Assessing the influence of immobilization remediation of heavy metal contaminated farmland on the physical properties of soil

Impact of particle size separation on the stabilisation efficiency of heavy-metal-contaminated soil: a meta-analysis

The separation of heavy-metal-contaminated soil by particle size is crucial for minimising the volume of contaminated soil because of the pronounced variability in the heavy-metal distribution among different soil particle sizes. However, relevant analyses on the effect of soil particle size sorting on stabilisation are limited. Therefore, we screened 2766 peer-reviewed papers published from January 2010 to April 2022 in the Web of Science database, of which 117 met the screening requirements, and conducted a meta-analysis to explore how soil particle size sorting and the interaction between sorting particle size and soil properties affect the stabilisation of heavy metals. The results showed that: (1) For fractionations ≤0.15 mm and from 0.15-2 mm, the materials demonstrating the highest average unit stabilisation efficiency were phosphate (45.0%/%) and organic matter (59.5%/%), respectively. (2) The smaller the size of soil particles, the greater the effect of the initial pH on stabilisation efficiency. (3) Similarly, for soil organic matter, smaller particle sizes (≤0.15 mm) combined with a lower initial content (≤1%) significantly increased the heavy metal stabilisation efficiency. (4) The impact of soil particle size fractionation on unit stabilisation efficiency was observed to be similar for typical heavy metals, specifically Cd and Pb. The relationship between particle size and unit stabilisation efficiency shows an inverted U shape. Particle size sorting can affect the distribution of heavy metals, but the type of stabilisation agent should also be considered in combination with the soil properties and heavy metal types.

Sustainable soil rehabilitation with multiple network structures of layered double hydroxide beads: Immobilization of heavy metals, fertilizer release, and water retention

The remediation of heavy metal-contaminated soils necessitated a holistic approach that encompassed water and fertilizer conservation alongside soil property restoration. This study introduced the synthesis of (poly)acrylamide-layered double hydroxide gel spheres (PAM-LDH beads), which were designed to simultaneously immobilize heavy metals, control the release of fertilizers, and enhance soil water retention. Laboratory soil experiments under diverse conditions highlighted the superior performance of PAM-LDH beads in the immobilization of hexavalent chromium (Cr(VI)). The layered double hydroxide (LDH) component was identified as the key player in Cr(VI) immobilization, with anion exchange being the predominant mechanism. Notably, the encapsulated urea within the beads was released independently of environmental influences, governed by a concentration gradient across the beads surface. This release process was characterized by an initial phase of absorptive swelling followed by a diffusive phase. The impact on plant growth was assessed, revealing that PAM-LDH beads significantly curtailed Cr(VI) accumulation and alleviated its phytotoxic effects. Changes in the carbon (C) and nitrogen (N) content of the plants suggested that the urea encapsulated within the beads served as a nutrient source, contributing to soil fertility. Moreover, the water-holding capacity and soil-water characteristic curves of PAM-LDH beads suggested that these superabsorbent beads could delay soil water evaporation. The observed shifts in microbial community structure provided evidence for the enhancement of soil carbon and nitrogen cycles, indicative of improved soil properties.

Combined use of biochar and phosphate rocks on phosphorus and heavy metal availability: A meta-analysis

Biochar (BC) and phosphate rocks (PR) are alternative nutrient sources with multiple benefits for sustainable agriculture. The combination of these soil amendments serves two main purposes: to increase soil phosphorus (P) availability and to remediate heavy metal (HM) contamination. However, a further demonstration of the benefits and risks associated with the combined use of BC and PR (BC + PR) is needed, considering the specific characteristics of raw materials, soil types, experimental conditions, and climatic contexts. This meta-analysis is based on data from 28 selected studies, including 581 paired combinations evaluating effects on extraction and fractionation of cadmium (Cd) and lead (Pb), and 290 paired combinations for soil labile and non-labile P. The results reveal that BC, PR, and BC + PR significantly increase soil labile and non-labile P, with BC + PR showing a 150% greater increase compared to BC alone. In tropical regions, substantial increases in P levels were observed with BC, PR, and BC + PR exhibiting increments of 317, 798, and 288%, respectively. In contrast, temperate climate conditions showed lower increases, with BC, PR, and BC + PR indicating 54, 123, and 88% rises in soil P levels. Moreover, BC, PR, and BC + PR effectively reduce the bioavailability of Cd and Pb in soil, with BC + PR demonstrating the highest efficacy in immobilizing Cd. The synergistic effect of BC + PR highlights their potential for Cd remediation. BC + PR effectively reduces the exchangeable fraction of Cd and Pb in soil, leading to their immobilization in more stable forms, such as the residual fraction. This study provides valuable insights into the remediation potential and P management benefits of BC and PR, highlighting their importance for sustainable agriculture and soil remediation practices.

