Desorption Patterns Research Articles

Zinc desorption kinetics as influenced by pH and phosphorus in soils

The rate of zinc (Zn) desorption from soil surfaces into soil solution is a dynamic factor that regulates its continuous supply to growing plants. To ascertain the pattern of Zn desorption as a function of phosphorus (P) application rate and pH, kinetics of Zn desorption from three surface soils representative of three major benchmark soil series were investigated using four equations. Zinc desorption decreased continuously with increase in pH from 4.25 to 8.00 in Oxisol. By contrast, Alfisol and Vertisol exhibited maximum Zn desorption at pH 5.50. Thereafter, Zn desorption decreased abruptly at pH 6.75 and finally steady state condition was obtained in both sodium chloride (NaCl) and calcium chloride (CaCl2) medium. The Elovich equation described Zn desorption kinetics much better at pH 4.25 and 5.50 followed by Parabolic diffusion equation at pH 6.75 and 8.00 for all the soils in NaCl medium. Whereas in the CaCl2 medium, the Elovich equation was superior in fitting the Zn desorption data irrespective of P level, pH, and soil. The Elovich constant (β) indicated that P affects Zn desorption inversely up to pH values 6.75.

Coating Carboxylic Acids on Amorphous Iron Nanoparticles

Nanophased amorphous iron particles were coated by various carboxylic acids and their physical properties (differential scanning calorimetry, thermogravimetric analysis, temperature-programmed desorption, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and magnetism) were measured. The various properties were measured as a function of the alkyl chain length. The desorption pattern at 400 °C revealed a keto derivative.

Phenol adsorption on Si(111)7×7 studied by synchrotron radiation photoemission and photodesorption

Phenol adsorption on Si(111)7×7 has been studied by synchrotron radiation photoemission and photodesorption in the valence band Si 2p energy ranges both at room temperature and at ≈50 K. The assignment of the phenol adsorption features has been made by comparison of the gas-phase spectrum. At low exposures the surface rest atoms are selectively quenched, while the adatoms are involved in the adsorption process only for higher exposures. The comparison of the spectra at different temperatures allows us to infer that the adsorption on the rest atoms is molecular. However, the relative intensity of the spectral features changes when different sites are involved in the adsorption process. In particular, the feature attributed to the σ O–H orbital does not increase in intensity, although the other features do, suggesting that the adsorption is dissociative. The photon-stimulated desorption pattern shows a D + yield in the Si 2p energy range, due to the enhanced production of secondary electrons in correspondence with the ionization threshold, similarly to the system benzene–Si(111)7×7.

Development of resistance models to describe measurements of bi-directional ammonia surface–atmosphere exchange

Ammonia fluxes over vegetated land are bi-directional, with differences between emission and deposition occurring in relation to environmental conditions (temperature and wetness), plant community (e.g. arable land, unfertilized ecosystems), as well as other factors such as plant phenology. In the present paper, measurements of net NH 3 fluxes over arable cropland are reported in the range −50 to 40 ng m -2s -1. These are used as examples to highlight the factors affecting the exchange process. Net fluxes are expected to depend on the competition between deposition to leaf surfaces and bi-directional exchange with a “stomatal compensation point” for leaf tissues ( χ s). Current models describe this effect poorly, and two new resistance models are described here. These estimate a “canopy compensation point” ( χ c), as the net potential for NH 3 emission from the canopy. In the simpler of the two models, leaf surface uptake is parametrized using a resistance ( R w). This model is able to reproduce bi-directional fluxes, though there is also evidence that both cuticular NH 3 adsorption and desorption may occur, dependent on previous fluxes. A more complex dynamic approach is therefore developed, treating cuticular uptake as a capacitance ( C d). The dynamic model is able to reproduce the pattern of desorption, but further development of both models is required to provide descriptions valid over longer periods and for a range of ecosystem types.

