Decolorization of the azo dye Reactive Violet 5 by UV-A-Fenton and Ultrasound-Fenton Processes ^†

Abstract

The textile dye Reactive Violet 5 (RV5) is mutagenic and teratogenic as well as carcinogenic and must be degraded before the release of textile wastewater into the environment. The aim of this work was to assess UV-A LED and ultrasound (US) reactors in RV5 degradation. Different AOPs were tested, Fenton and UV-A-Fenton processes showed the highest RV5 degradation with 86.6 and 95.5%, respectively. The UV-A-Fenton operational conditions were optimized varying initial pH (3.0–7.0), H₂O₂ (2.0–16.0 mM) and Fe²⁺ (0.05–0.20 mM) concentrations. The best deduced operational conditions (pH = 3.0, [RV5] = 0.28 mM, [H₂O₂] = 4.0 mM, [Fe²⁺] = 0.15 mM) were applied to the US-Fenton process, achieving a RV5 removal of 95.7%. The lowest values of electric energy per order (${\mathrm{E}}_{\mathrm{EO}}$) = 11 kWh m⁻³ order⁻¹ and specific applied energy (${\mathrm{E}}_{\mathrm{SAE}}$) = 38 kWh mol⁻¹ order⁻¹ were obtained with the treatment of RV5 aqueous solution by the UV-A-Fenton process. This work shows that textile dyes can be degraded by UV-A-Fenton and US-Fenton processes and the UV-A LED reactor presented the lowest operational costs.

1. Introduction

The textile industry is one of the highest water consuming industries, utilizing a large amount of dyes, organic and inorganic chemicals and additives in the production process [1]. Among the textile dyes, the azo dye Reactive Violet 5 (RV5) is a dye used in the dyeing and printing of natural, synthetic, man-made and mixed textile materials such as wool, silk, nylon, polyester, acrylic, polyacetate and polyurethane [2]. Due to washing operations, the wastewater produced from textile dyeing mills is not only toxic, but it is also enriched by the presence of mutagenic and teratogenic as well as carcinogenic chemicals. For instance, the well-known carcinogen, benzidine, is the parent component of most of the azo dyes, which poses a threat to living organisms [3]. In order to treat these types of wastewater, advanced oxidation processes (AOPs) can be applied, in which catalytic and non-catalytic processes generate hydroxyl radicals (${\mathrm{HO}}^{•}$) with high oxidation potential (2.80 V), degrading the azo dyes [4,5]. Among the AOPs, the application of the Fenton process can be an efficient method, in which ferrous iron (Fe²⁺) reacts with hydrogen peroxide (H₂O₂) to generate ${\mathrm{HO}}^{•}$ radicals (Equation (1)). The ferric iron (Fe³⁺) produced in the reaction also reacts with the H₂O₂ (Equation (2)), generating hydroperoxyl radicals (${\mathrm{HO}}_2^{•}$) with lower oxidation potential (1.65 V) [6,7].

$${\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{Fe}}^{3+}+{\mathrm{HO}}^{•}+{\mathrm{HO}}^{-}$$

(1)

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2 \to {\mathrm{Fe}}^{2+}+{\mathrm{HO}}_2^{•}+{\mathrm{H}}^{+}$$

(2)

To enhance the Fenton process, UV radiation with “near-UV to visible region” of light up to a wavelength of 600 nm can be applied to improve the ${\mathrm{HO}}^{•}$ radical production (Equation (3)) and reducing rapidly the Fe³⁺ back to Fe²⁺ (Equation (4)) [8].

$${\mathrm{H}}_2{\mathrm{O}}_2+\mathrm{UV} \to {2\mathrm{HO}}^{•}$$

(3)

$${\mathrm{Fe}}^{3+}+{\mathrm{H}}_2\mathrm{O}+\mathrm{UV} \to {\mathrm{Fe}}^{2+}+{\mathrm{HO}}^{•}$$

(4)

Another interesting approach is the use of ultrasounds, which promise high reaction rates and short treatment times. In the high temperature and high-pressure region of bubbles, water molecules are prone to react, producing strong oxidizing substances such as ${\mathrm{H}}^{•}$ and ${\mathrm{HO}}^{•}$ radicals (Equation (5)) [9]. In addition, the application of ultrasound allows the regeneration of Fe³⁺ to Fe²⁺ (Equation (6)), with the generation of ${\mathrm{HO}}_2^{•}$ radicals [10].

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

(5)

$${\mathrm{FeHO}}_2^{2+}+\mathrm{US} \to {\mathrm{Fe}}^{2+}+{\mathrm{HO}}_2^{•}$$

(6)

The aim and novelty of this work lies in the application of a self-made UV-A LED reactor and an ultrasound reactor for the removal of RV5 from an aqueous solution.

