Mobility Of Cd Research Articles

Growth and Element Uptake by Salt-Sensitive Crops under Combined NaCl and Cd Stresses.

To test an assumption that organic soil can ameliorate nutritional disorders associated with metal and salinity stresses, we exposed salt-sensitive strawberry and lettuce to four salinity (0–60 mM NaCl) and three contamination (0.3–5 mg Cd/kg) rates in peat (pHH2O = 5.5). The results showed that, even at 20 mM NaCl, salinity stress exerted a dominant effect on rhizosphere biogeochemistry and physiological processes, inducing leaf-edge burns, chlorosis/necrosis, reducing vegetative growth in crops; at ≥40 mM, NaCl mortality was induced in strawberry. Signifiacntly decreased K/Na, Ca/Na and Mg/Na concentration ratios with raising salinity were confirmed in all tissues. The combined CdxNaCl stresses (vs. control) increased leaf Cd accumulation (up to 42-fold in lettuce and 23-fold in strawberry), whereas NaCl salinity increased the accumulation of Zn (>1.5-fold) and Cu (up to 1.2-fold) in leaves. Lettuce accumulated the toxic Cd concentration (up to 12.6 mg/kg) in leaves, suggesting the strong root-to-shoot transport of Cd. In strawberry Cd, concentration was similar (and sub-toxic) in fruits and leaves, 2.28 and 1.86 mg/kg, respectively, suggesting lower Cd root-to-shoot translocation, and similar Cd mobility in the xylem and phloem. Additionally, the accumulation of Cd in strawberry fruits was exacerbated at high NaCl exposure (60 mM) compared with lower NaCl concentrations. Thus, in salinized, slightly acidic and organically rich rhizosphere, pronounced organo- and/or chloro-complexation likely shifted metal biogeochemistry toward increased mobility and phytoavailability (with metal adsorption restricted due to Na+ oversaturation of the caton exchange complex in the substrate), confirming the importance of quality water and soils in avoiding abiotic stresses and producing non-contaminated food.

Biomineralization of Cd2+ and inhibition on rhizobacterial Cd mobilization function by Bacillus Cereus to improve safety of maize grains

Reducing cadmium (Cd) bioavailability and rhizobacterial Cd mobilization functions in the rhizosphere via the inoculation of screened microbial inoculum is an environmental-friendly strategy to improve safety of crop grains. In this study, Bacillus Cereus, a model Cd resistant strain, was selected to explore its effects on Cd bioavailability and uptake, bacterial metabolic functions related to Cd mobilization. Results indicated that inoculation of Bacillus Cereus in maize roots of sand pot with water-soluble Cd (0.06–0.15 mg/kg) and soil pot with high Cd-contaminated soil (total Cd: 2.33 mg/kg; Cd extracted by NH4NO3: 38.6 μg/kg) could decrease water-soluble Cd ion concentration by 7.7–30.1% and Cd extracted with NH4NO3 solution by 7.8–22.5%, inducing Cd concentrations in maize grains reduced by 10.6–39.9% and 17.4–38.6%, respectively. Even for a single inoculation in soil, Cd concentration in maize grains still satisfy food safety requirements (Cd content: 0.1 mg/kg dry weight) due to its successful colonization on root surface of maize. Bacillus Cereus could enrich more plant growth promotion bacteria (PGPB) and down-regulate the expression of genes related to bacterial motility, membrane transports, carbon and nitrogen metabolism in the rhizosphere soil, decreasing Cd bioavailability in soil. Approximately 80% Cd2+ in media was transferred into intracellular, meanwhile Cd salts (sulfide and/or phosphate) were produced in Bacillus Cereus through biomineralization process. Overall, this study could provide a feasible method for improving safety of maize grains via the inoculation of Bacillus Cereus under Cd pollution.

Salinization depresses soil enzyme activity in metal-polluted soils through increases in metal mobilization and decreases in microbial biomass.

