Separation and Purification Technology 34 (2004) 35–42

Sonocatalytic oxidation processes for the removal of contaminantscontaining aromatic rings from aqueous effluents

Maria Papadaki a,∗, Richard J. Emery a, Mohd A. Abu-Hassan b,Alex Dı́az-Bustos a, Ian S. Metcalfe b, Dionissios Mantzavinos c

a Department of Chemical Engineering, University of Leeds, Clarendon Road, Leeds LS2 9JT, UKb Department of Chemical Engineering, UMIST, PO Box 88, Manchester M60 1QD, UK

c Department of Environmental Engineering, Technical University of Crete, Polytechneioupolis, GR-73100 Chania, Greece

Abstract

The effect of low frequency (20 kHz) ultrasonic irradiation on the removal of sodium dodecylbenzene sulfonate (SDBS),phenol, 2-chlorophenol and 3,4-dichlorophenol from aqueous solutions has been investigated. Sonolytic degradation of SDBS(at an initial concentration of 1 g/l) results in the formation of lower molecular weight compounds and is accompanied bylow total oxidation rates. In parallel, water sonolysis results in the formation of hydrogen peroxide. Several heterogeneouscatalysts (three noble metals and a metal oxide) were tested with respect to their effect on SDBS sonolysis. Of these, a CuO·ZnOsupported on alumina catalyst appears to enhance both SDBS fragmentation and total oxidation rates as well as hydrogen peroxideformation. Phenolic compounds (at an initial concentration of 0.1 g/l) are only partially removed by ultrasonic irradiation with2-chlorophenol being more susceptible to degradation than phenol and 3,4-dichlorophenol. However, the presence of Fe2+ ionsat concentrations as low as about 10−3 g/l generally increases the rate of the uncatalysed sonolytic degradation. This is attributedto iron being capable of readily decomposing hydrogen peroxide in a Fenton-like process to form reactive hydroxyl radicals aswell as being an effective oxidation catalyst.© 2003 Elsevier B.V. All rights reserved.

Keywords: Catalysts; Fenton oxidation; Phenols; Surfactants; Ultrasound; Wastewaters

1. Introduction

The second half of the last century has witnesseda rapidly deteriorating water environment as the out-come of enormous use of complex organic com-pounds, the spent parts of which were dischargedinto conventionally designed and operated wastewater

∗ Corresponding author. Tel.: +44-113-343-2420;fax: +44-113-343-2405.

E-mail address: m.papadaki@leeds.ac.uk (M. Papadaki).

treatment systems. Given that biological processeshave little effect in successfully treating biorefractorywastewaters, several thermochemical and advancedoxidation processes have been developed for eitherthe partial or complete destruction of hazardous andtoxic organic pollutants.

Only very recently, has considerable interest beenshown in the application of an innovative treatmentfor hazardous chemical destruction based on the useof ultrasound. The chemical effects of ultrasoundderive from acoustic cavitation i.e. the formation,

1383-5866/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S1383-5866(03)00172-2

36 M. Papadaki et al. / Separation and Purification Technology 34 (2004) 35–42

growth and implosive collapse of cavitation bub-bles in a liquid. Extreme temperatures of severalthousand degrees and pressures of several hundredatmospheres are developed locally within the bub-bles during their collapse with these bubbles servingas hot spot microreactors in an otherwise cold liq-uid. Destruction of chemicals is usually achievedthrough a combination of pyrolytic reactions oc-curring inside or near the bubble and hydroxylradical-mediated reactions occurring in the liquidbulk [1–3].

In previous studies, the beneficial effect of ultra-sonic irradiation on the removal of several targetcompounds from model aqueous solutions has beendemonstrated. Such compounds include, amongstothers, phenol, chlorophenols, nitrophenols, polychlo-rinated biphenyls, pesticides, polycyclic aromatic hy-drocarbons and surfactants [4–10]. Typical treatmentconditions include dilute aqueous solutions of initialconcentrations less than about 10−3 M (such concen-trations are typical of refractory compounds found innatural waters or in already treated effluents), low tomedium ultrasound frequencies from 20 to 500 kHzand irradiation times from several minutes to severalhours. These studies are comprehensively analysed ina recent review paper [3].

