Chihiro Yoshimura

Effects of Molecular Composition of Natural Organic Matter on Ferric Iron Complexation at Circumneutral pH

Thermodynamic and kinetic parameters for ferric iron (Fe[III]) complexation by well-characterized humic substances (HS) from various origins were determined by a competitive ligand method with 5-sulfosalicylic acid at circumneutral pH (6.0-8.0) and an ionic strength of ∼0.06 M. The measured Fe binding properties including conditional stability constants and complexation capacities ranged over more than 2 orders of magnitude, depending on the origin and the particular operationally defined fraction of HS examined. Statistical comparison of the complexation parameters to a range of chemical properties of the HS indicated a strong positive correlation between Fe(III) complexation capacity and aromatic carbon content in the HS at all pHs examined. In contrast, the complexation capacity was determined to be up to a few orders of magnitude smaller than the concentration of carboxylic and phenolic groups present. Therefore, specific functional groups including those resident in the proximity of aromatic structures within the HS are likely preferable for Fe(III) coordination under the conditions examined. Overall, our results suggest that the concentration of dissolved Fe(III) complexes in natural waters is substantially influenced by variation in HS characteristics in addition to other well-known factors such as HS concentration and nature and concentration of competing cations present.

Chemical properties, microbial respiration, and decomposition of coarse and fine particulate organic matter

Abstract Fine particulate organic matter (FPOM) plays a critical role in structuring and sustaining stream food webs by providing an essential resource for various organisms. Our goal was to elucidate FPOM dynamics by determining chemical properties, microbial respiration, and in situ decomposition rates of different FPOM fractions in relation to the parent coarse particulate organic matter (CPOM). FPOM (100–500 μm) of defined quality was produced by feeding 5 types of CPOM to shredding amphipods (Gammarus spp.): wood, filamentous green algae, and conditioned leaves of ash, alder, and oak. Feeding and defecation of Gammarus homogenized POM of the different origins in terms of proximate lignin and nutrient content. FPOM had higher lignin content (20.5–45.6%) than did parental CPOM (5.7–26.8%), whereas molar C:N decreased during the conversion of CPOM (12–109) to FPOM (10–34). Microbial respiration rates on leaf-derived FPOM were lower (0.13–0.45 mg O2 g−1 C h−1) compared to rates measured for parent CPOM (0.37–0.80 mg O2 g−1 C h−1). Furthermore, microbial decomposition over 2 mo in a stream was slower for leaf-derived FPOM (k < 0.0015/d) than for the parent CPOM (k = 0.0013–0.0049/d), and this pattern resulted in a positive correlation between rates of microbial respiration and decomposition. Overall, our data indicate that transformation of CPOM to FPOM has a homogenizing effect toward lower C quality, which, in turn, reduces microbial activity and decomposition rate.

2020s scenario analysis of nutrient load in the Mekong River Basin using a distributed hydrological model

A distributed hydrological model, YHyM, was integrated with the export coefficient concept and applied to simulate the nutrient load in the Mekong River Basin. In the validation period (1992-1999), Nash-Sutcliffe efficiency was 76.4% for discharge, 65.9% for total nitrogen, and 45.3% for total phosphorus at Khong Chiam. Using the model, scenario analysis was then performed for the 2020s taking into account major anthropogenic factors: climate change, population, land cover, fertilizer use, and industrial waste water. The results show that the load at Kompong Cham in 2020s is 6.3 x 10(4)tN a(-1) (+13.0% compared to 1990s) and 4.3 x 10(3)tP a(-1) (+24.7%). Overall, the noticeable nutrient sources are cropland in the middle region and urban load in the lower region. The installation of waste water treatment plants in urban areas possibly cut 60.6%N and 19.9%P of the estimated increase in the case without any treatment.

Mechanism and kinetics of ligand exchange between ferric citrate and desferrioxamine B.

