Influence of electrochemical pre-treatment in dyeing wastewater reuse for five reactive dyes

Abstract

In this study, the influence of an electrochemical pre-treatment was evaluated in dyeing wastewater reuse for five reactive dyes for cotton fabrics. The most proper dyeing method was established and the influence of alkali was also studied. Wastewater was treated in an electrochemical cell at 20 Lh⁻¹ and 40 mA/cm² for between 3 and 15 minutes before being reused. During the electrochemical treatment the evolution of the dye degradation was evaluated by COD measurements. From the results, it can be concluded that 70% of the wastewater can be satisfactorily reused by direct bath reuse with most of the reactive dyes obtaining DE (CMC (2:1)) values below 1 with respect to the reference. Differences of DE (CMC (2:1) can be improved up to 75% by applying an electrochemical pre-treatment. In DE (CMC (2:1)) values, DL* was found to be the most influential parameter. Finally, the influence of the level of dye degradation in the wastewater was observed as non-relevant in the direct reuse of the bath for most of the studied dyes.

Keywords

Direct bath reuse (DBR)reactive dyes textile dyeing organic matter removal electrochemical pre-treatment direct bath reuse (DBR)

Introduction

The textile industry uses and rejects high amounts of water, wastewaters being the main way by which dyes are discharged into the environment. Textile effluents have high concentrations of organic and inorganic compounds and strong color, caused by residual dyes that were not fixed to the fibers during the dyeing process, and become evident because of the following main characteristics: high variations of flow and composition, the presence of non-biodegradable compounds and toxic substances, high temperature and high pH value.¹ In the last few years, the use of bifunctional reactive dyes in the cotton dyeing process has increased rapidly. These dyes produce a higher exhaustion level which implies less residual hydrolyzed dye in solution after the dyeing process.

Environmental pressures on the textile industry and the consumption of water and energy in this sector, lead to the adoption of measures that directly influence the industrial processes. In order to reduce the environmental impact, optimizing the process to improve exhaustion level and/or the reuse of final baths with or without intermediate treatment, are some of the measures that have been applied.² Two fundamental methods of control are wastewater treatment and source reduction. The latter is very attractive in many cases since it avoids cost and liability involved in waste disposal.

On one hand, the most common wastewater treatments to remove color consist of oxidation techniques such as electrochemical treatments alone or assisted by UV,^3–6 ozone treatments,^7,8 Fenton processes;^9,10 physico-chemical techniques such as the use of adsorbents like alumina, silica gel and activated carbon^11–13 and coagulation-floculation treatments;¹⁴ and membranes, mainly nanofiltration membranes.^15,16 These treatments usually focus on color removal from the textile dyeing wastewaters.

On the other hand, in recent years, the textile industry has increased the efforts to optimize the processes and also to reuse water.

Most of the studies that improve or optimize textile effluents are focused on the current dyeing processes¹⁷ mainly for cotton dyeing,^18–20 wool,²¹ cotton-wool mixtures²² and acrylics.²³

The studies on water reuse are based on nanofiltration, ultrafiltration, reverse osmosis and their combinations.^24–33 These techniques usually allow the reuse of salts.^34,35 Other techniques used for the wastewater reuse are ozonation,^36,37 physico-chemical treatments^38–40 and electrochemical treatments.^41,42

Other important source reduction treatment is the direct bath reuse (DBR). DBR was studied by Cook and Tincher for disperse-dye mixtures in polyester.⁴³ Their experiments demonstrated that multicolor sequences that are not in a color line can be dyed satisfactorily if at least one of the dyes overlaps and if the shade series are chosen judiciously. Further investigations carried out by Cook et al.⁴⁴ concluded that DBR can be feasible in multishade sequences (light-medium-dark) with an acceptable color correlation to their standards.⁴⁵ When performing DBR, it must be taken into account that some water and auxiliaries must be added to the dyeing to obtain a specific volume because there are some losses when the dyed material is taken out of the exhausted dye bath as some liquor is physically retained in the fabric.⁴⁶ Besides this, certain substances can accumulate in the dye bath.⁴⁷

Despite the numerous studies of wastewater treatment, only a few propose specific uses for the reused water. Wastewater is proposed to be reused as process and cleaning water^48,49 or in new dyeing processes⁵⁰ although no dyeing results are given.

