Recent studies in environmental applications ofultrasound1

Thuy Duong Pham, Reena Amatya Shrestha, Jurate Virkutyte, and Mika Sillanpaa

Abstract: As a young, new, and rapidly growing science, the applications of ultrasound in environmental technology holda promising future. Compared with conventional methods, ultrasonication can bring several benefits such as environmen-tally friendly (no toxic chemicals are used or produced), low cost, and compact (allowing on-site treatment). Beside anoverview on ultrasonic background, this paper summarizes main findings and innovations of recent studies that used ultra-sound in environmental analysis, water and sludge treatment, soil and sediment remediation, and air purification.

Key words: ultrasound, environmental analysis, water treatment, sludge stabilization, soil remediation, air purification.

Resume : Meme s’ils sont consideres comme une science jeune, nouvelle et en croissance rapide, les ultrasons appliques ala technologie environnementale montrent un avenir prometteur. Par rapport aux methodes conventionnelles, l’ultrasoni-cation peut presenter plusieurs avantages tels que la convivialite environnementale (aucun produit chimique n’est utilise ouproduit), le faible cout et la compacite, permettant un traitement sur place. Cet article presente un survol de l’historiquedes ultrasons et resume les principales conclusions et innovations des etudes recentes utilisant les ultrasons dans l’analyseenvironnementale, le traitement des eaux et des boues, la restauration des sols et des sediments jusqu’a la purification del’air.

Mots-cles : ultrasons, analyse environnementale, traitement de l’eau, stabilisation des boues, restauration des sols, purifica-tion de l’air.

[Traduit par la Redaction]

1. Background

Ultrasound refers to inaudible sound waves with frequen-cies in the range of 16 kHz to 500 MHz, greater than theupper limit of human hearing. It can be transmitted throughany elastic medium including water, gas-saturated water,and slurry. Ultrasound has been used for diverse purposesin many different areas. In terms of frequency, ultrasoundcan be categorized into two main strands: (1) high frequency(2–10 MHz) — low power diagnostic ultrasound, involvingmedical imaging, nondestructive testing, and (2) low to me-dium frequency (20–1000 kHz) frequency — high power ul-trasound, involving other applications in industry,nanotechnology, ultrasonic therapy, and sonochemistry.Among these various applications, this paper will focus onthe uses of ultrasound in five main areas of environmentalscience and technology: water treatment, sludge treatment,soil and sediment remediation, air purification, and environ-mental analysis (Fig. 1).

Like any sound wave, ultrasound is propagated via a ser-

ies of compression and rarefaction waves induced in themolecules of the medium through which it passes. Compres-sion cycles push molecules together, while expansion cyclespull them apart. At sufficiently high power, the rarefactioncycle may exceed the attractive forces of the molecules ofthe liquid and cavitation bubbles will form. Cavitation bub-ble collapse is a remarkable phenomenon induced through-out the liquid. Cavitational collapse produces intense localheating (*5000 K) and pressures (*1000 atm) with veryshort lifetimes, implying the existence of extremely highheating and cooling rates (>109 K/s). It has been shown thattransient supercritical water is obtained during the collapseof cavitation bubbles generated sonolytically (Hoffmann etal. 1996). Acoustic cavitation provides a unique interactionof energy and matter, and ultrasonic irradiation of liquidscauses high energy chemical reactions to occur (Suslick2001).

According to Adewuyi (2001), so far four theories havebeen proposed to explain the sonochemical events: hot-spottheory, electrical theory, plasma discharge theory, and super

Received 12 February 2008. Revision accepted 24 April 2009. Published on the NRC Research Press Web site at cjce.nrc.ca on25 November 2009.

T.D. Pham2 and M. Sillanpaa. Laboratory of Applied Environmental Chemistry, Department of Environmental Sciences, University ofKuopio, Patteristonkatu 1, FI-50100 Mikkeli, Finland.R.A. Shrestha. Ecole des Mines de Nantes, GEPEA, UMR CNRS 6144, 4 rue Alfred Kastler, BP 20722, 44307 Nantes cedex 03,France.J. Virkutyte. Department of Environmental Sciences, University of Kuopio, Patteristonkatu 1, FI-50100, Mikkeli, Finland.

Written discussion of this article is welcomed and will be received by the Editor until 31 March 2010.

1A paper submitted to the Journal of Environmental Engineering and Science.2Corresponding author (e-mail: Duong.Pham@uku.fi).

