Oxygen Mass Balance Research Articles

Oxygen transfer during aerobic biodegradation of pollutants in a dense activated sludge slurry bubble column: Actual volumetric oxygen transfer coefficient and oxygen uptake rate in p-nitrophenol degradation by acclimated waste activated sludge

Volumetric oxygen transfer coefficient and oxygen uptake rate in a dense activated sludge slurry bubble column were measured by varying activated sludge concentrations (from 2000 to 8000mg/L) and/or aeration rates (from 0.3 to 1.5Lmin−1). They were separately determined by the dynamic methods. The endogenous oxygen uptake rate of the activated sludge was estimated by monitoring the dissolved oxygen concentration change after turning off the air sparging in a stirred tank. The volumetric oxygen transfer coefficient in a dense activated sludge slurry bubble column was determined using the dissolved oxygen concentration profiles and the oxygen uptake rates predetermined from the separate measurements. While the oxygen uptake rate almost linearly increased with increasing the activated sludge concentration, the volumetric oxygen transfer coefficient decreased with an increase in the activated sludge concentration. Their empirical correlations were obtained as functions of activated sludge concentration by fitting the experimental data.For the aerobic biodegradation of p-nitrophenol (PNP) by acclimated waste activated sludge, the dynamic models for the dissolved oxygen mass balance and PNP degradation kinetics with the proposed correlations for volumetric oxygen transfer coefficient and oxygen uptake rate could successfully simulate the concentration profiles of dissolved oxygen and PNP during the time course of an aerobic biodegradation of PNP.

Net biological oxygen production in the ocean—II: Remote in situ measurements of O 2 and N 2 in subarctic pacific surface waters

Net community biological production in the euphotic zone of the ocean fuels organic matter and oxygen export from the upper ocean, which has a large influence on the atmospheric pressure of carbon dioxide and is the driving force for metabolite distributions in the sea. We determine the net annual biological oxygen production in the mixed layer of the northeast subarctic Pacific Ocean from in situ O 2 and N 2 measurements. Temperature, salinity, total gas pressure and O 2 were measured every 3 h for 9 months in 2007 at about 3 m depth on a surface mooring at Station P (50°N, 145°W). The concentration of nitrogen gas, N 2, determined from separate total gas pressure and pO 2 measurements, was used as an inert tracer of the physical processes that induce gas departure from thermodynamic equilibrium with the atmosphere. We use a simple model of the ocean’s mixed layer along with the nitrogen concentration to constrain the importance of bubbles, gas exchange and horizontal advection, which are then used in the oxygen mass balance to derive net biological oxygen production. The mixed-layer oxygen mass balance is dominated by exchange with the atmosphere, and we determine a mean summertime oxygen production of 24 mmol O 2 m −2 d −1. The annual pattern in the difference between the supersaturation of oxygen and nitrogen in the surface waters reveals very little net oxygen production during the winter at this location. The calculated annual net community production (NCP) of carbon from this new method, 2.5 mol m −2 yr −1, agrees to within its error of about×40% with previous determinations at this location from oxygen mass balance, NO 3 − draw down and 234Th measurements. This value is either indistinguishable from or lower than annual NCP measurements in the subtropical North Pacific, indicating that there is no experimental evidence for differences in annual NCP between the subarctic and subtropical North Pacific Ocean.

Role of hydrogen peroxide and hydroxyl radical in pyrite oxidation by molecular oxygen

Hydrogen peroxide and hydroxyl radical are readily formed during the oxidation of pyrite with molecular oxygen over a wide range of pH conditions. However, pretreatment of the pyrite surface influences how much of the intermediates are formed and their fate. Acid-washed pyrite produces significant amounts of hydrogen peroxide and hydroxyl radical when suspended in air-saturated water. However, the hydrogen peroxide concentration shows an exponential decrease with time. Suspensions made with partially oxidized pyrite yield significantly lower amounts of hydrogen peroxide product. The presence of Fe(III)-oxide or Fe(III)-hydroxide patches facilitates the conversion of hydrogen peroxide to oxygen and water. Hence, the degree to which a pyrite surface is covered with patches of Fe(III)-oxide or Fe(III)-hydroxide patches is an important control on the concentration of hydrogen peroxide in solution. Hydrogen peroxide appears to be an important intermediate in the four-electron transfer from pyrite to molecular oxygen. Addition of catalase, an enzyme that decomposes hydrogen peroxide to water and molecular oxygen, to a pyrite suspension reduces the oxidation rate by 40%. By contrast, hydroxyl radical does not appear to play a significant role in the oxidation mechanism. It is estimated on the basis of a molecular oxygen and sulfate mass balance that 5–6% of the molecular oxygen is consumed without forming sulfate.

