Cd Species Research Articles

Mechanical Insights into Thiol-Mediated Synergetic Biotransformation of Cadmium and Selenium in Nematodes.

Cadmium ion (Cd2+) is a common environmental pollutant with high biotoxicity. Interestingly, the Cd2+ biotoxicity can be alleviated by the coexisting selenite (SeO32-), which induces the formation of cadmium selenide-rich nanoparticles (CdSe NPs) under the function of thiol-capping peptides. However, the detailed biochemical mechanisms by which Cd and Se are synergistically transformed into CdSe NPs in living organisms remain unclear so far. Here, we shed light on the molecular basis of such biotransformation processes in Caenorhabditis elegans by focusing on the roles of several key thiol-capping peptides. By monitoring the compositional and structural changes of the Cd and Se species and the genetic-level responses of nematodes, we revealed the specific roles of glutathione (GSH) and phytochelatins (PCs) in mediating the CdSe NP formation. With the aid of in vitro bioassembly assay and density functional theory calculations, the detailed Cd-Se interaction pathways were further deciphered: the ingested Cd binds predominantly to GSH and PCs in sequence, then further interacts with selenocysteine to form tetrahedral-structured PC2-Cd2-Sec2 complex, and ultimately grows into CdSe NPs. This work provides molecular-level insights into the Cd-Se interaction in C. elegans and lays a basis for controlling the ecological and health risks of heavy metals in polluted environment.

Trends of nutrients and metals in precipitation in northern Germany: the role of emissions and meteorology

We analyzed the precipitation chemistry for a maritime region in northern Germany (Schleswig–Holstein) from 1997 to 2017 in order to reveal temporal and spatial patterns and to evaluate the role of meteorological factors relative to emission reductions in Germany and Europe. Therefore, we applied several statistical methods such as time series decomposition, principal component, and redundancy analysis. We extracted two main groups: (i) a marine group (Cl, Na, Mg) that was related to natural processes like sea spray input and (ii) an anthropogenic group (Pb, Cd, As, Zn, and nitrogen species) with a terrestrial subgroup (Fe, Al, Mn), which were both related to emissions. These groups were valid for the spatial, seasonal, and annual trend data. Other elements, like Ca, K, total P, and sulfate, were influenced by natural and anthropogenic processes. The seasonal variation of ammonium deposition was caused primarily by ammonia emissions and ancillary by precipitation. Most heavy metals as well as sulfate, nitrate, and ammonium showed decreasing trends in concentrations and deposition fluxes. Only Hg did not show any trend. The decreasing depositions of sulfate and total nitrogen were correlated to emission reductions in Germany. The deposition of most heavy metals was influenced by emission reductions on European scale and meteorological factors such as wind speed and humidity. Hg did not show any correlation with the emission time series in Europe. Instead, it was correlated to the NAO index and wind, implying that global emissions and transport pathways determine the temporal development of Hg depositions. Overall, the study reveals that emission reductions positively influence regional depositions for most investigated substances. The regional spatial patterns of depositions were also influenced by local meteorological factors.

Changes of Cadmium Storage Forms and Isotope Ratios in Rice During Grain Filling.

Rice poses a major source of the toxic contaminant cadmium (Cd) for humans. Here, we elucidated the role of Cd storage forms (i.e., the chemical Cd speciation) on the dynamics of Cd within rice. In a pot trial, we grew rice on a Cd-contaminated soil in upland conditions and sampled roots and shoots parts at flowering and maturity. Cd concentrations, isotope ratios, Cd speciation (X-ray absorption spectroscopy), and micronutrient concentrations were analyzed. During grain filling, Cd and preferentially light Cd isotopes were strongly retained in roots where the Cd storage form did not change (Cd bound to thiols, Cd–S = 100%). In the same period, no net change of Cd mass occurred in roots and shoots, and the shoots became enriched in heavy isotopes (Δ114/110Cdmaturity–flowering = 0.14 ± 0.04‰). These results are consistent with a sequestration of Cd in root vacuoles that includes strong binding of Cd to thiol containing ligands that favor light isotopes, with a small fraction of Cd strongly enriched in heavy isotopes being transferred to shoots during grain filling. The Cd speciation in the shoots changed from predominantly Cd–S (72%) to Cd bound to O ligands (Cd–O, 80%) during grain filling. Cd–O may represent Cd binding to organic acids in vacuoles and/or binding to cell walls in the apoplast. Despite this change of ligands, which was attributed to plant senescence, Cd was largely immobile in the shoots since only 0.77% of Cd in the shoots were transferred into the grains. Thus, both storage forms (Cd–S and Cd–O) contributed to the retention of Cd in the straw. Cd was mainly bound to S in nodes I and grains (Cd–S > 84%), and these organs were strongly enriched in heavy isotopes compared to straw (Δ114/110Cdgrains/nodes–straw = 0.66–0.72‰) and flag leaves (Δ114/110Cdgrains/nodes–flag leaves = 0.49–0.52‰). Hence, xylem to phloem transfer in the node favors heavy isotopes, and the Cd–S form may persist during the transfer of Cd from node to grain. This study highlights the importance of Cd storage forms during its journey to grain and potentially into the food chain.

