Fe Oxide Content Research Articles

Spatial statistical modeling of arsenic accumulation in microsites of diverse soils

Determining reaction mechanisms that control the mobility of nutrients and toxic elements in soil matrices is confounded by complex assemblages of minerals, non-crystalline solids, organic matter, and biota. Our objective was to infer the chemical elements and solids that contribute to As binding in matrices of soil samples from different pedogenic environments at the micrometer spatial scale. Arsenic was reacted with and imaged in thin weathering coatings on eight quartz sand grains separated from soils of different drainage classes to vary contents of Fe and Al (hydr)oxides, organic carbon (OC), and other elements. The grains were analyzed using X-ray fluorescence microprobe (µ-XRF) imaging and microscale X-ray absorption near edge structure (μ-XANES) spectroscopy before and after treatment with 0.1 mM As(V) solution. Partial correlation analyses and regression models developed from multi-element µ-XRF signals collected across 100 × 100 µm2 areas of sand-grain coatings inferred augmenting effects of Fe, Zn, Ti, Mn, or Cu on As retention. Significant partial correlations (r′ > 0.11) between Fe and Al from time-of-flight secondary ion mass spectrometry (TOF-SIMS) analysis of most samples suggested that Fe and Al (hydr)oxides were partially co-localized at the microscale. Linear combination fitting (LCF) results for As K-edge μ-XANES spectra collected across grain coatings typically included >80% of As(V) adsorbed on goethite, along with varying proportions of standards of As(V) adsorbed on boehmite, As(V) or As(III) bound to Fe(III)-treated peat, and dimethylarsinic acid. Complementary fits for Fe K-edge μ-XANES spectra included ≥50% of the Fe(III)-treated peat standard for all samples, along with goethite. Our collective results inferred a dominance of Fe and possibly Al (hydr)oxides in controlling As immobilization, with variable contributions from Zn, Ti, Cu, or Mn, both across the coating of a single sand grain and between grains from soils developed under different pedogenic environments. Overall, these results highlight the extreme heterogeneity of soils on the microscale and have implications on soil management for mitigating the adverse environmental impacts of As.

Bioavailability of antimony and arsenic in a flowering cabbage–soil system: Controlling factors and interactive effect

Soil contamination with antimony (Sb) and arsenic (As) has become a well-recognized environmental and human health issue. Consumption of vegetables, especially leafy vegetables, is one of the most important sources of Sb and As exposure in humans. Accordingly, it is necessary to understand the behaviors of Sb and As in the vegetable–soil system. Moreover, although Sb and As are often assumed to have similar biogeochemical behavior, identified differences in the controlling factors affecting mobility and bioavailability of Sb and As in soils need further investigation. In this study, 112 pairs of soil and flowering cabbage samples were collected from typical farmland protection areas and vegetable-producing regions across the Pearl River Delta (PRD), South China. The contamination levels of Sb and As in soils and harvested cabbages across the PRD were investigated. The main factors affecting the mobility and bioavailability of Sb and As in the cabbage–soil system were disentangled using a random forest model. The contamination levels of Sb in the cabbages and soils of the PRD were generally low, but the soils were moderately polluted by As. Increased concentrations of Fe oxides could decrease Sb accumulation in cabbages but increased the mobilization of As in soils to some extent. In contrast, Al oxides contributed strongly to the mobilization of Sb and the immobilization of As. Moreover, an increased sand content promoted the mobility of Sb and As, whereas increased silt and clay contents showed inhibitory effects. The interactions of As and Sb with Fe oxides decreased the mobility of Sb but moderately increased the mobility of As in soils. Overall, the behaviors of Sb and As in the cabbage–soil system under the effect of several important environmental factors showed some differences indicating that these differences should be considered in the remediation of co-contaminated soils.

