Heavy Species Research Articles

Collisional radiative coarse-grain model for ionization in air

We present a reduced kinetic mechanism for the modeling of the behavior of the electronic states of the atomic species in air mixtures. The model is built by lumping the electronically excited states of the atomic species and by performing Maxwell-Boltzmann averages of the rate constants describing the elementary kinetic processes of the individual states within each group. The necessary reaction rate coefficients are taken from the model compiled by Bultel et al. [“Collisional-radiative model in air for earth re-entry problems,” Phys. Plasmas 13, 043502 (2006)10.1063/1.2194827]. The reduced number of pseudo-states considered leads to a significant reduction of the computational cost, thus enabling the application of the state of the art collisional radiative models to bi-dimensional and three-dimensional problems. The internal states of the molecular species are assumed to be in equilibrium. The rotational energy mode is assumed to quickly equilibrate with the translational energy mode at the kinetic temperature of the heavy species as opposed to the electronic and the vibrational modes, assumed to be in Maxwell-Boltzmann equilibrium at a common temperature TV. In a first step we validate the model by using simple zero- and one-dimensional test cases for which the full kinetic mechanism can be run efficiently. Finally, the reduced kinetic model is used to analyze the strong non-equilibrium flow surrounding the FIRE II flight experiment during the early part of its re-entry trajectory. It is found that the reduced kinetic mechanism is capable of reproducing the ionizational non-equilibrium phenomena, responsible for the drastic reduction of the radiative heat loads on the space capsules during the re-entry phase.

Chronic ingestion of cadmium and lead alters the bioavailability of essential and heavy metals, gene expression pathways and genotoxicity in mouse intestine

Chronic ingestion of environmental heavy metals such as lead (Pb) and cadmium (Cd) causes various well-documented pathologies in specific target organs following their intestinal absorption and subsequent accumulation. However, little is known about the direct impact of the non-absorbed heavy metals on the small intestine and the colon homeostasis. The aim of our study was to compare the specific bioaccumulation and retention of Cd and Pb and their effect on the essential metal balance in primary organs, with those occurring specifically in the gastrointestinal tract of mice. Various doses of Cd (5, 20 and 100mgl(-1)) and Pb (100 and 500mgl(-1)) chloride salts were provided in drinking water for subchronic to chronic exposures (4, 8 and 12weeks). In contrast to a clear dose- and time-dependent accumulation in target organs, results showed that intestines are poor accumulators for Cd and Pb. Notwithstanding, changes in gene expression of representative intestinal markers revealed that the transport-, oxidative- and inflammatory status of the gut epithelium of the duodenum, ileum and colon were specifically affected by both heavy metal species. Additionally, in vivo comet assay used to evaluate the impact of heavy metals on DNA damage showed clear genotoxic activities of Cd, on both the upper and distal parts of the gastrointestinal tract. Altogether, these results outline the resilience of the gut which balances the various effects of chronic Cd and Pb in the intestinal mucosa. Collectively, it provides useful information for the risk assessment of heavy metals in gut homeostasis and further disease's susceptibility.

Mathematical model for the removal of trace metal by complexation-ultrafiltration

Complexation-ultrafiltration is emerging as the potential method for the removal of dissolved trace contaminant species from water in an energy efficient manner. The study incorporates the pore-size distribution of the membrane and the molecular weight distribution of the complexing ligand into the irreversible thermodynamic model by modifying the coefficients σ and ω to predict the rejection behavior of trace elements in the presence of complexing ligand necessitated due to the fact that the rejection behavior at trace concentrations is more dependent on probability with reference to a particular pore (related to pore diameter) and the size of the complexed species (mw of the particular species). A set of transport equations have been derived from Kedem Katachalsky’s irreversible thermodynamic model, by incorporating pore-size distribution of the membrane and molecular weight distribution (derived from size distribution) of ligand (polyethyleneimine). The validation of model has been done through the experimental data with copper–polyethyleneimine system using commercially available 6, 20, and 100 KD molecular wt. cutoff hollow fiber ultrafiltration membrane in-bench scale systems. The model developed is in good agreement with the experimental results as long as the stoichiometric concentration of ligand is equal or excess compared with the heavy metal species. Further, the studies also have underlined the importance of considering the pore-size distribution for predicting the performance characteristics. The model was not found suitable when the metal species is in excess, a situation we may not normally encounter in the removal trace metal species.

