Application of the photo-Fenton process to the treatment of wastewaters contaminated with diesel

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Abstract

The application of the photo-Fenton process for the treatment of wastewaters contaminated with diesel oil was investigated. This particular process has been widely studied for the photochemical degradation of highly toxic organic pollutants. Experiments were performed according to a factorial experimental design at two levels and two variables: H2O2 concentration (5–200 mM) and Fe2+ concentration (0.01–1 mM). Experimental results demonstrated that the photo-Fenton process is technically feasible for the treatment of wastewaters containing diesel oil constituents, with total mineralization. A combination of factorial experimental design and gradient descent techniques was employed to optimize the amount of the Fenton reagents, resulting in Fe2+ (0.1 mM) and H2O2 (50 mM). These optimized levels did not exceed the limit for disposal of ferrous ions (0.27 mM) proposed at the local environmental legislation.

Introduction

The increasing number of vehicles has provoked the appearance of a corresponding number of gas stations for the distribution of fuels such as gasoline and diesel oil. In Brazil, there is a significant occurrence of fuel spills from storage tanks as a result of corrosion or structural failure, resulting in a contamination of soil and underground waters (Manzochi, 2001, Corseuil and Marins, 1994, Corseuil, 1994). The development of adequate techniques for the remediation of these contaminated waters by fuels is thus very important. The utilization of photo-oxidative methods for the treatment of wastewaters offers several advantages, particularly the feasibility of total elimination of the organic pollutants (Krutzler and Bauer, 1999, Legrini et al., 1993, Andreozzi et al., 1999, Espulgas et al., 1994, Mansilla et al., 1997, Ince, 1999, Salah et al., 2004). In spite of the proven efficiency of biological processes for the treatment of many types of wastewaters, they do exhibit limitations with respect to the range of concentration and toxicity of the pollutants (Piras, 1993, Braun and Oliveros, 1997, Sýkora et al., 1997). In the case of waters contaminated by derivative hydrocarbons of petroleum, for example, significant amounts of benzene, toluene, ethyl-benzene and xylenes (BTEX) can be observed. Among the photo-oxidative processes, the photo-Fenton system, which consists in the combination of hydrogen peroxide (H2O2), ferrous ions (Fe2+) and UV irradiation (Will et al., 2004), has been largely studied for the oxidation of wastewaters containing highly toxic organic compounds (Moraes et al., 2004a, Moraes et al., 2004b, Legrini et al., 1993, Mansilla et al., 1997).

The photo-Fenton process can be divided into the following stages (Pignatello, 1992, Bossmann et al., 1998): the first step is the so-called Fenton reaction, in which ferrous ions (Fe2+) are oxidized to ferric ions (Fe3+) in acidic aqueous solution, as shown in Eq. (1), giving rise to hydroxyl radicals (HO).Fe2+ + H2O2  Fe3+ + HO

The ferric ions, represented by the complex [Fe(OH)]2+, is reduced back to Fe2+ by UV irradiation, according to Eq. (2).[Fe(OH)]2+hνFe2++HO

The ferric species can also form complexes with the initial organic compounds and/or degradation products, leading to photo-reduction back to Fe2+ according to Eq. (3).[Fe(OOCR)]2+hνFe2++R+CO2

The generated hydroxyl radicals oxidize the target organic compounds (RH), shown in Eq. (4).HO + RH  H2O + R

Hydrogen peroxide concentration plays a more crucial role in deciding the overall efficacy of the degradation process. Usually, it has been observed that the percentage degradation of the pollutant increases with the amount of hydrogen peroxide (Lin and Lo, 1997, Rivas et al., 2001). The hydrogen peroxide is a well-known radical scavenger and, in concentrations above to 3%, is a biocidal agent (Costa et al., 1990), therefore, should be careful with high concentrations in the cases where Fenton oxidation is used as a pre-treatment to biological processes (Ito et al., 1998).

