Elsevier

Desalination

Volume 249, Issue 2, 15 December 2009, Pages 878-883
Desalination

Advanced oxidation of catechol: A comparison among photocatalysis, Fenton and photo-Fenton processes

https://doi.org/10.1016/j.desal.2009.02.068Get rights and content

Abstract

The aim of this work was to compare the behaviour of Fenton, photo-Fenton and photocatalysis processes to treat catechol solutions which are pollutants occurring in wastewaters from many industries. The effect of different process parameters, such as initial catechol concentration, H2O2/FeSO4 ratio in Fenton and photo-Fenton oxidation, TiO2 loadings in photocatalysis and irradiation times has been studied.

Fenton and photo-Fenton (H2O2/FeSO4 = 600/500 (w/w) and 30 min reaction time) processes allowed us to achieve a high efficiency in the mineralization of catechol (COD removals up to 83% and 96% respectively), and removal of aromaticity (UV280) (up to 93% and 98% respectively), for an initial catechol concentration of 110 mg/l. On the opposite, photocatalysis was not effective in the removal of higher catechol concentrations (110 and 200 mg/l), whereas a significant removal of aromaticity versus time was observed for 50 mg/l. Gas chromatography–mass spectrometry analysis, performed under selected treatment conditions, showed that total removal of catechol can occur after Fenton (2000/500 w/w; 30 min), photo-Fenton (600/500 w/w; 30 min), and photocatalysis (3 g TiO2/l; 240 min) treatments.

Introduction

Catechol, pyrocatechol or o-hydroxyphenol, is used in many industrial fields such as photographic developer, lubricating oils, polymerization inhibitors and pharmaceutical [1]. Therefore effluents from many industries are frequently contaminated by catechol. Moreover, catechol is one of the most abundant compounds in wastewaters from olive mills [2], [3], [4]. Due to the aromatic structure, which is also responsible for its toxicity, catechol is a persistent water pollutant under environmental conditions [5]. The International Agency for Research on Cancer (IARC) classified catechol as possibly carcinogenic to humans (Group 2B) [6]. Council Directive 67/548/EEC, emended by Directive 2006/121/EC, which is the main European Union law concerning chemical safety, lists catechol among substances or preparations which are considered to be harmful [7].

The compound is readily absorbed from the gastrointestinal tract, causes hemolysis, renal tube degeneration, diminished liver function, and it accumulates in the bone marrow [1]. Bukowska et al. [8] proved that catechol is even more toxic than phenol since it provokes changes in the function of erythrocytes at doses as low as 50 g/l compared to 250 g/l of phenol. However no limit values are established for catechol according to the European Union (Urban Water Directive 91/271/EC), whereas the maximum amount of phenol allowed in wastewater was set as 1 mg/l.

Several technologies have been attempted for the removal of catechol from wastewater and aqueous solutions, including biological methods [9], [10], adsorptive micellar flocculation [11], ultra-filtration [12], adsorption [1], [13], ionizing radiation, ozone and combined process ozone–electron-beam [14]. Anyway, except for the last which are advanced oxidation processes (AOPs), the above mentioned processes only transfer the contaminant from liquid to solid phase.

Over the last few decades AOPs have gained growing importance for the removal of organic pollutants from water. AOPs include a combination of oxidants (e.g., H2O2; ozone), UV radiation, catalysts (e.g., Fe2+, TiO2) and ultrasounds. Although a complete classification of AOPs is quite difficult due to a wide range of combinations among the above mentioned systems, all processes are characterized by the generation of hydroxyl radicals which are the strongest oxidants (E0 = 2.8 V) after fluorine. The main advantage of AOPs compared to other technologies is the capacity to completely remove organic contaminants from the environment, not only from the aqueous phase, by transforming them into the simplest organic compounds and finally into harmless inorganic species.

Although an extensive literature on the degradation of phenols and other substituted phenolic compounds using AOPs is available [15], [16], [17], only a few studies on the removal of catechol from aqueous solutions by means of AOPs can be found [18], [19], [20]. Among the AOPs, Fenton, photo-Fenton and photocatalysis are very promising technologies since they have been found to be effective in the removal of a wide group of organic pollutants.

