Arsenic In Sediments Research Articles

Impact of sedimentary arsenic through irrigated groundwater on soil, plant, crops and human continuum from Bengal delta: Special reference to raw and cooked rice

Existence of arsenic-rich (exceeds 2000 mg/kg) iron pyrite in Bengal delta sediments is responsible for arsenic release in aquifer. Ingestion of contaminated drinking water is not the only elevated source of arsenic to the diet. Irrigation of agricultural fields with arsenic-contaminated groundwater has led to arsenic build-up in soil, with subsequent elevation of arsenic in crops grown on these soils. About 50.7 and 11.2 kg of arsenic are withdrawn per year from 23 and 18 shallow tubewells, used for agricultural irrigation in the studied two districts Murshidabad and Nadia, respectively and deposited on lands throughout the year. Considerable higher amounts of arsenic are accumulated in paddy plants. Mean arsenic concentration in food categories is 107 μg/kg. Arsenic concentrations are high in cooked food and skin of the vegetables. Arsenic concentration in cooked rice increases with water arsenic concentration. About 70.2% of arsenic is recovered by using the analytical speciation extraction method. Inorganic arsenic and DMA contribute 89.5% and 10.5% of the total content of arsenic in food, respectively. About 0.081% and 1.57% of arsenic are accumulated in rice grain from arsenic-deposited lands and contaminated soils. The daily dietary intake of inorganic arsenic (μg/kg body wt./day) by an adult from rice grain itself (2.32) is higher than the WHO recommended PTDI value of inorganic arsenic (2.1) and inorganic arsenic contributes 96.8% of the total dietary intakes of arsenic. Population exposed to arsenic toxicity has been identified.

Geographical distribution of arsenic in sediments within the Rio Conchos Basin, Mexico

Arsenic (As) content of sediments from the Rio Conchos and Rio San Pedro in northern Mexico were measured to determine if this toxic metalloid had accumulated to unsafe levels to humans and aquatic life. The spatial distribution of As in each of the six clusters of river and arroyo sediments was analyzed to determine variations with respect to background levels and to infer about potential As sources and sinks. In the northern part of the study area, background concentrations varied little throughout the area and concentrations in river sediments were close to background levels. In the southern part, however, the content of As in arroyo sediment contained a wider range of values and anomalous concentrations. The latter could be traced in part to the presence of mine tailings. As concentrations were below the limit in all studied river stretches and thus do not pose an immediate threat to the river environment, but As content in reservoir sediments exceeded the guideline values. Reservoirs seem to act as a sink for As, warranting closer observation and monitoring.

Evaluating mobilization and transport of arsenic in sediments and groundwaters of Aquia aquifer, Maryland, USA

Groundwater and sediment samples were collected along a flow path in the Aquia aquifer (Paleocene), Maryland in order to examine and study the factors influencing “evolution” of arsenic (As) in these groundwaters. The Aquia crops out near Washington, DC, where it is unconfined, and extends approximately 90 km down dip to the south and east towards and beneath the Chesapeake Bay. The studied flow path was chosen owing to (i) the number of accessible wells, (ii) differences in total dissolved As concentrations in groundwaters from some of the sampled wells, which reach values ≥667 nmol kg-1 or ≥50 ppb, and (iii) the distinct difference in total dissolved As concentrations in Aquia groundwaters between the northern and southern portions of the study area. In groundwater samples, in situ separation of inorganic As species [As(III) and As(V)] were performed by using anion exchange chromatography. Subsequently, As concentrations were determined by inductively coupled plasma mass spectrometry. In situ measurements of Fe concentrations and speciation, dissolved S(-II) concentrations, pH, alkalinity, and oxidation-reduction potential (Eh) were determined to establish the oxidation-reduction conditions and solution chemistry along the flow path. Concentrations of As in 12 analyzed groundwater samples range from ∼0.75 to 1 072 nmol kg-1, and As(III) concentrations ranging from 0.24 to 980 nmol kg-1 appears to be the dominant form of As in solution. 50% of the studied wells yielded groundwaters with concentrations that exceed the US EPA's Maximum Contaminant Level for As in drinking water of 133 nmol kg-1 or 10 ppb. In order to examine the solid phase speciation of As within the aquifer sediments, we collected a number of Aquia sediment samples from a drill core that was archived at the Maryland Geological Survey. These sediment samples were evaluated using a previously established sequential extractions procedure. Solid phase As concentrations range between 973 and 2,012 nmol kg-1. Additionally, petrographic, X-Ray diffraction and diffuse reflectance spectroscopy analyses of the Aquia sediments reveal presence of glauconite, and smectite along with goethite and hematite within the samples. Here, we present the possible mechanisms responsible for the elevated As concentrations in the studied groundwaters of the Aquia aquifer.

