Evaluation of cadmium transfer from soil to leafy vegetables: Influencing factors, transfer models, and indication of soil threshold contents

Development and application of machine learning models for prediction of soil available cadmium based on soil properties and climate features

Identifying the key influencing factors in soil available cadmium (Cd) is crucial for preventing the Cd accumulation in the food chain. However, current experimental methods and traditional prediction models for assessing available Cd are time-consuming and ineffective. In this study, machine learning (ML) models were developed to investigate the intricate interactions among soil properties, climate features, and available Cd, aiming to identify the key influencing factors. The optimal model was obtained through a combination of stratified sampling, Bayesian optimization, and 10-fold cross-validation. It was further explained through the utilization of permutation feature importance, 2D partial dependence plot, and 3D interaction plot. The findings revealed that pH, surface pressure, sensible heat net flux and organic matter content significantly influenced the Cd accumulation in the soil. By utilizing historical soil surveys and climate change data from China, this study predicted the spatial distribution trend of available Cd in the Chinese region, highlighting the primary areas with heightened Cd activity. These areas were primarily located in the eastern, southern, central, and northeastern China. This study introduces a novel methodology for comprehending the process of available Cd accumulation in soil. Furthermore, it provides recommendations and directions for the remediation and control of soil Cd pollution.

Uptake of zinc from the soil to the wheat grain: Nonlinear process prediction based on artificial neural network and geochemical data

Trace elements in plants primarily derive from soils, subsequently influencing human health through the food chain. Therefore, it is essential to understand the relationship of trace elements between plants and soils. Since trace elements from soils absorbed by plants is a nonlinear process, traditional multiple linear regression (MLR) models failed to provide accurate predictions. Zinc (Zn) was chosen as the objective element in this case. Using soil geochemical data, artificial neural networks (ANN) were utilized to develop predictive models that accurately estimated Zn content within wheat grains. A total of 4036 topsoil samples and 73 paired rhizosphere soil-wheat samples were collected for the simulation study. Through Pearson correlation analysis, the total content of elements (TCEs) of Fe, Mn, Zn, and P, as well as the available content of elements (ACEs) of B, Mo, N, and Fe, were significantly correlated with the Zn bioaccumulation factor (BAF). Upon comparison, ANN models outperformed MLR models in terms of prediction accuracy. Notably, the predictive performance using ACEs as input factors was better than that using TCEs. To improve the accuracy, a two-step model was established through multiple testing. Firstly, ACEs in the soil were predicted using TCEs and properties of the rhizosphere soil as input factors. Secondly, the Zn BAF in grains was predicted using ACE as input factors. Consequently, the content of Zn in wheat grains corresponding to 4036 topsoil samples was predicted. Results showed that 85.69 % of the land was suitable for cultivating Zn-rich wheat. This finding offers a more accurate method to predict the uptake of trace elements from soils to grains, which helps to warn about abnormal levels in grains and prevent potential health risks.

Mitigating Toxic Metal Exposure Through Leafy Greens: A Comprehensive Review Contrasting Cadmium and Lead in Spinach

Metals and metalloids (hereafter, metal(loid)s) in plant-based foods are a source of exposure to humans, but not all metal(loid)-food interactions are the same. Differences exist between metal(loid)s in terms of their behavior in soils and in how they are taken up by plants and stored in the edible plant tissue/food. Thus, there cannot be one consistent solution to reducing toxic metal(loid)s exposure to humans from foods. In addition, how metal(loid)s are absorbed, distributed, metabolized, and excreted by the human body differs based on both the metal(loid), other elements and nutrients in the food, and the nutritional status of the human. Initiatives like the United States Food and Drug Administration's Closer to Zero initiative to reduce the exposure of young children to the toxic elements cadmium, lead, arsenic, and mercury from foods warrant careful consideration of each metal(loid) and plant interaction. This review explores such plant-metal(loid) interactions using the example of spinach and the metals cadmium and lead. This review highlights differences in the magnitude of exposure, bioavailability, and the practicality of mitigation strategies while outlining research gaps and future needs. A focus on feasibility and producer needs, informed via stakeholder interviews, emphasizes the need for better analytical testing facilities and grower and consumer education. More research should focus on minimization of chloride inputs for leafy greens to lessen plant-availability of Cd and the role of oxalate in reducing Cd bioavailability from spinach. These findings are applicable to other leafy greens (e.g., kale, lettuce), but not for other plants or metal(loid)s.

Multicontamination Toxicity Evaluation in the Model Plant Lactuca sativa L

Many contaminated soils contain several toxic elements (TEs) in elevated contents, and plant-TE interactions can differ from single TE contamination. Therefore, this study investigated the impact of combined contamination (As, Cd, Pb, Zn) on the physiological and metabolic processes of lettuce. After 45 days of exposure, TE excess in soil resulted in the inhibition of root and leaf biomass by 40 and 48%, respectively. Oxidative stress by TE accumulation was indicated by markers-malondialdehyde and 5-methylcytosine-and visible symptoms of toxicity (leaf chlorosis, root browning) and morpho-anatomical changes, which were related to the change in water regime (water potential decrease). An analysis of free amino acids (AAs) indicated that TEs disturbed N and C metabolism, especially in leaves, increasing the total content of free AAs and their families. Stress-induced senescence by TEs suggested changes in gas exchange parameters (increase in transpiration rate, stomatal conductance, and intercellular CO2 concentration), photosynthetic pigments (decrease in chlorophylls and carotenoids), a decrease in water use efficiency, and the maximum quantum yield of photosystem II. These results confirmed that the toxicity of combined contamination significantly affected the processes of lettuce by damaging the antioxidant system and expressing higher leaf sensitivity to TE multicontamination.

How exogenous ligand enhances the efficiency of cadmium phytoextraction from soils?

Many experiments showed that exogenous ligands could enhance cadmium (Cd) phytoextraction efficiency in soils. Previous studies suggested that the dissociation and the apoplastic uptake of Cd complex could not fully explain the increase of root Cd uptake. Two hypotheses are evaluated to explain enhanced Cd uptake in the presence of ligand: i) enhanced apoplastic uptake of complex due to reduced apoplastic resistance and ii) complex internalization by membrane transporters. RESULTS: show that the ligand affinity for Cd is a key characteristic determining the potential mechanism for enhanced Cd uptake. When low molecular weight organic acids are applied, the complex dissociation could generally be fast (> 10-3.3 s-1) and result in the increased Cd uptake. When hydrophilic aminopolycarboxylic acids (APCAs) are applied in experiments without water or temperature stresses to the plant, the root water uptake flux could very likely be high (> 10-7.8 dm s-1), and the strong apoplastic complex uptake could enhance the root Cd uptake. When lipophilic APCAs are applied, the strong internalization of the complex by membrane transporters could result in the increased Cd uptake if the maximum internalization rate is high (> 10-12 mol dm-2 s-1). However, the complex internalization by membrane transporters must be experimentally confirmed.

Evaluation of the Effects of Wastewater Irrigation on Heavy Metal Accumulation in Vegetables and Human Health in the Cauliflower Example : Heavy Metal Accumulation in Cauliflower

The goals of the present research were to determine the heavy metal contents in the water-soil-cauliflower samples in industrial wastewater irrigated areas and to assess the health risks of these metals to the people. Metal analyses were carried out using the atomic absorption spectrophotometer equipped with a graphite furnace. The metal readings in the cauliflower specimens ranged from 1.153 to 1.389, 0.037 to 0.095, 0.61 to 0.892, 0.625 to 0.921, 1.165 to 2.399, 0.561 to 0.652, 0.565 to 0.585, 0.159 to 0.218 and 1.268 to 1.816 mg/kg for Cd, Co, Cr, Cu, Fe, Ni, Pb, Zn and Mn, respectively. Statistics revealed that, with the exception of Pb and Co (p > 0.05), there was no statistically significant variation in the metal concentrations in the cauliflower samples according to the irrigation type. Pb, Ni, and Cr had HRI values below 1.0 and did not seem to be a hazard to human health, in contrast to Cd, Co, Cr, Cu, Fe, Ni, Pb, Zn and Mn, which glanced to constitute a health risk. Regular monitoring of vegetables irrigated with wastewater is strongly advised to reduce health hazards to people.