Relations between soil organic carbon content and the pore size distribution for an arable topsoil with large variations in soil properties

AbstractSoil organic carbon (SOC) in arable topsoil is known to have beneficial effects on soil physical properties that are important for soil fertility. The effects of SOC content on soil aggregate stability have been well documented; however, few studies have investigated its relationship with the soil pore structure, which has a strong influence on water dynamics and biogeochemical cycling. In the present study, we examined the relationships between SOC and clay contents and pore size distributions (PSDs) across an arable field with large spatial variations in topsoil SOC and clay contents by combining X‐ray tomography and measurements of soil water retention. Additionally, we investigated the relationships between fractionated SOC, reactive Fe and Al oxide contents and soil pore structure. We found that porosities in the 0.2–720 μm diameter class were positively correlated with SOC content. A unit increase of SOC content was associated with a relatively large increase in porosity in the 0.2–5 and 480–720 μm diameter classes, which indicates that enhanced SOC content would increase plant available water content and unsaturated hydraulic conductivity. On the other hand, macroporosities (1200–3120 μm diameter classes) and bioporosity were positively correlated with clay content but not with SOC content. Due to strong correlations between soil texture, carbon‐to‐nitrogen ratios and reactive iron contents, we could not separate the relative importance of these soil properties for PSDs. Reactive aluminium and particulate organic carbon contents were poorer predictors for PSDs compared with clay and SOC contents. This study provides new insights on the relations between SOC and soil pore structure in an arable soil and may lead to improved estimations of the effects of enhanced SOC sequestration on soil water dynamics and soil water supply to crops.Highlights Relations between soil organic carbon (SOC) and pore size distribution (PSD) in an arable soil were explored. We used X‐ray tomography and soil water retention to quantify a wide range of PSD. There were positive correlations between SOC and porosities in 0.2–720 μm diameter classes. Porosities in 0.2–5 and 480–720 μm diameter classes were more strongly correlated with SOC than clay. Our results have implications for improved estimates of effects of SOC sequestration on soil water dynamics.

Sulfur accumulation in soil in a forested watershed historically exposed to air pollution in central Japan

Forest soils that have accumulated anthropogenic sulfur (S) as a result of historical air pollution may gradually or suddenly release inorganic sulfate either by desorption of inorganic sulfate or by mineralization of organic S, even after mitigation of previous air pollution. The release of inorganic sulfate from soil to surface water leads directly to watershed acidification. Thus, whether the organic S compounds that have accumulated in soils are resistant to microbial activity, due to formation of organo-mineral complexes, is essential information for management of forest soil ecosystem services. A nationwide air pollution monitoring survey by the Japanese Ministry of the Environment identified acidification of both surface water and soil in the Lake Ijira watershed of central Japan, presumably sourced from the Chukyo Industrial Area which lies upwind of the watershed. We compared the amount of S accumulated in the Ijira soils (Dystrudepts) with that in comparable soils in other watersheds in central Japan that were less affected by industrial air pollution. To determine how much mineral-associated organic S had accumulated in the Ijira soils, we measured pyrophosphate- extractable S. We used high-resolution x-ray fluorescence (HRXRF) spectroscopy with a double-crystal type high energy resolution spectrometer to estimate the oxidation states of both mineral-associated and free organic S. We also measured S isotope compositions to explore S dynamics in the soils. We found that more S had accumulated in the Ijira soils than in the comparable soils from other watersheds in central Japan. Concentrations of total S in the Ijira soils varied among soil horizons, reaching more than 1000 mg S kg−1 in some horizons. Mineral-associated organic S accounted for more than half of the total accumulated S. High-resolution x-ray fluorescence spectrum analyses showed that mineral-associated organic S was retained at both high and low oxidation states. The δ34S values of total S in the deeper horizons tended to be higher than those of the uppermost horizons of the Ijira soils and correlated positively with concentrations of extractable Al and Fe oxides (which characterize pedogenic minerals). Our results suggest that about half of total accumulated S in Ijira soils is stabilized in pedogenic minerals and the remainder comprises free organic S and adsorbed inorganic sulfates. There might have been repetitive mobilization and immobilization of S and the degraded organic S might have greater affinity with pedogenic minerals. Further, the ratio of total C to mineral-associated organic S correlated negatively with concentrations of extractable Al and Fe oxides, suggesting that organic S is more resistant to mineralization than organic C because S is preferentially associated with the pedogenic minerals. Monitoring of the chemical states of S compounds in soils historically exposed to air pollution is recommended to assist in prediction of the fate of the S compounds that have accumulated in them.