Evaluation of artificial neural networks and kriging for the prediction of arsenic in Alaskan bedrock-derived stream sediments using gold concentration data

The detection of arsenic in sediments of placer gold mining areas is critical for planning future controls on migration and mitigation, or tapping uncontaminated groundwater resources for public water use. Arsenic (As) is often found to be collocated and correlated with gold in sediments. However, due to biogeochemical processes, arsenic can partition between the solid and the dissolved fractions in sediments and their interstitial waters. Such partitioning can mobilize arsenic into areas away from the co-located gold distribution in the sediments. In such cases, it is critical to detect the dispersed arsenic concentration. In this paper, neural network (NN) and kriging techniques were used to predict the presence of arsenic in the sediments of Circle City, Alaska using the gold concentration distribution within the sediments. The results obtained using kriging were more promising than those using NNs, albeit a statistically low correlation existed between the observed and the predicted arsenic concentrations. However, irrespective of the method used, the prediction of arsenic value without using gold concentration data was extremely poor.

Groundwater chemistry and occurrence of arsenic in the Meghna floodplain aquifer, southeastern Bangladesh

Dissolved major ions and important heavy metals including total arsenic and iron were measured in groundwater from shallow (25–33 m) and deep (191–318 m) tube-wells in southeastern Bangladesh. These analyses are intended to help describe geochemical processes active in the aquifers and the source and release mechanism of arsenic in sediments for the Meghna Floodplain aquifer. The elevated Cl− and higher proportions of Na+ relative to Ca2+, Mg2+, and K+ in groundwater suggest the influence by a source of Na+ and Cl−. Use of chemical fertilizers may cause higher concentrations of NH 4 + and PO 4 3− in shallow well samples. In general, most ions are positively correlated with Cl−, with Na+ showing an especially strong correlation with Cl−, indicating that these ions are derived from the same source of saline waters. The relationship between Cl−/HCO 3 − ratios and Cl− also shows mixing of fresh groundwater and seawater. Concentrations of dissolved HCO 3 − reflect the degree of water–rock interaction in groundwater systems and integrated microbial degradation of organic matter. Mn and Fe-oxyhydroxides are prominent in the clayey subsurface sediment and well known to be strong adsorbents of heavy metals including arsenic. All five shallow well samples had high arsenic concentration that exceeded WHO recommended limit for drinking water. Very low concentrations of SO 4 2− and NO 3 − and high concentrations of dissolved Fe and PO 4 3− and NH 4 + ions support the reducing condition of subsurface aquifer. Arsenic concentrations demonstrate negative co-relation with the concentrations of SO 4 2− and NO 3 − but correlate weakly with Mo, Fe concentrations and positively with those of P, PO 4 3− and NH 4 + ions.

