Indirect Estimation of Heavy Metal Contamination in Rice Soil Using Spectral Techniques

The rapid growth of industrialization and urbanization in China has led to an increase in soil heavy metal pollution, which poses a serious threat to ecosystem safety and human health. The advancement of spectral technology offers a way to rapidly and non-destructively monitor soil heavy metal content. In order to explore the potential of rice leaf spectra to indirectly estimate soil heavy metal content. We collected farmland soil samples and measured rice leaf spectra in Xushe Town, Yixing City, Jiangsu Province, China. In the laboratory, the heavy metals Cd and As were determined. In order to establish an estimation model between the pre-processed spectra and the soil heavy metals Cd and As content, a genetic algorithm (GA) was used to optimise the partial least squares regression (PLSR). The model's accuracy was evaluated and the best estimation model was obtained. The results showed that spectral pre-processing techniques can extract hidden information from the spectra. The first-order derivative of absorbance was more effective in extracting spectral sensitive information from rice leaf spectra. The GA-PLSR model selects only about 10% of the bands and has better accuracy in spectral modeling than the PLSR model. The spectral reflectance of rice leaves has the capacity to estimate Cd content in the soil (relative percent difference [RPD] = 2.09) and a good capacity to estimate As content in the soil (RPD = 2.97). Therefore, the content of the heavy metals Cd and As in the soil can be estimated indirectly from the spectral data of rice leaves. This study provides a reference for future remote sensing monitoring of soil heavy metal pollution in farmland that is quantitative, dynamic, and non-destructive over a large area.

[Ecological Risk Assessment and Migration and Accumulation Characteristics of Heavy Metals in Farmland Soil-crop System from Typical Pyrite Mining Area]

To evaluate the ecological risk of heavy metals in the soil-crop system in the superimposed high background and human activities from pyrite mining, the heavy metal contents and chemical speciation in soil and crop samples were analyzed, and these data were used to assess the potential ecological risk and factors affecting the migration ability of heavy metals using bioconcentration factors(BCF), potential ecological risk index(RI), risk assessment code(RAC), and correlation analysis. The results indicate that the average Cd, Cu, Pb, and Zn concentrations exceeded the background values of soils in Zhejiang Province and China. Cd had the greatest potential ecological harm, followed by that of Hg. The bioactive components and potential bioactive components of Cd accounted for 46% and 33%, respectively, indicating relatively high bioavailability. Cu and Pb were mainly in potential bioactive components accounting for 60% and 73%, respectively. The As, Cr, Hg, Ni, and Zn were predominantly residual and accounted for >60%, which indicated low biological activity. The RAC levels were in the following order:Cd>Zn>Cu>Pb>Ni>As>Cr>Hg; soil Cd had the highest ecological risk, mainly with high and extremely high levels, whereas other elements had no risk or low risk. Compared with Cd content in soil, only eight rice samples had Cd contents exceeding the safety limit, and sweet potato samples did not exceed the standard. The migration and enrichment capability of rice in order from strong to weak was s follows:Cd>Zn>Cu>Hg>As>Ni>Cr>Pb; the bioactive component of Cd played a significant role in promoting Cd absorption by rice. Soil OM had a bi-directional effect on Cd bioavailability, whereas soil texture had an indirect effect. This comprehensive study shows that the total amount of heavy metals in soil, chemical speciation, biological activities, absorption, and enrichment of heavy metals by crops should be taken into consideration when assessing the ecological risks in the superimposed areas affected by high background and human activities, such as the pyrite mining area.

