The associations of heavy metals with crystalline iron oxides in the polluted soils around the mining areas in Guangdong Province, China

Cd mobilization in mining-impacted soils with different bedrock lithology: Insights from stable Cd isotopes

The environmental risk of Cd in soils strongly depends on the mobilization of Cd in soils. However, limited knowledge exists on the redistribution of exogenic Cd inputs in soils, especially across diverse lithological regions. Herein, we aimed to investigate the fate of Cd in soils from two mining areas with contrasting lithologies (siliceous and calcareous) using stable Cd isotopes. The isotope tracing results confirm that mining activities are the main Cd source in both areas. The positive correlation between δ114/110Cd values and goethite/dolomite content indicates the release of heavy Cd isotopes during the dissolution of exogenetic minerals. Additionally, high contents of exchangeable Cd (11 % to 36 %) and Fe oxide-bound Cd (29 % to 42 %) drive plant pumps to transport heavy Cd isotopes from the deeper to upper horizons of the soils from the siliceous area. In the calcareous area, the total organic carbon content is positively correlated with the Cd concentration and δ114/110Cd value, suggesting potential complexation of Cd with organic matter due to the stabilizing effect of carbonate minerals on soil organic matter. This study highlights the different redistributions of exogenous Cd in soils from diverse lithological regions, emphasizing the need to consider regional lithology when developing soil quality standards for Cd.

Soil amendments alter cadmium distribution and bacterial community structure in paddy soils

Soil amendments play a pivotal role in ensuring the safety of food production by inhibiting the transfer of heavy metal ions from soils to crops. Nevertheless, their impact on soil characteristics and the microbial community and their role in reducing cadmium (Cd) accumulation in rice remain unclear. In this study, pot experiments were conducted to investigate the effects of three soil amendments (mineral, organic, and microbial) on the distribution of Cd speciation, organic components, iron oxides, and microbial community structure. The application of soil amendments resulted in significant reductions in the soil available Cd content (16 %-51 %) and brown rice Cd content (16 %-78 %), facilitating the transformation of Cd from unstable forms (decreasing 10 %-20 %) to stable forms (increasing 77 %-150 %) in the soil. The mineral and organic amendments increased the soil cation exchange capacity (CEC) and plant-derived organic carbon (OC), respectively, leading to reduced Cd accumulation in brown rice, while the microbial amendment enhanced OC complexity and the abundances of Firmicutes and Bacteroidota, contributing to the decreased rice Cd uptake. The synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy indicated that soil amendments regulated soil Cd species by promoting iron oxides and OC coupling. Moreover, both organic and microbial amendments significantly reduced the diversity and richness of the bacterial communities and altered their compositions and structures, by increasing the relative abundances of Bacteroidota and Firmicutes and decreasing those of Acidobacteria, Actinobacteria, and Myxococcota. Soil microbiome analysis revealed that the increase of Firmicutes and Bacteroidota associated with Cd adsorption and sequestration contributed to the suppression of soil Cd reactivity. These findings offer valuable insights into the potential mechanisms by which soil amendments regulate the speciation and bioavailability of Cd, and improve the bacterial communities, thereby providing guidance for agricultural management practices.

Characteristics of iron (hydr)oxides and Cr(VI) retention mechanisms in soils from tropical and subtropical areas of China

Though both iron (hydr)oxides and soil organic matter (SOM) significantly influence heavy metal behaviors in soils, studies on the characteristics of natural minerals and the synergic effects of the two on Cr(VI) transformation are limited. This study investigated Cr(VI) retention mechanisms in four soils from tropical and subtropical regions of China based on a comprehensive characterization of Fe (hydr)oxides. These soils exhibited varying quantities of hematite, ferrihydrite and goethite, with distinct Al substitution levels and varied exposed crystallographic facets. Adsorption experiments revealed a positive correlation between Fe (hydr)oxide content and Cr(VI) fixation amount on colloid, which was influenced by the mineral types, Al substitution levels and facet exposures. Further, Cr(VI) was sequestered on soil by adsorption and reduction. In soils enriched with crystalline Fe (hydr)oxides, Cr(VI) reduction was primarily governed by SOM, while in soils enriched with poorly crystalline Fe (hydr)oxides, mineral-associated Fe(II) also contributed to Cr(VI) reduction. Aging experiments demonstrated that SOM and mineral-associated Fe(II) expedited Cr (VI) passivation and diminished the Cr leaching. These results improve our understanding of natural Fe (hydr)oxide structures and their impact on Cr(VI) behavior in soils, and shed light on complex soil-contaminant interactions and remediation of Cr(VI) polluted soils.

The Immobilization Mechanism of Inorganic Amendments on Cu and Cd in Polluted Paddy Soil in Short/Long Term

This study investigated the impact of soil colloidal characteristics on the transfer patterns of different Cu and Cd speciation in contaminated soil treated with three different amendments: lime (L), zero-valent iron (ZVI), and attapulgite (ATP). It seeks to clarify the activation hazards and aging processes of these modifications on Cu and Cd. Compared with the control (CK), the available Cu concentrations treated with amendments reduced in the short term (6 months) by 96.49%, 5.54%, and 89.78%, respectively, and Cd declined by 55.43%, 32.31%, and 93.80%, respectively. Over a 12-year period, there was no significant change in the immobile effect with L, while Cu and Cd fell by 19.06% and 40.65% with ZVI and by 7.63% and 40.78% with ATP. Short- and long-term increases in the readily reducible iron and manganese oxide fraction of Cu and Cd were accompanied by a considerable rise in the concentrations of amorphous iron oxide in the soil and colloid after amendment treatment. This suggested that Cu and Cd were immobilized and stabilized in part by amorphous iron oxide.

A critical review on the migration and transformation processes of heavy metal contamination in lead-zinc tailings of China

The health risks of lead-zinc (Pb-Zn) tailings from heavy metal (HMs) contamination have been gaining increasing public concern. The dispersal of HMs from tailings poses a substantial threat to ecosystems. Therefore, studying the mechanisms of migration and transformation of HMs in Pb-Zn tailings has significant ecological and environmental significance. Initially, this study encapsulated the distribution and contamination status of Pb-Zn tailings in China. Subsequently, we comprehensively scrutinized the mechanisms governing the migration and transformation of HMs in the Pb-Zn tailings from a geochemical perspective. This examination reveals the intricate interplay between various biotic and abiotic constituents, including environmental factors (EFs), characteristic minerals, organic flotation reagents (OFRs), and microorganisms within Pb-Zn tailings interact through a series of physical, chemical, and biological processes, leading to the formation of complexes, chelates, and aggregates involving HMs and OFRs. These interactions ultimately influence the migration and transformation of HMs. Finally, we provide an overview of contaminant migration prediction and ecological remediation in Pb-Zn tailings. In this systematic review, we identify several forthcoming research imperatives and methodologies. Specifically, understanding the dynamic mechanisms underlying the migration and transformation of HMs is challenging. These challenges encompass an exploration of the weathering processes of characteristic minerals and their interactions with HMs, the complex interplay between HMs and OFRs in Pb-Zn tailings, the effects of microbial community succession during the storage and remediation of Pb-Zn tailings, and the importance of utilizing process-based models in predicting the fate of HMs, and the potential for microbial remediation of tailings.

Zn isotope signatures in soil FeMn nodules with karst high geochemical background

Zn isotope has the potential to be used as an environmental tracer, due to its role in fingerprinting specific sources and processes. However, few studies have focused on Zn isotope system in terrestrial ferromanganese (FeMn) nodules, which is important on understanding the behaviors of Zn in soils. In this study, we analyse the isotopic composition in soil FeMn nodules and surrounding materials from a typical karst region in Guangxi Province, southwestern China and use advanced synchrotron-based methods to characterize Zn speciation. The Zn isotope compositions of the FeMn nodules range from 0.09 to 0.66 ‰, with an average value of 0.24 ‰. Pb isotope fingerprinting reveals that the major material sources contributing to the FeMn nodules are the surrounding soil (δ⁶⁶Zn: ~0.36 ‰) and partly weathered carbonate bedrock (δ⁶⁶Zn: ~0.58 ‰), which contain heavier Zn isotopes than the nodules. Synchrotron-based X-ray fluorescence (μ-SXRF) shows that Zn is well correlated with both Fe and Mn. X-ray absorption near edge spectroscopy (XANES) measurements reveal that Zn is associated with both goethite and birnessite phases, with goethite-sorbed Zn accounting for ~76 % of the total Zn and birnessite-sorbed Zn accounting for ~24 %. By combining these new results, the isotopically light Zn in the FeMn nodules compared to their sources can be explained by equilibrium sorption of Zn on goethite and birnessite, during which light Zn is preferentially sorbed. Our study provides important new data on Zn isotope compositions in terrestrial soil FeMn nodules and constrains associated mechanisms, and have implications for using Zn isotopes as environmental tracers.