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critical theory. These have led to several modes of reactivitybeing proposed: pyrolytic decomposition, hydroxyl radicaloxidation, plasma chemistry, and super critical water oxida-tion. Generally, most studies in environmental sonochemis-try have adopted the hot-spot concepts to explainexperimental results. In the hot-spot model (Adewuyi 2001),three regions are postulated (Fig. 2): (1) a hot gaseous nu-cleus, (2) an interfacial region, and (3) bulk solution at am-bient temperature. Reactions involving free radicals canoccur within the collapsing bubble, at the interface of thebubble, and in the surrounding liquid.

Within the center of the bubble, high temperatures andpressures generated during cavitation provide the activationenergy required for bond breakage, dissociation of solvents,and other vapors or gases, leading to the formation of freeradicals or excited species. The radicals generated either re-act with each other to form new molecules and radicals ordiffuse into the bulk liquid to serve as oxidants.

The second reaction site is the liquid shell immediatelysurrounding the imploding cavity, which has been estimatedto heat up to approximately 2000 K during cavity implosion.In this solvent layer surrounding the hot bubble, both com-

bustion and free-radical reactions (involving �OH derivedfrom the decomposition of H2O) occur. Reactions here arecomparable to pyrolysis reactions. Pyrolysis in the interfa-cial region is predominant at high solute concentrations,while at low solute concentrations, free-radical reactions arelikely to predominate. It has been shown that the majority ofdegradation takes place in the bubble-bulk interface region.

In the bulk liquid, no primary sonochemical activitytakes place, although subsequent reactions with ultrasoni-cally generated intermediates may occur. A small numberof free radicals produced in the cavities or at the interfacemay move into the bulk-liquid phase and react with thesubstrate present there in secondary reactions to form newproducts. Depending on their physical properties and con-centrations, molecules present in the medium will beburned in close to the bubble (pyrolysis) or will undergoradical reactions.

These chemical effects (sonochemistry) explained aboveare used in most of ultrasonic applications in environmen-tal remediation, especially in organic decontamination. Inaddition to that, the physical (mechanical) effects of ultra-sound are also useful in some environmental applicationslike air purification, sludge dewatering, and metal leaching.Although ultrasonic applications in environmental areas arestill in developing stage, they are growing rapidly, attract-ing more and more interests, because of many advantagesthey offer: environmentally friendly (no toxic chemicalsare used or produced), low energy demands, and compactand transportable method that can be used on-site (Mason2003, 2007). Recent studies that applied ultrasound in en-vironmental science and engineering will be summarizedand discussed in more details in the next sections of thispaper.

2. Water treatment

In water treatment technology, the application of ultra-sound (ultrasonication) can be useful in various processeslike organic decontamination, disinfection, electrocoagula-tion, and membrane filtration.

Fig. 1. Ultrasound in environmental applications.

Fig. 2. Three reaction zones in the cavitation process (adapted fromAdewuyi 2001).

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2.1. Decontamination

2.1.1. Researches with various organic pollutantsBecause of cavitation phenomenon, the formation of free

radicals and high localized temperatures and pressures, ultra-sonic irradiation (ultrasonication) appears to be an effectivemethod for the destruction of hazardous organic compoundsin water (Hoffmann et al. 1996; Joseph et al. 2000). Thebeneficial effect of ultrasonication on the removal of severaltarget compounds from aqueous solutions has been demon-strated in many studies. These compounds include phenol(Entezari et al. 2003), chlorophenols, nitrophenols, aniline(Emery et al. 2003; Teo et al. 2001; Jiang et al. 2002a; Pa-padaki et al. 2004; Goskonda et al. 2002), trichloroethylene(Drijvers et al. 1996), ethylbenzene (De Visscher et al.1997), chlorobenzene (Dewulf et al. 2001), chloronaphtha-lene (Jiang et al. 2002b), polychlorinated biphenyls, pesti-cides, polycyclic aromatic hydrocarbons, azobenzene, textiledyes (Joseph et al. 2000; Tezcanli-Guyer and Ince 2003),carbofuran (Hua and Ulrike 2001), nitroaromatics (Abramovet al. 2006), hydrazine (Nakui et al. 2007), detergents andsurfactants (Adewuyi 2001; Abu-hassan et al. 2006; Bel-giorno et al. 2007). Among these various organic contami-nants, phenol and phenolic compounds are the most widelyinvestigated. Many studies on sonodegradation of phenoliccompounds are summerized carefully in two interesting re-views of Kidak and Ince (2006) and Gogate (2008). In gen-eral, the optimum range for the frequency lies between 200and 540 kHz, while the best pH is in the acid region (Kidakand Ince 2006).

