Sonochemical environmental remediation ... in environmental remediation biological .... methodology was found to decrease the heat resistance of Staphylococcus aureus .... filtration (acoustic filtration) and in conjunction with an electric field.
Sonochemical environmental remediation T.J.MASON School of Natural and Environmental Sciences Coventry University Coventry CV1 5FB U.K.
1. Introduction At the time of writing this is one of the most rapidly expanding areas of research in sonochemistry with the majority of investigations focusing on the harnessing of cavitational effects for the destruction of chemical pollutants. The field is, of course, much broader than this and in this brief review we will explore several aspects in addition to chemical decontamination (Table 1). Table 1: The uses of power ultrasound in environmental remediation biological decontamination chemical decontamination acoustic deliquoring ultrasonic dispersion
1.
Biological remediation.
There are essentially three main areas of interest in this domain; surface disinfection, potable water purification and food sterilisation. 1.1
SURFACE DECONTAMINATION
The use of power ultrasound for surface cleaning, is a long-established and efficient technology. Ultrasound is particularly effective in this type of decontamination because the cleaning action is induced by cavitational collapse on and near surfaces which will dislodge bacteria adhering to them. The particular advantage of ultrasonic cleaning in this context is that it can reach crevices that are not easily reached by conventional cleaning methods. Objects for cleaning can range from large crates used for food packaging and transportation to delicate
surgical implements such as endoscopes. One recent example of surface cleaning in food technology is the use of a combination of a bactericide and ultrasound to decontaminate chicken eggs in hatcheries1. In hatcheries the eggs are normally laid within the nest and relatively clear of contamination but a number are laid (or dislodged to) outside of the nest (some 10%) and generally have contaminated surfaces. These dirty eggs are removed from the hatchery because egg shells are porous and the contamination can penetrate to the inside and kill the developing embryo. The result of this is that these eggs "explode" after about 10 days due to the build-up of gas inside due to decomposition. Ultrasonically assisted cleaning has proved to be more efficient than the currently used spraying or gassing techniques for the decontamination of "clean" eggs but, more significantly, ultrasonic decontamination of the "dirty" eggs is so good that they can also be used in the hatchery. 1.2
DISINFECTION OF LIQUIDS
Power ultrasound is known to disrupt biological cell walls and thereby destroy bacteria however for complete disinfection of water very high acoustic intensities are required. Conventional methods of disinfection involve the use of a bactericide e.g. chlorine or ozone in the water industry. Despite the fact that chlorine has proved to be successful in combating a range of water borne diseases there are problems associated with using it, these include: - Micro-organisms (especially bacteria) are capable of producing strains that are tolerant to normal chlorine treatment levels. This can be overcome by increasing the chlorine dose, however, this can lead to the generation of unpleasant flavours and odours due to the formation of chlorophenols and other halocarbons through reaction with chemical contaminants in the water. - Certain species of micro-organisms produce colonies and spores which agglomerate in spherical or large clusters. Chlorination of such clusters may destroy micro-organisms on the surface leaving the innermost organisms intact. - Fine particles such as clays are normally removed by flocculation using chemicals such as aluminium sulphate. The flocs can entrap bacteria and spores and although the vast majority of floc particles are removed during processing it is possible that one or two may pass through the system and the bacteria protected by the floc material may well be unaffected by further disinfection. Current trends are towards the reduction in use of chlorine as a disinfectant either by replacement with other biocides or by a reduction in the concentration required for treatment. Low power ultrasound offers the latter possibility since is capable of enhancing the effects of chemical biocides. The
effect is thought to be due in part to the break-up and dispersion of bacterial clumps and flocs which renders the individual bacteria more susceptible to chemical attack. In addition cavitation induced damage to bacterial cell walls will allow easier penetration of the biocide. The results of a study of the combined effect of low power ultrasound and chlorination on the bacterial population of raw stream water is shown in Table 2. Neither chlorination alone nor sonication alone was able to completely destroy the bacteria present. It is significant however that extending the time of chlorination or sonication from 5 minutes to 20 minutes seems to double the biocidal effect of the individual techniques. When sonication is combined with chlorination however the biocidal action is significantly improved. These results suggest that ultrasound could be used in conjunction with chemical treatments to achieve a reduction in the quantity of bactericide required for water treatment 2. Table 2: The effect of ultrasound and chlorination on bacterial growth treatment
% bacteria kill after 5 minutes
% bacteria kill after 20 minutes
no treatment
0
0
chlorine 1 ppm
43
86
ultrasound alone
19
49
ultrasound + chlorine
86
100
Conditions: 1:10 dilution of raw stream water, using ultrasonic bath (power input to system = 0.6 W cm-2), T = 20C.
