WO2025169022A1 - Process for the recovery and reuse of dyes from aqueous effluents - Google Patents

Abstract

The present invention relates to a process for the recovery and reuse of dyes from aqueous effluents. The process comprises the formation of an alcohol/salt aqueous biphasic system by the addition of at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and a salt, to aqueous effluents, followed by the separation of the alcohol/salt aqueous biphasic system into an alcohol/dye-rich phase and a salt-rich phase. The alcohol/dye-rich phase can then be used in an alcohol-assisted dyeing process. The disclosed process can be carried out cyclically, with subsequent formation of an alcohol/salt aqueous biphasic system, followed by phase separation and use of the alcohol/dye-rich phase to dye different types of materials.

Description

Process for the recovery and reuse of dyes from aqueous effluents

This application relates to a process for the recovery and reuse of dyes from industrial effluents.

The widespread production and use of dyes in various industries, such as pharmaceutical, food, paper and pulp, or textile, has led to major environmental issues associated with the discharge of untreated or only partially treated industrial effluents into the environment [1][2]. The release of these improperly treated dye-contaminated wastewaters into aquatic bodies pose serious threat to environmental safety, harming the fauna, flora and entire human communities depending on these fresh-water resources [1][2].

Water pollution caused by textile effluents stands out as one of the most notable and widely documented instances of this environmental concern. Global population growth and respective increase in textile demands, have spread the already concerning production of hazardous effluents from textile industry. New technological approaches for the treatment of these wastewaters are paramount for the transition from a linear to a circular textile industry.

In recent decades, the textile industry has witnessed unprecedented growth, boosted by an ever-expanding global demand for garments and fabrics. Unfortunately, this remarkable growth has been accompanied by a concerning environmental challenge – the discharge of hazardous textile dye effluents into fresh-water resources [3][4] Among the textile wet processes, the dyeing step generates the highest amount of wastewaters, loaded with synthetic dyes and complex chemical additives. As these effluents find their way into water bodies, not only they disrupt aquatic life but also compromise the integrity of hydric resources which are essential for human well-being and overall environmental health [5][6][7]. The pressing need to address these adverse impacts of dye effluents has catalyzed extensive research efforts to develop effective treatment strategies.

Conventional wastewater treatment methods, including physical, chemical, and biological processes, have been extensively investigated for mitigating the environmental impacts of textile dye effluents [8][9][10]. While they have demonstrated some success in reducing dye concentration, a significant drawback of many existing water treatment methods is their inherent dye-destructive nature. Moreover, they often generate harmful byproducts, leading to a transfer of the environmental impact rather than its mitigation [1][10].

Aqueous biphasic systems (ABS) have recently emerged as new, promising approaches for the recovery of textile dyes from industrial effluents, revolutionizing textile effluent treatment [1][11]. ABS are characterized by the formation of two immiscible liquid phases when at least two water-miscible compounds are mixed in water above a certain concentration range. These biphasic systems allow the design of unique platforms for the selective extraction, recovery, and recycling of dye compounds, thereby aligning with the principles of circularity and minimizing the generation of harmful waste [11][12][13].

Within ABS, alcohol-salt ABS have gained attention as a particularly promising alternative [14][15][16]. The combination of alcohols and salts creates phase separation conditions, such as low viscosity and short separation time, that enhance the efficiency of the partitioning of a wide range of solutes [15][17]. This advantage extends to the selective extraction of textile dyes from wastewater, offering a versatile and environmentally benign alternative to conventional dye removal methods, as the later often involve harsh chemicals and energy-intensive, costly processes. The distinctive properties of alcohol-salt ABS, including their tunability and compatibility with various dyes emphasize their potential as an environmentally friendly solution for dye recovery from textile effluents [1][18].

Moreover, recent years have witnessed a growing interest in novel textile dyeing techniques, such as alcohol-assisted dyeing [19][20][21][22][23]. These new approaches represent a paradigm shift in the dyeing scenario, offering a more environmentally responsible way to achieve vivid and durable colors on textiles. Unlike conventional dyeing methods, which heavily rely on water and harmful chemical additives as a medium for dye dispersion and fixation, alcohol-assisted dyeing takes advantages of the unique properties of alcohol to enhance dye penetration and adherence to textile fibers. This ecologically advanced process aims to replace the use of synthetic dyeing auxiliaries, such as wetting and swelling agents, dispersants, retardants, and softeners, with sustainable, low-impact alternatives [19].

The present invention relates to a process for the recovery and reuse of dyes from an aqueous effluent comprising dyes, comprising the following steps:

Mixing at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, with a primary aqueous effluent in sufficient amount to obtain a primary alcohol/salt aqueous biphasic system comprising between 0.1 and 99.9 wt% of at least one alcohol and between 0.1 and 99.9 wt% of at least one salt;

Separating a primary dye/alcohol-rich phase and a primary salt-rich phase of the primary alcohol/salt aqueous biphasic system;

Adjusting the pH of the primary dye/alcohol-rich phase;

Using the pH-adjusted primary dye/alcohol-rich phase in an alcohol-assisted dyeing process obtaining a secondary aqueous effluent comprising dyes;

Mixing between 0.1% and 99.9% wt% of the primary salt-rich phase with the secondary aqueous effluent;

Adding at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, to the previous mixture in sufficient amount to obtain a secondary alcohol/salt aqueous biphasic system with the same concentration of the primary alcohol/salt aqueous biphasic system;

Separating a secondary dye/alcohol-rich phase and a secondary salt-rich phase of the secondary alcohol/salt aqueous biphasic system;

Mixing the secondary dye/alcohol-rich phase with a primary effluent to obtain a tertiary alcohol/salt aqueous biphasic system with the same concentration of the previous alcohol/salt aqueous biphasic systems;

Repeating the steps of separating the dye/alcohol-rich phase and the salt-rich phase, and using a pH-adjusted dye/alcohol-rich phase in subsequent alcohol-assisted dyeing process.

In one embodiment at least one short-alkyl chain alcohol is selected from methanol, ethanol, propanol, isopropanol or isopropyl alcohol, butanol, isobutanol, sec-butanol, or tert-butanol.

In one embodiment at least one salt is selected from triammonium citrate, diammonium sulfate, trisodium citrate, dipotassium hydrogen phosphate, tripotassium citrate, sodium dihydrogen phosphate, tripotassium phosphate, potassium carbonate, sodium chloride, sodium thiosulfate, sodium sulfate, sodium carbonate, disodium hydrogen citrate, disodium tartrate, diammonium hydrogen citrate, dipotassium oxalate, sodium acetate, potassium acetate, potassium chloride, ammonium acetate, calcium chloride, lithium sulfate, lithium acetate, or lithium nitrate.

