Kinetic degradation of the pollutant guaiacol by dark ... - Springer Link

Environ Sci Pollut Res (2011) 18:1497–1507 DOI 10.1007/s11356-011-0514-4

RESEARCH ARTICLE

Kinetic degradation of the pollutant guaiacol by dark Fenton and solar photo-Fenton processes Youssef Samet & Ines Wali & Ridha Abdelhédi

Received: 4 March 2011 / Accepted: 14 April 2011 / Published online: 3 May 2011 # Springer-Verlag 2011

Abstract This work is first intended to optimize the experimental conditions for the maximum degradation of guaiacol (2-methoxyphenol) by Fenton’s reagent, and second, to improve the process efficiency through the use of solar radiation. Guaiacol is considered as a model compound of pulp and paper mill effluent. The experiments were carried out in a laboratory-scale reactor subjected or not to solar radiation. Hydrogen peroxide solution was continuously introduced into the reactor at a constant flow rate. The kinetics of organic matter decay was evaluated by means of the chemical oxygen demand (COD) and the absorbance measurements. The experimental results showed that the Fenton and solar photo-Fenton systems lead successfully to 90% elimination of COD and absorbance at 604 nm from a guaiacol solution under particular experimental conditions. The COD removal always obeyed a pseudo-first-order kinetics. The effect of pH, temperature, H2O2 dosing rate, initial concentration of Fe2+, and initial COD was investigated using the Fenton process. The solar photo-Fenton system needed less time and consequently less quantity of H2O2. Under the optimum experimental conditions, the solar photo-Fenton process needs a dose of H2O2 40% lower than that used in the Fenton process to remove 90% of COD. Keywords Guaiacol . Degradation kinetics . Fenton . Solar photo-Fenton . COD removal

Responsible editor: Philippe Garrigues Y. Samet (*) : I. Wali : R. Abdelhédi UR Electrochimie et Environnement, Ecole Nationale d’Ingénieurs de Sfax, Sokra Street Km 3.5, BPW 3038, Sfax, Tunisia e-mail: [email protected]

1 Introduction The pulp and paper industry produces large quantities of wastewater that contain significant concentrations of contaminants like phenols, guaiacols, catachols, and vanillins. These wastewaters represent a risk factor for human health and the environment because they are difficult to degrade toxic compounds that generally end up in aquatic sources, where they can persist for long periods. Consequently, the remediation of these wastewaters has received much attention in the last decades. Among the several processes used in the treatment of these wastes, the so-called advanced oxidation processes (AOPs) appear to be a promising field of study due to the effective complete mineralization of organic contaminants under mild conditions (Ruppert and Bauer 1994; Andreozzi et al. 2000). AOPs are based on the use of a very strong oxidizing agent such as hydroxyl radical (HO·) with E° (HO·/H2O)=2.8 V/ NHE, which is generated in situ in the reaction medium (Hirvonen et al. 1996; Flotron 2004). Fenton (H2O2/Fe2+) and photo-Fenton (UV/H2O2/Fe2+) processes have proved to be effective and economical AOPs used for the detoxification and degradation of many organic compounds (Lin et al. 2000; Sabhi and Kiwi 2001; Catastini et al. 2002; Samet et al. 2009; Walling 1975). Oxidation with Fenton’s reagent is based on ferrous ion and hydrogen peroxide, and exploits the reactivity of the hydroxyl radical produced in acidic solution by the catalytic decomposition of H2O2 (Walling 1975; Chen and Pignatello 1997; Chamarro et al. 2001; Kang et al. 2002): þ

þ

Fe2 þ H2 O2 ! Fe3 þ OH þ HO

k1 ¼ 63 L mol1 s1

ð1Þ

1498

Environ Sci Pollut Res (2011) 18:1497–1507

Ferrous iron is slowly regenerated through the so-called Fenton-like reaction between ferric iron and H2O2 in acidic aqueous medium (Walling 1975; Chen and Pignatello 1997; Kang et al. 2002):

Eq. 11 (Walling 1975; Bergendahl and Thies 2004; Venkatadri and Peters 1993; Kang and Hwang 2000; Flotron et al. 2005; Sun et al. 2007), or by hydroxyl addition, as shown in Eq. 12 (Flotron et al. 2005).

