Munetsugu Kawashima

The role of Mn2+-rich hydrous manganese oxide in the accumulation of arsenic in lake sediments

Arsenic is present at high concentrations in the upper layer of Lake Biwa sediments and shows a depth profile similar to that of Mn. Adsorption experiments of As onto synthetic hydrous Mn oxide (HMO) in the presence of Mn2+ and the speciation of Mn in the sediment cores, suggest that the accumulation of As at the sediment surface results from post-depositional migration of arsenite in the sediment pore water followed by oxidation to arsenate at the sediment surface and adsorption onto Mn2+-rich HMO.

Phosphate adsorption onto hydrous manganese(IV) oxide in the presence of divalent cations

Abstract Alkaline earth cations, Ba2+, Sr2+, Ca2+, Mg2+ and transition metal ions, Mn2+, Co2+, Ni2+, cause hydrous manganese(IV) oxide (HMO) to strongly adsorb phosphate between pH 6 and 9 depending on the cation. The effectiveness of the alkaline earth cations to cause P adsorption was Ba2+ > Sr2+ > Ca2+ > Mg2+, which is the same order as their affinities for the oxide. Changes with time were found in the abilities of the transition metals to cause P adsorption onto HMO and this may be due to conversion of the adsorbed cation to its oxide. A new potential role for HMO as an adsorbent of phosphate in natural waters was indicated.

Mechanisms of precipitation of manganese(II) in Lake Biwa, a fresh water lake

Abstract The precipitation mechanisms of Mn(II) were studied from the viewpoint of adsorption onto suspended solids (SS) and microbial oxidation of Mn in Lake Biwa. In aerobic water samples maintained in the laboratory, the concentration of dissolved Mn [Mn(II)] decreased at pH > 7 by microbially mediated oxidation. Filtration, autoclaving and irradiation of u.v. light of the water samples and the addition of NaN 3 inhibited the precipitation of Mn(II). The adsorption of Mn(II) onto SS was also appreciable at pH > 7, and attained equilibrium within at least 30 min. The oxidation rate of Mn(II) was much less than the adsorption rate. In various environments of the lake (bottom water, sediment surface, river mouth, etc.) the initial adsorption of Mn(II) and subsequent slow oxidation mediated by bacteria may be essential to the mechanisms of Mn(II) precipitation.

The budget of dissolved trace metals in Lake Biwa, Japan

A comprehensive study on the dynamics of dissolved elements (Mg, Al, Si, P, Ca, V, Cr, Mn, Fe, Ni, Zn, As, Sr, Y, W, and U) in Lake Biwa was carried out using a clean technique. Lake water samples (n = 523) were collected from six stations in the North Basin and three stations in the South Basin. River water samples (n = 178) were collected from 14 major rivers flowing into the North Basin. Rainwater samples (n = 89) were collected at Otsu. The river water was enriched with Mn, Al, Fe, P, and Zn and the rainwater was enriched with Zn, Al, Fe, and Mn compared to North Basin water during winter mixing. The residence times of dissolved species were estimated on the basis of input through the rivers and rain. The residence times for Ca, Mg, and Sr were about 8 years, the same as that for water. Mn, Al, Fe, and Zn showed the shortest residence times (0.05–0.19 year). A budget calculation suggested that more than 60% of the input of dissolved Si, P, V, Cr, Mn, Fe, Ni, and Zn was scavenged and retained in the lake sediments and/or discharged as suspended particles.

