Impact of Flooding-Drainage Alternation on Fe Uptake and Transport in Rice: Novel Insights from Iron Isotopes.

Iron (Fe) isotopes were utilized to provide insights into the temporal changes underlying Fe uptake and translocation during rice growth (tillering, jointing, flowering, and maturity stages) in soil-rice systems under typical flooding-drainage alternation. Fe isotopic composition (δ56Fe values) of the soil solution generally decreased at vegetative stages in flooding regimes but increased during grain-filling. Fe plaques were the prevalent source of Fe uptake, as indicated by the concurrent increase in the δ56Fe values of Fe plaques and rice plants during rice growth. The increasing fractionation magnitude from stem/nodes I to flag leaves can be attributed to the preferred phloem transport of light isotopes toward grains, particularly during grain-filling. This study demonstrates that rice plants take up heavy Fe isotopes from Fe plaque and soil solution via strategy II during flooding and the subsequent drainage period, respectively, thereby providing valuable insights into improving the nutritional quality during rice production.

Intelligent monitoring of the available lead (Pb) and cadmium (Cd) in soil samples based on half adder and half subtractor molecular logic gates.

The available heavy metals in soil samples can cause the direct toxicity on ecosystems, plants, and human health. Traditional chemical extraction and recombinant bacterial methods for the available heavy metals assay often suffer from inaccuracy and poor specificity. In this work, we construct half adder and half subtractor molecular logic gates with molecular-level biocomputation capabilities for the intelligent sensing of the available lead (Pb) and cadmium (Cd). The available Pb and Cd can cleave DNAzyme sequences to release the trigger DNA, which can activate the hairpin probe assembly in the logic system. This multifunctional logic system can not only achieve the intelligent recognition of the available Pb and Cd according to the truth tables, but also can realize the simultaneous quantification with high sensitivity, with the detection limits of 2.8 pM and 25.6 pM, respectively. The logic biosensor is robust and has been applied to determination of the available Pb and Cd in soil samples with good accuracy and reliability. The relative error (Re) between the logic biosensor and the DTPA + ICP-MS method was from -8.1 % to 7.9 %. With the advantages of programmability, scalability, and multicomputing capacity, the molecular logic system can provide a simple, rapid, and smart method for intelligent monitoring of the available Pb and Cd in environmental samples.

Distribution and correlation of iron oxidizers and carbon-fixing microbial communities in natural wetlands.

Most microaerophilic Fe(II)-oxidizing bacteria (mFeOB) belonging to the family Gallionellaceae are autotrophic microorganisms that can use inorganic carbon to drive carbon sequestration in wetlands. However, the relationship between microorganisms involved in Fe and C cycling is not well understood. Here, soil samples were collected from different wetlands to explore the distribution and correlation of Gallionella-related mFeOB and carbon-fixing microorganisms containing cbbL and cbbM genes. A significant positive correlation was found between the abundances of mFeOB and the cbbL gene, as well as a highly significant positive correlation between the abundances of mFeOB and the cbbM gene, indicating the distribution of mFeOB in co-occurrence with carbon-fixing microorganisms in wetlands. The mFeOB were mainly dominated by Sideroxydans lithotrophicus ES-1 and Gallionella capsiferriformans ES-2 in all wetland soils. The structures of the carbon-fixing microbial communities were similar in these wetlands, mainly consisting of Alphaproteobacteria, Betaproteobacteria, and Gammaproteobacteria. The extractable Fe(II) concentrations affected the community composition of mFeOB, resulting in a significant difference in the relative abundances of the dominant FeOB. The main factors affecting cbbL-related microbial communities were dissolved inorganic carbon and oxygen, soil redox potential, and sodium acetate-extracted Fe(II). The composition of cbbM-related microbial communities was mainly affected by acetate-extracted Fe(II) and soil redox potential. In addition, the positive correlation between these functional microorganisms suggests that they play a synergistic role in Fe(II) oxidation and carbon fixation in wetland soil ecosystems. Our results suggest a cryptic relationship between mFeOB and carbon-fixing microorganisms in wetlands and that the microbial community structure can be effectively altered by regulating their physicochemical properties, thus affecting the capacity of carbon sequestration.

