Processing Contaminated Soils And Sediments By High Power Ultrasound
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Processing Contaminated Soils And Sediments By High Power Ultrasound as PDF for free.
More details
- Words: 2,074
- Pages: 4
Minerals Engineering 19 (2006) 450–453 This article is also available online at: www.elsevier.com/locate/mineng
Processing contaminated soils and sediments by high power ultrasound A.F. Collings
a,b,*
, A.D. Farmer a, P.B. Gwan a, A.P. Sosa Pintos
a,b
, C.J. Leo
b
a
b
CSIRO Industrial Physics, P.O. Box 218, Bradfield Road, West Lindfield, Sydney, NSW 2070, Australia School of Engineering and Industrial Design, University of Western Sydney, Penrith South DC, NSW 1797, Australia Received 20 June 2005; accepted 24 July 2005 Available online 21 September 2005
Abstract Following the successful application of high power ultrasound to some areas of mineral processing, attention was directed towards the remediation of contaminated soils and sediments. Laboratory experiments have produced high contaminant destruction rates with surprisingly low energy costs and work at pilot plant scale is under way. The process relies on the phenomenon of cavitation to destroy contaminants such as PCBs, polycyclic aromatic hydrocarbons and organochlorides which adsorb to the surface of soil particles because of their inherent hydrophobicity. Such chemicals persist in the environment because of their chemical stability and they bioaccumulate, posing a serious health threat to animals and humans. High power ultrasound allows low-cost, onsite remediation and circumvents many of the shortcomings posed by conventional remediation technologies. Results are presented for a range of contaminants and the underlying physics of the technology is explained. Crown Copyright Ó 2005 Published by Elsevier Ltd. All rights reserved. Keywords: Environmental; Pollution
1. Introduction Modern technology has incurred a huge growth in recent decades in the manufacture and use of synthetic chemicals. The incidence of polycyclic aromatic hydrocarbons (PAHs) which are not synthetic is a consequence of the burning of coal, gas and oil to satisfy the burgeoning energy demands of industry and society. Part and parcel of such developments has been an environmental risk and the minerals industry, like others, must now devote effort and expense to the remediation of waste. The chemical properties of these materials are such that they can persist in the environment, they are readily transported by air and water currents, and *
Corresponding author. Address: CSIRO Industrial Physics, P.O. Box 218, Bradfield Road, West Lindfield, Sydney, NSW 2070, Australia. Tel.: +61 2 9413 7148; fax: +61 2 9413 7200. E-mail address: [email protected] (A.F. Collings).
they can accumulate in food chains. A simple, cost-effective solution to waste disposal is essential. This paper describes the development of high power ultrasound to destroy persistent organic pollutants (POPs) in soils and sediments. This development builds upon experience obtained with the application of ultrasound in areas of mineral processing (Farmer et al., 2000a,b). Since the principles of the use of high power ultrasound are not widely appreciated, the physics underlying the technology is first explained and results are presented for a range of contaminants.
2. Experimental 2.1. High power ultrasound The destruction of contaminants in soils relies on the process of cavitation, which is the formation, growth
0892-6875/$ - see front matter Crown Copyright Ó 2005 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.07.014
A.F. Collings et al. / Minerals Engineering 19 (2006) 450–453
and collapse of bubbles due to the propagation of a mechanical wave through a liquid. When a sound wave or any mechanical wave propagates through a liquid, it does so as a series of rarefactions and compressions. If the pressure in the rarefaction phase is sufficiently reduced, micro-bubbles form as a result of three processes: the vapour pressure of the liquid is greater than the hydrostatic pressure; dissolved gas in the liquid is released; or liquid molecules are simply pulled apart forming voids. The bubbles grow with successive cycles of the wave because the rate of mass transfer is proportional to the surface area of the bubble, and the transfer into the bubble in the rarefaction phase (when the bubble is expanded) is greater than the transfer out when the bubble diameter is reduced in the compression phase. The bubble diameter increases up to a critical value at which it undergoes near-instantaneous collapse. The resultant shock wave generates highly localised temperatures and pressures as high as 5000 K and 1000 atm, respectively (Mason and Lorimer, 1991; Suslick, 1989). Such highly localised but extreme conditions have led to the development of the field of sonochemistry where the collapsing bubble is essentially used as a microreactor. While the collapse of a bubble in the body of a liquid is symmetrical, this is not the case at a solid surface. Surface tension results in the formation of a dome-shaped vapour bubble, the collapse of which generates a high speed jet directed towards the solid surface. This has been confirmed theoretically (Plesset, 1973) and experimentally (Coleman et al., 1987) as seen in Figs. 1 and 2. In the case of a slurry, the solid particles will preferentially nucleate bubble formation because of the disruption in free energy at the solid–liquid interface, and the extreme conditions generated by the shock wave are localised on the solid surface. Since POPs are hydrophobic and are adsorbed on the particle surface, they bear the brunt of the release of energy. A further advantage of the experimental conditions is that the extreme conditions are localised on the adsorbed POP but the bulk solution temperature is relatively unaffected. The
451
Fig. 2. Experimental confirmation by Coleman et al. (1987).