Slurry sampling for arsenic determination in sediments by hydride generation atomic absorption spectrometry

A simple and robust method for the determination of arsenic in sediments by hydride generation atomic absorption spectrometry (HG-AAS) using slurry sampling is proposed. Aqua regia (5% v/v) and hydrofluoric acid (1% v/v) were mixed with an aliquot of the sediment with particle size of 50 µm in an ultrasonic bath for 30 min. The slurry was allowed to stand for 48 h for an effective extraction of the analyte to the aqueous phase. The final volume of the slurry was made up with HCl 1 mol L-1. A NaBH4 solution stabilized with NaOH was used as reducing agent. The arsine was transported to the quartz T-tube heated at 900 °C for detection. Five certified reference materials of sediment were analyzed and the obtained concentrations were in good agreement with the certified values, according to the t-test for a 95% confidence level with RSD (relative standard deviation) lower than 13%. The limit of detection (LOD) in the sample was 0.6 µg g-1. The proposed procedure for arsenic determination in sediment is precise, accurate and adequate for the environmental samples routine analyses.

Interactions between the Fe(III)-Reducing Bacterium Geobacter sulfurreducens and Arsenate, and Capture of the Metalloid by Biogenic Fe(II)

Previous work has shown that microbial communities in As-mobilizing sediments from West Bengal were dominated by Geobacter species. Thus, the potential of Geobacter sulfurreducens to mobilize arsenic via direct enzymatic reduction and indirect mechanisms linked to Fe(III) reduction was analyzed. G. sulfurreducens was unable to conserve energy for growth via the dissimilatory reduction of As(V), although it was able to grow in medium containing fumarate as the terminal electron acceptor in the presence of 500 muM As(V). There was also no evidence of As(III) in culture supernatants, suggesting that resistance to 500 muM As(V) was not mediated by a classical arsenic resistance operon, which would rely on the intracellular reduction of As(V) and the efflux of As(III). When the cells were grown using soluble Fe(III) as an electron acceptor in the presence of As(V), the Fe(II)-bearing mineral vivianite was formed. This was accompanied by the removal of As, predominantly as As(V), from solution. Biogenic siderite (ferrous carbonate) was also able to remove As from solution. When the organism was grown using insoluble ferrihydrite as an electron acceptor, Fe(III) reduction resulted in the formation of magnetite, again accompanied by the nearly quantitative sorption of As(V). These results demonstrate that G. sulfurreducens, a model Fe(III)-reducing bacterium, did not reduce As(V) enzymatically, despite the apparent genetic potential to mediate this transformation. However, the reduction of Fe(III) led to the formation of Fe(II)-bearing phases that are able to capture arsenic species and could act as sinks for arsenic in sediments.

Iron(II)-Catalyzed Oxidation of Arsenic(III) in a Sediment Column

Arsenic contamination in aquatic systems is a worldwide concern. Understanding the redox cycling of arsenic in sediments is critical in evaluating the fate of arsenic in aquatic environments and in developing sediment quality guidelines. The direct oxidation of inorganic trivalent arsenic, As(III), by dissolved molecular oxygen has been studied and found to be quite slow. A chemical pathway for As(III) oxidation has been proposed recently in which a radical species, Fe(IV), produced during the oxidation of divalent iron, Fe(II), facilitates the oxidation of As(III). Rapid oxidation of As(III) was observed (on a time scale of hours) in batch systems at pH 7 and 7.5, but the extent of As(III) oxidation was limited. The Fe(II)-catalyzed oxidation of As(III) is examined in a sediment column using both computational and experimental studies. A reactive-transport model is constructed that incorporates the complex kinetics of radical species generation and Fe(II) and As(III) oxidation that have been developed previously. The model is applied to experimental column data. Results indicate that the proposed chemical pathway can explain As(III) oxidation in sediments and that transport in sediments plays a vital role in increasing the extent of As(III) oxidation and efficiency of the Fe(II) catalysis.