Machine learning-based prediction of cadmium bioaccumulation capacity and associated analysis of driving factors in tobacco grown in Zunyi City, China.

Tobacco grown in areas with high-geochemical backgrounds exhibits considerably different cadmium (Cd) bioaccumulation abilities due to regional disparities and environmental changes. However, the impact of key factors on the Cd bioaccumulation ability of tobacco grown in the karst regions with high selenium (Se) geochemical backgrounds is unclear. Herein, 365 paired rhizospheric soil-grown tobacco samples and 321 topsoil samples were collected from typical karst tobacco-growing soil in southwestern China and analyzed for Cd and Se. XGBoost was used to predict and evaluate the Cd bioaccumulation ability of tobacco and potential influencing factors. Results showed that regional geochemical characteristics, such as soil Cd and Se contents, soil type, and lithology, have the highest influence on the Cd bioaccumulation ability of tobacco, accounting for 46.5% of the overall variation. Moreover, soil Se contents in high-geochemical background areas considerably affect Cd bioaccumulation in tobacco, with a threshold for the mutual suppression effects of Cd and Se at a soil Se content of 0.8 mg/kg. According to the results of bivariate local indicators of spatial association analysis, tobacco cultivated in the central, northeast, and southeast regions of Zunyi City carries a lower risk of soil Cd contamination. This study provides new insights for managing tobacco cultivation in karst regions.

Humic acid controls cadmium stabilization during Fe(II)-induced lepidocrocite transformation.

Abiotic reduction of iron (oxyhydr)oxides by aqueous Fe(II) is one of the key processes affecting the Fe cycle in soil. Lepidocrocite (Lep) occurs naturally in anaerobic, clayey, non-calcareous soils in cooler and temperate regions; however, little is known about the impacts of co-precipitated humic acid (HA) on Fe(II)-induced Lep transformation and its consequences for heavy metal immobilization. In this study, the Fe(II)-induced phase transformation of Lep-HA co-precipitates was analyzed as a function of the C/Fe ratio, and its implications for subsequent Cd(II) concentration dynamic in dissolved and solid form was further investigated. The results revealed that secondary Fe(II)-bearing magnetite commonly formed during the Fe(II)-induced transformation of Lep, which further changed the mobility and distribution of Cd(II). The co-precipitated HA resulted in a decrease in the Fe solid phase transformation as the C/Fe ratios increased. Magnetite was found to be a secondary mineral in the 0.3C/Fe ratio Lep-HA co-precipitate, while only Lep was observed at a C/Fe ratio of 1.2 using X-ray diffraction (XRD) and Mössbauer spectroscopy. Based on XRD, scanning electron microscopy (SEM), Mössbauer, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) results, newly formed magnetite may immobilize Cd(II) through surface complexes, incorporation, or structural substitution. The presence of HA was beneficial for binding Cd(II) and affected the mineralogical transformation of Lep into magnetite, which further induced the distribution of Cd(II) into the newly formed secondary minerals. These results provide insights into the behavior of Cd(II) in response to reaction between humic matter and iron (oxyhydr)oxides in anaerobic environments.

Flooding-drainage alternations impact mobilization and isotope fractionation of cadmium in soil-rice systems.

Sequential flooding and draining substantially alter Cd mobilization in paddy fields, primarily due to redox-driven changes in Fe-Mn (hydro)oxides and Cd-sulfides. However, the impacts of carbonates on Cd mobilization during flooding-drainage alternations remain poorly understood. In this study, Cd isotope compositions were analyzed in soils and plants at three growth stages, and the results show a pH-dependent Cd mobilization and isotope fractionation. Sequential extraction shows the Cd mainly binds to the exchangeable fraction and carbonates, and their amounts vary with pH. Exchangeable Cd with light isotopes coprecipitates into carbonates due to increased pH during flooding (tillering and panicle initiation). Whereas in drained soils (maturity), the carbonate-bound Cd releases with decreased pH. Light isotopes are enriched in rice compared with exchangeable Cd, but this enrichment is insignificant at maturity. This difference is mainly caused by the change in Cd isotope composition of exchangeable Cd pool due to carbonate coprecipitation during flooding. Limited isotope fractionation between roots and aboveground tissues is found at tillering, whereas significant isotope fractionation is observed at two other stages, suggesting the nodes might work during Cd translocation between tissues. These findings demonstrate alternating flooding-drainage impacts the mobilization of carbonate-bound Cd and, consequently, isotope fractionation in soil-rice systems.

