Heavy Metal-contaminated Soils Research Articles

Effect of polyethylene microplastics on antibiotic resistance genes: A comparison based on different soil types and plant types

Microplastics (MPs) and antibiotic resistance genes (ARGs) are two types of contaminants that are widely present in the soil environment. MPs can act as carriers of microbes, facilitating the colonization and spread of ARGs and thus posing potential hazards to ecosystem safety and human health. In the present study, we explored the microbial networks and ARG distribution characteristics in different soil types (heavy metal (HM)-contaminated soil and agricultural soil planted with different plants: Bidens pilosa L., Ipomoea aquatica F., and Brassica chinensis L.) after the application of MPs and evaluated environmental factors, potential microbial hosts, and ARGs. The microbial communities in the three rhizosphere soils were closely related to each other, and the modularity of the microbial networks was greater than 0.4. Moreover, the core taxa in the microbial networks, including Actinobacteriota, Proteobacteria, and Myxococcota, were important for resisting environmental stress. The ARG resistance mechanisms were dominated by antibiotic efflux in all three rhizosphere soils. Based on the annotation results, the MP treatments induced changes in the relative abundance of microbes carrying ARGs, and the G1–5 treatment significantly increased the abundance of MuxB in Verrucomicrobia, Elusimicrobia, Actinobacteria, Planctomycetes, and Acidobacteria. Path analysis showed that changes in MP particle size and dosage may indirectly affect soil enzyme activities by changing pH, which affects microbes and ARGs. We suggest that MPs may provide surfaces for ARG accumulation, leading to ARG enrichment in plants. In conclusion, our results demonstrate that MPs, as potentially persistent pollutants, can affect different types of soil environments and that the presence of ARGs may cause substantial environmental risks.

Bacterial–fungal interactions and response to heavy metal contamination of soil in agricultural areas

IntroductionLong-term heavy metal contamination of soil affects the structure and function of microbial communities. The aim of our study was to investigate the effect of soil heavy metal contamination on microorganisms and the impact of different heavy metal pollution levels on the microbial interactions.MethodsWe collected soil samples and determined soil properties. Microbial diversity was analyzed in two groups of samples using high-throughput sequencing technology. Additionally, we constructed microbial networks to analyze microbial interactions.ResultsThe pollution load index (PLI) < 1 indicates that the area is not polluted. 1 < PLI < 2 represents moderate pollution. PLI was 1.05 and 0.14 for the heavy metal contaminated area and the uncontaminated area, respectively. Cd, Hg, Pb, Zn, and Cu were identified as the major contaminants in the contaminated area, with the contamination factors were 30.35, 11.26, 5.46, 5.19, and 2.46, respectively. The diversities and compositions of the bacterial community varied significantly between the two groups. Compared to the uncontaminated area, the co-occurrence network between bacterial and fungal species in the contaminated area was more complex. The keystone taxa of the co-occurrence network in the contaminated area were more than those in the uncontaminated area and were completely different from it.DiscussionHeavy metal concentrations played a crucial role in shaping the difference in microbial community compositions. Microorganisms adapt to long-term and moderate levels of heavy metal contamination through enhanced interactions. Bacteria resistant to heavy metal concentrations may play an important role in soils contaminated with moderate levels of heavy metals over long periods of time.

Adsorption Potential, Speciation Transformation, and Risk Assessment of Hg-, Cd-, and Pb-Contaminated Soils Using Biochar in Combination with Potassium Dihydrogen Phosphate

The objective of this study is to develop a remediation technology for composited heavy metal-contaminated soil. Biochars (BC300, BC400, and BC500) derived from corn were combined with potassium dihydrogen phosphate (KH2PO4) to immobilize and remove heavy metal ions, including mercury (Hg2+), cadmium (Cd2+), and lead (Pb2+). The adsorption kinetics of metal ions in aqueous solutions with different concentrations was tested, and the fitting effects of the two models were compared. The findings demonstrate that the joint application of biochar and KH2PO4 could markedly enhance the immobilization efficacy of Pb2+, whereas the utilization of KH2PO4 on its own exhibited a more pronounced immobilization impact on Cd2+. Furthermore, the present study underscores the shortcomings of various remediation techniques that must be taken into account when addressing heavy metal-contaminated soils. It also emphasizes the value of comprehensive remediation techniques that integrate multiple remediation agents. This study offers a novel approach and methodology for addressing the intricate and evolving challenges posed by heavy metal contamination in soil. Its practical value and potential for application are significant.

