Electrode-solution Interface Research Articles

Contribution of Electrochemistry to the Biomedical and Pharmaceutical Analytical Sciences

All analytical techniques have experienced major progress since the last ten years and electroanalysis is also involved in this trend. The unique characteristics of phenomena occurring at the electrode-solution interface along with the variety of electrochemical methods currently available allow for a broad spectrum of applications. Potentiometric, conductometric, voltammetric and amperometric methods are briefly reviewed with a critical view in terms of performance of the developed instrumentation with special emphasis on pharmaceutical and biomedical applications.

Flat-Band Potentials of Molecularly Thin Metal Oxide Nanosheets.

Exfoliated nanosheets derived from Dion-Jacobson phase layer perovskites (TBAxH1-xA2B3O10, A = Sr, Ca, B = Nb, Ta) were grown layer-by-layer on fluorine-doped tin oxide and gold electrode surfaces. Electrochemical impedance spectra (EIS) of the five-layer nanosheet films in contact with aqueous electrolyte solutions were analyzed by the Mott-Schottky method to obtain flat-band potentials (VFB) of the oxide semiconductors as a function of pH. Despite capacitive contributions from the electrode-solution interface, reliable values could be obtained from capacitance measurements over a limited potential range near VFB. The measured values of VFB shifted -59 mV/pH over the pH range of 4-8 and were in close agreement with the empirical correlation between conduction band-edge potentials and optical band gaps proposed by Matsumoto ( J. Solid State Chem. 1996, 126 (2), 227-234 ). Density functional theory calculations showed that A-site substitution influenced band energies by modulating the strength of A-O bonding, and that subsitution of Ta for Nb on B-sites resulted in a negative shift of the conduction band-edge potential.

Square wave anodic stripping voltammetry of copper(II) at a MWCNT paste electrode modified with a tetracarbonylmolybdenum(0) nanocomposite

The article reports on a MWCNT paste electrode modified with the fenchone diazine tetracarbonylmolybdenum(0) complex, and on its application to sensitive determination of Cu(II) by using square wave anodic stripping voltammetry. Under optimal conditions and at a typical working voltage of 33 mV (vs. Ag/AgCl), response is linearly related to the concentration of Cu(II) in the 0.1 nM to 1.0 μM range, and the limit of detection is 80 pM. The relative standard deviation [for n = 5 and at a 1.0 μM concentration of Cu(II)] is 3.6 %. Unlike Pb(II), the ions Ni(II), Mg(II), Cd(II), Co(II), Hg(II), and Zn(II) do not strongly interfere. Electrochemical impedance spectroscopy reveals a fast charge transfer at the electrode-solution interface. The method was applied to the analysis of Cu(II) in water samples, and the results agreed well with those obtained by ICP-OES.

π-Conjugated Microporous Polymer Films: Designed Synthesis, Conducting Properties, and Photoenergy Conversions.

Conjugated microporous polymers are a unique class of polymers that combine extended π-conjugation with inherent porosity. However, these polymers are synthesized through solution-phase reactions to yield insoluble and unprocessable solids, which preclude not only the evaluation of their conducting properties but also the fabrication of thin films for device implementation. Here, we report a strategy for the synthesis of thin films of π-conjugated microporous polymers by designing thiophene-based electropolymerization at the solution–electrode interface. High-quality films are prepared on a large area of various electrodes, the film thickness is controllable, and the films are used for device fabrication. These films are outstanding hole conductors and, upon incorporation of fullerenes into the pores, function as highly efficient photoactive layers for energy conversions. Our film strategy may boost the applications in photocatalysis, energy storage, and optoelectronics.

π‐Conjugated Microporous Polymer Films: Designed Synthesis, Conducting Properties, and Photoenergy Conversions

AbstractConjugated microporous polymers are a unique class of polymers that combine extended π‐conjugation with inherent porosity. However, these polymers are synthesized through solution‐phase reactions to yield insoluble and unprocessable solids, which preclude not only the evaluation of their conducting properties but also the fabrication of thin films for device implementation. Here, we report a strategy for the synthesis of thin films of π‐conjugated microporous polymers by designing thiophene‐based electropolymerization at the solution–electrode interface. High‐quality films are prepared on a large area of various electrodes, the film thickness is controllable, and the films are used for device fabrication. These films are outstanding hole conductors and, upon incorporation of fullerenes into the pores, function as highly efficient photoactive layers for energy conversions. Our film strategy may boost the applications in photocatalysis, energy storage, and optoelectronics.