Ecotoxicological monitoring of potentially toxic elements contamination in Eucalyptus forest plantation subjected to long-term irrigation with recycled wastewater

Afforestation is an evergreen technology for restraining greenhouse gases (GHGs) emission and improving soil carbon sink in arid and semi-arid regions. Nonetheless, the long-term impact of woody forests irrigation using recycled wastewater resources remains inconclusive so far. For this purpose, the ecological risk benchmarks of potentially toxic elements (PTEs) were investigated on Eucalyptus forest plantation in order to gauge their bioavailability in the rhizospheric layer of Typic Torripsammentsoil and their accretion capacity in the biosphere. Water quality guidelines pointed to a moderate degree of restriction on use with elevated levels of PTEs. Notably, concentrations of As, B, Cd, Cr, Cu, Mn, Ni, V and Zn were above the permissible limits for irrigation. The geospatial mapping of PTEs concentration in soil pointed to elevated levels of most PTEs, particularly in the deforestated areas. Some of PTEs (Cd, Cu, Hg and Zn) showed values above the permissible limits. A spectrum of ecological risk indices showed considerable to high degree of contamination. Among PTEs, the water-soluble and exchangeable fractions showed high values of As, Cd and Hg (20.7, 17.2 and 11.0%, respectively). Sequential extraction showed variations among PTEs in their tendency to bind with different soil geochemical fractions: (i) carbonate (Cd, Zn and Cu), (ii) Fe-Mn oxides (Pb, Zn and Mn) and (iii) organic matter (B, Pb and Hg). Eight fungal species including Aspergillus flavus, Fusarium solani, Cephalosporimsp., Penicilliumsp., Rhizoctonia solani, Aspergillus niger, Botrytissp. and Verticilliumsp. were dominated in soil. Meanwhile, Agrobacteriumsp., phosphate solubilizing bacteria, nitrogen fixing bacteria and Escherichia coli were the dominant bacterial strains. Values of bioaccumulation index varied among PTEs, wherein B (5.15), Ni (1.98), Mn (1.62) and Cd (1.02) exhibited higher phytoextraction potentials. Other PTEs, however, exhibited values below 1.0 confirming their low phytoextraction potentials. Findings of this investigation, therefore, provide insights into biochemical signals of PTEs contamination in woody forest plantations and the urgent need to contextualize the large-scale utilization of recycled wastewater resources in such vulnerable areas.

Elevated temperature induces contrasting transformation of exogenous copper to soil solution and solid phases in an arable soil

Heavy metal contamination of soils has been a global environmental issue over the past decades, threatening food security and human health. Understanding the migration and transformation of heavy metals in soils is critical for restoring an impaired environment and developing sustainable agriculture, particularly in the face of global warming. However, little effort has been devoted to investigating the impact of elevated temperatures on the migration and distribution of exogenous heavy metals in soils. This study experimented with a 180-day incubation at 15 °C, 30 °C, and 45 °C with an arable soil (Alfisol) of Huang-Huai-Hai River Basin, China, which was initially spiked with copper (Cu). A comparison of the results revealed that the percentage of soil water-soluble Cu doubled at 45 °C compared with 15 °C. The percentage of protein-like substances in dissolved organic matter (DOM) was the highest at 45 °C, suggesting that proteinaceous components play a more significant role in controlling the dissolution of Cu into DOM. Moreover, by sequential extraction and micro-X-ray fluorescence (μ-XRF), Cu was facilitatively transformed from exchangeable, and specifically adsorbed fractions, to iron (Fe)/manganese (Mn) oxides bound species by 7.75%23.63% with the elevation of temperature from 15 °C to 45 °C. The conversion of Cu speciation is attributed to the significant release of organic carbon from Fe/Mn oxides, especially the Mn oxide components, which are available for Cu binding. The findings of this work will provide an in-depth understanding of the fate of Cu in soils, which is fundamental for the risk assessment and remediation of Cu-polluted soils in the Huang-Huai-Hai River Basin under the context of global warming.

Predictive and estimation model of Cd, Ni, and Zn bioaccumulations in maize based on diffusive gradients in thin films

Consumption of maize contaminated with heavy metals such as cadmium, nickel, and zinc threaten human health. For situ measuring the bioavailability of heavy metals, the diffusive gradients in thin films (DGT) is superior to other traditional methods. It is also important to find a method for predicting heavy metal enrichment in maize based on the DGT method. In this study, field surveys were conducted in the main maize producing areas of Tianjin, China. Heavy metal concentrations in maize grains were predicted by coupling DGT with traditional extraction methods. The results show that coupling DGT with soil solution can significantly improve prediction accuracy (Cd-R² = 0.908, Ni-R² = 0.903, and Zn-R² = 0.904). This indicated that DGT and soil solution were feasible predictors of heavy metal concentration in maize. The DGT induced fluxes in soil/sediment (DIFS) model was used to simulate the uptake process of heavy metals by DGT, and better reveal the desorption processes of heavy metals in soils. DIFS-based desorption processes were employed to characterize the resupply ability of heavy metals in soils. The coupling of DGT and DIFS parameters provided the best prediction accuracy in this study (Cd-R² = 0.920, Ni-R² = 0.928, and Zn-R² = 0.908). Predictions are slightly weaker for Zn than for Cd and Ni (Cd-P < 0.01, Ni-P < 0.01, and Zn-P < 0.05). The reason is that the average resupply type of Cd and Ni in soil is partially sustained while Zn is resupplied via diffusion only. The desorption rate k_-1 can excellently improve the prediction accuracy of DGT, which avoids the disadvantage that soil solution does not consider desorption. The coupling of DGT and DIFS parameters provides an accurate and reliable method for predicting heavy metal enrichment in maize.

Study on release and occurrence of typical metals in corrosion products of drinking water distribution systems under stagnation conditions

Metal contaminants in corrosion products of drinking water distribution systems (DWDS) can be released into potable water under specific conditions, thereby polluting drinking water and posing a health risk. Under stagnation conditions, the release characteristics, occurring forms, and environmental risks of ten metals were determined in loose and tubercle scale solids of an unlined cast iron pipe with a long service history, before and after immersion. Most Al, As, Cr, Fe, and V in corrosion scales existed in the residual fraction, with the released concentration and pollution risk being low. Since more than 59% of Ca in pipe scales existed in the exchangeable fraction, Ca release was high. Although the Pb and Cd content of corrosion solids was low, a high proportion of Pb and Cd was present in non-residual fractions with high mobility. Sudden severe Pb or Cd pollution events in DWDS could result in high pollution and environmental risk levels. The total content and released amount of Mn and Zn in corrosion scales were both high. Therefore, while special attention should be paid to Mn and Zn, Pb and Cd also present a high risk in pipe scales, despite their low concentrations. During stagnation immersion, metal release from powdered pipe scales occurred via the processes of mass release, re-adsorption into scales, and slow release until equilibrium was reached. The levels of metal re-adsorption into scales were much higher than the concentrations dissolved into bulk water. However, the amount of metal re-adsorption into tubercle scale blocks was less. Importantly, these findings highlight that during DWDS operation, the sudden release of metal pollutants caused by pipe scale breakage should be avoided.

The geochemical and mineralogical controls on the release characteristics of potentially toxic elements from lead/zinc (Pb/Zn) mine tailings

Large quantities of lead/zinc (Pb/Zn) mine tailings were deposited at tailings impoundments without proper management, which have posed considerable risks to the local ecosystem and residents in mining areas worldwide. Therefore, the geochemical behaviors of potentially toxic elements (PTEs) in tailings were in-depth investigated in this study by a coupled use of batch kinetic tests, statistical analysis and mineralogical characterization. The results indicated that among these studied PTEs, Cd concentration fluctuated within a wide range of 0.83-6.91 mg/kg, and showed the highest spatial heterogeneity. The mean Cd concentrations generally increased with depth. Cd were mainly partitioned in the exchangeable and carbonate fractions. The release potential of PTEs from tailings was ranged as: Cd > Mn > Zn > Pb > As, Cd > Pb > Zn > Mn > As and Cd > Pb > Mn > Zn > As, respectively, under the assumed environmental scenarios, i.e. acid rain, vegetation restoration, human gastrointestinal digestion. The results from mineralogical characterization indicated that quartz, sericite, calcite and pyrite were typical minerals, cumulatively accounting for over 80% of the tailings. Sulfides (arsenopyrite, galena, and sphalerite), carbonates (calcite, dolomite, cerussite and kutnahorite), oxides (limonite) were identified as the most relevant PTEs-bearing phases, which significantly contributed to PTEs release from tailings. A combined result of statistical, geochemical and mineralogical approaches would be provided valuable information for the alteration characteristics and contaminant release of Pb/Zn mine tailings.