Influencing Factors, Risk Assessment, and Source Identification of Heavy Metals in Purple Soil in the Eastern Region of Guang’an City, Sichuan Province, China

Soil heavy metal contamination poses a significant threat to both environmental health and ecological safety. To investigate the influencing factors, ecological hazards, and sources analysis of heavy metals in purple soil, 27 sets of soil samples were collected from varying genetic horizons within Guang’an City, and the contents of As, Cd, Cu, Cr, Hg, Ni, Pb, and Zn were analyzed. The results indicated higher concentrations of heavy metals in soil A horizon, compared to that of C horizon. The relevance analysis indicated that the soil’s heavy metals were strongly correlated with the soil’s physicochemical properties. The enrichment factor, pollution load index, and potential risk index highlighted slightly to severely polluted levels of soil Cd and Hg, which significantly contribute to the ecological hazards posed by soil heavy metals. The potential source of heavy metals analyzed using the APCS-MLR model identified both anthropogenic inputs and natural sources as primary contributors to heavy metal presence in soils. The Cu, Cr, Ni, Pb, and Zn contents in the samples from different genetic horizons were chiefly influenced by natural sources, such as soil matrix erosion and weathering, while the concentrations of Cd and Hg were largely affected by anthropogenic activities, specifically coal combustion and agriculture. Conversely, the As content was found to be influenced by a combination of both factors. Anthropogenic activities greatly impacted soil heavy metals at various depths within the study area, thereby underscoring the importance of monitoring these heavy metals. The findings gained from this research can give a scientific basis for the potential utilization of purple soil.

Effect of voltage gradients on EK-PRB remediation: Experimental and molecular dynamics simulations

Electrokinetic-Permeable Reaction Barrier (EK-PRB) coupled remediation technology can effectively treat heavy metal-contaminated soil near coal mines. This study was conducted on cadmium (Cd), a widely present element in the soil of the mining area. To investigate the impact of the voltage gradient on the remediation effect of EK-PRB, the changes in current, power consumption, pH, and Cd concentration content during the macroscopic experiment were analyzed. A three-dimensional visualized kaolinite-heavy metal-water simulation system was constructed and combined with the Molecular Dynamics (MD) simulations to elucidate the migration mechanism and binding active sites of Cd on the kaolinite (001) crystalline surface at the microscopic scale. The results showed that the voltage gradient positively correlates with the current, power consumption, and Cd concentration during EK-PRB remediation, and the average removal efficiency increases non-linearly with increasing voltage gradient. Considering power consumption, average removal efficiency, and cost-effectiveness, the voltage range is between 1.5 and 3.0 V/cm, with 2.5 V/cm being the optimal value. The results of MD simulations and experiments correspond to each other. Cd2+ formed a highly stable adsorption structure in contrast to the Al–O sheet on the kaolinite (001) crystalline surface. The mean square displacement (MSD) curve of Cd2+ under the electric field exhibits anisotropy, the total diffusion coefficient DTotal increases and the Cd2+ migration rate accelerates. The electric field influences the microstructure of Cd2+ complexes. With the enhancement of the voltage gradient, the complexation between Cd2+ and water molecules is enhanced, and the interaction between Cd2+ and Cl− in solution is weakened.

