Nobuyuki Yanase

A natural attenuation of arsenic in drainage from an abandoned arsenic mine dump

Abstract At the abandoned As mine in Nishinomaki, Japan, discharged water from the mining and waste dump area is acidic and rich in As. However, the As concentration in the drainage has been decreased to below the maximum contaminant level (0.01 mg/l for drinking water, Japan) without any artificial treatments before mixing with a tributary to populated areas. This implies that the As concentration in water from the waste dump area has been naturally attenuated. To elucidate the reaction mechanisms of the natural attenuation, analysis of water quality and characterization of the precipitates from the stream floor were performed by measuring pH, ORP and electric conductivity on-site, as well as X-ray diffraction, ICP-mass spectrometry and ion-chromatography. Selective extractions and mineral alteration experiments were also conducted to estimate the distribution of As in constituent phases of the precipitates and to understand the stability of As-bearing phases, respectively. The water contamination resulted from oxidation of sulfide minerals in the waste rocks, i.e., the oxidation of pyrite and realgar and subsequent release of Fe, SO4, As(V) and proton. The released Fe(II) transformed to Fe(III) by bacterial oxidation; schwertmannite then formed immediately. While the As concentrations in the stream were lowered nearly to background level downstream, those in the ochreous precipitates were up to several tens of mg/g. The As(V) was effectively removed by the formed schwertmannite and had been naturally attenuated. Although schwertmannite is metastable with respect to goethite, the experiments show that the transformation of schwertmannite to goethite may be retarded by the presence of absorbed As(V) in the structure. Therefore, the attenuation of As in the drainage and the retention of As by schwertmannite are expected to be maintained for the long term.

Arsenate sorption on schwertmannite

Abstract The sorption mechanism of arsenate [As(V)] on schwertmannite was investigated by means of batch sorption experiments as a function of As(V) concentration in acidic solution at 25 °C. Structural simulation indicated that the surface sites of schwertmannite comprised various O atom (or hydroxyl) and SO4 groups. Sorption experiments showed that the reactive sites for As(V) sorption are surfacecoordinated SO4 groups rather than surface hydroxyl groups, as reported in earlier studies. The As(V) sorption mechanism involves ligand exchange with surface-adsorbed and structural SO4. The results of the sorption experiments also suggested monodentate As(V) coordination at the surface-adsorbed SO4 sites [(Fe1)2SO4] and bidentate As(V) coordination at the structural SO4 sites [(Fe3)2SO4]. The overall ligand-exchange reaction was 0.61 (Fe1)2SO4 + 0.39 (Fe3)2SO4 + 1.61 H2AsO4- → 1.22 Fe1H2AsO4 + 0.39 (Fe3)2HAsO4 + 0.39 H+ + SO42- where the 1 and 3 in Fe1 and Fe3 are coordination numbers. The equilibrium constant derived for the exchange reaction, log KEX = 4.96, describes the observed As(V) sorption behavior. Nanocrystalline materials like schwertmannite are widespread in nature and typically contain significant amounts of anionic impurities, such as sulfate and silicate. Our results indicate that the effects of impurities can be significant and should be considered in order to gain a realistic understanding of sorption processes in natural systems.

Discharge of dissolved and particulate137Cs in the Kuji River, Japan

Abstract The discharge of fallout137Cs in the water of the Kuji River was studied with the aim of understanding the migration behavior of137Cs in the river watershed. Under normal flow conditions, the radioactivity of dissolved137Cs in the water ranged from 0.05 to 0.12 mBq/l, while that of the particulate137Cs ranged from 0.02 to 0.09 mBq/l. A large increase in the particulate137Cs, up to 0.77 mBq/l, was observed under high flow conditions caused by a 40 mm precipitation event. It was found that the dominant speciation of137Cs changed from the dissolved to the particulate mode according to the flow conditions related to the precipitation event. The annual budget of137Cs in the watershed was estimated based on empirical relation between the load of137Cs and the flow rate. The accumulated137Cs in the watershed was evaluated as 4.2 × 103 MBq/km2. On the other hand, the annual losses were 1.7 MBq/km2 by the discharge into the river and 1 × 102 MBq/km2 by radioactive decay. This budget has proved, qualitatively, that the surface soil acts as a significant adsorbant for atmospherically-derived137Cs.

Particulate and dissolved elemental loads in the Kuji River related to discharge rate.

In order to investigate influences of discharge rates on fluvial transport behavior of elements in the Kuji River, Japan, suspended and dissolved phase concentrations in the river water were measured together with the water flow rates. The concentrations of suspended particulate matter (SPM) increased by two or three orders of magnitude with the water discharge, and also had seasonal variations. Adsorptive elements or heavy metal elements, which tend to form insoluble compounds, were present generally as suspended forms in the river waters, and their dissolved form concentrations tended to increase with the water discharge. On the other hand, non-adsorptive elements such as alkali and alkaline-earth elements were present as dissolved forms under a normal flow rate condition, but equivalent quantity of suspended species also occurred under a high flow condition. In this case, the dissolved form concentrations decreased with the water discharge. Characterization of SPM with analyses of chemical compositions, scanning electron microscopy (SEM) and X-ray diffractometry (XRD) indicated that clay mineral such as montmorillonite would be the main carrier material of trace and major elements during the fluvial transport in the Kuji River. Variations of contents of selected elements in SPM with the water discharge indicated that river bottom sediments, which are probably potential sources of SPM added due to high water flow rates, would be different in chemical compositions from SPM floating under a normal flow condition. Variations of chemical compositions of SPM as well as of suspended or dissolved form concentrations of elements in the river waters were formulated as a function of the water discharge rates.