[Ecological Risk and Migration Patterns of Heavy Metals in Soil and Crops in the Lead-Zinc Mining Area in Guizhou, China]

An accumulation of heavy metals in soil poses a risk to the ecological environment and human health. In this study, the concentrations of heavy metals in soil and crops were examined in a lead-zinc mining area in Guizhou Province, China. The distribution and sources of heavy metals were analyzed using GIS spatial mapping. The potential ecological risks of heavy metals were assessed using the potential ecological risk index (RI), and the human health risk assessment method recommended by the United States Environmental Protection Agency (USEPA) was used to quantify the health risk of residents exposed to heavy metals in the soil around lead-zinc mines. According to the results, the average of concentrations of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in the soil were 58, 7.9, 175, 64, 0.461, 65, 1539, and 2513 mg·kg^-1, respectively, which were significantly higher than the background values in Guizhou Province. It was found that the As, Cd, Cu, Hg, Pb, and Zn concentrations were extremely irregular in the soil and that the concentrations decreased significantly with the distance to the smelters, which were greatly disturbed by human activities. Comprehensive evaluation of soil heavy metals using the potential ecological risk index revealed that the risks of soil heavy metals were pole-strength and strong levels, and Cd constituted the primary ecological risk factor. A total of 22% and 10% of the corn samples contained Pb and As above the heavy metal pollution thresholds in the national food safety standards. According to human health risk assessments, heavy metals in the soil present potential non-carcinogenic risks to adults or children, and pose a potential carcinogenic risk to children. Soil pH was an important controlling factor affecting the bioavailability, migration, and accumulation of Cd in soil-crop systems. This study provides data and theoretical support for the prevention and control of soil pollution in lead-zine mining areas.

[Heavy Metal Pollution Characteristics and Health Risk Assessment of Soil-crops System in Anhui Section of the Yangtze River Basin]

In order to explore the pollution characteristics and health risks of heavy metals in a soil-crop system in the Anhui section of the Yangtze River basin, a total of 338 groups of soil samples from rice, wheat, and their roots were collected, and the contents of eight types of heavy metals were determined. Additionally, the pollution index method, potential ecological hazard, multivariate statistical analysis, and health risk were used to evaluate the heavy metal content in rice and wheat root soil. The results showed that Cd, Cu, Cr, and Ni in the soil had an obvious accumulation effect, and the pollution of rice root soil was more serious than that of wheat soil. The potential ecological risk was mild to moderate, which mainly came from the elements Cd and Hg. Multivariate statistical analysis showed that Cu, Pb, Zn, and Cd were industrial sources and agricultural sources; Cr and Ni were natural sources; and As and Hg were agricultural sources. The absorption and enrichment intensity of heavy metals by rice was in the order of Cd>Zn>Cu>Hg>Ni>As>Cr=Pb, and those in wheat were ordered as Zn>Cd>Cu>Hg>Ni>As=Pb>Cr. The root soil heavy metal health risk assessment indicated that oral intake was the main exposure route of non-carcinogenic risk, and children were more vulnerable to heavy metal pollution. Non-carcinogenic risk assessment showed that wheat root soil posed non-carcinogenic risk to children but no carcinogenic risk. Intake of rice and wheat had some degree of non-carcinogenic risk and unacceptable carcinogenic risk for both adults and children.

[Accumulation Characteristics and Health Risk Assessment of Heavy Metals in Soil-Crop System Based on Soil Parent Material Zoning]

This study used both the element occurrence form analysis and the chronic health risk assessment method to investigate the accumulation characteristics of heavy metals in the soil-crop system and the health risk assessment of agricultural products in northeastern Yunnan, which is a typical area of Southwest China where heavy metals are enriched in soil. Based on the study of 1137 groups of agricultural products and corresponding root soils, the results showed that the soil cadmium (Cd) and lead (Pb) in the lead-zinc ore concentration area were higher than the risk-intervention values of the "Soil Environmental Quality Risk control standard for soil contamination of agricultural land" (GB 15618-2018), whereas the soil Cd in the other parent material areas was within the screening-intervention buffer zone, and Pb was below the minimum risk-screening value. According to the National Food Safety Standard of China (GB 2762-2017), the heavy metal Cd in potatoes and soybeans in the area seriously exceeded the standard, the heavy metal Pb in tartary buckwheat and walnut exceeded the limit value, and the exceeding rate of heavy metal Cd in crops from these parent material areas showed: clastic rock>basalt>lead-zinc ore>carbonate ≈ Quaternary sedimentary>sand (mud) rock. According to the U.S. Environmental Protection Agency's assessment method for the chronic health risk of heavy metal intake by humans, the grains and potatoes, staple foods, and fruits had low chronic health risks of heavy metal intake. Agricultural products from the parent material area of clastic rock, sand (mud) rock, Quaternary sedimentary, and lead-zinc ore concentration showed health risks; with the change in soil physical and chemical properties and the increase in the types of edible crops, the risk will gradually increase. Based on this research, it is urgent to carry out real-time monitoring of agricultural products in the area.

