Environmental Protection

Historical Perspective

Biological Decontamination

The biological effects of “supersound” were first reported by Wood and Loomis in 1927 who identified two contrasting effects a stimulation or a lethal effects on unicellular organisms, tissues, small fish, and animals [1]. Two years later Harvey and Loomis examined the reduction in light emission (a factor related to bacterial kill) from a seawater suspension of rod shaped Bacillus Fisheri caused by sonication at 375kHz and 19^oC  [2]. The final sentence of the paper predicted a poor future for the commercial exploitation of sonication (which they referred to as “raying”) and this read: “In conclusion we can state that, under proper conditions of raying, luminous bacteria can be broken up and killed by sound waves of approximately 400,000 cycles per second and the solutions sterilized, but the method is not one of any practical or commercial importance because of the expense of the process”. At the time this was written the conclusion was probably valid in that ultrasonic equipment was specialist, large and expensive. Today that situation has changed, ultrasonic technology is more commonplace, capital costs have been reduced and applications are more economic. In the following year Harvey went on to suggest  that the biological effects of these sound waves may be grouped in five categories [3]:

·         Whirling of the protoplasm.

·         Displacement of small particles.

·         Cytolysis of cells.

·         Disintegration (emulsification?) of small bodies like chloroplasts.

·         Stimulation of cells.

Ultrasound has certainly been found to be effective in removing microbiological contamination although it would seem to be mainly applied in conjunction with other techniques e.g. chemical biocides, ozone, uv light etc [4]. It has also been used for the treatment of algae contamination, particularly the harmful blue-green type [5].

Chemical Decontamination

The basis for the use of ultrasound to remove chemical contamination dates back almost as far as investigations into the biological effects.  In 1929 cavitation was found to induce oxidation when iodine was liberated during sonication of aqueous potassium iodide  [6]. The oxidation was thought to arise from the formation of hydrogen peroxide and in 1964 Anbar and Pecht investigated the location of the sonochemical formation of hydrogen peroxide and found that H₂O₂ was produced in cavitation bubbles and not in the liquid phase [7].

It was in more recent years that the oxidation was found to be routed in the generation of the highly oxidizing HO^. free radical which was neatly demonstrated in 1994 in a study of the oxidative degradation of phenol in water at 514kHz  [8]. This report was significant in that it showed not only the complete destruction of phenol in water but also the production of intermediate hydroxylated benzenes e.g. catechol which themselves were further oxidized.

There is an active interest in the use of sonochemical methods in the presence of catalysts and chemical additives for treatment of organic pollutants in wastewater [9]

1.     Wood, R.W. and A. Loomis, The physical and biological effects of high frequency sound waves of great intensity. Phil. Mag., 1927. 4: p. 417.

2.     Harvey, E.N. and A.L. Loomis, The destruction of luminous bacteria by high frequency sound waves. Journal of Bacteriology, 1929. 17: p. 373-379.

3.     Harvey, E.N., Biological aspects of ultrasonic waves a general survey. Biological Bulletin, 1930. 59: p. 306 - 325.

4.     Joyce, E.M. and T.J. Mason, Sonication used as a biocide A review. Chimica Oggi - Chemistry Today, 2008. 26(6): p. 12-15.

5.     Wu, X., E.M. Joyce, and T.J. Mason, The effects of ultrasound on cyanobacteria. Harmful Algae, 2011. 10(6): p. 738-743.

6.     Schmitt, F.O., C.H. Johnson, and A.R. Olson, Oxidations promoted by ultrasonic radiation. Journal of the American Chemical Society, 1929. 51(2): p. 370 - 375.

7.     Anbar, M. and I. Pecht, On the Sonochemical Formation of Hydrogen Peroxide in Water. The Journal of Physical Chemistry, 1964. 68(2): p. 352-355.

8.     Berlan, J., et al., Oxidative degradation of phenol in aqueous media using ultrasound. Ultrasonics Sonochemistry, 1994. 1(2): p. S97-S102.

9.     Pang, Y.L., A.Z. Abdullah, and S. Bhatia, Review on sonochemical methods in the presence of catalysts and chemical additives for treatment of organic pollutants in wastewater. Desalination, 2011. 277(1-3): p. 1-14.

General Overview

Research into the use of ultrasound in environmental protection has received a considerable amount of attention with the majority of investigations focusing on the harnessing of cavitational effects for the destruction of biological or chemical pollutants in water and the processing of sewage. The field is much broader than this however and a summary of topics is given in the Table.