Heavy metal pollution in agricultural soils of a typical volcanic area: Risk assessment and source appointment

Heavy metals are naturally occurring elements with high natural background levels in the volcanic area. Therefore, it is necessary to conduct a risk assessment and identify potential sources of heavy metals. In this study, 4488 soil samples (0-20 cm) were collected in Chengmai County, a typical volcanic area in Hainan Province, and analyzed for eight heavy metals and major oxides. Pollution level, ecological risks, and health risks were evaluated by geo-accumulation index (I_geo), pollution index (PI), potential ecological risk index (RI), hazard index (HI), and carcinogenic risks (CR). The positive matrix factorization (PMF) model was further used to determine the priority source of heavy metals. The average values of heavy metal concentrations in soil were 7.06, 0.07, 156.88, 33.43, 0.05, 72.47, 19.48, and 67.51 mg kg^-1 for As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn, respectively. Except for Pb, the average concentrations of all heavy metals exceeded background concentration in Hainan soils, indicating different degrees of heavy metal enrichment. The I_geo and PI showed that the main pollutant element in volcanic soils was Ni, followed by Cr and Cu. The RI shows that the percentage of soil samples with considerable or worse potential ecological risk was 44.4% of the total samples, with Hg, As, Cd, and Ni causing high ecological risks. The estimated average daily doses of heavy metals were below the tolerable limits and the HI values were below one for both adults and children, indicating that the residents had an acceptable potential non-carcinogenic risk. However, the potential carcinogenic risk of exposure to Cr, Ni, and As was unacceptable for residents, with high CR values exceeding 10^-4, especially for children. Based on the PMF, five major sources of heavy metals were found in the study area: Ni, Cu, and Zn mainly from parent materials, As and Pb from daily life and vehicle emissions, Cd from agricultural activities, Hg from industrial activities, and Cr from parent materials under different environmental conditions. Significant positive correlations between Al₂O₃, TFe₂O₃, Mn, soil organic carbon (SOC), and heavy metals reflect that aluminium-rich minerals, Fe-Mn oxides, and SOC are the most critical factors affecting heavy metal accumulation in volcanic agricultural soils.

The impacts of aging pH and time of acid mine drainage solutions on Fe mineralogy and chemical fractions of heavy metals in the sediments

Fe (oxyhydr)oxides are the main components that accumulate heavy metals (HMs) in the acid mine drainage (AMD) sediments, but how the aging pH and time of AMD solution affects the Fe mineralogy and HMs speciation remains ambiguous. Herein, we determined the impacts of aging pH and time on the Fe mineralogy and chemical fractions of HMs in the sediments from Dabaoshan mining area using mineral characterizations, chemical extraction, and AMD solution incubation. For the natural AMD sediments, jarosite and goethite are the major Fe (oxyhydr)oxides in sample S1 with solution pH 2.68, while schwertmannite is dominant in sample S2 with solution pH 6.78, co-existing minor ferrihydrite. With increasing the AMD solution pH, the total contents of HMs (expect for As) and the reducible fraction of HMs (expect for Pb) in the sediments both increase. The HMs of Mn, Zn, Ni, and Cd are mainly associated with Fe (oxyhydr)oxides, while Pb possibly exists as Pb-bearing minerals (e.g., PbSO₄) in the sediments. The oxidizable fraction of all HMs is negligible in both sediments. When the AMD solution of S1 was aged at different pHs, schwertmannite is dominant initially at all pHs, with a higher crystallinity being at a lower pH. With increasing aging time, the pre-formed schwertmannite transforms to goethite and jarosite at pH ≤ 3, while it keeps stable at pH 5 and 7 due to the accumulation of more HMs. These new insights are essential to assess the mobility and availability of HMs in the AMD-affected areas.

Contamination and source of metals in surface sediments from the Nandu River of Hainan Island, China

Concentrations of eight metals, organic carbon (Corg), and eight oxides were measured in 33 river sediment samples from the Nandu River. Sediment quality guidelines (SQGs) showed no severe pollution at all stations. However, according to the geo-accumulation index (Igeo), enrichment factor (EF), and potential ecological risk index (EI and RI), As and Cd were the primary pollutants in the survey area and caused low to moderate potential ecological risk. The positive correlations between Al₂O₃, TFe₂O₃, Mn, Corg, and metals indicated that clay, FeMn oxides, and organic carbon content were the main factors for metal accumulation in the study area. From the results of correlation (CA) and principal component analysis (PCA), we inferred that Cr, Ni, Cu, and Zn were mainly from natural sources, while As, Cd, Hg, and Pb were from anthropogenic activities in the Nandu River basin. This was the first study of metal pollution in the surface sediments of the Nandu River, which will serve as a reference for future research.

Lithologic controls on the mobility of Cd in mining-impacted watersheds revealed by stable Cd isotopes

Cd-rich wastes from open-pit mining can be transported into rivers, which are often followed by deposition in river sediments and/or further transfer into agricultural soils. The lithology of bedrock exerts a huge effect on physicochemical properties (e.g., buffering capacities, metal species, mineral phases, etc.) of the river system, thereby potentially impacting the Cd mobility in watersheds. However, to date, little is known about the microscopic processes (e.g., dissolution, adsorption, and precipitation) controlling the migration of Cd from mines to varied watersheds. This study, therefore, aims to determine the controlling factors on Cd mobilization in two mining-impacted watersheds with contrasting bedrock lithology using both Cd and Pb isotopes. The Pb isotope ratios of sediments and soils in both watersheds fall into a binary mixing model with two isotopically distinct sources, i.e., mining wastes and bedrock. These results indicate that mining activities are the main sources of Cd in sediments and soils. However, the Cd isotope ratios reveal different Cd migration processes between the two watersheds. In the siliceous watershed, the δ^114/110Cd values of sediments decrease from -0.116‰ in the upper reach to -0.712‰ in the lower reach, with a concomitant increase in Cd concentration, which may result from Cd adsorption by goethite due to the increased pH. In contrast, in the calcareous watershed, the Cd isotope compositions of sediments (-0.345 to -0.276‰) and the pH of river water are nearly invariable, suggesting that the adsorption and release of Cd in sediments are limited. This may result from the strong pH buffering effect due to the presence of carbonate rocks. This study highlights the different fates of Cd in siliceous and calcareous watersheds and suggests that the development of Cd pollution control policies must consider regional lithology.

Immobilization of soil Cd by sulfhydryl grafted palygorskite in wheat-rice rotation mode: A field-scale investigation

The safe utilization of heavy metal contaminated farmland has attracted extensive attention of the whole society, and there is an urgent need to develop novel high-efficiency amendments. To clarify the actual remediation effect and potential for large-scale application of sulfhydryl grafted palygorskite (SGP) in Cd polluted soil in wheat-rice rotation mode, a field-scale experiment was conducted. SGP at the dosages of 0.5 g/kg-2.0 g/kg could reduce gain Cd contents by 27.15-59.05% and 16.16-79.47% for wheat and rice, respectively. The maximal decreases of soil available Cd figured out by DTPA extraction in wheat and rice season were 58.18% and 33.67%, respectively. The immobilization ratio for Cd was much more than that of trace elements, including Fe, Mn, Cu, and Zn, Ni. SGP showed an effective immobilization rate for soil Cd under the interference of many elements in the soil, pointing to the targeting and selectivity of its high-efficiency immobilization. It had no lifting effect on soil pH but decreased zeta potentials of soil particles. The sorption of Cd²⁺ on SGP amended soil could be fitted by the second-order kinetic model and Langmuir isotherm, and the changes of thermodynamic parameters showed SGP strengthened the fixation. SGP made the biological accumulation coefficient and transfer factor of rice grain drop dramatically but had no noticeable effect on these parameters of winter wheat, indicating different botanical responses. SGP as a novel immobilization amendment may provide an efficient and sustainable solution for the remediation of contaminated soil in wheat-rice rotation mode in field-scale.

