Mathematical modelling and numerical simulation of cyclic voltammetry at an electrode covered with an insulating film containing cylindrical micropores

A general mathematical model for cyclic voltammetry at a planar electrode modified with an insulating surface coating containing non-interconnected cylindrical pores is developed and solved using numerical simulation. Electroactive species are assumed to diffusionally access the electrode only via the pores. The model allows us to explore the effects of different physical and electrochemical parameters on the mass transport properties of such systems and to highlight how they deviate from linear diffusion to a macroelectrode. © 2005 Elsevier B.V. All rights reserved

Recent advance in the electrochemical detection of sulphide and sulphhydryl species

This article aims at providing a critical review of some most recent developments in the electrochemical detection and measurement of hydrogen sulphide and the related species, which are of great significance to a variety of industries and in environmental monitoring. The molecular recognition processes are initiated by using either an organic precursor or a catalytic complex, leading to extensive ranges of detection. A series of advanced chemical and simulation techniques are used to probe the mechanistic details of the analytical chemistry involved

Mathematical modelling and numerical simulation of adsorption processes at microdisk electrodes

A general mathematical model for electrosorption under planar and microdisk diffusion conditions has been developed and solved using numerical simulation. This model allowed us to explore the electrochemical behaviour of such systems and to highlight some of their main properties. © 2004 Elsevier B.V. All rights reserved

Membrane fouling during the harvesting of microalgae using static microfiltration

A challenge in the filtration of algal suspensions is the reduction in filtrate flow rate with time, associated with membrane fouling and filtercake formation. These two fouling mechanisms may occur simultaneously and have different effects on the scaling relationship of filtration rate with time, making quantitative predictions of the filtration behaviour complex. The foulants include suspended microalgal cells and their presumed Extracellular Polymeric Substances (EPS). To investigate this problem, we perform static microfiltration experiments to harvest oil-rich marine microalgae Nannochloropsis oculata. Batch filtration experiments of microalgal suspensions using glass-fibre membranes are conducted under filtration pressures varying between 0.5 and 200 kPa. Here we investigate the relative importance of potential fouling mechanisms, including pore plugging, entrance blocking, and filtercake formation. We examine variations in filtrate flux, using a novel approach involving numerical differentiation of the filtrate volume to identify how its scaling relationship with time and compare with the traditional root-time behaviour. These results are analysed with reference to the blocking filtration laws to determine the potential fouling mechanism. Rheology tests of both filtration feeds and filtrate are performed, and both optical and scanning electron microscopy are used to observe the filtercake. Our results show a significant drop in the filtrate flux after a spurt loss phase under pressure. The scaling analysis demonstrates a power law relationship between cumulative filtrate volume and time in the post-spurt phase. We show here, for the first time, that the scaling exponent varies with time, approaching a value close to 0.5 (i.e. the root-time behaviour) after a long period of time. Using the analysis, we conclude that the filtration process can be divided into various stages; a spurt loss phase when the pressure is initially applied, followed by a stage in which both internal and external filtercake formation occurs, and a final stage which is dominated by external filtercake formation (at which point root-time behaviour is observed). Furthermore, our findings suggest that fresh microalgal suspensions experienced a larger spurt loss compared to aged suspensions in microfiltration. This difference may be attributed to the limited production of EPS and microalgal debris during shorter cultivation periods. The microscopic observations reveal the invasion of microalgal cells into the membrane, which verifies the formation of an internal filtercake suggested by our scaling analysis. The contamination of the membrane increases with higher filtration pressures. Finally, we demonstrate that Fe3+ coagulant can be used to increase the microalgal particulate size before filtration, resulting in a much higher filtrate flux compared to uncoagulated suspensions. This result provides opportunities to reduce membrane contamination to a negligible level

The thermodynamics of sequestration of toxic copper(II) metal ion pollutants from aqueous media by L-cysteine methyl ester modified glassy carbon spheres