2.1.2. Effects of pHStudying the effect of pH, Jiang et al. (2002a) con-

cluded that pH of a solution plays an important role in de-grading the rate of polar aromatic compounds by sonolysisbecause it affects the charge of the substances (negativelycharged under alkaline condition like 4-nitrophenol, or pos-itively charged at acidic pH like aniline). For these hydro-philic compounds, the neutral species are more easy todiffuse and accumulate at the hydrophobic interface ofliquid–gas bubbles in comparison with their correspondingionic forms (thus, the rate of 4-nitrophenol degradation de-creases with increasing pH, while the rate of aniline de-struction exhibits a maximum under alkaline conditions).The ultrasonic induced formation of H2O2 also affected bypH as the yield of H2O2 has a maximum at a pH of ap-proximately 3 and decreases with increasing pH (Jiang etal. 2002a).

2.1.3. Effects of frequenciesUltrasonic irradiation of carbofuran (C12H15NO3) was per-

formed at 16 and 20 kHz by Hua and Ulrike (2001), whichshowed that the rate of carbofuran decomposition increasedwith higher power density applied (1.65–5.55 W/mL), lowerinitial carbofuran concentrations (25 versus 130 mmol/L),and when sparging with an Ar/O2 mixture.

Low frequency at 20 kHz also used for sonodegradation oflinear alkylbenzene sulfonate (LAS) solutions (Abu-hassan etal. 2006). Ultrasound of 20 kHz demonstrated capability ofdegrading the sodium dodecylbenzene sulfonate (SDBS, arepresentative LAS molecule) but complete mineralisation

may not be possible. Degradation rates increased with increas-ing power and decreasing temperature and volume of samples.

Further study on the effect of low frequency (20 kHz) ul-trasonication for removal of SDBS and phenolic compoundsin the addition of several heterogeneous catalysts (Pt, Pd,Ru, CuO.ZnO) was investigated by Papadaki et al. (2004).Such process was called sonocatalytic oxidation. The per-formance indicated that among these catalysts, a CuO.ZnOsupported on alumina catalyst, appears to enhance bothSDBS decomposition, total oxidation rates, as well as hydro-gen peroxide formation. By ultrasonication alone, phenoliccompounds at initial concentration of 0.1 g/L were removedonly partially, that is, about 10%–20% removals after180 min of irradiation. However, in the presence of Fe2+

ions at concentration as low as 10–3 g/L, the rate of sono-lytic degradation increased generally more than 2.5 times.

Studying frequency effect on sonochemical remediation ofalachlor, a widely employed herbicide, Wayment and Casa-donte (2002) concluded that, in general, the 300 kHz pro-vides a faster rate of degradation than either higher(446 kHz) or lower (20 kHz) frequency under comparableenergy input. The study also examined effect of dissolvedgases, and the results indicated that Argon-saturated solu-tions displayed an enhancement in rate by a factor of 2 com-pared with either oxygen- or air-saturated solutions uponsonication at 300 kHz (Wayment and Casadonte 2002).

Ultrasonic irradiation at approximately 500 kHz had beeninvestigated for degradation of textile dyestuff by Joseph etal. (2000) and Tezcanli-Guyer and Ince (2003). Both studiespresented that oxidation by ultrasonically generated hy-droxyl radicals were main mechanisms responsible for dyedegradation. In general, sonochemical bleaching is a fastprocess as complete decolorization was achieved within 40–150 min, while mineralization is attained after a longer pe-riod of time (Joseph et al. 2000). However, total detoxifica-tion could be achieved within shorter contact than completemineralization (Tezcanli-Guyer and Ince 2003). Summary ofmany studies with ultrasound use in decolouration and min-eralization of textile dyes can be seen in Vajnhandl andMajcen Le Marechal (2005).

The application of high frequency ultrasound at 2.4 and1.7 MHz was perfomed successfully to remove ammoniafrom simulated industrial wastewater, within a short time of1.5–2 h, by Matouq and Al-Anber (2007).

Entezari et al. (2003) conducted ultrasonication experi-ments with same ultrasonic power 50 W in three differentequipments that operated at 20, 35, and 500 kHz to removephenol from water. The 35 kHz-reactor was a new cylindri-cal tube called SonitubeTM. The results of this reactor werecompared with the other two classical equipments. The re-sults showed that, without oxidant addition, the rate of phe-nol destruction was higher at 500 kHz than at 35 or 20 kHz.However, when hydrogen peroxide was added, the rate ofphenol decomposition was higher for 35 kHz reactor than500 or 20 kHz reactors. It was explained that such differentbehaviour was not necessarily a pure frequency effect butcould be due to a response to other factors like surface areaof the sonicator (acoustic field) and intensity (Entezari et al.2003). The coupling oxidant-ultrasound method provedmore effective than the ultrasound or the oxidant alone. Thesonochemical degradation (20 kHz) of phenol and other phe-

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nolic pollutants such as chlorophenol and dichlorophenolwere also carried out in the work of Emery et al. (2003).Results showed that the removal rates did increase in thepresence of Fe2+ ions and the relative degradation decreasedin the order: 2-chlorophenol > 3,4-dichlorophenol > phenol.

