This article was downloaded by: [Marshall University] On: 02 May 2013, At: 21:02 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Ozone: Science & Engineering: The Journal of the International Ozone Association Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/bose20 Degradation of Recalcitrant Surfactants in Wastewater by Ozonation and Advanced Oxidation Processes: A Review Keisuke Ikehata a & Mohamed Gamal El-Din a a Department of Civil and Environmental Engineering, Edmonton, Alberta, Canada Published online: 10 Aug 2010. To cite this article: Keisuke Ikehata & Mohamed Gamal El-Din (2004): Degradation of Recalcitrant Surfactants in Wastewater by Ozonation and Advanced Oxidation Processes: A Review, Ozone: Science & Engineering: The Journal of the International Ozone Association, 26:4, 327-343 To link to this article: http://dx.doi.org/10.1080/01919510490482160 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.
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This article was downloaded by: [Marshall University]On: 02 May 2013, At: 21:02Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,37-41 Mortimer Street, London W1T 3JH, UK
Ozone: Science & Engineering: The Journal of theInternational Ozone AssociationPublication details, including instructions for authors and subscription information:http://www.tandfonline.com/loi/bose20
Degradation of Recalcitrant Surfactants in Wastewaterby Ozonation and Advanced Oxidation Processes: AReviewKeisuke Ikehata a & Mohamed Gamal El-Din aa Department of Civil and Environmental Engineering, Edmonton, Alberta, CanadaPublished online: 10 Aug 2010.
To cite this article: Keisuke Ikehata & Mohamed Gamal El-Din (2004): Degradation of Recalcitrant Surfactants in Wastewaterby Ozonation and Advanced Oxidation Processes: A Review, Ozone: Science & Engineering: The Journal of the InternationalOzone Association, 26:4, 327-343
To link to this article: http://dx.doi.org/10.1080/01919510490482160
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematicreproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form toanyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contentswill be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses shouldbe independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly inconnection with or arising out of the use of this material.
Ozone: Science and Engineering, 26:327–343, 2004Copyright 2004 International Ozone AssociationISSN: 0191-9512 print / 1547-6545 onlineDOI: 10.1080/01919510490482160
c
Degradation of Recalcitrant Surfactants inWastewater by Ozonation and Advanced
Oxidation Processes: A Review
Keisuke Ikehata and Mohamed Gamal El-Din Department of Civil and Environmental Engineering, University of Alberta, Edmonton, Alberta, Canada
Received for review: 27 January 2004 Accepted for publication: 24 February 2004
Address correspondence to Mohamed Gamal El-Din, Department of Civil and Environmental Engineering, 304 Environmental Engineering Building, University of Alberta, Edmonton, Alberta, Canada T6G 2M8. E-mail: [email protected]
Keisuke Ikehata, Department of Medicinal Chemistry, 4048 Malott Hall, 1251 Wescoe Hall Drive, University of Kansas, Lawrence, Kansas 66045–7582, USA. E–mail:[email protected]
ABSTRACT Surfactants are used in varieties of industrial cleansingprocesses as well as in consumer products. Spent surfactants normallyenter domestic or industrial wastewater and are treated biologically.However, some of them are resistant to biodegradation and are releasedinto the environment. Thus, the toxicity and environmental persistenceof these surfactants are emerging concerns. Based on extensive review ofthe literature, ozonation and advanced oxidation using various combi-nations of ozone, hydrogen peroxide, ultraviolet light irradiation, andiron salts were found effective in degrading recalcitrant surfactants,including linear alkylbenzene sulfonates, alkylphenol ethoxylates, andquaternary ammonium surfactants. Biodegradability of these surfac-tants was improved after the treatment to some extent in the aqueoussolution as well as in real wastewaters.
KEY WORDS Ozone; Advanced Oxidation Processes; Alkylphenol Ethoxylates;Fenton Process; Hydrogen Peroxide; Linear Alkylbenzene Sulfonates; Ozonation;Quaternary Ammonium; Surfactant; Wastewater Treatment
urfactants, or surface-active compounds, are the organic mole-cules containing both hydrophilic and hydrophobic moieties, andare heavily used in detergents, pesticides, shampoos, cosmetics, and
other consumer product formulations around the world. The surfactantsused for a variety of industrial and domestic cleansing processes are foundin wastewaters. Biological treatment processes such as activated sludgeare probably the most economical means to remove these surfactantsfrom the wastewater; however, some surfactant molecules are not readilybiodegradable, and relatively high residuals and partially degraded prod-ucts are found in the treated effluents and receiving waters (Swisher,1987). Consequently, concerns over the globally distributed surfactants
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are growing because these surfactants are environ-mentally persistent and bioaccumulative, and becauseof their potential to cause adverse health and envi-ronmental effects. Effective treatment of wastewaterscontaining recalcitrant surfactants before their releaseto the environment, therefore, is necessary to preventthese potential problems.
Chemical oxidation can be used for the degrad-ation of recalcitrant organic pollutants includingsurfactants found in water and wastewater. Oxidantscommonly used for this purpose include potassiumpermanganate, chlorine, hydrogen peroxide (H2O2),Fenton’s reagent (H2O2/Fe2+) and ozone (O3)(Eckenfelder, 2000). In addition, the processesusing a combination of these oxidants as well asultraviolet (UV) irradiation, such as H2O2/UV,ozone/UV, and ozone/H2O2, forming hydroxylradicals (HO·) have also been actively investigated.Together with Fenton processes, which also utilizehydroxyl radicals as their primary oxidizing species,these processes are called advanced oxidation pro-cesses (AOPs). Hydroxyl radicals produced by AOPshave a higher oxidation potential (2.8V; relative tothe hydrogen electrode) than the parent oxidantsalone, and consequently improve the oxidation ofrecalcitrant organic pollutants. Although completemineralization of organic pollutants by AOPs is gen-erally possible, partial degradation of the pollutantsto more readily biodegradable products is likely amore feasible option because it can save oxidantrequirements (Alvares et al., 2001). The oxidationproducts subsequently can be removed by biologicaltreatment, which is more economical than the chem-ical oxidation.