Utilizing machine learning algorithm for finely three-dimensional delineation of soil-groundwater contamination in a typical industrial park, North China: Importance of multisource auxiliary data

Intensive industrial activities cause soil contamination with wide variations and even perturb groundwater safety. Precision delineation of soil contamination is the foundation and precondition for soil quality assurance in the practical environmental management process. However, spatial non-stationarity phenomenon of soil contamination and heterogeneous sampling are two key issues that affect the accuracy of contamination delineation model. Taking a typical industrial park in North China as the research object, we constructed a random forest (RF) model for finely characterizing the distribution of soil contaminants using sparse-biased drilling data. Results showed that the R2 values of arsenic and 1,2-dichloroethane predicted by RF (0.8896 and 0.8973) were greatly higher than those of inverse distance weighted model (0.2848 and 0.2908), indicating that RF was more adaptable to actual non-stationarity sites. The back propagation neural network algorithm was utilized to establish a three-dimensional visualization of the contamination parcel of subsoil-groundwater system. Multiple sources of environmental data, including hydrogeological conditions, geochemical characteristics and anthropogenic industrial activities were integrated into the model to optimize the prediction accuracy. The feature importance analysis revealed that soil particle size was dominant for the migration of arsenic, while the migration of 1,2-dichloroethane highly depended on vertical permeability coefficients of the soil. Contaminants migrated downwards with soil water under gravity-driven conditions and penetrated through the subsoil to reach the saturated aquifer, forming a contamination plume with groundwater flow. Our findings afford a new idea for spatial analysis of soil-groundwater contamination at industrial sites, which will provide valuable technical support for maintaining sustainable industry.

Analysis of soil fertility and toxic metal characteristics in open-pit mining areas in northern Shaanxi

The study specifically focused on the Hongliulin mining area, where a total of 40 soil samples were meticulously collected and analyzed from within a 1000 m radius extending from the tailings dam. The findings revealed that soil pH within the 0-1000 m range generally leaned towards the alkaline side. In terms of soil nutrient content, encompassing factors such as soil organic matter (SOM), total nitrogen (TN), total phosphorus (TP), total potassium (TK), alkali nitrogen (AK), available phosphorus (AP), and quick-acting potassium (AK), the variations fell within the following ranges: 2.23-13.58 g/kg, 0.12-0.73 g/kg, 0.18-1.15 g/kg, 9.54-35.82 g/kg, 2.89-6.76 mg/kg, 3.45-11.25 mg/kg, and 5.86-130.9 mg/kg. Collectively, these values indicate relatively low levels of soil nutrients. Within the 0-500 m range of soil samples, the average concentrations of Cd, Hg, Pb, and As were 0.778, 0.198, 24.87, and 17.92 mg/kg, respectively. These concentrations exceeded the established soil background values of Shaanxi Province and emerged as the primary pollutants in the study area. Within this same range, the mean values of eight toxic metals (Pi) were ranked in the following descending order: 1.726 (Hg), 1.400 (As), 1.129 (Cr), 1.109 (Pb), 0.623 (Zn), 0.536 (Cd), 0.309 (Cu), and 0.289 (Ni). With the exception of Hg, As, Cr, and Pb, which exhibited slight pollution, the other toxic metals were found to be within acceptable pollution limits for this sampling range, in line with the results obtained using the geo-accumulation index method. The average potential ecological risk index for the eight toxic metals in the study area stood at 185.0, indicating a moderate overall pollution level. When assessing individual elements, the proportions of ecological risk attributed to Hg, As, Pb, and Cd were 34.57%, 27.44%, 25.11%, and 23.11%, respectively. This suggests that the primary potential ecological risk elements in the study area are Hg and As, followed by Cd and Pb. Notably, toxic metals Hg and Pb, as well as As and Pb, exhibited significant positive correlations within the sampling area, suggesting a common source. An analysis of the relationship between soil physicochemical properties and toxic metals indicated that soil pH, SOM, TN, and TP were closely linked to toxic metal concentrations. The toxic metal elements in the research area's soil exhibit moderate variability (0.16 < CV < 0.36) to high variability (CV > 0.36). Within the range of 0-200 m, the CV values for Cd and Hg exceed 1, indicating a high level of variability. The coefficient of variation for SOM, TP, AP, AK and TK is relatively high with the of 2.93, 2.36, 2.36, 21.01, 7.54. The soil in the sampling area has undergone significant disturbances due to human activities, resulting in toxic metal pollution and nutrient deficiencies.

