Short Mean Residence Time Research Articles

Numerical Analysis of The Effect of Hydrodynamics and Operating Conditions on Biodiesel Synthesis in a Rotor-Stator Spinning Disk Reactor

AbstractA rotor-stator spinning disk reactor for intensified biodiesel synthesis is described and numerically simulated in the present research. The reactor consists of two flat disks, located coaxially and parallel to each other with a gap ranging from 0.1 mm to 0.2 mm between the disks. The upper disk is located on a rotating shaft while the lower disk is stationary. The feed liquids, triglycerides (TG) and methanol are injected into the reactor from centres of rotating disk and stationary disk, respectively. Fluid hydrodynamics in the reactor for synthesis of biodiesel from TG and methanol in the presence of a sodium hydroxide catalyst are simulated, using convection-diffusion-reaction multicomponent transport model with the CFD software ANSYS©Fluent v. 13.0. Effect of operating conditions on TG conversion is particularly investigated. Simulation results indicate that there is occurrence of back flow close to the stator at the outlet zone. Small gap size and fast rotational speed generally help to intensify mixing among reagents, and consequently enhance TG conversion. However, increasing rotational speed of spinning disk leads to more backflow, which decreases TG conversion. Large flow rate of TG at inlet is not recommended as well because of the short mean residence time of reactants inside the reactor.

Rapid and continuous synthesis of methacrolein with high selectivity by condensation of propanal with formaldehyde in laboratory

AbstractAn experimental device with a micro‐tubular reactor for continuous reaction process was designed and constructed based on the reaction mechanisms and characteristics of condensation of propanal with formaldehyde, a short mean residence time (a few seconds) for the reactive mixture was acquired to increase selectivity to methacrolein (MAL) by reducing the side reactions. Diethylamine and acetic acid aqueous solution was used as catalysts for the reaction. The reactor temperature ranged between 100 and 210 °C. The reactor pressure was set at 4.5–6.0 MPa to make sure that the reaction can be carried out in liquid phase. The results indicated that the reaction rate was heavily influenced by the concentration of the catalysts and the feed preheating temperature. When the feed preheating temperature was about 132 °C, the mass concentration of diethylamine was about 6 %, the reaction pressure was 4.5–5.0 MPa, and the mean residence time varied from 4.5 to 5.5 s, high conversion of propanal (99.9 %) and high selectivity to MAL (98.7 %) were acquired. High catalytic activity and high selectivity were maintained after the catalysts had been recycled six times.

In‐line measurement of residence time distribution in melt extrusion via video analysis

Residence time distribution (RTD) is an important factor in melt extrusion as it strongly influences the product quality. It can cause degradation of the active pharmaceutical ingredient (API), which can be reduced by shorter mean residence times, and the content uniformity, which can be improved by broadening the RTD caused by axial mixing. In our study, we developed an inline video analysis that to date has mainly been applied offline. For extrusion, hydrophilic polymer and a surfactant were used as a molten matrix, in which an API was embedded. The video analysis consisted of four steps: (1) segmentation of the image to mask the strands, (2) calculation of average color values for the segmented pixels of each frame, (3) fitting of the obtained color curve with an analytical solution, and (4) calculating several RTD measures (e.g., the minimal and mean RT) for better comparison of the results. The RTD measures were responses in the design of experiments (DoE), which included the process parameters screw design, screw speed, and throughput. This analysis clearly showed that screw design had the strongest effect on the RTD measures. The results suggest that video analysis is a suitable tool for inline RTD measurements. POLYM. ENG. SCI., 58:170–179, 2018. © 2017 Society of Plastics Engineers

Residence time distribution and flow pattern of reduced-gluten wheat-based formulations in a twin–screw extruder

