Fenton and Photo-Fenton oxidation of tannery wastewater

Fenton and Photo-Fenton processes are attractive alternatives in effluent treatment, especially when applied to recalcitrant compounds. The aim of this work was to evaluate the efficiency of Fenton and Photo-Fenton processes for the treatment of wastewater from leather industry, investigating the reduction of COD, ammoniac nitrogen concentration and toxicity in treated wastewaters. The results showed that the kinetic of degradation by Fenton and Photo-Fenton reactions can be divided in two stages: an initial fast process, where approximately 70% of the COD reduction takes place, followed, by a slow process, where a reaction takes up to 4 hours, resulting in about 90% of COD reduction. Different mass ratios of Fe/H2O2 were tested and the results showed that the efficiency of the Fenton and photo-Fenton reactions increases from 65 to 90% as the concentration of hydroxyl radicals (OH) increases. No significant difference in the ammoniac nitrogen amount reduction for the Fenton and Photo-Fenton processes was observed, either before or after coagulation. The ammonia removal was ascribed to the oxidation of nitrogen organic compounds, possibly forming N2 and nitrate ions. The toxicity biossays using Artemia salina decreased as the wastewater was degraded and increased if the hydrogen peroxide residue at the end of the reaction was high.


Introduction
The leather industry is associated with the generation of huge amounts of liquid effluents (30-35L/kg of raw material processed) (Suresh et al., 2001).The wastewater may be characterized by several key parameters such as sulfide, chromium, oil and grease, BOD and COD (Kabdasli et al., 1993).
It has been demonstrated that a single treatment unit is not sufficient to achieve legal parameters.In fact, after conventional treatment (i.e., chromium precipitation, primary sedimentation, biological oxidation, secondary sedimentation), effluents still do not have the required limits, at least for some parameters, such as COD, salinity, ammonia and surfactants (Iaconi et al., 2002).
Acta Scientiarum. Technology Maringá, v. 25, no. 1, p. 91-95, 2003 Advanced oxidation processes (AOP's) based on the chemistry of hydroxyl radicals ( • OH) are currently used for destroying organic pollutants (Oturan et al., 2001).These radicals react in a nonselective way on organic compounds, leading finally to mineral end products.Fenton's reagent is a system based on the generation of very reactive free radicals, especially hydroxyl radicals, which have a stronger oxidation potential than ozone (De Heredia et al., 2001).Fenton's oxidation process has been successfully employed to treat textile wastewater (Kang and Hwang, 2000;Pérez et al., 2002a;Kang et al., 2002), paper pulp wastewater (Pérez et al., 2002b) and single contaminants in aqueous solution (Maletzky and Bauer, 1998;Fallmann et al., 1999;Oturan et al., 2001).
In Fenton reaction, hydroxyl radicals • OH are produced by interaction of H 2 O 2 with ferrous salts according to Equation 1.
Fe HO FeOOH (3) When the system is irradiated with UV illumination, the degradation rate of the organic pollutants by Fenton reaction can increase through the involvement of high valence iron intermediates, responsible for the direct attack on organic matter (Pérez et al., 2002b).The absorption of visible light by the complex formed between Fe 3+ and H 2 O 2 could be the cause of these intermediates.
This paper evaluates the Fenton and Photo-Fenton processes to treat tannery wastewater, removing chemical oxygen demand and ammonia nitrogen.The possible toxicity of the reaction products was also investigated, using Artemia salina bioassays.

