The performance and mechanism of sulfidated nano-zero-valent iron for the simultaneous stabilization of arsenic and cadmium.

Co-contamination of soil and groundwater with arsenic (As) and cadmium (Cd) is widespread. Sulfidized Nanoscale Zero-Valent Iron (S-nZVI) is effective in removing As and Cd from contaminated environments. However, the mechanisms governing As and Cd removal from systems containing both species are still unclear. This study investigated the effectiveness of S-nZVI in the simultaneous removal of Cd(II) and As(III) from contaminated solutions and their interaction mechanisms. Adsorption experiments were conducted under aerobic conditions to investigate the effect of Cd(II) and As(III) on their co-immobilisation at different As(III) and Cd(II) concentrations. S-nZVI was characterised before and after the reaction to elucidate the mechanism of its simultaneous immobilisation of As(III) and Cd(II). Batch experiments revealed that the presence of Cd(II) and As(III) together considerably promotes the passivation of S-nZVI. The adsorption of Cd(II) at Cd:As = 1:3 was 198.37 mg/g, which was 27.6 % higher than that in Cd(II)-only systems, and the adsorption of As(III) at As:Cd = 1:3 was 204.05 mg/g, which was 175 % higher than that in As(III)-only systems. The results of X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy indicated that the removal of Cd(II) and As(III) by S-nZVI involves electrostatic adsorption, complexation and oxidation reactions, amongst which electrostatic adsorption and ternary-complex generation are responsible for the synergistic effect. As and Cd ions can form two types of surface complexes with FeOH or FeS on the outer layer of S-nZVI: anionic bridging to form Fe-As-Cd and cationic bridging to form Fe-Cd-As. This investigation elucidates the synergistic action of Cd(II) and As(III) during their removal using S-nZVI. Thus, S-nZVI is a promising material for the combined removal of Cd(II) and As(III), which can mitigate environmental pollution.

Effects of water deficit at different stages on growth and ear quality of waxy maize.

Introduction: Extreme weather has occurred more frequently in recent decades, which results in more frequent drought disasters in the maize growing season. Severe drought often decreases remarkably plant growth and yield of maize, and even reduces significantly the quality of maize production, especially for waxy maize. Results: To study the changes in plant growth, fresh ear yield, and fresh grain quality of waxy maize under water deficits occurring at different growth stages, and further strengthen the field water management of waxy maize, water deficit experiments were carried out under a rain shelter in 2019 and 2020. Water deficit treatments were imposed respectively at the V6-VT (DV6-VT), VT-R2 (DVT-R2), and R2-R3 (DR2-R3) stages of waxy maize, and treatment with non-water deficit in the whole growing season was taken as the control (CK). The lower limit of soil water content was 50% of field capacity for a water deficit period and 65% of field capacity for a non-water deficit period. Results: In this study, water deficits imposed at V6-VT and VT-R2 stages decreased plant growth rate and leaf gas exchange parameters, accelerated leaf senescence, and limited ear growth of waxy maize, which resulted in 11.6% and 23.1% decreases in grains per ear, 19.4% and 7.3% declines in 100-grain weight, 20.3% and 14.2% losses in fresh ear yield in 2019 and 2020 growing seasons, respectively, while water deficit at R2-R3 stage had no significant effect on ear traits and fresh ear yield, but the fresh ear yield with husk of DR2-R3 decreased by 9.1% (P<0.05). The obvious water deficit imposed at the V6-VT and VT-R2 stages also lowered grain quality. Water deficits at the V6-VT and VT-R2 stages led to accelerated maturity, resulting in increased total protein, starch, and lysine content in grains at the R3 stage and decreased soluble sugar content. Principal component analysis revealed that when water deficits occurred in the waxy maize growing season, they firstly altered maize physiological processes, then affected ear characteristics and yield, and finally resulted in significant grain quality changes. In conclusion, a water deficit during V6-VT and VT-R2 not only reduced fresh ear yield but also adversely affected grain quality. However, water deficit during R2-R3 had little effect on total protein, starch, and soluble sugar content,but increased obviously lysine content. Discussion: The above results suggested that avoiding serious water deficits at the V6-VT and VT-R2 stages of waxy maize while imposing a slight water deficit at the R2-R3 stage has not only little effects on fresh ear yield but also a remarkable improvement in grain quality.