Competition of Organic and Mineral Phases in Radiocesium Partitioning in Organic Soils of Scotland and the Area near Chernobyl

Radiocesium (RCs) partitioning has been studied in four organic soils, with an organic matter (OM) content from 46 to 99%. For each soil, two fractions were derived after removing fats/waxes and humic/fulvic acids (humin+mineral) and after further removal of the mineral matter by an HF/ HCl treatment (humin). Characterization by ICP-OES, FTIR, and XRD showed that the removal of mineral matter by the HF treatment was complete. Illitic material was observed in all the soils, with the exception of that with 99% OM. Two patterns of adsorption and desorption were observed. For the 99% OM soil and all the humin fractions, the increase of NH 4 in the solution did not affect the RCs distribution coefficient (K D ), and no differences were observed between the desorption yields obtained with CH 3 COONH 4 and CaCl 2 (around 80% for each extractant). For the other three soils and their humin+mineral fractions, an increase of NH 4 led to a decrease in K D , and desorption yields were lower with CaCl 2 than with CH 3 COONH 4 . This indicated that, in the latter soils, RCs was associated with specific sites in the illitic material. Finally, the increase in K D in the humin+mineral fraction as compared with the initial soil suggested that humic/fulvic acids influence RCs interaction with the mineral phase.

Factors affecting resolution in scanning electron beam induced patterning of surface adsorption layers

The monoatomic hydride layer on silicon was used as a prototype for resistless electron beam lithography. Arbitrary patterns with linewidths below 60 nm have been achieved. The variation of the linewidth with electron energy, electron dose, and substrate thickness was studied to determine the mechanisms that govern surface hydrogen desorption and subsequent pattern formation. Unlike in resist based lithography, no resolution enhancement was observed with decreasing substrate thickness. The experimental data in combination with Monte Carlo simulations of the backscattered and transmitted electrons suggest that surface hydrogen desorption and pattern formation are not strongly related to the backscattered electrons and the secondary electrons (energies <50 eV) associated with the backscattered electrons.

Synchrotron radiation photoemission and photostimulated desorption of deuterated methanol on Si(111)7 × 7 and Si(100)2 × 1

Deuterated methanol adsorption on Si(111)7 × 7 and Si(100)2 × 1 surfaces has been investigated, at room temperature, by synchrotron radiation photoemission and photostimulated desorption. Photoemission experiments as a function of methanol coverage show that deuterated methanol adsorption is dissociative on both surfaces and occurs via б O-D bond breakage and б SiO bond formation. Site selectivity is displayed by the methoxy species on Si(111)7 × 7 surface, since t S 2 surface state related to the rest atoms is quickly quenched upon low coverage adsorption. The S 1 surface state is quenched only subsequently at much higher coverages. The photostimulated desorption pattern is similar for the two surfaces and strongly depends on the investigated energy range. D + is always the most abundant ion produced, but higher mass fragments are present too in the energy range including the C 1s threshold, where multiple desorption processes are possibly taking place.

Advanced Lithography for Nanofabrication

AbstractA novel method for generating lateral features by patterning the naturally forming surface hydride layer on Si is described. Because of the relatively strong chemical bonding between silicon and hydrogen, the hydride layer acts as a robust passivation layer with essentially zero surface mobility at ordinary temperatures. A focused electron beam from a scanning electron microscope was used for patterning. Upon losing the hydrogen passivation the silicon surface sites become highly reactive. Ideally, the lifetime of such a pattern in a clean environment should be infinite. Deliberate exposure of the entire wafer to a suitable gas phase precursor results in selective area film growth on the depassivated pattern. Linewidths and feature sizes of silicon dioxide on silicon below 100 nm were achieved upon exposure to air. The silicon dioxide is robust and allows effective pattern transfer by anisotropic wet-chemical etching. In this paper, the mechanism of hydrogen desorption and subsequent pattern formation, and the factors that govern the ultimate pattern resolution will be discussed.

