Kinetics of oxidative degradation/mineralization pathways of the antibiotic tetracycline by the novel heterogeneous electro-Fenton process with solid catalyst chalcopyrite

https://doi.org/10.1016/j.apcatb.2017.03.034Get rights and content

Highlights

  • A new electro-Fenton (EF) process using chalcopyrite as heterogeneous catalyst at neutral pH.

  • Synergistic effect of Fe2+ and Cu2+ ions released by chalcopyrite.

  • BDD anode is more efficient than Pt anode for tetracycline mineralization.

  • EF/Chalcopyrite is more efficient than conventional EF process.

  • Proposed oxidation pathway based on the identification of 19 intermediates.

Abstract

The degradation of solutions of the antibiotic tetracycline (TC) has been studied by a novel electrochemical advanced oxidation process, consisting in electro-Fenton (EF) process using chalcopyrite as heterogeneous catalyst. In fact, chalcopyrite powder was the source of Fe2+ and Cu2+ ions instead of a soluble catalyst salt used in conventional EF. Experiments were performed in an undivided cell equipped with a Pt or boron-doped diamond (BDD) anode and a carbon felt cathode, where TC and its oxidation intermediate products were destroyed by hydroxyl radicals (radical dotOH) formed both, in the bulk solution from electrochemically induced Fenton’s reaction (Fe2+ and H2O2) and Fenton’s-like reaction (Cu+ and H2O2), and at the anode surface from water oxidation. The effects of operating parameters such as applied current, chalcopyrite concentration and anode material were investigated. TC decay followed pseudo-first-order reaction kinetics. The absolute rate constant for TC oxidation by radical dotOH was found to be 3.2 × 109 M−1 s−1, as determined by the competition kinetic method. EF process using chalcopyrite as heterogeneous catalyst showed to be more efficient than conventional EF, achieving almost total mineralization of the TC solution (98% of total organic carbon removal) after 360 min under optimum operating conditions. A plausible mineralization pathway for mineralization of TC aqueous solution by radical dotOH was proposed based on the identification of different oxidation by-products. Moreover, toxicity tests pointed out that this heterogeneous EF process was able to detoxify the TC solutions.

Introduction

The presence of pharmaceuticals in the aquatic environment is now well recognized as an important issue, causing long-term adverse impacts on the ecosystems and human health [1], [2]. Pharmaceutical industries, hospitals and urban wastewater effluents are important points of drug discharge into the environment, leading to a significant effect on the physical, chemical and biological composition of water bodies [3]. Among drugs, antibiotics are widely used in human and veterinary medicine to treat diseases. The presence of low levels of antibiotics and their transformation products in the environment could have adverse effects, such as bacterial resistance and disruption of key cycles critical to aquatic ecology or crop and animal production [4], [5]. Because of their recalcitrant character, antibiotics, and drugs in general, are ineffectively removed in conventional wastewater treatment plants [2], [5], [6]. Nevertheless, advanced oxidation processes (AOPs) have proved to be a suitable alternative for rapid degradation of recalcitrant and non-biodegradable compounds in water [7], [8]. Among them, electrochemical advanced oxidation processes (EAOPs) have received great attention over the last decade as an effective and suitable technology for the remediation of wastewater contaminated with toxic and persistent organic pollutants [9], [10], [11].

One of the most attractive EAOPs is the electro-Fenton process (EF), in which H2O2 is formed by the two-electron reduction of dissolved O2 (Eq. (1)) at a suitable carbonaceous cathode, such as graphite, carbon felt, reticulated vitreous carbon, gas diffusion electrodes (GDE), boron-doped diamond (BDD), carbon nanotubes, activated carbon fiber, and so on [12], [13], [14], [15], [16], [17], [18].O2 + 2H+ + 2e  H2O2