Influence of tree species on selenium and iodine partitioning in an experimental forest ecosystem

Storage of selenium and iodine can greatly vary between forest ecosystems, but the influence of tree species on partitioning and recycling of those elements remains elusive. In this study, contents of Se and I were measured in tree compartments, litterfall, humus, and soil horizons in monospecific stands of Douglas fir, pine, spruce, beech, and oak under identical climatic and edaphic conditions. The cycle of each element was characterized in terms of stocks and fluxes. Lowest concentrations were in wood (Se: 8–13 μg kg−1; I: <16.5 μg kg−1). Senescing organs had higher Se and I content, than the living parts of trees due to direct exposure to atmospheric deposition, with some variation between coniferous and deciduous trees. For all stands, low amounts of Se and I were involved in biological cycle as reflected by low root uptake. In humus, the enrichment of elements greatly increased with the stage of organic matter (OM) degradation with average factors of 10 and 20 for Se and I. OM degradation and element persistence in humus was influenced by tree species. Deciduous trees, with low biomass, and fast degradation of OM stored less Se and I in humus compared to fir and spruce with high humus biomass. Interestingly, tree species did not affect soil reserves of Se and I. Concentration ranges were 331–690 μg Se kg−1 and 4.3–14.5 mg I kg−1. However, the divergent vertical profiles of the elements in the soil column indicated greater mobility of I. Selenium concentrations regularly decreased with depth in correlation with OM and Fe oxides content. For iodine, the maximum iodine concentration at a soil depth of 15 to 35 cm was caused by a parallel precipitation/sorption behavior of aluminium and organic iodine dissolved in the topsoil.

Shale weathering profiles show Hg sequestration along a New York-Tennessee transect.

Shale-derived soils have higher clay, organic matter, and secondary Fe oxide content than other bedrock types, all of which can sequester Hg. However, shales also can be Hg-rich due to their marine formation. The objectives of this study were to determine the concentration and phase partitioning of Hg in seven upland weathering profiles from New York to Tennessee USA and use geochemical normalization techniques to estimate the extent of Hg inheritance from weathering of shale bedrock or sequestration of atmospheric Hg. Total Hg concentrations in unweathered shale ranged from 3 to 94ng/g. Total Hg concentrations decreased with depth in the Ultisols and Alfisols, with total Hg concentrations ranging from 18 to 265ng/g. Across all shale soils and rocks, the oxidizable fraction of Hg (15% H2O2 extraction) comprised a large portion of the total Hg at 68% ± 8%. This fraction was dominated by organic matter as confirmed with positive correlations between Hg and %LOI, but could also be impacted by Hg sulfides. Across all sites, the reducible fraction of Hg (citrate-bicarbonate-dithionite extraction) was only 10% ± 4% of the total Hg on average. Thus, secondary Fe oxides did not contain a significant portion of Hg, as commonly observed in tropical soils. Although colder sites had a higher organic matter and sequestered more Hg, τ values for Hg indexed to Ti suggest that atmospheric deposition, such as pollution sources in Ohio River Valley, drove the highest enrichment of Hg along the transect. These results demonstrate that shale-derived soils have a net accumulation and retention of atmospheric Hg, primarily through stabilization by organic matter.

Quantifying the influence of soil factors on the migration of chromium (VI)

Identification of the factors controlling the migration of hexavalent chromium (Cr (VI)) in soil can provide valuable information for pollution control and risk assessment. In this study, we assessed the processes of transportation and migration of Cr (VI) in soil by batch and column experiments. Breakthrough curves were used to examine the mobility of Cr (VI) in soil columns. Most of them could be described by a typical S-shaped curve. The Yoon–Nelson model and Thomas model were applied to predict the behavior of Cr (VI) in soil and obtained parameters to characterize the rate and capacity of adsorption. It was observed that all obtained parameters changed along with soil properties. Moreover, stepwise multiple regression and grey correlation analysis were used to determine the main control factors by evaluating the relationship between soil properties and dynamic or static parameters. The results showed Fe oxide content was the key factor no matter in static or dynamic adsorption while soil texture had a great influence on the transportation of Cr (VI) in soil columns. Also, in the actual situation, the external input condition should not be ignored. Our research provides meaningful information for authorities to develop better soil management and remediation strategies.