Temperature Programmed Desorption of n-nonane; the method to measure the micropore size distribution

TPD of n-nonane gives a micropore size distribution of micropores in the range of .4 to 2 nm. This method is independent of the shape of the pore entrances. It is proven that n-nonane fills these micropores. The Temperature Programmed Desorption patterns of five activated microporous carbons and three zeolites will be shown. The difference in adsorption behaviour of CO2 and SF6 at 273 K and of CH4 at 315 K on the carbons is explained with the aid of the obtained micropore size distributions.

Kinetics of Zinc Desorption from Vertisols

AbstractThe rate of Zn desorption from soil surfaces into soil solution is a dynamic factor that regulates its continuous supply to growing plants. To ascertain the pattern of Zn desorption, the soil characteristics affecting it, and whether cropping alters its rate, the kinetics of Zn desorption from the <2‐mm fraction of 14 Vertisols by diethylenetriaminepentaacetic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA) were investigated using soil samples taken before and after one wheat (Triticum aestivum L.) crop. Nine kinetic models were evaluated to describe the rate of desorption of soil Zn by DTPA, which was rapid initially but gradually declined with time. The parabolic double diffusion, the two‐constant rate, and the simple Elovich equation adequately described Zn desorption from Vertisols. Rate constants for the parabolic double diffusion equation (k'σ), the two‐constant (a and b) rate equation, and the initial Zn desorption rate constant (α) from the simple Elovich equation were closely associated with clay content, soil pH, and amorphous Fe and Al contents of the soil—the soil characteristics that affect solubility, sorption and desorption, and diffusion of Zn in Vertisols. Rate constants from the latter two equations for Zn desorption by DTPA from 14 Vertisols were highly correlated with those from Zn desorption by EDTA and also with those for Zn desorption by DTPA on soil samples taken after one wheat crop. Thus, the rate constants obtained on initial samples can be used to predict Zn availability for at least two cropping seasons.

On why do different carbons show different gasification rates: A transient isotopic CO 2 gasification study

A suite of nine different carbons have been investigated by step-response experiments (SRE) using 13CO 2 at 10 and 100 kPa and, predominantly, at 1200 and 1250 K. Although the gasification activities vary by a factor of twenty, all carbons behave similarly in their CO desorption patterns upon a step change from a reactive to an inert gas phase. This CO desorption can be described by a two-stage exponential decay, irrespective of the type and structure of the carbon used. The amounts of desorbable surface oxygen complexes differ between the various carbons and increase with increasing CO 2 pressure and temperature. The complexes are restored by a reversed step from an inert to a reactive atmosphere. Clear indications are found that a distribution in desorption activation energies exists. The variation in gasification rate between the carbons and as a function of the CO 2 pressure and temperature is mainly due to the variation in desorbable surface oxygen complex concentration. The transient CO desorption is best accounted for by a recent model of Chen, et al., where two types of oxygen complexes contribute to the gasification rate. The largest contribution comes from highly reactive intermediates that contribute five times more than the more stable complexes, but which are present in 3–4 times smaller amounts. The associated rate constants are fairly similar for the different carbons, averaged 0.6 and 0.034 s −1, respectively and exhibit only a weak temperature dependency due to the surface heterogeneity. Correlation of the gasification rate with the total amounts of desorbable surface complexes is incompatible with the decay curves. The use of 13CO 2 demonstrated that carbon from the gas phase reactant is inserted in the carbon structure according to an activated process, which is evidence of the reversibility of the gasification reaction. Three carbons show a sharp maximum in the decaying CO desorption curve at 100 kPa CO 2, a peculiar phenomenon not yet explained.

Potential mechanistic routes for the oxidative disintegration of ketones on carbon adsorbents

Solvent recovery and air emission control are among the many industrial uses of activated carbon. The efficient use of activated carbon technology for these applications requires some understanding of carbon-solvent interactions and associated system behavior, especially in an oxidizing atmosphere. The oxidative activity characteristics of a number of organic solvents adsorbed on activated carbon were determined using temperature programmed oxidation (TPO) and temperature programmed desorption (TPD) techniques. Thermogravimetry (TG), differential thermal gravimetry (DTG), differential scanning calorimetry (DSC), and Fourier transform infrared (FTIR) spectroscopy were employed to provide some insights on the thermal behavior of the carbon-solvent systems in oxidative and inert atmospheres. The thermal desorption patterns and oxidation stabilities of selected ketones were monitored, and the formation of products due to incomplete combustion was established. Based on the observed results, plausible mechanisms associated with the decomposition of these oxidizable organic solvents on activated carbon are discussed to delineate the types of reactions that initiate carbon bed combustion and/or carbon degradation.

