Chemical Residence Time Research Articles

Coke formation pathway under various reaction conditions during the production of syngas in a dielectric barrier discharge plasma environment using CeO2 nanorods supported Ni catalysts

Plasma-assisted dry reforming of methane (DRM) was investigated using CeO2 nanorods (NR) supported Ni catalysts in a fixed-bed coaxial dielectric barrier discharge (DBD) reactor. This study individually examined the catalyst support, packing material, and 10 wt% Ni-CeO2 NR catalyst under pure thermal and plasma-assisted thermal DRM environments to explore the plasma-thermal synergy. Support and packing material displayed no discernible reactivity under thermal DRM within 450 °C; however, the catalyst exhibited reactivity beginning around 350 °C. Alternatively, plasma-assisted DRM experiments revealed a distinct reaction occurrence at significantly lower temperatures across all sample types. Introducing plasma demonstrated a significant enhancement in CH4 conversion compared to CO2 conversion. The 10 wt% Ni-CeO2 NR catalyst in plasma-assisted DRM testing showed increasing conversions of CH4 and CO2 with rising temperatures, peaking at 52.1 % and 46.1 %, respectively, at 450 °C. The catalyst’s improved performance in the plasma environment can be attributed to the enhanced metal-support interaction between NiO and the CeO2 NR support, a consequence of the rich surface oxygen vacancy concentration. However, this increase in conversion was accompanied by an escalation in carbon deposition. Considering contributions from the thermal DRM process, the optimal operating temperature was determined as 350 °C. Increasing plasma power at this temperature led to enhanced conversion. However, beyond a threshold of 23.8 W power, the reaction path altered, resulting in reduced syngas output. Increasing the CH4:CO2 feed gas flow ratio, while maintaining constant plasma power and temperature, elevated H2/CO generation. Moreover, increasing the total flow rate under identical conditions, with a constant feed gas flow ratio, reduced both conversions due to decreased chemical residence time. Consequently, under a constant feed gas ratio, a higher syngas yield is achievable with lower overall flow rates.

Avermectin Toxicity to Benthic Invertebrates is Modified by Sediment Organic Carbon and Chemical Residence Time.

Chemicals used in sea lice management strategies in salmonid aquaculture include the avermectin class of compounds that can accumulate and persist in the sediments underneath salmon farms and directly impact nontarget benthic fauna. The effects of sediment organic carbon content and chemical residence time (CRT) on the lethal and sublethal toxicity of emamectin benzoate (EB; formulation: Slice®) and ivermectin (purified) and a combination of both were examined in two benthic invertebrates, the amphipod Eohaustorius estuarius and the polychaete Neanthes virens. In both species, increased sediment organic carbon content significantly reduced lethal toxicity, a modulation that was more pronounced for ivermectin and combination exposures. At a CRT of 4 months, lethal toxicity was reduced in E. estuarius but was unaffected in N. virens. Sublethal toxicity in N. virens (burrowing behavior) was modulated by sediment organic carbon and CRT in a similar manner to the trend in lethal toxicity. Inconsistencies in behavior (phototaxis) in E. estuarius made conclusions regarding toxicity modification by sediment organic carbon or CRT inconclusive. Our results indicate that environmental factors including sediment organic carbon content and the time compounds reside in sediments are important modifiers of chemotherapeutant toxicity in nontarget benthic species and should be considered when regulatory decisions regarding their use are made.Environ Toxicol Chem 2022;41:1918-1936. © 2022 SETAC.

Effects of injection parameters, boost, and swirl ratio on gasoline compression ignition operation at idle and low-load conditions

In this work, we study the effects of injector nozzle inclusion angle, injection pressure, boost, and swirl ratio on gasoline compression ignition combustion. Closed-cycle computational fluid dynamics simulations using a 1/7th sector mesh representing a single cylinder of a four-cylinder 1.9 L diesel engine, operated in gasoline compression ignition mode with 87 anti-knock index (AKI) gasoline, were performed. Two different operating conditions were studied—the first is representative of idle operation (4 mg fuel/cylinder/cycle, 850 r/min), and the second is representative of a low-load condition (10 mg fuel/cylinder/cycle, 1500 r/min). The mixture preparation and reaction space from the simulations were analyzed to gain insights into the effects of injection pressure, nozzle inclusion angle, boost, and swirl ratio on achieving stable low-load to idle gasoline compression ignition operation. It was found that narrower nozzle inclusion angles allow for more reactivity or propensity to ignition (determined qualitatively by computing constant volume ignition delays) and are suitable over a wider range of injection timings. Under idle conditions, it was found that lower injection pressures helped to reduce overmixing of the fuel, resulting in greater reactivity and ignitability (ease with which ignition can be achieved) of the gasoline. However, under the low-load condition, lower injection pressures did not increase ignitability, and it is hypothesized that this is because of reduced chemical residence time resulting from longer injection durations. Reduced swirl was found to maintain higher in-cylinder temperatures through compression, resulting in better ignitability. It was found that boosting the charge also helped to increase reactivity and advanced ignition timing.

