Electrochemical impedance spectroscopy study of a surface confined redox reaction: The reduction of azobenzene on mercury in the absence of diffusion

The kinetics of azobenzene reduction on mercury electrodes in the absence of diffussional mass transport is studied by electrochemical impedance spectroscopy (EIS) in acetic acid/acetate buffered solutions at different pH values. Cyclic voltammetry experiments confirm the absence of diffusion effects and provide the values of the surface equilibrium potential. The analysis of the impedance frequency spectrums at every potential within the faradaic region conforms well the model and provides the global rate constant of the process, k_f. The potential dependence of k_f suggests the existence of an EE mechanism, with two electron transfers controlling the overall rate. The kinetic parameters of every step are obtained and their pH dependences clarify the role played by the protonation steps.     

Scale‐up of adsorptive styrene drying

This work is focused on the analysis of the scale‐up process of styrene drying by adsorption onto activated alumina as a necessary step in the manufacture of styrene–butadiene rubber. In a previous work, the mathematical model was developed and the design parameters were estimated from the fitting of the model to experimental results obtained in a laboratory set‐up. The obtained model and parameters have been used to simulate the behaviour of a pilot plant that contains up to 56 times more sorbent stating their validity after comparison with experimental results. Finally, the behaviour of an industrial plant presently at work, was also satisfactorily described. The ability of the model to simulate experimental results obtained on three different scales checks its adequacy for process design and optimization. © 2002 Society of Chemical Industry     

Characterisation of the main enzymatic activities present in six commercial macerating enzymes and their effects on extracting colour during winemaking of Monastrell grapes

Numerous studies have reported on the use of enzymes in wineries to facilitate the extraction of grape phenolics and to aid colour stability, although no clear conclusions were reached. Information concerning the enzymatic activities of industrial preparations is very difficult to obtain despite its importance. The aim of this work was to characterise and quantify the activities of six macerating enzyme preparations to optimise their use, and to study their effect on the chromatic characteristics of a Monastrell wine. The results indicate that differences exist in the type of enzymatic activities present in the preparations and in their concentration. The chromatic characteristics of the resulting wines, both at the end of alcoholic fermentation and after 12 months of storage in stainless steel tanks, showed differences with the use of the different maceration enzymes when comparing with control wine, although very little differences were observed between the different enzyme treatments. The enzyme-treated wines showed, at the end of alcoholic fermentation, a higher OD₂₈₀ and tannin content, and after 12 months, a higher colour intensity too. The main effect of all the preparations was an increase in the total phenol content, probably by facilitating the release of skin tannins.     

Changes in phytoplankton composition in a Mediterranean coastal lagoon in the Cullera Estany (Comunitat Valenciana,Spain)

The Cullera Estany is a coastal lagoon located in a highly intensified agriculture and tourist area in Valencia. This coastal lagoon has connections with the sea that produce marine intrusion and generate a freshwater interface. Four sampling campaigns were carried out during 2010 in order to analyse the phytoplankton composition and its relation to nutrient content through a Redundancy Analysis. Temperature, dissolved oxygen, nitrite and salinity are the main factors controlling the dynamics of phytoplankton community. During July and October, there is water column stratification; meanwhile in March, there is a well‐mixed water column. In addition, in May and July campaigns, hypoxia/anoxia conditions are detected at the bottom. The most abundant phytoplankton groups are Diatoms and Cryptophyceae. Diatoms and Cyanophyceae respond positively to temperature while Cryptophyceae, Prasinophyceae and Dinophyceae respond to high salinity and dissolved oxygen values. Furthermore, picoplankton is correlated inversely with nutrient concentrations.     

Zinc recovery and waste sludge minimization from chromium passivation baths

This work reports the feasibility of applying emulsion pertraction technology (EPT) aiming at zinc recovery and waste minimization in the zinc electroplating processes that include Cr (III) passivation. The assessment consists of firstly the lifetime extension of the passivation baths by selective removal of the tramp ions zinc and iron, and secondly, the recovery of zinc for further reuse. Spent passivation baths from a local industry were tested, being the major metallic content: Cr³⁺ 9000 mg L⁻¹, Zn²⁺ 12,000 mg L⁻¹, Fe³⁺ 100 mg L⁻¹. Working in a Liqui-Cel hollow fiber membrane contactor and using the extractant bis(2,4,4-trimethylpentyl) phosphinic acid, reduction of zinc and iron concentrations below 60 mg L⁻¹ and 2 mg L⁻¹, respectively were obtained, while trivalent chromium, the active metal that generates the passivation layer, was retained in the baths. Zinc was selectively transferred to an acidic stripping phase that in the experimental time reached a concentration of 157,000 mg L⁻¹. Zinc recovery by electrowinning from the acidic stripping phase without any pretreatment of the electrolyte solution provided a purity of 98.5%, matching the lower commercial zinc grade. As a result of the extension of the life time of the passivation bath, significant environmental advantages are derived such as minimization of the volume of hazardous wastes and savings in the consumption of raw materials.     

