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Monosodium methanearsonate (MSMA), a sodium salt of monomethylarsonic acid (MMA), is a selective contact herbicide used for the control of a broad spectrum of weeds. In water, MSMA dissociates to ions of sodium (Na+) and monomethylarsonate (MMA−) that is stable and does not transform abiotically. In soils characteristic of MSMA use, several simultaneous processes can occur: (1) microbial methylation of MMA to dimethylarsinic acid (DMA), (2) microbial demethylation of MMA to inorganic arsenic (iAs), (3) methylation of iAs to MMA, and (4) sorption and sequestration of MMA and its metabolites to soil minerals. Sequestered residues are residues that cannot be desorbed from soil in environmental conditions. Sequestration is rapid in the initial several days after MSMA application and continues at a progressively slower rate over time. Once sequestered, MMA and its metabolites are inaccessible to soil microorganisms and cannot be transformed. The rate and extent of the sorption and sequestration as well as the mobility of MMA and its metabolites depend on the local edaphic conditions. In typical MSMA use areas, the variability of the edaphic conditions is constrained. The goal of this research was to estimate the amount of iAs potentially added to drinking water as a result of the use of MSMA, with models and scenarios developed by the US Environmental Protection Agency for pesticide risk assessment. In this project, the estimated drinking water concentrations (EDWCs) for iAs were assessed as the average concentration in the reservoir over a 30‐year simulation with annual applications of MSMA at maximum label rates. When the total area of suitable land was assumed to be treated, EDWCs ranged from <0.001 to 0.12 µg/L. When high estimates of actually treated acreage are considered, the EDWCs are below 0.06 µg/L across all scenarios. Integr Environ Assess Manag 2024;20:2076–2087. © 2024 The Author(s). Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC). Key Points: Inorganic arsenic (iAs) potentially added to drinking water from the use of monosodium methanearsonate (MSMA) was estimated with models (Pesticide Root Zone Model, version 5 [PRZM5], Variable Volume Water Model [VVWM], and aerial spray prediction model [AgDRIFT]) and based on scenarios developed by the USEPA for pesticide risk assessment.Sequestered residues (residues that could not be extracted by water or weak acids) were considered as losses to the modeled systems in PRZM5 and VVWM.Rate constants used for modeling sequestration and transformation were derived from empirical data representative of MSMA use environments.The modeled estimated drinking water concentration for iAs ranged from <0.001 to 0.12 µg/L when 100% of the suitable area of a watershed is treated with MSMA and below 0.06 µg/L across all scenarios when the actual area treated with MSMA is considered. [ABSTRACT FROM AUTHOR]

Monosodium methanearsonate (MSMA), the sodium salt of monomethylarsonic acid (MMA), is used as a selective, broad‐spectrum contact herbicide to control weeds in cotton and a variety of turf. In water, MSMA dissociates into ions of sodium (Na+) and of MMA−, which is the herbicide's active component. Certain soil microorganisms can methylate MMA to dimethylarsinic acid (DMA) other microorganisms can demethylate MMA to inorganic arsenic (iAs). To predict the groundwater concentration of iAs that may result from MSMA application, the processes affecting the environmental behavior of MSMA must be quantified and modeled. There is an extensive body of literature regarding the environmental behavior of MSMA. There is a consensus among scientists that the fate of MMA in soil is controlled by microbial activity and sorption to solid surfaces and that iAs sorption is even more extensive than that of MMA. The sorption and transformation of MMA and its metabolites are affected by several factors including aeration condition, temperature, pH, and the availability of nutrients. The precise nature and extent of each of these processes vary depending on site‐specific conditions; however, such variability is constrained in typical MSMA use areas that are highly managed. Monomethylarsonic acid is strongly sorbed on mineral surfaces and becomes sequestered into the soil matrix. Over time, a greater portion of MMA and iAs becomes immobile and unavailable to soil microorganisms and to leaching. This review synthesizes the results of studies that are relevant for the behavior of MSMA used as a herbicide to reliably predict the fate of MSMA in its use conditions. Integr Environ Assess Manag 2024;20:1859–1875. © 2024 The Author(s). Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC). Key Points: The environmental fate of monosodium methanearsonate (MSMA) used as a herbicide should be evaluated based on studies conducted with experimental conditions and setup that are relevant to MSMA use.The environmental fate of MSMA in soil is controlled by sorption and sequestration to soil minerals and by microbial activity.The rate and extent of sorption and sequestration of MSMA vary depending on site‐specific conditions; however, such variability is constrained in typical MSMA use areas.With time, the majority of MSMA and its metabolites become immobile and unavailable to soil microorganisms and transport in the environment. [ABSTRACT FROM AUTHOR]

