A multi-surface model to predict Cd phytoavailability to wheat (Triticum aestivum L.)

Roles of the SOM and clay minerals in alleviating the leaching of Pb, Zn, and Cd from the Pb/Zn smelter soil: Multi-surface model and DFT study

Soil organic matter (SOM) and clay minerals are important sinks for reactive heavy metals (HMs) and exogenous hydrogen ions (H+). Therefore, HMs are likely to be released into soil porewater under acid rainfall conditions due to the competitive adsorption of H+. However, negligible Lead, Zinc, and Cadmium (<6 ‰) in the Pb/Zn smelter soil were leached, and the effects of SOM and clay minerals on HMs leaching were unclear. Herein, the H+ consumption and HMs redistribution on SOM and clay minerals were quantitated by the multi-surface model and density functional theory calculations to reveal the roles of SOM and clay minerals in alleviating HMs' leaching. Clay minerals consumed 43.2 %-52.0 % of the exogenous H+, serving as the dominant sink for the exogenous H+ due to its high content and hindering H+ competitive adsorption on SOM. Protonation of the functional groups constituted >90 % of the total H+ captured by clay minerals. Meanwhile, some H+ also competed with HMs for adsorption sites on clay minerals due to its 0.497-fold to 1.54-fold higher binding energies than HMs, resulting in the release of HMs. On the contrary, SOM served as an accommodator for taking over the released HMs from clay minerals. The HMs complexation on the low-affinity sites (R-L-) of SOM was responsible for the recapture of HMs. In Ca-enriched soil, the released HMs were also recaptured by SOM via ion exchange on the R-L-Ca+ and the high-affinity sites (R-H-Ca+) sites due to the 30.8 %-178 % higher binding energies of HMs on these sites than those of Ca. As a result, >63.4 % of the released HMs from clay minerals were transferred to the SOM. Thus, the synergy of SOM and clay minerals in alleviating the leaching of HMs in Pb/Zn smelter soils cannot be ignored in risk assessment and soil remediation.

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.

Impact of meteorological factors on Cd availability and average concentration prediction in rice growth cycle

During the rice growth cycle, the average available cadmium concentration (CA-Cd) in the soil determines the Cd content in rice plant. Given defined soil properties and rice varieties, the meteorological factors play a crucial role in soil's available cadmium concentration (CCd) during the rice growth cycle. Thus, it is significant to investigate the influence of meteorological factors in CCd during the rice growth cycle and develop a predictive model for CA-Cd. The rice was cultivated under seven different sowing dates in Cd and As-contaminated soil in Hunan Province. Studied the impact of meteorological factors on paddy soil. The results showed that accumulated temperature (AT) and total precipitation (TP) were key factors affecting the soil CCd. The correlation coefficients between AT and TP with soil CA-Cd were 0.98 and -0.94 (p < 0.01), respectively. However, there was no significant correlation with CAs. AT mainly influenced the CCd during the grouting and maturity stages. A straightforward empirical prediction model was developed, capable of accurately forecasting CA-Cd during the rice growth cycle by considering meteorological factors and the initial soil CCd. This study supported a novel foundation for the precise prediction of Cd content in rice.

Prediction of cadmium and zinc phytoextraction by the hyperaccumulator Sedum plumbizincicola using a dynamic geochemical mechanical combination model

Phytoextraction with hyperaccumulators is an environmentally friendly and cost-effective technique for soil remediation but remediation time is largely dependent on experience due to variations in soil properties which restrict the application of this technique. Here, a novel dynamic multi-surface model (MSM) framework is proposed to predict the efficiency and duration of cadmium (Cd) and zinc (Zn) phytoextraction using the hyperaccumulator Sedum plumbizincicola. First, the application of MSM to S. plumbizincicola was investigated using 95 naturally contaminated soils. Using the 'default' settings and considering the pH and DOC content in the rhizosphere, the dissolved Cd/Zn predicted by MSMs showed strong correlations with metal uptake by shoots (R2 = 0.825/0.802 for Cd/Zn, n = 95) and outperformed chemical extraction methods. Then the MSMs were further integrated with time and Cd and Zn interactions to form dynamic-MSM combined (D-MSM-C) models to evaluate and predict phytoextraction efficiency and duration based on a six-season continuous pot experiment. The D-MSM-C models well predicted metal contents remaining in soils after each season with mean absolute percentage error (MAPE) = 20.4 % (Cd) and 2.46 % (Zn) (n = 66). This model is a powerful tool for assessing and predicting phytoremediation efficiency and duration and is applicable across diverse soil properties and multiple metal-contamination scenarios.