However, it is notable that relatively few studiesreport the use of ultrasonic irradiation in conjunctionwith an effective oxidation catalyst which could syner-gistically enhance degradation rates and consequentlyallow milder treatment conditions to be employed,thus reducing capital and operating costs. Ultrasonicirradiation in the presence of Cu2+ ions was em-ployed as an effective pretreatment step coupled withsubsequent wet air oxidation to treat effluents fromthe petrochemical [11] and textile [12] industries. Infurther studies [8], the beneficial synergy betweenFe2+ ions and ultrasound to enhance 3-chlorophenoldegradation rates in aqueous solutions has beendemonstrated.

The scope of this work is to study the effect oflow frequency ultrasound on the removal of variousorganic pollutants, such as surfactants and chlori-nated phenols, typically found in wastewaters pro-duced by chemical processing. In particular, thesynergy between ultrasonic irradiation and severalheterogeneous and homogeneous catalysts has beeninvestigated.

2. Experimental and analytical

Sodium dodecylbenzene sulfonate (SDBS), phenol,2-chlorophenol (CP) and 3,4-dichlorophenol (DCP)were supplied by Fluka and used without further pu-rification. Aqueous solutions containing either 1 g/lof SDBS or 0.1 g/l of the phenolic compound wereprepared and loaded into a glass vessel. A horn-typesonifier (Ultrason 250 manufactured by Labplant, UK)capable of operating either continuously or in a pulsemode at a fixed frequency of 20 kHz and a variableelectric output power up to 250 W was used for son-ication experiments. In all cases, sonication was per-formed continuously at an output power of 125 W.

Sonication of 1 l SDBS solutions was carried outwith or without catalysts. Several heterogeneous cat-alysts were used in catalytic experiments as follows:(a) noble metals: platinum (Pt), palladium (Pd) andruthenium (Ru) at a noble metal concentration of 5%w/w supported on alumina, and (b) a CuO·ZnO sup-ported on alumina. In all cases, 0.05 g/l of catalystwas used in the form of slurry. All experiments withSDBS were performed at ambient conditions withouttemperature control.

0.05 l solutions containing phenolic compoundswere subject to various treatments as follows: (a)sonication alone (US), (b) sonication in the presenceof Fe2+ ions (US + Fe2+), (c) oxidation by the Fen-ton reagent (Fe2+ + H2O2), and (d) a combinationof US and the Fenton reagent (US + Fenton). In allcases where iron ions were used as a homogeneouscatalyst, the appropriate amount of FeSO4·7H2O wasadded to the solution to give a metal concentration of0.81 × 10−3 g/l; in those experiments where hydrogenperoxide was added to the solution, its initial concen-tration was 0.35 g/l. All experiments with phenoliccompounds were carried out at a constant liquid phasetemperature of 35 ◦C in a temperature-controlled wa-ter bath. Samples periodically drawn from the vesselwere analysed as follows.

High performance liquid chromatography (HPLC)was used to follow concentration profiles of the orig-inal organic substrates. Different analytical methodswere developed to analyse SDBS and phenols as fol-lows: (a) SDBS was analysed on a 250 × 4.6 mmThermoHypersil C18 column using a gradient mobilephase comprising a 70% acetonitrile solution (bufferedwith 0.15 M sodium perchlorate) and pure water (also

M. Papadaki et al. / Separation and Purification Technology 34 (2004) 35–42 37

buffered with 0.15 M sodium perchlorate) at a flowrateof 1 ml/min and ambient temperature. The injectionvolume was 20 �l and detection was through a UVdetector set at 210 nm, (b) phenols were analysed ona 150 × 3 mm Waters RP8 SymmetryShield columnusing 40:60 acetonitrile:water as an isocratic mobilephase at 1 ml/min and ambient temperature. The in-jection volume was 20 �l and detection was througha UV detector set at 276 nm.

The liquid phase total organic carbon content (TOC)was measured by catalytic combustion/non-dispersiveinfra red (NDIR) gas analysis with a Shimadzu5050 TOC analyser. Total carbon was measured firstfollowed by inorganic carbon and TOC was deter-mined by subtracting inorganic carbon from totalcarbon. The relative standard deviation of three sep-arate measurements was always less than 2%. TOCproved to be useful in assessing the extent of to-tal oxidation of organics to carbon dioxide that hadoccurred.

