USE OF FENTON REAGENT TO IMPROVE ORGANIC CHEMICAL BIODEGRADABILITY doc

ESPLUGAS*M Departament d’Enginyeria Quı´mica i Metal.lu´rgia, Universitat de Barcelona C/Martı´ i Franque`s 1, 08028 Barcelona, Spain First received 9 November 1999; accepted in revised

2001 Elsevier Science Ltd All rights reserved

Printed in Great Britain 0043-1354/01/$ - see front matter PII: S0043-1354(00)00342-0

USE OF FENTON REAGENT TO IMPROVE ORGANIC

CHEMICAL BIODEGRADABILITY

E CHAMARRO, A MARCO and S ESPLUGAS*M

Departament d’Enginyeria Quı´mica i Metal.lu´rgia, Universitat de Barcelona C/Martı´ i Franque`s 1,

08028 Barcelona, Spain (First received 9 November 1999; accepted in revised form 15 January 2000) Abstract}Fenton reagent has been used to test the degradation of different organic compounds (formic acid, phenol, 4-chlorophenol, 2,4-dichlorophenol and nitrobenzene) in aqueous solution A stoichiometric coefficient for the Fenton reaction was found to be 0.5 mol of organic compound/mol of hydrogen peroxide, except for the formic acid where a value of approximately one was obtained (due to the direct formation of carbon dioxide) The treatment eliminates the toxic substances and increases the biodegradability of the treated water (measured as the ratio BOD 5 /COD) Biodegradability is attained when the initial compound is removed # 2001 Elsevier Science Ltd All rights reserved

Key words}Fenton reagent, advanced oxidation technologies (AOT)

INTRODUCTION Water quality regulations are becoming stricter in the

late decades due to an increasing social concern on

environment A very interesting field of concern is

what to do with wastewater that contains soluble

organic compounds that are either toxic or

non-biodegradable Advanced oxidation technologies

(AOT) for water and wastewater treatment show

high efficiency but work at a high cost of both

investment (complex installations) and operation

(higher consume of energy and/or reagents) This

makes these processes only useful when the cheaper

options are not effective Experiences with different

oxidation technologies and different substrates

have shown that a partial chemical oxidation of a

toxic wastewater may increase its biodegradability up

to high levels (Kiwi et al., 1994; Scott and Ollis,

1995)

One of the most effective technologies to remove

organic pollutants from aqueous solutions is the

Fenton’s reagent treatment (Bidga, 1995) It is well

known that organic compounds can easily be

oxidized It consists in a mixture of hydrogen

peroxide and iron salts There are chemical

mechan-isms that propose hydroxyl radicals as the oxidant

species (Pignatello, 1992; Walling et al., 1974), that

are generated in the following chemical equation:

Fe2þþH2O2! Fe3þþOHÿþOH ð1Þ

Hydroxyl radicals may be scavenged by reaction with another Fe2+:

OHþFe2þ! OHÿþ Fe3þ ð2Þ

Fe3+ catalytically decomposes H2O2 following a radical mechanism that involves hydroxyl and hydroperoxyl radicals, including (1) and (2):

Fe3þþ H2O2Ð Fe2OOH2þþ Hþ ð3Þ

Feÿ OOH2þ! HO2þFe2þ ð4Þ

Fe2þþ HO2! Fe3þþ HOÿ2 ð5Þ

Fe3þþHO2! Fe2þþHþþO2 ð6Þ

OHþH2O2! H2Oþ HO2 ð7Þ Fenton reagent shows to be a very powerful oxidizing agent (Sedlak and Andren, 1991; Potter and Roth, 1993) There are, however, species that show resistance to oxidation by Fenton reaction (Bidga, 1995) These species are small chlorinated alkanes (tetrachloroethane, trichloroethane), n-paraffins and short-chain carboxylic acids (maleic, oxalic, acetic, malonic) These last compounds are indeed a very interesting kind because they are typical oxidation products of larger molecules after fragmentation Even more interesting is that the cited compounds are known to be primary metabolites, which act in energetic cycles of most living organisms Partial chemical oxidation yields biodegradable products, together with destruction of inhibitory species (Marco et al., 1997) The objective of this paper

*Author to whom all correspondence should be addressed.