Speciation of heavy metals in soils and their immobilization at micro-scale interfaces among diverse soil components

Heavy metal (HM) pollution of soils is a globally important ecological and environmental problem. Previous studies have focused on i) tracking pollution sources in HM-contaminated soils, ii) exploring the adsorption capacity and distribution of HMs, and iii) assessing phyto-uptake of HMs and their ecotoxicity. However, few reviews have systematically summarized HM pollution in soil-plant systems over the past decade. Understanding the mechanisms of interaction between HMs and solid soil components is consequently key to effectively controlling and remediating HM pollution. However, the compositions of solid soil phases are diverse, their structures are complex, and their spatial arrangements are heterogeneous, all leading to the formation of soil micro-domains that exhibit different particle sizes and surface properties. The various soil components and their interactions ultimately control the speciation, transformation, and bioavailability of HMs in soils. Over the past few decades, the extensive application of advanced instrumental techniques and methods has greatly expanded our understanding of the behavior of HMs in organic mineral assemblages. In this review, studies investigating the immobilization of HMs by minerals, organic compounds, microorganisms, and their associated complexes are summarized, with a particular emphasis on the interfacial adsorption and immobilization of HMs. In addition, methods for analyzing the speciation and distribution of HMs in aggregates of natural soils with different particle sizes are also discussed. Moreover, we also review the methods for speciating HMs at mineral-organic micro-scale interfaces. Lastly, developmental prospects for HM research at inorganic-organic interfaces are outlined. In future research, the most advanced methods should be used to characterize the interfaces and in situ characteristics of metals and metal complexes. In particular, the roles and contributions of microorganisms in the immobilization of HMs at complex mineral-organic interfaces require significant further investigation.

Remediation of Pb-diesel fuel co-contaminated soil using nano/bio process: subsequent use of nanoscale zero-valent iron and bioremediation approaches

The effectiveness of the nano/bio process was investigated as a remediation option for co-contaminated soils. Nano/bio process is a hybrid treatment method that may be defined as the use of nanoscale zero-valent iron (nZVI) and bioremediation approaches subsequently/concurrently. Different bioremediation approaches (bioattenuation, biostimulation, and/or bioaugmentation) were performed together with nZVI application to remediate Pb- and diesel fuel-spiked soils. Nutrient (N and P) and activated sludge amendment were made to realize biostimulation and bioaugmentation, respectively. The nZVI application decreased the total percentage of the most mobile and bioavailable soil Pb fractions (exchangeable and carbonate-bound) from 68.3 to 31.7%. The biodegradation levels of nZVI-applied co-contaminated soils were significantly higher than the soils without nZVI indicating the positive effect of the reduced mobility, bioavailability, and toxicity of Pb content. The use of nano/biostimulation or nano/bioaugmentation treatments resulted in higher than 60% total n-alkane degradation, whereas 89.5% degradation was obtained by using nano/biostimulation + bioaugmentation. Hydrocarbon-degrader strains belonging to phyla Actinobacteria, Proteobacteria, or Firmicutes were identified from samples subjected to nano/bio process and the strains from biostimulation and bioaugmentation treatments were different. These results indicate that the stress on the microbial population caused by the co-contamination might be subsided and the biodegradation of alkanes might be improved by using the nano/bio process.

Spatial distribution of trace elements in surface sediments of Hooghly (Ganges) river estuary in West Bengal, India

The spatial distribution of trace elements in surface sediments of the Hooghly estuary was studied over the monsoons in 2014-2017. As, Cd, Ni, Pb and U were two- to sixteen-fold the crust means with increasing levels toward the estuary, with Ni peak during the post-monsoon. Pearson's correlation matrix, cluster analysis, enrichment factors and pollution index revealed the anthropic source and association of trace elements with Fe, Mn and Al and of Pb with U. Geoaccumulation index revealed for Ni an extremely contaminated situation at the estuary water during monsoon and for Cd a heavily contaminated situation at freshwater location. The potential contamination index was >6; thus, sediments were very severely contaminated by As, Cd and Ni with worst situation for As and Cd at fresh and brackish water and during post-monsoon. The overall ecological risk was severe, 300≤RI<600 at all sites and seasons, especially after the monsoon, at fluvial and brackish locations.

An efficient biochar synthesized by iron-zinc modified corn straw for simultaneously immobilization Cd in acidic and alkaline soils

Synthetic functional biochar using agricultural waste as raw materials not only serves as an effective means for recycling waste but can also be employed for the remediation of heavy metal contaminated soil. However, the associated effect and mechanism underlying the immobilization of functional biochar in acidic and alkaline soils remain unclear. In this study, a novel iron-zinc oxide composite modified corn straw (Fe/Zn-YBC) was prepared and applied for the remediation of cadmium-contaminated acidic and alkaline farmland soils. The results showed that the addition of Fe/Zn-YBC increased the pH, cation exchange capacity (CEC), and dissolved organic carbon (DOC) in acidic soil, while increased the pH and DOC in alkaline soil. After immobilization for 42 d, the DTPA-Cd content in acidic and alkaline soils treated with Fe/Zn-YBC decreased by 12.77 %-57.45 % and 23.73 %-52.50 %, respectively. Fe/Zn-YBC treatment promoted the transformation of the exchangeable fraction into the Fe/Mn oxyhydroxide fraction of Cd, and increased the abundance and diversity of bacterial communities in the two soils. Furthermore, the SEM-EDS, XRD and FTIR results for Fe/Zn-YBC separated from the test soils showed that the distribution of Cd adsorbed on Fe/Zn-YBC was positively correlated with Fe, Zn, and O. Additionally, the Cd complexes (CdCO₃, CdZnFe₂O₄ and CdO) detected on Fe/Zn-YBC indicated that the primary immobilization mechanism of Fe/Zn-YBC involved the complexation of Cd and Fe, Zn oxides, and the precipitation of Cd and CO₃^2- in acidic and alkaline soils. The efficient remediation capacity and associated mechanism for this novel functional biochar provide insights for improved remediation of heavy metal contaminated farmland soil.

Formation Process of Silicate-Iron Oxyhydroxide Complex and Its Influence on the Distribution of Heavy Metals in Mining Area

Silicate-iron oxyhydroxide complex formed by mineral weathering has an important influence on the geochemical reactions of heavy metals in mining areas. In this work, tailings were collected from an abandoned iron tailings pond, and the physicochemical properties and distribution of heavy metals were studied under natural weathering and hydraulic processes. The results showed that Fe²⁺ in the iron tailings were transported to the surface during the weathering process, and then the iron oxyhydroxide formed by mineralization adsorbed Cu²⁺ and Zn²⁺. Silicic acid and exchangeable acid were released during the formation of binary agglomerates between hydroxy iron oxide and kaolin, then they migrated to the lower area of a tailing pond via surface runoff. Finally, silicate-iron oxyhydroxide complex were formed. The heavy metals were replaced by H⁺ and penetrated to the bottom layer with water. This research provides an important scientific basis for the prevention and control of heavy metal pollution in mining areas.

Phytoremediation of cadmium contaminated soils by Amaranthus Hypochondriacus L.: The effects of soil properties highlighting cation exchange capacity

Phytoremediation is a cost-effective method to remedy Cd-contaminated soils. However, it is difficult to predict the performance of a given (hyper)accumulator at different soils due to the divergent plant-soil mutual fitness. Soil properties could be quite influential in determining plant growth and Cd uptake and therefore affect phytoremediation efficiency. To explore the primary soil factors that regulate the efficiency of phytoremediation, a phytoextraction experiment with grain amaranth (Amaranthus Hypochondriacus L.) was conducted in six long-term Cd-contaminated agricultural soils from southern China. The results showed that besides the soil available Cd, the soil cation exchange capacity (CEC) greatly affected plant growth and the amount of total Cd extraction. The deficiency of available Ca and Mg in low CEC soil caused insufficient uptake of Ca and Mg by grain amaranth, which was adverse to plant growth and Cd detoxification. The impaired plant biomass production sharply influenced plant total Cd accumulation, despite the relatively high Cd concentration in plants. While for the grain amaranth grown in soils with higher CEC, the increases in plant Ca and Mg promoted plant photosynthesis and plant tolerance to Cd stress, as indicated by the increase of leaf chlorophyll content and antioxidant enzyme activities, which contributed to the higher plant biomass and phytoremediation efficiency. These findings highlight that maintaining regular plant biomass production is vital to ensure the efficiency of phytoremediation, and low CEC of soil is a substantial barrier that needs to be concerned and further addressed for efficient phytoremediation of Cd-contaminated soils.