In this work, bath reuse was studied for five reactive dyes as well as the influence of an electrochemical pre-treatment of the reused wastewater for new dyeing. The electrochemical process permits the treatment of the wastewater at the original pH, contrary to Fenton process, there is no loss of salts like with some filtration processes, it does not generate sludge as in the case of coagulation-flocculation processes and uses a ‘clean reactive’ electron transfer, differing for instance from the activated sludge. Moreover, electrochemical treatment allows fast and high organic matter degradation of the effluents.

DBR was performed for each dye in the study. Electrochemical treatments were conducted to determine the influence of the presence of organic matter removal in subsequent dyeing. For that purpose, the effluents to be reused were treated in the electrochemical cell at different time expositions. The extent of dye degradation was determined by COD measurements and its influence was evaluated in dyed fabrics, as mentioned before, by color measurement comparisons with a standard. All experiments were performed reusing 70% of the wastewater, the other 30% is assumed to be lost during the dyeing process.

The goal of this study is to minimize the water and salt consumption in the dyeing process by DBR in the 70% reused wastewater and to establish whether color removal, dye degradation or both are required to reuse dyeing wastewater.

Material and methods

Dyes and fiber

Five reactive dyes, kindly provided by DyStar, were selected: Remazol Black B 133% (C. I. Reactive Black 5), reactive group bissulfatoethylsulfone and a diazo chromophore; Procion Crimson H-EXL, Procion Navy H-EXL, Procion Yellow H-EXL and Procion Blue H-EXL, all of them with a bismonochlorotriazine reactive group and an azo chromophore. These five dyes will be referred to in the text as: RB5, PC, PN, PY and PB, respectively.

Figure 1 shows the available structures of the dyes.^51–54 PN and PY are a blend of two monomers. The formula corresponding to PB has not been published currently.

Dyeings were performed on 100% cotton plain interlock knitting fabrics (10 g) kindly provided by TIPSA.

Reactive dye bath effluent preparation

Simulated dye bath effluents were prepared in the laboratory in soft water at 0.1 gL⁻¹ of the commercial dyes, previously hydrolyzed to simulate an industrial effluent. The hydrolysis was carried out as follows: pH was adjusted to 12 with NaOH (Fluka) and then the solution was heated at 80 °C for two hours. After the hydrolysis, it was allowed to cool down to room temperature and the pH was adjusted to 9 with sulfuric acid (Scharlau).

Sodium sulfate (Scharlau), 40 gL⁻¹, was added to the solution to simulate the conductivity of industrial effluents after the dyeing process with reactive dyes.

Dye degradation in the effluent

Effluents to be reused underwent an electrochemical treatment in order to degrade the organic matter present in them. These treatments were conducted in an undivided electrolytic cell, ECO 75 (ELCHEM, Germany). In Figure 2, the outline system associated with the electrolytic cell ECO 75, and the cell itself, can be seen. Cathodes were constituted of titanium and the anodes were made of titanium covered by platinum oxides. The total surface of each electrode was 486 cm². The volume of the vessel was 1 L. The plant operated continuously at 20 Lh⁻¹ and 40 mA/cm². Each effluent underwent the electrochemical treatment five times. The retention time in the reactor was calculated as 3 minutes.

The evolution of the dye degradation was followed by COD measurements⁵⁵ as follows:

(1)

where COD_i corresponds to the initial value and COD_t is the value at time t.

It must be taken into account that dyes will not be the only organic compounds in the wastewater. Therefore COD differences can be attributed to the dyestuff degradation and also degradation of other organic compounds released from the fabric, namely polysaccharides and residual lubricants.

Dyeing method and chlorine removal

Dyeing experiments were performed in a Linitest apparatus (Original Hanaut Heraeus) under the following conditions: 10 g of cotton fabric; 3% o.w.f. of dyestuff; liquor ratio 1/10 and 40 gL⁻¹ of Na₂SO₄ in the effluent.

Two different dyeing methods were studied, as well as the influence of the alkali, used to adjust the dyeing pH. Table 1 summarizes the evaluated conditions. All experiments were run in triplicate. In Figure 3, the dyeing method conditions are schematized.

Table 1.