Tel.: +34-3-402-12-90; fax: +34-3-402-12-91; e-mail:

esplugas@angel.qui.ub.es

1047

was the degradation of small organic molecules by

the Fenton reagent

MATERIALS AND METHODS

Different organic compounds (acetic acid, formic acid,

phenol, 4-chlorophenol, 2,4-dichlorophenol and

nitroben-zene) were choosen to study their degradation in aqueous

solution using Fenton’s reagent All chemicals used were

produced by Panreac (Spain) and were of analytical grade.

The experiments were carried out at a ratio Fe 2+ /compound

equal to 1, 0.1 and 0.01 Initial concentration of organic

pollutants was set to 300 mg/L.

Total organic carbon (TOC) analysis were performed in

order to know the amount of organic compounds that were

depleted to CO 2 during the chemical oxidation The TOC

content of the samples was determined by Dohrman

DC-190 high-temperature TOC analyzer.

Concentration of organic compounds was followed by

HPLC (high-performance liquid chromatography)

Chro-matograms were made with Millennium software using a

Waters 600 Controller with a Waters 996 Photodiode Array

Detector The column (spherisorb ODS2; 5m; 25  0.46 cm)

was washed with methanol before analysis A mixture of

50% acetonitrile in 50% water was chosen as the optimal

mobile phase.

Biodegradability was measured by 5-day biochemical

oxygen demand (BOD 5 ) and by 21-day biochemical oxygen

demand (BOD 21 ) analysis of samples at different times of

treatment As bacterial seed (this synthetic water is sterile) a

small amount of filtered activated sludge from a municipal

wastewater plant was used This kind of seed was chosen

because it comes from the most common and cheap

biological treatment, and it means that no special or

adapted bacteria are required to reproduce these results.

Chemical oxygen demand (COD) is also an important

parameter that was followed in order to know the degree of

oxidation changes.

EXPERIMENTAL RESULTS AND DISCUSSION

The experimental work was oriented towards

studying how the amount of oxidant applied affects

the biodegradability of initially non-biodegradable

different organic compounds Figure 1 shows the

BOD/COD ratio (the standard for 5 and 21 days) for

six organic compounds BOD/COD constitutes a

good measure of the biodegradability of a

waste-water Contaminants with a ratio of BOD5/

COD>0.4 may be considered thoroughly

biodegrad-able It can be observed that acetic acid (ACH) and phenol (PHE) are quite biodegradable, formic acid (FOR) is lightly biodegradable, but 4-chlorophenol (4-CP), 2,4-dichlorophenol (DCP) and nitrobenzene (NB) are refractory to the biological treatments Two kinds of experiments were developed First type were conducted to the search of the stoichio-metric coefficients, that is to know the moles of organic compound removed by 1 mol of hydrogen peroxide Thirty mililiter vials, at room temperature, were filled with organic/Fe2+solution at Fe/organic ratios of 1 : 1, 0.1 : 1 and 0.01 : 1 Different doses of peroxide were added to these vials (from 0.1–50 mol

H2O2molÿ 1 organic) After 24 h organic remaining was analyzed by HPLC, TOC, COD and BOD Second type were kinetic experiments and were carried out in a stirred reactor of 1.5 L capacity at batch operation, isothermal conditions and refriger-ated by water For these experiments the measured variables were: redox potential, pH, temperature, concentration of organic and TOC

The stoichiometric coefficients for the five organic compounds studied are shown in Table 1 They have been obtained through linear fitting of the experi-mental results until 90% degradation The behavior

of the organic compounds is similar with the exception of the formic acid The explanation is that hydroxyl radical generated oxidize the main com-pound and its intermediates, but in the case of the formic acid only the main compound is oxidized because formic acid is already highly oxidized, little additional oxidation by Fenton reagent is required before conversion to carbon dioxide

