Reaction Kinetics

Although there have been numerous studies on metal sulphide precipitation, the research field as a whole is not well integrated. This paper reviews the disparate areas of study into metal sulphide precipitation in an attempt to summarise the current work, as well as to suggest potential for future consolidation in the field. The review encompasses (1) fundamental studies into metal sulphide precipitation, which usually focus on mechanisms and are carried out at very low (micromolar) concentrations; (2) applied studies focussing on metal removal and reaction kinetics (mostly via the aqueous phase); (3) studies that focus on the solid phase and address the crystallization kinetics of the formed particles; (4) studies into precipitation of metal sulphide nanocrystals and lastly, (5) applications of metal sulphide precipitation to effluent treatment processes such as Acid Mine Drainage (AMD) treatment as well as industrial hydrometallurgical processes. The review found that, besides lack of integration between the fundamental and applied areas of study, the applied studies have often used flawed methods to evaluate the efficiency of the metal sulphide precipitation process. Applying classical crystallization theory has also not been entirely successful because of the sparingly soluble nature of the systems. The studies that investigate nanocrystal formation tend to focus mostly on recipes and characterization of the formed particles. The industrial and effluent treatment studies form another area of research that stands relatively isolated from its more scientific counterparts. One of the key insights obtained from the summary of these disparate areas of work is that the level of scientific understanding in each of the fields is vastly different. The fundamental studies into mechanistic aspects of metal sulphide precipitation are far advanced of the other areas. However, they are restricted to very low concentrations, which are of limited value in most process-based hydrometallurgical applications. Most of the applied studies are still at a relatively empirical level, with the findings being highly systemdependent. Truly generic findings are still to be realised in these areas. Developing robust measurement techniques to be able to collect consistent data and thus model the simultaneous nucleation, growth, aggregation and attrition of the nano to micro scale particles is one of the challenges in the field. Understanding and characterising the complex aqueous chemistry, taking into account multiple sulphide and metal species interactions, is another. It is proposed that, by integrating the different priorities of the various study areas (chemistry, reaction mechanisms, crystallization mechanisms, particle characterization and industrial applications) the research field as a whole would benefit.

The kinetics of oxygen reduction was investigated in acid solutions on Pt monolayers deposited on modified carbon-supported PdIr nanoparticles using the rotating diskelectrode technique. Iridium is introduced into the Pd substrate in order to fine-tune the Pt-Pd interactions and to improve Pd stability under operating conditions of the fuel cell. The kinetics of the oxygen reduction reaction shows enhancement with the Pt monolayer on the PdIr nanoparticle surfaces in comparison with the reaction on Pt/

Sarcoplasmic reticulum (SR), Ca *+ + Mg *+-ATPase, and Ca *+-ioncphore were obtained from white rabbit skeletal muscles. Methylmercury inhibited the Ca *+ + Mg*+-ATPase an d Ca**-transport but had no effect on the Ca*+ionophore. Mercuric chloride inhibited all three functions (i.e., ATPase, transport and ionophoric activity). The mechanism of HgC12 inhibition of the Ca*+ionophore was by competition with Ca*+ for Ca*'-ionophoric site whereas its inhibition of the enzyme ar,d Ca*+-transport was due to the blockage of essential sulfhydryl (-SH) groups. Ca*' + Mg*+-ATPase and Ca*+-transport were more sensitive to methylmercury than to HgC12. Acetylcholine receptor (AChR) was obtained for the electric organ of T.

We investigated the wetting behavior and reactions of different metals on Si3N4 using sessile drop measurements, analysis of reaction layers, and measuremeints of strengths of joined bars. Active metals, such as A1 and Ti, and alloys that contain them react with Si3N4 and cause ,wetting and spreading at the interface. AI-Si3N4 reaction at 900°C produced a thin layer of A1203 at the interface. Reaction between Si3N4 and Ag-Cu-Ti braze alloys at 900°C resulted in a complex microstructure in the reaction zone that contained TIN and titanium silicides. Breaking strengths of Si3N4 bars joined with the Ag-Cu-Ti braze alloys were higher than those for Si3N4 joined with Al, primarily because of the better wetting by the Ag-Cu-Ti alloys. Nonreactive metals and alloys such as Sn, In, Ag-Cu, and Ag-Cu-Sn neither wet, spread, nor adhere to Si3N4 substrates. [

Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth's oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H 2 S $ 200 lM in the bottom waters (or about 300 lM total sulfide: RS 2À = H 2 S + HS À + S 2À ). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a d 98 Mo oxic = 0.9 ± 0.1&, which matches that of the riverine input, d 98 Mo river = 0.9 ± 0.1&. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1&, but the dissolved Mo is even heavier at d 98 Mo sulfidic = 1.8&. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with d 98 Mo values significantly fractionated at À0.8& to À1.2& both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by $0.3-1.5&. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive d 98 Mo trend with depth, from d 98 Mo = 1.2& to 1.4& while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of d 98 Mo = 0.9-1.0&.