Soil particle size fractions affect arsenic (As) release and speciation: Insights into dissolved organic matter and functional genes.

Soil particle size fractions (PSFs) are important for arsenic (As) partitioning, migration, and speciation transformation. However, information is lacking about the environmental fate of As and its distribution on different PSFs. In the present study, two types of soils from mining areas were divided into four PSFs, including coarse sand (2-0.25 mm), fine sand (0.25-0.05 mm), silt (0.05-0.002 mm), and clay (< 0.002 mm) fractions. The results showed that As was enriched in the coarse sand, which was primarily affected by the content of organic carbon (OC), followed by iron (Fe), aluminum (Al), and manganese (Mn) (hydr)oxides. The elevated total As (TAs), As(III), organic As, Fe(II), and dissolved organic carbon (DOC) concentrations were mainly originated from the clay fraction. The intensified humification degree of DOM and promoted bacterial metabolism related to As/iron bioreduction were also exhibited in the clay fractions. The dynamics of As fractions in soils indicated the potential formation of secondary minerals and re-adsorption of As in the PSFs. The highest abundances of arrA, arsC, arsM, and Geo genes were found in the clay fraction, implying that the clay fraction potentially released more As, including As(III) and organic As. Results from the correlation analysis showed that elevated DOC concentrations promoted the catabolic responses of iron-reducing microorganisms and triggered microbial As detoxification. Overall, this study provides valuable information and guidance for the remediation of As-contaminated soils.

[Effect of Modified Chitosan Loaded with Silica on Arsenic Uptake and Transport in Rice].

Chitosan loaded with silica foliar spraying material (NCSI) was prepared using natural biomass chitosan and organic silicon and was applied to Lindao 16, Nanjing 2728, Zhenghan 10, and Nongken 58 at the rice filling stage to study the effects on arsenic (As) absorption in rice. The results showed that under a 100 µmol·L-1 pentavalent arsenic[As(Ⅴ)] hydroponic environment, foliar spraying of 30 mL NCSI on leaves promoted the accumulation of As in leaves and reduced the accumulation of As in roots and grains. Among them, the As contents of Nanjing 2728, Zhenghan 10, and Nongken 58 grains were decreased by 14.44%, 21.66%, and 10.85%, respectively, compared with those of the blank group on the 3rd day after spraying NCSI. Meanwhile, foliar spraying of NCSI improved the activities of CAT and SOD in rice leaves and increased the GSH content, indicating that the spraying of NCSI alleviated the stress of As on rice, improved the tolerance of rice to As, and reduced the toxicity of As to rice. The main mechanism of foliar application of NCSI to reduce As accumulation in rice grains may have been that NCSI induced GSH production in leaves, reducing more As(Ⅴ) to trivalent arsenic[As(Ⅲ)], thereby promoting As chelating in leaves and inhibiting As migration to grains. Therefore, foliar spraying of NCSI can be used as a foliar control technology to solve the problem of excessive As in rice and provide a scientific basis for the safe production of rice in China.

The stabilizing mechanism of cadmium in contaminated soil using green synthesized iron oxide nanoparticles under long-term incubation.

Despite numerous studies having been conducted on the stabilization of heavy metal contaminated soil, our understanding of the mechanisms involved remains limited. Here green synthesized iron oxide nanoparticles (GION) were applied to stabilize cadmium (Cd) in a contaminated soil. GION not only stabilized soil Cd, but also improved soil properties within one year of incubation. After GION application both the exchangeable and carbonate bound Cd fractions decreased by 14.2-83.5% and 18.3-85.8% respectively, and most of the Cd was translocated to the residual Cd fraction. The application of GION also strongly altered soil bacterial communities. In GION treatments, the abundance of Gemmatimonadetes, Proteobacteria, and Saccharibacteria increased which led to a shift in the dominant bacterial genera from Bacillus to Candidatus koribacter. The variation in bacteria confirmed the restoration of the contaminated soil. The most abundant bacterial genus and species found in GION treatments were related to (i) plant derived biomass decomposition; (ii) ammoxidation and denitrification; and (iii) Fe oxidation. GION application may enhance the formation of larger soil aggregates with anaerobic centers and coprecipitation coupled Fe (II) oxidization, ammoxidation and nitrite reduction followed by Fe mineral ripening may be involved in Cd stabilization. The predominant stabilization mechanism was thus coprecipitation-ripening-stabilization.