In one embodiment the alcohol/salt aqueous biphasic system comprises isopropyl alcohol between 5 and 50 wt% and sodium sulfate between 5 and 25 wt%.

In one embodiment more alcohol and/or salt is added to the subsequent aqueous effluents in sufficient amount to obtain an alcohol/salt aqueous biphasic system with the same concentration of the primary alcohol/salt aqueous biphasic system.

In one embodiment the primary aqueous effluent is an aqueous effluent obtained from an industrial process using dyes.

In one embodiment the phases are separated using any processing operation able to separate two liquid immiscible phases.

In one embodiment all steps of mixing of the aqueous effluent with the at least one short alkyl-chain alcohol, or the at least one short alkyl-chain alcohol and at least one salt, to obtain the alcohol/salt aqueous biphasic system, occur at a temperature between 10 and 90 ºC and a time between 30 seconds and 60 minutes.

In one embodiment all steps of mixing of the salt-rich phase with the aqueous effluent occur at a temperature between 10 and 90 ºC and a time between 30 seconds and 60 minutes.

In one embodiment the pH of the dye/alcohol-rich phases is adjusted between 2 and 13.

General description

Herein described are alcohol/salt ABS compositions comprising at least one short-alkyl chain alcohol and at least one salt formed in aqueous effluents containing dyes which are suitable to recover said dyes for further use.

Here is also described a process for the recovery and reuse of dyes from aqueous effluents. In general terms, the process comprises obtaining an alcohol/salt aqueous biphasic system by adding at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, to an aqueous effluent comprising dyes. After the alcohol/salt ABS is obtained, the dye/alcohol-rich phase and the salt-rich phase are separated, and the pH of the dye/alcohol-rich phase is adjusted to be suitable for use in an alcohol-assisted dyeing process. The effluent, comprising dyes, from said dyeing process is then mixed with the aforementioned salt-rich phase to obtain another alcohol/salt ABS, which can then also be separated in a dye/alcohol-rich phase and a salt-rich phase. The later dye/alcohol-rich phase can then be mixed with primary dye-contaminated effluent (as well as fresh alcohol and salt, or just alcohol, if necessary to form a new ABS), forming a new alcohol/salt ABS and beginning a new cycle of recovered dyes’ concentration and alcohol-assisted dyeing. This sequence of steps can be repeated successively, making this process cyclic.

The aqueous effluent comprising salt and dyes, or only dyes, can be an effluent from a textile dyeing process or any other type of industrial effluent comprising dyes. The effluent can comprise one type of dye or mixtures thereof.

The salt-rich phase obtained after mixing with the secondary effluent can be directed for water treatment. A notable challenge in this context consists in the treatment of the residual effluents, which will predominantly consist of saline solutions. It is imperative to address this additional step, as it plays a pivotal role in comprehending the sustainability of the overall process.

Thus, the dyes recovered from the aqueous effluents can be recovered with an efficiency equal or above 97% and reused to produce new dyed materials, such as textiles, yarns, or plastics. Industries such as food and beverage, cosmetics, paper, paint and coatings, printing and publishing and leather, can benefit from the dyes recovered with the presently discloses process.

For proof of concept, alcohol/salt ABS were used in the design of a circular recovery and reuse process for textile dyes present in aqueous solutions. Remazol brilliant blue R (RBBR), acid orange (AO) and a mixture of both of these dyes was used as probes for proof of concept. Both ABS compositions tested presented similar phase separation hydrodynamics and almost complete extraction efficiencies of all dyes (i.e., extraction efficiency (EE) > 90%), present whether individually or as a mixture in aqueous solutions. The ABS systems were further compared according to the ability of their dye/alcohol-rich phases being used for dyeing wool under the same operation conditions (i.e., 40ºC, pH 4 and no agitation). The pH of all the dye-rich phases was corrected to pH 4 using a diluted aqueous solution of acetic acid (5 %(v/v)). The isopropyl alcohol/sodium carbonate (IPA/SC) ABS presented itself as the most suitable system for the recovery and reuse of the removed textile dyes in wool dyeing, with approximately 90% of all recovered textile dyes concentrated in the dye/alcohol-rich phase of the biphasic mixture (composed of 10.6 wt% alcohol, 15 wt% salt and 74.4 wt% water) being adsorbed by the wool fabrics after only 60 min of dyeing. The unfixed dyes in the dye bath after the alcohol-assisted dyeing process were reextracted by forming the same ABS and mixture point using that dye-exhausted dye bath, the salt-rich phase of the first ABS’ extraction and small alcohol and salt make-ups. The top, dye/alcohol-rich phase from this second extraction was mixed with a fresh dye-contaminated aqueous solution, forming a new ABS and extracting the textile dyes present in this fresh effluent. The dye/alcohol-rich phase obtained from the third extraction cycle was used in a new dyeing process, with the wool adsorbing near 90% of all textile dyes present in the dye/alcohol-rich phase, as occurred in the first dyeing cycle.

The attained results proved the potential of alcohol/salt ABS as circular recovery and reuse platforms of dyes from dye-contaminated aqueous solutions such as industrial effluents, presenting enhanced phase separation hydrodynamics, high EE and dyeing performance in consecutive extraction and dyeing cycles. This approach offers innovative and promising features capable of helping the transition of industries into a more environmentally sustainable and circular economy model, while maintaining, or even improve, the financial profits of their industrial activity.

Within this scenario, the present invention proposes the promising and innovative integration of alcohol/salt ABS and sustainable dyeing techniques. The use of alcohol/salt ABS for the recovery and partitioning of dyes from effluents allows to obtain a phase rich in alcohol, dyes, and a minimal quantity of salts.

Herein, are also described the details of alcohol/salt ABS determination, characterization, and hydrodynamics, uncovering the fundamental principles underlying their behavior. The optimal conditions for the efficient extraction and concentration of dyes, namely RBBR and AO within these biphasic systems were investigated, with the suitability using the ABS dye/alcohol-rich phase in the formation of dye baths for dyeing wool being evaluated. A circular process was proposed to reuse the remaining dyes and the ABS’ components from the new alcohol-assisted dye baths in subsequent cycles. Through this multifaceted approach, are provided valuable insights into the potential of ABS as a sustainable and efficient solution for industries that use dyes in their processes. This offers a full perspective on the journey from recovery to reapplication of dyes, ultimately contributing transition toward environmentally conscious practices and a more circular economy. This strategy not only eliminates the need for conventional, potentially harmful, dyeing auxiliary compounds but also facilitates an efficient dye recovery process that is integrated into the dyeing process itself.