Prediction of cadmium bioavailability in the rice-soil system on a county scale based on the multi-surface speciation model

Relative to total cadmium (Cd) content, bioavailable Cd in paddy soil is regarded as a more reasonable indicator for the risk of Cd bioaccumulation in rice. However, there is still a lack of approach to accurately predict the content of bioavailable Cd in paddy soil due to its heterogeneity and complexity. Here, multi-surface speciation model (MSM) was employed to predict the bioavailable Cd and Cd immobilization effect. Moreover, a precise remediation strategy was designed based on screening and scenario simulation of the sensitive factors with MSM. The results demonstrated that MSM can well predict Cd bioaccumulation risk in rice. The contribution of pH to Cd bioavailability was quantified under three analysis scenarios, accounting for 87.51% of the total variance of bioavailable Cd. In addition, the pH alert value (6.31 ± 0.52) for Cd risk was acquired for each rice field on a county scale. A precise map for the application amount of lime materials was constructed by taking CaCO₃ (3.38-15.75 t ha^-1) as a recommended economical and green immobilization agent. This study provides a potentially effective approach for risk assessment of Cd contamination in rice and important reference for precise Cd remediation in paddy soil.

Tracking cadmium pollution from source to receptor: A health-risk focused transfer continuum approach

Cadmium (Cd) exposure poses a substantial risk to human health. Despite this, the multi-stage process through which Cd is released to the environment before being taken up and impacting human receptors has rarely been investigated. Here we utilized an integrated model involving Cd emissions, atmospheric transport, deposition, uptake by rice, receptor ingestion and metabolic processing in quantifying the critical emission sources and human health risks of Cd. Atmospheric Cd emissions in the study area in southeastern China were estimated at 147 kg (2016), with >53 % of emissions from non-ferrous metals (NFM) smelting activities. Atmospheric Cd depositions caused elevated Cd content in soil and rice, accounting for 3-79 % and 50-85 % of, respectively, soil and rice Cd. Cumulative frequency analysis showed that an estimated 1.3 % of predicted urine Cd through the consumption of Cd-contaminated rice and exceeded existing safety standards (1 μg g^-1), thus highlighting the risks posed to health from high levels of Cd pollution. Applying stricter industrial emission standards to the NFM sector in particular and effective soil management practices could substantially reduce exposure to Cd pollution. The results contribute to understanding of the Cd transfer process and draw attention to the relative health benefits of interventions aimed at mitigating Cd levels and exposure risks at different stages along the Cd transfer continuum from source to receptor.

Distribution and accumulation of cadmium in soil under wheat-cultivation system and human health risk assessment in coal mining area of China

The soil contamination caused by the discharge of cadmium (Cd) from coal mining activities has aroused continuous attention due to the detrimental effects on the human health. This study aimed to investigate the characteristics on distribution of Cd in soils and its accumulation in wheat grains under wheat-cultivation system, and further assess the human health risks to adults and children. 58 soils and wheat samples in pairs from Linhuan coal mining area, Anhui Province were collected and analyzed. Results showed that the concentrations of Cd in 17.24% of soil samples exceeded the limit value established by the Ministry of Ecology and Environment. The ordinary kriging interpolation displayed that the spatial variability of Cd concentrations in soils was mainly influenced by coal mining activities. The transfer capacity of Cd from soils to wheat roots was greater than that from the wheat roots to grains. Multiple linear regression model clarified that soil pH and exchangeable Cd fraction in soils were the critical factors affecting the Cd accumulation in wheat grains. The carcinogenic risk of Cd levels in our studied wheat grains was a concern but still within the acceptable range, while their non-carcinogenic hazard was negligible for adults and children. The calculation results were in accord with the uncertainty analysis conclusion based on Monte Carlo simulation. The study was expected to promote the source management and control strategy of reducing tailing discharge, and providing scientific references for current soil remediation and land degradation prevention.

