Field Fluxes and Speciation of Arsines Emanating from Soils

Abstract

The biogeochemical cycle of arsenic (As) has been extensively studied over the past decades because As is an environmentally ubiquitous, nonthreshold carcinogen, which is often elevated in drinking water and food. It has been known for over a century that micro-organisms can volatilize inorganic As salts to arsines (arsine AsH(3), mono-, di-, and trimethylarsines, MeAsH(2), Me(2)AsH, and TMAs, respectively), but this part of the As cycle, with the exception of geothermal environs, has been almost entirely neglected because of a lack of suited field measurement approaches. Here, a validated, robust, and low-level field-deployable method employing arsine chemotrapping was used to quantify and qualify arsines emanating from soil surfaces in the field. Up to 240 mg/ha/y arsines was released from low-level polluted paddy soils (11.3 ± 0.9 mg/kg As), primarily as TMAs, whereas arsine flux below method detection limit was measured from a highly contaminated mine spoil (1359 ± 212 mg/kg As), indicating that soil chemistry is vital in understanding this phenomenon. In microcosm studies, we could show that under reducing conditions, induced by organic matter (OM) amendment, a range of soils varied in their properties, from natural upland peats to highly impacted mine-spoils, could all volatilize arsines. Volatilization rates from 0.5 to 70 μg/kg/y were measured, and AsH(3), MeAsH(2), Me(2)AsH, and TMAs were all identified. Addition of methylated oxidated pentavalent As, namely monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA), to soil resulted in elevated yearly rates of volatilization with up to 3.5% of the total As volatilized, suggesting that the initial conversion of inorganic As to MMAA limits the rate of arsine and methylarsines production by soils. The nature of OM amendment altered volatilization quantitatively and qualitatively, and total arsines release from soil showed correlation between the quantity of As and the concentration of dissolved organic carbon (DOC) in the soil porewater. The global flux of arsines emanating from soils was estimated and placed in the context of As atmospheric inputs, with arsines contributing from 0.9 to 2.6% of the global budget.

Arsenic Speciation and Volatilization from Flooded Paddy Soils Amended with Different Organic Matters

“…abundance and activity (indicated by 16S rDNA and 16S rRNA copy number, respectively) of DT2 were significantly higher than those of CT (Figure 3a). There was no clear relationship between As speciation in soil solution and arsine species, which was consistent with the finding of Mestrot et al 10 Arsenic Speciation in Soil and Soil Solution. The accepted prevailing hypothesis is that inorganic As is transformed to TMAs through the "Challenger pathway", 24 and the rate-limiting steps of the pathway under specific conditions may be different.…”

Arsenic Methylation in Soils and Its Relationship with Microbial arsM Abundance and Diversity, and As Speciation in Rice

“…21,23 In general, volatilization of methylarsine species represents only a very small proportion of the total As in soils. 20,21 Demethylation of DMA was also observed in soils or soil extracts. 52 Gao and Burau 53 showed that both MMA and DMA were mineralized to As(V) when added to a soil, with the latter being mineralized more rapidly than the former.…”

“…Triplicate of 500 g soil were placed in plastic pots and flooded with deionized water to maintain a 2 cm layer of standing water above the soil surface. A Rhizon soil solution sampler was inserted to each pot for the collection of soil pore water at 5,10,15,20,30, and 54 days after flooding. Soil pore water was acidified to pH <2 with HCl (10 mL solution with 0.1 mL concentrated HCl) and filtered through a sterilized 0.2 μm filter before analysis of As speciation.…”

Effects of microbial processes on the fate of arsenic in paddy soil

“…About 419-1252 t/year As are emitted into the atmosphere from paddy soils, which are 15-to 45-times higher than from sea spray (26.5 t/year), 3-to 10-times higher than from forest fires (125 t/year), even up to 240 mg ha −1 year −1 arsines were released from paddy soils with only 11.3 mg/kg As [20,103,104].…”