Salinity may increase metal mobilization with a potentially significant consequence for soil enzymatic activity and nutrient cycling. The goal of this study was to investigate changes in soil enzyme activity in response to salinization of a clay loam soil artificially polluted with cadmium (Cd) and lead (Pb) during a 120-day incubation experiment. Soil samples were polluted with Cd (10 mg Cd kg-1), Pb (150 mg Pb kg-1), and a combination of Cd and Pb, then preincubated for aging and eventually salinized with three levels of NaCl solution (control, low and high). NaCl salinity consistently increased the mobilization of Cd (12-22%) and Pb (5-16%) with greater increases at high (17-22% for Cd, 9-16% for Pb) than low (12% for Cd, 5-7% for Pb) salinity levels. While the increased Cd mobilization was greater in co-polluted (22%) than Cd-polluted (17%) soils, the increase of Pb mobilization was lower in co-polluted (9%) than Pb-polluted (16%) soils at high salinity level. The salinity-induced increases in metal mobilization significantly depressed soil microbial respiration (up to 43%), microbial biomass content (up to 63%), and enzymatic activities (up to 87%). The multivariate analysis further supported that the increased soil electrical conductivity, Cd mobilization, and pH after salinization were the most important factors governing microbial activity and biomass in metal-polluted soils. Results showed that changes in microbial biomass and mobile metal pool with increasing salinity had a major effect on enzyme activities, particularly under the combined metals. This study indicated that the secondary salinization of metal-polluted soils would impose an additional stress on enzymatic activities as biochemical indicators of soil quality, and therefore should be avoided for the maintenance of soil microbial and biochemical functions, especially in arid regions. In metal-polluted soils, the observed responses of extracellular and intracellular enzymes to salinity can be used to advance our knowledge of microbial processes when modeling the carbon and nutrient cycling.

Nitrogen fertilizer affects rhizosphere Cd re-mobilization by mediating gene AmALM2 and AmALMT7 expression in edible amaranth roots

In-situ stabilization of Cd-contaminated farmland is a commonly used remediation technology. Yet, rhizosphere metabolites (e.g., organic acids) during crop cultivation may cause Cd re-mobilization and over-accumulation. Here, we identified four pivotal cytomembrane-localized genes underlying Cd accumulation difference between two contrasting edible amaranth cultivars based on root gene expression profile, studied their subcellular localization and functional characteristics, and then investigated effects of nitrogen fertilizer on their expression and rhizosphere Cd re-mobilization. Results showed that more Cd accumulated by edible amaranth was due to rhizosphere Cd mobilization by mediating high expression of AmALMT2 and AmALMT7 genes, not Cd transporters in roots. This was confirmed by heterologous expression of AmALMT2 and AmALMT7 genes in Arabidopsis thaliana, since they mediated malic, fumaric, succinic, and aspartic acids efflux. Furthermore, nitrogen influencing rhizosphere acidification might be closely associated with organic acids efflux genes. Compared with N-NO3- application, N-NH4+ was massively assimilated into glutamates and oxaloacetates through up-regulating glutamine synthetase and alanine-aspartate-glutamate metabolic pathways, thereby enhancing TCA cycle and organic acids efflux dominated by binary carboxylic acids via up-regulating AmALMT2 and AmALMT7 genes, which finally caused Cd re-mobilization. Therefore, N-NO3--dominated nitrogen retarded rhizosphere Cd re-mobilization via inhibiting organic acids efflux function of AmALMT2 and AmALMT7 proteins.

Cadmium transport in red paddy soils amended with wheat straw biochar.