A number of studies have been carried out toinvestigate the application of advanced oxidationand ozonation processes for the degradation ofrecalcitrant surfactants during the past two decades.Due to increasing public attention to the potentialhealth and environmental problems of trace organicpollutants in water, study on the effective removalof environmentally persistent surfactants is becominga more and more active research area these days. Inthis review, the surfactants that are potentially persis-tent in the environment and may cause health andenvironmental problems are identified, and the
molecular and environmental characteristics of thesesurfactants are summarized. Subsequently, the cur-rent status of research on the advanced oxidation andozonation of the recalcitrant surfactants is reviewedand discussed in the light of future perspectives.
CHARACTERISTICS OF SURFACTANTS
Structure of Surfactants
There are three classes of surfactants, includinganionic, cationic, and nonionic surfactants. As ahydrophobic moiety, a typical surfactant moleculecontains an either linear or branched alkyl chainwith a total of 10 to 20 carbon atoms (Figure 1).Some surfactants such as alkylbenzene sulfonates(ABSs) and alkylphenol ethoxylates (APEs) alsohave an aryl group in the hydrophobic moiety. Onthe other hand, the hydrophilic moiety of surfac-tant molecules varies among the classes.
Anionic surfactants are negatively charged inaqueous solution, usually originating in sulfonate,sulfate, or carboxylate groups. Anionic surfactantsrepresent a major fraction of surfactants used today(Scott and Jones, 2000). There are several commonvarieties of anionic surfactants, including linearalkylbenzene sulfonates (LASs) and ABSs, aliphaticsulfonates, fatty alcohol sulfates, and alcohol ethersulfates.
Most of the cationic surfactants contain a quater-nary ammonium group as the hydrophilic moiety.The quaternary ammonium surfactants commonlyused are alkyl trimethylammonium, alkyl benzyldimethylammonium, and dialkyl dimethylammo-nium compounds. There are also nonquaternaryammonium cationic surfactants such as alkyl dime-thylamine oxides, which are often used in detergentformulations (Swisher, 1987).
There are roughly two types of nonionic surfac-tants, APEs and fatty alcohol ethoxylates (AEs). Bothtypes of the nonionic surfactants have a chain ofpolyethoxylate as the hydrophilic moiety. Com-mercial APE or AE products are normally a mixtureof those molecules having different lengths of poly-ethoxylate chain. The nonionic surfactants with
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various average lengths of polyethoxylate chain,ranging from 7 to 20, are produced for desired physi-cochemical and detersive properties (Swisher, 1987).
Biodegradation and Environmental Impact of Surfactants
Most of the commercial surfactants currentlyused are synthetic organic compounds; thus, theyare xenobiotic compounds. Biodegradation of thesurfactants therefore, is limited because of the lackof enzyme systems in the microorganisms for theirutilization, although not all types of surfactants areresistant to biodegradation. The biodegradabilityvaries among the surfactant classes, depending onthe structure of the surfactant molecules, that is,types of functional groups, length of hydrophobicand hydrophilic chains, and branching structures,as well as among the microbes with different meta-bolic characteristics and acclimation (Swisher, 1987).
It used to be considered that surfactants areessentially nontoxic and harmless. However, recentstudies have revealed that some of the synthetic sur-factants and their biodegradation products indeedpossess potential health and environmental effects.Environmental or ecological effects associated withthe presence of synthetic surfactants in the aquaticenvironment are considered a serious problem becausethese compounds are often toxic to aquatic verte-brates and invertebrates, such as fish and aquatic
insects, have a potential to disrupt hormonal systemsof aquatic organisms, and may subsequently alterthe ecosystem.
The majority of surfactants currently used are con-sidered fairly biodegradable. Especially, nonbranched,aliphatic surfactants, such as alcohol sulfates, alco-hol ether sulfates, and alcohol ethoxylates, can bedegraded both aerobically and anaerobically, andthe typical biological treatment processes are oftencapable of almost completely removing these sur-factants from the wastewater (Scott and Jones, 2000).On the other hand, those surfactants containing anaryl ring such as LASs and APEs, as well as somequaternary ammonium surfactants are more resis-tant to ultimate biodegradation and are known topersist either in the treated wastewater or in thedigested sludge. Thus, effective pretreatment methodsof these compounds need to be explored, and oneof the promising approaches is chemical oxidationby ozonation and advanced oxidation processes.Some characteristics of these surfactants are sum-marized below.
Linear Alkylbenzene Sulfonates
LASs (Figure 1, compound 1) are the most widelyused anionic surfactants because of their excellentdetersive property and low cost (Swisher, 1987).LASs are obtained by reaction of the parent alkyl-benzene with sulfuric acid or sulfur trioxide to give
n + m = 7 11
H3C (CH2)n CH (CH2)m CH3
SO3
1
R = nonyl or octyl group, linear or branchedn = 7 20
R
(OCH2CH2)n
OH2
NH3C
R1
CH3
R2R1 = methyl, alkyl or aryl groupR2
= alkyl group (C12 to C18)
3
FIGURE 1 Molecular structure of surfactants: 1 linear alkylbenzene sulfonates (LASs), 2 alkyl phenol ethoxylates (APEs),3 quaternary ammonium surfactants (QASs).