2.1.4. Effects of ultrasonic irradiation modes (continuous /pulse)

Since nearly most of the researches on sonochemistry us-ing continuous ultrasonic irradiation, the work of Casadonteet al. (2005), though only still an initial study, bring an in-teresting and new perspective to the use of pulse mode ultra-sonic irradiation. They explored the application of power-moduleated pulsed (PMP) ultrasound in the degradation ofacid orange, a common azo-dye used as an industrial colo-rant. The performances indicated that PMP ultrasound maybe more effective in terms of producing hydroxyl radicalsand other oxidants in aqueous media, thus, the degradationrate increased by a factor of three compared with continuousirradiation under equal acoustic input power (Casadonte etal. 2005).

Sonication could be used as a pretreatment step to en-hance the biodegradability of the effluent by transformingthe molecules into simpler ones, which are further degradedby microorganisms in following biological treatment step(Sangave and Pandit 2004). Alternatively, sonication couldpossibly be employed as a post-treatment step for the re-moval of several refractory compounds formed commonlyduring thermochemical and advanced oxidation processes(Emery et al. 2003). Ultrasonication can further increase thedegradation of intermediates into smaller molecules eveninto carbon dioxide and water (Teo et al. 2001). If environ-mental applications of ultrasonic techniques emerge as post-treatment schemes for destructive removal of refractorycompounds in effluent streams, they will require mostly theuse of medium-frequency ultrasound, since such chemicalsare usually macromolecules with complex molecular struc-tures and hydrophilic properties. It is fortunate that reactorsystems designed for medium-frequency irradiation are rela-tively easier to maintain than those operated with power ul-trasound because the drawbacks associated with the lattersuch as noise and cavitational erosion. Such problems, how-ever, may be overcome by sound-proof material and properselection, configuration, and maintenance of the equipment(Ince et al. 2001).

2.1.5. Ultrasonic-based hybrid techniquesMost studies indicate that ultrasonication alone cannot be

an economical technique for wastewater treatment, but itshould be combined with or worked as an enhancement tosome conventional methods. Hybrid methods such as ultra-sound / H2O2 or O3, ultrasonication assisted by catalysts /additives, sonophotocatalytic oxidation, sonoelectrochemis-try, and ultrasonication coupled with biological oxidationhave been discussed carefully in Gogate’s review (2008).

Ozonation combined with ultrasonication in the degrada-tion of p-Aminophenol (PAP) in aqueous solution was in-vestigated recently by He et al. (2007). This combination ofultrasound and ozone resulted in a synergetic increase in theoverall process rate. Although ozonation (72% and 90%PAP removal at 10 and 30 min, respectively) was more ef-

fective than ultrasonication (3% and 4% at 10 and 30 min),the efficiency of the combination of ozone and ultrasound(88% and 99% at 10 and 30 min) exceeded even the sum ofthose using ozone and ultrasound alone. It was explainedthat the synergy observed in combined treatment was mainlydue to the effects of sonolysis in enhancing the decomposi-tion of ozone in collapsing bubbles to yield additional freeradicals (He et al. 2007).

Yasman et al. (2004) developed a new method for detox-ification of hydrophilic chloroorganic pollutants (commonherbicide 2,4-dichilorophenoxyacetic acid (2,4-D), and itsderivative 2,4-dichlorophenol) in effluent water, using acombination of ultrasound waves, electrochemistry, and Fen-ton’s reagent. The high degradation power of this process isdue to the large production of oxidizing hydroxyl radicalsand high mass transfer made by sonication. Application ofthis sono-electrochemical Fenton process (SEF) treatment(at 20 kHz) with quite a small current density, accomplishedalmost 50% oxidation of 2,4-D solution (300 ppm,1.2 mmol/L) in just 60 s. Similar treatments ran for 10 minand resulted in practically full degradation of the herbicide.Thus, the efficiency of the SEF process is much higher thanother methods with the time required for full degradationconsiderably shorter. However, the oxidative degradation of2,4-D was accompanied by the production of the highlytoxic intermediate 2,4-dichlorophenol. Therefore, Yasmanet al. (2006) reported another study in enhanced sono-electro-degradation of these chloroorganic compounds bycatalysts Pd or Pd/Fe (promoters for reduction chlorineatom abstraction). The bimetallic Pd/Fe catalyst performedsuperiorly to the pure Pd catalyst and were more economi-cal. Their findings demonstrate that coupling ultrasound toelectro-catalytical reduction of 2,4-D results in completemineralization of the substrate and the reaction times areeven greatly shortened in comparison with traditionalelectro-catalytic processes. It was concluded that thismethod is promising for the remediation of both fresh andwastewater contaminated by chloroorganic compounds(Yasman et al. 2006).

