Chapter 5: Background on advanced

Transcription

1 Chapter 5: Background on advanced oxidation processes Advanced oxidation processes are a series o methods that produce highly oxidative radicals to degrade persistent molecules. Several techniques are involved and they can be broadly divided in two: 1) methods that do not use radiation and 2) methods that use radiation (photochemical). Table 5-1 shows examples o advanced oxidation processes (APs) rom each category. Table 5-1. assiication o advanced oxidation processes [Domènech et al., 2004] on-photochemical processes Photochemical processes zonation in an alkali medium ( 3 / - ) zonation with hydrogen peroxide ( 3 /H 2 2 ) Fenton and related processes Sub and supercritical water oxidation Water photolysis in vacuum ultraviolet (UVV) UV/hydrogen peroxide (Fe 2+ /H 2 2 ) Electrochemical oxidation UV/ 3 Radiolysis γ and treatment with electron beam on-thermal plasma Photo-Fenton y related processes Heterogeneous photocatalysis Electrohydraulic-ultrasound discharge Many o the APs use the radical as their primary tool in the breaking down o recalcitrant molecules. The redox or oxidation potential is a measure o the voltage to which electrons 35

2 outside a system would have to be exposed to equal the electrochemical energy per mole that the electrons have in the system o interest [Benjamin, 2002]. Thus this parameter is an estimation o the energy o chemical species expressed in volts. The hydroxyl radical has the second largest redox potential (2.80 V at 25 C) (although Bard (1985) reports it at 2.38 V and Stanbury (1989) at 2.72 V) with the most potent being luorine (3.03 V at 25 C) and over the second choice or oxidation, 3 (2.07 V at 25 C). Thus this species is able to attack most organic compounds and it reacts times aster than ozone [Domènech et al, 2004]. In order to be eicient, APs must generate high concentrations o radicals in a steady state [Domènech et al, 2004]. Some APs (heterogeneous photocatalysis or radiolysis) use chemical reducers or species not subject to oxidation, like metallic ions or halogenated compounds. Many o the products mineralized by APs are toxic to microorganisms, which makes these processes suitable or pretreatment o water that can aterwards be treated by conventional biological technologies. For pesticides, Mabury and Crosby (1994) have reported the order or relative reactivity toward hydroxyl radicals or several pesticides: carbaryl> carbouran> mephanac> 2,4-D> propanil> molinate> atrazine> hexazinone> quinclorac. Some degradation pathways have been oered or 2,4-D and atrazine. Presumably, all APs would present similar rupture routes, since most are based on the hydroxyl radical to begin breakup. Figures 5-1 and 5-2 are proposed pathways or the mineralization o 2,4-D and atrazine. 36

3 H H H H C 2 Figure.5-1. Proposed degradation pathway o 2,4-D by ozonolysis. [Ikehata and El-Din, 2005] This model proposed by Ikehata and El-Din (2005) was studied using 3 as the radical inductor. The irst step is the separation o aromatic and aliphatic components o 2,4-D. C 2 is obtained ater continuous oxidation o the aliphatic chain. Regarding the aromatic ring, dierent oxidation pathways substitute or and eventually lead to the breakup o the ring. 37

4 For atrazine, Pelizzetti et al. ( ) have proposed the ollowing sketch to describe its mineralization. a H 3 C H H CH 3 H 3 C a H 3 C H 3 C H H 2 b H 2 H CH 3 b a a H 3 C H 3 C H H 2 a H 2 H CH 3 b H 2 H 2 H 2 H 2 c c H H 2 b H H 2 c c b H H Figure 5-2. Proposed degradation pathway or atrazine by heterogeneous photocatalysis. [Pelizzetti et al, ] 38

5 Pelizzetti et al. ( ), identiy dierent chemical reactions by letters. All arrows labeled a show lateral alkyl chain oxidation, b reers to dechlorination and c indicates deamination. As ar as the bibliographic research ound, cyanuric acid is the last species known or the mineralization by APs o atrazine. Cyanuric acid is toxic, but its degradation has been achieved by stable mixed bacterial cultures [Mandelbaum et al., 1993]. 5.1 on-photochemical processes /H 2 2 zone is a powerul oxidant, being the ourth most reactive species in Domènech s list in Table I [Domènech et al., 2004]. It has bactericidal properties and is commonly used or drinking water disinection. zone may be ormed in situ by supplying electrical current to oxygen and it is an unstable species, thus its lie is short. Since 3 is a gas injected to a liquid, mass transer resistances must be acquainted or. Used with hydrogen peroxide, the combined oxidizing power may result in rapid mineralization o compounds. It is an expensive process, but it can treat very diluted organic compounds (ppb). It works well in a ph range o 7-8 and the optimal molar relationship 3 /H 2 2 is 2:1 [Domènech et al., 2004]. zonation o organic compounds may go through two types o pathways, molecular ozone reactions (ozonolysis) or hydroxyl radical reactions. The path taken by the reaction depends on ph. At low ph ozonolysis is avored, while at phs > 8 ozone molecules are decomposed into ree radicals, - 2 and H 2, which in turn produce the hydroxyl radical [Ikehata and El-Din, 2004]. The general equation as written by Domènech is 39