Carbon Nanotubes Labeled with Aptamer and Horseradish Peroxidase as a Probe for Highly Sensitive Protein Biosensing by Postelectropolymerization of Insoluble Precipitates on Electrodes.

Carbon nanotubes (CNTs) labeled with aptamer and horseradish peroxidase (HRP) were used as a probe to amplify the impedimetric sensing of the aptamer-protein (with thrombin as the model) interaction. The HRP-biocatalyzed oxidation of 3,3-diaminobenzidine (DAB) in the presence of H2O2 and the postelectropolymerization of insoluble precipitates produced on the electrode supports were used as a signal amplification route for the sensing process. Thrombin was sensed by aptamer 1 immobilized on a glassy carbon electrode. The multiwalled CNT-aptamer 2-HRP probe was linked to the aptamer 1-thrombin complex through the thrombin-aptamer 2 interaction. The postelectropolymerization of biocatalyzed precipitates of DAB on the electrode greatly increased the electron-transfer resistance at the electrode-solution interface. Cyclic voltammetry and electrochemical impedance spectroscopy were employed to follow the stepwise fabrication of the aptasensor and impedimetric detection of thrombin. Thrombin concentration as low as 0.05 pM could be detected by this method. In addition, the proposed impedimetric aptasensor exhibits good sensitivity (5195 Ω decade(-1)), selectivity, and reproducibility. The aptasensor also has acceptable recovery for thrombin detection in complex protein sample.

Magnetic Fields That Enhance the Rates of Multistep Reactions Important in Energy Storage and Conversion

Reactions at the electrode solution interface are ubiquitous and integral in electrochemical generation and storage systems. As the number of steps in the electron transfer process increases, the probability of reduce energy efficiency increases through undesirable side reactions and slow steps. When the electron transfer step slows the overall reaction rate, electrocatalysts can increase net efficiency. Incorporation of magnetic microparticles to the electrode has been shown to increase the efficiency of various electrochemical energy systems that include fuel cells, batteries, photovoltaics, and supercapacitors. It is generally observed that energy and power are increased by 40 % on incorporation of magnetic microparticles. Every interfacial electron transfer process is composed of multiple steps. The simplest model is mass transport of the reactant to the electrode surface, the electron transfer, and mass transport of the product into the solution. In multistep processes, additional steps may be important such as adsorption, multiple electron transfers, and chemical reaction on the surface or in the bulk. The electron transfer step is usually envisioned as a simple charge transfer. However, the electron transfer includes transfer of charge and spin. Magnetic properties of the electron and molecular entities arise through the spin. Addition of magnetic microparticles alters the rates of interfacial electron transfers. Here, three examples of magnetoelectrocatalysis in reactions important to electrochemical generation and storage are presented. In each of these systems, chemically inert micromagnets are held to the electrode surface in a coating of Nafion (R). In these reactions, there are components of mass transport, adsorption, interfacial spin and charge transfer, and surface reactions. The hydrogen evolution reaction (HER) is facilitated at magnetically modified, single crystal p-Si. Overpotential is reduced >500 mV on magnetic modification.Hydride storage in palladium is improved as the interfacial rates of H∙ transfer are increased by several orders of magnitude and overpotentials reduced by several hundred millivolts.CO oxidation on platinum occurs near the thermodynamic potential at room temperature in water. The overpotential is reduced 600 mV with micromagnets. In each of these technologically important reactions, magnetic microparticles increase the rates of inherently slow, interfacial reactions. The effect is a physical interaction wherein magnetoelectrocatalysis effects substantially increased rates of electron transfer.

Mediated Electron Transfer at Vertically Aligned Single-Walled Carbon Nanotube Electrodes During Detection of DNA Hybridization.