Long-term effects of phytoextraction by a poplar clone on the concentration, fractionation, and transportation of heavy metals in mine tailings.

Mine tailings are one of main causes of diffused heavy metal pollution since the heavy metals in there may acquire mobility. The current knowledge of the processes at work in long-term phytoremediation by woody species remains insufficient. Through a 4-year field study, we evaluated the phytoextraction efficiency of Populus deltoides CL. 'Xianglin 90' grown on a mine tailing co-polluted by Cd, Cu, Cr, Ni, Pb, and Zn. The concentrations of Cd, Cu, Ni, Pb, and Zn in the rhizospheric soil were reduced by amounts ranging from 12.86 to 42.19% during the study period. Bioconcentration factors and translocation factors showed that the accumulation of Cd and Zn occurring in the shoots was the most effective. Combined with the considerable biomass produced by poplar, the extracted amounts of Cd and Zn could reach 0.61 g and 10.66 g plant-1, respectively, in which the shoots account for 77.3% (Cd) and 89.0% (Zn) of the overall extraction amounts. Acid-soluble Cd and Zn increased by 5.49% and 4.29%, respectively, in the rhizosphere compared to the bulk soil, indicating that poplar enhanced the mobility of Cd and Zn in the rhizosphere, which explained its ability for bioaccumulation and root-shoot translocation. Moreover, calculated time required to address the issue of Cd and Zn pollution was theoretically shortened by more than half from 2015 to 2019. This study brings new insights into the long-term effects of phytoextraction on the concentration, fractionation, and transportation of heavy metals and confirms the potential of poplar as a Cd and Zn remediation species.

Measurement and modeling of heavy metal behaviors during the incineration of RDF in a pilot-scale kiln incinerator-Part 1: Modeling using multizonal thermodynamic equilibrium calculation

Multizonal thermodynamic equilibrium calculation is one of the methods used in estimating or explaining the fate of elements such as heavy metals during waste incinerations and probably only one method that can estimate the fate of trace elements, but the ability of the calculation as a fate-estimation tool was not fully examined. This study examines the performance of multizonal thermodynamic equilibrium calculation applied to waste incineration by comparing the calculated results with the results from the incineration test and literature information. High temperature behaviors of the heavy metals were also reviewed to correctly understand their gas phase speciation in the incinerator, and the thermodynamic data of gaseous Cd species were modified to better reproduce the high temperature vaporization behavior. The calculation represented the high volatility of Hg, Cd, Pb, and low volatility of Cr in the incineration, whereas the measured high volatility of Zn and As could not be obtained by the calculation. The calculation succeeded in reproducing the possible gas phase chemical speciation of heavy metals in the incinerator expected from the review on the high temperature properties, whereas the calculated fly ash speciation does not match with the measured speciation. Possible reasons for these inconsistencies were considered.

