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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.52 no.1 Concepción Mar. 2007 

 J. Chil. Chem. Soc., 52.No. 1 (2007)



Carolina Baeza1,4, , Claudia Oviedo5, Claudio Zaror4, Jaime Rodríguez1,3, and Juanita Freer1,2*

1Biotechnology Center, Renewable Resources Laboratory, 2Faculty of Chemical Sciences, 3Faculty of Forestry Sciences, 4Department of Chemical Engineering, University of Concepción, Concepción, Chile.5Department of Chemistry, Universidad del Bío-Bío, Concepción, Chile.*


Degradation of EDTA in a Total Chlorine Free (TCF) cellulose pulp bleaching effluent, using UV and UV/H2O2 in presence and absence of iron, was studied. All experiences were conducted in an annular photolytic reactor at pH 9, 20°C, and 0.38 mM (110 mgL-1) EDTA concentration. EDTA degradation followed a first order apparent kinetics, with rate constant in the range of 0.02–0.72 min-1, depending on the initial hydrogen peroxide concentration and iron content. UV treatment without peroxide yielded 100 % degradation of Fe(III)-EDTA in 40 minutes of reaction, but low COD (8 %) and TOC (13 %) removal. Peroxide addition (3.3 mM)increased Fe(III)-EDTA degradation rates by UV treatment, resulting in 100% removal after 20 minutes, and greater COD (28 %) and TOC (36 %) reductions. In absence of iron, peroxide addition was required to obtain EDTA degradationyields above 50% after 1 h treatment. Results presented here show that UV/H2O2 treatment could be suitable as a preliminary stage before a biological treatment.

Keywords: AOTs, UV/H2O2 treatment, EDTA, effluent treatment, TCF effluent.



Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid containing six donor atoms, that acts as a hexadentate ligand. This compound forms strong and very stable complexes with many metal cations (divalent and trivalent ions), increasing the metal ion solubility in aqueous solution as water-soluble complexes [1,2], being one of the cheapest and most suitable multipurpose chelating agents [3]. The use of chelating agents has become widespread over the past decade, mainly, tomoderate the adverse effects of transition metal ions on the performanceon laundry detergents, cosmeticsproducts, and photochemicals.Chelating agents are also used in textile manufacture, hydrogen peroxideand ozone based cellulose bleaching processes, and treatment of soils contaminated with heavy metal [4,5].

EDTA is poorly biodegradable and has been found in many water bodies, leading to restrictions to its use and disposal in many countries [1,6,7,8]. Photodegradation has been identified as a natural removal mechanism of Fe(III)-EDTA species [9]. EDTA’s resistance to bacterial biodegradation is widely documented, and its presence affects the efficiency of biological wastewater treatment [10-16]. Indeed, EDTA has been found to reduce COD removal capacity in activated sludge treatment [17], although some bacterial species.Al laboratory scale, selected bacterial strains have been reported to degrade metal-EDTA complexes [10-18].

The pulp and paper industry has been identified as a major contributor of EDTA waste water emissions [19]. EDTA is extensively used in totally chlorine free (TCF) bleaching sequences, to prevent transition metals ions (Mn(II), Cu(II) and Fe(III)) acting as catalysts in free radical forming reactions, which attacks the cellulose affecting pulp quality [20, 21].