Thallium content in vegetables and derivation of threshold for safe food production in soil: A meta-analysis

Soil thallium (Tl) pollution is a serious environmental problem, and vegetables are the primary pathway for human exposure to Tl. Therefore, it is important to investigate the characteristics of soil Tl uptake by vegetables. In this study, the meta-analysis approach was first applied to explore the relationship between Tl content in vegetables and soil environment, as well as key factors influencing soil physical-chemical properties, and to derive soil thresholds for Tl. The results indicated that various types of vegetables have different capabilities for Tl accumulation. Vegetables from contaminated areas showed high Tl accumulation, and the geomean Tl content in different types of vegetables was in the following order: leafy > root-stalk > solanaceous vegetables. Taro and kale had significantly higher capability to accumulate soil Tl among the 35 species studied, with Tl bioconcentration factor values of 0.060 and 0.133, respectively. Pearson correlation analysis and meta-analysis revealed that the Tl content in vegetables was significantly correlated with soil pH and Tl content in soil. The linear predictive model for Tl accumulation in vegetables based on soil Tl content described the data well, and the fitting coefficient R2 increased with soil pH value. According to potential dietary toxicity, the derived soil Tl thresholds for all, leafy and root-stalk vegetables increased with an increase in soil pH, and were in the range of 1.46-6.72, 1.74-5.26 and 0.92-6.06 mg/kg, respectively. The soil Tl thresholds for kale, lettuce and carrot were in the range of 0.24-4.89, 2.94-3.32 and 3.77-14.43 mg/kg, respectively. Ingestion of kale, beet, sweet potato, potato, taro, pepper, turnip, Chinese cabbage, eggplant and carrot poses potential health risks. The study provides scientific guidance for vegetable production in Tl-contaminated areas and can help with the selection of vegetable species suitable for avoiding the absorption of Tl from contaminated soil.

Influencing factors of selenium transformation in a soil-rice system and prediction of selenium content in rice seeds: a case study in Ninghua County, Fujian Province

Selenium (Se) is an essential element for human and animal health and has antioxidant, anticancer, and antiviral effects. However, more than 100 million people in China do not have enough Se in their diets, resulting in a state of low Se in the human body. Since the absorption of Se by crop seeds depends not only on the Se content in soil, there are many omissions and misjudgments in the division of Se-rich producing areas. Soil pH, total iron oxide content (TFe2O3), soil organic matter (SOM), and P and S contents were the main factors affecting Se migration and transformation in the soil-rice system. In this study, we compared the performance of the back propagation neural network (BP network) and multiple linear regression (MLR) using 177 pairs of soil-rice samples. Our results showed that the BP network had higher accuracy than MLR. The accuracy and precision of the prediction data met the requirements, and the prediction data were reliable. Based on the Se data of surface paddy fields, 26,900 ha of Se-rich rice planting area was planned using this model, accounting for 77% of the paddy field area. In the planned Se-rich area for rice, the proportion of soil Se content greater than 0.4 mg·kg-1 was only 5.29%. Our research is of great significance for the development of Se-rich lands.

Prediction of cadmium contents in rice grains from Quaternary sediment-distributed farmland using field investigations and machine learning

The Quaternary sediment-distributed regions of South China are suitable for rice cultivation, which is crucial for ensuring food security. Spatial correlations between soil cadmium (Cd) and rice Cd contents are generally poor, making the evaluation of rice quality and associated health risks challenging. In this study, we developed machine learning algorithms for predicting rice Cd contents using 654 data pairs of soil-rice samples collected in Guangxi province, China. After a comprehensive comparison, our results showed that the random forest (RF) had the better performance than artificial neural network (ANN) based on all the data (RMSEtesting 0.066 vs. 0.099 and R2testing 0.860 vs. 0.688). The feature importance analysis showed that soil CaO, Cd, elevation, and rainfall were the four most important features affecting the rice Cd contents in the study area. Using the established RF-predicated model, the rice Cd contents were predicted at the provincial level with an additional dataset of 1176 paddy soil samples. The prediction result revealed about 23 % of farmland cultivated rice with Cd content over 0.2 mg kg-1 in the study area. Therefore, it is recommended to implement strict measures by local agricultural departments to reduce rice Cd contents and ensure food safety in these areas. Our study provides valuable insights into the prediction of rice Cd contents, thus contributing to ensuring food safety and preventing Cd exposure-associated health risks.

Potential functions of engineered nanomaterials in cadmium remediation in soil-plant system: A review

Soil cadmium (Cd) contamination is a global environmental issue facing agriculture. Under certain conditions, the stable Cd that bound to soil particles tend to be remobilized and absorbed into plants, which is seriously toxic to plant growth and threat food safety. Engineering nanomaterials (ENMs) has attracted increasing attentions in the remediation of Cd pollution in soil-plant system due to their excellent properties with nano-scale size. Herein, this article firstly systematically summarized Cd transformation in soil, transport in soil-plant system, and the toxic effects in plants, following which the functions of ENMs in these processes to remediate Cd pollution are comprehensively reviewed, including immobilization of Cd in soil, inhibition in Cd uptake, transport, and accumulation, as well as physiological detoxication to Cd stress. Finally, some issues to be further studied were raised to promote nano-remediation technology in the environment. This review provides a significant reference for the practical application of ENMs in remediation of Cd pollution in soil, and contributes to sustainable development of agriculture.

Quantitative relationship between soil pH and electrical conductivity values and cadmium phytoavailability for Chinese cabbage under simulated conditions

Pot experiments were conducted to investigate the impacts of continuous addition of different concentrations of calcium chloride (CaCl2) and/or low-molecular-weight organic acids (LMWOAs) on soil pH, electrical conductivity (EC), and cadmium (Cd) transformation. These factors subsequently affected Cd phytoavailability in a system consisting of Cd-contaminated soil and Chinese cabbage (Brassica chinensis L.). The results indicate that CaCl2 addition had a greater impact on reducing soil pH value, increasing soil EC value, and enhancing Cd phytoaccumulation in Chinese cabbage compared to LMWOAs. When soil pH dropped by 0.3 unit and the soil EC increased by 500 µS cm-1, the Cd concentration in the Chinese cabbage shoots was 3 times higher than that in the control group. Throughout two planting terms of Chinese cabbage, the addition of CaCl2 (1.6-3.2 g kg-1) and LMWOAs (≤ 1.0 g kg-1) led to phytoextracted Cd concentration exceeding exchangeable Cd concentration in soil samples before the pot experiment. Regarding phytoextracted Cd, desorption from carbonate-bound Cd contributes more than desorption from bound to organic matter Cd and adsorption to Fe/Mn oxide Cd. This study underscores the influence of soil pH and EC value variations and Cd transformation on Cd phytoavailability. Special attention should be given to leafy vegetables grown in Cd-contaminated soil, as the phytoavailable Cd concentration reaches approximately 2.0 µg kg-1, which may lead to Cd levels surpassing acceptable limits for Chinese cabbage.

