Re-dissolution Processes Research Articles

An integrated process by ultrasonic enhancement in the deep eutectic solvents system for extraction and separation of chlorogenic acid from Eucommia ulmoides leaves

This study established an integrated process for the extraction and enrichment of chlorogenic acid(CGA)from Eucommia ulmoides leaves in a deep eutectic solvent system via ultrasonic wave-enhanced adsorption and desorption practices utilizing macroporous resins. Although deep eutectic solvents (DESs) have the advantages of chemical stability, good dissolving capacity, and nonvolatilization, routine solvent recovery operations are not suitable for subsequent separation in this solvent system. Based on the above characteristics, this study integrated the extraction and enrichment processes, in which DESs extracts directly loaded onto the macroporous adsorption resin, avoiding the loss of target components in solvent recovery and redissolution processes. The screening results of solvents and resin types further showed that choline chloride-malic acid (1:1) was the optimal DES, and the NKA-II resin had high adsorption and elution performance for CGA. The viscosities of the DESs were much higher than those of water and conventional organic solvents; thus, the mass transfer resistance was large, which could also affect the adsorption behaviour of the macroporous resin. The thermal and mechanical effects of ultrasound could effectively enhance the efficiency of the mass transfer, adsorption, and desorption in the DES systems. When compared to no sonication treatment, the CGA adsorption at various ultrasonic powers (120–600 W) was examined. At optimal ethanol concentration (60%), the effect of the ultrasonic treatment on the recovery of the DESs (water eluting process) and the desorption capability of CGA were confirmed. The use of three volumes of water elution could recover the DESs without loss of CGA. The adsorption process significantly differed depending on the ultrasonic settings, and the absorption balance time and experimental adsorption capacity at equilibrium were enhanced. Additionally, the adsorption procedure of the NKA-II macroporous resin for CGA under ultrasonic treatment could be clarified by the pseudo second order kinetic equation and the Freundlich isotherm model. Thermodynamic and dynamic parameters indicated that physical adsorption was the main process of the entire procedure, and it was a spontaneous, exothermic, and entropy-reducing physical adsorption process. This study potentially indicates that the use of ultrasonication, as a high-efficiency, environmentally friendly method, can enhance the features of the macroporous resin to better purify target chemicals from a DES extract.

Dissolution Behavior of Different Alumina Phases within Plasma Electrolytic Oxidation Coatings

The influence of chemical redissolution in the PEO layer-growth equilibrium on aluminum is evaluated differently in literature. In order to estimate whether and to what extent various alumina modifications could be affected by redissolution processes during PEO, immersion experiments were carried out on PEO coatings in model electrolytes of different alkalinity and silicate concentration. Their composition was determined spectroscopically before and after the experiments. The layers were characterized by XRD before and after the tests and examined at affected and unaffected zones using SEM, EDX, and EBSD. The results show that chemical oxide dissolution can only be observed at the layer/substrate interface and that primarily amorphous alumina is affected. This process is intensified by higher alkalinity and inhibited by silicate additives. The crystalline Al2O3 modifications show no significant attack by the electrolytes used. Transferring these observations to plasma electrolytic oxidation, they allow the conclusion that the electrochemically active zone in the pore ground of discharge channels interacts with the electrolytic and electrical process parameters throughout the entire PEO procedure. Influences of bath alkalinity and silicate content on layer growth rates are to be understood as impact on the passivation processes at the layer/substrate interface rather than chemical redissolution.

Interplay between abiotic and microbial biofilm-mediated processes for travertine formation: Insights from a thermal spring (Piscine Carletti, Viterbo, Italy).