Chemical oxygen demand (COD) was determinedby the dichromate method. The appropriate amountof sample was introduced into commercially availabledigestion solution (Hach Europe, Belgium) containingpotassium dichromate, sulfuric acid and mercuric sul-phate. The mixture was then incubated for 120 min at150 ◦C in a COD reactor (Model 45600-Hach Com-pany, USA) and the COD concentration was measuredcolorimetrically using a DR/2010 spectrophotometer(Hach Company, USA). The average value of six sep-arate measurements was taken, and the deviation ofthem never exceeded 1.5% for the range of COD con-centrations measured.

The concentration of hydrogen peroxide generatedduring sonication experiments was determined spec-trophotometrically at 410 nm using the titanium com-plexing method according to the procedures describedin detail elsewhere [13]. In some cases, SDBS sampleswere also analysed with respect to active detergency(AD) content using the crystal violet method [14].

3. Results and discussion

3.1. Degradation of SDBS

Fig. 1 shows the normalised TOC–time profilesduring the sonication of SDBS with and without

Fig. 1. TOC-time profiles during the sonication of SDBS solu-tions. -�-, uncatalysed; -�-, CuO·ZnO/Al2O3; -�-, Pt/Al2O3;-�-, Pd/Al2O3; -�-, Ru/Al2O3.

various catalysts. Of the various catalysts tested,CuO·ZnO/Al2O3 appears to be the most effective interms of TOC removal. In all cases though, the final(i.e. after 120 min) TOC removal was never greaterthan about 20%. The pH of the solution dropped fromits initial value of 8 to 6.5–7 within 10–20 min of son-ication and then remained almost constant throughoutthe course of reaction up to 120 min. These resultsimply that SDBS sonolysis is accompanied by theformation of acidic compounds, and that their concen-tration remains quite stable. At the conditions underconsideration, these acidic compounds appear to berefractory to total oxidation; however, interestingly,AD analysis showed that sonified solutions had de-creased levels of detergency. For instance, a 30% ADdecrease was recorded after 180 min of uncatalysedsonication. Fig. 2 shows the molecular weight (MW)distribution of original SDBS solution as well as ofsolutions obtained after 120 min of sonication withand without CuO·ZnO/Al2O3, where the UV detectorresponse is plotted against the elution time from thechromatographic column. All peaks of both sonicationruns have been magnified by a factor of 2.4 for clar-ity. As can be seen, peaks originally present at elutiontimes greater than about 8.6 min which correspond tohigher mass fractions of SDBS, do not appear in thetreated solutions, thus implying that partial fragmen-tation of SDBS has occurred. (Similar results werealso obtained for those experiments performed with

38 M. Papadaki et al. / Separation and Purification Technology 34 (2004) 35–42

Fig. 2. The MW distribution of original and sonified SDBS solutions.

noble metal catalysts). It has to be pointed out thatseparation of peak clusters to narrower MW rangeswas not possible with the present equipment and tech-niques; consequently, quantification based on peakareas was found to be inaccurate. However, for theCuO·ZnO/Al2O3-catalysed run most of the remainingpeaks are clearly smaller than those of the originalsolution as well as of the corresponding uncatalysedrun, and this is also consistent with the TOC profilesshown in Fig. 1.

Aqueous phase sonolysis is likely to result in theformation of hydrogen peroxide which may be formedthrough the recombination of hydroxyl radicals at thegas–liquid interface and/or in the solution bulk. More-over, if the solution is saturated with air or oxygen,peroxyl (O2H) and more hydroxyl radicals are formedin the bubble and the recombination of the former atthe interface and/or in the solution bulk results in theformation of additional hydrogen peroxide [1,15,16].

To test this, 0.15 l of pure water was subject to un-catalysed sonication and hydrogen peroxide concen-tration was recorded as a function of the sonicationtime. As can be seen in Fig. 3, hydrogen peroxide con-centration appears to increase linearly with sonicationtime (a correlation coefficient r2 = 0.996 is obtainedfor the linear regression of the concentration againsttime). These results are in agreement with those re-ported in previous studies [15,16], where the concen-tration of hydrogen peroxide generated during watersonication (carried out at various experimental condi-tions such as frequencies between 20 and 800 kHz,water saturated with various gases (i.e. air, oxygen,nitrogen, argon)) was found to increase linearly withincreasing sonication time.

