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

Mobilization of grassland soil arsenic stores due to agronomic management

The fate of arsenic in mineral soil stores over time is poorly understood. Here we examined arsenic loss over five decades from a managed grassland soil profile through analysing archived material from a long-term slurry (LTS) experiment at Hillsborough, Northern Ireland. A randomized block experiment was established in 1970 where a perennial ryegrass sward was seeded onto the site and subjected to control (no fertilization) and fertilization treatments using conventional (i.e. at farmers recommended application rate) mineral NPK fertilizer, and pig and cow slurry treatments. Soil (0-5, 5-10 and 10-15 cm), slurry applied, and sward off-take was archived each year. A mass-balance calculation found that control soils lost no arsenic down the 15 cm depth soil profile, the NPK treatment had a 10 % loss, while cow slurry caused 25 % loss, and a 35 % loss was observed for pig slurry. For treatments with arsenic loss, removal was linear over the 50 years of study in 2 out of the 3 blocks, with the 3rd block showing little or no change. Principal Component Analysis (PCA) found that arsenic was most positively associated with soil magnesium, manganese and nickel, while negatively associated with pH, organic carbon, phosphorus and silicon. Laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) of soil found that arsenic association with lead mineralogy could potentially explain why there was a gradient in arsenic loss across the experimental plots. Slurry and atmospheric inputs, and sward off-take had little impact on the soil arsenic mass-balance. The findings suggest that leaching loss down the soil profile was the mechanism of loss of arsenic. The applicability of the LTS experimental site arsenic findings to other soils is discussed, as is the implication for the global biogeochemical cycling of those soils.

Migration, transformation of arsenic, and pollution controlling strategies in paddy soil-rice system: A comprehensive review

Arsenic pollution in paddy fields has become a public concern by seriously threatening rice growth, food security and human health. In this review, we delve into the biogeochemical behaviors of arsenic in paddy soil-rice system, systemically revealing the complexity of its migration and transformation processes, including the release of arsenic from soil to porewater, uptake and translocation of arsenic by rice plants, as well as transformation of arsenic species mediated by microorganism. Especially, microbial processes like reduction, oxidation and methylation of arsenic, and the coupling of arsenic with carbon, iron, sulfur, nitrogen cycling through microbes and related mechanisms were highlighted. Environmental factors like pH, redox potential, organic matter, minerals, nutrient elements, microorganisms and periphyton significantly influence these processes through different pathways, which are discussed in this review. Furthermore, the current progress in remediation strategies, including agricultural interventions, passivation, phytoremediation and microbial remediation is explored, and their potential and limitations are analyzed to address the gaps. This review offers comprehensive perspectives on the complicated behaviors of arsenic and influence factors in paddy soil-rice system, and provides a scientific basis for developing effective arsenic pollution control strategies.

Marine and terrestrial contributions to atmospheric deposition fluxes of methylated arsenic species

Arsenic, a toxic element from both anthropogenic and natural sources, reaches surface environments through atmospheric cycling and dry and wet deposition. Biomethylation volatilizes arsenic into the atmosphere and deposition cycles it back to the surface, affecting soil-plant systems. Chemical speciation of deposited arsenic is important for understanding further processing in soils and bioavailability. However, the range of atmospheric transport and source signature of arsenic species remain understudied. Here we report significant levels of methylated arsenic in precipitation, cloud water and aerosols collected under free tropospheric conditions at Pic du Midi Observatory (France) indicating long-range transport, which is crucial for atmospheric budgets. Through chemical analyses and moisture source diagnostics, we identify terrestrial and marine sources for distinct arsenic species. Estimated atmospheric deposition fluxes of methylated arsenic are similar to reported methylation rates in soils, highlighting atmospheric deposition as a significant, overlooked source of potentially bioavailable methylated arsenic species impacting plant uptake in soils.

Seasonal and spatial variations of arsenic and its species in particulate matter in an urban environment of Brno, Czech Republic

The present paper deals with an analysis of total arsenic concentration using ICP-MS/MS and an analysis of concentration of several arsenic species, arsenite (AsIII), arsenate (AsV), monomethylarsonate (MMA), dimethylarsenite (DMA), and trimethylarsine oxide (TMAO), using HPLC-ICP-MS/MS in the PM10 fraction of airborne urban aerosol. The samples were collected during two campaigns, in the autumn of 2022 and in the winter of 2023, at three locations within the central European city of Brno, with the aim to evaluate the seasonal and spatial variations in the PM10 composition. The results confirmed only the seasonal variability in the content of the methylated arsenic species in PM10 influenced by biomethylation processes. To gain better understanding of the possible arsenic origin, a supplementary analysis of the total arsenic concentrations was performed in samples of different size fractions of particulate matter collected using ELPI + . Local emissions, including industrial activities and heating during the winter season, were suggested as the most likely predominant source contributing to the total As content in PM10.

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.

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.