Active hydrothermal travertine systems are ideal environments to investigate how abiotic and biotic processes affect mineralization mechanisms and mineral fabric formation. In this study, a biogeochemical characterization of waters, dissolved gases, and microbial mats was performed together with a mineralogical investigation on travertine encrustations occurring at the outflow channel of a thermal spring. The comprehensive model, compiled by means of TOUGHREACT computational tool from measured parameters, revealed that mineral phases were differently influenced by either abiotic conditions or microbially driven processes. Microbial mats are shaped by light availability and temperature gradient of waters flowing along the channel. Mineralogical features were homogeneous throughout the system, with euhedral calcite crystals, related to inorganic precipitation induced by CO2 degassing, and calcite shrubs associated with organomineralization processes, thus indicating an indirect microbial participation to the mineral deposition (microbially influenced calcite). The microbial activity played a role in driving calcite redissolution processes, resulting in circular pits on calcite crystal surfaces possibly related to the metabolic activity of sulfur-oxidizing bacteria found at a high relative abundance within the biofilm community. Sulfur oxidation might also explain the occurrence of gypsum crystals embedded in microbial mats, since gypsum precipitation could be induced by a local increase in sulfate concentration mediated by S-oxidizing bacteria, regardless of the overall undersaturated environmental conditions. Moreover, the absence of gypsum dissolution suggested the capability of microbial biofilm in modulating the mobility of chemical species by providing a protective envelope on gypsum crystals.

The geochemical evolution of brines from phosphogypsum deposits in Huelva (SW Spain) and its environmental implications

The present study focuses on the geochemistry of large phosphogypsum deposits in Huelva (SW Spain). Phosphogypsum slurry waste from fertiliser production was disposed in large ponds containing aqueous waste (i.e. brines) and exposed to weathering. These evaporation ponds were found to be dynamic environments far from attaining steady state conditions where a number of trace pollutants are subjected to temporal variations in response to changing environmental conditions. Chemical, mineralogical and morphological data were used to improve our understanding on the dynamics of a large number of elements in the phosphogypsum-brine-evaporation deposits system. Weekly sampling of brines over the course of 1 yr indicated a substantial enrichment in potentially harmful elements (e.g. As, Cr, Cu, F, Ni, U, V, Zn) present in time-dependent concentrations. The evaporation deposits formed multi-layered precipitates of chlorides, sulphates, phosphates and fluorides containing a large number of pollutants in readily soluble forms. The precipitation sequence revealed a time-dependent composition reflecting alternating precipitation and re-dissolution processes associated with seasonal changes in the local weather conditions. Concatenation of precipitation/re-dissolution stages was found to progressively enrich the brines in pollutants. These findings were supported by the observations from a tank experiment simulating the phosphogypsum-brine-evaporation deposits system under laboratory conditions. Given the substantially high concentrations of pollutants present in mobile forms in the brine-salt system, actions to abate these compounds should be implemented.

Phase-specific bioactivity and altered Ostwald ripening pathways of calcium carbonate polymorphs in simulated body fluid.

Calcium carbonate is an abundant biomineral, and already archeological records demonstrate its bioactivity and applicability for osseo-integrative implants. Its solubility, which is generally higher than those of calcium phosphates, depends on its polymorph turning calcium carbonate into a promising biomaterial with tunable bioresorption rate. However, the phase-dependent bioactivity of calcium carbonate, i.e., its osteoconductivity, is still insufficiently characterized. In this study, we address this issue by monitoring the behavior of the four most important calcium carbonate phases, i.e., calcite, aragonite, vaterite, and amorphous calcium carbonate, in simulated body fluid solution at 37 °C. Our results demonstrate that the thermodynamically stable calcite phase is essentially inert. In contrast, the metastable phases aragonite and vaterite are bioactive, thus promoting the formation of calcium phosphate. Amorphous calcium carbonate (ACC) shows prominent bioactivity accompanied by pronounced redissolution processes. Mg-stabilized ACC was additionally tested since its increased stability eases formulation and handling in future applications. It is highly bioactive and, moreover, the additional release of Mg promotes cell viability. Overall, our results demonstrate that bioactivity of calcium carbonate is phase-dependent, allowing tailored response and bioactivity of future calcareous biomaterials. Our results also reveal that phosphate ions strongly interfere with Ostwald–Lussac step ripening of calcium carbonate, kinetically stabilizing metastable polymorphs such as vaterite and aragonite; this is a distinctive feature of the calcium carbonate mineral system which clearly has to be considered in future applications of calcium carbonate as a bioceramic.