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the sulfonic acid, which is then neutralized to givethe desired salt, often the sodium salt. LASs areintroduced as an alternative to ABSs, such as tetra-propylbenzene sulfonate, which is more resistant tobiodegradation (Swisher, 1987). A representativemember of LASs is dodecylbenzene sulfonate thathas 12 alkyl carbon atoms. Typical LAS productscontain a mixture of homologues such as differentlength of the alkyl chain and different isomers.
In recent years, acute toxicity of LASs to variousaquatic organisms has been reported, including themidge Chironomus riparius (Hwang et al., 2003), fat-head minnow Pimephales promelas (Mori et al., 2002;Rosen et al., 2001; Traina et al., 1996), giltheadSparus aurata sperm (Rosety et al., 2001), marinemicroalgal species (Moreno-Garrido et al., 2001), roti-fer Brachinous calyciflorus (Versteeg et al., 1997), juvenilerainbow trout (Buhl and Hamilton, 2000), and seaurchin Paracentrotus lividus sperm (Ghirardini et al.,2001). In addition, the biological effects of LASs onterrestrial organisms including soil microorganisms,plants, and soil fauna have also been evaluated( Jensen, 1999). Relationships between molecularstructure, physicochemical property, and toxicity(quantitative structure-activity relationships; QSARs)of different homologues and isomers of LASs alsohave been studied. These studies have indicatedthat the toxicity of LASs generally increases withincreasing number of carbon atoms in the alkylchain (Ghirardini et al., 2001; Moreno-Garrido et al.,2001), and interfacial properties of the surfactantmolecules correlate with bioconcentration and tox-icity of LASs (Rosen et al., 1999, 2001).
It has been proven that LASs are readily biodegrad-able by aerobic processes; however, these compoundsinhibit biodegradation in some types of biologicaltreatment processes, especially when they exist athigh concentrations (Swisher, 1987). It is also knownthat the degradation of LASs is inhibited underanaerobic conditions (García-Morales et al., 2001;Mösche and Meyer, 2002; Scott and Jones, 2000),and accumulation of LASs in digested sewage sludgehas been observed ( Jensen, 1999; Scott and Jones,2000). Although potential ecological impacts of theapplication of the LASs containing sludge to landsis worrisome (Elsgaard et al., 2001), fairly rapid deg-
radation of LASs in the applied soil has beenreported, and the impacts on the soil organisms andplants have been considered minimal (Jensen, 1999;Petersen et al., 2003; Scott and Jones, 2000).
Fate of the biodegradation products of LASs inthe aquatic environment also has drawn considerableattention in recent years (Eichhorn and Knepper,2002; Eichhorn et al., 2001; Trehy et al., 1996).During their biodegradation, LASs are subjected toω-oxidation that leads to the formation of carboxy-lic acid (Scott and Jones, 2000; Swisher, 1987). Thecarboxylic acid can then undergo β-oxidation to losecarbon atoms from the alkyl chain. The occurrenceof sulfophenyl carboxylates, the major biodegradationproduct of LASs, in the surface water has been con-firmed. Although sulfophenyl carboxylates are morepolar than parent LASs and less toxic (Swisher,1987), their persistence in surface water and drink-ing water sources may cause a potential problembecause of their limited biodegradability (Eichhornet al., 2002). It should be noted that unlike alkyphe-nols, which are biodegradation products of alkyl-phenol ethoxylates, sulfophenyl carboxylates do notappear to have an estrogenic activity (Routledge andSumpter, 1996).
Alkylphenol Ethoxylates
APEs (Figure 1, compound 2) are nonionic sur-factants used widely in various detergents, paint,pesticides, textile and petroleum recovery chemi-cals, and other commercial products. APEs are thepolyethers produced from alkyl phenols, such asnonylphenol and octylphenol, reacting with ethyleneoxide (Swisher, 1987). Due to the polymerizationreaction, commercial APEs contain a mixture of mol-ecules having various lengths of the polyethoxylatechain. The average number of the ethoxylate unit inAPE products may range from 7 or less up to 20 ormore, depending on the required physicochemicalproperties (Swisher, 1987). The alkyl chain of an APEis often highly branched. Although APEs are havingbeen replaced with more readily biodegradable non-ionic surfactants, alcohol ethoxylates, they still are inuse in many industrial applications around the world.
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In the presence of oxygen, biodegradation ofAPEs primarily involves shortening of the poly-ethoxylate chain, which gives eventually nonyl- andoctylphenols (Kravetz, 1981; Patoczka and Pulliam,1990). Ultimate biodegradation of these alkylphe-nols is considerably slower than the primary bio-degradation due to the presence of the branchedalkyl chain and the aromatic ring. As a result theproducts of primary biodegradation, alkylphenols,tend to build up in biologically treated wastewater.Because alkylphenols have a bulky hydrophobicalkyl chain, which shows an affinity for particulatematters, these compounds often have been foundin the primary and activated sludge at high concen-trations (Scott and Jones, 2000). It also has beenreported that the concentrations of APEs in anaero-bically digested sludge (900 to 1100mg/kg−1) appearto be higher than those in aerobically digestedsludge (0.3mg/kg−1). The land application of digestedsludge containing APEs and alkylphenols, however,removed these compounds fairly quickly and didnot show adverse effects on the soil fertility (Petersenet al., 2003).