The traditional method of sterilising foodstuffs is thermal and some efforts have been made to improve such treatment with the concurrent use of ultrasonic irradiation. This process has been named thermosonication and has been shown to be effective against a range of biopollutants e.g. Bacillus subtilis spores3. This treatment provides a substantial reduction in treatment time using a small scale reactor operating at 20kHz and 150W. It can be envisaged that process times and/or temperatures could be reduced to achieve the same lethality values. A modified treatment process for food sterilisation involving a combination of heat and sonication under pressure has also been reported4. This methodology was found to decrease the heat resistance of Staphylococcus aureus by 63% and Bacillus subtilis by 43% as compared to heat treatment alone. Under pressure the boiling point is raised and lethality of the microbes above boiling point is maintained with values 5 to 30 times greater than those achieved with heat treatment alone. Spores appeared to be most resistant and yeasts the most susceptible to this type of treatment.
2.
Chemical Remediation
2.1
DECONTAMINATION OF WATER
Ultrasonic irradiation of aqueous solutions results in the formation of free radicals due to the homogeneous sonolysis of water, if the pollutants enter the bubble they may also suffer decomposition. A study of sonochemical treatment of chlorinated hydrocarbons in water demonstrated the homogeneous destruction of CH 2Cl2, CCl4, MeCCl3, and ClCH=CCl2 in solution at concentrations of 100-1000 ppm by volume5. The method appears to be potentially quite powerful for the purification of contaminated water. In a separate study when saturated aqueous solutions of CH2Cl4 (110 ppm) and MeCCl3 (1300 ppm) were sonicated for 20 minutes some 75% of the contaminant was degraded6. The sonochemical removal of chlorinated aromatic compounds from water is attracting considerable attention. The degradation of the aromatic species is generally initiated by an attack on the aromatic ring by HO. radicals generated through the sonochemical break-down of water itself. Using phenol as a model substrate it has been shown that degradation takes place via sequential oxidation and the intermediate formation of hydroquinone and catechol7. The final products of the process using ultrasound at 541kHz are low molecular weight carboxylic acids. The process for the degradation of phenol (100mgl-1) requires long reaction times of from 1 to three hours depending on the entrained gas used. Table 3: Sonophotochemical dechlorination of aqueous pentachlorophenol conditions
Cl- yield (50 mins)
Cl- yield (120 mins)
uv
40%
no change
uv/ultrasound
60%
100%
Pentachlorophenol (2.4x10-4M) in water containing 0.2% TiO2
There are a few reports on the combined application of ultrasound and ultraviolet light for the destruction of chemical pollutants. A great potential exists for synergistic improvements in technology e.g. the removal of 1,1,1trichloroethane from aqueous solutions using the combined application of ultrasound and light is more efficient than the application of either technique individually8. In the presence of an aqueous suspension of TiO2 powder, uv irradiation causes the breakdown of polychlorobiphenyls (PCB's). Using pentachlorophenol as a model substrate in the presence of 0.2% TiO2, uv irradiation induces dechlorination but only to a limit of 40% completion probably due to surface contamination of the powder 9. When ultrasound is used in conjunction with the photolysis the dechlorination is dramatically improved
(Table 3). This improvement can be ascribed largely to the mechanical effects of cavitation involving surface cleaning and increased mass transport to the powder surface. The advantages of using ultrasound in conjunction with electrochemistry have been referred to elsewhere10. This combination has been particularly beneficial in the destruction of phenols by electrochemical oxidation. Ultrasound (25kHz, 104 W/m2) when applied to a solution containing phenol (100gl-1) and NaCl (2gl-1) achieves better than 80% oxidation to maleic acid11. In the absence of ultrasound only 50% decomposition was obtained under the same conditions. In a more recent study an almost complete sonochemical destruction of phenol in saline solution at pH6 was obtained at a current density of 170 Am-2 in 10 minutes12. The reaction was shown to proceed via intermediate chlorinated phenols. A long-time environmental problem has been the removal of colour from the effluent streams of textile factories since the presence of residual dyes is rather obvious even at low dilutions. There are several conventional approaches to the solution of this problem including absorption onto activated charcoal, flocculation, chemical oxidation, ozonolysis and irradiation with uv light. Sonication can be added to any of these and the combination of ultrasound with ozonolysis seems particularly efficient13. We have investigated the sonoelectrochemical destruction of dyes and in particular the destruction of Sandolan Yellow. The process entails the electrolysis of aqueous NaCl solution which involves the liberation of chlorine at the anode and hydroxide ion at the cathode. The overall cell reaction is: 2OH- + Cl2 --> Cl- + OCl- + H2O The improvement using ultrasound in conjunction with electrolysis is evident (Figure 1). Electrolysis was carried out in an open beaker immersed in an ultrasonic bath (38kHz, 1.1 Wcm-2) at 75mA using 1.5 mgl-1 dye in 0.1 M NaCl solution. The results are normalised to an initial dye concentration of 100% at zero time.