Adsorption of cadmium on clay-organic associations in different pH solutions: The effect of amphoteric organic matter.

Clay minerals are important soil components and usually coexist with organic matter, forming mineral-organic associations (MOAs), which control the speciation, mobility, and bioavailability of heavy metals. However, the adsorption mechanism of cadmium (Cd) by MOAs is still unclear, especially for the associations of amphotericorganic matter and clay minerals. In this study, 12-aminododecanoic acid (ALA) and montmorillonite (Mt) were chosen to prepare MOAs via intercalation (Mt-ALA composite) and physical mixing (Mt-ALA mixture). Batch experiments were conducted to investigate the adsorption mechanism of Cd(II) by MOAs under different pH values and initial Cd(II) concentrations. The results showed that the Cd(II) adsorption capacities followed as Mt > Mt-ALA mixture > Mt-ALA composite under acidic conditions, Mt-ALA mixture > Mt > Mt-ALA composite under neutral conditions, and Mt-ALA mixture > Mt-ALA composite > Mt under alkaline conditions, suggesting the adsorption behaviors of Cd(II) by MOAs were primarily constrained by the speciation of ALA and solution pH. Under acidic conditions, cationic HALA+ could intercalate into the interlayer of Mt and occupy the adsorption sites, reducing the adsorption capacity of Cd(II). As pH increased to neutral, HALA+ decreased and changed to a zwitterionic state, which caused ALA to release out from the interlayer of Mt-ALA composite or not easily enter into Mt-ALA mixture and promoted Cd(II) adsorption. Under alkaline conditions, the increase of anion ALA- would cause ALA to be mainly adsorbed on the surface of Mt and chelate with Cd(II), enhancing the adsorption of Cd(II). Further analysis by Fourier transform infrared and X-ray photoelectron spectroscopy indicated that the carboxyl and amino groups of ALA both participated in the adsorption of Cd(II). These findings could extend the knowledge on the mobility and fate of Cd in clay-based soils and be used as a basis for understanding the biogeochemical behavior of Cd in the environment.

Source and Strategy of Iron Uptake by Rice Grown in Flooded and Drained Soils: Insights from Fe Isotope Fractionation and Gene Expression.

Rice can simultaneously absorb Fe2+ via a strategy I-like system and Fe(III)-phytosiderophore via strategy II from soil. Still, it remains unclear which strategy and source of Fe dominate under distinct water conditions. An isotope signature combined with gene expression was employed to evaluate Fe uptake and transport in a soil-rice system under flooded and drained conditions. Rice of flooded treatment revealed a similar δ56Fe value to that of soils (Δ56Ferice-soil = 0.05‰), while that of drained treatment was lighter than that of the soils (Δ56Ferice-soil = -0.41‰). Calculations indicated that 70.4% of Fe in rice was from Fe plaque under flooded conditions, while Fe was predominantly from soil solution under drained conditions. Up-regulated expression of OsNAAT1, OsTOM2, and OsYSL15 was observed in the root of flooded treatment, while higher expression of OsIRT1 was observed in the drained treatment. These isotopic and genetic results suggested that the Fe(III)-DMA uptake from Fe plaque and Fe2+ uptake from soil solution dominated under flooded and drained conditions, respectively.

Zinc regulation of iron uptake and translocation in rice (Oryza sativa L.): Implication from stable iron isotopes and transporter genes.