Chemical pretreatment to degrade EDTA into biodegradable species represents an interesting option to increase the efficiency of biological effluent treatment systems [22,23]. EDTA degradation using Advanced Oxidation Technologies (AOTs) have been studied [24]. H202/UV treatment of air-saturated solution of EDTA achieved 78 % mineralization after 9 hours, using a molar EDTA: H202 ratio 1: 40, at pH 3[25]. Electro-oxidation has been used to treat Cu-EDTA complex in an electrolytic cell system, removing 79% EDTA after 8 hours treatment in presence of H202 [6]. Under alkaline conditions, O3/UV and O3/H202/UV treatments achieved 80% EDTA degradation after 15 minutes [26]. Combined UV/H202/microwave treatment yielded more than 90% EDTA mineralization, using a molar EDTA: H202 ratio1:10, for 6 min, at acid pH [27].Recently, a method for EDTA degradation induced by oxygen activation in a zerovalent iron/air/water system has been published (ZEA) [28]. Oviedo et al [23] reported a degradation of 90% ofFe(III)-EDTA complex by Fenton reaction without theadditional supplement of iron in the system. The Fenton reaction was carried out by 1,2-dihydroxibenzene like a Fe(III) reductant, in the presence of H2O2. The EDTA is not displaced from the complex by 1,2-dihydroxybenzene due to the lower stability constant of the complex formed by this, implying a not clear way for the Fenton (or Fenton-like) reaction in this system.

The aim of this article is assess the capacity of UV/H202 to remove EDTA present in TCF cellulose bleaching effluents. The effect of the iron complex on the EDTA degradation was studied, considering a possible role of the Fe(III) in the autocatalysis of Fe-EDTA degradation.


2.1 Materials and methods

 Preparation of TCF synthetic effluent: A model EDTA containing effluent was prepared from eucalypt Kraft pulp, using a Totally Chlorine Free bleaching sequence based on oxygen, ozone and peroxide (O/Z/PO) [29].This model effluent was composed of formic acid (500 mg L-1), acetic acid (100 mg L-1), vanillin (8 mg L-1), glyoxal (3 mg L-1), EDTA (110 mgL-1) and, in some runs, iron chloride (102 mg L-1).

2.2 Chemical treatment

The experimental system is illustrated in Figure 1. The annular photoreactor featured a total volume of 1.8 L (750 mm long and 60 mm external diameter), and was built of borosilicate glass (2.20 mm thickness). The UV source was a crossed fluorescent germicide tube (254 nm, 30 W, diameter of the tube of 26 mm). The reactor feed was stored in a 5 L glass tank, featuring a bottom air bubbling through a circular diffuser. Model feedstock was fed to the reactor and fully recirculated to the feed tank using a peristaltic pump, at flow rate of 425 ± 2 mL min-1.

A total volume of 3 L EDTA, Fe(III)-EDTA, Effluent-Fe(III)-EDTAor Effluent-EDTAsolutions was used at pH 9, and 20ºC. Experiments were conducted in absence and presence of hydrogen peroxide (1 mM, 3.3 mM, and 16 mM; i.e. at a molar EDTA: H2O2 ratio 1:2.6, 1:8.6 and 1:43, respectively).

 2.3HPLC analysis

EDTA was monitored by HPLC analysis [30]. A HPLC pump (Merck-Hitachi L-6200A) with an autosampler (Merck-Hitachi AS-4000) of 20 μL loop was used, equipped with a Lichrospore 100 RP-18 column (Merck) and connected to a UV Merck-Hitachi L-4250 detector (258 nm). Buffer formate/formic acid (0.02 M, pH 3.3) - 10% (v/v) methanol mixture, was used as eluent. The flow rate was set at 0.6 mL min-1.

2.4 COD and TOC determination

The chemical oxygen demand (COD) was measured by the procedure described in Standard Methods for Water Examination [31]. Total organic carbon (TOC) was measured using a total carbon analyzer (TOC- 5000; Shimadzu Co.; Ltda. Japan).


3.1 Mathematical modeling

Residence time distribution (RTD) tests showed that a 6 equal-volume stirred tank reactor in series could be used to model the photoreactor. The feed tank could be modeled as a continuous stirred tank without chemical reaction. Therefore, the EDTA corresponding mass balance is given by:

With initial conditions:t = 0C = C(0)(i.e. at the time UV light was switched on).