Phytostabilization of Heavy Metals and Fungal Community Response in Manganese Slag under the Mediation of Soil Amendments and Plants

The addition of soil amendments and plants in heavy metal-contaminated soil can result in a significant impact on physicochemical properties, microbial communities and heavy metal distribution, but the specific mechanisms remain to be explored. In this study, Koelreuteria paniculata was used as a test plant, spent mushroom compost (SMC) and attapulgite (ATP) were used as amendments, and manganese slag was used as a substrate. CK (100% slag), M0 (90% slag + 5% SMC + 5% ATP) and M1 (90% slag + 5% SMC + 5% ATP, planting K. paniculata) groups were assessed in a pilot-scale experiment to explore their different impacts on phytoremediation. The results indicated that adding the amendments significantly improved the pH of the manganese slag, enhancing and maintaining its fertility and water retention. Adding the amendments and planting K. paniculata (M1) significantly reduced the bioavailability and migration of heavy metals (HMs). The loss of Mn, Pb and Zn via runoff decreased by 15.7%, 8.4% and 10.2%, respectively, compared to CK. K. paniculata recruited and enriched beneficial fungi, inhibited pathogenic fungi, and a more stable fungal community was built. This significantly improved the soil quality, promoted plant growth and mitigated heavy metal toxicity. In conclusion, this study demonstrated that the addition of SMC-ATP and planting K. paniculata showed a good phytostabilization effect in the manganese slag and further revealed the response process of the fungal community in phytoremediation.

Effective immobilization and biosafety assessment of antimony in soil with zeolite-supported nanoscale zero-valent iron

Antimony (Sb) contamination in certain areas caused by activities such as antimony mining and smelting poses significant risks to human health and ecosystems. In this study, a stable composite material consisting of natural zeolite-supported nanoscale zero-valent iron (Z-ZVI) was successfully prepared. The immobilization effect of Z-ZVI on Sb in contaminated soil was investigated. Experimental results showed that Z-ZVI exhibited superior performance compared to pure nano zero-valent iron (nZVI) in terms of stability, with a lower zeta potential (−25.16 mV) at a pH of 7 and a higher specific surface area (54.54 m2/g). It can be easily applied and dispersed in contaminated soils. Additionally, Z-ZVI demonstrated a more abundant porous structure. After 60 days of treatment with 3% Z-ZVI, the leaching concentration of Sb in the contaminated soil decreased from 1.32 mg/L to 0.31 mg/L (a reduction of 76%), and the concentration of available Sb species decreased from 19.84 mg/kg to 0.71 mg/kg, achieving a fixation efficiency of up to 90%. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis confirmed the effective immobilization of Sb in the soil through reduction of antimonate to antimonite, precipitation, and adsorption processes facilitated by Z-ZVI. Moreover, the addition of Z-ZVI effectively reduced the bioavailability of Sb in the contaminated soil, thereby mitigating its toxicity to earthworms. In conclusion, Z-ZVI can be utilized as a promising material for the safe remediation and antimony and other heavy metal-contaminated soils.

The Hotspots and Trends of Patented Technologies for Heavy Metal-Contaminated Soil Remediation: A Systematic Review

Heavy metal soil pollution severely threatens human health and food safety. This study used PRISMA to systematically review heavy metal-contaminated soil remediation patents in the Derwent Patent Database from 2003 to 2023. A total of 1744 patents were selected. The results of the analysis show that related patent applications are growing around the world. Among them, China has the most significant number of patents, but the layout of transnational patents needs to be revised. Countries have different preferences in transnational patent technology. Technological development is generally balanced, and there is no apparent monopoly. However, the need for continuous in-depth research on inventors is an obstacle to technological development. In addition, the technology in this field is concentrated in chemistry and engineering. Currently, the mainstream technology is soil remediation agents, and thermal desorption technology has also attracted much attention. Future technologies will use new polymer materials and advanced machinery to improve efficiency and control repair costs. In addition, remediation has shifted from the total amount of heavy metals to the control of practical parts. This study summarizes the current status of heavy metal-contaminated soil remediation technology and analyzes future development trends, providing a reference for technology development.