The photochemical behavior of oxyfluorfen [2-chloro-1-(3-etoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene] on two Greek soils was investigated. Soils were sampled from Nea Malgara and Preveza regions, characterized by a different organic matter content. Soils were spiked with the diphenyl-ether herbicide and irradiation experiments were performed either in the laboratory with a solar simulator (xenon lamp) or outside, under natural sunlight irradiation; other soil samples were kept in the dark to control the retention reaction. Kinetic parameters of both retention and photochemical reactions were calculated using zero-, first- and second- (Langmuir-Hinshelwood) order equations, and best fit was checked through statistical analysis. The soil behaviors were qualitatively similar but quantitatively different, with the soil sampled from the Nea Malgara region much more sorbent as compared with Preveza soil. All studied reactions followed second-order kinetics and photochemical reaction...

Pulsatile action can be used to mix two streams entering a tube from two separate branches of a bifurcation at low Reynolds numbers. The pulsatile action is provided by two pinch valves, which deform flexible tubing immediately upstream of the connection. The pinch valve action is controlled using a master-slave pulse generator setup. The quality of mixing is evaluated directly by measuring the fluorescence that results from the chemical reaction of species transported in the two streams, one containing native biotin and the other, fluorescein biotin bound to streptavidin. The reaction kinetics are accounted for by normalization using fluorescence measurements on well mixed solutions at the same residence time. The results show that the pulsatile micromixer provides almost complete mixing. Furthermore, the present measurements match results obtained in a previous experiment where flow visualization and image analysis were used to measure mixing quality in a scaled-up model.

Dedicated to Harry Kurreck on the occasion of his 65th birthday Abstraet. The mechanism of 4-chlorophenol (4CP) photolysis was investigated with the aid of Fou-¡ Transform Electron Paramagnetic Resonance (FT-EPR) and pulsed-laser photolysis combined with High Performance Liquid Chromatography (HPLC) deteetion of stable (diamagnetic) products. With FT-EPR transient free radicals produced by pulsed-laser excitation of solutions of 4CP in alcohols could be identified. Time profiles of the FT-EPR spectra provided information on reaction kinetics and Chemically Indueed Dynamic Electron Polarization (CIDEP) effects. Ir was found that 4CP photolysis in alcohols leads to the simultaneous formation of the phenoxyl radical and radieals produced by hydrogen abstraction from the solvent. CIDEP pattems establish that these radicals are formed in a reaction sequence involving a triplet state precursor and radical pair intermediate. Results of earlier transient optical absorption measurements indicate that the t¡ precursor must be the carbene 4-oxocyclohexa-2,5-dienylidene. This assignment is supported by the finding that photolysis of quinone diazide in a hydrogen-donating solvent gives the same free radical products as those obtained from 4CP. The formation of the phenoxyl radical intermediate accounts for the finding that photolysis of deoxygenated solutions of 4CP in alcohols gives phenol as stable diamagnetic product. By contrast, photolysis of aerated and deoxygenated aqueous solutions of 4CP produces benzoquinone and hydroquinone as primary produets, respectively.

The curing reaction kinetics of an epoxy based on the diglycidyl ether of bisphenol A (DGEBA) with an inorganic complex based on nickel(II) chelate with ethylenediamine (en) as a ligand were studied using DSC in dynamic mode. The complex curing agent was synthesized and characterized by the elemental analysis, FT-IR, and ICP-Plasma techniques. Thermal dissociation behavior of curing agent was also studied using thermogravimetric (TG) analysis in isolated form. Three kinetic models, Kissinger, Ozawa-Flynn-Wall, and Expanded Freeman-Carrole, were used to determine the kinetic parameters. The effect of hardener concentration on the kinetic parameters and the shape of DSC thermograms of the DGEBA/Ni(en) 3 Br 2 system were investigated. Finally, the previous proposed mechanism by another researcher was used to explain the DSC data in detail.

