Arsenic Flux Research Articles

Heavy metals and trace elements in atmospheric fall-out: Their relationship with topsoil and wheat element composition

The objectives of this study were to determine the average concentrations and deposition rates of 28 elements in atmospheric bulk deposition and to elucidate associations among topsoil, bulk deposition and wheat element composition. The fluxes of arsenic (As), copper (Cu), lead (Pb) and zinc (Zn) deposition in Córdoba were higher than in other agro-ecosystems, which reflects both natural (geochemistry and topsoil removal) and anthropogenic sources. High lanthanide, uranium (U) and thorium (Th) concentrations revealed the impact of an open cast uranium mine. The highest enrichment factors (EF) were those of Cu, Pb, Zn and nickel (Ni), with calcium (Ca) being the most prominent in the surroundings of a cement plant. Industries and the transport of airborne urban pollutants were the main anthropogenic sources for Ca, Cu, Ni, Pb, Zn, cadmium (Cd), iron (Fe), manganese (Mn) and antimony (Sb). The concentrations of metals in wheat grain were predicted using the topsoil and atmospheric fall-out composition with R2=0.90, with the latter being the best explanatory variable. The present study highlights the potential health hazards of wheat consumption (Environmental Protection Agency) by the assessment of heavy metals in bulk atmospheric deposition.

Land–ocean contributions of arsenic through a river–estuary–ria system (SW Europe) under the influence of arsenopyrite deposits in the fluvial basin

Water was sampled monthly from September 2005 to August 2006 at 14 stations distributed throughout the coastal system of Anllóns–Laxe, from where 30 surface sediment samples were also taken. After filtration through 0.22 μm polycarbonate filters, dissolved inorganic and total arsenic (UV oxidation) concentration was determined by HG-AFS. After microwave digestion, the arsenic in SPM and sediment was determined by AAS. Ultra-clean procedures were adopted during sampling, handling and analysis and the analytical accuracy was checked using certified reference material. Spatial distribution of As in water (0.2–4.0 μg L − 1 ), SPM (21–169 mg kg − 1 ) and sediment of the river reservoir was altered by the presence of arsenopyrite deposits in the middle fluvial basin that increases 2.1 ± 0.5 and 1.7 ± 0.5 times the concentrations of inorganic dissolved (DI-As) and particulate (P-As) arsenic, respectively. At the termination fluvial zone As fluxes can be calculated to be: [DI-As] = 7.09 · Q − 0.69 . The Anllóns River exports to its estuary 460 kg a − 1 of dissolved (< 7% as organic) arsenic annually. It is higher (i.e. 0.83 kg s − 1 km − 2 of DI-As) than that of most of European rivers. In the estuary reservoir, the influence of arsenopyrite is also evident as the river concentration of DI-As, which was lower than in seawater during the wet season and higher during the dry season. Arsenic has non-conservative behaviour, as in other European estuaries, but the Anllóns shows an ambivalent pattern: as it usually gains DI-As during the wet season and loses it during the dry season, whilst P-As seems to behave contrary to the DI-As. When the fluvial arsenic reaches the ria its concentration varies due to the estuarine processes. In the wet season DI-As increases its concentration by one third whilst in the dry season it decreases by one fifth and the annual contribution to the ria is 10% higher than the fluvial output. In the case of P-As more data are necessary to quantify its behaviour; however, the estuarine sediments are contaminated by arsenic. In the ria reservoir DI-As levels were similar to those of the ocean, DO-As comprises 9–22% of the inorganic, P-As ranges from 3 to 40 mg kg − 1 , and As-sedimentary can be classified as uncontaminated (4–18 mg kg − 1 ), except in the fishing ports. The ria circulation, reinforced by upwelling favours the exportation of arsenic to the ocean. In the Anllóns–Laxe system as a whole, the freshwater–saline interface processes do not lead to a decrease in the dissolved fluvial arsenic flux to the ocean.