Residence time distribution (RTD) and flow pattern in a co-rotating twin screw extruder were evaluated using a color tracer method. A reduced-gluten formulation was prepared by mixing wheat flour, sesame protein concentrates and Tef flour in the proportion of 50:20:30, respectively. The effects of screw speed (150, 180 and 220 rpm), feed moisture content (17, 19 and 21 g/100 g wet basis) and feed rate (40 and 90 g/min) on RTD and flow pattern were investigated. All tested conditions were found to have a significant (P < 0.05) effect on the mean residence time and other RTD parameters. Higher screw speed, higher moisture content and higher feed rate resulted in shorter mean residence times and lower RTD spread due to enhanced barrel fill, mass flow rate and system pressure. A dispersion number was used to define the level of axial mixing in the extruder and, as defined, the axial mixing was more influenced by the feed rate than screw speed. Increasing feed rate resulted in the mixing pattern to become more like plug flow, whereas, mixed flow condition achieved when the screw speed was increased. Feed moisture had no significant effect on mixing pattern.

Groundwater responses to recharge in the Gacka Area, Croatia, as revealed by stable isotopes, tritium, CFCs and noble gases

Due to their extreme anisotropy and heterogeneity, the hydrogeological characterization of karst terrains is notoriously difficult. Approximately 50% of Croatia consist of karst aquifers, in particular the touristically important region along the Adriatic coast. Being an important water reservoir for this area, we have chosen the Gacka area of Croatia as a typical example. We studied the mixing of groundwater and the responses to recharge in several karst springs by using environmental tracers, including stable isotopes in water (O-18, H-2), tritium (H-3), chlorofluorocarbons (CFCs) and noble gases (He-3, He-4, Ne, Ar, Kr, Xe). The variation of the stable isotopic signal (delta O-18 and delta H-2) in the spring discharges provides qualitative information on the mixing of waters with different transit time. Assuming piston flow in the system, the CFC and H-3 data indicate that the waters contain mainly post-1980 recharge. The CFC concentrations in the spring waters were found to be significantly different between wet and dry weather periods. The higher CFC values during the wet weather period indicate that the waters contained some water components that have relatively short mean residence times. An important finding of the study was that the sole application of tritium-helium methods does not permit an accurate age determination. This is due to mass exchange between liquid and gaseous phases occurring in karst aquifer systems particularly in dry weather periods. During such a dry weather period, more space in the karst groundwater system is not filled with water compared with a wet weather period. It is shown in this study, however, that the combined use of stable isotopes (O-18, H-2), tritium, CFCs and noble gases is able to account for groundwater mixing and responses to recharge in this specific scenario. Compared to single-tracer studies, multi-tracer techniques will allow a much better understanding and characterization of these systems. This information is vital for the assessment of resources and potential contamination of the groundwater in this special environment.

Preclinical pharmacokinetics and ADME characterization of a novel anticancer chalcone, cardamonin.

Cardamonin (CRD), a chalconoid obtained from several medicinal plants of Zingiberaceae family, had shown promising potential in cancer prevention and therapy. For further development and better pharmacological elucidation, we performed a series of in vitro and in vivo studies to characterize its preclinical pharmacokinetics. The study samples were analyzed using validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) and high performance liquid chromatography-ultra violet (HPLC-UV) methods. CRD is partially soluble (<10 μM) and possess high permeability (>0.2 × 10-4 cm/sec). It is moderately bound to plasma proteins (<50%). It shows partitioning in red blood cell (RBC) compartment with the partition coefficient between RBCs and plasma (KRBC/P ) of 0.95 at 0 min to 1.39 at 60 min, indicating significant but slow RBC uptake. In mice, CRD is poorly absorbed after oral administration with 18% oral bioavailability. It possesses high clearance, short mean residence time, and high volume of distribution in mice. It exhibited multiple peak phenomena both after oral and intravenous administration and is excreted both as conjugated and unchanged CRD in bile. It is majorly excreted in faeces and negligibly in urine. The preclinical absorption, distribution, metabolism, and excretion data are expected to succour the future clinical investigations of CRD as a promising anticancer agent. Copyright © 2016 John Wiley & Sons, Ltd.