Contribution of components in natural soil to Cd and Pb competitive adsorption: Semi-quantitative to quantitative analysis

Cadmium (Cd) and lead (Pb) are two of the most common elements found in contaminated sites. The behavior of specific metals in the soil may be affected by other metals because of the competition for adsorption sites. In this study, adsorption experiments after chemical extraction, multi-surface models, and advanced spectroscopy technology were jointly used to explain the adsorption mechanism of Cd and Pb and to determine the contribution of each component in the competitive system. The results show that pH is the key factor in determining the contribution of soil components to metal adsorption. Soil organic matter (SOM) is the dominant adsorbent for both Cd and Pb. Clay minerals play an adsorption role at low pH, whereas Fe/Al oxides adsorb metals primarily in the high pH range. Further, the competitive effect of Pb on Cd occurred primarily on SOM rather than on clay minerals. When the Pb concentration increased from 0 to 500 mg/L, the adsorption of Cd on SOM decreased by 132.0 mg/kg, whereas it decreased only by 1.9 mg/kg on clay minerals. Therefore, the competitive effect of Pb on Cd cannot be ignored in soils with high organic matter content.

Prediction of Cd Accumulation in Wheat (Triticum aestivum L.) and Simulation Calculation of Lime or Zn Fertilizer Remediated Soil

Soil Cd contamination to wheat raise wide concerns over food safety. It is essential to find the key factors affecting Cd accumulation in wheat and to establish a predictive model. The effects of pH, Zn, Ca, and DOM on the accumulation of Cd in wheat were investigated using hydroponic experiments. The results showed that Zn was the most important factor inhibiting Cd uptake in wheat. Models were developed to predict the Cd contents in wheat tissues based on the ion concentration. Meanwhile, the available Cd contents in soil were predicted using a geochemical multi-surface model (MSM) which is suitable for various soils and conditions. The combination of the hydroponic accumulation model and MSM exhibits good predictions of wheat-Cd (R² = 0.822-0.862, RMSE = 0.317-0.533). The results of this study can quantitatively predict the accumulation of Cd in wheat and provide a reference for soil remediation and safe wheat production.

Effect of land use pattern on the bioavailability of heavy metals: A case study with a multi-surface model

In modern agricultural practice, the land use pattern has been changing due to economic reasons and related policies, which significantly affects the basic physical and chemical properties of soils, thereby influencing the speciation and distribution of heavy metals (HMs) in soils. In this study, we selected three typical types of land use patterns (vegetable field, paddy field and forest field) in Shaoguan City, Guangdong Province, to analyze the content and distribution of HMs, screen the sensitive physicochemical properties, and predict the phytoavailability of HMs under different land use patterns with the multi-surface model (MSM). The forest field had relatively lower levels of labile and free HM ions than both paddy and vegetable fields, which may be attributed to the lower HM content in forest field. The modeling results revealed that organic matter (OM) is the primary carrier of HMs, accounting for 0.19%-97.92% of labile HMs. The sensitivity of soil physicochemical properties to free HM ions followed the order of pH > SOM > goethite > clay. Besides, the conversion of paddy field into vegetable or forest field increased the environmental risk of HMs. Our results may help better decision making in agricultural restructuring to reduce the risk of HM-contaminated soils, as well as give a demonstration for the application of the MSM in predicting the phytoavailability of HMs as a powerful technique.

Rape straw application facilitates Se and Cd mobilization in Cd-contaminated seleniferous soils by enhancing microbial iron reduction

Many naturally seleniferous soils are faced with Cd contamination problem, which severely limits crop cultivation in these areas. Straw returning has been widely applied in agricultural production due to its various benefits to soil physicochemical properties, soil fertility, and crops yield. However, effects of straw application on the fates of Se and Cd in Cd-contaminated seleniferous soils remain largely unclear. Therefore, the effects of straw application on the fates of Se and Cd in Cd-contaminated seleniferous soils were investigated in this study. The results showed that iron reduction driven by Clostridium and Anaeromyxbacter was responsible for the variations in Se and Cd fates in soil. Straw application respectively increased the gene copy numbers of Clostridium and Anaeromyxbacter by 19.5-56.3% and 33.6-39.8%, thus promoting iron reductive dissolution, eventually resulting in a high release amount of Se and Cd from Fe(III) (oxyhydr) oxides. Under reducing conditions, the released Cd was adsorbed by the newly formed metal sulfides or reacted with sulfides to generate CdS precipitates. Straw application decreased the soil exchangeable Se and soil exchangeable Cd concentration during flooding phase. However, straw application significantly increased Se/Cd in soil solution which had the highest bioavailability during flooding. In addition, straw application increased soil exchangeable Se concentration, but it had no significant effects on soil exchangeable Cd concentration after soil drainage. Taken together, straw application increased Se bioavailability and Cd mobility. Therefore, straw application is an effective method for improving Se bioavailability, but it is not suitable for the application to Cd-contaminated paddy soils. In the actual agricultural production, straw could be applied in seleniferous soils to improve Se bioavailability. At the same time, straw application should be cautious to avoid the release of Cd from Cd-contaminated soil.

