Role of Reduced Sulfur in the Transformation of Cd(II) Immobilized by δ-MnO2

Speciation and Possible Origins of Organosulfur Compounds in Rice Paddy Soils Affected by Acid Mine Drainage

Although sulfur cycling in acid mine drainage (AMD)-contaminated rice paddy soils is critical to understanding and mitigating the environmental consequences of AMD, potential sources and transformations of organosulfur compounds in such soils are poorly understood. We used sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy to quantify organosulfur compounds in paddy soils from five AMD-contaminated sites and one AMD-uncontaminated reference site near the Dabaoshan sulfide mining area in South China. We also determined the sulfur stable isotope compositions of water-soluble sulfate (δ34SWS), adsorbed sulfate (δ34SAS), fulvic acid sulfur (δ34SFAS), and humic acid sulfur (δ34SHAS) in these samples. Organosulfate was the dominant functional group in humic acid sulfur (HAS) in both AMD-contaminated (46%) and AMD-uncontaminated paddy soils (42%). Thiol/organic monosulfide contributed a significantly lower proportion of HAS in AMD-contaminated paddy soils (8%) compared to that in AMD-uncontaminated paddy soils (21%). Within contaminated soils, the concentration of thiol/organic monosulfide was positively correlated with cation exchange capacity (CEC), moisture content (MC), and total Fe (TFe). δ34SFAS ranged from -6.3 to 2.7‰, similar to δ34SWS (-6.9 to 8.9‰), indicating that fulvic acid sulfur (FAS) was mainly derived from biogenic S-bearing organic compounds produced by assimilatory sulfate reduction. δ34SHAS (-11.0 to -1.6‰) were more negative compared to δ34SWS, indicating that dissimilatory sulfate reduction and abiotic sulfurization of organic matter were the main processes in the formation of HAS.

The dissolution characteristics of cadmium containing birnessite produced from paddy crusts

The cadmium (Cd) accumulates in birnessite as it forms on the surface of paddy crusts (PC). The stability of Cd-containing birnessite is influenced by environmental factors, and destabilized birnessite releases dissolved Cd. We report the effects of pH, oxalic acid, and light on the dissolution of Cd-containing birnessite. We found that at pH 4.0, with light and 0.20 mol/L oxalic acid, the ratio of dissolved Cd and manganese (Mn) peaked after 24 h at 2978.0 μg/g and 326.8 mg/g, respectively. The three environmental factors affected the dissolution of Cd-containing birnessite in the following order: pH > oxalic acid > light. During dissolution process, Cd and Mn did not dissolve simultaneously, and the dissolved Cd/Mn ratio in the solution was significantly lower than that of the pristine mineral (33.5 × 10-3). Compared with Mn, Cd dissolution was inhibited by strong acidity (pH 4.0-5.0), and the dissolved Cd/Mn ratio was 5-10 × 10-3. Mild acidity (pH 6.0) was weakly inhibitory, with a Cd/Mn ratio of 6-15 × 10-3. In an alkaline (pH 8.0) oxalate environment, light illumination inhibited Cd dissolution, and the Cd/Mn ratio decreased over time due to the stability of the products formed by oxalate and carbonate, with Cd being more stable than those formed by Mn. Our findings would provide insights into the migration and transformation of PC-associated Cd in paddy fields.

Mercapto-palygorskite efficiently immobilizes cadmium in alkaline soil and reduces its accumulation in wheat plants: A field study

Cadmium (Cd) contamination in wheat fields has become a major environmental issue in many regions of the world. Mercapto-palygorskite (MPAL) is a high-performance amendment that can effectively immobilize Cd in alkaline wheat soil. However, MAPL as an in-situ Cd immobilization strategy for alkaline wheat soil remains to be evaluated on a field-scale and the underlying mechanisms requires further evaluation. Here, MPAL were used as soil amendment to evaluate their immobilization efficiency on Cd-contaminated alkaline soil in the field experiments. The field experiments showed that MPAL application significantly reduced wheat grain Cd concentration from 0.183 mg/kg to 0.056 mg/kg, with Cd concentration in wheat grain treated with MPAL all falling below the limit value of 0.1 mg/kg as defined in China's food safety standard (GB 2762-2022). The maximal immobilization efficiency of MPAL on soil Cd figured out by diethylenetriaminepentaacetic acid (DTPA) extraction was 61.5%. The mechanisms involved in Cd immobilization by MPAL were mainly related to the enhanced sorption of Cd onto Fe oxides, and the removal of amorphous or free Fe oxides from soil had a substantial impact on Cd immobilization efficiency by MPAL. Furthermore, the antagonistic effect between Mn and Cd uptake may also contribute to the reduction of wheat Cd accumulation after MPAL application. The current research can provide theoretical and technical support for the large-scale application of MPAL in Cd-contaminated wheat fields.