Kinetics of Precipitation of Non-ideal Binary Solid-solutions in an Aqueous Medium

We have developed a formalism which accounts for the kinetics of precipitation of binary non-ideal solid-solutions, and includes nucleation, growth and/or re-dissolution processes. It allows prediction of the time evolution of the aqueous ion activities together with all characteristics of the solid phase: crystal size distribution function and composition profiles of particles. We show that different scenarios of precipitation are possible depending on the thermodynamic properties of the solid solution end-members, the degree of non-ideality of the solid solution and the initial conditions of precipitation. We illustrate the various scenarios by simulations performed under a few simplifying assumptions for some carbonate solid solutions of geochemical interest. To our knowledge, this is the first time in the field of geochemistry that these out-of-equilibrium precipitation processes of non-ideal solid solutions are fully described.

In vivo evaluation of supersaturation/precipitation/re-dissolution behavior of cinnarizine, a lipophilic weak base, in the gastrointestinal tract: the key process of oral absorption

The aim of this study is to evaluate how supersaturation, precipitation, and re-dissolution processes influence the intestinal absorption of cinnarizine (CNZ), a lipophilic weak base, by monitoring its plasma and luminal concentration–time profile, after oral administration as a HCl solution containing fluorescein isothiocyanate dextran (FD-4), a non-absorbable marker. In the in vitro pH shift experiment, the supersaturation stability was significantly lower when the higher-concentration solution of CNZ (pH1.5) was added to the simulated intestinal fluid. However, although the in vivo bioavailability after oral administration of high and low dose as HCl solutions was greatly improved compared to those as neutral suspensions, the difference in the supersaturation stability was not reflected in the improvement of the in vivo bioavailability. Analysis of CNZ and FD-4 concentrations in each segment of the gastrointestinal tract revealed that most of the CNZ precipitated in the duodenum after gastric emptying, and supersaturation was observed only in the duodenum. Thereafter, the precipitate was rapidly re-dissolved and absorbed in the upper and middle small intestine. The rapid re-dissolution may be caused by smaller particles of the precipitate. In this case, it is considered that the key process for the absorption of CNZ was re-dissolution, not supersaturation. Therefore, different supersaturation stabilities in different doses observed in in vitro precipitation experiment was not reflected to in vivo absorption. These findings may be useful to design efficient supersaturable formulations and to validate and improve current prediction methods.

Distribution and potential health risk of groundwater uranium in Korea

Chronic exposure even to extremely low specific radioactivity of natural uranium in groundwater results in kidney problems and potential toxicity in bones. This study was conducted to assess the potential health risk via intake of the groundwater containing uranium, based on the determination of the uranium occurrence in groundwater. The groundwater was investigated from a total of 4140 wells in Korea. Most of the groundwater samples showed neutral pH and (sub-)oxic condition that was influenced by the mixing with shallow groundwater due to long-screened (open) wells. High uranium contents exceeding the WHO guideline level of 30 μg L−1 were observed in the 160 wells located mainly in the plutonic bedrock regions. The statistical analysis suggested that the uranium component was present in groundwater by desorption and re-dissolution processes. Predominant uranium phases were estimated to uranyl carbonates under the Korean groundwater circumstances. These mobile forms of uranium and oxic condition facilitate the increase of potential health risk downgradient. In particular, long-term intake of groundwater containing >200 μg U L−1 may induce internal exposure to radiation as well as the effects of chemical toxicity. These high uranium concentrations were found in twenty four sampling wells of rural areas in this study, and they were mainly used for drinking. Therefore, the high-level uranium wells and neighboring areas must be properly managed and monitored to reduce the exposure risk for the residents by drinking groundwater.