Fig. 3 also shows hydrogen peroxide concentration–time profiles during the sonication of SDBS with andwithout catalysts. As can be seen, the concentrationof hydrogen peroxide reaches a maximum within

M. Papadaki et al. / Separation and Purification Technology 34 (2004) 35–42 39

Fig. 3. H2O2 concentration–time profiles during the sonicationof SDBS solutions or pure water. -�-, uncatalysed, SDBS; -�-,CuO·ZnO/Al2O3, SDBS; -�-, Pt/Al2O3, SDBS; -�-, Pd/Al2O3,SDBS; -�-, Ru/Al2O3, SDBS; -�-, uncatalysed, H2O (secondaryaxis).

the first 30 min of sonication with CuO·ZnO/Al2O3,Pt/Al2O3 and Ru/Al2O3 after which it decreasespresumably due to its decomposition. Conversely,hydrogen peroxide concentration remains relativelylow throughout the experiments with Pd/Al2O3 andwithout catalyst, which implies that it is not formedat appreciable concentrations or, if it is formed, itreadily decomposes. In a recent study [6], the mecha-nisms of uncatalysed sonolytic (at 200 kHz) degrada-tion of various surfactants including SDBS amongstothers, have been investigated. It was found thatSDBS mainly decomposes at the gas–liquid interfacethrough both hydroxyl radical oxidation and pyrolyticreactions (for instance, methane and carbon monoxidewere identified as the primary pyrolysis by-products)rather than in the solution bulk. This was attributedto the fact that SDBS, although a water soluble andnon-volatile molecule, tends to accumulate at thegas–liquid interface. Interestingly, it was also foundthat the concentration of hydrogen peroxide formedduring SDBS sonication experiments was substan-tially lower than that formed during water sonication(i.e. in the absence of substrate). Moreover, hydrogenperoxide formation decreased with increasing initialsubstrate concentration and was nearly completelysuppressed at SDBS initial concentration of 10−3 M(this is about a third of the initial concentration em-ployed in this study), thus implying the scavenging be-

haviour of SDBS against hydroxyl radicals. From theresults presented so far, it could be hypothesised thatinteractions between ultrasound and the metal cata-lyst (i.e. CuO·ZnO/Al2O3) result in increased rates offormation of hydroxyl radicals that partly recombineto hydrogen peroxide. Moreover, radicals are capableof oxidising the organic substrate to lower organicsand eventually to carbon dioxide and water, a processpromoted by the catalytic activity of copper, whichis known [17] to be an effective oxidation catalyst.It could be argued that the beneficial effects arisingfrom increased hydroxyl radical formation are com-promised by the fact that radicals are wasted throughrecombination reactions. This is so since hydrogenperoxide in its molecular form is a rather ineffectiveoxidant for the aqueous phase oxidation of organics.However, copper is known [18] to be an effective cat-alyst for hydrogen peroxide decomposition to severalreactive species (hydroxyl radicals mainly) and thisis consistent with the concentration profiles shown inFig. 3.

3.2. Degradation of phenol, CP and DCP

Figs. 4–6 show the normalised concentration–timeprofiles of phenol, CP and DCP, respectively, duringvarious treatments. As can be seen, all phenols are onlypartially removed by US treatment alone with about10, 15 and 20% removals being recorded after 180min for phenol, DCP and CP, respectively. Given that(a) sonolytic degradation of phenol and chlorophenols

Fig. 4. Concentration–time profiles of phenol during various treat-ments. -�-, US; -�- US + Fe2+; -�- US + Fenton; -�-, Fenton.

40 M. Papadaki et al. / Separation and Purification Technology 34 (2004) 35–42

Fig. 5. Concentration–time profiles of CP during various treat-ments. -�-, US; -�- US + Fe2+; -�-, US + Fenton; -�-, Fenton.

is known to occur predominantly through reactionsinvolving hydroxyl radicals [5,8,16], and (b) watersonolysis generates hydroxyl radicals partial recom-bination of which would result in the formation ofhydrogen peroxide, it was decided to investigate theeffect of Fe2+ ions on the degradation of phenols. Therational behind this is associated with the fact thatiron (a) is an effective oxidation catalyst for severalclasses of organics capable of degrading the organicradicals formed via reactions of free radicals with theorganic substrate, and (b) readily decomposes hydro-gen peroxide to regenerate more hydroxyl radicals(classic Fenton chemistry), thus promoting the oxi-

Fig. 6. Concentration–time profiles of DCP during various treat-ments. -�-, US; -�-, US + Fe2+; -�-, US + Fenton; -�-, Fenton.