Dyeing methods, alkali used and replicates

Replicate	Method	Alkali
1, 2, 3	Isothermic	NaOH + Na₂CO₃
4, 5, 6	Isothermic	NaOH
7, 8, 9	All in	NaOH + Na₂CO₃
10,11,12	All in	NaOH

After electrochemical treatment, free chlorine present in the effluents was measured with a Chlorine Kit (Chemetrics) and removed by the stoichiometric addition of NaHSO₃. The chlorine detection limit of the method was 0.1 ppm.

The cleaning process of the fabrics was divided in ten steps as indicated in Figure 4. All steps (rinsing and soaping) were performed at a liquor ratio of 1:10. Steps 4 and 7 correspond to soaping with 2 gL⁻¹ of Cotemoll T-LTR kindly provided by Color Center.

Color evaluation

Fabric color was measured using a Macbeth Color Eye 7000A spectrophotometer, with the following measuring conditions: illuminant D65 and 10° standard observer.

The CMC (l:c) color difference formula was used as a tolerance system, which allows tolerance ellipsoids to be calculated around the target where the dimensions of the ellipsoid are functions of the position in color space of the target. The standard corresponds to the fabric dyed in the original dye bath without reuse.

The color difference is composed of three components that compromise the difference between the standard and the reused dyeing. These are:

Lightness component that is weighted by the lightness tolerance (ΔL*/lS_L). This is represented as ΔL_cmc. If the ΔL_cmc is positive, the reused dyeing is lighter than the standard. If the ΔL_cmc is negative, the reused dyeing is darker than the standard.

Chroma component that is weighted by the chroma tolerance (ΔC*_ab/cS_c). This is represented as ΔC_cmc. If the ΔC_cmc is positive, the reused dyeing is more chromatic than the standard. If the ΔC_cmc is negative, the reused dyeing is less chromatic than the standard.

Hue component that is weighted by the hue tolerance (ΔH*_ab/S_H). This is represented as ΔH_cmc. If the ΔH_cmc is positive, the hue difference of the reused dyeing is anti-clockwise from the standard in the CIELAB a*, b* diagram. If the ΔH_cmc is negative, the hue difference of the reused dyeing is clockwise from the standard in the CIELAB a*, b* diagram.

The CMC color difference between the standard and another fabric (reused dyeing) is indicated in equation 2:

(2)

Calculate the ellipsoid semi-axes from the L*_R, C*_ab,R and the h_ab,R of the standard as follows:

S_L = 0.040975L*_R/(1 + 0.01765L*_R) if L*_R ≥ 16

Or S_L = 0.511 if L*_R < 16;

S_c = (0.0638C*_ab,R/(1 + 0.0131C*_ab,R)] + 0.638;

S_H = (FT + 1 − F)S_c

where

F = ((C*_ab,R)⁴/((C*_ab,R)⁴ + 1900))^1/2;

T = 0.36 + |0,4cos(35 + h_ab,R)| if h_ab,R ≥ 345° or h_ab,R ≤ 164°

Or T = 0.56 +| 0,2 cos(168 + h_ab,R)| if 164° < h_ab,R < 345°.

The values l and c used in the CMC (l:c) color difference formula are l = 2, c = 1 according to the recommendation for acceptability color differences,⁵⁶ and the values of ΔE*_ab = 1, 2 and 3 are the limits used to calculate the tolerance ellipsoid.²

Results and discussion

Dyeing method and alkali selection – determination of reference fabrics

In this section, the dyestuff, PC, was selected to determine the best way to reuse dyeing waste water. For that purpose, two different dyeing methods, Isothermic and All-in and two different alkalis systems, NaOH and a mixture of NaOH + Na₂CO₃ (5 gL⁻¹ both) were studied. All experiments were run in triplicate and numbered as indicated in Table 1. Prior to establishing the final proper dyeing conditions, the standard replicate for each condition must be determined. For this purpose, color coordinates (L*, C*_ab, h_ab) were measured and color differences (DE*, DL*, DC*_ab and DH*_ab) were calculated and compared between replicates. DE*, DL*, DC*_ab and DH*_ab were respectively compared between reused dyeing and the standard. As an example, Table 2 shows the values of these parameters for the isothermic method when a mixture of NaOH + Na₂CO₃ was used as the alkali.

Table 2.