Binding of Cd(II) to birnessite and fulvic acid organo-mineral composites and controls on Cd(II) availability

Manganese oxide minerals (MnOs) are major controls on cadmium (Cd) mobility and fate in the environment. However, MnOs are commonly coated with natural organic matter (OM), and the role of this coating in the retention and availability of harmful metals remains unclear. Herein, organo-mineral composites were synthesized using birnessite (BS) and fulvic acid (FA), during coprecipitation with BS and adsorption to preformed BS with two organic carbon (OC) loadings. The performance and underlying mechanism of Cd(II) adsorption by resulting BS-FA composites were explored. Consequently, FA interactions with BS at environmentally representative (5 wt% OC) increase Cd(II) adsorption capacity by 15.05-37.39% (q_m = 156.5-186.9 mg g^-1), attributing to the enhanced dispersion of BS particles by coexisting FA led to significant increases in specific surface area (219.1-254.8 m² g^-1). Nevertheless, Cd(II) adsorption was notably inhibited at a high OC level (15 wt%). This might have derived from the supplementation of FA decreased pore diffusion rate and generated Mn(II/III) competition for vacancy sites. The dominant Cd(II) adsorption mechanism was precipitation with minerals (Cd(OH)₂), and complexation with Mn-O groups and acid oxygen-containing functional groups of FA. In organic ligand extractions, the exchange Cd content decreased by 5.63-7.93% with low OC coating (5 wt%), but increased to 33.13-38.97% at a high OC level (15 wt%). These findings help better understand the environmental behavior of Cd under the interactions of OM and Mn minerals, and provide a theoretical basis for organo-mineral composite remediation of Cd-contaminated water and soil.

Cd(II) adsorption on earth-abundant serpentine in aqueous environment: Role of interfacial ion specificity

Adsorption of heavy metal ions (e.g., Cd(II)) on clay minerals significantly affects their transport and fate in natural and engineered waterbodies. To date, the role of interfacial ion specificity in the adsorption of Cd(II) on earth-abundant serpentine remains elusive. In this work, the adsorption of Cd(II) on serpentine at typical environment conditions (pH 4.5-5.0), particularly under the complex influence of common environmental anions (e.g., NO₃^-, SO₄^2-) and cations (e.g., K⁺, Ca²⁺, Fe³⁺, Al³⁺) was systemically investigated. It was found that the adsorption of Cd(II) on serpentine surface due to the inner-sphere complexation could be negligibly affected by the anion type, yet the cations specifically modulated the Cd(II) adsorption. The presence of mono- and divalent cations moderately enhanced the Cd(II) adsorption by weakening the electrostatic double layer (EDL) repulsion between Cd(II) and Mg-O plane of serpentine, while trivalent cations significantly suppressed the adsorption of Cd(II) due to the competitive adsorption. Based on the spectroscopy analysis, Fe³⁺ and Al³⁺ were found to robustly bind the surface active sites of serpentine, thereby preventing the inner-sphere adsorption of Cd(II). The density functional theory (DFT) calculation indicated that Fe(III) and Al(III) exhibited the larger adsorption energy (E_ad = -146.1 and -516.1 kcal mol^-1, respectively) and stronger electron transfer capacity with serpentine compared to Cd(II) (E_ad = -118.1 kcal mol^-1), thus resulting in the formation of more stable Fe(III)-O and Al(III)-O inner-sphere complexes. This study provides valuable insights into the influence of interfacial ion specificity on the Cd(II) adsorption in terrestrial and aquatic environments.