Fig. 7. COD–time profiles during the sonication of phenolic com-pounds. -�-, phenol; -�-, CP; -�-, DCP.

dation of the organic substrate. As can be seen, withthe exception of DCP, iron (at a concentration as lowas 0.81 × 10−3 g/l) substantially increases the rate ofsubstrate removal. This was more pronounced in thecase of CP, where a 55% removal was recorded after180 min of sonication. These results are in line withthose reported by Nagata et al. [8] who studied thesonolysis of 3-chlorophenol (initial concentration of10−4 M) at 200 kHz and found that degradation ratesincreased by as much as 2.4-times in the presence of56 × 10−3 g/l of Fe2+.

It has to be pointed out that, in this series of ex-periments, hydrogen peroxide concentration was notdetermined analytically. However, an indication ofits presence in the reaction mixture came throughmeasuring the COD content of various samples takenduring a series of screening experiments with phenoland several chlorophenols (including CP and DCPamongst others). Fig. 7 shows normalised COD–timeprofiles obtained during the sonication of phenol, CPand DCP at an initial substrate concentration of 0.2g/l. In contrast to what would have been expected,COD values are scattered showing no clear decreas-ing trend; interestingly, COD values, in several cases,exceed that of the original solution, implying the pres-ence in the reaction mixture of compounds that inter-fere with the COD test. It is well documented in theliterature [19,20] that hydrogen peroxide has a posi-tive effect on the COD content of organic-containingsamples which is due to the reduction of potas-sium dichromate in an acidified solution to Cr3+

M. Papadaki et al. / Separation and Purification Technology 34 (2004) 35–42 41

as follows:

K2Cr2O7 + 3H2O2 + 4H2SO4

→ K2SO4 + Cr2(SO4)3 + 7H2O + 3O2

It has to be pointed out that increased COD valuesare not likely to be due to chloride interference as thiswas prevented by the addition of mercuric sulphatein the digestion solution. HPLC analysis also showedthat the degradation of phenolic compounds is accom-panied by the formation of several reaction interme-diates; however, the identity of such compounds wasnot checked for.

Phenols were also treated by the Fenton reagentwith and without ultrasonic irradiation. As can beseen, all phenols are readily oxidised by the Fen-ton process with about 85% substrate removal be-ing recorded after 130, 40 and 30 min for phenol,DCP and CP, respectively. However, coupling the Fen-ton reagent with ultrasonic irradiation had an unex-pectedly detrimental effect on the efficiency of theFenton process. It can be hypothesised that iron ionsand the ultrasound compete for hydrogen peroxidedecomposition through different routes which couldpossibly involve (a) catalytic decomposition to hy-droxyl radicals, and (b) thermal decomposition (whichmay occur in the vicinity of the cavitation bubblewhere extreme local conditions exist) to water andoxygen rather than to reactive radical species. If thisis the case, hydrogen peroxide is partly consumedthrough wasteful reactions and this would explain thedecreased efficiency of the combined Fenton and ultra-sound treatment. However, further investigations areneeded to understand the mechanisms involved in theprocess.

4. Conclusions

The sonolytic degradation of an anionic surfac-tant and three phenolic compounds with and withoutseveral catalysts has been investigated. Sonolysis oforganic compounds in aqueous phase occurs throughcomplex reaction mechanisms and pathways de-pending on the physicochemical properties of thesubstrate as well as the operating conditions underconsideration. Such mechanisms are believed to in-clude free radical reactions involving the participation

of hydroxyl radicals and hydrogen peroxide bothof which are formed during water sonolysis or/andpyrolytic reactions occurring in the vicinity of thebubble. A stable and inexpensive material capableof promoting the oxidation of organic substrate andeffectively utilising hydrogen peroxide would make apromising catalyst for the sonocatalytic degradation oforganic pollutants. In this respect, a copper-containingheterogeneous catalyst and homogeneous iron werecapable of promoting the sonolytic degradation ofSDBS and phenols, respectively.

Current research focuses on the effect of varioustreatment conditions such as ultrasound frequency,output power and mode of operation (i.e. continuousvs. pulse mode sonication), volume and concentrationof the liquid phase and type of catalyst on the chem-ical and biological properties of the waste stream.Such fundamental studies are needed to thoroughlyunderstand and rationally develop the proposedprocess.

Acknowledgements

The financial support of this work from Glaxo-SmithKline (UK), EPSRC and the Royal Society isgreatly acknowledged.

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