APEs having a long polyethoxylate chain are highlypolar molecules and exhibit relatively low toxicitytoward aquatic organisms. However, the aquatic tox-icity of APEs increases with decreased length of thepolyethoxylate chain, and biodegradation of APEsgives less polar, more toxic APEs having ethoxylategroups shorter than the parent compounds (Patoczkaand Pulliam, 1990; Yoshimura, 1986). Nonethoxy-lated alkylphenols, the final product of primary bio-degradation of APEs, are the most toxic as well asenvironmentally persistent. Because of these facts,restrictions on the use of APEs have been put inplacein some countries (Scott and Jones, 2000).
Even more environmental concerns about thisclass of nonionic surfactants were provoked by reportsof weak estrogenic activities of nonylphenol as wellas mono- and diethoxylated alkylphenols in early1990s (Soto et al., 1991; White et al., 1994). Numer-ous reports on the ecotoxicology of these com-pounds have been published since then. Althoughthe contribution of APEs and alkylphenols to thexenoestrogen burden of humans is unknown, it hasbeen reported that these chemicals are present in
certain sewage outlets in concentrations sufficient tofeminize sentinel fish (Sonnenschein and Soto, 1998).Other adverse effects on various aquatic organismsdue to long-term exposure to APEs and alkylphenols,typically 4-nonylphenol, also have been reported(Bettinetti and Provini, 2002; Brown et al., 1999).
Quaternary Ammonium Surfactants
Quaternary ammonium surfactants (QASs;Figure 1, compound 3) are cationic molecules with atleast one hydrophobic long alkyl chain attached toa positively charged nitrogen atom. Several varietiesof QASs exist, including alkyl trimethylammonium,alkyl benzyl dimethylammonium, and dialkyl dime-thyl ammonium salts, as well as esterquats in whichhydrophobic alkyl chains are connected with esterbonds, and alkyl pyridinium and alkyl imidazoliumsalts. One of the major uses of QASs is as fabric soft-ener formulations, and the other uses include anti-septics, germicides, and detergents (Swisher, 1987).
In wastewater, cationic QASs are strongly adsorbedby particulate matters that are mostly negativelycharged. Thus, the large portion of QASs entered inwastewater has been found in primary and activatedsludges (Games et al., 1982; Scott and Jones, 2000).Aerobic biodegradability of QASs varies notablydepending on the molecular structure of the surfac-tants (Baleux and Caumette, 1977). Whereas straight-chain alkyl ammonium and some heteroaromaticammonium (hexadecyl pyridinium bromide and analkyl imidazolium chloride) compounds are fairlybiodegradable, other aromatic QASs and petroleumderived cationic surfactants have not degraded.Because of the strong tendency of QASs to beingadsorbed on sludge particles, anaerobic biodegrada-tion is considered to be more important than aerobic.It has been reported that anaerobic biodegradabilityof QASs also varies among the surfactant types aswell as the length of hydrophobic alkyl chain (Garcíaet al., 1999, 2000). However, because no anaerobicbiodegradation pathway for QASs has been sug-gested to date, more studies may be needed to eluci-date the overall biodegradation mechanisms for these
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compounds and their toxic effects on anaerobicmicroorganisms (Scott and Jones, 2000).
Reports on environmental impacts of cationicsurfactants including QASs are relatively scarce ascompared with those of anionic and nonionicsurfactants. Several reports have been publishedrecently on the toxicity of QASs to various aquaticorganisms (Mori et al., 2002; Roberts and Costello,2003; Sandbacka et al., 2000). The comparativetoxicity studies showed that alkyl QASs were moretoxic than anionic surfactants such as LAS andsodium dodecyl sulfate and nonphenolic nonionicsurfactants on fathead minnow cells (Mori et al.,2002), rainbow trout gill epithelial cells, and Daph-nia magna (Sandbacka et al., 2000). It also wasshown that the toxicity of the same type of QASs(i.e., alkyl trimethyl ammonium salts) increasedwith increased length of the alkyl chain (Sandbackaet al., 2000). A recent QSAR study for the toxicityof QSAs to fathead minnow demonstrated the rela-tionship between the modified log P (P = octanol/water partition coefficient) values, which were takenon account of micellization potential of the cat-ionic surfactants, and their toxicities (Roberts andCostello, 2003).
ADVANCED OXIDATION PROCESSES AND OZONATION
AOPs and ozonation have been extensively stud-ied for the removal of recalcitrant xenobioticorganic compounds as well as natural organic com-pounds from waster and wastewater. These pro-cesses have several advantages over conventionalchemical oxidation processes using potassium per-manganate or chlorine, including higher oxidationpotential, no production of potentially carcinogenicchlorinated by-products, and no persistence of theoxidant, which is toxic to microorganisms in subse-quent biological treatment processes (Alvares et al.,2001; Langlais et al., 1989).
The ozonation of organic pollutants involvestwo types of oxidation reactions, either molecularozone reactions (ozonolysis) or hydroxyl radicalreactions, depending on the reaction conditions
such as pH. At lower pH, molecular ozone reac-tions are predominant where organic compoundsare subjected to the electrophilic attack of ozonemolecules and decomposed into carboxylic acids asfinal end products. The molecular ozone reactionsare selective to the organic molecules having nucleo-philic moieties such as carbon-carbon double bonds,�OH, �CH3, �OCH3, and other nitrogen, oxygen,phosphorus, and sulfur bearing functional groups(Alvares et al., 2001). It is also known that aromaticcompounds are selectively decomposed throughozonolysis. On the other hand, in the presence ofhydroxyl anion (HO−) at high pH (>8), ozone mol-ecules are decomposed into free radicals (· and
·), and subsequently produces hydroxyl radical( ·), which will attack organic compounds. Theradical reactions are nonselective and very powerfulchain reactions, which can lead organic compoundsto the ultimate mineralization. The complete sets ofradical reactions can be found in the literature(Langlais et al., 1991; Wang et al., 2003).
