Chemosphere 54 (2004) 1005–1010

www.elsevier.com/locate/chemosphere

Short Communication

Fenton’s pre-treatment of mature landfill leachate

Antonio Lopez a,*, Michele Pagano a, Angela Volpe a,Appio Claudio Di Pinto b

a Consiglio Nazionale delle Ricerche, Istituto di Ricerca Sulle Acque, Reparto di Chimica e Tecnologia delle Acque,

Via Francesco De Blasio, 5, 70123 Bari, Italyb Via Reno, 1, 00198 Roma, Italy

Received 14 April 2003; received in revised form 18 July 2003; accepted 11 September 2003

Abstract

The aim of this study was to check the effectiveness of the Fenton’s reagent (Fe2þ +H2O2 +Hþ) for the pre-treatment

of a municipal landfill leachate with the objective of improving its overall biodegradability, evaluated in terms of BOD5/

COD ratio, up to a value compatible with biological treatment. The leachate came from a municipal sanitary landfill

located in southern Italy and the average values of its main parameters were: pH¼ 8.2; COD¼ 10 540 mg l�1;BOD5¼ 2300 mg l�1; TOC¼ 3900 mg l�1; NH4-N¼ 5210 mg l�1; conductivity¼ 45350 lS cm�1; alkalinity¼ 21 470mg l�1 CaCO3. The effect of initial pH value on the pre-treatment effectiveness was evaluated by titrating the amount of

acidic by-products formed. The extent of leachate oxidation was monitored and controlled by both pH and redox

potential measurements. The best operational conditions for achieving the desired goal (i.e., BOD5/CODP 0.5) re-

sulted: Fe2þ ¼ 275 mg l�1; H2O2¼ 3300 mg l�1; initial pH¼ 3; reaction time¼ 2 h. At the end of the Fenton’s pre-treatment, in order to permit a subsequent biological treatment, residual ferric ions were removed increasing the pH up

to 8.5 by adding 3 g l�1 of Ca(OH)2 and 3 mg l�1 of a cationic polyelectrolyte, the latter as an aid to coagulation. This

final step also resulted in a further modest removal of residual COD due to co-precipitation phenomena.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Fenton’s reagent; Landfill leachate; Biodegradability enhancement; Oxidation processes

1. Introduction

Although landfill leachates have been proved to be

toxic and recalcitrant, landfilling still remains one of the

main methods for municipal and industrial solid waste

disposal. There are many factors affecting the quality

and the quantity of such leachates, i.e., seasonal weather

variation, landfilling technique, piling and compaction

method, waste type and composition, structure of the

landfill, etc. In particular, the composition of landfill

leachates varies greatly depending on the age of the

*Corresponding author. Tel.: +39-080-582-0506; fax: +39-

080-531-3365.

E-mail address: an.lopez@area.ba.cnr.it (A. Lopez).

0045-6535/$ - see front matter � 2003 Elsevier Ltd. All rights reserv

doi:10.1016/j.chemosphere.2003.09.015

landfill (Baig et al., 1999). Accordingly, several treat-

ment technologies are used in practice (Haapea et al.,

2002).

To remove the bulk of pollutants, biological treat-

ments are usually preferred over physico-chemical (Im

et al., 2001). However, good performances are obtained

with biological processes only treating ‘‘young’’ biode-

gradable leachates. In the case of ‘‘old’’ leachates,

instead, COD (chemical oxygen demand) maximum

allowable concentration (MAC) for direct or indirect

discharge cannot be met because of the occurrence

of pollutants that inhibit biomass activity and/or are

recalcitrant to biological treatments. In such instances,

MAC values are usually achieved by more expen-

sive physico-chemical treatments such as flocculation–

precipitation, adsorption on activated carbon,

ed.

1006 A. Lopez et al. / Chemosphere 54 (2004) 1005–1010

evaporation, chemical oxidation, incineration. Among

them, growing interest has been focused on advanced

oxidation processes, AOP, (Huang et al., 1993; Sch-

roder, 1996) which, exploiting the strong oxidation po-

tential of hydroxyl radicals (HO�), can achieve two

alternative goals: (i) the reduction of the COD content

of wastewater up to the desired MAC value through the

mineralization of recalcitrant pollutants (i.e., their

transformation into CO2); (ii) the enhancement of the

biodegradability of treated effluents with the aim of

making their subsequent biological treatment possible.