Evaluation of Mixing Performance in Baffled Screw Channel Using Lagrangian Particle Calculations

Different kind of discontinuous baffles, which had the larger geometric ratios and higher height than the previous ones, were inserted in the unwound channel of a single-screw extruder to generate chaotic mixing in the screw channel. The periodic unit of the flow channel was modeled as a dynamic system of complex duct flow. The finite-volume method was used to solve the three-dimensional flow of purely viscous, non-Newtonian fluid obeying the power law constitutive model. Lagrangian particle calculations along with the statistical methods were performed by a fourth-order Runge–Kutta scheme. The effect of the baffle's geometric ratio on the mixing kinematics was investigated numerically. Poincare sections were applied to reveal the different patterns and sizes of the KAM tori. Period points were analyzed on the lines of the perturbative theory. Distributive mixing was then visualized by the evolution of passive tracer particles initially located at different positions. Shannon entropy index and cumulative residence time distribution (RTD) were then used to evaluate the statistical results. The growth of the tracer's interface with time was also obtained to reveal the intensity of the chaotic mixing. The cases with the geometric ratios being 0.5, 0.6, and 0.8 had the similar RTD performance, with shorter mean residence time and the narrower broadening of RTD. Among all the test cases, case B with a geometric ratio equal to 0.5 had the best mixing ability. The results also imply there is likely an optimal baffle geometric ratio between 0.5 and 0.6 that can achieve more efficient mixing.

The influence of C&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and C&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; vegetation on soil organic matter dynamics in contrasting semi-natural tropical ecosystems

Abstract. Variations in the carbon isotopic composition of soil organic matter (SOM) in bulk and fractionated samples were used to assess the influence of C3 and C4 vegetation on SOM dynamics in semi-natural tropical ecosystems sampled along a precipitation gradient in West Africa. Differential patterns in SOM dynamics in C3/C4 mixed ecosystems occurred at various spatial scales. Relative changes in C / N ratios between two contrasting SOM fractions were used to evaluate potential site-scale differences in SOM dynamics between C3- and C4-dominated locations. These differences were strongly controlled by soil texture across the precipitation gradient, with a function driven by bulk δ13C and sand content explaining 0.63 of the observed variability. The variation of δ13C with soil depth indicated a greater accumulation of C3-derived carbon with increasing precipitation, with this trend also being strongly dependant on soil characteristics. The influence of vegetation thickening on SOM dynamics was also assessed in two adjacent, but structurally contrasting, transitional ecosystems occurring on comparable soils to minimise the confounding effects posed by climatic and edaphic factors. Radiocarbon analyses of sand-size aggregates yielded relatively short mean residence times (τ) even in deep soil layers, while the most stable SOM fraction associated with silt and clay exhibited shorter τ in the savanna woodland than in the neighbouring forest stand. These results, together with the vertical variation observed in δ13C values, strongly suggest that both ecosystems are undergoing a rapid transition towards denser closed canopy formations. However, vegetation thickening varied in intensity at each site and exerted contrasting effects on SOM dynamics. This study shows that the interdependence between biotic and abiotic factors ultimately determine whether SOM dynamics of C3- and C4-derived vegetation are at variance in ecosystems where both vegetation types coexist. The results highlight the far-reaching implications that vegetation thickening may have for the stability of deep SOM.