Influence of Industrial Wastewater Irrigation on Heavy Metal Content in Coriander (Coriandrum sativum L.): Ecological and Health Risk Assessment

The primary objective of this study was to determine the heavy metal contents in the water-soil-coriander samples in an industrial wastewater irrigated area and to assess the health risks of these metals to consumers. Sampling was done from areas adjoining the Chistian sugar mill district Sargodha and two separate sites irrigated with groundwater (Site 1), and sugar mill effluents (Site 2) were checked for possible metal contamination. The water-soil-coriander continuum was tested for the presence of cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), iron (Fe), manganese (Ni), lead (Pb), and zinc (Zn). The mean concentrations of all metals were higher than the permissible limits for all studied metals except for Mn in the sugar mill wastewater, with Fe (8.861 mg/L) and Zn (9.761 mg/L) exhibiting the highest values. The mean levels of Fe (4.023 mg/kg), Cd (2.101 mg/kg), Cr (2.135 mg/kg), Cu (2.180 mg/kg), and Ni (1.523 mg/kg) were high in the soil at Site 2 in comparison to the groundwater irrigated site where Fe (3.232 mg/kg) and Cd (1.845 mg/kg) manifested high elemental levels. For coriander specimens, only Cd had a higher mean level in both the groundwater (1.245 mg/kg) and the sugar mill wastewater (1.245 mg/kg) irrigated sites. An estimation of the pollution indices yielded a high risk from Cd (health risk index (HRI): 173.2), Zn (HRI: 7.012), Mn (HRI: 6.276), Fe (HRI: 1.709), Cu (HRI: 1.282), and Ni (HRI: 1.009), as all values are above 1.0 indicating a hazard to human health from consuming coriander irrigated with wastewater. Regular monitoring of vegetables irrigated with wastewater is strongly advised to reduce health hazards to people.

Foliar application of salicylic acid inhibits the cadmium uptake and accumulation in lettuce (Lactuca sativa L.)

Introduction

Salicylic acid (SA) is a multi-functional endogenous phytohormone implicated in the growth, development, and metabolism of many plant species.

Methods

This study evaluated the effects of different concentrations of SA (0, 25, 100, 200, and 500 mg/L) on the growth and cadmium (Cd) content of lettuce (Lactuca sativa L.) under Cd stress. The different concentrations of SA treatments were administered through foliar application.

Results

Our results showed that 100-200 mg/L SA significantly increased the plant height and biomass of lettuce under Cd stress. When SA concentration was 200 mg/L, the plant height and root length of lettuce increased by 19.42% and 22.77%, respectively, compared with Cd treatment alone. Moreover, 200 mg/L and 500mg/L SA concentrations could reduce peroxidase (POD) and superoxide dismutase (SOD) activities caused by Cd stress. When the concentration of exogenous SA was 500 mg/L, the POD and SOD activities of lettuce leaves decreased by 15.51% and 19.91%, respectively, compared with Cd treatment. A certain concentration of SA reduced the uptake of Cd by the lettuce root system and the transport of Cd from the lettuce root system to shoots by down-regulating the expression of Nramp5, HMA4, and SAMT, thus reducing the Cd content of lettuce shoots. When the concentration of SA was 100 mg/L, 200 mg/L, and 500 mg/L, the Cd contents of lettuce shoots were 11.28%, 22.70%, and 18.16%, respectively, lower than that of Cd treatment alone. Furthermore, principal component and correlation analyses showed that the Cd content of lettuce shoots was correlated with plant height, root length, biomass, antioxidant enzymes, and the expression level of genes related to Cd uptake.

Discussion

In general, these results provide a reference for the mechanism by which SA reduces the Cd accumulation in vegetables and a theoretical basis for developing heavy metal blockers with SA components.

Derivation and validation of soil cadmium thresholds for the safe farmland production of vegetables in high geological background area

Excessive dietary intake of cadmium (Cd) poses toxicity risks to human health, and it is therefore essential to establish accurate and regionally appropriate soil Cd thresholds that ensure the safety of agricultural products grown in different areas. This study investigated the differences in the Cd accumulation in 32 vegetable varieties and found that the Cd content ranged from 0.01 to 0.24 mg·kg^-1, and decreased in the order of stem and bulb vegetables > leafy vegetables > solanaceous crops > bean cultivars. A correlation analysis and structural equation model showed that pH, soil organic matter, and the cation exchange capacity had significant effects on Cd accumulation in the vegetables and explained 72.1 % of the variance. In addition, species sensitivity distribution (SSD) curves showed that stem and bulb vegetables were more sensitive to Cd than other types of vegetables. Using the Burr Type III function for curve fitting, we derived Cd thresholds of 6.66, 4.15, and 1.57 mg·kg^-1 for vegetable soils. These thresholds will ensure that 20 %, 50 %, and 95 % of these vegetable varieties were risk-free, respectively. The predicted threshold of soil Cd was more than twice that of China's current National Soil Quality Standard (GB 15618-2018) for Cd values. Therefore, soil scenarios and cultivars should be considered comprehensively when determining farmland soil thresholds. The present results provide a new model for setting soil Cd criteria in high geological background areas.

Comparative transcriptomic analysis reveals the important process in two rice cultivars with differences in cadmium accumulation

To date, Cd remains a major contaminant in rice production. An in-depth exploration of the mechanism that causes genotypic differences in Cd enrichment in rice is necessary to develop strategies to regulate Cd enrichment in rice. Here, two rice cultivars (low grain Cd, ZZ143; and high grain Cd, YX409) displayed different transcriptomic profile patterns when subjected to 100μmol/L Cd stress. In fact, 18,721(9833 upregulated and 8888 downregulated) and 16,403 (8366 upregulated and 8037 downregulated) differentially expressed genes (DEGs) were found in ZZ143 and YX409, respectively. Gene ontology (GO) classification revealed 28 and 26 terms enriched in ZZ143 and YX409, respectively. ZZ143 had more enriched DEGs than YX409, primarily in cellular processes, metabolic processes, binding terms, catalytic activity, etc. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that 21 and 24 pathways were significantly enriched in ZZ143 and YX409, respectively. Based on the DEGs, ZZ143 had a stronger ability for sulfur assimilation and Cys synthesis, whereas YX409 had a stronger ability to maintain cell wall stability. A series of DEGs involved in metabolic pathways, biosynthesis of secondary metabolites, plant hormone signal transduction pathways, and mitogen-activated protein kinase signaling pathways were identified, which maybe closely related to Cd resistance and the different Cd concentrations between cultivars. The above pathways and the greater number of identified DEGs in more than half of the GO terms and KEGG pathways suggest a higher absorption and stronger tolerance of the roots of ZZ143 than YX409 to Cd.

A comprehensive assessment of heavy metal(loid) contamination in leafy vegetables grown in two mining areas in Yunnan, China-a focus on bioaccumulation of cadmium in Malabar spinach

Contamination of leafy vegetables grown in heavy metal(loid)-polluted mining areas pose serious health risks. This study aimed to explore the heavy metal(loid) contamination of leafy vegetables near two mining areas, by collecting samples from 14 different leafy vegetable species in Yunnan Province, China. The lead (Pb), cadmium (Cd), arsenic (As), and copper (Cu) contents of the samples were determined, and risks to human health were calculated using the hazard quotient and hazard index (HI). Moreover, Malabar spinach was identified as a leafy vegetable that exhibits low accumulation of heavy metal(loid)s. The accumulation capacity of different Malabar spinach varieties was verified, and a Cd soil safety threshold was determined using a pot experiment. Overall, Pb and Cd were the main soil and vegetable contaminants found in both study sites. The HI values for all leafy vegetables, apart from Malabar spinach, were greater than 1, indicating the presence of risks to human health; moreover, the health risks were greater for children than adults. The Malabar spinach pot experiment results showed that only some Cd forms exceeded China's maximum permissible standards. Furthermore, Malabar spinach varieties A (instant Malabar spinach), C (extra-large leaf green vine Malabar spinach), and F (large leaf Malabar spinach) displayed the lowest Cd accumulation. We calculated Cd total and bioavailable soil safety thresholds of 4.75 and 0.77 mg kg-1, respectively. However, further research is required to validate soil heavy metal safety thresholds for different vegetables. Ultimately, the heavy metal(loid) contamination of leafy vegetables described here was more serious than anticipated. Finally, the results of this study can inform residents living near these mining areas of a low-risk leafy vegetable, which will reduce the harm caused by heavy metal(loid) contamination in the area.