Iron (Fe) is an essential nutrient for living organisms and Fe deficiency is a worldwide problem for the health of both rice and humans. Zinc (Zn) contamination in agricultural soils is frequently observed. Here, we studied Fe isotope compositions and transcript levels of Fe transporter genes in rice growing in nutrient solutions having a range of Zn concentrations. Our results show Zn stress reduces Fe uptake by rice and drives its δ56Fe value to that of the nutrient solution. These observations can be explained by the weakened Fe(II) uptake through Strategy I but enhanced Fe(III) uptake through Strategy II due to the competition between Zn and Fe(II) combining with OsIRT1 (Fe(II) transporter) in root, which is supported by the downregulated expression of OsIRT1 and upregulated expression of OsYSL15 (Fe(III) transporter). Using a mass balance box model, we also show excess Zn reduces Fe(II) translocation in phloem and its remobilization from senescent leaf, indicating a competition of binding sites on nicotianamine between Zn and Fe(II). This study provides direct evidence that how Zn regulates Fe uptake and translocation in rice and is of practical significance to design strategies to treat Fe deficiency in rice grown in Zn-contaminated soils.

Enrichment and environmental availability of cadmium in agricultural soils developed on Cd-rich black shale in southwestern China.

The enrichment of cadmium (Cd) in black shale-derived soils is of increasing concern due to its wide occurrence, high Cd concentrations, and potential risks. However, characteristics of enrichment and environmental availability of Cd in these soils are not well understood, which has restricted pollution control and land management. In this study, agricultural soils with elevated Cd concentrations resulting from weathering of Cd-bearing black shale in southwestern China were collected and analyzed. The results showed that Cd could be retained in soils through mechanical inheritance and/or associated with secondary minerals and organic materials. Cd concentrations in soils of the study area ranged between 0.83 and 21.6 mg/kg (average of 5.20 mg/kg), exceeding the risk screening value for agricultural land in China. The heterogeneity of Cd in these soils was highly related to geochemical composition of parent rock and other natural factors. The 0.01 M CaCl2 and 0.05 M EDTA extraction showed that Cd in these soils had high environmental availability and potential risks. Mobile Cd pool (CaCl2 extractable Cd, average: 0.24 mg/kg) accounted for 0.07-38.9% of the total Cd, depending on soil pH. Mobilizable Cd pool (EDTA extractable Cd, average: 2.18 mg/kg) accounted for 22.0-100%. These results showed the significance of geochemical background on enrichment of Cd in soils, documented high environmental availability of Cd in black shale-derived soils, and influence of soil pH.

Coincorporation of N and S into Zero-Valent Iron to Enhance TCE Dechlorination: Kinetics, Electron Efficiency, and Dechlorination Capacity.

Sulfidated zero-valent iron (S-ZVI) enhances the degradation of chlorinated hydrocarbon (CHC) in contaminated groundwater. Despite numerous studies of S-ZVI, a versatile strategy to improve its dechlorination kinetics, electron efficiency (εe), and dechlorination capacity is still needed. Here, we used heteroatom incorporation of N(C) and S by ball-milling of microscale ZVI with melamine and sulfur via nitridation and sulfidation to synthesize S-N(C)-mZVIbm particles that contain reactive Fe-NX(C) and FeS species. Sulfidation and nitridation synergistically increased the trichloroethene (TCE) dechlorination rate, with reaction constants kSA of 2.98 × 10-2 L·h-1·m-2 by S-N(C)-mZVIbm, compared to 1.77 × 10-3 and 8.15 × 10-5 L·h-1·m-2 by S-mZVIbm and N(C)-mZVIbm, respectively. Data show that sulfidation suppressed the reductive dissociation of N(C) from S-N(C)-mZVIbm, which stabilized the reactive Fe-NX(C) and reserved electrons for TCE dechlorination. In addition to lowering H2 production, S-N(C)-mZVIbm dechlorinated TCE to less reduced products (e.g., acetylene), contributing to the material's higher εe and dechlorination capacity. This synergistic effect on TCE degradation can be extended to other recalcitrant CHCs (e.g., chloroform) in both deionized and groundwater. This multiheteroatom incorporation approach to optimize ZVI for groundwater remediation provides a basis for further advances in reactive material synthesis.