Fe3þ þ H2 O2 ! Fe2þ þ HO2 þ Hþ k2 ¼ 0:01 L mol1 s1

ð2Þ

RH þ HO ! R þH2 O ! Products k11 ¼ 107 L mol1 s1

ð11Þ

ð3Þ

R þ HO ! HOR ! Products k12 ¼ 107 L mol1 s1

ð12Þ

Fe3þ þ HO2 ! Fe2þ þ O2 þ Hþ k3 ¼ 3:1 105 L mol1 s1

Nevertheless, numerous competitive reactions can also occur, namely the following ones, which negatively affect the oxidation process (Walling 1975; Kang et al. 2002; Bielski et al. 1985; Buxton et al. 1988; Bergendahl and Thies 2004; Burbano et al. 2005).

In photo-Fenton process, in addition to the above reactions the formation of hydroxyl radical also occurs by the following reactions (Eqs. 13 and 14) (Muruganandham and Swaminathan 2004; Will et al. 2004; Tamimi et al. 2008).

HO þ H2 O2 ! HO2 þ H2 O

Fe3þ þ H2 O þ hn ! HO þ Fe2 þ Hþ

þ

ð13Þ

H2 O2 þ hn ! 2HO ðl < 300 nmÞ

ð14Þ

k4 ¼ 1:2 107 L mol1 s1 HO þ Fe2þ ! Fe3þ þ OH k5 ¼ 4:3 108 L mol1 s1

HO2 þ H2 O2 ! O2 þ HO þ OH k6 ¼ 0:5 L mol1 s1

ð4Þ

ð5Þ

At acidic pH (2.5–5), the main compounds absorbing light in the photo-Fenton system are ferric ion complexes, e.g., Fe(OH)2+ and Fe(RCO2)2+, which produce additional Fe2+ (Eqs. 15 and 16) (Sagawe et al. 2001). þ

ð6Þ

FeðOHÞ2þ þ hn ! Fe2 þ HO ðl < 450 nmÞ

ð15Þ

þ

FeðRCO2 Þ2þ þ hn ! Fe2 þ R þ CO2 ðl < 500 nmÞ HO2 þ Fe2þ ! Fe3þ þ HO 2 k7 ¼ 1:2 106 L mol1 s1 HO þ HO ! H2 O2 k8 ¼ 5:3 109 L mol1 s1

ð7Þ

ð8Þ

HO þ HO2 ! O2 þ H2 O k9 ¼ 5:3 1010 L mol1 s1 HO2 þ HO2 ! H2 O2 þ O2 k10 ¼ 8:3 105 L mol1 s1

ð9Þ

ð10Þ

In the presence of substrate, as a target contaminant, the generated hydroxyl radicals are able to attack most of contaminants either by hydrogen abstraction, as shown in

ð16Þ The rate of organic pollutant degradation could be increased by irradiation of Fenton with UV or visible light (photo-Fenton process). This leads not only to the formation of additional hydroxyl radicals but also to recycling of ferrous catalyst by reduction of Fe(III). In this way, the concentration of Fe2+ is increased and the overall reaction is accelerated. Solar technology can be used as alternative to UV lamps to reduce the degradation process costs. Thus, photo-Fenton degradation of contaminants using solar light has been successfully used being an economically viable process since solar energy is an abundant natural energy source and can be used instead of artificial light sources which are costly and hazardous (Robert et al. 2004; Gumy et al. 2005; Monteagudo et al. 2009). The aim of this work is the optimization of Fenton process for the degradation of guaiacol as model compound of pulp and paper mill wastewater. In order to improve the reaction rate, solutions were subjected to solar radiation


1499

using a laboratory-scale reactor. The influence of different operational parameters such as H2O2 dosing rate, initial concentrations of guaiacol and Fe2+, temperature, and pH was investigated.