The Effect of Temperature and Pressure on the Reversibility of Asphaltene Precipitation

A lot of hindrances are seen in petroleum operation, production, and transportation as a results of factors that related to asphaltene precipitation. It has great importance to investigate the reversibility of asphaltene precipitation under changes of effective factors on thermodynamic conditions such as pressure, temperature, and composition. In the present work the reversibility of asphaltene precipitation under changes of pressure and temperature was investigated for two kind of Iranian heavy oil. The stability test shows these samples are located at unstable region in aspect of asphaltene precipitation. The experimental procedure includes two parts, (a) decreasing pressure from initial reservoir pressure to near saturation pressure and surveying asphaltene content hysteresis with redissolution process at reservoir temperature, and (b) investigation of precipitated asphaltene in both precipitation and redissolution processes at different temperature and reservoir pressure. At each step IP143 standard test was used to measure precipitated asphaltene. It was concluded that above bubble point pressure, asphaltene precipitation is nearly reversible with respect to pressure for both samples and it was partially reversible with respect to the temperature for sample A, and accordingly pressurizing is acceptable method for solving the problem in both heavy asphaltenic crude oil samples and increasing temperature is acceptable method for solving asphaltene problem in crude oil sample A. Also density measurement of flashed oil confirmed that there is a little hysteresis in asphaltene content during redissolution and precipitation processes.

Statistical terpolymers with thermo-responsive fluorescence response in an ionic liquid: effects of solvatophilicity on LCST phase separation and reversibility

Well-defined terpolymers of benzyl methacrylate (BzMA) and 9-(4-vinylbenzyl)-9H-carbazole (VBK) with either methyl methacrylate (MMA) or oligo(ethylene glycol)methacrylate (OEGMA) were synthesized to study their LCST-type phase behavior (due to BzMA) and fluorescence properties (due to VBK) in a hydrophobic ionic liquid IL, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone)imide ([C2mim][NTf2]) as a function of solvatophilicity and the difference in terpolymer chain mobility (via glass transition differences due to using MMA versus OEGMA). Specifically, MMA and OEGMA provided solvatophilic groups (the ester in MMA or ethylene glycol units in OEGMA) to the terpolymers and their composition was varied to study the effect of solvatophilicity on the terpolymer phase separation and re-dissolution processes in the IL. The concentration of solvatophilic groups in the terpolymer was not only shown to affect phase separation temperature significantly, but also demonstrated to be crucial in driving the re-dissolution process. A minimum concentration of solvatophilic groups to facilitate reversible phase separation was in the range of 20–30 mol% for both terpolymer systems. Solution concentration (1–10 wt%) also showed significant effects on phase separation temperature but lower concentration did not guarantee improved re-dissolution. Phase separation temperature was also demonstrated to be inversely proportional to the polymer molecular weight (7.3–22.5 kg mol−1). Finally, the reversibility of phase separation and fluorescence responses in terpolymers with sufficient solvatophilicity was confirmed to be consistent by multiple heating/cooling cycles.

Epilithic Algae Distribution Along a Chemical Gradient in a Naturally Acidic River, Río Agrio (Patagonia, Argentina)

The epilithic algae distribution along a pH gradient and the relationship between the chemical gradient and biomass development were studied in Río Agrio, a naturally acidic river located in Patagonia (Argentina). The epilithic community was monitored during the summer of three consecutive years in sites located above and below the entrance of tributaries. The epilithic community showed differences between sites based on the chemical composition of the water and the precipitates that appear on the streambed of the river. The lowest biomass, diversity, and number of species were found at the most extreme part of the river in terms of pH (ca. 2) and element concentrations. Euglena mutabilis was the dominant species in this section of the river. As pH increased (ca. 3), the community changed to be dominated by filamentous green algae (Ulothrix spp., Mougeotia sp., Klebsormidium sp.) showing luxuriant growths in terms of biomass. With the inflow of a neutral tributary, the pH of Río Agrio increased above 3, and the precipitates of orange-red iron hydroxides appeared. The algal community was not affected by these precipitates or the low P concentrations, along the next 30 km of river downstream from this site. The apparent physical stress that the precipitates impose on algae is in fact a dynamic reservoir of P because diel cycle of Fe could be promoting precipitation and redissolution processes that binds and releases P from these precipitates. Where the pH increased above 6, precipitates of aluminum hydroxides appeared. At this site, the epilithic biomass and density decreased, some algae species changed, but the diversity and the number of species in general remained consistent with the upstream values. The physical stress of the Al precipitates on the algae is added to the chemical stress that represents the sequestering of P in these precipitates that are not redissolved, resulting P a limiting nutrient for algae growth.

