Oxidation of Acetaminophen by Fluidized-bed Fenton Process:

Optimization using Box-Behnken Design

M.C. Lu*, R.M. Briones**, and M.D.G. de Luna**, ***

*Department of Environmental Resources Management, Chia Nan University of Pharmacy and Science, Tainan 717,

Taiwan

(E-mail: mmclu@mail.chna.edu.tw)

** Environmental Engineering Graduate Program, University of the Philippines, 1011 Diliman, Quezon City, Philippines

(Email: rowenambriones@yahoo.com)

*** Department of Chemical Engineering, University of the Philippines, 1011 Diliman, Quezon City, Philippines

Abstract

One of the most frequently used over-the-counter analgesic and antipyretic is acetaminophen

(ACT). This drug finds its way into effluent wastewaters in concentrations that still pose an

environmental threat even after conventional treatment. This study demonstrates the effectiveness

of a fluidized-bed Fenton process in the degradation of acetaminophen in synthetic wastewater.

Box-Behnken experimental design was employed to optimize initial pH, Fe2+

and H2O2

concentrations. The best ACT removal was achieved at pH 3.00 with both initial Fe2+

and H2O2

concentrations at their maximum. At this operating condition, almost 98% degradation was attained

within only 20 minutes of reaction time. Optimization of operating conditions gave the best

removal efficiency at pH=3.22, [Fe2+

] = 0.06mM and [H2O2] = 19.87mM. Verification studies

resulted in a 97.83% ACT removal with an initial rate of 0.234 mM/min. COD and TOC removals

of 38.18% and 59.62%, respectively, were achieved. Parametric studies showed that a single stage

slow degradation occurs at very low FH ratio. At higher FH ratio, a fast initial degradation followed

by slow degradation occurs.

Keywords: Fluidized-bed Fenton process; Box-Behnken design; acetaminophen

INTRODUCTION

Pharmaceuticals and personal care products (PPCPs) from pharmaceutical companies and human

use reach surface waters usually unaltered. PPCPs are persistent and mostly resistant to microbial

attack. Most of these substances are found to be endocrine disrupting compounds (EDCs), an

emerging class of pollutants. Even at low concentrations, PPCPs have shown severe effects to the

environment. A recent study attributed the rapid decline of vulture population in Northern India to

diclofenac, a non-steroidal anti-inflammatory drug (NSAID) used as analgesic to treat arthritic and

rheumatic conditions (Rizzo, et al., 2009). PPCPs were not only detected in effluents from

wastewater treatment plants (WWTPs) but also in surface and ground waters (Larsen, et al., 2004).

The discharge of these substances into sewers and WWTPs poses serious problems and challenges

in their removal. Existing water and WWTPs are not designed to remove these unregulated micro-

pollutants. Many studies have shown that the reduction of pharmaceutical compounds in

conventional WWTPs is usually incomplete. Though incomplete, WWTPs with tertiary treatment

have better removal of PPCPs (Zhou, et al., 2009). To mitigate environmental contamination from

PPCPs, complete removal of these compounds in wastewaters is the best solution.

Acetaminophen (ACT) is a non-steroidal anti-inflammatory drug (NSAID) used as an over-the-

counter analgesic and antipyretic. It is one of the most frequently used drugs and ranked as the 5th

most used drug in the Philippines in 2006 (IMS, 2007). When ingested, 58-68% of this drug is

excreted by the body unchanged (Zhang, et al., 2008). It is present in wastewaters in concentrations

exceeding 1000 ng/L (Wiegel, et al., 2004).

Figure 1. Structure of acetaminophen

Advanced oxidation processes (AOPs) can achieve total degradation of target pollutants producing

organic acids, inorganic salts and CO2 as by-products. These processes generate hydroxyl radicals

(•OH) in solution. Hydroxyl radicals are very powerful nonselective oxidizing agents which attack

organic matter in wastewater thereby promoting its degradation. Fenton oxidation, one of the most

important AOPs known, utilizes Fenton’s reagent, a combination of H2O2 and Fe2+

, to produce

hydroxyl radicals for complete mineralization of organics (Khataee, et al., 2008).

(1)

(2)

(3)

Values of rate constants suggest that ferric ions are produced (Equation 1) more rapidly than they

are reduced to Fe2+

(Equation 3) resulting in the formation of excess Fe3+

at the end of the process.

Neutralization of the final solution leads to the formation and accumulation of Fe(OH)3 sludge -a

major disadvantage of the Fenton process.