Cadmium (Cd) can be leached from soil into the groundwater and exhibit its adverse effect on the health of animals and humans. While previous studies have studied the process of Cd transport in water-saturated sand columns, literature regarding Cd transport in soil is scarce. The aim of this experiment was to investigate the transport of Cd in soil columns and biochar application rate effects on the mobility and distribution of Cd in soil. The red paddy soil was collected from the paddy of Changsha County, Hunan Province in southern China. Batch sorption and column experiments were conducted to study the adsorption isotherms of Cd2+ and its mobility at different biochar application rate treatments (0, 0.5, 1, 1.5, and 2%) referenced here as A0, A10, A20, A30, and A40, respectively. The Cd concentration of in effluent samples and digestion solutions was measured by inductively coupled plasma optical emission spectrometer (ICP-OES, Thermo Fisher Scientific, USA). After finishing the column experiment, columns were dissected into five layers (1-cm segments), the Cd fractions in soil were performed by the European Community Bureau of Reference (BCR). The amount of Cd sorption among treatments decreased in the order of A40 > A30 > A20 > A10 > A0, and the Langmuir model was more suitable to study the Cd2+ adsorption on biochar-amended soil than Freundlich model. Breakthrough curves showed that increasing biochar application rate increased the initial breakthrough time, whereas the pore-water velocity and dispersion coefficient were 81.0 and 99.8% lower in the A40 treatments than in the A0 treatments, respectively. Increasing biochar application rate enhanced the pH but reduced redox potential (Eh) in the most of effluents. Compared with A0, the concentration of Cd retained in soil columns increased by 86.6% in the A40 treatments. However, BCR sequential extractions showed that biochar addition in A40 treatments increased the acid soluble fraction but reduced the reducible fraction. In A40 treatments, compared with the 0-1-cm soil layer, the relative Cd concentration (N/Ni) in the 1-2-, 2-3-, 3-4-, and 4-5-cm soil layers increased by 5.4, 10.9, 14.3, and 21.9%, respectively. Biochar application in A40 treatments showed strong capacity for retarding Cd transport in soil, while the potential mobility of Cd in soil should be considered.

Effect of biochar amendment on mobility and plant uptake of Zn, Pb and Cd in contaminated soil

This study aimed to assess the effect of rice straw biochar application as a soil amendment on the mobility, availability, speciation and plant uptake of Zn, Pb and Cd in contaminated soil. A pot experiment with maize (Zea mays L.) was conducted using different rates 0, 1, 2, and 5% (w/w) of rice straw biochar. The soil pore water properties; pH, EC, and DOC concentration, the dissolved metal concentrations in soil pore water as well as plant metals uptake were determined at the end of the experiment. The BCR sequential extraction procedure was adopted to determine the effect of biochar on speciation and partitioning of the studied metals.Results showed that the application of biochar is significantly increased the plant shoots biomass by 94.5% with 5% biochar rates compared to untreated soil. Similarly, the soil pore water properties pH, EC, and DOC concentration were also increased with biochar addition compared to untreated soil. The dissolved metal concentrations were decreased in soil pore water with the increasing of biochar rates by 92%, 81.5%, and 90% for Zn, Pb and Cd, respectively at 5% biochar rate. In the same trend, the plant metals uptake reduced significantly with the increasing of biochar dose. Compared to untreated soil, the BCR sequential extraction showed that the biochar addition induced the transformation of the exchangeable metal fractions to oxidizable and residual fractions. These results confirmed the ability of rice straw biochar to immobilize the studied metals and therefore reducing their bioavailability and their uptake by plant.

Effect of Triton X-100 on the wheat and lettuce growth and contaminant absorption

This study was set to study the effects of surfactants on crops using Triton X-100, one of widely used surfactants for various purposes including agricultural uses, as a target surfactant. The effects of Triton X-100 on the growth of wheat and lettuce were studied and the germination and shoot growth of wheat were not significantly affected by Triton X-100. With lettuce, the increasing Triton X-100 concentrations tend to negatively affect the growth, possibly due to the absorption of Triton X-100 by lettuce. The average lettuce fresh mass was reduced by 31% when Triton X-100 concentration increased from 0 to 240 mg L−1. This may mean that chemicals dissolved or mobilized by Triton X-100 can be absorbed by lettuce. The Cd mobilization was facilitated with Triton X-100, and the absorption of procymidone in soil by lettuce was greater when Triton X-100 was applied (i.e., 0.18 mg kg−1) than when water was applied (i.e., 0.15 mg kg−1), although they were statistically not different (p-value > 0.05). The average lettuce masses in the presence of residual procymidone in soil and Triton X-100 (16 g) were lower than that of the control soils (20 g), although they were statistically not different (p-value > 0.05). The results suggest that surfactants contained in pesticide formulations can potentially affect crop growth and absorption of other contaminants. Therefore, the residual surfactants and active ingredients in pesticide formulations need to be properly managed to protect the environment and to produce crops free of contaminants.

Hazardous metal additives in plastics and their environmental impacts.