Vertically aligned single-walled carbon nanotube (VASWCNT) assemblies are generated on cysteamine and 2-mercaptoethanol (2-ME)-functionalized gold surfaces through amide bond formation between carboxylic groups generated at the end of acid-shortened single-walled carbon nanotubes (SWCNTs) and amine groups present on the gold surfaces. Atomic force microscopy (AFM) imaging confirms the vertical alignment mode of SWCNT attachment through significant changes in surface roughness compared to bare gold surfaces and the lack of any horizontally aligned SWCNTs present. These SWCNT assemblies are further modified with an amine-terminated single-stranded probe-DNA. Subsequent hybridization of the surface-bound probe-DNA in the presence of complementary strands in solution is followed using impedance measurements in the presence of Fe(CN)63−/4− as the redox probe in solution, which show changes in the interfacial electrochemical properties, specifically the charge-transfer resistance, due to hybridization. In addition, hybridization of the probe-DNA is also compared when it is attached directly to the gold surfaces without any intermediary SWCNTs. Contrary to our expectations, impedance measurements show a decrease in charge-transfer resistance with time due to hybridization with 300 nM complementary DNA in solution with the probe-DNA attached to SWCNTs. In contrast, an increase in charge-transfer resistance is observed with time during hybridization when the probe-DNA is attached directly to the gold surfaces. The decrease in charge-transfer resistance during hybridization in the presence of VASWCNTs indicates an enhancement in the electron transfer process of the redox probe at the VASWCNT-modified electrode. The results suggest that VASWCNTs are acting as mediators of electron transfer, which facilitate the charge transfer of the redox probe at the electrode–solution interface.

On the modeling of electrochemical phenomena at the electrode-solution interface in a PEF treatment chamber: Methodological approach to describe the phenomenon of metal release

In pulsed electric field (PEF) processing of liquid foods, unavoidable electrochemical reactions occur, including those involving metal release from the electrodes, which may seriously affect food safety and quality as well as the electrode’s lifetime.The aim of this paper was to set up a mathematical model describing the phenomenon of the metal release from stainless steel (type AISI 316L) electrodes of a continuous flow parallel plate PEF treatment chamber into model liquid foods. The effect of PEF processing variables such as electric field strength, total specific energy input and treatment medium composition on the dynamics of the spatial distribution of the main metallic elements (Fe, Cr and Ni) released from the electrodes in the treatment volume as well as their concentration in the bulk of the product exiting the treatment chamber were simulated. All simulations were performed using a FEM based commercial software (COMSOL Multiphysics™). A validation of the predicted results was carried out and it was demonstrated that the model set up accurately describes phenomenon of metal release at the electrode solution interface.

On the modelling of the electrochemical phenomena at the electrode-solution interface of a PEF treatment chamber: Effect of electrical parameters and chemical composition of model liquid food

In this work, a Multiphysics model based on the Finite Element Method (FEM), was used to study the effect of different electrical parameters as well as chemical composition of the treatment medium on the metal release from the stainless steel (type AISI 316L) electrodes of a PEF chamber. A total of 15 processing conditions obtained by coupling different field strength (12–31kV/cm) and total specific energy input (20–100kJ/kg) were simulated with two model liquid food solutions with the same pH (7) and electrical conductivity (2mS/cm at 25°C) but different halides concentration.The results showed that the concentration of metallic elements (Fe, Cr and Ni) increased with decreasing the field strength applied or increasing the total specific energy input. These results were attributed to the predominant effect of the pulse frequency, rather than the applied voltage, on the average Faradaic current density values which, in turn, determine the extent of the metal release. Moreover, it was shown that the presence of halides in the composition of the processed product, may lower the breakdown potential and increase the Faradaic current density value which lead to an intense localized metal release from the electrode’s surface.