Potential role of the Fe(III)-reducing bacteria Geobacter and Geothrix in controlling arsenic solubility in Bengal delta sediments

AbstractPrevious studies from our laboratory have suggested a role for indigenous metal-reducing bacteria in the reduction of sediment-bound As(V), and have also shown that a stable enrichment culture of Fe(III)-reducing bacteria was able to mobilize arsenic (as As(III)) from sediments collected from West Bengal (Islam et al., 2004). To identify the Fe(III)-reducing bacteria that may play a role in the reduction of As(V) and mobilization of As(III), we made a detailed molecular analysis of this enrichment culture. It was dominated by a close relative of Geothrix fermentans, but the type strain of this organism was unable to conserve energy for growth via the dissimilatory reduction of As(V), or reduce As(V) present in a defined medium containing fumarate as the electron acceptor. Furthermore, when the cells were grown using soluble Fe(III)-citrate as an electron acceptor in the presence of As(V), bacterial Fe(III) reduction resulted in the precipitation of the Fe(II)-bearing mineral vivianite in 2 weeks. This was accompanied by the efficient removal of As from solution. These results demonstrate that Geothrix fermentans, in common with other key Fe(III)-reducing bacteria such as Geobacter sulfurreducens, does not reduce As(V) enzymatically, but can capture arsenic in Fe(II) minerals formed during respiration using Fe(III) as an electron acceptor. Thus, the reduction of arsenic-bearing Fe(III) oxide minerals is not sufficient to mobilize arsenic, but may result in the formation of Fe(II) biominerals that could potentially act as sinks for arsenic in sediments. Additional mechanisms, including dissimilatory As(V) reduction by other specialist anaerobic bacteria, are implicated in the mobilization of arsenic from sediments.

Preservation and XANES determination of the oxidation state of solid-phase arsenic in shallow sedimentary aquifers in Bengal and Cambodia

AbstractDetermination of the solid-phase arsenic speciation in sediments hosting high-arsenic groundwaters, utilized for drinking and irrigation in Bengal, SE Asia and elsewhere is important in order to understand the biogeochemistry of arsenic. Despite this, there is a relative paucity of speciation data for solid-phase arsenic in such systems, due to preservation difficulties, low arsenic concentrations in the sediments, multiple coordination environments and sample heterogeneity. In this study, X-ray absorption near edge structure spectroscopy was used in conjunction with linear least-squares fitting of model compounds to determine the oxidation state of arsenic in sediments from West Bengal and Cambodia. Whatever the collection and storage method used, substantial oxidation of arsenic was commonly observed over periods of weeks to several months. Sands were particularly susceptible to changes in arsenic oxidation state during storage. Analysis within two or three weeks of collection is therefore recommended, whilst on-site storage under a nitrogen atmosphere immediately after collection is particularly recommended for the preservation of sandy samples. Both muds and sands from West Bengal and Cambodia were dominated by arsenite (As(III)) with <35±10% arsenate (As(V)). Complete oxidation to arsenate was never observed suggesting that a significant proportion of the sedimentary arsenic is inaccessible within crystalline phases. Centrifuging under anaerobic conditions enabled more detailed information about a variety of arsenic coordination environments to be determined.

Fractionation of sedimentary arsenic from Port Kembla Harbour, NSW, Australia

The binding of arsenic in sediments of the heavily industrialised Port Kembla Harbour, NSW, Australia, has been investigated. Both dredge and core samples have been used to develop a sieving/sequential extraction (SE) procedure. Dredge samples included oxic surficial and deeper anoxic sediment. The main core sample analysed was 18 cm deep, sliced at 2 cm intervals. Sediment was sieved to three size ranges (<63 microm, 63-250 microm, >250 microm) and each of these was then subjected to a four step SE, sequentially solubilizing arsenic as ion exchangeable, 1 M HCl soluble, NH(2)OH.HCl soluble, and strong oxidising acid soluble. Concentrations of 50-500 mg As kg(-1) were found, elevated well above local background values. The core sample showed elevated concentrations of arsenic within the top 6-8 cm (300-500 mg As kg(-1)), relative to the deeper sediment (100-200 mg As kg(-1)). Substantial portions of the total arsenic present in the 0-8 cm sediments of core and dredge samples, were found to be soluble in 1 M pH 5 phosphate buffer or 1 M HCl. Arsenic in the lower 8-18 cm of the core displayed different solubility, the fourth stage SE strong acid digestion being required to solubilize >90% of the deep sediment arsenic. It appears that diagenesis had resulted in remobilisation of weakly bound arsenic with subsequent diffusion and deposition in surficial layers. Strong acid soluble arsenic present in deeper sediments has two possible origins: sedimented as strongly bound remaining untouched by diagenetic events, or subjected to diagenetic reactions such as pyritization, which lead to more stable crystalline forms of minerals.