Soil potassium regulation by changes in potassium balance and iron and aluminum oxides in paddy soils subjected to long-term fertilization regimes

Southern China’s paddy soils are poor in potassium (K) and rich in iron (Fe) and aluminum (Al) oxides, both of which are affected by fertilizer application. However, the response of soil K budget to long-term K fertilization and Fe and Al oxides remains unclear, especially in the subsurface horizons in different soil types. Here, four long-term fertilization treatments (no fertilizer, CK; inorganic nitrogen and phosphorus fertilizers, NP; NPK; and the combined NPK and manure, NPKM) were selected to determine the effects of K input and different forms of Fe and Al oxides on soil K status at two soil layers (0–20, surface; and 20–40 cm, subsurface) in red (Ferralsols) and purple (Cambisols) paddy soils across China. Overall, treatments where K fertilizer application was withheld had lower surface soil exchangeable K (EK), non-exchangeable K (NEK), and total K contents than treatments applied with K fertilizer. In contrast, the treatment including K with manure fertilizer increased EK and NEK contents. Regardless of fertilization regimes, the contents of EK and NEK in both soil depths of purple soil were significantly higher than those in their corresponding depths of red soil. Moreover, there were significantly lower EK and NEK contents in the subsurface layer than those in the surface layer of red soil, while no significant differences were observed in purple soil. A positive correlation was obtained between K balances and soil EK contents (P < 0.05) and the slopes of linear regressions in red soil was higher than that in purple soil. A three-way ANOVA showed that the lone and interactive effects of experimental site, fertilization regime, and soil depth significantly influenced the contents of Fe and Al oxides in both soils. The application of NPKM can inhibit the decrease of free Fe and Al oxides in the surface layer of soil and increase the amorphous and chelated Fe and Al oxides, especially in red soil. Redundancy analysis showed that the amorphous Fe and Al oxides were the most important factors for regulating surface soil EK content. The free and chelated oxides were the most important factors for regulating NEK contents in red and purple soils, respectively, particularly in subsurface soil. Our results imply that the combined application of inorganic fertilizer and manure is a viable strategy for improving soil K availability by increasing K balance and regulating the contents and forms of Fe and Al oxides in different depths of paddy soil.

Aluminous clay and pedogenic Fe oxides modulate aggregation and related carbon contents in soils of the humid tropics

Abstract. Aggregation affects a wide range of physical and biogeochemical soil properties with positive effects on soil carbon storage. For weathered tropical soils, aluminous clays (kaolinite and gibbsite) and pedogenic Fe (oxyhydr)oxides (goethite and hematite; termed “Fe oxides”) have been suggested as important building units for aggregates. However, as aluminosilicates, aluminum hydroxides, and Fe oxides are part of the clay-sized fraction it is hard to separate how certain mineral phases modulate aggregation. In addition, it is not known what consequences this will have for organic carbon (OC) persistence after land-use change. We selected topsoils with unique mineralogical compositions in the East Usambara Mountains of Tanzania under forest and cropland land uses, varying in contents of aluminous clay and Fe oxides. Across the mineralogical combinations, we determined the aggregate size distribution, aggregate stability, OC contents of aggregate size fractions, and changes in aggregation and OC contents under forest and cropland land use. Patterns in soil aggregation were rather similar across the different mineralogical combinations (high level of macroaggregation and high aggregate stability). Nevertheless, we found some statistically significant effects of aluminous clay and pedogenic Fe oxides on aggregation and OC storage. An aluminous clay content &gt; 250 g kg−1 in combination with pedogenic Fe contents &lt; 60 g kg−1 significantly promoted the formation of large macroaggregates &gt; 4 mm. In contrast, a pedogenic Fe content &gt; 60 g kg−1 in combination with aluminous clay content of &lt; 250 g kg−1 promoted OC storage and persistence even under agricultural use. The combination with low aluminous clay and high pedogenic Fe contents displayed the highest OC persistence, despite conversion of forest to cropland causing substantial disaggregation. This indicates that aggregation in these tropical soils is modulated by the mineralogical regime, causing moderate but significant differences in aggregate size distribution. Nevertheless, aggregation was little decisive for overall OC persistence in these highly weathered soils, where OC storage is more regulated by direct mineral–organic interactions.