Desorption of Chlorobenzenes from Natural Suspended Solids and Sediments

The desorption of chlorobenzenes from heterogeneous suspended solid mixtures and sediments at nonequilibrium conditions was investigated, using a gas purge method. This technique offers the possibility to measure equilibrium and rate data from adsorbent suspensions, without phase separation. The desorption patterns were interpreted and fitted in terms of multiple box models. The results show that when the equilibration time for sediment suspensions contaminated in the laboratory is longer, desorption is slower. At nonequilibrium conditions, the more hydrophobic chlorobenzenes are released faster than the less hydrophobic ones. For ten suspended solid samples the model parameters were related to adsorbent characteristics, such as organic matter, phytoplankton and clay content by means of correlation analysis, and compared to similar desorption data for algal cultures obtained in earlier studies in our laboratory. It was found that all desorption data could be described with a two-compartment desorption model. The partition coefficient was proportional to the content of organic matter, which in turn was mainly determined by the concentration of phytoplankton in the samples. The desorption rate was positively correlated with the phytoplankton content as well. This suggests that phytoplankton cells form an easily accessible fraction of the total adsorbent.

A TDS study on the reaction intermediates in the catalyzed gasification of activated charcoal

Catalytic coal gasification was studied by using thermal desorption spectroscopy (TDS) under high vacuum conditions. Activated charcoal was used as a model compound of coal, and Na2CO3 and K2CO3 as catalysts. Catalysts enhanced oxygen adsorption and decomposed at temperatures below their own decomposing temperatures. After pretreatment of the catalyst at high temperature the CO2 desorption pattern represented characteristics of reaction intermediates. There were two peaks, α-peak and β-peak, in CO2 TDS curves. From the experimental results, α-peak was interpreted to the result from intercalated metal-carbon (M-C) complex, and β-peak from metal-oxygen (M-O-C) complex. A higher pretreatment temperature led to decrease both the number of M-O-C complex and its thermal stability. The increase of catalyst loading in Na2CO3-containing sample resulted in relative increase of the number of M-O-C complex than that of intercalated M-C complex, and also increase of thermal stabilities of both reaction intermediates. Major difference between K2CO3 and Na2CO3 appeared in α-peak, intercalated M-C complex. That is, the contribution of intercalated metalcarbon complex to total reaction intermediates was larger in K2CO3 than Na2CO3. This could be attributed mainly to the better intercalating ability of potassium due to its lower ionization potential.

Sorption and Desorption of Dyes by Sulfonated Coal

In this technical note, the results on adsorption of acid and basic dyes by sulfonated coal are presented. A mathematical model based on adsorption isotherm was developed to predict the desorption of dyes from the sorbent. There was a good agreement between the predicted and experimentally determined values for basic dyes only. An attempt to understand the nature of binding of dyes was made from the desorption pattern of dyes from sulfonated coal using different eluants

Root-uptake of radioiodine by rice plants.

Root-uptake of iodine by rice plants from two different soil types was studied using an 125I tracer. At harvest, the 125I concentration was lowest in brown rice (hulled rice) followed by the rachis, stem and leaves. The transfer factors of iodine for brown rice grown on Andosol and Gray lowland soil (fine texture) respectively were 0.007 and 0.002. The ratio of the radioiodine concentrations between flag leaf (first leaf blade) and brown rice was about 400. Transfer factors in plants grown on Andosol were higher than those grown on Gray lowland soil. This may be related to the high 125I concentration in the Andosol soil solutions, particularly after the flowering period. The adsorption and desorption of iodine in the soil was effected by flooding with water and also by the cultivation of rice plants. The radioiodine concentration in the soil solutions first increased in the upper soil layer than in the lower layer. The desorption pattern of iodine was influenced by the soil types.

