Short review: Current trends and future challenges in the ... - SER

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Sustain. Environ. Res., 22(5), 271-278 (2012)

Short review: Current trends and future challenges in the application of sono-Fenton oxidation for wastewater treatment Ying-Shih Ma* Department of Environmental Engineering and Health Yuanpei University Hsinchu 300, Taiwan

Key Words: Hydroxyl radicals, intermediate, mineralization, sono-Fenton oxidation, ultrasound ABSTRACT

Currently, ultrasonic process has been widely used for research purposes, mostly in synthetic solutions spiked with one or several contaminants. Practical application of the ultrasonic process for wastewater treatment is often limited by its low mineralization efficiency on the target compound, formation of intermediate product(s), setup cost(s), and noise in operation. However, the combination of ultrasound and Fenton reagent, i.e., sono-Fenton oxidation, has great potential for rapid destruction of refractory organics in a short span of time through the mechanisms of thermal destruction and removal of free hydroxyl radicals. This review addresses the theory and the effects of operational parameters involved in sono-Fenton oxidation on various target pollutants. Finally, challenges in the application of sono-Fenton oxidation for wastewater treatment are critically assessed for future research. . INTRODUCTION Theoretically, advanced oxidation processes (AOPs) are known to generate highly reactive and nonselective hydroxyl radicals (·OH), which are able to oxidize almost all toxic organic compounds and nonbiodegradable pollutants. Among the ·OH producing AOPs, ultrasound is a novel method in which water molecules undergo molecular fragmentation and releases ·OH owing to high-frequency acoustic cavitation. In the past several years, ultrasound has been utilized extensively for the removal/degradation of organic pollutants from water/wastewater [1-8]. The major disadvantage of water sonolysis is the insufficient generation of ·OH. Thus, ultrasound is often supplemented with other oxidants such as H2O2, O2 and various AOPs for developing advanced hybrid techniques to increase the pollutant degradation efficiency . and also to decrease treatment time [9,10]. Several sonolysis studies conducted with chemicals addition such as Fenton's reagent with three states of iron (i.e., Fe0, Fe2+ and Fe3+) have proven the improvement of pollutant degradation [10-14]. Although the efficacies of these hybrid techniques are significantly greater, their fundamental concepts and reaction *Corresponding author Email: [email protected]

mechanisms still need to be researched. Furthermore, coupling of hydrodynamic cavitation in conjunction with the Fenton process is an effective method for the continuous remediation of aqueous organic compounds and non-biodegradable pollutants. Therefore, a proper understanding of sono-Fenton process, i.e., theory, optimal operating conditions, and future research challenges, will definitely improve its real-time application for the treatment of wastewaters or effluent containing organic pollutants. . THEORY OF SONO-FENTON OXIDATION Sono-Fenton oxidation generates two mechanisms to degrade organic compounds: (1) reaction with ·OH and (2) thermal cleavage. During sonication, extremely high temperature and pressure are generated with the collapse of the cavitation bubbles, which causes the rupture of O-H bond and results in the formation of ·OH (Eq. 1), where [)))] denotes the ultrasound. The effects of sonolysis arise mainly from acoustic cavitation, namely the formation, growth, and implosive collapse of bubbles in a liquid, which produces unusual chemical and physical environments. The collapse of bubbles generates localized “hot spots” with transient

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Ma, Sustain. Environ. Res., 22(5), 271-278 (2012)