For easier understanding of this application, figures are attached in the annex that represent the preferred forms of implementation which nevertheless are not intended to limit the technique disclosed herein.

Fig.1

shows phase diagrams of IPA/SC (A) and IPA/SS (B) at 25ºC and atmospheric pressure, in weight fraction (wt%). The black circles (●) and lines (−) correspond to experimental data and adjusted binodal curves, with the blue squares corresponding to the tie-lines’ (TLs) relation and the remaining squares and lines corresponding to the respective tie-lines (i.e., TL1, TL2, TL3, TL4 and TL5). The critical point for both systems (red square, ■) was determined through the extrapolation the TLs’ slopes of distinct systems.

Fig.2

shows Ts values of IPA/SC (A) and IPA/SS (B) according to the tie-line length (TLL) of each biphasic mixture studied, at 25ºC. The colour of each group of data corresponds to the dye present in each biphasic mixture (■ – AO, ▲ – RBBR, ♦ – Mix, ● – without dye (reference)).

Fig.3

shows partition coefficients (A and B) and extraction efficiencies (C and D) of IPA/SC and IPA/SS for all the studied biphasic mixtures, and respective TLL. White, grey and black colours correspond to the AO, RBBR and 50/50 of both of these dyes, respectively.

Fig.4

shows wool dyeing performance using the extracted textile dyes present in the IPA-rich phases of IPA/SC (A) and IPA/SS (B) systems at 40ºC, with no agitation (■ – AO, ▲ – RBBR, □ – AO in the dye mixture, ∆ – RBBR in the dye mixture). For this dyeing experiments it was used the initial biphasic mixtures correspondent to TL1 of both ABS (also used for the previous hydrodynamic studies). All dyeing assays were performed at 40ºC, without agitation.

Fig.5

shows wool dyeing performance using the extracted textile dyes present in the TL5 IPA-rich phase from the IPA/SC system, at 40ºC, with no agitation (■ – AO, ▲ – RBBR, □ – AO in the dye mixture, ∆ – RBBR in the dye mixture). All dyeing assays were performed at 40ºC, without agitation.

Fig.6

the wool dyeing performance using a dyeing media composed of the IPA-rich phase from the demixing of the TL1 new biphasic mixture of IPA/SC systems (■ – AO, ▲ – RBBR, □ – AO in the dye mixture, ∆ – RBBR in the dye mixture). All dyeing assays were performed at 40ºC, without agitation.

Fig.7

shows a scheme of one embodiment of the alcohol-based process proposed for the recovery and reuse of dyes. The example composition of each system at any point of the process is indicated below each respective system.

Fig.8

shows the detailed composition of all mixtures.

Fig.9

shows wool dyeing performance (A) using the recovered compounds from the previous dyeing cycle and newly extracted dye from fresh dye-contaminated solution (■ – AO, ▲ – RBBR, □ – AO in the dye mixture, ∆ – RBBR in the dye mixture).

Fig.10

shows one embodiment of the disclosed process.

Now, preferred embodiments of the present application will be described in detail with reference to the annexed drawings. However, they are not intended to limit the scope of this application.

In one embodiment, an alcohol/salt ABS is an aqueous solution formed by the addition of at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, to an aqueous effluent comprising dyes.

The process for the recovery and reuse of dyes from an aqueous effluent comprising dyes, comprises the following steps:

Mixing at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, with a primary aqueous effluent in sufficient amount to obtain a primary alcohol/salt aqueous biphasic system comprising between 0.1 and 99.9 wt% of at least one alcohol and between 0.1 and 99.9 wt% of at least one salt;

Separating a primary dye/alcohol-rich phase and a primary salt-rich phase of the primary alcohol/salt aqueous biphasic system;

Adjusting the pH of the primary dye/alcohol-rich phase;

Using the pH-adjusted primary dye/alcohol-rich phase in an alcohol-assisted dyeing process obtaining a secondary aqueous effluent comprising dyes;

Mixing between 0.1% and 99.9% wt% of the primary salt-rich phase with the secondary aqueous effluent;

Adding at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, to the previous mixture in sufficient amount to obtain a secondary alcohol/salt aqueous biphasic system with the same concentration of the primary alcohol/salt aqueous biphasic system;

Separating a secondary dye/alcohol-rich phase and a secondary salt-rich phase of the secondary alcohol/salt aqueous biphasic system; Mixing the secondary dye/alcohol-rich phase with a primary effluent to obtain a tertiary alcohol/salt aqueous biphasic system with the same concentration of the previous alcohol/salt aqueous biphasic systems;

Repeating the steps of separating the dye/alcohol-rich phase and the salt-rich phase, and using a pH-adjusted dye/alcohol-rich phase in subsequent alcohol-assisted dyeing process.

The proportions of the at least one short alkyl-chain alcohol and at least one salt are such as to ensure the formation of an aqueous biphasic system in accordance with solubility data. It is important to note that the amount of at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, added to the aqueous effluents is defined by the solubility curve of the alcohol/salt ABS intended to obtain within its biphasic region. Said solubility curve also differs depending on the alcohols and salts used. Thus, the alcohol/salt ABS is only obtained when the concentrations of alcohol and salt are above said curve.

In one embodiment, at least one short-alkyl chain alcohol is selected from, including but not limited to, methanol, ethanol, propanol, isopropanol, or isopropyl alcohol (IPA), butanol, isobutanol, sec-butanol, or tert-butanol.

In one embodiment, at least one salt is selected from, including but not limited to, triammonium citrate, diammonium sulfate, trisodium citrate, dipotassium hydrogen phosphate, tripotassium citrate, sodium dihydrogen phosphate, tripotassium phosphate, potassium carbonate, sodium chloride, sodium thiosulfate, sodium sulfate (SS), sodium carbonate (SC), disodium hydrogen citrate, disodium tartrate, diammonium hydrogen citrate, dipotassium oxalate, sodium acetate, potassium acetate, potassium chloride, ammonium acetate, calcium chloride, lithium sulfate, lithium acetate, or lithium nitrate.

In one embodiment, the alcohol/salt ABS comprises IPA between 5 and 50 wt% and SS between 5 and 25 wt%. Notably, IPA and SS must be used in concentrations above their respective solubility curves within the disclosed range to ensure the establishment of a stable aqueous biphasic system.

In another embodiment, the alcohol/salt ABS comprises IPA and SC. Notably, IPA and SC must be used in concentrations above their respective solubility curves within the disclosed range to ensure the establishment of a stable aqueous biphasic system.

The addition of at least one salt is contingent upon the existing salt content in the aqueous effluent, and as such, the amount of at least one salt added may vary or may not be necessary at all.

