Junji Akai

Geochemical occurrence of arsenic in groundwater of Bangladesh: sources and mobilization processes

Abstract Total arsenic, dissolved organic carbon (DOC), iron, major cations and anions were measured in tube-well waters used for drinking by people to investigate the geochemical condition of subsurface aquifer, source of arsenic and mechanism of arsenic release to groundwater of Bangladesh. Solid phases of total arsenic, iron, manganese and aluminum oxide and organic carbon (TOC) content in sediments were determined to find out their interrelationships. Arsenic concentrations in groundwaters vary from 0.03 to 0.75 mg/l with the mean value of 0.41 mg/l that exceed the maximum permissible limit of WHO (0.01 mg/l) and Bangladesh (0.05 mg/l) for drinking water. Arsenic concentrations demonstrate negative covariation with the concentrations of sulfate and nitrate but correlate weakly with iron concentrations and positively with those of ammonium ions. Dissolved iron exhibits a negative covariance with the concentrations of sulfate. Very low concentrations of nitrate and sulfate and high concentrations of dissolved iron and ammonium ions demonstrate the reducing condition of subsurface aquifer. These relationships suggest that oxidation of arsenic-rich pyrite is not responsible for the increased concentrations of arsenic but reflect the dependence of As concentration on the reductive processes. Arsenic is strongly correlated with DOC concentrations. Borehole data demonstrate the arsenic enrichment in organic matter-rich fine-grained clayey silt and silty sand than in sandy sediments. Arsenic contents are well correlated with those of iron, manganese and aluminum oxides in fine particle fraction of sediments, whereas arsenic is significantly correlated with TOC in large particle fraction. Sequential chemical leaching exhibits that arsenic is mainly present in three phases: (1) oxide phase of Fe and Mn, (2) organic matter and (3) sulfide and silicate phases. Since the distribution of arsenic in the subsurface sediments is not solely controlled by a single solid phase, the dissolution–desorption from different phases contributes to the total arsenic concentrations in groundwater. Microbial oxidation of organic matter (including localized peat layer) and reductive dissolution of Fe and Mn oxyhydroxide are the important processes to mobilize arsenic. The combined effects of NaHCO3 and high pH values also play a significant role to mobilize arsenic from surface of iron oxides, other minerals and subsurface sediments. The very strong relationship between arsenic and bicarbonate concentrations in groundwater of Bangladesh and high pH values (8.03–8.7) of the Ganges sediments support the hypothesis. This paper reports first about arsenic leaching by the combined effects of NaHCO3 and high pH values in Bangladesh.

Arsenic poisoning in groundwater: Health risk and geochemical sources in Bangladesh

Of the 2508 water samples analyzed in 10 districts of Bangladesh, 51%, on an average, contained arsenic levels of 0.05 to 2.50 mg/l. 95% of nail, 96% of hair, and 94% of urine samples contained arsenic above the normal level. Approximately 3.58 million people out of a total of 17.92 million who are drinking water containing arsenic levels >0.20 mg/l are potentially exposed to high risk of health hazard. Eight thousand and five hundred arsenic patients are identified; they are suffering from various skin lesions, gangrene in leg, skin, lung, bladder, liver, and renal cancer. A big portion of the total population is highly vulnerable to various internal cancers. Lowest arsenic concentration in drinking water producing dermatological disease is found to be 0.103 mg/l. However, the exposure time to develop arsenicosis varies from case to case reflecting its dependence on arsenic level in drinking water and food, nutritional status, genetic variant of human being, and compounding factors. This study has determined the high intensity of fluorescent humic substances in drinking water containing elevated concentrations of arsenic and very low concentrations of heavy metals. The synergistic/antagonistic effect of fluorescent compounds present in drinking water may aggravate the toxicity of arsenic. Geochemical study suggests that arsenic may be released from both reductive dissolution of Fe and Mn (oxy)hydroxide and microbial oxidation of organic matter.

Incompletely transformed serpentine-type phyllosilicates in the matrix of antarctic CM chondrites

Abstract Recent HREM (high resolution electron microscopy) investigation has shed light on the characterization of matrix phyllosilicates in CM chondrites. In examining the Antarctic CM chondrite, Yamato-793321, by HREM, mineral grains which show an ED (electron diffraction) pattern consisting of both olivine and serpentine spots show disordered and defect structures in patchy HREM images. These data are interpreted as representing an ‘intermediate phase’ in the transformation from serpentine to olivine. This is the first description of such an ‘intermediate phase’. In Belgica-7904, phyllosilicates seem to have been almost completely transformed to olivine. It is suggested that it formed by heating of matrix phyllosilicates. Preliminary heating experiments on Murchison (also a CM chondrite) showed that the matrix phyllosilicates transformed to a phase which gave an ED pattern and HREM image similar to those for Yamato-793321. The results of these heating experiments support the formation of the ‘intermediate phase’ by transformation from phyllosilicate to olivine during a heating event at a late stage.

