Degradation of phenol and m-cresol in aqueous solutions using indigenously developed microwave-ultraviolet reactor

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1 Journal of Scientific KARTHIKEYAN & Industrial & GOPALAKRISHNAN Research : DEGRADATION OF PHENOL AND m-cresol IN AQUEOUS SOLUTIONS Vol. 70, January 2011, pp Degradation of phenol and m-cresol in aqueous solutions using indigenously developed microwave-ultraviolet reactor S Karthikeyan* and A Navaneetha Gopalakrishnan Centre for Environmental Studies, Anna University, Chennai , India Received 13 July & 26 August 2010; revised 20 November 2010; accepted 23 November 2010 A MW-UV reactor developed indigenously for degradation of aqueous phenol and m-cresol (degradation efficiency, MW-UV- > UV- > MW- > MW-UV > UV > MW). Maximum degradation of phenol (85%) and m-cresol (92%) by MW-UV- and MW-UV- three folds faster in reaction time and catalyst requirement than UV- method. Degradation of phenol and m-cresol in reactor followed pseudo first order reaction and reaction rate (k) for MW-UV- 2.2 times greater than UV- and greater (17 times for phenol and 22 times for m-cresol) than other processes. MW-UV- 2.3 times more efficient in removal of TOC than conventional photocatalytic method. Keywords: m-cresol, Microwave-ultraviolet treatment, Phenol, Photocatalytic degradation, Wastewater treatment Introduction Advanced oxidation processes (AOPs) such as UV, UV-1, UV-H 2 O 22, UV-Fe-H 2 O 23, O 34, UV-O 3 5 etc., have been demonstrated as efficient treatment methods for recalcitrant pollutants like phenolic compounds. AOPs are destructive, non selective in nature and produce no sludge 6. Most AOPs use ferric or ferrous salts, which might cause secondary pollution 7, or ozone and hydrogen peroxide, which are costly. Moreover, their efficiencies need to be further improved from an economic point of view, especially when techniques are to be used for largescale operations 8. Depending on targeted tewater quality, extended treatment duration may involve unaffordable, high operating costs, mostly due to high energy and chemical requirements. Combination of microwave irradiation with ultraviolet irradiation (MW-UV) is reported 9 an efficient treatment method for destruction of organics in tewater. Although MW accelerates photocatalytic reaction, it is reported 10,11 that metal electrodes in conventional UV lamps in that reactors were easily damaged under MW irradiation and hence conventional UV lamps are not suitable for such reaction. Though microwave electrodeless lamp (MEL), which comprises an envelope or bulb containing plasma-forming medium, substitutes *Author for correspondence ksingaram@gmail.com conventional lamp as light source, mechanism of photolytic degradation of organic pollutants under MEL irradiation is uncertain. In MELs, polar solvents absorb most of MW energy at a low power; lamp does not have enough power to operate, it is impossible to carry out photochemical reactions at temperatures below solvent boiling point and overheating causes lamp failure. Special attention is also required when working with flammable and toxic material with such reactors 12. This study presents an indigenously developed MW-UV reactor for phenol and m-cresol degradation in aqueous solutions in presence/absence of under controlled temperature. Experimental Section Development of MW-UV Reactor MW-UV reactor (Fig. 1) developed using a domestic microwave oven (Kenstar-Ken chef, Model No. MWO 9807, Serial No. V807K00498, made in India; output power, 300W; frequency, 2450 MHz), a mercury lamp (SANKYODENKI 7B, made in Japan, supplied by Heber Scientific-India; 352 nm & 6 W; length, 24 cm) and a quartz cylinder [Air Blow Equipments Pvt Ltd, India; outer diam, 50 mm; height, 20 cm; inlet and outlet openings at opposite directions at top and bottom with a hole of 18 mm diam at center of cylinder (350 ml)]. Lamp inserted into quartz tube and both

