BCR Method Research Articles

Assessing soil remediation effect of Cr and Pb based on bioavailability using DGT, BCR and standardized determination method

In the field of soil remediation, the importance of bioavailability of pollutants has not received adequate attention, leading to the excessive application of remediation measures. Therefore, to ensure the safe use of farmland soil, a scientific method is needed to assess labile contaminants and their translocation in plants. To evaluate soil remediation effect based on bioavailability, the concentrations of these heavy metals in soil were analyzed using by the method for total metal content, the Community Bureau of Reference (BCR) extraction, and the diffusive gradients in thin films (DGT) technique. The results reveal that the correlation coefficients between metal concentrations measured by DGT and those accumulated in rice grains are the highest (Cr-R2 = 0.8966, Pb-R2 = 0.9045). However, the capability of method for total metal content to evaluate the remediation effect of heavy metals is very limited. In contrast, although Cr and Pb measured by BCR show a high correlation with HMs in rice plants, the method still falls short in precisely assessing bioavailability. Significantly, DGT proves to be more effective, successfully distinguishing the remediation effects of different treatments. Generally, DGT offers a more accurate and simpler assessment method, underscoring its practical significance for monitoring soil remediation and environmental management.

Analysis of heavy metals in sediments with different particle sizes and influencing factors in a mining area in Hunan Province

Heavy metals mainly exist on the surface of sediment particles and are transported using particulate matter as carriers. Therefore, the particle size of sediment particles can affect the adsorption, release, and migration of heavy metals. This study aim to investigate the distribution characteristics and chemical fraction of Cd, Pb, and As contents in sediments of different particle sizes using the BCR method, and to determine the key factors affecting the distribution of heavy metals through mineralogical methods such as XRD and EDS. The results revealed that the overall content of various forms of heavy metals increases with the decrease of particle size, mainly presents in fine particles. The mineralogical analysis results indicated that fine particles predominantly contained clay minerals such as chlorite and illite and coarse particles mainly include primary minerals. Due to the mining areas in the middle-upstream, Cd, Pb and As were primarily associated with galena, sphalerite and pyrite, respectively. The distribution of heavy metals is jointly influenced by sediment particle size and sediment material composition.

Investigating the elemental composition of Egyptian glauconite sediments by applying BCR sequential extraction procedure and some single extractants

This study assesses the elemental composition of Egyptian glauconite sediments, focusing on potentially toxic elements (PTEs) and macronutrients. The primary aim is to evaluate the feasibility of utilizing these sediments as a natural source of potassium for agricultural purposes, besides conventional chemical fertilizers like potassium sulfate. To quantify elemental content, chemical analysis was employed across five distinct grain size fractions after grinding glauconite rock. The assessment included potassium, calcium, sodium, and PTE concentrations, utilizing potassium chloride (KCl) and ammonium acetate lactate (AL Solution) as single extractants, and the BCR extraction protocol, in addition to measuring the pseudo-total content of these elements. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) facilitated a comparative analysis of elemental concentrations. Results indicate PTE concentrations within European Union regulations, with an absence of cadmium. Glauconite samples contain approximately 3–3.3% potassium by weight, alongside significant amounts of essential macronutrients (calcium, magnesium) and micronutrients (copper, nickel, zinc) crucial for agriculture. BCR sequential extraction protocol results closely align with or slightly surpass pseudo-total content results. Notably, the AL Solution demonstrates high efficiency compared to KCl or acetic acid in the first step of the BCR method. BCR sequential protocol provides valuable insights into various elemental forms and potential mobility. Overall, this study reveals that glauconite has the potential to serve as a promising alternative potassium fertilizer without causing adverse environmental impacts.