Changes in speciation, mobility and bioavailability of Cd, Cr and As during the transformation process of pig manure by black soldier fly larvae (Hermetia illucens)

The black soldier fly larvae (BSFL) (Hermetia illucens) are a good candidate for poultry and livestock manure treatment. The harvested insect bodies and feces can be used as animal feed and organic fertilizer. However, heavy metals have a negative impact on the transformation process of the manure by BSFL. Here we evaluated the effects of Cd, Cr and As in pig manure on the growth of BSFL, as well as the mobility and changes in speciation of the metals during the transformation process. The results showed that As significantly reduced the weight of BSFL, but Cr and Cd had no significant effects on BSFL growth. The bioaccumulation factors (BAFs) at various concentrations of Cd (2.8–3.7) were significantly higher than those of Cr (0.44–0.62) and As (0.43–0.45). The heavy metals in pig manure were mainly transferred to BSFL feces, and the distribution percentages of Cd, Cr and As in the BSFL feces were 52.6–62.0%, 90.3–94.2% and 93.0–93.3%, respectively. Cd concentrations in BSFL feces were significantly lower than those in the pig manure, while there were no significant differences in the concentrations of Cr and As between BSFL feces and pig manure (except for the treatment with the addition of 150 mg Cr kg−1). The metal speciation (weak acid soluble, reducible, oxidizable and residual fractions) in BSFL feces obviously changed when compared with pig manure. In BSFL feces, the reducible proportion of Cr decreased while the oxidizable proportion increased. Cd mainly existed in the weak acid soluble and reducible states in pig manure and BSFL feces. For As, the proportions of weak acid soluble and reducible states decreased in BSFL feces. After the BSFL transformation process, the bioavailable fraction of Cr decreased by 17.3–23.1%, but those of Cd and As did not change significantly, except for As in the CK group. These findings contribute to our understanding of the roles of BSFL during the biotransformation process of livestock manure as well as the safe utilization of transformed products.

Laboratory versus field soil aging: Impacts on cadmium distribution, release, and bioavailability.

To date, most studies about the aging of metals in soil were based on the controlled laboratory experiments, and few works have attempted to investigate how aging process influences the distribution and bioavailability of metals in soil under the field condition. The purpose of this study was to compare the aging of cadmium (Cd) in soils under the controlled laboratory and the field by monitoring time-dependent soil Cd speciation changes, Cd release kinetics, and Cd bioavailability to plant through the 438-day aging experiments. During the aging process, the proportions of Cd associated with the most weakly bound fraction tended to decrease, with corresponding increases in the more stable binding fractions. After aging, a higher concentration of available Cd was found in the field aging soil (0.74mgkg-1) than the laboratory aging soil (0.65mgkg-1). The Elovich equation was the best model to describe the soil available Cd aging process. The constant b in the Elovich equation, which was defined as the transformation rate, was in the order of laboratory aging soil > field aging soil. Moreover, higher Cd release amounts were found for the field aging soil (2.74mgkg-1) than the laboratory aging soil (2.57mgkg-1) at the end of aging. Additionally, higher body Cd concentrations were found for the vegetables grown in the field aging soils (1.49mgkg-1, fresh weight) than those grown in the laboratory aging soils (1.32mgkg-1, fresh weight). Therefore, this study indicated that the metal distribution process and its bioavailability may be overestimated or underestimated if research data from the laboratory experiments are used to derive soil quality criteria or investigate soil metal bioavailability.

Simple models efficiently predict free cadmium Cd2+ in the solutions of low-contaminated agricultural soils.

Speciation of Cd in soil solutions strongly determines the fate of this toxic metal in the environment. Generally, in soil solutions, Cd predominantly binds to the dissolved organic matter (DOM). The determination of the quantity and reactivity of DOM that actually complexes Cd in soil solutions is challenging for operational purposes. Therefore, this study tested whether Cd2+ concentration in soil solutions could effectively be predicted by considering complexation with a single mean organic ligand assumed to be a fraction of DOM of unspecified nature or assumed to be purely fulvic acids (FA) with reactivity as described in WHAM VII. The reactivity of the unspecified ligand and the concentration of FA were modelled and fitted to experimental data from 76 agricultural soils with low Cd contents. The optimal reactivity and FA concentration that minimized the relative error (RE) of predictions of the concentration of Cd2+ in soil solutions were either considered constant across soils or modelled from soil properties by multiple linear regressions (MLR) or random forests (RF), giving 6 models, the predictive value of which was assessed by 10-folds cross-validation. When the reactivity of the mean ligand and the optimal FA concentration were considered constant across soils, the models were biased and 66.9% of predictions had relative errors below a factor of 2. By contrast, if the reactivity of the mean ligand or the optimal FA concentration were allowed to vary with soil characteristics, these performances increased to 95.5%, soil pH being the main predictor and RF being slightly more efficient than MLR. With more than 95% of the relative errors of prediction below a factor of 2, the models developed in this work could be valuable for assessing Cd speciation in the solution of soils having a low Cd content.