L-Cysteine Me ester-modified glassy carbon powder (CysOMe-GC) has been shown to have potential for the removal of toxic heavy metal ions, such as copper(II), cadmium(II), and arsenic(III), from real water samples. To develop this material for environmental applications, we must develop an understanding of the thermodn. parameters controlling the uptake of metal ions by the modified carbon powder. Here, we characterize the material using XPS, before investigating the effect of varying the soln. pH, the concn. of copper(II) ions, and the mass of CysOMe-GC powder added to the soln. using square wave voltammetry (SWV). These data allow us to understand the thermodn. controlling the copper(II) ion uptake and elucidate that the adsorption of copper(II) onto the CysOMe-modified surface is controlled by a Freundlich isotherm. [on SciFinder(R)

Mathematical modelling and numerical simulation of adsorption processes at microdisk electrodes

A general mathematical model for electrosorption under planar and microdisk diffusion conditions has been developed and solved using numerical simulation. This model allowed us to explore the electrochemical behaviour of such systems and to highlight some of their main properties. © 2004 Elsevier B.V. All rights reserved

Electrochemical characterization of sulfide tagging via its reaction with benzoquinone derivatives

The voltammetric response of a range of structurally diverse substituted benzoquinones species (p-benzoquinone, 2,3-dimethoxy-5-methyl-p-benzoquinone, 2,6-dimethoxy-p-benzoquinone, and 3,5-ditert-butyl-o-benzoquinone) to increasing additions of sulfide has been examined using square wave voltammetry. In all cases the response shows that the quinone undergoes a reaction with sulfide to form a new redox active species. It is shown that by judicious choice of the pH of the solution analytical signals can be obtained for each species; the corresponding analytical parameters are detailed and the results critically appraised

The oxidation of cysteine by aqueous ferricyanide: A kinetic study using boron doped diamond electrode voltammetry

The oxidation of cysteine by ferricyanide is well established. However, electrochemical studies of the mechanism have to date been underdeveloped due to the lack of resolution between the oxidation waves of the ferrocyanide and cysteine on most electrode materials. It is shown that on boron doped diamond electrode, unlike for example platinum electrodes, the voltammetric responses of the ferrocyanide wave and the targets are sufficiently different, with the former at a lower oxidizing potential, to permit examination of the kinetics of the homogeneous oxidation via voltammetric methods. Accordingly conventional cyclic voltammetry using a BDD electrode is used to show that an EC′ mechanism occurs and rate constants deduced. Both the protonated and de-protonated forms of cysteine (pKa = 10.5) are shown to undergo oxidation by ferricyanide. The cyclic voltammetry data obtained is corroborated by use of channel flow cell experiments with excellent agreement shown between the two techniques for deduced values of the homogeneous electrocatalytic rate constants: Channel electrodes are seen to provide a convenient means to allow quantitative mechanistic hydrodynamic voltammetry at BDD electrods

Influence of the block geometry on the voltammetric response of partially blocked electrodes: Application to interfacial liquid-liquid kinetics of aqueous vitamin B<inf>12S</inf> with random arrays of femtolitre microdroplets of dibromocyclohexane

The cyclic voltammetric response of electrodes modified with catalytically reactive blocks is simulated using finite difference methods. The responses of three different models using various block geometries are studied. The results are used to determine kinetic parameters of coupled liquid|liquid interfacial reactions. First, we examine the liquid-liquid reaction between aqueous vitamin B12S and pure trans-dibromocyclohexane (DBCH) microdroplets immobilized on a basal plane pyrolytic graphite (bppg) surface, immersed in an aqueous solution of vitamin B12. Second, cyclic voltammetry on electrodes modified with microdroplets of DBCH diluted in dodecane is employed to determine the apparent bimolecular interfacial rate constant for the initial step in the DBCH(oil)/B12S(aq) reaction. The results are compared with a previous SECM/ITIES study of a similar reaction. © 2005 Elsevier B.V. All rights reserved