The kinetics of ligand exchange between ferric citrate and desferrioxamine B (DFB) was investigated at pH 8.0 and high citrate/Fe molar ratios (500-5000) with particular attention given to understanding the precise mechanism of ligand exchange. Ferric citrate complexes present in a test solution and therefore involved in the reaction with the incoming ligand (DFB) were initially examined by evaluating ferric citrate speciation on the basis of published thermodynamic constants. The speciation analysis indicated that mononuclear (mono- and dicitrate) ferric complexes are the major species responsible for the ligand exchange with DFB under the conditions examined in the present work. Given the tendency of DFB to adjunctively associate with the ferric citrate complexes, we propose a kinetic model containing the following three mechanisms: (i) direct association of DFB to the ferric dicitrate complex prior to any dissociation of citrate molecules from the Fe center, (ii) adjunctive association of DFB toward ferric monocitrate complex following dissociation of one molecule of citrate from the parent complex, and (iii) complexation of hydrated Fe by DFB after sequential dissociation of two molecules of citrate from the Fe center. Overall rates for the ligand exchange were determined by spectrophotometrically monitoring the formation of ferrioxamine B. Further analysis in quantifying the rate of each mechanism by use of published and determined rate constants of relevant elemental reactions suggested that the first and second mechanisms were significant under our experimental conditions where [Cit] ≫ [DFB] with the relative importance of these two pathways depending on citrate concentration.

Correlations between aromaticity of dissolved organic matter and trace metal concentrations in natural and effluent waters: A case study in the Sagami River Basin, Japan

Chemical speciation, reactivity, and bioavailability of trace metals in aqueous systems arestrongly influenced by dissolved organic matter (DOM). DOM is a mixture of diverse components, so a range of organic molecules potentially participates in the occurrence of dissolved trace metals. In this study, we investigated water quality variables that influence dissolved trace metal concentrations in natural and effluent water systems with a particular attention given to the relationship between DOM optical properties and dissolved copper and iron concentrations. We found that specific UV absorbance (SUVA254: an indicator of DOM aromaticity) has a significant correlation with dissolved trace metal to dissolved organic carbon concentration ratios ([Me]T/[DOC]) for copper and iron in natural freshwaters and treated municipal wastewater in the Sagami River basin, Japan. This trend was also prevalent for other freshwaters in temperate climates except for Fe-rich waters. Our findings indicate that the concentrations of dissolved copper and iron in natural and effluent waters are significantly influenced not only by DOM concentration, but also by aromaticity of DOM, and that this DOM property can be inferred from spectrophotometric measurements.

A simulation-based suitability index of the quality and quantity of agricultural drainage water for reuse in irrigation.

The suitability of agricultural drainage water (ADW) for reuse in irrigation was indexed based on a simulation of quality and quantity. The ADW reuse index (DWRI) has two components; the first one indicates the suitability of water quality (QLT) for reuse in irrigation based on the mixing ratio of ADW to canal irrigation water without violating the standards of using mixed water in irrigation, while the second indicates the available water quantity (QNT) based on the ratio of the available ADW to the required reuse discharge to meet the irrigation requirements alongside the drain. The QLT and QNT values ranged from 0 to ≥3 and from 0 to ≥0.40, respectively. Correspondingly, five classes from excellent to poor and from high scarcity to no scarcity were proposed to classify the QLT and QNT values, respectively. This approach was then applied to the Gharbia drain in the Nile Delta, Egypt, combined with QUAL2Kw simulations in the summer and winter of 2012. The QLT values along the drain ranged from 1.11 to 2.91 and 0.68 to 1.73 for summer and winter, respectively. Correspondingly, the QLT classes ranged from good to very good and from fair to good, respectively. In regard to QNT, values ranged from 0.10 to 0.62 and from 0.10 to 0.88 for summer and winter, respectively. Correspondingly, the QNT classes ranged from medium scarcity to no scarcity for both seasons. The demonstration of DWRI in the Gharbia drain suggests that the proposed index presents a simple tool for spatially evaluating the suitability of ADW for reuse in irrigation.

Evaluation of Spatial Pattern of Altered Flow Regimes on a River Network Using a Distributed Hydrological Model

Alteration of the spatial variability of natural flow regimes has been less studied than that of the temporal variability, despite its ecological importance for river ecosystems. Here, we aimed to quantify the spatial patterns of flow regime alterations along a river network in the Sagami River, Japan, by estimating river discharge under natural and altered flow conditions. We used a distributed hydrological model, which simulates hydrological processes spatiotemporally, to estimate 20-year daily river discharge along the river network. Then, 33 hydrologic indices (i.e., Indicators of Hydrologic Alteration) were calculated from the simulated discharge to estimate the spatial patterns of their alterations. Some hydrologic indices were relatively well estimated such as the magnitude and timing of maximum flows, monthly median flows, and the frequency of low and high flow pulses. The accuracy was evaluated with correlation analysis (r > 0.4) and the Kolmogorov–Smirnov test (α = 0.05) by comparing these indices calculated from both observed and simulated discharge. The spatial patterns of the flow regime alterations varied depending on the hydrologic indices. For example, both the median flow in August and the frequency of high flow pulses were reduced by the maximum of approximately 70%, but these strongest alterations were detected at different locations (i.e., on the mainstream and the tributary, respectively). These results are likely caused by different operational purposes of multiple water control facilities. The results imply that the evaluation only at discharge gauges is insufficient to capture the alteration of the flow regime. Our findings clearly emphasize the importance of evaluating the spatial pattern of flow regime alteration on a river network where its discharge is affected by multiple water control facilities.