2.2. Improving disinfectionStudies have shown that high power ultrasound, operated

at low frequencies is an effective means for disintegration ofbacterial cells (Blume and Neis 2004). However, disinfec-tion by ultrasonication alone requires very high energy.Thus, generally it cannot be considered as an alternative toconventional disinfection for economical aspects. Then, ul-trasonication should be used together with other techniques.For instance, the combination of a short ultrasonication anda subsequent ultraviolet treatment is even cost-efficient andmeaningful (Blume and Neis 2004). Ultrasonication com-bined with chlorination improved significantly the biocidalaction. These results suggest that ultrasound could be usedin conjunction with chemical treatments to achieve a reduc-tion in the quantity of bactericide required for water treat-ment (Mason et al. 2003).

In another study by Joyce et al. (2003), 40 kHz ultrasoundwas used in conjunction with electrolysis to disinfect salinesolution. The results show that sonication appears to amplifythe effect of electrolysis: (i) ultrasound enhanced mixing ofbacterial suspensions in the vicinity of the electrode surfacewhere the hypochlorite is being generated; (ii) the mechani-

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cal action of cavitation damage and weaken the bacterialcell wall, thus make them more susceptible to attack by hy-pochlorite; (iii) the cleaning action of ultrasound on theelectrode surface prevents fouling build up, thus maintainsmore efficient electrolysis. Obviously, a combination ofboth treatments is significantly better than sonication orelectrolysis alone.

2.3. Enhancing membrane filtrationAccording to review of Kyllonen et al. (2005), ultrasound

irradiation can provide enhancement in membrane filtrationof wastewaters. It increases the flux primarily by breakingthe cake layer at the membrane surface. Liquid jets pro-duced by cavitation served as a basis for ultrasonic mem-brane cleaning. Lower ultrasound frequencies have highercleaning efficiencies than higher frequencies (Lamminen etal. 2004; Kobayashi et al. 2003). Intermittent ultrasound ir-radiation resulted in the same flux obtained as continuous ir-radiation but intermittent ultrasound consumed less energyand prolonged the lifetime of the membranes used, thus canbe considered as a cost effective method of membranecleaning (Tarleton and Wakeman 1990; Matsumoto et al.1996; Muthukumaran et al. 2005).

3. Sludge stabilization

Anaerobic digestion is the most commonly applied proc-ess for stabilization of sewage sludge. The process is morebeneficial than several other sludge stabilization methodsbecause it has the ability to produce a net energy gain inthe form of methane gas leading to cost-effectiveness. How-ever, anaerobic digestion is a very slow process and largefermenters are necessary. Enhanced performance of theanaerobic process could be achieved by finding a pretreat-ment to accelerate the slow and rate-determining hydrolysis.Ultrasound can produce various effects on biological materi-als, for example, stimulating enzyme activity, cell growth,biosynthesis, etc., which enhances the bio-activity of the ac-tivated sludge. Thus, the improvement in efficiency of en-hanced biological removal of phosphorus (Xie et al. 2008)and nitrogen (Zhang et al. 2008). Low frequency (25 kHz)was more effective than higher ones (80 and 150 kHz), orin other term, higher energy ultrasound was more efficientthan lower energy ultrasound for the sludge treatment, indi-cating that mechanical effects, instead of free radicals, wereresponsible for the bio-activity enhancement (Zhang et al.2007, 2008). Comparing with other pre-treatment methods,ultrasonication exhibits a great potential of not being hazard-ous to environment and for being economically competitive(Mao et al. 2004).

SonixTM is a new technology using high-power, concen-trated ultrasound for conditioning sludges prior to furthertreatment (Hogan et al. 2004). The studies (Hogan et al.2004), which used ultrasound energy at frequencies above20 kHz to create cavitation in secondary sludge, haveproved that the use of ultrasound to enhance anaerobic di-gestion can be achieved at full scale and it results effectivelyin the thickened waste activated sludge (typically difficult todigest), after sonication. The technology presents benefits interms of improved solids destruction, substantial increases ingas production and better residual solids dewatering.

Mao et al. (2004) had conducted ultrasound treatment ofprimary and secondary sludges to improve the quality ofsludges for the anaerobic digestion. Experiment results indi-cate that the significant reduction in particle size and in-crease in soluble organics could be achieved. Thus,ultrasonication could offer a feasible treatment method todisintegrate sludge efficiently. They also found that, ultra-sound treatment could be influenced by sonication densityand solid concentration. High power density ultrasound canimprove sludge disintegration, cell lysis, and inactivation(Zhang et al. 2007). The higher the sonication power em-ployed, the more the rupture of sludge particles and themore complete the deterioration of the structure.. Also, ahigher ultrasound density required less specific energy to de-rive a better sonication treatment.