FeNX(C)-Coated Microscale Zero-Valent Iron for Fast and Stable Trichloroethylene Dechlorination in both Acidic and Basic pH Conditions.

FeNX in Fe single-atom catalysts can be the active site for adsorption and activation of reactants. In addition, FeNX species have been shown to facilitate electron transfer between Fe and the carbon supports used in newly developed metal-air batteries. We hypothesized that the combination of FeNX species with granular zero-valent iron (ZVI) might result in catalyzed reductive decontamination of groundwater contaminants such as trichloroethylene (TCE). Here, such materials synthesized by ball milling microscale ZVI with melamine and the resulting N species were mainly in the form of pyridinic, pyrrolic, and graphitic N. This new material (abbreviated as N-C-mZVIbm) dechlorinated TCE at higher rates than bare mZVIbm (about 3.5-fold) due to facilitated electron transfer through (or around) the surface layer of iron oxides by the newly formed Fe-NX(C). N-C-mZVIbm gave higher kTCE (0.4-1.14 day-1) than mZVIbm (0-0.4 day-1) over a wide range of pH values (4-11). Unlike most ZVI systems, kTCE for N-C-mZVIbm increased with increasing pH values. This is because the oxide layer that passivates Fe0 at a high pH is disrupted by Fe-NX(C) formed on N-C-mZVIbm, thereby allowing TCE dechlorination and HER under basic conditions. Serial respike experiments gave no evidence of decreased performance of N-C-mZVIbm, showing that the advantages of this material might remain under field applications.

Arsenic detoxification by iron-manganese nodules under electrochemically controlled redox: Mechanism and application.

Iron-manganese binary oxides are characterized by high oxidation and adsorption capability and widely applied to arsenic (As) detoxification in contaminated waters. Despite of their lower preparation cost relative to synthesized iron-manganese binary oxides, the low adsorption capacity of natural iron-manganese oxides largely hinders their application. Here, electrochemically controlled redox was employed to improve the As(III,V) removal performance of iron-manganese nodules in a symmetric electrode system, and the removal mechanism and electrode reusability were also examined. Experimental results showed that both the electrochemical reduction and oxidation of birnessite in iron-manganese nodules contributed much to As(III,V) removal. Higher cell voltage facilitated a higher removal efficiency of total As within 0-1.2 V, which reached 94.7% at 1.2 V for actual As-containing wastewater (4068 µg L-1). The efficiency was obviously higher than that at open circuit (81.4%). Under electrode polarity reversal, the alternating reduction dissolution and oxidation recrystallization of birnessite in iron-manganese nodules promoted their contact with As, enhancing the total As removal efficiency from 75.6% to 91.8% after five times of repeated adsorption. This research clarifies the effect of electrochemical redox on As(III,V) detoxification by iron-manganese oxides, and expands the application of natural iron-manganese nodules in the treatment of As-contaminated wastewaters.

Contamination, oral bioaccessibility and human health risk assessment of thallium and other metal(loid)s in farmland soils around a historic TlHg mining area.

In this study, tweenty-nine soil samples were collected from a historic TlHg mining area, located in southwest Guizhou, China. Total concentrations of metal(loid)s in soils and in vitro extracts were analysed by ICP-MS, and the bioaccessibility of metal(loid)s was conducted by two often used in vitro extraction methods, Simplified bioaccessibility Extraction Test (SBET) and Physiologically Based Extraction Test (PBET). The health risk assessment based on total concentrations of metal(loid)s, bioaccessibility of SBET and PBET through soil ingestion were investigated. Results indicated that the collected cultivated soils contained elevated concentrations of Tl (44.8 ± 67.7 mg kg-1), Hg (110 ± 193 mg kg-1), As (84.4 ± 89.2 mg kg-1) and Sb (14.8 ± 24.8 mg kg-1), exceeding the regional background values of Guizhou province, China and the Chinese farmland risk screening values. However, the bioaccessibility of Tl, Hg, As and Sb were relatively low, usually less than 30% for most samples and varied greatly among metal(loid)s and sampling sites. The average bioaccessibility values of Tl, Hg, As and Sb by SBET were lower than those by PBET. The non-carsinogenic risk (HQ and HI) and Carcinogenic Risk (CR) values were significantly reduced when incorporating the bioaccessibiltiy of metal(loid)s into health risk assessment. It is worth noting that the health risk to children exceeded adults. Moreover, Tl and As contributed the most to the risk, indicating that more attention should be paid on Tl and As during the daily environmental regulation and management of contaminated soils in Lanmuchang.

