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Reducing the bioavailability of both cadmium (Cd) and arsenic (As) in paddy fields is a worldwide challenge. The authors investigated whether ridge cultivation combined with biochar or calcium-magnesium-phosphorus (CMP) fertilizer effectively reduces the accumulation of Cd and As in rice grains. Field trial showed that applying biochar or CMP on the ridges was similar to the continuous flooding, which maintained grain Cd at a low level, but grain As was reduced by 55.6%, 46.8% (IIyou28) and 61.9%, 59.3% (Ruiyou 399). Compared with ridging alone, the application of biochar or CMP decreased grain Cd by 38.7%, 37.8% (IIyou28) and 67.58%, 60.98% (Ruiyou399), and reduced grain As by 38.9%, 26.9% (IIyou28) and 39.7%, 35.5% (Ruiyou 399). Microcosm experiment showed that applying biochar and CMP on the ridges decreased As in soil solution by 75.6% and 82.5%, respectively, and kept Cd at a comparably low level at 0.13-0.15 μg L -1 . Aggregated boosted tree (ABT) analysis revealed that ridge cultivation combined with soil amendments altered soil pH, redox state (Eh) and enhanced the interaction of Ca, Fe, Mn with As and Cd, which promoted the concerted reduction of As and Cd bioavailability. Application of biochar on the ridges enhanced the effects of Ca and Mn to maintain a low level of Cd, and enhanced the effects of pH to reduce As in soil solution. Similar to ridging alone, applying CMP on the ridges enhanced the effects of Mn to reduce As in soil solution, and enhanced the effects of pH and Mn to maintain Cd at a low level. Ridging also promoted the association of As with poorly/well-crystalline Fe/Al and the association of Cd on Mn-oxides. This study provides an effective and environmentally friendly method to decrease Cd and As bioavailability in paddy fields and mitigate Cd and As accumulation in rice grain., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