ABSTRACT Management of Hg-contaminated sites poses particular challenges because methylmercury (MeHg), a potent bio-accumulative neurotoxin, is formed in the environment, and concentrations are not generally predictable based solely on total Hg (THg) concentrations. In this review, we examine the state of knowledge regarding the chemical, biological, and physical controls on MeHg production and identify those most critical for contaminated site assessment and management. We provide a list of parameters to assess Hg-contaminated soils and sediments with regard to their potential to be a source of MeHg to biota and therefore a risk to humans and ecological receptors. Because some measurable geochemical parameters (e.g., DOC) can have opposing effects on Hg methylation, we recommend focusing first on factors that describe the potential for Hg bio-accumulation: site characteristics, Hg and MeHg concentrations, Hg availability, and microbial activity, where practical. At some sites, more detailed assessment of biogeochemistry may be required to develop a conceptual site model for remedial decision making. Integr Environ Assess Manag 2017;13:249-263. © 2016 SETAC [ABSTRACT FROM AUTHOR]

Formation of Fe(II)-As(III) solids is suspected to limit dissolved As concentrations in anaerobic environments. Iron(II) precipitates enriched in As(III) have been observed after microbial reduction of As(V)-loaded lepidocrocite (γ-FeOOH) and symplesite (Fe(II)3(As(V)O4)2]•8H2O) and upon abiotic reaction of Fe(II) with As(III). However, the conditions favorable for Fe(II)-As(III) precipitation and the long-term stability (relative to dissolution) of this phase are unknown. Here we examine the composition, local structure, and solubility of an Fe(II)-As(III) precipitate to determine environments where such a solid may form and persist. We reveal that the Fe(II)-As(III) precipitate has a composition of H7Fe4(AsO3)5 and a log Kso of 34 for the dissolution reaction defined as: H7Fe4(AsO3)5 + 8H+ = 4Fe2+ + 5H3AsO3. Extended X-ray absorption fine structure spectroscopic analysis of H7Fe4(AsO3)5 shows that the molecular environment of Fe is dominated by edge-sharing octahedra within an Fe(OH)2 sheet and that As is dominated by corner-sharing AsIIIO3 pyramids, which are consistent with previously published structures of As(III)-rich Fe(II) solids. The H7Fe4(AsO3)5 solid has a pH-dependent solubility and requires millimolar concentrations of dissolved Fe(II) and As(III) to precipitate at pH <7.5. By contrast, alkaline conditions are more conducive to formation of H7Fe4(AsO3)5; however, a high concentration of Fe(II) is required, which is unusual under alkaline conditions. [ABSTRACT FROM AUTHOR]

Soils are an aggregate-based structured media that have a multitude of pore domains resulting In varying degrees of advective and diffusive solute and gas transport. Consequently, a spectrum of biogeochemical processes may function at the aggregate scale that collectively, and coupled with solute transport, determine element cycling in soils and sediments. To explore how the physical structure impacts biogeochemical processes influencing the fate and transport of As, we examined temporal changes in speciation and distribution of As and Fe within constructed aggregates through experimental measurement and reactive transport simulations. Spherical aggregates were made with As(V)-bearing ferrihydrite-coated sand inoculated with Shewanella sp. ANA-3; aerated solute flow around the aggregate was then induced. Despite the aerated aggregate exterior, where As(V) and ferrihydrrte persist as the dominant species, anoxia develops within the aggregate interior. As a result As and Fe redox gradients emerge, and the proportion of As(III) and magnetite increases toward the aggregate interior. Arsenic(III) and Fe(II) produced in the interior migrate toward the aggregated exterior and result in coaccumulation of As and Fe(III) proximal to preferential flow paths as a consequence of oxygenic precipitation. The oxidized rind of aggregates thus serves as a barrier to As release into advecting pora-water, but also leads to be a buildup of this hazardous element at preferential flow boundaries that could be released upon shifting geochemical conditions. [ABSTRACT FROM AUTHOR]