Experiments in water-macrophyte systems to uncover the dynamics of pesticide mitigation processes in vegetated surface waters/streams.

Knowledge on the dynamics and the durability of the processes governing the mitigation of pesticide loads by aquatic vegetation in vegetated streams, which are characterized by dynamic discharge regimes and short chemical residence times, is scarce. In a static long-term experiment (48 h), the dissipation of five pesticides from the aqueous phase followed a biphasic pattern in the presence of aquatic macrophytes. A dynamic concentration decrease driven by sorption to the macrophytes ranged from 8.3 to 60.4% for isoproturon and bifenox, respectively, within the first 2 h of exposure. While the aqueous concentrations of imidacloprid, isoproturon, and tebufenozide remained constant thereafter, the continuous but decelerated concentration decrease of difenoconazole and bifenox in the water-macrophyte systems used here was assumed to be attributed to macrophyte-induced degradation processes. In addition, a semi-static short-term experiment was conducted, where macrophytes were transferred to uncontaminated medium after 2 h of exposure to simulate a transient pesticide peak. In the first part of the experiment, adsorption to macrophytes resulted in partitioning coefficients (logK D_Adsorp) ranging from 0.2 for imidacloprid to 2.2 for bifenox. One hour after the macrophytes were transferred to the uncontaminated medium, desorption of the compounds from the macrophytes resulted in a new phase equilibrium and K D_Desorp values of 1.46 for difenoconazole and 1.95 for bifenox were determined. A correlation analysis revealed the best match between the compound affinity to adsorb to macrophytes (expressed as K D_Adsorp) and their soil organic carbon-water partitioning coefficient (K OC) compared to their octanol-water partitioning coefficient (K OW) or a mathematically derived partitioning coefficient.

Detection of herbicides in the urine of pet dogs following home lawn chemical application

Exposure to herbicide-treated lawns has been associated with significantly higher bladder cancer risk in dogs. This work was performed to further characterize lawn chemical exposures in dogs, and to determine environmental factors associated with chemical residence time on grass. In addition to concern for canine health, a strong justification for the work was that dogs may serve as sentinels for potentially harmful environmental exposures in humans. Experimentally, herbicides [2,4-dichlorophenoxyacetic acid (2,4-D), 4-chloro-2-methylphenoxypropionic acid (MCPP), dicamba] were applied to grass plots under different conditions (e.g., green, dry brown, wet, and recently mowed grass). Chemicals in dislodgeable residues were measured by LC-MS at 0.17, 1, 24, 48, 72h post treatment. In a separate study, 2,4-D, MCPP, and dithiopyr concentrations were measured in the urine of dogs and in dislodgeable grass residues in households that applied or did not apply chemicals in the preceding 48h. Chemicals were measured at 0, 24, and 48h post application in treated households and at time 0 in untreated control households. Residence times of 2,4-D, MCPP, and dicamba were significantly prolonged (P<0.05) on dry brown grass compared to green grass. Chemicals were detected in the urine of dogs in 14 of 25 households before lawn treatment, in 19 of 25 households after lawn treatment, and in 4 of 8 untreated households. Chemicals were commonly detected in grass residues from treated lawns, and from untreated lawns suggesting chemical drift from nearby treated areas. Thus dogs could be exposed to chemicals through contact with their own lawn (treated or contaminated through drift) or through contact with other grassy areas if they travel. The length of time to restrict a dog's access to treated lawns following treatment remains to be defined. Further study is indicated to assess the risks of herbicide exposure in humans and dogs.