Material and methods
Ferrous sulfate FeSO 4 .7H 2 O, sulphuric acid and hydrogen peroxide (30% wt.) of analytical grade (Merck) were used.The effluent was obtained from a specific tannery (Centro de Tecnologia do Couro e do Calçado Albano Franco -Campina Grande, Paraíba, Brazil), without any pre-treatment.The wastewater characteristics are given in Table 1.The relation COD/BOD is nearly 18, indicating that most of the contaminants are not biodegradable.Fenton process: The Fenton reactions were carried out, at 25 o C, in a 1-L batch reactor using a wastewater volume of 500mL.In the oxidation step, hydrogen peroxide and ferrous sulfate were added together in the reactor.In the coagulation step, the oxidized sample above mentioned was slowly mixed and then left still for sedimentation.The pH for Fenton oxidation was controlled at 2.5 with H 2 SO 4 and the pH was adjusted to 7.0 for the coagulation step, using NaOH 6M.At the end of the process, the residual hydrogen peroxide was analyzed using the method described by Malick and Saha (2003).
Photo-Fenton process: The Photo-Fenton reactions were carried out in a 1-L batch reactor under strong solar irradiation, from 11:00 am to 02:00 p.m., at 25 o C, in the city of Campina Grande, State of Paraíba, northeastern Brazil, from October 1 st to February 20, 2003.The UV-B index was in the range 12. 2 -12.8 (Inpe, 2003).The pH was controlled at 2.5 using H 2 SO 4 , and adjusted at 7.0 for coagulation, using NaOH 6M.
Chemical analyses: The COD was measured according to procedures described in Standard Methods (APHA, 1995).Since the residual H 2 O 2 interferes with the measurement of COD (Kang et al., 2002), the residual amount of H 2 O 2 was also measured, using the permanganate titration with KMnO 4 0.1N.This method is suitable for measuring solutions of hydrogen peroxide in the range 0.25 to 70% wt (Morita and Assumpção, 1995).According Lin and Lo (1997), 1mg.L -1 of H 2 O 2 contributes 0.27mg.L -1 COD concentration.Since no H 2 O 2 residual concentration higher than 0.25% was measured, no correction was perfomed to Acta Scientiarum.Technology Maringá, v. 25, no. 1, p. 91-95, 2003 COD analysis.All other analyses were performed according to Standard Methods (APHA, 1995).Artemia salina bioassays: Artemia salina cysts were incubated in artificial seawater illuminated by a tungsten filament light and gently sparged with air at 25°C.After 24 hours, hatched A. salina cysts were transferred to fresh artificial seawater and incubated for further 24h under artificial light with air sparging (Metcalf and Lindsay, 2002).Subsequently, an aliquot of 0.5mL of treated wastewater and 5mL of fresh seawater was poured into wells in polyestyrene microtiter plates, and then 7 to 10 A. salina nauplii were placed in each well.Deaths were periodically recorded after incubating at 25ºC for 24 hours.

Kinetics of Fenton and Photo-Fenton processes
Previous results showed that tannery wastewater did not suffer decomposition when only hydrogen peroxide or ferrous sulfate were added (Dantas, 2003).
The COD removal/time rate by Fenton and photo-Fenton processes is shown in Figure 1, where it can be observed that the tannery wastewater is significantly oxidized by the Fenton and Photo-Fenton processes.No significant difference is apparent between Fenton and Photo-Fenton kinetics, possibly due to insufficient UV irradiation.The whole reaction can be divided into a two-stage reaction for both processes.The degradation rate in the first 20 minutes of the reaction was more rapid than after.Almost 70% of the degradation occurs in the first 20 minutes for both processes.The rest of the reaction occurs slowly, i.e., it takes 240 minutes for almost 90% of degradation.These two-stage reaction can be explained considering that ferrous ions react very quickly with hydrogen peroxide to produce large amounts of hydroxil radicals (Malick and Saha, 2003).The hydroxil radicals produced can react rapidly with organic matter.The ferric ions produced in the first stage react with hydrogen peroxide to produce hydroperoxyl radicals (HO 2 • ) and ferrous ions, according to reactions 2 and 3 and, finally, the reproduced ferrous ions react with hydrogen peroxide to produce more hydroxil radicals.
The hydroxyl and hydroperoxyl radicals thus formed are apt to continue the wastewater oxidation.These two-stage reactions have been observed also by other authors (Lu et al., 1999;Malik and Saha , 2003).Since the hydroperoxyl radicals have a lower oxidation capability than the hydroxyl radicals, this results in a rapid first-stage reaction, referred to as the Fe 2+ /H 2 O 2 stage, followed by the slow second step, referred to as the Fe 3+ /H 2 O 2 stage.
Despite the high COD removal, the acute toxicity of treated wastewater for different times is high (Figure 2).The mortality of Artemia salina in control tubes was zero in all tests.As shown in Figure 2, the wastewater treated by the Fenton and photo-Fenton processes presented the same acute toxicity, indicating only partial oxidation or residual H 2 O 2 concentration, as discussed below.In the firststage reactions, since hydroxyl radicals were in high number, the toxicity was also measured, indicating that, for industrial applications, the reaction time should be higher than 120 minutes to achieve toxicity decreasing.