Application of the gas tracer method for measuring oxygen transfer rates in subsurface flow constructed wetlands

The oxygen transfer rate (OTR) has a significant impact on the design, optimal operation and modelling of constructed wetlands treating wastewater. Oxygen consumption is very fast in wetlands and the OTR cannot be determined using an oxygen mass balance. This problem is circumvented in this study by applying the gas tracer method. Experiments were conducted in an unplanted gravel bed (dimensions L × W × d 125 × 50 × 35 cm filled with a 30-cm layer of 10–11-mm gravel) and a planted horizontal subsurface flow constructed wetland (HSSFCW) (L × W × d 110 × 70 × 38 cm filled with a 30-cm layer of 3.5-mm gravel with Phragmites australis). Tap water saturated with propane as gas tracer (pure or commercial cooking gas, depending on the test) was used. The mass transfer ratio between oxygen and commercial propane gas was quite constant and averaged R = 1.03, which is slightly lower than the value of R = 1.39 that is usually reported for pure propane. The OTR ranged from 0.31 to 5.04 g O 2 m −2 d −1 in the unplanted gravel bed and from 0.3 to 3.2 g O 2 m −2 d −1 in the HSSFCW, depending on the hydraulic retention time (HRT). The results of this study suggest that the OTR in HSSFCW is very low for the oxygen demand of standard wastewater and the OTR calculations based on mass balances and theoretical stoichiometric considerations overestimate OTR values by a factor that ranges from 10 to 100. The gas tracer method is a promising tool for determining OTR in constructed wetlands, with commercial gas proving to be a viable low-cost alternative for determining OTR.

Oxygen transfer in marsh-pond-marsh constructed wetlands treating swine wastewater

Oxygen transfer efficiencies of various components of the marsh-pond-marsh (M-P-M) and marsh-floating bed-marsh (M-FB-M) wetlands treating swine wastewater were determined by performing oxygen mass balance around the wetlands. Biological oxygen demand (BOD) and total nitrogen (TN) loading and escaping rates from each wetland were used to calculate carbonaceous and nitrogenous oxygen demands. Ammonia emissions were measured using a wind tunnel. Oxygen transfer efficiencies of the aerated ponds were estimated by conducting the ASCE standard oxygen transfer test in a tank using the same aeration device. Covering pond water surface with the floating bed slightly decreased oxygen transfer efficiency. The diffused membrane aeration (26.7 kg O2 ha-1 d-1) of M-P-M was surprisingly not as effective as plant aeration in the marsh (38.9 to 42.0 kg O2 ha-1 d-1). This unusually low oxygen transfer efficiency of the diffused aeration was attributed to its low submergence depth of 0.8 m compared to typical depth of 4.5 m. The wetlands consisting entirely of marsh removed similar amounts of C and N without investing additional equipment and energy costs of aerating ponds in the middle of wetlands.

Pressurized Chemical-Looping Combustion of Chinese Bituminous Coal: Cyclic Performance and Characterization of Iron Ore-Based Oxygen Carrier