Controlling the formation of quantum dot pairs using nanohole templates

Modifying the shape of nanoholes formed by arsenic debt epitaxy by the overgrowth of a thin GaAs buffer is shown to provide a simple and robust method to grow low density lateral In(Ga)As quantum dot pairs (QDPs). We present here a systematic study of the effect of GaAs buffer thickness, InAs deposition amount, substrate temperature and arsenic overpressure on dot nucleation and QDP formation. A (10–30) nm GaAs buffer over nanoholes initially ∼ 10.5 nm deep, (60–80) nm wide results in up to 80% of the nanoholes containing QDPs. The QD pairs are aligned along the [110] direction and have centre-to-centre separation of ∼ 38 nm . These QDPs form following InAs deposition between 1.3 ML and 1.6 ML at 490 °C under an arsenic arrival flux of 0.6 ML/s. From the infilling of the hole prior to QD formation, we estimate a net indium surface flux towards the hole of ∼ 7 times the incident flux. The substrate temperature does not significantly alter the dot distribution over the range (470–510) °C. However, the QDP formation is very sensitive to the arsenic overpressure over the range (0.6–1.2) ML/s because of a partial collapse of the nanohole, due to mass transport as the substrate passes through the (2×4) to c(4×4) surface reconstruction around 500 °C.

External sources vs internal processes: What control inorganic As speciation and concentrations in the Penzé estuary?

Over one year study (12 campaigns), particulate arsenic and dissolved inorganic arsenic (total inorganic Asi and arsenite As(III)) levels were determined along the whole salinity gradient of the Penzé estuary (12 stations) in Brittany (France). Concentrations, in the range 2–26nM and 3–56μgg−1 for dissolved inorganic and particulate arsenic, respectively, were relatively high in comparison to pristine systems and were attributed to pig and poultry farming activities taking place in the catchment area. Our results show that variations of dissolved inorganic and particulate arsenic up to mid-estuary were significantly linked to fluvial inputs, these latter being controlled by water discharge and precipitations. Arsenite variations over a large salinity range (S>4) were controlled by those of the marine end-member mainly through bacterial processes. Removal of dissolved species within the water column by adsorption on oxide material was found to be minimal while arsenic fluxes from the sediment control arsenate and arsenite levels in the upper part of the estuary.

Growth conditions effects on optical properties of InAs quantum dots grown by molecular beam epitaxy on GaAs (1 1 3)A substrate

We have investigated the optical properties of InAs/GaAs (1 1 3)A quantum dots grown by molecular beam epitaxy (MBE) with different growth rates by photoluminescence spectroscopy (PL) as a function of the excitation density and the sample temperature (10–300 K). Reflection high-energy electron diffraction (RHEED) is used to investigate the formation process of InAs quantum dots (QDs). A redshift of the InAs QDs PL band emission was observed when the growth rate was increased. This result was explained by the increase of the InAs quantum dot size with increasing growth rate. A significant redshift was observed when the arsenic flux was decreased. The evolution of the PL peak energy with increasing temperature has showed an S-shaped form due to the localization effects and is attributed to the efficient relaxation process of carriers in different InAs quantum dots and to the exciton transfer localized at the wetting layer.

Arsenic mass balance in a paper mill and impact of the arsenic release from the WWTP effluent on the Moselle River

Rivers used for drinking water production might be subject to anthropogenic pollution discharge upstream of the intake point. This problem was investigated in the case of the Moselle River, used for water production in Nancy (350,000 inhabitants) and which might be impacted by industrial activities 60 km upstream. The arsenic flux of a pulp and paper mill discharging in the Moselle River at this location has been more specifically investigated. The main sources of arsenic in that mill seemed to be the recovered papers and the gravel pit water used as feed water. The arsenic input related to wood and bark was limited. The main arsenic outputs from the plant were the paper produced on site and the deinking sludge. The arsenic concentration in the effluent of the wastewater treatment plant (WWTP) was not correlated to the one in the gravel pit water, but may depend on the operating conditions of the WWTP or the changes in processes of the mill. The impact of this anthropogenic source of arsenic on the Moselle River was slightly larger in summer, when the flowrate was lower. Globally the impact of the paper mill on the Moselle River water quality was limited in terms of arsenic.

Weathering fluxes of arsenic from a small catchment in Slovak Republic

Inputs of As to a small catchment due to chemical weathering of bedrock, mechanical weathering of bedrock, and atmospheric precipitation were 71.53, 23.98 and 0.02 g ha−1 year−1, respectively. The output fluxes of As due to mechanical erosion of soil, biological uptake, stream discharge, and groundwater flow were 6.32, 4.77, 0.37 and 0.02 g ha−1 year−1, respectively. The results indicate that arsenic accumulates in soil and regolith with a very high rate. This is attributed to the selective weathering and erosion with respect to arsenic and fixation of arsenic in the secondary solids produced by weathering. The output fluxes of As in stream and groundwater in Vydrica catchment in Slovak Republic (0.39 g ha−1 year−1) based on muscovite–biotite granites and granodiorites were much lower compared to catchments in a gold district in the Czech Republic. These results may be ascribed to the low levels of arsenic pollution measured in Vydrica catchment. The arsenic fluxes were estimated by calculation of mechanical and chemical weathering rates of the bedrocks in Vydrica catchment from mass balance data on sodium and silica. The justification of the steady state of Na and Si is that neither of the elements is appreciably accumulated in plants and in exchangeable pool of ions in soil.