The pyrolysis of switchgrass (Panicum virgatum) of the cultivar, ''Cave-in-Rock'' harvested at three stages of physiological maturity was studied in a PY-GC/MS system at the 600-1050 8C temperature range. Under these conditions, the decomposition was complete within 20 s yielding char, and two sets of pyrolysis gas, condensable and non-condensable. The former consisted of acetaldehyde (CH 3 CHO), acetic acid (CH 3 COOH) and higher molecular weight compounds possibly from the hydroxyl group and from the methoxy groups of the cell wall components. The non-condensable gases were mainly CO, CO 2 and C 1 -C 3 hydrocarbons. For these, there was a 900 8C temperature boundary where dramatic change occurred in their evolution rates. Below this temperature, CO 2 decreased but CO and the C 1 -C 3 hydrocarbons increased almost linearly with temperature. Above this temperature boundary, the hydrocarbons leveled off but there was a rapid rise in CO and CO 2 evolution at a constant CO/CO 2 ratio. These suggest the appearance of secondary or tertiary pyrolysis reactions involving rearrangement and release of CO and hydrocarbons prior to this temperature boundary and the release of CO and CO 2 from the tightly bond oxygen functionalities including C-C bonds thereafter. At <750 8C, there were modest increases in condensable gas yield and decrease in non-condensable gas due to differences in plant maturity at harvest. However, the effect of switchgrass physiological maturity on gas yield was statistically insignificant at high temperatures. The energy content of the non-condensable gas measured was about 68% of the gross energy content of the biomass for the early harvest crop and 80% for the mature crop. The activation energy for the decomposition, estimated assuming first order reaction kinetics, showed a linear increase with plant physiological maturity. The results demonstrate that physiological maturity at harvest of switchgrass biomass can result in different concentrations of pyrolysis products at different temperatures. These results also demonstrate the need for additional research with a broader array of herbaceous biomass materials to develop a better understanding of the synergies of crop cultivation, harvesting and processing of dedicated herbaceous biomass energy crops during their thermochemical conversion. #

Information about the times of thermal breakthrough and subsequent rates of thermal drawdown in enhanced geothermal systems (EGS) is necessary for reservoir management, designing fracture stimulation and well drilling programs, and forecasting economic return. Thermal breakthrough in heterogeneous porous media can be estimated using conservative tracers and assumptions about heat transfer rates; however, tracers that undergo temperature-dependent changes can provide more detailed information about the thermal profile along the flow path through the reservoir. To be effectively applied, the thermal reaction rates of such temperature sensitive traces must be well characterized for the range of conditions that exist in geothermal systems. Reactive tracers proposed in the literature include benzoic and

The reaction of an equimolar mixture of H2 and CO was studied over ZrOr at 350 to 45O"C, 35 atm total pressure, and varying residence times. The isosynthesis reaction mechanisms were studied by monitoring the incorporation of oxygenated compounds into the isosynthesis products. Propionaldehyde and [t3C]acetone, [i3C]isopropanol, and [W]methanol were added to the CO/Hz reactant stream. Their effect on the isosynthesis product distributions and on the isotopic distribution of the products was monitored as a function of oxygenate feed rate. Propionaldehyde, acetone, and methanol were found to incorporate into the isosynthesis products. The isotopic distribution of the products was used to support an isosynthesis reaction scheme that involves CO insertion into a bound aldehyde or ketone as the major chain growth step and a second chain growth step that involves condensation between methoxide and a surface bound enolate. Q 1988 Academic press, IK.

The unfolding of a protein by the application of an external force pulling two atoms of the protein can be detected by atomic force and optical tweezers technologies as have been broadly demonstrated in the past decade. Variation of the applied force results in a modulation of the free-energy barrier to unfolding and thus, the rate of the process, which is often assumed to have single exponential kinetics. It has been recently shown that it is experimentally feasible, through the use of force clamps, to estimate the distribution of unfolding times for a population of proteins initially in the native state. In this Letter we show how the analysis of such distributions under a range of forces can provide unique information about the underlying free-energy surface such as the height of the free-energy barrier, the preexponential factor and the force dependence of the unfolding kinetics without resorting to ad hoc kinetic models.

Numerical solutions have been obtained for the nonlinear heat conduction equations arising in the theory of thermal explosions. Explosion times are calculated for externally heated spheres, cylinders, and slabs of several explosive materials, and the results are shown to agree with experiment.