The Fenton oxidation process is one of the oldest AOPs which is being increasingly used in the treatment of industrial wastewater [21], [22], [23], [24]. Although the Fenton reagent has been known for more than a century and is shown to be a powerful oxidant, the mechanism of the Fenton reaction is still under intense and controversial discussion. However, it is generally accepted that the reaction between H2O2 and Fe2+ in an acidic aqueous medium (pH  3) produces HO radicals and involves the following steps: pH adjustment to low acidic values, oxidation reaction, neutralization, and coagulation.

The photo-Fenton reaction involves irradiation with UV light which significantly increases the rate of contaminant degradation by stimulating the reduction of Fe3+ to Fe2+. Further hydroxyl radicals are produced via direct H2O2/UV photolysis (slow reaction) and the reaction of H2O2 with Fe2+ produced by photoreduction of Fe3+. Arana et al. [15] applied photo-Fenton reaction to phenol degradation suggesting that phenol is oxidized by hydroxyl radicals and converted in catechol and hydroquinone which may be degraded to aliphatic acids and finally to CO2. The same authors studied the photocatalytic degradation of mixtures of phenolic compounds including catechol, resorcinol, m-cresol, o-cresol and phenol, focusing on possible competition for adsorption and photoactive centres as well as the radicals photogenerated [17].

Photocatalysis is successfully used in water and wastewater treatment for the removal of organic and inorganic pollutants as well as in disinfection systems [25], [26], [27], [28]. The formation of hydroxyl radicals occurs as a catalytic semiconductor is illuminated with near UV radiation (λ < 400 nm). Although several semiconductors have been tested as photocatalysts, TiO2 is the widest used one because of its high stability, low energy band-gap, toxicity and cost [29]. Just a few studies documented the photocatalytic degradation of catechol [17], [20] and only one photocatalyst loading (2 g TiO2/l) was investigated, the main purposes of the works being the photocatalytic oxidation of a mixture of phenolic compounds [17] and the effect of acetic acid on the photocatalytic oxidation of catechol [20].

In the present experimental work, a comparative study was carried out with the aim of investigating the effect of Fenton, photo-Fenton and photocatalysis processes on the degradation of catechol in aqueous solutions. The photocatalytic degradation of catechol (50, 110 and 200 mg/l initial concentrations) was investigated under different photocatalyst loadings (0.5–3.0 g TiO2/l) and irradiation times (0–120 min). Fenton and photo-Fenton oxidation experiments were carried out using 110 mg/l catechol solutions varying H2O2/FeSO4 ratios at pH 3 (reaction time: up to 30 min).

Section snippets

Reagents and supplies

Catechol (purity, ≥ 98.0%, solubility in water: 43 g/100 ml; molecular weight C6H6O2 = 110.1 g/mol; melting point 105 °C; boiling point 245.5 °C; density 1.344 g/cm³, solid appearance: white solid) was purchased from Sigma Aldrich. Ferrous sulphate heptahydrate (FeSO4 7H2O), used as a source of Fe2+, was purchased from Merck. Hydrogen peroxide solution (30% w/w) in stable form was purchased from Carlo Erba (Italy). Sulfuric acid (H2SO4) and sodium hydroxide (NaOH) (from Carlo Erba) solutions were used

Fenton experiments

An initial set of experiments was carried out keeping constant the concentration of H2O2 at the value of 2000 mg/l while the concentration of FeSO4 was varied from 75 to 600 mg/l. However, no oxidation occurred at FeSO4 < 200 mg/l. The maximum efficiency of the degradation process was achieved at 500 mg/l FeSO4 concentration.

The efficiency of the Fenton process in terms of both catechol mineralization (COD) and aromaticity removal (UV280) obtained after 30 min treatment at varying initial FeSO4

Conclusion

In this study, the degradation of catechol by Fenton, photo-Fenton and photocatalysis processes was studied. Treatment behaviour was evaluated according to catechol mineralization rate (COD measurements) and aromaticity removal (UV280). The optimum H2O2/FeSO4 ratio (83 and 98% of COD removal, 93% and 96% of UV280 removal for Fenton and photo-Fenton respectively) was determined as 600/500 (w/w) at pH 3.0, 30 rpm mixing for 30 min. Although a good removal was achieved by the photocatalysis process

Acknowledgement

The technical assistance of Mr. Paolo Napodano is highly acknowledged.

References (32)

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