The symbiotic system of sulfate-reducing bacteria and clay-sized fraction of purplish soil strengthens cadmium fixation through iron-bearing minerals

The microbe-clay mineral system is widely known to reduce the fluidity of heavy metals through biomineralization, thus mitigating soil pollution stemming from heavy metals. Here, we investigated the effect of mineral distinction on the solidification of cadmium (Cd) using sulfate-reducing bacteria (SRB) to construct symbiotic systems with purplish soil, clay-sized fraction of purple soil (Clay_-csp), clay particles of amorphous iron (Fe) oxide (Clay_-ox), clay particles removing crystalline Fe oxide (Clay_-CBD), and residues of Clay_-CBD treated by hydrochloric acid (Clay_-HCl). The difference in Cd morphology among purplish soil, Clay_-csp, and Clay_-ox indicated that the fixation of Cd in soil was largely determined by Fe oxides. The content of Cd in Clay_-csp decreased by 66.7% after the removal of amorphous Fe, confirming that clay easily adsorbed infinitive Fe oxides in purple soil. In the system of SRB and Clay_-ox, carbonate-bound Cd (F2) decreased by 14.85% and residual Cd (F5) increased by 14% from the retardation to late decline phase, eventually forming iron-sulfur (Fe-S) compounds. Based on the correlation analyses of Cd and Fe in amorphous-bound state and Fe-manganese (Mn) oxidation state in simulation experiments, it is demonstrated that Fe-Mn oxides control the behavior of Cd in soil clay, and SRB-mediated Fe-bearing minerals promote the transformation of Cd from activated to stable state.

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.

Fe(III) reduction due to low pe+pH contributes to reducing Cd transfer within a soil-rice system

Effect of Fe redox state caused by low soil pe+pH levels on Cd uptake by rice is unclear. Rice grown in pots of Cd-contaminated paddy soil were subjected to different irrigation regimes: flooding, intermittent flooding (Int-FL), and sustained soil moisture at 70% water holding capacity (WHC). Results showed low pe+pH (5.52 and 7.09) in flooding treatment significantly increased relative abundances of Fe-reducing bacteria (FeRB) (6.29% and 4.51%), especially members within the Clostridium, Geobacter and Desulfuromonadia genera. Stimulation of FeRB activity induced Fe(III) reduction and increased Fe²⁺ content in flooded soils, which promoted Cd sequestration in low-crystalline fraction of IP (IP-Feh-Cd) and Cd bonded to amorphous Fe-oxides (amFeox-Cd). The 24.9-62.4% higher amFeox-Cd content was the important factor for 20.4-44.2% lower CaCl₂-extractable Cd content in flooding treatment than those in other treatments. Soil submergence reduced Cd uptake by rice at tillering and booting stages, the critical periods of Cd transport in the soil-rice system, which was attributed to the increases in dissolved Fe²⁺ and IP-Feh-Cd contents and decrease in CaCl₂-Cd content. Therefore, maintaining flooding during the tillering and booting stages may be an effective strategy to reduce Cd uptake by rice cultivated in Cd-contaminated soil.

Redistribution of iron oxides in aggregates induced by pe + pH variation alters Cd availability in paddy soils

In this study, the effect of unstable pe + pH levels on the transformation of Fe oxides in different-sized soil fractions and its impact on Cd speciation were explored. Paddy soil samples collected from two locations in China were cultivated for two months under one of four pe + pH conditions: flooding + N₂ (T1), flooding (T2), 70% water holding capacity (T3), and 70% water holding capacity + O₂ (T4). Chemical analysis and X-ray diffraction (XRD) were used to identify the mineralogical phases and species of Fe and Cd in paddy soils. The results show that the decrease of soil pe + pH level favored the transformation of well-crystallized Fe oxides (Fe_c), such as hematite and goethite, into poorly-crystallized (Fe_o) and organically-complexed (Fe_p) forms. The transformation promoted the binding of Cd to Fe oxides and was primarily responsible for up to a 41.8% decrease of soil DTPA (diethylenetriaminepentaacetic acid)-extractable-Cd content. In addition, the decline in pe + pH value reduced Fe concentrations in soil particle fractions of 0.2-2-mm (17.8%-30.6%) and <0.002-mm (20.7%-31.7%) of the two flooding treatments. The decreased Fe concentrations were closely associated with less Fe_c contents in these same fractions and more Fe_o and Fe_p in coarser aggregates (P < 0.01). Importantly, the increase in contents of Fe_o and Fe_p in the 0.002-2 mm fraction were significantly correlated with content of Fe-/Mn-oxide-bound Cd (OX-Cd) in larger particle-size fractions (P < 0.01). Furthermore, the increasing content of OX-Cd played a crucial role in reducing DTPA-Cd content. This study demonstrates that low pe + pH values favor the transformation of crystalline Fe oxides into a poorly-crystallized and organically-complexed phase, which facilitates Cd accumulation in coarser aggregates and enhances Cd stability in paddy soils.

