Electro-Fenton catalyzed by Fe-rich lateritic soil for the treatment of food colorant Bordeaux Red (E123): Catalyst characterization, optimization of operating conditions and mechanism of oxidation
Graphical abstract
Introduction
Electrochemical advanced oxidation processes (EAOPs) are oxidation processes with great ability for efficient and effective mineralization of a large varieties of toxic and/or biorefractory organic pollutants [1], [2], [3], [4], [5]. These processes are based on the electrochemical/photoelectrochemical generation of hydroxyl radicals (•OH), either directly at the anode surface from water oxidation (Eq. (1)) (electrooxidation) or indirectly by total or partial in-situ production of Fenton’s reagent (H2O2 + Fe2+) (Eq. (2)) (EF process) [4], [6], [7], [8], [9]. The generated •OH is a very strong oxidant (E° = 2.8 V/SHE) which reacts non-selectively with organic pollutants by oxidation until their final mineralization to CO2 and H2O [10], [11], [12], [13].M + H2O → M(•OH) + OH–H2O2 + Fe2+ + H+ → •OH + Fe3+ + H2O
In particular, Fenton-based EAOPs is known for the production of large quantities of •OH and possess some exciting advantages such as high efficiency, amenability and environmental compatibility compared to classical Fenton process [14], [15], [16]. However, the use of soluble Fe2+ to catalyze EF process has some inherent disadvantages such as narrow pH working windows (pH 2.8 – 3.5), need for catalyst optimization and low reusability [14], [17], [18].
Recently, bare or supported iron-bearing solid catalysts have been investigated to catalyse the activation of H2O2 to •OH in the so called HEF process [14], [17], [19], [20]. Although, both synthetic and natural iron-bearing compounds have been studied as heterogeneous catalyst, natural iron bearing minerals are usually preferred because of cost implication, complex synthetic routes as well as possible hazardous effect associate with synthetic iron catalyst. However, these minerals (magnetite, hematite and pyrite) have more important engineering role, specifically in iron and steel industries which make them relatively not available. The advantages, mechanisms and different classes of heterogeneous catalysts as well as summary of recent studies and current advances in the HEF process have been reported in some reviews and a chapter of book [14], [17], [18]. Iron rich laterite can be an interesting alternative to catalyse EF oxidation of organic pollutants since it is relatively abundant and has less industrial values. In fact, laterite soil constitutes a significant portion of the land crust in West Africa and only use for sand filling of buildings, road and other civil engineering works. Laterite soil has reddish color owing to high oxides of iron content, low stability at strong acidic pH, and specific gravity of 2.7 [21]. It is relatively thermally stable but there is removal of volatile organic matter during calcination, which concentrates the oxides in the soil. Indeed, some studies have investigate iron-rich laterite soil as catalyst in classical Fenton process, [21], [22], [23] but till date no report have been published about the application of this material in HEF process.
Wastewater containing synthetic dyestuffs has become one of the biggest challenge and threat to ecosystem due to the potentially toxic, mutagenic, teratogenic and carcinogenic effects of the parent compounds as well as their transformation by-products like aromatic amines [24], [25]. They constitute one of the largest volumes of wastewater stream that is introduced into the environment because of their wide application across all industrial sectors. Colorants dyes are a class of food additives without nutritional values, added to food stuffs with the aim of impacting color to the product in order to increase its attractiveness and enhancing its acceptability by the consumers [26], [27]. Bordeaux Red (E123) is a sulfonated azo-dye extensively used in the food industry to give red coloring to various foodstuffs. Recently, the used of these azo-dyes, especially the artificial ones as colorants in food industry have raised several serious health issues because of their hazardous effect on human and ecosystems [28], [29]. Based on the results of international research and recommendation of Codex Committee on Food Additives and Contaminants, several regulations have been made to control the use of food dyes and reduced the Acceptable Daily Intakes (ADI) of the additives [30], [31]. In the past decade, E123 is among dyes with controversial toxicity because its toxicological evaluations in country like England and Brazil did not show significant cytotoxicity in tests with rodents; thus this dye continues to be marketed freely, whereas the bioassay assessments showed that it is mutagenic and carcinogenic and therefore ban in country like USA, Japan, Canada, Norway and Finland [30], [32]. Consequently, wastewater resulting from its manufacturing and usage plants becomes a potential threat to the ecosystem.
In this study, we investigated the potential of Fe-rich laterite catalyzed HEF for the treatment of E123 aqueous solution using an undivided reactor equipped with Pt and carbon-felt cathode. The laterite was characterized by scanning electron microscopy-energy dispersive (SEM-EDS), X-ray diffraction spectroscopy (XRD), Fourier tansform Infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) to determine its structural and physicochemical properties. The effect of catalyst dosage, calcination temperature, applied current, initial E123 solution concentration and pH on the degradation and mineralization of dye solution was carefully studied. Iron ions leached into the treated solution at different pH values were assessed by colorimetric method. The short-chain carboxylic acids formed during the EF treatment were identified and quantified by chromatographic analysis.
Section snippets
Chemicals
Bordeaux Red 2R (E123) (C20H15N2Na2O7S2) was purchased from Sigma Aldrich and used without further purification. Fe-rich laterite was obtained from one of its large deposit in Cameroon, crushed and sieved to less than 150 μm and calcinated at 200, 400 and 700 °C in EDG3P-S 3000 muffle furnace (EDG Equipamentos, Brazil) at 10 °C heating rate in oxygen/air atmosphere. Sodium sulfate and iron II sulfate heptahydrate (supporting electrolyte and Fe2+ source), oxalic, oxamic, maleic, malic,
Characterization of the laterite soil
The SEM image and EDS of the Fe-rich laterite is shown in Fig. 1a and 1b. The laterite has coarse and rough particles of the different sizes and morphologies. The EDS analysis showed the Kα peaks of oxygen, aluminum, titanium, iron and germanium, indicating that the laterite is composed of oxides of aluminum, titanium, and iron. Besides, no peaks of hazardous metals and toxic anions such as lead (Pb), cadmium (Cd), chromium (Cr), arsenic and selenium were found in the EDS, which imply that the
Conclusions
The potential and feasibility of using Fe-rich laterite soil as Fe source to catalyze HEF oxidation of organic pollutants has been investigated. The laterite composition is mainly oxides of Al and Fe with small proportion of oxides of Ti and Si without any trace of toxic heavy metal and anions. Optimized amount of laterite soil which gives maximum activation of H2O2 to •OH and COD removal efficiency was 8.5 g L-1, whereas lower or higher laterite dosage led to lower COD removal at similar
Declaration of Competing Interest
All the authors declared that there is not conflict of interest
Acknowledgement
Financial support from National Council for Scientific and Technological Development (CNPq – 465571/2014-0; CNPq - 446846/2014-7 and CNPq - 401519/2014-7) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP 2014/50945-4) are gratefully acknowledged. Soliu O. Ganiyu gratefully acknowledges CAPES for the financial support under PNPD post-doctoral grant awarded and Mr. Jean Baptiste for providing the laterite soil.
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