Comparison of DE*, DL*, DC*_ab and DH*_ab values between the three replicates for the isothermic method and NaOH + Na₂CO₃ alkali

DE*	replicate			DL*	replicate
replicate	1	2	3	replicate	1	2	3
1		0.57	0.18	1		−0.52	−0.08
2	0.58		0.46	2	0.53		0.44
3	0.18	0.46		3	0.09	−0.44

DC*_ab	replicate			DH*_ab	replicate
replicate	1	2	3	replicate	1	2	3
1		0.11	0.01	1		0.2	0.16
2	−0.11		−0.11	2	−0.21		−0.05
3	−0.01	0.11		3	−0.16	0.05

From this table, it can be concluded that color differences between replicates are very low and due to experimental system limitations in the measurements. Besides this, DE* variations are mainly due to the influence of DL*. Taking all this into account, replicates number 1 and 3 are the most alike, with number 1 being selected as the standard for these dyeing conditions. The same criterion was used to select the other references for each dyeing process and alkali system. Replicates 1, 5, 8 and 11 were finally selected. Figure 5 shows a photograph of the standards selected.

From the photograph, the influence of the alkali on the final result can be appreciated. Standards 1 and 8 were obtained when a mixture of NaOH + Na₂CO₃ was used while standards 5 and 11 were obtained when only NaOH was used as alkali. These last two standards show a higher intensity of color.

At this point, the comparison between methods and alkalis was done. Table 3 compares the results of these four samples. Following the same methodology as stated before, from the comparison of these results it can be concluded that different results are obtained depending on the dyeing method and alkali used. Again, DL* is the most influential parameter in DE*. Apart from this, it is possible to state that the dyeing method (Isothermic or All-in) has a greater influence on the results than the alkali system (NaOH or NaOH + Na₂CO₃). To sum up, the best results in terms of color intensity were obtained when the All-in method and NaOH alkali were used, which correspond to sample 11.

Table 3.

Comparison of DE*, DL*, DC*_ab and DH*_ab values between standards for each method

DE*	Isothermic		All-in		DL*	Isothermic		All-in
	1	5	8	11		1	5	8	11
1		5.25	2.33	6.04	1		−3.94	−1.82	−4.61
5	5.7		3.22	0.89	5	4.37		2.34	−0.75
8	2.41	3.07		3.9	8	1.91	−2.21		−2.91
11	6.67	0.91	4.16		11	5.23	0.76	3.16

DC*_ab	Isothermic		All-in		DH*_ab	Isothermic		All-in
	1	5	8	11		1	5	8	11
1		1.83	0.99	1.93	1		2.96	1.06	3.4
5	−1.76		−0.81	0.1	5	−3.2		−2.05	0.48
8	−0.97	0.82		0.92	8	−1.1	1.97		2.43
11	−1.85	−0.1	−0.9		11	−3.71	−0.48	−2.55

Once the proper dyeing method and alkali system are selected, the standards of the other four dyes were also determined using the same methodology indicated at the beginning of this section (comparison of three replicates).

Dye degradation and energy consumption

Degradation of the hydrolyzed residual dye was evaluated by COD measurements for all the dyes. In Table 4, COD values are listed and their % removal with respect to the initial values.

Table 4.

COD values and % of dye removal for PY, PN, PC, RB5 and PB

Electr.	Yellow		Navy		Crimson		Black		Blue
Time (min)	COD	% COD removal	COD	% COD removal	COD	% COD removal	COD	% COD removal	COD	% COD removal
0	83.2	−	59.1	−	159.1	−	154.6	−	173.0	–
3	73.3	12	49.2	17	138.7	13	94.5	39	60.8	65
6	55.5	33	46.5	21	106.0	33	74.1	52	46.9	73
9	39.6	52	44.7	24	73.4	54	56.3	64	38.3	78
12	29.7	64	43.2	27	57.1	64	43.6	72	29.6	83
15	25.8	69	29.5	50	40.8	74	34.5	78	26.1	85

As can be seen, over 70% of organic matter removal was achieved after 15 minutes of electrochemical treatment for almost all dyes. Short electrochemical treatments remove residual color of the wastewater and longer treatments imply the gradual mineralization of the dye into CO₂ and water. In this study, it will also be important to establish whether color removal, dye mineralization or both are required to reuse dyeing wastewater.

From the results, it is important to note that different behavior was appreciated for the dyes. PY, PN and PC were initially difficult to degrade, between 12% and 17% of dye removal was reached in three minutes, while 39% and 65% were achieved in the case of RB5 and PB, respectively. This can be understood as PY, PN and PC structures are very stable in comparison to RB5 and PB that are easily degraded.