A numerical model of non-equilibrium thermal plasmas. I. Transport properties

A self-consistent and complete numerical model for investigating the fundamental processes in a non-equilibrium thermal plasma system consists of the governing equations and the corresponding physical properties of the plasmas. In this paper, a new kinetic theory of the transport properties of two-temperature (2-T) plasmas, based on the solution of the Boltzmann equation using a modified Chapman–Enskog method, is presented. This work is motivated by the large discrepancies between the theories for the calculation of the transport properties of 2-T plasmas proposed by different authors in previous publications. In the present paper, the coupling between electrons and heavy species is taken into account, but reasonable simplifications are adopted, based on the physical fact that me/mh ≪ 1, where me and mh are, respectively, the masses of electrons and heavy species. A new set of formulas for the transport coefficients of 2-T plasmas is obtained. The new theory has important physical and practical advantages over previous approaches. In particular, the diffusion coefficients are complete and satisfy the mass conversation law due to the consideration of the coupling between electrons and heavy species. Moreover, this essential requirement is satisfied without increasing the complexity of the transport coefficient formulas. Expressions for the 2-T combined diffusion coefficients are obtained. The expressions for the transport coefficients can be reduced to the corresponding well-established expressions for plasmas in local thermodynamic equilibrium for the case in which the electron and heavy-species temperatures are equal.

Ion–Neutral Collision Modeling Using Classical Scattering With Spin-Orbit Free Interaction Potential

A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high-velocity xenon ions incident to a quiescent xenon gas at low neutral pressures. The range of neutral pressure and collisionality examined are applicable for electric propulsion as well as plasma processing devices; therefore, the computational technique described herein can be applied to more complex simulations of those devices. Momentum and resonant charge-exchange collisions between ions and background neutrals are implemented using two different models, classical scattering with spin-orbit free potential and variable-hard-sphere model, depending on the incident particle energy. The primary and charge-exchange ions are tracked separately, and their trajectories within a well-defined “Test Cell” domain are determined. Predicted electrode currents as a function of the Test Cell pressure are compared with electrode currents measured in an ion gun experiment. The simulation results agree well with the experiment up to a Test Cell pressure corresponding to a mean free path of the Test Cell length and then start to deviate with increasing collisionality at higher pressures. This discrepancy at higher pressures is likely due to the increasing influence of secondary electrons emitted from electrodes due to the high-velocity impacts of heavy species (i.e., beam ions and fast neutrals created by charge-exchange interaction) at the electrode surfaces.

유기성 폐기물 장기시용 후 토양에서 무 (Raphanus sativus cv. sodamaltari)의 중금속 흡수

This study was carried out to understand the long-term effects of organic waste treatments on the fate of heavy metals in soils originated from the organic wastes and consequent uptake of heavy metals by plant, together with examination of changes in soil properties and plant growth performance. In this study, the soils treated with three different organic wastes (municipal sewage sludge, alcohol fermentation processing sludge, pig manure compost) at three different rates (12.5, 25.0, 50.0 ton <TEX>$ha^{-1}yr^{-1}$</TEX>) for 7 years (1994 - 2000) were used. To see the long-term effect, plant growth study and soil examination were conducted twice in 2000 and 2010, respectively. There was no additional treatments of organic wastes for 10 years after the organic waste treatment for 7 years. Compared to plant growth examination conducted in 2000 using radish (Raphanus sativus cv. sodamaltari), it appeared that height, root length and diameter, fresh weight of radish grown in 2010 decreased in the plots treated with municipal sewage sludge and alcohol fermentation processing sludge and that the extent of decrease was higher with increase of sludge application rates. On the other hand, pig compost treatment increased plant height, root length and diameter, fresh weight with increasing application rates. Cu and Pb concentrations in radish root and leaves increased in 2010 compared to those in 2000 while Ni concentrations in root and leaves decreased. Zn concentration was increased only in the soils treated with pig manure compost. Multiple regression analysis among heavy metal species fractions in soils, soil pH, and metal concentrations in radish root and leaves indicated that the metal uptake by radish was governed mainly by the soil pH and subsequent increase of available heavy metal fractions in soils with organic waste treatments.