Re-dissolution and de-compaction of DNA–cationic surfactant complexes using non-ionic surfactants

Addition of a cationic surfactant to a solution of DNA causes the formation of compacted DNA-cationic surfactant complexes which precipitate from aqueous solution. It has been shown previously that addition of anionic surfactant will re-dissolve and de-compact the DNA-cationic surfactant complexes and we find that addition of non-ionic surfactants of the alkylpolyoxyethylene type can be used similarly. In principle, these de-compaction and re-dissolution processes could occur either by stripping of the cationic surfactant from the DNA into mixed micelles with the non-ionic surfactant or by solubilisation of the DNA-cationic surfactant complexes within the non-ionic micelles. Solubility phase-boundary measurements, fluorescence microscopy observations of the de-compaction process and light scattering results indicate that de-compaction and re-dissolution occur by the stripping mechanism, even for non-ionic surfactants where the favourable attractive electrostatic interaction between the two surfactants is absent. Using measurements of critical micelle concentrations and calculations based on regular solution mixed micelle theory, we show that re-dissolution and de-compaction of the DNA-cationic surfactant complexes occurs when the concentration of free monomeric cationic surfactant is reduced (by incorporation into mixed micelles) below a critical value.

Layer-by-layer assembly of mucin and chitosan—Influence of surface properties, concentration and type of mucin

Bovine submaxillary mucin (BSM) and chitosan were used to build layer-by-layer structures on solid substrates. The build-up was monitored using in situ ellipsometry to obtain time resolved values of the thickness and adsorbed amount. Additionally surface morphology during build-up was studied by atomic force microscopy (AFM). It was found that the adsorbed amount of the film increases approximately linearly with each deposition cycle on hydrophobized silica whereas construction on silica was found not to be possible at the experimental conditions used. We conclude that sufficient amount of the first mucin layer is crucial for the subsequent multilayer formation. The complex build-up kinetics on hydrophobized silica is characterized by adsorption and redissolution processes and the overall growth is the sum of both processes. AFM imaging on hydrophobized silica also confirmed the presence of redissolution processes and chitosan addition led to a reduction both in the number of surface aggregates and in the roughness of the surface. The present work also shows that by adjusting the relative concentrations of the polyelectrolytes it is possible to change the growth rate considerably. The final structures after deposition of 8 bilayers were found to have a high content of water and film stability test revealed that a substantial amount dissolves when increasing electrolyte concentration or pH of the ambient solution. Human mucin from saliva (MUC5B) was also used to create multilayers with chitosan on hydrophobized silica and it was revealed that no redissolution appears to be present in this system.

Decomposition of diclofenac by solar driven photocatalysis at pilot plant scale

Pilot plant experiments applying solar titanium dioxide photocatalysis and solar photo-Fenton treatment at different pH and iron concentrations with an initial diclofenac concentration of 50 mg L −1 are described. In preliminary experiments absence of hydrolysis and slow photolysis under solar irradiation of diclofenac solutions were observed. Solar photo-Fenton treatment with freshly precipitated iron at pH around 7 showed first order kinetics, the reaction taking place on the surface of the iron precipitate. Simultaneous oxidation, precipitation and re-dissolution processes of diclofenac governed photo-Fenton decomposition kinetics at pH 2.8. The use of different iron concentrations (0.03–0.75 mM) showed no influence on the reaction rate in a neutral medium due to reactor geometry. Similar behaviour (no influence of iron concentration) was observed at pH 2.8, due to precipitation problems. A pH of around 4, close to the p K a of diclofenac, showed promising results, partly overcoming both iron and diclofenac precipitation. Solar titanium dioxide photocatalysis with Degussa P-25 followed first order kinetics and no precipitation or adsorption occurred. Decomposition of diclofenac took around 100 min under all photo-Fenton treatment conditions employed. Decomposition by titanium dioxide photocatalysis took about 200 min. In photo-Fenton treatment, hydrogen peroxide consumption to diclofenac decomposition was about 80–110 mM at pH 2.8 and 40 mM in the other two treatments (neutral pH and pH = 4).