Impact of Vanadium-Containing Stone Coal Smelting on Trace Metals in an Agricultural Soil-Vegetable System: Accumulation, Transfer, and Health Risks

Dietary exposure to trace metals (TMs) through vegetable consumption has been identified as a potential risk to human health. Fifty-one paired agricultural soil and leaf vegetable samples were collected around V-containing stone coal smelting sites in Hunan Province, China, to study the contamination and transfer characteristics of TMs (Cd, Cr, Cu, Pb, V, and Zn) in the soil-vegetable system. The health risk to local residents through vegetable ingestion was evaluated using Monte Carlo simulations. The results showed that 96.2%, 23.1%, 53.8%, 30.8%, 96.2%, and 69.2% of the soil samples had Cd, Cr, Cu, Pb, V, and Zn contents exceeding their related maximum allowable values, respectively. Cadmium and V were the primary pollutants based on the I_geo values. Moreover, 46.9% and 48.4% of vegetable samples exceeded the maximum permissible levels for Cd and Pb, respectively. There was a negative correlation between the bioaccumulation factors for Cd and V of the vegetable and soil physicochemical properties, including pH, organic matter, and free Fe₂O₃ content. Ingestion of garland chrysanthemum and pak choi posed high health risks, and Cd, V, and Pb were the primary contributors. These findings will help design strategies to minimize contamination and human exposure to soil-vegetable systems caused by V-containing stone coal smelting.

The determination of regulating thresholds of soil pH under different cadmium stresses using a predictive model for rice safe production

Regulating soil pH becomes a crucial practice to alleviate cadmium (Cd) contamination. However, little is known about the threshold of soil pH for the safe production of rice at various soil Cd levels. In this paper, the relationships between soil pH values and the contents of available Cd extracted by calcium chloride (CaCl₂-Cd) in neutral and acidic soils were studied by mandatory acidification with H⁺ addition or neutralization with lime at various soil Cd levels. The results showed that the soil CaCl₂-Cd contents dramatically decreased with increasing soil pH, and a logarithmic function could well describe the relations of soil CaCl₂-Cd contents and soil pH at constant total Cd (CaCl₂-Cd model). The Cd contents in rice grain (grain-Cd) in relation to soil CaCl₂-Cd was further established through modified rice pot experiments. A model for the prediction of Cd content in rice grains (grain-Cd model) was set up, though which the grain-Cd content could be predicted based on soil pH and total Cd content. 122 data pairs of rice grain-Cd contents obtained at various soil total Cd contents and pH were employed from the literature to verify the reliability of the established model, approximately 95.08% of those data favorably located within the 1:1 line ± 0.5 unit area of the grain-Cd model. Notably, this model can be applied to determine the thresholds of soil pH at a specific Cd pollution level. For instance, to achieve a rice grain-Cd contents matching the Chinese national food safety limit of 0.2 mg kg^-1, the soil pH thresholds were estimated to be 5.05, 5.70, and 6.02 at soil Cd contents of 0.3, 0.6, and 0.8 mg kg^-1, respectively. In addition, the established model can also be used to estimate the health risk from rice in broad regions with various soil pH values and Cd contents.

A New Sight of Influencing Effects of Major Factors on Cd Transfer from Soil to Wheat (Triticum aestivum L.): Based on Threshold Regression Model

Due to the high toxicity and potential health risk of cadmium (Cd), the influencing effects of major factors (like pH, OM, and clay, etc.) on Cd bioaccumulation and transfer from soil to crop grains are highly concerned. Multiple linear regression models were usually applied in previous literature, but these linear models could not reflect the threshold effects of major factors on Cd transfer under different soil environmental conditions. Soil pH and other factors on Cd transfer in a soil-plant system might pose different or even contrary effects under different soil Cd exposure levels. For this purpose, we try to apply a threshold regression model to analyze the effects of key soil parameters on Cd bioaccumulation and transfer from soil to wheat. The results showed that under different soil pH or Cd levels, several factors, including soil pH, organic matter, exchangeable Cd, clay, P, Zn, and Ca showed obvious threshold effects, and caused different or even contrary impacts on Cd bioaccumulation in wheat grains. Notably, the increase of soil pH inhibited Cd accumulation when pH > 7.98, but had a promotional effect when pH ≤ 7.98. Thus, threshold regression analysis could provide a new insight that can lead to a more integrated understanding of the relevant factors on Cd accumulation and transfer from soil to wheat. In addition, it might give us a new thought on setting regulatory limits on Cd contents in wheat grains, or the inhibitory factors of Cd transfer.

Use of artificial neural network to evaluate cadmium contamination in farmland soils in a karst area with naturally high background values

In recent years, the naturally high background value region of Cd derived from the weathering of carbonate has received wide attention. Due to the significant difference in soil Cd content and bioavailability among different parent materials, the previous land classification scheme based on total soil Cd content as the classification standard, has certain shortcomings. This study aims to explore the factors influencing soil Cd bioavailability in typical karst areas of Guilin and to suggest a scientific and effective farmland use management plan based on the prediction model. A total of 9393 and 8883 topsoil samples were collected from karst and non-karst areas, respectively. Meanwhile, 149 and 145 rice samples were collected together with rhizosphere soil in karst and non-karst areas, respectively. The results showed that the higher CaO level in the karst area was a key factor leading to elevated soil pH value. Although Cd was highly enriched in karst soils, the higher pH value and adsorption of Mn oxidation inhibited Cd mobility in soils. Conversely, the Cd content in non-karst soils was lower, whereas the Cd level in rice grains was higher. To select the optimal prediction model based on the correlation between Cd bioaccumulation factors and geochemical parameters of soil, artificial neural network (ANN) and linear regression prediction models were established in this study. The ANN prediction model was more accurate than the traditional linear regression model according to the evaluation parameters of the test set. Furthermore, a new land classification scheme based on an ANN prediction model and soil Cd concentration is proposed in this study, making full use of the spatial resources of farmland to ensure safe rice consumption.

Concentration of potentially harmful elements (PHEs) in eggplant vegetable (Solanum melongena) irrigated with wastewater: a systematic review and meta-analysis and probabilistic health risk assessment

The main objectives of this study were PHEs concentration meta-analysis (Fe, Zn, Cr, Ni, Cu, Pb, and Cd) in eggplant irrigated by wastewater and the following estimation of non-carcinogenic (n-CR) risk for the consumers based on countries. According to the results, the rank order of PHEs concentration in eggplant was Fe (88.3 mg/kg -dry weight) > Zn (10.1 mg/kg -dry weight) > Pb (3.0 mg/kg -dry weight) > Ni (2.7 mg/kg -dry weight) > Cu (1.1 mg/kg -dry weight) > Cd (0.9 mg/kg -dry weight) > Cr (0.05 mg/kg -dry weight). Moreover, n-CR risk showed that all investigated countries (China, India, Pakistan, Turkey, and Jordan) except for United Arab Emirates (UAE) had a considerable n-CR in both age groups (adults and children).