A toxicokinetics approach using Enchytraeus crypticus to evaluate the efficiency of hydroxyapatite to remediate soils contaminated with rare earth elements

Extensive rare earth element (REE) mining activities pose threats to agricultural soils surrounding the mining areas. Here, low and high REE-contaminated soils from farmlands around mine tailings were remediated with hydroxyapatite. A toxicokinetic approach was applied to assess whether the use of hydroxyapatite reduced the bioavailability of REEs and thus inhibited their accumulation in the terrestrial organism Enchytraeus crypticus. Our results showed that addition of hydroxyapatite increased soil pH, DOC and anion contents. CaCl2-extractable REE concentrations significantly decreased, indicating the stabilization by hydroxyapatite. The influence of hydroxyapatite on the REE accumulation in enchytraeids was quantified by fitting a toxicokinetic model to dynamic REE body concentrations. The estimated uptake (Ku) and elimination rate constants (Ke), and bioaccumulation factor (BAF) for REEs were in the range of 0.000821 - 0.122 kgsoil/kgworm day-1, 0.0224 - 0.136 day-1, and 0.00135 - 1.96, respectively. Both Ku and BAF were significantly reduced by over 80% by hydroxyapatite addition, confirming the decreased REE bioavailability. Low atomic number REEs had higher BAFs in slightly contaminated soil, suggesting a higher bioaccumulation potential of light REEs in soil organisms. Overall, chemical stabilization with amendments can attenuate the bioavailability of REEs and reduce the potential ecological risk of contaminated agricultural soils near REE mining areas.

The spatial distribution, health risk, and cytotoxicity of metal(loid)s in contaminated field soils: The role of Cd in human gastric cells damage

The spatial distribution and pollution level of heavy metal(loid)s in soil (0-6 m) from a typical industrial region in Jiangmen City, Southeast China was investigated. Their bioaccessibility, health risk, and human gastric cytotoxicity in topsoil were also evaluated using an in vitro digestion/human cell model. The average concentrations of Cd (87.52 mg/kg), Co (106.9 mg/kg), and Ni (1007 mg/kg) exceeded the risk screening values. The distribution profiles of metal(loid)s showed a downward migration trend to reach a depth of 2 m. The highest contamination was found in topsoil (0-0.5 m), with the concentrations of As, Cd, Co, and Ni being 46.98, 348.28, 317.44, and 2395.60 mg/kg, respectively, while Cd showed the highest bioaccessibility in the gastric phase (72.80 %), followed by Co (21.08 %), Ni (18.27 %), and As (5.26 %) and unacceptable carcinogenic risk. Moreover, the gastric digesta of topsoil suppressed the cell viability and triggered cell apoptosis, evidenced by disruption of mitochondrial transmembrane potential and increase of Cytochrome c (Cyt c) and Caspases 3/9 mRNA expression. Bioaccessible Cd in topsoil was responsible for those adverse effects. Our data suggest the importance to reduce Cd in the soil to decrease its adverse impacts on the human stomach.

New insight into the mechanisms of preferential encapsulation of metal(loid)s by wheat phytoliths under silicon nanoparticle amendment

Silicon nanoparticles (SiNPs) have been widely used to immobilize toxic trace metal(loid)s (TTMs) in contaminated croplands. However, the effect and mechanisms of SiNP application on TTM transportation in response to phytolith formation and phytolith-encapsulated-TTM (PhytTTM) production in plants are unclear. This study demonstrates the promotion effect of SiNP amendment on phytolith development and explores the associated mechanisms of TTM encapsulation in wheat phytoliths grown on multi-TTM contaminated soil. The bioconcentration factors between organic tissues and phytoliths of As and Cr (> 1) were significantly higher than those of Cd, Pb, Zn and Cu, and about 10 % and 40 % of the total As and Cr that bioaccumulated in wheat organic tissues were encapsulated into the corresponding phytoliths under high-level SiNP treatment. These observations demonstrate that the potential interaction of plant silica with TTMs is highly variable among elements, with As and Cr being the two most strongly concentrated TTMs in the phytoliths of wheat treated with SiNPs. The qualitative and semi-quantitative analyses of the phytoliths extracted from wheat tissues suggest that the high pore space and surface area (≈ 200 m² g^-1) of phytolith particles could have contributed to the embedding of TTMs during silica gel polymerization and concentration to form PhytTTMs. The abundant SiO functional groups and high silicate-minerals in phytoliths are dominant chemical mechanisms for the preferential encapsulation of TTMs (i.e., As and Cr) by wheat phytoliths. Notably, the organic carbon and bioavailable Si of soils and the translocation of minerals from soil to plant aerial parts can impact TTM sequestration by phytoliths. Thus, this study has implications for the distribution or detoxification of TTMs in plants via preferential PhytTTM production and biogeochemical cycling of PhytTTMs in contaminated cropland following exogenous Si supplementation.

