Atmospheric stability of arsine and methylarsines

Seasonal variation of arsenic in PM10 and PMx in an urban park: The influence of vegetation-related biomethylation on the distribution of its organic species and air quality

Arsenic (As) levels in particulate matter (PM) are routinely monitored in cities of developed countries. Despite advances in the knowledge of its inorganic species in PM in urban areas, organic species are often overlooked with no information on their behaviour in urban parks - areas with increased potential for As biomethylation. Therefore, the aim of this study was to characterize As distribution, bioaccessibility, seasonal variation and speciation (AsIII, AsV, MMA, DMA and TMAO) in PMx-PM10 of an urban park. Two sites with different distance from the road were selected for winter and summer sampling. From the PM samples, we gravimetrically determined PM10 concentrations in the air and via ICP-MS the total As content there. To assess the portion of bioaccessible As, water extractable As content was analysed. Simultaneously, the As species in PM10 water extracts were analysed via coupling of HPLC with ICP-MS method. There was no seasonal difference in PM10 concentration in the park, probably due to the increased summer PM load related to recreational activities in the park and park design. Spatial distribution of total As in PM10 and As fractional distribution in PMx suggested that As mostly didn't originate from traffic although highest As content was observed in the fine fraction (PM2.5) related to combustion processes. However, significant winter increase of As (determined by AsIII and AsV) despite the unchanged concentration of PM10 indicated a decisive influence of household heating-related combustion and possibly influence of reduced vegetation density. As present in the PM10 was mostly in bioaccessible form. Seasonal influence of As biomethylation was clearly demonstrated on the TMAO specie during the summer campaign. Except the significant summer TMAO increase, the results also indicated the biomethylation influence on DMA. Therefore, an increased risk of exposure to organic As species in urban parks can be expected during summer.

Contaminant containment for sustainable remediation of persistent contaminants in soil and groundwater

Contaminant containment measures are often necessary to prevent or minimize offsite movement of contaminated materials for disposal or other purposes when they can be buried or left in place due to extensive subsurface contamination. These measures can include physical, chemical, and biological technologies such as impermeable and permeable barriers, stabilization and solidification, and phytostabilization. Contaminant containment is advantageous because it can stop contaminant plumes from migrating further and allow for pollutant reduction at sites where the source is inaccessible or cannot be removed. Moreover, unlike other options, contaminant containment measures do not require the excavation of contaminated substrates. However, contaminant containment measures require regular inspections to monitor for contaminant mobilization and migration. This review critically evaluates the sources of persistent contaminants, the different approaches to contaminant remediation, and the various physical-chemical-biological processes of contaminant containment. Additionally, the review provides case studies of contaminant containment operations under real or simulated field conditions. In summary, contaminant containment measures are essential for preventing further contamination and reducing risks to public health and the environment. While periodic monitoring is necessary, the benefits of contaminant containment make it a valuable remediation option when other methods are not feasible.

Increasing temperature and flooding enhance arsenic release and biotransformations in Swiss soils

Reductive dissolution is one of the main causes for arsenic (As) mobilisation in flooded soils while biomethylation and biovolatilisation are two microbial mechanisms that greatly influence the mobility and toxicity of As. Climate change results in more extreme weather events such as flooding and higher temperatures, potentially leading to an increase in As release and biotransformations. Here, we investigated the effects of flooding and temperature on As release, biomethylation and biovolatilisation from As-rich soils with different pH and source of As (one acidic and anthropogenic (Salanfe) and one neutral and geogenic (Liesberg)). Flooded soils incubated at 23 °C for two weeks showed a ~ 3-fold (Liesberg site) and ~ 7-fold (Salanfe site) increase in the total As concentration of soil solution compared to those incubated at 18 °C. Methyl- and thio-As species were found in the acidic soil and soil solution. High temperatures enhanced thiolation and methylation although inorganic As was predominant. We also show that volatile As fluxes increased more than 4-fold between treatments, from 18 ± 5 ng/kg/d at 18 °C to 75 ± 6 ng/kg/d at 23 °C from Salanfe soil. Our results suggest that high As soils with acidic pH can become an important source of As to the surrounding environment according to realistic climatic scenarios, and that biovolatilisation is very sensitive to increases in temperature. This study provides new data and further justifies further investigations into climate-induced changes on As release and speciation and its links to important factors such as microbial ecology and sulfate or iron biogeochemistry. SYNOPSIS: In the studied Swiss soils, elevated temperature increases arsenic mobility through volatilisation and methylation.