Co-pyrolysis of sewage sludge and phosphate tailings: Synergistically enhancing heavy metal immobilization and phosphorus availability

Phosphate tailings (PT) was used to reduce the release of heavy metals (HMs) during pyrolysis and the leachable rate of residual HMs, and simultaneously improve the bioavailability of phosphorus in the sludge-based biochar. The concentration of heavy metals and the fractions determined by BCR method was used to investigate the release and the transformation of Zn, Pb, Mn, Ni and Cu during pyrolysis involved with the effects of temperature and the addition of PT. The respective pyrolysis experiments shows that the release of Zn and Pb increases with temperature for both sewage sludge (SS) and PT, and the bioavailable fractions (F1 + F2) of Mn, Ni, and Cu increases with temperature for PT. During co-pyrolysis, blended samples released lower quantities of Zn and Pb and presented lower bioavailability of HMs than the individual SS or PT. A synergistic effect of co-pyrolysis was evident for volatile Zn and Pb. The decomposition of CaMg (CO3)2 from PT produced CaO, by which the volatile ZnCl2 and PbCl2 were transformed into ZnO and PbO with less volatility and higher reactivity with SiO2 and Al2O3 than the chlorides. Then SiO2 and Al2O3 from SS acted as the final stabilizer to immobilize the oxides. The final product combined with SiO2 and Al2O3, such as ZnSiO4 and ZnAl2O4, were detected. The addition of PT also introduced more Ca and P into sludge to produce biochar with higher concentration of apatite phosphorus with higher bioavailability.

Effect of terminal temperature on the morphology and heavy metal concentrations of typical rural domestic biochar and its potential use in ammonia nitrogen adsorption

In this study, straw, kitchen waste and pine needles were used as raw materials to prepare biochar at terminal temperatures of 300, 500 and 700 °C (SB, KB and PB), respectively. The morphology and concentrations of heavy metals extracted from biochar at different synthetic temperatures were analyzed by BCR (European Community Bureau of Reference) method. The effects of temperature and pH on the adsorption of ammonia nitrogen (NH4-N) in landfill leachate with SB, KB and PB were studied. The results show that the yield of PB300 was the highest (86.47%). Further analysis presents that SB, KB and PB contain basic functional groups (pH > 8.50) and the heavy metal content of SB, KB and PB is low (<2.564 mg∙kg−1). Moreover, it is found that terminal temperature is negatively correlated with heavy metal concentration. The F3 and F4 are the primary states of zinc (Zn), lead (Pb) and barium (Ba) of SB, KB and PB, indicating relatively high biological stability of the products. The potential ecological risk (RI) values of SB, KB and PB was lower than 1, illustrating SB, KB and PB are non-hazardous waste (<1 mg∙L−1). When pH was alkaline (pH = 9), SB500 had the best ability for NH4-N adsorption in landfill leachate (19.14 mg∙g−1). Further improvement of adsorption performance with SB500 can be achieved at temperature of 35 °C (21.88 mg∙g−1). Overall, SB, KB and PB can be used as environmentally friendly remediation materials.

Analysis of the Effect of Soil Remediation Processes Contaminated by Heavy Metals in Different Soils

Heavy metal pollution in China’s soil is very serious, and soil remediation is urgent. At present, most of the domestic and foreign research is aimed at one soil type for soil heavy metal pollution remediation. However, the distribution of heavy metals and the effect of remediation with chemical agents are different for different soils. This study is committed to investigating the effect of WTF on the remediation of heavy metal contamination in different soils based on the existing research in the laboratory. The influence of soil quality on remediation efficiency was analyzed by TCLP leaching of heavy metals, and different forms of heavy metals were extracted from the soil using the BCR method. The experimental results showed that the soil environment was alkaline, and the response to a low addition of WTF was more obvious. The Pearson correlation coefficient analysis yielded that the increase in the organic matter content led to an increase in the oxidizable Cd content. The WTF remediation did not change the overall acidity and alkalinity of the soil so that the overall soil environment remained stable; it increased the organic matter content and added fertility to the soil, and it increased the activity of most enzymes in the soil and promoted the circulation of the soil elements, making the soil fertile.

