Chaoyi Yin, Jingyuan Ma, Jian Qiu, Ruifang Liu, Long Ba,*

1 State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering Southeast University, Nanjing 210096, China

2 State Key Laboratory of Materials-Oriented Chemical Engineering, National Engineering Research Center for Special Separation Membrane, College of Chemical Engineering,Nanjing Tech University, Nanjing 210009, China

Keywords:

ABSTRACT

1. Introduction

Azo dyes have excellent water solubility and color fixation since one or more azo groups (R1—N=N—R2) attached to the aromatic ring and other functional groups (—NH2, —OH, —COOH, —NO2, —Cl, —SO3) are present in the structure of azo dyes, so they can be widely used in the printing and dyeing industry [1]. And it is also the most widely used synthetic dye in the world, accounting for nearly 70% of the market share of all dyestuffs consumed [2]. In the printing and dyeing process, about 20% of the water-soluble dyes are lost to the printing and dyeing wastewater, which is one of the industrial pollutant headstreams that seriously pollute the environment [3,4]. The dye effluents discharged by the textile industry contain a variety of toxic, carcinogenic, mutagenic, and non-biodegradable substances, which explains why they can wreak havoc on the receiving water body, affect the growth of aquatic organisms and microorganisms, as well as pose a major threat to other organisms, such as humans, animals and plants[5,6].

Biological methods [7–11], physical methods [12–14], and chemical methods [15–19] are currently the conventional treatment methods for azo dyes wastewater. The biological method can not only generate flocculants and oxidizing substances indirectly through microbial metabolism like fungi [8], bacteria [9]and yeast[10]to destroy the unsaturated bonds and chromophore groups of dye molecules,but also use algae[11]to perform directly biological adsorption and flocculation of chromogenic substances in wastewater, thereby achieving the separation and degradation of dye effluents. For instance, adsorption [12], membrane separation [13], and extraction [14] are examples of physical methods for separating and removing insoluble compounds from wastewater. On the one hand, the chemical method is to degrade wastewater by adding organic or inorganic flocculants [15], but on the other, through advanced oxidation processes (AOPs), such as electrochemical oxidation [16], Fenton oxidation [17], photocatalytic oxidation [18] and ozone oxidation [19], the active substance hydroxyl radical with strong oxidizing property is generated to degrade the refractory organic matter in the azo dye wastewater into low-toxic or non-toxic small molecular substances,and effectively destroy the structure of dye molecules. Each method has advantages and disadvantages, and it is vital to pick a suitable way to degrade sewage by considering the level of water contamination and the cost of treatment [20].

AOPs offer the benefits of excellent operability and a better removal rate compared to other strategies for degrading dye effluent including complex organic components and high chroma concentrations [21]. In particular, as an electrochemical oxidation approach, electrocoagulation technology has a long history of study in wastewater treatment and has attracted considerable interest [22]. The British constructed the first electrocoagulation treatment of seawater workshop in 1889,followed by A.E.Dietrich in 1906, who successfully got the patent for electrocoagulation technology in the United States and applied it to the treatment of cabin sewage[23].Electrocoagulation is a wastewater treatment technique that combines flocculation adsorption with redox and air flotation separation and often employs soluble metals such as iron plates, aluminum plates, or stainless-steel plates as electrode materials [24]. The anodic dissolution generates in-situ metallic ions (M(s)) coagulants (Eq. (1)) in conjunction with the generation of hydroxyl ions(Eq.(2))and the hydrogen gas at the cathode(Eq.(3)).Although it is possible to improve wastewater treatment efficiency by combining different materials, such as metal and metal or metal and non-metal,and electrodes modes,such as monopolar electrodes or multipolar electrodes [22,25], there is currently no study showing that wastewater degradation can be achieved by combining metal fibers with fabrics to form fully flexible electronic fabrics.