[Characteristics of Typical Soil Acidification and Effects of Heavy Metal Speciation and Availability in Southwest China]

Using a spatial instead of temporal approach, soil samples were collected from the main types and different stages of acidification in Southwest China, and the characteristics of soil physicochemical properties, acid-buffering properties, and heavy metal fugacity patterns were analyzed, combined with biological experiments in small cabbage pots, to explore the coupling relationship between soil acidification and changes in heavy metal morphological activity. The results showed that the exchangeable salt-based ions of the soil decreased with increasing acidification in purple and yellow soils, caused by the loss of exchangeable Ca²⁺. The acid-buffering capacity of purple and yellow soils was higher at pH>7.50 and pH<4.50. The acid-buffering capacity of yellow soils was strongly correlated with the content of soil exchangeable salt-based ions, and the increase in acid-buffering capacity was related to the rate of depletion of soil salt-based ions with the increase in acid addition. The distribution of Cd and Pb in the soil was closely related to the soil type and degree of acidification: in the purple and yellow soils, Cd and Pb were mainly in the exchangeable and residue states, and the proportions of the exchangeable state and residue state increased and decreased, respectively, with increasing acidification; in the red soils, the residue state and Fe-Mn bound state were predominant; the Fe-Mn bound state of Cd was 2.15 and 1.73 times higher than that of the purple and yellow soils, respectively, and the Fe-Mn bound state of Pb was 4.30 and 3.91 times higher than that of purple and yellow soils, which was related to the higher iron content in red soils. Pot experiments showed that soil acidification inhibited the growth of Chinese cabbage to a certain extent, and the biomass of Chinese cabbage in the heavily acidified yellow soil (pH<5.70) was significantly lower than that in the non-acidified yellow soil.

[Potential Ecological Risk Assessment and Source Analysis of Heavy Metals in Soil-crop System in Xiong'an New District]

In order to evaluate the potential ecological risk of heavy metals in the soil-crop system in the Xiong'an New District, the heavy metal contents and forms in wheat seed and root soil samples are analyzed, and the comprehensive pollution index (IPI_N), potential ecological risk index (RI), bio-enrichment coefficient (BFC), risk assessment code (RAC), principal component analysis, and correlation analysis are used to assess the potential ecological risk of heavy metals and analyze their sources. The results indicate that the average content of Cd, Cu, Pb, and Zn in the root soil is significantly higher than the soil background value in Hebei province. The IPI_N ranges from 0.2 to 5.18, 94.83% of the soil samples are in the safe and pollution-free grade, and the potential ecological risks of heavy metal are slight and moderate, accounting for 64.66% and 30.17%, respectively. Cd has the greatest potential ecological harm, followed by Hg, Cr, Ni, and Zn. All the heavy metal elements besides Cd in the root soil are dominated by the residual form, which accounts for 60%, and the bioactive form (ion-exchange and water-soluble state) of Cd accounts for 33.43%, indicating relatively high bio-availability. The risk assessment code can be ranked as Cd > Ni > Hg > As > Cu > Cr > Zn > Pb, and the risk of Cd is moderate, while other elements are of low or no risk. The leading potential source of heavy metals is human activity combined with the geological background. The migration and enrichment capability of the wheat seeds is in order from strong to weak of Zn > Cu > Cd > Hg > As > Ni > Pb > Cr, and the biological effective components of As, Cd, Pb, and Zn plays an substantial role in promoting the absorption of heavy metals. The content of heavy metals in the wheat seeds has a negative correlation with the soil pH, and the physical and chemical indices, such as OM and CEC, has bi-directional influence on the biological effective state of the heavy metals.