Metal contamination in a sediment core from Osaka Bay during the last 400 years

Osaka Bay adjacent to the Kyoto–Osaka–Kobe metropolitan area was affected by severe metal pollution during the twentieth century; yet little is known about the trace metal sources and pre-industrial human activities. We have determined the elemental concentrations and zinc stable isotope ratios (δ66Zn) in bulk sediments and the trace metal concentrations in chemical fractions of a 9-m-long sediment core from Osaka Bay. Our goals were (1) to reconstruct the historical trace metal contamination, and (2) to identify anthropogenic Zn sources and the solid phases of anthropogenic trace metals. The core provided a continuous environmental record of the last 2300 years based on radiocarbon dating of molluscan shells. Copper, Zn, and Pb showed an initial enrichment from the 1670s AD, which could be caused by human activities due to an increasing population. In agreement with previous findings, the trace metal concentrations slightly increased from the 1870s, strongly increased from the beginning of the twentieth century, and peaked around 1960 before environmental pollution control laws were enacted. Increasing trace metal concentrations in the acid-labile and reducible fractions obtained by the Community Bureau of Reference (BCR) sequential extraction procedure toward the surface indicate carbonates and Mn oxyhydroxides were the primary fractions for anthropogenic trace metals. The δ66Zn values (1) were constant until the 1940s, suggesting that the average δ66Zn of industrial sources was indistinguishable from that value of the natural background, (2) showed a slight decrease from the 1950s and remained constant until the present, and (3) fell in a binary mixing process between a lithogenic (~ + 0.27‰) and an anthropogenic endmember (~ + 0.17‰), the latter likely representing a mixture of various Zn sources such as road dust, tire wear, industrial effluents, and effluents from wastewater treatment plants. We conclude the combination of Zn stable isotopes together with chemical fractions obtained by the BCR method represents a promising approach to assess the trace metal sources and their potential mobility in sediment cores from anthropogenically affected coastal areas.

Accelerated screening of lead fractions and bioavailability in coal and its combustion products by ultrasound and microwave assistant procedure

Coal-fired power plant is one of the main anthropogenic sources of lead emission and most of lead in coal can be transferred to its combustion products (fly ash, desulfurization gypsum and slag) after burning. The fraction distribution of lead in coal, fly ash (FA), desulfurization gypsum and slag can reveal their environmental stability and migration ability. However, the conventional sequential extraction method (CSE) for screening lead fractions is time-consuming and labor-intensive. In this paper, two high efficiency methods including ultrasonic-assisted sequential extraction (UASE) method and microwave-assisted sequential extraction (MASE) method were developed. The working parameters were optimized by comparing the concentration obtained from the UASE and MASE with the optimized BCR method. The whole extraction time shorten from 48 h to 20 min. The recoveries of the first three fractions were on average of 85% for the UASE method and 95% for the MASE method in coal, fly ash, desulfurization gypsum and slag. The accuracies of the developed methods were verified by the analysis of the coal CRM (GBW11156) and FA CRM (GBW08401). The high reproducibility and applicability of the methods were demonstrated by analyzing lead in coal, fly ash, desulfurization gypsum and slag samples from 12 different power plants in China. The developed UASE and MASE were more efficient methods for accelerated screening of lead fractions in coal and its combustion products.

Remediation of the soil contaminated by heavy metals with nano-hydroxy iron phosphate coated with fulvic acid

ABSTRACT Heavy metals pose a serious threat and damage to ecological health when released into the environment. n-HFP is usually used to remediate soils contaminated with heavy metals, but its ability to solidify heavy metals is limited. FA has good ability to trap heavy metals due to its abundant oxygen-containing functional groups. However, the solubility of FA in water limits its application in the field of heavy metal removal. In this paper, n-HFP@FA was prepared by co-precipitation method. Through FT-IR and BET analysis, the oxygen-containing functional groups and specific surface area of n-HFP@FA increased due to the addition of FA. The adsorption behaviour of n-HFP@FA on Pb, Cd, and Cu followed the pseudo-second-order and Langmuir isotherm models. In addition, the maximum adsorption capacities of n-HFP@FA for Pb, Cd, and Cu were 371.1, 190.5, and 129.75 mg/g, respectively. As shown by FT-IR and XPS analysis, the main mechanisms of Pb, Cd and Cu removal by n-HFP@FA are: complexation, electrostatic and precipitation. The n-HFP@FA showed high removal rates of Pb, Cd, and Cu in soil leachates of different pH. In the soil remediation experiments, the BCR method and Pearson correlation analysis showed that the acid-soluble, reducible and oxidizable fractions of Pb, Cd, and Cu in the soil were effectively converted into a more stable residual fraction. This study opens up a prospect for the application of n-HFP@FA composites in the remediation of contaminated soil.