Coriander leaves are widely used in cooking throughout the world. Thermal degradation kinetics of chlorophyll a, b, and total chlorophyll in coriander leaf puree was investigated at varying levels of pH (4.5-8.5) and processing temperature (80-145°C). Coriander puree at pH 4.5 was processed at 80°to 100°C, whereas that at pH 5.5 to 8.5 was processed at 105°to 145°C. Chlorophyll degradation followed first-order reaction kinetics. Good agreement was found between estimated and experimental chlorophyll retention in all cases (R 2 >0.80). Activation energies ranged from 6.57 to 96.00 kJ/mol. Reaction rate and activation energy data indicated that chlorophylls were more stable at alkaline pH. Transition state theory was applied to estimate the enthalpy, entropy, and Gibbs free energy of activation. Enthalpy of activation (ΔH # ) ranged from 3.46 to 91.99 kJ/mol, whereas entropy of activation (ΔS # ) ranged from −0.265 to −0.047 kJ/(mol K). The overall free energy change was 107.55 kJ/mol. Results indicated that, the compensation effect did not exist for chlorophyll degradation in coriander puree during thermal processing.

The paper reviews the pyrolysis of biomass constituents and possible secondary reactions. Biomass pyrolysis yields mostly liquid and solid fuel, easy to store and transport. Relevant working conditions for experiments and large-scale operation are: (i) biomass particles < 200 mm, (ii) a particle heating rate of at least about 80 K min À1 and (iii) a reactor environment where the internal resistance to heat penetration is smaller than the external resistance to heat transfer (Biot-number, Bi < 1). The circumstances of TGA and DSC experiments meet these requirements and fully determine the reaction kinetics and endothermicity of the pyrolysis reaction. The reaction rate constant and the heat of reaction are essential parameters in the design of a pyrolysis reactor. For most of the biomass species tested, the first order reaction rate constant is large and >0.5 s À1. The heat of reaction ranges from 207 to 434 kJ kg À1. All results tie in with literature data, although the reader is cautioned in using literature data since experiments were not always performed under relevant testing conditions.

Dynamic adsorption and reaction studies were made for the catalytic oxidation of sulfur dioxide in aqueous slurries of activated carbon at room temperature and atmospheric pressure. For the small particles (d, = 0.03 mm) of carbon used, mass transfer resistances were of minor importance SO that the data could be analyzed in terms of the kinetics of the chemical reaction. Quantitative evaluation of mass transfer effects, including intraparticle diffusion, is presented in Part 11.

The effectiveness of wetland treatment of acid mine drainage (AMD) was assessed using three measures of performance: treatment efficiency, area-adjusted removal, and first-order removal. Mathematical relationships between these measures were derived from simple kinetic equations. Area-adjusted removal is independent of pollutant concentration (zero-order reaction kinetics), while first-order removal is dependent on concentration. Treatment efficiency is linearly related to area-adjusted removal and exponentially related to first-order removal at constant hydraulic loading rates (flow/area). Examination of previously published data from 35 natural AMD wetlands revealed that statistically significant correlations exist between several of the performance measures for both iron and manganese removal, but these correlations are potentially spurious because these measures are derived from, and are mathematical rearrangements of, the same operating data. The use of treatment efficiency as a measure of performance between wetlands is not recommended because it is a relative measure that does not account for influent concentration differences. Area-adjusted removal accounts for mass loading effects, but it fails to separate the flow and concentration comp-* Corresponding author. Tel.: +1-570-961-7862; fax: +1-570-961-7858. 0925-8574/99/$ -see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 -8 5 7 4 ( 9 8 ) 0 0 1 1 4 -1 W.J. Tarutis Jr. et al. / Ecological Engineering 12 (1999) 353-372 354

Terephthalic acid (TPA) is widely applied as a raw material in making polyester fiber, polyethylene terephthalate (PET) bottles, polyester films, etc. TPA is toxic and is known to act as endocrine disruptor. TPA wastewater is traditionally treated by biological process and this study aims to evaluate the effectiveness of several advanced oxidation processes on TPA removal. The oxidation processes studied were: . The results indicate that the time required for the complete destruction of 50 ppm of TPA can be minimized from 10 h using UV-TiO 2 system, to less than 10 min by UV-H 2 O 2 -Fe-O 3 system. Some of the likely organic intermediates identified during TPA destruction include, benzoquinone, benzene, maleic acid and oxalic acid. Possible destruction pathway of TPA has been proposed. TPA degradation by various systems was also analyzed based on the reaction kinetics and operating costs.