The co-existence of antibiotics and heavy metals in soil with manure application poses high risk to both environment and human health, and thus effective remediation methods are in urgent need. This study investigated the synergistic effects of electrokinetic remediation (EKR) on antibiotic resistance and arsenic (As) in co-contaminated paddy soils. EKR treatments in soil amended with pig manure (EKR-PD) showed better remediation efficiency compared with that without pig manure. In detail, the content of available As and the abundance of antibiotic-resistant bacteria (ARB) decreased by 25.2 %-41.4 % and 9.5 %-21.1 % after 7-d remediation, respectively, due to a relatively higher current density for EKR-PD. The role of the electric field contributed to 33.9 % of antibiotic degradation. Antibiotic resistance genes (ARGs) with ribosomal-protection and enzymatic-deactivation types were easier to remove, with the removal ratio of 37.8 %-41.6 % in EKR-PD. Brevundimonas was the most significantly different species during remediation. Bacillus and Clostridium_ sensu_stricto_1 were potential host bacteria of ARGs in the electric field. Membrane transport might be an effective strategy for microorganisms to respond to the stress of both electric field and co-contaminated environments. This study supports the potential role of EKR in the co-contamination of heavy metals and antibiotic resistance under manure application., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Arsenic (As) and cadmium (Cd) are two highly toxic elements. In recent years, many newly synthesized chemical materials have been used widely for treatments of As- and Cd-contaminated effluents. However, most materials do not exhibit high efficiencies for simultaneous removal of As and Cd from water systems. Our study established a simple scheme for synthesizing a sepiolite (SEP)-modified nanoscale zero-valent iron (S-nZVI) for simultaneous removal of coexisting As and Cd from water and illuminated a possible underlying mechanism. Batch experiments showed that the maximum capacities for adsorption of As(III) and Cd(II) by S-nZVI were 230.29 mg/g and 11.37 mg/g, respectively, which represented better effects than those of other materials, as reported previously. Removal of Cd(II) depended on pH, but As(III) removal showed little dependence on pH. Coexisting ions such as phosphate (PO 4 3- ) and the conjugate base of humic acid (HA) significantly inhibited simultaneous removal of As(III) and Cd(II). In the mixed As(III)-Cd(II) system, the presence of As(III)-pretreated S-nZVI significantly enhanced Cd(II) adsorption by a factor of four over that seen for aqueous solution without As(III). XRD and XPS results showed that CdFe 2 O 4 (Fe-O-Cd), Fe 2 As 2 O 14 or FeAsO 4 (Fe-O-As) were formed after As(III) and Cd(II) were captured by S-nZVI. However, a further zeta (ζ) potential analysis showed that the mechanism for As(III) and Cd(II) adsorption by S-nZVI is not just simple formation of the above chemicals, since the adsorbed As(III) increased the negative charge of S-nZVI; this suggested an electrostatic attraction between S-nZVI and Cd(II) and indicated that adsorbed As(III) created new sorption sites for Cd(II), which enhanced Cd(II) sorption via formation of ternary complexes (Fe-As-Cd). These results suggested that S-nZVI is a promising material for in situ remediation of heavy metal-contaminated groundwaters or paddy soils., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Manure application increases the transfer risk of antibiotic resistance to farmland. Especially, its impact remains unclear when it occurs in arsenic (As)-contaminated paddy soils, which is considered as a global environmental problem. In this work, we investigated the fate of antibiotic resistance genes (ARGs) in As-antibiotic co-contaminated paddy soils under the application of manure from different sources (pig manure, cow dung, and chicken manure). Differences in the aliphatic carbon and electron-donating capacities of these dissolved organic matters (DOM) regulated the transformation of iron and As by both biotic and abiotic processes. The regulation by pig manure was stronger than that by cow dung and chicken manure. DOM regulation increased the abundance of As-related functional genes (arsC, arrA, aioA, and arsM) in the soil and accelerated the transformation of As speciation, the highest proportion of As(III) being 45%-61%. Meanwhile, the continuous selection pressure provided by the highly toxic As(III) increased the risk of ARGs and mobile genetic elements (MGEs) via horizontal gene transfer. As-resistant bacteria, including Bacillus, Geobacter, and Desulfitobacterium, were finally considered as potential host bacteria for ARGs and MGEs. In summary, this study clarified the synergistic mechanism of As-antibiotic on the fate of ARGs in co-contaminated paddy soils, and provided practical guidance for the proper application of organic fertilizers., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Soil management and cultivar selection are two strategies to reduce the accumulation risk of heavy metals in crops. However, it is still an open question which of these two strategies is more efficient for the safe utilization of contaminated soil. In this study, the available bio-concentration factors (aBCF) of arsenic (As) and cadmium (Cd) among 39 maize cultivars were determined through a field experiment. The effect of soil management was mimicked by choosing diverse sampling sites having different soil available contents of As and Cd. The aBCF of As and Cd in grain ranged from 0.02 to 0.13 and 1.17 to 42.2, respectively. The accumulation ability of As and Cd was classified among different maize cultivars. Soil pH and total As controlled the level of available As in soils, while soil pH dominated available Cd in soil. A soil pH of 6.5 was recommended to simultaneously minimize soil available As and Cd by managing soil conditions. The quantitative effects of cultivar and soil management on grain As and Cd were expressed as Q [Grain As] = 0.746Q [Cultivar]-0.126Q [pH]+0.276Q [As available ] (R 2  = 0.648, P = 1.00 × 10 -37 ) and Q [Grain Cd] = 0.913Q [Cultivar]-0.192Q [pH]+0.071Q [SOC] (R 2  = 0.782, P = 1.00 × 10 -37 ), respectively. Cultivar selection contributed stronger than soil management to decrease the As and Cd levels in maize grains. A feasible method to seek for a more efficient strategy was proposed for the safe utilization of contaminated soil., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Bioavailability and speciation of arsenic (As) are impacted by fertilization and bacteria in the rice rhizosphere. In this study, we investigated the effects of long-term manure application on As bioavailability, microbial community structure, and functional genes in a rice paddy field. The results showed that manure application did not affect total As in the soil but increased soluble As forms by 19%, increasing arsenite (As(III)) accumulation in rice grains and roots by 34 and 64% compared to a control. A real-time quantitative polymerase chain reaction (qPCR) and high-throughput sequencing analysis demonstrated that manure application increased the relative abundance of Rhizobium , Burkholderia , Sphingobium , and Sphingomonas containing arsenate reductase genes ( arsC ) in the rhizosphere soil, consistent with the 529% increase in arsC , which may have promoted arsenate (As(V)) reduction and increased As availability in pore water. In addition, manure application significantly altered the iron (Fe)-plaque microbial community structure and diversity. The microbes, particularly, Bradyrhizobium , Burkholderia , and Ralstonia , were mostly associated with As, Fe, and sulfur (S) cycles. This result was consistent with changes in the functional genes related to As, Fe, and S transformation. Although manure application promoted As(V) reduction ( arsC) in Fe-plaque by 682%, it inhibited Fe and S reduction by decreasing Fe III reduction bacteria (Geobacteraceae) and the sulfate-reducing gene ( dsrA ) abundance. Further, manure application changed the composition of the microbial community that contained the arsC gene. In short, caution needs to be excised even in the soil with a low As concentration as manure application increased As(III) accumulation in rice grains.

During the aging process, ferrihydrite was transformed into mineral mixtures composed of different proportions of ferrihydrite, goethite, lepidocrocite and hematite. Such a transformation may affect the fixed ability of arsenic. In this study, the stability of Fe-As composites formed with As(V) and the minerals aged for 0, 1, 4, 10 and 30 days of ferrihydrite were systematically examined, and the effects of molar of ratios Fe/As were also clarified using kinetic methods combined with multiple spectroscopic techniques. The results indicated that As(V) was rapidly adsorbed on minerals during the initial polymerization process, which delayed both the ferrihydrite conversion and the hematite formation. When the Fe/As molar ratio was 1.875 and 5.66, the As(V) adsorbed by ferrihydrite began to release after 6 hr and 12 hr, respectively. The corresponding release amounts of As(V) were 0.55 g/L and 0.07 g/L, and the adsorption rates were 92.43% and 97.50% at 60 days, respectively. However, the As(V) adsorbed by the transformation products aged for 30 days of ferrihydrite began to release after adsorbed 30 days. The corresponding release amounts of As(V) were 0.25 g/L and 0.03 g/L, and the adsorption rates were 84.23% and 92.18% after adsorbed 60 days, for the Fe/As=1.875 and 5.66, respectively. Overall, the combination of As(V) with ferrihydrite and aged products transformed from a thermodynamically metastable phase to a dynamically stable state within a certain duration. Moreover, the aging process of ferrihydrite reduced the sorption ability of arsenate by iron (hydr)oxide but enhanced the stability of the Fe-As composites., (Copyright © 2020. Published by Elsevier B.V.)

