Elsevier

Electrochimica Acta

Volume 54, Issue 2, 30 December 2008, Pages 173-182
Electrochimica Acta

Reaction sequence for the mineralization of the short-chain carboxylic acids usually formed upon cleavage of aromatics during electrochemical Fenton treatment

https://doi.org/10.1016/j.electacta.2008.08.012Get rights and content

Abstract

Electrochemical Fenton treatment of aromatic pollutants in aqueous medium always leads to the formation of short-chain carboxylic acids, which account for the slower degradation rate at the final stages of the process. In order to gain further insight into the fate of such compounds, bulk electrolyses of 200 ml aqueous solutions of eleven C1–C4 carboxylics, namely formic, glyoxylic, oxalic, acetic, glycolic, pyruvic, malonic, maleic, fumaric, succinic and malic acid have been carried out by electro-Fenton process with 0.1 mM Fe2+ as catalyst, at room temperature and pH 3.0, applying a constant current and using an open and undivided cell equipped with a carbon-felt cathode and a Pt anode. In situ cathodic electrogeneration of Fenton's reagent leads to the formation of the very oxidizing species hydroxyl radical (radical dotOH) in the medium, allowing the degradation and total mineralization of all carboxylics studied. Various goals have been accomplished: (a) identification of the degradation intermediates for each carboxylic acid and study of their time course, (b) discussion and proposal of the reaction mechanisms under the action of radical dotOH/O2, (c) analysis of the decay kinetics and determination of the absolute rate constants, which agree well with those available in literature for processes involving radical dotOH, (d) verification of the great oxidation ability of the process to degrade mixtures containing all the carboxylics, upon variation of some experimental parameters such as current, concentration and cathode dimensions and, finally, (e) elucidation of a detailed reaction sequence for their mineralization, indicating the plausible pre-eminent pathways.

Introduction

The electrochemical approach for water treatment has favored the development of emerging and very promising technologies (i.e., electrochemical advanced oxidation processes, EAOPs) on behalf of environment preservation. These EAOPs show many positive aspects, such as the ease to automation, versatility, environmental compatibility (because the electron is a clean reagent), high efficiency and cost effectiveness, what explains the increasing interest for the use of these technologies in the treatment of water pollutants [1]. Their performance relies on the great oxidation ability of the large amounts of hydroxyl radicals (radical dotOH, E°(radical dotOH/H2O) = 2.80 V/NHE) that can be generated under control of applied current. Direct electrochemical methods such as anodic oxidation allow the production of radical dotOH adsorbed on the surface of high O2-overpotential anodes [2], [3], whereas indirect electrochemical techniques are based on the generation of radical dotOH in the bulk solution [4], [5], [6], [7].

Major efforts have been done with the indirect electrochemical process called electro-Fenton (EF), in which one or both species involved in Fenton's reaction (1) can be electrogenerated in situ from the cathodic reduction of O2 (2) and/or Fe3+ (3):Fe2+ + H2O2 + H+  Fe3+ + H2O + radical dotOHO2(g) + 2H+ + 2e  H2O2Fe3+ + e  Fe2+Different carbonaceous materials have been shown to yield good current efficiencies for the H2O2 production. Thus, 2D electrodes such as graphite [8], [9] or gas-diffusion [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20] and 3D electrodes including carbon-felt [18], [19], [21], [22], [23], [24], [25], [26], carbon-sponge [27], RVC [28], [29] or activated carbon fibre [30] have been used as cathode during the last decade, evidencing the outstanding performance of these electrochemical Fenton treatments to degrade a plethora of organic pollutants like herbicides and pesticides [10], [12], [24], [29], [31], dyes [9], [15], [23], [25], [26], [27], [30], pharmaceutical drugs [11], [13], [14], [17], [18], [19], [32], [33] and other aromatics [16], [20], [21], [22], [28].

Whatever the pollutant degraded or the cathode used, prolonged oxidation weaken the bonds and make these compounds prone to oxidative ring opening reactions [4], [5], [6]. The cleavage of the initial aromatic structures involves the formation of short-chain (i.e., low-molecular weight) carboxylic acids, which are usually the same ones [13], [14], [15], [17], [18], [19], [20], [23], [24], [25], [26], [27]. Since the degradation of carboxylic acids by radical dotOH is much slower than that of aromatics [34], they are responsible for a notorious decrease of the oxidation rate at the final stages of the treatment and account for the residual total organic carbon (TOC) that remains in solution long time. Besides this, the large production level of these acids makes it evident their commercial and industrial relevance, and leads to their presence in industrial wastewaters [35]. They are usually released during the manufacture of polymer products, the finishing and dyeing of textiles [36], the synthesis of soaps and pharmaceuticals and even from cleaning processes of nuclear power plants [37]. Most of these acids are biodegradable, but they are often mixed with other persistent compounds in wastewaters. Moreover, although their intrinsic toxicity is low, some of them can cause acute adverse effects to skin and mucous membranes [38], and their potential environmental impact cannot be neglected, due to their tendency to complex heavy metals and their significant contribution to rainwater acidity [39].