Abiotic and biotic regulation on carbon mineralization and stabilization in paddy soils along iron oxide gradients

Iron (Fe) oxides regulate soil organic carbon (C) content via balancing C processes of stabilization and mineralization. However, abiotic and biotic mechanisms are involved in stabilization (e.g., by adsorption and/or co-precipitation) and decomposition (e.g., by shifting the microbial community) of paddy soil rich in iron oxides remains poorly understood. We examined the mineralization and stabilization of maize-straw-derived C (δ13C = 5000‰), soil priming effects (PE), and soil microbial community structure in four paddy soils, along with Fe oxide concentrations gradient ranging from 13.7 to 55.8 g kg−1 soil (Fe-13, Fe-25, Fe-42, and Fe-55). The paddy soil with the highest Fe content (Fe-55) stabilized 20.5 mg 13C kg−1 soil of the maize-straw-derived C, being significantly greater (P < 0.05) than Fe-13 (5 mg 13C kg−1 soil). The high C:Fe molar ratio of Fe-55 suggests the main pathway of stabilizing the maize-straw-derived C via co-precipitation as Fe-OM. Larger stabilization in Fe-55 led to less CO2 emission from maize and SOM, e.g., Fe-55 had 12–16% lower straw mineralization and 8–11% lower PE than Fe-13 during the first 7 days of incubation. Random forest analysis further revealed that Proteobacteria and Actinobacteria (the keystone species, i.e., Gaiella) gave the largest contribution to maize-straw mineralization and PE, while microbial diversity and some microorganisms featured with filamentous hyphae contributed to C stabilization. This study confirmed that the concentration of Fe oxide in paddy soils plays a central role in C sequestration via biotic and abiotic processes, including i) modulation of microbial community diversity and composition, especially the abundance of fungi and Actinobacteria, and ii) physicochemical stabilization of maize-straw-derived C through the formation of Fe-OM complexes via co-precipitation.

Soil Olsen P response to different phosphorus fertilization strategies in long‐term experiments in NW Italy

AbstractSoil available P for crops, derived from P inputs and regulated by soil chemical, physical and biological factors, represents a small fraction of the total P content. Soil P balance (P input minus P output) is potentially a good descriptor of fluctuations in the available P pool, making it possible to compare experiments that differ in management and duration. Using a data set compiled from three long‐term experiments in NW Italy that tested different P fertilization types (nil, mineral, organic and manures), cropping systems and P balances in different soils, this paper focused on (a) proposing an empirical exponential model to describe the relationship between variables and (b) providing an agronomical interpretation of optimized parameter values, to set the basis for a unified interpretation of long‐term dynamics of soil plant‐available P. The pooled data fit well in an exponential model that identified three factors supportive of an agronomic interpretation, two linked to soil attributes and one that differentiated fertilizer types. Soil characteristics were found to affect minimum soil Olsen P values in depletion and response rates with additions, while fertilizer type had a multiplier effect on cumulative P balance. Farmyard manure was shown to build the available P pool better than other organic sources (bovine slurry, commercial organic fertilizers, green manure) and inorganic fertilizers, owing to a high C/P. The exponential model provided a good theoretical basis, but further studies are needed to correlate parameter values to soil (e.g., carbonates, Al and Fe oxide content) and fertilizer (e.g., C:P, lignin:P) characteristics and to verify the model applicability to different conditions.

