Treatment of a Textile Effluent by Electrochemical Oxidation and Coupled System Electooxidation–Salix babylonica

Centro Interamericano de Recursos del Agua, Universidad Autónoma del Estado deMéxico, Carretera Toluca-Atlacomulco, Km 14.5 Unidad San Cayetano, 50200 Toluca, MEX, Mexico Centro de Investigación en Ciencias Biológicas Aplicadas, Universidad Autónoma del Estado de México, Carretera TolucaAtlacomulco, Km 14.5 Unidad San Cayetano, 50200 Toluca, MEX, Mexico Facultad de Geografía, Universidad Autónoma del Estado de México, Cerro de Coatepec s/n, Ciudad Universitaria, 50110 Toluca, MEX, Mexico


Introduction
The textile industry is one of the greatest generators of liquid effluent pollutants due to the high quantities of water used in the dyeing processes.The chemical composition involves a wide range of pollutants: inorganic compounds, polymers, and organic products [1][2][3].Treatment of textile dye effluent is difficult and ineffective with conventional processes because many synthetic dyes are very stable in light and high temperature, and they are also nonbiodegradable.Moreover, partial oxidation or reduction can generate very toxic byproducts [4][5][6].
Advanced oxidation processes (AOPs) have emerged as potentially powerful methods that can transform recalcitrant pollutants into harmless substances.AOPs rely on the generation of very reactive free radicals and very powerful oxidants, such as the hydroxyl radical, HO• (redox potentia l = 2 8 V) [7,8].These radicals react rapidly with most organic compounds, either by addition to a double bond or by the abstraction of a hydrogen atom from organic molecules [9,10].
The resulting organic radicals, then, react with oxygen to initiate a series of degradative oxidation reactions that lead to products, such as CO 2 and H 2 O [1,11].Electrochemical oxidation is carried out by indirect and/or direct anodic reactions in which oxygen is transferred from the solvent (water) to the product to be oxidized [12].The main characteristic of this treatment is that it uses electrical energy as a vector for environmental decontamination [13].During direct anodic oxidation, pollutants are initially adsorbed on the   2 International Journal of Photoenergy surface of the anode, where the anodic electron transfer reaction degrades them [6].In indirect anodic oxidation, strong oxidants, such as hypochlorite, chlorine, ozone, or hydrogen peroxide, are electrochemically generated.
The pollutants are degraded via the oxidation reactions with these strong oxidants [11].Boron-doped diamond (BDD) thin films are electrode materials that possess several technologically important characteristics, including an inert surface with low adsorption properties, an acceptable conductivity, and remarkable corrosion stability even in strongly acidic media and extremely high O 2 evolution overvoltage [14,15].
On the other hand, biotechnology continues to be used to solve environmental problems [16][17][18].Phytoremediation (PR) is a green technology that uses plant systems for the remediation and restoration of contaminated sites [19].PR's advantages are solar energy dependence and an esthetically pleasant method of treatment [20].Plants have inbuilt enzymatic characteristics that are capable of degrading complex structures, and they can be used for cleaning contaminated sites [17].
Another alternative is to use species of fast-growing woody plants with high biomass production and high genetic variability [25][26][27].Trees from the Salicaceae family with the genera Salix and Populus are suitable candidates for this purpose [28][29][30].Willows (Salix spp.) have several characteristics that make them ideal plant species for PR application, including easy propagation and cultivation, a large amount of biomass, a deep root system, a high transpiration rate, tolerance to hypoxic conditions, and high metal accumulation capability [30,31].
Salix babylonica has been used to solve the problems associated with aquifers contaminated with ethanol-blended gasoline [32] and studies of the biotransformation and   [33,34].In recent years, several authors have described dye removal by electrochemical (EC) or coupled electrochemical with other chemical, electrochemical, or biological procedures (Table 1).However, no studies have been conducted on the implementation of coupled electrochemical oxidationphytoremediation with weeping willow in the remediation of textile effluents, and because it is an introduced species, is noninvasive, and is widely distributed in Mexico, the aim of this study was to evaluate the removal of the pollutants of a textile effluent using an electrochemical oxidation process and to compare their performance with Salix babylonica.