Experimental Investigation of Removal of SO3 from Flue Gas with Modified Fly Ash Adsorbents

The removal of nonconventional pollutants in coal-fired power plants, such as SO₃, has been receiving more and more attention. However, due to its unique nature, it is difficult to remove SO₃ effectively with the widely used wet flue gas desulfurization systems. Nowadays, dry-sorbent injection technology has become a promising method for SO₃ emission control in coal-fired power plants. The removal characteristics of SO₃ from flue gas with modified fly ash adsorbents were investigated in a fixed-bed reactor. Factors affecting the adsorption efficiency of SO₃ were studied, including modification method, modified fly ash adsorbent particle size, reaction temperature, and flue gas component. Combined with adsorbent characterization analysis, the adsorption kinetics of SO₃ by modified fly ash adsorbents were carried out with four different adsorption kinetics models. The results show that the SO₃ adsorption efficiency of the fly ash samples increases after modification; the best SO₃ removal performance of fly ash was achieved when 1.5 mol/L NaOH solution was used, with the highest SO₃ adsorption efficiency of up to 98.3%. The modified fly ash adsorbent particle size, water vapor content, and the addition of NO have little effect on the adsorption efficiency of SO₃. As the reaction temperature increases from 250 to 450 °C, the SO₃ adsorption efficiency first increases and then decreases, with an optimal reaction temperature of 350 °C. The addition of SO₂ would compete with SO₃ for adsorption and inhibit the uptake of SO₃ by the adsorbent. Adsorption kinetics data show that external mass transfer and chemical adsorption are the main critical mechanisms affecting the adsorption efficiency of the modified fly ash adsorbent in the SO₃ removal process compared to internal diffusion.

Mechanisms of S cooperating with Fe and Mn to regulate the conversion of Cd and Cu during soil redox process revealed by LDHs-DGT technology

The activity changes of Cd and Cu in paddy field were strongly influenced by the transformation of S, Fe and Mn species. However, in the process of soil redox, how S cooperates with Fe/Mn to regulate the law and mechanism of Cd and Cu speciation transformation still needs to be studied. In this study, we used DGT technology based on layer double hydroxides (LDHs) combined with pore water sampling to investigate soil redox changes, rice growth, and the effects of different forms of sulfur (S⁰, SO₄^2-) on soil Cd and Cu activities. The results showed that the concentrations of C_DGT-Cd and Cu in the soil decreased rapidly in the anaerobic stage, but increased slowly in the oxidative stage. Multiple regression analysis showed that the changes of Cu and Cd concentrations mainly depended on the changes of Fe/Mn morphology. Sulfur treatment promoted the dissolution of Fe/Mn oxides in the short term (<48 h), and the activities of C_DGT-Fe, Mn, and Cd increased simultaneously, but C_DGT-Cu was not affected. However, after long-term anaerobic conditions (>10 d), sulfur addition reduced the activities of C_DGT-Cd and Cu, and decreased the uptake of Cd and Cu by rice. During sulfate reduction, the sulfur addition treatment group resulted in a 24.5-50.2 % decrease in C_DGT-Fe, indicating that sulfur addition may delay the release of Cd and Cu after rice planting by promoting the formation of FeS/FeS₂. In addition, in the anaerobic stage, Cu formed sulfide before Cd and was fixed, and the higher thermodynamic stability of CuS would promote the dissolution of CdS in the oxidation stage. Overall, soil flooding with sulfur to enhance the generation of metal sulfides and secondary iron ores provides an opportunity to use sulfur as an environmentally friendly modifier to coordinate Fe, Mn to improve heavy metal-contaminated soils.