Microbes play a crucial role in arsenic (As) biogeochemical cycling and show great potential for environmental detoxification and bioremediation. Efflux, transformation, and compartmentalization are key processes in microbial As resistance. However, organelle specific As detoxification and fate during intracellular transfer and compartmentalization is not well understood. We conducted a time course experiment (2-5 days) of the organelle separation for fungal strains to explore subcellular As distributions. After exposure to 10 mg L -1 of arsenate (As(V)), the As accumulation among fungal organelles was generally in the order of extracellular (65 %) > cell wall (15 %) > vacuole (10 %) > other organelles (8 %). The vacuole As accounted for 55 % of the protoplast As. Extracellular bonding and vacuole compartmentalization were the main mechanisms of As resistance in the fungal strains tested. Glutathione (GSH) increases in fungal protoplast in response to As toxicity, acting as a reasonable indicator of As tolerance. Fourier transform infrared (FT-IR) spectroscopy indicated that carboxyl and amines groups within fungal cell walls potentially bind with As preventing As influx. Further analysis using scanning transmission X-ray microscopy (STXM) identified that fungal septa besides vacuole could also immobilize As., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The interplay between rice roots and manuring with respect to arsenic speciation, subsequent assimilation into roots, and translocation to shoots in paddy soil was investigated, alongside bacterial diversity characterization. Planting increased soil Eh and decreased soil solution arsenic species: inorganic arsenic, monomethylarsonic acid, trimethylarsenic oxide, and dimethylarsinic acid. Presence of plant roots increased the copy number of Clostridium and Tumebacillus 16S rRNA as well as Streptomyces arsenic methylating gene ( arsM), but decreased Acidobacteria_GP1 16S rRNA and Rhodopseudomonas. palustris BisB5 arsM. Sum of arsenic species decreased under root influence due to the interplay of inorganic arsenic mobilization in bulk soil under anaerobic and immobilization under oxygenated rhizospheric conditions. Manuring increased all soil solution arsenic species (>90%), shoot total arsenic (60%), copy number of Geobacter 16S rRNA, and R. palustris TIE-1 arsM, indicative of a shift towards microbes with iron reduction and oxidation as well as arsenic methylation capabilities.

Arsenic (As) accumulation in agricultural soils is prone to crop uptake, posing risk to human health. Passivation shows potential to inactivate soil labile As and lower crop As uptake but often contributes little to improving the microbiota in As-contaminated soils. Here, the combined addition of ferrihydrite and Trichoderma asperellum SM-12F1 as a potential future application for remediation of As-contaminated soil was studied via pot experiments. The results indicated that, compared with the control treatment, the combined addition of ferrihydrite and T. asperellum SM-12F1 significantly increased water spinach shoot and root biomass by 134 and 138%, respectively, and lowered As content in shoot and root by 37 and 34%, respectively. Soil available As decreased by 40% after the combined addition. The variances in soil pH and As fractionation and speciation were responsible for the changes in soil As availability. Importantly, the combined addition greatly increased the total phospholipid fatty acids (PLFAs) and gram-positive (G+), gram-negative (G-), actinobacterial, bacterial, fungal PLFAs by 114, 68, 276, 292, 133, and 626%, respectively, compared with the control treatment. Correspondingly, the soil enzyme activities closely associated with carbon, nitrogen, and phosphorus mineralization and antioxidant activity were improved. The combination of ferrihydrite and T. asperellum SM-12F1 in soils did not reduce their independent effects.

This study investigated arsenic (As) availability and uptake by rape (Brassica campestris L.) during two harvest periods of carboxymethyl cellulose (CMC)-stabilized ferrihydrite (HFO) nanoparticles for in situ treatment As-contaminated soil. Application of modified HFO nanoparticles in soils not only provided a larger specific surface area but also markedly improved stability against aggregation and recrystallization. For 90-day incubation, bare HFO particles were gradually converted to the crystalline Fe(III) oxide form, although this was not observed for the 0.5% CMC-HFO nanoparticles. CMC-modified HFO nanoparticles could be more effective in lowering the As uptake by rape and available As in soils than bare HFO particles. Compared the control without amendments, As contents in rape and available As in soils decreased 69.7 and 59.0%, respectively, during the second harvest when soils were amended with 0.5% HFO nanoparticles. And the soil-solution distribution coefficient (K d ) increased by 2.6 and 2.8 times for the first and second harvest. Furthermore, the ratio of amorphous and free Fe-oxides (Fe o /Fe d ) showed significant negative linear correlations with As plant (P < 0.01), available As (P < 0.05), and nonspecifically sorbed As in soil (P < 0.01). In contrast, Fe o /Fe d was positively correlated with K d and amorphous crystalline Fe/Al oxide-sorbed As, which suggests that a larger amount of As is associated with Fe(hydr)oxide in the amorphous phase or smaller particles.