Nitrogen-Rich Sewage Sludge Mineralized Quickly, Improving Lettuce Nutrition and Yield, with Reduced Risk of Heavy Metal Contamination of Soil and Plant Tissues

Sewage sludge should primarily find use in agriculture, reducing the quantity directed towards alternative disposal methods like incineration or deposition in municipal landfills. This study evaluated the agronomic value and the risk of soil and plant tissue contamination with heavy metals in sewage sludge obtained from two wastewater treatment plants (WWTP). The experiment was arranged as a 2 × 5 factorial (two sewage sludges, five sanitation treatments), involving lettuce cultivation in pots over two growing cycles. The two sewage sludges were sourced from the WWTPs of Gelfa and Viana do Castelo and underwent five sanitation and stabilization treatments (40% and 20% calcium oxide, 40% and 20% calcium hydroxide, and untreated sewage sludge). The Gelfa sewage sludge, characterized by a higher initial nitrogen (N) concentration, resulted in greater dry-matter yield (DMY) (12.4 and 8.6 g plant−1 for the first and second growing cycles, respectively) compared to that from Viana do Castelo (11.0 and 8.1 g plant−1), with N release likely being a major factor influencing crop productivity. The high N concentration and the low carbon (C)/N ratio of sewage sludge led to rapid mineralization of the organic substrate, which additionally led to a higher release of other important nutrients, such as phosphorus (P) and boron (B), making them available for plant uptake. Alkalizing treatments further stimulated sewage sludge mineralization, increasing soil pH and exchangeable calcium (Ca), thereby enhancing Ca availability for plants, and indicating a preference for use in acidic soils. Cationic micronutrients were minimally affected by the sewage sludge and their treatments. The concentrations of heavy metals in the sewage sludge, soils, and lettuce tissues were all below internationally established threshold limits. This study highlighted the high fertilizing value of these sewage sludges, supplying N, P, and B to plants, while demonstrating a low risk of environmental contamination with heavy metals. Nevertheless, the safe use of sewage sludge by farmers depends on monitoring other risks, such as toxic organic compounds, which were not evaluated in this study.

Feasibility of Microbially Induced Carbonate Precipitation to Enhance the Internal Stability of Loess under Zn-Contaminated Seepage Conditions

Loess is widely distributed in Northwestern China and serves as the preferred engineering construction material for anti-fouling barriers. Heavy metal contamination in soil presents significant challenges to the engineering safety of vulnerable loess structures. Hence, there is an urgent need to investigate the impact of heavy metal ions on their percolation performance. In order to investigate the effectiveness of microbially induced carbonate precipitation (MICP) using Sporosarcina pasturii (CGMCC1.3687) bacteria in reducing internal seepage erosion, a saturated permeability test was conducted on reshaped loess under constant water head saturation conditions. The response of loess to deionized water (DW) and ZnCl2 solution seepages was analyzed by monitoring changes in cation concentration over time, measuring Zeta potential, and using scanning electron microscopy (SEM). The results indicate that the hydrolysis of Zn2+ creates an acidic environment, leading to the dissolution of carbonate minerals in the loess, which enhances its permeability. The adsorption of Zn2+ ions and the resulting diffusion double-layer (DDL) effect reduce the thickness of the diffusion layer and increase the number of free water channels. Additionally, the permeability of loess exposed to ZnCl2 solution seepage significantly increased by 554.5% compared to loess exposed to deionized water (DW) seepage. Following the seepage of ZnCl2 solutions, changes in micropore area ratio were observed, decreasing by 48.80%, while mesopore areas increased by 23.9%. MICP treatment helps reduce erosion and volume shrinkage in contaminated loess. Carbonate precipitation enhances the erosion resistance of contaminated loess by absorbing or coating fine particles and creating bridging connections with coarse particles. These research results offer new perspectives on enhancing the seepage properties of saturated loess in the presence of heavy metal erosion and the geochemical mechanisms involved.