Visualization the fixation of cadmium on manganese dioxide in sulfur reduction environments

Manganese oxides as common soil components were considered as an important sink for the cadmium pollution, which, however, would be affected by the reductive sulfide introduced during the flooding period of paddy soil. In this study, the phase transitions caused by the reactions among S^2-, MnO₂ and Cd²⁺ were visualized by atomic force microscopy (AFM). The dissolution of MnO₂ was in-situ studied by AFM in the S^2-containing environments. Moreover, in the ternary system (S^2-, MnO₂ and Cd²⁺), the pre-adsorption of Cd²⁺ by the MnO₂ nanosheets would promote the subsequent precipitation of CdS on the surface of MnO₂, while the pre-formed CdS nanoparticles in the aquatic phase would tend to suspense rather than precipitating on MnO₂. The kinetic study results indicated that the CdS crystallite generation rate was faster than the MnO₂ dissolution rate in the aquatic environments with different sulfide contents. In the macroscopic Cd²⁺ fixation test, the introduction of S^2- dramatically improved the fixation of the pre-adsorbed Cd²⁺ on the MnO₂ nanosheets by forming the CdS precipitate. This study provided a fundamental understanding of the interactions among the S^2-, MnO₂ and Cd²⁺ ternary system and shed light on the development of Cd pollution remediation methods for paddy soils.

Co-incorporation of Chinese milk vetch (Astragalus sinicus L.), rice straw, and biochar strengthens the mitigation of Cd uptake by rice (Oryza sativa L.)

Soil cadmium (Cd) contamination is becoming a widespread concern because of its threat to global ecosystem health and food security. Co-incorporation of Chinese milk vetch (MV) and rice straw (RS) is a common agricultural practice in Southern China; however, the effects of combining these two materials with biochar on Cd bioavailability remain unclear. This study investigated the effects of MV, RS, rape straw biochar (RB), iron-modified biochar (FB), and their combinations on Cd uptake by rice through incubation and field experiments. The results showed that compared with the control without material input (CK), MV + RS (MR), MV + RS + RB (MRRB), and MV + RS + FB (MRFB) considerably reduced the Cd concentration in brown rice by 61.20 %, 65.38 %, and 62.65 %, respectively. Furthermore, the treatments increased the formation of iron‑manganese plaque (IMP) at different growth stages; MRRB and MRFB exhibited the highest increase rates among the treatments. Quantitatively, the Fe plaque and Mn plaque were increased by 20.61 %-47.23 % and 80.18 %-172.74 %, respectively. Compared with CK, the MRRB and MRFB treatments reduced the soil available Cd by 35.09 %-54.45 % and 38.20 %-50.20 %, respectively, at all stages. This decrease was substantially lower than that observed in the MV, RS, and MR treatments. Similar trends were observed in the incubation experiment. Additionally, the Community Bureau of Reference Sequential Extraction Analysis indicated that the MRRB and MRFB treatments converted the bioavailable Cd fractions into a stable form. Partial least squares path model and redundancy analysis revealed that pH was the major factor influencing Cd bioavailability. This study emphasized that the dual impact factors from the enhancement of Cd passivation capability and IMP formation jointly result in the reduction of Cd uptake by rice. Consequently, the co-incorporation of MV, RS, and biochar is promising for remediating Cd-contaminated paddy soils in Southern China.

Immobilization of cadmium by mercapto-functionalized palygorskite under stimulated acid rain: Stability performance and micro-ecological response

The interaction of cadmium (Cd) pollution and acid rain stress has seriously threatened soil ecosystem and human health. However, there are still few effective amendments for the in-situ remediation in the Cd-contaminated acidified soil. In this study, the performance and mechanisms of palygorskite (PAL) and mercapto-functionalized PAL (MPAL) on Cd immobilization were investigated, and the stability as well as effects on soil micro-ecology under stimulated acid rain were also explored. Results showed that MPAL could react with Cd to form stable Cd-sulfhydryl and Cd-O complexes. The reduction of bioavailable Cd by MPAL was 121.19-164.86% higher than that by PAL. Notably, the Cd immobilization by MPAL remained stable within 90 days in which the concentrations of HOAc-extractable Cd were reduced by 18.28-25.12%, while the reducible and residual fractions were increased by 9.26-18.53% and 54.16%-479.01%, respectively. The sequential acid rain leaching demonstrated that soil after MPAL treatments had a strong H⁺ resistance, and the immobilized Cd showed prominent stability. In addition, activities of acid phosphatase, catalase and invertase in MPAL treated soil were significantly enhanced by 34.60%, 22.09% and 48.87%, respectively. After MPAL application, bacterial diversity was further improved with diversified sulfur metabolism biomarkers. The decreased abundance of Cd resistance genes including cadA, cadC, czcA, czcB, czcR and zipA also indicated that soil micro-ecology was improved by MPAL. These results showed that MPAL was an effective and eco-friendly amendment for the immobilization of Cd in contaminated soil.