The mobility of arsenic (As) in soil depends on its sorption/desorption processes on soil particles. Plant uptake locally lowers As concentration in soil pore water, which would trigger resupplies of As from soil solid phase. To better understand the fate of As in soil system after its inputs into soil and its subsequent dynamic processes, diffusive gradients in thin films (DGT) technique along with DGT-induced fluxes in soils (DIFS) model were introduced to study the kinetic information of As in soils, including its response time (T C ) and resupply rate constant (k -1 ). To achieve a series of soils with gradient As level, two different types of soils with similar As level (total As in soil JL is 7.4 mg kg -1 , while in soil BJ is 6.5 mg kg -1 ) were collected and amended with exogenous As. Then, DGT deployments were carried out following a period of 90-day soil incubation. The simulated T C values in non-amended soil JL and soil BJ were 0.036 and 0.001 s -1 , respectively. The difference may due to the properties of these two soils, including pH values and contents of adsorption materials, such as Fe and Al compounds. After As inputs into soils, the intrinsic rate of As release from the solid phase to the solution phase in As-amended JL soil was much higher than that in non-amended soil. While for soil BJ, a decreasing trend was observed after As spiking. The redistribution of As may responsible for the different variation trends of As kinetics in these two soils after As spiking. The results indicated that the distribution coefficient of As (K d ) in soil was mainly affected by soil Olsen-P content due to an ubiquitous competition between P and As on soil particles.

The effect of exogenous phosphate (P, 200 mg⋅kg -1 soil) on the lability and phytoavailability of arsenic (As) was studied using the diffusive gradients in thin films (DGT) technique. Lettuce were grown on the As-amended soils following the stabilization of soil labile As after 90 days incubation. Phosphate (P) application generally facilitated plant growth except one grown on P-sufficient soil. Soil labile As concentration increased in all the soils after P application due to a competition effect. Plant As concentration increased in red soils collected from Hunan Province, while decreases were observed in the other soils. Even though, an overall trend of decrease was obtained in As phytoavailability along with the increase of DGT-measured soil labile P/As molar ratio. The functional equation between P/As and As phytoavailability provided a critical value of 1.7, which could be used as a guidance for rational P fertilization, thus avoiding overfertilization., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Although specific soil properties controlling the arsenic (As) aging process have been studied extensively, few investigations have attempted to determine how soil types influence As bioavailability and fractionations in soils. Nine types of soil were selected from typical grain producing areas in China, and the bioavailability and fractionations of As during aging were measured. Results showed that available As in all soils rapidly decreased in the first 30 days and slowly declined thereafter. In spiked soils, As easily became less available and less toxic in low pH soils compared to high pH soils, demonstrating the importance of soil pH on As availability. Results from fitting kinetic equations revealed that the pseudo-second-order model described the As aging processes well in all soils (R 2  = 0.945-0.999, P < 0.01, SE = 0.09-4.25), implying that the mechanism for As aging combined adsorption, external diffusion, and internal diffusion. Fe oxides were more important than Al oxides for determining the As aging rate (|k|). Based on these results, we are the first to propose the approximate aging equilibrium time (T) for As, which was mainly influenced by soil clay content. The shortest time for approximate stabilization of As aging was 28 d in latosol soils (LS), while the longest approximate equilibrium time was 169 d in cinnamon soils (CS). Individual soil properties controlling the variation in different As fractionations further confirmed that the influences of soil types on As aging were the result of the combined effects of soil properties and a time-consuming redistribution process., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Microbial methylation and demethylation are central to arsenic's (As) biogeochemical cycling. Here, the transformations of monomethylarsonic acid (MMA(V)) (50 mg L -1 ) for 15 days in cells of As-methylating fungi, Fusarium oxysporum CZ-8F1, Penicillium janthinellum SM-12F4, and Trichoderma asperellum SM-12F1, were evaluated, and trace concentrations of As(III) and As(V) were observed in fungal cell extracts. Trace amounts of DMA(V) were also detected in MMA(V) and P. janthinellum SM-12F4 incubations. In situ X-ray absorption near edge structure (XANES) indicated that after exposure to MMA(V) (500 mg L -1 ) for 15 days, 28.6-48.6% of accumulated As in fungal cells was DMA(V), followed by 18.4-30.3% from As(V), 0-28.1% from As(III), and 4.8-28.9% from MMA(V). The concurrent methylation and demethylation of As occurs in fungal cells. Furthermore, a majority of proteins involved in metabolism, transport, ATP activity, biosynthesis, signal transduction, DNA activity, translation, and oxidative stress were upregulated in T. asperellum SM-12F1 cells after MMA(V) exposure compared to As(III), As(V), and DMA(V). The detoxification process of T. asperellum SM-12F1 was As species-specific. Methylenetetrahydrofolate reductase (R7YMH0) donation of a methyl group for S-adenosylmethionine (SAM) generation significantly increased following MMA(V) exposure., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Arsenic (As)-contaminated agricultural soils threaten crop yields and pose a human health risk. Augmentation of exogenous microorganisms exhibiting plant-growth promoting and As speciation changing shows potential to improve crop growth and change soil As availability. Trichoderma asperellum SM-12F1 exhibiting both traits was developed into chlamydospores to improve its persistence in contaminated soils. After inoculation, As availability and enzyme activity in two types of soils and the growth as well as As uptake of water spinach (Ipomoea aquatic Forsk.) were investigated. The results indicated that inoculation significantly improved water spinach growth in both soils. Inoculating chlamydospores at 5% significantly increased As concentration (139%), bioconcentration factor (150%), and translocation factor (150%) in water spinach grown in Chenzhou (CZ) soils, while no significant change for these in Shimen (SM) soils. Inoculating chlamydospores at 5% caused a significant increase (16%) of available As content in CZ soils, while a significant decrease (13%) in SM soils. Inoculation significantly caused As methylation in both soils, while significant As reduction merely observed in CZ soils. The differential changes in available As contents in both soils were attributed to the soil pH, As fractionations and speciation characteristics. Furthermore, Inoculating chlamydospores at 5% significantly improved the activities of β-glucosidase (155%), chitinase (211%), and phosphatase (108%) in SM soils, while significant decreases in β-glucosidase (81%), phosphatase (54%), aminopeptidase (60%), and catalase (67%) in CZ soils. Bioaugmentation and As availability change were responsible for this result. These observations will be helpful for the application of fungal chlamydospores in the future bioremediation., (Copyright © 2017. Published by Elsevier Ltd.)