At the anode:

At the cathode:

Fibers and fabrics are mainly combined by sewing and embroidery. From thousands of years ago to the present, various sewing products and embroidery products have appeared in our country,but large-scale mechanized production of sewing fabrics and embroidery fabrics of machines originate from abroad.Embroidery enables combining more metal fibers on the surface of the fabric to form a flat structure, as compared to the sewing process [26,27].And this flexible electronic fabric with the ability to degrade wastewater could embroider electrode structures of varying sizes on the fabric’s surface in vast numbers using a high-speed and programmable-design industrial embroidery machine to treat wastewater in varying volumes.

In this paper, a flexible electronic fabric integrating 316L stainless steel fibers(316L SSF)on the surface of an insulating fabric by embroidery is proposed for catalytic degradation of wastewater.And the embroidered electrode structure with monopolar arrangement can achieve 99.25% methyl orange (MO) removal rate in 120 min. The kinetic equations and correlation coefficients are used to demonstrate the consistency of our designed structures,and various parameters such as electrode spacing,current density,electrolyte concentration, and pH value are studied to determine the optimal conditions for degradation in a laboratory.The flexible electronic fabric with cathode and anode arranged on the fabric surface can not only realize the degradation of wastewater, but also has the advantages of small footprint, low production cost,and suitable for large-scale wastewater treatment.

2. Materials and Methods

2.1. Raw materials

The 316L SSF was purchased from Dongguan Mingpu Rope Industry Co., Ltd. The white polyester insulating fabric was purchased by Yaner Computer Embroidery Factory. The MO was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.Sodium chloride (NaCl) solid, sodium hydroxide (NaOH) solid,and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals used in the entire treatment were of analytical grade. Ultrapure water (～18 MΩ) from a Milli-Q Plus system (Millipore) was utilized throughout the experiments.

2.2. Experimental procedure

2.2.1. Flexible electronic fabric manufacture

The WILCOM program generates the embroidery layout, which is subsequently imported into the embroidery machine via a flash memory drive (BARUDAN embroidery machine). After threading 316L SSF into the machine,immediately position a white polyester fabric at the embroidery needle and begin stitching. To study the impact of varying numbers of electrodes and interelectrode distance on the MO removal efficiency.Table 1 shows twelve different embroidery flexible electronic fabrics images of monopolar arrangement are 2, 3 and 4 with the interelectrode distance is 0.5-2 cm (step size 0.5 cm), respectively. Here, the horizontal dimensions of all embroidered flexible electronics device electrodes are 6 cm in length and 5 mm in breadth. Theoretically,the length of all samples should be same in the perpendicular direction. So that the area and weight of the underlying embroidered cloth are uniform and the number of current density calculations could be simplified. However, considering the production cost in practical applications, the smaller the electrode spacing,the shorter the vertical electrode length. To save the amount of 316L SSF used in the embroidery process.In addition,the two ver-tical electrodes are primarily used to link the lateral electrodes without regard to their impact on removal efficiency. And compared to other connection techniques, this connection technique has a better rate of pollution removal [28].

Table 1 Flexible electronic fabrics with different embroidery specifications

2.2.2. Electrocoagulation degradation device

Prior research has shown that stainless steel plate as a metal electrode material has a positive impact on wastewater degradation [29,30]. We present the flexible electronic fabric with 316L SSF embroidery for wastewater degradation, which not only has the rapid degradation effect of stainless-steel plate but is also more appropriate for mass-producible low-cost manufacturing. Various types of conductive fibers,such as metal fibers,carbon black fibers,conductive metal compound fibers,and conductive polymer fibers,are now available on the market to meet a variety of applications.According to the mechanism of electrocoagulation degradation of azo dye wastewater, just metal fibers can only be used (including iron, aluminum, copper, etc.). In addition, the 316L SSF is less expensive per kilogram than other varieties of metal fibers, and have been found that other kinds of metal fibers are more prone to breaking during the embroidery process, which stresses the embroidery work.