A field study to predict Cd bioaccumulation in a soil-wheat system: Application of a geochemical model

An accurate model to predict Cd accumulation in crops based on soil properties would facilitate evaluations of soil quality and the potential risk posed by metals. However, given the heterogeneity of soil, such models are difficult to establish on large regional scales. This study for the first time examined the applicability of a multi-surface speciation model (MSM) in predicting Cd accumulation in wheat at a regional field scale, based on 140 soil-wheat paired samples collected from a 205-km² field. The MSM resulted in a better correlation between Cd accumulation in wheat grain (R² = 0.75) and roots (R² = 0.74) than obtained with chemical extraction methods (total Cd in soil, 0.01 M CaCl₂, and 0.43 M HNO₃). In addition, while the performance of the MSM was comparable to that of a traditional multiple regression model, a parameter-fitting process was not required. The predictive ability of the MSM was further used to assess and predict the soil Cd risk and to develop a soil Cd sensitivity map to better localize areas of greatest sensitivity to Cd contamination. The results showed that the MSM can serve as a useful tool for regional soil risk assessments and thus in the development of soil protection measures.

Iron fractions responsible for the variation of Cd bioavailability in paddy soil under variable pe+pH conditions

Iron (Fe) in soil is closely related to cadmium (Cd) uptake by rice plants, and soil pe + pH significantly influences Fe redox behavior. This study aimed to explore the influential mechanisms of varying pe + pH conditions on the transformation of iron oxides in the rhizosphere and the subsequent effect on Cd accumulation in rice plants. A two-month pot experiment was conducted to investigate the effect of soil pe + pH on the fractions of iron oxides and formation of iron plaque (IP), as well as the effect of these changes on Cd uptake by rice plants (Oryza sativa L.). Different irrigation strategies, 70% water holding capacity (DY), continuous flooding (FL), and alternate flooding/drying weekly (AWD), were used to achieve various soil pe + pH levels. The results showed that low pe + pH conditions (under the FL and AWD treatments) were more beneficial to the transformation of crystalline iron oxides into amorphous forms in rhizosphere soil and the precipitation of IP on rice roots. The increase of amorphous iron oxides resulted in the reduction of Cd availability in rhizosphere soil by immobilizing more Cd on Fe oxides. Moreover, Cd adsorbed on rice root surfaces reacted with IP, inhibiting Cd soil-to-root transport. The two mechanisms combinatively functioned at decreasing Cd concentration in rice shoots by 14.1-33.1% at low pe + pH conditions compared to that of the high pe + pH (DY treatment). These results indicate that lowering soil pe + pH effectively reduced Cd accumulation in rice plants, probably through the immobilization of amorphous Fe oxides on Cd and sequestration of iron-plaque on Cd.

Prediction of the uptake of Cd by rice (Oryza sativa) in paddy soils by a multi-surface model

The uptake of Cd by crops from soils is predominantly determined by the concentration and speciation of Cd in soil solution, which is controlled by soil physicochemical properties such as soil pH, soil minerals and organic matter, etc. Water management significantly affects soil pH, and especially soil pH is driven to be neutral under continuous flooding treatment. In the present study, the multi-surface models (MSMs) were modified to determine Cd partitioning in soils for prediction of Cd uptake by rice grain, with multiple parameter or setup changes including: (1) soil pH was considered as variables to improve the accuracy of model prediction for paddy soils; (2) practical co-existing cation concentration and ionic strength were derived from electron conductivity to improve the universality of model. Our results suggested that the modified MSMs model provided a better prediction for the actual uptake by rice grain and a more consistent Cd distribution pattern in paddy soils.