Transcriptomic and physiological analyses of Symphytum officinale L. in response to multiple heavy metal stress

Soil heavy metal contamination has become a global environmental issue, which threaten soil quality, food security and human health. Symphytum officinale L. have exhibited high tolerance and restoration capacity to heavy metals (HMs) stress. However, little is known about the mechanisms of HMs in S. officinale. In this study, transcriptomic and physiological changes of S. officinale response to different HMs (Pb, Cd and Zn) were analyzed and investigated the key genes and pathways involved in HMs uptake patterns. The results showed that phenotypic effects are not significant, and antioxidant enzyme activities were all upregulated. Transcriptome analysis indicated that 1247 differential genes were up-regulated, and 1963 differential genes were down-regulated under Cd stress, while 3752 differential genes were up-regulated, and 7197 differential genes were down-regulated under Pb stress; and 527 differential genes were up-regulated; and 722 differential genes were down-regulated under Zn stress. Based on their expression, we preliminarily speculate that different HMs resistance of S. officinale may be regulated by the differential expression of key genes. These results provide a theoretical basis for determining the exact expression of genes in plants under different heavy metal stress, the processes involved molecular pathways, and how they can be efficiently utilized to improve plant tolerance to toxic metals and improve phytoremediation efficiency.

Determination of Heavy Metal Sources in an Agricultural Catchment (Poland) Using the Fingerprinting Method

This study investigates the heavy metal contamination of soils and suspended sediments in the Nurzec River catchment (Eastern Poland), focusing in particular on the effects of catchment land use. The fingerprinting technique has been combined with the classic, Igeo, and EF index-supported contamination analysis to identify heavy metals sources. A wide range of elements (31 chemical elements including heavy metals) allowed the author to analyse the pathways and to identify the pressures of heavy metal contamination. The developed statistical models of heavy metal source distribution displayed results at a statistically significant level. The results have revealed the significant impact of land use connected with urban areas (URBAN) and pastures (PAST), which together constitute less than 20% of the river catchment area. These results are relevant to the local authorities and stakeholders, as they highlight the significant impact of low-density urban areas that are not locally considered as the major sources of heavy metal pollution. These results will contribute to sustainable decisions in the field of contaminated catchment area remediation.

Improving Lead Phytoremediation Using Endophytic Bacteria Isolated from the Pioneer Plant Ageratina adenophora (Spreng.) from a Mining Area.

This study aimed to isolate and characterise endophytic bacteria from the pioneer plant Ageratina adenophora in a mining area. Seven strains of metal-resistant endophytic bacteria that belong to five genera were isolated from the roots of A. adenophora. These strains exhibited various plant growth-promoting (PGP) capabilities. Sphingomonas sp. ZYG-4, which exhibited the ability to secrete indoleacetic acid (IAA; 53.2 ± 8.3 mg·L-1), solubilize insoluble inorganic phosphates (Phosphate solubilization; 11.2 ± 2.9 mg·L-1), and regulate root ethylene levels (1-aminocyclopropane-1-carboxylic acid deaminase activity; 2.87 ± 0.19 µM α-KB·mg-1·h-1), had the highest PGP potential. Therefore, Sphingomonas sp. ZYG-4 was used in a pot experiment to study its effect on the biomass and Pb uptake of both host (Ageratina adenophora) and non-host (Dysphania ambrosioides) plants. Compared to the uninoculated control, Sphingomonas sp. ZYG-4 inoculation increased the biomass of shoots and roots by 59.4% and 144.4% for A. adenophora and by 56.2% and 57.1% for D. ambrosioides, respectively. In addition, Sphingomonas sp. ZYG-4 inoculation enhanced Pb accumulation in the shoot and root by 268.9% and 1187.3% for A. adenophora, and by 163.1% and 343.8% for D. ambrosioides, respectively, compared to plants without bacterial inoculation. Our research indicates that endophytic bacteria are promising candidates for enhancing plant growth and facilitating microbe-assisted phytoremediation in heavy metal-contaminated soil.