The electrochemical degradation experiments were carried out in a beaker,where 50 mg MO dissolved in NaCl solution was added.The current was maintained constant by employing a precision DC power supply(HSPY-200-01,0–200 V,0–1 A adjustable).As shown in Fig.1,immerse the embroidered flexible electronic fabric in MO solution, and directly clamp both ends of the embroidered 316L SSF electrodes through the alligator clips connected to the positive and negative electrodes of the DC power supply. Throughout the testing procedure, the MO solution was stirred with a magnetic stirrer at the same speed to accelerate the rate of electrolysis. For the sake of simplifying the experimental process and mainly exploring the effect of electrode distance,electrolyte concentration and pH value on the degradation of MO by electrocoagulation of the designed flexible electronic fabric, the research was carried out at a rate of 200 r?min-1and room temperature (22-25 °C).

2.3. Characterization

SEM(Zeiss)was used to evaluate the surface and morphologies of 316L SSF before and after degradation.During the electrocoagulation treatment analysis, 2 ml of treated MO was collected every 10 min for solution analysis, and the absorbance of the solution after centrifugation was determined using an ultraviolet–visible spectrophotometer 3600Plus (Shimadzu). The change of organic matter content in the process of degrading azo dye wastewater by total organic carbon(TOC)analyzer under optimal external conditions. The TOC results of wastewater treated by electrocoagulation were calculated by Eq. (5) [31]:

Fig. 1. Configuration of the electrocoagulation reactor system.

In addition, the removal efficiency was introduced to examine the degradation of target pollutants, which is defined as follows in Eq. (6) [32]:

where C0represents the initial concentration of the MO solution(mg?L-1), Ctis the MO concentration at arbitrary time t during the treatment.

And Eq. (7) [33] is used to estimate the electrical energy consumption (EEC) associated with the electrocoagulation process per treated volume of wastewater.

where U(V)is the applied voltage,I(A)is the electric current,T(h)is the electrolytic time, and V (L) is the volume of wastewater.

3. Results and Discussion

3.1. Effect of interelectrode distance

Before conducting experiments, each flexible electronic fabric was washed with distilled water and then air-dried at room temperature. In the electrocoagulation process, the distance between the cathode and anode has a considerable impact on the mixing and transport processes. The results in Fig. 2(a) show that when the number of electrodes is 2 and the corresponding electrode spacing is 0.5-2 cm (step size 0.5 cm), the MO removal rates of 400 ml within 120 min are 96.92%,99.07%,92.14%,91.33%,respectively. At a narrow interelectrode distance, the MO removal efficiency can be relatively low due to the generated metal hydroxides,which usually act to form flocs with the MO and aggregate near the anodes and cathodes causing a consequent higher electrical resistance. With the increase in interelectrode distance the hindrance effects may reduce gradually, but after the optimal distance, increasing the distance between electrodes increases electric resistance against the current flowing between anode and cathode, and the MO removal performance decreases because the interaction between hydroxide and MO decreases due to the lower electron transfer rate [34,35]. Owing to we first proposed to integrate 316L SSF fibers onto flexible electronic device fabrics for MO degradation, extensive repeated experiments were performed to verify the feasibility of the designed fabric devices,which is why we embroidered electrodes of different specifications. Here, the quantity of degraded wastewater was determined based on the number of transverse electrodes for embroidery to make the entire transverse electrode completely immersed in the solution, the dye removal efficiency of the same material was tested twice under identical conditions, and error bars were utilized to highlight the consistency of the results. While evaluating the rate of wastewater degradation based only on electrode spacing, the same number of electrodes must maintain the same current density, electrolyte concentration, and pH when degrading the same volume of wastewater. Fig. 2(b) and (c) shows the removal efficiency of 500 ml MO degraded by 3 electrodes and 600 ml MO degraded by 4 electrodes within 120 min. Similarly,when the distance between electrodes is 1 cm, MO removal is the best in both cases, reaching 99.25% and 99.13%, respectively.Previous studies also demonstrated the validity of the results[36,37].Furthermore,we can reasonably embroider the number of electrodes for the amount of wastewater that needs to be degraded in practical applications to reduce the production cost according to the experimental results in Fig. 2(a)–(c).

Fig. 2. Effect of interelectrode distance on MO removal in different number of electrodes (a) 2, (b) 3, and (c) 4 (pH=7.6, [NaCl]=0.1 mol?L-1, current density=15 mA?g-1, room temperature=24-25 °C).