Fluidized-bed (FB) Fenton minimizes sludge production as the carrier itself acts as a seeding

material for crystallization of Fe3+

ions. Aside from the homogenous catalytic reaction between

H2O2 and Fe2+

in solution, heterogeneous chemical reaction also occurs as the iron oxide-coated

carrier also acts as a catalyst to produce •OH (Chou, Huang, & Huang, 1999).

This study focused on the degradation of synthetic acetaminophen wastewater by chemical

oxidation using FB-Fenton process. It is a preliminary research in advanced wastewater treatment

technology particularly in sewage and pharmaceutical industries.

MATERIALS AND METHODS

Chemicals and analytical methods All chemicals used, including 35% H2O2, 4-hydroxy acetanilide (ACT), FeSO4•7H2O, HClO4, HCl,

H2SO4, NH4C2H3O2, C12H8N2•H2O, K2TiO4, acetonitrile, NaOH, were purchased from Merck. All

solutions were prepared using Millipore system deionized water with a resistivity of 18.2 MΩ.

Residual H2O2 in solution was analyzed using titanium oxalate method. Residual ferrous

concentration was determined by complexation with 1,10-phenanthroline. Both methods were

analyzed using a Thermo Spectronic Genesys 20 spectrophotometer at 400 nm for H2O2 and 510

nm for ferrous ions. ACT concentration was determined with SpectraSYSTEM SN4000 HPLC

equipped with Asahipak ODP-50 6D using 20 mM phosphoric acid and acetonitrile at 85:15, flow

rate of 1 mL/min and at 220 nm. COD was measured using closed reflux titrimetric method,

Standard Methods 5220 C. Total iron concentration was determined by Perkin Elmer AAnalyst 200

AAS.

Fluidized-bed Fenton and Fenton experiments The reactor with a working volume of 1.45 L was made of a cylindrical glass with inlet, outlet and

recirculating sections. It was equipped with a Suntex portable pH meter. All batch experiments

were done at room temperature. Synthetic acetaminophen wastewater at 5 mM was poured into the

reactor and the pump was turned on. The initial pH was adjusted by adding concentrated HClO4 or

0.1N NaOH. The desired amount of FeSO4•H2O was added into the solution as ferrous source.

Glass beads of diameters 4 mm and 2 mm were added as support followed by the addition of SiO2

carrier with a diameter of 0.5 mm. The pH of the solution was further adjusted. Samples were taken

for analyses of initial conditions. Samples taken were immediately injected into tubes containing

sodium hydroxide solution to quench Fenton reaction and were filtered through a 0.22 µm syringe

microfilters. Hydrogen peroxide was finally added to start the reaction. The pH of the solution was

not further adjusted as the reaction proceeded. Samples were taken at different time intervals of 0,

3, 5, 10, 20, 40, 60, 90 and 120 minutes.

Fenton experiments were done following the same procedure as in fluidized-bed Fenton process but

without the addition of glass bead support and SiO2 carriers.

Design of experiment

Optimization of operating conditions was done using Box-Behnken design (BBD), a three-level

design used to fit second-order models. It can be expanded to estimate the combinations of third

order terms, i.e. x12x2, x1

2x3 and x1x2

2(Davis & Draper, 1998). This design has the advantage of

having very efficient number of required runs to fit the model. Design-Expert 7.0 software (Stat-

Ease, Inc., Minneapolis, USA) was used to determine the number of experiments needed to

optimize and analyze the system.

Three important parameters namely: pH, Fe2+

and H2O2, were studied and optimized using BBD. A

total of 17 experimental runs were conducted with five replicates at the center point. All runs were

conducted at room temperature. Based on previous FB-Fenton studies (Muangthai, Ratanatamsakul,

& Lu, 2010), the amount of carrier material has little significant impact on removal efficiency.

Hence, its amount and size were fixed at 100g and 0.5mm respectively. Table 1 shows the levels

for each factor used in the BBD.

Table 1. Levels of factors used in Box-Behnken Design

Levels Factors Symbol

Low (-1) Center (0) High (+1)

pH A 2 3 4

Fe2+

(mM) B 0.01 0.055 0.1

H2O2 (mM) C 5 15 25

RESULTS AND DISCUSSION

Box-Behnken design Aside from fitting a model for ACT removal, initial rate was also included as a response to predict

the efficiency of the process at optimum conditions. Using Design-Expert software, a reduced cubic

model best fits ACT removal. For initial rate, a full quadratic model is sufficient and gives an

insignificant lack of fit. Results of ANOVA gives adjusted R2 values of 0.9979 and 0.9764 for ACT

removal and initial rate, respectively.