Wastewater Sampling.
A textile wastewater sample was collected from a textile industry whose business is the dyeing and washing of denim garments in Almoloya del Río, State of Mexico, Mexico.The wastewater that this industry discharges does not receive any treatment and is discharged into the sewage system, so it is necessary to give it some kind of treatment to improve its quality.The textile wastewater sample was placed in plastic containers and transported to the laboratory, where it was refrigerated at 4 °C for analysis and for conducting the electrochemical oxidation and coupled system electrooxidation-Salix babylonica.

Electrochemical
Reactor.In this study, a batch electrochemical reactor was used.The reactor contained five vertical parallel electrodes of BDD (titanium/BDD) that CONDIAS DIACHEM manufactured, two as cathodes and three as anodes.Each electrode was 20.5 cm long and 2.5 cm wide, resulting in an area of 102.5 cm 2 for each electrode and a total anodic area of 307.5 cm 2 .A schematic diagram of the electrochemical reactor is shown in Figure 1.The tests were carried out in a 1 L cylindrical reactor.The reactor was operated at different pH values (5.23, 7, and 10).A current density power supply provided 1, 2, and 3 A and 5-6.75 V, corresponding to a current density of 3.5, 7, and 10 mA•cm −2 .
The experiment design used included the two factors of pH and current density.The levels of each of the factors are listed in Table 2. Different aliquots were taken, and the chemical oxygen demand (COD), biochemical oxygen demand (BOD 5 ), color, turbidity, and conductivity were analyzed.The boron-doped diamond electrodes (BDD) were cleaned for one hour in Na 2 SO 4 (0.03 M) after each experiment to remove adsorbed molecules at the electrode surface, and then they were rinsed with distilled water.

Coupled System with Salix babylonica Treatment. For
Salix babylonica treatment, secondary branches of weeping willows located in five regions near the discharge site were collected based on some defined phenotypic characteristics: intense green color, wide coverage, height greater than 8 meters, absence of pests, and straight shaft.Branch cuttings of 20 cm in length were placed in hydroponics [28] in 1 L containers with 300 mL of distilled water.They were kept at room temperature (19-22 °C) for a normal photoperiod (12 h light, 12 h dark).Ten willows per region were placed in jars of 1 L and were then placed in 500 mL of textile wastewater.They remained in contact with wastewater for 15 days, and water aliquots were taken at baseline and at intervals of eight days.Likewise, the development of plants during those time periods was assessed.

Methods of Analysis
3.1.Physicochemical Characterization.The characterization of textile wastewater was performed.During both electrochemical and phytoremediation treatment, COD, BOD 5 , color, turbidity, pH, and electrolytic conductivity analyses were performed as indicated in the standard methods procedures by the American Public Health Association [38].In addition, infrared spectroscopy of the raw and treated water was performed.

Biological Parameters.
Once roots and leaves were developed in hydroponics, they were weighed on an analytical balance (BEL Engineering), and the lengths of the plants and roots were measured using a vernier.The numbers of roots and leaves were counted, and the leaf areas and photosynthetic pigments were measured by using the method that Val et al. [39] and Moisés et al. [40] established.These measurements were performed at the beginning of biological treatment and every eight days.

Results and Discussion
4.1.Wastewater Characterization.Physicochemical chacarcterization of textile wastewater is show in Table 3.
The organic parameters indicate for the BOD 5 a value of 1400 mg/L.According to Mexican regulation, the allowed limit for discharging wastewater into rivers is 150 mg/L.The COD was 2022 mg/L; in this situation, the BOD/COD ratio (0.7) indicates good biodegradability [9].The TOC was 1396.6 mg/L, and the color was 3000 Pt-Co U; this high level of color stemmed from the indigo blue dye in the textile effluent.Regarding inorganic matter, different ions contribute to high conductivity (2.811 mS/cm).This parameter could be beneficial to the electrooxidation process because it was not necessary to add any support electrolyte.However, the presence of ions as nitrates, phosphates, and alkalinity could reduce the oxidation speed of organic compounds; on the other hand, chlorides (843.71mg/L) could improve the indirect organic oxidation.