3.2. Materials characterization of 316L SSF

Fig.3 depicts SEM images of the 316L SSF as received and after 120 min of electrolysis, respectively, as well as EDS data corresponding to the fibers in a certain region in both cases. The 316L SSF needs to be rinsed with deionized water before observation to prevent the resulting flocs from adhering to the fiber surface.There are grooves and a small number of attachments on the surface of the electrolyzed 316L SSF,which is caused by the precipitation of metal after electrolysis. And the metal crystals of chloride attached to the fiber surface and some impurities in the medium.The EDS results suggest that the 316L SSF contains the elements C, Cr, Si, Mn, Ni, Cu, Mo, S, and Fe elements. Comparing Fig. 3(a)with (b) reveals that the metal content of 316L SSF after 120 min of MO degradation is lower than that of the original 316L SSF,confirming the premise that the electrocoagulation method degrades MO via metal ions, primarily Fe. By applying a certain magnitude of current density on the fiber electrode, the metal is dissolved in the solution to form metal ions, which is also consistent with the results of the previous study using metal plates as the electrode material in the electrocoagulation process [38]. Fe turns into Fe(OH)nduring electrocoagulation, where n = 2 or 3, includes two mechanisms [39]:

Fig. 3. SEM images and EDS data of 316L SSF, (a) original material, (b) electrocoagulation for 120 min (pH=7.6, [NaCl]=0.1 mol?L-1, interelectrode distance=1 cm,electrodes number=3, current density=15 mA?g-1, room temperature = 24-25°C).

Mechanism 1

At the anode:

Overall:

Mechanism 2

At the anode:

At the cathode:

Overall:

For the metal plate electrode,it is unnecessary to explain from a microscopic perspective that the appropriate voltage and current density can be used to recycle the electrode that employs the electrocoagulation principle to degrade azo dye wastewater. The electrocoagulation function will be lost unless the electrode is completely dissolved. But for the 316L SSF, macroscopic observation is impossible due to the selection of insulating fibers as the substrate during the production process. It is necessary to analyze the material through microscopic analysis to further illustrate that the selected degradation conditions can enable the material to be recycled. Through the SEM images and EDS data of the observed region, we would like to highlight the 8% decrease in iron content in 316L SSF (the errors of repeated observations in other regions are within 8%±1%)under ideal degradation circumstances,suggesting the reasonableness of our choice of current density while it is being recycled as we first proposed to use 316L SSF as fabric electrodes for wastewater degradation.

Fig. 4. Effect of current density on MO removal efficiency (pH=7.6, [NaCl]=0.1 mol?L-1, interelectrode distance=1 cm, electrodes number=3, room temperature=23-24 °C).

3.3. Effect of current density

The current density was found to be one of the most important parameters of the electrolytic process as it determines the coagulant dosage, bubble production rate, size and growth of the flocs,and the kinetics of the reaction. That is to say, the current density can influence the treatment efficiency of the electrocoagulation technology.As shown in Fig.4(a),we investigated the degradation rate as a function of time at different current densities from 5-25 mA?g-1(step size 5 mA?g-1). Unsurprisingly, the best removal efficiency was obtained at the same degradation time when the maximum current density was 25 mA?g-1. According to Faraday’s law, which describes the relationship between the current density and the dissolved metal in the electrode, the greater the current density is, the more metal is solved on the anodes[35,40]. After 60 min, increasing the current density above 15 mA?g-1did not increase the MO removal efficiency, because a sufficient number of metal hydroxide flocs are available for the sedimentation of the pollutant.

According to the experimental results and similar studies[36,41],the reaction rate r of MO can be represented by the following nth order reaction kinetics Eq. (8) [42,43]:

When n = 1,the relationship between the removal efficiency of MO with time at different current densities was further characterized by the results of the first-order kinetic model, the kinetic model of Eq. (8) becomes

in which k is the reaction rate constant,C0is the initial MO concentration,and Ctis the MO concentration at arbitrary time t during the treatment.According to Eq.(9),a plot of ln(C0/Ct)versus t will yield a straight line with a slope of k. Fig. 4(b) shows that the first-order kinetic model corresponding to the MO removal efficiency at different current densities is well fitted to the experimental data under the condition of fixed solution pH, electrolyte concentration, interelectrode distance, and electrode number. All correlation coefficients R2are steady within the range of 0.9725–0.9946, and the rate constant values of k exhibit a positive correlation (Table 2).

