Efecto de la temperatura sobre la oxidación del fenol con fenton

7/31/2019 Efecto de la temperatura sobre la oxidacin del fenol con fenton

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Intensification of the Fenton Process by Increasing the Temperature

Juan A. Zazo,* Gema Pliego, Sonia Blasco, Jose A. Casas, and Juan J. Rodriguez

Ingeniera Qumica, Facultad de Ciencias, UniVersidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain

The effect of temperature on the Fenton process has been studied within the range of 25-130 C using

phenol (100 mg/L) as target compound, 10 mg/L Fe

2+

, and a dose of H2O2 corresponding to the theoreticalstoichiometric amount (500 mg/L) for mineralization. The TOC reduction was considerably improved astemperature increased. Whereas at 25 C the TOC decreased less than 28%, a reduction of almost 80% wasachieved at 90 C. Beyond this temperature no significant improvement of mineralization was observed,although the rate of the process was considerably enhanced. Increasing the temperature leads to a more efficientconsumption of H2O2 which indicates an enhanced iron-catalyzed H2O2 decomposition into radicals astemperature increases rather than the generally accepted thermal breakdown of H 2O2 into O2 and H2O. Therefore,working at a temperature well above the ambient provides a way of intensifying the Fenton process since itallows a significant improvement of the oxidation rate and the mineralization percentage with reduced H2O2and Fe2+ doses. Furthermore it would not represent a serious drawback in the case of many industrialwastewaters which may be already at that temperature. Besides, partial recovery of heat from the treatedoff-stream would always allow saving energy. The TOC time-evolution was well described by a kinetic modelbased on TOC lumps with apparent activation energy values in the range of 30-50 kJ/mol.

1. Introduction

The need to develop technical solutions capable of fulfilling

increasingly stringent discharge limits for industrial wastewaters

or allowing water recycling or reuse promotes research efforts

toward either the implementation of new treatments or the

intensification of those already available. The Fenton process

emerges as a suitable way of treating a wide variety of industrial

effluents.1 This process implies the generation of OH radicals

(a strong and nonselective oxidant) from catalytic H2O2decomposition by means of Fe2+ at acidic pH. The overall

mechanism also includes several secondary reactions,2-5 among

them the regeneration of Fe2+ by reaction between Fe3+ and

H2O2 and competitive scavenging reactions involving Fe2+

,H2O2, and OH.

This treatment has shown some significant advantages with

respect to other processes, as the fact that iron is a widely

available and nontoxic element and hydrogen peroxide is easy

to handle and the excess decomposes to environmentally safe

products.6 Besides, it requires relatively mild operating

conditions and simple equipment.7,8 However its application

to the treatment of real wastewaters has been so far limited

mainly due to the high requirements of H2O2 and iron which

results in high operational cost, and finally leads to the

generation of high volumes of Fe(OH) 3 in the neutralization

step.9,10

Several alternatives have been proposed in order toovercome these drawbacks. On one hand, the combination

of the Fenton process with biological treatment is one of the

most developed.11-15 The effluent is first chemically oxidized

for the sake of reducing the toxicity and increasing the

biodegradability. Other possibilities such as semicontinuous

H2O2 addition,16,17 or integrated Fenton-coagulation/floc-

culation18 have been proposed. On the other hand, hetero-

geneous Fenton,19-23 where iron is fixed on the surface of a

support, is considered as a feasible solution for minimizing

iron loss and the consequent generation of Fe(OH)3 sludge.

The possibility of increasing the operating temperature as a

way of improving the efficiency of the Fenton process has been

so far scarcely investigated, because the idea of thermal

decomposition of H2O2 into O2 and H2O24,25 seems to be widely

accepted as a serious drawback. However, higher temperatures

could lead to a more efficient use of H2O2 (defined as the amount

of TOC removed per unit weight of H2O2 decomposed) upon

enhanced generation of OH radicals at low Fe2+ concentration.