Heavy metal behaviour at mineral-organo interfaces: Mechanisms, modelling and influence factors

The mineral-organo composites control the speciation, mobility and bioavailability of heavy metals in soils and sediments by surface adsorption and precipitation. The dynamic changes of soil mineral, organic matter and their associations under redox, aging and microbial activities further complicate the fate of heavy metals. Over the past decades, the wide application of advanced instrumental techniques and modelling has largely extended our understanding on heavy metal behavior within mineral-organo assemblages. In this review, we provide a comprehensive summary of recent progress on heavy metal immobilization by mineral-humic and mineral-microbial composites, with a special focus on the interfacial reaction mechanisms of heavy metal adsorption. The impacts of redox and aging conditions on heavy metal speciations and associations with mineral-organo complexes are discussed. The modelling of heavy metals adsorption and desorption onto synthetic mineral-organo composites and natural soils and sediments are also critically reviewed. Future challenges and prospects in the mineral-organo interface are outlined. More in-depth investigations are warranted, especially on the function and contribution of microorganisms in the immobilization of heavy metals at the complex mineral-organo interface. It has become imperative to use the state-of-the-art methodologies to characterize the interface and develop in situ analytical techniques in future studies.

Using cadmium bioavailability to simultaneously predict its accumulation in crop grains and the bioaccessibility in soils

Single extraction procedures (SEPs) have been extensively conducted to determine Cd bioavailability (Cd-Bav) in soils. However, whether SEPs can simultaneously predict Cd accumulation in crop grains and bioaccessibility (Cd-Bac) in soils remains unclear. To assess their suitability, the Cd-Bav in 20 contaminated soils (containing 0.27-56.59 mg/kg Cd) determined by four SEPs (including DTPA, EDTA, HOAc and HCl) was compared with Cd concentrations in crop grains (wheat and rice) and Cd-Bac in soils (based on SBET and PBET assays). The results indicated that both Cd-Bav (0-103.2%) and Cd-Bac (0-110.4%) in soils varied greatly with the methods used. The Cd-Bav obtained from chelators (DTPA and EDTA) was generally greater in low-Cd soils but lower in high-Cd soils as compared to those obtained from acid solutions (HOAc and HCl). Regression analysis revealed that bioavailable Cd concentrations in soils were linearly correlated with Cd concentrations in wheat grains (R² = 0.88-0.91); however, no significant correlation was found for rice grains. The Cd-Bac in soils was significantly correlated with Cd-Bav obtained from HOAc (R² = 0.55-0.59) or HCl (R² = 0.60-0.68), but not with those obtained from chelators (DTPA and EDTA). Our data suggest that SEPs, particularly the HCl method, have great potential to simultaneously predict Cd accumulation in wheat grains and Cd-Bac in contaminated soils.

Heterologous expression of TuCAX1a and TuCAX1b enhances Ca2+ and Zn2+ translocation in Arabidopsis

TuCAX1a and TuCAX1b improved Ca²⁺ and Zn²⁺ translocation and TuCAX1b enhanced Ca²⁺, Zn²⁺, Mn²⁺ and Fe²⁺ content when exposed to Cd²⁺; Cd²⁺ translocation was inhibited under Ca²⁺ and Zn²⁺. Cation/H⁺ antiporters (CAXs) are involved in the translocation of Ca²⁺ and various metal ions in higher plants. In the present study, TuCAX1a and TuCAX1b, two cation/H⁺ antiporters, were isolated from the diploid wheat Triticum urartu, and their metal cation translocation functions investigated. TuCAX1a and TuCAX1b showed abundant tissue-specific expression in the internode and beard, respectively, and their expression levels were increased in shoots exposed to Cd²⁺, Zn²⁺ and Ca²⁺. Plant phenotype analysis showed that overexpression of TuCAX1a and TuCAX1b could improve the tolerance of Arabidopsis to exogenous Ca²⁺ and Zn²⁺. In the plant shoots and roots, the contents of Ca²⁺ and Zn²⁺ were higher than wild-type plants under Ca²⁺ and Zn²⁺ treatments, indicating that TuCAX1a and TuCAX1b can enhance Ca²⁺ and Zn²⁺ translocation. Ca²⁺, Zn²⁺, Mn²⁺ and Fe²⁺ contents showed higher accumulation in TuCAX1b-transgenic Arabidopsis shoots than in wild-type plants exposed to Cd²⁺, and the translocation of Cd²⁺ was inhibited under Ca²⁺ and Zn²⁺. Overall, the present study provides a novel genetic resource for improving the uptake of microelements and reducing accumulation of toxic heavy metals in wheat.