The effects of aging time and soil parent materials on the bioavailability and fractionations of arsenic (As) in five red soils were studied. The results indicated that As bioavailability in all soils decreased during aging, especially with a sharp decline occurring in the first 30 days. After aging for 360 days, the highest available As concentration, which accounted for 12.3% of the total, was observed in soils derived from purple sandy shale. While 2.67% was the lowest proportion of the available As in soils derived from quaternary red clay. Furthermore, the best fit of the available As changing with aging time was obtained using the pseudo-second-order model (R(2) = 0.939-0.998, P < 0.05). Notably, Al oxides played a more crucial role (R(2) = 0.89, P＜0.05) than did Fe oxides in controlling the rate of As aging. The non-specially and specially absorbed As constituted the primary forms of available As., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Arsenic (As) biomethylation is increasingly being regarded as a promising method to volatize As from the environment; however, the As volatilization efficiency of most microorganisms is low. Here, the speciation transformation of dimethylarsinic acid (DMA) as an important methylation intermediate in the cells of Fusarium oxysporum CZ-8F1, Penicillium janthinellum SM-12F4, and Trichoderma asperellum SM-12F1 were investigated. These fungal strains have been certified to volatilize As from As-loaded environment. In situ X-ray absorption near edge structure (XANES) indicated that demethylation of DMA with methylarsonic acid (MMA), arsenate [As(V)], and arsenite [As(III)] as intermediates or products occurred in fungal cells after exposure to DMA for 15 days. 36.7-55.7% of the original DMA could lose one or two methyl groups and be changed into MMA or inorganic As. Chromatographic separation of the cell lysates also supported these findings. Thus it comes that demethylation might be a remarkable internal factor limiting As volatilization efficiency., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

The investigation of arsenic (As) re-accumulation in an area previously remediated by soil dressing will help in sustainable controlling the risks of As to local ecosystems and should influence management decisions about remediation strategies. In this study, As content in an area remediated by soil dressing and the possible As accumulation risk in agricultural products were investigated. The results indicated that after 7 years of agricultural activities, the average As content (24.6 mg kg(-1)) in surface soil of the investigated area increased by 83.6% compared with that (13.4 mg kg(-1)) in clean soil. Of the surface soil samples (n = 88), 21.6% had As levels that exceeded the limits of the Environmental Quality Standard for Soils of China (GB 15618-1995) and 98.9% of the surface soil samples with As contents exceeding that in clean soil was observed. Soil dressing might be not a remediation method once and for all in some contaminated areas, even though no significant difference in available As content was found between clean (0.18 mg kg(-1)) and surface (0.22 mg kg(-1)) soils. The foreign As in surface soil of the investigated area mainly specifically sorbed with soil colloid or associated with hydrous oxides of Fe and Al, or existed in residual fraction. The upward movement of contaminated soil from the deeper layers and the atmospheric deposition of slag particles might be responsible for the re-accumulation of As in the investigated area. Decreases in soil pH in the investigated soils and the fact that no plant samples had As levels exceeding the limits of the National Food Safety Standards for Contaminants of China (GB 2762-2012) were also observed.

Microorganisms dominated speciation of arsenic (As) play an important role in the biogeochemical cycling of As. In the study, species transformation of arsenite [As(III)] and As(III) influx and efflux across the cell membranes of Trichoderma asperellum SM-12F1, Penicillium janthinellum SM-12F4, and Fusarium oxysporum CZ-8F1 cells were studied using a cellular lysis plus chromatographic separation method and further the in-situ X-ray absorption near edge structure (XANES) analysis. The results indicated that As(III) can enter into fungal cells and that a portion of the As(III) can be exuded out of cells. For both As sequestrated into fungal cytoplasm and As adsorbtion onto cell walls, As(III) was found to be the dominated form of As. XANES analysis showed that As(III) accounted for 58.4%, 59.5%, and 73.0% of the total As in the cells of T. asperellum SM-12F1, P. janthinellum SM-12F4, and F. oxysporum CZ-8F1, respectively. Among these fungal strains, however, there were obvious differences in the relative proportions of arsenate [As(V)], monomethylarsonic acid (MMA), and dimethylarsinic acid (DMA). For T. asperellum SM-12F1, the proportion (%) of MMA was 31.1%, and no As(V) or DMA was detected. For F. oxysporum CZ-8F1, the proportions of As(V) and MMA were 15.8% and 8.8%, respectively, but no DMA was observed. As(V), MMA, and DMA accounted for 4.2%, 29.5%, and 8.1%, respectively, of the P. janthinellum SM-12F4 cells. Some of the intracellular As(III) can be oxidated and methylated by these fungal strains and yield As(V), MMA, and DMA as products., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