For the enhanced production of hydroxyl radi-cals, AOPs utilize ozone, hydrogen peroxide(H2O2), or both as the primary oxidant(s) and UVirradiation, ferrous ion (Fe2+), or both as the cata-lyst(s). Ozonation under a basic condition (pH>8)is also considered to be an advanced oxidation pro-cess because it employs hydroxyl radical reactionsas described above. Ferrous ions are normally usedin combination with H2O2, and this type of AOP iscalled the Fenton process (H2O2/Fe2+). Ferrous sul-fate (FeSO4) is commonly used to supply Fe2+ ionfor the Fenton process. The Fenton process pro-duces sludge containing iron hydroxide as a by-product; thus, the sludge disposal has to be takeninto account for the cost assessment of this process.The Fenton process combined with UV irradiationis often referred as photo-Fenton process (H2O2/Fe2+/UV). This process involves photoreduction ofFe3+ to Fe2+ allowing a lower catalyst dosage andavoiding sludge formation. The cost for reagentsand equipment, energy requirements, and efficiencyof hydroxyl radical generation vary among theAOPs. The removal efficiency of organic pollutantsis also affected by the reaction conditions, includ-ing the presence of color, suspended solids, and
O2 –
HO2
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radical scavenging compounds such as humic sub-stances, carbonate, and bicarbonate, in the waste-water, pH, and temperature (Wang et al., 2003).
SURFACTANT DEGRADATION BY ADVANCED OXIDATION
AND OZONATION
There are roughly three approaches for theremoval of recalcitrant organic compounds, surfac-tants in this case, from wastewater using ozonationor AOPs: ultimate mineralization by the chemicaloxidation alone, the chemical oxidation as a polish-ing step for biologically treated wastewater, and thechemical pretreatment followed by biological treat-ment. The first approach likely is not feasible totreat wastewaters with high organic content, becausevery high dosages of oxidant will be required todestroy the pollutants completely. The use of AOPsor ozonation as a polishing step may be suitable if thepresence of surfactant(s) does not trigger inhibitory,toxic effects on the microorganisms in biologicaltreatment at the concentrations found in wastewa-ter. This approach may not work if the compoundsto be removed or their biodegradation products haveaffinity to suspended solids in biological treatmentprocesses, because such compounds will not appearin the treated effluent but in the disposed sludge athigh concentrations. Although the toxicity may notbe a problem in the case of surfactant removal, allof the recalcitrant surfactants identified in thisreview, including LASs, APEs, and QASs, and somedegraded products such as alkylphenols have sometendency to be adsorbed on the surface of suspendedsolids (Scott and Jones, 2000). Therefore, the lastapproach, chemical oxidation as a pretreatment, maybe a suitable option for wastewaters containing thesesurfactants, although the presence of additional oxi-dant demands such as radical scavengers and theorganic compounds other than the target compounds,which compete with each other for the oxidant, needsto be considered.
A number of articles on the surfactant degrada-tion by ozonation and/or AOPs have been publishedto date. The major surfactants of concern here are
LASs and APEs, as the trends in biodegradation andtoxicity studies also indicate. Degradation of QASshas yet to be studied intensively; only two articles,one published in 1989 and the other in 2000, areaccessible. These studies are summarized below. Tab-ulated summary of these studies is also presented inthe Appendix. Most of the studies have employedozonation or advanced oxidation as a pretreatmentmethod, with a few exceptions. It should be notedthat the “removal” of surfactants could have severalimplications: destruction of surfactant molecules sothat they could not be detected by surfactant spe-cific assays such as methylene blue active substances(MBAS) test; reduction in organic content meas-ured by chemical oxygen demand (COD), dissolvedorganic carbon (DOC), or total organic carbon(TOC); or mineralization to carbon dioxide (CO2)and water. Thus, the removal data cannot becompared directly if they have been reported in dif-ferent ways.
Removal of Linear Alkylbenzene Sulfonates
As far as the authors are aware, the first ozona-tion study on the removal of anionic surfactant waspublished in the early 1990s (Gasi et al., 1991). Inthis study, ozonation was actually evaluated as apolishing step of the effluent from upflow anaero-bic sludge blanket (UASB) reactors treating domes-tic sewage in Brazil in order to remove pathogenicmicroorganisms. Surfactant removal was monitoredas one of the parameters in the wastewater. Theeffluent from UASB reactors contained 4.63 to5.30mg/L−1 of anionic surfactants, presumably LASs,reported as MBAS along with other organic com-pounds, suspended solids, and microorganisms. Afterozonation for 30 to 50min with total applied ozonedosage of 15.9 to 16.7mg/L−1, 67% to 90% removalof surfactants was achieved. Data for the consumedozone dose are not available in this report.
The compatibility of advanced oxidation usingozonation followed by UV irradiation was studiedfor the treatment of laundry wastewater in nuclearpower plants, at which secondary waste generation
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such as spent activated carbon and other solidwastes was not desirable (Matsuo, 1997). The syn-thetic wastewater contained 7×10−3mol/L−1 (approx.84 mg/L−1) of LAS (alkyl C11) or potassium laurate,a component of liquid soap, as TOC was treatedwith ozonation followed by UV irradiation in a cir-culating batch reactor, and the effects of UV lampvapor pressure (1Pa and 105 Pa), initial dissolvedozone concentration (0 to 72mg/L−1), wastewatertemperature (10 to 40°C), and the type of surfactant(LAS and potassium laurate) on the surfactantremoval as TOC reduction were studied. The studyshowed that with a high-pressure UV lamp,3.3 ×10−4 mol/L−1 (= 15.6mg/L−1) initial dissolvedozone concentration were the optimum reactionconditions for the synthetic laundry wastewater.Data for the total applied and consumed ozonedoses are not available. Higher temperature (40°C)did not improve the treatment efficiency due to thelimited ozone solubility in wastewater. The TOCoriginated from the surfactants was removed effec-tively to the desired concentration, 8×10−4 mol/L−1
(= 9.6 mg/L−1), by the ozonation followed by UVtreatment. The effect of initial pH apparently wasnot studied here, although it would strongly governthe performance of the AOPs in general.