AbstractÐThe in¯uence of an extended set of metals (Ca 2+ , Mg 2+ , Pb 2+ , Cu 2+ , Cd 2+ , Ni 2+ , Zn 2+ , Mn 2+ , Fe 3+ ) on the reduction rates of chromium (VI) with H 2 S have been measured in NaCl solutions. Ca 2+ and Zn 2+ (0.01 to 0.02 M and 0.5 to 3.0 mM) did not show any signi®cant eect. Mg 2+ and Mn 2+ (0.05 to 0.2 M and 1.8 to 14.2 mM) produced only moderate increases in the rates in 0.5 M NaCl that were not proportional to the metal concentrations. The metals studied (Pb 2+ , Cu 2+ , Cd 2+ , Ni 2+ ) caused large increases in the reduction rates at micromolar concentrations. The eect was attributed to the formation of MeCrO 4 complexes which react faster with sul®de than free chromate. From measurements of the rates as a function of the added metal the rate constants for the metal chromate complexes were determined. The rates increased in the order k(NiCrO 4 )>k(PbCrO 4 )> k(CuCrO 4 )>k(CdCrO 4 ). The addition of Fe(III) (0.2 to 1.8 mM) increased the rates considerably due to the catalytic cycling of the redox reactions of iron. #

The kinetic characteristics have been studied for noncircularly permuted variants of the human hepatitis delta virus antigenomic ribozyme to find out the cause of the two-phase kinetics of the self-cleavage reaction. Different ways of reaction initiation, suboptimal conditions, and jumpwise changes of reaction conditions have been used, and the temperature dependences have been studied. A correlation has been shown between

Sulfated metal oxides have attracted considerable inter-Reaction kinetics measurements, selective poisoning, and mi-est because they are more environmentally benign and crocalorimetry were used to study the activity, selectivity, and more active for the transformation of hydrocarbons at low deactivation of acid sites on a sulfated zirconia catalyst for ntemperatures than current systems (3). In particular, sulbutane isomerization at 423 K. The sulfated zirconia catalyst fated zirconia has been studied for the isomerization of nhas a distribution of acid site strengths, containing 50 mol/ butane to isobutane, a hydrocarbon used in the production g of strong acid sites characterized by heats of ammonia adsorpof oxygenates and alkylates. Measurements of acid tion from 125 to 165 kJ/mol. The strongest acid sites (heats strength using Hammett indicators and studies of the catafrom 145 to 165 kJ/mol) are responsible for the high initial lytic activity of sulfated zirconia for hydrocarbon reactions activity of the catalyst, but these sites deactivate rapidly under at low temperatures (4, 6, 7) have indicated that this catareaction conditions. The acid sites exhibiting heats from 125 to 145 kJ/mol are less active than the stronger sites, but deactivate lyst may be superacidic. Several factors have been found more slowly. Weaker sites have low activity and deactivate to be important in determining the activity of sulfated more rapidly after the intermediate sites are deactivated or zirconia catalysts, including the preparation method, sulpoisoned. Brønsted acidity is necessary for extended catalytic fate loading, activation temperature, and moisture content activity, although Lewis acid sites may play a role in generating of the catalyst (4, 7-12). While both Brønsted and Lewis the initial high activity. Acid sites with heats of ammonia acid sites have been suggested to be important for catalytic adsorption from 125 to 165 kJ/mol show good selectivity activity , the roles of these sites in determining for production of isobutane (ca. 93%), while sites with heats the activity, selectivity, and deactivation of the catalyst from 120 to 125 kJ/mol show lower isobutane selectivity (ca.

The aim of this work was to study and to model the effects of time-temperature conditions for sterilization on the overall quality of tomato puree. Tomato puree was produced from raw tomatoes in a pilot plant. Three sterilization tests were carried out at 90, 95 and 100°C to obtain kinetic constants of quality properties. Heat and oxidative damage was observed in samples throughout sterilization. The furosine content increased, both the ascorbic acid content and the antioxidant activity of the hydrophilic fraction decreased, and a variation in colour (DE) occurred. No significant variations were found in the lycopene content, antioxidant activity of lipophilic fraction and sensory descriptors.