In one embodiment, if necessary, more alcohol and/or salt can be added to the aqueous effluents in sufficient amount to obtain an alcohol/salt ABS with the same concentration of the primary alcohol/salt ABS.

In the context of the present invention, the primary aqueous effluent is an aqueous effluent obtained from an industrial process using dyes, for example a textile dyeing process.

In one embodiment, the phases are separated using any processing operation able to separate two liquid immiscible phases, for example, a mixer-settler unit.

In one embodiment, all steps of mixing of the aqueous effluent with the at least one short alkyl-chain alcohol, or the at least one short alkyl-chain alcohol and at least one salt, to obtain the alcohol/salt aqueous biphasic system, occurs at a temperature between 10 and 90 ºC and a time between 30 seconds and 60 minutes.

In one embodiment, all steps of mixing of the salt-rich phase with the aqueous effluent occur at a temperature between 10 and 90 ºC and a time between 30 seconds and 60 minutes.

In one embodiment, the pH of the dye/alcohol-rich phases is adjusted between 2 and 13 to make the dye/alcohol-rich phases suitable for use in alcohol-assisted dyeing processes. The pH of the dye/alcohol-rich phases is dependent on the material intended to dye.

Proof of concept

For this invention, the alcohol/salt ABS comprising IPA, SC or SS was experimentally tested for proof of concept with a focus on their hydrodynamics, and dye extraction efficiency. Both alcohol/salt systems present similar, enhanced hydrodynamics for all the tested mixture points, with phase settling occurring in less than 60 seconds. All dyes and mixture of dyes partitioned almost completely to the alcohol-rich, top phase, enabling the extraction and concentration of textile dyes from aqueous solutions contaminated with 100 ppm of these compounds. The wool dyeing performances of the dye-rich phases from the IPA/SS and IPA/SC systems were assessed and compared, with the best mixture point being composed of 10.6 wt% alcohol, 15.0 wt% salt and 74.4 wt% water in the TL1 of IPA/SC system. For this system, a dye uptake of near 90% was achieved after only 60 minutes, at 40ºC and without any agitation. The formation of sodium acetate after the addition of diluted acetic acid solution to correct the dye bath to pH 4 appeared to have improved the dyeing performance. Lower alcohol contents in the dye baths also resulted in higher dye uptake by the wool fabrics. The recyclability of the process was evaluated, with the compounds present in the final dye bath of the first dyeing process being reused in a new extraction and dyeing cycle, without any loss of wool dyeing performance after the same operation time when compared to the first dyeing cycle. These results confirmed the consistency of extraction and dyeing performances, enabling the design of a circular process for the recovery of textile dyes from textile effluents and marking a promising step towards sustainable and efficient textile dyeing processes. This research underscores the potential of using alcohol-based ABS as extraction, concentration, and reuse of textile dyes from industrial effluents, offering valuable insights for the textile industry's transition towards more environmentally conscious and economically viable practices.

Experimental data

1. Experimental section

1.1 Chemicals and materials

For this study, isopropyl alcohol (IPA) (Panreac (France), >99.8% purity), sodium sulfate anhydrous (SS) (Merck (Germany), >99% purity) and sodium carbonate anhydrous (SC) (Panreac (France), >99.5 – 100.5%) were used to form the ABS. The dyeing fabric was 100% crude wool yarn, purchased in a local store (Meraki Store, Coimbra, Portugal), without any type of treatment before the dyeing process. Acid orange (AO) (Tokyo Chemical Industry (Japan), >97% purity) and Remazol Brilliant Blue R (RBBR) (Thermo Scientific (Switzerland), pure). The aqueous solution of acetic acid (AA) at 5% (v/v) used for the pH correction of the dyeing assays was obtained using distilled water and glacial AA (Chem-Lab (Belgium), >99%). Double distilled water was used to form the ABS and the dyeing experiments, being obtained from a reverse osmosis system and further treatment with a Milli-Q plus 185 water purification apparatus.

1.2 ABS determination and characterization

Aqueous solutions containing SS or SC (both at approximately 25%wt.) were individually formulated and used to determine the ABS binodal curves and respective phase diagrams. Using the cloud point titration method, the phase diagrams for IPA/SS and IPA/SC systems were established under atmospheric pressure and at 25ºC, with the specific experimental methodology employed being outlined elsewhere [24]. The composition of the system was determined by precise weighing of all components contributing to phase separation, with an accuracy of ±10-4 g, and subsequently, the corresponding binodal curves were constructed. The binodal curves of each ABS were described by the means of the empiric equation proposed by Merchuk and collaborators [24] (Equation 1):

Where X is the mass fraction of SS or SC and Y is the mass fraction of IPA, both expressed as weight percentages. A, B and C are fitting parameters.

The determination of the system’s tie-lines (TLs) was accomplished by using the gravimetric method described by Merchuk et al. [24]. For the TLs determination, five different mixture points within the biphasic region were prepared for each ABS with a total mass of 10 g in 15 mL conical bottom glass centrifuge tubes, vigorously stirred and left to equilibrate in a thermostatic for 3 h at 25ºC. After the equilibration time, to clear separate coexisting phases were observed (a top (IPA-rich) phase and a bottom (salt-rich) phase), with both being carefully separated, collected, and weighed. Using the mass compositions of the salt-rich phase and of the overall system, each individual TL was determined by applying the lever rule and solving the following four equations (Equations 2 to 5). From the simultaneous resolution of these equations, the composition of respective phase-forming compound (i.e., Xb, Xt, Yb e Yt) in each of the coexisting phases were determined.

where X _t and X _b represent SS or SC, Y _t and Y _b correspond to IPA, at the IPA-rich and salt-rich phases, respectively. X _m is the initial concentration of SS or SC, in the mixture, while Y_m is the initial concentration of IPA in the same solution. The parameter α is the ratio between the bottom phase and the total weight of the mixture.

The pH values (±0.02) of the coexisting phases were determined using the pH-meter Metrohm – Model 914 from Metrohm® (Switzerland), with the calibration being performed using two standard buffers with pH values of 7.00 and 4.00±0.02.

To determine each ABS hydrodynamics and give further insights regarding their phase separation, the procedure used by Jorge et al. [25] was employed with small modifications. The phase settling time (Ts) (i.e., the time required to obtain to clear, emulsion-free separated phases) of each mixture point with a total mass of 10g analysed was determined by agitating the systems at the same conditions used in that work (1200 rpm for 300 seconds using a 25 mL glass flask and a 22×5 mm magnetic stirrer). The glass flask with the studied mixtures and the magnetic stirrer was inserted in a jacketed glass reactor vessel in order to maintain the temperature at 25ºC during all experiments, with the agitation of the systems being assured by a magnetic stirrer plate. After the 300 seconds of constant agitation, the stirrer was stopped and the phases’ settling time was recorded using a chronometer. All biphasic mixtures presented the same amount of dye used to determine the textile dyes extraction performance using these ABS, in order to assess the influence of the different dyes on the ABS phase separation hydrodynamics.