Uncovering geochemical fractionation of the newly deposited Hg in paddy soil using a stable isotope tracer

The newly deposited mercury (Hg) is more readily methylated to methylmercury (MeHg) than native Hg in paddy soil. However, the biogeochemical processes of the newly deposited Hg in soil are still unknown. Here, a field experimental plot together with a stable Hg isotope tracing technique was used to demonstrate the geochemical fractionation (partitioning and redistribution) of the newly deposited Hg in paddy soils during the rice-growing period. We showed that the majority of Hg tracer (²⁰⁰Hg, 115.09 ± 0.36 μg kg^-1) was partitioned as organic matter bound ²⁰⁰Hg (84.6-89.4%), followed by residual ²⁰⁰Hg (7.6-8.1%), Fe/Mn oxides bound ²⁰⁰Hg (2.8-7.2%), soluble and exchangeable ²⁰⁰Hg (0.05-0.2%), and carbonates bound ²⁰⁰Hg (0.04-0.07%) in paddy soils. Correlation analysis and partial least squares path modeling revealed that the coupling of autochthonous dissolved organic matter and poorly crystalline Fe (oxyhydr)oxides played a predominant role in controlling the redistribution of the newly deposited Hg among geochemical fractions (i.e., fraction changes). The expected aging processes of the newly deposited Hg were absent, potentially explaining the high bioavailability of these Hg in paddy soil. This study implies that other Hg pools (e.g., organic matter bound Hg) should be considered instead of merely soluble Hg pools when evaluating the environmental risks of Hg from atmospheric depositions.

Cadmium immobilization in lake sediment using different crystallographic manganese oxides: Performance and mechanism

Cd pollution in sediments poses severe threats to environmental safety and human health. Mn oxides have potential merit for the remediation of Cd pollution in sediment but have not received enough attention. Although Mn oxides have proven effective as adsorbents for removing heavy metals from water/wastewater, the performance and the underlying mechanism of Cd immobilization in sediments by Mn oxides remain unclear. Here, three crystallographic Mn oxides δ-MnO₂, γ-MnOOH, and Mn₃O₄ were used as amendments to investigate their potential for the in situ immobilization of Cd in lake sediment. Experimental data showed that when the sediment samples were treated with synthesized Mn oxides at dosages of 2% and 6% (w/w) for 56 days, the TCLP (toxicity characteristic leaching procedure) leachable Cd in the sediment decreased by 43.9-66.81%, and the PBET (physiologically based extraction test) extractable Cd decreased by 45.16-99.40%. Additionally, the acid-soluble fraction of Cd was partially transformed to a residual fraction, resulting in a 27.55-35.49% decrease in acid-soluble Cd and a 25.16-30.36% increase in the residual Cd fraction. Sediment pH and oxidation-reduction potential were important factors affecting the bioavailability of Cd in the remediation process. Furthermore, scanning electron microscopy, X-ray diffractometer, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis illustrated that the interaction between the amendment and Cd mainly involved complexation with O-containing groups, ion-exchange as > OCd⁺, and precipitation with carbonate. The efficient remediation capacity and associated mechanism for Mn oxides provide insights for the improved restoration of heavy metal-contaminated sediment.