Geospatial evaluation and bio-remediation of heavy metal-contaminated soils in arid zones

Introduction: Soil pollution directly impacts food quality and the lives of both humans and animals. The concentration of heavy metals in Egypt’s drain-side soils is rising, which is detrimental to the quality of the soil and crops. The key to reducing the detrimental effects on the ecosystem is having accurate maps of the spatial distribution of heavy metals and the subsequent use of environmentally sustainable remediation approaches. The objective of this work is to assess soil contamination utilizing spatial mapping of heavy metals, determine contamination levels using Principal Component Analysis (PCA), and calculate both the contamination severity and the potential for bioremediation in the soils surrounding the main drain of Bahr El-Baqar. Furthermore, evaluating the capacity of microorganisms (bacteria, fungi, and “Actinomycetes) to degrade heavy elements in the soil.Methodology: 146 soil sample locations were randomly selected near the Bahr El-Baqar drain to examine the degree of soil pollution Ordinary Kriging (OK), method was used to map and analyze the spatial distribution of soil contamination by seven heavy metals (Cr, Fe, Zn, Cd, Pb, As, and Ni). Modified contamination degree (mCd) and PCA were used to assess the research area’s soil pollution levels. The process involved isolating, identifying, and classifying the microorganisms present in the soil of the study area. The study findings showed that variography suggested the Stable model effectively matched pH, SOM, and Cd values. Furthermore, the exponential model proved suitable for predicting Fe, Pb and Ni, while the spherical model was appropriate for Ni, Cr, and Zn.Results: The study revealed three levels of contamination, with an extremely high degree (EHDC) affecting approximately 97.49% of the area. The EHDC exhibited average concentrations of heavy metals: 79.23 ± 17.81 for Cr, 20,014.08 ± 4545.91 for Fe, 201.31 ± 112.97 for Zn, 1.33 ± 1.37 for Cd, 40.96 ± 26.36 for Pb, 211.47 ± 13.96 for As, and 46.15 ± 9.72 for Ni. Isolation and identification of microorganisms showed a significant influence on the breakdown of both organic and inorganic pollutants in the environment. The study demonstrated exceptionally high removal efficiency for As and Cr, with a removal efficiency reached 100%, achieved by Rhizopus oryzae, Pseudomonas aeruginosa, and Bacillus thuringiensis.Conclusion: This study has designated management zones for soil contamination by mapping soil pollutants, geo-identified them, and found potential microorganisms that could significantly reduce soil pollution levels.

Effects of sulfur nanoparticles on rhizosphere microbial community changes in oilseed rape plantation soil under mercury stress

In the present study, experiments were conducted to assess the influence of nanoscale sulfur in the microbial community structure of metallophytes in Hg-contaminated rhizosphere soil for planting rapeseed. The results showed that the richness and diversity of the rhizobacteria community decreased significantly under Hg stress, but increased slightly after SNPs addition, with a reduction in the loss of Hg-sensitive microorganisms. Moreover, all changes in the relative abundances of the top ten phyla influenced by Hg treatment were reverted when subjected to Hg + SNPs treatment, except for Myxococcota and Bacteroidota. Similarly, the top five genera, whose relative abundance decreased the most under Hg alone compared to CK, increased by 19.05%–54.66% under Hg + SNPs treatment compared with Hg alone. Furthermore, the relative abundance of Sphingomonas, as one of the dominant genera for both CK and Hg + SNPs treatment, was actively correlated with plant growth. Rhizobacteria, like Pedobacter and Massilia, were significantly decreased under Hg + SNPs and were positively linked to Hg accumulation in plants. This study suggested that SNPs could create a healthier soil microecological environment by reversing the effect of Hg on the relative abundance of microorganisms, thereby assisting microorganisms to remediate heavy metal-contaminated soil and reduce the stress of heavy metals on plants.