This paper describes the pervaporation performance of film-like zeolite NaA membranes synthesized in our laboratory by template-free seeded hydrothermal towards the dehydration of n-pentanol/water and n-pentanol/water/DNPE mixtures (DNPE = din -pentyl ether, C 10 H 22 O). DNPE is a linear symmetric ether that can be used as blending additive in reformulated diesel fuels and that can be produced from C 4 feedstocks via n-pentanol, obtained in its turn by selective hydroformylation of linear butenes. The membranes showed selectivities up to 3000 in the dehydration of n-pentanol/water binary mixtures, and water/DNPE and n-pentanol/DNPE selectivities approaching infinity (i.e. DNPE does not pervaporate) in the dehydration of n-pentanol/water/DNPE ternary mixtures. On the basis of the separation results presented in this work, and using experimental reaction kinetics data, we discuss the feasibility of zeolite NaA membrane reactors to carry out the liquid-phase etherification reaction of n-pentanol to din -pentyl ether (DNPE) catalyzed by ion-exchange sulfonated resins. These catalysts suffer from strong deactivation in the progress of the reaction due to the generation of water.

The stability of polymer inclusion membranes (PIMs) relative to other liquid membranes is amongst the major reasons for the recent rejuvenation of interest in carrier-mediated transport for selective separation and recovery of metal ions as well as numerous organic solutes. This is reflected by an increasing number of PIM investigations reported in the literature over the last two decades. Given the outstanding performance of PIMs compared to other types of liquid membranes particularly in terms of membrane lifetime, it has been predicted that practical industrial applications of PIMs will be realized in the near future. This review provides a comprehensive summary of the current knowledge relevant to PIMs for the extraction and transport of various metal ions and small organic solutes. PIM studies reported to date are systematically summarized and outlined accordingly to the type of carriers used, i.e. basic, acidic and chelating, neutral or solvating, and macrocyclic and macromolecular. The paper reviews the various factors that control the transport rate, selectivity and stability of PIMs. The transport phenomena observed by various authors are related to the membrane characteristics, physicochemical properties of the target solutes as well as the chemistry of the aqueous solutions making up the source and receiving phases. The results from these studies reveal an intricate relationship between the above factors. Furthermore, while the interfacial transport mechanisms in PIMs are thought to be similar to those in supported liquid membranes (SLMs), the bulk diffusion mechanisms in PIMs governing their permeability and selectivity require better understanding. This review also delineates two mathematical modeling approaches widely used in PIM literature: one uses a set of assumptions that allow the derivation of analytical solutions valid under steady-state conditions only; the other takes into account the accumulation of the target species in the membrane during the initial transport state and therefore can also be applied under non-steady-state conditions. The latter is essential when the interfacial complexation reaction kinetics is slow. It involves more complex mathematics and requires the application of numerical techniques. The studies included in this review highlight the potential of PIMs for various niche applications on a practical scale. The discussions provided, however, also emphasize the need for more fundamental research before any such practical applications of PIMs can be realized. This is specifically important for small organic compounds because to date scientific investigation involving the extraction and transport of these compounds remains limited. Transport mechanisms of small organic compounds are less well understood and are likely to be more complex than those observed with the transport of metal ions.

Ethanol steam reforming has been studied over Ni/SiO 2 catalysts under a wide range of operating conditions, and the effect of Ni loading on catalyst properties and H 2 yield and selectivity has been analyzed. Surface area and pore volume significantly decrease with nickel content, but there is only a slight decrease in mean pore diameter. XRD patterns show peaks corresponding to NiO but no evidence of nickel silicate species has been found. Ni/SiO 2 catalysts are active for H 2 production by ethanol steam reforming over 400 ºC. Ethanol conversion and H 2 yield considerably increase with temperature and Ni loading. Up to 3.9 mol H2 /mol EtOH is obtained at 600 ºC and 8.3 g catalyst min/g EtOH , although lower temperatures (500 ºC) are more suitable for minimizing CO selectivity. An improvement in catalyst performance is observed for high values of time-on-stream, which is explained by coke deposition in the form of carbon fibers.