A decrease of the iron dose is important since it reduces the

amount of Fe(OH)3 sludge and also improves the efficiency of

H2O2 by minimizing competitive scavenging reactions. There-

fore, increasing the temperature can be considered as a way of

intensification of the conventional Fenton process. On the otherhand, working above ambient temperature would not represent

any drawback in the case of many industrial wastewaters.26

Besides, partial recovery of heat from the treated off-stream

would always allow saving energy.

The aim of this work was to investigate in depth the effect

of temperature on the performance of the Fenton process using

phenol as target pollutant. The influence of this variable on the

rate of mineralization as well as on the efficiency of H 2O2 at

low Fe2+ concentration was analyzed attempting to optimize

this treatment. The results were compared to those obtained in

previous works with higher H2O2 and Fe2+ doses at lower

temperatures as commonly used in the conventional Fenton

process. Finally, a kinetic lumped model was used, which

described well the evolution of TOC thus providing a useful

tool for design purposes. So far the effect of temperature on

the performance of Fenton process has been scarcely investi-

gated. Lopez et al.26 studied that effect at 25 and 70 C on the

evolution of TOC and the oxidation byproducts but these authors

used H2O2 and Fe2+ doses substantially higher than ours as well

as much longer reaction times. In the present work a wider

temperature range (up to 130 C) has been tested and the

efficiency of H2O2 consumption is carefully considered given

its critical importance on the economy of the Fenton process.* To whom correspondence should be addressed. E-mail: juan.zazo@

uam.es. Tel: +34 914978774. Fax: +34 914973516.

Ind. Eng. Chem. Res. 2011, 50, 866870866

10.1021/ie101963k 2011 American Chemical SocietyPublished on Web 12/16/2010


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2. Experimental

The experiments were carried out in stoppered glass batch

reactors (Buchi, inertclave Type I) equipped with a backpressure

controller. The reaction volume was 500 mL and the starting

concentrations were 100 mg/L phenol, 10 mg/L Fe2+, and 500

mg/L H2O2 (which corresponds to the theoretical stoichiometric

amount for complete oxidation of phenol up to CO2 and H2O,

i.e., mineralization). Initially, an aqueous solution containing

the aforementioned concentrations of phenol and Fe

2+

wasplaced into the reactor. Once the desired temperature and

pressure were achieved, 1.04 mL of a 30% w/v H2O2 solution

was added, considered as the starting time for the reaction. The

temperature effect was tested within the 25 to 130 C range.

Experiments at different pressures between atmospheric and 6

bar (achieved by pumping air into the reactor) were also carried

out at 50 C. The initial pH value was 3.0, which was not

controlled along the process. Nevertheless, no significant

changes in this value were observed during the experiments.

Blank experiments with phenol in absence of H 2O2 and Fe2+

were also performed.

The progress of the reaction was followed by periodically

taking samples from the reactor throughout 4 h. The samples

were immediately analyzed. Phenol and aromatic byproducts

were quantified by means of HPLC (Varian Pro-Star 240) using

a diode array detector (330 PDA). A Microsorb C18 5 m

column (MV 100, 15 cm long, 4.6 mm diameter) was used as

stationary phase and 1 mL/min of 4 mM aqueous sulfuric

solution was used as mobile phase. Short-chain organic acids

were analyzed by an ion chromatograph with chemical sup-

pression (Metrohm 790 IC) using a conductivity detector. A

Metrosep A supp 5-250 column (25 cm long, 4 mm diameter)

was used as stationary phase and 0.7 mL/min of an aqueous

solution 3.2 mM of Na2CO3 and 1 mM of NaHCO3 was used

as mobile phase. Total organic carbon (TOC) was measured

using a TOC analyzer (Shimadzu, model TOC VSCH) and

hydrogen peroxide concentration was determined by colorimetrictitration using the TiOSO4 method.

27 All of the chemicals except

H2O2 (Panreac, Hydrogen Peroxide 30% w/v PA) and formic

acid (Fluka, puriss. p.a., w98%), were purchased from Sigma-

Aldrich (>99% pure).