Optimization of Solidification and Stabilization Efficiency of Heavy Metal Contaminated Sediment Based on Response Surface Methodology

Solidification and stabilization (S/S) by agents and stabilizers is an effective way to treat heavy metal-contaminated sediments. Optimization of curing condition is crucial to minimize the consumption of reagents on the base of effective S/S. In this work, the synergistic effects of cement and stabilizer on mechanical strength and leaching toxicity of contaminated sediments were investigated, and the S/S conditions were optimized using response surface methodology. On the basis of a single-factor test, multi-factor experiments were conducted to fit the relationship between the S/S effect of contaminated sediments and the amount of cement and stabilizer. The mechanism of stabilization was investigated by the results from the revised BCR method. The results indicate that the optimal curing conditions were 44.29% of cement content with 2.05% of trimercapto-s-triazine trisodium salt (TMT). After 28 days of curing, the compressive strength reached 2.07 MPa and the leaching concentrations of Cd, Cu, and Pb were 0.094 mg/L, 0.031 mg/L, and 0.173 mg/L, respectively, which met the requirement of in-situ resource recycling standard. The stability of heavy metals was significantly improved as a result of the removal of acid extractable fraction (15.58~69.92%) and an increase in the residual fraction (18.27~49.07%).

Heavy metal mobility, bioavailability, and potential toxicity in sediments of the Korle lagoon in Ghana

ABSTRACT This study aimed to identify the mobility, bioavailability, and potential toxicity of heavy metals (HM) in sediments of the Korle lagoon using metal partition. Sediments were analysed using the BCR (Bureau of reference) method. As (60%), Pb (50%), Zn (57%) and Fe (40%) were found in high concentration in the ion exchangeable fraction. The oxidisable fraction was dominated by Cu (51%), Cd (95%), Cr (61%), Ni (63%), As (35%), Fe (21%), Hg (30%), Pb (19%) and Zn (11%). The reducible fraction was dominated by Zn (29%), Pb (24%) and Hg (19%). The residual fraction was dominated by Hg (49%) and Fe (34%). PCA was used to identify pH and organic matter as the most important factors controlling the mobility and bioavailability of HM in the sediments. Arsenic, Pb, Zn and Fe were found to have the highest mobility. Chromium, Cu and Ni have medium mobility. Hg has low mobility. Most of the HM show potential bioavailability and toxicity.

Arsenic in Santa Catarina soils

Arsenic (As) is one of the most harmful chemical elements known to man and to the environment, mainly due its high toxicity and wide distribution; the content of this element within the soils is a genuine concern, thus making it paramount to know its natural contents in a regional context. The present study aimed to determine the natural Arsenic content in the A horizon of 31 soil profiles from the state of Santa Catarina, Brazil, which is useful in determining reference values, monitoring, remediation of contaminated areas, legal regulation and Brazilian laws. Soil samples were prepared following the USPEA 3051A SW-846 method and were previously chemically reduced from As(V) to AS(III) by using the BCR method. The determination was performed in an Inductively Coupled Plasma - Optical Emission Spectrometry - Hydride Generation (ICP-OES-HG at cold vapor). Results obtained from the soil groups reveal the materials of basaltic origins as the ones with more As content while those of sediment origins had lesser content. Evaluated soil profiles fit into the following descending order regarding their As content: Latossolos, according to EMBRAPA (Oxisols according to Soil Taxonomy) &gt; Nitossolo (Ultisols, Oxisols (Kandic), Alfisols) &gt; Chernossolos (---) = Cambissolo (Inceptisols) = Argissolo (Ultisols) &gt; Neossolos (Entisols).&#x0D; Keywords: arsenic content, reference value, soils, trace element.