Wastewater treatment facilities and agricultural production are potential sources of steroid hormones in the environment. The fate of steroid hormones in the environment has not been extensively studied. Important degradation pathways must be identified to predict the long-term impact of steroid hormones and their metabolites. Chemical oxidation, biodegradation, and photodegradation have been identified as potential mechanisms for the transformation of

The aqueous thermal degradation of oxalic acid (a naturally occurring dicarboxylic acid) and its anions has been examined experimentally under variable conditions of temperature (160-23O"Q time (24-96 h), buffered pH (4-7), and ionic strength (0.2-0.5 M). Decomposition of oxalate species follows a first-order rate law. Degradation rate increases with decreasing pH; the effects of ionic strength are not significant. Changes in reaction rate over the temperature range studied give activation energies that range from 29-50 kcal/mol over a starting solution pH range of 5-7. Extrapolation of these data to sedimentary basin temperatures suggests that oxalate species may be long-lived in formation waters. At 8O"C, half-lives range from 2500 to 28,000 years at pH values of 5 and 7, respectively. Formate species are relatively stable degradation products over the range of conditions studied. Comparison of these data with results of other thermal degradation studies of carboxylic acids and their anions indicate that acetate stability $ formate stability > oxalate stability > gallate and malonate stability. These results are consistent with the high concentrations of acetate relative to oxalate and malonate observed in formation waters.

Society, at large, has to realise the gravity of environmental degradation and participate fully in the mitigation of environmental problems. This article discusses the Indian initiatives and the important milestones in the path of environmental protection and pollution abatement. The commitment of the Indian Government to the cause of public awareness of environment conservation is reflected in its outreach and educational programmes involving its ministries, environmental monitoring agencies,NGOs, academic and research institutions. The Ministry of Environment and Forests (MoEF)has played a leading role in the national priority programmes of environmental monitoring, assessment and pollution control.All the stake holders have succeeded to some extent in creating public interest in environmental issues but much more needs to be done. National level institutions like NEERI,NIO and TERI are actively involved in research to find feasible solutions to our environmental problems and in ...

We analyze the steady-state behavior of a general mathematical model for reversible galvanic cells, such as redox flow cells, reversible solid oxide fuel cells, and rechargeable batteries. We consider not only operation in the galvanic discharging mode, spontaneously generating a positive current against an external load, but also operation in two modes which require a net input of electrical energy: (i) the electrolytic charging mode, where a negative current is imposed to generate a voltage exceeding the open circuit voltage, and (ii) the "super-galvanic" discharging mode, where a positive current exceeding the short-circuit current is imposed to generate a negative voltage. Analysis of the various (dis-)charging modes of galvanic cells is important to predict the efficiency of electrical to chemical energy conversion and to provide sensitive tests for experimental validation of fuel cell models. Notably, we consider effects of diffuse charge on electrochemical charge transfer rates by combining a generalized Frumkin-Butler-Volmer model for reaction kinetics across the compact Stern layer with the full Poisson-Nernst-Planck transport theory, without assuming local electroneutrality. Since this approach is rare in the literature, we provide a brief historical review. To illustrate the general theory, we present results for a monovalent binary electrolyte, consisting of cations, which react at the electrodes, and non-reactive anions, which are either fixed in space (as in a solid electrolyte) or are mobile (as in a liquid electrolyte). The full model is solved numerically and compared to analytical results in the limit of thin diffuse layers, relative to the membrane thickness. The spatial profiles of the ion concentrations and electrostatic potential reveal a complex dependence on the kinetic parameters and the imposed current, in which the diffuse charge at each electrode and the total membrane charge can have either sign, contrary perhaps to intuition. For thin diffuse layers, simple analytical expressions are presented for galvanic cells valid in all three (dis-)charging modes in the two subsequent limits of the ratio δ of the effective thicknesses of the compact and diffuse layers: (i) the "Helmholtz limit" (δ → ∞) where the compact layer carries the double-layer voltage as in standard Butler-Volmer models, and (ii) the opposite "Gouy-Chapman limit" (0 δ →) where the diffuse layer fully determines the charge-transfer kinetics. In these limits, the model predicts both reaction-limited and diffusion-limited currents, which can be surpassed for finite positive values of the compact-layer, diffuse-layer and membrane thicknesses.