The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions.

The chemical speciation of arsenic in sediments and porewaters of aquifers is the critical factor that determines whether dissolved arsenic accumulates to potentially toxic levels. Sequestration of arsenic in solid phases, which may occur by adsorption or precipitation processes, controls dissolved concentrations. We present synchrotron x-ray absorption spectra of arsenic in shallow aquifer sediments that indicate the local structure of realgar (AsS) as the primary arsenic-bearing phase in sulfate-reducing conditions at concentrations of 1-3 mmol.kg(-1), which has not previously been verified in sediments at low temperature. Spectroscopic evidence shows that arsenic does not substitute for iron or sulfur in iron sulfide minerals at the molecular scale. A general geochemical model derived from our field and spectroscopic observations show that the ratio of reactive iron to sulfur in the system controls the distribution of solid phases capable of removing arsenic from solution when conditions change from oxidized to reduced, the rate of which is influenced by microbial processes. Because of the difference in solubility of iron versus arsenic sulfides, precipitation of iron sulfide may remove sulfide from solution but not arsenic if precipitation rates are fast. The lack of incorporation of arsenic into iron sulfides may result in the accumulation of dissolved As(III) if adsorption is weak or inhibited. Aquifers particularly at risk for such geochemical conditions are those in which oxidized and reduced waters mix, and where the amount of sulfate available for microbial reduction is limited.

Bioaccumulation of arsenic in marine fish and invertebrates from Alaska and California.

Past studies determined that concentrations of arsenic in the liver of flathead sole from Alaska were generally higher than those found in fish from other locations sampled along the west coast of the United States (Meador et al. 1994). A study was conducted to examine arsenic concentrations and patterns of bioaccumulation in fish and potential prey species from two geographic locations. Flathead sole were collected from four sites in the Gulf of Alaska and white croaker and English sole were collected from five sites in California. Potential prey species from each site were also examined and found to contain high concentrations of arsenic. In California, the sites with the lowest sediment concentrations of arsenic, total organic carbon, and acid-volatile sulfides (AVS) contained invertebrates with the highest tissue concentrations. Regression analysis determined that arsenic in polychaetes was highly correlated to sediment concentrations of arsenic normalized to AVS but was higher overall for the California samples. Even though invertebrates from several of the California sites exhibited much higher concentrations of arsenic than invertebrates from the Alaska sites, liver and muscle tissue from flathead sole collected in Alaska usually exhibited higher concentrations than fish from the California sites. When concentrations of arsenic in fish liver were plotted against concentrations of arsenic in sediment normalized to AVS levels, a very high correlation was obtained for all sites. This suggests that AVS, or some factor correlated with AVS, may have been responsible for controlling arsenic bioaccumulation in these fish species through dietary uptake and exposure to arsenic in water. Based on the available data, it appears that dietary uptake may be related to fish tissue concentrations, but uptake of aqueous arsenic may be responsible for the higher tissue concentrations in fish from Alaska.