Measurement of 90Sr in environmental samples by cation-exchange and liquid scintillation counting

Abstract A new method for the measurement of 90Sr in environmental samples by cation-exchange and liquid scintillation counting is described. Strontium carbonate is purified by precipitation and ion-exchange, weighed for the determination of chemical yield, dissolved in hydrochloric acid and mixed with the liquid scintillator, Aquasol-2. Two channels of a low-background liquid scintillation counter are used to determine 90Sr, 90Y and 89Sr, free from the effects of environmental tritium. The values of 90Sr obtained by this method are in good agreement with those from ordinary 90Y milking and the gas proportional counting method. The concentration of 90Sr in the air at Tokai-mura in Japan has been measured by the new method.

Accumulation and potential dissolution of Chernobyl-derived radionuclides in river bottom sediment.

Areas contaminated with radionuclides from the Chernobyl nuclear accident have been identified in Pripyat River near the Chernobyl Nuclear Power Plant. The river bottom sediment cores contained 137Cs (10(5)-10(6) Bq/m2) within 0-30 cm depth, whose concentration is comparable to that in the ground soil in the vicinity of the nuclear power plant (the Exclusion Zone). The sediment cores also accumulated 90Sr (10(5) Bq/m2), (239,240)Pu (10(4) Bq/m2) and 241Am (10(4)Bq/m2) derived from the accident. Several nuclear fuel particles have been preserved at 20-25 cm depth that is the peak area of the concentrations of the radionuclides. These inventories in the bottom sediments were compared with those of the released radionuclides during the accident. An analysis using a selective sequential extraction technique was applied for the radionuclides in the sediments. Results suggest that the possibility of release of 137Cs and (293,240)Pu from the bottom sediment was low compared with 90Sr. The potential dissolution and subsequent transport of 90Sr from the river bottom sediment should be taken into account with respect to the long-term radiological influence on the aquatic environment.

The geochemistry of uranium in pore waters from lake sediments

The concentration of pore water uranium in six sediments from oligotrophic, mesotrophic, acidotrophic and dystrophic lakes was measured by inductively coupled plasma-mass spectrometry. Profiles of pore water U can be divided into two groups such as low (17±7 ng/l) and high concentration (69±30 ng/l). These values were 1–2 orders of magnitude higher than that of lake waters (6±4 ng/l) due to the release of U from the sediments by decomposition of organic materials. Variations in pore water U concentrations seem to relate to the differences in pore water pH, the association forms, and their contents of U in the lake sediments.

The role of suspended particles in the discharge of 210Pb and 7Be within the Kuji River watershed, Japan

Abstract Fluvial discharge of atmospherically-derived 210Pb and 7Be was studied in the Kuji River watershed in Japan. The daily load of 210Pb and 7Be, associated with suspended particles, greatly increased during high-flow conditions. However, the activity concentrations of 210Pb and 7Be in the suspended particles were found to be rather independent of the flow rate. Due to this feature, the daily load of the activities associated with the suspended particles was mainly controlled by that of the suspended particles. Compared with the other reported values, the estimated mean residence time of atmospherically-derived 210Pb in the present study was rather short (about 800 years), possibly reflecting the high availability of the suspended particles eroded from the top soil in the watershed. The annual load of 210Pb and 7Be was predominantly of particulate form (approx 90%). These results suggest that the fluvial discharge of atmospherically-derived, highly adsorptive (kd ≳ 104 mg l−1) radionuclides in a river watershed is primarily due to the discharge of suspended particles, which strongly depends on the flow rate of river water and also on the location of the watershed.

Temporal variations in metal enrichment in suspended particulate matter during rainfall events in a rural stream

We studied enrichment of heavy metals (V, Zn, Cr, Ni, Cu, Pb, As, Sb, and Cd) in a rural stream of the Kuji River basin in central Japan in suspended particulate matter, and associated transport flux during two rainfall events (in November 2003 and in April 2008). The concentration of heavy metals in suspended particulate matter (SPM) exhibited a distinctive temporal variation, wherein the concentrations decreased with increasing water discharge and then increased as the discharge decreased. Concentration of dissolved metal forms showed a slight increase with scatters around the flow rate peak. Enrichment factors for those metals in the SPM decreased sharply as the flow rate increased, making an obvious concave shaped curve (the November 2003 rainfall event). The metal enrichment factors under low flow conditions had a similarity to those found in atmospheric deposits at a foot of the Kuji River basin, suggesting atmospheric source would contribute to enriching the SPM with those metals in part. Mineralogical analyses and carbon content analysis (the April 2008 rainfall event) of the SPM suggests the SPM matrix became more lithological as the flow rate increased. The changes observed in the matrix are thought to be directly related to progressive changes in metal enrichment within the SPM. Concerning the transport phase of several heavy metals, a dynamic change in transport phase partitioning within a rainfall event was suggested. The present enrichment factor study and the SPM matrix characterization implied the partitioning change is due to an increase in lithologic solids during high flow conditions.

Characterization of hot particles in surface soil around the Chernobyl NPP

Surface soil samples (0–1 cm) were collected to understand the role of hot particle (HP) in migration of radionuclide approximately 11 years after the Chernobyl accident. Three types of HPs were found by the analyses using alpha-track technique and scanning electron microscopy/energy dispersive X-ray spectrometer (SEM/EDX). The dissolution of HPs has been affected by the existing type of HPs. Six-stage sequential selective extraction procedure was carried out to define the chemical forms of uranium, 241Am and 137Cs. The chemical form of radionuclides in surface soil is significantly different between the west and the north traces of radioactive plumes approximately 11 years after the accident.