The best ACT removal was achieved at pH 3 and with the highest initial Fe2+

and H2O2

concentrations of 0.01 and 25 mM, respectively. Almost 98% degradation was attained within only

20 min of reaction time. The worst removal was 20.91% at pH=2, [Fe2+

]=0.01mM and

[H2O2]=15mM.

Correlation of each factor on ACT removal and initial rate. Table 2 shows the correlation of each

parameter studied on ACT removal efficiency and initial rate. All three parameters have significant

positive effects on both responses and must be considered in the analysis of the effect of each

factor.

Table 2. Correlation values of each factor on ACT removal and initial rate

Correlation Factor

ACT removal Initial rate

pH 0.304 0.481

Fe2+ 0.429 0.654

H2O2 0.555 0.211

Although all parameters have positive effect on ACT removal, 3D surface plots reveal pH level and

Fe2+

concentration levels where removal of ACT start to decrease.

Figure 2. 3D surface plots of the two parameter

interaction effects of initial pH, [Fe2+

] and

[H2O2] on ACT removal: (a) [H2O2]=25mM,

(b) pH=3, (c) [Fe2+

]=0.055mM.

Figure 3. 3D surface plots of the two parameter

interaction effects of initial pH, [Fe2+

] and

[H2O2] on initial rates: (a) [H2O2]=25mM, (b)

pH=3, (c) [Fe2+

]=0.055mM.

(a)

(b)

(c)

(a)

(b)

(c)

Optimization using BBD. As generated by Design Expert 7.0 software, the equations for ACT

removal and initial rate, respectively, by fluidized-bed Fenton process in terms of coded factors are

as follows:

% ACT removal= 92.99 + 3.15A + 5.51B + 16.89C – 12.68AB + 3.17AC + 4.25BC – 10.72A

2 - 12.71B

2 –

7.57C2 + 15.09A

2B + 12.21AB

2

Initial rate, mM/min = 0.22 + 0.21A + 0.29B + 0.094C + 0.28AB – 0.0079AC + 0.099BC + 0.073A2 +

0.12B2 + 0.014C

2

where A, B and C are initial pH, initial Fe2+

concentration and initial H2O2 concentration,

respectively with values of (-1) to (1) indicating the level.

For a cost effective operation, the amount of chemicals added were kept at a minimum while ACT

removal was set to a target of 95-100% for best removal efficiency. The software generated only

one solution with the criteria as shown in Table 3.

Table 3. Optimum condition factors and responses as predicted by Design-Expert 7.0

Factor Condition

pH 3.22

[Fe2+] 0.06 mM

[H2O2] 19.87 mM

ACT removal 100%

Initial rate 0.3185 mM/min

Comparison between conventional Fenton and fluidized-bed Fenton process at the optimum

condition. To validate the model generated by the software, FB-Fenton reaction was carried out at

optimum conditions and the resulting efficiency was compared to that of conventional Fenton

process using the same parameters. As shown in Table 4, the experimental results obtained for the

model at optimum conditions were close to the predicted values indicating a good fit for the range

of concentrations investigated.

Table 4. Comparison between actual and predicted values

Response Actual Predicted Difference

ACT removal, % 97.83 100 2.17

Initial rate, mM/min 0.2343 0.3185 0.0842

Figure 4 shows the comparison between Fenton and fluidized-bed Fenton using optimum

conditions of the latter. The trends for both methods were almost the same for residual H2O2,

residual ACT and residual COD. The marked difference is evident in total residual iron.

Total residual iron from the Fenton process was much higher than that from the FB-Fenton process.

FB-Fenton process resulted to 62.92% iron removal compared to only 9.06% using Fenton process.

This was expected since FB-Fenton was developed to reduce sludge formation in the form of iron

precipitates. The presence of SiO2 in the reactor provides a site for crystallization of iron oxides

onto the surface of these carriers thereby reducing the amount of iron in solution. SEM/EDS

analysis supports this as results showed an increase in iron from 0.83% to 2.10%.

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0[H

2O

2]

(C/C

o)

Time (min)

Fenton process

FB-Fenton process

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

[AC

T]

(C/C

o)

Time (min)

Fenton process

FB-Fenton process

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

To

tal

iro

n (

C/C

o)

Time (min)

Fenton process

FB-Fenton process

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

CO

D (

C/C

o)

Time (min)

Fenton process

FB-Fenton process

Figure 4. Comparison of residual (a) H2O2, (b) ACT (c) total iron and (d) COD between Fenton

and Fluidized-bed Fenton processes at optimum condition: [ACT]=5mM, pH=3.22,

[Fe2+

]=0.06mM, [H2O2]=19.87mM

Parametric Studies To be able to discuss in detail the effect of each factor on the removal efficiency, parametric studies

were done. This involved changing the value of the parameter being studied while keeping other

variables constant.