Fingerprinting vanadium in soils based on speciation characteristics and isotope compositions

Vanadium (V) can have toxic effects on human organs and physiological systems, yet tracing V sources remains challenging. Here, two methods were used for V source tracing in soil based on speciation characteristics and isotope compositions. According to the sequential extraction method of the European Communities Bureau of Reference (BCR), the analysis of speciation distributions offers a possible means of distinguishing V sources. Here, the isotope compositions of polluted soils around a coal-fired power plant and smelter in China were used to identify the sources of V. Significant V isotope variation (δ51V range = −0.74 ± 0.07; mean ± 2SD = −0.52 ± 0.05‰) was observed in the soil samples, attributed to coal-burning (Δ51VCoal−Fly ash 1 = −0.31 ± 0.05‰; mean ± 2SD; n = 1) and smelting processes (Δ51VSlag-Fly ash 2 = −0.31 ± 0.07‰; mean ± 2SD; n = 1). All of the soil V isotope ratios plotted within the range of end-member components corresponding to potential V contributors in the environment. Among these, δ51V ranged from −0.74 ± 0.07 to −0.55 ± 0.02‰ in topsoil, the average δ51V was −0.52 ± 0.05‰ in the deep soils, and the δ51V of the end-member components ranged from −0.52 ± 0.05 to −0.94 ± 0.11‰. The primary anthropogenic source of V in the topsoil was fly ash from coal-burning that was consistent with the BCR method results. Furthermore, the downward migration of V was identified in the soil profile adjacent to the smelting plant, and V in the deep soils was dominated by natural sources relative to anthropogenic sources in the surface soils.

The distribution and speciation characteristics of vanadium in typical cultivated soils

ABSTRACT The contents and chemical speciation of vanadium(V) in different types of soil and soils with different particle were studied by the modified BCR method. The results indicated that the V content was in the order of as red soil > cinnamon soil > chernozem soil, and the content of V in red soil was as high as 264.9 mg/kg, which posesa potential risk to the environment and biosecurity. In cinnamon soil, V exhibitedan obvious gradation effect; that is, the content of V in fine particles was higher than in large particles. In addition, V(IV), with low toxicity, accounted for more than 87% of the total V in the three types of soil, and most V existed in the residual fraction. The pH was the main factor affecting the V valence state in the soil, and when the environmental conditions change due to various agricultural measures, such as applying fertiliser, long-term cultivation and intensive irrigation, V(IV) is easily oxidised to V(V) with high toxicity, and reduceable and oxidisable V is converted into an acid-soluble fraction, which leads to increased bioavailability and a potential threat to the environment. Therefore, it is necessary to strengthen environmental risk assessments and monitoring of heavy metals in cultivated soils, especially in areas with high background values.

Influence of thermal hydrolysis treatment on chemical speciation and bioleaching behavior of heavy metals in the sewage sludge.

In this study, the transformation of chemical speciation of Cr, Mn, As and Cd in the sewage sludge before and after thermal hydrolysis treatment was investigated using modified BCR method. The effect of thermal hydrolysis treatment and chemical speciation change on the subsequent bioleaching behavior was also researched. The results showed that the concentrations of Cr, Mn, As and Cd in oxidizable fraction decreased in the sludge treated by thermal hydrolysis. Meanwhile, the proportions of Cr, Mn and As in the mobile fractions (acid-soluble/exchangeable and reducible fraction) all decreased, while Cd was concentrated in the sludge treated by thermal hydrolysis. The final pH value of bioleached sludge treated by thermal hydrolysis was lower than that in the bioleached raw sewage sludge. And faster increase of oxidation-reduction potential (ORP) was also found in the bioleaching process of the sludge treated by thermal hydrolysis. The removal percentage of Mn and Cd increased in the bioleached sludge treated by thermal hydrolysis. Thermal hydrolysis treatment can promote the bioleaching to some extent. Furthermore, the environmental risk of Cr, Mn, As and Cd in the bioleached sludge treated by thermal hydrolysis was all alleviated according to risk assessment analysis compared with the bioleached raw sewage sludge.