X-ray absorption spectroscopy evidence of sulfur-bound cadmium in the Cd-hyperaccumulator Solanum nigrum and the non-accumulator Solanum melongena.

It has been proposed that non-protein thiols and organic acids play a major role in cadmium phytoavailability and distribution in plants. In the Cd-hyperaccumulator Solanum nigrum and non-accumulator Solanum melongena, the role of these organic ligands in the accumulation and detoxification mechanisms of Cd are debated. In this study, we used X-ray absorption spectroscopy to investigate Cd speciation in these plants (roots, stem, leaves) and in the soils used for their culture to unravel the plants responses to Cd exposure. The results show that Cd in the 100mgkg-1 Cd-doped clayey loam soil is sorbed onto iron oxyhydroxides. In both S.nigrum and S.melongena, Cd in roots and fresh leaves is mainly bound to thiol ligands, with a small contribution of inorganic S ligands in S.nigrum leaves. We interpret the Cd binding to sulfur ligands as detoxification mechanisms, possibly involving the sequestration of Cd complexed with glutathione or phytochelatins in the plant vacuoles. In the stems, results show an increase binding of Cd to -O ligands (>50% for S.nigrum). We suggest that Cd is partly complexed by organic acids for transportation in the sap.

Cadmium species in marine short-necked clam analyzed by size exclusion chromatography hyphenated respectively with inductively coupled plasma mass spectrometry and electrospray ionization quadrupole - time of flight mass spectrometry

To date, the speciation analysis of heavy metals in organs is still a challenge because these species are labile in vitro and scarce of the reference materials. In the present study, cadmium (Cd) species in marine shellfish were investigated with size exclusion chromatography (SEC) hyphenated with inductively coupled plasma mass spectrometry (ICP-MS), by which three Cd species in marine short-necked clam were observed. After pre-fractionation with a home-made Sephadex column, the Cd binding species were further characterized with SEC coupled with electrospray ionization quadrupole - time of flight mass spectrometry (ESI-Q-TOF-MS). The results showed that Cd species with 32,695 Da was the maximum Cd-binding molecule in the clam. Several metallothionein (MTs)-like Cd-binding proteins including a main species with molecular weight of 19,236 Da were observed. Some small Cd species with molecular weights less than 1000 Da were also observed in the clam. In order to verify the reliability of the two hyphenated methods, a standard reference material of Zn-binding MTs was analyzed at the same conditions. The results demonstrated that the two hyphenated methods of SEC-ICP-MS and SEC-ESI-Q-TOF-MS were practical to characterize the Cd species in marine shellfish.

Sequestration of cadmium ions from aqueous phase using magnetic chitosan composite

Present communication reports on the synthesis, characterization and application of Fe3O4-chitosan composite (MCC). The magnetic chitosan composite are crafted by mixing Fe3O4 nanoparticles solution to chitosan solution. Fe3O4 nanoparticles have been synthesized by co-precipitation method. The synthesized composite have been extensively characterized using various techniques such as FTIR, TEM, XRD and BET analysis. The surface area analysis suggested that the materials synthesized is mesoporous in nature. Incorporation of chitosan crosslinked actively with Fe3O4 nanoparticles that results in reduction of surface area and narrower pore size distribution of the composite than pure Fe3O4 nanoparticles. TEM study demonstrated that Fe3O4 nanoparticles were found in the range of 20–25 nm. The synthesized composite is able to capture more than 89% Cd(II) from synthetic Cd(II) solution within 80 min of contact time. The adsorption equilibrium data better conformed with the Langmuir isotherm. The maximum capacity determined from Langmuir isotherm was 149.92 mg/g at 323 K. Adsorption kinetics of cadmium on MCC could be well analyzed with pseudo-second-order kinetic model. Thermodynamics investigation reveals that with the increase of solution temperature 303 K, 313 K and 323 K the ΔGo values decreases correspondingly to −87.76, −90.36 and -93.36 KJ/mol, suggesting endothermic nature of the adsorption process. The adsorbent could be successfully reused for more than five cycles. Thus, MCC can be serves as a reliable and sustainable material for efficient sequestration of Cd(II) species from aqueous environment.