Cadmium Isotope Fractionation during Adsorption and Substitution with Iron (Oxyhydr)oxides

Cadmium (Cd) isotopes have great potential for understanding Cd geochemical cycling in soil and aquatic systems. Iron (oxyhydr)oxides can sequester Cd via adsorption and isomorphous substitution, but how these interactions affect Cd isotope fractionation remains unknown. Here, we show that adsorption preferentially enriches lighter Cd isotopes on iron (oxyhydr)oxide surfaces through equilibrium fractionation, with a similar fractionation magnitude (Δ^114/110Cd_{solid-solution}) for goethite (Goe) (-0.51 ± 0.04‰), hematite (Hem) (-0.54 ± 0.10‰), and ferrihydrite (Fh) (-0.55 ± 0.03‰). Neither the initial Cd²⁺ concentration or ionic strength nor the pH influence the fractionation magnitude. The enrichment of the light isotope is attributed to the adsorption of highly distorted [CdO₆] on solids, as indicated by Cd K-edge extended X-ray absorption fine-structure analysis. In contrast, Cd incorporation into Goe by substitution for lattice Fe at a Cd/Fe molar ratio of 0.05 preferentially sequesters heavy Cd isotopes, with a Δ^114/110Cd_{solid-solution} of 0.22 ± 0.01‰. The fractionation probably occurs during the transformation of Fh into Goe via dissolution and reprecipitation. These results improve the understanding of the Cd isotope fractionation behavior being affected by iron (oxyhydr)oxides in Earth's critical zone and demonstrate that interactions with minerals can obscure anthropogenic and natural Cd isotope characteristics, which should be carefully considered when applying Cd isotopes as environmental tracers.

The regulating effects and mechanism of biochar and maifanite on copper and cadmium in a polluted soil-Lolium perenne L. system

Arable land polluted by copper (Cu) and cadmium (Cd) is a widespread problem. The use of biochar and/or clay mineral as a soil amendment can effectively solidify heavy metals in the soil. We applied biochar (BC), iron modified biochar (Fe-BC), maifanite (MF, a kind of clay minerals), a combination of BC with MF (BC:MF), and Fe-BC with MF (Fe-BC:MF) at a 2 wt % dose as soil amendments to study their ability to prevent Cu and Cd from accumulating in ryegrass (Lolium perenne L.). We found that after 90 days of cultivation, the Cd and Cu content both significantly decreased in ryegrass shoots from 2.06 and 209.3 mg kg⁻¹ (control) to 1.44–2.01 and 51.50–70.92 mg kg⁻¹, respectively, across treatments (p < 0.05). Similarly, the bioconcentration factor (BCF) for Cd/Cu was significantly smaller (P < 0.05) in all amendments versus control soil. This trend differed among the shoot, BCF, and transportation factor (TF). Combining BC:MF or Fe-BC:MF did not significantly improve the Cd/Cu stabilization in the soil compared to the corresponding single amendment (p > 0.05). Our adsorption balance experiment showed that BC, Fe-BC, and MF physically and chemically adsorbed Cd and Cu by complexation with functional groups (mesoporous nanomaterials) whose porosity measurements ranged from 0.68 to 78.57 m² g⁻¹. Furthermore, the amorphous crystalline iron oxide binding Cd and Cu was the key to immobilizing these metals in the soil. The amendments applied in our study show promise for enhancing immobilization of Cu and Cd in contaminated paddy soils.

Assessment of heavy metal content, distribution, and sources in Nansi Lake sediments, China

Much attention has been paid to the heavy metal contamination of lake sediments in rapidly developing regions. In this study, heavy metal (Cd, Cr, Co, Ni, Mn, Pb, As, Cu, and Zn) concentrations in sediment surface samples and cores from the Nansi Lake were investigated to ascertain the potential sources and environmental risks of heavy metals. The average concentration of heavy metals was 0.16-16.04 times background concentrations. The enrichment factor, Tomlinson pollution load index, geo accumulation index, positive definite matrix factor analysis (PMF), and potential ecological risk index were used to assess heavy metal concentrations and explore the evolution of heavy metal sources, and result indicated that Cd reached moderate pollution levels, which is the most polluted heavy metal in the history and present, while the remaining heavy metals are at low or no pollution levels. The contribution of Cd to RI exceeded 76%, which is the decisive factor in the ecological risk of Nansi Lake. The result of ecological risk showed that the risk level for most of Nansi Lake is medium, and some areas of Zhaoyang Lake and Weishan Lake reach high levels. The PMF results showed that there are four main factors influencing heavy metal concentrations in Nansi Lake sediments, including industrial sources, fertilizers, and herbicides used in agricultural production, traffic-related emissions, and mineral mining. Among these factors, industrial and mineral mining sources were found to be the most important, and the highest contribution rate occurred in the -10cm (1960s). Although the contribution of fertilizers and herbicides is lower than that of other sources, increasing trend should be a warning sign that Cd has reached a high ecological risk level in Nansi Lake sediments.

Schwertmannite transformation to goethite and the related mobility of trace metals in acid mine drainage

In acid mine drainage (AMD), precipitated schwertmannite can reduce the trace metal concentration by adsorption and co-precipitation. With the geochemical changes in the water, the precipitated minerals are transformed into more stable goethite. However, no detailed systematic studies have been performed on the mobility changes of trace metals during iron-mineral transformation. The behaviors of trace metals during the transformation of schwertmannite to goethite are investigated for core samples from an AMD treatment. Schwertmannite had gradually transformed to goethite from the top to the bottom of the core samples. Among trace metals, Pb was highly retained in schwertmannite during precipitation, probably by co-precipitation with schwertmannite. Arsenate and chromate were also relatively well retained in schwertmannite, probably because of the substitution of sulfate during precipitation. Sequential extraction results showed that during the transformation of schwertmannite to goethite, most trace metals decreased their mobility by decreasing their exchangeable fraction. However, only Pb increased its mobility during transformation. Some elements, such as Cd and Co, had higher contents of exchangeable fractions compared to other metals and can be relatively easily released into water with slight geochemical changes, greatly affecting the environments of ecological systems.

Using Zn isotopes to trace Zn sources and migration pathways in paddy soils around mining area

Paddy soils around mining areas suffer from the great threat of heavy metal pollution. The traditional source-tracing methods based on metal concentrations limit our ability to quantify the sources of heavy metals and trace their transport processes to paddy soils. In this study, Zn isotope compositions of paddy soils in Dabaoshan mine area, a typical sulfide deposit in southern China, have been systematically studied. According to a plot between 1/Zn (i.e. inverse concentration) and δ⁶⁶Zn value, all the polluted paddy soils fall on the mixing line between acid mine drainage precipitate (AMD-precipitate) and fertilizer while the unpolluted paddy soil falls on the mixing line between fertilizer and bedrock. This indicates the mixing of Zn sources at least three end-members: the mining end-member (i.e. AMD-precipitate), the agricultural end-member (i.e. fertilizer), and bedrock whose geochemical signature is often overprinted by the former two sources around the mining area. The quantitative calculations to apportion the end-member's contributions show that the mining activity contributes most Zn in the paddy soils with an average of ∼66.2%. The contribution of mining activities has significant spatial variations. Specifically, the mining activities have relatively low impacts on the lower reach and the deep soil. Additionally, the apparent Zn isotope fractionation between AMD and AMD-precipitate (Δ⁶⁶Zn_{AMD-precipitate} - _AMD of -0.35 to -0.08‰) in the tailings dam suggests that Zn cations in AMD coprecipitated with the secondary Fe-bearing minerals (e.g. jarosite and goethite). After being discharged from the tailings dam, Zn is mainly carried by the Fe-oxide minerals and migrated during surface runoff. Our study highlights the contribution of human activities to the Zn pollution in the paddy soils and the key role of Fe-bearing minerals in the migration of Zn. These findings provide a scientific base for the development of policy for pollution control in mining-affected region.