Simultaneous catalytic oxidation of elemental mercury and arsine over CeO2(111) surface: a density functional theory study

Ceria (CeO₂)-based materials are potential catalysts for the removal of the Hg⁰ and AsH₃ present in reducing atmospheres. However, theoretical studies investigating the Hg⁰ and AsH₃ removal capacity of ceria remain limited. In this study, the adsorption behavior and mechanistic pathways for the catalytic oxidation of Hg⁰ and AsH₃ on the CeO₂(111) surface, including the calculation of optimized adsorption configurations and energies, were investigated using density functional theory calculations. The results suggest that Hg⁰ and AsH₃ are favorably adsorbed on the CeO₂(111) surface, whereas CO is not, which is crucial for selective removal when CO is a desirable gas component. Furthermore, AsH₃ is adsorbed more favorably than Hg⁰. In addition, the calculations revealed that the Hg atom is initially adsorbed on the surface and then oxidized by lattice oxygen to form HgO. Concerning AsH₃ decomposition, the stepwise dehydrogenation of AsH₃ followed by bonding with lattice O atoms to form the As-O bond seems the most plausible. Finally, the adsorbed As-O bond is further forms elemental As and As₂O₃. Therefore, CeO₂ can adsorb and remove Hg⁰ and AsH₃, making it a promising catalyst for the simultaneous catalytic oxidation of Hg⁰ and AsH₃ in strongly reducing off-gas.

Effect of Fe-Mn-La-modified biochar composites on arsenic volatilization in flooded paddy soil

As can be volatilized naturally; however, this has adverse environmental effects. In this study, we investigated As volatilization in flooded paddy soil with the addition of biochar (BC) and Fe-Mn-La-modified BC composites (FMLBCs). The addition of BC and FMLBCs caused decreases in total As volatilization in the soil over 7 weeks. Maximum volatilization was achieved in the third week followed by stabilization. Volatilization decreased by 21.9%, 18.8%, 20.8%, and 31.1% with the addition of BC, FMLBC₁, FMLBC₂, and FMLBC₃ (BC/Fe/Mn/La weight ratios different), respectively, in lightly contaminated soil, and by 15.2%, 20.5%, 17.6%, and 25.4%, respectively, in highly contaminated soil. The FMLBCs decreased the exchangeable As fractions and increased the non-swappable As in the soil. Furthermore, the addition of FMLBCs significantly reduced the As(III) concentration in a suspended solution (P < 0.05), whereas no significant changes were observed in the As(V) or methyl arsenic acid concentrations. Soil enzyme activity increased and the relative abundances of Proteobacteria and Actinobacteria changed with the addition of FMLBCs. Therefore, the mechanism by which FMLBCs affected As volatilization likely included the following two aspects: (1) FMLBCs affected the transformation and distribution of soil As and decreased As dissolution, crystallization, and methylation; (2) FMLBCs influenced soil properties, which directly affected microorganism activity, thereby affecting As volatilization. FMLBCs therefore can decrease As volatilization properties and be used to control As volatilization in As-contaminated paddy soils.

Arsenic: Various species with different effects on cytochrome P450 regulation in humans

Arsenic is well-recognized as one of the most hazardous elements which is characterized by its omnipresence throughout the environment in various chemical forms. From the simple inorganic arsenite (iAsIII) and arsenate (iAsV) molecules, a multitude of more complex organic species are biologically produced through a process of metabolic transformation with biomethylation being the core of this process. Because of their differential toxicity, speciation of arsenic-based compounds is necessary for assessing health risks posed by exposure to individual species or co-exposure to several species. In this regard, exposure assessment is another pivotal factor that includes identification of the potential sources as well as routes of exposure. Identification of arsenic impact on different physiological organ systems, through understanding its behavior in the human body that leads to homeostatic derangements, is the key for developing strategies to mitigate its toxicity. Metabolic machinery is one of the sophisticated body systems targeted by arsenic. The prominent role of cytochrome P450 enzymes (CYPs) in the metabolism of both endobiotics and xenobiotics necessitates paying a great deal of attention to the possible effects of arsenic compounds on this superfamily of enzymes. Here we highlight the toxicologically relevant arsenic species with a detailed description of the different environmental sources as well as the possible routes of human exposure to these species. We also summarize the reported findings of experimental investigations evaluating the influence of various arsenicals on different members of CYP superfamily using human-based models.