temperature of about 5,000 K and the pressure about 6 x 104 kPa. During collapse, gas molecules are thermally fragmented to generate a variety of reactive species, including ·OH. Alternatively, organic compounds in the vicinity of a collapsing bubble may undergo pyrolysis degradation (Eq. 2). The generated ·OH can react with organic compounds and mineralize them to carbon dioxide and water (Eq. 3) [15]. As the Fenton's reagent is added, ·OH can be easily produced (Eq. 4) and the Fe2+ is oxidized to Fe3+. Then the Fe3+ will react with H2O2 to produce a complex intermediate (Fe-OOH2+) as shown in Eq. 5. Although the Fe-OOH2+ can be decomposed to Fe2+ and ·OOH (Eq. 6), the reaction rate is much lower [4,16,17]. When the sonoFenton process is applied to organic pollutant degradation, a synergetic effect between Fenton's reagent and ultrasound takes place, which spontaneously decomposes Fe-OOH2+ to Fe2+ and ·OOH (Eq. 7). This isolated Fe2+ can react subsequently with H2O2, produce ·OH again, and thus establish a cyclic mechanism. The combination of ultrasound and Fenton's reagent not only enhances the reaction rate of Fe2+ isolation from Fe-OOH2+ but also accelerates the formation of OH by the reaction between Fe0/H2O, Fe2+/H2O2 or Fe3+/H2O [2,10,16-18] as shown in Eqs. 8-11. Therefore, as the sono-Fenton process is carried out for the degradation of organic pollutants, two mechanisms such as (1) more ·OH are produced than sole ultrasound or Fenton process to react and oxidize the organic pollutant, and (2) thermal cleavage oxidation in ultrasonic cavitational bubbles takes place so that the degradation efficiency of organic pollutants can be improved. . H2O + )))

·OH + H·

(1)

X (organic compounds)(g) ·OH + X

CO2 + H2O

2+

Fe + H2O2 3+

Fe + H2O2 Fe-OOH

2+

Fe + ·OH + OH 2+

Fe-OOH + H

(4)

+

(5)

2+

(6)

2+

Fe + ·OOH (fast) (7)

Fe-OOH + ))) (or hv) Fe + H2O2 + ))) 3+

Fe + H2O + ))) Fe + 2H2O + ))) 0

-

Fe + ·OOH (slow)

2+

3+

Fe + ·OH +OH2+

Fe + ·OH +H

+

2+

Fe + H2 + 2·OH

2Fe + O2 + 2H2O + )))

(2) (3)

3+

2+

0

products

(8) (9) (10)

2+

2Fe + 2H2 + 4·OH (11)

OPERATION PARAMETERS IN SONOFENTON OXIDATION 1. Effect of Sonication Frequency

.

Most of the sono-Fenton oxidation studies have proven that the degradation of organic pollutants is increased when the ultrasonic power output increases [17,21,22]. Even though an increase in sonolysis in-

tensity will lead to greater sonochemical effects in the collapsing bubbles, a serious attention must be given to understanding the effect of ultrasonic frequency on . the degradation of organic compounds. Table 1 summarizes the operational conditions of several sonochemical studies adopted on wastewater treatments. Most studies in Table 1 used a fixed high ultrasonic frequency (> 200 kHz) [1,3,6,23,24] or lower ones (20-40 kHz) [4,13,17,25-28]. Nevertheless, these studies showed distinct effect of ultrasonic frequency on the degradation of pollutant. Oturan et al. [24] applied both high frequency (460 kHz) and low frequency (28 kHz) sonoelectro-Fenton method to degrade 2,4-dichlorophenoxyacetic acid (2,4-D). Experimental observations indicated no destruction of 2,4-D after 5 h of high frequency sonication and almost 100% of 2,4-D was degraded within 75 min reaction at low frequency (28 kHz) ultrasound. Ghodbane and Hamdaoui [6] investigated the sonochemical degradation of anthraquinonic dye, C.I. Acid Blue 25 (AB25) at frequencies of 22.5 and 1,700 kHz and the authors found that the initial degradation rates were 0.042 mg L-1 min-1 at 22.5 kHz and 0.15 mg L-1 min-1 at 1,700 kHz, where the best sonochemical degradation rate of . AB25 was observed at 1,700 kHz. Based on above descriptions, the enhancement of treatment efficiencies could be concluded as follows. At first, higher ultrasound power inputs in solution would increase the number of active cavitational bubbles and the subsequent generation of more ·OH. Furthermore, the increased number of acoustic cycles and cavitational collapses at high frequency lead to the increase of free ·OH so that the Ghodbane and Hamdaoui [6] found a higher ultrasound frequency was better than the lower one. However, the high ultrasonic frequency would not improve the wastewater treatment efficiency, due to the extremely rapid collapse of the ·OH producing cavitation bubbles [24]. In lower ultrasonic frequency condition, the longer lifetime of ·OH takes place and these free ·OH could effectively degrade the pollutants. Secondly, the chemical/physical properties such as boiling point, volatility and solubility of organic compounds would affect the oxidation or the destruction of complex organic compounds. For example, if the compounds are easily vaporized into the cavitation bubbles, a high frequency ultrasound process is useful for enhancing the degradation efficiency of organic pollutant, due to cavitational thermal cleavage. Otherwise, the lower ultrasound frequency process is a feasible method for the degradation of soluble organic compounds through ·OH oxidation . [3,29]. 2. Addition of Iron in Sono-Fenton System