2 Materials and methods The experiments were performed in an experimental device, whose scheme is shown in Fig. 1. This device consists of an aluminum frame (0.5 m length, 0.5 m width, and 0.6 m height), which supports: (1) a platform in aluminum placed 34° to the horizontal on which was fixed a solar reactor consisting of a borosilicate glass tube (4.5 m length, an inner diameter of 6 mm, and outer diameter of 8 mm) snake-shaped; and (2) an Erlenmeyer flask (Pyrex 1 L) at which the guaiacol solution was prepared with bidistilled water. This Erlenmeyer flask was immersed in water bath to control the working temperature by a thermostat (Julabo Labortechnik GMBH, Sellback, Germany). The treated volume was 0.4 L, the solution was circulated through the reactor using a peristaltic pump (Cole-Parmer Instrument, Chicago, Illinois 60648 USA)

3

2 1 4 6

5

Fig. 1 Scheme of the experimental installation. (1) Erlenmeyer flask (guaiacol solution), (2) H2O2 solution, (3) solar reactor, (4) peristaltic pump, (5) thermostatic bath, (6) magnetic stirrer

with a flow rate of 140 mL min−1. This pump is simultaneously used for the flow of the H2O2 solution in the Erlenmeyer flask with a constant rate of 0.4 mL min−1 (more details were given in the top of the Section 3). The quantity of the ferrous sulfate was introduced into the solution at startup. The solutions were continuously stirred using a magnetic stirrer (Tacussel, France). The pH values were adjusted using a prepared 1 M sulfuric acid solution and measured using a pH meter (PHM multi-parameter analyzer, pH/mV Belgium Kingdom). The hydraulic system used was a closed circuit which prevented the evaporation of the solution. The solar photo-Fenton experiments were performed at the Electrochemical and Environmental Laboratory, National Engineering School of Sfax (approximately 3 m above sea level, latitude 34°44′ N; longitude 10°45′ E), Tunisia. All tests were conducted between 11:00 a.m. and 3:00 p.m. on sunny days from July to October 2009. The global solar radiation intensity was approximately 850 W/m2. For tests using only the Fenton reagent, the experimental device was kept away from solar radiation by covering with a black plastic film and an aluminum foil. Samples (0.5 mL) were withdrawn from the reactor at selected intervals for COD and absorbance analysis. COD was measured using a spectrophotometer (Shimadzu UVMini 1240 UV/Vis Spectrophotometer) using a dichromate solution as the oxidant in strong acid media (Kolthof et al. 1969). The absorbance was measured with the same spectrophotometer. Guaiacol is colorless; however, its oxidation products (dihydroxylated rings such as catechol, methoxyhydroquinone, methoxycatechol, and their quinonic forms) have dark color may be at low concentrations. The absorption maximum was detected at a wavelength of 604 nm. Therefore, the variation of absorbance of the treated solutions was followed with time at this wavelength. Samples were previously centrifuged using a Micro-12 Hanil centrifuge. All samples were tested in duplicate, and the test was reproduced three times for each sample, so that the relative errors could be minimized. All the figures show the average values. Reaction intermediates were detected by the use of high-performance liquid chromatography (HPLC) analysis system (model 1100, Hewlett-Packard) equipped with a Hamilton PRP ×300 column (Metrohm) as described in our previous paper (Samet et al. 2002). Guaiacol, catechol, and methoxyhydroquinone (analytical grade) were purchased from Aldrich (Gillingham, Dorset, UK) and were used as received. Ferrous sulfate heptahydrate (FeSO4 7H2O) was obtained from Riedel-de Haën (Seelze-Hannover, Germany) and used as the Fe(II) catalyst. Hydrogen peroxide (35% v/v) and sulfuric acid

1500


Ln (CODO/COD)

800

−1

COD (mg L )

