Catalytic Ozonation of Melanoidin in Aqueous Solution over CoFe₂O₄ Catalyst

Abstract

In this work, cobalt ferrite (CoFe₂O₄) was synthesized by solvothermal route for application as a catalyst in the ozonation reaction for the decolorization and mineralization of melanoidin from aqueous solution. The structural properties of CoFe₂O₄ sample were investigated by X-ray diffraction (XRD), nitrogen adsorption-desorption isotherms, Fourier-transform infrared spectroscopy (FTIR), particle-size distribution, scanning electron microscopy (SEM) and X-ray dispersive energy spectroscopy (EDS). Single-phase CoFe₂O₄ particles with a predominantly mesoporous structure containing a high specific surface area were obtained. Results showed that the CoFe₂O₄-catalyzed ozonation reaction has higher activity for the decolorization and mineralization of melanoidin when compared with the ozonation reaction without the presence of catalyst. Therefore, this material can be very promising for the application in catalytic ozonation systems for the melanoidin removal from liquid effluents.

Keywords
CoFe₂O₄; solvothermal; catalytic ozonation; melanoidin; decolorization; mineralization

1. Introduction

The industrial distillation and fermentation processes for the production of ethanol, aminoacids and yeasts produce high volumes of wastewater containing high load of organic molecules, being characterized by high concentrations of biochemical oxygen demand (BOD5) and chemical oxygen demand (COD), and a dark brown color due to the presence of melanoidin molecule^1-3. The biological treatment is generally used with a combination of anaerobic-aerobic processes in order to reduce BOD₅ and COD of these wastewaters to acceptable levels. However, the dark brown color persists because only 6-7% of melanoidin is biodegraded by these conventional processes⁴. Melanoidin molecules are amino-carbonyl complex polymers containing a dark brown color, and formed from the non-enzymatic amino-carbonyl reactions taking place between the amino acid and sugars⁵. Therefore, colored compounds such as melanoidins when disposed in water bodies without an effective pre-treatment can reduce the penetration of light, preventing the photosynthesis of aquatic vegetation⁶. In addition, their mineralization towards CO₂ and water is necessary in order to reduce the organic load, avoiding a damage to aquatic life^4,5. Thus, recent efforts by researchers have been sought towards to more efficient treatment methods.

Advanced oxidative processes (AOPs) are currently known to be efficiently used for the degradation of organic pollutant molecules^7-9. These processes are based on the generation of hydroxyl radicals (^●OH), which are highly reactive and can degrade many organic compounds¹⁰. Among the several existing AOPs methodologies^11,12, ozonation process has been shown to be highly efficient in the degradation of several recalcitrant organic contaminants^13-15. The chemical oxidation with ozone presents a high oxidative character, which is a fundamental property for the effluents treatment containing recalcitrant compounds, reaching satisfactory efficiency in the decomposition of organic pollutants, as in the case of melanoidin¹⁶.

The use of solid catalysts in the ozonation process (known as heterogeneous catalytic ozonation) promotes significant improvements in the organic matter degradation¹⁷. Furthermore, a solid catalyst can produce more powerful and unselective oxidant radicals, allowing that a high degradation rate to be achieved through the catalytic ozonation¹⁸. In the catalytic ozonation of organic compounds, ozone is firstly adsorbed onto the catalyst surface, and then decomposed to produce hydroxyl radicals (•OH) on the solid surface, where the organic pollutants may (or not) be adsorbed^19,20. The adsorption of O₃ and/or the organic molecule on the catalyst surface is a fundamental step in the catalytic ozonation^21-23. Although there is experimental evidence to assume that heterogeneous catalytic ozonation involves the generation of hydroxyl radicals (•OH), some researchers reported the occurrence of direct reactions between molecular ozone and organic compounds adsorbed on the solid surface of the catalyst²⁴.

Several materials have been reported as alternative catalysts for the ozonation process aiming the degradation of organic pollutants, such as Al₂O₃²⁵, MnO₂²⁶, MgO²⁷, ZSM-5²⁸, SnO₂²⁹ as well as catalysts supported on matrices in order to improve their catalytic properties^30-32. However, a very few studies using CoFe₂O₄ as a catalyst in ozonation reaction have been reported in literature. CoFe₂O₄ particles were employed in Fenton/ozone oxidation process for the treatment of wastewater containing cytotoxic drugs³³ and on the oxalic acid ozonation³⁴. Moreover, so far, there is no report on the degradation of melanoidin using CoFe₂O₄ as catalyst in ozonation process.