Theoretical investigation of the decay of an SF6 gas-blast arc using a two-temperature hydrodynamic model

The behaviour of a decaying SF6 arc, which is representative of the approach to the final current-zero state of switching arcs in a high-voltage circuit breaker, is theoretically investigated by a two-temperature hydrodynamic model, taking into account the possible departure of the plasma state from local thermodynamic equilibrium (LTE). The model couples the plasma flow with electromagnetic fields in a self-consistent manner. The electrons and heavy species are assumed to have different temperatures. The species composition, thermodynamic properties and transport coefficients of the plasma under non-LTE conditions are calculated from fundamental theory. The model is then applied to a two-dimensional axisymmetric SF6 arc burning in a supersonic nozzle under well-controlled conditions; for this configuration, experimental results are available for comparison. The effect of turbulence is considered using the Prandtl mixing-length model. The edge absorption of the radiation emitted by the arc core is taken into account by a modified net emission coefficient approach. The complete set of conservation equations is discretized and solved using the finite volume method. The evolution of electron and heavy-particle temperatures and the total arc resistance, along with other physical quantities, is carefully analysed and compared with those of the LTE case. It is demonstrated that the electron and heavy-particle temperature diverge at all times in the plasma–cold-flow interaction region, in which strong gas flow exists, and further in the transient current-zero period, in which case the collision energy exchange is ineffective. This study quantitatively analyses the energy exchange mechanisms between electrons and heavy particles in the high-pressure supersonic SF6 arcs and provides the foundation for further theoretical investigation of transient SF6 arc behaviour as the current ramps down to zero in gas-blast circuit breakers.

Tailoring the hydrogen storage properties of Li4BN3H10 by confinement into highly ordered nanoporous carbon

The effects of nanoconfinement on the H2 release and uptake, and the emission of toxic ammonia (NH3) and diborane (B2H6) during the decomposition of quaternary phase Li4BN3H10 have been comprehensively investigated. Li4BN3H10 confined in highly ordered nanoporous carbon (Li4BN3H10@NPC) with cylindrical pores and an average pore size of 4.4 nm (NPC-4.4 nm) did not show Bragg peaks or melting of the Li4BN3H10 due to nanoconfinement-induced amorphization. Li4BN3H10 confined in NPC starts to decompose at 110 °C, lower by more than 160 °C with respect to bulk and the dehydrogenation kinetics was significantly enhanced. More importantly, in comparison with the exothermic decomposition of bulk Li4BN3H10, a distinct two-step endothermic decomposition process was observed in the nanoconfined Li4BN3H10 system indicating the decomposition pathway was altered by nanoconfinement. A direct line-of-site residual gas analyzer mass-spectrometer (RGA-MS) analysis indicated NH3 was released earlier than H2 during the decomposition of nanoconfined Li4BN3H10 but it evolved slowly and the overall emissions of NH3 and B2H6 were significantly suppressed by nanoconfinement. More interestingly, some other possible gases, diimine (N2H2) and triborane or heavier species, produced during the decomposition of Li4BN3H10 or its constituent parts (LiBH4 and LiNH2), were identified by RGA-MS for the first time. The observation of diimine in the reaction product may provide insight into the reaction mechanisms for rehydriding of compounds in the LiNH2 and Li2NH systems as well.