Historically, many additives and catalysts used in plastics were based on compounds of toxic metals (and metalloids), like arsenic, cadmium, chromium(VI), and lead. Despite subsequent restrictions, hazardous additives remain in plastics in societal circulation because of the pervasiveness of many products and the more general contamination of recycled goods. However, little is understood about their presence and impacts in the environment, with most studies focusing on the role of plastics in acquiring metals from their surroundings through, for example, adsorption. Accordingly, this paper provides a review of the uses of hazardous, metal-based additives in plastics, the relevant European regulations that have been introduced to restrict or prohibit usage in various sectors, and the likely environmental impacts of hazardous additives once plastics are lost in nature. Examination of the literature reveals widespread occurrence of hazardous metals in environmental plastics, with impacts ranging from contamination of the waste stream to increasing the density and settling rates of material in aquatic systems. A potential concern from an ecotoxicological perspective is the diffusion of metals from the matrix of micro- and nanoplastics under certain physico-chemical conditions, and especially favorable here are the acidic environments encountered in the digestive tract of many animals (birds, fish, mammals) that inadvertently consume plastics. For instance, in vitro studies have shown that the mobilization of Cd and Pb from historical microplastics can greatly exceed concentrations deemed to be safe according to migration limits specified by the current European Toy Safety Directive (17mgkg-1 and 23mgkg-1, respectively). When compared with concentrations of metals typically adsorbed to plastics from the environment, the risks from pervasive, historical additives are far more significant.

Mechanistic insights into the (im)mobilization of arsenic, cadmium, lead, and zinc in a multi-contaminated soil treated with different biochars.

The effect and mechanistic evidence of biochar on the (im)mobilization of potentially toxic elements (PTEs) in multi-contaminated soils, with respect to the role of surface-functional groups and organic/inorganic compounds of biochar, are poorly understood. Herein, biochars produced from grass residues, rice straw, and wood were applied to a mining-soil contaminated with As, Cd, Pb, and Zn for 473-d. Biochars did not reduce the mobilization of Cd and Zn, whereas they simultaneously exhibited disparate effects on As and Pb mobilization. The phenolic hydroxyl and carboxylic groups on the wood biochar's surfaces promoted the conversion of Pb2+ into PbCO3/Pb(OH)2 and/or PbO, minimally by the rice and grass biochars. Rice and grass biochars led to the dissolution of scorodite and the formation of less stable forms of Fe-oxide-bound As (i.e., goethite and ferrihydrite); furthermore, it resulted in the reduction of As(V) to As(III). The PTEs mobilization and phytoavailability was mainly governed by the release of dissolved aliphatic- and aromatic-carbon, chloride, sulfur chemistry, phosphate competition, and the electrostatic repulsion in biochar-treated soils. In conclusion, pristine-biochar has a limited impact on the remediation of multi-contaminated soils, and the use of modified-biochar, possessing higher surface areas and functionality and active exchange sites, are preferred under such conditions.

Mechanisms of arsenate and cadmium co-immobilized on ferrihydrite inferred from ternary surface configuration

Elucidating the interfacial interactions of arsenate (AsO43-) and cadmium (Cd) on minerals can better reveal their environmental behavior under various geological settings. Herein, the adsorption of Cd on ferrihydrite (Fh) in the presence of arsenate was investigated through batch experiments, surface complexation, and spectroscopic techniques. We determined the concentration-dependent interactions of Cd (II) and As (V) on each other’s behavior on Fh. Cadmium and arsenate tended to co-precipitate at high Cd/As ratios, whereas adsorption dominated at low Cd/As ratios. Compared with the single-ion system, the coexistence of Cd(II) and As on Fh significantly reduces the respective mobility of Cd(II) and As(V). The results of in situ attenuated total reflection–Fourier-transform infrared spectroscopy and charge distribution multisite complexation modeling revealed that the synergistic effects originated from electrostatic interactions and the formation of a ternary complex. The curve fitting of the spectra revealed three deconvolution peaks at ~ 765 cm−1, ~813 cm−1, and ~ 870 cm−1, which were assigned to As–OX (X = H+, cations, and H2O), As–O–Fe, and As–Oumcomplexed, respectively. After the addition of Cd(II), the intensity of As–OX increased at different pH values, indicating the formation of an As–O–Cd ternary complexes. We infer that two types of surface complex were assembled on the surface of Fh: namely anion-bridged Fe–As–Cd and cation-bridged Fe–Cd–As. These results contribute to our understanding of the behavior of multiple contaminates in an iron-rich environment and are helpful for the development of the corresponding restoration technology.