Effect of H 2 O 2 dosage on the COD removal and on the toxicity by Fenton and Photo-Fenton processes
Figure 3 shows the COD removal at different H 2 O 2 dosages after Fenton and Photo-Fenton processes (oxidation + coagulation) for a reaction time of 3 hours.As the H 2 O 2 dosage increased from Acta Scientiarum.Technology Maringá, v. 25, no. 1, p. 91-95, 2003 0 to 30g.L -1 , the COD removal increased from 65% to 90% after the Fenton process.According to reaction 1, the concentration of • OH is expected to increase with the increasing H 2 O 2 dosage, leading to increased oxidation rates of organic compounds.The toxicity of the treated wastewater is shown in Figure 4 for different H 2 O 2 dosages.It may be observed that the toxicity decreases as the H 2 O 2 dosage increases up to 9g.L -1 , and then the toxicity increases.No residual H 2 O 2 concentration was measured after reaction and coagulation, although the detection limit for residual H 2 O 2 concentration by permanganate titration method is 0.25% wt.However, the LC 50 value for H 2 O 2 reported by Twiner et al. (2001) is 2.6x10 -2 mol/L (0.08% wt) and the presence of iron incresases the toxicity, depending on the iron concentration.The partial oxidation of organic compounds is able to lead to less or more toxic components in the wastewaster, depending on the products formed.Also, the residual H 2 O 2 concentration below detection limit by permanganate titration could increase the toxicity of treated wastewater.

Ammonia nitrogen removal
During the experiments, the ammonia concentration was measured.The ammonia removal by Fenton and Photo-Fenton processes is shown in Figure 5.No significant differences were observed for either of the processes, and the ammonia removal reached 90% after treatment.
Organic nitrogen can be decomposed to form ions and gases, for example, as nitrate, nitrite, ammonia, molecular nitrogen or nitrogen monoxide (Maletsky and Bauer, 1998).Malestky and Bauer (1998) reported that the Fenton degradation of nitrogen contained in organic compounds produces ammonia and traces of nitrate.In this case, the ammonia concentration should increase, because ammonia can be oxidized to nitrite, but only under special conditions and in small concentrations (Meyer and Pietsch, 1996).
To H 2 O 2 concentration higher than 5g/L, Figure 5 shows that is possible to eliminate ammonia up to 80%.Several authors have reported that only for high H 2 O 2 concentration it is possible to eliminate ammonia nitrogen and the main byproducts are gaseous N 2 and nitrate (Zoh and Stenstrom, 2002;Goi and Trapido, 2002).Then it should be expected that the ammonia concentration should decrease continuously as the wastewater is oxidized.

Conclusion
No significant differences were observed after Fenton and photo-Fenton processes for the treatment of tannery wastewater, in relation to kinetics, toxicity or ammoniac nitrogen removal.The kinetic of COD removal for both processes showed similar results.The degradation reaction can be divided in two periods for both processes and the degradation rate in the first 20 minutes is much faster than it is after this period.About 70% of the degradation occurs in the first 20 minutes for the Acta Scientiarum.Technology Maringá, v. 25, no. 1, p. 91-95, 2003 two processes and the remainder of the reaction occurs slowly.Thus 4 hours are necessary for a degradation of 90% of the organic matter.
Using ferrous ions at a dosage of 1g.L -1 and a concentration of 15g.L -1 of hydrogen peroxide, the Fenton process reached 83% of COD removal and the Photo-Fenton process reached 87% of COD removal for 3 hours of reaction.No appreciable difference in the reduction of the ammonia nitrogen amount for the two processes was found and the removal of ammonia can be ascribed to the oxidation reaction.

Table 1 .
Characterization of the tannery wastewater.