Another factor in dye degradation could be the size of the dyes. From Figure 1, it is possible to see that PY, PN and PC are large dye molecules while RB5 is smaller. In this sense, larger dyes are probably more difficult to degrade than small ones. For the rest of the dyes, PN and PC can be seen to be large as they contain many benzene groups. The structure of PB is not available; it is, however, probably an anthraquinone dye, which are easily degraded.

In the electrochemical treatments, the degradation of the dyes is due to the direct and indirect oxidation. Direct oxidation takes place on the anode surface while indirect oxidation is due to the action of radical species formed at the anode.⁵⁷ Indirect oxidation is usually more effective than direct oxidation.⁵⁸ The main radical species involved in the indirect oxidation are the hydroxyl radical (E° = 2.8 V); chlorine species such as hypochlorite (E° = 1.49 V), chlorine (E° = 1.36 V) and chlorine dioxide (E° = 1.27 V); and hydrogen peroxide (E° = 1.87 V).

Although the hydroxyl is the most oxidizing radical, the chlorine radicals play a very important role in the electrochemical treatment as their presence is needed for the electrochemical treatment to be efficient [57, 6]. Therefore, the degradation mechanism of the dye is mainly due to the action of hydroxyl radicals and chlorine radicals. The influence of dye degradation in the reuse of wastewater will be discussed in the following section.

The proposed general mechanism for the electrochemical treatment of the dye is the following:

(3)

The OH^• is the most oxidizing and is the main radical of the degradation reaction of the dye. However, the chlorine radicals play a very important role in the electrochemical treatment, as the presence of chlorine is needed for the treatment to be efficient.^6,57 Therefore, the degradation mechanism of the dye is mainly due to the action of hydroxyl radicals and chlorine radicals. The influence of dye degradation in the reuse of wastewater will be discussed in the following section.

In order to estimate the energy in terms of Wh⁻¹L⁻¹ for dye solution consumption in the electrochemical treatment of the dye solution, the following statistical study was performed.^59,60 Seven variables were evaluated:

X1: flow (5–25 Lh⁻¹)

X2: potential (10–15–20 V)

X3: pH (5, 7, 9)

X4: conductivity (20000 μScm⁻¹, 35000 μScm⁻¹, 50000 μScm⁻¹)

X5: temperature (25 °C; 37.5 °C; 50 °C)

X6: dye concentration (0.1 gL⁻¹; 1 gL⁻¹; 2 gL⁻¹)

X7: electrolyte (Na₂SO₄ or NaCl)

The conclusion of the study was that the energy consumption to treat 1 liter of the dye solution was dependent on potential, conductivity and flow. The energy consumption equation is the following:

(4)

As can be observed the energy consumption of the electrochemical treatment increases when potential and/or conductivity increase and decreases when flow increases.

Behavior of reused dyeing baths with the five dyestuffs

In this section, the possibility of reusing dyeing wastewater was studied. For that purpose, the influence of an electrochemical pre-treatment was evaluated. In all experiments, 70% of wastewater was were reused, and to the remaining 30% of clean water, the dye and required salts were added. The dyeing conditions were the same for the standards and the later replicates. References were dyed in 100% soft water.

In Figure 6a and 6b , the evolution of DE (CMC (2:1)) for every dye, at different electrochemical pre-treatment times, with respect to the reference, can be seen. In the figure, the DE* tolerance limit value of 1 is dotted. Value differences, between the reference and the sample, of below 1 are considered acceptable.

From Figure 6a, it can be seen that in the majority of dyes: PC, PN and RB5, the DBR and all electrochemical pre-treatment times are below a DE (CMC(2:1)) of 1 and therefore, all samples are acceptable and not different from the standards.

An important fact of the evolution of DE* versus electrochemical treatment residence times, in the cases of PC, RB5 and PN, is their trend versus time. For these dyes, after 3 minutes of electrochemical pre-treatment DE* values tend to remain constant and lower than when no electrochemical pre-treatment was applied to the reused wastewater.