Generally, the degradation rate increases with an increase of the iron ions concentration (Lin and Lo, 1997), though the extent of increase is sometimes observed to be marginal above a certain concentration as reported by Rivas et al. (2001). In fact, an excess of ferrous ions can provoke an inhibition of the photochemical degradation process, because, at these levels, the Fe2+ can compete with the organic compounds by the oxidant agents (hydroxyl radicals), as indicated in Eq. (5) (Torrades et al., 2003, Pérez et al., 2002).Fe2+ + HO  Fe3+ + OH

Other factor that can be detrimental for the performance of the degradation process, in high iron ions concentration, it is the reduction of the penetration of the UV radiation in the system, caused by the significant opacity of the solution in these conditions. Furthermore, the local environmental legislation (CONAMA 20) establishes a maximum limit of ferrous ions content for disposal (0.27 mM). These ions are classified as secondary or aesthetic pollutants because they can affect the color and the flavor of the water. Furthermore, in excess, iron poses a threat to cells and tissues, because iron can catalyze the generation of radicals, which attack and damage cellular macromolecules and promote cell death and tissue injury (Papanikolaou and Pantopoulos, 2005, Gurzau et al., 2003), causing problems for the health of the aquatic life. Because of these all cited reasons, it is very important to establish the optimum loading of iron ions under similar conditions unless data are available in the literature.

In this work, the application of the photo-Fenton process was studied for the treatment of waters contaminated by diesel oil. The amounts of the Fenton reagents were optimized by a combination of experimental design methodology and the gradient descent techniques.

Section snippets

Materials and methods

Ferrous sulfate heptahydrate (FeSO4·7H2O) and hydrogen peroxide (H2O2; 30%) were used as the source of hydroxyl radicals. A quenching solution containing potassium iodide (KI; 0.1 M), sodium sulfite (Na2SO3; 0.1 M) and sodium hydroxide (NaOH; 0.1 M) was used to interrupt the oxidation reactions in the samples. The pH was adjusted by addition of concentrated sulfuric acid (H2SO4). All chemical compounds were analytical grade. Commercial diesel oil was employed as the model pollutant.

Preparation of the synthetic wastewater

The

Comparison of different degradation systems

In order to study and compare the performances of the photo-Fenton system and others degradation processes, four experiments were specially carried out in the treatment of water containing diesel oil constituents: UV-photolysis (exclusive use of UV irradiation), the thermal Fenton reaction ([Fe2+] = 0.1 mM and [H2O2] = 50 mM; without UV irradiation), UV/H2O2 ([H2O2] = 50 mM; with UV irradiation) and the photo-Fenton process ([Fe2+] = 0.1 mM and [H2O2] = 50 mM; with UV irradiation). The results of these

Conclusions

The application of the photo-Fenton process for the treatment of wastewaters contaminated with diesel oil resulted in conversions of up to 99%, in terms of removal of the total organic carbon content. By comparison, UV photolysis alone and the thermal Fenton reaction resulted in degradation of only 28% and 26%, respectively. Furthermore, the UV/H2O2 process was able to mineralize 71%, but requesting a significant irradiation time; in other words, a high cost related to the power consumption.

Acknowledgments

Financial support and fellowships from the CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo), ANP (Agência Nacional do Petróleo) and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) are gratefully acknowledged. The authors are also grateful to Prof. Frank Quina for his manuscript comments.

Osvaldo Chiavone-Filho. Chemical Engineer in 1985 (FEI — São Paulo/BR). Master in Chem. Engineering in 1988 (COPPE–UFRJ — Rio de Janeiro/BR). PhD in Chem. Engineering in 1993 (IKT–DTU — Lyngby/Denmark). Professor since 1994 (Chem. Engineering Dept. — UFRN — Natal/BR). Teaching: Chemical Process Design, Chemical Thermodynamics and Phase Equilibria. Research group and topics of interest: FOTEQ — photodegradation of industrial wastewaters and phase equilibrium behavior applied to process design.

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Osvaldo Chiavone-Filho. Chemical Engineer in 1985 (FEI — São Paulo/BR). Master in Chem. Engineering in 1988 (COPPE–UFRJ — Rio de Janeiro/BR). PhD in Chem. Engineering in 1993 (IKT–DTU — Lyngby/Denmark). Professor since 1994 (Chem. Engineering Dept. — UFRN — Natal/BR). Teaching: Chemical Process Design, Chemical Thermodynamics and Phase Equilibria. Research group and topics of interest: FOTEQ — photodegradation of industrial wastewaters and phase equilibrium behavior applied to process design.

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