Agglomeration of Ni-rich hydroxide crystals in Taylor vortex flow

The agglomeration of Ni-rich hydroxide crystals (Ni0.9Co0.05Mn0.05)(OH)2 was studied in a continuous Couette-Taylor (CT) crystallizer. Due to the excellent mixing and periodic fluid motion of the Taylor vortex flow in the CT crystallizer, the formation of spherical and uniform agglomerate particles of Ni-rich crystals was promoted as the rotation speed and the mean residence time increased. Thus, for a rotation speed of 1500rpm and mean residence time of 60min, with a high feed concentration of 3.0mol/L produced a high tap-density of agglomerate particles of 2.13g/cm3, which is higher than any previously reported values. These results were due to the excellent micromixing effects of Taylor vortex in the CT crystallizer. In addition, the long mean residence time contributed to the formation of spherical particles due to the extended exposure of the agglomerates to the fluid shear. Furthermore, the formation of spherical particles was optimized at a pH of 12.0 and required a high concentration of ammonia due to the high composition of Ni ions in the Ni-rich hydroxide crystals and the short mean residence time in the continuous CT crystallizer. Moreover, when comparing the crystallization in a continuous MSMPR crystallizer, which required a long mean residence time of 12h for a high tap-density of above 2.0g/cm3, the continuous CT crystallizer was clearly much more effective for the formation of uniform spherical particles due to the more efficient hydrodynamic fluid motion of the Taylor vortex in the CT crystallizer in contrast to the random turbulent eddy motion in the MSMPR crystallizer.

Pharmaceutical crystallisation processes from batch to continuous operation using MSMPR stages: Modelling, design, and control

In pharmaceuticals manufacturing, the conversion of conventional batch crystallisations to continuous mode has the potential for intensified, compact operation and more consistent production via quality-by-design. A pragmatic conversion approach is to utilise existing stirred tank batch crystallisers as continuous mixed-suspension mixed-product removal (MSMPR) stages. In this study, a rigorous and general mathematical model is developed for a pharmaceutical crystallisation process under continuous MSMPR operation. In the proposed changeover from batch to continuous operation, concentration control (C-control), which has been well accepted in batch crystallisation operation, is further extended to facilitate the convenient design of the steady-state operating point of a continuous MSMPR crystalliser; an objective is to ensure that the start-up procedures and on-line control conditions fall within the design-space of the original batch operation. Both single-stage and cascaded two-stage MSMPR crystallisers were investigated and compared to the conventional batch operation. It was observed that despite the production of a smaller number-based mean crystal size, the proposed continuous MSMPR operation achieved higher production capacity with shorter mean residence time and comparable product yield to the batch operation. Lastly, the robustness of C-control strategy against uncertainties in crystallisation kinetics was also demonstrated for the proposed continuous MSMPR operation.

Characterisation of stable isotopes to identify residence times and runoff components in two meso-scale catchments in the Abay/Upper Blue Nile basin, Ethiopia

Abstract. Measurements of the stable isotopes oxygen-18 (18O) and deuterium (2H) were carried out in two meso-scale catchments, Chemoga (358 km2) and Jedeb (296 km2) south of Lake Tana, Abay/Upper Blue Nile basin, Ethiopia. The region is of paramount importance for the water resources in the Nile basin, as more than 70% of total Nile water flow originates from the Ethiopian highlands. Stable isotope compositions in precipitation, spring water and streamflow were analysed (i) to characterise the spatial and temporal variations of water fluxes; (ii) to estimate the mean residence time of water using a sine wave regression approach; and (iii) to identify runoff components using classical two-component hydrograph separations on a seasonal timescale. The results show that the isotopic composition of precipitation exhibits marked seasonal variations, which suggests different sources of moisture generation for the rainfall in the study area. The Atlantic–Indian Ocean, Congo basin, Upper White Nile and the Sudd swamps are the potential moisture source areas during the main rainy (summer) season, while the Indian–Arabian and Mediterranean Sea moisture source areas during little rain (spring) and dry (winter) seasons. The spatial variation in the isotopic composition is influenced by the amount effect as depicted by moderate coefficients of determination on a monthly timescale (R2 varies from 0.38 to 0.68) and weak regression coefficients (R2 varies from 0.18 to 0.58) for the altitude and temperature effects. A mean altitude effect accounting for −0.12‰/100 m for 18O and −0.58‰/100 m for 2H was discernible in precipitation isotope composition. Results from the hydrograph separation on a seasonal timescale indicate the dominance of event water, with an average of 71 and 64% of the total runoff during the wet season in the Chemoga and Jedeb catchments, respectively. Moreover, the stable isotope compositions of streamflow samples were damped compared to the input function of precipitation for both catchments. This damping was used to estimate mean residence times of stream water of 4.1 and 6.0 months at the Chemoga and Jedeb catchment outlets, respectively. Short mean residence times and high fractions of event water components recommend catchment management measures aiming at reduction of overland flow/soil erosion and increasing of soil water retention and recharge to enable sustainable development in these agriculturally dominated catchments.