Influence of soil properties on cadmium accumulation in vegetables: Thresholds, prediction and pathway models based on big data

Soil properties, such as soil pH, soil organic matter (SOM), cation exchange capacity (CEC), are the most important factors affecting cadmium (Cd) accumulation in vegetables. In this study, we conducted big data mining of 31,342 soil and vegetable samples to examine the influence of soil properties (soil pH, SOM, CEC, Zn and Mn content) on the accumulation of Cd in root, solanaceous, and leafy vegetables in Hunan Province, China. Specifically, the Cd accumulation capability was in the following order: leafy vegetables > root vegetables > solanaceous vegetables. The soil property thresholds for safety production in vegetables were determined by establishing nonlinear models between Cd bioaccumulation factor (BCF) and the individual soil property, and were 6.5 (pH), 30.0 g/kg (SOM), 13.0 cmol/kg (CEC), 100-140 mg/kg (Zn), and 300-400 mg/kg (Mn). When soil property values were higher than the thresholds, Cd accumulation in vegetables tended to be stable. Prediction models showed that pH and soil Zn were the leading factors influencing Cd accumulation in root vegetables, explaining 87% of the variance; pH, SOM, soil Zn and Mn explained 68% of the variance in solanaceous vegetables; pH and SOM were the main contributors in leafy vegetables, explaining 65% of the variance. Further, variance partitioning analysis (VPA) revealed that the interaction effect of the corresponding key soil properties contributed mostly to BCF. Meanwhile, partial least squares (PLS) path modeling was employed to analyze the path and the interactive effects of soil properties on Cd BCF. pH and SOM were found to be the biggest two players affecting BCF in PLS-models, and the most substantial interactive influence paths of soil properties on BCF were different among the three types of vegetables.

Zinc concentration prediction in rice grain using back-propagation neural network based on soil properties and safe utilization of paddy soil: A large-scale field study in Guangxi, China

Zn is an essential nutrient for humans, with crucial biological functions. However, Zn concentration in rice grains is generally low. Therefore, a cereal-based diet may lead to Zn deficiency in people, further leading to a series of health problems, such as immune and brain dysfunction. Previous studies seldom focused on the accumulation of Zn in rice grains based on large-scale field research. In the present study, a large-scale field survey of paddy (n = 40,853) and paired soil-rice samples (n = 1332) was conducted in Guangxi, China. Zn concentration in soil and rice grains was determined, and the associations of its spatial distributions with lithology, soil properties, and Mn nodules were investigated. According to the daily rice intake of different age and sex groups and the values of recommended Zn intake and tolerable Zn upper intake level recommended by National Health Commission of China, the Zn threshold value of the rice grain is 15.47-24.49 mg·kg^-1. Moreover, a back-propagation neural network (BPNN) model was used to predict the Zn bioaccumulation factor (BAF) of rice grains with high accuracy. Soil Zn concentration, Mn concentration, pH, and total organic carbon derived from Pearson's correlation analysis were used as input variables in the BPNN model. Compared with the multiple linear regression model, the developed BPNN model using the training (1198 samples) and testing (134 samples) datasets showed better performance in estimating rice Zn BAF, with R² = 0.93, normalized mean error of 0.009, normalized root mean square error of 0.21. When the BPNN model was applied to the 40,853 paddy soil samples, 85.7% of the agriculture lands were within the rice threshold values. These findings further our understanding of the development and utilization of Zn-rich rice and soil.

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.

A risk-based approach for the safety analysis of eight trace elements in Chinese flowering cabbage (Brassica parachinensis L.) in China

BACKGROUND

Most countries set regulatory values for the total trace element (TE) concentrations in soil, although there is growing interest in using a risk-based approach to evaluate the bioavailable TE using dilute salt extractants or other soil parameters, including pH and organic carbon_. The present study compares the current regulatory system (based on total TEs and pH) and a risk-based approach using 0.01 mol L^-1 CaCl₂ to estimate the bioavailable fraction.

RESULTS

In total, 150 paired samples of Chinese flowering cabbages (Brassica parachinensis) and their growth soils were collected, and the total and extractable concentrations of chromium (Cr), cadmium (Cd), lead (Pb), nickel (Ni), copper (Cu), zinc (Zn), arsenic (As) and mercury (Hg), as well as soil pH and organic matter content, were measured. No more than 3.33% of the edible parts exceeded Chinese food safety standards, even when growing in soils exceeding the current regulatory thresholds by over 50%. The total soil Cd (1.5 mg kg^-1 ), as well as the extractable concentrations of Cd (0.1 mg kg^-1 ), Ni (0.03 mg kg^-1 ) and Zn (0.1 mg kg^-1 ), are the key factors affecting the TE concentrations in B. parachinensis.

CONCLUSION

Our findings suggest that the current soil regulatory guidelines for safe production of B. parachinensis are overly strict and conservative. A risk-based approach based on the extractable TE concentrations would provide a better indication for plant uptake of soil TEs and avoid the waste of farmlands that can still produce safe vegetables. Future research should focus on providing crop-specific available TE concentration guidelines to promote effective utilization of farmlands. © 2021 Society of Chemical Industry.

Evaluation of Cadmium Transfer from Soil to the Human Body Through Maize Consumption in a Cadmium Anomaly Area of Southwestern China

Evaluating the bioavailability, bioaccessibility, and transferability of cadmium (Cd) in soil-grain-human systems is essential in areas with a Cd anomaly in the karst region of southwestern China. In the present study, the main controlling factors and prediction models for Cd transfer in a soil-grain-human system were investigated in a typical area where natural processes and anthropogenic activities interact in the karst region of southwestern China. The environmental availability of Cd (diethylenetriaminepentaacetic acid- and CaCl₂ -extractable Cd [ CdCaCl2 ]) in the soil varies significantly because of the diversity of soil properties. However, Cd concentrations in the maize grain were significantly related only to the CdCaCl2 concentrations in the soil (r = 0.595, p < 0.01), indicating that soil CdCaCl2 is a good indicator for evaluating Cd uptake by maize grain. Of all the measured soil properties, the soil cation exchange capacity (CEC) and the soil calcium (Ca_soil ) were the most important factors influencing Cd accumulation in the soil-maize grain system. A transfer model combining CdCaCl2 , soil CEC, and Ca_soil was sufficiently reliable for predicting Cd accumulation in the maize grain (R²  = 0.505). Although there is room for improvement regarding the prediction performance of the chain model combining soil CdCaCl2 with Ca_soil to predict the bioaccessible Cd concentration in maize grain (R²  = 0.344 for the gastric phase and R²  = 0.356 for the gastrointestinal phase), our findings provide a useful reference to further explore a model that can be used for a relatively rapid and reliable estimation of dietary Cd exposure for specific regions prior to crop harvest. Environ Toxicol Chem 2021;40:2923-2934. © 2021 SETAC.

Heavy Metals Presence in the Soil and Their Content in Selected Varieties of Chili Peppers in Slovakia

Capsicum chili peppers are popular vegetables in Slovakia. They provide a supply of health-promoting substances, but contaminated vegetables can pose a serious health risk to the people who consume them. Therefore, the aim of this study was to determine the content of heavy metals (Mn, Zn, Cr, Cu, Ni, Cd, Pb and Hg) in the soil as well as in selected varieties of the genus Capsicum grown in southern Slovakia. The results were compared with the limit values given by the Law no. 220/2004 (valid in the SR) as well as threshold values proposed by the European Commission (EC) (2006). The gained result showed that the total content of Cd (1.64 mg/kg) as well as the available mobile forms of Cd (0.12 mg/kg) and Pb (0.26 mg/kg) was exceeded on the soil on which Capsicum cultivars were grown. The limit values of other monitored heavy metals (Mn, Zn, Cr, Cu, Ni, and Hg) were not exceeded. The studied species of the genus Capsicum did not accumulate monitored heavy metals. It can be stated that consumption of chili peppers does not pose any risk for human health.