The Pedosphere as a Sink, Source, and Record of Anthropogenic and Natural Arsenic Atmospheric Deposition

The Anthropocene has led to global-scale contamination of the biosphere through diffuse atmospheric dispersal of arsenic. This review considers the sources arsenic to soils and its subsequent fate, identifying key knowledge gaps. There is a particular focus on soil classification and stratigraphy, as this is central to the topic under consideration. For Europe and North America, peat core chrono-sequences record massive enhancement of arsenic depositional flux from the onset of the Industrial Revolution to the late 20th century, while modern mitigation efforts have led to a sharp decline in emissions. Recent arsenic wet and dry depositional flux measurements and modern ice core records suggest that it is South America and East Asia that are now primary global-scale polluters. Natural sources of arsenic to the atmosphere are primarily from volcanic emissions, aeolian soil dust entrainment, and microbial biomethylation. However, quantifying these natural inputs to the atmosphere, and subsequent redeposition to soils, is only starting to become better defined. The pedosphere acts as both a sink and source of deposited arsenic. Soil is highly heterogeneous in the natural arsenic already present, in the chemical and biological regulation of its mobility within soil horizons, and in interaction with climatic and geomorphological settings. Mineral soils tend to be an arsenic sink, while organic soils act as both a sink and a source. It is identified here that peatlands hold a considerable amount of Anthropocene released arsenic, and that this store can be potentially remobilized under climate change scenarios. Also, increased ambient temperature seems to cause enhanced arsine release from soils, and potentially also from the oceans, leading to enhanced rates of arsenic biogeochemical cycling through the atmosphere. With respect to agriculture, rice cultivation was identified as a particular concern in Southeast Asia due to the current high arsenic deposition rates to soil, the efficiency of arsenic assimilation by rice grain, and grain yield reduction through toxicity.

Unraveling the role of plant growth-promoting rhizobacteria in the alleviation of arsenic phytotoxicity: A review

The toxic metalloid arsenic (As), is a major pollutant of soil and water, imposing severe health concerns on human lives. It enters the food chain mainly through As-contaminated crops. The uptake, translocation and accumulation of As in plant tissue are often controlled by certain soil-inhabiting microbial communities. Among them, indigenous, free-living As-resistant plant growth-promoting rhizobacteria (PGPR) plays a pivotal role in As-immobilization. Besides, the plant's inability to withstand As after a threshold level is actively managed by these PGPR increasing As-tolerance in host plants by a synergistic plant-microbe interaction. The dual functionality of As-resistant PGPR i.e., phytostimulation and minimization of As-induced phytotoxic damages are one of the main focal points of this review article. It is known that such PGPR having the functional arsenic-resistant genes (in ars operon) including As-transporters, As-transforming genes contributed to the As accumulation and detoxification/transformation respectively. Apart from assisting in nutrient acquisition and modulating phytohormone levels, As-resistant PGPR also influences the antioxidative defense system in plants by maneuvering multiple enzymatic and non-enzymatic antioxidants. Furthermore, they are effective in reducing membrane damage and electrolyte leakage in plant cells. As-induced photosynthetic damage is also found to be salvaged by As-resistant PGPR. Briefly, the eco-physiological, biochemical and molecular mechanisms of As-resistant PGPR are thus elaborated here with regard to the As-exposed crops.