Oxygen Reduction Reaction Kinetics at Elevated Temperatures and Pressures

Fundamental understanding of the oxygen reduction reaction (ORR) in aqueous medium at temperatures between 100 and 200 °C is challenging due to practical limitations related to the harsh experimental conditions. By using a high-temperature, pressurized electrochemical cell, we obtain a 150-fold improvement in the kinetics of the electrocatalytic reduction of O2 on Pt relative to room temperature and pressure under an O2 pressure of 3.4 MPa at 200 °C in basic solution. Figure 1 demonstrates that the ORR kinetics is a function of both temperature and pressure. To deconvolute their combined effect, we examine the underlying variables that dictate the observed ORR kinetics individually. In the Temkin region, we observe temperature-insensitive Tafel slopes, suggesting that the transfer coefficient (α) is temperature-dependent. Another factor contributing to this temperature dependence is a change in the surface morphology of Pt at 200 °C, likely reflecting instability of Pt at elevated temperatures during ORR. O2 availability at the electrode-solution interface is controlled by the interplay between the diffusion coefficient and the concentration of O2 in bulk solution. ORR kinetics measured on a glassy carbon (GC) electrode yields similar temperature-dependent behavior of the Tafel slopes and α values. GC electrode reduces O2 via a 2 e– process even at elevated temperatures. We conclude that correctly accounting for the many temperature- and pressure-dependent variables is necessary to accurately describe the enhanced ORR kinetics observed at intermediate temperatures under pressurized conditions. Acknowledgements. E.C.M.T. acknowledges a Croucher Foundation Scholarship. We thank the US Department of Energy (DE-FG02-95ER46260) for support of this research. This work was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, which are partially supported by the US Department of Energy (DE-FG02-07ER46453 and DE-FG02-07ER46471). Figure 1

Effect of temperature and pressure on the kinetics of the oxygen reduction reaction.

Fundamental understanding of the oxygen reduction reaction in aqueous medium at temperatures above 100 °C is lacking due to the practical limitations related to the harsh experimental conditions. In this work, the challenge to suppress water from boiling was overcome by conducting the electrochemical investigation under pressurized conditions. A striking improvement in the kinetics of the electrocatalytic reduction of O2 by about 150 fold relative to room temperature and pressure was recorded under an O2 pressure of 3.4 MPa at 200 °C in basic aqueous environment. To deconvolute the combined effect of temperature and pressure, the underlying variables that dictate the observed O2 reduction kinetics of Pt and carbon electrodes were examined individually. O2 availability at the electrode-solution interface was controlled by the interplay between the diffusion coefficient and concentration of O2. Accurate knowledge of the temperature and pressure dependence of O2 availability at the electrode surface, the Tafel slope, the transfer coefficient, and the electrochemical active surface area was required to correctly account for the enhanced O2 reduction kinetics.

Mechanism and Experiment of Planar Electrode Sensors in Water Pollutant Measurement

This paper investigates the detecting mechanism of planar electrode sensor in water pollutant detection. A system composed of electrodes, bulk media, and electrode-solution interface is built up, and the corresponding equivalent circuit containing bulk and interfacial impedances is established. The influence of each part is discussed while the impedance at interface is especially focused on. The impedance spectroscopy of the circuit is divided into three parts by the cutoff frequencies caused by interfacial and bulk impedances, and each of the frequency range can be used for the measurement of different physical quantities in water. The effect of electrode, especially the combination of interdigital and coils, is analyzed, and nine different sensors were designed and fabricated. Experiments validate the accuracy and validity of the model, and the sensor with best sensitivity is found. The stability of the planar sensor is proven in the experiment too.

Interaction study of paraquat and silver electrode using electrochemical impedance spectroscopy: Application in milk and tomato samples

Abstract This paper reports on the use of electrochemical impedance spectroscopy (EIS) for analytical determination of paraquat herbicide at a silver electrode. The electrochemical technique was applied to aqueous solution, at pH 7.0, containing different concentrations of paraquat. The impedance measurements were carried out in the frequency range from 100 mHz to 100 KHz at a potential of 0.0 mV versus Ag/AgCl. The Nyquist plots were modeled with a Randle equivalent circuit, by identifying the charge transfer resistance as the relevant concentration dependent parameter. The silver surface adsorbs paraquat and the transfer process is controlled by diffusion. The response of the working electrode was attributed to the electron-exchange mechanism at the electrode–contact interface and the ion exchange at the electrode–solution interface. The proposed methodology offers good repeatability (RSD = 1.03%) with (n = 8) and impedimetric response to paraquat in the range from 1.0 × 10−14 to 8.0 × 10−4 mol L−1. The detection limit (DL, 3σ) and quantification limit (QL, 10σ) were 7.37 × 10−16 mol L−1 and 24.59 × 10−16 mol L−1 respectively. The method was satisfactorily applied to determine paraquat in milk and tomato samples.