.

Three types of irons, such as Fe2+, Fe3+ and Fe0, have been applied in sono-Fenton systems. Many researchers proposed that the increase of Fe2+/Fe3+ and

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Table 1. Studies utilizing sono-Fenton/sonocatalysis/sonolysis for the degradation of pollutants

DNOC: 4,6-dinitro-o-cresol

H2O2 in ultrasonic system provided higher oxidation capacity and enhanced the degradation efficiency of 2-chlorophenol and its intermediate [30], 2,4-dinitrophenol [16], AB1 dye [4] or dye reactive Brilliant Red [25]. This proved that Fe-OOH2+ formed in the Fenton process was easily decomposed to Fe2+ and ·OOH (Eq. 7) by ultrasonic irradiation, and the Fenton process (Eq. 4) was cycled to enhance the treatment efficiency, which could be named as the synergic effect . by ultrasound and Fenton process. Other researchers proposed the presence of Fe0 in the ultrasonic oxidation might improve the production of the ·OH as shown in Eqs. 10 and 11, and subsequently, increase the pollutant degradation efficiency [19,23,26]. Three mechanisms have been suggested to prove the above description. Firstly, the addition of Fe0 enhanced the mass transport as a result of transient cavitations created by the turbulent flow conditions within the reaction system [27]. Secondly, the presence of Fe0 led to an increase in the cavitation intensity by acting as nuclei for surface cavitation. This can proportionately increase the number of cavitation events occurring in the reactor [26]. Finally, the presence of

Fe0 in combination with H2O2 resulted in Fenton-like . chemistry leading to enhanced degradation. 0 However, Son et al. [19] found that the Fe could hinder the degradation of 1,4-dioxane (1,4-D) in ultrasonic system by competing with 1,4-D in the reaction with ·OH radical through Eq. 12. Therefore, as the particular iron is introduced into the ultrasonic system, the quenching effect of overdosed Fe0 on the . ·OH formation should be taken in consideration. Fe0 + ·OH

Fe2+ + ·OH- + e-

(12)

The effect of iron dosage on the sono-Fenton process was investigated in several studies and most researchers proposed that degradation of pollutants by sono-Fenton process was noticeably increased with the increasing amount of iron-salts [1,6,13,17,31]. The addition of iron-salts is generally useful in the increment of organic compound degradation; however, excess dosage of iron-salts will decrease the treatment efficiency, due to the reduction of ·OH by excessive addition of Fe2+ [5,6,28]. Therefore, reasonably higher iron dosages will only be beneficial under certain . conditions.

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.