Arsenolysis and Thiol-Dependent Arsenate Reduction

Conversion of arsenate to arsenite is a critical event in the pathway that leads from inorganic arsenic to a variety of methylated metabolites. The formation of methylated metabolites influences distribution and retention of arsenic and affects the reactivity and toxicity of these intermediates. Indeed, some of the toxic and carcinogenic effects associated with exposure to arsenate or arsenite are probably mediated by methylated arsenicals. Recent work has demonstrated a biologically plausible role for phosphorolytic-arsenolytic enzymes in a reaction scheme in which an "activated" arsenate ester is readily reduced by thiols to arsenite. Thiol-dependent reduction of arsenate esters formed by arsenolysis may be one of several functionally reductant processes that control the flux of arsenic into the cellular pathway for arsenic methylation. Integrating these reductive processes into a conceptual model for arsenic metabolism may provide new insights into the cellular machinery for handling this toxic metalloid.

Removal of arsenic from contaminated groundwater by solar-driven membrane distillation using three different commercial membranes

Investigations on solar-driven membrane distillation (SDMD) were carried out for removal of arsenic from contaminated groundwater. Three different types of hydrophobic membranes made of polytetrafluoroethylene (PTFE) and polypropylene (PP) with surface area of 120 × 10 −4 m 2 were used as flat sheet in a direct contact membrane distillation (DCMD) set up in a cross flow module. Effects of initial arsenic concentration in the feed, feed velocity, feed temperature and distillate inlet temperature on arsenic removal efficiency and flux were studied where temperatures of feed and distillate were found to have significant effect on the flux. Almost 100% arsenic separation was achieved without wetting membrane pore even after 120 h of operation. The PTFE membrane with a flux of 49.80 kg/m 2 h was found to the best one out of the tested membranes. The study shows that solar-driven DCMD can effectively separate arsenic from groundwater using a cross flow membrane module with PTFE hydrophobic membrane.

Geochemical processes influencing arsenic mobility at Bullendale historic gold mine, Otago, New Zealand

The historic Bullendale gold mine processed arsenopyrite-rich ore from mesothermal deposits in the South Island of New Zealand from 1864 to 1907. No site rehabilitation was undertaken upon mine closure and processing waste contains arsenic concentration of up to 40 wt%. Originally, all of this arsenic was present as arsenopyrite and with time it has been replaced by secondary arsenic phases. Waste evolution has resulted in a mineralogical gradation of arsenic phases between the ferrihydrite with varying amounts of adsorbed arsenic, amorphous iron arsenates and crystalline kankite, scorodite and zykaite. The stability of the secondary arsenic phases is favoured at low pH (pH 3, <0.1 mg/L) and, at present, reactions associated with iron arsenate dissolution ensure a prevailing pH window of c. 2.5 to 3.5. However, an increase in pH through flooding could destabilise the secondary arsenic minerals resulting in a substantial increase of arsenic solubility (>100 mg/L). In addition to the battery site, an upstream adit is a continuous source of dissolved arsenic to the environment. Due to the alkaline pH conditions in catchment water, arsenic remains mobile and dissolved As of c. 0.03 mg/L is detectable in Skippers Creek more than 2km downstream. Even when this anomaly has been diluted, the arsenic flux of the river is still predominantly controlled by the adit input.

Arsenic incorporation in MBE‐grown HgCdTe studied with the use of ion milling

AbstractIon milling was employed for the study of arsenic incorporation in molecular beam epitaxy‐grown Hg0.8Cd0.2Te films doped in situ. The behaviour of specific defects, which formed after injection of mercury interstitials provided by the milling, allowed for estimating density of electrically active arsenic atoms. It is confirmed that the most efficient incorporation of As occurs when arsenic flux cracking is used, at the cracking zone temperature Tcr &gt; 700 °C. Growth with doping without cracking was found to be accompanied by formation of Te‐related defects, which ion milling activated electrically. Films doped from the cracker cell with Tcr &gt; 700 °C appear to be almost free of these defects. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Different shape of GaAs quantum structures under various growth conditions