2. Materials and Methods

2.1. Reagents

The reactive dye, Reactive Violet 5 (RV5, Color Index 18097), was provided by Sigma-Aldrich Co. (St. Louis, MO, USA) and used as received without further purification. The molecular structure of RV5 in non-hydrolyzed form is illustrated in

Table 1. Chemical structure, maximum absorption wavelength and molecular weight of Reactive Violet 5 (RV5) [11].

Name	Chemical Structure	λmax (nm)	Molecular Weight (g/mol)
Reactive Violet 5 (azo dye)		560 and 320 nm	735.59

. Iron (II) sulfate heptahydrate (FeSO₄•7H₂O) was acquired by Panreac and hydrogen peroxide (H₂O₂ 30% w/w) was acquired by Sigma-Aldrich. NaOH and H₂SO₄ (95%) were both obtained from Analar Normapur. Deionized water was used to prepare the respective solutions.

2.2. Analytic Techniques

The maximum absorbance wavelength (λ_max) of RV5 was found at 560 nm, and the concentration of the residual dye in solution was calculated by Beer–Lambert’s law (Equation (7)), using the optical density and molar extinction observed at the characteristic wavelength [12]:

A = lεC

(7)

where A is the absorbency, l is the path length (cm), ε is the molar extinction coefficient (L/mol/cm) and C the dye concentration at time t (mol/L). Dye removal was determined as follows (Equation (8)) [13,14]:

$$\mathrm{Dye} \mathrm{degradation} (\%)=(\frac{{\mathrm{C}}_{\mathrm{dye},0}{-\mathrm{C}}_{\mathrm{dye},\mathrm{t}}}{{\mathrm{C}}_{\mathrm{dye},0}}) \times 100$$

(8)

where ${\mathrm{C}}_{\mathrm{dye},0}$ and ${\mathrm{C}}_{\mathrm{dye},\mathrm{t}}$ are the concentrations of RV5 at reaction time t and 0, respectively.

2.3. Experimental Process

The photo-Fenton process was carried out in a lab-scale batch reactor, which was illuminated with a UV-A LED photo-system. The photo-system consisted of a matrix of 12 InGaN LEDs lamps (Roithner APG2C1-365E LEDs) with a maximum emission wavelength of λ = 365 nm.

Batch experiments for ultrasound-Fenton process were performed with a VCX 500 Watt Ultrasonic Processor (SONICS Vibra Cell™, Newtown, CT, USA) in a cylindrical reactor of 500 mL capacity. All the experiments were performed in triplicate and the observed standard deviation was always less than 5% of the reported values.

3. Results and Discussion

AOP Application

Considering the difficult treatment of wastewaters contaminated by RV5, several AOPs were performed to evaluate the efficiency of different experimental conditions on dye degradation. Figure 1a shows the RV5 removal obtained by different AOPs as a function of time (min) under the following operational conditions: pH = 3.0, [RV5] = 0.28 mM, [H₂O₂] = 4.0 mM, [Fe²⁺] = 0.15 mM, radiation UV-A (365 nm), I_UV = 32.7 W m⁻², time = 7 min. The results showed the highest removal of 95.5 and 86.6% with the application of photo-Fenton and Fenton processes, respectively. These results can be explained by the high generation of ${\mathrm{HO}}^{•}$ radicals by these processes, leading to the removal of RV5 from aqueous solution. The remaining AOPs were observed to have a low capacity for ${\mathrm{HO}}^{•}$ radical production, thus explaining the low efficiency in RV5 degradation. These results are supported by the findings of Teixeira et al. [15] who observed a high Acid red 88 removal with the application of Fenton and photo-Fenton processes. It was also observed that other AOPs showed low efficiency in textile dye removal.

The selection of the optimum pH is important to achieve high efficiency in RV5 degradation. The initial pH of the wastewater was varied from 3.0 to 7.0 and the results showed a RV5 removal of 95.5, 90.2, 84.4 and 80.6%, respectively, for pH 3.0, 4.0, 6.0 and 7.0 (Figure 1b). Clearly, as the pH increased above 3.0, the degradation efficiency decreases due to (1) iron precipitation as hydroxide derivate, reducing the Fe²⁺ availability, and (2) the dissociation and auto-decomposition of H₂O₂ [16].

The next step in photo-Fenton optimization is the variation of the H₂O₂ concentration (2.0–16.0 mM). The results showed the highest RV5 removal with the application of 4.0 mM H₂O₂ (95.5%). The application of 2.0 mM H₂O₂ was insufficient to generate ${\mathrm{HO}}^{•}$ radicals in sufficient amounts to degrade the RV5 (Figure 1c). Above 4.0 mM H₂O₂, radical scavenging was observed by the excess of H₂O₂ present in the solution (Equation (9)), thus explaining the decrease in RV5 removal [17].