Evaluating the fate and behaviour of cyclic volatile methyl siloxanes in two contrasting North American lakes using a multi-media model

The behaviour of cyclic volatile methyl siloxanes (cVMS) in lakes was explored using a fugacity-based steady-state non-equilibrium multimedia fate and transport model (a modified version of QWASI). Three substances were investigated: Octamethylcyclotetrasiloxane (D4), Decamethylcyclopentasiloxane (D5) and Dodecamethylcyclohexasiloxane (D6) in two contrasting North American lakes: Lake Ontario and Lake Pepin (a natural shallow lake on the Mississippi river between Minnesota and Wisconsin). Values for the principal partition coefficients and the hydrolysis rate constant were adjusted for the mean annual temperatures of each lake. Hydrolysis rate constants were also adjusted for the fraction of chemical calculated to be in the freely dissolved phase. Half-lives in sediment were calculated from partitioning theory, assuming hydrolysis can occur only in the dissolved phase. Best estimates of substance-specific emissions were obtained by combining current per capita approximations of usage and fraction lost to domestic waste water, the population of the lake catchment and cVMS removal in waste water treatment. Concentrations were generally lower and chemical residence times longer in Lake Ontario than in Lake Pepin, owing to greater depth, a higher hydraulic retention time and a higher degree of dilution. Overall persistence in Lake Pepin is significantly influenced by the high rate of sediment burial assumed in the model, as well as by a relatively high rate of water discharge. Despite the many similarities of the compounds considered, the dominant loss mechanisms vary significantly and are not the same in each lake system. This highlights the pitfalls of subjective evaluation of chemical fate and illustrates the important role which models have to play in providing a quantitative framework for assessing chemical behaviour objectively under the influence of a complex and interacting set of factors.

Using streamlines to simulate stochastic reactive transport in heterogeneous aquifers: Kinetic metal release and transport in CO2 impacted drinking water aquifers

A Lagrangian streamline approach that stochastically represents uncertainty in spatial hydraulic conductivity distribution is coupled to kinetic reactive transport in a heterogeneous 3-D domain. This methodology is designed to efficiently account for uncertainties inherent in subsurface reactive transport while maintaining hydro-geochemical processes. A hypothetical CO2 leak from a geological carbon storage site into an overlying aquifer is used to simulate reactive transport where contamination may occur. Uncertainty in subsurface hydraulic conductivity is accounted for using correlated, Gaussian random fields in a Monte Carlo approach. In this approach 100 realizations of each ensemble were simulated with variances of the natural log of hydraulic conductivity (σ2lnK) of 1, 3.61, and 16. Peak ensemble lead concentrations were found at σ2lnK of 3.61, the middle of the variances simulated. σ2lnK within an aquifer was found to influence chemical residence time, which in turn determined the equilibrium state of the plume along the flow path and at the pumping well thus driving geochemical conditions. However, macrodispersion due to heterogeneous flow paths caused lower contaminant concentrations at the pumping well due to dilution with uncontaminated water. Furthermore, a strong link between σ2lnK and the probability of well-capture was found, suggesting that proper characterization of the σ2lnK within an aquifer will help to quantify the impact of uncertainty on risks of groundwater contamination.

Chemical Residence Time and Hydrological Conditions Influence Treatment of Fipronil in Vegetated Aquatic Mesocosms

Fipronil, a phenyl-pyrazole insecticide, is often used in rice (Oryza sativa L.) production agriculture, with elevated runoff concentrations and loads having potential toxicological effects on downstream aquatic environments. This study evaluated two species of aquatic plants-broadleaf cattail (Typha latifolia L.) and powdery alligator-flag (Thalia dealbata Fraser ex Roscoe)-placed in series against a nonvegetated mesocosm in reducing concentrations and loads of fipronil, and associated metabolites. Vegetation type and hydrological condition (inundated vs. dry) were treatment effects used for comparison. The vegetated mesocosms significantly reduced higher loads and concentrations of fipronil, fipronil sulfone, and sulfide in both inundated and dry hydrological conditions over nonvegetated nesocosms. Under inundation conditions, vegetated mesocosms reduced >50% of influent fipronil concentrations and betweeen 60 and 70% of fipronil loads, which was significantly higher than the dry conditions (10-32% concentration and load). These results show that agricultural management strategies usingephemeral aquatic zones, such as drainage ditches, can be optimized to couple chemical applications with vegetation presence and hydrology to facilitate the reduction in chemical waste loads entering downstream aquatic ecosystems. Such reduction is critical for use with fipronil, where negative impacts have been demonstrated with several nontarget species.

