271 research outputs found
Speciation and Redox Chemistry of Selenium and Arsenic in Wetland Soils and Sediments.
Studies dealing with the speciation, species transformations, and solubility of selenium and arsenic as affected by soil or sediment redox potential and pH were initiated because of a lack on information and the need for a better understanding of selenium and arsenic chemistry in wetland soils and sediments. Analytical techniques were developed that permitted the determination of selenium and arsenic species commonly encountered in soils and sediments. The speciation and redox chemistry of selenium and arsenic was studied in selected soils and sediments. Under reduced conditions (200 mV), selenium solubility in sediments from Kesterson Reservoir (CA) and Hyco Reservoir (NC) was low and (elemental selenium + selenides) comprised 80-100% of the total soluble selenium. Experimental data and equilibrium thermodynamic calculations suggest that insoluble metal selenides, particularly FeSe, controlled selenium solubility under reduced conditions. Upon oxidation form 200 to 500 mV selenium solubility increased approximately 20 times in both the Kesterson and Hyco Reservoir sediments. Under moderately reduced conditions (0-200 mV), selenite was the dominant (45 to 100%) soluble selenium species. At redox levels above 200 mV, selenite was further oxidized to selenate and selenium solubility reached a maximum. An alkaline pH resulted in greater dissolved selenium concentrations. The experimental data also illustrated the importance of biomethylation in selenium chemistry. Under oxidized conditions, dimethyl selenide constituted 15 to 36% of the total soluble selenium. Redox potential and pH were also shown to exhibit a major impact on arsenic speciation, and solubility in Hyco Reservoir (NC) sediments and in an arsenic contaminated soil from Kolin (La). In contrast to selenium, arsenic solubility increased with decreasing redox. In both studies, arsenate was the major (80%) arsenic species present under oxidized conditions. Upon reduction, arsenite became the major dissolved arsenic species, and arsenic solubility increased. Upon reduction from 500 to 200 mV, total arsenic in solution increased 25 and 13 times in the Hyco Reservoir and Kolin soil, respectively. The importance of adsorption-desorption and precipitation-dissolution reactions in controlling arsenic chemistry was illustrated in the Kolin soil
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Impact of the earthworm Lumbricus terrestris (L.) on As, Cu, Pb and Zn mobility and speciation in contaminated soils
To assess the risks that contaminated soils pose to the environment properly a greater understanding of how soil biota influence the mobility of metal(loid)s in soils is required. Lumbricus terrestris L. were incubated in three soils contaminated with As, Cu, Pb and Zn. The concentration and speciation of metal(loid)s in pore waters and the mobility and partitioning in casts were compared with earthworm-free soil. Generally the concentrations of water extractable metal(loid)s in earthworm casts were greater than in earthworm-free soil. The impact of the earthworms on concentration and speciation in pore waters was soil and metal specific and could be explained either by earthworm induced changes in soil pH or soluble organic carbon. The mobilisation of metal(loid)s in the environment by earthworm activity may allow for leaching or uptake into biota
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Atmospheric electricity influencing biogeochemical processes in soils and sediments
The Earth’s subsurface represents a complex electrochemical environment that contains many electro-active chemical compounds that are relevant for a wide array of biologically driven ecosystem processes. Concentrations of many of these electro-active compounds within Earth’s subsurface environments fluctuate during the day and over seasons. This has been observed for surface waters, sediments and continental soils. This variability can affect particularly small, relatively immobile organisms living in these environments. While various drivers have been identified, a comprehensive understanding of the causes and consequences of spatio-temporal variability in subsurface electrochemistry is still lacking. Here we propose that variations in atmospheric electricity (AE) can influence the electrochemical environments of soils, water bodies and their sediments, with implications that are likely relevant for a wide range of organisms and ecosystem processes. We tested this hypothesis in field and laboratory case studies. Based on measurements of subsurface redox conditions in soils and sediment, we found evidence for both local and global variation in AE with corresponding patterns in subsurface redox conditions. In the laboratory, bacterial respiratory responses, electron transport activity and H2S production were observed to be causally linked to changes in atmospheric cation concentrations. We argue that such patterns are part of an overlooked phenomenon. This recognition widens our conceptual understanding of chemical and biological processes in the Earth’s subsurface and their interactions with the atmosphere and the physical environment
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