This study was conducted to evaluate the feasibility and accuracy of available arsenic (As) measurement in soils using the diffusive gradients in thin films (DGT) technique and to shed further light on the risk evaluation of As-contaminated soils. A bio-indicator method was used to evaluate the dependence of As concentrations in plant tissues (edible rape) on the As concentrations measured by other methods. DGT, extraction of soil pore water, 0.5 M NaHCO3, 1 M HCl, 1 M NH4Cl, 0.5 M NH4F, 0.1 M NaOH and 0.25 M H2SO4 extraction methods, as well as total As content in the soil were employed in this study. Edible rape was grown in 43 soils containing different concentrations of As. A better correlation was obtained between the As concentrations in plant tissues and the As concentrations measured by DGT than between plant concentrations and those measured by other methods. The coefficients of determination (R(2)) before and after growth of edible rape were 0.84 and 0.83, respectively. Levels of As in plants and in soil pore water were also closely related: the coefficients of determination (R(2)) between these variables before and after growth of edible rape were 0.83 and 0.81, respectively. Measurements from the chemical extraction methods were inferior to those from DGT and soil pore water. Levels of As in plants were more scattered and nonlinear with respect to total soil As content (R(2) = 0.27). The As measured by DGT closely reflected the pool of plant-adsorbed As. From the data obtained in this study, we can conclude that DGT is an effective technique for the prediction and assessment of As bioavailability in soils.

Bioremediation of arsenic (As)-contaminated soil using microorganisms has been a focus of research because it is environment friendly and cost-effective. The As-resistant fungi Trichoderma asperellum SM-12F1, Penicillium janthinellum SM-12F4, and Fusarium oxysporum CZ-8F1 were exposed to 50 mg l(-1) of As(V), and the biotransformation of As and the concomitant variance of Eh and pH in the media were studied. Fresh weights of all three isolates increased and then decreased depended on cultivation period. After cultivation for 2 or 3 days, the As(V) added to the media had been completely changed into As(III), whilst As(V) was predominate in fungal cells with concomitantly little As(III) during cultivation. After 15 days, little monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) besides of As(V) and As(III) were found in the cells of T. asperellum SM-12F1, and the total As content was the highest in cells of P. janthinellum SM-12F4 (about 41.5 μg) according to the quantitative analysis of As speciation in cultures. Moreover, when cultivation period reached 3 days, the Eh and pH in the media of T. asperellum SM-12F1 (312.5 mV and 4.8), P. janthinellum SM-12F4 (411.1 mV and 4.2), and F. oxysporum CZ-8F1 (269.4 mV and 4.8) might not responsible for the reduction of As(V) based on the previous study. Therefore, it is speculated that import/export, reduction, and methylation of As are conducted in fungal cells. Future studies investigating the biochemical behaviour of fungi responding to As are needed to gain a better understanding of bioremediation of As-contaminated soils., (Copyright © 2011. Published by Elsevier B.V.)

Some fungi are able to control and remediate arsenic (As)-contaminated soil, sediment, or water. Here, we investigate potential accumulation and volatilisation of As by three fungi strains. Results indicated that the highest level of As was accumulated by Penicillin janthinellum with 39.54 μg after 10 days in the culture system amended with 2,500 μg As(V), which represents 50 mg/l As. Fusarium oxysporum showed the highest amount of volatilised As with 304.06 μg after 15 days. The As content in the treated system (filter paper + As + fungi) was significantly higher than that in the control (filter paper + As; filter paper + fungi; filter paper). Trichoderma asperellum and F. oxysporum showed superior abilities for the absorption of extracellular As and accumulation of intracellular As, which accounted for 82.2 and 63.4% of the total accumulated As, respectively. However, P. janthinellum presented an equal distribution of intracellular and extracellular As. Scanning electron microscope (SEM) analysis suggested that little impact on mycelium growth of the three fungal strains was seen after exposure to 50 mg/l As(V) for 5 days, while the growth of fungi in the control was inhibited. The present results demonstrate that P. janthinellum, F. oxysporum, and T. asperellum would be expected to tackle As-contaminated environments.

The mobility of arsenic (As) in soil depends on its sorption/desorption processes on soil particles. Plant uptake locally lowers As concentration in soil pore water, which would trigger resupplies of As from soil solid phase. To better understand the fate of As in soil system after its inputs into soil and its subsequent dynamic processes, diffusive gradients in thin films (DGT) technique along with DGT-induced fluxes in soils (DIFS) model were introduced to study the kinetic information of As in soils, including its response time (TC) and resupply rate constant (k−1). To achieve a series of soils with gradient As level, two different types of soils with similar As level (total As in soil JL is 7.4 mg kg−1, while in soil BJ is 6.5 mg kg−1) were collected and amended with exogenous As. Then, DGT deployments were carried out following a period of 90-day soil incubation. The simulated TC values in non-amended soil JL and soil BJ were 0.036 and 0.001 s−1, respectively. The difference may due to the properties of these two soils, including pH values and contents of adsorption materials, such as Fe and Al compounds. After As inputs into soils, the intrinsic rate of As release from the solid phase to the solution phase in As-amended JL soil was much higher than that in non-amended soil. While for soil BJ, a decreasing trend was observed after As spiking. The redistribution of As may responsible for the different variation trends of As kinetics in these two soils after As spiking. The results indicated that the distribution coefficient of As (Kd) in soil was mainly affected by soil Olsen-P content due to an ubiquitous competition between P and As on soil particles. [ABSTRACT FROM AUTHOR]