Manganese facilitates cadmium stabilization through physicochemical dynamics and amino acid accumulation in rice rhizosphere under flood-associated low pe+pH

Periodic flooding in paddy soils impacts redox behavior and induces variations in pe+pH levels. Manganese (Mn) is capable of reducing cadmium (Cd) uptake by rice. However, the processes involved in how Mn alters Cd mobilization under different pe+pH environments remain poorly understood. To investigate the mechanisms of Mn-mediated soil Cd-stabilization and subsequent inhibition of Cd uptake from flooded soils, we examined Cd immobilization in soil pot incubations, transcriptional changes in Cd-transport genes, and metabolomic analyses of roots and rhizosphere soils with or without Mn application. We found a decrease in extractable Cd concentration largely depended on irrigation-associated low pe+pH, exogenous Mn enhancement of Fe−Mn (oxyhydro)oxide-mediated Cd transformation, and Cd deposition in rice Fe/Mn plaques. Mn application led to striking effects on the expression of Cd-related genes eg. IRT, HMA, and NRAMP in rice root tissue. Exposure to Mn under variable pe+pH levels resulted in metabolic reprogramming of soil and rice roots. Mn induced amino acid synthesis in rice roots, leading to rhizosphere accumulation of free L-lysine, glycine, and glutamine, which can reportedly bind metal ions, forming complexes with Cd. Thus, secreted amino acids, low pe+pH, and free Mn can together comprise a multi-faceted approach to managing Cd toxicity in rice.

Cadmium mobility, uptake, and accumulation in spinach, kale, and amaranths vegetables as influenced by silicon fertilization

Using silicon (Si) can reduce heavy metal stress in plants, which have been grown in soil contaminated with cadmium (Cd). This study aimed at evaluating the influence of Si on Cd uptake, mobility, and compartmentation in leafy vegetables: spinach, kale, and amaranths. The study was conducted in both greenhouse and field experiments, which included four treatments: Cd, Cd + Si, Si, and control. Data were collected on root, stem, and leaves Cd concentration, transfer index (TI), translocation factor (TF), and Cd uptake by leafy vegetables. The Cd concentration was higher in roots than in stems and leaves in all the treatments whereas its TI and TF were less than one with spinach having the highest TF. Generally, Cd transfer was highest in roots and least in leaves in all treatments and it increased with age of the vegetables. Spiking the soils with Cd in the greenhouse resulted in higher Cd concentration in leafy vegetable tissues than in the field experiment. With application of Si, there was a significant reduction in the concentration of Cd in plant tissues of between 20 and 40%, which further resulted in strong and positive relationships between the Cd concentration in the soil and plant tissues. Therefore, the study recommends against the production of leafy vegetables in Cd contaminated soils for human consumption. Nonetheless, under unavoidable circumstances, we recommend application of Si to alleviate the transfer of Cd to edible tissues of leafy vegetables.

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.

The effects of biochars produced in different pyrolsis temperatures from agricultural wastes on cadmium uptake of tobacco plant.