Table 2 R2 and k values of various influencing variables under different parameters

3.4. Effect of electrolyte concentration

To maintain the consistency of the experiment,the flexible electronic fabric with 3 electrodes and 1 cm interelectrode spacing was chosen to degrade 500 ml MO in two hours. Similar to the approach used to analyze the removal efficiency of MO based on the current density. As shown in Fig. 5(a), the color removal efficiency did not increase significantly with the increase in NaCl concentration throughout the degradation process. Because decrease the applied voltage with the increase of the solution conductivity at a constant current density[36,43].That is to say,we can reasonably choose the applied voltage according to the conductivity of the solution to reduce the energy consumption in the actual application process. An increase in the NaCl concentration not only increases the ionic conductivity but also produces strong oxidative by-products (such as hypochlorous acid and hypochlorite formation) due to the presence of chloride ions, thus corresponding to the higher removal efficiency in a relatively short time [44]. The experimental results fitting to the first-order reaction kinetics of the MO degradation are presented in Fig.5(b).To analyze the effect of different electrolyte concentrations on the removal efficiency of MO under the conditions of a constant current density and solution pH value, the experimental results summarized in Table 2 reveal that the k values are essentially identical, and R2exhibits a very strong correlation.

3.5. Effect of initial pH

Fig. 5. Effect of NaCl concentration on MO removal (pH = 7.6, interelectrode distance = 1 cm, electrodes number = 3, current density = 15 mA?g-1, room temperature = 24-25 °C).

Fig.6. Effect of initial pH on MO removal([NaCl]=0.1 mol?L-1,interelectrode distance=1 cm,electrodes number=3,current density=15 mA?g-1,room temperature=22-23°C).

The pH of MO solution is a crucial factor that affects electrocoagulation processes. Here, the solution pH without any treatment here is 7.6 and the desired pH range is 3–11 adjusted by 0.1 mol?L-1NaOH and 0.1 mol?L-1HCl to study the effect on the removal efficiency.As shown in Fig.6(a),the removal effect is rather modest at pH 3 and 11, which corresponds to 89.23% and 87.74%, respectively. In addition, a higher color removal efficiency of 99.25%was obtained in a neutral solution. On the one hand, in acidic and alkaline solutions, the reduction of protons in the solution at the cathode produces a large amount of hydrogen, resulting in the reduction of hydroxide ions in the solution and the formation of fewer flocs. On the other hand, the removal effectiveness is diminished as a result of the combination of dissolved metals with a significant number of hydroxyl radicals to produce additional hydroxides without adsorption. The results are consistent with previous studies [30,45]. And according to experimental results presented in Fig. 6(b) and Table 2, the k value is 0.0373 when the pH value is 7.6 without correction.The electrocoagulation reaction was inhibited by the addition of NaOH and HCl, resulting in a somewhat considerable change in the k value. Similarly, there is a strong association between the degradation period under varied pH and the removal effectiveness. Therefore, the ideal pH range is between 5 and 9, which can give a cost-effective way of treating sewage in practice.

3.6. Performance analysis of the fabric electrode under optimal degradation conditions

The MO solution has a maximum absorption band at ～464 nm.Fig. 7(a) shows the corresponding UV–vis spectra of MO during electrocoagulation with time. Combined with Fig. 7(b), it can be seen that it gradually becomes a colorless solution within 2 h(step size 10 min), which is also confirmed by the gradual reduction of the absorption band at 464 nm.As shown in Fig. 7(c),the removal efficiency of the designed flexible electronic fabric electrode slightly decreased from 99.25% to 95.26% after 10 repeated cycles,which may be due to the adsorption of flocs on the electrode surface to reduce the precipitation of electrode metal ions.It is worth noting that due to the degradation of MO at the same current density, the applied voltage also increases with the increase in the number of degradations, which increases energy consumption.But we can provide enough metal ion precipitation to prevent energy loss by embroidering wider and more electrodes or using multiple identical devices for degradation treatment in the same sewage environment.