reaction products formed by destruction of the POP are very rapidly quenched avoiding back reactions which could lead to the formation of dioxins. To summarise the physics of the process, when a contaminated slurry is exposed to high power ultrasound, cavitation occurs at the particle surface where a POP is concentrated; the cavitation energy is preferentially directed to the destruction of the contaminant and the breakdown products are essentially instantaneously quenched. Consequently, the energy budget is very favourable, unlike high temperature incineration where all of a contaminated solid must be heated to high temperatures and all quenched to minimise back reactions. 2.2. Experimental procedure Experimental studies for several POPs have been made using fine glass beads as a model matrix, sand ‘‘spiked’’ with a particular contaminant, and with contaminated soils and sediments. These have been done on three scales: (i) a small laboratory scale circulating 100 ml of 40 wt.% slurry through a glass reaction tube of total volume of 30 ml with a 12.5 mm diameter ultrasonic horn typically operating at 150 W; (ii) a larger (45 mm diameter) reaction tube insonated at 1–1.5 kW by a 38 mm diameter horn; and (iii) a 4 kW pilot plant. The larger laboratory unit and pilot plant are shown in Figs. 3 and 4. Slurry samples were collected over a range of sonication times, filtered, and the solids contaminations were analysed by GC/MS at the Australian Government Analytical Laboratory. Liquid samples were also analysed for traces of the POP and for breakdown products. 3. Results
Fig. 1. Theoretical predictions of bubble collapse by Plesset (1973).
The results of the preliminary investigation with PCB 1260 in sand are shown in Fig. 5 (Mason et al., 2004) and indicate that some 85% of the contaminant was destroyed after 1 h of sonication. In Fig. 6, a much
452
A.F. Collings et al. / Minerals Engineering 19 (2006) 450–453
Percentage Reduction (%)
0% 10% 20%
28%
30% 40%
48%
50% 60%
69%
70% 80%
85%
90% 0
20
40
60
80
Time in Reaction Zone (min) Fig. 5. Destruction of PCB 1260 in sand.
Percentage Reduction (%)
0
Fig. 3. 1.5 kW laboratory unit.
10 20 30 40 50
Test 1
60 70 Test 2
80
Test 3
90 100 0
1
2
3
4
5
6
7
8
9 10 11
Time in Reaction Zone (min) Fig. 6. Destruction of PAH contaminated riverine sediment.
A significant improvement in the rate and completeness of destruction has been achieved by ensuring that all the contaminated solid passes through the zone of most intense cavitation (Collings, submitted for publication). The dramatic improvement in destruction is shown in Fig. 7 for two experiments involving PCB 1254 adsorbed onto a model soil consisting of 90 lm diameter glass beads. Ninety percent of the PCB is reduced in 1–2 min and 99% in 7–10 min of sonication. GCMS analysis of the liquid phase on the completion 100%
Fig. 4. 4 kW pilot plant.
faster rate of destruction rate is seen for a nominal 400 ppm level of PAH contamination in riverine sediment. A series of organochloride compounds in sand was also exposed to the inserted 12.5 mm horn resulting in destruction rates of the order of 70%, with most of the contaminant being destroyed in the first two minutes of treatment (Collings and Gwan, submitted for publication).