3. Results and Discussion

Figure 1 shows the evolution of TOC and H2O2 concentration

upon Fenton oxidation of phenol at different temperatures.

Phenol and the aromatic byproducts were almost completely

converted within the first 5 min of reaction time, even at the

lowest temperature tested but a dramatic improvement of

mineralization was observed as temperature increased, especially

within the range of 25 to 100 C.

Short-chain organic acids (mainly formic and oxalic but also

maleic and traces of acetic acid) were the only byproducts

detected beyond the first 5 min of reaction. The differences

between the measured TOC values and the amount of carbon

in the identified compounds reveal the presence of unidentified

byproducts, which are usually assessed to condensation

species.28-30 Figure 2 shows the evolution of the estimated

overall amount of those species as well as formic and oxalic

acids, by far the two major identified byproducts.

As can be seen in Figure 2 the amount of condensation

byproducts formed decreases as the temperature increases, being

almost negligible above 110 C. The evolution of organic acids

(Figure 2b and c) allows concluding that the condensationcompounds are mostly mineralized rather than converted into

organic acids. This behavior is different from that observed in

a previous work29 at 25 C using higher H2O2 and Fe2+ doses,

where much higher concentrations of formic and oxalic acids

were obtained.

The amount of formic acid diminished monotonically as the

temperature increased, and beyond 100 C this compound

disappeared almost completely after 2 h of reaction time. This

systematic behavior with temperature was not observed in the

case of oxalic acid. Within the range of 25-100 C, this acid

appeared quite resistant to Fenton oxidation as a significant

remaining concentration was measured even after 4 h. At higher

temperatures the concentration of oxalic acid clearly decreased

upon reaction time and at 130 C even completely disappear

after 2 h.

This fact might not be related with the oxidation of oxalic

acid but is probably related with thermal decomposition of ferric

oxalate.31 The thermal decomposition of ferric oxalate must be

accompanied by ferric oxide precipitation as suggested by the

slight development of turbidity in the reaction media. This solid

was identified as Fe2O3 by means of polycrystal X-ray diffrac-

tion. The presence of remaining amounts of oxalic acid must

be considered as recent works using respirometric techniques

have pointed out the lower biodegradability of this compound

by conventional wastewater activated sludge in spite of its verylow ecotoxicity.32

Figure 1. Influence of temperature on TOC (a) and H2O2 (b) conversion.([phenol]0: 100 mg/L, [H2O2]0: 500 mg/L, [Fe

2+]0: 10 mg/L, pH: 3).

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Considering an increase of the temperature affects the pressure

of the system it is necessary to learn more about the effect of

this variable. Working at 50 C and different pressures within

the range of 1-6 bar we obtained fairly similar results so that

we can conclude that pressure has no influence on the efficiency

of the performance of the Fenton process within the range tested.

Figure 1 shows a faster decomposition of H2O2 as temperature

increases. Thermal instability of this reagent which provokes

its decomposition into O2 and H2O has been claimed in the

literature.6 However, a higher temperature could also enhance

H2O2 decomposition toward OH radicals. This last hypothesisis supported by the results gathered in Figure 3, which shows

the TOC vs H2O2 conversion values upon 4 h reaction time at

different temperatures. For the sake of comparison, it also

includes the results obtained at 25 C in a previous work28 with

different Fe2+

doses. As can be seen, the efficiency of H 2O2(), defined as the amount of TOC converted per unit of H 2O2decomposed (w/w), does not decrease when increasing the

temperature as should be expected if H2O2 would be decom-

posed into O2 and H2O due to thermal instability. On the

contrary, a higher temperature implies a faster iron-catalyzed

H2O2 conversion into radicals, which enhances mineralization.

The TOC reduction reaches around 80% at 100 C and almost

90% at 130 C in 20 min.