Cadmium Water Pollution Associated with Motor Vehicle Brake Parts

With increasing industrial growth, there is a greater need to understand factory production processes, the resulting products, and the pollution caused by the fabrication processes leading to these products. Cadmium (Cd) is used in the electro-less Nickel-Cadmium bath phase of the brake manufacturing process, which provides the brake coating that produces corrosion-resistant brake parts. During the operation, the friction created during braking corrodes the Cd layer and releases Cd particles into the environment. Cd particles can enter water bodies and drinking water supplies through stormwater runoff. This research will first examine Cd pollution associated with motor vehicle brake discs from cradle to grave. Following this comprehensive look into the role of Cd in the brake manufacturing process as well as Cd speciation in natural waters, three interventions are proposed to prevent Cd pollution associated with brake parts: (i) Carbon-reinforced silicon carbide as an alternative for metal based brake parts; (ii) bacteria “coating” instead of Cd coating; (iii) permeable roads that can effectively remove Cd from runoff with nearly 98% reduction. A discussion into the advantages and disadvantages of each proposition are provided with this presentation.

Speciation and Potential Ecological Risk of Heavy Metals in Soils from Overlapped Areas of Farmland and Coal Resources in Northern Xuzhou, China.

Contamination caused by heavy metals (HMs) in soil of overlapped area of farmland and coal resources (OAFCR) has impact on crops. The concentrations and speciation of As, Cd, Cr, Cu, Pb and Zn were investigated in topsoil of an OAFCR in Xuzhou, China. The results showed that mean concentrations of all six metals were higher than the background values of Xuzhou city and Cd was moderate accumulated with the maximum Igeo equalled to 2.13. Cd showed moderate contamination level (IPi = 1.75) and potential ecological risk (Er = 44.06). Most of the total Cr, Cu, Ni, Pb, and Zn were presented in the residual forms (above 60%), and the percentages of reducible, oxidisable and residual forms of Pb were 23%, 21% and 43% respectively. Pb and Cd reflected a moderate degree of potential ecological risk and a considerable migration risk and ecotoxicity.

From hazardous agriculture waste to hazardous metal scavenger: Tobacco stalk biochar-mediated sequestration of Cd leads to enhanced tobacco productivity

Tobacco is a super-enriched plant for heavy metals, and its productivity is sensitively affected by Cd. In this study, tobacco stalk was converted to biochar (TS-biochar) for the sequestration of Cd in soils to enhance the productivity of tobacco. FTIR, SEM-EDX, and XPS characterizations of TS-biochar together with Cd2+ adsorption kinetics revealed that oxy-containing functional groups (‒OH, C˭O, and ‒COOH) in TS-biochar played a crucial role on Cd2+ adsorption. The changes of soil property and Cd speciation by adding TS-biochar in red (acidic) and cinnamon (alkaline) soils was evaluated. Effects of TS-biochar on tobacco growth and development under Cd stress was also investigated. Results indicated that a 2 wt% addition of TS-biochar in red soil could significantly increase the soil pH value (from 5.21 to 7.39) and reduce exchangeable Cd fractions (from 40% to 23%), but those were not obvious in cinnamon soil. Under the stress of Cd, TS-biochar could obviously improve the tobacco dry biomass, and decrease the accumulation of Cd in the middle and upper leaves, thus reducing economic loss. Overall, the application of TS-biochar on Cd contaminated soil can transform bioavailable Cd into low hazardous forms, so as to repair soils and improve the productivity of tobacco.