1.3 Textile dyes extraction and concentration

The partitioning of AO, RBBR and a 50/50 (w/w) mixture of both dyes were determined for the biphasic mixtures previously characterized. Stock solutions of 0.5% wt. of each textile dye and of the mixture of both dyes were produced, with the 10 g biphasic systems of each ABS previously characterized being reproduced and with whether each dye or the mixture of dyes being added to these systems. The amount of dye added to the system was the needed to have a concentration of 100 ppm of the respective dye in the total mass of water and salt used to form the ABS, aiming to mimic a dyed effluent (composed of water and salt) with 100 ppm concentration of that textile dye and the posterior addition of IPA to produce the required ABS mixture point.

Afterwards, the dyed two-phase systems were left to equilibrate in the thermostatic bath at 25ºC for 3 hours, with the phases being separated and the concentration of each dye in the salt-rich phase was measured using a P9 Double Beam UV-Visible Spectrophotometer (Avantor®, USA). The determination of the dyes concentration in the IPA-rich phase was not reliable, as the considerable amount of IPA in this phase interfered with the obtained measurements and was not possible to correctly assess the concentration of each dye in the systems’ top phase where mixtures with same concentration of AO and different concentrations of IPA resulted in different peaks’ intensity). Thus, the amount of each dye present in the top phase was calculated by the difference between the initial mass of dye added to the system and the amount of dye present in the salt-rich phase. Considering the mass of each phase, the concentration of dye in each one of them was calculated, being determined the partitioning coefficient (K) and extraction efficiency (EE%) of each biphasic systems for each textile dye and their mixture using Equations 7 and 8, respectively:

Eq. 7

Eq. 8

Where is the total mass of dye added to the system, and is the mass of dye present in the bottom and top phases, respectively; and are the mass of the bottom and top phases, respectively; and are the absorbance recorded for each respective dye in the bottom phase of the biphasic system and the calibration curve slope for the dye tested, respectively; and are the concentrations of the dye in the top and bottom phases, respectively.

Analysing the results obtained regarding K, EE and the ABS phase separation hydrodynamics, the best ABS was selected. For this biphasic system, two TL were analysed concerning their dyeing performance and the TL with the best results was chosen in order to concentrate the extracted dyes in the IPA-rich phase. Thus, the slope of the best TL was obtained from points in the binodal curve corresponding to the composition of the top and bottom phases, with the new mixture point presenting more salt and less IPA, but with the composition of the coexisting phases (and respective system’s equilibrium and dye extraction performance) being maintained. The wool dyeing performance of this new mixture point was also assessed.

1.4 Wool dyeing assays and dyeing performance

After the extraction of the textile dyes, the dye-rich phase (i.e., the IPA-rich phase) was incorporated in an innovative, circular alcohol-assisted dyeing process.

After the separation of the ABS phases, the dye-rich phases’ pH was corrected to 4 using a 5 %(v/v) solution of acetic acid (AA), in the same 15 mL conical bottom glass centrifuge tubes used for the dyes’ extraction. The pH-corrected system was used in the isothermal dyeing of the wool yarn at 40ºC and using a material to liquor ratio of 1:30. The wool was added to the dyeing system using a clamp and the final systems were maintained at 40ºC using a Stuart® SI-600 incubator, with no agitation.

The dye adsorption kinetics for each dye, whether individually used or as a mixture, was recorded for a total of 6 hours, with measurements being made every 10 minutes during the first hour and then hourly until the completion of 6 hours. As the absorbance values for all dyes and all dyeing systems were > 1.0 (indicating that the solution has an high dye concentration and with the Lambert Beer’s Law not being valid, as the absorbance does not linearly increase with increased concentration of solute for absorbance values > 1.0 [26], it would be required the dilution of the collected aliquots used to assess the dyes’ adsorption kinetics, which would introduce variations on the dyeing systems’ composition (i.e., the collected samples could not be retrieved to the dyeing system, otherwise would alter the concentration of dye in solution and the overall system’s composition). Thus, the dye adsorbed by the wool was determined as indicated in Equation 9:

Where Absi and Abst are the absorbance values before the addition of wool and after a certain time of dyeing operation (t), respectively. Moreover, the dye uptake after the 6h dyeing process was recorded by washing-off the yarns with water, to remove any unfixed dye and the absorbance of the washing water was recorded to calculate the real dye uptake.

The dyeing performance of the ABS dye-rich phases were also compared with the control samples composed of dye + water, dye + water + IPA and dye + salt + water. The concentrations of IPA and salt in these systems was the same as the ones presented in the ABS dye-rich phase, with the dyeing temperature, pH and total mass of the systems being the same for all systems.

To prove the circularity of the idealized dyeing process, the effluent from this IPA-based dyeing process (still containing residual textile, IPA, and salts) was used in the formation of a second ABS extraction. This dyeing effluent was mixed with the salt-rich phase from the first extraction performed, and IPA and salt was added as make-up to achieve the same mixture point used in the first extraction. Afterwards, the IPA-rich top phase (containing any residual dye from the alcohol-assisted dyeing process) was incorporated with fresh dye-contaminated effluent containing 100 ppm of the same dye, with more salt and IPA being added as make-up to reproduce the same ABS mixture point used in the previous extractions. From this last extraction, the dye-rich phase (i.e., IPA-rich phase) was used in a new alcohol-assisted dyeing cycle and their dyeing performance analysed as previously.

1.5 Statistical analysis

All experiments were performed in triplicate and the results are expressed as the average of, at least, three independent assays with the corresponding errors at a 95% confidence level. Statistical analyses were performed using the JMP® Pro 17 software (USA). Values of p ≤ 0.05 were considered statistically.

2. Results and Discussion

In order to provide deeper insights concerning the selected alcohol-based ABS, the phase diagrams of the biphasic systems IPA/SS and IPA/SC were determined (at 25ºC and atmospheric pressure). The obtained experimental data was posteriorly adjusted using the Merchuk’s equation (Equation 1), with the experimental TLs and respective TLL being determined using Equations 2 and 3, respectively. The attained phase diagrams and correlations allow to identify the regions where the systems are monophasic (below the binodal curve) and where they are biphasic (above the binodal curve), enabling the characterization of the different biphasic mixtures. This information is of paramount importance in the understanding for the textile dyes’ extraction and wool dyeing performance, as it will be further discussed in the upcoming sections.