Rapid turnover of organic acids in a Dystric Brunisol under a spruce–lichen forest in northern Saskatchewan, Canada

Fujii, K., Morioka, K., Hangs, R., Funakawa, S., Kosaki, and Anderson, D. W. 2013. Rapid turnover of organic acids in a Dystric Brunisol under a spruce–lichen forest in northern Saskatchewan, Canada. Can. J. Soil Sci. 93: 295–304. Organic acids released by lichen play an important role in mineral weathering and podzolization in the Boreal–Tundra transition zone of Canada; however, importance of low-molecular-weight organic acids in the soil carbon (C) cycle in the black spruce–lichen forests remains unclear. We examined soil solution composition and mineralization kinetics of 14C-radiolabelled oxalate and citrate to quantify the C fluxes from organic acid mineralization in a Dystric Brunisol under a spruce–lichen forest in northern Saskatchewan. Oxalate concentration in soil solution was greatest in the lichen layer, while the high levels of citrate were observed in the lichen and organic (O) layers to the Ae horizon with the lowest sorption capacity. Oxalate and citrate were rapidly mineralized within the lichen and O layers and had short mean residence times (0.5 to 2.7 h). Substantial C fluxes due to citrate mineralization were observed both within the lichen and O layers, but oxalate mineralization led to C flux in the lichen layer only. The contribution of citrate and oxalate to microbial respiration was large (up to 57%) in the surface soil layers. Citrate was the dominant substrate for microbial respiration of the surface soil; however, it appears that oxalate could also be an important microbial substrate within the lichen layer, at least in summer months. We conclude that the exudation of low-molecular-weight organic acids by lichenous fungi, followed by their rapid mineralization, could play an important role in the C cycles of the sandy soils under spruce–lichen forest.

Fate and transport of ethanol-blended dissolved BTEX hydrocarbons: a quantitative tracing study of a sand tank experiment

To estimate the behavior of ethanol-blended dissolved BTEX hydrocarbons in groundwater, a quantitative tracing study instead of qualitative analysis was carried out by using a large sand tank, into which 2-L solution including bromide, ethanol and dissolved BTEX was injected under a controlled hydraulic condition. Mean residence time (MRT), pore volume swept by solute (Vp), retardation coefficient (R) and biodegradation rate constant (k) of injected solutes were estimated. Compared with bromide that was used as a conservative tracer, ethanol and BTEX had shorter MRT and smaller Vp with the sequence of EtOH T > E > m/p-X > o-X > B was just opposite to the sequences of MRT and Vp. The relationship among above sequences implies that MRT and Vp can be used as indicators to assess in situ biodegradability of a solute. Biodegradation of a reactive solute can make its MRT shortened and Vp shrunk. In addition, the sorption of ethanol could be neglected (R = 1.0), whereas BTEX compounds were adsorbed (R = 1.04–1.15). It should be noted that biodegradation of a solute can affect the estimation of its retardation coefficient. To our knowledge, this paper provides an available route to quantitatively estimate biodegradability of a solute in groundwater.

Reprint of “Biodegradation of low molecular weight organic acids in rhizosphere soils from a tropical montane rain forest”