1000

3 Results and discussion 3.1 Oxidation of guaiacol by the Fenton process 3.1.1 H2O2 injection mode In most studies, hydrogen peroxide is added at once at startup. If a sufficiently high concentration of reagents is added ([H2O2]0 >[Fe2+]0>> [substrate]0), it will form a large amount of hydroxyl radicals in the early response. Therefore, substrates are rapidly degraded. However, some of hydroxyl radicals will be consumed by hydrogen peroxide in excess (Eq. 4), and by the termination reaction between hydroxyl radicals (Eq. 8), which has a relatively high constant rate (5.3×109 L mol−1 s−1). Furthermore, the total ferrous ions will be oxidized in a few minutes, and the slow regeneration of these ions according to Eq. 2 will limit the Fenton process. The several or continuous addition of hydrogen peroxide can overcome all these problems. Indeed, a quasi-stationary concentration of radicals HO· can thus be maintained throughout the reaction. Several authors have shown that continuous addition of H2O2 in the Fenton or photo-Fenton processes is more effective than the addition of all the quantity of H2O2 at the startup (Monteagudo et al. 2009; Yalfani et al. 2009; Silva et al. 2007). So in the present work, H2O2 was injected continuously in the solution from the beginning to the end of reaction at a constant flow rate of 0.4 mL min−1. Since the residual H2O2 interferes with the measurement of COD (Kang et al. 2002), the residual amount of H2O2 was also measured, using the permanganate titration. This method is suitable for measuring solutions of hydrogen peroxide in the range 0.25 to 70 wt.% According to Lin and Lo (1997), 1 mg L−1 of H2O2 contributes 0.27 mg L−1 COD concentration. Since no H2O2 residual concentration higher than 0.25 wt.% was measured, no correction was performed to COD analysis. 3.1.2 Kinetics of COD removal Figure 2 shows the decrease of COD during the oxidation of guaiacol (COD0 =1,080 mg L−1) by Fenton’s reagent. It can be seen that COD values decreased almost exponentially and 90% of COD removal was obtained after approximately 45 min. If we suppose that the oxidation of guaiacol and its by-products by the hydroxyl radicals HO· is of a first order with respect to COD and the hydroxyl radicals concentration is constant during the

kapp = 0.07 min−1

5

1200

were provided by Merck (Darmstadt, Germany). All solutions were prepared with bidistilled water.

600

4

2

R = 0.983

3 2 1 0 0

20

400

40

60

Time (min)

200

0 0

10

20

30

40

50

Time (min)

Fig. 2 Trend of COD during the oxidation of guaiacol solution by Fenton process. The inset panel shows its kinetic analysis assuming that COD follows a pseudo-first-order reaction. Dosing rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM, pH=3, and T=40°C

treatment, the oxidation rate (r) can be given by the following equation: r¼

dCOD ¼ k½HO a COD ¼ kapp COD dt

ð17Þ

where k is the reaction rate constant, α is the reaction order related to the hydroxyl radicals, and kapp ¼ k ½HO a the apparent rate constant. The integration of Eq. 17 subject to the initial condition COD=COD0 at t=0 leads to the following equation: COD ¼ COD0 expðkapp tÞ

ð18Þ

kapp could be calculated from the plot of Ln (COD0/COD) versus t (inset of Fig. 2). As it can be seen, points lie satisfactory in straight line with correlation coefficient greater than 0.96. kapp was used to study the effect of different concentrations of H2O2, Fe2+, guaiacol, and for different temperatures and pH. 3.1.3 Effect of the dosing rate of hydrogen peroxide The dosing rate of H2O2 is considered as one of the most important factors which should be considered in the Fenton process. The effect of the dosing rate of hydrogen peroxide on the efficiency of the oxidation process was investigated under the operating conditions (COD 0 = 1,080 mg L−1, [Fe2+]0 = 8 mM pH = 3 and T = 40°C) (Fig. 3). It was found that COD removal efficiency increases with increasing the dosing rate of hydrogen peroxide from 3 to 60 mg min−1. The higher percent removal of COD was attained at 45 min when using 60 mg min−1 H2O2 dosing rate, so further addition of

1.6

−1

−1

H2O2 (mg min )

1.2 1.0

2

3 15 24 30 60

1.4

COD/CODO

kapp x 10 (min )


1501

10 8 6 4 2 0

0

20

0.8

40

60

H 2 O 2 (mg min

80 −1

100

)

0.6 0.4 0.2 0.0 0

10

20

30

40

50

Time (min)

Fig. 3 Effect of the dosing rate of H2O2 on the COD removal by the Fenton process. The inset panel shows kapp evolution at different dosing rate of H2O2. COD0 =1,080 mg L−1, [Fe2+]0 =8 mM, pH=3, and T=40°C