A pressurized chemical-looping combustion combined cycle (PCLC) system is proposed for solid fuels combustion with potential high system efficiency, improving the fuel conversion and lowering the cost for CO2 sequestration. In this study, pressurized CLC of coal with Companhia Valedo Rio Doce (CVRD) iron ore was investigated in a laboratory fixed bed reactor focusing on cyclic performance. CVRD iron ore particles were exposed alternately for 20 cycles to reduction by 0.4 g of Chinese Xuzhou bituminous coal gasified with 87% steam/N2 mixture and oxidation with 5% O2 in N2 at 970 °C at atmospheric pressure (0.1 MPa) and a typical elevated pressure of 0.5 MPa. With increasing number of redox cycles, more pyrolysis gases are oxidized by the oxygen carrier. At elevated pressure, the char gasification is intensified with negligible gasification intermediate products released. The CO2 fraction increases from 80% to approximate 90% after 10 cycles at atmospheric pressure. At elevated pressures, the average CO2 fraction stabilizes at 95.75%, approximate to the equilibrium value. The carbon conversion at 0.1 MPa and 0.5 MPa is 76.48 and 84.65%, respectively, and maintains approximately the same during the cycles because excessive steam gasification agent used in this study. The oxygen carrier conversion determined from the oxygen mass balance verifies that reduction level increases with the cycle number. The physical and chemical properties of oxygen carrier particles were characterized. X-ray diffraction (XRD) analysis verifies the extent of reduction level increases with cycles. No detectable formation of compound of iron oxide and coal ash was observed. Scanning electron microscope (SEM) analyses show that the iron ore particles become porous and that more pores formed with cycles. Agglomeration of particles was not observed in all experiments at both pressures. Energy-dispersive X-ray spectroscopy (EDX) analysis show an increasing amount of coal ash on the oxygen carrier particles with increasing numbers of cycles. Pore size analyses show that the oxygen carrier particles maintained mesopores for both atmospheric and elevated pressure. The increase of both surface area and pore volume illustrates that the particles become more porous with redox cycles. This study show that pressurized CLC of coal is promising and a low-cost iron ore-based oxygen carrier may be suitable for pressurized CLC of coal.

METHODS FOR STUDYING THE PHYSIOLOGY OF KIDNEY OXYGENATION

1. An improved understanding of the regulation of kidney oxygenation has the potential to advance preventative, diagnostic and therapeutic strategies for kidney disease. Here, we review the strengths and limitations of available and emerging methods for studying kidney oxygen status. 2. To fully characterize kidney oxygen handling, we must quantify multiple parameters, including renal oxygen delivery (DO2) and consumption (VO2), as well as oxygen tension (Po2). Ideally, these parameters should be quantified both at the whole-organ level and within specific vascular, tubular and interstitial compartments. 3. Much of our current knowledge of kidney oxygen physiology comes from established techniques that allow measurement of global kidney DO2 and VO2, or local tissue Po2. When used in tandem, these techniques can help us understand oxygen mass balance in the kidney. Po2 can be resolved to specific tissue compartments in the superficial cortex, but not deep below the kidney surface. We have limited ability to measure local kidney tissue DO2 and VO2. 4. Mathematical modelling has the potential to provide new insights into the physiology of kidney oxygenation, but is limited by the quality of the information such models are based on. 5. Various imaging techniques and other emerging technologies have the potential to allow Po2 mapping throughout the kidney and/or spatial resolution of Po2 in specific renal tissues, even in humans. All currently available methods have serious limitations, but with further refinement should provide a pathway through which data obtained from experimental animal models can be related to humans in the clinical setting.

Net community production in the deep euphotic zone of the subtropical North Pacific gyre from glider surveys

Seagliders, deployed through most of 2005 in the subtropical North Pacific gyre, made measurements of temperature, salinity, and dissolved oxygen to quantify net community production (NCP) at Station A Long‐ Term Oligotrophic Habitat Assessment (ALOHA) of the Hawaii Ocean Time‐series (HOT) using an oxygen mass balance approach. A ‘bowtie’‐shaped pattern, 50 km by 50 km in size was repeatedly traversed at two week intervals with the goal of observing the influence of Rossby waves and eddies on the productivity of the study region. Rossby waves and eddies in the region cause a vertical displacement of isopycnal depth of ~±50 m at the base of the euphotic zone. Shoaling of isopycnals is demonstrated to drive productivity in the deep euphotic zone. Four mesoscale shoaling events were observed between February and November in 2005. During each event when isopycnals shoaled, oxygen concentrations on isopycnals increased, fluorescence in the deep euphotic zone was higher, and net community production was elevated. Productivity in the deep euphotic zone was strongly influenced by Rossby waves and eddies, but this influence was not observed to extend into the mixed layer.