The composition of organic carbon molecules (OCMs) impacts microbial diversity and in turn regulates the microbial transformation of elements in soils. However, the potential association of OCMs characteristics with arsenic methylation mediated by arsenite S-adenosylmethionine (SAM) methyltransferase gene (arsM)-harboring microbes in paddy soils remains unclear. Here, we investigated the impacts of OCMs on microbial methylators and arsenic methylation by identifying OCMs with Fourier transform ion cyclotron resonance mass spectrometry. The results showed that arsenic methylation efficiency increased by 57.4–260.1% in pig manure- or rice straw-treated paddy soils compared to the control. Molecules containing carbon, hydrogen, oxygen, and nitrogen (CHONs) with the atomic oxygen to carbon ratio (O/C) ranging from 0.5 to 0.8 had a more significant correlation (p < 0.01) with the abundance of the arsM gene than with that of 16S rRNA. This was confirmed by a significant positive correlation (p < 0.001) between consumed CHONs with O/C of 0.5–0.8 and increased methylated arsenic. CHONs (with O/C of 0.5–0.8) increased the intracellular SAM concentration and methyltransferase activity, stimulating arsenic methylation in vivo. The specialization of arsM -harboring taxa in CHONs with different O/C were identified. These observations provide a basis for predicting the risk of straight-head disease in paddy soils. [Display omitted] • Microbial arsenic methylation in paddy soils is regulated by OCMs. • Priming effects of CHONs with O/C of 0.5–0.8 on arsenic methylation was found. • Increased SAM concentration and ArsM activity in methylators stimulate arsenic methylation. • Specialization of arsM -harboring taxa in CHONs with different O/C were identified. [ABSTRACT FROM AUTHOR]

Ferrihydrite has been prevalently used in soil remediation as an effective amendment for in situ immobilization of arsenic (As). However, its poorly crystalline structure is unstable and can pose the risk of re-releasing As into soil. In this study, sequential extraction was coupled with a newly-established method, chelex-metsorb diffusive gradient in thin films (DGT), to study the ferrihydrite transformation/dissolution process and its effect on the mobility of arsenic in soil. Experiments in this work found that high soil moisture (70% SWHC) with a low soil redox potential (Eh) can significantly increase the rate of the ferrihydrite transformation/dissolution process compared to 30% SWHC. Soils with low pH and high available iron (Fe) content may also accelerate ferrihydrite transformation/dissolution, while soils with high clay fraction and high soil total organic matter (STOM) may inhibit the process. The amount of arsenic adsorption can also affect ferrihydrite transformation, even exceeding the effect of soil pH and dissolved Fe. Arsenic release was clearly observed in all three soils across all treatments, and it was also affected by changes in soil redox potential. More arsenic was released at high soil moisture (70% SWHC) roughly 7–15 d after the release of Fe. In addition, a partial arsenic fraction was transformed, along with ferrihydrite, from a combined As (F1 As) amorphous phase to combined As (F2 As) well-crystallized phase. These results suggested that ferrihydrite transformation/dissolution can affect the mobility of arsenic and that this phenomenon is more extreme at higher soil moisture levels. Unlabelled Image • Soil redox condition significantly regulated the ferrihydrite transformation/dissolution process. • Ferrihydrite transformation rate in soils was also controlled by the soil properties and amount of arsenic adsorption. • Ferrihydrite transformation/dissolution may lead to a short time release of the labile As. • Labile As release had a strong correlation with Fe(II) release. • Arsenic fractionation was also transformed with transformation of ferrihydite. [ABSTRACT FROM AUTHOR]

Ferrihydrite is commonly used as an amendment to remove or stabilize arsenic (As) in water, soils, and sediments due to its high specific surface area (>200 m 2 /g). However, its instability under oxic or anoxic conditions sometimes limits its efficiency for arsenic adsorption. This study employed a two-step sequential extraction method and the diffusive gradients in thin films (DGT) technique to investigate the effect of soil properties and water management on ferrihydrite transformation in soils. The results of sequential extraction and DGT indicated that ferrihydrite transformation and dissolution occurred within all treatments in three red soils derived from plate shale (PS), sandstone (SS) and quaternary red clay (QR). Ferrihydrite transformation and dissolution rate under 30% soil water holding capacity (SWHC) treatments were the fastest in PS followed by SS and QR, while in 70% SWHC treatments the order was SS > PS > QR. Ferrihydrite in 70% SWHC was transformed about 2–7 times faster than in 30% SWHC for the various treatments. The soil pH may be the main factor controlling ferrihydrite transformation in 30% SHWC, and in 70% SHWC it may work synergistically with Fe(II) as a result of continuous, partial anaerobic conditions formed after about 22 days. Variation among the three soils was controlled by different amounts of soil total organic matter, pH, available Fe, and the content of clay fraction (particulate size <0.02 mm). In the present study, SWHC was expected to be the direct factor controlling ferrihydrite transformation and dissolution process. However, the factor controlling these processes was actually variation in soil moisture, which promotes the dissolution of H + solutes and formation of Fe(II) in partial anaerobic conditions. The results obtained in this study have demonstrated the potential of using ferrihydrite on contaminated farmland as an amendment for arsenic stabilization. [ABSTRACT FROM AUTHOR]