H2O2 is not necessary. Excessive H2O2 reacts with HO· competing with organic pollutants and consequently reducing treatment efficiency. In all tests, the drop in COD was more significant during the first minutes of reaction where the concentration of organic matter is high. This observation clearly appeared in the case of 60 mg min−1 H2O2 dosing rate. The inset of Fig. 3 shows the variation of the apparent rate constants (kapp) values, at different H2O2 dosing rate, calculated from the straight lines considering a pseudo-firstorder reaction. kapp increased when the dosing rate of H2O2 increased. This increase becomes more significant for H2O2 dosing rate higher than 30 mg min−1, due to the effect of the additional HO· radicals produced. Given that the concentration of Fe2+ introduced initially in the solution is sufficient to react with H2O2, the competitive reactions (Eqs. 4, 6, and 8–10) did not affect significantly the COD removal rate. Moreover, in order to follow the change in solution color during guaiacol oxidation, the absorbance measurements were carried out at a wavelength of 604 nm. In the first minutes, the guaiacol solution undergoes a fast color change from colorless to dark brown, reaching a peak level. Later, the solution begins to slowly clear up to a light brown, and even turns a pale yellow residual color in some experimental conditions. The kinetic pathway followed by the guaiacol oxidation reaction and the experimental results reported here show that color is not a fortuitous result depending on trace components or parameters with low significance, but depends directly on the main reaction intermediates. Indeed, in any experiment, color shows the path of a reaction intermediate which follows a slow kinetics that can continue for many minutes. So it

might be possible to establish a relationship between the color level observed and the intermediate compounds generated during the oxidation. Based on HPLC analysis results for the determination of the reaction intermediates, the mechanism that we proposed for guaiacol oxidation shows that during the first stages of oxidation, highly colored intermediate compounds such as methoxy-p-benzoquinone (yellow) and o-benzoquinone (red) are generated (Fig. 4). Their color comes from their quinoidal structure, which contains chromophore groups substituted in benzene rings. Benzoquinones achieve their peak level during the first minutes, and then disappear slowly because they are very stable species due to the conjugated carbonyl groups contained in their internal structure. The effect of the dosing rate of hydrogen peroxide on color evolution was tested in a set of assays with constant catalyst concentration [Fe2+]0 =8 mM at pH 3. Results were reported in Fig. 5, which shows the temporal absorbance at 604 nm using different dosing rate of H2O2. In all cases, the degradation reactions were much slower than color generation. The rate of this decolorization stage increased with the increase of the dosing rate of H2O2 and the solution being fully decolorized at shorter reaction times. With 60 mg min−1 H2O2 dosing rate, the absorbance increased rapidly and has a maximum (A604 =0.517) after about 6 min of treatment and then decreased and tended to zero beyond 35 min leading to the almost complete disappearance of color and so of quinine-type intermediates. The relationship between the dosing rate of hydrogen peroxide and the final color of the solution is therefore established, and it is concluded that the color observed depends on the level of oxidation reached. Consequently, it can be said that current color is a good indicator of the degree of oxidation achieved during the reaction. Using at least 60 mg min−1 H2O2 dosing rate is required in these conditions to remove completely the toxicity associated with aromatic intermediates and that of guaiacol itself, but some biodegradable acids would remain in the solution. 3.1.4 Effect of the initial concentration of ferrous iron Ferrous ion acts as a catalyst in Fenton’s reactions. To choose the optimal amount of Fe2+ added in the reaction solution, a set of tests was performed. Figure 6 illustrates the decrease of COD with time, during the oxidation of guaiacol solution (COD0 =1,080 mg L−1) using different initial concentrations of Fe2+. As can be seen, Fe2+ dosage has a significant effect on the degradation of guaiacol. The COD percent removal increased from 65% to 90% within 45 min reaction when the initial concentration of Fe2+ increased from 2 to 8 mM. As a catalyst, ferrous ion

1502


Fig. 4 Reaction pathway of the degradation of guaiacol by Fenton and solar photo-Fenton processes

OCH3

OCH3

.

OH OH - H2O

.

OCH3

.

O

.

Guaiacol

O

O

O

. - CH3OH

OH

OCH3

OCH3

.