In general, AOP are defined as oxidation processes

which generate hydroxyl radicals in sufficient quantity to

affect water and wastewater treatment (Huang et al.,

1993). The hydroxyl radical is one of the most reactive

free radicals and one of the strongest oxidants (HO� +

Hþ + e� )H2O; E0¼ 2.33 mV). Many systems can be

classified as AOP and most of them use a combination

of: two oxidants (e.g., O3 plus H2O2); catalyst plus

oxidant (e.g., Fe2þ +H2O2); oxidant plus irradiation

(e.g., H2O2 plus UV); oxidant plus photo-catalyst (e.g.,

H2O2 plus TiO2 plus hm); oxidant plus ultrasounds (e.g.,H2O2 plus ultrasounds). One common feature of such

systems is the high demand of electrical energy for de-

vices such as ozonizers, UV lamps, ultrasounds, and this

results in rather high treatment costs. The only excep-

tion is the Fenton’s process. In such a process, in fact,

under acidic condition, a Fe2þ/H2O2 mixture produces

OH� radicals in a very cost-effective way. The major

advantages of the Fenton’s reagent (Fe2þ +H2O2 +Hþ)

are: (i) both iron and hydrogen peroxide are cheap and

non-toxic; (ii) there is no mass transfer limitation due to

its homogeneous catalytic nature; (iii) there is no form of

energy involved as catalyst; (iiii) the process is techno-

logically simple.

Because of these features, Fenton’s process has been

applied in many areas (Prousek, 1995) including that of

recalcitrant wastewater and/or landfill leachates treat-

ment (Koyama and Nakamura, 1994; Gau and Chang,

1996; Tang and Huang, 1996; Bae et al., 1997; Kim

et al., 1997; Steensen, 1997; Kwong et al., 1999; Rivas

et al., 2001; Zhu et al., 2001). However, at least in the

case of landfill leachates, Fenton’s process has been used

mainly as a post-treatment to achieve desired COD

MAC values rather than as a pre-treatment in order to

permit a subsequent economical biological treatment

(Chamarro et al., 2001).

On the basis of the above considerations, the inves-

tigation described in the present work was carried out

specifically to check the effectiveness of the Fenton’s

process for pre-treating a municipal landfill leachate

with the aim of improving its overall biodegradability up

to a value compatible with subsequent aerobic biological

treatment. Measuring biodegradability as the ratio be-

tween the biochemical oxygen demand measured after 5

days (BOD5) and COD, such a value must beP 0.5.

2. Experimental

2.1. Fenton’s treatment procedure

Fenton’s treatment of landfill leachate was carried

out at ambient temperature according to the following

sequential steps. (1) Leachate sample was put in a beaker

and magnetically stirred; its pH was adjusted to fixed

values by H2SO4 95–97% (w/w). (2) The scheduled Fe2þ

dosage was achieved by adding the necessary amount of

solid FeSO4 Æ 7H2O. (3) A known volume of 35% (w/w)H2O2 solution was added in a single step. (4) After fixed

reaction time (2 h), before carrying out BOD tests, 3

g l�1 of calcium hydroxide and 3 mg l�1 of a cationic

polyelectrolyte (Dryfloc 652) were added to treated

leachate samples to precipitate residual ferric ions and to

better coagulate the resulting sludge. (5) At the end of

Fenton’s treatment, stirring was turned off and the

sludge was allowed to sediment. In every case, all the

analyses of treated leachate were carried out on filtered

samples.

2.2. Analyses and chemicals

Total organic carbon (TOC) analyses were per-

formed by a TOC 5050 analyzer from Shimadzu,

Japan. H2O2 concentration was determined by iodo-

metric method (Kolthoff et al., 1974). Metals were

analysed by inductively coupled plasma emission spec-

troscopy with an Optima 3000 ICP-OES system from

Perkin Elmer, Norwalk-CT/USA. Redox potential and

pH were measured by a Mettler DL 40 RC system

equipped with DG 111-SC and platinum DM 140

combined electrodes from Mettler-Toledo, Milano, It-

aly. BOD5, COD, TS, TSS, PO4-P, NH4-N, alkalinity

and chlorides were measured according to standard

methods (APHA, 1995). In Fenton treated leachate

samples, COD content was calculated as difference be-

tween the measured total COD and the COD due to

residual H2O2 (Kang et al., 1999). All the used chemicals

were analytical grade from Baker, Germany. Cationic

polyelectrolyte Dryfloc 652 was from Nymco-Acque,

Cormano, Italy.

3. Results and discussion

The composition of the investigated leachate is re-

ported in Table 1. Taking into account the low con-

centration of heavy metals, the pH value (8.2), the low

value of the BOD5/COD ratio [(2300/10540)¼ 0.2] andthe high contents of NH4-N (5210 mg l

�1) and alkalinity

(21 470 mg l�1), the leachate was classified as ‘‘old’’ and

non-biodegradable (Baig et al., 1999).

