S.S. Phull

The development and evaluation of ultrasound for the treatment of bacterial suspensions. A study of frequency, power and sonication time on cultured Bacillus species☆

Some species of bacteria produce colonies and spores which agglomerate in spherical clusters (Bacillus subtilis) and this serves as a protection for the organisms inside against biocidal attack. Flocs of fine particles e.g. clay can entrap bacteria which can also protect them against the biocides. It is because of problems such as these that alternative methods of disinfecting water are under active investigation. One such method is the use of power ultrasound, either alone or in combination with other methods. Ultrasound is able to inactivate bacteria and deagglomerate bacterial clusters or flocs through a number of physical, mechanical and chemical effects arising from acoustic cavitation. The aim of this study was to investigate the effect of power ultrasound at different powers and frequencies on Bacillus subtilis. Viable plate count techniques were used as a measure of microbial activity. Results showed a significant increase in percent kill for Bacillus species with increasing duration of exposure and intensity of ultrasound in the low-kilohertz range (20 and 38 kHz). Results obtained at two higher frequencies (512 and 850 kHz) indicated a significant increase in bacteria count suggesting declumping. In assessing the bacterial kill with time under different sonication regimes three types of behaviour were characterized: High power ultrasound (lower frequencies) in low volumes of bacterial suspension results in a continuous reduction in bacterial cell numbers i.e. the kill rate predominates. High power ultrasound (lower frequencies) in larger volumes results in an initial rise in cell numbers suggesting declumping of the bacteria but this initial rise then falls as the declumping finishes and the kill rate becomes more important. Low intensity ultrasound (higher frequencies) gives an initial rise in cell numbers as a result of declumping. The kill rate is low and so there is no significant subsequent decrease in bacterial cell numbers.

Potential uses of ultrasound in the biological decontamination of water.

In the past there was a prevailing feeling in industry that power ultrasound would be too expensive to use for water treatment on an industrial scale. This was based on calculations involving the direct scale up of power consumption in small-scale (generally batch) laboratory experiments. In recent times this attitude has changed somewhat as a result of the installation of a number of ultrasonic devices in operational water or sewage treatment plants. In our laboratories we have investigated the decontamination of water under the influence of ultrasound alone and in conjunction with other treatments. The results, particularly when applied to flowing systems, indicate a real future for sonochemistry in water treatment.

Degradation of dye effluent

Solutions of both basic and acidic dyes were subject to sonolysis, electrolysis, and sonoelectrolysis. Only basic dyes were decolorized by ultrasound alone. Removal of the acidic dye Sandolan Yellow required the use of an electrooxidation process. The rate of electrochemical decolorization in the absence of ultrasound was dependent on the type of electrolyte, the electrolyte concentration, the reaction temperature, and the current density. The sonoelectrooxidation of Sandalon Yellow needed to be performed in a sealed cell to minimize the effects of ultrasonic degassing.

Dye effluent decolourisation using ultrasonically assisted electro-oxidation.

Solutions of the acidic dye, Sandolan Yellow, were subjected to sonolysis, electrolysis and sonoelectrolysis. Decolourisation did not take place using ultrasound alone but was achieved using an electro-oxidation process. The rate of electro-chemical decolourisation in the absence of ultrasound was dependent on the type of electrode used, electrolyte concentration, reaction temperature and the current density. Electro-oxidation of Sandolan Yellow using platinum electrodes was enhanced using ultrasound when carried out in a semi-sealed cell, which minimised the effects of ultrasonic degassing.

The development and evaluation of electrolysis in conjunction with power ultrasound for the disinfection of bacterial suspensions

There is an increasing incidence in health problems related to environmental issues that originate from inadequate treatment of potable waters. This has compelled scientists and engineers to engage in innovative technologies to achieve a maximum disinfection at affordable costs. Some species of bacteria produce colonies and spores that can agglomerate in spherical clusters and thus protect organisms on the inside of the cluster against biocidal attack. Flocs of fine particles (e.g., clay) can entrap bacteria and this can also protect them against the biocides. Other bacteria have the ability to mutate, thus building up resistance to conventional biocides (e.g., chlorine). Ultrasound has been shown to be effective in improving the effectiveness of biocides such as chlorine. The aim of this present study was to investigate the effect of electrolysis and power ultrasound as a disinfection treatment and to provide a greater knowledge of the fundamentals of disinfection through the production of hypochlorite in situ from saline solution via electrolysis. The electrode materials investigated were, carbon (felt and graphite), copper and stainless steel rods. The results show that sonication appears to amplify the effect of electrolysis. A combination of both treatments is significantly better than sonication or electrolysis alone.