Influence of net ecosystem metabolism in transferring riverine organic carbon to atmospheric CO2 in a tropical coastal lagoon (Chilka Lake, India)

Studies on biogeochemical cycling of carbon in the Chilka Lake, Asia’s largest brackish lagoon on the east coast of India, revealed, for the first time, strong seasonal and spatial variability associated with salinity distribution. The lake was studied twice during May 2005 (premonsoon) and August 2005 (monsoon). It exchanges waters with the sea (Bay of Bengal) and several rivers open into the lake. The lake showed contrasting levels of dissolved inorganic carbon (DIC) and organic carbon (DOC) in different seasons; DIC was higher by ∼22% and DOC was lower by ∼36% in premonsoon than in monsoon due to seasonal variations in their supply from rivers and in situ production/mineralisation. The DIC/DOC ratios in the lake during monsoon were influenced by physical mixing of end member water masses and by intense respiration of organic carbon. A strong relationship between excess DIC and apparent oxygen utilisation showed significant control of biological processes over CO2 production in the lake. Surface partial pressure of CO2 (pCO2), calculated using pH–DIC couple according to Cai and Wang (Limnol and Oceanogr 43:657–668, 1998), exhibited discernable gradients during monsoon through northern (1,033–6,522 μatm), central (391–2,573 μatm) and southern (102–718 μatm) lake. The distribution pattern of pCO2 in the lake seems to be governed by pCO2 levels in rivers and their discharge rates, which were several folds higher during monsoon than premonsoon. The net CO2 efflux, based on gas transfer velocity parameterisation of Borges et al. (Limnol and Oceanogr 49(5):1630–1641, 2004), from entire lake during monsoon (141 mmolC m−2 d−1 equivalent to 2.64 GgC d−1 at basin scale) was higher by 44 times than during premonsoon (9.8 mmolC m−2 d−1 ≈ 0.06 GgC d−1). 15% of CO2 efflux from lake in monsoon was contributed by its supply from rivers and the rest was contributed by in situ heterotrophic activity. Based on oxygen and total carbon mass balance, net ecosystem production (NEP) of lake (−308 mmolC m−2 d−1 ≈ −3.77 GgC d−1) was found to be almost in consistent with the total riverine organic carbon trapped in the lake (229 mmolC m−2 d−1 ≈ 2.80 GgC d−1) suggesting that the strong heterotrophy in the lake is mainly responsible for elevated fluxes of CO2 during monsoon. Further, the pelagic net community production represented 92% of NEP and benthic compartment plays only a minor role. This suggests that Chilka lake is an important region in biological transformation of organic carbon to inorganic carbon and its export to the atmosphere.

Conversion of Solid Carbonaceous Fuels in a Fluidized Bed Fuel Cell

A fluidized bed direct carbon fuel cell was employed to achieve direct conversion of solid fuels into electricity. Power was generated from pulverized Lower Kittanning (bituminous) coal, synthetic carbon, and biomass in a single process step. Current-voltage characteristics exhibited typical fuel cell behavior. Fluidization in flowing CO{sub 2} overcomes the difficulty of attaining solid fuel-to-anode contact and generates CO in situ via the Boudouard reaction. A mechanistic reaction pathway is proposed for anodic oxidation of the solid fuel. Conversion was verified by gas analysis of oxidation products in the flue stream and by oxygen mass balance.

Direct carbon conversion in a helium fluidized bed fuel cell

A solid oxide fuel cell based fluidized bed configuration is employed for the first time to convert solid carbon directly into electricity. Current–voltage characteristics exhibited typical fuel cell behavior with large ohmic losses. Chromatographic analysis of the product gases in the flue stream confirmed conversion of carbon. This is also verified by oxygen mass balance. Peak power densities for carbon compared well with benchmarking tests that employed only gaseous fuels. A possible mechanistic pathway is proposed to explain the observed behavior.