3. Effect of pH

Several studies adopted acidic conditions for the sono-Fenton process for attaining satisfactory degradation efficiency [2,7,13,23,24,26], and the solution pH is an important parameter affecting the kinetics of the sono-Fenton process. Based on pH changes, the nature of organic compound, the scavenge of ·OH by H+ and the solubility of iron can affect the pollutant . treatments. Since 1990, it has been well accepted that the ultrasonic degradation of organic pollutants in aqueous solution depends strongly on the nature of the organics. At acidic condition, the organic compounds could easily pass through the cavitation bubbles, decomposed by ·OH in film zone and thermal pyrolysis in gas bubbles [7,16,32,33]. As the compound exists in molecular nature, it is easier to pass through the cavitation bubbles thus giving better degradation result. Otherwise, the ionic type compound can only be . oxidized by ·OH in aqueous solution. The presence of [H+] in the solution phase is also as an important factor on the formation of ·OH. Liu et al. [27] proposed the reaction of Fe0 in solutions with different pH values, which are shown in Eqs. 13 and 14. Based on Eq. 14, the Fe0 can react with H+ and form Fe2+ to produce more ·OH radicals under Fenton and sono-Fenton systems, which indicates that low pH level is superior to neutral and basic levels. However, additional [H+] was worthless for the treatment; the ·OH would react with H+ leading to less available ·OH thus lowering the organic compound degradation rate. Moreover, H2O2 might capture a proton to form oxonium ion H3O2+ in extreme pH values. Subsequently, H3O2+ would make H2O2 to be electrophilic and enhance its stability, which would reduce the reactivity between H2O2 and Fe2+. Therefore, very low pH levels will hinder the ·OH formation in . solution phase [4,17]. Fe0 + 2H2O Fe2+ + H2 + 2OHFe0 + 2H+ Fe2+ + H2

(13) (14)

When the solution's pH is greater than 4.0, Fe will precipitate in the form of hydroxide, which inhibits the ·OH formation and decreases the degradation of pollutants. Therefore, better degradation efficiency is generally found at acidic pH levels ranging from 2 to 4 [25]. Nevertheless, the wastewater itself seldom reaches the suitable pH levels. Therefore, determination of optimal reaction pH value is still important for degradation of different target compounds under sono-Fenton system. . 3+

FUTURE RESEARCH CHALLENGES 1. Applicability of Sono-Fenton to Treat Emerging Contaminants

Recently, increasing attention has been paid in the application of sono-Fenton process for the treatment of wastewaters containing many types of pollutants. However, applying the sono-Fenton process in emerging contaminants such as pharmaceutically active chemicals and endocrine-disrupting chemicals are not yet explored completely. First, as shown in Table 1, all investigations regarding sono-Fenton process with different types of ultrasonic producer (cup-horn or horn with tip) were carried out in lab-scale (i.e., reactor volumes were generally smaller than 300 mL). The maximum volume for a commercial ultrasonic producer was in litre-scale [13,34]. Even though, most researches shown in Table 1 propose that the sonoFenton degradation of chemicals is satisfactory, to scale-up the treatment capacity from a lab-scale to pilot or full-scale still needs to be investigated. As a solution, modifying the shape of sonicator from tip or horn-type to plate-type is a useful method. In fact, the plate-form sonicator has been practically used in real wastewater treatment plant in Taiwan to oxidize the industrial wastewaters. However, the expensive setup and hardware costs as well as operational skills are still limiting the applicability of a full-scale sonicator. Therefore, sono-Fenton method may be the answer to effectively shorten the necessary treatment time and reduce the operational cost for real wastewater treat. ment plant. Secondly, most of the investigations shown in Table 1 were focused on synthetically prepared single pollutant. When the researchers tried to investigate the degradation of organic pollutant, they always prepared the solution in distilled water and without cations, anions or other interferences. The effects of anions such as SO32-, CH3COO-, Cl-, CO32-, HCO3-, SO42- and NO3- (in combinations) on pollutant degradation are not investigated in detail [4], and the simultaneous effects of several anions on pollutant degradation need to be further studied. Using only one pollutant in sonoFenton system with constant temperature is to simplify the reaction for understanding the reaction kinetics and oxidation mechanisms. However, if several pollutants are presented in wastewater, the reaction kinetics and oxidation mechanisms are significantly different. Therefore, the well-known zero-order, first-order and pseudo first-order reaction kinetics are not suitable for estimating the kinetics of real wastewater treatment. . Finally, many types of pharmac eutically active chemicals and endocrine-disrupting chemicals are often present in hospital, medicine and industrial wastewaters [35]. Kasprzyk-Hordern et al. [36] proposed the amount of pharmaceuticals, personal care products, endocrine disruptors and illicit drugs in raw sewage was 10 kg d-1 and 55% of them could be removed by trickling filter while over 85% could be removed by activated sludge treatment. However, instead of being readily oxidized to intermediates or mineralized to CO2, these removed chemicals were retained