In the case of PY, Figure 6a shows that DBR without electrochemical pre-treatment provides DE* values, with respect to the reference, of below 1, which is acceptable. In this case, the application of only 3 minutes of electrochemical pre-treatment, to the wastewater to be reused, reduces DE* values up to 56%. Longer electrochemical pre-treatments are not advisable as DE* values rise over 1 and their tendency was to increase.

For the PB dye, similar results as for PY were obtained. The best DE* value was also achieved after 3 minutes of electrochemical pre-treatment. Neither DBR without electrochemical pre-treatment nor longer electrochemical pre-treatments provided acceptable DE* values and results did not exhibit a clear trend.

The different behavior of PY and PB, with respect to PC, RB5 and PN, can be attributed to their high sensitivity to chlorinated water in their dyeing processes.⁶¹ It must be considered that chlorine was removed from the wastewater by the use of a kit based on a visual comparison and therefore, this can imply an experimental error. The influence of small amounts of residual chlorine in the dyeing liquor is much greater for PY and PB, compared to PC, RB5, and PN, as expected taking into account the chlorinated water sensitivity.

Another possibility is that the residual organic products coming from the chemical destruction of the dye interfere in new dyeing as they can physically fix to the fiber which may hinder their chemical fixation. It is also thought that the new species and/or radicals, products of dye degradation, could react with dyes added in the new process. A further study is required to evaluate the type of interference.

However, with respect to the dye degradation extent, it was observed that, in general, this is not an influential factor in DE* values, therefore the removal of color from the wastewater is enough. However, the formation of species and/or radicals during the electrochemical pre-treatment that can react with the dye, modifying its characteristics and properties, could be further studied especially in the case of PY and PB.

As mentioned previously, DL* has a greater influence on DE* than DC*_ab and DH*_ab. As an example, Figure 7 shows the contribution of DL*, DC*_ab and DH*_ab values to DE* for RB5.

From this figure, it is possible to state that DE* and DL* parameters present very similar and almost parallel tendency. The rest of the parameters that influence DE*, DC*_ab and DH*_ab, have lower values and, for instance, in the case of DH*_ab also have a different tendency than DE*. When electrochemical pre-treatment is short, DH*_ab has a very low influence on DE*, but increases its influence for longer treatments.

Further characterization of the dyed fabrics and replicates is shown in the next section.

Fabric characterization

In order to evaluate the quality of the fabrics dyed with the reused wastewater, the washing and rubbing color fastness tests and the color uniformity of the fabrics were determined.

Washing color fastness tests were conducted according to ISO 105-C06:2010⁶² and in all the cases, for all the dyes and time of electrochemical treatments, all the changes in color and staining on cotton and wool were the same; 4–5 on the gray scales, which means no influence in the wash fastness of dyeing. Rubbing color fastness tests were conducted according to ISO 105-X12:2001.⁶³ Results are shown in Table 5, where a little difference can be found and of the order of 0.5 points on the gray scale for staining. Color uniformity of the fabric was determined for each replicate by measuring the color difference from the mean at four different corners of each fabric. The mean color difference from the mean (MCDM) was calculated for each dye and time interval and the results are shown in Table 6.

Table 5.

Rubbing color fastness tests values of the dyed fabrics with electrochemical pre-treated wastewater for the PC, PN, PY, RB5 and PB dyes

Dye	Electr. treatment duration (min)	Staining
		Dry	Wet
PC	0 (ref)	4–5	4–5
	0	4–5	4
	3	4–5	4
	6	4–5	4
	9	4–5	4
	12	4–5	4–5
	15	4–5	4–5
PN	0 (ref)	4–5	4–5
	0	4–5	4
	3	4–5	4
	6	4–5	4
	9	4–5	4–5
	12	4–5	4–5
	15	4–5	4–5
PY	0 (ref)	4–5	4–5
	0	4–5	4
	3	4–5	4
	6	4–5	4–5
	9	4–5	4–5
	12	4–5	4–5
	15	4–5	4–5
RB5	0 (ref)	4–5	4–5
	0	4–5	4
	3	4–5	4–5
	6	4–5	4–5
	9	4–5	4–5
	12	4–5	4–5
	15	4–5	4–5
PB	0 (ref)	4–5	4–5
	0	4–5	4–5
	3	4–5	4–5
	6	4–5	4–5
	9	4–5	4–5
	12	5	4–5
	15	5	4–5

Table 6.