The consumption of H2O2 is a critical issue of Fenton process

since it is by far the main component of the operating cost.33

For the assessment of reagent consumption we have considered

the amount of TOC converted per unit weight of hydrogen

peroxide decomposed () and fed (), respectively. The secondvariable is more representative as the residual H2O2 cannot be

recovered and, moreover, needs to be eliminated before

discharge due to its toxicity. Table 1 gathers the values obtained

for both parameters after 4 h reaction time at different

temperatures using the stoichiometric H2O2 dose and 10 mg/L

Fe2+.

The values of increase with temperature up to around 90

C and at that temperature no residual H2O2 remained after 4 h

reaction time so that it can be considered around the optimum

from this point of view. The maximum values of at complete

TOC conversion when using the stoichiometric H2O2 dose would

be 153 mg TOC/g H2O2. Therefore, at 90 C, 77% of that

maximum value was achieved, almost 2.8 times the value

observed at 25 C. Beyond 90 C, although this parameter hardlyvaried, the oxidation rate increased (Figure 1), thus allowing a

Figure 2. Evolution of reaction byproducts with temperature. ([phenol]0:100 mg/L, [H2O2]0: 500 mg/L, [Fe

2+]0: 10 mg/L, pH: 3).

Figure 3. Evolution of TOC vs H2O2 conversion. ([phenol]0: 100 mg/L,

[H2O2]0: 500 mg/L, pH: 3).

Table 1. Values of and at Different Temperatures Using theStoichiometric H2O2 Dose (500 mg/L) and 10 mg/L Fe

2+. ([Phenol]0:100 mg/L, pH: 3, Reaction Time: 4h)

25 C 50 C 70 C 90 C 100 C 110 C 120 C 130 C

XTOC 0.28 0.54 0.58 0.77 0.79 0.80 0.81 0.81

a 64 103 113 118 121 122 124 124

b 43 83 89 118 121 122 124 124

a mg TOC/g H2O2 converted.b mg TOC/g H2O2 fed.

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lower reactor volume. Lowering the H2O2 dose up to one-half

the stoichiometric hardly affected the values of, although the

percentage of TOC removed decreased markedly (Table 2). The

dose of Fe2+ can be also significantly lowered with increasing

the temperature that would reduce the sludge volume resulting

upon neutralization.

Thus, increasing the temperature is a better strategy than

stressing the H2O2 and Fe2+ doses beyond the stoichiometric

amount and around 10 mg/L, respectively, as can be seen fromthe results of Table 3 obtained at 25 C with different H2O2and Fe2+ doses upon 4 h reaction time.

3.1. Kinetic Analysis. The evolution of TOC upon reaction

time was adjusted to a simplified kinetic model described in a

previous work.17 Briefly, TOC was lumped into three blocks

depending on the degradability. Thus, TOCA which lumps the

easily oxidizable compounds (phenol and aromatic intermedi-

ates), is converted to TOCB (which includes condensation

byproducts and maleic and formic acid) and/or mineralized to

CO2. Depending on the operating conditions TOCB can be

oxidized up to TOCC (those compounds that are refractory to

this treatment, mainly oxalic acid) and/or to CO2. Scheme 1

summarizes the TOC pathway.The model assumes second-order kinetics with respect to TOC

and first-order kinetics with respect to H2O2, the evolution of

which is directly related to the generation of OH. Fitting of

the experimental TOC vs time values to the proposed model

can be seen in Figure 1 (lines). The model describes fairly well

the time-evolution of TOC. Table 2 reports the values of the

rate constants obtained by fitting the model to the experimental

results using Scientist 3.0 software. The value of k5 corresponds

to the first order rate constant of H2O2 decomposition into

radicals. The correlation coefficients are also included. The

increase of temperature favors TOCA oxidation up to CO2 rather

than to TOCB, according to the values of k1 and k2. This fact

significantly affects the distribution of byproducts. Thus, at

temperatures below 90 C, oxidation proceeds through theclassical Fenton pathway, where direct mineralization of

aromatics hardly occurs, unlike what happens at higher tem-

peratures. This justifies the lower concentration of condensation

byproducts as well as the lower production of formic and oxalic

acid as a consequence of the oxidation of TOCB up to TOCC.

