Application of Ferrocene in the Treatment of Winery Wastewater in a Heterogeneous Photo-Fenton Process ^†

Abstract

Recent studies in the literature have shown successful applications of the nanomaterial precursor ferrocene (Fc) as a catalyst in heterogeneous photo-Fenton; however, ferrocene has never been used for the treatment of winery wastewater (WW). Therefore, the aim of this work is to: (1) characterize Fc by Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM); (2) optimize heterogeneous photo-Fenton in WW treatment; and (3) study the kinetic rate and regeneration of Fc. The FTIR analysis confirmed the presence of Fe²⁺ in its composition and SEM images showed that Fc is a porous compound. Under the best operational conditions—[Fc] = 0.50 g/L, [H₂O₂] = 194 mM, pH = 3.0, agitation = 350 rpm, T = 298 K, radiation UV-C, t = 240 min—significant TOC removal was achieved (82.7%). The results showed that the Fc catalyst decreased its stability across three consecutive cycling processes, with the TOC removals decreasing to 82.7, 76.2 and 63.9% for the first, second and third cycles. Fermi’s non-linear kinetic model was applied, observing a k_TOC of 4.77 × 10⁻² min⁻¹. In conclusion, ferrocene is a suitable compound for the treatment of WW with the heterogeneous photo-Fenton process.

1. Introduction

Winery wastewater (WW) is a major waste stream resulting from a number of activities including tank cleaning, washing floors and equipment, rinsing transfer lines, barrel cleaning, off wine and product losses, bottling, filtration, and the management of rainwater diverted into or captured in the wastewater management system. The effluent produced contains various contaminants, such as ethanol, sugars, organic acids, phenolic compounds, etc. [1,2]. To treat this type of wastewater, several methods can be applied, such as thermocatalytic [3], ozone [4], homogeneous Fenton [5], and heterogeneous photo-Fenton processes [6]. The homogeneous Fenton process has several disadvantages, such as: (1) a low pH is needed to prevent metal precipitation; (2) the homogeneous catalyst needs to be recovered from the treated wastewater; and (3) the resulting wastewater needs to be neutralized to meet the legal discharge limits imposed (pH 6.0–9.0) [6]. To overcome these disadvantages, in this work, we performed the treatment of WW with heterogeneous photo-Fenton with the application of ferrocene. Ferrocene has several advantages, such as high stability, a special structure, and strong ultraviolet absorbance [7]. In addition, in the work of Nie et al. [8], it was observed that ferrocene could be reused in four cycles. Therefore, the aim of this work is: (1) to characterize ferrocene by FTIR and SEM; (2) to optimize the heterogenous photo-Fenton process; and (3) to study the kinetic rate and regeneration of ferrocene.

2. Materials and Methods

2.1. Reagents and Winery Wastewater Sampling

Ferrocene was supplied by Alfa Aesar (Haverhill, MA, USA), and hydrogen peroxide (H₂O₂ 30%) was supplied by Sigma-Aldrich (St. Louis, MO, USA). For pH adjustment, sodium hydroxide (NaOH) was used from Labkem, Barcelona, Spain, as well as sulphuric acid (H₂SO₄, 95%) from Scharlau, Barcelona, Spain. Deionized water was used to prepare the respective solutions. The winery wastewater was collected from a cellar located in the Douro region (Northern Portugal).

2.2. Analytical Determinations

Different physical–chemical parameters were determined to characterize the WW, including turbidity, total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD₅), total organic carbon (TOC), and total polyphenols. The main winery wastewater characteristics are shown in

Table 1. Characterization of winery wastewater.

Parameters	Winery Wastewater	Portuguese Decree Law No. 236/98
pH	4.0	6.0–9.0
Conductivity (μS/cm)	62.5
Turbidity (NTU)	296
Total suspended solids (mg/L)	750	60
Chemical Oxygen Demand—COD (mg O2/L)	2145	150
Biochemical Oxygen Demand—BOD5 (mg O2/L)	550	40
Total Organic Carbon (mg C/L)	400
Total polyphenols (mg gallic acid/L)	22.6	0.5
Ferrous iron (mg Fe/L)	0.05	2.0
Biodegradability—BOD5/COD	0.26

.