Sludge disintegration was the most significant at low fre-quencies. Low-frequency ultrasound created large cavitationbubbles, upon which collapse initiates powerful jet streamsexerting strong shear forces in the liquid. Short sonicationtime resulted in sludge floc deagglomeration without the de-struction of bacteria cells. Longer sonication brought aboutthe break up of cell walls, disintegration of the sludge sol-ids, and release of dissolved organic compounds (Yin et al.2004; Nickel and Neis 2007).

4. Sediment and soil remediation

4.1. Heavy metals removalUltrasound has been used previously to enhance precious

metals recovery process by the cleaning action that removesan unwanted clay coating from raw ore and acceleratesleaching of minerals from the ore as well as improved filtra-tion rates (Newman et al. 1997). Nowadays, the method ofpower ultrasound is applied in mining process and soil re-mediation of heavy metals (Kyllonen et al. 2004). Themechanism of metal removal is based on the mechanical de-tachment promoted by the collapse of cavitation bubblesnear a solid, which can produce microjets that cause thesolid surface to pit and erode. Furthermore, shock wavesfrom cavitation in liquid-solid slurries can result in high-ve-locity inter-particle collisions that can also contribute to par-ticle size reduction. In addition, the ‘‘cavities’’ or areas oflow pressure in ultrasonic cavitation provide a sink of lowconcentration or partial pressure of the contaminant whereadsorbed material will desorb (Meegoda and Perera 2001).

An investigation of ultrasonic treatment of polluted solidmedium had been done by Newman et al. (1997) more than10 years ago. In that research, granular pieces of brick con-taminated with copper oxide were used as a model for con-taminated soil. Soil-washing was conducted by passingwater across the substrate on an ultrasonically (20 kHz)shaken tray. This ultrasonic treatment considerably enhancedthe process with 40% reduction in copper content comparedwith only 6% reduction by conventional shaking (Newmanet al. 1997).

Meegoda and Perera (2001) studied the 20 kHz sonicationcoupled with extraction using vaccum pressure in an inte-grated multi-step technology to remove heavy metal contam-inants from dredged residues. Important factors thatinfluence the treatment process were considered to be ultra-sound power, soil-to-water ratio, dwell time, and vaccum

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pressure. The study showed that the proposed treatmenttechnique was effective and economical for sediments withlower clay contents (only the silt fraction had a considerablemetal removal, while the clay fraction was insensitive to thetreatment). A maximum removal of 83% was obtained forsilt fraction at 1200 W power, 1:50 soil-to-water ratio and90 min of dwell time (Meegoda and Perera 2001).

Kyllonen et al. (2004) did a research on power ultrasoundas aiding method for the mineral processing technique,which has recently been developed for the remediation ofsoil contaminated by heavy metals. Power ultrasound wasused to disperse the soil to remove metals and metal com-pounds from soil particle surfaces instead of attrition condi-tioning. The soil diluted with water was treated using22 kHz ultrasound power of 100 W up to 500 W. The effectof different ultrasonic treatment time and pulsation of ultra-sound were studied on the purity of sink and float fractionsin heavy medium separation process, screen fractions, andmineral concentrates and tailings from flotation process. Ul-trasound enhanced the remediation of soil fractions in all thestudied cases (Kyllonen et al. 2004).

4.2. Organic decontaminationFollowing the successful application of high power ultra-

sound in areas of mineral processing, attention has been di-rected towards the remediation of contaminated soil andsediment (Collings et al. 2006). Ultrasonic leaching hasbeen investigated for the decontamination of different typesof soils from landfills, mining spills, and river sediments aswell as various types of contaminants like organic com-pounds.

An ultrasonically enhanced soil-flushing method for insitu remediation of the ground contaminated by nonaqueousphase liquid (NAPL) hydrocarbons was investigated by Kimand Wang (2003). Crisco Vegetable Oil was chosen as themodel compound. The soil-flushing tests were conducted intwo conditions — without ultrasound and with 20 kHz ultra-sonic waves. Experimental results indicated that ultrasonica-tion can enhance oil removal considerably. The degree ofenhancement depends on factors such as ultrasonic power,water washing flow rate, and soil type. Increasing ultrasonicpower will increase pollutant extraction only up to the levelwhere cavitation occurs. The effectiveness of ultrasonicationdecreases with flushing rate but eventually becomes constantunder higher flow rates (Kim and Wang 2003).