Role of metal-reducing bacteria in arsenic release from Bengal delta sediments

The contamination of ground waters, abstracted for drinking and irrigation, by sediment-derived arsenic threatens the health of tens of millions of people worldwide, most notably in Bangladesh and West Bengal. Despite the calamitous effects on human health arising from the extensive use of arsenic-enriched ground waters in these regions, the mechanisms of arsenic release from sediments remain poorly characterized and are topics of intense international debate. We use a microscosm-based approach to investigate these mechanisms: techniques of microbiology and molecular ecology are used in combination with aqueous and solid phase speciation analysis of arsenic. Here we show that anaerobic metal-reducing bacteria can play a key role in the mobilization of arsenic in sediments collected from a contaminated aquifer in West Bengal. We also show that, for the sediments in this study, arsenic release took place after Fe(III) reduction, rather than occurring simultaneously. Identification of the critical factors controlling the biogeochemical cycling of arsenic is one important contribution to fully informing the development of effective strategies to manage these and other similar arsenic-rich ground waters worldwide.

Speciation of arsenic and selenium by capillary electrophoresis

Capillary electrophoresis (CE) with direct UV detection, both on-capillary and with a high-sensitivity detection cell (HSDC), were used for the simultaneous determination of arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), selenate [Se(VI)], selenite [Se(IV)], selenocystine (SeC), selenomethionine (SeM) and selenocystamine (SeCM). These anionic and cationic species were separated with negative separation voltage polarity in a capillary coated with poly(diallydimethylammonium chloride) (PDDAC). The background electrolyte (BGE) providing optimal separation was 15 mM phosphate buffer at pH 10.6. Arsenic and selenium species were detected at 195 and 200 nm, respectively. The limit of detection (LOD) values with on-capillary detection (75 μm i.d.) for the individual analytes were 1.02, 1.50, 1.08, 1.35, 19.5, 0.36, 0.32, 1.11 and 1.47 mg/L, respectively (expressed as arsenic or selenium). The method precision for peak area was from 2.1 to 3.4% relative standard deviation. HSDC was applied to increase the detection sensitivity and gave LOD improvement factors from 4.7 to 8.2, yielding LODs for the individual target analytes ranging from 0.049 to 2.38 mg/L. When the normal sample stacking mode was employed using a sample plug up to 4.6% of capillary volume, both the cationic and anionic analytes were stacked simultaneously with LOD improvement factors of 2.6–4.5 yielding LODs for the individual analytes ranging from 0.11 to 7.42 mg/L. The method was applied for the speciation of arsenic in sediment and determination of SeM in a selenium nutrition supplement. The achieved separation selectivity also gives the method a more general application potential for simultaneous arsenic and selenium speciation when hyphenated with inductively coupled plasma MS.

Speciation of arsenic and selenium by capillary electrophoresis

Capillary electrophoresis (CE) with direct UV detection, both on-capillary and with a high-sensitivity detection cell (HSDC), were used for the simultaneous determination of arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), selenate [Se(VI)], selenite [Se(IV)], selenocystine (SeC), selenomethionine (SeM) and selenocystamine (SeCM). These anionic and cationic species were separated with negative separation voltage polarity in a capillary coated with poly(diallydimethylammonium chloride) (PDDAC). The background electrolyte (BGE) providing optimal separation was 15 mM phosphate buffer at pH 10.6. Arsenic and selenium species were detected at 195 and 200 nm, respectively. The limit of detection (LOD) values with on-capillary detection (75 μm i.d.) for the individual analytes were 1.02, 1.50, 1.08, 1.35, 19.5, 0.36, 0.32, 1.11 and 1.47 mg/L, respectively (expressed as arsenic or selenium). The method precision for peak area was from 2.1 to 3.4% relative standard deviation. HSDC was applied to increase the detection sensitivity and gave LOD improvement factors from 4.7 to 8.2, yielding LODs for the individual target analytes ranging from 0.049 to 2.38 mg/L. When the normal sample stacking mode was employed using a sample plug up to 4.6% of capillary volume, both the cationic and anionic analytes were stacked simultaneously with LOD improvement factors of 2.6–4.5 yielding LODs for the individual analytes ranging from 0.11 to 7.42 mg/L. The method was applied for the speciation of arsenic in sediment and determination of SeM in a selenium nutrition supplement. The achieved separation selectivity also gives the method a more general application potential for simultaneous arsenic and selenium speciation when hyphenated with inductively coupled plasma MS.