ABS characterization

The phase diagrams, biphasic mixtures, and respective TLs, for both IPA/SS and IPA/SC, are displayed in . The experimental binodal curve data, correlation coefficients and composition of the phases in each biphasic mixture were also determined.

Since there is no intersection between the TLs and R2 ≈1.0 for all the ABS, a good adjustment of the binodal curve and the experimental data are obtained with the Merchuk’s method [25]. By examining the binodal curve of each ABS, it is possible to observe that the one composed of SC presents a wider biphasic region than the one formed with SS. This is related with the salting-out ability of both salts, as salts possessing anions with higher hydration capacities (i.e., bigger Gibbs’ hydration energy, -ΔGhyd (kJ/mol)) have stronger interactions with water molecules than the water molecules between themselves, which result in stronger salting-out effects, higher ABS formation and larger biphasic regions [1], [25] Since SC is more kosmotropic than SS (i.e., SC is more easily hydrated than SS), the ability of the first to salt-out the IPA in the system is higher and a wider biphasic region for the system IPA/SC is obtained when compared to the IPA/SS system [27].

Regarding the composition of the ABS’ phases in equilibrium, the top and bottom phases of both systems are mainly composed by IPA and sodium salts, respectively. Furthermore, the pH of each phase was recorded, with the mean values for SC and SS being 11.52 ± 0.11 and 7.35 ± 0.94, respectively. The importance of this parameter on the extraction of the studied textile dyes and the wool dyeing performance will be further highlighted in the next sections.

ABS hydrodynamics

To design and effectively implement ABS-based extraction processes at industrial scale, regarding of the target compound or molecule desired, it is necessary the full understanding of both the systems’ hydrodynamics and partitioning of the target compounds [1]. ABS with better K or EE do not necessarily have the best phase separation hydrodynamics, which can reduce their potential for industrial implementation and financial feasibility by requiring additional equipment to reduce operation time and increase the process productivity [1], [25]. Thus, it is crucial to know the ABS formation hydrodynamics when selecting the best system for a required process.

The phase separation hydrodynamics of both IPA-based systems studied in this work was analysed by determining the phase settling time (Ts) of the biphasic mixtures formed within the biphasic region of each ABS. To provide insights of the impact of textile dyes in the ABS hydrodynamics, the biphasic mixtures used to determine Ts possessed the same amount of dye used in the partitioning studies. The obtained results of Ts are displayed in .

It is possible to observe in that the phase separation hydrodynamics of both IPA-based ABS are very rapid, requiring less than a minute to achieve a complete phase separation and ABS formation (i.e., for IPA/SS, Ts varied from 30 to 40 seconds, while IPA/SC presented a settling time between 25 and 40 seconds). It is also noticed that the systems with and without dyes presented similar results of Ts for biphasic mixtures with the same composition, which indicates that the presence of textile dyes in these systems does not have a significant impact on their formation hydrodynamics.

The increase in TLL led to a decrease in Ts, indicating a better hydrodynamics performance of biphasic mixtures with longer TLL.

Although they both have very good and similar hydrodynamics performances, the phase settling times recorded for each studied ABS are slightly different, with this tendency being more noticeable when the Ts of the biphasic mixtures with longer TLL are compared (i.e., TL5 of both systems). The values of Ts for TL5 of IPA/SS and IPA/SC were around 30 and 25 seconds, respectively.

Thus, it is possible to conclude that the differences between the Ts values of both ABS are not very significant from an industrial implementation point-of-view (i.e., both systems required less than 40 seconds to achieve a complete phase separation, presenting exceptionally fast demixing mechanisms and not limiting the process design). Since the ABS hydrodynamics were this similar and none of the ABS could be selected as the best to continue the experimental planning, the textile dyes’ partitioning was studies in both systems for the same biphasic mixtures used in the hydrodynamic studies (see section 3.3).

Dyes extraction performance using ABS

To assess the dyes extraction efficiency of IPA/SS and IPA/SC systems, 100 ppm of AO, RBBR and a 50/50 mix of both dyes was added to the biphasic mixtures previously used for the hydrodynamic studies. The amount of dye added was calculated based on the total mass of water and salt present in the biphasic mixtures, with the aim to simulate the use of a textile effluent composed of 100 ppm of dye, water, and sodium salts, in the formation of the studied ABS. The obtained results for K and EE for both ABS are presented in .

As displayed in , the partition coefficients for all systems were superior to 1, indicating a higher migration of the dyes to the alcohol-rich phase. As TLL increases, it is expected to obtain an increased partition of dyes into the alcohol-rich phase, increasing both the partition coefficient and the extraction efficiency, since the discrepancy between the phases is more pronounced. In other words, as the TLL increases, there is a greater difference in the alcohol and salt concentrations in the top and bottom phases, respectively, leading to a more significant salting-out effect and higher dye solubility in the alcohol-rich phase. However, there was no significant difference between the results obtained for different TLL as, for both salts, since TL1 it is already possible to obtain extraction efficiencies higher than 99% for AO, 98% for RBBR and 97% for the 50/50 mix. This means that the salting out effect in the first tie-line is already enough to induce the migration of almost all the dye molecules to the alcohol-rich phase.

Given that the results indicate no substantial differences in terms of both hydrodynamics and extraction efficiencies among the various tie lines, TL1 was selected for further studies in systems involving both SS and SC. This decision is based on the ability to achieve similar outcomes with reduced concentrations of alcohol and salt. The systems were subsequently employed to evaluate their dyeing ability on wool, aiming to determine if there are any notable differences in terms of dyeing performance when using different salts.

Comparison between IPA-based ABS dyeing performances

Furthermore, the wool dyeing performance of the systems was analysed using their IPA-rich phase (with the extracted dyes) from TL1. The IPA-rich phase of IPA/SC system was the first to be used in the dyeing tests, with the 5 %(v/v) aqueous solution of AA being used to correct the phase’s pH to 4. As SC induced a very alkaline environment in the biphasic mixture (pH ≈ 11), a considerable amount of water was required to achieve the desired pH value. Furthermore, SC is a very reactive salt and the pH change resulted in the protonation of SC and the production of sodium acetate (SA), as it can be seen in the following reaction:

2 CH3COOH + Na2CO3 --> 2 CH3COONa + CO2 + H2O

(i.e., 2 mol of acetic acid + 1 mol of sodium carbonate --> 2 mol of sodium acetate + 1 mol of carbon dioxide + 1 mol of water).