Application of 3A Zeolite Prepared from Venezuelan Kaolin for Removal of Pb (II) from Wastewater and Its Determination by Flame Atomic Absorption Spectrometry

This work consists in the use of a 3A zeolite (K-LTA) obtained by a process of exchange of sodium for potassium (4A zeolite), synthesized from Venezuelan kaolin for the removal of Pb (II) ions from aqueous solutions by batch process mode in order to consider its application in treating industrial wastewaters. The 3A zeolite was characterized for X-ray powder diffraction (XRD), Fourier Transform infrared spectroscopy (FTIR) and scanning electron microscopy and energy dispersive X-ray microanalysis (SEM-EDX). The metal concentration in the equilibrium Ce (mg·L-1) after adsorption with 3A zeolite was analyzed using flame atomic absorption spectrometry (FAAS). The influences of the solution pH, contact time, metal initial concentration and adsorbent dosage have been studied. The retention of metal occurring at pH values around 6.5 and the adsorption equilibrium was obtained at 60 min. The equilibrium process was well described by Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherm models. The Langmuir parameters qm (mg·g-1) and b (L·mg-1) (which are related to the sorption capacity and constant of sorption energy) obtained were 14.64 and 5.42 respectively. The Pb (II) experimental uptake was about 14.56 mg·g-1, a little smaller than the theoretical one given by Langmuir isotherm model. The regression parameters and correlation coefficients (R) indicate that the adsorption data for Pb (II) removal fit better the Langmuir isotherm model. Moreover, 0 1/n 1 (1/n is 0.13), indicating that adsorption of metal ions on the zeolite, is a favorable physical process. The application of removing of the metal lead from real samples was examined by industrial wastewater samples. For all samples, the percentage of recovery was found with accuracy of more than 98%. The present work suggests 3A zeolite used as a sorbent material with relatively low cost, obtained from Venezuelan raw material; it is a candidate for removal lead ion and probably other cationic heavy metal species from wastewater.

Significant correlation of Cd, Cu, Pb and Zn in sediments and oysters (Saccostrea cucullata) collected from Qeshm Island, Persian Gulf, Iran

The relationships are evaluated between the heavy metal species in sediments and the accumulation by the soft tissues of Saccostrea cucullata collected from the northern and southern coasts of Qeshm Island, Persian Gulf. The sequential extraction technique was employed to fractionate the sediment into non resistant and resistant fractions. Data from sequential extractions indicated that the metals were mainly associated with the residual fraction at the north coast and with non residual fractions at the south coast. The results showed that the best correlations were observed between heavy metals concentrations in soft tissues and the fractions in the southern sediment. Note that the bioavailability of trace metals is influenced by the chemical characteristics and properties of the sediment. Therefore, the present results generally supported the use of soft tissues of S. cucullata as a more accurate biomonitoring organism for Cu, Pb and Cd in sediments from the southern coast of Qeshm Island, Persian Gulf.

Domination of Grain Bearing on the Strength Properties of Engkabang Jantong as Fast Growing Timber in Sarawak

Nowadays, the hardwood category which refers to heavy hardwood species no longer sustains to the market demand. The harvesting of this wood category takes longer period of time. Thus, several studies in timber engineering have been carried out to investigate the fast-growing indigenous species as the alternative to heavy hardwood categories. For this purpose, fast-growing indigenous species of Engkabang jantong were selected and also the test samples were prepared in small clear specimens. The British Standard, BS373.1957 method of testing small clear specimens of timber was adopted to evaluate the strength properties of the species. The strength properties determinations were referred to hardness, cleavage and shear parallel to grain. The results were represented into two different grain directions namely tangential and radial at two different conditions, i.e. green and air-dry condition. It was found that, hardness strength at tangential was higher than radial for both condition green and air-dry with 4.71and 8.86%, respectively. At cleavage strength, it was reported that tangential possessed higher strength values than the radial for both conditions green and air-dry with 23.06 and 27.22%, respectively. Shear strength at tangential was higher than radial for both conditions green and air-dry with 11.70 and 8.58%, respectively.