Release of Legacy Pollutants from Melting Glaciers: Model Evidence and Conceptual Understanding

Recently, increasing concentrations of persistent organic pollutants (POPs) have been observed in the sediment of glacier-fed Lake Oberaar, Switzerland. Melting glaciers have been suggested as a secondary source of POPs released to Alpine lakes. Here we further investigate whether climate warming may accelerate the release of POPs previously deposited to Alpine glaciers ("glacier hypothesis"). To test this hypothesis, a dynamic multimedia mass balance model is developed for the catchment area of Lake Oberaar and is applied to polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs), and dichlorodiphenyl trichloroethane (DDT). This lake model is combined with two other models. The first is a dynamic multimedia mass balance model parametrized for the Swiss lowlands that is used to calculate (on the basis of historical emission data) the atmospheric concentrations that are an advective input into the model of the lake catchment. The second is a flow model of Oberaar Glacier that determines the residence time of persistent chemicals in the glacier after their deposition to the glacier surface. According to results from these three models in combination, the release of POPs by the glacier is currently increasing and accounts for the observed increase in concentrations in the lake sediment. The models indicate that approximately half of the amount of PCBs, PCDD/Fs, and DDT initially incorporated into the glacier ice is still stored in the glacier. Under the assumption that the climate is warming, accelerated release of POPs is to be expected for the future; in a model run where no climate warming is assumed, the period of time required for release of the same amount of chemicals is longer by several decades than in the scenario with a changing climate.

Abstract A40: Exposure of children and pet dogs to chemicals in lawn care products

Abstract Exposure to lawn herbicides and pesticides has been associated with increased risk for invasive urinary bladder cancer in pet dogs. There is concern of potential risks for humans as well. Among many steps to further investigate this finding, two were addressed here. The purpose of this work was to determine: (1) length of time for potential exposure to chemicals after lawn treatment, i.e. residence time of chemicals on grass, and (2) lawn chemical concentrations in the urine of pet dogs and children living in households that used commercial lawn products. Marker chemicals commonly used in lawn products were selected to track potential exposures. Methods: The work included 2 parts. 1. Residence chemical times were studied in experimental grass plots to define methods and sampling times for the subsequent work. Briefly, a product containing 0.169% 2,4-dichlorophenoxyacetic acid (2,4-D), 0.045% 4-chloro-2-methylphenoxypropionic acid (MCPP), and 0.015% Dicamba was applied by calibrated sprayer at manufacturer recommended amounts under different lawn conditions (green, dry brown, wet, recently mowed grass). Chemicals in the dislodgeable residue were measured by LC-MS (0.17, 24, 48, 72 hrs post treatment). 2. A household study was conducted in which chemical (2,4-D, MCPP) residence times and chemical concentrations in urine of children and dogs were determined at 0, 24, and 48 hrs post lawn treatment. Questionnaires regarding lawn exposure for dogs and children were completed for treated and for untreated (control) households. Results: In the experimental grass plot study, minimal amounts of 2,4-D, MCPP, and dicamba were still detected at 48 hours post chemical application to green lawns, although chemical residence time was prolonged (P &amp;lt; 0.05) on dry brown grass. In the household study, chemical concentrations were more than 5 fold higher in some lawns, and chemical residence times were considerably longer (up to 196 µg/m2 of 2,4-D at 48 hrs) than for the experimental grass plot study. No chemicals were detected in the urine of children, although children in this study spent little time on the lawn. Chemicals (0.69 – 6.12 mg/g creatinine 2,4-D, 0.20 – 0.68 mg/g creatinine MCPP) were detected in the urine of dogs in 4 of 9 treated households. No chemicals were detected in the urine of dogs in 5 control households. Conclusions: The findings of much higher concentrations and longer residence times (&amp;gt;48 hrs) of chemicals on commercially treated lawns compared to experimental grass plots treated with recommended amounts of herbicide suggest that excessive amounts of chemicals are applied in some commercial settings, thun increasing exposure potential. Detection of chemicals in the urine of dogs in 4 of 9 treated households confirms that chemical uptake and internalization occurs, and that a clear exposure risk exists for pet animals. Further study is needed to assess the exposure risk to humans. Citation Information: Cancer Prev Res 2010;3(1 Suppl):A40.