This reaction was also confirmed and macroscopically seen by the liberation of bubbles (i.e., gaseous CO2) upon the addition of AA in the IPA-rich phase for pH correction, with the total mass of the dyeing assays being recorded after their pH was 4.

For the IPA-rich phases of the IPA/SS systems, the amount of AA added was considerably lower than for the IPA/SC system, as the intrinsic pH of the system was much lower (pH ≈ 7) and SS does not react with the AA as SC does. Therefore, the amount of AA added was much less to correct the pH and the total mass of the systems afterwards was also lower. To reduce the differences between the dyeing assays of both biphasic systems studied (i.e., the assays with SC had considerably more total mass than SS, possessing more water, lower total concentration of dye and which would result in the addition of much more wool for the same amount of dye), ultrapure water was added to the assays of SS with corrected pH until both dyeing assays (whether obtained from IPA/SC or IPA/SS) possessed the same total mass for the subsequent dyeing process.

Once the assays were uniformized for both ABS, and the same amount of wool was added to all systems to remove the same amount of textile dyes, the dyeing process began, and the dyeing performance recorded. The obtained results are presented in .

Observing the results displayed in , it is possible to conclude that the dyeing performance using the IPA-rich phase of IPA/SC is higher than using the IPA-rich phase of IPA/SS for all the tested dyes (whether individually or mixed). Comparing the results for RBBR it is possible to see that the systems comprising SC obtained a maximum dyeing performance of around 80% after 120 minutes, while the ones with SS attained approximately 50% after the same time of operation. These differences are also observed for AO and were related to the intrinsic composition of each dyeing assay and their influence in dyeing the wool fibers, namely the amount of IPA and the types of salts present in the dyeing system. The dye uptake after 6 hours does not significantly differ from the final value of dye adsorption registered, which indicates effectively strong interactions between the wool fibers and the textile dyes used.

Effect of IPA in dyeing performance

When the IPA-rich phase of both ABS of this work and respective dyeing performances are compared, it is possible to see that the dyeing assay containing more IPA (i.e., the initial top phase of IPA/SC containing near 40% of IPA) presents higher dyeing adsorption than the dyeing assay containing a lower amount of this compound (i.e., the initial top phase of IPA/SS containing around 35% of IPA). Although, since the total mass of all the dyeing systems was around 12 g and the mass of the top phase was near 3 g, the dilution factor was roughly 4 and the final concentration of IPA was approximately 10% for IPA/SC and 8.75% for IPA/SS. Such a small difference cannot fully explain the near 30% difference between the dyeing performance of IPA/SS and IPA/SC. This indicates that other compounds present in the dye baths of IPA/SC were also improving their dyeing performance in comparison to IPA/SS, such as the sodium salts and AA present in the dye bath as well.

Effect of SS and sodium acetate in dyeing performance

Salts are very common compounds used in textile industries for dyeing fabrics, with sodium chloride, SS and SC being some of the most used. Sodium acetate is formed from sodium carbonate when the pH is adjusted with acetic acid. These compounds are typically called as exhausting agents, as they facilitate the movement of the dye molecules into the fiber and “exhaust” the dye bath [28], [29], [30].

SS (also called Glauber’s salt) is normally used as levelling agent in wet dyeing processes, reducing the negative charges on fibers and enabling the dyes to penetrate evenly into the fabrics. Although, it also has a negligible effect [31] or even a detrimental effect [28] on the absorption of acid dyes by wool. These results were attributed to the reduction of positive charges in the surface of the fibers by the presence of salts, leading to a lower affinity of the negatively charged anions of acid dyes towards the wool fibers [28].

SA is a salt commonly used in the dyeing process as well, also acting as a levelling agent and a buffer that maintains the pH throughout the dyeing operation and keeps the fibers soft and damage free [32], [33]. This compound is usually used in combination with AA to maintain the dye baths neutral/acidic and improve the wool dyeing performance, with their beneficial properties in dyeing this type of fabric being even reported in several patents [34], [35].

By analysing the results obtained in the present invention for the dye baths presenting SS or SA (i.e., from IPA/SS or IPA/SC, respectively), it is possible to observe that the dyeing systems presenting SA had much higher dye absorption than SS, for both types of dyes. Moreover, the European Commission (in the best available techniques for textile industry) recommends the application of SA and AA when using acid dyes for dyeing fabrics, with SS being avoided due to its little effect on migration and possible promotion of uneven dye adsorption [30].

Moreover, for both types of dyeing assays, it is noted that the dye bath with only RBBR led to the highest dye uptakes. Since RBBR is a reactive dye, it possesses more binding sites with the wool than AO, which means that a higher number of bonds between the wool and the textile dye can be achieved for RBBR and ease the dyeing of the fabric with this dye [36], [37]. Also, reactive dyes can establish stronger covalent linkages with the fibers molecules than acid dyes, with these last interacting electrostatically with the yarns chemical structures. This results in stronger interactions between RBBR and the wool than between AO and the same fabric, leading to a more prominent exhaustion of RBBR from the dye bath than AO [1], [38].

These results confirm the potential of the ABS-based process developed in this work, as the IPA/SC system can be used to both effectively extract the textile dyes from dye-contaminated aqueous solutions and use these recovered dyes in new dyeing processes. The proposed method in this work also comprises less additives and auxiliary chemicals in the dyeing process than that of conventional processes, which results in lower environmental impact, higher cost-efficiency, and techno-economic viability of these type of platforms.

Optimization of IPA/SC dyeing performance

With the data from clearly indicating that the dye baths obtained from the IPA/SC system have better dyeing performance than the ones attained from IPA/SS, the first was selected to continue the idealized experimental planning. Thus, the TL5 of IPA/SC was also evaluated according to its dyeing performance and the results compared to TL1, in order to select the best TL for the recovery and reuse of textile dyes in dyeing wool. The obtained results are presented in .

Comparing the results of TL5 in with the ones from TL1 in for the system IPA/SC, it is possible to observe that TL1 presented considerably higher dye exhaustion. It should also be noted that the concentration of IPA in the IPA-rich phase for TL1 is lower than for TL5, which can improve the effect of SA and increase the wool dye uptake. The amount of alcohol used in alcohol-assisted dyeing is a very important variable which should the optimized, as to much alcohol can also have detrimental influence on the dyeing performance of the studied fabrics[18]. For instance, the presence of too much alcohol can reduce the interfacial tension between the dyes in the bath and the yarns, acting like a surfactant [18].