Immobilization of cadmium in polluted soils by phytogenic iron oxide nanoparticles.

While phytogenic nanomaterials have been successfully used to remove heavy metals in wastewater, the potential to successfully use such materials to immobilize heavy metals in soils is still unclear. In this study, phytogenic iron oxide nanoparticles (PION) were used to immobilize cadmium (Cd) in six soils. Amendment with PION effectively immobilized Cd, with a concomitant increase in the concentrations of iron oxides, soil pH and dissolved organic carbon (DOC) under both oxic and anoxic conditions. However, observed changes in soil properties and Cd fractions were different under oxic and anoxic conditions. After PION application, the exchangeable Cd fraction decreased by up to 91 and 69%, while the carbonate bound Cd fraction decreased by up to 61 and 75%, under oxic and anoxic conditions, respectively. Pearson correlation analysis revealed that under both oxic and anoxic conditions, Cd fractions were significantly and positively correlated with free iron oxide content and pH, where free iron oxide content was positively correlated with amorphous iron oxide, DOC and pH. The Cd immobilization mechanisms potentially involved either (1) formation of insoluble hydroxides at elevated pH; (2) participation of biomolecules released from PION in ligand complexation with Cd and (3) co-precipitated of Cd during the formation of iron oxides. This study provided new insights into the potential effects of PION applications for practical Cd immobilization in contaminated soils.

Biosynthesized iron oxide nanoparticles used for optimized removal of cadmium with response surface methodology.

To effectively reuse adsorbent in removal of Cd (II), magnetic modification was considered as an alternative. In this study, iron oxide nanoparticles (IONPs) synthesized from the extract of Excoecaria cochinchinensis Lour leaves were modified by low-temperature calcination, and used to remove Cd (II). Transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and magnetic properties analysis confirmed the successful synthesis of nanoscale magnetic FeOC composite. Response surface methodology (RSM) served to optimize the adsorption of Cd (II) by IONPs based on Box-Behnken design (BBD). According to the quadratic model, the effect of each factor on the removal of Cd (II) by IONPs was: pH > dosage > ionic strength > temperature. In percentage terms, 98.50% of Cd (II) (10 mg L-1) was removed when the pH, absorbent dosage, temperature and ionic strength conditions were 8.07, 2.5 g L-1, 45 °C, and 0.07 mol L-1, respectively. The adsorption of Cd (II) by IONPs is consistent with pseudo-second order kinetics and Langmuir adsorption isotherm models, indicating that the process of adsorption of Cd (II) by IONPs belongs to monolayer chemical adsorption. The -COOH, -COH, Cπ electron and ≡FeOH may be the binding sites for Cd (II) on the surface of IONPs. Overall, IONPs can be used to remove Cd (II) effectively from aqueous solution in a wide range of conditions.

Functional kaolin supported nanoscale zero-valent iron as a Fenton-like catalyst for the degradation of Direct Black G.

Kaolin supported nanoscale zero-valent iron (K-nZVI) is synthesized and applied as the Fenton-like oxidation catalyst to degrade a model azo dye, Direct Black G (DBG). The characterization of K-nZVI by the high resolution transmission electronmicroscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Energy Diffraction Spectrum (EDS) and X-ray diffraction (XRD) show that kaolin as a support material not only reduces the aggregation of zero-valent iron (nZVI) but also facilitates the Fenton-like oxidation by increasing the local concentration of DBG in the vicinity of nZVI. Pseudo first-order and pseudo second-order kinetic models are employed to reveal the adsorption and degradation of the DBG using K-nZVI as the catalyst. A better fit with pseudo second-order model for the adsorption process and equal excellent fits with pseudo first-order and pseudo second-order models for the degradation process are observed; the adsorption process is found to be the rate limiting step for overall reactions. The adsorption, evaluated by isotherms and thermodynamic parameters is a spontaneous and endothermic process. High-performance liquid chromatography-mass spectrometry (LC-MS) analysis was used to test degraded products in the degradation of DGB by K-nZVI. A removal mechanism based on the adsorption and degradation is proposed, including (i) prompt adsorption of DBG onto the K-nZVI surface, and (ii) oxidation of DBG by hydroxyl radicals at the K-nZVI surface. The application of K-nZVI to treat real wastewater containing azo dyes shows excellent degradation efficiency.