Sonovoltammetry at platinum electrodes: surface phenomena and mass transport processes

Application of simultaneous ultrasound to representative solution-phase reversible voltammetric couples produce a step-shaped voltammogram at platinum electrodes of both ‘macro’ and ‘micro’ dimensions. The limiting current increases with ultrasonic power, but is not markedly affected by ultrasonic frequency in the 20–800kHz region. In contrast the complex voltammetry of a platinized platinum electrode surface within the hydrogen adsorption regime in aqueous acid medium is very little affected by sonication. Factors affecting the reproducibility of sonoelectrochemical experiments when employing ultrasonic sources are discussed.

The effect of ultrasonic frequency and intensity upon limiting currents at rotating disc and stationary electrodes

Abstract The limiting current from Fe(CN) 6 3− Fe(CN) 6 4− redox in dilute aqueous KCl at a platinum electrode appears to be little affected by the frequency of simultaneous ultrasonic irradiation in the range 20–800 kHz, and is not influenced by choice of bath or probe as sonic source, provided measurements are made at constant ultrasonic intensity. The limiting current is dependent on ultrasonic intensity at constant frequency. In contrast, the peak potential of the redox couple varies with ultrasonic frequency in the same range, but does not vary with ultrasonic intensity at constant frequency. Application of ultrasound to a rotating disc electrode provides an increase in limiting current across the frequency range, the magnitude of which varies with both concentration and rotation speed. The average maximum increase is some 1.5-fold which corresponds to an effective rotation rate of 100 000 rpm (1667Hz) for a silent solution in order to achieve the same transport rates. It is shown that changes in macroscopic temperature throughout the experiment are insufficient to cause the observed enhanced diffusion.

The effect upon limiting currents and potentials of coupling a rotating disc and cylindrical electrode with ultrasound

The reduction of the silver cation, Ag+, was studied in dilute aqueous Na2S2O3/NaHSO3 solution at a platinum and stainless steel rotating electrode in the absence and presence of ultrasound. Under silent conditions, electrode potential measurements indicate that silver deposition is the main reaction occurring at the cathode during the electrolysis of a model photographic processing solution. Under insonation conditions at 20 kHz the reduction wave shifts anodically with increasing ultrasonic power. Similarly the discharge of hydrogen shifted anodically with increasing power. It is also shown that below 1000 r.p.m. insonation leads to improved limiting currents when compared to the silent system. As the rotation speed of the electrode increases above 1000 r.p.m., the limiting current decreases as the ultrasonic power increases.

The effect of ultrasound upon the oxidation of thiosulphate on stainless steel and platinum electrodes

Ultrasound was found to increase the oxidation peak current and hence the decomposition rate of thiosulphate 50-fold compared to silent conditions. The effects of the ultrasonic frequency (20 and 38 kHz) and power upon the electrochemical oxidation of thiosulphate in aqueous KCl (1 mol dm-3) at stationary stainless steel and platinum electrodes were studied chronoamperometrically and potentiostatically (at various scan rates). No sigmoidal-shaped voltammograms were observed for the redox couple S4O6(2-)/S2O3(2-) in the presence of ultrasound. However, application of ultrasound to this redox couple provided an increase in the oxidation peak current at the frequencies employed, the magnitude of which varied with concentration, scan rate and ultrasonic power. Under sonication at 20 and 38 kHz, the oxidation peak potential shifted anodically with increasing ultrasonic power. This anodic shift in potential may be due to the formation of hydroxyl radicals, changes in electrode surface composition and complex adsorption phenomena. The large increase in oxidation peak currents and the rates of decomposition of thiosulphate, in the presence of ultrasound, are explained in terms of enhanced mass transfer at the electrode due to cavitation and acoustic streaming together with microstreaming coupled with adsorption phenomena. It is also shown that changes in macroscopic temperature throughout the experiment are insufficient to cause the observed enhanced diffusion.

Sonoelectrochemical effects in electro-organic systems.

This paper describes recent studies in organic sonoelectrochemistry at Coventry University, including the oxidation of thiophene monoxides, degradation of dye pollutants, formation of conducting polymers and electrosynthetic modification of proteins.