Research progress of bio-slurry remediation technology for organic contaminated soil

Bio-slurry remediation technology, as a controllable bioremediation method, has the significant advantage of high remediation efficiency and can effectively solve the problems of high energy consumption and secondary pollution of traditional organic pollution site remediation technology. To further promote the application of this technology in the remediation of organically polluted soil, this paper summarizes the importance and advantages of bio-slurry remediation technology compared with traditional soil remediation technologies (physical, chemical, and biological). It introduces the technical infrastructure and its technological processes. Then, various factors that may affect its remediation performance are discussed. By analyzing the applications of this technology to the remediation of typical organic pollutant-(polycyclic aromatic hydrocarbons(PAHs), polychlorinated biphenyls(PCBs), total petroleum hydrocarbons(TPH), and pesticide) contaminated sites, the following key features of this remediation technology are summarised: (1) the technology has a wide range of applications and can be used in a versatile way in the remediation projects of various types of organic-contaminated soil sites such as in clay, sand, and high organic matter content soil; (2) the technology is highly controllable. Adjusting environmental parameters and operational conditions, such as nutrients, organic carbon sources (bio-stimulation), inoculants (bio-augmentation), water-to-soil ratio, etc., can control the remediation process, thus improving the restoration performance. To sum up, this bio-slurry remediation technology is an efficient, controllable and green soil remediation technology that has broad application prospects.

Heavy metal stabilization remediation in polluted soils with stabilizing materials: a review

The remediation of soil contaminated by heavy metals has long been a concern of academics. This is due to the fact that heavy metals discharged into the environment as a result of natural and anthropogenic activities may have detrimental consequences for human health, the ecological environment, the economy, and society. Metal stabilization has received considerable attention and has shown to be a promising soil remediation option among the several techniques for the remediation of heavy metal-contaminated soils. This review discusses various stabilizing materials, including inorganic materials like clay minerals, phosphorus-containing materials, calcium silicon materials, metals, and metal oxides, as well as organic materials like manure, municipal solid waste, and biochar, for the remediation of heavy metal-contaminated soils. Through diverse remediation processes such as adsorption, complexation, precipitation, and redox reactions, these additives efficiently limit the biological effectiveness of heavy metals in soils. It should also be emphasized that the effectiveness of metal stabilization is influenced by soil pH, organic matter content, amendment type and dosage, heavy metal species and contamination level, and plant variety. Furthermore, a comprehensive overview of the methods for evaluating the effectiveness of heavy metal stabilization based on soil physicochemical properties, heavy metal morphology, and bioactivity has also been provided. At the same time, it is critical to assess the stability and timeliness of the heavy metals' long-term remedial effect. Finally, the priority should be on developing novel, efficient, environmentally friendly, and economically feasible stabilizing agents, as well as establishing a systematic assessment method and criteria for analyzing their long-term effects.

Microplastics distribution and microbial community characteristics of farmland soil under different mulch methods

The widespread use of plastic film in agricultural production has resulted in the accumulation of large amounts of residual plastic film in the soil, and most of the plastic residuals eventually break up into microplastics (MPs). However, the effects of different film mulching methods on the soil ecosystems are largely unexplored. Therefore, we investigated the MPs distribution and the physicochemical properties and microbial community structure in the farmland soil tillage layer covered with different mulching method of film. The results indicate that the film mulching method had no significant effect on the soil pH and organic matter content, however, the respiration intensity of the soil covered with mulching film (MF) (60.11-84.99 μg/g) and shed film (SF) (56.10-65.68 μg/g) was significantly higher than that covered with shed film & mulching film (SMF) (17.25-39.16 μg/g). The MPs abundance in the soil covered with MF (1367 particles/kg soil) was significantly higher than that covered with SF (800 particles/kg soil) and slightly higher than that with SMF (1000 particles/kg soil). The small-sized (0-0.5 mm) MPs abundance was increased with the tillage layer depth (0-20 cm), while the large-sized (1-5 mm) MPs abundance was the opposite. In addition, in the soil covered with agricultural film, the dominant phylum and genera of the bacteria were Proteobacteria (relative abundance was 64.06 %) and Pseudomonas (13.16 %), respectively. In the soil without agricultural film application as a control treatment, the diversity of the soil bacterial community was higher than that in the soil covered with agricultural film, and the relative abundances of the top 10 genera were all less than 5 %. Overall, this study provides essential information for understanding the effects of different film mulching methods on the agricultural systems. Overall, this study provides essential information for understanding the effects of different film mulching methods on the distribution of MPs and the biogeochemical properties of farmland soils.