Mason et al. (2004) had reported some laboratory researchon ultrasonic soil washing of organic contaminants like pes-ticide DDT, PCB and PAH. Initial concentrations of DDT(250 ppm), PCB (250 ppm), and PAH (400 ppm) in sand(200 g contaminated fine sand in 200 g water) were re-moved ultrasonically (20 kHz, 170 W) by 70% after 10,25, and 3 min, respectively. The potential for the scale-upof this soil washing using acoustic energy was also reportedthere. Two basic mechanisms for acoustically enhanced soilwashing that have been suggested are abrasion of surfacecleaning and leaching out of more deeply entrenched mate-rial. According to Mason, factors that contribute towards im-provement in efficiency by the influence of ultrasoundinclude (i) the high-speed microjets formed during asymmet-ric cavitation bubble collapse in the vicinity of the solid sur-face enhance transport rates and also increase surface area

through surface pitting; (ii) particles fragmentation throughcollisions increase surface area; and (iii) diffusion is en-hanced by the ultrasonic capillary effect.

Collings et al. (2006) have developed high power ultra-sound to destroy persistent organic pollutants (POPs) in soilsand sediments. They have worked successfully on majorcontaminants, atrazine, simazine, total petroleum hydrocar-bons, DDT, lindane, endosulfan, 2,4,5-T, tetrachloronaphtha-lene and TBT. The range of contaminants they have studiedis broad sufficiently to suggest that high power ultrasoundwill be effective for most adsorbed large molecules. The re-sults indicates several advantages of high power ultrasonictechnology compared with conventional methods. These in-clude high destruction rates, the lack of dangerous break-down products, and low energy demands leading to low-cost. Moreover, the technology can be made quite compactand transportable while allowing on-site treatment.

4.2.1. Ultrasonically assisted advanced oxidative soilremediation

A new process for remediation of soil contaminated withorganic compounds (toluene and xylenes) has been proposedby Flores et al. (2007). The innovation combined the ad-vanced oxidation method using Fenton-type catalyst, withthe application of ultrasonic energy (47 kHz, 147 W, 10 minduration time for 20 g soil in 40 g aqueous solution). Exper-imental results showed that the application of ultrasound notonly assists the desorption of the contaminants from the soilbut also promotes the formation of hydroxyl radicals, whichare the main oxidant agent involved in the decontaminationprocess. The global efficiency of the process was noticeablyenhanced when applying ultrasonic energy because of a syn-ergistic effect in conjunction with the hydrogen peroxideconcentration and Fenton catalyst (Flores et al. 2007).

4.2.2. Ultrasonically enhanced electrokinetic remediationPrevious studies showed that electrokinetic technique was

applied to remove mainly heavy metals and the ultrasonictechnique was applied to remove mainly organic substancein contaminated soil. Thus, combination of the two techni-ques can be predicted to be helpful. Chung and Kamon(2005) have studied electrokinetic and ultrasonic remedia-tion technologies for the removal of heavy metal and poly-cyclic aromatic hydrocarbon (PAH) in contaminated soils.The study emphasized the coupled effects of electrokineticand ultrasonic techniques on migration as well as clean-upof contaminants in soils. Natural clay was used as a testspecimen, Pb and phenanthrene were used as contaminants.On one hand, Pb is a positive charged ionic contaminant; onthe other hand, phenanthrene is a neutrally charged nonioniccontaminant. The ultrasonic processor has a maximumpower output of 200 W with a frequency of excitation equalto 30 kHz.

When ultrasonic energy is applied into contaminated soil,the viscosity of fluid phase decreased and flow rate in-creased, the molecular movement increased, sorbed contam-inants mobilized, the cavitation developed, and porosity andpermeability increased. The removal efficiency of contami-nant is higher for combined electrokinetic-ultrasonic test(91% for Pb and 90% for phenanthrene) than for simpleelectrokinetic test alone (88% for Pb and 85% for phenan-

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threne). Therefore, the introduction of enhancement techni-que like ultrasonic process into electrokinetic process, couldbe effective for increasing thef contaminant removal ratefrom the contaminated soil.

5. Air pollution controlThe application of ultrasound in air pollution control is

based on acoustic agglomeration phenomenon that makessmall particles precipitated for easy removal. Acoustic ag-glomeration is a process in which high intensity soundwaves produce relative motion and collisions among fineparticles suspended in gaseous media. In an acoustic field,fine particles suspended in the air will migrate to the nodesof the sound wave, becoming concentrated. Once the par-ticles collide, they tend to adhere together to form larger ag-glomerated particles (Mason 2007). More details on acousticagglomeration mechanisms can be found in Hoffmann’s(2000) article . In general, acoustic agglomeration can beconducted in two approaches, with low frequency and highfrequency (ultrasound) sonication. While low frequencyacoustic field is more cost and energy efficient, high fre-quency acoustic (ultrasonic) agglomeration might achievebetter particle retention efficiency, especially for very smallparticles in submicron range (Hoffmann 2000).