Modifying Effect of Soil Properties on Bio-Accessibility of As and Pb from Human Ingestion of Contaminated Soil

Exposure to soils contaminated with heavy metals can pose human health risk to children through ingestion of contaminated soil. Soil properties such as soil pH, reactive Fe and Al oxide content, clay content, soil organic matter (SOM), and cation exchange capacity (CEC) can reduce contaminant bio-accessibility and exposure. In vitro bio-accessibility (%IVBA) of As and Pb in 19 soils was determined using U.S. EPA Method 1340. Soil properties reduced the bio-accessibility of As by 17–96.5% and 1.3–38.9% for Pb. For both As and Pb, bio-accessibility decreased with increasing Al and Fe oxide content. Al oxides were found to be the primary driver of As and Pb bio-accessibility. Multiple regressions with AlOx, soil pH, %clay and/or FeOx predicted %IVBA As (p &lt; 0.001). The multiple regression including log (FeOx + AlOx) and %clay explained 63% of the variability in %IVBA Pb (p &lt; 0.01). Fe and Al oxides were found to be important drivers of As and Pb bio-accessibility, regardless of in vitro method. These findings suggested soil pH should be used in addition to reactive oxides to predict bio-accessible As. Risk-based adjustments using soil properties for exposure via incidental ingestion should be considered for soils contaminated with As and/or Pb.

Distribution and secondary enrichment of heavy metal elements in karstic soils with high geochemical background in Guangxi, China

Karst environments are globally significant, and the soils that develop on these landscapes often have unique geochemical features, including elevated–levels of metals. However, it has not been determined whether the enrichment of metals in soils is mainly controlled by parent rocks or secondary enrichment during weathering. In this paper, we studied the control of the accumulation of heavy metals in soils by parent materials and/or the process of soil formation. We examined the un–weathered carbonate and clastic bedrocks, as well as surface soils in a typical karst area in Guangxi of China. The spatial distribution of heavy metal (Cd, As, Cr, Cu, Hg, Ni, Pb, and Zn) concentrations in soils was mainly controlled by the geological background. Except for Cd, the concentrations of heavy metals in carbonate rocks were much lower than those in clastic rocks, whereas the soils derived from the carbonate rocks were highly enriched with heavy metals than those in clastic area. The enrichment of Cd in surface soils from the carbonate substrates was the result of the dual effect of secondary enrichment and parent rock inheritance, whereas the enrichment of other metals was mainly the result of secondary enrichment. During the weathering process of carbonate bedrock, great majority of the quantities of inherent heavy metals were leached, with only 2% of Cd being retained in situ. The high concentrations of soil Fe, Al, and Mn oxides, organic carbon and clay content (Al2O3/SiO2) were closely related to the enrichment of heavy metals in studied surface soils.

Interplay between the composition of the passive film and the corrosion resistance of citric acid‐passivated AISI 316L stainless steel

Citric acid passivation is an attractive alternative to increase the corrosion resistance of stainless steels. In this work, the correlation of passive film composition and the corrosion properties of citric acid‐passivated AISI 316L stainless steel samples was assessed. The concentration of citric acid in the passivating solution was varied from 10 to 30 wt.%. Passive film composition was examined by X‐ray photoelectron spectroscopy. The corrosion behavior was investigated by potentiodynamic polarization curves. The citric acid passivation treatment led to Cr2O3 enrichment of the passive film, especially for the 20‐wt.% concentration. By increasing the citric acid concentration to 30 wt.%, the corrosion resistance decreased. In addition to chromium compounds, the passivation treatment also affected the relative concentrations of Fe, Mo, and Ni compounds in the passive films. The formation of Mo oxides was triggered at citric acid concentrations of 10 and 20 wt.%, contributing to slower anodic dissolution rate. The corrosion resistance of the passivated samples was favored as the relative concentration of Fe oxides (FeO and Fe2O3) increased with respect to FeOOH.