Figure 4 compares the experimental and predicted TOC

values, confirming the validity of the model. In addition, the

TOCi simulated profiles validate the nature of the lumping

groups. The values of the kinetic constants obey the Arrhenius

equation which allows obtaining the apparent activation energy

for each step (Table 4). These values are comparable to those

reported by Guedes et al.34 and Bautista et al.35 working with

different industrial wastewaters.

4. Conclusions

Increasing the temperature clearly improves both the oxidationrate and the degree of mineralization of phenol by Fenton

oxidation allowing working with reduced amounts of H2O2 and

Fe2+. Thus, it can be considered a way of intensification of the

Fenton process. The temperature and the H2O2 dose can be

conveniently adjusted for the sake of achieving a high miner-

alization (TOC reduction) at complete H2O2 conversion with a

low Fe2+ concentration, which would reduce the amount of

Fe(OH)3 sludge produced upon neutralization. Working above

ambient temperature would not represent a serious drawback

in the case of many industrial wastewaters which may be already

at that temperature. Besides, partial recovery of heat from the

treated off-stream would always allow saving energy. The TOC

time-evolution was well described by a kinetic model based on

TOC lumps with apparent activation energy values in the rangeof 30-50 kJ/mol.

Table 2. Values of and at 90 C and 10 mg/L Fe2+ UsingSubstoichiometric H2O2 Dose ([Phenol]0: 100 mg/L, pH: 3, ReactionTime: 4h)

H2O2 (mg/L) XTOC a

b

250 0.40 122 122

375 0.55 112 112


Table 3. Values of and at 25 C Using Different H2O2 and Fe2+

Doses ([Phenol]0: 100 mg/L, pH: 3, Reaction Time: 4h)H2O2 (mg/L) Fe

2+ (mg/L) XTOC a

b

500 1 0.08 38 125 0.20 66 31

10 0.28 64 43

100 0.28 43 432500 100 0.37 11 11

5000 100 0.55 8 8


Scheme 1. TOC Pathway Oxidation by Fenton Reagent

Figure 4. Parity plot for TOC (mg/L).

Table 4. Values of the Rate Constants (k1-k4: L2 mg-2 min-1; k5:min-1) and Activation Energies (kJ/mol)

T (C) k1 105 k2 10

5 k3 107 k4 10

7 k5 r2

25 0.18 0.04 0.02 0.03 0.04 0.9950 0.14 0.07 0.08 0.11 0.12 0.99

70 0.71 0.28 0.55 0.50 0.13 0.99

90 1.59 0.81 0.55 0.82 0.23 0.99100 0.70 2.34 7.47 1.53 0.47 0.99

110 1.80 5.92 12.2 8.87 0.58 0.99

120 4.00 13.54 37.09 23.13 0.67 0.99130 11.97 34.62 158.5 130.3 0.74 0.99

Ea (kJ/mol) 31.8 43.2 53.4 47.4

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A lumped kinetic model has been used which describes well

the time-evolution of TOC. The values of the kinetic constants

show that phenol and aromatic intermediates are mainly

transformed into organic acids at temperatures below 90 C

whereas they are mainly oxidized up to CO2 beyond that

temperature.

Acknowledgment

We thank the financial support from the Spanish PlanNacional I+D+i through the projects CTQ2007-61748/PPQ and

CTQ2008-03988/PPQ and from the CAM through the project

S2009/AMB-1588.

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ReceiVed for reView September 24, 2010ReVised manuscript receiVed November 25, 2010

Accepted December 2, 2010

IE101963K

870 Ind. Eng. Chem. Res., Vol. 50, No. 2, 2011

Efecto de la temperatura sobre la oxidación del fenol con fenton

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