Measuring and modelling temporal trends of 226Ra in waters of a Spanish estuary affected by the phosphate industry

The presence and temporal evolution (1990-2001) of (226)Ra in a tidal estuary affected by the phosphate industry has been investigated. Water samples collected in the course of four separate sampling campaigns were analysed for (226)Ra content using a gas flow proportional counter following Ba coprecipitation. Two (226)Ra sources have been identified: direct discharges from the industrial complex and run-off from a phosphogypsum pile. Although activity levels are similar, or even higher, than those found in other environments affected by the phosphate industry, there has been a general decrease in contamination since direct discharges ceased in 1998 due to new regulations from the EU. However, sediments are now acting as a source of Ra to the water column due to redissolution processes. A numerical model of the estuary has been developed to describe quantitatively the experimental results. The model solves the hydrodynamics and the dispersion equation of (226)Ra including interactions with sediments. Model results are, in general, in good agreement with observations.

Modelling the physico-chemical speciation of plutonium in the eastern Irish Sea: a further development

A numerical full three-dimensional model previously developed to simulate the physico-chemical speciation of plutonium in the eastern Irish Sea has been improved. The model solves simultaneously the hydrodynamic equations, the suspended matter equation and the equations that give the time evolution of Pu concentrations in water, suspended matter and bottom sediments. It is considered that Pu may exist in each phase in two different oxidation states. Redox reactions are also considered. In the earlier version of the model, a one-step kinetic model was used to describe the transfers of radionuclides between the dissolved and solid phases. Although with this kind of model the contamination of the sediments can be properly simulated, it is clearly not able to describe the re-dissolution of radionuclides from a contaminated sediment once the external source to the sea is reduced. Thus, the model has been improved by substituting the one-step model with a two-step kinetic model consisting of two consecutive reversible reactions. Now it is possible to simulate both the sediment contamination and the re-dissolution processes.

The influence of mixing timescales and re-dissolution processes on the distribution of radionuclides in Northeast Irish Sea sediments

The sediments of the northeast Irish Sea contain substantial inventories of contaminant radionuclides as a consequence of past discharges of low-level liquid radioactive waste from the Sellafield nuclear fuel reprocessing plant. Previous work has indicated that onshore transport of this contaminated sediment is the dominant mechanism of supply of Sellafield waste radionuclides to surrounding intertidal and floodplain areas, but an inherent, and previously unsubstantiated, assumption in this work was that past discharges from Sellafield are effectively homogenised in the surface offshore sediment, with respect to 238Pu 239,240Pu and 241Am 239,240Pu activity ratios, as the result of mixing on a timescale of a few years. Results are presented here for a study of three cores, of lengths ranging from 10 to 38 cm, collected from the deposit of contaminated offshore sediment in 1992. The activity ratios of the longlived radionuclides 238Pu, 239,240Pu and 241Am in the cores exhibited variations that were small relative to those in the Sellafield discharge, indicative of a high degree of mixing of the sediment. Moreover, the sediment activity ratios were found to be compatible with those of the Sellafield time- integrated discharge and with those of sediment deposited in surrounding coastal areas during the year after sample collection. In view of the heterogeneous nature of the sediments in the northeast Irish Sea, caution must obviously be applied in any generalisation based upon the analysis of only three cores, but for the locations sampled, the results are consistent with the surface sediment acting as a well mixed source of long-lived Sellafield waste radionuclides. 60Co and 134Cs were detected in the sediment to depths ranging from 10 to 18cm, implying transport to these levels by mixing on a timescale commensurate with their half-lives. Activities of the short-half-life nuclides 106Ru and 144Ce decreased with depth in the sediment, suggesting a timescale for mixing of the surface (< 13 cm depth) sediment of the order of 1 year. On the basis of the distribution of these short-half-life radionuclides, biodiffusion coefficients of the order of 10 −7 cm 2 s −1 were derived for the top 10 cm of sediment. The trends in 137Cs 241Am activity ratio with depth were consistent with re-dissolution of radiocaesium that had initially been taken up in the sediment during the period of high discharges from Sellafield. Comparison of the results with 137Cs 241Am activity ratios for Irish Sea sediment in the late 1970s suggests that up to 77% of the 137Cs had been re-dissolved from surface (< 10 cm depth) sediment and some 15% from deeper layers. Finally, it was observed that the 137Cs 241Am activity ratios for the three cores studied implied a K d of the order of 10 4–10 5 for radiocaesium in this system.