2.3. Heterogeneous Photo-Fenton Experimental Set-Up

The heterogeneous photo-Fenton experiments were performed in a batch cylindrical photoreactor (600 cm³) equipped with a UV-C low-pressure mercury vapour lamp (TNN 15/32) with a working power of 15 W (795.8 W/m²) and a λ_max of 254 nm (Heraeus, Germany). The optimization of the heterogeneous photo-Fenton process was performed in a 500 mL stirred glass reactor under UV-C radiation, the temperature was maintained constant to 298 K for 240 min, and the WW samples were treated as follows: (1) variation of pH (3.0–7.0); (2) variation of H₂O₂ addition (single and multiple dosing steps); (3) variation of H₂O₂ concentration (97–291 mM); (4) variation of Fc concentration (0.25–1.0 g/L); (5) performance of 3 recovery cycles.

After the reaction had started, 20 mL of solution was withdrawn and filtrated with a 0.20 µm filter for TOC measurements at different reaction times, completing a total period of 240 min. The percentage of organic carbon removed through adsorption was calculated according to Equation (1) [9]:

$${\mathrm{TOC}}_{\mathrm{rem}.}\left(\%\right)\frac{{\mathrm{TOC}}_{\mathrm{i}}{-\mathrm{TOC}}_{\mathrm{t}}}{{\mathrm{TOC}}_{\mathrm{i}}} \times 100$$

(1)

where TOC_i is the initial TOC content (mg C/L) and TOC_t corresponds to the TOC value at instant t (mg C/L).

2.4. Statistical Analysis

All the experiments were performed in triplicate and the observed standard deviation was always less than 5% of the reported values. The statistical analysis was performed with OriginLab 2019 software (Northampton, MA, USA).

3. Results

3.1. Characterization of Ferrocene

Ferrocene, an organometallic compound with the formula Fe(C₅H₅)₂, has a chemical assembly of two cyclopentadienyl rings bound to a central iron atom in a “sandwich” or “double-cone”-type structure. By the performance of FTIR analysis (Figure 1), it was observed that the main bands exhibited by ferrocene were associated with C–H stretching at 3093 cm⁻¹, C=C stretching at 1631 cm⁻¹, C–C stretching at 1404.18 and 1105.21 cm^–1, C–H deformation at 999.13 and 812.03 cm^–1 [10], and the Fe peak at 476 cm⁻¹. The iron center in ferrocene should be assigned the (+2) oxidation state. Each cyclopentadienyl (Cp) ring should then be allocated a single negative charge. Thus, ferrocene could be described as iron(II) bis(cyclopentadienide), Fe²⁺[C₅H₅]₂ [11].

In Figure 2, the SEM images of the ferrocene can be observed. The ferrocene catalyst in its initial form has an irregular shape with a lot of free space in between the particles; thus, the Fc catalyst has adsorption capacity.

3.2. Heterogeneous Photo-Fenton Optimization

To achieve maximum TOC removal from the WW, the heterogeneous photo-Fenton process was optimized. The pH varied from 3.0 to 7.0, and the results showed that the heterogeneous photo-Fenton process was highly dependent on pH (3.0 > 4.0 > 6.0 > 7.0) with 53.3, 42.1, 35.0 and 22.8%, respectively (240 min). In the work of Wang et al. [12], it was observed that ferrocene had a complete degradation of methylene blue (MB) at pH 3.0 (Equations (2) and (3)), but at a higher pH (4.0 and 5.0) the ferrocene had a decrease in its efficiency, which could be explained by the fact that Fc achieved a higher dissolution at pH 3.0, decreasing as the pH increased, thus decreasing the heterogeneous photo-Fenton efficiency, as observed in Equation (4), as follows:

$$\mathrm{Fc}+{\mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{Fc}}^{+}+{\mathrm{HO}}^{•}+{\mathrm{HO}}^{-}$$

(2)

$${\mathrm{Fc}}^{+}+{\mathrm{H}}_2\mathrm{O} \to \mathrm{Fc}+{\mathrm{HO}}^{•}+{\mathrm{H}}^{+}$$

(3)

2Fc⁺ + H₂O₂ → 2Fc + O₂ + 2H⁺

(4)