Optimized EDTA Washing Procedure to Decontaminate Heavy Metals from Soils in Iron and Steel Works Sites

The removal of heavy metal ions from the soil of iron and steel work site with EDTA enhanced washing was investigated through a series of laboratory tests to examine the effect of EDTA concentration, washing time, washing cycles and physical sieving. The heavy metal species in the soil before and after EDTA washing were examined by the sequential extraction procedure . The results are based on comprehensive consideration of associated factors such as high heavy metal removal rates, low loss of soil mineral elements and proper pH value of washing solution, EDTA concentration was controlled around 0.075 mmol g(-1). Considering the heavy metal removal rates and its increasing extent, the washing time and cycles were kept at about 3 h and 3 cycles respectively. Screening out the soil with size of 1-2 mm from the buck soil reduced soil volume by 21.7%. However, the heavy metal removal rates were not significantly reduced. This made EDTA washing economically viable. Under the optimum conditions, a total of 91.7, 35.5 and 21.4 % of Zn, Cd and As, respectively, could be removed from the soil, mainly came from the Fe/Mn fraction (48-82 %). With the lower initial concentration in contaminated soil, Cd and As were more tightly bound to soil particles, which resulted in greatly lower removal rates of Cd and As than Zn. The operationally defined fraction results indicated that heavy metals in the soil after EDTA washing occurred mainly in residue bound fraction (62-68 %) and lowest in exchangable fraction (lower than 0.4 %), to explain that the heavy metal availability in the washed soil did not increase. The results implied that EDTA washing could be used to remediate heavy metal contaminated soil of the iron and steel work site. However, careful control of the process parameters of EDTA washing should be considered to minimize the mobility of the remaining heavy metals.

Removal of Cadmium and Chromium Ions Using Modified α, β, and γ-Cyclodextrin Polymers

The adsorption of cadmium and chromium from aqueous solutions with epichlorohydrin cross-linked α, β, and γ-cyclodextrin polymers (CDPs) was investigated under a wide range of experimental conditions including pH, metal concentration, and CDP amounts. Recent studies have shown that α, β, and γ-CDPs follow the Freundlich, Langmuir, and Brunauer–Emmett–Teller models and are capable of reducing heavy metal ion concentrations in water to desirable levels with enhanced sorption capabilities. The removal of cadmium and chromium achieved equilibrium in 4 h. The pH of effective cadmium removal was 6.5 to 7.5, and phosphate buffer favored the removal of cadmium in the presence of potassium ion. Chromium removal was optimal at pH 3.6 to 6.5. Current technologies can remove 99% cadmium ions from solution, yet this process is not very efficient for chromium ions. Furthermore, a small amount of CDPs can remove large quantities of heavy metal ions and can then be regenerated with acid for reuse. The present results are promising for using inexpensive CDPs as a low-cost material that is effective in remediating waters contaminated with heavy metal species. The sorption kinetics of CDPs along with the conditions to adsorb cadmium and chromium are reported here for the first time.

Separation of some heavy metal species from electroplating rinsing solutions by ion exchange resin

A strongly basic anion exchange lewatit MP 600 resin was tested to separate metal species from electroplating rinsing wastewater. An azo resorcinol 4-(2-pyridylazo) dyestuff was used as a complexing agent. Adsorption isotherms were modelled in an ethanol/acetic acid/water media. The loaded resin is regenerated using 4 M HCl whereby the eluted metal species are recovered. Results revealed that mere anion exchangers could not uptake metal ions unless they reacted with the dye to form negatively charged complexes. Trivalent chromium showed significant uptake at pH range of 3.5-4.5. Cadmium, zinc and nickel ions formed complex compounds in the alkaline medium. The electrostatic interaction and formation of complex compounds identified the major adsorption mechanisms. The uptake capacity of the sorbent is directly proportional to the electro negativity of the metal-dye complexes.