Time-resolved patterns for electron-stimulated desorption of metastable atoms from thin Ar(111) films.

Time-resolved desorption patterns of metastable argon atoms have been obtained from thin Ar(111) films condensed on Pt(111) at 15 K. The desorption is stimulated by monochromatic electrons with a fixed energy of 14.5 eV. Combining the information on time-of-flight and angular distributions, five distinct metastable-atom components can be identified. The maxima of their kinetic energies are located at 19, 36, 53, 85, and 345 meV (\ifmmode\pm\else\textpm\fi{}10%), with the two lowest-energy signals being more intense. The 53-meV component has a narrow angular distribution of 8\ifmmode^\circ\else\textdegree\fi{} at half maximum with respect to the surface normal, while the others are broad. Different aspects of the basic desorption mechanisms are discussed. The sharpness of the angular distribution of the 53-meV component is interpreted as due to nuclear zero-point motion in an undistorted crystalline environment. The data for the 19-, 36-, and 53-meV components are in qualitative agreement with recent molecular-dynamics studies.

Desorption kinetics of selected volatile organic compounds from field contaminated soils

Batch experiments conducted with five, low organic carbon content (0.04-1.12%), field contaminated soils (three silty-clay and two sandy soils) for the simultaneous desorption of trichloroethylene, tetrachloroethylene, toluene, and xylene showed that a substantial portion (48-94%) of the sorbed contaminant mass resisted desorption in deionized water-soil mixtures after 7 days of contact time. The desorption pattern was biphasic, a slow desorption following an initial fast phase

Decomposition of perfluoropolyethers during XPS measurements

Decomposition during XPS measurements of a perfluoropolyether (PFPE) lubricant applied to a gold film has been studied by XPS and mass spectrometry. The decomposition is probably initiated inside the sample either by X-ray photons or secondary electrons created by the photoelectric effect, or by both. The desorption patterns and XPS spectra show that the decomposition behavior for X-ray irradiation is similar to that for electron bombardment. According to XPS results, under X-ray exposure the -O-CF 2-O- unit is the most fragile part in the polymer. Both the X-ray-induced and the electron-induced desorption spectra reveal that OCF 2, CO 2 and C 2F 6 are the primary gas products. A flurocarbonaceous layer with little oxygen content is formed on the gold film surface after X-ray irradiation. The fluorocarbonaceous layer is composed of a straight-chain structure and a cross-linked structure, and probably contains conjugated carbons.

Activity and selectivity of 1-hexene conversion on SAPO-11 and Pd/SAPO-11

The activity and selectivity patterns for 1-hexene reactions have been compared in the temperature range of 150-450°C for four different kinds of acidic zeolites (H-mordenite, HZSM-5, SAPO-11 and Pd/SAPO-11) differing both in acid properties and geometric structure. At lower reaction temperatures (150-175°C) the predominate reaction pathway was double bond shift (DBS) and was not influenced by the type of catalyst. At higher temperatures significant shifts in selectivity were observed from DBS to cracking (c) or skeletal rearrangement (SR) depending on the type of catalyst. H-mordenite maintained DBS even at the highest temperature, HZSM-5 produced significant quantities of cracked alkenes, and SAPO-11 was intermediate. The Pd/SAPO-11 gave enhanced DBS selectivity and less cracking at higher temperatures in comparison to unloaded SAPO-11. In general, hydrogen was found to inhibit DBS and promote cracking reactions. Temperature-programmed desorption patterns were also determined. Desorption peaks for all materials were found at around 170°C, 240°C and 330°C, with a significant loss of strong Brønsted acidity upon palladium loading on SAPO-11. A selectivity correlation at 350°C showed strong dependence of DBS and cracking on total acidity, with a shift from DBS to cracking with increasing acidity. The reaction seems well suited for investigation of the interrelation between pore structure and surface acidity in terms of these selectivity factors.