Stable and efficient immobilization of lead and cadmium in contaminated soil by mercapto iron functionalized nanosilica

Mercapto as an effective functional group for immobilizing heavy metal ions is usually grafted into various materials. However, mercapto is easily oxidized, which limits its practical application to a great extent. Therefore, mercapto iron complex functionalized nanosilica (RNS-SF) was prepared to improve the stability of amendments and the immobilization efficiencies in Pb/Cd-contaminated soil. RNS-SF was characterized with SEM, TEM, FTIR. The long-term stability and immobilization governing factors of RNS-SF were investigated. The results showed that the immobilization efficiencies of Pb and Cd in contaminated soil via RNS-SF were 98.7% and 97.9% with the dosage of 3% (w/w), respectively. Moreover, its immobilization efficiencies remains unchanged from −20 °C to 50 °C. And after 360 days storage at room temperature, the immobilization efficiencies of Pb and Cd were all about 90%. RNS-SF had the best passivation effect on Pb and Cd when the soil moisture maintained at 60% after 6 days aging. Sequential extraction experiments demonstrated that the fractions of EXC species and Carb species of Pb and Cd in the contaminated soil before and after RNS-SF immobilization declined from 22.62% and 43.12% to 0.321% and 0.94%, resulting in substantially reduced environmental risk of Pb and Cd.

Alleviation of Cadmium Toxicity to Medicago Truncatula by AMF Involves the Changes of Cd Speciation in Rhizosphere Soil and Subcellular Distribution

Arbuscular mycorrhizal fungi (AMF) can improve plant tolerance to several abiotic stresses, including heavy metals, drought or salinity exposure. However, the role of AMF in alleviation of soil cadmium (Cd)-induced toxicity to plants is still largely unknown. In this study, Cd speciation in soil and subcellular distribution of Cd were used to characterize the roles of application AM fungi in the alleviation of Cd toxicity in alfalfa plants. Our results showed that the addition of <i>Glomus mosseae</i> in Cd contaminated soil (10 mg/Kg) significantly increased soil pH, cation exchange capacity (CEC) and organic matter in rhizosphere soil with <i>Medicago truncatula</i> L., and then account for significantly decreased contents of exchangeable and carbonate-bounded Cd speciation in rhizosphere soil, indicating alleviation of plant toxicity by reduction of bioavailable fractions of Cd. Although there is no significant difference found in Cd accumulation by roots and shoots respectively between Cd and AM-Cd treatments, more portion of Cd was recorded compartmentalization in cell wall fraction of both root and shoot in treatment of Cd with AM application, indicating alleviation of Cd toxicity to plant cell. Herein, application of AM fungi in Cd treatments performed to inhibit the appearance of Cd toxicity symptoms, including the improvement of leaf electrolyte leakage, root elongation, seedling growth and biomass. This information provides a clearer understanding of detoxification strategy of AM fungi on Cd behavior with development and stabilization of soil structure and subcellular distribution of plant.

Speciation and Release Kinetics Simulation of Zn and Cd from River Sediment Contaminated by Gold Mining

Heavy metals release from contaminated sediments is one of the most important chemical processes affecting overlying water quality in river, lake, and ocean. The objective of this study was to determine the leaching properties, kinetic rate, and leaching amount of zinc (Zn) and cadmium (Cd) from the river sediment contaminated by gold mining. Speciation of Zn and Cd in the sediments was extracted by a modified BCR extraction procedure. Release kinetics of Zn and Cd were studied by a simulated leaching experiment using a stirred-flow reactor and a two-site equilibrium-kinetic model. The sediments we studied were significantly contaminated by Zn (620–5878 mg kg−1) and Cd (2–67 mg kg−1), and both have high content of weak acid extractable forms. There were much more smaller particles on the surface of sediment JH01 and JH02 than sediment JH03 and JH04. The two-site equilibrium-kinetic model fits the release data of Zn and Cd well, and it was demonstrated as an effective tool to describe the kinetic release of Zn and Cd from river sediments. Kinetic rates obtained from curve fitting showed large variation among sediments indicating different reaction mechanisms. The rapid release stage (before the second stop-flow) of Zn and Cd was controlled both by the equilibrium sites and the kinetic sites, while the slow release stage (after the second stop-flow) was mainly controlled by the kinetic sites. The total leaching amount of Zn and Cd in JH01 (27.1 mg kg−1, 0.4 mg kg−1), JH02 (474 mg kg−1, 7.5 mg kg−1), JH03 (320 mg kg−1, 7.6 mg kg−1), and JH04 (52.4 mg kg−1, 2.0 mg kg−1) demonstrated that large amount of Zn and Cd in sediments can be leached into solution. Thus, effective measures should be taken to prevent leaching of heavy metals from river sediment.