The Effects of Mine Waste and Rain Water on the Structure of Bacteria Community of Paddy Soils

ABSTRACT Heavy metal contamination of soil has become one of the most important environmental problems worldwide. Heavy metals can affect growth and lead to changes in microbial community composition, which could influence the growth of crops in the soil. In this study, we conducted an experiment to investigate the distribution profile of heavy metals and bacterial communities in soil with the effects of slag and/or water on the top of the soil. Extra heavy metals, acting as selective pressure, led to the loss of sensitive taxa, with tolerant or less sensitive taxa being more abundant in the community. The results showed that the addition of slag on the top of soils did not change the diversity of bacteria but influenced the relative abundance of indigenous members in the community. Water addition, however, improved the homogeneity of the environment vertically in the soil, which consequently led to a lower variation in the bacterial community. This study mimicked the scenario of tailings and abandoned mines in nature, and aimed to study the effects of mines on the soil microbial community with the influence of precipitation on the distribution of heavy metals in the soil. With the effect of precipitation, the transportation of heavy metals in the slag may influence the local bacterial community and subsequently undermine the quality of soil for agriculture. These observations provide evidence of the effects of heavy metals on the soil around abandoned mines without any treatment. Mining causes both heavy metal and water pollution, which may lead to the accumulation of heavy metals in plants and agricultural production. Therefore, it is necessary to take care of tailings with proper procedures instead of exposing them to nature.

Genomic characterization of a novel ureolytic bacteria, Lysinibacillus capsici TSBLM, and its application to the remediation of acidic heavy metal-contaminated soil

Soil heavy metal contamination is an essential challenge in ecological and environmental management, especially for acidic soils. Microbially induced carbonate precipitation (MICP) is an effective and environmentally friendly remediation technology for heavy metal contaminated sites, and one of the key factors for its realization lies in the microorganisms. In this study, Lysinibacillus capsici TSBLM was isolated from heavy metal contaminated soil around a gold mine, and inferred to be a novel ureolytic bacteria after phylogenomic inference and genome characterization. The urease of L. capsici TSBLM was analyzed by genetic analysis and molecular docking, and further applied this bacteria to the remediation of Cu and Pb in solution and acidic soils to investigate its biomineralization mechanism and practical application. The results revealed L. capsici TSBLM possessed a comprehensive urease gene cluster ureABCEFGD, and the encoded urease docked with urea at the lowest binding energy site (ΔG = −3.43 kcal/mol) connected to three amino acids threonine, aspartic, and alanine. The urease of L. capsici TSBLM is synthesized intracellularly but mainly functions extracellularly. L. capsici TSBLM removes Cu/Pb from the solution by generating heavy metal carbonates or co-precipitating with CaCO3 vaterite. For acidic heavy metal-contaminated soil, the carbonate-bound states of Cu and Pb increased significantly from 7 % to 16 % and from 23 % to 35 % after 30 days by L. capsici TSBLM. Soil pH improved additionally. L. capsici TSBLM maintained the dominant status in the remediated soil after 30 days, demonstrating good environmental adaptability and curing persistence. The results provided new strain resources and practical application references for the remediation of acidic heavy metal contaminated soil based on MICP.

Does livestock-Manure-Derived Biochar Suitable for the Stabilization of Cadmium and Zinc in Contaminated Soil?

Both livestock-manure and livestock-manure-derived biochar have been used to remediate heavy metal-contaminated soil. However, direct comparisons of the heavy metal stabilization efficiency of livestock-manure and EQC-manure-biochar (derived from an equal quantity of corresponding livestock-manure) are limited. In the present study, the effect of livestock-manures and EQC-manure-biochars on soil properties and heavy metal bioavailability and leachability were compared using two contrasting soils (Ferralsols and Fluvisols). The results showed that both the livestock-manures and EQC-manure-biochars significantly changed soil pH, available phosphorus, available potassium, alkaline nitrogen and organic matter content (p < 0.05), but the trends were variable. In Ferralsols, the DTPA-extractable Cd and Zn decreased by -0.38%~5.70% and - 3.79%~9.98% with livestock-manure application and by -7.99%~7.23% and - 5.67%~7.17% with EQC-manure-biochars application. In Fluvisols, the DTPA-extractable Cd and Zn decreased by 13.39%~17.41% and - 45.26%~14.24% with livestock-manure application and by 10.76%~16.90% and - 36.38%~16.37% with EQC-manure-biochar application. Furthermore, the change in TCLP-extractable Cd and Zn in both soils was similar to that of DTPA-extractable Cd and Zn. Notably, the Cd and Zn stabilization efficiency of the EQC-manure-biochars was no better than that of the corresponding livestock-manures. These results suggest that the use of livestock-manure-derived biochar is not cost-effective for the remediation of heavy metal-contaminated soil.