Root exudation of organic acids could be an important strategy for plant acquisition of phosphorus (P) from P-deficient soils in tropical rain forests. However, the efficacy of organic acids on P mobilization in the rhizosphere could be reduced due to their rapid biodegradation by rhizosphere microorganisms. To assess the dynamics and function of organic acids in the rhizosphere soils in tropical rain forests, we examined the concentrations of oxalate, citrate, and malate in soil solution and the mineralization kinetics of 14C-radiolabelled oxalate and citrate in the rhizosphere and bulk soil fractions. We compared two tropical montane rain forests from Mt. Kinabalu, Borneo that share similar parent material (i.e., sedimentary rocks) and climate but differ in terms of soil age. The older soil (Tertiary age materials) was affected by podzolization and had less inorganic labile P compared to the younger soil (Quaternary colluvial deposits). In the P-deficient older soil, the rhizosphere soil solution contained markedly higher concentrations of oxalate, citrate, and malate than did the bulk soil, whereas in the P-rich younger soil, the levels of organic acids in the rhizosphere were lower. The higher levels of organic acids in the rhizosphere of P-deficient soils are caused by greater root exudation and the lower sorption capacity for organic acids. The results of mineralization kinetics showed that oxalate and citrate in soil solution were rapidly mineralized in both rhizosphere and bulk fractions of both P-rich and P-deficient soils, having short mean residence times (2.3–13.1 h for oxalate and 0.8–1.6 h for citrate). The mineralization rates of oxalate and citrate were highest in the rhizosphere fraction of the P-deficient soil, where the pool of organic acids was largest and rapidly replenished by root exudation. Our data indicate that consumption as well as production of organic acids in the rhizosphere could be enhanced in P-deficient soil. The efficacy of organic acids on P mobilization in the rhizosphere in tropical montane rain forests appears to vary depending on the level of soil P availability and the anion sorption capacity, attributable to soil aging with podzolization.

Rate of belowground carbon allocation differs with successional habit of two afromontane trees.

BackgroundAnthropogenic disturbance of old-growth tropical forests increases the abundance of early successional tree species at the cost of late successional ones. Quantifying differences in terms of carbon allocation and the proportion of recently fixed carbon in soil CO2 efflux is crucial for addressing the carbon footprint of creeping degradation.MethodologyWe compared the carbon allocation pattern of the late successional gymnosperm Podocarpus falcatus (Thunb.) Mirb. and the early successional (gap filling) angiosperm Croton macrostachyus Hochst. es Del. in an Ethiopian Afromontane forest by whole tree 13CO2 pulse labeling. Over a one-year period we monitored the temporal resolution of the label in the foliage, the phloem sap, the arbuscular mycorrhiza, and in soil-derived CO2. Further, we quantified the overall losses of assimilated 13C with soil CO2 efflux.Principal Findings 13C in leaves of C. macrostachyus declined more rapidly with a larger size of a fast pool (64% vs. 50% of the assimilated carbon), having a shorter mean residence time (14 h vs. 55 h) as in leaves of P. falcatus. Phloem sap velocity was about 4 times higher for C. macrostachyus. Likewise, the label appeared earlier in the arbuscular mycorrhiza of C. macrostachyus and in the soil CO2 efflux as in case of P. falcatus (24 h vs. 72 h). Within one year soil CO2 efflux amounted to a loss of 32% of assimilated carbon for the gap filling tree and to 15% for the late successional one.ConclusionsOur results showed clear differences in carbon allocation patterns between tree species, although we caution that this experiment was unreplicated. A shift in tree species composition of tropical montane forests (e.g., by degradation) accelerates carbon allocation belowground and increases respiratory carbon losses by the autotrophic community. If ongoing disturbance keeps early successional species in dominance, the larger allocation to fast cycling compartments may deplete soil organic carbon in the long run.

Residence Time Distribution of Soy Protein Isolate and Corn Flour Feed Mix in a Twin-Screw Extruder