We report the influences of growth parameters on the characteristics of GaAs quantum rings (QRs) and quantum dots (QDs) formed on AlGaAs/GaAs by the droplet epitaxy (DE) method. After forming Ga droplets on the AlGaAs/GaAs surface, varying amounts of arsenic (As) flux were introduced to fabricate the GaAs quantum structures. By decreasing the As flux from 8 × 10 − 5 to 3 × 10 − 5 Torr, the shape of the GaAs quantum structures was changed from QDs to elongated QRs. With further decreasing As flux, the shape of the elongated QRs became symmetric. The formation characteristics of the GaAs QRs from the QDs with the amount of As flux were discussed in terms of migration behaviors of the gallium (Ga) atoms on the GaAs(001)- c(4 × 4) surface. The effects of the amount of Ga supply and the growth temperature for the deposition of Ga droplets on the formation of the GaAs quantum structures were also considered.

Removal of arsenic from contaminated groundwater by membrane-integrated hybrid treatment system

Experimental investigations were carried out to separate arsenic from contaminated groundwater by using three different types of nanofiltration membranes in a flat sheet cross flow membrane module with a pre-oxidation step for conversion of trivalent arsenic to pentavalent form. KMnO 4 was used as an oxidising agent in the pre-oxidation step. Effects of pressure, iron concentration in groundwater, pH, and pre-oxidation on the performance of the membranes in terms of flux as well as percentage rejection of arsenic were studied. While transmembrane pressure was found to have strong impact on flux and retention of arsenic, pH and pre-oxidation exhibited strong influence on percentage removal of arsenic. Introduction of pre-oxidation step remarkably enhanced arsenic separation in a largely fouling-free membrane module. Overall arsenic removal increased from 50–63% to 97–100% for all the three types of membranes over a transmembrane pressure range of 5–12 kgf/cm 2 on pre-oxidation of trivalent arsenic. With increase in pH from 3 to 10, arsenic rejection increased by 23% for NF-1, 33% for NF-2 and 26% for NF-20 membranes. Polyamide composite nanofiltration membrane fitted in a cross flow membrane module could successfully remove arsenic from contaminated groundwater with a pre-oxidation step.

Quadra-quantum dots grown on quantum rings having square-shaped holes: Basic nanostructure for quantum dot cellular automata application

Preferable quantum dot (QD) nanostructure for quantum computation based on quantum cellular automata (QCA) is laterally close-packed quantum dot molecular (QDM) having 4 QDs at the corners of square configuration. We called this 4 QDs-set as quadra-quantum dots (QQDs). Aligned QQDs with 2 electron-confinements work like a wire for digital information transmission by Coulomb repulsion force which is high speed and consumes little power. Combination of QQDs in line and their cross-over work as logic gates and memory bits. We have developed a Molecular Beam Epitaxial (MBE) growth technique call “Droplet Epitaxy” for several quantum nanostructures such as quantum rings (QRs), quantum dot rings (QDRs). In this presentation, we prepare QRs with 20 ML In-Ga (15:85) droplet at 390 °C and droplet growth rate of 1ML/s. Arsenic flux (7–8 × 10 −6 Torr) is then exposed for InGaAs crystallization at 200 °C for 5 min. During droplet epitaxy at high value of droplet thickness and high temperature, out-diffusion from the centre of droplets occurs under anisotropic stain. This leads to quantum ring structures having non-uniform ring stripes and deep square-shaped nanoholes. Using these peculiar QRs as templates, 4 QDs situating at each corner of square shape are regrown. Two of these 4 QDs are aligned either [1 1 ¯ 0] or [1 1 0] which are preferable crystallographic directions of QD alignment in general.

Unusual role of the substrate in droplet‐induced GaAs/AlGaAs quantum‐dot pairs

AbstractThe structural complexity of GaAs quantum‐dot pairs has been revealed by cross‐sectional transmission electron microscopy. As a result of high‐temperature droplet epitaxy, the AlGaAs substrate beneath the quantum‐dot pairs is no longer immobile and its reconstruction is observed to define the crystallization of gallium droplets under an arsenic flux. The GaAs quantum‐dot pairs are immersed into the substrate and further confined by the re‐distributed AlGaAs materials above the substrate plane. There are two underlying mechanisms responsible for the final nanostructure configuration, melt‐back etching by the gallium droplets and preferential crystallization of gallium around reconstructed sidewalls. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Influence of arsenic flux on the annealing properties of GaInNAs quantum wells for long wavelength laser applications around 1.6 μm