Agricultural planning via substitution with crops of low arsenic (As) accumulation can efficiently reduce the dietary exposure from As to human health through food chain. In this study, in total 157 crop cultivars, including rice, vegetable and maize, were used and their risk of As accumulation in edible parts was studied via pot and field experiments. When soil As level was 56.7 mg kg−1, equivalent to a level of slight contamination, inorganic As (iAs) in rice grain ranged from 0.44 to 0.61 mg kg−1 Fresh weight (FW), which was 2.2–3.0 times higher than its limit in China (0.2 mg kg−1). When soil As enhanced to 156.2 mg kg−1 (moderate contamination), mater convolvulus and crown daisy had the over-limit risk of As accumulation in their edible parts. Green tender, pakchoi cabbage, rape and amaranth emerged with the over-limit risk of As, when soil As reached 238.3 mg kg−1 (severe contamination). Comparatively, maize and tuber or fruit vegetables had the lower levels of As in their edible parts and sweet potato, peanut, pepper and potato had less variation of As accumulation among cultivars of the same crop. A recommended list was provided for preferable food crops under different contamination level of As in soils. Switching paddy soil into upland soils and planting in priority with tuber or fruit vegetables and maize are encouraged. Agricultural planning resulted in significant decreases for the target hazard quotient (THQ) and target cancer risk (TCR) of iAs via dietary exposure to the residents near the realgar mine in Shimen County of China. [Display omitted] • 7-year field experiments were conducted by employing 157 cultivars or varieties of food crops. • Arsenic accumulation in food crops under different degree of soil contamination was investigated. • Food crops in priority with tuber or fruit vegetables and maize was strongly suggested. • Agricultural planning resulted in significant decrease in human dietary exposure to As. [ABSTRACT FROM AUTHOR]

• DOM characteristics influence As methylation and biovolatilization in paddy soils. • Arsine production is markedly enhanced following pig manure DOM application. • Among DOMs, Pig manure DOM addition yields much higher As-methylation in paddy soil. • Pig manure DOM addition increases the OTU number of arsM -carrying bacteria. • High labile C amount is responsible for the enhanced As-methylation in paddy soils. The effect of dissolved organic matter (DOM), derived from composted pig manure or rice straw, on arsenic methylation and subsequent biovolatilization in paddy soils was investigated. Arsine production following pig manure DOM application was 2.7- and 9.6-fold higher than that of soils treated with rice straw DOM and the control, respectively. Trimethylarsine was the dominant arsine at 54 %, followed by dimethylarsine at 22 %, arsine at 21 %, and monomethylarsine at 3 %. The copy numbers of the total and As-methylating bacteria were significantly enhanced in paddy soils treated with DOM. Pig manure DOM altered soil bacterial profile by increasing the OTU number of As methylation-inducing bacteria, such as Proteobacteria, Bacteroidetes, Geobacter , Sphingomonas , Streptomyces , and Rhodopseudomonas , thereby promoting As volatilization and methylation in paddy soils. The higher relative content of alkyl-C, N-alkyl C, and carboxyl-C in pig manure DOM was responsible for the increase in total and arsM -carrying bacteria in paddy soils, leading to enhanced As methylation. These observations will promote a better understanding of the role of DOM in mediating As methylation and volatilization, along with how organic fertilization affects straighthead disorder of rice, a condition caused by methylated arsenic species. [ABSTRACT FROM AUTHOR]

• Arsenic is electrochemically adsorbed by birnessite from mining wastewater. • Electrochemical redox reactions of birnessite contribute to arsenic adsorption. • H 2 O 2 formation and high potential of birnessite anode facilitate As(III) oxidation. • Mn2+ in wastewater can also be effectively removed during arsenic adsorption. • Cell voltage reversal facilitates the continuous use of birnessite electrode. Manganese oxides have been extensively investigated for arsenic (As) adsorption from aqueous solution. However, the effect of electrochemical redox reactions on the adsorption performance and underlying mechanism remain elusive. Herein, birnessite was used for electrochemical adsorption of As from mining wastewater at a constant cell voltage, and the effect of cell voltage and the continuous use (without desorption) performance of birnessite electrode were also evaluated. At 1.2 V for 24 h, the concentrations of total As (As(T)) and As(III) in wastewater decreased from 3808.7 to 73.7 μg L−1 and 682.8 to 21.4 μg L−1, respectively. The As(T) removal ratio increased with increasing cell voltage and reached 98.1% at 1.2 V, which was higher than that at open circuit (84.1%). The Mn2+ concentration also significantly decreased in wastewater during As adsorption. The high potential of birnessite anode and the generation of H 2 O 2 on cathode facilitated As(III) oxidation, and the electrochemical redox reactions of birnessite contributed to the enhancement of As(T) removal. The application of cell voltage reversal could improve the utilization rate of birnessite electrodes by dissolution-recrystallization during continuous use, and the As(T) removal ratio was increased from 73.5% to 85.1% after five cycles of voltage alteration. The present work indicates that birnessite is a promising absorbent for the electrochemical adsorption of As from real wastewaters. [ABSTRACT FROM AUTHOR]