Study of the Substrate Effect in the KCl Adsorption onto Single-Crystal and Polycrystalline Gold Electrodes

Electron transfer kinetics in electrochemical systems is ubiquitous to natural and industrial processes. Thus a deep knowledge of all stages involved in the charge transfer at the electrode solution interface, which includes mass transport, adsorption and the role of the electrical double layer is highly desirable [1]. In particular, adsorption and the electrical double layer play a distinctive role in the charge transfer at electrode/solution interfaces, since they are essential part of reaction mechanisms, especially for reactions where more than one electron is transferred. Fueled by the importance of adsorption and the electric double-layer in the electrode kinetics, there has been published in the literature several works, but mainly focused on thermodynamic rather than kinetics [2-4].Here a qualitative study of the effect of the substrate and surface heterogeneities in the adsorption of chloride ions on gold surfaces is presented. With this goal in mind, we have performed a study of the adsorption of chloride ions at different concentrations (1 mM, 10mM 0.1M) disolved in HClO40.5 M on the surfaces of polycrystalline (pc) and gold (111) single crystal. Our results include the characterization of the system by using classic electrochemical techniques, such as cyclic voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) and the novel intermodulated techniqued called Interfacial Capacitance Modulation (MIC) [5]. Figure 1 shows a comparison between the typical voltammetric responses as well as the double-layer capacitance vs. polarization behavior for polycristalline and (111) gold electrodes in contact with HClO4in absence and presence of KCl, respectively. After this characterization, EIS and MIC measurements are carried out. All our results are discussed qualitatively based on the relaxation of the double layer capacitance as a function of chloride ion concentration on the electrode solution interface.REFERENCES:[1] F.C. Anson, Acc. Chem. Res. 8 (1975) 400.[2] D.C. Grahame, Chem. Rev. 41 (1947) 441.[3] J. Lipkowski, W. Schmickler, D.M. Kolb, R. Parsons, J. Electroanal. Chem. 452 399, (1998) 193.[4] T. Pajkossy, D.M. Kolb, Electrochim. Acta 54 (2009) 3594.[5] E.R. Larios-Durán, R. Antaño-López, M. Keddam, Y. Meas, H. Takenouti, V. Vivier. Electrochim. Acta 55, (2010) 6292.

Electrocatalytic determination of captopril in real samples using NiO nanoparticle modified (9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboximido)-4-ethylbenzene-1,2-diol carbon paste electrode

Synthesis and application of NiO nanoparticles (NiO/NPs) and (9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboximido)-4-ethylbenzene-1,2-diol (DED) as high sensitive sensors for voltammetric determination of captopril (CAP) using carbon paste electrode has been reported in this work. The synthesized nanoparticles were characterized by different methods such as TEM and XRD. The electrochemical oxidation of CAP on the new NiO/NPs (9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboximido)-4-ethylbenzene-1,2-diol carbon paste electrode (NiO/NPs/DED/CPE) has been carefully studied. Under the best experimental conditions selected, the calibration curve for CAP was linear in the concentration range from 0.035 to 550μmolL−1 and a detection limit of 0.007μmolL−1 was obtained. The influence of pH and potential interfering substances on the determination of CAP were studied. Electrochemical impedance spectroscopy (EIS) was used to study the charge transfer properties at the electrode–solution interface. Finally, the sensor was examined as a selective, simple, and precise new electrochemical sensor for the determination of CAP in real samples, such as drugs and urine.

Quantification of metal release from stainless steel electrodes during conventional and pulsed ohmic heating