The manner of addition of H₂O₂ was varied (single and multiple addition) to evaluate the TOC removal. The final TOC values after 240 min of treatment were 82.7% using multiple dosing and 53.3% using a single dose. The same effect was also observed by Rodríguez-Chueca et al. [13] in the treatment of WW by multiple dosage steps. By the application of multiple dosing steps, the H₂O₂ concentration was varied from 97 to 291 mM, and the results showed a TOC removal of 48.5, 82.7 and 81.4% for 97, 194 and 291 mM H₂O₂, respectively. Clearly, increasing the H₂O₂ concentration above 194 mM led to radical scavenging by the excess of H₂O₂ present in solution, as observed in Equation (5), as follows:

$${\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{HO}}^{•} \to {\mathrm{H}}_2\mathrm{O}+{\mathrm{HO}}_2^{•}$$

(5)

The ferrocene catalyst concentration was varied (0.25 to 1.0 g/L) and the results showed a TOC removal of 83.1, 82.7 and 54.2% for 0.25, 0.50 and 1.0 g/L Fc, respectively (Figure 3a). These results showed that the application of 0.50 g/L was ideal because more hydroxyl radicals were produced; however, increasing to 1.0 g/L may have contributed to increasing the solution turbidity, hampering the UV radiation penetration, as previously observed by Guimarães et al. [6]. Without the penetration of UV radiation, there was a lower reduction of ferric iron to ferrous iron, decreasing the heterogeneous photo-Fenton efficiency, which explains the decrease in TOC removal with the application of 1.0 g/L Fc.

Finally, the durability of the Fc catalyst was examined by recovering the material and re-using it under the best operational conditions, as follows: [Fc] = 0.50 g/L, [H₂O₂] = 194 mM, pH = 3.0, agitation = 350 rpm, T = 298 K, radiation UV-C, t = 240 min, for three consecutive cycles. The results showed a TOC removal of 82.7, 76.2 and 63.9% for the first, second and third cycles, respectively (Figure 3b). Therefore, the FC catalyst can be reused.

3.3. Kinetic Analysis

Fermi’s non-linear kinetic model was used to determine the behavior of the ferrocene catalyst. Fermi’s model provides a single fit for experimental results showing a transition between the induction period (slow degradation) and the subsequent rapid degradation step of an organic compound (inverted S-shaped transient curve), as observed in Equation (6) [14,15], as follows:

$$\frac{\mathrm{TOC}}{{\mathrm{TOC}}_0}=\frac{{1 - \mathrm{X}}_{\mathrm{TOC}}}{{1+\exp [\mathrm{k}}_{\mathrm{TOC}}\left({\mathrm{t} - \mathrm{t}}_{\mathrm{TOC}}^{\ast }\right)]}{+\mathrm{X}}_{\mathrm{TOC}}$$

(6)

where ${\mathrm{k}}_{\mathrm{TOC}}$ corresponds to the apparent reaction rate constant; ${\mathrm{t}}_{\mathrm{TOC}}^{\ast }$ represents the transition time related to the TOC content curve’s inflection point; and ${\mathrm{X}}_{\mathrm{TOC}}$ corresponds to the fraction of non-oxidizable compounds that are formed during the reaction. The results showed that a higher ${\mathrm{k}}_{\mathrm{TOC}}$ was obtained under the following operational conditions: pH 3.0, Fc dosage 0.50 g/L, H₂O₂ concentration 194 mM (addition in six steps) (${\mathrm{k}}_{\mathrm{TOC}}$ = 4.77 × 10⁻² min⁻¹; 82.7% TOC removal).

4. Conclusions

Considering the results obtained in this work, it is concluded that:

(1)

Ferrocene can be used as a source of iron in heterogeneous catalysis in winery wastewater treatment;

(2)

Under the optimal conditions, the heterogeneous photo-Fenton achieves an 82.7% TOC removal;

(3)

Fermi’s kinetic model shows the best operational condition at ${\mathrm{k}}_{\mathrm{TOC}}$ = 4.77 × 10⁻² min⁻¹;

(4)

Ferrocene can be reused for three consecutive cycles with a TOC removal of 82.7, 76.2 and 63.9% for the first, second and third cycles, respectively.