[Factors Affecting the Translocation and Accumulation of Cadmium in a Soil-Crop System in a Typical Karst Area of Guangxi Province, China]

To understand the main factors influencing the translocation and accumulation of cadmium (Cd) in soil-crop systems in typical karst areas, 68 sets of paddy soil and rice grain samples were collected in Guangxi Province. These were used to analyze Cd concentrations and soil properties (pH, organic matter (OM) content, oxide content, and texture). Spearman's correlation coefficients and principal component analysis (PCA) were used to examine the effects of soil properties on Cd concentrations and identify the main influencing factors. The studied soils were highly enriched in iron oxide (TFe₂O₃), aluminum oxide (Al₂O₃), and manganese oxide (MnO) compared to background levels, with average concentrations of 20.2%, 19.0%, and 0.2%, respectively. However, the soils are relatively depleted in silica (SiO₂), with an average concentration of 41.0%. The soils are strongly weathered and leached in study area, giving rise to rich occurrences of Fe-Mn nodules. The concentrations of TFe₂O₃ and MnO in the study soils were significantly correlated with soil Cd, rice seed Cd, and the Cd bioconcentration factor (BCF). The PCA analysis further showed that TFe₂O₃ and MnO in soils were the main factors affecting the migration and enrichment of Cd while soil pH, OM, and Al₂O₃ had less of an influence. Furthermore, SiO₂ and soil texture indirectly affected the migration and enrichment of Cd. It is suggested that the Fe-Mn nodules effectively adsorb and immobilize Cd in the study area soils, acting as a heavy metal scavenger that reduced the biological accessibility of Cd.

[Effects of Biochar on Bioavailability of Two Elements in Phosphorus and Cadmium-Enriched Soil and Accumulation of Cadmium in Crops]

Using phosphorus and cadmium enriched soil (total Cd is 0.94 mg·kg^-1, total phosphorus is 0.86 g·kg^-1) and low cadmium accumulation genotype Jinqiuhong 3 as experimental materials, we set up four treatments: absolute control (only NK inorganic fertilizer), relative control (CKp, NPK inorganic fertilizer), biochar (only NK inorganic fertilizer), and biochar+phosphate fertilizer (BC-CKp). The availability of phosphorus and heavy metal Cd in soil, the biomass, Cd accumulation characteristics of edible parts of plants, and the basic characteristics of soil were investigated. The results showed that the content of available Cd in soil decreased by 8.23% and 5.68% by BC and BC-CKp treatment with biochar compared with CK0 and CKp treatment without biochar, respectively. At the same time, the content of available phosphorus in soil significantly increased 11.60-16.26 mg·kg^-1 of biochar. The content of available Cd in CKp and BC-CKp treated with exogenous phosphate fertilizer was significantly lower than that in CK0 and BC treatments without phosphate fertilizer by 31.43% and 33.29%, respectively. In addition to CK0 treatment, the Cd content of edible parts of Brassica campestris bolting crops in the other three treatment groups (CKp, BC, and BC-CKp) did not exceed the limit value of Cd of the China Food Safety National Standard (GB 2762-2017) of 0.1 mg·kg^-1. The results showed that the dual functions of heavy metal Cd passivation and phosphorus activation could be realized by injecting biochar into moderate and mild Cd contaminated soil with phosphorus enrichment at the same time. Under the condition of no additional use of phosphorus fertilizer, planting vegetable crop genotypes with weak absorption and low accumulation of Cd can not only ensure an increase in biomass of edible parts but can also ensure that the heavy metal Cd content in edible parts meets the national standard of food safety.