Lin et al. (1999) studied the effect of pH,amounts of ferrous sulfate, and H2O2 and tempera-ture on the batch Fenton oxidation of LAS (alkyC12) and ABS (alkyl C12) as well as the reactionkinetics of the chemical oxidation. The optimumreaction conditions for > 95% removal of 10mg/L−1
of surfactant include 90mg/L−1 ferrous sulfate,60 mg/L−1 H2O2, 50min of treatment time, and ini-tial pH around 3. It appears from the data presentedin Lin et al. (1999) that the type of anionic sur-factants has no significant effect on the Fenton oxi-dation, although statistics of the presented datawere not shown in the report.
Beltrán et al. (2000a) studied the kinetics of aLAS, sodium dodecylbenzenesulfonate (DBS), decom-position by ozonation. Their preliminary studyshowed that DBS decomposition was accelerated atbasic pH suggesting the involvement of hydroxylradical reactions. The calculated kinetic rate constantfor the reaction between hydroxyl radical and DBS
( · =1.6×1010 M−1 s−1) was much higher than thatfor direct ozone reaction (kd =3.68 M−1 s−1). Ozona-tion of synthetic and real domestic wastewaters con-taining 15 mg/L−1 DBS as well as other organic andinorganic compounds showed that the organiccompounds in the real wastewater compete withDBS for ozone and thus reduce the DBS removal.The highest reductions in COD and TOC removals(about 30% and 15%, respectively) were also achievedat pH 10 in both wastewaters, and biodegradabilityof the wastewater constituents were improved bythe ozonation. The kinetic rate constants of bio-degradation of DBS pretreated by ozonation in anintegrated ozone-activated sludge system were alsodetermined for its operation to treat synthetic andreal wastewaters (Beltrán et al., 2000b).
Cuzzola et al. (2002) evaluated the use of varioushomologous and heterogeneous iron catalysts forFenton-like oxidation of a commercial LAS mixture(alkyl C10 to C15) at pH 2 to 3 in the presence andin the absence of solar irradiation for 2h. Theauthors found that heterogeneous iron catalystsincluding FeO(OH), Fe(III)/γ-Al2O3, Fe(III)/SiO2had very little effect on Fenton-like degradation of1g/L−1 LASs in the absence of solar radiation (up to4% mineralization), but the effect increased in thepresence of solar radiation. A silica-based catalystFe(III)/SiO2 (3%) showed comparable, but inferiorphoto-Fenton activity (27% mineralization) to fer-rous sulfate (36% to 62%) without high metal pol-lution. They suggested a photo-Fenton treatmentscheme for wastewater treatment in which Fe(III)/SiO2 catalyst could be recycled. They also suggestedthat longer exposure to the sunlight might improvethe mineralization using the silica-based hetero-geneous catalyst further. It also can be suggested touse more stable light sources such as a UV lamp forthe heterogeneous photo-Fenton treatment.
A UV/H2O2 process was also examined recentlyfor the degradation of LASs (Sanz et al., 2003). Veryhigh concentration of a LAS mixture (2500mg/L−1,alkyl C10 to C13) was treated with various oxidant(H2O2) to surfactant molar ratios (rM) with a low-pressure UV lamp emitting monochromatic radiationat 254nm with a power output of 8W/L−1 in a recir-culated batch reactor. The authors found that the
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increasing H2O2 concentration improved the LASremoval efficiency up to a certain point (rM =15, inthis case), and the optimum pH was 2.2 to 3, whichis close to the pKa of LASs. Some kinetic parame-ters including quantum yields for LAS photolysisand rate constants for the radical reactions were alsodetermined in this study. The authors found highercontribution of radical reactions (up to 97%) thanin direct photolysis during the LAS degradation bythe H2O2/UV treatment, especially under the reactionconditions with high oxidant to surfactant molarratios.
Removal of Alkylphenol Ethoxylates
The application of ozonation to improve thebiodegradability of APEs was studied as early as thelate 1970s (Narkis and Schneiderrotel, 1980). Althoughthe ozonation alone caused only minor reductionin the organic content of the aqueous solution of40 mg/L−1 nonylphenol ethoxylates (NPEs; 2 in Fig-ure 1; average n =13 with a branched nonyl group)as measured by COD (up to 39%) or TOC (up to16%), it enhanced biodegradability of this nonionicsurfactant by transforming its molecular structure.Overall removal of 70% COD and 62.5% TOCwas achieved by biodegradation after ozonation(50.8 mg/L−1 of ozone consumed), in contrast with8% to 25% COD and 23% TOC reduction withoutozonation. The effect of pH and the length of poly-ethoxylate chain of NPEs (average n =4 to 30) onthe ozonation has also been studied (Narkis et al.,1985). The optimum pH for ozone degradation ofthe nonionic surfactants was determined to be 9.First-order ozonation reaction rate constants withrespect to surfactant concentration, COD, and TOCwere also determined for each surfactant. It was shownthat the reaction rates were higher for the NPEswith a longer polyethoxylate chain. The results indi-cated that ozone molecules appeared to attack pri-marily the polyethoxylate chain, and the oxidationof the aromatic ring occurred to a relatively smallerextent. Ozonation products of NPEs (average n=9.5)have been analyzed using high-performance liquid
chromatography (HPLC) and fast atom bombard-ment-mass spectrometry (FAB-MS) to investigatesome mechanisms of the surfactant degradation(Calvosa et al., 1991). From the analysis, the pro-gressive fragmentation of polyethoxylated sidechain was observed, which is consistent with thedegradation mechanism suggested by Narkis et al.(1985). Faster degradation of the polyethoxylatedside chain was observed at pH 4 than at pH 9,which is contradictory to the results reported byNarkis et al. (1985). Hydroxylation of the phenol ringwas also suggested to occur because of the appearanceof more polar compounds after the ozonation.