Electrooxidation Treatment
4.2.1.Current Density Effect.An important operating variable of the electrochemical process is the current density, which is the input current divided by the surface area of the electrode [11].From other variables effective in the electrochemical process is current density as the rate of electrochemical reactions is controlled by this parameter.Further, the performance of electrodes is highly dependent on this parameter.Three different current densities were applied (3.5, 7, and 10 mA•cm −2 ) to investigate the effect in the oxidation process.All experiments were carried out at pH 5.23 (sample pH), and in all cases, a direct effect of the current densities was observed: If the current increases,  5 International Journal of Photoenergy the removal efficiency increases.This could be due to the increased rate of the generation of oxidants, such as hydroxyl radicals and chlorine/hypochlorite at higher current densities [6].The results at different densities are shown in Figure 2. The best removal efficiency was when 10 mA•cm −2 was applied.BOD 5 was reduced considerably from 1400 mg/L to 114 mg/L with 92% of efficiency; COD was 2022 mg/L and was reduced to 356.05 mg/L with 82% of removal efficiency; color was reduced from 3000 Pt-Co U to 5.5 Pt-Co U (99.8% removal efficiency); and initial turbidity was 735 NTU and at the end of the process was 0.65 NTU, achieving 99.99% of removal efficiency.Efficiency was measured during 300 min of treatment time (Table 4).
The instantaneous current efficiency (ICE) for the anodic oxidation was calculated from the values of COD using where F is the Faraday constant (96487 C/mol), V is the volume (L), COD i and COD t are the chemical oxygen demand (g/L) at initial time and time t, I is the applied current (A), Δt is the treatment time (s), and 8 is the equivalent mass of oxygen (g•eq −1 ).The instantaneous current efficiency (ICE) decreased during the electrolysis as wastewater was oxidized.This behavior is shown in Figure 3.The best ICE percentage was when the lowest current was applied 1 A (3.5 mA•cm −2 ) in the middle stage of electrooxidation (15-45 min).This may be attributed to the presence of a higher concentration of organics near the electrodes.This indicates that the electrooxidation was under the current control regime at least in the middle stage of electrooxidation.The ICE decreased after 60 min of the electrooxidation process.This may be due to the depletion of the concentration of organics on the electrode surface.
The energy consumption per volume of treated effluent was estimated and expressed in kWh•m −3 .The average cell voltage during the electrolysis (cell voltage is reasonably constant with just some minor oscillations, and for this reason, the average cell voltage was calculated) was measured to calculate the energy consumption by using [15] where t is the time of electrolysis (h); ΔE c (V) and I (A) are the average cell voltage and the electrolysis current, respectively; and V is the sample volume (m 3 ).According to the results, 5.87 kWh•m −3 is required to oxidize the pollutants in the textile wastewater.In another study, a real textile effluent was treated using a BDD anode, applying a current density of 20 mA•cm −2 .The energy consumption was 20 kWh•m −3 [15].The specific energy consumption (Ec) in kWh•(kg COD) −1 removed was determined according to [37] where U is the mean applied voltage (V), I is the current (A), t is the treatment time (min), V is the liquid volume (L), and COD 0 and COD are the COD values (g O 2 L −1 ) at times 0 and t.The results showed that 21.87 kWh•(kg COD) −1 was required in the electrooxidation process.In a previous work, 95 kWh•(kg COD) −1 was applied for the same COD removal [37].

pH Effect.
The studies were performed at three different initial pH values (5.23, 7, and 10) to investigate their effects as depicted in Figure 4.The current density applied in these experiments was 3.5 mA•cm −2 .At alkaline pH (10), the best efficiencies were achieved: COD (85.9%), color (99.6%),BOD 5 (70.3%), and turbidity (99.7%).However, an addition of NaOH was required to adjust the pH, and this could be a disadvantage in the oxidation process.
The pH solution was an important factor for wastewater treatment.In anodic oxidation, many reports exist on the influence of pH solution, but the results are diverse and even contradictory due to different organic structures and electrode materials [1].In an acidic solution, the degradation process of azo dyes is higher than in a basic solution, as in acidic solutions, chlorides are reduced to free chlorine, which is a dominant oxidizing agent [6].During all experiments, the initial pH decreased during the treatment time (2.3-2.75).This could be attributed to the fragmentation of organic matter into carboxylic acids, carbonic acid, and ions as by-products of mineralization.Figure 5 shows the behavior of the conductivity during the treatment time; it increased at the end of the process probably as a result of the mineralization in the electrochemical oxidation process.