Abiotic stresses caused by cadmium (Cd) contamination in soil retard plant growth and decline the quality of food. Amendment of biochar was reported effective in reduction of mobility, plant uptake and toxicity of Cd in plants. The aim of this study was to investigate the effect of biochar applications produced from corn cob and rice husk at three different pyrolysis temperatures (400, 500 and 600 °C) on Cd uptake of tobacco plants. The results showed that the shoot Cd concentration and content of tobacco plants significantly increased with the application of Cd in increasing doses. The results showed that increasing Cd dosescaused significant increase (P < 0.01) in shoot Cd concentration and content of the tobacco plant at three different pyrolysis temperatures of both corn cob and rice husk biochars. The concentration of Cd was 0.48 mg kg−1 in Cd0 dose of corn cob biochar produced at 500 °C and increased to 61.6 mg kg−1 at Cd5, while Cd concentration increased to 72.3 mg kg−1 with rice husk biochar. Despite the increase in Cd concentrations and content, shoot Cd concentrations and contents were significantly (P < 0.01) reduced with the treatments of corn cob and rice husk biochars produced at different pyrolysis temperatures. The Cd concentration at Cd5 dose in the absence of biochar addition was 90.5 mg kg−1, while Cd concentration at Cd5 dose in 400, 500 and 600 °C treatments of corn cob biochar was reduced to 66.5, 61.6 and 67.3 mg kg−1 respectively, and to 77.0, 72.3 and 70.2 mg kg−1 in rice husk biochar. The results also revealed that corn cob biochar treatments were more effective in reducing Cd uptake of tobacco plants compared to rice husk biochar. Higher specific surface area of corncob biochar compared to rice husk biochar caused to the difference between two biochar sources on Cd uptake of tobacco plants.

Laboratory and simulation study on the Cd(Ⅱ) adsorption by lake sediment: Mechanism and influencing factors

Sediments are the major sinks for Cd(Ⅱ) in the aquatic environment. Here, the detailed binding mechanisms and effects of environmental factors on Cd(Ⅱ) adsorption onto lake sediment were tested by a batch of adsorption and characteristic experiments. Sediment samples and sediment-Cd complexes were characterized using Scanning electron microscopy, Energy dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectral analyses. The interactive and main effect of parameters such as pH, flow velocity, Cd(II) concentration, sediment particle size, humic acid, fulvic acid and adsorption time involved in the adsorption process were determined using two models based on response surface methodology (RSM) and a back-propagation neural network with genetic algorithm (GABP). Results showed that Cd(II) adsorption onto sediment was mainly achieved through surface complexation with O-containing groups and precipitation with carbonate and sulfide. RSM was favorable for modeling Cd(II) adsorption in lake systems because it intuitively reflected the influence of the factors and had a good fitting precision (R2 = 0.8838, RSME = 2.5496) close to that of the GABP model (R2 = 0.8959, RSME = 2.5410). pH, sediment particle size, and humic acid exerted strong influences on Cd(II) immobilized by the sediment. Overall, our findings facilitate a better understanding of Cd(II) mobility in lakes and provide a reference for controlling heavy metals derived from both aqueous and sediment sources.

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.

Effect of amendments (lime–zeolite–biochar) on the immobilization of Cd and Pb in a contaminated acidic soil

An incubation experiment was carried out to study the remediation effect of lime, zeolite and biochar under different ratios on cadmium (Cd) and lead (Pb) contaminated soil. Soil pH, organic carbon (SOC), bioavailability and mobility of Cd and Pb were investigated. The results indicated that combined application of lime, zeolite and biochar can significantly increase soil pH, the increment was from 0.15 to 0.76. Compared to the control treatment, the concentrations of CaCl2-, TCLP-Cd and Pb significantly decreased in all amended treatments. Considering the soil environmental quality and immobilization efficiency, the ratio of lime, zeolite and biochar was determined to be 1:2:2. The application rate of combined amendment (lime:zeolite:biochar=1:2:2) had obvious effect on soil pH, SOC and removal efficiency of Cd and Pb, promoting the acid soluble fractions of Cd and Pb to residual fractions, especially in higher application rate (5.0% and 10.0%). However, there was no significant difference when the application rate exceeded 5.0%. Therefore, it was suggested that the application rate of combined amendment (lime:zeolite:biochar=1:2:2) could be 5.0% in short-term soil remediation.

Double-edged effects of polyvinyl chloride addition on heavy metal separation and biochar production during pyrolysis of Cd/Zn hyperaccumulator