The performance of emerging materials derived from waste organism blood and saponified modified orange peel for immobilization of available Cd in soil

Waste organism blood (WOB) and orange peel are emerging stabilization materials obtained as by-products from agricultural processes, which are quite suitable for heavy metal immobilization in soil. In this work, waste organism blood and chemically modified orange peel (SOP) were investigated as potential sorbents for immobilization of available Cd in soil. Application of 5% WOB and SOP effectively immobilized cadmium (Cd) with an associated regulation of soil pH, among which the pH of acidic soil increased most significantly. While the application of 3% SOP alone stabilized almost the same amount of available Cd compared to WOB, it caused the highest stabilization rate of 58.85% when applied at 5%. By contrast, SOP combined with WOB (the mass ratio of the material is 1 : 1) at a 5% addition rate stabilized the available Cd in soils remarkably, with a stabilization rate of 57.74%. This study revealed that the soil particles after stabilization have a more compact and flaky structure, and the SOP and WOB had a particular pore structure, which was helpful for the adsorption of available Cd in soil. This study put forward new insights into the potential effects of Cd immobilization in contaminated soil by newly emerging stabilization biomass materials (WOB and SOP).

Effects of hematite on the stabilization of copper, cadmium and phosphorus in a contaminated red soil amended with hydroxyapatite

Iron (Fe) oxides are intimately coupled with phosphorus and closely associated with the bioavailability of potential toxic elements (PTEs) in soil. Thus, Fe oxides may influence the stabilization of PTEs in contaminated soils amended by phosphorus. To evaluate the effects of hematite (HMT) on the stabilization of PTEs, 1-5% (by weight) of HMT was added into a contaminated red soil amended with hydroxyapatite (HAP) to simulate naturally occurring Fe oxides. The stabilization efficiencies of soil copper (Cu) and cadmium (Cd) amended with HAP in soils with low, moderate, and high content of HMT were assessed after a 60-day incubation. HAP treated the soil with high rate HMT decreased the CaCl₂-extractable and acid-soluble fractions of Cu and Cd than that of HAP alone. In particular, CaCl₂-extactable Cu and Cd in the soil with 5% HMT amended by HAP were 91-95% and 41-68% lower than those amended with only HAP. High content of HMT in soil could decrease the concentration of labile phosphorus in the presence of HAP, but it did not increase the concentration of NaOH-extractable inorganic phosphorus (the fraction bound to Fe oxides). The concentrations of free and crystalline Fe oxides were significantly increased by adding high dosages of HMT with or without HAP. High content of HMT in soil amended by HAP reduced metal phytotoxicity and uptake by wheat shoots than the soil containing HAP without HMT. The results indicate that HMT can promote Cu and Cd stabilization while decrease labile phosphorus in red soil amended with HAP, suggesting that phosphorus-based amendments combined with Fe oxides can be used to stabilize PTEs in contaminated red soils.

Distribution and mobilization of heavy metals at an acid mine drainage affected region in South China, a post-remediation study

Dabaoshan Mine Site (DMS) is the largest polymetallic mine in South China. The Hengshi River flowing next to DMS receives acid mine wastes leaching from the tailings pond and run-off from a treatment plant, which flows into the Wengjiang River. This study focuses on spatiotemporal distribution and mobilization of As, Cd, Pb, and Zn along the Hengshi River, groundwater, fluvial sediments, and soils, with a focus on As due to its high toxicity and the fact that mining is one of the main sources of contamination. Geochemical analyses (heavy metals, grain-size, X-ray diffraction, organic carbon and sulfur content) followed by geochemical modeling (PHREEQC) and statistical assessment were done to determine the physicochemical characteristics, toxicity risks, and behavior of heavy metals. Near the tailings pond, heavy metal concentrations in surface water were 2-100 times higher than the Chinese surface water standard for agriculture. Although water quality during the dry season has improved since the wastewater treatment plant started, heavy metal concentrations were high during rainy season. In groundwater, heavy metal concentrations were low and pose little risks. Soils along the Hengshi River were disturbed and they did not show any specific trends. The potential ecological risk of heavy metals was ranked as Cd > As > Cu > Pb > Zn in sediments and Cd > Cu > Pb > As > Zn in soils indicating multi-metal contamination and toxicity. As(III) was the predominant species in surface water during the dry season, whereas As(V) dominated during the rainy season. Arsenic levels in most sites exceeded the Chinese soil standard. Although As is assumed to have a moderate ecological risk in sediments and low risk in soils, anthropogenic activities, such as mining and land-use changes contribute to the release of As and other heavy metals and pose a risk for local residents.

Adsorption of Cr(VI) on Al-substituted hematites and its reduction and retention in the presence of Fe2+ under conditions similar to subsurface soil environments

Aluminum substitution is common in iron (hydr)oxides in subsurface environments, and can significantly modify mineral interactions with contaminants. However, few studies investigate Cr(VI) adsorption and its subsequent mobility on Al-substituted iron (hydr)oxide surfaces. Here shows that Al substitution gradually modifies hematite crystals from {101}, {112}, {110} and {104} faceted rhombohedra to {001} faceted plates, resulting in a general decrease in Cr(VI) adsorption density and favoring of monodentate mononuclear over bidentate binuclear Cr(VI) adsorption complexes. Consequently, the mobility of Cr(VI) might be increased in environments with an abundance of Al-containing iron (hydr)oxides. However, pre-adsorption of Fe²⁺ on hematite promotes Cr(VI) adsorption, reduction and fixation, and Al-substituted hematite removes more Cr(VI) than pure hematite. Similarly, although addition of Fe²⁺ to Cr(VI)-adsorbed hematite remobilizes a small proportion of Cr, it greatly increases the proportion of Cr fixed. As the coexistence of Fe²⁺ and iron (hydr)oxides is common in subsurface environments, Al-containing iron (hydr)oxides will promote Cr(VI) uptake and retention, with a significant proportion fixed as Cr(III), limiting Cr mobility and toxicity. These results offer new insights into how iron (hydr)oxides might control the behaviors of other high-valence redox-sensitive contaminants, and provide a platform for modeling such processes in complex soil and sediment systems.

Chronic trace metals effects of mine tailings on estuarine assemblages revealed by environmental DNA

Mine tailing disasters have occurred worldwide and contemporary release of tailings of large proportions raise concerns of the chronic impacts that trace metals may have on the aquatic biodiversity. Environmental metabarcoding (eDNA) offers an as yet poorly explored opportunity for biological monitoring of impacted aquatic ecosystems from mine tailings and contaminated sediments. eDNA has been increasingly recognized to be an effective method to detect previously unrecognized small-sized Metazoan taxa, but their ecological responses to environmental pollution has not been assessed by metabarcoding. Here, we evaluated chronic effects of trace metal contamination from sediment eDNA of the Rio Doce estuary, 1.7 years after the Samarco mine tailing disaster, which released over 40 million m3 of iron tailings in the Rio Doce river basin. We identified 123 new sequence variants environmental taxonomic units (eOTUs) of benthic taxa and an assemblage composition dominated by Nematoda, Crustacea and Platyhelminthes; typical of other estuarine ecosystems. We detected environmental filtering on the meiofaunal assemblages and multivariate analysis revealed strong influence of Fe contamination, supporting chronic impacts from mine tailing deposition in the estuary. This was in contrast to environmental filtering of meiofaunal assemblages of non-polluted estuaries. Here, we suggest that the eDNA metabarcoding technique provides an opportunity to fill up biodiversity gaps in coastal marine ecology and may become a valid method for long term monitoring studies in mine tailing disasters and estuarine ecosystems with high trace metals content.

Effect of physical and chemical properties of vanadium slag from stone coal on the form of vanadium

Vanadium mining and smelting activities were increasing extensively and causing serious vanadium pollution in soil around the mining area. Different existing forms of vanadium had different biological effects and the exchangeable state had been recognized as a severe threat to biodiversity and ecosystem functioning. At present, the research on vanadium morphology had not received much attention. In this study, the area that we researched had been severely polluted with vanadium due to mining and smelting activities. The changes in the morphology of vanadium in soil were studied by adjusting the organic matter content, clay mineral content, pH value, and Eh value. The results showed that at pH 8 and for 1% of humic acid added, the exchangeable fraction of vanadium in the slag was 10% and 9%, respectively, which was 5% and 6% lower than the control group. The addition of kaolin and the redox change had little effect on the exchangeable fraction of vanadium, with a change of only about 2%. To control the soil pollution caused by slag and to repair its ecological characteristics, kaolin and humic acid were used for the repair test. The results showed that after 1% humic acid mixed with 8% kaolin was added in soil, the germination rate of ryegrass reached 95% and grew flourishingly which is significantly better than other treatment groups. Our research can provide a reference for future vanadium pollution control, especially in the morphology of vanadium research.