Assessment of extraction methods for studying the fractional composition of Cu and Zn in uncontaminated and contaminated soils

This study is aimed at elucidating the fractionation of Cu and Zn in Haplic Chernozem and its alteration under the contamination to evaluate the extraction selectivity of different extractants and thus the efficiency of three sequential extraction schemes (Tessier and McLaren five-step and modified BCR three-step methods). General trends in the distribution of Cu and Zn speciations revealed with these three fractionation schemes suggest that they can be applied for the soils and the obtained results can be compared. Low mobility of potentially toxic metals (PTM) in the studied soil is suggested not only by their high content in residual fraction, but also by low contents of exchangeable and carbonate bound fractions (not more than 4-5 % Cu and 5-7 % Zn in the first two fractions). The highest contribution to the absorption and retention of Cu delivered from anthropogenic sources is made by organic matter and sesquioxides (up to 29 %); for Zn, by the nonsilicate Fe and Mn compounds (up to 25 %). However, the pattern of PTM extraction from soils varies during the application of different fractionation methods. The Tessier method is distinguished by a higher extractability relative to organic matter and sesquioxides. Therefore, this method is more informative for the contaminated soils. The McLaren method makes it possible to track the weakly bound species of compounds without the risk of involving other soil components. The BCR method is marked by simplicity of application and, therefore, recommended only for the noncalcic or low-calcic soils.

Reconstructed Soil Vertical Profile Heavy Metal Cd Occurrence and Its Influencing Factors

Aiming at the problem of heavy metal Cd pollution in mining wasteland under the high background value and superposition area of the historical mining source, a total of 30 samples were collected from the reclaimed soil of a historical sulfur mine in Southwest China. The total contents of Cadmium (Cd), Chromium (Cr), Nickel (Ni), Arsenic (As), and Mercury (Hg) in soil samples were analyzed. Scanning electron microscopy-electron diffraction spectroscopy (SEM-EDS) and BCR method for continuous extraction of heavy metals were used to analyze the occurrence of Cd under different soil thicknesses. The effectiveness of heavy metal Cd was evaluated by the risk assessment coding method (RAC), and the ratio of secondary phase and primary phase (RSP) was obtained. In addition, the changes in Cd morphology under different physicochemical properties were discussed. The results show that the concentration factor of Cd in the five heavy metals is the highest over 4. Through the morphological characteristics, it can be found that there are mainly subangular and angular particles in the samples. The resulting energy spectrum characteristics at different depths were similar, and all sampled contained Cd, Fe, S, and As. Isomorphs of Cd element were observed. According to the potential risk assessment of Cd, the RAC results show that the risk is medium or below, while the RSP results may lead to serious pollution except for the bottom layer. The total amount of heavy metals is the most important factor affecting the distribution of Cd forms, followed by pH, organic matter, and CEC. The results of this study can provide scientific basis for further understanding the activity, migration rule, biological toxicity, and occurrence form of Cd in profile soil.

Heavy metal removal from sewage sludge under citric acid and electroosmotic leaching processes