In this context, this work aims to produce the cobalt ferrite and to evaluate its activity in heterogeneous catalytic ozonation for the degradation of melanoidin from aqueous solution.

2. Materials and Methods

2.1 Preparation of synthetic melanoidin

Synthetic melanoidin was prepared based on the methodology described by Dahiya et al.³⁵. Firstly, 1 M glucose, 1 M amino acid and 0.5 M sodium bicarbonate were dissolved in distilled water under magnetic stirring. After, the solution was placed into a Teflon-lined stainless steel autoclave and treatedat 120 °C for 3h. Posteriorly, the obtained solution containing a dark brown color was filtered using a hollow fiber membrane (SLP-1053-10 kDa) (Pall Corporation, USA) coupled to the micro/ultrafiltration module (TE-0198, Tecnal, Brazil) to obtain melanoidin molecules with a molecular weight in the range of 1,000-10,000 Daltons. Then, the resultant filtrate of synthetic melanoidin was lyophilized (Liofilizator L101, Liobras, Brazil) in order to obtain the powdered melanoidin.

2.2 Preparation of CoFe₂O₄

Cobalt ferrite (CoFe₂O₄) was synthesized by the solvothermal method using ethylene glycol as a solvent. Cobalt chloride (4 mmol; CoCl₂.6H₂O, Sigma-Aldrich) and ferric chloride (8 mmol; FeCl₃.6H₂O, Sigma-Aldrich) were used in molar ratio of 1:2 = Co:Fe. The cobalt and ferric chlorides salts were dissolved in 120 mL of ethylene glycol (C₂H₄O₂, Vetec) under magnetic stirring, followed by the addition of 60 mmol of sodium acetate (NaC₂H₃O₂.3H₂O, Sigma-Aldrich). After, the resulting solution was transferred into the Teflon-lined stainless steel autoclave and subjected to a temperature of 200 °C for 10 h, and then cooled to room temperature. The solid particles were washed with distilled water and dried at 110 °C.

2.3 Characterization of CoFe₂O₄

The CoFe₂O₄ particles were characterized by X-ray diffraction (XRD), using a Rigaku Miniflex model 300 diffractometer, being operated with Cu-Kα radiation (λ = 1.5418 Å), 30 kV, 10 mA, step size of 0.03º and a count time of 0.5 s per step. The pore properties were determined using a Micromeritics ASAP 2020 apparatus. The particle-size distribution was measured using a laser particle size analyzer (Malvern Mastersizer 2000). The morphology and chemical analysis of the sample were obtained by scanning electron microscopy (SEM), using a FEI Inspect S50 apparatus coupled to an auxiliary Energy-Dispersive X-ray spectrometer (EDS) with secondary electron detector. FTIR spectrum of sample pressed into KBr pellet (10 mg CoFe₂O₄/300 mg KBr) was recorded by a Shimadzu IR-Prestige-21 spectrometer.

2.4 Melanoidin degradation essays

Melanoidin degradation essays were performed in a 300 mL glass reactor at 25 ºC. Ozone gas was generated from the atmospheric air by an ozone generator apparatus (Ozone Generator, China). The ozone was fed into the reactor through a porous silica diffuser at the flow rate of 10 mL min^-1. The residual ozone in the off-gas from the reactor was absorbed by a 10% Na₂S₂O₃ aqueous solution. The schematic diagram of reaction system is shown in Figure 1. The reactor was filled with 200 mL of melanoidin aqueous solution (initial melanoidin concentration of 300 mg L^-1; natural pH of solution = 6.85) and 0.1 g of catalyst, and posteriorly submitted at magnetic stirring until the equilibrium adsorption was reached. Subsequently, ozone gas was fed into the reactor, and aliquots of the aqueous solution were collected at various time intervals and centrifuged for the separation of catalyst. For comparison purposes, the ozonation reaction without the presence of catalyst (non-catalytic ozonation) was also performed.

Figure 1
Schematic diagram of reaction system.

The decolorization efficiency of melanoidin was determined using a UV-Vis spectrophotometer (Bel Photonics, SP1105), with a maximum wavelength of 475 nm³⁵. The decolorization efficiency was expressed by the ratio C/C₀= (A/A₀) as a function of reaction time t, where: C is the absorbance after a reaction time t, and C₀ is the initial absorbance before the reaction.

Total Organic Carbon (TOC) was measurement on a Shimadzu TOC-L CPH/CPN analyzer to determine the mineralization efficiency of melanoidin. The mineralization efficiency was expressed in terms of TOC/TOC₀ as a function of reaction time t, where: TOC is the carbon concentration after a reaction time t, and TOC₀ is the initial concentration before the reaction.