Synthesis of Iron-Based Carbon Microspheres with Tobacco Waste Liquid and Waste Iron Residue for Cd(II) Removal from Water and Soil

Herein, a novel magnetic iron-based carbon microsphere was prepared by cohydrothermal treatment of tobacco waste liquid (TWL) and waste iron residue (WIR) to form WIR@TWL. After that, WIR@TWL was coated with sodium polyacrylate (S.P.) to fabricate WIR@TWL@SP, whose removal efficiency for bivalent cadmium (Cd(II)) was studied in water and soil. As a result, WIR@TWL@SP possessed a high Cd(II) removal efficiency, which could reach 98.5% within 2 h. The adsorption process was consistent with the pseudo-second-order kinetic model because of the higher value of adjusted R2 (0.99). The thermodynamic data showed that the adsorption process was spontaneous (ΔG° < 0) and exothermic (ΔH° = 32.42 KJ·mol-1 > 0). Cd(II) removal mechanisms also include cation exchange, electrostatic attraction, hydrogen-bond interaction, and cation-π interaction. Notably, pot experiments demonstrated that WIR@TWL@SP could effectively reduce Cd absorption by plants in water and soil. Thus, this study offers an effective method for remediating Cd(II)-contaminated water and soil and may have a practical application value.

Differences and mechanism of dynamic changes of Cd activity regulated by polymorphous sulfur in paddy soil

Sulfur (S) can decrease the bioavailability of Cd in paddy soil, and therefore reduce Cd uptake by rice. However, the dynamic influence mechanism of different forms of sulfur on the bioavailability of cadmium in soil has not been systematically studied. In this study, we used Diffusive gradients in thin films (DGT) technology and soil pore water sampling technology to investigate the effects of different types of S application on the bioavailability of Cd. The three forms of S are elemental sulfur (S⁰), sodium sulfate (SO₄^2-), and mercapto-grafted palygorskite (MP), which have been treated in six ways according to CK, SL, SH, SO₄²L, SO₄²H, MP (L and H are the low and high levels of treatments). The results showed that soluble and labile Cd concentration was quickly fixed after flooding but activated after rice transplantation. Both MP and SL treatments increased the content of dissolved organic carbon (DOC) and significantly reduced the accumulation of Cd in roots and grains. The Cd content of roots treated with MP was only 0.50 mg kg^-1, which was 77.6% lower than the Cd content in CK (2.22 mg kg^-1). S promotes the reduction and dissolution of Fe, and the formation of FeS/FeS₂ is also one of the reasons for the low bioavailability of Cd, while the SH treatment has a greater effect on lowering pH, so the effect of fixing Cd is not obvious. In addition, SO₄^2- treatment delays the soil reduction process, so it cannot effectively fix Cd. At the stage of rice maturity, dissolved Cd increased in S treatment, but MP treatment did not have this phenomenon. The possible reason was the consumption of DOC or the oxidation of CdS which caused Cd to enter the soil solution. Taken together, Fe-S play a critical role in controlling the mobilization of Cd in paddy systems.

Theoretical insights into the adsorption mechanism of Cd(II) on the basal surfaces of kaolinite

Retardation of Cd(II) migration is an ongoing concern for environmental remediation, but a prevalent obstacle of the procedure originates from the lack of an atomic-scale description of the inherent mechanism for Cd(II) adsorption at mineral-water interfaces. Herein, we performed first-principles calculations and ab initio molecular dynamics (AIMD) simulations to explore the adsorption mechanism of Cd(II) on the basal surfaces of kaolinite. Representative monodentate and bidentate Cd(II) complexes were constructed on the Kln-Al(001) and Kln-Si(001̅) surfaces. The results showed that bidentate coordination of Cd(II) on the Kln-Al(001) surface was superior to all other studied models due to the favorable formation energy and better agreement with EXAFS data. The calculated electron density difference revealed the charge transfer from surface oxygen (O_s) to Cd(II) upon adsorption. In particular, partial density of states (PDOS) analysis indicated that the Cd-O_s bond exhibited covalent characteristics, attributed to the overlaps of Cd-5p and O_s-2p orbitals in the valence band. Furthermore, radial distribution functions supported by AIMD simulations were employed to confirm the structural features of Cd(II) coordination shell at kaolinite-water interfaces. This theoretical study provides insightful guidance for future Cd(II) research to improve current assessments of contaminant remediation.