Emerging contaminants: Evaluation of degradable chelators towards enhancing cadmium phytoextraction efficiency of bioenergy crop grown on polluted soil

Exogenous application of chelators towards enhancing heavy metals extraction efficiency of bioenergy crop has received considerable attention in recent time. However, little is known about optimal application rate, comparative evaluation of degradable versus non-degradable chelators and their impact on cadmium (Cd) speciation and uptake, physiological and biochemical activity of sweet sorghum grown under Cd stress. Four chelators namely, Nitrilotriacetic acid (NTA), tetrasodium N,N-diacetate (TDA), Ethylenediamine tetraacetic acid (EDTA), iminodisuccinic acid (IDA) and control (CK) were applied and compared at rates of 2, 5, and 10 mmol kg−1 towards augmenting phytoextraction efficiency of sweet sorghum for Cd remediation in a screen house study. Results showed that sweet sorghum augmented with TDA significantly (P < 0.05) increased biomass, enhanced Cd uptake and accumulation when compared to EDTA at application rate of 2–5 mmol kg−1. TDA influenced Cd speciation via increasing acid-soluble, reducible, oxidizable, and residual fractions of Cd, thus increasing Cd bioavailability. NTA and TDA increased proline concentrations, antioxidant enzymes and net photosynthetic activity rate (Pn) comparably to EDTA in sweet sorghum, thus enhancing stress tolerance and stabilizing photosynthetic activities. Overall, exogenous application of degradable TDA at 2–5 mmol kg−1 could be recommended as replacement for non-degradable EDTA in a chelate assisted approach towards augmenting bioenergy crop for phytoremediation of Cd polluted soil.

Marked Changes in Biochar's Ability to Directly Immobilize Cd in Soil: Implication for Biochar Remediation of Cd Contaminated Soil

To investigate the change in biochar’s ability to directly immobilize Cd in soil，a successive wheat cultivation with experiment was conducted. Three biochar with different Cd adsorption mechanisms were added into soils and a mesh bag was used to separate the soil particles (> 1 μm) from biochar. The results showed that the ash contents and anionic contents (CO3 2- and PO4 3-) of the biochar decreased with the cultivation time, while the oxygen-containing functional groups content and CEC of the biochar increased. Resultly, the Cd concentration on biochar decreased, while unstable Cd species (acid soluble and reducible fraction of Cd) on biochar increased with successive cultivation. Correspondingly, the ability of biochar to inhibit Cd accumulation in wheat decreased. The results of this study illustrated that the ability of biochar to directly immobilize Cd in soil is not permanent, it gradually decreases with aging in soil. The adsorption mechanism of Cd on biochar changed from precipitation to complexation and ion exchange processes could be the main reason.

Cadmium accumulation, subcellular distribution and chemical fractionation in hydroponically grown Sesuvium portulacastrum [Aizoaceae

Sesuvium portulacastrum is a well-known halophyte with considerable Cd accumulation and tolerance under high Cd stress. This species is also considered as a good candidate of Cd phytoremediation in the polluted soils. However, the mechanism of Cd accumulation, distribution and fractionation in different body parts still remain unknown. Seedlings of Sesuvium portulacastrum were studied hydroponically under exposure to a range of Cd concentrations (50 μM or μmol/L to 600 μM or μmol/L) for 28 days to investigate the potential accumulation capability and tolerance mechanisms of this species. Cd accumulation in roots showed that the bio-concentration factor was > 10, suggesting a strong ability to absorb and accumulate Cd. Cd fractionation in the aboveground parts showed the following order of distribution: soluble fraction > cell wall > organelle > cell membrane. In roots, soluble fraction was mostly predominant than other fractions. Cd speciation in leaves and stems was mainly contained of sodium chloride and deionised water extracted forms, suggesting a strong binding ability with pectin and protein as well as with organic acids. In the roots, inorganic form of Cd was dominant than other forms of Cd. It could be suggested that sodium chloride, deionised water and inorganic contained form of Cd are mainly responsible for the adaption of this plant in the Cd stress environment and alleviating Cd toxicity.