Biologically induced iron ore at Gunma iron mine, Japan

Abstract The mineralogy of sedimentary iron ores from the Gunma iron mine are described to evaluate the role of microorganisms and plants in ore formation. The iron ore is composed of nanocrystalline goethite, well-crystallized jarosite and very small amounts of strengite. The ore characteristically occurs as thick-bands of alternating goethite and jarosite bands, thin-bands of different goethite grain sizes, and fossil-aggregate ore rich in moss and/or leaves. Algal fossils are clearly preserved in the goethite bands in the thick-banded ore. Lattice imaging showed characteristic crystallographic orientations of the goethite nanocrystals. The thin-banded iron ores consist of micrometer-sized chestnut-burr-like goethite aggregates, probably formed by bacterial iron biomineralization. The bands may be attributed to biological or seasonal rhythms. Various products of biomineralization are found in the present-day pH 2-3, Fe2+-, and SO42--rich streams. Bacterial precipitation had var- 4 iations from amorphous Fe-P-(S) precipitates near the outlet of mineral spring to Fe-P-S precipitates and to Fe-S-(P) (schwertmannite-like) precipitates in the midstream. Mosses and green algae are also collecting Fe precipitates in and around the living and dead cells. Comparison of the processes occurring in the present-day streams and the iron-ore specimens supports the interpretation of these ores as the product of biomineralization.

Icosahedral domain structure of framboidal pyrite

Abstract A new type of framboidal pyrite, with icosahedral domains, is described in this study. Examining the microcrystals on sections of framboids from various localities using a scanning electron microscope, we found pentagonal and trigonal patterns. These are made up of rectangular and fan-shaped domains, and octahedral microcrystals are regularly linked by sharing of edges in each domain. These symmetrical arrangements are interpreted to be different sections of icosahedrally arranged framboids which are composed of twenty tetrahedral domains. Thus some pyrite framboids are not spherical, but are fundamentally icosahedral both in appearance and internal structure. The formation of the icosahedral framboids might be related to the initial nucleation rate and the number of microcrystals within each framboid.

Growth controls in colloform pyrite

Abstract Primary colloform textures preserved in ore deposits can be a useful tool in understanding changing conditions of ore formation due to the sequential development of the colloform layers. However, the growth controls that influence formation of these textures are poorly understood. To try to address this problem, samples from two ore deposits, Greens Creek in Alaska and Ezuri in Japan, have been systematically analyzed for grain size and shape, crystal preferred orientation (CPO), sulfur isotope composition, and trace element content. Grain size and shape varies between layers of equant, ~20 μm crystals to acicular and elongate crystals up to several millimeters in length. Electron backscatter diffraction (EBSD) reveals that both samples have an initial random orientation of crystals with CPO in subsequent layers developed either about <100>, <110>, or <111> crystallographic axes. Despite similarity in texture, the sulfur isotope results from Greens Creek colloforms have a very negative, open-system bacteriogenic δ34S between -40 and -32‰, whereas the Ezuri colloform has a positive δ34S of ~+5‰, typical of hydrothermal sulfur in Kuroko ores. Trace element results indicate variability in As, Sb, and Cu distribution. Whereas trace element variability at Greens Creek appears to be related to changes in δ34S, with a heavier signature correlating with sequestration of Sb in outer layers, overall the detailed analyses reveal that in both Greens Creek and Ezuri, there is no systematic correlation between sulfur source or trace element sequestration and CPO. This suggests that the abrupt changes in CPO recorded appear most likely to be influenced by changes in degree of supersaturation.

Contrasting Fe-Ca distributions and related microtextures in syenite alkali feldspar from the Patagonian Andes, Chile

Abstract Contrasting distribution patterns of Fe and Ca have been found by electron microprobe analysis (EMPA) mapping of alkali feldspar in a quartz syenite from the Patagonian Andes, Chile. They comprise mainly mantle zoning (Fe-rich, Ca-poor rims and Fe-poor, Ca-rich interiors) and corresponding patchy zoning in grain interiors. The rims are dominantly of turbid, patch microperthites associated with abundant micropores, but there remain clear, optically featureless regions almost free of micropores. The interiors are intricate mixtures of optically clear, featureless regions, and turbid, patch microperthite regions. The clear, featureless regions (Or31−47) are of remaining exsolution lamellar cryptoperthites. The zoning patterns of Fe and Ca formed by large-scale transport over the feldspar grain during the high-temperature fluid stage. They have been modified by successive transport of Fe and Ca during the later hydrothermal development of patch microperthites and finally by K- feldspathization and albitization. Cathodoluminescence images correspond to the spatial distribution patterns of Fe overprinted by these multi-stage reactions. The original composition of the alkali feldspar before the subsolidus reactions is estimated to have been ~Or34Ab65An1, and the present bulk composition after the reactions is Or40Ab59An0.5.