.

.

OH

OH OCH3

OCH3

OH

OH

OH Polymeric products

OH

HO

.

- 2 H2O

2 OH

OH

.

O

.

- 2 H2O

2OH

2 OH

- 2 H2O OCH3

OCH3 O

O

O

O

O

.

OH

HOOC

COOH

COOH HOOC

.

OH

HOOC

COOH

HCOOH

.

OH

CO 2

0.6

−1

H2O2 (mg min ) 60 15 3

0.5

A604

0.4

0.3

0.2

0.1

0.0 0

10

20

30

40

50

Time (min)

Fig. 5 Temporal results of color evolution at 604 nm (A604). Effect of the dosing rate of H2O2 upon color formation and abatement. COD0 = 1,080 mg L−1, [Fe2+]0 =8 mM, pH=3, and T=40°C

+

H2O

initiates the decomposition of hydrogen peroxide to generate the very reactive HO· in Fenton’s reactions. Therefore, higher initial Fe2+ concentration lead to higher generation of HO· (Banerjee et al. 2008) and better degradation of guaiacol and its by-products. However, for Fe2+ doses higher than 8 mM, the COD percent removal decreased slightly; it passed from 90% to 82% when the Fe2+ concentration increased from 8 to 40 mM. This decrease is essentially due to competitive consumption of HO· and HO2· radicals (Eqs. 5 and 7) (Bielski et al. 1985; Buxton et al. 1988) It is worth noting that, in the Fenton process, the amounts of Fe2+ ions should be as low as possible for economic and environmental reasons; high amounts of Fe2+ ions might produce a larger quantity of Fe3+ sludge. The removal/treatment of the sludge-containing Fe3+ at the end of the wastewater treatment is expensive and needs large amount of chemicals and manpower (Ramirez et al. 2007).

2+

1.0

1503

10

2

[Fe ]0 (mM) 2 4 8 25 40

1.2

COD/CODO

−1

1.4

kapp x 10 (min )


0.8

8 6 4 2 0

0

10

20

30

40

50

2+

[Fe ]O (mM)

0.6 0.4

modified by iron (III) that appears in the reacting medium due to the oxidation of Fenton reagent. Quinone-type compounds are the main contributors to the color observed during the reaction, although the contribution of iron (III) and its complexes is by no means negligible because the residual color of fully oxidized water increases with the initial concentration of iron. This suggestion is also supported by other authors (Federico et al. 2006). 3.1.5 Effect of the initial concentration of guaiacol

0.2 0.0 0

10

20

30

40

50

Time (min)

Fig. 6 Effect of the initial concentration of Fe2+ on the COD removal by Fenton process. The inset panel shows kapp evolution at different Fe2+ concentration. COD0 =1,080 mg L−1, dosing rate of H2O2 60 mg min−1, pH=3, and T=40°C

On the other hand, the inset of Fig. 6 shows that kapp values, calculated from the straight lines, considering a pseudo-first-order reaction, increased as a function of Fe2+ dosage and reached a maximum around 8 mM of Fe2+. Therefore, 8 mM Fe2+ was selected as the optimum Fe2+ dosage in this work. Moreover, Fig. 7 shows the change in absorbance of the solution versus time measured for four initial concentrations of Fe2+ ions. In all cases the curves showed the same shape. They passed as from the first minutes through a maximum whose intensity increased proportionally with the amount of Fe2+ ions. The final color can be deepened or

It is important from an application point of view to study the dependence of removal efficiency on the initial concentration of the pollutant. Therefore, the effect of guaiacol concentration on the degradation efficiency was investigated at different initial concentrations (COD0, 453, 710, 1,080, and 1,435 mg L−1) and presented in Fig. 8. It can be observed that the COD removal decreased with the increase of the initial concentration of the pollutant. Almost 90% of COD removal was achieved after about 15, 25, and 35 min time of reaction for COD 0 453, 710, and 1,080 mg L−1, respectively. However, at high guaiacol concentration, the removal of COD needs more time and so more quantity of H2O2 (e.g., the percent removal of COD is about 75% after 45 min when using COD0 =1,435 mg L−1). This is because when the concentration of guaiacol increases, the quantity of hydroxyl radicals produced continuously with time does not increase accordingly and hence the removal rate decreases. Also, from the inset of Fig. 8, it can be seen that kapp decreased linearly with COD0. This behavior was similar to those reported by many researchers (Tamimi et al. 2008; Lucas and Peres 2006; Modirshahla et al. 2007).