Comparison of nanoparticle-selenium, selenium-enriched yeast and sodium selenite on the alleviation of cadmium-induced inflammation via NF-kB/IκB pathway in heart

Cadmium (Cd) has been confirmed as an environmental contaminant, which potential threats health impacts to humans and animals. Selenium (Se) as a beneficial element that alleviates the negative effects of Cd toxicity. Se mainly exists in two forms in food nutrients including organic Se usually as (Se-enriched yeast (SeY)) and inorganic Se (sodium selenite (SSe)). Nanoparticle of Se (Nano-Se), a new form Se, which is synthesized by the bioreduction of Se species, which attracted significant attention recently. However, compared the superiority alleviation effects of Nano-Se, SeY or SSe on Cd-induced toxicity and related mechanisms are still poorly understood. The purpose of this study was to compare the superiority antagonism effects of Nano-Se, SeY and SSe on Cd-induced inflammation response via NF-kB/IκB pathway in the heart. The present study demonstrated that exposed to Cd obviously increased the accumulation of Cd, disruption of ion homeostasis and depressed the ratios of K⁺/Na⁺ and Mg²⁺/Ca²⁺ via ion chromatography mass spectrometry (ICP-MS) detecting the heart specimens. In the results of histological and ultrastructure observation, typical inflammatory infiltrate characteristics and mitochondria and nuclear structure alterations in the hearts of Cd group were confirmed. Cd treatment enhanced the inducible nitric oxide synthase (iNOS) activities and NOS isoforms expression via NF-kB/IκB pathway to promote inflammation response. However, the combined treatment of Cd-exposed animals with Nano-Se was more effective than SeY and SSe in reversing Cd-induced histopathological changes and iNOS activities increased, reducing Cd accumulation and antagonizing Cd-triggered inflammation response via NF-kB/IκB pathway in chicken hearts. Overall, Se applications, especially Nano-Se, can be most efficiently used for relieving cardiotoxicity by exposed to Cd compared to other Se compound.

Derivation of empirical model to predict the accumulation of Pb in rice grain

Lead contamination in soil has become a worldwide threat on food security and human health. To assess the Pb bioavailability and evaluate the safe use of low Pb polluted soil for food production, the speciation of Pb in 19 types of paddy soil were investigated by chemical extraction and X-ray absorption near-edge structure (XANES), and the uptake and accumulation characteristics of Pb in different soil-rice systems were investigated. Moreover, an empirical model was established to predict the content of Pb in rice grain, and field validation was conduct to evaluate model performance. Results showed that the proportion of available Pb in different soil satisfied normal distribution N (0.47, 0.23). Pb(CH₃COO)₂, GSH-Pb, PbO, PbHPO₄ and Pb₃(PO₄)₂ performed well in characterizing the speciation of Pb in different rhizosphere soils, and PbHPO₄ accounted for more than 70%. The exceedance of Pb in grain in CK, 0.5X and 1X treatment were 10.5%, 36.1% and 42.1%, respectively, and the accumulation of Pb in grain was significantly related with Pb content in root. Carbonate and organic bound Pb in rhizosphere soil were two major Pb species that influenced the accumulation of Pb in rice. Moreover, content of total Pb, clay and SOM performed well in predicting the Pb content in grain, both for pot and field samples. Above all, our predicting model worked well in evaluating Pb accumulation in rice grain among low polluted paddy farmland (Total Pb < 300 mg/kg).

Hazardous Heavy Metals Accumulation and Health Risk Assessment of Different Vegetable Species in Contaminated Soils from a Typical Mining City, Central China

Heavy metal poisoning has caused serious and widespread human tragedies via the food chain. To alleviate heavy metal pollution, particular attention should be paid to low accumulating vegetables and crops. In this study, the concentrations of five hazardous heavy metals (HMs), including copper (Cu), chromium (Cr), lead (Pb), cadmium (Cd), and arsenic (As) were determined from soils, vegetables, and crops near four typical mining and smelting zones. Nemerow’s synthetical pollution index (P_n), Potential ecological risk index (RI), and Geo-accumulation index (I_geo) were used to characterize the pollution degrees. The results showed that soils near mining and metal smelting zones were heavily polluted by Cu, Cd, As, and Pb. The total excessive rate followed a decreasing order of Cd (80.00%) > Cu (61.11%) > As (45.56%) > Pb (32.22%) > Cr (0.00%). Moreover, sources identification indicated that Cu, Pb, Cd, and As may originate from anthropogenic activities, while Cr may originate from parent materials. The exceeding rates of Cu, Cr, Pb, Cd, and As were 6.7%, 6.7%, 66.7%, 80.0%, and 26.7% among the vegetable and crop species, respectively. Particularly, vegetables like tomatoes, bell peppers, white radishes, and asparagus, revealed low accumulation characteristics. In addition, the hazard index (HI) for vegetables and crops of four zones was greater than 1, revealing a higher risk to the health of local children near the mine and smelter. However, the solanaceous fruit has a low-risk index (HI), indicating that it is a potentially safe vegetable type.

Accumulation of potentially harmful elements (PHEs) in lettuce (Lactuca sativa L.) and coriander (Coriandrum sativum L.) irrigated with wastewater: a systematic review and meta-analysis and probabilistic health risk assessment

Water shortage and stress around the world lead to increasing wastewater reuse for the agricultural sector. In addition to its benefits, it can be a way to transfer pollutants such as potentially harmful elements (PHEs) to the human food chain. Many studies have been conducted for this purpose on various vegetables; however, no comprehensive study has been performed on lettuce (Lactuca sativa L.) and coriander (Coriandrum sativum L.). In this respect, the study was aimed to meta-analyze the PHEs concentration in the edible part of lettuce and coriander vegetables irrigated by wastewater. Carcinogenic risk (CR) and noncarcinogenic risk (non-CR) assessments were also done for consumers. After reviewing 32 included articles (41 studies), the rank order of the PHEs in lettuce was obtained as Fe (194.76 mg/kg) > Zn (133.47 mg/kg) > Cu (55.70 mg/kg) > Ni (26.96 mg/kg) > Pb (12.80 mg/kg) > Cr (9.68 mg/kg) Cd (8.24 mg/kg) > As (1.13 mg/kg) and for coriander Fe (1056 mg/kg) > Zn (79.80 mg/kg) > Cr (28.34 mg/kg) > Ni (24.71 mg/kg) > Cu (17.46 mg/kg) > Pb (13.23 mg/kg) > Cd (2.23 mg/kg). Total target hazard quotient (TTHQ) for adults in all countries except UAE, France, and Kenya and for children all countries except Kenya was more than 1 value. The carcinogenic risk for adult groups in Nigeria, France, China, and Iran countries was not acceptable (CR > 1E-4). As a consequence, it can be noted that wastewater reuse in the agriculture sector can endanger the health of consumers.