Polymerization Process of Biopyribole in Metasomatism at the Akatani Ore Deposit,Japan

Clinopyroxene transforms to triple chain silicate, double chain silicate (amphibole) and sheet silicate (talc) in the metasomatic process of the Akatani ore deposit. The triple chain silicate is contained in fibrous amphibole-like phase (“amphibole”). It is of electron microscopic size of 1,000 Å at maximum width in b-direction, and is a calciferous analogue of clinojimthompsonite. Various kinds of fine textures formed in metasomatic reaction process were found in clinopyroxenes. A large amount of triple chains and a small amount of double chains were transformed from single chains in the host clinopyroxene, maintaining topotactic relation. The kinetics of the structural change of biopyriboles at the Akatani ore deposit was discussed from the viewpoint of the mode of occurrence of triple and double chain silicates. The nucleation of triple chain structure slab with one triple chain width is apt to occur rather than the nucleation of double chain structure slab with width of two double chains in clinopyroxene host. Various fine textures in clinopyroxenes and amphibole-like phase were interpreted as corresponding to the propagation of metasomatic reactions.

Fluorite particles inducing butterfly aggregates of incipient microperthite in alkali feldspar from a syenite, the Patagonian Andes, southern Chile

Abstract Alkali feldspar grains found in a syenite from the Patagonian Andes, southern Chile, have bulk compositions of about Or40Ab59An0.5, and consist of two parts when viewed under an optical microscope: a clear part and a turbid part. Grain interiors are mixtures of the two parts, whereas the rims are mainly turbid. The microscopically clear part, which is almost free of micropores, is cryptoperthitic, whereas the turbid part is microperthitic. The microperthite is of the patch type, and the turbidity is due to abundant micropores that are polygonal and generally less than 1 mm in diameter. The patch microperthite has been formed by coarsening of primary cryptoperthite by hydrothermal reactions. An incipient stage of the microperthite formation is recorded as the segregation of the Or-rich feldspar with diagonal elongation and Ab-rich feldspar into aggregates that have a “butterfly” shape. Each butterfly aggregate of microperthite is generally less than 10 mm in length. The centers of the butterfly aggregates are usually occupied by round fluorite particles about 1 mm in diameter, which were identified by EPMA and TEM analyses. The fluorite particles may have been formed at the fluid stage. The microperthite formation may have started as butterfly aggregates along the interfaces with the fluorite particles at the hydrothermal stage. The butterfly aggregates have changed to patch microperthite with further coarsening. The timing and process of the formation of the fluorite particles are important in relation to the evolution of feldspar microtextures, and the behavior of fluorine in alkaline igneous rocks.

Sources of organic carbon and depositional environment in the Bengal delta plain sediments during the Holocene period

This study investigated the sources of organic matter and sediment depositional environment within fluviatile sediments of the Ganges–Meghna (GM) delta plains. The very low contents of trace metals e.g., chromium (Cr), cobalt (Co), scandium (Sc), and vanadium (V), organic carbon content, and cerium (Ce)-anomaly data of sediments indicate the redox conditions that fall within the boundary of oxic–anoxic condition, with dominantly oxic conditions in the sediment deposition environment. The higher atomic carbon nitrogen (C/N)a ratios and depleted stable carbon isotope ratio (δ13C) values for sediments from three study areas indicated the terrestrial sources of organic matter derived from C3 plant materials, whereas the contribution of organic materials from C4 vegetation and riverine productivity is low. Some silty sand samples exhibited lower (C/N)a ratios and enriched δ13C values in Sonargaon and Faridpur areas that are attributed to the adsorption of ammonium ions on clay minerals and the contribution of organic matter from C4 plants. Total sulfur over total organic carbon (TS/TOC) ratios in sediments of the Ganges delta reflect the nonmarine environments of sediment deposition. The lower ratios of syringyl to vanillyl phenols (S/V), cinnamyl to vanillyl phenols (C/V), and acid to aldehyde in vanillyl phenols (Ad/Al)v observed in Daudkandi indicate that the lignin in sediments derived from dominant woody gymnosperm sources and is very highly degraded. By contrast, the S/V ratio, C/V ratios, and [Ad/Al]v ratios in Faridpur suggest that the lignin in sediments derived from a mixture of woody and nonwoody angiosperm plant tissue contribution that underwent high degradation as well.