Spectroscopic characterization of nitrogen plasma generated by waveguide-supplied coaxial-line-based nozzleless microwave source

In this paper, results of spectroscopic study of microwave (2.45 GHz) plasma at atmospheric pressure and high flow rate are presented. The plasma is generated by waveguide-supplied coaxial-line-based microwave plasma source (MPS). Pure nitrogen was used as working gas. An additional nitrogen flow rate of 50 l/min was introduced to the plasma by four gas ducts which formed a swirl flow inside the quartz tube. The swirl concentrated near the quartz cylinder wall, stabilized plasma generation and protects the quartz tube wall from overheating. Working gas flow rate and microwave absorbed power varied from 50 up to 200 l/min and from 600 up to 5500 W, respectively. The emission spectra in the range of 300 – 600 nm were recorded. The rotational and vibrational temperatures of N2+ ions and N2 molecules, as well as the rotational temperature of OH radicals were determined by comparing the measured and simulated spectra. The plasma gas temperature inferred from rotational temperature of heavy species ranged 4000 to 6200 K. It depended on location in plasma, microwave absorbed power and working gas flow rate. The MPS can be used in various gas processing applications.

Plasma Spray-PVD: Plasma Characteristics and Impact on Coating Properties

Typical plasma characteristics of the plasma spray-physical vapour deposition (PS-PVD) process were investigated by optical emission spectroscopy. Electron temperatures were determined by Boltzmann plots while temperatures of the heavy species as well as electron densities were obtained by broadening analysis of spectral lines. The results show how the plasma properties and thermodynamic equilibrium conditions are affected by the admixture of hydrogen and the ambient chamber pressure. Some experimental examples of PS-PVD coatings demonstrate the impact on feedstock treatment and deposited microstructures.

Seasonal Variations of Heavy Metals in the &lt;i&gt;Bivalve’s mollusks&lt;/i&gt; from Ta-Peng Bay Lagoon in Southwestern Taiwan

Ta-Peng Bay lagoon in southwestern coast of Taiwan belonging to a shallow, productive, coastal lagoon was heavily polluted by the effluent of agriculture from fisheries and domestic sewage. This area provided a plentiful bivalves mollusks for regional market, including Anadara antiquate, Katelysia hiantina, Perna viridis and Anomalocardia squamosa. For understanding correlation between heavy metals existed in aquatic product and variation of heavy metal varied with season, during Dec., 2005- Sep., 2006, six heavy metals, such as Cu, Zn, Pb, Ni, Cr and Cd were analyzed by an air-acetylene flame / Graphite Atomizer atomic absorption spectrophotometer. The variations of different heavy metals existed in aquatic product in season were evaluated using multivariate analysis. Results show that the mean concentration of heavy metal in soft tissues for six aquatic products, Cd species had the lowest value than the others, and Cu and Zn were significantly higher than the other elements. The relationships of all heavy metal species in seasonal variations showed significant dependence. High concentrations of heavy metals were observed in winter. The negative correlation coefficient (p &lt; 0.05) between heavy metals content and season variations statistically revealed decreased with season series for Ni, Pb, and Cr. An interesting finding that Cd bioaccumulation among the species had a significant discrepancy was observed. It means that bivalves mollusks could be develped as a potetial tool for monitoring Cd species.

One-dimensional hybrid-direct kinetic simulation of the discharge plasma in a Hall thruster

In order to model the non-equilibrium plasma within the discharge region of a Hall thruster, the velocity distribution functions (VDFs) must be obtained accurately. A direct kinetic (DK) simulation method that directly solves the plasma Boltzmann equation can achieve better resolution of VDFs in comparison to particle simulations, such as the particle-in-cell (PIC) method that inherently include statistical noise. In this paper, a one-dimensional hybrid-DK simulation, which uses a DK simulation for heavy species and a fluid model for electrons, is developed and compared to a hybrid-PIC simulation. Time-averaged results obtained from the hybrid-DK simulation are in good agreement with hybrid-PIC results and experimental data. It is shown from a comparison of using a kinetic simulation and solving the continuity equation that modeling of the neutral atoms plays an important role for simulations of the Hall thruster discharge plasma. In addition, low and high frequency plasma oscillations are observed. Although the kinetic nature of electrons is not resolved due to the use of a fluid model, the hybrid-DK model provides spatially and temporally well-resolved plasma properties and an improved resolution of VDFs for heavy species with less statistical noise in comparison to the hybrid-PIC method.