PCB Remaining (%)
90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0
1
2
3
4
5
6
7
8
9 10 11
Time in Reaction Zone (min) Fig. 7. Destruction of PCB 1254 39 ppm m and 45 ppm beads.
on glass
Percentage Reduction (%)
A.F. Collings et al. / Minerals Engineering 19 (2006) 450–453
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0
1 2 3 4 5 Time in Reaction Zone (min)
Fig. 8. Destruction of atrazine j, simazine
, and TPH
6
on sand.
of the experiment showed only 5 ppb of PCB and no traces of breakdown products. Similar results were obtained for slurries contaminated with hexachlorobenzene contaminated slurries (Collings, submitted for publication). Results are shown in Fig. 8 for a contaminated soil from an industrial site where a mixture of organochlorine compounds, petroleum hydrocarbons and PAHs were present in a clay substrate. This experiment was conducted on the larger scale (1.5 kW) laboratory unit. Ultrasound was effective in destroying the major contaminants, atrazine, simazine and total petroleum hydrocarbons. In addition to the compounds mentioned above, we have worked successfully on DDT, lindane, endosulfan, 2,4,5-T, tetrachloronaphthalene and TBT (Collings and Gwan, submitted for publication). The range of materials we have studied is sufficiently broad to suggest that high power ultrasound will be effective for most adsorbed large molecules. The destruction rates for all materials were qualitatively similar and follow a first order (exponential) decay.
4. Discussion It is important to compare this technology with current remediation technologies. These have proven neither cost-effective nor efficacious. Most require the removal and transport off-site to a remediation facility adding to both the cost and risk. High temperature
453
incineration has become politically untenable, whereas in situ technologies such as thermal conduction or electrical resistance heating, bioremediation or phytoremediation are limited to shallow soil and sediment contamination. Recent evaluations of the state-of-theart technologies summarise developments in Europe (Vijgen, 2002) and Australia (Rae, 2001). The high power ultrasonic technology offers several advantages compared with current methods. These include high destruction rates, the lack of dangerous breakdown products and the low energy demands, assuming that similar rates of destruction to those obtained in these studies can be obtained at an industrial scale. A further advantage is that the technology can be made quite compact and transportable, allowing on-site treatment and the convenience of hydraulic transport of soil in slurries typically 40% by weight.
References Coleman, A.J., Saunders, J.E., Crum, L.A., Dyson, M., 1987. Acoustic cavitation generated by an extra-corporeal shockwave lithotripter. Ultrasound in Medicine and Biology 13, 69–76. Collings, A.F. Destruction of organochloride contaminants in slurries by high power ultrasonics. Science, submitted for publication. Collings, A.F., Gwan, P.B. Ultrasonic destruction of pesticide contaminants in slurries. Ultrasonics Sonochemistry, submitted for publication. Farmer, A.D., Collings, A.F., Jameson, G.J., 2000a. Effect of ultrasound on the cleaning of mineral particles. International Journal of Mineral Processing 60, 101–113. Farmer, A.D., Collings, A.F., Jameson, G.J., 2000b. The application of power ultrasound to the surface cleaning of silica and heavy mineral sands. Ultrasonics Sonochemistry 7, 243–247. Mason, T.J., Lorimer, J.P., 1991. Sonochemistry, Theory, Applications and Uses of Ultrasound in Chemistry. Ellis Horwood Publishers, London. Mason, T.J., Collings, A.F., Sumel, A., 2004. Sonic and ultrasonic removal of contaminants from soil in the laboratory and on a large scale. Ultrasonics Sonochemistry 11, 205–210. Plesset, M.S., 1973. Bubble dynamics and cavitation erosion. In: Bjorno, L. (Ed.), Proceedings of 1973 Symposium Finite Amplitude Effects in Fluids. IPC Science and Technology Press Ltd., Copenhagen. Rae, I.D., 2001. Management of chlorinated wastes in Australia. In: 6th International HCH and Pesticides Forum, Poland. Suslick, K., 1989. The chemical effects of ultrasound. Scientific American, 62–89. Vijgen, J., 2002. Evaluation of demonstrated and emerging remedial action technologies for the treatment of contaminated land and groundwater. NATO/CCMS Pilot Study, December.