Intrinsic soil property effects on Cd phytotoxicity to Ligustrum japonicum 'Howardii' expressed as different fractions of Cd in forest soils

A better comprehensive understanding of the influence of soil/solution properties on cadmium (Cd) phytotoxicity is essential for soil Cd ecological risk assessment. The toxicity of soil spiked Cd to Ligustrum japonicum 'Howardii' seedling growth was conducted by the greenhouse pot experiments using 13 typical forest soils selected from mainland of China. The results showed that the ranges of Cd toxicity thresholds of 10% seedling growth inhibition (EC₁₀) and 50% inhibition (EC₅₀) followed the order: soil pore water Cd (EC₁₀ on average 0.88 mg L^-1 with the variation of 54.9 folds and EC₅₀ on average 2.28 mg L^-1 with variation of 41.8 folds), DTPA extractable Cd (EC₁₀ on average 5.4 mg kg^-1 with 20.9 folds variation and EC₅₀ on average 17.86 mg kg^-1 with 6.6 folds variation), total added Cd (EC₁₀ on average 6.55 mg kg^-1 with 16.7 folds variation and EC₅₀ on average 22.11 mg kg^-1 with 5.1 folds variation), which suggested that whatever the available Cd expressed, its toxicity is largely affected by soil properties. The empirical multiple equations were well developed between different fractions of Cd toxicity thresholds ECx (x = 10 or 50) and soil/solution. The results also showed that the pH inversely correlated with EC₁₀ (r² = 0.54, P < 0.01) and EC₅₀ (r² = 0.63, P < 0.001) based on soil pore water, indicating the ECx decreased with more toxicity as pH increased. No single significant soil solution properties were found for ECx in DTPA extractable Cd. For the ECx of DTPA extractable and total Cd, the content of aluminum oxides in soil and soil pH were the two significant factors inversely related with ECx, which explained 68%-79% of the inter-soil variation, respectively. Overall, soil or solution pH was the most important factor controlling Cd toxicity thresholds. Meanwhile, significant negative correlations existed between the soil solution pH and the slopes of parameter (b) of the dose-response curves for different fractions of Cd, implying that the growth of toxic effect enhanced as unit Cd dosage increased in low pH soils. These results will be helpful to evaluate the metal ecological risk in forest soils.

Cadmium adsorption behavior of porous and reduced graphene oxide and its potential for promoting cadmium migration during soil electrokinetic remediation

In this study, a porous reduced graphene oxide (PRGO) carbon nanomaterial was successfully obtained by activation of natural graphite with KOH at high temperature and was applied as an auxiliary electrode in soil electrokinetic remediation to investigate the promoting effect on Cd migration. We found that PRGO contained a large amount of oxygen-containing groups (hydroxyl and carboxyl groups) and exhibited high Cd²⁺ adsorption efficiency at pH values above 4, achieving a maximum adsorption capacity of 434.78 mg/g for Cd. In addition, PRGO could selectively adsorb Cd, Pb, Cu, and Zn but not K, Na, or Mg from soil solution. The electrokinetic remediation experiment showed that the PRGO auxiliary electrode promoted the migration of Cd and effectively controlled the increase in soil pH near the cathode, possibly due to ion exchange between the surface functional groups on the auxiliary electrode and Cd²⁺. In addition, the location of the PRGO auxiliary electrode strongly influenced the migration of Cd. For instance, the soil Cd concentration of treatment H-5 was 57.86% lower than that of H-0 at a distance of 5-10 cm from the electrode; however, the soil Cd concentration measured at 0-5 cm for treatment H-5 was 34.84% higher than that of treatment H-0. Our study demonstrated that PRGO could be applied as an auxiliary electrode to promote Cd migration during electrokinetic remediation of Cd-contaminated soil.

Predicting accumulation of Cd in rice (Oryza sativa L.) and soil threshold concentration of Cd for rice safe production

Rice contamination by cadmium (Cd) poses a serious threat to human health, which has attracted widespread concerns in China. It is imperative to determine major soil factors influencing the accumulation of Cd in rice and develop prediction models to derive the threshold concentration of Cd in soil for rice food safety. In this study, the bioavailability, accumulation, and transfer of Cd in the 18 typical paddy soil-rice systems with a wide range of soil properties was investigated using pot experiments. The regression-based models incorporated with total or extractable Cd and soil properties were constructed to predict Cd content of rice grain. Pot experimental results indicated that rice showed a high accumulation potential for Cd, while rice grains grown in acid soils displayed larger Cd contents than those in neutral and alkaline soils. The pH and MnO content were major soil factors influencing the Cd accumulation of rice. Multiple regression models based on the total Cd, extractable Cd, pH, and MnO content in soils could well describe the Cd content in rice grain. Measured Cd content of rice grains from field samples demonstrated that the empirical models could quantitatively predict the Cd content of rice grains. The threshold concentrations of Cd in soils for rice food safety could be back-calculated by both EDTA-extractable and total Cd contents in soils. The EDTA-extractable Cd in soils could use as an indication to derive the threshold concentrations of Cd for rice food safety. In conclusions, multiple regression models proved reliable and practical in predicting Cd accumulation in rice grain. These empirical models could well predict the content of Cd in rice grain and deduce soil Cd threshold criteria. These results could help to quantitatively evaluate the health risk of Cd accumulation in rice crop and provide a useful reference for safe production of rice.

Cadmium uptake from soil and transport by leafy vegetables: A meta-analysis

Cadmium (Cd) pollution in soil is a serious problem affecting environmental safety and human health, and the majority of Cd in human body comes from edible vegetables, especially leafy vegetables. Therefore, it is necessary to understand the absorption and transport of Cd soil by leafy vegetables. In this study, the meta-analysis method was firstly employed to study the relationship of Cd in leafy vegetables and soil systems. The results showed that different kinds of leafy vegetables have different abilities of Cd accumulation (measured by bioconcentration factor (BCF)) and transportation (measured by translocation factor (TF)): Brassica juncea (BCF = 5.10) and Brassica pekinensis (BCF = 1.90) had significantly higher ability to absorb cadmium in soil among the 19 studied species, Brassica pekinensis (TF = 2.52), Coriandrum sativum (TF = 2.18) had significantly higher cadmium transport capacity than other 11 species. To further clarify the influence of the three main factors of soil pH, Cd content and leafy vegetable species on the Cd enrichment ability of leafy vegetables, the regression equation was obtained by meta-regression analysis. BCF is affected by species, soil pH, soil cadmium content in the order from high to low. It was found that the estimated range of SOM for safe production of leafy vegetables is 20-30 g/kg. It could also be observed that soil cation exchange capacity (CEC) had a negative correlation with BCF, while soil salinity had a strong positive correlation with BCF. This study can provide a reliable reference for leafy vegetable security production in the Cd polluted field and aids in selecting species suitable for avoiding the absorption of heavy metals from polluted soil.

Evaluation of cadmium (Cd) transfer from paddy soil to rice (Oryza sativa L.) using DGT in comparison with conventional chemical methods: derivation of models to predict Cd accumulation in rice grains

The consumption of rice contaminated with soil cadmium (Cd) threatens human health. It is essential to ensure the production of rice that meets food quality standards. Therefore, a large-scale field survey was conducted in Zhejiang province, southeastern China, to investigate the relationship between Cd accumulation in rice grains and Cd bioavailability in soil, and thus to establish a model to predict Cd contents in rice grains based on soil properties. For this purpose, a total of 156 paired rice and soil samples were collected. Pearson's correlation analysis revealed that Cd measurements obtained by diffusive gradient in thin films (DGT) had a higher correlation (r = 0.818, p < 0.001) with the Cd in rice grains as compared to the Cd measured by the DTPA, CaCl₂, EDTA, and HCl extraction methods, which indicated that the DGT technique was a reliable method for the assessment of Cd bioavailability in soils. In addition, among the four extraction methods, the DTPA-extractable Cd showed the highest correlation with the Cd contents in rice grains. Therefore, we developed two predictive models (model_DGT and model_DTPA) to predict Cd levels in rice grains via Cubist multivariate mixed linear regression, using "soil DGT-measured Cd, pH, and oxide contents of Ca, Si, and Fe" or "soil DTPA-extractable Cd, pH, OM, and oxide contents of Ca and Fe" as explanatory variables, respectively. The overall model_DGT and model_DTPA had R² values of 0.95 and 0.93, respectively, and relative error values of 0.30 and 0.33, respectively. Simple correlation analysis showed direct and close relationships between the measured Cd in rice grains and the Cd concentrations predicted by the Cubist model_DGT and model_DTPA, with R² values of 0.979 and 0.922, respectively. Therefore, Cd levels in rice grains could be predicted very well based on the two prediction models, and thus, the two models derived in this study are effective in identifying soils in which the Cd in rice grains will exceed food safety standards, thereby helping to ensure safe rice production.