In acidic medium, the oxidizing power of H2O2 is strongly enhanced by the addition of a catalytic amount of Fe2+ (or Fe3+) ions, which promotes the generation of homogeneous radical dotOH via Fenton’s reaction (Eq. (2)). Furthermore, Fe2+ ions are rapidly regenerated from reduction of Fe3+ ion at the cathode according to Reaction (3) [12].Fe2+ + H2O2 + H+  Fe3+ + H2O + radical dotOHFe3+ + e  Fe2+

When a non-active anode material (M) with a high O2 evolution overpotential such as BDD is used in EF process, heterogeneous M(radical dotOH) radicals are also formed at the anode surface by water oxidation (Eq. (4)), whence enhancing the efficiency of the process [19], [20]. BDD electrodes are currently the most powerful and preferred anodes for electrochemical oxidation [10], [21], [22].M + H2O  M(radical dotOH) + H+ + e

EF process is typically performed using soluble ferric salts as catalyst (Fe2+) source. However, other transitions metals have also been used to promote Fenton like reaction, such as Cu2+ ions, showing good results [23], [24]. On the other hand, the use of alternative heterogeneous catalyst has been proposed aiming to enhancing the effectiveness of Fenton’s-based processes with parallel sustainability purposes. These “green” catalysts containing Fe and/or Cu solids include pyrite (FeS2), chalcocite (Cu2S), bornite (Cu5FeS4), magnetite (Fe2O3) or wustite (FeO) [25], [26], [27], [28].

On the other hand, the chalcopyrite is one of the most important copper sulfide minerals in the world [29], [30]. It can releases Cu2+ and Fe2+ ions in aqueous solution according to Reactions (5)–(7) [30]. In the present work, we explore the novel possibility of using this mineral for catalyzing the EF process (EF/Chalcopyrite) during the degradation of synthetic aqueous solutions of antibiotic tetracycline (TC).CuFeS2(s) + 4O2  Cu2+ + Fe2+ + 2SO42−CuFeS2(s) + 4H+ + O2  Cu2+ + Fe2+ + 2S(s)0 + 2H2OCuFeS2(s) + 16 Fe3+ + 8H2O  Cu2+ + 17Fe2+ + 2SO42− + 16H+

In addition of self-regulation of catalyst, chalcopyrite allows also the self-regulation of solution pH. Taking into account that pH values around 3 constitute the optimum values for Fenton’s reaction, reaction (Eq. (7)) becomes highly important being as the release of H+ contributes to the acidification of the solution, thus the use of mineral acids for pH adjustment becomes unnecessary.

TC is an antibiotic extensively used for disease control due to their great therapeutic values. It is also widely used in livestock feed to prevent illness and promote growth. Its detection in aquatic systems and soils has raised concern about its biological impacts and potential risks to the environment, as well as to public health [31]. Due to its large global consumption, it was chosen as a model molecule for assessing the efficiency of the innovative EF/Chalcopyrite heterogeneous process.

Interestingly, in a recent study, synthesized chalcopyrite nanocrystals supported on sawdust were applied for the degradation of organic dyes in a filtration column in which H2O2 was externally supplied [32]. Although high efficiency was obtained for the discoloration of dye solutions (via Fenton’s reaction), any data on mineralization of the dyes to CO2, H2O and inorganic ions was not reported. However, these results highlighted the potential of chalcopyrite as catalyst for Fenton-based processes, among which EF presents several advantages as discussed above.

This work hence presents a thorough assessment of the performance of the EF/Chalcopyrite process through examination of the main operating parameters affecting degradation and mineralization efficiencies, such as catalyst concentration, applied current, and the effect of the anode material. Additionally, a comparative study with homogeneous EF using Fe2+ and Cu2+ ions as catalyst is presented. The identification of aromatic intermediates, as well as short-chain carboxylic acids and inorganic ions formed during EF/Chalcopyrite treatment of TC, allowed the proposal of a mineralization pathway of the drug with radical dotOH. Moreover, toxicity evaluation throughout electrolysis by means of the Microtox method is presented.