Determination of arsenic species in fish, crustacean and sediment samples from Thailand using high performance liquid chromatography (HPLC) coupled with inductively coupled plasma mass spectrometry (ICP-MS).

Suitable techniques have been developed for the extraction of arsenic species in a variety of biological and environmental samples from the Pak Pa-Nang Estuary and catchment, located in Southern Thailand, and for their determination using HPLC directly coupled with ICP-MS. The estuary catchment comprises a tin mining area and inhabitants of the region can suffer from various stages of arsenic poisoning. The important arsenic species, AsB, DMA, MMA, and inorganic arsenic (As III and V) have been determined in fish and crustacean samples to provide toxicological information on those fauna which contribute to the local diet. A Hamilton PRP-X100 anion-exchange HPLC system employing a step elution has been used successfully to achieve separation of the arsenic species. A nitric acid microwave digestion procedure, followed by carrier gas nitrogen addition- (N2)-ICP-MS analysis was used to measure total arsenic in sample digests and extracts. The arsenic speciation of the biological samples was preserved using a Trypsin enzymatic extraction procedure. Extraction efficiencies were high, with values of 82-102%(As) for fish and crustacean samples. Validation for these procedures was carried out using certified reference materials. Fish and crustacean samples from the Pak Pa-Nang Estuary showed a range for total arsenic concentration, up to 17 microg g(-1) dry mass. The major species of arsenic in all fauna samples taken was AsB, together with smaller quantities of DMA and, more importantly, inorganic As. For sediment samples, arsenic species were determined following phosphoric acid (1 M H3PO4) extraction in an open focused microwave system. A phosphate-based eluant, pH 6-7.5, with anion exchange HPLC coupled with ICP-MS was used for separation and detection of AsIII, AsV, MMA and DMA. The optimum conditions, identified using an estuarine sediment reference material (LGC), were achieved using 45 W power and a 20 minute heating period for extraction of 0.5 g sediment. The stability and recovery of arsenic species under the extraction conditions were also determined by a spiking procedure which included the estuarine sediment reference material. The results show good stability for all species after extraction with a variability of less than 10%. Total concentrations of arsenic in the sediments from the Pak Pa-Nang river catchment and the estuary covered the ranges 7-269 microg g(-1)and 4-20 [micro sign]g g(-1)(dry weight), respectively. AsV was the major species found in all the sediment samples with smaller quantities of AsIII. The presence of the more toxic inorganic forms of arsenic in both sediments and biota samples has implications for human health, particularly as they are readily 'available'.

Arsenic extractability with phosphate and citrate from peat collected in Bangladesh

Arsenic extractability with various solutions from peat samples collected in Deuli Village, Bangladesh, was investigated to reveal the leaching mechanisms of arsenic in sediments to groundwater. Peat layer was distributed at the depth of 7–8 m in the sediments, and the peat samples collected and used in this experiment contained 137 mg kg−1 arsenic. A wide range of pH values and concentrations of phosphoric acid and trisodium citrate was used for the extracting solutions. Arsenic extractability with Hel (25–1,000 mM) or NaOH (20–500 mM) at different pH values ranged for 30 to 60% in strong acid or alkali, while was less than 0.1% in the neutral pH range against the total content. Extraction of arsenic with 100 mM phosphoric acid and 100 mM trisodium citrate was about 20% even at a neutral pH, while the extractability with these solutions at concentrations below 1 mM was similar to that with distilled water. These results suggest that competitive exchange and / or chelating action are / is the prior possible mechanism for arsenic leaching from peat. However, the experimental conditions such as pH value or phosphate anion concentration differed from those in the study area where the pH of groundwater was almost neutral and the phosphate anion concentration was only 0.01 mM. Moreover, since the experiment was conducted using a batch system under oxidative conditions and not using a continuous flow system, the importance of the oxidation-reduction potential in the sediments and organic matter content in the groundwater should be investigated.