Effect of initial pH. Previous studies have shown that the pH of the contaminated solution is a very

important parameter that should be controlled in Fenton processes to achieve effective removal.

The concentration of Fe2+

in solution was found to be maximum at pH=2.8 (Brillas, Sires, &

Oturan, 2009). However, the operative optimum pH depends on the pollutant/s in solution.

As shown in Fig. 5, only a small amount of ACT was removed at pH=2 (20.9%) as compared to the

removal at pH=3 (68.0%) and pH=4 (77.0%). But if pH of the solution was not adjusted (pH=6.5),

ACT removal decreased drastically to 8.65%. At a very low pH, the high H+ concentration in

solution scavenges •OH as shown in Equation 4 thereby decreasing the degradation rate (Devi, et

al., 2010). Also, at low pH conditions, reduction of ferric to ferrous is inhibited (Equation 5).

(4) (5)

(a) (a)

(b)

(c)

(d)

2 4 60

20

40

60

80

AC

T r

em

ov

al

(%)

pH

ACT removal

0.0

0.1

0.2

0.3

Initial rate

In

itia

l ra

te (

mM

/min

)

Figure 5. Effect of pH: [ACT]=5 mM, [Fe

2+]=0.01 mM, [H2O2]=15 mM

At pH between 3 and 5, the predominant species is Fe2+

and degradation occurs at a faster rate in

this region. However, at a very high pH, Fe2+

is unstable and is easily oxidized to Fe3+

in solution.

This precipitates out as Fe(OH)3 and reduces the amount of free Fe2+

in solution to catalyze Fenton

reaction. Also, hydrogen peroxide is unstable at higher pH levels as it decomposes to water and

oxygen.

(6)

Effect of initial ACT concentration. To determine the effect of acetaminophen on the degradation

rate and removal, different concentrations of 2.5, 5, 7.5 and 10mM ACT were treated while fixing

other factors constant at pH=3, [Fe2+

]=0.01mM and [H2O2]=5mM. Results showed a 50% decrease

in ACT removal after 2 hours of reaction time as the concentration was increased 4 times from 2.5

to 10mM. The decrease in the rate of degradation at higher concentrations was observed because

there is lower hydroxyl radical available in the solution compared to the target organic compounds.

A lower oxidant to pollutant ratio resulted to a decrease in removal efficiency.

It is also important to note that the degradation rate follows the trend of H2O2 concentration in

solution. This implies that the amount of H2O2 available is directly related to the removal rate. If

there is no or little observed decline in H2O2 concentration, it is possible that ACT degradation has

stopped and that the process has reached its maximum removal effectiveness. In Fig. 7 (a) and (b),

ACT concentration is almost constant at 40 min which coincides with the slow H2O2 disappearance.

3 6 90

15

30

45

60

AC

T r

em

ov

al

(%)

[ACT] (mM)

ACT removal

0.00

0.05

0.10

0.15

0.20

Initial rate

In

itia

l ra

te (

mM

/min

)

Figure 6. Effect of ACT: pH=3, [Fe

2+]=0.01mM, [H2O2]=5mM

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

1.2

[AC

T]

(C/C

o)

Time (min)

ACT=5mM

ACT=10mM

0 20 40 60 80 100 1200.0

0.2

0.4

0.6

0.8

1.0

[H2O

2]

(C/C

o)

Time (min)

ACT=5mM

ACT=10mM

Figure 7. Effect of ACT on FB-Fenton process: pH=3, [Fe2+

]=0.1mM, [H2O2]=25mM, (a) ACT

remaining and (b) residual H2O2

Effect of initial [Fe2+

]:[H2O2] ratio. Removal of ACT increased as initial Fe2+

concentration

increased from 0.01 to 0.1 mM as depicted in Fig. 8. However, at Fe2+

concentration of 0.1 mM,

ACT degradation started to decrease. This suggests that there is a competing reaction which

involves Fe2+

aside from its catalytic role shown in Equation 1. Fe2+

does not act merely as a

catalyst to speed up Fenton reaction, it also reacts with •OH (Equation 7) (Kang, Lee, & Yoon,

2002).There is a certain Fe2+

:H2O2 ratio wherein scavenging of •OH in the solution manifests as

observed by a decrease in removal efficiency.