Notwithstanding the interest of a detailed study on the behavior of these compounds when applying promising electrochemical Fenton treatments, research has always been focused on the fate of aromatic pollutants and, sometimes, the identification of carboxylic acids as ultimate by-products. Much more efforts have been devoted to the action of radical dotOH on carboxylic acids by means of other processes, including anodic oxidation [40], [41], [42], photo-Fenton [37], H2O2 photolysis [43] and TiO2 photocatalysis [35], [37], [44], [45], [46].

Our large background on the application of electrochemical Fenton processes to the degradation of aromatic pollutants, as well as the existence of many other related works, has encouraged us to try to fill the gap by analyzing with high accuracy several aspects concerning the short-chain carboxylic acids studied:

  • -

    To identify the intermediates formed during the treatment of each carboxylic acid and follow their time course, what has allowed us to analyze the reaction mechanisms that rule the process and, in the end, to propose a detailed reaction sequence for their mineralization (i.e., transformation into CO2 and H2O).

  • -

    To determine their absolute rate constants by a competition method, which we can compare with those already established.

  • -

    To verify the great ability of EF process to completely mineralize mixtures containing all these carboxylic acids when varying some experimental parameters.

Bulk electrolyses were performed at constant current by EF process with a carbon-felt cathode. Such high-surface material ensured the efficient reduction reactions (2), (3), so in such conditions the Fe3+/Fe2+ system is catalytic because Fe2+ can be regenerated at the cathode [8], [25], [26] and in the medium [47], thus avoiding the production of iron sludge.

Section snippets

Chemicals

Eleven carboxylic acids, namely formic (FOR), glyoxylic (GLYOX), oxalic (OXA), acetic (ACE), glycolic (GLYC), pyruvic (PYR), malonic (MLO), maleic (MLE), fumaric (FUM), succinic (SUC) and malic (MLI) were selected because they have been usually reported as ultimate products during the cleavage of aromatics by electrochemical Fenton treatments.

All the carboxylic acids studied were reagent grade with purity ≥98% and were provided by Acros Organics. Some relevant physicochemical constants of each

Degradation of carboxylic acids by electro-Fenton process and proposed mechanisms

Firstly, the fate of the eleven carboxylic acids was explored to verify their complete destruction and follow the time course of the carboxylic by-products. In order not to complicate in excess, we have not included iron complexes in the reaction schemes, but note that in systems where Fe2+ is not easily regenerated some acids tend to be accumulated at long term in the form of Fe3+ complexes. An Fe2+ concentration as low as 0.1 mM was used in order to minimize parasitic reactions according to

Conclusions

Eleven linear C1–C4 carboxylic acids, found among the most usually reported ones during the treatment of organic pollutants by electrochemical Fenton treatments, have been carefully examined in this study. The destruction of all the acids by radical dotOH attack fits well to a pseudo-first order kinetics. The carboxylics formed during the EF degradation of each acid have been identified and their time course has been followed by ion-exclusion chromatography to know the pre-eminent oxidation pathways. The

Acknowledgments

I. Sirés acknowledges support from the Laboratoire Géomatériaux et Géologie de l’Ingénieur to obtain the ATER position in the Université Paris-Est Marne-la-Vallée.

References (56)

  • K. Jüttner et al.

    Electrochim. Acta

    (2000)
  • J. Iniesta et al.

    Electrochim. Acta

    (2001)
  • I. Sirés et al.

    J. Electroanal. Chem.

    (2008)
  • Z. Qiang et al.

    Water Res.

    (2003)
  • G. Zhang et al.

    Electrochim. Acta

    (2008)
  • B. Boye et al.

    J. Electroanal. Chem.

    (2003)
  • C. Flox et al.

    Appl. Catal. B- Environ.

    (2006)
  • I. Sirés et al.

    Chemosphere

    (2007)
  • I. Sirés et al.

    Appl. Catal. B- Environ.

    (2007)
  • I. Sirés et al.

    Electrochim. Acta

    (2007)
  • C. Flox et al.

    Appl. Catal. B- Environ.

    (2007)
  • M. Panizza et al.

    Water Res.

    (2001)
  • S. Meinero et al.

    Chemosphere

    (2006)
  • S. Hammami et al.

    J. Electroanal. Chem.

    (2007)
  • M. Diagne et al.

    Chemosphere

    (2007)
  • I. Sirés et al.

    Chemosphere

    (2008)
  • M.A. Oturan et al.

    Appl. Catal. B- Environ.

    (2008)
  • A. Özcan et al.

    J. Electroanal. Chem.

    (2008)
  • C. Badellino et al.

    J. Hazard. Mater.

    (2006)
  • M.A. Oturan et al.

    J. Electroanal. Chem.

    (1992)
  • N. Serpone et al.

    J. Photochem. Photobiol. A- Chem.

    (2005)
  • M.I. Franch et al.

    Appl. Catal. B- Environ.

    (2004)
  • N. Quici et al.

    Catal. Today

    (2005)
  • G. Brooks Avery et al.

    Atm. Environ.

    (2006)
  • C.A. Martínez-Huitle et al.

    Electrochim. Acta

    (2004)
  • X.-Y. Li et al.

    Water Res.

    (2005)
  • J.-M. Herrmann et al.

    Catal. Today

    (1999)
  • I. Mazzarino et al.

    Chem. Eng. Sci.

    (1999)
  • Cited by (166)

    View all citing articles on Scopus
    1

    ISE member.

    View full text