The influence of the arsenic flux during molecular beam epitaxy growth of 1.5–1.6-μm-emitting GaInNAs quantum wells on the annealing properties is studied. Reducing the arsenic flux results in an increase of the time needed for optimum annealing and offers an easy possibility for tailoring the annealing behavior of GaInNAs layers. Investigation of the annealing behavior of complete laser structures shows that over-annealing can already occur after the growth of complete laser structures due to the high substrate temperature during growth of cladding layers. Applying these observations to the growth of laser structures results in laser diodes covering the wavelength range from 1460 to 1610 nm with threshold current densities in the range 1.3–3.3 kA/cm 2.

The Kinetics of (2×4) --&gt; (3×1(6)) Reconstruction Transition on GaAs Surface

The features of (2×4) --&gt; (3×1(6)) superstructure transition on GaAs(100) surface realized during rapid change of arsenic flux density were studied using RHEED method. The time dependences of ISB RHEED were obtained during this transition. The measurement data were analyzed in the framework of JMAK (Johnson – Melh – Avrami – Kolmogorov) kinetic model. The complex (double-state) structure of the transition process has been detected. This process is realized through the intermediate disordered state with different reconstructions domains coexisting on the surface. Activation energies and rates of the phase transition are defined for each stage. The procedure for precise temperature determination for GaAs(001) surface is proposed. This procedure uses the features of (2×4) --&gt; (3×1(6)) reconstruction transition kinetics.

Modeling Arsenic in the Patuxent Estuary

A water quality model was developed to track the fate and transport of four arsenic species in the Patuxent Estuary: arsenate (As(V)), arsenite (As(III)), methylarsonate (MMA), and dimethylarsinate (DMA). Processes simulated include mass transport, solid-liquid partitioning with suspended solids, uptake and transformation of As(V) by phytoplankton, oxidation of As(III), demethylation of MMA and DMA, and settling/deposition/ resuspension of particulate arsenic in the water column. A sediment module was also developed and linked with the water column to generate fluxes of inorganic arsenic from the sediment bed. The arsenic model was calibrated using water quality data from the Patuxent Estuary over a period ranging from May 24, 1995 to October 29, 1997. Model results indicated that transformation of arsenic by phytoplankton is not a significant source of DMA to the lower Patuxent. Instead, results suggested that the primary source of methylated arsenic (DMA and MMA) to the lower estuary is beyond the downstream boundary (Chesapeake Bay). However, model results supported the hypothesis that flux of arsenic from the sediment is a significant source of inorganic arsenic to the lower estuary.

Room temperature photoluminescence at 4.5μm from InAsN

Nitrogen incorporation in InAsN epilayers grown by radio-frequency plasma-assisted molecular beam epitaxy was investigated as a function of growth conditions. Reduced growth rate, growth temperature, and arsenic flux significantly enhance the nitrogen incorporation. Optimal growth conditions allowed us to obtain high quality InAsN with nitrogen composition of up to 2.5%. The epilayers exhibit intense 4K photoluminescence (PL) with double-peak features, which were attributed to free carrier recombination and localized carrier recombination. Strong room temperature PL emission up to a wavelength of 4.5μm is obtained.

Fluxes of inorganic and organic arsenic species in a Norway spruce forest floor

To identify the role of the forest floor in arsenic (As) biogeochemistry, concentrations and fluxes of inorganic and organic As in throughfall, litterfall and forest floor percolates at different layers were investigated. Nearly 40% of total As total input (5.3 g As ha −1 yr −1) was retained in Oi layer, whereas As total fluxes from Oe and Oa layers exceeded the input by far (10.8 and 20 g As ha −1 yr −1, respectively). Except dimethylarsinic acid (DMA), fluxes of organic As decreased with depth of forest floor so that <10% of total deposition (all <0.3 g As ha −1 yr −1) reached the mineral soil. All forest floor layers are sinks for most organic As. Conversely, Oe and Oa layers are sources of As total, arsenite, arsenate and DMA. Significant correlations ( r ≥ 0.43) between fluxes of As total, arsenite, arsenate or DMA and water indicate hydrological conditions and adsorption–desorption as factors influencing their release from the forest floor. The higher net release of arsenite from Oe and Oa and of DMA from Oa layer in the growing than dormant season also suggests microbial influences on the release of arsenite and DMA.