Figure 1 Sonoelectrochemical removal of Sandolan Yellow in saline solution
2.2
DECONTAMINATION OF SOIL
Power ultrasound can be used for the rehabilitation of polluted industrial sites through the removal of chemical and biological contamination from soil. Conventional soil washing processes are based on the principle that many pollutants adsorb onto the fine fractions of soil such as silt, clay and humic matter which tend to be attached to coarser sand and gravel particles which make up the majority of the soil content. Therefore the primary aim in soil washing is to separate these fine components from the bulk soil. Isolation of the fine materials will result in a "concentrated" volume of polluted soil which may be treated or disposed of, and a large volume of residual soil which requires relatively little treatment and can be returned to the site as back fill. A comparison has been made of the efficiencies of conventional and ultrasonically assisted pollutant extraction procedures using model soil samples (granular pieces of brick) which had been deliberately contaminated with copper oxide at 51 ppm14. Analysis of the brick particles after 30 minutes sonication on a Vibrating Tray15 revealed an average reduction in copper content to 31 ppm, a reduction of about 40%. Using a conventional mechanically shaken tray for the same time period the residual contamination was 48 ppm representing a reduction of only 6% (Table 4). Table 4: Ultrasonic washing of brick particles Washing with mechanical shaking residual brick = 746.5g 48 ppm
< 20 mesh = 2.9g 310 ppm
fines = 0.63g 3200 ppm
water = 12.6 l 0.22 ppm
Reduction in copper contamination in treated brick = 6% Washing with Vibrating Tray residual brick = 744.7g 31 ppm
< 20 mesh = 3.4g 96 ppm
fines = 1.89g 4700 ppm
water = 13.5 l 0.49 ppm
Reduction in copper contamination in treated brick = 40%
Initial mass of brick 750g, copper contamination 51.4 ppm
3.
Acoustic Deliquoring
The subject of acoustic filtration has been covered elsewhere10. Within this section the particular advantages of combining ultrasound alone with
filtration (acoustic filtration) and in conjunction with an electric field (electroacoustic filtration) waere discussed. The former technique is currently being successfully applied to a specific environmental separation problem - the removal of fine particulate and cellular material from the white water outflow of paper mills. This is a particularly difficult filtration process since the filters readily clog under conventional filtration conditions. Acoustic filtration appears to offer an extremely good means of separation when applied to the type of membranes used in filter cartridges15.
4.
Ultrasonic Dispersion
On offshore oil drilling rigs an environmental problem exists related to the disposal of drilling cuttings. This material is particulate and must be cleaned of oil thoroughly before being dumped at sea. Cleaning technologies exist for the first phase of this process but when the clean material is dumped it sinks to the sea bed and accumulates around the platform legs presenting a problem to sea life and eventually rig maintenance. An ultrasonic device has been built which is suitable for use on an oil rig and provides a flow treatment for the drilling mud which substantially reduces particle size and, as a result, the cleaned mud disperses across the sea bed and does not accumulate around the legs 17.
References 1. Slapp P., Production Line Cleaning, Diss, U.K. British Patent 95 00587 2. 2. Phull,S.S., Newman,A.P., Lorimer,J.P., Pollet,B. and Mason, T.J. (1997) Ultrasonics Sonochemistry, 4, 157. 3. Ordonez J.A., Sanz B., Burgos J. and Garcia M.L. (1989) Journal of Applied Bacteriology, 67, 619. 4. Effect of Heat and Ultrasound on Microorganisms and enzymes, Sala F.J., Burgos J., Condon S., Lopez P. and Raso J. (1995) New Methods of Food Preparation, ed G.W.Gould, Blackie (London). 5. Cheung, H.M., Bhatnagar, A. and Jansen, G. (1991) Environ. Sci.Technol., 25, 1510. 6. Nagata, Y., Kurosaki, Y., Nakagawa, M. and Maeda, Y. (1993) Chem.Express, 8, 657. 7. Berlan, J., Trabelsi, F., Delmas, H., Wilhelm, A-M. and Petrignani, J.F. (1994) Ultrasonics Sonochemistry, 1, S97. 8. Toy, M.S., Carter, M.K. and Passell, T.O. (1990) Environmental Technology, 11, 837. 9. Information produced by the Stanford Research Institute, 333 Ravenswood Ave., Menlo Park, CA, U.S.A. 10. See chapter on "Industrial Applications of Sonochemistry and Power Ultrasonics" by T.J.Mason in this volume. 11. Mizera, J. (1992) Chem. Abs., 99, 127904x, (Polish). 12. Berlan, J., presented at Industrial Sonochemistry Symposium, ENSIGC, Toulouse, France, 1994. 13. See chapter on "Cavitational Environmental Remediation" by J.R.Russell in this volume. 14. Newman, A.P., Lorimer, J.P., Mason, T.J. and Hutt, K.R. (1997) Ultrasonics Sonochemistry, 4, 153. 15. Vibrating Tray is a product of Lewis Corporation, 102 Willenbrock Road, Oxford, Connecticut 06478-1033, U.S.A. 16. Information provided by VTT Energy, Dewatering Research Group, PO Box 1603, FIN-40101 Jyväskylä, Finland. 17. Information provided by Sonic Process Technologies Ltd., P.O. Box 82, Astley, Shrewsbury SY4 4WP, U.K.