6 3 + H H 2 (Eq. 5-1) There are commercial equipments available that use this technology, which is suitable or water with high-turbidity where radiation would be impeded Fenton and Fenton-like techniques The Fenton process is a useul technique to treat wastewater with high concentration o recalcitrant compounds due to its eiciency in producing hydroxyl radicals. It is based on the coupling o a transition metal with an oxidant. Fenton couples iron(ii) with hydrogen peroxide. Domènech et al. (2004) suggested that was ormed by the ollowing general reaction: Fe 2+ +H 2 2 Fe 3+ +H - +H (Eq. 5-2) Hydroxyl radical could then react through one o two pathways: oxidation o Fe(II) or the attack o organic matter. The process is limited to ph less than 3, and causes some iron sludge that needs post-treatment [Jeong and Yoon, 2005]. Pignatello (1992) describes the reactions that allow the regeneration o Fe 2+. The more oxidized species Fe 3+ acts as a catalyst or the decomposition o peroxide to 2 and H 2, regenerating Fe 2+ or the continued Fenton reaction: Fe 3+ + H 2 2 H+ Fe- 2 H 2+ +H + (Eq. 5-3) Fe- 2 H 2+ Fe 2+ +H 2 (Eq. 5-4) Fe 2+ +H 2 Fe H 2 (Eq. 5-5) Fe 3+ +H 2 Fe 2+ + H (Eq. 5-6) 40

7 + H 2 2 H 2 + H 2 (Eq. 5-7) When excess H 2 2 is used, Fe 2+ is in a lower concentration than Fe 3+ since reaction 5-5 is slower than reaction 5-2 [Pignatello, 1992]. There are, however, other compounds that are used as transition metal and oxidants. Anipitakis and Dionysiou (2004) reported the testing o Ag(I), Ce(III), Co(II), Fe(II), Fe(III), Mn(II), i(ii), Ru(III) and V(III) coupled with three oxidants; hydrogen peroxide (H 2 2 ), potassium peroxymonosulate (KHS 5 ) and potassium persulate (K 2 S 2 8 ). They conclude that the most eicient metals or the activation o each oxidant are: Fe(II) and Fe(III) or H 2 2, Co(II) or KHS 5 and Ag(I)or K 2 S 2 8. The reactions concerning the systems Co-KHS 5 and Ag-K 2 S 2 8 are presented: Co HS 5 Co 3+ + S (Eq. 5-8) Ag + + S Ag 2+ + S S 4 (Eq. 5-9) The Co-KHS 5 system operates well at a wider ph range, (2-8), while the Ag-K 2 S 2 8 needs to work under ph o 3 to avoid metallic precipitation and sludge ormation [Anipitakis et al., 2004]. The above mentioned authors reported that the S 4 - radical has a stronger oxidizing power than H at elevated ph (>5) with a redox potential between 2.5 and 3.1 V Electrochemical oxidation By applying electrical current to water the hydroxyl radical is ormed, which can then oxidize organic matter. Domènech et al. (2004) gives two equations that describe the process: 41

8 H 2 H + H + + e - Anodic oxidation (Eq. 5-10) 2 + 2H + + 2e - H 2 2 Catodic reduction (Eq. 5-11) Suggested electrical current is between 2 and 20 A [Domènech et al., 2004] and the material or the electrodes determines the production rate. Quiroz et al., (2005) report that depending on the electrode material, the oxidation mechanism proceeds through either the preliminary introduction o oxygen into the oxide lattice, which results in a change o the oxidation state o the metal (Ir 2 and Ru 2 electrodes) or the electrochemical combustion, that yields C 2 by a direct surace oxidation o the organic compound (Sn 2 and Pb 2 anodes). 5.2 Photochemical processes Photocatalysis has been deined by Plotnikov (1936) as the acceleration o a photoreaction by the presence o a catalyst. n the other hand, photocatalysis has been viewed as the enhancement o catalytic processes by their exposition to radiation. Many authors have assayed the dierence in reaction eiciencies and velocities under dark and radiated conditions. In this sense we have photoelectrochemical oxidation (Enea et al., 1999; Boye et al., 2003), photo-fenton and Fenton-like processes (Anipsitakis and Dionisyou, 2003; Jeong and Yoon, 2005; Pignatello, 1992; Sun and Pignatello, 1993;), photo-ozonolysis (Müller et al., 1998), and photo-hydrogen peroxide (De Laat et al., 1999) UV/H 2 2 In systems where one strong oxidant is exposed to radiation, the principle is that energy rom light is strong enough to 42