An epoxy resin system based on a triglycidyl p-amino phenol (MY0510) was crosslinked using stoichiometric amounts of 4,4 0 -diaminodiphenyl sulfone. The epoxy was modified with random copolymers, polyethersulfone-poly(ether-ethersulfone) (PES:PEES), with either amine or chlorine end groups, at 10 and 20 wt %. The reaction kinetics for both unmodified and modified epoxy systems were studied using differential scanning calorimetry in isothermal and dynamic conditions. The results show that the degree of conversion in thermoplastic-modified epoxies at any reaction time is smaller compared with the unmodified resin. Gel point (GP) determination was done from rheological measurements. The modified system containing 20% of the PES:PEES additive showed considerable increase in the GP. The reaction rate shows the characteristic of an autocatalytic reaction where the product acts as catalyst. The activation energy, E a calculated from the isothermal reaction depends on the extent of conversion and increases with increasing PES:PEES content. For unmodified epoxy system, the average E a is 67.8 AE 4.1 kJ mol À1 but for systems modified with 20 wt % of amine and chlorine PES:PEES, the value increased to 74.1 AE 3.3 and 77.9 AE 4.4 kJ mol À1 , respectively. V

Simultaneous membrane based solvent extraction (MBSE) and membrane based solvent stripping (MBSS) in HF contactors of a heterocyclic carboxylic acid (HCA) of industrial importance from sulphate solutions with solvent containing trioctylamine (TOA) were studied. An effective transport of HCA was achieved. The proposed reactiondiffusion model gives a much better fit to experimental values than the diffusion model. The mean values of the lumped rate constants of extraction and stripping reactions for HCA are 2.19.10" and 5.92. lo4 m.s-', respectively. The mass-transfer resistances based on reaction kinetics play a decisive role in MBSE and MBSS of HCA and participate in their overall resistances by about 65 and 40 %, respectively.

Alkanolamines are the most popular absorbents used to remove CO 2 from process gas streams. Therefore, the CO 2 reaction with alkanolamines is of considerable importance. The aim of this article is to provide an overview on the kinetics of the reaction of CO 2 with aqueous solutions of alkanolamines. The various reaction mechanisms that are used to interpret experimental kinetic data -zwitterion, termolecular and base-catalyzed hydration -are discussed in detail. Recently published data on reaction kinetics of individual amine systems and their mixtures are considered. In addition, the kinetic behavior of several novel aminebased solvents that have been proposed in the literature is analyzed. Generally, the reaction of CO 2 with primary, secondary and sterically hindered amines is governed by the zwitterion mechanism, whereas the reaction with tertiary amines is described by the base-catalyzed hydration of CO 2 .

This work is focused on the optimization of reaction parameters for the synthesis of ascorbyl palmitate catalyzed by Candida antarctica lipase in different organic solvents. The sequential strategy of experimental designs proved to be useful in maximizing the conditions for product conversion in tert-butanol system using Novozym 435 as catalyst. The optimum production were achieved at ascorbic acid to palmitic acid mole ratio of 1:9, stirring rate of 150 rpm, 70°C, enzyme concentration of 5 wt.% at 17 h of reaction, resulting in an ascorbyl palmitate conversion of about 67%. Reaction kinetics for ascorbyl palmitate production showed that very satisfactory reaction conversions (∼56%) could be achieved in short reaction times (6 h). The kinetic empirical model proposed showed ability to satisfactory represents and predict the experimental data.

Pressure-assisted thermal processing (PATP) at T \ 100°C can be used when enzyme inactivation and pasteurization by high pressure processing (HPP) is not feasible due to long processing times while PATP at T [ 100°C can be used when bacterial spores inactivation ...

The dilute acid hydrolysis of grass and cellulose with phosphoric acid was undertaken in a microwave reactor system. The experimental data and reaction kinetic analysis indicate that this is a potential process for cellulose and hemi-cellulose hydrolysis, due to a rapid hydrolysis reaction at moderate temperatures. The optimum conditions for grass hydrolysis were found to be 2.5% phosphoric acid at a temperature of 1758C. It was found that sugar degradation occurred at acid concentrations greater than 2.5% (v/v) and temperatures greater than 1758C. In a further series of experiments, the kinetics of dilute acid hydrolysis of cellulose was investigated varying phosphoric acid concentration and reaction temperatures. The experimental data indicate that the use of microwave technology can successfully facilitate dilute acid hydrolysis of cellulose allowing high yields of glucose in short reaction times. The optimum conditions gave a yield of 90% glucose. A pseudo-homogeneous consecutive first order reaction was assumed and the reaction rate constants were calculated as: k 1 ¼ 0.0813 s 21 ; k 2 ¼ 0.0075 s 21 , which compare favourably with reaction rate constants found in conventional non-microwave reaction systems. The kinetic analysis would indicate that the primary advantages of employing microwave heating were to: achieve a high rate constant at moderate temperatures: and to prevent 'hot spot' formation within the reactor, which would have cause localised degradation of glucose.