Referring to the Fenton process, it is well known that

higher hydrogen peroxide to substrate ratios result in

Table 1

Composition of the investigated landfill leachate

Parameter Unit Value

pH 8.2

Conductivity lS cm�1 45 350

Total COD mg l�1 10 540

TOC mg l�1 3900

BOD5 mg l�1 2300

Total solids (TS) mg l�1 20 255

Total suspended solids (TSS) mg l�1 1666

Alkalinity (CaCO3) mg l�1 21 470

NH4-N mg l�1 5210

PO4-P mg l�1 32

Cl� mg l�1 4900

Metal ions

Ca mg l�1 15.7

Mg mg l�1 24.1

Na mg l�1 3970

K mg l�1 3460

Fe mg l�1 2.7

Zn mg l�1 0.16

Ni mg l�1 0.31

Cr mg l�1 2.21

Mn mg l�1 0.04

Cd mg l�1 <0.02

Pb mg l�1 <0.03

01

23

45

67

0 250 500 750 1000

Fe2+ added (mg/l)

Res

idua

l H2O

2 (g

/l)

60

70

80

90

100

Rem

aini

ng C

OD

(%

)

50

60

70

80

90

100

Rem

aini

ng T

OC

(%

)

Fig. 1. Effect of Fe2þ dosage on selected parameters measured

in Fenton treated leachate’s samples. All the experiments were

carried out in batch at initial pH¼ 3.0, constant H2O2 dosage(6300 mg l�1), reaction time¼ 2 h.

A. Lopez et al. / Chemosphere 54 (2004) 1005–1010 1007

more extensive substrate degradation, while higher

concentrations of iron ions yield faster rates. However,

in order to maximise the effectiveness of the process,

it is preliminarily necessary to determine the optimal

operational H2O2/Fe2þ mass ratio. In fact, it must be

pointed out that Fenton’s reagent (Fe2þ +H2O2 +Hþ )

Fe3þ +H2O+HO�) is used to produce the hydroxyl

radicals necessary to oxidise organic substances (OM)

according to the following reaction (Edwards and Curci,

1992; Huang et al., 1993):

HO� þOM) Oxidation Products

ðk20 �C � 107–1010 M�1 s�1Þ ð1Þ

Particular attention must be paid to Fe2þ and H2O2dosages in order to avoid the following undesired HO�

radicals scavenging reactions occurring in the presence

of an excess of each of the two reagents (Tang and

Huang, 1997).

Fe2þ þHO� ) Fe3þ þOH� ðk20 �C ¼ 3� 108 M�1 s�1Þð2Þ

H2O2þHO� )H2Oþ �O2H ðk20 �C ¼ 2:7� 107 M�1 s�1Þð3Þ

Previous studies, in fact, have demonstrated that the

best oxidation efficiency is achieved by reaction (1) when

neither H2O2 nor Fe2þ is overdosed, so that the maxi-

mum amount of HO� radicals is available for the oxi-

dation of organic compounds (Tang and Huang, 1997).

In other words, an optimal ratio between H2O2 and Fe2þ

must be fixed in order to minimize scavenging effects. In

order to assess such a ratio, maintaining constant the

concentration of H2O2 as 6300 mg l�1, an arbitrary

value, a set of experiments was carried out progressively

increasing the dosage of Fe2þ. According to previous

studies, all the experiments were carried out fixing a

reaction time of 2 h and an initial pH of 3.0 (Prousek,

1995).

Fig. 1, that shows the results of such experiments,

demonstrates that in the treated leachate the remaining

COD values regularly decreased with increasing iron

dosages. However, when Fe2þ was overdosed (Fe2þ >

500 mg l�1) HO� radicals were scavenged through reac-

tion (2) and an opposite trend was recorded. Referring

to remaining TOC and residual H2O2 values, their

trends showed a clear decrease only up to a Fe2þ con-

centration of 250 mg l�1. The fact that, in the Fe2þ

concentration range 250–500 mg l�1, the decrease of

COD resulted much more sharp than that of TOC in-

dicates that, within this range, organic matter still con-

tinued to be oxidised but the extent of its transformation

into CO2 is lower than previously. Fig. 1 also shows that

1008 A. Lopez et al. / Chemosphere 54 (2004) 1005–1010

at Fe2þ concentrations higher than 500 mg l�1 residual

TOC values essentially remain constant although the

correspondent COD values regularly increase. This

means that the progressive organic matter mineraliza-

tion (i.e., its transformation into CO2), caused by in-

creasing Fe2þ concentration, just stops at a Fe2þ dosage

of 500 mg l�1. Any further addition of ferrous ions only

causes a progressive reduction of the oxidation degree of

the residual organic matter measured as TOC. In fact,

for a same amount of TOC, the lower its oxidation de-

gree the higher its correspondent COD. Once again,

such a trend can be explained taking into account that,

in the presence of an excess of Fe2þ, reaction (2) pre-

dominates over reaction (1). Therefore, the higher the

Fe2þ concentration, the lower the amount of OH� radi-

cals available to oxidise organic matter according to

reaction (1).