The adsorbent capacity of growing media does not constrain myo-inositol hexakiphosphate hydrolysis but its use as a phosphorus source by plants

The hydrolysis of organic P in soils is a relevant aspect contributing to the supply P to plants, which is affected by adsorbent capacity and biological properties of soils. This work aimed at studying the contribution of phytate to plant nutrition as affected by Fe oxides and phosphohydrolases releasing microorganisms in the growing medium. An experiment with cucumber and myo-inositol hexakiphosphate (myo-Ins6P) as P source was performed involving two factors: Fe oxide –ferrihydrite– rates (0, 100, 300 mg kg−1 of citrate–ascorbate extractable Fe), and microbial inoculation (Trichoderma asperellum T34, Bacillus subtilis QST713, and non-inoculated). P uptake decreased with increased Fe oxides in the growing media. Phytase activity and organic anions concentration increased with increased Fe oxides in the media. Most of the P supplied was recovered as inorganic P at the highest Fe oxide concentration. Inoculants did not improve P uptake by plants, despite B. subtilis promoted an enhanced hydrolytic activity at the highest Fe oxide concentration. An increased adsorption capacity of the growing media restricts the use of myo-Ins6P as P source by plants. This was not the result of its stabilization through adsorption or a decreased hydrolytic activity, but of the adsorption of inorganic P on Fe oxides after hydrolysis.

Effects of Heavy Metal-immobilizing Bacteria on Reducing Wheat Uptake Cd by Regulation Soil Iron Oxides

Iron oxides are widely distributed in soils and have large specific surface areas and strong affinities with heavy metals, and thus play a significant role in the transformation of heavy metals in soils. Pot experiments were conducted to study the effects of heavy metal-immobilizing bacteria Bacillus megaterium N3, Serratia liquefaciens H12, and strain N3 combination with H12 on the absorption of Cd by ZM-3 (Triticum aestivum L.). The effects of different treatments on soil pH, iron oxide content, and relative abundance of iron oxide bacteria, clay particle mineral composition, and aggregates in wheat rhizosphere were also measured. The results showed that strains N3, H12, and N3+H12 significantly (P<0.05) increased the dry weight (36.5%-75.3%) of wheat grains, and significantly (P<0.05) decreased the content of Cd (30.2%-84.9%) in wheat grains. Furthermore, compared with the single application of strain N3 or H12, the application of strain N3 combined with H12 had a stronger ability to reduce the absorption of Cd by ZM-3. The results suggested that strain N3, combined with H12, decreased the uptake of Cd in wheat by:①increasing the pH (from 6.74 to 7.08) of wheat rhizosphere soil; ② increasing the Fe oxide (67.9%) and goethite contents of wheat rhizosphere soil; ③ increasing the relative abundance of iron-oxidizing bacteria (Leptothrix spp. and Gallionella spp.), and ④ increasing the content of small particle size (<0.25 mm) aggregates. The results provide a theoretical basis and technical support for the use of heavy metal-immobilizing bacteria to repair farmland contaminated by heavy metals.

Effect of the grain size on the corrosion behavior of CoCrFeMnNi HEAs in a 0.5 M H2SO4 solution

This work investigated the corrosion behavior of the equiatomic CoCrFeMnNi high entropy alloy (HEA) in a 0.5 M H2SO4 solution. The grain size of the HEA was adjusted by annealing. The electrochemical analysis showed that the corrosion resistance of the CoCrFeMnNi HEA increases first and then decreases with the increase of grain size. When the grain size is lower than a certain value, increasing the grain size can improve the corrosion resistance. Fine-grained (≤1.24 μm) HEA forms abundant grain boundaries, which promotes the occurrence of galvanic corrosion at the grain boundaries with inter-grains, resulting in reduced corrosion resistance. However, coarse-grained (≥145.9 μm) HEA provides too few ionic diffusion channels to form a stable and continuous passive film, which also reduces the corrosion resistance. Nanoindentation and XPS tests showed that a high content of Fe (Ⅲ) oxides and Cr (Ⅲ) oxides are formed on the medium-grained HEA, which improves the chemical stability and mechanical strength of the passive films. Thus, the corrosion resistance of medium-grained HEA is much better than those of fine-grained and coarse-grained HEA.