Spatial Pattern Evolution and Influencing Factors on Agricultural Non-Point Source Pollution in Small Town Areas under the Background of Rapid Industrialization

To promote sustainable agricultural development in small town areas during rapid industrialization, it is important to study the evolution of agricultural non-point source pollution (ANSP) and its influencing factors in small town areas in the context of rapid industrialization. The non-point source inventory method was used to study the characteristics of ANSP evolution in 14 small town areas in Gongyi City from 2002 to 2019. Using the spatial Durbin model and geographical detectors, the factors influencing ANSP in small town areas were analyzed in terms of spatial spillover effects and the spatial stratified heterogeneity. The results showed a zigzagging downward trend of ANSP equivalent emissions over time. Spatially, the equivalent emissions of ANSP showed a distribution pattern of being high in the west and low in the east. There was a significant positive global spatial autocorrelation feature and there was an inverted "U-shaped" Environmental Kuznets Curve relationship between industrialization and ANSP. Affluence, population size, and cropping structure positively contributed to the reduction of ANSP. Population size, land size, and industrialization were highly influential factors affecting the spatial variation of ANSP and the interaction of these factors was bivariate or nonlinearly enhanced. This study provides a feasible reference for policymakers and managers to develop reasonable management measures to mitigate ANSP in small town areas during rapid industrialization.

Preparation of phosphorus-modified biochar for the immobilization of heavy metals in typical lead-zinc contaminated mining soil: Performance, mechanism and microbial community

The use of modified biochar for the remediation of heavy metal (HM) has received much attention. However, the immobilization mechanism of biochar to multiple HMs and the interaction of different forms of HMs with microorganisms are still unclear. K₂HPO₄-modified biochar (PBC) was produced and used in a 90-days immobilization experiment with soil collected from a typic lead-zinc (Pb-Zn) mining soil. Incubation experiments showed that PBC enhanced the transformation of Cd, Pb, Zn and Cu from exchangeable (Ex-) and/or carbonate-bound forms (Car-) to organic matter-bound (Or-) and/or residual forms (Re-). After scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDS), X-ray diffractometry (XRD), fourier transform infrared (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) analysis, the mechanisms of HM immobilization by PBC were proposed as precipitation (PO₄^3-, HPO₄^2-, OH^- and CO₃^2-), electrostatic attraction, complexation (-COOH, -OH and R-O-H) and the indirect roles of soil parameter variations (pH, moisture and microbial community). Microbial community analysis through high-throughput sequencing showed that PBC reduced bacterial and fungal abundance. However, addition of PBC increased the relative proportions of Proteobacteria by 15.04%-42.99%, Actinobacteria by 4.74%-22.04%, Firmicutes by 0.76%-23.35%, Bacteroidota by 0.16%-12.34%, Mortierellomycota by 4.00%-9.66% and Chytridiomycota by 0.10%-13.7%. Ex-Cd/Pb/Zn, Car-Cd/Zn and Re-Cd/Pb/As were significantly positively (0.001<P≤0.05) correlated with bacterial phyla of Crenarchaeota and Methylomirabilota, and Re-Cu and Ex-/Car-/Fe-Mn oxide-bound (Fe-Mn-)/Or-As were significantly positively correlated (0.001<P≤0.05) with the bacterial phyla of Proteobacteria and Bacteroidota. While Car-Cd/Zn and Re-Pb/As were positively correlated (0.01<P≤0.05) with fungal phyla of Ascomycota, Glomeromycota, Kickxellomycota, Basidiomycota and Mucoromycota. The bacterial network contained more complex interactions than the fungal network, suggesting that bacteria play a larger role in HMs transformation processes. The results indicate that PBC is an effective agent for the remediation of HMs polluted soil in Pb-Zn mining areas.

Initial Studies on the Effect of the Rice-Duck-Crayfish Ecological Co-Culture System on Physical, Chemical, and Microbiological Properties of Soils: A Field Case Study in Chaohu Lake Basin, Southeast China

Rice-duck and rice-crayfish co-culture patterns can increase soil productivity and sustainability and reduce the use of chemical pesticides and fertilizers, thereby reducing the resulting negative environmental impacts. However, most studies have focused on the rice-duck and rice-crayfish binary patterns and have ignored integrated systems (three or more), which may have unexpected synergistic effects. To test these effects, a paddy field experiment was carried out in the Chaohu Lake Basin, Hefei city, Southeast China. Four groups, including a rice-duck-crayfish ecological co-culture system (RDC), idle field (CK), single-season rice planting system (SSR), and double-season rice planting system (DSR), were established in this study. The results showed that the RDC improved the soil physical properties, fertility, humus content, and enzyme activity. In the RDC system, the soil total nitrogen content ranged from 8.54% to 28.37% higher than other systems in the 0-10 cm soil layer. Similar increases were found for soil total phosphorus (8.22-30.53%), available nitrogen (6.93-22.72%), organic matter (18.24-41.54%), urease activity (16.67-71.51%), and acid phosphatase activity (23.41-66.20%). Relative to the SSR treatment, the RDC treatment reduced the total losses of nitrogen and phosphorus runoff by 24.30% and 10.29%, respectively. The RDC also did not cause any harm to the soil in terms of heavy metal pollution. Furthermore, the RDC improved the yield and quality of rice, farmer incomes, and eco-environmental profits. In general, the RDC can serve as a valuable method for the management of agricultural nonpoint-source pollution in the Chaohu Lake area and the revitalization of the countryside.