Organic compoundþ Fe=H2O2! Oxidized products

Oxidized productsþ Fe=H2O2

! Other oxidized products Figure 2 shows the organic remaining after 24 h for a ratio Fe2+/organic of 0.01 : 1 and different hydrogen peroxide doses A similar behavior can be seen for the four compounds (PHE, 4-CP, DCP and NB) tested For a ratio H2O2/organic above 3 the organic reduction is practically complete in all cases It is not necessary to add a large quantity of hydrogen peroxide to the system to remove the organic compounds Other ratios Fe2+/organic tested (0.001 : 1 and 0.1 : 1) have given the same results

Fig 1 BOD 5 /COD and BOD 21 /COD ratios for the tested

organic compounds.

Table 1 Stoichiometric coefficients for FOR, PHE, 4-CP, DCP and

NB (confidence coefficient 95%) Compound Mol removed/mol H 2 O 2

4-Chlorophenol 0.601  0.044 2,4-Dichlorophenol 0.520  0.031

The analysis of total organic carbon for these

experiments with a ratio Fe2+/organic 0.01 : 1 shows

a similar behavior for the four compounds In all the

cases, for a ratio H2O2/organic equal to 3 the

degradation of organic compound was practically

complete However, the TOC decreased more slowly

and there was no complete mineralization of the

compounds Figure 3 shows the TOC reduction for

these compounds (Fe2+/organic 0.01 : 1) Similar

results were obtained for the other ratios studied

The total organic carbon consumed was also

determined for three different ratios Fe2+/organic

(1, 0.1 and 0.01) In these experiments it can be seen

that mineralization increases with iron concentration

Figure 4 shows the TOC consumed in the case of

4-chlorophenol for these three Fe2+concentrations at

a different H2O2/4-CP ratio (0–50) According to

Fig 4, it can be seen that there is a limiting TOC

value at high concentrations of hydrogen peroxide

In order to reduce the TOC, the concentration of

Fe2+and H2O2show to be very important

In all the experiments, after 24 h of reaction, the

TOC decreased with the concentration of hydrogen

peroxide The COD values decreased too, and the

BOD values were seen to increase In Figure 5, it can

be seen the variation of BOD5/COD after 24 h for

4-chlorophenol experiments with the hydrogen

per-oxide doses operating at a Fe2+/4-CP ratio of 1 : 1

Figure 5 shows 4-chlorophenol solution with a

BOD5/COD ratio initially near zero (as it can be

seen in Fig 1) It becomes a biodegradable solution

when H2O2 is added (as the hydrogen peroxide

concentration increases, the BOD5/COD ratio also

increases to a value  0.4)

Figures 5 and 6 show that the Fenton reaction

with these organic compounds yields biodegradable

substances Only when the initial organic compounds

are depleted, microorganisms are able to degrade the

products Figure 6 shows the biodegradability of

4-chlorophenol and 2,4-di4-chlorophenol vs the

percen-tage of substance removed It can be seen that true

biodegradability is attained when the initial com-pound is removed

Kinetic experiments were carried out for formic acid and 4-chlorophenol In experiments with formic acid the H2O2/FOR ratio was 1.2 and the Fe2+/FOR ratio was 0.4 For these experiments, the TOC decreased very fast and after 2.5 h it was practically zero That is because the formic acid reacts with radicals to give carbon dioxide directly The pH increased when hydrogen peroxide was added de-creasing afterwards to a constant value Also, the redox potential increased when the oxidant was added and afterwards decreased to a constant value,

as it can be seen in Figure 7

The experiments with 4-chlorophenol were carried out with a Fe2+/4-CP ratio of 1 : 1 and with different amounts of H2O2 For these experiments the redox potentials increased when hydrogen peroxide was added to the system and after decreased For a same

Fig 2 Remaining PHE, 4-CP, DCP and NB for initial

concentrations of 300 ppm (Fe 2+ /organic=0.01 : 1).