The significance of side‐arm connectivity for carbon dynamics of the River Danube, Austria

Summary1. Side‐arms connected to the main stem of the river are key areas for biogeochemical cycling in fluvial landscapes, exhibiting high rates of carbon processing.2. This work focused on quantifying autochthonous and allochthonous carbon pools and, thereby, on comparing transport and transformation processes in a restored side‐arm system of the River Danube (Regelsbrunn). We established a carbon budget and quantified carbon processing from March to September 2003. In addition, data from previous studies during 1997 to 1999 were assessed.3. Gross primary production (GPP) and community respiration were estimated by diel oxygen time curves and an oxygen mass balance. Plankton primary production was determined to estimate its contribution to GPP under different hydrological conditions.4. Based on the degree of connectivity, three hydrological phases were differentiated. Most of the organic matter, dominated by allochthonous carbon, was transported in the main channel and through the side‐arm during floods, while at intermediate and low flows (and thus connectivity), transformation processes became more important and autochthonous carbon dominated the carbon pool. The side‐arm system functioned as a sink for particulate matter [total suspended solids and particulate organic carbon (POC)] and a source of dissolved organic carbon (DOC) and chlorophyll‐a.5. Autochthonous primary production of 4.2 t C day−1 in the side‐arm was equivalent to about 20% of the allochthonous inputs of 20 t C day−1 (POC and DOC) entering the area at mean flow (1% of the discharge of the main channel). Pelagic photosynthesis was generally high at mean flow (1.3–3.8 g C m−2 day−1), and contributed up to 90% of system productivity. During long stagnant periods at low discharge, the side‐arm was controlled by biological processes and a shift from planktonic to benthic activity occurred (benthic primary production of 0.4–14 g C m−2 day−1).6. The transformation of the organic matter that passes through the side‐arm under different hydrological conditions, points to the importance of these subsystems in contributing autochthonous carbon to the food web of the main channel.

Constraining bubble dynamics and mixing with dissolved gases: Implications for productivity measurements by oxygen mass balance

We used a dynamic mixed layer model to determine carbon export by the oxygen mass balance method from a time series of O-2/Ar, N-2/Ar and Ne measurements collected at station ALOHA near Hawaii from July 2000 to June 2001. The inert gas measurements constrain the flux of oxygen into the mixed layer from small, collapsing bubbles (injection) to be greater than or equal to the flux from larger bubbles (exchange), with mean estimates of the ratio in the range of 1-2. We also show that monthly observations of temperature and inert gases cannot constrain the rate of diapycnal mixing at this location, because of uncertainties in air-sea heat flux estimates and bubble dynamics. Organic carbon export from the mixed layer calculated from our dataset was 1.1 +/- 0.5 mol C m(-2) yr(-1), with most of the error deriving from uncertainties in the parameterization of diffusive gas exchange with wind speed. Our estimates of carbon export from the zone beneath the mixed layer but still in the euphotic zone ranged from 0 to 0.6 mol C m(-2) yr(-1) as the rate of background diapycnal mixing was increased from 0. 1 to 1.0 cm(2) s(-1). We conclude that the oxygen mass balance method has errors of about a factor of two in areas similar to the subtropical North Pacific, with the main uncertainties deriving from mixing rates and the parameterization of diffusive gas exchange.

Aerobic transformations in sewer systems: are they relevant?

In-sewer transformation processes affect wastewater quality. Especially during dwf the transformation processes can exert a significant influence on wastewater quality. The transformation rates under aerobic conditions were estimated from an oxygen mass balance over a sewer reach. Oxygen probes were installed at the upstream and downstream end of the sewer reach. Moreover, 14 wastewater samples, taken at the downstream end of the sewer reach, were used to measure the oxygen uptake rate and the water quality parameters COD(total), COD(dissolved) and ammonium. The results show that the rate of fluctuations in COD concentrations in sewer systems is an order of magnitude higher than the aerobic transformation rate. Consequently, it is concluded that the aerobic transformations in sewer systems are generally not relevant with respect to the influent fluctuations for Dutch wastewater systems. However, in situations with very long aerobic transport times, the aerobic conversions can be significant. An ASM1 based model concept for transformations in sewer systems was used to study the applicability of the model for Dutch sewer conditions. The difference between the measured and simulated values was rather low for the range of upstream dissolved oxygen and COD(total), COD(dissolved) and COD(suspended) levels. Therefore, it is concluded that the ASM1 based sewer model properly describes the changes in dissolved oxygen level in an aerobic sewer reach.