In practice, Fig. 1 indicates that the optimal (H2O2/

Fe2þ) mass ratio should be about 12 [i.e. (6300/500)].

However, as proved by other authors, this value can

greatly fluctuate according to the class of pollutants and

to the matrix effect in complex wastewaters (Prousek,

1995; Tang and Huang, 1997).

Maintaining constant the H2O2/Fe2þ ratio, the steady

state concentration of hydroxyl radicals depends upon

the absolute amounts of H2O2 and Fe2þ. Therefore, in

order to assess the relationship between the percentage

of removed COD and such amounts, a set of experi-

ments was carried out fixing the H2O2/Fe2þ mass ratio

(i.e., 12) and treating the leachate with increasing

amounts of both reagents. In Fig. 2, in addition to the

results of such experiments, it is shown also the further

COD removal due to the final polishing step (i.e., the

0

10

20

30

40

50

60

70

80

90

100

0 5000 10000 15000H2O2 added (mg/l)

Rem

aini

ng C

OD

(%

)

Fig. 2. Effect of increasing dosages of H2O2 and Fe2þ, at con-

stant mass ratio (H2O2/Fe2þ ¼ 12), on COD removal. Residual

COD values were measured at the end of Fenton treatment

before () and after () adding 3 g l�1 of Ca(OH)2 and 3 mg l�1of a cationic polyelectrolyte (Dryfloc 652). Experimental con-

ditions: initial pH¼ 3.0, reaction time¼ 2 h.

addition of calcium hydroxide and cationic polyelec-

trolyte) necessary to permit successive biological treat-

ment. Fig. 2 demonstrates that the COD values of the

treated leachate regularly decreased with increasing

amounts of H2O2 and Fe2þ up to a H2O2 value of 10 000

mg l�1; afterwards, greater dosages did not modify re-

sidual COD value. Such a trend can be explained con-

sidering that the end by-products of oxidation reactions

are mainly made of short chain organic acids that are

difficult to be further oxidised (Rivas et al., 2001). Fig. 2

also demonstrates that the more oxidised was the

leachate (i.e., the lower is its COD), the greater was the

additional COD removal due to the polishing step. This

result is due to the fact that HO� radicals, in addition to

oxidation reactions, induce secondary effects, such as:

(i) the formation of colloidal particles from dissolved

organic matter through condensation and/or polymeri-

sation reactions that shift organic matter size distribu-

tion towards larger sizes; (ii) the improvement of ferric

ions complexation by oxidised organic matter resulting

in easier precipitation of the latter (Lee et al., 1996;

Kang and Chang, 1997; Kang and Hwang, 2000; Lau

et al., 2001).

Once assessed the optimal H2O2/Fe2þ mass ratio (i.e.,

12), the maximum achievable COD removal (i.e., 60%)

and the absolute amounts of H2O2 and Fe2þ necessary

to achieve this maximum removal (i.e., H2O2¼ 10 000mg l�1 and Fe2þ ¼ 830 mg l�1), experiments were carriedout to evaluate leachate’s biodegradability enhance-

ment, in terms of BOD5/COD ratio increase, achieved

during the Fenton’s treatment. The results of these ex-

periments are shown in Fig. 3 and indicate that, to

achieve an exploitable biodegradability improvement

(BOD5/COD >0.5), it is sufficient to treat the leachate

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0 2500 5000 7500 10000

H2O2 added (mg/l)

BO

D5

/CO

D

Fig. 3. Effect of increasing dosages of H2O2 and Fe2þ, at con-

stant mass ratio (H2O2/Fe2þ ¼ 12), on BOD/COD ratio mea-

sured after adding 3 g l�1 of Ca(OH)2 and 3 mg l�1 of a cationic

polyelectrolyte (Dryfloc 652). Experimental conditions: initial

pH¼ 3.0, reaction time¼ 2 h.