Development and environmental implication of pedotransfer functions of Cd desorption rate coefficients in historically polluted soils

The desorption rate is an important factor determining cadmium (Cd) ecotoxicity and pollution remediation in soils. The pedotransfer functions (PTFs) of desorption rate coefficients of fresh Cd in soils have been developed in literature. We hypothesized that the aging of Cd pollution would alter Cd desorption process. Taking historically polluted soils as the object, this study aimed at testing the hypothesis and developing new PTFs of desorption rate coefficients for historical Cd. 15 d batch extraction experiments and 13 kinetic models were employed to define Cd desorption rate coefficients in 27 historically polluted soil samples. Compared with fresh Cd, the desorption rate coefficients of historical Cd were lower, and the break time of biphasic desorption processes was retarded to 3 d (4320 min). Different with the usual models for fresh Cd desorption (e.g. parabolic diffusion and two constant rate models), the best models to mimic the historical Cd desorption processes were the pseudo first order, logarithmic, Elovich, and simple Elovich models. The rate-limiting step controlling Cd desorption was changed from the intraparticle diffusion to the interface reaction with aging of pollution. New PTFs of desorption rate coefficients of historical Cd were established (R² ≥ 0.71). Cd desorption rate coefficients increased with organic matter and clay contents, but decreased with oxalate extractable Fe content, solution pH, cation exchange capacity, and silt content. The key soil properties influencing desorption rate coefficients were not altered by the aging of pollution. The developed PTFs could guide us to adjusting the ecotoxicity and pollution remediation of Cd in historically polluted field soils.

The effect of sepiolite application on rice Cd uptake - A two-year field study in Southern China

Sepiolite (SEP) is a clay mineral with great potential to stabilize soil heavy metals. A two-year field experiment was conducted to explore the optimum use of SEP to immobilize soil Cd and to promote the consumption safety of rice grown in a typical paddy field in Southern China. SEP was applied once or twice over the two-year study at three levels (0.1, 0.5, and 1%, w/w) before rice planting. The results showed that SEP effectively reduced rice grain Cd concentrations by 47-49% in the first year and by 44-50% in the second year due to the residue effect. Application of SEP for two consecutive years reduced the rice grain Cd concentration by up to 75%, achieving a safe level (<0.2 mg kg^-1). SEP also reduced Zn concentrations in rice grains (by 6-10%), while the Cd/Zn ratios of rice grains were decreased by 24-72% over the two years, implying it was much safer for consumption. SEP significantly increased the soil pH (0.9-1.8 units) and available phosphorus, and it reduced the soil available Cd (by 20-95%) and Zn concentrations (by 30-99%). In brief, SEP effectively stabilized soil Cd and reduced uptake by rice; the effect was dose-dependent and 0.5% (w/w) was optimum in the present study. The main mechanism of SEP to stabilize soil Cd is the increase in soil pH analogous to liming. This study shows that SEP application can be an efficient way to remediate Cd contaminated rice paddies and fulfill the goal of safe production of rice and thereby reduce the health risks associated with consuming rice.

Cd accumulation and transfer in pepper (Capsicum annuum L.) grown in typical soils of China: pot experiments

Food chain contamination by soil cadmium (Cd) through vegetable consumption poses a threat to human health. It is imperative to understand the Cd uptake and transfer in different soil-vegetable systems. The aims of this study were to understand the effect of soil characteristics on Cd accumulation and transfer in pepper and to derive regression models to predict Cd concentrations in the vegetable grown on a wide range of soils with different properties. The accumulation and transfer of Cd in the root, stem, and fruit of pepper (Capsicum annuum L.) grown in 18 typical soils of China were investigated through pot experiments. The bioavailability of Cd in soil was evaluated by using EDTA and HNO₃ extraction methods. The pot experiments included a control and two concentration levels of Cd salt added to soils according to Soil Environmental Quality Standards of China. The results showed that the Cd content in pepper fruits ranged from 0.007 to 0.049 for the control, 0.045 to 0.260 for the low Cd treatment, and 0.076 to 0.345 mg/kg for the high Cd treatment, respectively. The concentrations of Cd in the different parts of pepper decreased in the order of root > stem > fruit, and there were significant correlations among the Cd concentrations in pepper root, stem, and fruit tissues. Bioaccumulation factor (BCF) and transfer factor (TF) of Cd in pepper fruits exhibited a low accumulation of Cd in the fruit of pepper. The Cd accumulation in pepper fruit could be quantitatively predicted by EDTA-extractable Cd content in soils. Multiple linear regression models proved functional in predicting Cd accumulation in different parts of pepper. The Cd content in pepper tissues was well predicted using EDTA-extractable Cd and soil variables, such as pH, EC, CEC, total phosphorus, and CaCO₃ content. Soil pH and EC were major soil factors influencing Cd transfer from soil to pepper fruits, whereas total phosphorus content presented a negative effect on Cd accumulation in stem and root parts of pepper.

Adsorption of 17β-estradiol onto humic-mineral complexes and effects of temperature, pH, and bisphenol A on the adsorption process

The long-term use of animal manure in agriculture has resulted in estrogen pollution, which poses risks to facility vegetable soils. Owing to the complex soil composition, estrogen may exhibit a variety of behaviors at the water/soil interface. This study demonstrated the role of humic acid (HA) on the 17β-estradiol (E2) adsorption by clay minerals (montmorillonite, kaolinite, and hematite). The interfacial behaviors were investigated using adsorption kinetics and isotherms data. Then, the effects of temperature, pH, and bisphenol A (BPA) on the interactions between humic-mineral complexes and E2 were explored. The adsorption of E2 is an exothermic and spontaneous process, and the addition of HA to minerals significantly promoted their E2 adsorption capacities. Higher pH levels (>10) and the presence of BPA decreased the adsorption capacities of minerals and mineral complexes for E2. Moreover, intercalation, hydrophobic partitioning, π-π interactions and hydrogen bonding could dominate the E2 adsorption onto complexes. These results provided insight into the interfacial behaviors of E2 on the surfaces of humic-mineral complexes and promoted the understanding of the migration and transport of estrogens in soils.

Probabilistic Human Health Risk Assessment of Heavy Metal Intake via Vegetable Consumption around Pb/Zn Smelters in Southwest China

Vegetable contamination in mining and smelting areas has resulted in high dietary intakes of heavy metals, which pose potential health risks to local residents. In this study, paired soil-vegetable samples were collected around Pb/Zn smelters in Southwest China. Probabilistic risks to local residents via vegetable consumption were evaluated with a Monte Carlo simulation. The mean concentrations of As, Cd, Cu, Pb, and Zn in the soils were 116.76, 3.59, 158.56, 196.96, and 236.74 mg/kg, respectively. About 38.18%, 58.49%, and 52.83% of the vegetable samples exceeded the maximum allowable concentrations for As, Cd, and Pb, respectively. The daily dietary intake of As, Cd, and Pb exceeded the provisional tolerable daily intakes for local residents, with children showing the highest intake via vegetable consumption. The percentages of the target hazard quotients of As, Cd, and Pb for local residents exceeding the safe value of one were about 95%, 50%, and 25%, respectively. The 95th percentiles of the hazard index for children, adolescents, and adults were 15.71, 11.15, and 9.34, respectively, indicating significant risks to local residents, especially children. These results highlight a need to develop effective strategies to reduce heavy metal contamination and exposure to protect human health.