−1

0.6

0.8

COD/COD0

A604

0.4

0.2

−1

1.0

2 4 8 25

0.8

COD0 (mg L ) 1435 1080 710 453

2

2+

[Fe ]O (mM)

kapp x 10 (min )

1.0

0.6

12 8 4 0 500

1000

1500 −1

COD0 (mg L )

0.4

0.2

0.0

0.0

0

10

20

30

40

50

Time (min)

Fig. 7 Temporal results of color evolution at 604 nm (A604). Effect of the initial concentration of ferrous ions upon color formation and abatement. COD0 =1,080 mg L−1, dosing rate of H2O2 60 mg min−1, pH=3, and T=40°C

0

10

20

30

40

50

Time (min)

Fig. 8 Effect of the initial concentration of guaiacol on the COD removal by the Fenton process. The inset panel shows kapp evolution at different initial COD. Dosing rate of H2O2 60 mg min−1, [Fe2+]0 = 8 mM, pH=3, and T=40°C

1504


3.1.6 Effect of the initial pH value It has been illustrated that the pH affects significantly the degradation of organics by the Fenton reaction and acidic conditions are required to produce the maximum amount of hydroxyl radicals by the decomposition of hydrogen peroxide catalyzed by ferrous ions (Lin and Lo 1997; Tang and Huang 1996). Several investigations have indicated that the optimum pH for the degradation of organics by the Fenton process is in the range 2.5–3.5 and that the extent of degradation decreases with increasing pH for pH>3.5 (Samet et al. 2009; Kang and Hwang 2000; Dutta et al. 2002; Malik and Saha 2003; Hsueh et al. 2005). Figure 9 shows the COD decrease with time and the kapp curve, during the oxidation of guaiacol solution (COD0 = 1,080 mg L−1) as a function of the initial pH. Clearly, the COD removal is significantly influenced by the pH and the optimum pH was 3. The values of kapp increase when pH increases from 2 to 3, then decrease when pH is raised from 3 to 6. The contributing factors for the low kapp in lower pH range (3) was caused by the formation of ferrous and ferric hydroxide complexes with much lower catalytic capability than Fe2+ (Kang and Hwang, 2000). On the other hand, the influence of the initial pH on the evolution with time of the absorbance at 604 nm wave-

COD/CODO

The temperature plays an important role in chemical oxidation, because it represents a determinant parameter in the kinetics of homogeneous reactions. The influence of this parameter on the kinetic rate constants, kapp, for the guaiacol degradation was investigated in the range between 30°C and 70°C with tests conditions at COD0 1,080 mg L−1, H2O2 60 mg min−1, [Fe2+]0 8 mM, and pH 3. The obtained results shown in Fig. 11 indicate that kapp was significantly influenced by the temperature with an optimal value of 40°C. The values of kapp quickly increased when the temperature increased from 30°C to 40°C, suddenly decreased when the temperature was raised from 40°C to 50°C, and then gradually and slightly drop off with the increase of temperature in the range of 50–70°C. The decrease of kapp at temperature higher than 40°C is due to the accelerated decomposition of H2O2 into oxygen and water. Similar results were reported by Wang (2008). The data for temperatures between 25°C and 40°C exhibit an Arrhenius-type behavior with apparent activation energy of 17,543 Jmol−1 (Eq. 19) calculated from the usual Ln kapp versus 1/T (Fig. 12). kapp ¼ 58:78exp

10

min1

ð19Þ

0.7

−1

0.8

17; 543 RT

8

pH 0.6

6

2

1.0

kapp x 10 (min )

2 3 4 5 6

3.1.7 Effect of the temperature

4

2 3 5

0.5

2 0 0

0.6

2

4

6

8

pH

A 604

pH 1.2

length is shown in Fig. 10. As can be seen, initial pH has no significant effect on the absorbance formation and abatement because the Fenton treatment at pH values between 2 and 5 led to the almost complete disappearance of color in the end of the treatment.