Whole-system metabolism and CO<sub>2</sub> fluxes in a Mediterranean Bay dominated by seagrass beds (Palma Bay, NW Mediterranean)

Abstract. Planktonic and benthic incubations (bare and Posidonia oceanica vegetated sediments) were performed at monthly intervals from March 2001 to October 2002 in a seagrass vegetated area of the Bay of Palma (Mallorca, Spain). Results showed a contrast between the planktonic compartment, which was on average near metabolic balance (−4.6±5.9 mmol O2 m-2 d-1) and the benthic compartment, which was autotrophic (17.6±8.5 mmol O2 m-2 d-1). During two cruises in March and June 2002, planktonic and benthic incubations were performed at several stations in the bay to estimate the whole-system metabolism and to examine its relationship with partial pressure of CO2 (pCO2) and apparent oxygen utilisation (AOU) spatial patterns. Moreover, during the second cruise, when the residence time of water was long enough, net ecosystem production (NEP) estimates based on incubations were compared, over the Posidonia oceanica meadow, to rates derived from dissolved inorganic carbon (DIC) and oxygen (O2) mass balance budgets. These budgets provided NEP estimates in fair agreement with those derived from direct metabolic estimates based on incubated samples over the Posidonia oceanica meadow. Whereas the seagrass community was autotrophic, the excess organic carbon production therein could only balance the planktonic heterotrophy in shallow waters relative to the maximum depth of the bay (55 m). This generated a horizontal gradient from autotrophic or balanced communities in the shallow seagrass-covered areas, to strongly heterotrophic communities in deeper areas of the bay. It seems therefore that, on an annual scale in the whole bay, the organic matter production by the Posidonia oceanica may not be sufficient to fully compensate the heterotrophy of the planktonic compartment, which may require external organic carbon inputs, most likely from land.

Net plankton community production in the Arabian Sea based on O2 mass balance model

Monthly net community production (NCP) rates were computed using oxygen mass balance model for the euphotic zone in the eastern (EAS) and western Arabian Sea (WAS). NCP showed large seasonal and spatial variability in the Arabian Sea, and it is consistent with gross productivity. Net autotrophic conditions were found during high productive seasons, while net heterotrophic conditions were prevalent during non‐monsoon seasons in both EAS and WAS. Though gross productivity is higher during SW monsoon (June to September), NCP decreased from June to September due to increase in grazing pressure. This suggests that production to respiration ratio controls NCP in the Arabian Sea. NCP is found to be higher in the WAS compared to the EAS and it is consistent with gross productivity. Incubation experiments suggested that phytoplankton growth was higher than zooplankton grazing in the WAS due to high concentrations of nutrients and it is opposite in the EAS, suggesting that a different ecosystem exists in the east and west probably due to existence of different physical forcing. In general, picoplanktons are more dominant in the EAS, whereas diatoms are major contributors to primary production in the WAS due to low nitrate concentration in the former region. As a result, most of the produced carbon is recycled in the euphotic zone in the EAS, whereas it is exported to the deeper layers in the WAS. These results are consistent with the sinking fluxes of carbon at 100 m depth in both the regions. On an annual scale, WAS is net autotrophic, whereas EAS is net heterotrophic. NCP amounts to approximately one fourth of the gross primary production, and it is consistent with the new production estimated using f‐ratios in the Arabian Sea.

Oxygen Sag Models for Multiorder Biochemical Oxygen Demand Reactions

The empirical biochemical oxygen demand (BOD) equation is expressed as a multiorder reaction equation of order n, then is combined with the dissolved oxygen mass balance equation to give the differential form of an oxygen sag equation for small rivers and streams for which dispersion can be neglected. The value of n in the BOD reaction is restricted to values that are larger than one (first order). The dissolved oxygen sag equation is verified with two published dissolved oxygen sag models by setting n equal to 3/2 (three-halves order BOD reaction), and n equal to 2 (second order BOD reaction). The proposed dissolved oxygen sag equation may be applied to test the BOD and dissolved oxygen models in large, complex numerical models, such as models used in developing total maximum daily load recommendations. Examples show how the BOD reaction order affects the dissolved oxygen sag characteristics of a river.