by the biological sludge cells hence the pharmaceuticals would be still contained in the sludge. Consequently, the AOPs are necessary to be used before biological method. Chitra et al. [37] showed the feasibility of applying different AOPs such as sunlight, UV and ultrasonic in combination with Fenton's reagent towards the degradation of 1,4-D. Moreover, the surfactant and 17á-ethynylestradiol could be effectively degraded by ultrasonic AOP [38,39]. All of the above observations propose that the sono-Fenton method is useful in the degradation of pharmaceuticals. However, treatments of many other types of pharmaceuticals such as codeine, macrolide, chlofibric acid, tetracycline, and so on, have not been well-discussed until now. Application of sono-Fenton method to oxidize the pharmaceutically active chemicals and endocrine-disrupting chemicals should be further . studied. 2. Possibility of Combining Sono-Fenton Treatment . with Biological Treatment Sono-Fenton process has been proven to be able to transfer the target compounds to intermediates or mineralize to CO2. Nonetheless, complete mineralization of the complex compounds is still difficult [13,14, 17,40]. Hence, a large amount of oxidants or longer reaction times are required to reach the high mineralization efficiency. Considering the cost of ultrasonic operations, large addition of oxidant or longer reaction time are not favorable methods. Thus, before combining the sono-Fenton method and biological treatment, three important facts including (a) suitable pH levels for sono-Fenton and biological methods; (b) effect of residual H2O2 in sono-Fenton method on the biological unit; and (c) profiles of toxicity or biodegradability from physical-chemical stage to biological . stage, should be preliminarily discussed. First, it is well-known that the acidic conditions are beneficial for the sono-Fenton process, especially at pH 3 [2,11,13,24,26]. After sono-Fenton process, the pH levels of effluents are similar to the influents [13,34]; this very low pH is improper for most aerobic and anoxic/anaerobic biological treatments. Therefore, pre-adjusting to pH 6-8 before the sono-Fenton treatment effluent passing into the biological units is a necessary step. Even though this pre-adjusting step will increase the operational cost, this is the key-step for combining the sono-Fenton method and biological . treatment. Secondly, H2O2 is generally used as the oxidant and disinfectant in water and wastewater treatment processes. Besides, H2O2 can be easily decomposed to H2O and O2 in solution phase so that the addition of H2O2 in activated sludge system is a useful method to avoid the bulking of sludge. However, the detergent characteristics of H2O2 will inactivate the microorganisms in biological process. Therefore, the content