Color differences from the mean, and mean color difference from the mean, of the dyed fabrics with electrochemical pre-treated wastewater for the PC, PN, PY, RB5 and PB dyes

Dye	Electr. treatment duration (min)	Color difference from the mean				Mean color difference from the mean (MCDM)
		1	2	3	4
PC	0	0.20	0.14	0.17	0.21	0.18
	3	0.19	0.17	0.22	0.25	0.21
	6	0.18	0.19	0.13	0.18	0.17
	9	0.24	0.17	0.19	0.13	0.18
	12	0.14	0.16	0.19	0.19	0.17
	15	0.21	0.15	0.17	0.18	0.18
PN	0	0.12	0.11	0.15	0.09	0.13
	3	0.13	0.13	0.12	0.11	0.12
	6	0.16	0.12	0.14	0.17	0.15
	9	0.18	0.19	0.13	0.19	0.17
	12	0.14	0.16	0.18	0.12	0.15
	15	0.19	0.16	0.14	0.18	0.17
PY	0	0.21	0.18	0.17	0.22	0.19
	3	0.23	0.20	0.19	0.21	0.20
	6	0.19	0.24	0.17	0.21	0.20
	9	0.20	0.16	0.19	0.12	0.17
	12	0.17	0.14	0.19	0.20	0.18
	15	0.17	0.16	0.16	0.21	0.17
RB5	0	0.15	0.17	0.13	0.16	0.15
	3	0.18	0.23	0.23	0.21	0.21
	6	0.22	0.24	0.16	0.19	0.20
	9	0.14	0.18	0.17	0.19	0.17
	12	0.22	0.21	0.18	0.17	0.19
	15	0.13	0.16	0.18	0.14	0.15
PB	0	0.19	0.22	0.18	0.24	0.21
	3	0.16	0.18	0.21	0.22	0.19
	6	0.18	0.19	0.22	0.15	0.18
	9	0.21	0.22	0.17	0.13	0.18
	12	0.17	0.13	0.16	0.14	0.13
	15	0.20	0.17	0.21	0.11	0.17

In summary, both the washing and rubbing color fastness test values of the fabrics are over 4 and the majority of them are between 4–5 and 5 which indicates that the fabrics dyed with reused wastewater previously electrochemical treated, are resistant to washing and rubbing. The results in Table 6 indicate that fabrics are uniformly colored since the values of the mean color differences from the mean are very low, as also visually stated, and can be attributed to the system limitations in the measurements.

As a whole, the application of an electrochemical pre-treatment before reuse of the wastewater not only improves the results of dyeing since it reduces DE* values between fabrics dyed with reused water compared to the references, but also produces dyed fabrics which are washing and rubbing resistant, and uniformly colored.

Conclusions

From the obtained results, it can be concluded that 70% of wastewater can be reused by DBR, with most reactive dyes studied in this work providing acceptable DE values.

In general, the application of an electrochemical pre-treatment before the reuse of the wastewater, improved the results of dyeing, as it reduced DE* values of fabrics dyed with reused water with respect to the references.

In the case of PC, RB5 and PN, the DBR can be applied directly without electrochemical pre-treatment as DE* values are below 1 with respect to the standard. However, electrochemical pre-treatment reduces DE* values around 50% after only 3 minutes and up to 75% after 15 minutes.

For PY dye, the results showed that DBR can also be applied without electrochemical pre-treatment but contrary to before, only 3 minutes of electrochemical treatment improves DE* results and longer electrochemical treatments leads to unacceptable DE* values with respect to the reference. It can also be observed that the DE* tended to increase with time, and this phenomenon can be attributed either to the great influence of chlorinated water in the dyeing process and/or to the formation of new radical species, as previously discussed.

Finally, bath reuse can only be applied to PB after short electrochemical pre-treatment, 3 minutes, for the rest of treatment intervals the DE* values are greater than 1. In this case, there is no clear trend in the values, so further investigations should be done.

In all cases, DL* is the most influential parameter in DE* values. With respect to the influence of further dye degradation in the wastewater, it was observed that it is not an important parameter in DBR for most of the studied dyes.

Footnotes

Funding

This work was supported by the Spanish Ministry of Science and Innovation (MICINN) (grant numbers, CTM2007-66570-C02-01/TECNO and CTM2010-18842-C02-01).

Acknowledgements

The authors thank Ariadna Sáez for her help in the experimental work of this study.

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