[Bioavailability, Translocation, and Accumulation Characteristic of Heavy Metals in a Soil-Crop System from a Typical Carbonate Rock Area in Guangxi, China]

This study uses 68 sets of paddy soil and rice grain samples collected from an area of carbonate rocks in Guangxi Province, China, to explore the ecological risks of heavy metals (As, Cd, Cr, Cu, Hg, Pb, Ni, and Zn) in soils from a high background area. We analyzed the concentrations of these heavy metals in soil and grain samples, and their chemical speciation in soil, and use these data to assess the associated ecological risks by means of statistics, a geo-accumulation index, bioconcentration factors (BCF), and correlation analysis. The arithmetic mean values of heavy metals concentrations in soil samples from the study area were (75.8±50.1), (1.91±1.02), (467.0±253.1), (48.5±9.8), (0.21±0.08), (76.2±28.1), (84.2±25.0), and (258.0±122.6) mg·kg^-1 for As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn, respectively, which were remarkably higher than of those from other regions within Guangxi Province and China. In comparison to China's soil environmental quality standard risk screening values (GB 15618-2018), the over-standard rates of Cd, As, and Cr were 95.6%, 86.8%, and 69.1%, respectively. In comparison to risk intervention values, the over-standard rates of Cd, As, and Cr were 27.9%, 17.6%, and 5.9%, respectively. Speciation analysis on heavy metals indicated that As, Cr, Cu, Ni, Pb, and Zn were mainly found in a residual form, and accounted for>80% of the total concentrations, and had a low bioavailability. The bioactive components (F₁+F₂+F₃) of Cd accounted for 21%, and the bioactivity of Cd was higher than other elements. The potential bioavailable components (F₄+F₅+F₆) of Hg accounted for 44%, with low total concentrations, which are understood to have little potential ecological harm for crops. However, the over-standard rates of Pb, Cd, and Cr in rice grains were only 23.5%, 8.8%, and 2.9%, respectively. Correlation analysis showed that there was no significant correlation between the concentrations of heavy metals in soils and the corresponding rice grains. The mean BCFs of each heavy metal were <0.1, and the BCFs of Hg, Pb, As, Cr, and Ni were <0.05. Overall, we found relatively high concentrations, low activity, and low ecological risks for heavy metals in the study area. For high geological background materials such as carbonate rocks, factors such as metal speciation, biological activity, and crop over-standard rates should be taken into account along with the traditional use of the total amount of heavy metals in a soil as the evaluation standard when formulating pollution control policies.

[Simulation of the Absorption, Migration and Accumulation Process of Heavy Metal Elements in Soil-crop System]

Soil-crop system is an important way that heavy metals harm the ecological environment and human health. To research and understand the process of heavy metal absorption, migration and accumulation in soil-crop system is important for the prevention and control of heavy metal pollution and the health of human beings. In this paper, we established a model for crop uptake of heavy metals to calculate the heavy metals contents of wheat roots, stems, leaves and grains, and we analyzed the accumulation process of heavy metals in roots, stems, leaves and grains in the growth cycle of wheat. The predicted values were compared with the measured values to test the accuracy of the model. The results demonstrated that different parts of wheat had different heavy metal absorption capacity, the absorption of the roots was the strongest, followed by leaves, and the absorption capacity of stems and grains was weak. In addition, the contents of different heavy metals in each part of wheat were also significantly different. The content of Cu was the highest, followed by Ni, while the contents of Pb and Cd were small. In the process of wheat growth, the heavy metal accumulation rate of stem, leaf and grain began to slow down at 90, 60 and 100 days respectively, and the concentration of heavy metals reached the maximum gradually, while the accumulation rate of heavy metals in roots showed a growing trend. We studied the process of absorption, migration and accumulation of heavy metals in soil-crop system by using numerical simulation technology, which can provide scientific basis for preventing the ecological and health risks of heavy metal pollution.