9 break the bonds between molecules and thus generate radicals. xidants must be photosensitive or this degradation to occur. Domènech et al. (2004), has proposed the ollowing equation describing the rupture o H 2 2 with a λ=254 nm: H hν 2H (Eq. 5-12) Photolysis o hydrogen peroxide is achieved using mercury vapor UV lamps o low or moderate pressure. Generally, λ=254 nm lamps are used although maximum H 2 2 absorbance occurs at λ=220 nm. When there is excess peroxide and high concentrations, there are competing reactions between the that can lower overall eiciency [Domènech et al., 2004] Photo-Fenton and related processes Recalling the basic equation o the Fenton process: Fe 2+ +H 2 2 Fe 3+ +H - +H (Eq. 5-2) This equation tells us that iron(ii) oxidizes to iron(iii), producing a hydroxyl radical. Another important step in the Fenton process is iron(ii) regeneration through equations 5-2 to 5-6, summarized as: Fe 3+ +H 2 2 Fe 2+ H 2 +H + (Eq. 5-13) This regeneration is slow and the use o radiation greatly increases the speed o reaction as: Fe 3+ () 2+ +hν Fe 2+ + (Eq. 5-14) 43

10 The reason or increased velocity is that Fe() 2+, the main Fe species dissolved in water, is photosensitive and absorbs light at wavelengths o 400nm or less [Anipitakis and Dionysiou., 2004]. The increased degradation velocity o organic molecules by submitting Fenton and Fenton-like reagents to UV light has been extensively discussed (Anipitakis and Dionysiou, 2004, Sun et al., 1993, Kaichouh et al., 2004). The enhancement o the velocity o reaction seems to be comparable to other Fenton-like systems. Anipitakis and Dionysiou (2004) report that degradation o 2,4- dichlorophenol, UV/Co/KHS 5 was twice as ast as Co/KHS 5, and or UV/Ag/K 2 S 2 8 complete degradation was achieved ater 1 hour while Ag/K 2 S 2 8 could only attain 12% degradation Heterogeneous photocatalysis In heterogeneous photocatalysis, the catalyst is in a dierent phase, usually solid. The chemical reaction is then a surace phenomenon, and adsorption mechanisms and surace area become important parameters. Catalysts used are semiconductor metallic oxides or salts, and ew have been ound useul under UV lamp or solar conditions. Moreno (2005) mentions some o these: Ti 2, Zn, CdS, iron oxides, W 3, ZnS. Each catalyst has its own characteristics and operation ranges, and selection depends upon the radiation source and organic compound to degrade. The more deeply studied catalysts are the wide gap metallic oxides, being titanium dioxide (Ti 2 ) the most studied [Moreno, 2005]. Titanium dioxide is a ine powder and has been used in supported systems or in suspension, orming two phases. When a semiconductor is excited with light o shorter wavelengths than the band gap, electron-hole pairs are ormed. These may recombine (releasing heat) or may migrate to the surace o the semiconductor where they can undergo redox reactions with 44

11 molecule and ions near the surace. The positively charged holes react at great speed with water to produce hydroxyl radicals, which in turn, attack organic molecules. The ollowing equations summarize the process Ti 2 undergoes in the production o [Halmann, 1996]: Ti 2 +hν e - + h + (Eq. 5-15) h + + H 2 H + + (Eq. 5-16) h (Eq. 5-17) Turchi and llis (1990) put orward our possible general mechanisms or the photodegradation o organic molecules by radiated aqueous Ti 2 : 1) While both the radical and the organic molecules are adsorbed on the Ti 2 surace. 2) Between an adsorbed organic molecule and a non-bound radical (ree in solution). 3) Between an adsorbed radical and a ree organic molecule hitting the catalyst surace. 4) Between a ree radical and a ree organic molecule in solution. 45