Pollution, Ecological Risk and Source Identification of Heavy Metals in Sediments from the Huafei River in the Eastern Suburbs of Kaifeng, China

Rivers in urban environments are significant components of their ecosystems but remain under threat of pollution from unchecked discharges of industrial sewage and domestic wastewater. Such river pollution, particularly over the longer term involving heavy metals, is an issue of worldwide concern regarding risks to the ecological environment and human health. In this study, we investigate the long-term pollution characteristics of the Huafei River, an important urban river in Kaifeng, China. River sedimentary samples were analyzed, assessing the degree and ecological risk of heavy metal pollution using the geo-accumulation index and potential ecological risk index methods, whilst Pearson's correlation, principal component and cluster analyses were used to identify the sources of pollution. The results show that heavy metal concentrations are significantly higher than their corresponding fluvo-aquic soil background values in China, and the geo-accumulation indexes indicate that of the eight heavy metals identified, Hg is most prevalent, followed in sequence by Cd > Zn > Cu > Pb > Ni > As > Cr. The potential ecological risk index of the Huafei River is very high, with the potential ecological risk intensity highest in the midstream and downstream sections, where it is recommended that pollution control is carried out, especially concerning Hg and Cd. Long-term sequence analysis indicates that Cu and Pb dropped sharply from 1998 to 2017, but rebounded in 2019, and that Zn shows a continuous decreasing trend. Four main sources for the heavy metal contaminants were identified: Cr, Cu, Ni, Pb, Zn and Hg derived mainly from industrial activities, traffic sources and natural sources; Cd originated mainly from industrial and agricultural activities; whilst As was mainly associated with industrial activities. Thus, special attention should be paid to Hg and Cd, and measures must be taken to prevent further anthropogenic influence on heavy metal pollution.

Diversity and composition of soil bacteria between abandoned and selective-farming farmlands in an antimony mining area

Background and aims

Land abandonment and selective farming are two common management methods to restore the soil conditions of low-pollution farmland in mining areas. The soil bacterial community plays an important role in farmland soil restoration; however, few studies have compared the composition and diversity of soil bacteria between the abandoned farmlands (AFS) and selective-farming farmlands (FFS). Here, the effects of AFS and FFS on soil properties and bacterial diversity were evaluated in an antimony (Sb) mining area in southern China. This study aimed to identify effective land management methods in terms of positive or negative changes in soil environment and bacterial diversity.

Methods

16S rRNA high-throughput sequencing was used to compare the diversity and composition of soil bacteria between AFS and FFS in the Xikuangshan (the largest Sb mine in the world).

Results

Compared to AFS, FFS had higher Sb concentration and nutritional properties (e.g., available N, P, and K) and lower Zn concentration (p < 0.05). The bacterial alpha diversity including Chao1 index, Simpson index, Shannon index and Pielou₋e index in FFS was higher than AFS (p < 0.05). At the phylum level, FFS had higher relative abundances of Chloroflexi, Acidobacteria, Gemmatimonadetes, and Rokubacteria, and lower relative abundances of Firmicutes, Actinobacteria, and Bacteroidetes. At the genus level, FFS had higher relative abundances of Acidothermus, Haliangium, and Rokubacteriales, and lower relative abundances of Bacillus, Rhodococcus, Sphingomonas, and 67-14. Redundancy analysis indicated that soil heavy metal content and soil fertility were closely correlated with the soil bacterial community. Altogether, selective farming of low-pollution farmland in the mining area can improve soil properties and soil bacterial diversity.