Arsenic volatilization by Aspergillus sp. and Penicillium sp. isolated from rice rhizosphere as a promising eco-safe tool for arsenic mitigation

Arsenic (As) is a non-threshold human carcinogenic. This element can be volatilized either by nature or anthropogenic sources. In the present study, the analytical performance of an As volatile species trapping system was evaluated to assess the As volatilization promoted by Penicillium sp. and Aspergillus sp., both isolated from rice rhizosphere, and Aspergillus niger sp. considered as a reference. The study was conducted for 60 days (sampling of volatile As species from 1st to 30^th day and from 31st to 60^th day). The efficiency of As-volatilization was associated with the fungal growth. The highest As volatilization occurred from 31st to 60^th day. Penicillium sp., Aspergillus sp. and A. niger were capable of producing 57.8, 46.4, and 5.2% of volatile arsenic species, respectively. The speciation analysis has shown trimethylarsine (TMAs) as the main volatilized As-form, followed by mono- and dimethylarsine (MMAs and DMAs). The results are following the "Challenger pathway". Therefore, the tested fungi isolated from rice rhizosphere have shown promising properties concerning bio-volatilization with potential use for As-mitigation in paddy soils.

Impact of eutrophication on arsenic cycling in freshwaters

Many arsenic-bearing freshwaters are facing with eutrophication and consequent algae-induced anoxia/hypoxia events. However, arsenic cycling in eutrophic waters and its impact on public health are poorly understood. Laboratory simulation experiments are performed in this study to investigate the effect of algal blooms on the cycling of arsenic in a sediment-water-air system. We found that the anoxia induced by the degradation of algal biomass promoted an acute arsenic (mostly As(III)) release within two days from sediment to both the water and atmosphere, and the release effluxes were proportional to the algae dosage. The reduction and methylation of arsenic were enhanced at the sediment-water interface, owing to the significant increase in arsenate reductase genes (arrA and arsC), and arsenite methyltransferase genes (arsM) caused by increased anoxia. The analysis of synchrotron-based X-ray absorption spectroscopy indicated that the concomitantly released natural organic matter (NOM) and sulfur (S) at the sediment-water interface reduced the As(III) release to a certain extent in the later reducing period of incubation, by forming As₂S₃ (43-51%) and As(III)-Fe-NOM (28-35%). Our results highlight the needs for the in-situ assessment of volatile arsenic in eutrophic freshwaters with its risk to human and animal health.

Arsenic speciation in aerosols of a respiratory therapeutic cave: A first approach to study arsenicals in ultrafine particles

Arsenic is ubiquitous in the environment and of special concern due to its varying toxicity depending on the chemical form present. Less is known about arsenic in air, especially about organoarsenicals, their sources and fate. There is also a lack of knowledge regarding arsenic in airborne nanoparticles that are critical for understanding with respect to human health effects due to their size. Here we show results from an arsenic speciation analysis in size-resolved airborne particles with aerodynamic diameters down to 15 nm. Analysis of aerosols from a respiratory therapeutic cave showed temporarily higher concentrations of trimethylarsine oxide than inorganic arsenic and substantial amounts of organoarsenicals, especially in smaller particles. Our method provides guidance for future studies investigating arsenicals in ultrafine particles and their health implications. Furthermore, the method developed can be used to widely monitor particle-bound organoarsenicals to fully understand the importance of As biovolatilization in the environment.

Arsenic speciation dynamics in paddy rice soil-water environment: sources, physico-chemical, and biological factors - A review

Rice is the main staple carbohydrate source for billions of people worldwide. Natural geogenic and anthropogenic sources has led to high arsenic (As) concentrations in rice grains. This is because As is highly bioavailable to rice roots under conditions in which rice is cultivated. A multifaceted and interdisciplinary understanding, both of short-term and long-term effects, are required to identify spatial and temporal changes in As contamination levels in paddy soil-water systems. During flooding, soil pore waters are elevated in inorganic As compared to dryland cultivation systems, as anaerobism results in poorly mobile As(V), being reduced to highly mobile As(III). The formation of iron (Fe) plaque on roots, availability of metal (hydro)oxides (Fe and Mn), organic matter, clay mineralogy and competing ions and compounds (PO₄^3- and Si(OH)₄) are all known to influence As(V) and As(III) mobility in paddy soil-water environments. Microorganisms play a key role in As transformation through oxidation/reduction, and methylation/volatilization reactions, but transformation kinetics are poorly understood. Scientific-based optimization of all biogeochemical parameters may help to significantly reduce the bioavailability of inorganic As.