Recently, Riera-Franco de Sarabia et al. (2003) studiedthe effect of humidity on the acoustic agglomeration of sub-micron particles in diesel exhausts at 21 kHz ultrasound.The presence of humidity raised the agglomeration rate bydecreasing the number particle concentration up to 56%.The results confirmed the benefit of using high-power ultra-sound together with humidity to enhance the agglomerationof particles that are much smaller than 1 mm (Riera-Francode Sarabia et al. 2003).

6. Environmental analysisThe use of ultrasound in environmental analysis brings

many benefits such as shorter analysis time, simplified ma-nipulation, and higher purity of final product (Chemat et al.2004).

6.1. Assisting microwave digestionConventional digestion is often done by prolonged heating

and stirring in strong acid solution that takes at least severalhours. In recent decades, microwave heating has been usedin analytical and organic laboratory practices as a very ef-fective and non-polluting method of activation. Moreover,nowadays, the simultaneous microwave and ultrasound irra-diation has been recognized as a new technique for atmos-pheric pressure digestion of solid and liquid samples inchemical analysis. The coupling microwave-ultrasound givessignificant improvement such as reduction of digestion time,reduction of the quantity of reagents, and reduction of con-tamination. In addition, the process can be totally automaticand more safety. The combination of these two types of irra-diation in physical processes like digestion, dissolution, andextraction appears very promising (Chemat et al. 2004).

6.2. Assisting solvent extractionMecozzi et al. (2002) proposed an accelerated ultrasound

assisted procedure for the extraction of the available humic

substance from marine sediments. The main advantage ofthe ultrasonic method is the shorten times of extraction thattakes only 30 min in contrast to the 24 h required by theshaking method. In addition, the extracts obtained were ho-mogenous and qualitative.

Ultrasonic solvent extraction of the organochlorine pesti-cide (OCP) including DDT, DDE, Dieldrin, methoxychlor,and mirex from soil was developed by Tor et al. (2006).The results obtained indicated that the ultrasonic solvent ex-traction method could be applied efficiently to extract OCPfrom soils with the recovery rate can be up to over 92%.Moreover, the ultrasonic solvent extraction is more rapid, astime consumption was reduced approximately 75% and 82%compared with conventional shake-flash and soxhlet extrac-tion. The solvent consumption is also significantly lowerthan soxhlet extraction, with 67% reduction.

Another new sample pretreatment technique, ultrasound-assisted headspace liquid-phase microextraction was appliedsuccessfully by Xu et al. (2007) to determine chlorophenolsin real aqueous samples. With high recovery ranging couldbe up to 100%, the proposed method demonstrated verypromisingly suitable for analysis of trace volatile and semi-volatile pollutants. Moreover, its advantages over the con-ventional headspace liquid-phase microextraction includesimple setup. ease of operation, rapidness, sensitivity, preci-sion, and no cross-contamination (Xu et al. 2007).

7. ConclusionSonochemistry has been developing rapidly in recent

years. Its potential in environmental applications is drawingmore attention. Ultrasonic bath is used widely in analyticallaboratory as an efficient method for solubilization, extrac-tion assistance, and cleaning. Moreover, the use of ultra-sound in environmental protection covers a broad range ofapplications: water treatment, soil remediation, and aircleaning. Among these environmental remediations, perhapsorganic water decontamination is the most extensively re-searched method as chemically ultrasonic effects work bestin aqueous medium because of free radicals formation dur-ing water sonolysis. However, the physical effects of ultra-sound are recognized in membrane filtration, sedimentheavy metal removal, dewatering, and air cleaning. Gener-ally, it is accepted that ultrasonication alone cannot be avery cost-effective technique. Ultrasound should rather becombined with other specific methods or work as an assis-tant for enhanced performance. Moreover, other physicalimpacts like heating, noise during ultrasonic process,andeconomic factor should also be considered, especially inpractical scale-up systems. Although ultrasound has largepotential in environmental applications, at this stage of de-velopment in sonochemical reactors, there are still consider-able technical and economic limitations for scale-up issues(Gogate and Pandit 2004). In general, as a young and inter-esting science, the applications of ultrasound in environmen-tal technology hold a promising future.

AcknowledgementsThe authors would like to thank the Maj and Tor Nessling

Foundation, Helsinki, Finland, for their funancial support inthis research.

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