12 5.3 Figures- o-merit A igure-o-merit is a numerical quantity based on one or more characteristics o a system or device that represents a measure o its eiciency. Usually igures-o-merit are used to compare systems that result in the same eect and depend on certain common variables. Bolton et al. (2001) have suggested our igures-o-merit based on the amount o energy a cubic meter o water needs to achieve a certain level o pollutant mineralization. The authors organized their proposal by energy source (electrical, solar) and inluent contaminant concentration (high, low). They describe zero order kinetic behavior ( dc / dt =k) when dealing with high concentrations and irst order kinetic behavior ( dc / dt =kc) or lower ones. Electrical igures-o-merit are oered irst and are given in units o kilowattshour, since the cost o electricity is the major investment associated with these types o APs. Solar based igures are described next. They have units o square meter per cubic meter per order, since collector area is the main capital cost or these Electric energy per mass It is suitable or use with electric energy driven systems when the contaminant concentration is high (i.e. > 100 mg/l). It corresponds to a zero order kinetic behavior. Electric energy per mass (E EM ) is the electric energy in kilowatt-hours (kwh) required to reduce the concentration o a unit mass o a contaminant C in water or air. Equations 5-18 and 5-19 describe E EM : P E EM = (Eq. 5-18) FM ( c c ) i 46

13 E EM Pt1000 = VM ( c c i ) (Eq. 5-19) E EM Where = electric energy per mass (kwh) c = initial concentration o contaminant C i P= rated power o the system (kw) c = inal concentration o contaminant C 3 V= volume o luid treated (L) F= luid volumetric low rate (m /h) t = elapsed time (h) M= molar mass (g/mol) o contaminant C Electric energy per order Bolton et al. suggest the use o this igure when contaminant concentration is low and the AP is electric energy driven. It corresponds to a irst order kinetic behavior. Electric energy per order (EE) is the electric energy in kilowatt-hours (kwh) needed to degrade a contaminant C by one order o magnitude in a unit volume o polluted water or air. E E = P c F log c i (Eq. 5-20) E E = Pt1000 (Eq. 5-21) ci V log c Where E E = electric energy per order (kwh) c i = initial concentration o contaminant C P= rated power o the system (kw) c = inal concentration o contaminant C 3 V= volume o luid treated (L) F= luid volumetric low rate (m /h) 47

14 5.3.3 Collector area per mass When solar driven systems handle large inluent concentrations o a given pollutant, the collector area per mass (A CM ) is suggested. A CM is based on a zero order kinetic behavior. In other words it is the collector area required to bring about the degradation o a unit mass o a contaminant C in a polluted luid in a time when the incident solar irradiance is 1000 W/m 2. Equations 5-22 and 5-23 describe A CM : A CM = E o s t AE s t1000 o MV ( c c ) i (Eq. 5-22) A CM AE = s MF ( c c ) i (Eq. 5-23) Where A CM = collector area per mass (m 2 /kg) 2 E s= average solar irradiance (W/m ) M= molar mass (g/mol) o contaminant C c i = initial concentration o contaminant C c = inal concentration o contaminant C 3 V= volume o luid treated (L) F= luid volumetric low rate (m /h) 2 t = elapsed time (h) A= actual collector area (m ) t o=1 (h) E s o =1000 (W/m 2 ) 48

15 5.3.4 Collector area per order This igure-o-merit is used in solar driven APs when the concentration o contaminant C is less than 100 mg/l. It describes a irst order kinetic behavior. Collector area per order (A C ) stands or the collector area required to reduce the concentration o a contaminant C in a unit volume o polluted luid by one order o magnitude in a time t 0 (1 h) when the incident solar irradiance is 1000 W/m 2. A C is described by equations 5-24 and A C AEst = c V log c i (Eq. 5-24) A C AE s = ci Flog c (Eq. 5-25) Where A = collector area per order (m /m - C order) E s= average solar irradiance (W/m ) c = initial concentration o contaminant C i c = inal concentration o contaminant C 3 V= volume o luid treated (L) F= luid volumetric low rate (m /h) t = elapsed time (h) A= actual collector area (m 2 ) 49

16 Figures-o-merit proposed by the authors link kinetic reaction considerations with the energy received by the system and all are presented in the units o the largest economical cost. Thereore, the lower the result, the less energy must be supplied to the system and it will be more eicient. E EM, E E, A CM and A C are energy eiciency igures-o-merit; to ully evaluate the most costeective system other actors must also be considered. 50