Adams et al. (1996) evaluated the effect of ozone/H2O2 advanced oxidation on the biodegradability ofseveral nonionic surfactants including NPEs (averagen =9). The surfactants were oxidized with 0.5, 1.0,and 2.0 mg absorbed ozone per mg surfactant and0.35mg H2O2 per mg absorbed ozone. The authorsobtained NPEs with shorter polyethoxylate chainsafter the ozonation, which is similar to the resultsobtained by ozonation alone (Calvosa et al., 1991;Narkis et al., 1985), or by UV/TiO2 photocatalyticdegradation (Pelizzetti et al., 1989), although biode-gradability of the surfactant was not improved withthe oxidant dosages used. Further study has shownthat relatively high oxidant dosages were required toenhance biodegradability of NPEs by ozone/H2O2advanced oxidation pretreatment (Kitis et al., 2000).As much as 7 mg absorbed ozone per mg NPEs(average n =5) was required to achieve more than80% DOC removal by ozone/H2O2-activated sludgebioassays (0.35mg H2O2 per mg absorbed ozone).The NPEs having longer polyethoxylate chains (aver-age n =40) appeared to be more amenable to ozone/H2O2 pretreatment. This may be because the mole-cular mass of these surfactant molecules is mainlyattributed to the polyethoxylate chain, on whichhydroxyl radicals are thought to primarily attack.
Fenton oxidation has also been examined as amean of pretreatment of aqueous APEs (Kitis et al.,1999). Two types of NPEs (average n =12 and 40;1000mg/L−1 as COD) were treated with 250 to2750mg/L−1 H2O2 and equal molar of Fe2+ for 10to 30min at 22°C. Higher dosages of H2O2(>750 mg/L−1) increased the biodegradable fraction
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surfactants (from 60% to 80% for NPE n =12, from70% to 88% for NPE n =40), whereas the biode-gradability was initially decreased with lower dos-ages. These results are consistent with the results ofthe ozone/H2O2 treatment (Adams et al., 1996).
Ferrero (2000) investigated the application ofFenton and photo-Fenton oxidations for the degra-dation of NPEs (average n =10). It was shown thatUV irradiation with a low-pressure mercury lamp(250W/L−1) could greatly enhance the TOC removalfrom the surfactant solution by Fenton process(109 mg/L−1 FeSO4, 1190mg/L−1 H2O2). It was alsoshown that increasing temperature (from 25°C to60°C) enhanced the Fenton oxidation of the non-ionic surfactants, whereas the photo-Fenton oxidationwas nearly independent of the reaction temperature.The author also studied the Fenton and photo-Fenton treatments of a dyehouse wastewater, presum-ably containing nonionic and other types of sur-factant as well as varieties of dyes, although actualsurfactant concentrations were not determined. Asimilar TOC removal enhancement by UV irradia-tion was observed in the real wastewater treatmentas well, although an initial lag period in the TOCremoval was noticed possibly due to the presence ofinorganic and/or organic radical scavengers in thereal wastewater. It can be suggested that detailedcharacterization of the wastewater compositions aswell as the treatment of synthetic wastewater withknown chemical compositions will likely help tounderstand the effects of radical scavengers on theAOPs.
Ike et al. (2002) studied the ozonation and UV/TiO2 treatment of the biodegradation products ofNPEs, including nonylphenol and nonylphenolmonoethoxylate, and nonylphenol carboxylic acid.The authors found that the effectiveness of ozona-tion (17mg/L−1 applied) for the degradation ofthese compounds was on the order of nonylphenolcarboxylic acid >> nonylphenol > nonylphenolmonoethoxylate, which was different from thatachieved by UV/TiO2 oxidation: nonylphenol >nonylphenol monoethoxylate > nonylphenol car-boxylic acid. This result suggests that the degrada-tion mechanisms of these compounds by the twooxidation processes are substantially different.
Removal of Quaternary Ammonium Surfactants
Ozonation of a QAS mixture containing dime-thyl and trimethyl alkyl ammonium compoundshaving unsaturated or saturated alkyl chains (alkylC16 to C18) was investigated in the late 1980s (Corlesset al., 1989). The objective of this study was not thedegradation or removal of QASs, but the determi-nation of ozonation products during potable watertreatment containing QASs. Nonquantitative deter-mination of QASs and ozonation products wascarried out using gas chromatography mass spec-trometry (GC MS) and FAB-MS. After a 30-mintreatment of 500µg/L−1 QAS sample with an initialozone concentration of 5 mg/L−1 at 10°C and pH4.5–5, quaternary ammonium carboxylic acids,aliphatic aldehyde, and aliphatic carboxylic acidwere detected by FAB-MS, which implies that a 1,3-dipolar cycloaddition reaction of ozone withcarbon-carbon double bonds in unsaturated QASsoccurred. The consumed ozone dose was not deter-mined in this report. The saturated QASs werefound unreactive under the tested conditions; how-ever, this is likely due to the relatively low reactionpH selected, at which direct ozonation dominates theoxidation reactions.