All the experimental essays were carried out in triplicate.

3. Results and Discussion

3.1 Characterization of material

Figure 2 shows the X-ray diffractogram of CoFe₂O₄ sample. From Figure 2, it is possible to observe that the peaks position (2θ) and their intensities are similar to JCPDS card no. 22-1086, confirming the formation of single-phase CoFe₂O₄ particles. The diffraction peaks at 2θ of 18.1°, 30.0°, 35.5°, 37.0°, 43.0°, 53.0°, 57,0° and 62.6° can be attributed to the diffraction planes (111), (220), (311), (222), (400), (422), (511) and (440), respectively. In addition, no impurities peaks are observed in diffractogram, evidencing thus the formation of a single-phase CoFe₂O₄ sample.

Figure 2
XRD pattern of CoFe₂O₄ (Inset at figure: Barsrepresent the CoFe₂O₄ reference according to JCPDS card no. 22-1086).

Figure 3 shows the nitrogen adsorption-desorption isotherms (Figure 3a) and pore-size distribution curve (Figure 3b) of CoFe₂O₄ sample. The nitrogen adsorption-desorption isotherms (Figure 3a) of CoFe₂O₄ sample can be categorized as type IV with an H1 hysteresis loop (according to the IUPAC classification), which indicates the predominance of mesoporous particles. The behavior of size-pore distribution curve (Figure 3b) confirms the presence of mesoporous structure, since the distribution is predominantly located on the mesoporous region (2 nm < pore size < 50 nm). The found values for the specific surface area, total pore volume and average pore size were 116 m² g^-1, 0.282 cm³ g^-1 and 8.90 nm, respectively. Kalam et al.³⁶ found a surface area value of 76 m² g^-1 for the CoFe₂O₄ particles prepared by modified solvothermal process with polysaccharide, whereas Srivastava et al.³⁷ found a value of 41.3 m² g^-1 through the modified co-precipitation method. CoFe₂O₄ particles prepared by sol-gel³⁸ and hydrothermal³⁹ methods presented surface area values of 10 m² g^-1 and 85 m² g^-1, respectively. Some works in literature demonstrate that the synthesis of materials through the solvothermal route using diols as solvents promotes very interesting physical properties to the material^40,41. Therefore, the synthesis method employed herein promoted CoFe₂O₄ particles with a highlighted value of surface area, which is primordial for catalytic purposes.

Figure 3
N₂ adsorption/desorption isotherms (a), and pore-size distribution curve (b) of CoFe₂O₄ sample.

Figure 4 shows the particle-size distribution curve of CoFe₂O₄ sample. A wide range of particle-size was observed for the sample, being between 0.55 and 125 µm, resulting in a Sauter mean diameter of 27 µm.

Figure 4
Particle-size distribution curve of CoFe₂O₄.

FTIR spectrum of CoFe₂O₄ is shown in Figure 5. The broad band at about 3423 cm^-1 is ascribed to stretching mode of O-H group of the free and absorbed water. Bands at 1080 cm^-1 and 1600 cm^-1 are associated with the presence of humidity absorbed on the CoFe₂O₄ sample. The band at 2340 cm^-1 corresponds to adsorbed CO₂ from the air. A strong absorption near 575 cm^-1 is attributed to the octahedral cation group complex (Co²⁺-O^2-) vibration mode of cobalt ferrite nanoparticles^42,43.

Figure 5
FTIR spectrum of CoFe₂O_4.

Figure 6 shows SEM image (Figure 6a) and EDS analysis (Figure 6b) of CoFe₂O₄ sample. From Figure 6a, it is possible to observe that the particles have irregular shapes and are partially agglomerated. In addition, different particle sizes can be observed, corroborating the result from the particle-size distribution analysis (Figure 4). Elemental analysis (Figure 6b) of CoFe₂O₄ was obtained from the point marked on the SEM image (Figure 6a), where an approximate atomic ratio Co:Fe = 1:2 was found, which is according to the stoichiometric composition in the formula CoFe₂O_4. Therefore, this result evidences the formation of CoFe₂O₄ phase, corroborating the results from the XRD analysis (Figure 2).

Figure 6
(a) SEM image and (b) EDS analysis of CoFe₂O₄ sample_.

Figure 7 shows the image of CoFe₂O₄ particles attracted by a magnet. Due to their magnetic properties^42,44, CoFe₂O₄ particles can be easily separated and recovered from the aqueous solution by a magnetic field for further reutilization.