Elucidating Adsorptive Fractions of Natural Organic Matter on Carbon Nanotubes

Natural organic matter (NOM) is a heterogeneous mixture of organic compounds that is omnipresent in natural waters. To date, the understanding of the adsorption of NOM components by carbon nanotubes (CNTs) is limited because of the limited number of comprehensive studies in the literature examining the adsorption of NOM by CNTs. In this study, 11 standard NOM samples from various sources were characterized, and their adsorption behaviors on four different CNTs were examined side-by-side using total organic carbon, fluorescence, UV-visible spectroscopy, and high-performance size-exclusion chromatography (HPSEC) analysis. Adsorption was influenced by the chemical properties of the NOM, including aromaticity, degree of oxidation, and carboxylic acidity. Fluorescence excitation-emission matrix (EEM) analysis showed preferential adsorption of decomposed and terrestrial-derived NOM compared to freshly produced and microbial-derived NOM. HPSEC analysis revealed preferential adsorption of fractions in the molecular weight range of 0.5-2 kDa for humic acids but in the molecular weight range of 1-3 kDa for all fulvic acids and reverse-osmosis isolates. However, the smallest characterized fraction (MW < 0.4 kDa) in all samples did not adsorb on the CNTs.

Spatio-temporal patterns of soil erosion and suspended sediment dynamics in the Mekong River Basin

Understanding of the distribution patterns of sediment erosion, concentration and transport in river basins is critically important as sediment plays a major role in river basin hydrophysical and ecological processes. In this study, we proposed an integrated framework for the assessment of sediment dynamics, including soil erosion (SE), suspended sediment load (SSL) and suspended sediment concentration (SSC), and applied this framework to the Mekong River Basin. The Revised Universal Soil Loss Equation (RUSLE) model was adopted with a geographic information system to assess SE and was coupled with a sediment accumulation and a routing scheme to simulate SSL. This framework also analyzed Landsat imagery captured between 1987 and 2000 together with ground observations to interpolate spatio-temporal patterns of SSC. The simulated SSL results from 1987 to 2000 showed the relative root mean square error of 41% and coefficient of determination (R(2)) of 0.89. The polynomial relationship of the near infrared exoatmospheric reflectance and the band 4 wavelength (760-900nm) to the observed SSC at 9 sites demonstrated the good agreement (overall relative RMSE=5.2%, R(2)=0.87). The result found that the severe SE occurs in the upper (China and Lao PDR) and lower (western part of Vietnam) regions. The SSC in the rainy season (June-November) showed increasing and decreasing trends longitudinally in the upper (China and Lao PDR) and lower regions (Cambodia), respectively, while the longitudinal profile of SSL showed a fluctuating trend along the river in the early rainy season. Overall, the results described the unique spatio-temporal patterns of SE, SSL and SSC in the Mekong River Basin. Thus, the proposed integrated framework is useful for elucidating complex process of sediment generation and transport in the land and river systems of large river basins.

The relationship between molecular composition and fluorescence properties of humic substances

The quantity and quality of dissolved organic matters have been widely characterized by fluorescence spectroscopy, yet the relationship between the fluorescence properties of dissolved organic matters and its molecular composition remains poorly described in the literature. Here, we measured the fluorescence excitation–emission matrix of 17 well-characterized humic substance standards to determine a range of fluorescence parameters, including classical fluorescence indices (e.g., fluorescence index, biological index and humification index) and parameters derived from parallel factor analysis (e.g., component contribution). Relationships between humic substance’s fluorescence and compositional parameters were then statistically examined using canonical correspondence and simple correlation analyses. The canonical correspondence analysis generally suggested that most fluorescence parameters determined here are highly associated with the amount of aliphatic and aromatic compounds in humic substances. However, the correlation analysis between single molecular and fluorescence parameters indicated that the fluorescence properties of humic substances including the parallel factor analysis component contribution also significantly correlate well with several aspects of the molecular composition of humic substances, such as elemental composition, carbon species, acidic functional group and iron complexation. Overall, our results suggest that measurement of humic substance’s fluorescence is beneficial in understanding the molecular composition and environmental functions of dissolved organic matters in natural and engineered waters.