Poorly crystalline iron and aluminium oxides contribute to the carbon saturation and sorption of dissolved organic carbon in the soil

AbstractSoil carbon (C) saturation implies an upper limit to a soil's capacity to store C depending on the contents of silt + clay and poorly crystalline Fe and Al oxides. We hypothesized that the poorly crystalline Fe and Al oxides in silt + clay fraction increased the C saturation and thus reduced the capacity of the soil to sorb additional C input. To test the hypothesis, we studied the sorption of dissolved organic carbon (DOC) on silt + clay fractions (&lt;53 µm) of highly weathered oxic soils, collected from three different land uses (i.e., improved pasture, cropping and forest). Soils with high carbon saturation desorbed 38% more C than soils with low C saturation upon addition of DOC, whereas adsorption of DOC was only observed at higher concentration (&gt;15 g kg−1). While high Al oxide concentration significantly increased both the saturation and desorption of DOC, the high Fe oxide concentration significantly increased the desorption of DOC, supporting the proposition that both oxides have influence on the DOC sorption in soil. Our findings provide a new insight into the chemical control of stabilization and destabilization of DOC in soil.

Combined biochar and metal-immobilizing bacteria reduces edible tissue metal uptake in vegetables by increasing amorphous Fe oxides and abundance of Fe- and Mn-oxidising Leptothrix species

In this study, a highly effective combined biochar and metal-immobilizing bacteria (Bacillus megaterium H3 and Serratia liquefaciens CL-1) (BHC) was characterized for its effects on solution Pb and Cd immobilization and edible tissue biomass and Pb and Cd accumulation in Chinese cabbages and radishes and the mechanisms involved in metal-polluted soils. In the metal-containing solution treated with BHC, the Pb and Cd concentrations decreased, while the pH and cell numbers of strains H3 and CL-1 increased over time. BHC significantly increased the edible tissue dry weight by 17–34% and reduced the edible tissue Pb (0.32–0.46 mg kg−1) and Cd (0.16 mg kg−1) contents of the vegetables by 24–45%. In the vegetable rhizosphere soils, BHC significantly decreased the acid-soluble Pb (1.81–2.21 mg kg−1) and Cd (0.40–0.48 mg kg−1) contents by 26–47% and increased the reducible Pb (18.2–18.8 mg kg−1) and Cd (0.38–0.39 mg kg−1) contents by 10–111%; while BHC also significantly increased the pH, urease activity by 115–169%, amorphous Fe oxides content by 12–19%, and relative abundance of gene copy numbers of Fe- and Mn-oxidising Leptothrix species by 28–73% compared with the controls. These results suggested that BHC decreased edible tissue metal uptake of the vegetables by increasing pH, urease activity, amorphous Fe oxides, and Leptothrix species abundance in polluted soil. These results may provide an effective and eco-friendly way for metal remediation and reducing metal uptake in vegetables by using combined biochar and metal-immobilizing bacteria in polluted soils.

Effect of ferrolysis and organic matter accumulation on chromate adsorption characteristics of an Oxisol-derived paddy soil

How paddy cultivation influences the adsorption isotherms, envelopes, and the kinetics of hexavalent chromate (Cr(VI)) on Fe (hydro)oxide-rich paddy soil, as well as the mechanisms involved, remain largely unaddressed. To this end, the Cr(VI) adsorption characteristics on a paddy soil, in comparison with its parent upland Oxisol, were studied. The results showed that Cr(VI) adsorption capacities (Qmad) were higher in the surface Oxisol than in the same layer of paddy soil. The Qmad increased by 18.0% and 41.3% after removal of soil organic matter (SOM) from the surface Oxisol and paddy soil layers, respectively, indicating that Cr(VI) adsorption was considerably inhibited by SOM. The adsorption and desorption isotherms demonstrated that non-electrostatic adsorption was mainly responsible for Cr(VI) adsorption, accounting for 59.37%–83.42% of Cr(VI) adsorption capacities. The negative shift of the zeta potential–pH curves with Cr(VI) loading further corroborated the finding that non-electrostatic adsorption is largely responsible for Cr(VI) retention. Cr(VI) adsorption at equilibrium, obtained by the stirred flow chamber technique, and the free Fe (hydro)oxides (Fed) contents were in the same order, suggesting that Fed are the main adsorbents for Cr(VI). Therefore, paddy cultivation has had a profound impact on the electrochemical properties of the Oxisol and on subsequent Cr(VI) adsorption characteristics.