Biogenic arsenic volatilisation from an acidic fen

To quantify arsenic (As) volatilisation by peatlands and to elucidate the environmental factors governing As volatilisation, a series of anoxic incubations with acidic fen soil collected in northeast Bavaria in Germany were performed at 15°C for 4months. Arsenic volatilisation summed to 2.32ng As in the control, which was 1.6% and ~0.01% of the porewater As and the total As storage in the fen soil, respectively. Treatment with 10mM NaN₃ resulted in only 0.03ng As volatilisation. In comparison, addition of 10mM NaOAc stimulated microbial activity in fen soil and As volatilisation rose to 8.42ng As, indicating that As volatilisation by fen soil is primarily biogenic. Spiking with 67μM As(III) increased As volatilisation eightfold, supposedly caused by the largely enhanced As availability in porewater for microbes (~10 times higher than the control). Adding 10mM FeCl₃ and Na₂SO₄ decreased As volatilisation to 0.30 and 0.82ng As, respectively, apparently due to the change of microbial activity. Speciation of gaseous As in the headspace using GC-ICP-MS/EI-MS showed the predominance of arsine and trimethylarsine in treatments with low and high porewater As concentrations, respectively, suggesting different formation pathways of arsine and methylarsines. This study demonstrated the strong linkage between microorganisms and As volatilisation by peatlands and furthermore indicated the minor role of As volatilisation in the natural As biogeochemical cycle in the semi-terrestrial environment.

Elevated Trimethylarsine Oxide and Inorganic Arsenic in Northern Hemisphere Summer Monsoonal Wet Deposition

For arsenic speciation, the inputs for wet deposition are not well understood. Here we demonstrate that trimethylarsine oxide (TMAO) and inorganic arsenic are the dominant species in monsoonal wet deposition in the summer Indian subcontinent, Bangladesh, with inorganic arsenic dominating, accounting for ∼80% of total arsenic in this medium. Lower concentrations of both species were found in monsoonal wet deposition in the winter Indian subcontinent, Sri Lanka. The only other species present was dimethylarsinic acid (DMAA), but this was usually below limits of detection (LoD). We hypothesize that TMAO and inorganic arsenic in monsoonal wet deposition are predominantly of marine origin. For TMAO, the potential source is the atmospheric oxidation of marine derived trimethylarsine. For inorganic arsenic, our evidence suggests entrainment of water column inorganic arsenic into atmospheric particulates. These conclusions are based on weather trajectory analysis and on the strong correlations with known wet deposition marine derived elements: boron, iodine, and selenium. The finding that TMAO and inorganic arsenic are widely present and elevated in monsoonal wet deposition identifies major knowledge gaps that need to be addressed regarding the understanding of arsenic's global cycle.

Evaluation of bioaugmentation and biostimulation on arsenic remediation in soil through biovolatilization

Arsenic (As) removal through microbially driven biovolatilization can be explored as a potential method for As bioremediation. However, its effectiveness needs to be improved. Biostimulation with organic matter amendment and bioaugmentation with the inoculation of genetic engineered bacteria could be potential strategies for As removal and site remediation. Here, the experiments were conducted to evaluate the impacts of rice straw and biochar amendment, inoculation of genetic engineered Pseudomonas putida KT2440 (GE P. putida) with high As volatilization activity, on microbial mediated As volatilization and removal from three different arseniferous soils. In general, the addition of rice straw (5%) significantly enhanced As methylation and volatilization in comparison with corresponding non-amended soils. Biochar amendments and inoculation of the GE P. putida increased As methylation and volatilization, respectively, but less than that of rice straw addition. The effectiveness of As volatilizations are quite different in the various paddy soils. The combined amendments of rice straw and GE P. putida exhibited the highest As removal efficiency (483.2 μg/kg/year) in Dayu soil, with 1.2% volatilization of the total As annually. The highest water-soluble As concentration (0.73 mg/kg) in this soil could be responsible for highest As volatilization besides the rice straw and bacteria in this soil.