Vertical distribution of arsenic in ganges delta sediments in Deuli Village, Bangladesh

Major and trace elements, including total arsenic (T-As), 1 M HCl-extractable arsenic (Ex-As), total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS), in sediment columns collected in 2000 through 5 boring operations to a depth of 15 m in Deuli Village, Bangladesh were analyzed to reveal the source of arsenic contamination in groundwater. This area mainly consists of sandy, clayey, and peaty layers, and the chemical composition changes considerably with the sediment properties. The concentrations of T-As (20 to 111 mg kg−1) in the samples of peat and peaty clay sediments from a depth of 7 to 10 m were significantly higher than the concentrations in the samples of clayey sediments (4 to 18 mg kg−1) or sandy sediments (3 to 7 mg kg−1), and the concentrations of the other elements were also high in the samples of peaty sediments. T-As concentration shows a significant correlation with the contents of both TS and TOC (p < 0.05). These results suggest that arsenic is retained in the sediments in the form of sulfide or is combined with organic matter. On the other hand, the extraction rate (Ex-As / T-As) changed depending on the Toe content in the sediments, in spite of the high correlation between the concentrations of T-As and Ex-As. This indicates that arsenic speciation was different in the samples of peaty sediments from the sandy and clayey sediments with a relatively low TOC content. The peat layer, in which arsenic is concentrated as a pool, may be the key layer to elucidate the mechanism of solubilization of arsenic in the sediments by groundwater.

Arsenic pollution in groundwater: a self-organizing complex geochemical process in the deltaic sedimentary environment, Bangladesh

The presence of considerable concentrations of As (Sonargaon: below detection limit (bdl)—1.46 mg/l; Faridpur: bdl—1.66 mg/l) and some other elements (like B, F, U) in groundwater of the Ganges–Meghna–Brahmaputra (G–M–B) rivers flood plain indicate that several millions of people are consuming contaminated water. Conditions regulating the mobilization and diagenetic behavior of arsenic in sediments are not well characterized, although understanding these conditions is essential in order to predict the modes of transfer of this contaminant from sediments to groundwater. Analyses of vertical profiles of total arsenic and iron as well as easily soluble As and reducible (reactive) iron concentrations in sediments of the Ganges and Meghna flood plains show no arsenic-enriched layer up to 36-m depth. However, arsenic content in sediments is relatively higher than mean crustal concentration, showing some peaks (Sonargaon: 27.9 mg/kg; 3 m, 31.5 mg/kg; 9 m, 27.30 mg/kg; 16 m, 37.70 mg/kg; 29.5 m, Faridpur: 19.80 mg/kg; 6 m, 26.60 mg/kg; 14.5 m, 29.40 mg/kg; 25 m) depending on the periodical differences in sedimentary cycling of arsenic, metal (hydr)oxides and organic matter. Seasonal changes have no clear or consistent effect on the groundwater arsenic concentrations; with the exception of a small-scale localized irregular change (10–16%). However, easily reducible metal oxides and hydroxides were significant factors affecting the retention of arsenic by sediments during leaching. The biogeochemical cycling of arsenic and iron is closely coupled in deltaic systems where iron oxy-hydroxides provide a carrier phase for the deposition of arsenic in sediments. Analytical results of mimic leaching experiments strongly supported the reduction (Fe oxy-hydroxides) mechanism for arsenic mobilization in alluvial aquifer of deltaic sedimentary environment of G–M–B rivers flood plain.