Abstract Electrochemical reactions at the electrode-solution interface of an ohmic heater can be avoided or significantly limited by choosing appropriate processing conditions in relation to the food properties. In the present work the effect of the electrical parameters (electric field strength and frequency of the applied current signal) and product factors (halides concentration, electrical conductivity and pH) on metal release from stainless steel (type AISI 316 L) electrodes of a batch ohmic heater was investigated. In each experiment, the concentrations of the main constituents of stainless steel (iron, chromium and nickel) released in the heating medium were detected by Inductively Coupled Plasma Mass Spectrometry (ICP-MS) and Atomic Absorption Spectrophotometry (AAS). Results showed that the rate of metal release from the electrodes to the heating medium depends on frequency and applied field strength. However, the use of ohmic heating at a higher frequency than conventional (50 Hz) can significantly (p ≤ 0.05) reduce the flux of metal ions from stainless steel electrodes. Moreover, it was also demonstrated that electrochemical phenomena occurring at the electrode-solution interface strongly depend on the composition, pH and electrical conductivity of the heating medium. Industrial relevance The magnitude of electrode material released into the heating medium during ohmic processing depends on many factors, whose effects should be known in order to define optimal treatment conditions, electrode material and food properties able to avoid or minimize the undesired phenomenon of contamination of the food product, electrode-fouling and electrode corrosion. This paper contributes to clarifying the effects of electric field strength applied as well as electrical conductivity, pH, and presence of halides in the heating medium on electrode corrosion or release of electrode materials during high frequency (25 kHz, bipolar square wave) pulsed power OH in comparison with conventional (50 Hz sine wave) OH. Interestingly, the use of sufficiently large frequencies may avoid or reduce the extent of electrochemical reactions at the electrode interface, minimizing corrosion and leakage of metals to the heating medium, even when electrode material of low cost and electrochemically active like stainless steel is used.

Metal release from stainless steel electrodes of a PEF treatment chamber: Effects of electrical parameters and food composition

Abstract The effects of electrical parameters (field strength E, total specific energy input W T and pulse frequency) and product composition on the release of the main metallic elements (Fe, Cr, Ni and Mn) of stainless steel (type 316L) electrodes of a continuous flow parallel plate PEF chamber into the treatment medium were investigated. Experiments were carried out by subjecting two different buffer solutions (McIlvaine and Trizma-HCl) with the same values of pH (7) and electrical conductivity (σ = 2 mS/cm) to PEF treatments (mono-polar exponential decay pulses, lasting 3.1 μs) at different intensities (E = 12–21–31 kV/cm, W T = 20–60–100 J/mL) and flow rates (2–3–4 L/h). The results showed that, for each field strength applied, the concentration of metallic elements increased upon increasing the total specific energy input. At constant total energy input, it was noticed that the metal concentration decreased upon increasing the field strength applied. These results were mainly attributed to the key role played by the pulse frequency in the charging process of the double layer capacitors at the electrode–solution interface. Moreover, it was shown that the amount of metal released from the electrodes markedly depended on the presence of halides in the composition of the processed product. Industrial relevance Electrochemical reactions at the electrode solution interfaces are unavoidable when typical conditions for PEF processing are applied. For the acceptability of PEF processing as a non-thermal method for liquid food pasteurization, the occurrence of these reactions should be avoided or minimized since they may determine undesired phenomenon of contamination of the food product, electrode-fouling and electrode corrosion. In this paper, electrode corrosion was studied, for the first time, in a continuous flow parallel plate PEF treatment chamber. The present investigation contributes to clarifying the effects of some electrical parameters and food composition on electrode corrosion or release of electrode materials under real PEF treatment conditions.

Applications of Ellipsometry in the Investigations of Electrode-Solution Interface

Applications of Ellipsometry in the Investigations of Electrode-Solution Interface

A microbial fuel cell driven capacitive deionization technology for removal of low level dissolved ions

The microbial fuel cell (MFC) is an emerging technology, which uses exoelectrogenic microorganisms to oxidize organic matter in the wastewater to produce electricity. However, the low energy output limits its application in practice. Capacitive deionization (CDI), an electrochemically controlled method for deionization by the adsorption of ions in the electrical double layer region at an electrode–solution interface, requires a low external power supply. Therefore, in this study, we investigated the MFC driven CDI (MFC–CDI) technology to integrate deionization with wastewater treatment and electricity production. Taking advantage of the low potential requirement of CDI, voltage generated from a continuous flow MFC could be used to drive the CDI to achieve removal of the electrolyte to a stable status. The results indicated that among the three connection types of MFCs including single-, series-, and parallel-configuration, the parallel connection of two MFCs resulted in the highest potential (0.63V) applied to CDI and the conductivity removal of NaCl solution was more than 60%. The electrosorption capacities under different electrolyte concentrations of 50, 100 and 150mgL−1 were 150, 346 and 295μgg−1, respectively. These results suggest that the new MFC–CDI technology, which utilizes energy recovery from the wastewater, has great potential to be an energy saving technology to remove low level dissolved ions from aqueous solutions for the water and wastewater treatment processes.