Degradation Mechanism.
Previous research studies [41,42] indicated that the oxidation of organics with concomitant oxygen evolution assumes that both organic oxidation and oxygen evolution take place on a BDD anode surface via the intermediation of hydroxyl radicals generated from the reaction with water shown in Figure 3: Instantaneous current efficiency (ICE) for the anodic oxidation process: 3.5 mA/cm 2 (•), 7.0 mA/cm 2 (▲), and 10 mA/ cm 2 (X). 6 International Journal of Photoenergy Reaction ( 4) is in competition with the side reaction of hydroxyl radical conversion to O 2 without any participation of the anode surface as indicated in Textile wastewater was analyzed via infrared spectroscopy before and after the electrochemical oxidation process, and the spectra are shown in Figure 6.The principal functional groups found in the aqueous solution of dye were -NH-(3305 cm −1 ), the C-H aromatic bond (2910 and 2845 cm −1 ), -NH 3 + (2340 cm −1 ), aromatic -C=C-(1614 cm −1 ), sulfoxides (1101 and 1022 cm −1 ), and C-CO-C in ketones (611 cm −1 ).The spectra of oxidized water showed that the intensity of corresponding bands to sulfoxides and secondary amines diminished after treatment, whereas the bands of R-COOH and O-C=O increased.In accordance with the above, the proposed dye degradation mechanism is shown in Figure 6.

Phytoremediation with Salix babylonica
4.3.1.Textile Wastewater.After oxidation treatment, the oxidized water was placed in contact with plants for 8 and 15 days, as shown in Figure 7. Parameters of the COD, color, and turbidity were minimally reduced.However, at 15 days of contact time, a visible reduction in color and turbidity was noted.According to the results, the plants assimilated better with the pollutants in the raw water than in the oxidized water due to the structural changes that the compounds suffered with the electrooxidation treatment.The coupled system (electrooxidation + phytoremediation)    With respect to the contact time, willow plants reduced their photosynthetic metabolism and lost leaves at eight days of contact time, but after this time, such plants recovered their photosynthetic metabolism to some extent as shown in Figure 9.This could be because willow plants became adapted to the new environmental conditions.The mechanism by which Salix babylonica decreases color and pollutant concentration is unknown, but an increase in the concentration of chlorophylls indicates that the plant is photosynthesizing and thus absorbing nutrients from wastewater.     9 International Journal of Photoenergy Furthermore, the adsorption of contaminants in plant roots has been documented [20].The results may indicate that the willow phenotype of the western region is characterized by very dense foliage and root system, by genetics, or by environmental influence, but in terms of photosynthetic metabolism, they are the same as the willows of the other regions (Figure 10).

Conclusions
Textile wastewater composition was favorable for carrying out electrochemical oxidation due to the high salt content.All experiments were carried out at the original pH (5.23), and it was determined that if the current density was increased, the removal efficiency increased.However, the current efficiency decreased during this process.For this reason, the lower current density was chosen (3.5 mA/cm −2 ) as optimal.The infrared spectroscopy of the wastewater before and after electrooxidation showed a degradation of dye.The proposed degradation mechanism showed carboxylic acids and sulfates as degradation products.In the coupled system, a reduction of the COD was decreased by 14%, color by 85%, and turbidity by 93%.The biomass and pigment of willow Salix babylonica demonstrated that this species has the ability to adapt to adverse conditions very quickly.International Journal of Photoenergy

Figure 1 :
Figure 1: A schematic diagram of the electrochemical reactor.

Figure 5 :
Figure 5: Conductivity behavior during the treatment time at three different pH values.

Figure 6 :
Figure 6: Diagram of the degradation mechanism proposed.Within the figure are shown the infrared spectra of raw water and oxidized water.

Table 1 :
Percentage of removal performance of color, turbidity, nitrogen, and COD by electrochemical or electrochemical with other electrochemical or biological procedures.

Table 2 :
Experimental design of the electrooxidation process.

Table 3 :
Physicochemical characterization of textile wastewater.

Table 4 :
Removal efficiencies of different parameters in the electrooxidation process.