Modeling mass transfer in hepatocyte spheroids via cell viability, spheroid size, and hepatocellular functions

Hepatocyte aggregation into spheroids attributes to their increased activity, but in the absence of a vascular network the cells in large spheroids experience mass transfer limitations. Thus, there is a need to define the spheroid size which enables maximal cell viability and productivity. We developed a combined theoretical and experimental approach to define this optimal spheroid size. Hepatocyte spheroids were formed in alginate scaffolds having a pore diameter of 100 microm, in rotating T-flasks or spinners, to yield a maximal size of 100, 200, and 600 microm, respectively. Cell viability was found to decrease with increasing spheroid size. A mathematical model was constructed to describe the relationship between spheroid size and cell viability via the oxygen mass balance equation. This enabled the prediction of oxygen distribution profiles and distribution of viable cells in spheroids with varying size. The model describes that no oxygen limitation will take place in spheroids up to 100 microm in diameter. Spheroid size affected the specific rate of albumin secretion as well; it reached a maximal level, i.e., 60 microg/million cells/day in 100-microm diameter spheroids. This behavior was depicted in an equation relating the specific albumin secretion rate to spheroid size. The calculated results fitted with the experimental data, predicting the need for a critical number of viable hepatocytes to gain a maximal albumin secretion. Taken together, the results on mass transport in spheroids and its effects on cell viability and productivity provide a useful tool for the design of 3D scaffolds with pore diameters of 100 microm.

Ｎａ２ＣＯ３スラグによる溶銅からのひ素およびアンチモンの除去速度

The rate of As elimination from molten copper by the use of Na2CO3 slag was measured at 1523 K. The results obtained under the present experimental conditions show that As in molten copper is eliminated in a penta-valent form and that its elimination rate increases with increasing initial oxygen concentration in molten copper. Based on the results obtained in the present study, the overall rate of As elimination is considered to be controlled by mass transfer in molten copper. The mass-transfer coefficient of As in molten copper at 1523 K was determined to be 1.3 (±0.4)×10-4 m · s-1 based on the mass balances of As and oxygen in the molten copper and slag phases and the equilibrium relation of the As elimination reaction at the slag-metal interface. In addition, the behavior of the simultaneous elimination of As and Sb which coexist as impurities in molten copper at 1523 K were also investigated from the kinetic viewpoints. The results obtained show that the elimination rate of As is much faster than that of Sb, and two types of elimination behaviors are observed, depending on the initial oxygen concentration in molten copper, i.e. at relatively low initial oxygen concentration, As being preferentially eliminated with stagnation of the Sb elimination, while both elements being simultaneously eliminated at relatively high initial oxygen concentration. These behaviors were examined from the viewpoint of the oxygen concentration at the slag-metal interface during the process.

The rate of antimony elimination from molten copper by the use of Na2CO3 slag

The rate of Sb elimination from molten copper by the use of Na2CO3 slag was measured at 1523 K. The results obtained under the present experimental conditions show that Sb in molten copper is eliminated in a tri-valent or a penta-valent form, depending on the oxygen concentration at the slag-metal interface, and its elimination rate increases with increasing initial oxygen concentration in molten copper. The overall elimination rate of Sb is affected by the stirring condition of the molten copper, which indicates a rate control by mass transfer in that phase. The mass-transfer coefficients of Sb and oxygen in molten copper at 1523 K without external stirring were determined, respectively, to be $$k_{m,Sb} = 4.0( \pm 0.9) \times 10^{ - 5} m \cdot s^{ - 1} and k_{m,O} = 1.3( \pm 0.3) \times 10^{ - 4} m \cdot s^{ - 1} $$ based on the mass balances of Sb and oxygen in the molten copper and slag phases and the equilibrium relation of the Sb elimination reaction at the slag-metal interface.