Estimating the chromium concentration of farmland soils in an arid zone from hyperspectral reflectance by using partial least squares regression methods

Heavy metal contamination of farmland soils is a severe concern worldwide.Chromium (Cr), a major environmental carcinogen, is a crucial indicator of farmland soil pollution. Compared with conventional chemical analysis, the hyperspectral analysis is superior in accurately and rapidly surveilling heavy metals in soils. However, various heavy metals, and types or geochemical traits of soils often affect the accuracy of hyperspectral estimation models, which makes guaranteeing the concentration estimation precision for a designated heavy metal in different regions considerably challenging. We here propose an optimal hyperspectral model for determining the Cr concentration of farmland soils first time in the Yanqi Basin of China's northwest arid zone. In total, 171 soil samples were gathered, and their Cr concentrations and relevant spectral reflectance data were determined. The best spectral transformation for measuring the Cr concentration was determined by using 12 transformation types of soil spectral curves, and characteristic wavebands were obtained. Then, by using random forest (RF), support vector machine (SVM), and partial least squares regression (PLSR) models, we constructed hyperspectral estimation models for Cr concentration in farmland soils. The estimated results of these models were compared to identify the best estimation model. Results show that the spectral transformations allow an improvement in the association between the soil Cr concentration and spectra. The accuracy of PLSR was significantly higher than that of SVM and RF. Finally, the second-order differentiation (SD) combined with the PLSR model (R2 = 0.903, RMSE = 40.363, MAE = 30.631) was identified as the optimal model for estimating the Cr concentration of farmland soils in the investigated area. The study findings offer a technical reference for the hyperspectral assessment of the Cr concentration in arid zone farmland soils.

Lead and zinc chemical fraction alterations in multi-metal contaminated soil with pomelo peel biochar and biochar/apatite incubation

The issue of heavy metal soil contamination is widespread, and the negative effects of heavy metals on the ecosystem depend on their chemical speciation in contaminated soil. Pomelo peel-derived biochar produced at 300 °C (PPB300) and 500 °C (PPB500) and its combination with apatite ore (AP) was applied to immobilize lead (Pb) and zinc (Zn) in agricultural multi-metal polluted soil. Soil amendments including biochar (PPB300 and PPB500) at concentrations of 3%, 5%, and 10%, as well as a mixture of biochar and apatite (AP) at 3:3% and 5:5% weight ratios, were introduced into the soil matrix. The chemical forms of Pb and Zn in incubated soil samples and control soil (CS) were studied utilizing the Tessier’s sequential extraction procedure. The chemical partitioning of heavy metals was carried out using Tessier’s extraction protocol, yielding fractions representing exchangeable (F1), carbonate (F2), Fe/Mn oxide (F3), organic matter (F4), and residual (F5) forms. Quantification of Pb and Zn concentrations was accomplished via inductively coupled plasma mass spectrometry (ICP-MS). Observations revealed notable elevations in pH, organic carbon (OC), and electrical conductivity (EC) levels within the treated soil relative to the control soil (p < 0.05). After incubating PPB300, PPB500, and AP for 30 days, there was a reduction in the exchangeable fraction of Pb and Zn by approximately 64% and 58% respectively. This reduction was achieved by transforming heavy metals from mobile fractions to immobile fractions using five primary mechanisms: chemical and physical adsorption, electrostatic attraction, the formation of complexes via interactions with active groups, cation exchange processes, and precipitation events mediated by phosphate, carbonate ions, or hydroxyl ions within an alkaline environment. Therefore, pomelo peel-derived biochar and the blend of biochar/apatite show promise as materials for mitigating heavy metal pollution in soil.