(7)

(b) (a)

0.0 0.3 0.640

45

50

55

60

65

AC

T r

em

oval

(%)

[Fe2+

] (mM)

ACT removal

0.0

0.2

0.4

0.6

0.8

1.0

In

itia

l ra

te (

mM

/min

)

Initial rate

Figure 8. Effect of Fe2+

: pH=3, [ACT]=5 mM,

[H2O2]=5 mM

0.0 0.3 0.675

80

85

90

95

100

AC

T r

em

oval

(%)

[Fe2+

] (mM)

ACT removal

0.0

0.5

1.0

1.5

2.0 Initial rate

In

itia

l ra

te (

mM

/min

)

Figure 9. Effect of Fe2+

: pH=3, [ACT]=5mM,

[H2O2]=25mM

0 10 20 3050

60

70

80

AC

T r

em

ov

al (%

)

[H2O

2] (mM)

ACT removal

0.00

0.02

0.04

0.06

0.08

0.10

In

itia

l ra

te (

mM

/min

)

Initial rate

Figure 10. Effect of H2O2: pH=3, [ACT]=5 mM, [Fe

2+]=0.01 mM

Aside from organic compounds, •OH can react with other species present in solution (Ting, Lu, &

Huang, 2009) which depletes the amount of available •OH in solution.

(8)

(9)

(10)

For the concentration range of H2O2 studied between 5 and 25 mM at [Fe

2+]=0.01 mM, there was

no observed decrease in ACT removal. Figure 10 shows that an increase in H2O2 concentration

leads to better removal. However, as the concentration was increased from 20 to 25mM, there was

only a slight increase in removal. From previous studies made, a very high H2O2 concentration

resulted in a decreased reduction because this H2O2 will scavenge the •OH produced as in Equation

8. There is a certain threshold concentration wherein the effect of increasing H2O2 does not result to

an increase in removal. Beyond this optimum condition, it was observed that little or no change in

degradation occurs.

Another observation from this study was the difference in degradation rates at various Fe2+

concentrations not only at low H2O2 but also at high H2O2 concentrations. It was observed that for

some runs, degradation occurs in two stages (Fig. 7(a)). There is a fast initial degradation rate

followed by a slow rate in ACT removal. However, for other runs, degradation of ACT occurs in a

single slow rate all throughout the reaction time. The two-stage degradation was observed to occur

at [Fe2+

] higher than 0.02mM.

This two-stage degradation is due to the very fast reaction of ferrous ions with hydrogen peroxide

(Lu, Chen, & Chang, 1999). During the first minutes, there is a large amount or hydroxyl radical

produced that attacks organic compounds. This results to a rapid decomposition of ACT in solution

(Fig. 8 and 9) for the initial 20 minutes. This first stage is the Fe2+

/H2O2 stage. However, as the

reaction proceeds, ferrous ions are converted to ferric ions as shown in Equation 1. This limits the

amount of ferrous that can react with H2O2 to produce •OH. The Fe3+

ions produced reacts with

H2O2 at a slower rate to produce hydroperoxyl radicals (•OH2) as in Equation 3, which is a weaker

oxidant with E°=1.65 as compared to the oxidizing potential of •OH at 2.80. Thus, the degradation

rate is much slower. The second stage is known as the Fe3+

/H2O2 stage.

CONCLUSION This study applied Box-Behnken design in the optimization of fluidized-bed Fenton process on the

degradation of the drug, acetaminophen. The optimum condition was found to be at pH=3.22,

[Fe2+

]=0.06mM and [H2O2]=19.87mM. Actual ACT removal at the optimum was 97.83%.

Comparison of conventional Fenton and fluidized-bed Fenton process at optimum condition

showed that FB-Fenton has reduced sludge formation with total iron removal at 62.92% compared

to 9.06% of Fenton process and higher TOC removal, 59.62% vs. 16.37%.

Parametric studies revealed that ACT removal without pH adjustment was almost negligible at only

8.65% which proves that initial pH adjustment is crucial and necessary for this system. Increasing

Fe2+

concentration to 0.1mM at [H2O2]=5mM decreased ACT removal. Increasing H2O2

concentration always lead to an increase in ACT removal for the concentration range studied. A

two-stage degradation rate was observed at Fe2+

concentrations higher than 0.02mM. FB-Fenton

proves to be a promising method, requiring minimal space, complete degradation and

mineralization with reduced sludge production.

ACKNOWLEDGEMENT This research was financially supported by the National Science Council, Taiwan (Grant: NSC 99-

2221-E-041-012-MY3), the Department of Science and Technology, Philippines and the

Engineering Research and Development for Technology (ERDT), Philippines.

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