Where:n=apparent reaction order

k=apparent rate constant

tFR = photoreactor residence time = 4.3 min

tE.A. = feedtank residence time =4 min

Ci= EDTA concentration in the i reactor

These equations represent the mathematical model of the reaction system, and were used to estimate kinetic parameters using a 2nd order least square routine, coupled to POLYMATHTM. Estimations of corresponding rate constants are shown in Table 1. Reactions fitted an apparent first order kinetic model (n=1).

Model predictions are shown as lines in Figures 2-5. In all cases, fitting is acceptable and in agreement with previous findings by Sörensen [32] who reported a first orderkinetics, with an EDTAdegradation constant of 0.72 min-1, with a molar ratio 1:20 (chelate:H2O2) at pH 6.1, in an UV-H2O2 treatment, with initial EDTA concentration of 0.1 mM.

 3.2 Degradation of EDTA

Main results of EDTA degradation are shown in Figures 2-5.Corresponding model predictions are presented as solid lines. As seen in Figure 2, when the photolysis reaction takes place in absence of peroxide, the EDTA degradation rate increases if present as Fe(III)-EDTA complex. Figure 3 and

Figure 4 show that hydrogen peroxide has a significant effect on the rate of EDTA oxidation when is mixed with the synthetic TCF effluent.As seen in these figures, EDTA degradation increases as the concentration of hydrogenperoxide increases.

In all cases, greater EDTA degradation is observed when it is forming the Fe(III)-EDTA complex. In absence of Fe(III),EDTA is predominantly broken in the ethylenic groups between the nitrogen atoms. In presence of ionic iron, the reaction occurs also through a "split off" of one of the acetate groups, by an intracomplex load transfer reaction, which is photochemically induced, reducing the Fe(III) to Fe(II), followed by a intermolecular load thermal transfer reaction [32]. In addition, hydrogen peroxide acts as an OH·free radical source, reacting either by electrophilic addition and/or electron transfer reactions between the compound and the radical. Hence, hydroxyl radical mechanism accounts for the higher degradation of EDTA without Fe(III). With iron, EDTA degradation in absence of peroxide is very effective, and although the degradation rate increases in presence of H2O2, it is relatively less important than for EDTA (without Fe). When other organic contaminants are present, EDTA degradation slows down probably because OH· radicals are not selective and there is competition by the other compounds.

In UV/H2O2 treatment, the photolysis velocity of the peroxide solution depends on the pH, increasing at alkaline conditions, probably due to the higher absorption coefficient (e) of the peroxide anion at 253.7 nm [33]. The final pH of reactions reported here varied in the range of 7.6 – 8.2. This pH reduction can be attributed to the formation of carboxylic acids [32]. Since experiments were conducted at basic pH, it is assumed that the Fenton reaction, which requires acidic pH to occur significantly [33], is not contributing to the observed EDTA degradation.

There was a slight TOC reduction when UV treatment without peroxide was applied to the effluent-Fe(III)-EDTA. After 20 minutes TOC levels remained approximately constant, with a total removal of 13 % after 60 minutes treatment. When the initial peroxide concentration was 3.3 mM,TOC removal reached 36 %. It is therefore possible to speculate that those treatments generate relatively stable organic molecules which are hard to further oxidize to CO2.

COD removal was slightly greater in presence of iron (Figure 6), which is in agreement with the greater EDTA degradation observed under those conditions. COD removal was found to increase as the initial peroxide concentration increased, reaching a 90% removal when peroxide (16 mM) was used.


The apparent kinetics of EDTA peroxide-assisted photodegradation could be described by a first order model with respect to the chelate concentration.

The degradation of the ligand is more rapid when is forming the complex Fe(III)-EDTA. Results presented here show that UV/H2O2 treatment could be suitable as a preliminary stage before a biological treatment.

ACKNOWLEDGEMENTS. Support from Dirección de Investigación, Universidad de Concepción is acknowledged.


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(Received: July 31, 2006 - Accepted: October 31,2006)

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