Electrochemical adsorption of cadmium and arsenic by natural Fe-Mn nodules.

Fe-Mn nodules are widely distributed and regarded as excellent adsorbents for heavy metals. Their adsorption-desorption reactions with heavy metal ions are usually accompanied by redox processes. Herein, Fe-Mn nodules were used as adsorbents for Cd(II) and As(III,V) at a constant cell voltage under electrochemically controlled reduction and oxidation, respectively. The results showed that the adsorption performance for Cd(II) and As(III,V) was enhanced respectively due to the decrease and increase of Mn average oxidation state (Mn AOS) in Fe-Mn nodules. High birnessite content and Mn average oxidation state (Mn AOS) improved the adsorption of Cd(II) and As(III,V). The adsorption capacity for Cd(II) and total As increased with increasing voltage. With increasing pH, the adsorption capacity for Cd(II) increased first and then reached equilibrium, and that of total As decreased and then increased. The Cd(II) electrochemical adsorption capacity (129.9 mg g-1) and the removal efficiency for total As at 1.2 V (83.6 %) in As-containing wastewater at an initial concentration of 4.068 mg L-1 were remarkably higher than the corresponding inorganic adsorption performance (9.46 mg g-1 and 70.5 %, respectively). This work may further promote the application of natural Fe-Mn nodules in the adsorption of heavy metals from wastewaters.

A portable and quantitative biosensor for cadmium detection using glucometer as the point-of-use device.

As an ubiquitous heavy metal pollutant, cadmium ion (Cd2+) is detrimental to food and human health even at low concentrations. Conventional methods require costly instruments and cannot meet the requirements of on-site analysis. Here we report the use of a personal glucose meter (PGM) as the point-of-use (POU) device for portable and quantitative detection of Cd2+. The specific recognition between the aptamer and Cd2+ trigger the recycling signal amplification process by exonuclease III (Exo III). After successive hybridization and cleavage reactions, numerous single-stranded DNA were liberated on the surface of the magnetic bead. An invertase-conjugated DNA that is complementary to the single-stranded DNA is introduced into the sensing system. After magnetic separation, the invertase conjugates hydrolyze sucrose into glucose, thus establishing direct conversion of Cd2+ concentration to glucose amount, which can be directly quantified by a PGM. Thanks to the synergistic signal amplification of Exo III and invertase, the POU device greatly improves the sensitivity for Cd2+ analysis, with a detection limit of 5 p.M. With the advantages of portability, cost-effectiveness, wide availability, and ease of use, the PGM-based detector has the potential to be used by the public as a routine tool for reliable and quantitative detection of Cd2+.

Enhanced immobilization of arsenic and cadmium in a paddy soil by combined applications of woody peat and Fe(NO3)3: Possible mechanisms and environmental implications.