TOC is the quantity of organic matter in water as reflected by its carbon content. The fabric electrode with the electrode spacing of 1 cm and the number of electrodes of 3 was used to solve 500 ml MO under the optimal condition to observe the change of TOC with time,which further proved that the fabric electrode designed by us is suitable for the practical treatment of organic wastewater. As shown in Fig.8(a),TOC removal efficiency was close to 90%within 120 min.In addition,Fig.8(b)illustrates the rate of removal of MO with varying initial mass concentrations within 2 hours under the aforementioned settings.Results indicate that when mass concentration decreases, removal efficiency increases over the same period. In actual applications, extending the degradation period for high-concentration wastewater can improve degradation efficiency.

Fig. 7. Absorption spectra (a) and corresponding colors (b) of MO at different electrolysis times, reusability study of the fabric electrode (c) (pH=7.6, [NaCl]=0.1 mol?L-1, interelectrode distance=1 cm, electrodes number=3, current density=15 mA?g-1, room temperature=23-24 °C).

Fig. 8. (a) TOC removal of 500 ml 50 mg?L-1 MO, (b) effect of initial mass concentration on 500 ml MO removal (pH=7.6, [NaCl]=0.1 mol?L-1, interelectrode distance=1 cm,electrodes number=3, current density=15 mA?g-1, room temperature=23-24 °C).

Table 3 shows the performance comparison between our proposed structure and the previously proposed structure for the treatment of azo dye wastewater by electrocoagulation. In this case, the decomposition of 500 ml MO under optimal conditions corresponds to a current of 0.04 A and a voltage of 3 V, respectively. According to Eq. (7), the EEC under the condition of 2 h of total degradation time is 0.48 kW?h?m-3. Compared to the previously suggested electrocoagulation approach for degrading wastewater, the calculated energy consumption is very small.The total cost calculation for the degradation of azo dye wastewater of the designed fabric mainly includes EEC cost, embroidery cost,bottom insulation fabric cost,and 316L SSF cost.For the structure with 3 embroidery electrodes,because the overall surface area is relatively small, about 0.014 m2, the embroidery processing fee and the cost of the underlying insulating fabric are about USD 0.23,and the 316L SSF used is about 3 g,corresponding to the cost of 0.39 USD, and the local price is 0.079 USD?(kW?h)-1. Therefore,the total cost of treating MO is about 0.699 USD?m-3. In addition,the 316L SSF fabric proposed in this paper is much lighter than the metal plate electrode with the same structure [38,40], which may lower the transportation costs in practical applications.

4. Conclusions

In this study, 316L SSF was combined with insulating fabrics through an embroidery process and the MO solution was degraded using the principle of electrocoagulation. The degradation of 50 mg?L-1MO solution may reach 99.25%within two hours under the circumstances of 15 mA?g-1current density, 1 cm electrode spacing, 0.1 mol?L-1NaCl, and pH 7.6. And this low-cost flexible electronic fabric device can be mass-producible by machine. The results of the color removal efficiency over time under different influencing factors are similar to those obtained by the previously proposed structure, which further verifies the feasibility of the designed combination. Kinetic investigations were conducted,and the outcomes at various current densities, electrolyte concentrations, and pH revealed that the degradation followed pseudofirst-order kinetics with correlation values ranging from 0.9241 to 0.9946. Reasonable embroidery design electrode size and number enables the device to be recycled repeatedly,which has a high practical value.

Data Availability

Data will be made available on request.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Contributions

Chaoyi Yin: Methodology, Data curation, Writing. Jingyuan Ma:Provide equipment. Jian Qiu: Supervision. Ruifang Liu: Validation.Long Ba: Conceptualization.

Table 3 Performance comparison between the proposed structure and previously proposed structure for the treatment azo dye wastewater by electrocoagulation

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (31872901) and Major State Basic Research Development Program of China (2016YFA0501602).