Localized Intensification of Arsenic Release within the Emergent Rice Rhizosphere

Behavior of trace elements in flooded/lowland rice soils is controlled by root-zone iron oxidation. Insoluble iron species bind/capture toxic elements, i.e., arsenic. However, it was recently observed that within this territory of arsenic immobilization lies a zone of prolific iron release, accompanied by a significant flux of arsenic in close proximity to rice root apices. Questions still remain on how common this phenomenon is and whether the chemical imaging approaches or soils/cultivars used influence this event. Here, three types of ultrathin/high-resolution diffusive gradient in thin films (DGT) substrates were integrated with oxygen planar optodes in a multilayer system, providing two-dimensional mapping of solute fluxes. The three DGT approaches revealed a consistent/overlapping spatial distribution with localized flux maxima for arsenic, which occurred in all experiments, concomitant with iron mobilization. Soil/porewater microsampling within the rhizosphere revealed no significant elevation in the solid phase's total iron and arsenic concentrations between aerobic and anaerobic zones. Contrary to arsenic, phosphorus bioavailability was shown to decrease in the arsenic/iron flux maxima. Rice roots, in addition to their role in nutrient acquisition, also perform a key sensory function. Flux maxima represent a significant departure from the chemical conditions of the bulk/field environment, but our observations of a complete rhizosphere reveal a mixed mode of root-soil interactions.

Microbial Arsenic Methylation in Soil and Uptake and Metabolism of Methylated Arsenic in Plants: A Review

Arsenic (As) poses a risk to the human health in excess exposure and microbes play an important role in the toxicity of As. Arsenic methylation mediated by microbes is a key driver of As toxicity in the environment and this paper reviews the role of microbial arsenic methylation and volatilization in the biogeochemical cycle of arsenic. In specific, little is presently known about the molecular mechanism and gene characterization of arsenic methylation. The uptake of methylated arsenic in plants is influenced by microbial arsenic methylation in soil, thus enhancing the volatilization of methylated arsenic is a potential mitigation point for arsenic mobility and toxicity in the environment. On the other hand, the potential risk of methylated arsenic on organisms is also discussed. And the directions for future research, theoretical reference for the control and remediation of arsenic methylation, are presented.

Maritime Deposition of Organic and Inorganic Arsenic

The speciation of arsenic in wet and dry deposition are ambiguously described in current literature. Presented here is a 2 year study quantifying arsenic species in atmospheric deposition collected daily from an E. Atlantic coastal, semirural site, with comparative urban locations. Inorganic arsenic (As_i) was the principal form of arsenic in wet deposition, with a mean concentration of 0.54 μmol/m³. Trimethylarsine oxide (TMAO) was found to be the dominant form of organic arsenic, determined as above the LoD in 33% of wet deposition samples with a mean concentration of 0.12 μmol/m³. Comparison with codeposited trace elements and prevailing weather trajectories indicated that both anthropogenic and marine sources contribute to atmospheric deposition. Analysis of dry deposition revealed it to be a less significant input to the land-surface for As_i, contributing 32% of that deposited by wet deposition. Dry deposition had a larger proportion of As_i than that found in wet deposition, with TMAO making up only 12% of the sum of species. In comparison, urban sites showed large spatial and temporal variations in organic arsenic deposition, indicating that local sources of methylated species may be likely and that further understanding of biogenic arsine evolution and degradation are required to adequately assess the atmospheric arsenic burden and subsequent contribution to terrestrial ecosystems.

Modifying the Parboiling of Rice to Remove Inorganic Arsenic, While Fortifying with Calcium

Using village-based rice processing plants in rural Bangladesh, this study considered how parboiling rice could be altered to reduce the content of the carcinogen inorganic arsenic. Parboiling is normally conducted with rough rice (i.e., where the husk is intact) that is soaked overnight at ambient temperatures, and then either steamed or boiled for ∼10 min, followed by drying. Across 13 geographically dispersed facilities it was found that a simple alteration parboiling wholegrain, instead of rough rice, decreased the inorganic arsenic content by 25% ( P = 0.002) in the final polished grain. Also, parboiling wholegrain had little impact on milling quality of the final polished rice. The wholegrain parboiling approach caused statistically significant median enrichment of calcium, by 213%; and a reduction in potassium, by 40%; with all other nutrient elements tested being unaffected. Milled parboiled rough rice had an enriched inorganic arsenic compared to nonparboiled milled rice, but parboiling of wholegrain rice did not enrich inorganic arsenic in the final milled product. Polished rice produced from the parboiling of both rough and wholegrain rice significantly reduced cadmium compared to nonparboiled polished rice, by 25%. This study also identified that trimethylarsine oxide and tretramethylarsonium are widely elevated in the husk and bran of rice and, therefore, gives new insights into the biogeochemical cycling of arsenic in paddy ecosystems.

Inhibition of Microbial Methylation via arsM in the Rhizosphere: Arsenic Speciation in the Soil to Plant Continuum

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.

Biovolatilization of Arsenic as Arsines from Seawater

Marine sources of arsenic to the atmosphere are normally dismissed as minor. Here we show that arsenic can be biovolatilized from seawater, and that biovolatilzation is based on organic arsenic species present in the seawater. Even though inorganic arsenic is in great excess in seawaters, it is trimethylarsine (TMA) that is the primary biovolatilized product, with dimethylarsine (DMA) also observed if dimethylarsinic acid (DMAA) is spiked into seawaters. With respect to budgets, 0.04% of the total arsenic in the seawater was biovolatilized over a 2-week incubation period. To test the environmental significance of this finding, wet deposition was analyzed for arsenic species at coastal locations, one of which was the origin of the seawater. It was found that the oxidized product of TMA, trimethylarsine oxide (TMAO), and to a less extent DMAA were widely present. When outputs for arsines (0.9 nmol/m²/d) from seawater and inputs from wet deposition (0.3-0.5 nmol/m²/d) were compared, they were of the same order of magnitude. These findings provide impetus to reexamining the global arsenic cycle, as there is now a need to determine the flux of arsines from the ocean to the atmosphere.