Processing contaminated soils and sediments by high power ultrasound A.F. Collings
a,b,*
, A.D. Farmer a, P.B. Gwan a, A.P. Sosa Pintos
a,b
, C.J. Leo
b
a
b
CSIRO Industrial Physics, P.O. Box 218, Bradfield Road, West Lindfield, Sydney, NSW 2070, Australia School of Engineering and Industrial Design, University of Western Sydney, Penrith South DC, NSW 1797, Australia Received 20 June 2005; accepted 24 July 2005 Available online 21 September 2005
Abstract Following the successful application of high power ultrasound to some areas of mineral processing, attention was directed towards the remediation of contaminated soils and sediments. Laboratory experiments have produced high contaminant destruction rates with surprisingly low energy costs and work at pilot plant scale is under way. The process relies on the phenomenon of cavitation to destroy contaminants such as PCBs, polycyclic aromatic hydrocarbons and organochlorides which adsorb to the surface of soil particles because of their inherent hydrophobicity. Such chemicals persist in the environment because of their chemical stability and they bioaccumulate, posing a serious health threat to animals and humans. High power ultrasound allows low-cost, onsite remediation and circumvents many of the shortcomings posed by conventional remediation technologies. Results are presented for a range of contaminants and the underlying physics of the technology is explained. Crown Copyright Ó 2005 Published by Elsevier Ltd. All rights reserved. Keywords: Environmental; Pollution
1. Introduction Modern technology has incurred a huge growth in recent decades in the manufacture and use of synthetic chemicals. The incidence of polycyclic aromatic hydrocarbons (PAHs) which are not synthetic is a consequence of the burning of coal, gas and oil to satisfy the burgeoning energy demands of industry and society. Part and parcel of such developments has been an environmental risk and the minerals industry, like others, must now devote effort and expense to the remediation of waste. The chemical properties of these materials are such that they can persist in the environment, they are readily transported by air and water currents, and *
Corresponding author. Address: CSIRO Industrial Physics, P.O. Box 218, Bradfield Road, West Lindfield, Sydney, NSW 2070, Australia. Tel.: +61 2 9413 7148; fax: +61 2 9413 7200. E-mail address: [email protected] (A.F. Collings).
they can accumulate in food chains. A simple, cost-effective solution to waste disposal is essential. This paper describes the development of high power ultrasound to destroy persistent organic pollutants (POPs) in soils and sediments. This development builds upon experience obtained with the application of ultrasound in areas of mineral processing (Farmer et al., 2000a,b). Since the principles of the use of high power ultrasound are not widely appreciated, the physics underlying the technology is first explained and results are presented for a range of contaminants.
2. Experimental 2.1. High power ultrasound The destruction of contaminants in soils relies on the process of cavitation, which is the formation, growth
0892-6875/$ - see front matter Crown Copyright Ó 2005 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.07.014
A.F. Collings et al. / Minerals Engineering 19 (2006) 450–453
and collapse of bubbles due to the propagation of a mechanical wave through a liquid. When a sound wave or any mechanical wave propagates through a liquid, it does so as a series of rarefactions and compressions. If the pressure in the rarefaction phase is sufficiently reduced, micro-bubbles form as a result of three processes: the vapour pressure of the liquid is greater than the hydrostatic pressure; dissolved gas in the liquid is released; or liquid molecules are simply pulled apart forming voids. The bubbles grow with successive cycles of the wave because the rate of mass transfer is proportional to the surface area of the bubble, and the transfer into the bubble in the rarefaction phase (when the bubble is expanded) is greater than the transfer out when the bubble diameter is reduced in the compression phase. The bubble diameter increases up to a critical value at which it undergoes near-instantaneous collapse. The resultant shock wave generates highly localised temperatures and pressures as high as 5000 K and 1000 atm, respectively (Mason and Lorimer, 1991; Suslick, 1989). Such highly localised but extreme conditions have led to the development of the field of sonochemistry where the collapsing bubble is essentially used as a microreactor. While the collapse of a bubble in the body of a liquid is symmetrical, this is not the case at a solid surface. Surface tension results in the formation of a dome-shaped vapour bubble, the collapse of which generates a high speed jet directed towards the solid surface. This has been confirmed theoretically (Plesset, 1973) and experimentally (Coleman et al., 1987) as seen in Figs. 1 and 2. In the case of a slurry, the solid particles will preferentially nucleate bubble formation because of the disruption in free energy at the solid–liquid interface, and the extreme conditions generated by the shock wave are localised on the solid surface. Since POPs are hydrophobic and are adsorbed on the particle surface, they bear the brunt of the release of energy. A further advantage of the experimental conditions is that the extreme conditions are localised on the adsorbed POP but the bulk solution temperature is relatively unaffected. The
451
Fig. 2. Experimental confirmation by Coleman et al. (1987).