Remediation for trace metals in polluted soils by turfgrass assisted with chemical reagents

Trace metal pollution in soils is one of the universal environmental problems in the world. Phytoremediation is a green, safe, ecological, and economic method to achieve continuous reduction of soil pollutants. Turfgrass is a plant with great landscape value and has considerable biomass when used for remediation of trace metal contaminated soil. However, its remediation ability needs to be improved in future application. The combined application of turfgrass, citric acid (CA) and auxin (gibberellin, GA₃) were applied in the phytoremediation of an artificial nutritive soil derived from sludge, and a field scale orthogonal experiment (L₉) was conducted to understand the interaction effect and obtain the optimum phytoremediation. Experimental results showed that the types and cultural patterns of turfgrass mainly determined plant height, root length and trace metal concentration in turfgrass, however CA treatment was prone to increase the aboveground biomass and the concentrations of most trace metals in turfgrasses, especially the concentration of Ni in turfgrass. GA₃ spraying significantly increased the concentration of Cd in turfgrass. The culture patterns of turfgrass played 42.4% influence on acid-extractable Cd, while CA applying had 53.8% influence on the acid-extractable Ni. The annual phytoextraction amount of trace metals based on five mowing a year were proposed to assess the remediation ability of treatments, which of the combination treatment (T3, intercropping Zoysia matrella and Lolium perenne, and applying 400 mg kg^-1 CA and 30 mg kg^-1 GA₃) were 1.6-2.1 times higher CK group. This research provides technical reference for intercropping turfgrass for remediation of trace metals in sludge-derived nutritive soil.

Recent advances in nanoremediation: Carving sustainable solution to clean-up polluted agriculture soils

Agriculture is one of the foremost significant human activities, which symbolizes the key source for food, fuel and fibers. This activity results in a lot of ecological harms particularly with the excessive usage of chemical fertilizers and pesticides. Different agricultural practices have remained industrialized to advance food production, due to the growth in the world population and to meet the food demand through the routine use of more effective fertilizers and pesticides. Soil is intensely embellished by environmental contamination and it can be stated as "universal incline." Soil pollution usually occurs from sewage wastes, accidental discharges or as byproducts of chemical residues of unrestrained production of numerous materials. Soil pollution with hazardous materials alters the physical, chemical, and biological properties, causing undesirable changes in soil fertility and ecosystem. Engineered nanomaterials offer various solutions for remediation of contaminated soils. Engineered nanomaterial-enable technologies are able to prevent the uncontrolled release of harmful materials into the environment along with capabilities to combat soil and groundwater borne pollutants. Currently, nanobiotechnology signifies a hopeful attitude to advance agronomic production and remediate polluted soils. Studies have outlined the way of nanomaterial applications to restore the eminence of the environment and assist the detection of polluted sites, along with potential remedies. This review focuses on the latest developments in agricultural nanobiotechnology and the tools developed to combat soil or land and or terrestrial pollution, as well as the benefits of using these tools to increase soil fertility and reduce potential toxicity.

Spatial distribution of toxic metal(loid)s at an abandoned zinc smelting site, Southern China

Toxic metal(loid) (TM) soil pollution at large-scale non-ferrous metal smelting contaminated sites is of great concern in China, but there are no detailed reports relating to them. A comprehensive study was conducted to determine contamination characteristics and horizontal and vertical spatial distribution patterns of soils at an abandoned zinc smelting site in Southern China. The spatial distribution of TMs revealed that soil environmental quality was seriously threatened, with Cd, Zn, As, Pb and Hg being the main contaminants present. The distribution of all TMs showed strong spatial heterogeneity and were expressed as a "patchy aggregation" pattern due to strong anthropogenic and production activities. Vertical migration of TMs indicated that the pollutants were mainly concentrated in the fill layers. Different contaminants had various migration depths, with migration occurring as: Cd > Hg > As > Zn > Pb> Cu> Mn> Sb. Analysis of their spatial variability showed that As, Pb, Cd and Hg had strong regional spatial variability. This research provides a new approach to comprehensively analyze TM pollution characteristics of non-ferrous smelting sites. It provides valuable information for guiding post-remediation strategies at abandoned non-ferrous metal smelting sites.

Effect of site hydrological conditions and soil aggregate sizes on the stabilization of heavy metals (Cu, Ni, Pb, Zn) by biochar

Biochar is a popular material that would effectively immobilize heavy metals in soil, which can greatly decrease the health risk of heavy metals. Although many previous studies have studied the immobilization of heavy metals by biochar, the influence of hydrological conditions on the immobilization effect is still not clear. This paper carried out column experiments to study the effect of fluctuating groundwater table on Cu, Ni, Pb, Zn distribution and speciation with the addition of biochar from pyrolysis of swine manure. Experimental results showed that biochar could significantly decrease the leaching toxicity of Cu and Ni by 24.4% and 44.7% respectively, while the immobilization effect of Pb and Zn was relatively insignificant. The average reduction percentage of bioavailable Cu was 14.5%, 39.5% and 33.3% in the unsaturated zone, fluctuating zone and saturated zone respectively, showing the better immobilization effect in the fluctuating zone and saturated zone. The residual fraction of heavy metals increased significantly after the addition of biochar, and the increase of residual fraction was larger in small soil aggregates. This study helped illustrate the influence of hydrological conditions and soil aggregate sizes on the stabilization effect of heavy metals by biochar, which could be used to guide the remediation process of sites contaminated by heavy metals.