Pyrolysis is a promising technique to achieve the sustainable utilization of heavy metal hyperaccumulator derived from phytoremediation of contaminated soils. To investigate the feasibility of synergistic treatment of hyperaccumulator and plastic waste (i.e. polyvinyl chloride, PVC), co-pyrolysis of Sedum alfredii and different mass percentages of PVC (5–25 wt%) was conducted at 300–900 °C in the present study. High pyrolysis temperature and low PVC addition amount (5 wt%) effectively promoted the volatilization of Cd and Zn from S. alfredii, while high PVC addition amount (15 wt% and 25 wt%) caused a significant suppression effect at insufficient pyrolysis temperatures. After PVC addition, the yields of biochar increased by 5.18–37.19% as compared with the theoretical values. However, the concentrations of Cd and Zn leached from biochar significantly elevated with increasing PVC addition amount, indicating that the addition of PVC improved the mobility of Cd and Zn in biochar. Moreover, S. alfredii derived biochars showed considerable sorption capacity for Cd (87.6–198.3 mg/g). These results imply that the addition of PVC has double-edged effects on heavy metal separation and biochar production during pyrolysis of Cd/Zn hyperaccumulator, and low PVC addition amount and sufficient pyrolysis temperature are beneficial for the further utilization of biochar.

Immobilization of Cd and Pb in a contaminated acidic soil amended with hydroxyapatite, bentonite, and biochar

This study aims to investigate the dynamic changes over time in the immobilization efficiency of hydroxyapatite, bentonite, and biochar for Cd and Pb co-contaminated soil given the potential remobilization of immobilized heavy metals for long-term stabilized soils. Hydroxyapatite, bentonite, and biochar were applied only once at 2.5% (w/w) of soil during a 5-year pot experiment with pepper-cabbage (Capsicum annuum L.-Brassica pekinensis) rotation. The dynamic changes of soil pH, soil organic carbon (SOC), TCLP (toxicity characteristic leaching procedure)-, DTPA-, and PBET (simulated human gastric solution extraction)-Cd and Pb in soil, fractionation of Cd and Pb in soil, as well as the dynamic variations of bioaccessible Cd and Pb in vegetables were determined. The results indicated the application of hydroxyapatite, bentonite, and biochar significantly increased the proportion of residual fraction of Cd and Pb, and reduced the mobility, bioavailability, and bioaccessibility of Cd and Pb analyzed by TCLP, DTPA, and PBET, respectively. Moreover, the concentrations of TCLP-, DTPA-, and PBET-Cd and Pb gradually increased over time. The proportions of acid-extractable Cd and Pb in amended treatments in 2019 were higher than those in 2016. The increment for acid-extractable Cd fraction was 4.69%, 5.53%, and 4.19% in hydroxyapatite, bentonite, and biochar, respectively. While the increment for acid-extractable Pb fraction was 1.48%, 2.08%, and 1.91%, respectively. The bioaccessible Cd and Pb in pepper and cabbage significantly decreased after remediation while no remarkable changes were observed for Pb from 2017 to 2019. The three amendments were effective ameliorants for Cd and Pb co-contaminated acidic soil, but the immobilization efficiency decreased over time. Moreover, hydroxyapatite showed the highest efficiency on metal stabilization and vegetable safety.

Biogeochemical Control on the Mobilization of Cd in Soil

Cadmium (Cd) is a toxic element that can easily enter the human body through the food chain. Rice grain is the main contributor to dietary Cd intake, especially for those populations who consume rice as the staple. Therefore, reducing Cd accumulation in rice is of significance for food safety and human health. In this review, we summarized the major factors underlying the biogeochemical processes controlling Cd availability in soils. Soil pH is one of the most important factors affecting soil Cd availability. An increase in soil pH can dramatically decrease soil Cd solubility; thus, liming is often found to be an effective and economical method in acidic contaminated soils to reduce grain Cd accumulation. The voltaic effect between Cd-sulfide and other metal sulfides formed during soil flooding regulates the remobilization of Cd during soil drainage. Therefore, addition of small amounts of ZnSO4 or MnSO4 to soils can form a protective voltaic cell partner against the oxidative dissolution of Cd sulfides, thereby decreasing Cd remobilization during soil drainage and Cd accumulation in rice grain. The voltaic effect is expected to be more important than their direct competition between Zn2+ and Cd2+. Besides, water management practices markedly affect soil Cd availability by altering the redox status of soils; continuous flooding and delaying drainage of paddy soils during the grain filling period can effectively reduce grain Cd accumulation. These effective biogeochemical strategies can be used singly or combinedly to reduce soil Cd availability and subsequent Cd uptake/accumulation in rice to ensure food safety.