The residence time distribution (RTD) of soy protein isolate (SPI) and corn flour mixture through a twin-screw extruder under different conditions was evaluated. A full-factorial design of experiments was used to select the variables at different levels. The feed mixture was prepared by blending corn flour and SPI in the ratio of 4:1 (20% SPI content). The effect of screw speed (75, 100 and 125 rpm), feed mixture moisture content (25, 30 and 35%) and die diameter (3 and 5 mm) were investigated. All factors were found to have a significant (P < 0.05) effect on the mean resident time and its variance (square of standard deviation). Higher screw speed, higher initial moisture content and larger die diameter resulted in a shorter mean residence time. Two conventional flow models were used to represent the RTD profile in the extruder – the frequency model (F-distribution) and the cumulative RTD model (E distribution). The parameters of these models – the half-concentration internal age and particle accumulation rate – were determined by a nonlinear regression. The parameters of these models were responsive and related to process variables and both F- and E-distributions were well predicted. Practical Applications Residence time distribution (RTD) is an important parameter that describes the time spent by particles through any continuous processing system. RTD study is important with respect to extrusion processing because the properties of extruded product depend on the shear, pressure, time and temperature history of the product in the extruder. The study is aimed at evaluating RTD of a protein-rich formulation obtained by different blends of soy protein isolates and corn flour which are prepared for producing protein-rich soy-based products. The study is important because it examines the influence of selected process and product variables on the residence time of particles in the twin-screw extrusion system.

Input related microbial carbon dynamic of soil organic matter in particle size fractions

This paper investigated the flow of carbon into different groups of soil microorganisms isolated from different particle size fractions. Two agricultural sites of contrasting organic matter input were compared. Both soils had been submitted to vegetation change from C3 (Rye/Wheat) to C4 (Maize) plants, 25 and 45 years ago. Soil carbon was separated into one fast-degrading particulate organic matter fraction (POM) and one slow-degrading organo-mineral fraction (OMF). The structure of the soil microbial community were investigated using phospholipid fatty acids (PLFA), and turnover of single PLFAs was calculated from the changes in their 13C content. Soil enzyme activities involved in the degradation of carbohydrates was determined using fluorogenic MUF (methyl-umbelliferryl phosphate) substrates.We found that fresh organic matter input drives soil organic matter dynamic. Higher annual input of fresh organic matter resulted in a higher amount of fungal biomass in the POM-fraction and shorter mean residence times. Fungal activity therefore seems essential for the decomposition and incorporation of organic matter input into the soil. As a consequence, limited litter input changed especially the fungal community favoring arbuscular mycorrhizal fungi. Altogether, supply and availability of fresh plant carbon changed the distribution of microbial biomass, the microbial community structure and enzyme activities and resulted in different priming of soil organic matter.Most interestingly we found that only at low input the OMF fraction had significantly higher calculated MRT for Gram-positive and Gram-negative bacteria suggesting high recycling of soil carbon or the use of other carbon sources. But on average all microbial groups had nearly similar carbon uptake rates in all fractions and both soils, which contrasted the turnover times of bulk carbon. Hereby the microbial carbon turnover was always faster than the soil organic carbon turnover and higher carbon input reduced the carbon storage efficiency from 51% in the low input to 20%. These findings suggest that microbial community preferentially assimilated fresh carbon sources but also used recycled existing soil carbon. However, the priming rate was drastically reduced under carbon limitation. In consequence at high carbon availability more carbon was respired to activate the existing soil carbon (priming) whereas at low carbon availability new soil carbon was formed at higher efficiencies.

Biodegradation of low molecular weight organic acids in rhizosphere soils from a tropical montane rain forest