275

of H2O2 in the effluent of sono-Fenton process should be reduced before passing through the biological unit. To reduce the concentration of H2O2 in solution phase before biological unit, aeration by air or violent mixing is a simple method to lead H2O2 to disappear from water. Also, adjusting the solution to a basic pH level can transfer to H2O2 as H2O so that the effect of H2O2 on the microorganisms is reduced. Thus, knowing the optimal dosage of H2O2 in sono-Fenton system can reduce the operation cost of AOPs and lower the effect . of residual H2O2 on microbial activities. Finally, Microtox® test, average oxidation state, carbon oxidation state and the ratio of biochemical oxygen demand/chemical oxygen demand have been proven to be the suitable indicator to evaluate the biodegradability of wastewater pretreated by AOPs [34,41-43]. Therefore, if toxicity of wastewater is reduced or biodegradability is enhanced by AOPs, this wastewater can be further treated with biological methods and closely meet the principle of Green . Technology. 3. A Cost Analysis and Energy Requirement and . Potential Limitations In ultrasonic oxidation system, heat is produced and most researchers need to use a water bath or a water jacket reactor to maintain a constant temperature (20 °C or room temperature) for avoiding a very high temperature generation in the solution phase [2,3,13,20,27]. In lab-scale sono-Fenton system, electricity consumption for operation of sonicator and temperature controller combined with fundamental chemicals usage (including Fe2+, H2O2, NaOH and H2SO4) are the major operational costs. Joseph et al. [15] mentioned that the increasing treatment cost associated with energy consumption could be offset by reducing required treatment time. Therefore, shorter treatment time should be considered as the first priority in sono-Fenton system to lower the cost. Ma et al. [13] used the sono-Fenton process to degrade the carbofuran; the operational energy of sonicator and temperature controller were 280 and 124 W, respectively. If the above system is carried out for 1 h, the electricity bill in Taiwan should be USD 42 m-3 sample with the carbofuran degradation of 22%. When 5 mg L-1 Fe2+ and 100 mg L-1 H2O2 were added into the sample with sonication, the cost of reagent degree chemicals was about USD 19.3 m-3 but the treatment time could be shorten to 1 min and the degradation of carbofuran reached almost 100%. Under this condition, the total cost of sono-Fenton system was USD 20 m-3. In addition, the removal of carbofuran by ultrasound and sono-Fenton process should be USD 1.9 and USD 0.2 g-1 carbofuran, respectively. This indicates that the cost of sono-Fenton process is much lower than that of sole ultrasound, due to shorter treatment time in sono. Fenton system.

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Finally, the use of homogeneous iron catalysts leads to the ferric hydroxide sludge during neutralization stage of sono-Fenton process, which requires additional separation and disposal treatment costs. The application of heterogeneous catalysts is a new choice, which can overcome this drawback. However, the application of insoluble solid-phase iron powder in sono-Fenton process requires suitable stirring speed to keep the iron powder in suspension. In addition, simplification of the procedures for solid-phase iron particulates recycling after wastewater treatments needs to be further investigated. Other than Fe0, ultrasound enhanced heterogeneous Fenton-like process is proposed recently where CuO, Cu/Al2O3, CuO·ZnO/Al2O3, iron powder, Fe2O3/SBA-15, a mixed (Al-Fe) pillared clay and goethite were employed as heterogeneous catalysts. The extremely high experimental operation costs restricted the sono-Fenton process in real plant application. Further studies with scaled-up equipment including alternative ways of inputting cavitation are necessary to improve the costeffectiveness in real-time application of this process. . CONCLUSIONS The addition of Fenton's reagent contributes to the effectiveness of ultrasonic degradation by developing a cyclic OH production reaction used in the rapid and accelerated degradation of wide range organic contaminants in laboratory. Even though sono-Fenton process appears to be the most promising method for commercially viable decontamination process, literature survey indicates that most of the experimental works were carried out in artificial systems consisting one or two compounds as the model pollutants. It would be of great importance to see the feasibility of this system in real-time industrial wastewater treatment. From an engineering view point, further investigation is necessary for commercializing the sonoFenton system. . ACKNOWLEDGEMENT The writers would like to thank the National Science Council, Republic of China, for financial support (grant NSC 100-2221-E-264-003). . REFERENCES 1.

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Discussions of this paper may appear in the discussion section of a future issue. All discussions should be submitted to the Editor-in-Chief within six months of publication. . Manuscript Received: June 22, 2012 Revision Received: July 28, 2012 and Accepted: August 3, 2012