Figure 7
Photographic image of magnetic CoFe₂O₄ particles.

3.2 Catalytic and non-catalytic essays

Figure 8 shows the melanoidin decolorization results using non-catalytic (O₃ alone) and catalytic (O₃/CoFe₂O₄) ozonation processes. In addition, no melanoidin adsorption on the catalyst surface was observed, as shown in Figure 8. The data obtained in this study clearly indicate that the catalytic ozonation of melanoidin in the presence of CoFe₂O₄ shows remarkable decolorization efficiency (98%) when compared with the non-catalytic ozonation (75%).

Figure 8
Comparison of melanoidin decolorization by the non-catalytic (O₃) and catalytic (O₃/CoFe₂O₄) ozonation processes.

The apparent rate constants for the melanoidin decolorization by catalytic (O₃/CoFe₂O₄) and non-catalytic (O₃) ozonation processes were estimated by linear regression, as shown in Figure 9. Both the processes followed the pseudo-first-order kinetics, according to Equation (1).^45,46

(1) $\ln \left(\frac{\mathit{C}}{{\mathit{C}}_0}\right)={\mathit{k}}_{\mathit{d}}\mathit{t}$

where, k_d the apparent rate constant for the melanoidin decolorization, which is obtained from the slope of the respective linear plots.

Figure 9
Pseudo-first-order plots for the melanoidin decolorization by non-catalytic (O₃) and catalytic (O₃/CoFe₂O₄) ozonation.

From Figure 9, the apparent rate constants for the melanoidin decolorization from the ln (C/C₀) versus time for the non-catalytic (O₃) and catalytic (O₃/CoFe₂O₄) ozonation processes were obtained. The reaction constants values were 51 x 10^-3 min^-1 (R² = 0.97) and 101 x 10^-3 min^-1 ( R² = 0.99) for the non-catalytic (O₃) and catalytic (O₃/CoFe₂O₄) ozonation reactions, respectively. Therefore, the presence of CoFe₂O₄ on the reaction medium promoted a highest catalytic activity, exhibited a reaction rate about two times faster than that of the non-catalytic ozonation reaction.

Figure 10 shows the color of melanoidin solutions during the catalytic ozonation reaction (O₃/CoFe₂O₄). The respective figure was obtained by the digital camera. The first sample (indicated as “0”) corresponds to melanoidin solution before the reaction. The respective solution presents a dark brown color, which is characteristic of melanoidin molecule. The melanoidin decolorization progressively increased as function of reaction time, being that at the end of 30 min, a practically transparent solution is observed. Therefore, the color removal can be attributed to the fact that the O₃/CoFe₂O₄ system is able to cleavage the conjugated carbon-carbon double bonds presents in the melanoidin structure, which are responsible for the brown color⁴⁷.

Figure 10
Photographic image of melanoidin solutions collected during the heterogeneous catalytic ozonation (O₃/CoFe₂O₄) until 30 min of reaction time.

In order to verify the species responsible for the melanoidin decolonization, isopropanol, p-benzoquinone and triethanolamine were employed as scavengers for hydroxyl radicals (•OH), superoxide anion radicals (O₂ ^−•) and photogenerated holes (h⁺), respectively^48,49,50. The experiments were carried out under similar conditions those for the non-scavenging experiments. Through the preliminary essays using p-benzoquinone and triethanolamine in the reaction, no change in melanoidin color was observed when compared to the non-scavenging experiment. On the other hand, a significant change in melanoidin color occurred under the presence of isopropanol. Therefore, these results indicate that the hydroxyl radicals (•OH) are the oxidative species involved in the melanoidin degradation by catalytic ozonation.

Figure 11 shows the effect of scavenger (isopropanol) on the melanoidin decolorization at 30 min of reaction time. The results show that the presence of isopropanol leads to a decrease in the melanoidin decolorization around 26% for the catalytic ozonation process, indicating that •OH is an important active radical involved in this process. On the order hand, the addition of isopropanol did not impact the melanoidin decolorization for the O₃ alone. Therefore, these results indicate that in the absence of catalyst, the direct mechanism by molecular O₃ (direct reaction) occurred, whereas in the presence catalyst, there was a major contribution of oxidant specie (•OH). In addition, the results showed that the catalytic ozonation reaction with the scavenger (isopropanol) was not be completely quenched, indicating that the •OH is not the only species involved in melanoidin decolorization. In this case, direct ozonation reaction also occurred. Therefore, the presence of CoFe₂O₄ catalyst on the ozonation reaction promoted the formation of •OH radicals in the reaction medium, resulting in greater efficiency of melanoidin decolorization. Recently, it has been reported that the presence of solid catalyst in the ozonation process could increase the generation of radicals and the oxidation of organic compounds due to the formation of hydroxyl radicals (•OH) in the reaction medium, which has a higher oxidation potential (2.80 eV) compared to the molecular ozone (2.07 eV)¹⁹.

