decreased significantly. The interior of PMBS is equivalent to a highly concentrated bentonite–sand–water system. When the colloidal osmotic pressure in the restricted space is higher than the external hydraulic pressure, it will prevent infiltration from occurring. When the external hydraulic pressure exceeds the high concentration of bentonite colloid osmotic pressure, the hydraulic conductivity may increase rapidly. Therefore, the impermeability of PMBS depends on the colloidal osmotic pressure of bentonite. Finally, it was confirmed that PMBS had a self-healing capacity by simulating damage to PMBS.

Modeling of Permeability Coefficient Calculations Based on the Mesostructure Parameters of Tailings

The permeability coefficient of tailings in tailings ponds, which can affect the release and migration of heavy metals in tailings, also affects the stability of dams by affecting the variation of the height of the saturation line. In this paper, tailings at different levels in a tailings pond were taken as research objects to measure the particle size and permeability coefficient of the tailings. At the same time, CT scanning technology and three-dimensional reconstruction were used to establish the three-dimensional model of the tailings, and the permeability coefficient of the tailings was analyzed from a mesostructural point of view. The results show the following: (1) The particle size of the tailings in the tailings pond decreased rapidly with the increase of distance from the discharge port. When the distance exceeded 8 m, a sudden change occurred, and the decreasing trend obviously slowed down. The particle size of tailings decreased, the compactness increased, and the permeability coefficient decreased gradually. (2) Statistics and analysis of the mesostructure affecting the permeability coefficient of tailings: the error between the calculated value and the measured value of the particle size and porosity of the three-dimensional reconstruction model was small, which proved that the model had high reliability. The porosity, sphericity, and particle size of the tailings were consistent and decreased with the increase of distance from the discharge port. The number of pore branches and nodes of the tailings increased with the increase of the distance from the discharge port, while the average radius and length of the pores decreased. The fragmentation index can characterize the pore channel connectivity of the tailings, which has a high negative linear correlation with the number of pore branches and a positive quadratic curve correlation with the average branch length of the pores. (3) Based on the Kozeny-Carman equation and data regression analysis method and combined with the results of permeability coefficient measurements, the fragmentation index was introduced into the Kozeny-Carman equation. Also, a modified model for calculating the permeability coefficient of the tailings was established based on the mesostructure parameters. By comparing the measured values of the tailings' permeability coefficient, the error range was 1.91-13.24%. The research results have important theoretical significance for the prevention and control of heavy metal pollution and the stability of tailings ponds.

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.

Peat and bentonite amendments assisted soilless revegetation of oligotrophic and heavy metal contaminated nonferrous metallic tailing

Soilless revegetation is a promising method for ecological restoration of nonferrous metallic tailings because of its low-cost and eco-friendliness. However, revegetation is difficult to construct in the tailings due to the high heavy metal concentration, poor water retention capacity and low fertility. In this study, soilless revegetation was successfully carried out by using peat and bentonite amendments. The results showed that amendment addition significantly increased the F.elata seed germination percentage, plant length and fresh biomass by 14.9%-24.3%, 48.9%-90.4% and 51.9%-88.1%, respectively. Such improvements probably referred to the variation of rhizosphere tailing microecological characteristics. Amendment addition dramatically improved tailing available NPK by 39.76-102.13%, 2.69-40.81% and 2.42-20.02%, respectively, and reduced pH from alkaline to relative neutral. Besides, heavy metal bioavailability was significantly decreased that the acid soluble fraction decreased by 1.7%-11.5%, resulting in the reduction of heavy metal concentration in F.elata plant. Amendments also increased the rhizosphere tailing microbial species richness and the relative abundance of ecologically beneficial genera including Arthrobacter, Altererythrobacter and Bacillus. This study not only provided a green and efficient method for remediation of oligotrophic and high heavy metal contaminated nonferrous metallic tailing, but also demonstrated relevant mechanisms of amendment on promoting soilless revegetation.

Adsorption and migration of heavy metals between sediments and overlying water in the Xinhe River in central China

Long-term polluted rivers often lead to the accumulation of heavy metals in sediments. Anthropogenic activities or biological disturbances break the adsorption balance, causing them to return from the bottom mud to the overlying water and change the aquatic environment. In order to understand the variation of heavy metals between sediments and river water, we collected the riverbed sediments in the polluted Xinhe River and carried out static continuous infiltration and dynamic uninterrupted disturbance experiments. The leaching experiment shows that the absorbability of Cd and Pb is stronger than Cr in the sediment; at the same time, the properties of the medium have a great influence on the adsorption of heavy metals. The disturbance can prompt heavy metals in the sediment to resuspend into the overlying water. The impact is the greatest during the first 12 h, and the influence degree is stronger in the relatively static water than in the moving river. In addition, pH and other factors have different degrees of influence on the desorption of heavy metals.