reaction products formed by destruction of the POP are very rapidly quenched avoiding back reactions which could lead to the formation of dioxins. To summarise the physics of the process, when a contaminated slurry is exposed to high power ultrasound, cavitation occurs at the particle surface where a POP is concentrated; the cavitation energy is preferentially directed to the destruction of the contaminant and the breakdown products are essentially instantaneously quenched. Consequently, the energy budget is very favourable, unlike high temperature incineration where all of a contaminated solid must be heated to high temperatures and all quenched to minimise back reactions. 2.2. Experimental procedure Experimental studies for several POPs have been made using fine glass beads as a model matrix, sand ‘‘spiked’’ with a particular contaminant, and with contaminated soils and sediments. These have been done on three scales: (i) a small laboratory scale circulating 100 ml of 40 wt.% slurry through a glass reaction tube of total volume of 30 ml with a 12.5 mm diameter ultrasonic horn typically operating at 150 W; (ii) a larger (45 mm diameter) reaction tube insonated at 1–1.5 kW by a 38 mm diameter horn; and (iii) a 4 kW pilot plant. The larger laboratory unit and pilot plant are shown in Figs. 3 and 4. Slurry samples were collected over a range of sonication times, filtered, and the solids contaminations were analysed by GC/MS at the Australian Government Analytical Laboratory. Liquid samples were also analysed for traces of the POP and for breakdown products. 3. Results
Fig. 1. Theoretical predictions of bubble collapse by Plesset (1973).
The results of the preliminary investigation with PCB 1260 in sand are shown in Fig. 5 (Mason et al., 2004) and indicate that some 85% of the contaminant was destroyed after 1 h of sonication. In Fig. 6, a much
452
A.F. Collings et al. / Minerals Engineering 19 (2006) 450–453
Percentage Reduction (%)
0% 10% 20%
28%
30% 40%
48%
50% 60%
69%
70% 80%
85%
90% 0
20
40
60
80
Time in Reaction Zone (min) Fig. 5. Destruction of PCB 1260 in sand.
Percentage Reduction (%)
0
Fig. 3. 1.5 kW laboratory unit.
10 20 30 40 50
Test 1
60 70 Test 2
80
Test 3
90 100 0
1
2
3
4
5
6
7
8
9 10 11
Time in Reaction Zone (min) Fig. 6. Destruction of PAH contaminated riverine sediment.
A significant improvement in the rate and completeness of destruction has been achieved by ensuring that all the contaminated solid passes through the zone of most intense cavitation (Collings, submitted for publication). The dramatic improvement in destruction is shown in Fig. 7 for two experiments involving PCB 1254 adsorbed onto a model soil consisting of 90 lm diameter glass beads. Ninety percent of the PCB is reduced in 1–2 min and 99% in 7–10 min of sonication. GCMS analysis of the liquid phase on the completion 100%
Fig. 4. 4 kW pilot plant.
faster rate of destruction rate is seen for a nominal 400 ppm level of PAH contamination in riverine sediment. A series of organochloride compounds in sand was also exposed to the inserted 12.5 mm horn resulting in destruction rates of the order of 70%, with most of the contaminant being destroyed in the first two minutes of treatment (Collings and Gwan, submitted for publication).