Evaluating the influence of wetland vegetation on chemical residence time in Mississippi Delta drainage ditches

The presence of emergent vegetation within channelized aquatic environments has the capacity to provide a number of biological functions as well as alter the hydrology of the system. Vegetation within the channel exerts roughness, drag and friction on flowing water, reducing flow rates, increasing water depths and increasing hydraulic retention time. By increasing the hydraulic retention time, chemical residence time (CRT) is increased, thus improving the potential of pollutant mitigation. The study compared two geomorphologically similar drainage ditches, one vegetated and one non-vegetated to evaluate the effect obligate, in-stream wetland vegetation had on CRT. A fluoride (F −) tracer was amended to both ditches with nutrients and sediments to simulate stormwater runoff event. The measured CRT of the vegetated drainage ditch was at least twice that of the non-vegetated ditch. These results suggest that with the presence of vegetation increasing CRT, chemical removal rates will improve, and as a result increase the possibility of microbial transformation, adsorption, and macrophyte assimilation. By dredging or clear-scraping ditches and removing the vegetative component, farmers and managers alike will increase water flows, decrease CRT and potentially increase pollutant loads into aquatic receiving systems.

A method to incorporate ecology into residence time of chemicals in embayments: Local effect time

Residence times are defined classically by the physical and chemical aspects of water bodies rather than by their ecological implications. Therefore, a more clear and direct connection between the residence times and ecological effects is necessary to relate these timescales quantitatively to ecology. The concept of local effect time (LET) is proposed in this paper as a timescale with spatial resolution that relates to ecological components and their spatial distribution within an embayment. The LET can predict the susceptibility of real-world ecological components to change from one condition to another. It can provide an efficient way to allow managers and agencies to evaluate the degree of stress or relief from current or projected changes in the loading of contaminants or nutrients. The steps for calculating LETs and defining their correlation with the existing ecological components in an embayment are presented along with illustrative applications to loading from riverine inflow and a wastewater treatment plant. The LETsuccessfully identified the areas within the water body that could be prone to ecological changes due to perturbations in the loading rate of riverine water and its constituents. An example is given that shows how the LET method can be used to delineate the distribution and duration of high levels of coliform bacteria due to a pulsed effluent from the treatment plant.

The formation of cubic and monoclinic Y2O3 nanoparticles in a gas-phase flame process

Synthesis of phase-pure cubic and monoclinic Y2O3 nanoparticles (16–90 nm) was achieved in a gas-phase flame process. The effect of process parameters on the crystal structure of the Y2O3 nanoparticles was systematically investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Fuel gases, H2 or C2H4, at various flow rates, two yttrium precursors, an oxidant stream with varying degrees of N2 or Ar dilution and different burner diameters were used to examine the effects of chemical atmosphere, flame temperature, residence time and precursor concentration on the crystal structure of the synthesized Y2O3 nanoparticles. Regardless of the other process parameters, at diluent/O2 ratios of 0.25 or lower, monoclinic Y2O3 nanoparticles were obtained, whereas at diluent/O2 ratios of 1 or higher, cubic Y2O3 nanoparticles were obtained. A lower diluent/O2 ratio was related to higher flame temperature. Thermodynamic analyses suggest that high temperatures likely favour the formation of monoclinic Y2O3 nanoparticles in this flame process.

A Method to Incorporate Ecology into Residence Time of Chemicals in Embayments: Local Effect Time

A Method to Incorporate Ecology into Residence Time of Chemicals in Embayments: Local Effect Time

Sequential risk mitigation and the role of natural recovery in contaminated sediment projects

Sequential risk mitigation approaches the remediation of contaminated sediments in three phases designed to: (1) immediately reduce the ecological and human health risks associated with high levels of contamination, using methods such as the confinement or capping of high-risk materials; (2) reduce the risks associated with moderate levels of pollution to a minimum, on a less urgent schedule and at a lower cost; and (3) address areas of limited contamination through a combination of natural recovery and enhanced natural recovery (to aid or speed those natural processes). Natural recovery, the reduction of contaminant concentrations through natural processes, is based on the practical observation that overall ecosystem recovery appears to be largely a function of time. Sediment decomposition and the mixing of new and old sediments by bottom-dwelling organisms can both contribute to reduced contaminant concentrations. Knowledge of these processes--sediment decomposition, sediment mixing by bottom-dwelling organisms, and chemical residence time is critical in the development of appropriate ecosystem recovery and waste management strategies. Evaluations to support natural recovery predictions are designed to collect and evaluate information necessary to determine whether surface sediment chemical concentrations, with adequate source control, will reach the cleanup standards within a ten-year period.