Organic matter (OM) plays an important role in the mobility of heavy metal(loid)s. Peat containing abundant OM can be used as an organic fertilizer improving physical and chemical properties of soil. Previous studies indicated that the immobilization of heavy metal(loid)s by peat is affected by the presence of metal oxides and/or hydroxides and that Fe-enriched peat is very effective in immobilizing metal(loid)s. Accordingly, we hypothesize that simultaneous application of peat and Fe-containing compounds may pronouncedly immobilize heavy metal(loid)s. In this study, the effects of the combined applications of woody peat and Fe(NO3)3 on As and Cd mobilities and accumulations in rice during the whole growth period were investigated by a pot experiment. The combined applications of woody peat and Fe(NO3)3 significantly decreased As(III) and Cd in porewater due to pH increases induced by applications of Fe(NO3)3, and these decreases were enhanced with increasing Fe(NO3)3. In addition, simultaneous application of peat and Fe(NO3)3 significantly decreased mobile portions of As and Cd but significantly increased their immobile portions. Increasing Fe(NO3)3 increased the amount of As immobilized by poorly crystalline Fe oxides. The formation of Fe plaques and production of poorly crystalline Fe oxides were enhanced by Fe(NO3)3 addition, which also contributed to the immobilization of As and Cd in soil. Overall, the combined applications of woody peat and Fe(NO3)3 provided a strategy for simultaneously immobilizing As and Cd in soils and further alleviating their accumulations from soil to rice plants. In paddy soil, the frequent occurrence of iron redox activity due to the alternating wetting and drying cycles provided favorable conditions for interactions between Fe and OM, and this process and its associated metal(loid) immobilization may be more important than we thought and need further study.

Microbial iron reduction as a method for immobilization of a low concentration of dissolved cadmium.

Much attention has been paid to the relationship between microbial iron reduction and the behavior of cadmium (Cd) recently, but most previous research has employed unrealistically high Cd concentrations (e.g., 2-55 mg L-1) and has failed to consider the effects of iron oxides and microbial cells together. We investigated the reduction of lepidocrocite by Shewanella oneidensis MR-1 in the presence of a low concentration of Cd using batch reactor systems. The results showed that with 422 µg L-1 added dissolved Cd2+, an initial 137 µg L-1 decrease in aqueous Cd occurred due to adsorption onto lepidocrocite and that the further removal of remaining aqueous Cd occurred only in the system containing bacteria. This further decrease in aqueous Cd was unlikely to be caused by mineral transformation because the microbial reduction of lepidocrocite resulted in particle-size-increased (thus, specific-surface-area-decreased) lepidocrocite, and unlikely to be caused by the pH increase to 7.4 induced by iron reduction either because a pH-adsorption edge suggested that at pH 7.4, less than 60% of aqueous Cd can be adsorbed by lepidocrocite in the reactors. An adsorption isotherm showed a significant Cd adsorption capacity by S. oneidensis MR-1 cells, and we therefore attributed the further Cd removal to adsorption by S. oneidensis MR-1 cells. The results suggest that a realistically low concentration of Cd can be immobilized during microbial iron reduction by adsorption on iron oxides and microbial cells.

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

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

Cadmium availability in rice paddy fields from a mining area: The effects of soil properties highlighting iron fractions and pH value.

Cadmium (Cd) availability can be significantly affected by soil properties. The effect of pH value on Cd availability has been confirmed. Paddy soils in South China generally contain high contents of iron (Fe). Thus, it is hypothesized that Fe fractions, in addition to pH value, may play an important role in the Cd bioavailability in paddy soil and this requires further investigation. In this study, 73 paired soil and rice plant samples were collected from paddy fields those were contaminated by acid mine drainage containing Cd. The contents of Fe in the amorphous and DCB-extractable Fe oxides were significantly and negatively correlated with the Cd content in rice grain or straw (excluding DCB-extractable Fe vs Cd in straw). In addition, the concentration of HCl-extractable Fe(II) derived from Fe(III) reduction was positively correlated with the Cd content in rice grain or straw. These results suggest that soil Fe redox could affect the availability of Cd in rice plant. Contribution assessment of soil properties to Cd accumulation in rice grain based on random forest (RF) and stochastic gradient boosting (SGB) showed that pH value should be the most important factor and the content of Fe in the amorphous Fe oxides should be the second most important factor in affecting Cd content in rice grain. Overall, compared with the studies from temperate regions, such as Europe and northern China, Fe oxide exhibited its unique role in the bioavailability of Cd in the reddish paddy soil from our study area. The exploration of practical remediation strategies for Cd from the perspective of Fe oxide may be promising.