The heavy metals present in sludge are very harmful to the environment and the human body. It is necessary to remove them before sludge disposal. In this study, an ex situ method for removing heavy metals from sludge using electroosmosis was designed, and experiments were conducted to study the effects of electroosmosis voltages (30 V, 40 V and 50 V), citric acid concentrations (0.03 mol/kg, 0.06 mol/kg, 0.09 mol/kg and 0.12 mol/kg) and the power supply (continuous and interrupted) on the removal rate of heavy metals (Cu, Cr, Cd, Zn and Pb). The study found that the combination of the citric acid pretreatment and electroosmosis can effectively improve the removal rate of heavy metals. An appropriate increase in the voltage and citric acid concentration can improve the removal rate of heavy metals. The best combination was determined to be 40 V and 0.09 mol/kg. Under these conditions, the removal rates of Cu, Cr, Cd, Zn and Pb, were 14.39–41.28% (continuous power supply) and 21.78–42.36% (interrupted power supply). The interrupted power supply effectively improved the removal rates of Cd and Zn but reduced the removal rates of Cr and Pb. The power supply mode had no significant effect on the removal rate of Cu.Heavy metal speciation was analyzed by the BCR method, and the relationship between the ratio of easily removed metal speciation and the removal rate was studied. The increase in the ratio of easily removed speciation was accompanied by an increase in the removal rate, indicating that citric acid and electroosmosis increased the removal rate by increasing the ratio of easily removed speciation. However, the high pH value around the cathode caused by the electrochemical reaction was not conducive to the removal of heavy metals.

ENRICHMENT AND BIOAVAILABILITY OF TOXIC ELEMENTS IN INTENSIVE VEGETABLE PRODUCTION AREAS1

ABSTRACT Vegetable cultivation stands out for intensive use of agricultural inputs due to high nutritional requirements of plants within a short period and their high susceptibility to pests and diseases. This study aimed to identify the main factors responsible for changes in pseudo-total and bioavailable contents of Cd, Cu, Cr, Mn, Ni, Pb, and Zn in soils under intensive vegetable cultivation. Soil samples were collected from 146 sites in Petrópolis (RJ), Brazil, in 2017. The samples were collected at a depth of 0-20 cm in family farm systems during vegetable production period. Pseudo-total contents of toxic elements were determined by the EPA 3050B method. Geochemical fractionation procedure was used based on the BCR method. Pollution indices were also calculated. In general, vegetable producing areas presented low contamination by Cu, Zn, Pb, Mn, and Ni, except for Cd, which showed severe contamination. In areas with the lowest slopes, most of the toxic elements showed increasing contents, reflecting relief influence. Overall, inadequate soil management, intensive application of phosphate, potassium, and organic fertilization, and relief were the most influencing parameters on the enrichment and bioavailability of toxic elements in the soil.

Plant-induced insoluble Cd mobilization and Cd redistribution among different rice cultivars

Selecting rice varieties that adsorb less or more Cd from soil can be respectively applied to safety rice production and phytoremediation. Considering plant-induced Cd mobilization will contribute to the rice cultivar screening. We firstly executed a pot experiment to assess the effect of rice plants on soil Cd forms (BCR method). The results showed that the presence of rice significantly reduced residual-Cd (BCR4) concentrations, indicating the rice plant-induced insoluble Cd mobilization. Subsequent sand culture trial with four rice species (conventional and hybrid low-Cd rice, conventional and hybrid high-Cd rice) and four insoluble Cd compounds (CdS, [Cd3(PO3)2], CdCO3 and CdSe) were conducted to further discuss the interaction between insoluble Cd among different rice varieties. The results showed that rice plants do solubilize soil Cd especially insoluble Cd form due to the interaction among rhizosphere acidification, root secretion of organic matter ligand and other ligands, like phytosiderophore. High-Cd cultivar and hybrid rice cultivar had a greater ability to solubilize the insoluble Cd. Visual MINTEQ predicted that free Cd2+ (∼85.6%) were the dominant Cd speciation of mobilized Cd followed by Cd-DOM complexes (∼7.80%) and other ligand-complexes (∼6.51%) in the rhizosphere solution. Cd bound to protein and pectates and to undissolved phosphate were the major mobilized-Cd chemical forms in rice roots and shoots. In addition, the subcellular analysis suggested that half of mobilized-Cd precipitated in the cell wall of rice root and shoot and the other Cd entranced into the protoplast of rice cell. The fate of rice plant-induced insoluble Cd mobilization could be an indispensable factor in prospective phytoremediation and cleaner rice production.