Sequential risk mitigation and the role of natural recovery in contaminated sediment projects

Sequential risk mitigation approaches the remediation of contaminated sediments in three phases designed to: (1) immediately reduce the ecological and human health risks associated with high levels of contamination, using methods such as the confinement or capping of high-risk materials; (2) reduce the risks associated with moderate levels of pollution to a minimum, on a less urgent schedule and at a lower cost; and (3) address areas of limited contamination through a combination of natural recovery and enhanced natural recovery (to aid or speed those natural processes). Natural recovery, the reduction of contaminant concentrations through natural processes, is based on the practical observation that overall ecosystem recovery appears to be largely a function of time. Sediment decomposition and the mixing of new and old sediments by bottom-dwelling organisms can both contribute to reduced contaminant concentrations. Knowledge of these processes-sediment decomposition, sediment mixing by bottom-dwelling organisms, and chemical residence time is critical in the development of appropriate ecosystem recovery and waste management strategies. Evaluations to support natural recovery predictions are designed to collect and evaluate information necessary to determine whether surface sediment chemical concentrations, with adequate source control, will reach the cleanup standards within a ten-year period.

Biodegradation of triethanolamine

AbstractTriethanolamine production in the United States exceeds 100,000 metric tons per year. Given this magnitude of production and widespread distribution to consumer product, specialty chemical, and agricultural chemical markets, an understanding of the environmental fate and lifetime of this chemical is warranted. Studies were conducted to determine the fate and lifetime of [14C]triethanolamine in an aerobic surface soil, freshwater river systems, and in activated sludge waste treatment. The half‐life determined for biodegradation of triethanolamine ranged from 0.02 to 0.10 d in reactions containing 818 mg/L mixed liquor activated sludge solids from a municipal wastewater treatment facility. The half‐life of triethanolamine in a sandy loam surface soil ranged from 0.5 to 1.8 d for initial concentrations ranging from 0.01 to 13.4 mmol/kg (1.4–2,000 mg/kg dry wt). Biodegradation was also measured in two different river waters, with an average half‐life (±1 SD) of 1.2 ± 0.5 d determined for initial triethanolamine concentrations of 0.67 and 3.28 μmol/L (99 and 489 μg/L). The kinetics of both primary biodegradation and mineralization were shown to be generally first‐order in triethanolamine and biomass concentrations (second‐order overall) for the matrices and range of triethanolamine concentrations tested. When these relatively short half‐lives are related to typical chemical residence times in various environmental compartments, it can be concluded that biodegradation would prevent accumulation of triethanolamine and its associated degradation products in the environment.

Quantitative application of biodegradation data to environmental risk and exposure assessments

AbstractBiodegradation is an important removal mechanism for natural and synthetic organic chemicals released to aquatic, benthic, and terrestrial ecosystems. It results in a decrease in the overall mass or load of chemicals present in the environment and is key in preventing the accumulation and persistence of chemicals in specific environmental compartments. Although biodegradation is an important process for minimizing potential adverse impacts on environmental systems, it has not been traditionally considered in a quantitative fashion in environmental risk assessments. This article outlines an approach and provides simple kinetic criteria for incorporating biodegradation rate data into environmental exposure and risk assessments. The approach is a generic one that relates biodegradation half‐lives to chemical residence times in specific environmental compartments. It is broadly applicable to any organic chemical in a range of environmental compartments and has potential use as a technical and regulatory tool to better quantify environmental exposure and risk.

Modeling of tritium recovery from ctr solid breeder

A model describing tritium release from Li 2 O was presented to explain the results obtained in the in-situ tritium release experiments (TTTEx) performed at the University of Tokyo. This model includes the desorption process as well as diffusion process of tritium. Two different kinds of tritium chemical species (HT and HTO) are considered. This model can explain the dependences of the released chemical form and tritium residence time on the sweep gas composition used in the series of the experiments.

Mn, Fe, Cu and Cd distributions and residence times in closed basin Lake Vanda (Wright Valley, Antarctica)

Mn, Fe, Cu, and Cd concentrations are reported for Lake Vanda, a closed-basin, meromictic, Antarctic lake and for its single major inflow, the Onyx River. Trace metal distributions in the upper lake and annual metal fluxes from the Onyx River were used to estimate chemical residence times in the mixed zone above the chemocline. Residence times, based on total metal loads, were 9.4 years for Mn; 1.4 years for iron; 174 years for copper; and 82 years for cadmium. Controls on the steady state concentrations of metals in this system are likely to include: particle settling of Fe and Mn; scavenging of minor elements on metal oxide surfaces; sulfide precipitation from the anoxic brine; and possibly uptake of metals on the surface of benthic algal mats. Model calculations show that metal removal by sinking phytoplankton can account for only a small fraction of the annual loss.