2 72 J SCI IND RES VOL 70 JANUARY cm? 18m m? 50mm UV power supply 2 UV lamp (Hight 24 cm) 3 Quartz reactor (350 ml) 4 MW oven 5 MW power supply 6 Three way valve 7 Peristaltic pump 8 Coller Fig. 1 Experimental setup of combined UV-MW reactor are positioned in oven in such a way that electrode of lamp is not directly exposed to microwaves. Turntable, typically found in microwave cooking ovens, removed to prevent rotation of reactor and to avoid damage to quartz cylinder and mercury lamp. One end of silicon tube (diam, 8 mm) inserted at the bottom of quartz reactor and other end connected with top of the quartz reactor through a peristaltic pump (Model Watson Marlow 313S; made in England) and a condenser (supplied by Air Blow Equipments Pvt Ltd, India), which cooled by a cooler (Model LAUDA WK 1400; make-germany) to prevent excess heat generated and damage to reactor during microwave heating. Reactor checked for any possible leakages of microwave using an electrical field probe (C.A.43 Field Meter Model-CHAUVIN ARNOUX, make- France) - an antenna combined with high frequency detector (power, V-m; sensitivity, 100 KHz to 2.5 GHz). Reactor set up fully closed by an aluminum sheet to prevent any exposure of microwave through joints, door and especially from modified area to working environment. Microwave leakage absolutely nil in all directions after reactor closed by aluminum sheet. Estimation of Microwave Power and Control of Reacting Fluid Temperature Microwave power absorbed into water could be estimated as 13 P=cm T/t, where P is absorbed power of microwave (W), m is mass of water (g), c is heat capacity of water (4.184 J-g C), T is temperature rise ( C), and t is irradiation time (s). It found that water irradiated with input current (15 amp) presented C-s of temperature rise ( T/t) and of absorbed power. Temperature of water or reacting fluid 5 Efficiency, % Removal efficiency, % ph a) ph Fig. 2 Effect of ph on degradation at 120 minutes of: a) phenol; and m-cresol during experiments measured by a temperature indicator with a sensor (range C, type PT 100) immediately when stopping to irradiate microwave and maintained as 21 C. Chemicals Commercial reagents in analar (AR) were used for preparing samples without further purification. Phenol and m-cresol were purchased from Central Drug House Ltd., New Delhi, India. Titanium dioxide, Degussa P25 (mean particle size, 0.3µ; Anatase/rutile (3.6/1); crystalline form; surface area, 56 m 2 /g; band gap energy, 3.2 ev; procured from Germany) used as photocatalyst. Phenol / m- cresol (P/m-C) degradation carried out in following six phases: (i) Microwave irradiation in absence of (MW), (ii) Photodegradation with UV light (UV), (iii) Simultaneous microwave and ultraviolet radiations in absence of (MW-UV), (iv) Microwave irradiation in presence of (MW- ), (v) Photodegradation by UV and (UV- ) and (vi) Simultaneous MW and UV irradiations in presence of (MW-UV- ).

3 KARTHIKEYAN & GOPALAKRISHNAN : DEGRADATION OF PHENOL AND m-cresol IN AQUEOUS SOLUTIONS 73 Removal efficiency, % Efficiency, % Initial concentration of phenol, mg/l a) Removal efficiency, % Removal efficiency, % dose, g a) Initial concentration of m-cresol, mg/l Fig. 3 Effect of initial concentration on degradation at 120 minutes of: a) phenol; and m-cresol Catalyst dose, g Fig. 4 Effect of MW-UV on catalyst dose at 120 minutes on: a) phenol; and m-cresol Results and Discussion After 180 min of treatment, efficiencies of MW- for degradation of phenol (12%) and m-cresol (17%) were found slightly more than that of MW, MW-UV and UV processes (< 10%) due to adsorption of P/m-C over surface of particles. MW-UV- found superior to any other processes studied in degradation of P/m-C. It also found that efficiency decreased with increase in initial concentration. Degradation efficiency of MW-UV- at 180 min found maximum at natural ph as follows: 0.1 mm of phenol (natural ph 6.9±0.1), 85% (1.4 times more than UV- process) (Fig. 2a); and 0.1 mm of m-cresol (natural ph 5.0±0.1), 79% (1.6 times more than UV- process) (Fig. 2. With a low ph, surface of is occupied with H + ions. When ph increases, active hydroxyl groups on surface increase and accelerate P/m-C oxidation. As ph value increases further, oxidation power of photogenerated holes decreased and so decrease of P/m-C degradation occurred in alkaline solution. It also observed that MW-UV times higher than UV- for any initial P/m-C concentration studied (Fig. 3). Effect of MW-UV- on P/m-C degradation studied by varying dosage ( g) and compared with conventional UV- photocatalytic method (Fig. 4). Degradation efficiency found to increase with increase in catalyst dosage and attained maximum degradation at 0.6 g for phenol and 0.8 g for m-cresol. Amount of affects both number of active sites on and penetration of UV light through suspension. Excess suspended in aqueous solution could cause a shielding effect on light, resulting in decrease of P/m- C degradation for higher dosage. Phenol degradation efficiency of UV- at 0.6 g of 59%, and that of m-cresol at 0.8 g 70%, whereas this attained with only 0.2 g dose of by MW-UV-. It also observed that efficiency of UV- in