Immobilization of cadmium in polluted soils by phytogenic iron oxide nanoparticles

While phytogenic nanomaterials have been successfully used to remove heavy metals in wastewater, the potential to successfully use such materials to immobilize heavy metals in soils is still unclear. In this study, phytogenic iron oxide nanoparticles (PION) were used to immobilize cadmium (Cd) in six soils. Amendment with PION effectively immobilized Cd, with a concomitant increase in the concentrations of iron oxides, soil pH and dissolved organic carbon (DOC) under both oxic and anoxic conditions. However, observed changes in soil properties and Cd fractions were different under oxic and anoxic conditions. After PION application, the exchangeable Cd fraction decreased by up to 91 and 69%, while the carbonate bound Cd fraction decreased by up to 61 and 75%, under oxic and anoxic conditions, respectively. Pearson correlation analysis revealed that under both oxic and anoxic conditions, Cd fractions were significantly and positively correlated with free iron oxide content and pH, where free iron oxide content was positively correlated with amorphous iron oxide, DOC and pH. The Cd immobilization mechanisms potentially involved either (1) formation of insoluble hydroxides at elevated pH; (2) participation of biomolecules released from PION in ligand complexation with Cd and (3) co-precipitated of Cd during the formation of iron oxides. This study provided new insights into the potential effects of PION applications for practical Cd immobilization in contaminated soils.

Risk assessment of heavy metals in pipe scales and loose deposits formed in drinking water distribution systems

The accumulation of inorganic contaminants in drinking water distribution systems (DWDS) can greatly threaten water quality and safety. This work mainly focused on the accumulation, speciation and risk assessment of inorganic contaminants found in pipe scales and loose deposits in DWDS. Global contamination factor (GCF), risk assessment code (RAC) and consensus-based sediment quality guidelines (CBSQGs) were adopted for the potential health risk assessment of inorganic contaminants. The Tessier sequential extraction method was used to study the speciation distribution of inorganic contaminants in fourteen samples (six pipe scale samples, eight loose deposit samples) collected from real DWDS. The significant correlation between Al and Mn showed there was a co-occurrence behavior of Al and Mn in pipe scales and loose deposits. In addition to the possible interactions between Al and Mn, Ba, Cu and As were possibly accumulated during the formation of Al and Mn commixtures. Mn, Cu, Pb, Zn, Ni, Co and Ba in the samples were mainly associated with the Fe-Mn oxides fraction, which indicated Fe-Mn oxides might play an important role in the accumulation and release of these inorganic contaminants. Fe, Al, As, Cr, V and Cd mainly existed in the residual fraction, which indicated their low mobility. The GCF results demonstrated that most of the sample sites had a certain environmental risk. The RAC results showed that high risk mainly resulted from Cd both in pipe scales and loose deposits. According to the CBSQGs evaluation, heavy metals in loose deposits were more harmful, and Ba exhibited the highest risk among all heavy metals.

Reconstructing the historical pollution levels and ecological risks over the past sixty years in sediments of the Beijiang River, South China

Three sediment cores were collected from the Beijiang River to investigate the effects of human activities on the geochemical fractions of trace metals (Ni, Cr, Cu, Zn, Cd, Pb and Fe) and to reconstruct the ecological risks for the period 1951-2015. Cd had the highest concentration in exchangeable and carbonate fractions and was probably contributed by smelting wastewater. High Cu, Zn and Pb contents were observed in the iron oxide fraction (mean values of 32.2%, 38.2% and 43.9%, respectively), reflecting the influence of mining activities. Flood events led to coarser sediment grain sizes and higher trace metal residual fractions at upstream sites (S1 and S2). Similar to the mining history of the basin, the excess metal fluxes of Cu, Zn, Pb, and Fe in the ²¹⁰Pb-dating core (S3) increased slowly from 1951 to 1987, increased rapidly from 1988 to 1998 and decreased gradually after 1999 because of government intervention. However, the excess Cd flux decreased continuously from 1951 to 1961, increased from 1961 to 2005, and declined by approximately 78.2% from 2005 to 2014. The excess Ni and Cr fluxes increased noticeably after 1996 because of the increasing sedimentation rate after the construction of the Feilai Gorge Dam. The enrichment factor (EF) and ratio of secondary and primary phases (RSP) indicated that sediments (S3) were moderately to strongly polluted by Cu, Zn and Pb from 1961 to 2007 and extremely polluted by Cd from 1951 to 2011. Human activities increased the bioavailable metal concentrations and resulted in a high risk of toxicity to benthic organisms, especially during intense mining activity (1990s) and Cd pollution incidents (2005). Cd and Pb were primarily responsible for the sediment toxicity in the Beijiang River. The integrated pollution and risk assessment methods provided a clearer understanding of the aquatic environmental quality.

Influence of iron plaque on the uptake and accumulation of chromium by rice (Oryza sativa L.) seedlings: Insights from hydroponic and soil cultivation

The effects of iron plaque formation on chromium (Cr) uptake and accumulation by rice seedlings (Oryza sativa L.) were assessed using hydroponic and soil experiments, where each 3 levels of Fe supplementation were added to Hoagland solution (0, 30, and 100 mg Fe²⁺ L^-1) and a typical paddy soil (0, 1, and 2 g Fe²⁺ kg^-1). For each treatment, rice seedlings were exposed to different levels of Cr as chromate at 0, 0.5, 2, 5, 10, and 20 mg L^-1 in solution or 300 mg kg^-1 in soil. Low levels of Cr supply (0.5, 2, and 5 mg L^-1) promoted root biomass, while high levels (10 and 20 mg L^-1) decreased root and shoot biomass and undermined the density and integrity of iron plaque. Iron supply significantly increased the proportion of Cr in iron plaque, but decreased that in rice plants. The results of hydroponic experiment showed that iron plaque formed with Fe supply at 100 mg L^-1 markedly reduced Cr accumulation in shoots of rice seedlings when exposure to 10 and 20 mg L^-1 Cr. The soil culture experiment also demonstrated that exogenous Fe addition significantly decreased Cr concentration in leaf and stem of rice seedlings. These results suggested that iron plaque with appropriate amount was effective to reduce the uptake and accumulation of Cr in rice plants, which have strong implication for taking measures to regulate Cr accumulation in rice grains.

Effect of Cadmium, Copper and Lead on the Growth of Rice in the Coal Mining Region of Quang Ninh, Cam-Pha (Vietnam)

The goal of this study was to quantify the mobility and partitioning of trace elements originating from mine waste rocks derived from open pit coal extraction activities. The results showed that native rice plants were adapted to growing in metal contaminated soils, posing a severe health risk to local population. Sequential extraction procedures and bulk soil chemical analyses both suggest enrichment of Cd, Pb and Cu in rice paddy soils. Lead was shown to be evenly partitioned among all mineral and organic phases. Copper was associated with carbonates and organic matter. Smaller fractions of Pb and Cu were also bound to Fe and Mn oxides. Only 25% of Cd, 9% of Pb and 48% of Cu were associated with the exchangeable fraction, considered mobile and thus bioavailable for plant uptake. Effects of Cd, Cu and Pb on local Cam Pha Nep cai Hoa vang, and control Asia Italian rice, showed marked differences in growth. The local Vietnamese variety grew close to control values, even upon exposure to higher trace metal concentrations. Whereas the development of the control rice species was significantly affected by increasing trace metal concentrations. This result suggests toxic trace elements accumulation in the edible parts of crops.

Temporal variability in Cu speciation, phytotoxicity, and soil microbial activity of Cu-polluted soils as affected by elevated temperature

Global warming has obtained increasing attentions due to its multiple impacts on agro-ecosystem. However, limited efforts had been devoted to reveal the temporal variability of metal speciation and phytotoxicity of heavy metal-polluted soils affected by elevated temperature under the global warming scenario. In this study, effects of elevated temperature (15 °C, 25 °C, and 35 °C) on the physicochemical properties, microbial metabolic activities, and phytotoxicity of three Cu-polluted soils were investigated by a laboratory incubation study. Soil physicochemical properties were observed to be significantly altered by elevated temperature with the degree of temperature effect varying in soil types and incubation time. The Biolog and enzymatic tests demonstrated that soil microbial activities were mainly controlled and decreased with increasing incubation temperature. Moreover, plant assays confirmed that the phytotoxicity and Cu uptake by wheat roots were highly dependent on soil types but less affected by incubation temperature. Overall, the findings in this study have highlighted the importance of soil types to better understand the temperature-dependent alternation of soil properties, Cu speciation and bioavailability, as well as phytotoxicity of Cu-polluted soils under global warming scenario. The present study also suggests the necessary of investigating effects of soil types on the transport and accumulation of toxic elements in soil-crop systems under global warming scenario.