Root exudation of organic acids could be an important strategy for plant acquisition of phosphorus (P) from P-deficient soils in tropical rain forests. However, the efficacy of organic acids on P mobilization in the rhizosphere could be reduced due to their rapid biodegradation by rhizosphere microorganisms. To assess the dynamics and function of organic acids in the rhizosphere soils in tropical rain forests, we examined the concentrations of oxalate, citrate, and malate in soil solution and the mineralization kinetics of 14C-radiolabelled oxalate and citrate in the rhizosphere and bulk soil fractions. We compared two tropical montane rain forests from Mt. Kinabalu, Borneo that share similar parent material (i.e., sedimentary rocks) and climate but differ in terms of soil age. The older soil (Tertiary age materials) was affected by podzolization and had less inorganic labile P compared to the younger soil (Quaternary colluvial deposits). In the P-deficient older soil, the rhizosphere soil solution contained markedly higher concentrations of oxalate, citrate, and malate than did the bulk soil, whereas in the P-rich younger soil, the levels of organic acids in the rhizosphere were lower. The higher levels of organic acids in the rhizosphere of P-deficient soils are caused by greater root exudation and the lower sorption capacity for organic acids. The results of mineralization kinetics showed that oxalate and citrate in soil solution were rapidly mineralized in both rhizosphere and bulk fractions of both P-rich and P-deficient soils, having short mean residence times (2.3–13.1 h for oxalate and 0.8–1.6 h for citrate). The mineralization rates of oxalate and citrate were highest in the rhizosphere fraction of the P-deficient soil, where the pool of organic acids was largest and rapidly replenished by root exudation. Our data indicate that consumption as well as production of organic acids in the rhizosphere could be enhanced in P-deficient soil. The efficacy of organic acids on P mobilization in the rhizosphere in tropical montane rain forests appears to vary depending on the level of soil P availability and the anion sorption capacity, attributable to soil aging with podzolization.

Laboratory evaluation of a prototype photochemical chamber designed to investigate the health effects of fresh and aged vehicular exhaust emissions

Laboratory experiments simulating atmospheric aging of motor vehicle exhaust emissions were conducted using a single vehicle and a photochemical chamber. A compact automobile was used as a source of emissions. The vehicle exhaust was diluted with ambient air to achieve carbon monoxide (CO) concentrations similar to those observed in an urban highway tunnel. With the car engine idling, it is expected that the CO concentration is a reasonable surrogate for volatile organic compounds (VOCs) emissions. Varying the amount of dilution of the exhaust gas to produce different CO concentrations, allowed adjustment of the concentrations of VOCs in the chamber to optimize production of secondary organic aerosol (SOA) needed for animal toxicological exposures. Photochemical reactions in the chamber resulted in nitric oxide (NO) depletion, nitrogen dioxide (NO2) formation, ozone (O3) accumulation, and SOA formation. A stable SOA concentration of approximately 40 μg m−3 at a chamber mean residence time of 30 min was achieved. This relatively short mean residence time provided adequate chamber flow output for both particle characterization and animal exposures. The chamber was operated as a continuous flow reactor for animal toxicological tests. SOA mass generated from the car exhaust diluted with ambient air was almost entirely in the ultrafine mode. Chamber performance was improved by using different types of seed aerosol to provide a surface for condensation of semivolatile reaction products, thus increasing the yield of SOA. Toxicological studies using Sprague-Dawley rats found significant increases of in vivo chemiluminescence in lungs following exposure to SOA.

Continuous Mass Crystallization of Vitamin C in l(+)-Ascorbic Acid−Ethanol−Water System: Size-Independent Growth Kinetic Model Approach

Experimental results concerning continuous isohydrical drowning-out mass crystallization of vitamin C in an l(+)-ascorbic acid−ethanol−water system are presented. The process environment was created in a laboratory-scale draft tube, mixed suspension mixed product removal (DT MSMPR) crystallizer with internal circulation of suspension. Assuming a constant feed concentration of ethanol (20 mass %), the feed concentration of vitamin C was changed within the 30−50 mass % range. The mean residence time of suspension in working volume of a crystallizer was varied from 900 to 3600 s. Combinations of the presented above input process parameters resulted in a productivity 244−1692 kg/(m3 h) of crystal product, of mean size within 0.20−0.24 mm and of CV within the 50−60% range. The supersaturation level in mother solution reached relatively high values (up to ca. 6.5 mass %), in particular at a short mean residence time of suspension (900 s). With this time elongation to 3600 s, the supersaturation level decreased, however, by ca. 40% (to 3.9 mass %). The simplest size-independent growth (SIG) kinetic model was adopted for nucleation ((3.3−27.8) × 107 1/(s m3)) and crystal growth ((1.6−6.9) × 10−8 m/s) rates estimation. Kinetic relations and feedback between nucleation and crystals growth were identified and analyzed in detail.