Low-order harmonic generation in nanosecond laser ablation plasmas of carbon containing materials

In this work we report on a systematic study of the spatiotemporal behaviour of low-order harmonics generated in nanosecond laser ablation plasmas of carbon containing materials. Plasmas were generated from targets of graphite and boron carbide ablated with a nanosecond Q-switched Nd:YAG laser at 1064nm. Low-order harmonics (3rd and 5th) of the fundamental wavelength of a ns Nd:YAG driving laser, propagating perpendicularly to the ablation laser at variable time delays, were observed. The temporal study of the low-order harmonics generated under vacuum and atmospheres of Kr and Xe, revealed the presence of two populations that contribute to the harmonic generation (HG) at different times. It was found that under vacuum only small species contribute to the HG process, whereas under buffer gas, heavier species, such as clusters and nanoparticles, contribute to the HG at longer times. Optical emission spectroscopy, time of flight mass spectrometry and characterization of deposits collected on-line on a nearby substrate provided additional information that complemented the results of the spatiotemporal study of the generated harmonics. This approach to ablation plume analysis allows elucidating the identity of the nonlinear emitters in laser ablation plasmas and facilitates the investigation of efficient, nanoparticle-enhanced, coherent short wavelength generation processes.

The structure of the absolutely unstable regions in the near field of low-density jets

AbstractThe viscous spatiotemporal stability properties of low-density laminar round jets emerging from circular nozzles or tubes are investigated numerically providing, for the first time, a separate treatment of the two particular cases typically studied in experiments: a hot gas jet discharging into a quiescent cold ambient of the same species, and an isothermal jet consisting of a mixture of two gases with different molecular weight, discharging into a stagnant ambient of the heavier species. To that end, use is made of a realistic representation for the base velocity and density profiles based on boundary-layer theory, with account taken of the effect of variable transport properties. Our results show significant quantitative differences with respect to previous parametric studies, and reveal that hot jets are generically more unstable than light jets, in the sense that they have larger associated critical density ratios for values of the Reynolds number and momentum thickness typically used in experiments. In addition, for several values of the jet-to-ambient density ratio, $S$, the downstream evolution of the local stability properties of the jet is computed as a function of the two main control parameters governing the jet, namely the Reynolds number, $\mathit{Re}$, and the momentum thickness of the initial velocity profile, ${\theta }_{0} / D$. It is shown that, for a given value of $S$, the $(\mathit{Re}, {\theta }_{0} / D)$ parameter plane can be divided in three regions. In the first region, defined by low values of $\mathit{Re}$ or very thick shear layers, the flow is locally convectively unstable everywhere. In the second region, with moderately large values of $\mathit{Re}$ and thin shear layers, the jet exhibits a localized pocket of absolute instability, away from boundaries. Finally, in the third region, that prevails in most of the $(\mathit{Re}, {\theta }_{0} / D)$ parameter plane, the absolutely unstable domain is bounded by the jet outlet. All the experiments available in the literature are shown to lie in the latter region, and the global transition observed in experiments is demonstrated to take place when the absolutely unstable domain becomes sufficiently large. The marginal frequency of the resulting global self-excited oscillations is shown to be fairly well described by the absolute frequency evaluated at the jet outlet, in agreement with the numerical results obtained by Lesshafft et al. (J. Fluid Mech., vol. 554, 2006, pp. 393–409) for synthetic jets.