Evaluation of the cadmium phytoextraction potential of tobacco (Nicotiana tabacum) and rhizosphere micro-characteristics under different cadmium levels

In this study, a field-scale and pot experiment were performed to evaluate the remedial efficiency of Cd contaminated soil by tobacco and explore rhizosphere micro-characteristics under different cadmium levels, respectively. The results indicated that tobacco could remove 12.9 % of Cd from soil within a short growing period of 80 d. The pot experiment revealed that tobacco could tolerate soil Cd concentrations up to 5.8 mg kg-1 and bioaccumulate 68.1 and 40.8 mg kg-1 Cd in shoots and roots, respectively. The high Cd bioaccumulation in tobacco might be attributed to strong acidification in the rhizosphere soil and the increase in Cd bioavailability. Rhizobacteria did not appear to be involved in Cd mobilization. In contrast, tobacco tended to enrich sulfate-reducing bacteria (such as Desulfarculaceae) under high Cd treatment (5.8 mg kg-1) but enrich plant growth-promoting bacteria (such as Bacillus, Dyadobacter, Virgibacillus and Lysobacter) to improve growth under low Cd treatment (0.2 mg kg-1), suggesting that tobacco employed different microbes for responding to Cd stress. Our results demonstrate the advantages of using tobacco for bioremediating Cd contaminated soil and clarify the rhizosphere mechanisms underlying Cd mobilization and tolerance.

Facet-Dependent Photoinduced Transformation of Cadmium Sulfide (CdS) Nanoparticles

Microbial-mediated transformation of anthropogenic Cd²⁺ controls its distribution, bioavailability, and potential risks. However, the processes readily form CdS nanoparticles (CdS-NPs), which exhibit dissolution behavior different from that of larger sized particles. Here, we investigated the effects of morphologies and facets of CdS-NPs on their photoinduced dissolution. Three CdS-NPs, CdS-sphere, CdS-rod, and CdS-sheet, and one nanosized biogenic CdS (Bio-CdS) were synthesized with different dominant facets of {101}, {100}, {001}, and {111} and thus distinct surface chemistry. As explored by HRTEM, EPR, and DFT calculations, photogenerated e^-/h⁺ pairs were more likely to generate on CdS-sheet surfaces due to higher surface energies and a narrower band gap, facilitating the formation of •OH and thereby faster dissolution (k_obs = 6.126-6.261 × 10^-2 h^-1). The wider band gaps of CdS-sphere and CdS-rod caused less formation of O₂^•- and •OH, leading to slower oxidative dissolutions (k_obs = 0.090-0.123 and 2.174-3.038 × 10^-2 h^-1, respectively). Given the similar surface energy as that of CdS-sheet, the dissolution rate of Bio-CdS was close to that of CdS-rod and CdS-sheet, which was 1.6-3.5 times faster than that of larger sized CdS, posing higher environmental risks than thought. Altogether, this work revealed the facet effects on the dissolution of CdS-NPs, manifesting a deeper understanding of metal sulfides' environmental behaviors.

The use of mercapto-modified palygorskite prevents the bioaccumulation of cadmium in wheat

In this study, the mechanism by which mercapto-modified palygorskite (MPAL) mediates Cd and Mn absorption by wheat was elucidated. In the aqueous phase, MPAL can react with Cd to form Cd-thiol complexes and CdO and with Mn to form MnO. In the wheat-soil system, 0.1-0.3% MPAL application increased the biomass of wheat by 18.6-29.4% and decreased the Cd concentration in shoots and roots by 19.4-51.8% and 35.9-64%, respectively; however, MPAL application did not decrease the diethylenetriaminepentaacetic acid (DTPA)-extracted Cd concentration in soil, probably because the formed Cd-thiol complexes and CdO could not be taken up by plants but could be extracted by DTPA. MPAL appeared to increase the Mn concentration in plants and the DTPA-extracted Mn concentration in soil, possibly because of the reduction in soil Mn oxides to more soluble Mn(Ⅱ) by the thiol groups in MPAL. MPAL enriched plant growth-promoting rhizobacteria and Cd-immobilizing bacteria and strengthened the sulfate reduction metabolism in rhizosphere soil, which partly contributed to the improvement in plant growth and the reduction in Cd bioaccumulation in wheat. These findings highlight the importance of the thiol group in MPAL and the regulation of the rhizosphere bacterial community in mediating Cd and Mn bioaccumulation in wheat.