Boron-doped diamond anodic treatment of landfill leachate: evaluation of operating variables and formation of oxidation by-products

Landfill leachate with a low BOD/COD ratio was electrochemically oxidized by means of a boron-doped diamond anode. In addition to organic matter removal, this study addressed the issue of formation of both chlorinated organic compounds and nitrate ions as a result of organic matter and ammonia and/or organic nitrogen electro-oxidation in the presence of chloride ions. A factorial design methodology was implemented to evaluate the statistically important operating variables: treatment time (1-4 h), pH (5-8), current intensity (6.3-8.4 A) and addition of chloride (2500-4500 mg L⁻¹). The process was evaluated on COD, total nitrogen (TN) and colour removal, as well as on the formation of nitrate, nitrite and chlorinated organics. Of the four variables studied, treatment time and pH had a considerable influence on COD and colour removal. On the contrary, none of the variables had a significant effect on the elimination of TN for which an average removal of 61 mg L⁻¹ was obtained. The studied variables exhibited different effects on the four groups of organo-chlorinated compounds considered in this study, namely trihalomethanes (THMs), haloacetonitriles (HANs), haloketons (HKs) and 1,2-dichloroethane (DCA). Further analysis at more intense conditions, i.e. current intensity up to 18 A and reaction time up to 8 h revealed that high levels of decolourization (84%) could be achieved followed by low COD (51%) and ammonia (32%) removals. Apart from DCA, the concentration of chlorinated organics increased continuously with treatment time reaching values as high as 1.9 mg L⁻¹, 753 μg L⁻¹ and 431 μg L⁻¹ of THMs, HANs and HKs, respectively.     

Continuous photosynthetic production of hydrogen peroxide by the blue-green algae Anacystis nidulans R2 as a way to solar energy conversion

Continuous production of hydrogen peroxide by a culture of the blue-green algae Anacystis nidulans R2 is described as a model system for the conversion of solar energy. The redox power from the photolysis of water, promoted by the photosynthetic apparatus of algae, is driven by the redox mediator methyl viologen to oxygen which is reduced, in two successive steps, to superoxide and hydrogen peroxide. Hydrogen peroxide is a chemical compound with a high value since it is extensively used in industry, especially as bleaching agent. Moreover, this product is a powerful source of energy which releases more than 100 kJ per mol upon decomposition into water and oxygen; it is considered a fuel able to drive rockets and engines. Optimum pH and temperature of the photosystem, as well as the effect on H₂O₂ production of the concentration of methyl viologen, amount of cell, and intensity of light have been studied. Under saturation of light and methyl viologen, and and at the temperature of 45°C, a production of hydrogen peroxide higher than 200 μmol/mg Chl · h was reached. In these conditions, 2% of the light energy absorbed by the photosystem is converted in chemical energy as H₂O₂.     

Scientific evidences beyond the application of inactive dry yeast preparations in winemaking

In recent years the use of inactive dry yeast (IDY) preparations such as inactive yeast, yeast autolysates, yeast extracts and yeast hulls or walls have been widely used within the oenological industry to improve either technological processes or the sensory characteristics of wines. Some of these preparations have very specific applications and there are currently many of these products in the market under different brands that promise different ways of improving wine characteristics. Nevertheless, scientific information about the chemistry beyond their use and their action mode is still scarce.The objective of this review is to revise the different applications of specific IDY preparations in winemaking, on the basis of their action mechanisms taking into consideration the scientific information available, underlining the necessity of more scientific work in order to better characterize their chemical composition, their action mechanisms, and the establishment of better criteria for their oenological use.     

Hydrogenation of CO and CO2 on carbon black-supported Ru catalysts

The activity and selectivity of both unsupported Ru and carbon black-supported Ru catalysts toward the hydrogenation of CO and CO₂ have been investigated in order to learn about the effect of metal particle size on both reactions. The specific activity for both reactions decreases with metal particle size and the product distribution obtained in the hydrogenation of CO (the hydrogenation of CO₂ only yields methane) is also a function of metal dispersion; thus, the proportion of methane produced increases and the olefin/paraffin decreases with decreasing Ru particle size. This behaviour is attributed to changes in the electronic properties of the Ru crystallites in close contact with the graphitised carbon black used as support.     

Removal and recovery of Cr(VI) from polluted ground waters: a comparative study of ion-exchange technologies

The focus of this work has been the study of Cr(VI) removal from ground waters and the simultaneous concentration for its reuse using three different technological alternatives: anion-exchange resins, liquid-liquid extraction assisted by hollow fibre membranes and emulsion pertraction. The viability of the considered objectives, i.e., Cr(VI) separation (<0.5 g/m3) and concentration for reuse (>20,000 g/m3) has been checked and a comparative analysis of the three technologies has been performed. Although the flexibility and ease of operation of non-dispersive solvent extraction, anion-exchange resins and emulsion pertraction lead to higher velocities of chromium removal, yet still maintaining similar concentration efficiencies.