Concentrations, distribution, and risk assessment of heavy metals in the iron tailings of Yeshan National Mine Park in Nanjing, China

For the first time, a field survey was conducted to investigate the present situation, vertical distribution and ecological risks of heavy metals (Cu, Zn, Mn, Cr, Pb and As) from 21 in-situ samples drilled out from Yeshan iron mine tailings in the Jiangsu Province of China. The heavy metal contents obtained for the tailing wastes in decreasing order were as follows: Mn > Cu > Zn > As > Cr > Pb. The contents of heavy metals varied with depth, and the variation trends were not completely consistent. Vertical distribution profiles showed that heavy metals accumulated in certain strata. Both the monomial potential ecological risk factor (E) and the risk quotient (RQ) showed a high ecological risk for Cu, Mn and As. The comprehensive ecological risk index (RI) also indicated that the wastes presented a high ecological risk level, to which Cu, Mn and As were the key contributors. Our study showed that the health of individuals, especially children, living in the mining-impacted areas could be affected by the potential noncarcinogenic risk of copper, manganese and the carcinogenic risk of arsenic.

Mechanical properties of cemented tailings backfill containing alkalized rice straw of various lengths

The tailings and rice straw that are produced in large quantities each year in the mining and agricultural industries, respectively, have significant effects on the ecological environment. This study aimed to explore the mechanical properties of cemented tailings backfill (CTB) mixed with alkalized rice straw (ARS) of different lengths. A series of uniaxial compressive strength (UCS) and indirect tensile strength (ITS) tests were conducted on the CTB. The results indicated that as the length of the ARS increased from 3 to 15 mm, the UCS and ITS values initially increased and then decreased. The critical length of the ARS was 12 mm, for which the effect of strength increase was the most significant. From the overall analysis, the UCS of CTB samples with ARS (9 and 12 mm) demonstrated the better improvement (increased by 10.0 and 14.7%, respectively) at 28 d curing age, and the improvement effect of the CTB samples with ARS of other lengths was not ideal. The ITS of CTB samples with ARS increased (except for an ARS length of 3 mm) regardless of the curing age; the maximum increase was approximately 24.2% at 28 d. The integrity, residual strength, and toughness of CTB sample with the ARS (12 mm) were the largest after the UCS test. Scanning electron microscopy (SEM) tests indicated that the surface of the ARS was covered with cement hydration products, and the interior of the ARS was filled with cement tailings, which produced stronger adhesion between the ARS (12 mm) and CTB matrix; the ARS performed a bridging role and suppressed crack propagation, which effectively improving the mechanical properties of CTB. Significantly short ARS exhibited a lower adhesive force with the matrix, and significantly long ARS exhibited a lower filling rate. Thus, while improving the mechanical properties of CTB, ARS provides a new method for treating rice straw and decreasing its combustion pollution.

Application Research of Biochar for the Remediation of Soil Heavy Metals Contamination: A Review

Soil contamination by heavy metals threatens the quality of agricultural products and human health, so it is necessary to choose certain economic and effective remediation techniques to control the continuous deterioration of land quality. This paper is intended to present an overview on the application of biochar as an addition to the remediation of heavy-metal-contaminated soil, in terms of its preparation technologies and performance characteristics, remediation mechanisms and effects, and impacts on heavy metal bioavailability. Biochar is a carbon-neutral or carbon-negative product produced by the thermochemical transformation of plant- and animal-based biomass. Biochar shows numerous advantages in increasing soil pH value and organic carbon content, improving soil water-holding capacity, reducing the available fraction of heavy metals, increasing agricultural crop yield and inhibiting the uptake and accumulation of heavy metals. Different conditions, such as biomass type, pyrolysis temperature, heating rate and residence time are the pivotal factors governing the performance characteristics of biochar. Affected by the pH value and dissolved organic carbon and ash content of biochar, the interaction mechanisms between biochar and heavy metals mainly includes complexation, reduction, cation exchange, electrostatic attraction and precipitation. Finally, the potential risks of in-situ remediation strategy of biochar are expounded upon, which provides the directions for future research to ensure the safe production and sustainable utilization of biochar.

Pore Connectivity and Dewatering Mechanism of Tailings Bed in Raking Deep-Cone Thickener Process

Paste and thickened tailings (PTT) technology can improve the utilization and management of tailings from processing plants. The pore size distribution (PSD) and microstructure evolution affected by the rake shear in thickening tailings beds are essential to produce a high-density tailings underflow. Continuous thickening and computed tomography (CT) scanning tests were conducted to study the PSD with and without shear. The pore morphology was studied to reveal the shearing-dewatering performance of the tailings bed. The results show that at a flocculant solution concentration of 0.01 wt % and a feed slurry concentration of 10 wt%, the underflow concentration with and without shear can reach 58.5 wt %and 55.8 wt %, respectively. The CT image reconstruction models demonstrated that the porosity of the sheared tailings bed increased with the bed height. When the bed height increased from 2.5 to 10 cm, the porosity increased from 35.1% to 41.9%, the pore fractal dimension increased from the range 1.8–1.95 to the range 2.1–2.15, and the pore quantity decreased by 21.39%. The average pore volume increased with increasing height by 13.93%, 16.57% and 12.07%. The pore structure became more complex with the bed height, and the connectivity between pores increased to form water-flow channels, which were beneficial to the drainage of sealed water.