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

(9)

Finally, the optimization of the concentration of Fe²⁺ catalyst in the range of 0.05–0.20 mM was performed using the conditions obtained previously. The results in Figure 1d show a RV5 removal of 87.2, 90.4, 95.5 and 90.3%, respectively, for 0.05, 0.10, 0.15 and 0.20 mM Fe²⁺. As the Fe²⁺ concentration increases to 0.15 mM, a higher generation of ${\mathrm{HO}}^{•}$ radicals occurs and, simultaneously, a higher concentration of RV5 is degraded. However, increasing the Fe²⁺ above 0.15 mM leads to scavenging reactions, thus decreasing the RV5 removal (Equation (10)) [17].

$$\mathrm{Fe}{2}^{+}+{\mathrm{HO}}^{•} \to {\mathrm{Fe}}^{3+}+{\mathrm{HO}}^{-}$$

(10)

The US-Fenton process was applied under the operational conditions: pH = 3.0, [RV5] = 0.28 mM, [H₂O₂] = 4.0 mM, [Fe²⁺] = 0.15 mM, P = 500 W, E = 1400 J min⁻¹, A = 40% and time = 7 min (Figure 2).

The results show that the application of US and US + H₂O₂ was insufficient to generate a high amount of ${\mathrm{HO}}^{•}$ radicals, thus explaining the low removal efficiency. The application of US-Fenton reached 95.7% RV5 removal, like the UV-A-Fenton. These results are in agreement with the work of Thanekar and Gogate [18], who observed low COD removal with the application of US and US + H₂O₂ in industrial wastewater treatment, and Rahmani et al. [10] who observed a high COD removal with the application of US-Fenton in the treatment of activated sludge.

The results obtained by Fenton, UV-A-Fenton and US-Fenton fit a pseudo-first-order kinetic rate (ln([RV5]_t = −kt + ln[RV5]₀). The results showed that the application of UV-A and US radiation significantly increased the kinetic rate of RV5 removal (

Table 2. Pseudo-first-order kinetic rate (k), electric energy per order (${\mathrm{E}}_{\mathrm{EO}}$ ), specific applied energy (${\mathrm{E}}_{\mathrm{SAE}}$ ) and costs. Means in the same column with different letters represent significant differences (p < 0.05) within each parameter by comparing the treatment processes. n.q.—not quantified.

Process	k (min−1)	${\mathbf{E}}_{\mathbf{EO}}(\mathbf{kWh} {\mathbf{m}}^{-3} {\mathbf{order}}^{-1})$	${\mathbf{E}}_{\mathbf{SAE}}(\mathbf{kWh} {\mathbf{mol}}^{-1} {\mathbf{order}}^{-1})$	Cost (EUR m−3)
Fenton	0.270 ± 0.01 a	n.q.	n.q.	n.q.
UV-A-Fenton	0.477 ± 0.01 b	11 ± 0.32 a	38 ± 1.13 a	0.84 ± 0.03 a
US-Fenton	0.483 ± 0.02 b	159 ± 4.77 b	568 ± 17.04 b	12.72 ± 0.38 b

). Having an effective process is not sufficient, it must also be cost-effective, therefore the electric energy per order (Equation (11)) and the specific applied energy (Equation (12)) were evaluated [15].

$${\mathrm{E}}_{\mathrm{EO}}=\frac{38{.4 \times 10}^{-3}\times \mathrm{P}}{\mathrm{V}\times \mathrm{k}}$$

(11)

$${\mathrm{E}}_{\mathrm{SAE}}=\frac{{\mathrm{E}}_{\mathrm{EO}}}{{\mathrm{C}}_0{ \times 10}^3}$$

(12)

where P is the rated power of the system (kW), V is the reactor volume (m³) and C₀ is the initial dye concentration (mol L⁻¹). The results showed that, although US-Fenton has the highest kinetic rate, the energy consumption is higher, mainly, due to the power of the reactor, which is higher than the UV-A reactor. By applying the cost of electricity in Portugal (0.08 EUR/kWh⁻¹) [19], it was observed that US-Fenton is more expensive than UV-A-Fenton.

4. Conclusions

This research work addresses the treatment of a non-biodegradable textile dye (RV5). Different feasible and efficient technological alternatives are shown and compared, by means of a preliminary economic assessment, to find out the lower-cost process in terms of operation. An initial assessment shows that RV5 is very difficult to degrade and only Fenton and UV-A-Fenton show efficiencies of 95.5 and 86.6%, respectively. The US-Fenton is an efficient system with 95.7% RV5 removal. Finally, although US-Fenton achieves a higher kinetic rate in RV5 degradation, it is more expensive than UV-A-Fenton.