Source identification and risk assessment based on fractionation of heavy metals in surface sediments of Jiaozhou Bay, China

To identify sources and evaluate ecological risks of heavy metals in sediments of Jiaozhou Bay, contents and chemical fractions of Cd, Cr, Cu, Pb, Zn, Ni, Sb and Sn were studied. Results suggested that higher metal contents appeared at inner bay and near marine dumping area. Labile fractions of heavy metals accounted for 0.5-77% (~36%) of total contents indicating their significant anthropogenic sources. The enrichment levels of Cd and Sb were relatively higher. Cu, Ni, Cd and Zn were at low to medium risks. Cr, Pb, Sn and Sb were at no or low risks. Total contents of heavy metals were mainly controlled by natural sources, while anthropogenic inputs were important sources of labile fractions of heavy metals in sediments of Jiaozhou Bay with industrial and domestic activities as main contributors for Cu, Pb, Zn, Cr, Ni and Sn, and agricultural activities for Cd and regional coal combustion for Sb.

Arsenic and trace elements in soil, water, grapevine and onion in Jáchal, Argentina

Contamination by trace elements (TE) is an increasing concern worldwide. In some areas, crop production could be limited by the presence of metals and metalloids, so it is important to determine their concentrations and mobility. The region of Jáchal, province of San Juan, Argentina, has good growing conditions for onion and grapevine production, but their quality and yield are affected by high TE concentration in soils and water. Soils, water, grapevine and onion were sampled and TE content determined. In soils elevated As, B, Cr, Hg, and Tl concentrations were detected (506±46, 149±3, 2714±217, 16±7, and 12±3μgg^-1, respectively, for maximum values measured), and physicochemical properties of the soil promotes these elements mobility. Water samples had high As, B, Cr, and Fe concentrations (1438±400, 10,871±471, 11,516±2363, and 3071±257μgL^-1, respectively, for maximum values measured) while in onion bulbs and grapevine berries, As, Cr, Cu, and Fe (92±7 and 171±20, 1412±18 and 2965±32, 17±3 and 126±88, and 418±204 and 377±213μgg^-1, respectively, for maximum values measured) exceeded the limits for food consumption established by Argentinian law. Correlation analyses indicated that: i) there is a common source of TE in this area, ii) each elements concentration in plants is associated with different soil variables and different soils depths, and iii) the lack of correlation between soil and water indicates that concentration in water is not constant over the time and/or there exists a differential accumulation of elements in soils depending on their own properties. Data obtained demonstrate very high concentration of TE in soil, grapevines, and onion plants in Jáchal region, and different remediation techniques are necessary to stabilize and minimize the bioavailability of these elements.

Cadmium accumulation in edible flowering cabbages in the Pearl River Delta, China: Critical soil factors and enrichment models

Although many previous studies have reported the soil pH and organic matter to be the most critical factors that affect the transfer of Cd in soil-crop systems in temperate zones, the behavior of Cd transfer is different in the Pearl River Delta (PRD), which is located in a subtropical zone with different climate and soil conditions. Therefore, we must determine the critical environmental factors that influence the transfer of Cd in the soil-vegetable system in the PRD region. Such knowledge can improve the safety of vegetables. In this study, the soil geochemical properties are investigated to explore the key soil factors that control the uptake of Cd by flowering cabbage, a popular leaf vegetable in China, from soils in the PRD region. The Cd contents in vegetables were most positively correlated to soil oxalate-Cd (p < 0.01), which indicates that amorphous Cd is the most available form for uptake into the cabbages. With the characteristics of rich in Fe oxide and Al oxide in the PRD soils, soil Fe and Al oxides were found to be the most relevant to the transfer factors of Cd from the soils to the cabbages. Soil secondary minerals are the key factor that affects the transfer of Cd, thereby influencing the migration and fate of Cd in soil-cabbage systems, with DCB-Fe significantly decreasing the Cd accumulation in cabbages. Additionally, models were developed to predict the enrichment of Cd in flowering cabbages, in which oxalate-Cd, DCB-Fe, and NaOAc-Al in soils were determined to be the most important factors that affect the Cd enrichment in flowering cabbages. In this study, we determine the important role of soil secondary minerals in affecting the transfer of Cd in soil-cabbage systems in the PRD. These observations are important to evaluate the accumulation of Cd in vegetables in subtropical zones.

Effects of sulfur on toxicity and bioavailability of Cu for castor (Ricinus communis L.) in Cu-contaminated soil

The biogeochemical cycling of sulfur (S) in soil has an important impact on the bioavailability of heavy metals and affects the utilization of soil polluted by heavy metals. In addition, S-containing compounds are involved in heavy metal detoxification. This study investigated the effects of S on the toxicity and bioavailability of copper (Cu) in castor (Ricinus communis L.) grown in Cu-contaminated mine tailings. The results showed that the application of S reduced the accumulation of Cu in castor and promoted its growth. With the addition of S, the malondialdehyde (MDA) content of castor leaves decreased significantly compared with control plants, indicating the alleviation of oxidative stress. Superoxide dismutase (SOD) and catalase (CAT) activities and glutathione (GSH) content decreased significantly with the alleviation of oxidative stress. The sequential extraction of Cu fractions showed that the application of S significantly reduced the reducible Cu fraction, and increased the oxidizable Cu fraction. It also increased the residual Cu fraction in the soil. The transformation of chemical speciation reduced the bioavailability of Cu in soil, which then reduced the accumulation of Cu in castor. Our results demonstrated that S application was effective at promoting castor growth by reducing the bioavailability and uptake of Cu in Cu-contaminated mine tailings.

Environmental assessment of the effects of a municipal landfill on the content and distribution of heavy metals in Tanacetum vulgare L

Heavy metal pollution is an important concern because of its potential to affect human health. This study was conducted to analyze plants growing on a landfill body and in its surroundings to determine their potential for heavy metal accumulation. In addition, the enrichment coefficient (EC) for the plant/soil system was used for determining the environmental contamination from a landfill in terms of heavy metal accumulation. The samples were taken in 2013-2014. Of the analyzed metals, iron achieved the highest values in the samples, i.e. - stalk (103.4-6564.6 mg/kg DM), roots (6563.6-33,036.6 mg/kg DM), leaf (535.1-11,275 mg/kg DM) and soil (12,389-39,381.9 mg/kg DM). The highest concentrations were determined in 2013 for Fe, Mn and Zn. Iron achieved the highest concentrations in the years 2013-2014. Next, EC values were then calculated, with the highest noted for Cd. Cd, as well as Cr, Ni and Zn are accumulated mostly in the leaves, whereas Co, Cu, Fe, Hg, Mn and Pb are accumulated mainly in the roots of T. vulgare.

Factors controlling cadmium and lead activities in different parent material-derived soils from the Pearl River Basin

Labile metals in agricultural soils are available to crops and thus pose a great health risk for human beings. Therefore, factors influencing heavy metal activity are of interest to researchers. In this study, a total of 142 soil samples representing 5 typical parent materials in the Pearl River Basin (PRB), China were collected to investigate factors impacting the distribution of labile Cd and Pb in the soils. The results showed that the labile fractions accounted for 0.03%-14.7% for Cd and 0.01%-0.39% for Pb of the total metals, and the labile fractions were linearly correlated to their corresponding total contents. The step regression analyses suggested that the key factors impacting labile Cd and Pb varied in different parent material soils. Pb activity was highly sensitive to pH in alkaline limestone soils. The quartz sand remained in granite-produced soils enhanced Cd activity. And dissolved organic matter (DOM) compositions considerably influenced Cd and Pb activities in sand shale, diluvium, and alluvium soils. Land use impacts heavy metal activities. The labile Cd and Pb in paddy soils were higher than those in non-paddy soils, although total metals in the soils were comparable. It could be ascribed to the long-term equilibrium of metals between the solution and solid phases of the paddy soils. The results provide a theoretical basis for preliminary prediction of heavy metal activity and provide a technical support for heavy metal activity management and pollution control based on soil parent materials.