Fig 3 Total organic carbon after 24 h for a ratio

Fe 2+ /organic=0.01 : 1 (initial concentration of organics:

300 ppm).

Fig 4 TOC consumed vs H 2 O 2 doses at different Fe 2+

ratios.

reaction time, when the H2O2 concentration was

increased, the redox potential was also seen to

increase The pH decreased in all experiments due

to the formation of more acid products than

4-chlorophenol For the same reaction time, when concentration of hydrogen peroxide increased, the

pH decreased In Figure 8 the variation of the concentration of 4-chlorophenol vs time can be seen For a ratio H2O2/4-CP of 1 : 1, the concentration decreased to a constant value In the later case enough hydrogen peroxide was not available in solution to degrade all the 4-CP However, for a ratio of H2O2/4-CP 10:1 the concentration of 4-CP decreased until zero The reaction rate was seen to increase with Fe concentration From Fig 8 it can be concluded that hydrogen peroxide and iron concen-tration have an influence on the degradation rate The iron concentration was seen to be more important than the peroxide ratio For Fe2+

/4-CP ratios larger than 0.1 : 1 the reaction may be considered instantaneous

CONCLUSIONS There are two important factors affecting the rate

of Fenton’s reaction: peroxide dose and iron concentration The peroxide dose is important in order to obtain a better degradation efficiency, while the iron concentration is important for the reaction kinetics

The extension of the oxidation is determined by the amount of hydrogen peroxide present in the system

A total elimination of organic carbon requires large amount of oxidant and/or large residence times The partial oxidation of toxic compounds enhances biodegradability Total depletion of organic carbon requires huge amounts of oxidant and large residence times Oxidant may be wasted under these condi-tions, but subsequent low-cost biological treatment

of pre-treated wastewater is shown in this study as a effective alternative

Acknowledgements}The authors wish to express their gratitude for the financial support given by the Ministry

of Education of Spain (DGICYT, project AMB 96-0906).

Fig 5 BOD 5 /COD vs H 2 O 2 dose of an initial

concentra-tion of 4-CP of 300 ppm (Fe 2+ /4-CP=1 : 1).

Fig 6 Biodegradability of 4-CP and DCP vs fraction

removed.

Fig 7 Redox potential vs time Fe/4CP 1 : 1 different initial

amounts H 2 O 2

Fig 8 Reaction rate of 4-chlorophenol at different Fe 2+ /

H 2 O 2 ratios.

Bidga R J (1995) Consider Fenton chemistry for

waste-water treatment Chemical Engineering Progress 91(12),

62–66.

Kiwi J., Pulgarin C and Peringer P (1994) Effect of Fenton

and photo-Fenton reaction on the degradation and

biodegradability of 2- and 4-nitrophenols in water

treatment Applied Catalysis B: Environmental 3, 335–350.

Marco A., Esplugas S and Saum G (1997) How and why

combine chemical and biological processes for wastewater

treatment Water Science and Technology 35(4), 321–327.

Pignatello J J (1992) Dark and photoassisted Fe3+

-catalyzed degradation of chlorophenoxy herbicides by

hydrogen peroxide Environment Science and Technology

26, 944–951.

Potter F J and Roth J A (1993) Oxidation of chlorinated phenols using Fenton’s Reagent Hazardous Waste & Hazardous Materials 10(2), 151–170.

Scott J P and Ollis D F (1995) Integration of chemical and biological oxidation processes for water treatment: review and recommendations Environmental Progress 14(2), 88–103.

Sedlak D L and Andren A W (1991) Oxidation of chlorobenzene with Fenton’s reagent Environment Science and Technology 125, 777–782.

Walling C., El-Taliawi G M and Johnson R A (1974) Fenton’s reagent IV: structure and reactivity relations in the reaction of hydroxyl radicals and the redox reactions

of radicals Journal of Amercian Chemical Society 96, 133–139.