Section snippets

Chemicals

Analytical grade TC (C22H24N2O8) with purity >98%, was purchased from Fluka and was used in the electrolytic experiments without further purification. Anhydrous sodium sulphate, used as background electrolyte, as well as H2SO4 and NaOH were provided from Acros Organics. Heptahydrated iron (II) sulphate and copper (II) sulphate pentahydrate used as catalysts in conventional EF were of analytical grade from Acros Organics. All solutions were prepared with ultrapure water from a Millipore Milli-Q

Properties of aqueous solutions with chalcopyrite powder in suspension

Before applying the EF/Chalcopyrite process to removal of TC, preliminary experiments were performed in order to study the properties of chalcopyrite in aqueous solutions. Fig. 2 shows the change in pH starting from the natural pH value of TC solution (near 5.94), varying the initial load of chalcopyrite slurry from 0.25 to 2.0 g L−1. In all cases, the solution pH progressively dropped, reaching values between 4.69 and 3.33 within 25 min. It further declined reaching values in the range of

Conclusions

An innovative heterogeneous electro-Fenton process catalysed by the natural mineral chalcopyrite, “EF/Chalcopyrite process”, was successfully applied for the degradation and mineralization of TC aqueous solutions. This “green” process presents two remarkable advantages: firstly, the utilization of low cost and reusable solid catalyst as source of Fe2+ and Cu2+ ions, and secondly, the capacity of performing the EF process at natural pH, since chalcopyrite provides the optimal pH value of about

References (47)

  • F. Sopaj et al.

    Effect of the anode materials on the efficiency of the electro-Fenton process for the mineralization of the antibiotic sulfamethazine

    Appl. Catal. B: Environ.

    (2016)
  • S. Garcia-Segura et al.

    Role of sp3/sp2 ratio on the electrocatalytic properties of boron-doped diamond electrodes: a mini review

    Electrochem. Commun.

    (2015)
  • A. Kapałka et al.

    The importance of electrode material in environmental electrochemistry: formation and reactivity of free hydroxyl radicals on boron-doped diamond electrodes

    Electrochim. Acta

    (2009)
  • M. Pimentel et al.

    Phenol degradation by advanced electrochemical oxidation process electro-Fenton using a carbon felt cathode

    Appl. Catal. B: Environ.

    (2008)
  • S. Garcia-Segura et al.

    Effect of the Fe3+/Cu2+ ratio on the removal of the recalcitrant oxalic and oxamic acids by electro-Fenton and solar photoelectro-Fenton

    Sol. Energy

    (2016)
  • S. Ammar et al.

    Degradation of tyrosol by a novel electro-Fenton process using pyrite as heterogeneous source of iron catalyst

    Water Res.

    (2015)
  • N. Barhoumi et al.

    Pyrite as a sustainable catalyst in electro-Fenton process for improving oxidation of sulfamethazine. Kinetics, mechanism and toxicity assessment

    Water Res.

    (2016)
  • G.-D. Fang et al.

    Superoxide radical driving the activation of persulfate by magnetite nanoparticles: implications for the degradation of PCBs

    Appl. Catal. B: Environ.

    (2013)
  • W. Zeng et al.

    Electrochemical behaviour of massive chalcopyrite electrodes bioleached by moderately thermophilic microorganisms at 48 °C

    Hydrometallurgy

    (2011)
  • Y. Li et al.

    A review of the structure, and fundamental mechanisms and kinetics of the leaching of chalcopyrite

    Adv. Colloid Interface Sci.

    (2013)
  • D. Belkheiri et al.

    Combined process for removal of tetracycline antibiotic—coupling pre-treatment with a nickel-modified graphite felt electrode and a biological treatment

    Int. Biodeterior. Biodegrad.

    (2015)
  • D. Majuste et al.

    Applications of in situ synchrotron XRD in hydrometallurgy: literature review and investigation of chalcopyrite dissolution

    Hydrometallurgy

    (2013)
  • N. Oturan et al.

    Electrocatalytic destruction of the antibiotic tetracycline in aqueous medium by electrochemical advanced oxidation processes: effect of electrode materials

    Appl. Catal. B: Environ.

    (2013)
  • Cited by (0)

    View full text