PCB Remaining (%)
90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 0
1
2
3
4
5
6
7
8
9 10 11
Time in Reaction Zone (min) Fig. 7. Destruction of PCB 1254 39 ppm m and 45 ppm beads.
on glass
Percentage Reduction (%)
A.F. Collings et al. / Minerals Engineering 19 (2006) 450–453
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0
1 2 3 4 5 Time in Reaction Zone (min)
Fig. 8. Destruction of atrazine j, simazine
, and TPH
6
on sand.
of the experiment showed only 5 ppb of PCB and no traces of breakdown products. Similar results were obtained for slurries contaminated with hexachlorobenzene contaminated slurries (Collings, submitted for publication). Results are shown in Fig. 8 for a contaminated soil from an industrial site where a mixture of organochlorine compounds, petroleum hydrocarbons and PAHs were present in a clay substrate. This experiment was conducted on the larger scale (1.5 kW) laboratory unit. Ultrasound was effective in destroying the major contaminants, atrazine, simazine and total petroleum hydrocarbons. In addition to the compounds mentioned above, we have worked successfully on DDT, lindane, endosulfan, 2,4,5-T, tetrachloronaphthalene and TBT (Collings and Gwan, submitted for publication). The range of materials we have studied is sufficiently broad to suggest that high power ultrasound will be effective for most adsorbed large molecules. The destruction rates for all materials were qualitatively similar and follow a first order (exponential) decay.
4. Discussion It is important to compare this technology with current remediation technologies. These have proven neither cost-effective nor efficacious. Most require the removal and transport off-site to a remediation facility adding to both the cost and risk. High temperature
453
incineration has become politically untenable, whereas in situ technologies such as thermal conduction or electrical resistance heating, bioremediation or phytoremediation are limited to shallow soil and sediment contamination. Recent evaluations of the state-of-theart technologies summarise developments in Europe (Vijgen, 2002) and Australia (Rae, 2001). The high power ultrasonic technology offers several advantages compared with current methods. These include high destruction rates, the lack of dangerous breakdown products and the low energy demands, assuming that similar rates of destruction to those obtained in these studies can be obtained at an industrial scale. A further advantage is that the technology can be made quite compact and transportable, allowing on-site treatment and the convenience of hydraulic transport of soil in slurries typically 40% by weight.
References Coleman, A.J., Saunders, J.E., Crum, L.A., Dyson, M., 1987. Acoustic cavitation generated by an extra-corporeal shockwave lithotripter. Ultrasound in Medicine and Biology 13, 69–76. Collings, A.F. Destruction of organochloride contaminants in slurries by high power ultrasonics. Science, submitted for publication. Collings, A.F., Gwan, P.B. Ultrasonic destruction of pesticide contaminants in slurries. Ultrasonics Sonochemistry, submitted for publication. Farmer, A.D., Collings, A.F., Jameson, G.J., 2000a. Effect of ultrasound on the cleaning of mineral particles. International Journal of Mineral Processing 60, 101–113. Farmer, A.D., Collings, A.F., Jameson, G.J., 2000b. The application of power ultrasound to the surface cleaning of silica and heavy mineral sands. Ultrasonics Sonochemistry 7, 243–247. Mason, T.J., Lorimer, J.P., 1991. Sonochemistry, Theory, Applications and Uses of Ultrasound in Chemistry. Ellis Horwood Publishers, London. Mason, T.J., Collings, A.F., Sumel, A., 2004. Sonic and ultrasonic removal of contaminants from soil in the laboratory and on a large scale. Ultrasonics Sonochemistry 11, 205–210. Plesset, M.S., 1973. Bubble dynamics and cavitation erosion. In: Bjorno, L. (Ed.), Proceedings of 1973 Symposium Finite Amplitude Effects in Fluids. IPC Science and Technology Press Ltd., Copenhagen. Rae, I.D., 2001. Management of chlorinated wastes in Australia. In: 6th International HCH and Pesticides Forum, Poland. Suslick, K., 1989. The chemical effects of ultrasound. Scientific American, 62–89. Vijgen, J., 2002. Evaluation of demonstrated and emerging remedial action technologies for the treatment of contaminated land and groundwater. NATO/CCMS Pilot Study, December.
Related Documents
Contaminated Sediments
November 2019 16
Ultrasound
November 2019 27
Soils
November 2019 15
