Kinetics of oxidative degradation/mineralization pathways of the antibiotic tetracycline by the novel heterogeneous electro-Fenton process with solid catalyst chalcopyrite
Graphical abstract
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 OH 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 + OHFe3+ + 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(OH) 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(OH) + 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+ + 2 + 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 OH. 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
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