Volatilization of arsenic from polluted soil by Pseudomonas putida engineered for expression of the arsM Arsenic(III) S-adenosine methyltransferase gene

Even though arsenic is one of the most widespread environmental carcinogens, methods of remediation are still limited. In this report we demonstrate that a strain of Pseudomonas putida KT2440 endowed with chromosomal expression of the arsM gene encoding the As(III) S-adenosylmethionine (SAM) methyltransfase from Rhodopseudomonas palustris to remove arsenic from contaminated soil. We genetically engineered the P. putida KT2440 with stable expression of an arsM-gfp fusion gene (GE P. putida), which was inserted into the bacterial chromosome. GE P. putida showed high arsenic methylation and volatilization activity. When exposed to 25 μM arsenite or arsenate overnight, most inorganic arsenic was methylated to the less toxic methylated arsenicals methylarsenate (MAs(V)), dimethylarsenate (DMAs(V)) and trimethylarsine oxide (TMAs(V)O). Of total added arsenic, the species were about 62 ± 2.2% DMAs(V), 25 ± 1.4% MAs(V) and 10 ± 1.2% TMAs(V)O. Volatilized arsenicals were trapped, and the predominant species were dimethylarsine (Me2AsH) (21 ± 1.0%) and trimethylarsine (TMAs(III)) (10 ± 1.2%). At later times, more DMAs(V) and volatile species were produced. Volatilization of Me2AsH and TMAs(III) from contaminated soil is thus possible with this genetically engineered bacterium and could be instrumental as an agent for reducing the inorganic arsenic content of soil and agricultural products.

Earth Abides Arsenic Biotransformations

Arsenic is the most prevalent environmental toxic element and causes health problems throughout the world. The toxicity, mobility, and fate of arsenic in the environment are largely determined by its speciation, and arsenic speciation changes are driven, at least to some extent, by biological processes. In this article, biotransformation of arsenic is reviewed from the perspective of the formation of Earth and the evolution of life, and the connection between arsenic geochemistry and biology is described. The article provides a comprehensive overview of molecular mechanisms of arsenic redox and methylation cycles as well as other arsenic biotransformations. It also discusses the implications of arsenic biotransformation in environmental remediation and food safety, with particular emphasis on groundwater arsenic contamination and arsenic accumulation in rice.

Biovolatilisation: a poorly studied pathway of the arsenic biogeochemical cycle

It has been known for over a hundred years that microorganisms can produce volatile arsenic (As) species, termed "arsines". However, this topic has received relatively little attention compared to As behaviour in soils and biotransformation through the trophic level in the marine and terrestrial environment. We believe this is due to long-standing misconceptions regarding volatile As stability and transport as well as an absence, until recently, of appropriate sampling methods. First and foremost, an attempt is made to unify arsines' designations, notations and formulas, taking into account all the different terms used in the literature. Then, the stability of As volatile species is discussed and new analytical developments are explored. Further, the special cases of diffuse low-level emissions (e.g. soil and sediment biovolatilisation), and point sources with high-level emissions (geothermal environments, landfills, and natural gas) are comprehensively reviewed. In each case, future possible areas of research and unknown mechanisms are identified and their importance towards the global As biogeochemical cycle is explored. This review gathers new information regarding mechanisms, stability, transport and sampling of the very elusive arsines and shows that more research should be conducted on this important process.

Arsenic biomethylation by photosynthetic organisms

Arsenic (As) is a ubiquitous element that is widespread in the environment and causes numerous health problems. Biomethylation of As has implications for its mobility and toxicity. Photosynthetic organisms may play a significant role in As geochemical cycling by methylating it to different As species, but little is known about the mechanisms of methylation. Methylated As species have been found in many photosynthetic organisms, and several arsenite S-adenosylmethionine (SAM) methyltransferases have been characterized in cyanobacteria and algae. However, higher plants may not have the ability to methylate As. Instead, methylated arsenicals in plants probably originate from microorganisms in soils and the rhizosphere. Here, we propose possible approaches for developing 'smart' photosynthetic organisms with an enhanced and sensitive biomethylation capacity for bioremediation and safer food.