More recently, Adams and Kuzhikannil (2000)investigated the application of UV/H2O2 AOP forthe pretreatment of QASs to enhance the biodegrad-ability. Aqueous solutions containing 1000 mg/L−1
(as COD) of two types of QASs, alkyldimethylben-zyl ammonium chloride (alkyl C12 to C18) and dio-ctyldimethyl ammonium chloride, were treated witha low-pressure UV lamp with a principle wavelengthof 254nm and energy output of 0.56W/L−1 and1000mg/L−1 H2O2 at pH 8. After 30 to 120 min ofUV/H2O2 treatment, the authors found that thebiodegradability of one type of QAS with an aro-matic ring, alkyldimethylbenzyl ammonium chloride,was enhanced notably, whereas that of another,alkyl-based QAS, dioctyldimethyl ammonium chlor-ide, was not affected. Adams and Kuzhikannil alsoevaluated ozonation and ozone-based AOPs for thedegradation of QASs, and found that the rigid foamformation upon ozonation hampered the degradation
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study, although the foaming might have been avoidedby lowering surfactant concentration or increasinghead space in the reactor. The effect of initial pH onthe degradation of QASs by ozonation and AOPs maybe another problem of interest for future research.
CONCLUSION AND SUGGESTED FUTURE RESEARCH
As reviewed above, because of their potentialenvironmental impacts, such as aquatic toxicity,limited biodegradability, environmental persistence,and potential endocrine disrupting activities, effec-tive degradation or pretreatment of synthetic surfac-tants is becoming an active research area. Amongthe surfactants reviewed, LASs and APEs are the twomajor surfactants that have been studied for theirdegradation by ozonation and AOPs. Most types ofthe AOPs and ozonation were evaluated for thetreatment of these two surfactants, and the effec-tiveness of these processes was demonstrated, at leastto some extent.
Anionic surfactants, LASs, are relatively welldegraded in aqueous solution by ozonation andsome AOPs. The ozonation of LASs in syntheticand domestic wastewater as well as in the anaerobi-cally treated domestic wastewater has also beenreported (Beltrán et al., 2000a, 2000b; Gasi et al.,1991). Higher concentrations of LAS solutions (1 to2.5 g/L−1), which simulated high-loaded industrialwastewater, were also successfully treated with Fenton,photo-Fenton, and H2O2/UV processes (Cuzzolaet al., 2002; Sanz et al., 2003). Some aspects of thekinetics of LAS decomposition by ozonation andAOPs have also been studied. Possible future researchareas on the LAS degradation by ozonation andAOPs include: identification of degradation products,treatment of various real wastewaters, such as textile,wood, mining, and oil recovery, pulp and paperindustry wastewaters, containing high concentrationsof LASs to study the effects of wastewater matrices onthe chemical oxidation, and development of morestable catalysts for photo-Fenton processes.
Nonionic surfactants, APEs such as NPEs, incontrast, are relatively resistant to the degradation
by ozonation and AOPs. As a result, high oxidantconcentrations were often required to achieve a suf-ficient improvement in the biodegradability (Kitiset al., 1999, 2000). The majority of reaction pro-ducts of APEs by ozonation and AOPs have beenidentified as the APEs with shorter polyethoxylatedchains. This result is not encouraging because insuf-ficient ozonation or advanced oxidation pretreat-ment may result in the production of morerecalcitrant and toxic compounds, alkylphenols, inthe effluent. In order to improve biodegradabilityof APEs, methods for the efficient oxidation of theAPE benzene ring need to be developed. Studies onthe treatment of real wastewaters containing APEsas well as other organic and inorganic compoundscan also be suggested for the future investigation.
Unlike the two other surfactants listed above,ozonation and advanced oxidation of QASs havenot been well studied. This is probably due to therelatively low importance of this class of surfac-tants, that is, cationic surfactants accounted for lessthan 10% of all surfactants produced in 1982, andthis percentage remains almost unchanged to date(Scott and Jones, 2000). There also are some diffi-culties in analytical techniques for QASs becausethese compounds are not readily amenable to GCor HPLC analysis or colorimetric assays like theMBAS method for LAS determination. Neverthe-less, their high toxicity and limited biodegradabilitymay warrant more studies to explore the efficientdegradation or pretreatment by means of ozonationand AOPs.
Besides than the three types of surfactants reviewedhere, a few types of surfactant have also been treatedby ozonation or AOPs, including a type of nonionicsurfactant, linear alcohol ethoxylates (Adams et al.,1996; Brambilla et al., 1993; Kitis et al., 2000), anda component of liquid soap, potassium laurate(Matsuo et al., 1997). These compounds, however,are more readily biodegradable than LASs, APEs,and QASs (Scott and Jones, 2000). Another class ofsurfactants that has notable environmental and toxiceffects is perfluorinated alkyl surfactants, includingperfluorinated octane sulfonate and perfluorinatedoctanoic acid (Giesy and Kannan, 2002; Hekster et al.,2003; Schultz et al., 2003). These compounds are
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used widely in varieties of industrial and commercialproducts such as detergents, pesticides, protectivecoatings, and fire extinguishing agents. However,these fluorinated surfactants are very stable againstbiodegradation and persist in the environment (Giesyand Kannan, 2002). The treatment of these environ-mentally persistent fluorinated surfactants by ozo-nation or AOPs may be of interest for the futureresearch.
ACKNOWLEDGMENT
This work was funded by the Natural Sciencesand Engineering Research Council of Canada andAlberta Ingenuity Fund.
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