0.4 0.3

0.4

0.2

0.2

0.1

0.0

0.0

0

10

20

30

40

50

60

Time (min)

Fig. 9 Effect of the initial pH on the COD removal by the Fenton process. The inset panel shows kapp evolution at different pH. Dosing rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM, COD0 =1,080 mg L−1, and T=40°C

0

10

20

30

40

50

Time (min)

Fig. 10 Temporal results of color evolution at 604 nm (A604). Effect of the initial pH upon color formation and abatement. COD0 = 1,080 mg L−1, dosing rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM, and T=40°C


T (°C)

4

6

4 20

3

40

60

80

T (°C)

Fenton Solar photo-Fenton

1.0

4

Ln (COD0/COD)

Ln (COD0/COD)

2

5

25 30 35 40 50 60 70

0.8

COD/COD0

8

−1

kapp x 10 (min )

6

1505

0.6

0.4

y = 0.21 x 2 R = 0.992

3

y = 0.12 x 2 R = 0.988

2 1 0 0

2

10

20

30

Time (min)

0.2 1

0.0 0

10

20

30

40

Time (min)

0 0

10

20

30

40

50

Time (min)

Fig. 11 Plot of Ln (COD0/COD)-t at different temperatures. COD0 = 1,080 mg L−1, dosing rate of H2O2 60 mg min−1, [Fe2+]0 =8 mM, and pH=3

where R is the ideal gas constant (8.314 Jmol−1 K−1) and T is the reaction absolute temperature (K). 3.2 Oxidation of guaiacol by the solar photo-Fenton process In order to improve the reaction rate and COD abatement efficiency, solutions were subjected to solar radiation using a laboratory-scale reactor (Fig. 1). Figure 13 shows the trend of the COD/COD0 ratio during the treatment of guaiacol solution by the two processes under the optimum experimental conditions already found when using Fenton

Fig. 13 Effect of solar radiation on the kinetic abatement of COD during the treatment of guaiacol solution (COD0 =453 mg L−1). The inset panel shows the fitting of the experimental data to a first-order reaction kinetic model. Dosing rate of H2O2 60 mg min−1, [Fe2+]0 = 8 mM, pH=3, and T=40°C

process (COD0 453 mg L−1, H2O2 60 mg min−1, [Fe2+]0 8 mM, pH 3, and T 40°C). It can be seen that the solar photo-Fenton system needed less time and consequently less quantity of H2O2 to reach the same COD recent removal. In fact, under the optimum experimental conditions, the solar photo-Fenton process need a dose of H2O2 40% lower than that used in the Fenton process to remove 90% of COD. On the other hand, the COD removal rate is higher with the solar photo-Fenton process as shown the kapp values in the inset of Fig. 13.

4 Conclusion -2.6

-2.7

Ln kapp(min−1)

In this study, the degradation of guaiacol has been studied by applying homogeneous Fenton and solar photo-Fenton processes. The results showed that:

y = −2.1101 x + 4.0739 2 R = 0.9691

–

-2.8

-2.9

-3.0

–

-3.1

–

3.15

3.20

3.25

3.30 3

3.35

3.40

−

1/Tx 10 (K 1)

Fig. 12 Plot of Ln kapp–(1/T) for the degradation of guaiacol by Fenton oxidation process. Dosing rate of H2O2 60 mg min−1, COD0 = 1,080 mg L−1, [Fe2+]0 =8 mM, and pH=3

– –

The solar photo-Fenton process was more efficient than the Fenton process for COD removal. In the solar photo-Fenton process, the degradation rate was increased by 40% which reduced the operating cost. The COD and color removal increased with the increase of the dosing rate of hydrogen peroxide. The ferrous ion as catalyst accelerated the COD removal. Fe2+ concentration of 8 mM could be used as an optimum dosage for Fenton process. The optimum pH for both COD and color removal was 3. The degradation rate was significantly influenced by the temperature with an optimum value of 40°C.

1506 Acknowledgment This research is funded by the Tunisian Higher Education and Scientific Research Ministry.

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