Synthesis of ZSM-5 from Ethanol-Containing Systems. Influence of the Gel Composition

ZSM-5 zeolite has been synthesized using ethanol as a templating agent. The effects of composition on the physicochemical properties of the zeolite (Al content, crystal and particle sizes) and on the kinetics of the crystallization have been studied. The dilution of the starting gel with water leads to a decrease of both the nucleation and crystal growth rates, whereas the increase of the ethanol/SiO[sub 2] molar ratio favors both processes in agreement with the templating role of the alcohol. The variation of the SiO[sub 2]Al[sub 2]O[sub 3] ratio causes drastic changes in the crystallization: the presence of a high proportion of Al in the gel delays the nucleation period but accelerates the crystal growth step. A certain contribution of the autocatalytic mechanism to the nucleation is present at the lowest SiO[sub 2]/Al[sub 2]O[sub 3] ratio investigated, the autocatalytic nuclei being probably formed around Al species in the gel phase. Thus, pure ZSM-5 cannot be obtained at high SiO[sub 2]/Al[sub 2]O[sub 3] ratios. The enhancement of the basicity leads to a decrease of the nucleation period, whereas a maximum of the crystal growth rate is observed for the intermediate OH[sup [minus]]/SiO[sub 2] ratio of 0.12 due to the competition between crystallizationmore » and crystal redissolution processes at high OH[sup [minus]] concentrations. The Al content of the zeolite is only changed by variation of the SiO[sub 2]/Al[sub 2]O[sub 3] ratio in the gel, its proportion being higher in the zeolite phase.« less

A differential scanning calorimetry investigation of the effects of zinc and copper on solid state reactions in AlZnMgCu alloys

The effects of zinc and copper on the precipitation and redissolution processes in AlZnMgCu alloys were investigated by differential scanning calorimetry. The study was carried out on an Al-5wt.%Zn-2.3wt.%Mg alloy to which 2 wt.% of either zinc or copper was added. It was found that both zinc and copper promote nucleation of Guinier-Preston zones, the upper temperature limit for zone formation being higher in the alloys containing copper. However, the kinetics of phase redissolution in both naturally and artificially aged material, were largely independent of zinc and copper contents. Additions of zinc and of copper increased the quench sensitivity to a similar extent.

Application of semidifferential electroanalysis to anodic stripping voltammetry

A new voltammetric technique, Semidifferential electroanalysis, in which the semiderivative, e, of the current, i, is measured as a function of electrode potential, has been applied for detection in anodic stripping voltammetry. The semiderivative of the current is defined by [fx131-1.tif] Cd 2+, Pb 2+, and Tl + in 0.1 M KNO 3 at different pH values were tested as samples on a hanging mercury drop working electrode. Symmetrical sharp peaks were observed for the re-dissolution processes of metal amalgams formed during pre-electrolysis at -1.0 V vs. SCE. The peak potentials of e vs. E curves for the above three amalgams agreed well with the literature values for d.c. polarographic half-wave potentials. The peak heights were proportional to the pre-electrolysis time up to about 5 min, to the potential scan rate in the range 60–160 mV s -1, and to the concentrations of Cd 2+, Pb 2+, and Tl + in the original solution in the range 10 -6–10 -9 M. The relative standard deviation for the determination of Cd 2+ was about ±4% at the 2 × 10 -5 M level.