Air Cleaning

The inhalation of airborne particles is now recognized as a serious public health concern. Such fine particles originate in the emissions associated with carbon-fired power plants, cement factories, the chemical industry and diesel-powered vehicles have increasingly become the focus of stricter government regulations.

In recent years a new type of transducer has been developed in Madrid capable of putting high power ultrasound into the air and causing particle agglomeration and defoaming. It is shown schematically here.

Airborne ultrasound for the precipitation of smokes and powders and the destruction of foams , Riera E, Gallego-Juarez JA, Mason TJ, Ultrasonics Sonochemistry 13, 107-116 (2006)

Land Remediation

For contaminated soil wastes the currently available options for management and disposal are principally:

·         Permanent storage in a secure landfill. This will result in a permanent retained liability by the waste generator.

·         Incineration in a permitted waste incinerator. This is costly and entails the risk of atmospheric emissions.

·         Soil washing to produce bulk soil with low-level contamination. However the washing process itself will produce a volume of solvent that must be treated before disposal.

For many years ultrasound has been considered as a technology to promote the process of soil washing and if subsequent disposal of the washings was considered at all this was perhaps to be a separate treatment. Research has shown that sonication has the potential for not only cleaning soil but also to destroy organic the contaminants which are leached into the wash water.

Sonic and ultrasonic removal of chemical contaminants from soil in the laboratory and on a large scale, T.J.Mason, A. Collings, A. Sumel  Ultrasonics Sonochemistry 11,  , 205-210  (2004).

Water Remediation

Removal of biological contamination

Some species of bacteria produce colonies and spores, which agglomerate in spherical clusters (e.g. Bacillus subtilis). The use of a biocide can destroy microorganisms on the surface of such clusters but often leaves the innermost bacteria intact. Flocs of fine particles e.g. clay can entrap bacteria which can also protect them against disinfection. Due to these problems alternative methods of purifying water are being investigated and amongst these the application of ultrasound is proving to be of considerable interest. Ultrasound is able to inactivate bacteria, make them more susceptible to biocides and/or deagglomerate bacterial clusters or flocs depending upon the power and frequency applied through a number of physical, mechanical and chemical effects arising from acoustic cavitation.

Removal of Chemical contamination

The mechanical effects of cavitational collapse together with the production of radical species combine to provide the essential elements for water decontamination. The primary radicals produced during the sonication of water are OH^. and H^. and the fate of these is quite complex. The HO^. radical is extremely reactive and is capable of oxidising most chemical compounds dissolved in the water. This oxidation is mainly responsible for the degradation of organic pollutants in sonicated aqueous media. The efficient generation of HO^. is therefore an important goal in waste treatment and this is why ultrasound is often used in conjunction with AOP systems such as ozone.

Ultrasound Processes, T.J.Mason and C.Petrier, Chapter 8 in Advanced Oxidation Processes for Water and Wastewater Treatment, pp 185-208, ed S Parsons, IWA Publishing, (2004).

Oxygen-induced concurrent degradation of volatile and non-volatile aromatic compounds , Christian Pétrier, Evelyne Combet and T. J Mason, Ultrasonics Sonochemistry14, 117-121 (2007)

General effects of ultrasound in microbiology

The effect of ultrasound on biological systems and biotechnological processes depends strongly on frequency, intensity and sonication time.

Low intensity effects (i.e. under conditions which occur below the cavitation threshold) are the result of microstreaming and acoustic streaming. At these intensities, where no cavitation damage will occur, the beneficial effects are:

·         activation of enzymes in enzymatic reactions

·         improvements in microbial reactions (e.g. fermentation)

·         improvement of the bioavailability of microrganism feedstocks which are considered contaminants in environmental remediation

Higher intensity effects are the result of cavitational damage and may be summarised as follows:

·        destruction of cell walls and release of cell components into the surrounding solution (damage to cell components e.g. DNA, proteins is limited if sonication time is short)

·         extraction of organic substances from plants

·         emulsification of food (see Food section)

·         damage of cell walls and cell components at very high intensity

·         killing of microorganisms

·         improvement of the conventional bacterial decontamination (disinfection) of water

·         destruction of biological tissue e.g. tumours or kidney stones (see Therapeutic Ultrasound)

Sonication used as a biocide A review: Ultrasound a greener alternative to chemical biocides ? Eadaoin M. Joyce T. J. Mason Chimica Oggi - Chemistry Today 26 12-15 2008

The effects of ultrasound on cyanobacteria, X. Wu, E.M. Joyce and T. J. Mason, Harmful Algae 10(6) 738-743, 2011

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