Figure 11
Decolorization efficiency of melanoidin by CoFe₂O₄/O₃ and O₃ processes at 30 min of reaction time without and with scavenger (isopropanol).

Figure 12 shows the mineralization profiles of melanoidin as a function of reaction time by the non-catalytic (O₃) and catalytic (O₃/CoFe₂O₄) ozonation processes. It is known that a more extended reaction time is necessary to obtain a high mineralization of an organic molecule from aqueous solution^45,46. Therefore, the O₃/CoFe₂O₄ system showed about 80 % removal of TOC at 180 min, whereas the non-catalytic (O₃) process showed about 60 % removal of TOC. These mineralization results for both the processes corroborate those obtained for the melanoidin decolorization, where the presence of the CoFe₂O₄ catalyst is primordial for a superior activity in the melanoidin degradation.

Figure 12
Mineralization profiles of melanoidin as a function of reaction time by the non-catalytic (O₃) and catalytic (O₃/CoFe₂O₄) ozonation processes.

Based on the experimental observations as aforementioned and those reported in literature using other materials as catalysts^51,52, a simplified general mechanism for the melanoidin ozonation using CoFe₂O₄ as catalyst, included an possible indirect and a direct oxidation, could be depicted as follows. The reaction starts with the O₃ adsorption on the surface catalyst followed by its decomposition, producing free radicals (•O₂H, •O₂ ^-), as shown in Equation (2) ⁵³. •O₂ ^- radical is a highly selective catalyst for the decomposition of O₃ in water, so it reacts with another molecule of O₃ producing the radical •O₃ ^-, according to Equation (3) ⁵³. •O₃ ^- decomposes upon protonation into •OH radicals⁵², as show in Equations (4) and (5). Therefore, •OH radicals, which are highly reactive and non-selective species, quickly react with melanoidin leading to its degradation (Equation 6). Moreover, the O₃ that has not been decomposed into •OH radicals is probably dissolved in the aqueous medium and can oxidize directly the melanoidin.

(2) ${\mathit{CoFe}}_2{\mathit{O}}_{4^{-}}\mathit{OH}+{\mathit{O}}_3\to •{\mathit{O}}_2\mathit{H}+•{\mathit{O}}_{2^{-}}$

(3) ${\mathit{O}}_3+•{\mathit{O}}_{2^{-}}\to •{\mathit{O}}_{3^{-}}+{\mathit{O}}_2$

(4) $•{\mathit{O}}_{3^{-}}+\mathit{H}+\to •{\mathit{O}}_3\mathit{H}$

(5) $•{\mathit{O}}_3\mathit{H}\to {\mathit{O}}_2+•\mathit{OH}$

(6) $\begin{array}{c}•\mathit{OH}+\mathrm{Melanoidin}\to \mathrm{deg}\mathrm{radation}\mathrm{products}\hfill \\ \left({\mathit{CO}}_2+{\mathit{H}}_2\mathit{O}+\mathrm{int}\mathrm{ermediates}\right)\hfill \end{array}$

4. Conclusions

Solvothermal method employed in this work produced CoFe₂O₄ particles with a predominantly mesoporous structure containing a high surface area (116 m² g^-1). The ozonation process with CoFe₂O₄ showed a higher decolorization and mineralization of the melanoidin compared to O₃ alone, being attributed to generation of •OH radicals in reaction medium. A reaction mechanism was proposed in this work in order to elucidate the melanoidin degradation by catalytic ozonation. In summary, the CoFe₂O₄-catalyzed ozonation reaction proving to be a promising process for the application in the treatment of melanoidin present in several distillery wastewaters for its further safe disposal into aquatic ecosystems.

5. Acknowledgement

The authors wish to thank for the technical support of Postgraduate Program in Chemical Engineering at Federal University of Santa Maria (Brazil), and financial support of CNPq (Brazil).

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Publication Dates

• Publication in this collection
  08 Nov 2018
• Date of issue
  2019

History

• Received
  09 July 2018
• Reviewed
  20 Sept 2018
• Accepted
  08 Oct 2018