0

2

4

6

8

10

12

0 25 50 75 100 125 150

NaOH added (meq/l)

pH

pH0 =2pH0 =3

pH0=4

Fig. 5. Titration curves of leachate’s samples collected after

Fenton treatments carried out fixing different initial pH values

(pH0). Experimental conditions: H2O2¼ 6300 mg l�1, Fe2þ ¼500 mg l�1, reaction time¼ 2 h.

A. Lopez et al. / Chemosphere 54 (2004) 1005–1010 1009

with amounts of H2O2 and Fe2þ of 3300 and 275 mg l�1

respectively, avoiding the need to use greater amounts

only to reduce the COD (see Fig. 2).

Tests were also carried out to assess whether reaction

times greater than 2 h, i.e. the time fixed during the

whole investigation, would result beneficial to the extent

of leachate’s oxidation. The results shown in Fig. 4

confirmed that after two hours the oxidation was over

and, then, this is the value that has to be used during

reactor designing steps. Fig. 4 also demonstrates that the

progress of Fenton’s treatment can be monitored by

measuring pH or redox potential evolution during

leachate’s oxidation. This is particularly important from

the technological stand point as the possibility of mon-

itoring a process ‘‘on-line’’ is always desirable.

Finally, the effect of initial leachate’s pH on Fenton’s

oxidation effectiveness was evaluated in terms of acidic

by-products formation. In fact, as previously pointed

out, the end by-products of Fenton’s reactions are

mostly made by carboxylic acids. Therefore, the higher

the Fenton’s oxidation extent the greater the amount of

acid by-products formed. Fig. 5 shows the curves ob-

tained by titrating with 1 N NaOH Fenton treated

leachates whose initial pHs were different. This figure

clearly demonstrates that the highest amount of NaOH

necessary for neutralizing the treated leachate was re-

quired when the pH, before Fenton’s treatment, was

50

60

70

80

90

100

Rem

aini

ng C

OD

(%)

50

60

70

80

90

100

Rem

aini

ng T

OC

(%)

0

2000

4000

6000

8000

Res

idua

l H2O

2

(mg/

l)

400

450

500

550

600

650

0 100 200 300 400 500Time (min)

red

ox p

oten

tial

(m

V)

2.02.22.42.62.83.03.2

pH

Redox potential

pH

Fig. 4. Trends of selected parameters during leachate’s Fenton

treatment. Initial experimental conditions: pH¼ 3.0, H2O2¼6300 mg l�1, Fe2þ ¼ 500 mg l�1.

fixed at 3.0, thus confirming the appropriateness of the

value scheduled and fixed for carrying out the whole

investigation.

4. Conclusions

An investigation aimed at checking the effectiveness

of Fenton’s reagent (Fe2þ +H2O2 +Hþ) for the en-

hancement of the biodegradability of real old municipal

landfill leachate has been carried out at lab scale at

ambient temperature. The investigation has led to the

following results:

• Themaximum amount of COD that could be removed

by the Fenton’s pre-treatment was about 60% of the

initial value (i.e., 10 540 mg l�1). Such a maximum re-

moval was achieved using reagent dosages as high as

10 000 mg l�1 of H2O2 and 830 mg l�1 of Fe2þ.

• BOD5/COD ratio of the leachate could be increased

from 0.2, the initial value, up to 0.5, the minimum

value compatible with a subsequent biological post-

treatment, using the following optimum operational

conditions: initial pH¼ 3.0; Fe2þ ¼ 275mg l�1; H2O2¼3300 mg l�1; reaction time¼ 2 h.

• The progress of Fenton’s pre-treatment could be in-

strumentally monitored by measuring pH or redox

potential evolution during leachate oxidation, thus,

indicating the possibility of an on-line process moni-

toring.

• A simple acid–base titration of Fenton pre-treated

leachate proved that a relevant fraction of by-prod-

ucts formed during the treatment was made of acidic

compounds, thus demonstrating that the higher the

extent of Fenton’s oxidation the greater was the

amount of acids formed.

1010 A. Lopez et al. / Chemosphere 54 (2004) 1005–1010

• After the Fenton’s pre-treatment, in order to permit

subsequent biological treatment, residual ferric ions

were precipitated and coagulated with 3 g l�1 of

Ca(OH)2 and 3 mg l�1 of a cationic polyelectrolyte.

This final polishing step caused an additional modest

removal of residual COD.

In conclusion, the above results seem rather encour-

aging and worthwhile confirming on a larger scale as

they indicate a technologically simple and cost-effective

method for improving the biodegradability of recal-

citrant wastewater.

Acknowledgements

The authors thank Mr. Michele Cammarota and Mr.

Giuseppe Labellarte for their technical assistance during

the investigation.

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