Evaluation of potential ecological risks in potential toxic elements contaminated agricultural soils: Correlations between soil contamination and polymetallic mining activity

Extensive mineral exploitation activities in history have aggravated potential toxic elements (PTEs) contamination in agricultural soils in China. Comprehensive ecological risk assessment is of great significance to orientate the restoration of contaminated soils, especially for those with high background values and multiple sources. The study area is located in the major rice producing area of China. Historically, there was a silver mine and a lead-zinc mine in the area, which were successively closed during the investigation. The intensive mining activities caused serious PTEs pollution in the agricultural soils around the mining area. In this study, five PTEs (As, Cd, Cr, Hg and Pb) selected to assessed the potential of geoaccumulation index in assessing agricultural soil potential risk assessment by identifying ecological risk sources. 315 of soil samples collected in 2009, 2014, 2018 were comprehensively analyzed by single pollution index evaluation (single factor index, geoaccumulation index), comprehensive evaluation (Nemerow index, potential ecological risk index) and trend analysis. Single factor index analysis showed that geoaccumulation index considered the impact of natural diagenesis of background values and human activities on the environment, ensuring high evaluation accuracy comparing to other methods used in typical complex agricultural soils. The modified potential ecological risk index revealed that the high background area did not represent high risk area, which was consistent with the implementation effect of governance measures. This study can provide important insights for policymakers and environmental engineers to quantitatively recognize the soil pollution and the effectiveness of governance based on applicable and reasonable evaluation methods.

Kosteletzkya pentacarpos: A Potential Halophyte Candidate for Phytoremediation in the Meta(loid)s Polluted Saline Soils

Kosteletzkya pentacarpos (L.) Ledebour is a perennial facultative halophyte species from the Malvacea family that grows in coastal areas with high amounts of salt. The tolerance of K. pentacarpos to the high concentration of salt (0.5–1.5% salinity range of coastal saline land) has been widely studied for decades. Nowadays, with the dramatic development of the economy and urbanization, in addition to the salt, the accumulation of mate(loid)s in coastal soil is increasing, which is threatening the survival of halophyte species as well as the balance of wetland ecosystems. Recently, the capacity of K. pentacarpos to cope with either single heavy metal stress or a combination of multiple meta(loid) toxicities was studied. Hence, this review focused on summarizing the physiological and biochemical behaviors of K. pentacarpos that has been simultaneously exposed to the combination of several meta(loid) toxicities. How the salt accumulated by K. pentacarpos impacts the response to meta(loid) stress was discussed. We conclude that as a potential candidate for phytoremediation, K. pentacarpos was able to cope with various environmental constrains such as multiple meta(loid) stresses due to its relative tolerance to meta(loid) toxicity.

Sustainable remediation of lube oil-contaminated soil by low temperature indirect thermal desorption: Removal behaviors of contaminants, physicochemical properties change and microbial community recolonization in soils

Thermal desorption is widely adopted for the remediation of organic compounds, yet is generally considered a non-green-sustainable manner owing to its energy-intensive nature and potential to deteriorate soil reuse. Here, lube oil-contaminated soils were remediated at 200-500 °C in nitrogen atmosphere, upon which removal behaviors of lube oil and physicochemical properties of soils were explored. Illumina 16S ribosomal RNA (rRNA) and 18S rRNA amplicon sequencing were employed to determine the relative abundances and diversities of bacteria and fungi in soils, respectively. The results indicated that, after heating at 350 °C for 60 min, 93% of the lube oil was reduced, with the residual lube oil concentration lower than the Chinese risk intervention values (GB 36600-2018). The weakly-alkaline, multi-phosphorus and char-rich soils after indirect thermal desorption could provide a nutrient source and favorable habitat space for living organisms, and the decomposition of minerals in soils is more conducive to the survival of organisms. Microbial species in soils after heating at 350 °C became extinct, however, microbial species after 3 days of recolonization were enough to carry out DNA extraction when these soils were exposed to natural grass land. Though the microbial richness and diversity in heated soils after 3 days of recolonization were still little lower than those in contaminated soils, Firmicutes (29.41%) and Basidiomycota (9.33%) became dominant at phyla level, while Planomicrobium (16.37%), Massilia (10.09%), Jeotgalibaca (7.91%) and Psychrobacter (6.84%) were dominant at general level, whose ecological function was more conducive to nutrient cycling and ecological resiliency. Overall, this innovative research provides a new perspective: low temperature indirect thermal desorption may also achieve a sustainable remediation, due to its energy-saving (low temperature), favorable physicochemical properties and the rapid recolonization capacity of microbial communities in heated soils.