4 74 J SCI IND RES VOL 70 JANUARY 2011 Ln (Ct/Co) In (Ct/Co) a) a) TOC removal, % TOC removal % Fig. 5 Kinetics of removal of: a) 0.1 mm phenol (actual ph,, 0.6g/l); and 0.1 mm m-cresol (actual ph,, 0.8g/l) Fig. 6 Mineralization of: a) 0.1 mm phenol (actual ph,, 0.6g/l); and 0.1 mm m-cresol (actual ph,, 0.8g/l) 3 h achieved by MW-UV- within 1 h. Thus, effect of integrated MW-UV- method three folds more effective in reaction time and catalyst requirement (four fold in case of m-cresol) when compared with conventional UV- method. Degradation of phenol in MW-UV reactor follows pseudo first order reaction (Fig. 5). Reaction rate (k) of degradation due to MW-UV- found more than double for phenol (13.2 x 10-3 min -1 ; only 5.2 x 10-3 min -1 for UV- ) and m-cresol (15.0 x 10-3 min -1 ; only 6.7 x 10-3 min -1 for UV- ) compared to conventional UV- method at same experimental conditions. Similarly, time requirement to reduce pollutant concentration to half of its initial concentration (half life t 0.5 ) by MW-UV- less (60% for phenol and 56% for m-cresol) when comparing to UV-. Sequence of reaction rate of degradation : MW-UV- > UV- > MW- > MW-UV > UV > MW. TOC not influenced significantly by the effect of UV, MW, MW-UV and MW- (Fig. 6). However, 68% TOC of phenol and 76% TOC of m-cresol were removed by MW-UV- as against only 30-32% removal of TOC by UV-. It also found that TOC reduction obtained at 3 h by UV- method achieved at 1 h by MW-UV- method. After 3 h of irradiation, 30% and 68% of phenol mineralized into CO 2 and H 2 O by UV- and MW- UV- methods respectively. During degradation of phenol, aromatic compounds such as hydroquinone, benzoquinone and catechol were identified as main intermediates and evolution of intermediates confirms high efficiency of MW-UV- method. Hydroquinone, benzoquinone and catechol, which are OH adducts of phenol, are also toxic as well as phenol and are desirable to remove. These intermediates were detected from beginning of reaction upto 110 min for UV- method, whereas, they disappeared after 40 min of reaction for MW-UV-. The intermediates detected can be rationalized 14 assuming existence of an activation of phenol molecule by reaction with an *OH radical, forming an adduct that evolves to give a phenoxy radical and to form hydroxylated compounds such as catechol,

5 KARTHIKEYAN & GOPALAKRISHNAN : DEGRADATION OF PHENOL AND m-cresol IN AQUEOUS SOLUTIONS 75 Table 1 Unit operating cost for degradation of phenol by treatment alternatives Method of Time electrical energy per Cost treatment h order (EE/O) kwh/m 3 Rs/ m 3 Phenol m-cresol Phenol m-cresol Phenol m-cresol MW MW MW-UV MW-UV UV UV hydroquinone or benzoquinone. A complete mineralization to CO 2 as a final product involves breaking of C-C bonds and decarboxylation of intermediates by further reaction with *OH radicals. Earlier disappearance of identified aromatic compounds in MW-UV- may be attributed to generation of greater quantity of *OH radicals when dispersion of organic substrate is irradiated by MW and UV simultaneously 15. Hence, though degradation mechanism of phenol by UV- and MW-UV- seems to be same, elimination of phenol, its intermediates and their toxicity is speeded up in MW-UV- significantly. Operating cost (inclusive of energy cost) of UV and MW radiations and cost of photocatalyst estimated for degradation of 0.1 mm P/m-C in 1 m 3 solutions by treatment alternatives based on their electrical energy per order (EE/O, kwh/m 3 ) calculated as 16 EE/O = P x t x 1000/V x 60 x log(c i /C f ) where, P is rated power (kw) of AOP system, V is volume (l) of effluent treated in time t (min), C i and C f are initial and final concentrations (mol l -1 ) of phenol and conversion factor of 1000 (converting g to kg). Operating time required minimum (3 h) for MW- UV- compared with other processes studied (Table 1). It also found that cost of photocatalyst major contributor of unit operating cost than energy costs. Though unit operating costs of MW-UV- and UV- are almost same, operating time required by MW-UV- significantly less than any other treatment alternatives for degradation of phenol at same experimental conditions. Conclusions A MW-UV reactor, developed indigenously, found to be effective in degradation of P/m-C at controlled temperature conditions (21 C) in order to avoid risks while handling such materials in microwave radiations and significant degradation could be obtained even at low reacting temperature in MW-UV reactor. Degradation efficiency (MW-UV- >UV- >MW- >MW-UV >UV >MW). MW-UV- three folds faster in reaction time and catalyst requirement when compared with conventional UV- method for P/m-C. Degradation of phenol in MW-UV reactor followed pseudo first order reaction. MW-UV- 2.3 times more efficient in removal of TOC of P/m-C than conventional photocatalytic method. Unit operating time for MW-UV- 50% less than UV-. References 1 Zhifeng G, Ma R & Li G, Degradation of phenol by nanomaterial in tewater, Chem Engg J, 119 (2006) Chen H W, Ku Y & Irawan A, Photodecomposition of o- cresol by UV-LED/ process with controlled periodic illumination, Chemosphere, 69 (2007) Kavitha V. & Palanivelu K, Destruction of cresols by Fenton oxidation processes, Wat Res, 39 (2005) Huang C R & Shu H Y, The reaction kinetics, decomposition pathways and intermediate formations of phenol in ozonation, UV/O 3, and UV/H 2 O 2 processes, J Haz Mat, 41 (1995) Esplugas S, Gimenez J, Contreras S, Pascual E & Rodriguez M, Comparison of different advanced oxidation processes for phenol degradation, Wat Res, 36 (2002) Melian P E, Dýaz G O, Arana J, Rodrýguez J M, Rendon T E & Melian J A, Kinetics and adsorption comparative study on the photocatalytic degradation of o-, m- and p-cresol, Catal Today, 129 (2007)

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