Urea degradation by picophytoplankton in the euphotic zone of Lake Biwa

Abstract The ability of photoautotrophic picoplankton Synechococcus to degrade urea was examined in the euphotic zone of Lake Biwa. Samples were divided into pico (0.2–2.0 μm) and larger (>2.0 μm) size fractions by filtration. The rates of urea degradation (the sum of the rates of incorporation of carbon into phytoplankton cells and of liberation of CO2 into water) measured by radiocarbon urea were 8 and 17 μmol urea m−3 day−1 in June and July, respectively, for the picophytoplankton in the surface water, and 196 and 96 μmol urea m−3 day−1, respectively for the larger phytoplankton. The rates decreased with depth, somewhat similar to the vertical profiles of the photosynthetic rate. The urea degradation rates were obviously high under light conditions. In daylight, urea was degraded into two phases, carbon incorporation and CO2 liberation, whereas in the dark it was degraded only into the CO2 liberation phase. The contribution of picophytoplankton to total phytoplankton in urea degradation was high in the subsurface to lower euphotic layer. Urea degradation activity was higher in the picophytoplankton fraction than in the larger phytoplankton fraction. Shorter residence times of urea were obtained in the upper euphotic zone. The contribution of picophytoplankton to urea cycling was 4% to 35%. The present results suggest that the picophytoplankton Synechococcus is able to degrade urea and effectively makes use of regenerated urea as a nitrogen source in the euphotic layer, and that picophytoplankton play an important role in the biogeochemical nitrogen cycle in Lake Biwa.

Characterization of suspended solids in Lake Biwa by measuring their elemental composition of Al, Si, P, S, K, Ca, Ti, Mn, and Fe

Abstract We investigated the concentrations of particulate Al, Si, P, S, K, Ca, Ti, Mn, and Fe at various areas of Lake Biwa, such as the limnetic zone in the northern basin, the offshore zone, the dredged area and Akanoi Bay in the southern basin, and reed community areas in the shore. In the wide and deep northern basin, the density of suspended solids was low, and substantial fractions of particulate Si, P, S, and Ca were biogenic matter produced by phytoplankton. In the offshore zone in the narrow and shallow southern basin, the concentration ratios of particulate Si, K, Ca, Ti, and Fe against particulate Al were almost constant throughout the year and were similar to those found in crusts. Therefore, these elements are most likely derived from terrigenous clay matter. In the dredged area where the hypolimnion became anoxic during the stratification period, the concentrations of particulate Fe, Mn, P, S, and Ca increased because of the formation of hydrous Fe and Mn oxides and Fe sulfide. In Akanoi Bay, the ratios of Mn/Al and Fe/Al were high because of the stirring of sediments enriched with Mn and Fe oxides. In the reed community areas, the concentration of suspended solids correlated well with particulate Fe and P, but not with Al or chlorophyll a; therefore these areas seem to be rich in humic organic matter enriched with P and Fe due to microbial activity. Thus, the elemental composition of suspended solids reflects the chemical, biological, and physical processes in the lake and gives us useful indices of the characteristics of the water.

A new method for temperature compensation of electrical conductivity using temperature-fold dependency of fresh water

A new method for temperature compensation of electrical conductivity (EC) is proposed. A relationship between temperature and EC was investigated in detail by a simple experiment using natural and artificial fresh water. Results showed that the temperature dependency of EC varied from 0.03 to 0.02, with temperature increasing from 3°C to 35°C. This means that a traditional method for compensating for EC using a constant coefficient is invalid, so that a temperature-fold dependency must be taken into consideration to standardize EC to a common temperature.

Mineralogy and geochemistry of sediments from Lakes Taupo and Waikaremoana, New Zealand

Abstract Sediments from Lakes Taupo and Waikaremoana, New Zealand, have been analysed to give background concentrations of trace elements in lakes relatively uninfluenced by anthropogenic contributions. Lake Waikaremoana sediments have higher contents of illite, chlorite, smectite, plagioclase, and quartz as well as higher contents of Al, Fe, Na, K, Mg, Ti, P, V, Cr, Co, Ni, Zn, Rb, Sr, Ba, Sc, U, Th, and Rare Earth Elements (REE), than Lake Taupo sediments. Diagenetic enrichment of Mn, Ni, Cu, Zn, P, As, and Fe was observed in surface sediments of certain cores from Lake Taupo where the surface sediment layer was preserved during coring.