After the selection of TL1 as the best TL to provide better dyeing performances, a new biphasic mixture was formed on TL1 (i.e., with the same TLL of the initial biphasic mixture on TL1) and further studied in this invention. This new biphasic mixture was on TL1 and was composed of 10.6 %wt. of IPA, 15 %wt. of SC and the reminiscent of ultrapure water, with the extraction efficiency being similar to the first biphasic mixture of TL1 studied (i.e., the new biphasic mixture presented approximately EE = 98% and Ts = 38.86 ± 0.9 seconds, within the same range of the results obtained for the initial biphasic mixture of TL1). These results confirmed that both the partitioning efficiency of the textile dyes and the ABS formation hydrodynamics were maintained for the new biphasic mixture, and it was possible to concentrate the same amount of textile dyes in a smaller volume of IPA-rich phase. Afterwards, the wool dyeing performance of the IPA-rich phase from this new biphasic mixture was analysed, with the obtained dye adsorption profiles being exhibited in .

The wool dyeing performance displayed in for the new biphasic mixture of TL1 was better than the dye uptake recorded inf for the initial biphasic mixture of TL1 of the IPA/SC system. Although the concentration of alcohol in the top-phases for both biphasic mixtures are equal, the volume of the phase is reduced for the new point. For instance, while the mass of the top-phase in the initial biphasic mixture was around 3 g, for the new biphasic system it was only around 1.7 g. Since the total mass of all the dyeing systems, after the addition of water and AA, was around 12 g, the alcohol in the dyeing systems for the new biphasic mixture was more diluted, resulting in a less concentrated dyeing media. To confirm this hypothesis, the dyeing performance of the systems were compared with control samples with and without the presence of alcohol. The results confirm that, although the presence of alcohol increases the dyeing rate, a lower concentration of alcohol leads to a better dyeing performance.

With the increased dilution of the new biphasic mixture, the dyeing medium not only becomes less concentrated in alcohol but also less concentrated in SC and, consequently, less concentrated in SA. However, even with a lower concentration of SA in the dyeing medium of the new mixture, an improved dyeing performance was observed, further affirming the detrimental influence that a higher alcohol concentration has on the dyeing system. Another plausible hypothesis to account for the observed trend is that, in addition to the lower concentration of alcohol, the presence of more water could have played a significant role in improving the dyeing rate of the dyeing system composed of the new biphasic mixture.

Clearly, due to the influence of various factors, the dyeing medium employing the new biphasic mixture in TL1 demonstrates the most favorable dyeing performance. Consequently, further investigations into this system were pursued, without implementing any additional optimizations, seeking to validate the viability of the idealized process.

Process circularity and dyeing cycle performance

Aiming at a circular process, recycling studies were performed in order to reuse the remaining unfixed dyes (i.e., 10% of the initial amount of dye used in the dyeing process was unfixed) and auxiliary compounds present in the dye bath. illustrates a schematic representation of the idealized circular process to facilitate the process overview.

As depicted in , the dyeing wastewater contains approximately 8 wt% salt, 5 wt% alcohol, and a residual amount of dyes. In order to concentrate and reuse this lower amount of dyes in subsequent processes, a second extraction was carried out. To achieve this, the salt-rich phase from the initial dyes’ extraction was added to the dyeing effluent in order to reuse the salt it contains in the formation of a new biphasic mixture, able to recover the remaining dyes in the dye bath. A makeup solution of salt and alcohol was added to attain the same biphasic mixture composition used in the first dyes’ extraction (i.e., concentrated TL1 of IPA/SC system) and promote the phase separation. The obtained two-phase system comprised an alcohol/dye-rich phase, containing the majority of the remaining dyes in the final dye bath, and a salt-rich phase with the same composition as the phase obtained in the first extraction step. The alcohol and dye-rich solution, with residual salts, were posteriorly used as the top phase of a new extraction step for recovering textile dyes in a new dye-contaminated aqueous solution, presenting the same amount of dye used in the first extraction step at the beginning of this process. To obtain the required mixture point composition used in the previous extraction steps, a makeup of alcohol and salt was also made. The detailed composition of all mixtures is disclosed in .

After the phase separation and extraction of the dyes from the new dyed aqueous solution, the dye-rich phase was used in a new dyeing process, as previously performed for the first extraction of textile dyes. presents the wool dyeing performance using the recovered compounds from the previous dyeing process and the dyes extracted from the fresh dyed effluent.

As illustrated in , the wool dyeing performance in the successive dyeing cycle presented approximately 90% of dye adsorption after 60 min of process and stabilizing afterwards. Herein, the dyeing process was only evaluated for 180 min, as previous experiments have already established that the adsorption rate stabilizes in roughly 60 min. Comparing Figures 6 and 9, it is possible to observe that the dye uptake for both dye baths result in similar dyeing performance, which demonstrates the reliability of the envisioned process as a circular platform for the extraction, concentration and reuse of textile dyes from dye-contaminated solutions. These results indicate that this process has potential to consistently deliver remarkable extraction and dyeing results across multiple cycles under optimized conditions, while making use of recovered compounds and reducing the amount of textile dyes present in the final textile effluents.

Considering the collective data presented here, it becomes clear that this innovative alcohol-based dyeing process for wool, applied for two distinct types of textile dyes, acid dyes (AO) and reactive dyes (RBBR), has demonstrated remarkable effectiveness. This pioneering approach, which employs alcohol/salt aqueous biphasic systems (ABS) with high extraction efficiencies and enhanced hydrodynamic behavior, not only showcases its potential but also addresses critical environmental concerns in the textile industry. The success of the process disclosed herein in achieving consistent dye uptake and performance across cycles establishes a strong foundation for potential industrial applications. The scalability of this process holds significant promise for large-scale textile production, where resource efficiency and sustainability are vital concerns. In this context, this innovative approach not only offers enhanced dyeing performance but also aligns with the growing necessity of environmentally conscious and economically viable textile manufacturing practices. A comprehensive overview of the industrial application for the proposed process is displayed in , comprising both the circular dyeing process and the subsequent wastewater treatment.

As observed in , the proposed process can be fully integrated within the well-established textile wet processing, as the textile dyes recovered from effluents produced in the conventional dyeing process can be recovered and reused, producing new dyed fabrics. This enables, not only the financial benefits from the reuse of otherwise wasted textile dyes, but also the higher profits resulting from the alcohol-assisted dyeing fabrics and the presumably lower cost in the treatment of wastewater with lower organic loads.

Overall, the present invention provides a pioneer approach for the recovery and reuse of textile dyes from dye-contaminated effluents and can be paramount for the transition of the textile industry economy model from linear to circular, increasing its environmental and financial benefits.

This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.

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Claims (10)

1. A process for the recovery and reuse of dyes from an aqueous effluent comprising dyes, comprising the following steps:
   Mixing at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, with a primary aqueous effluent in sufficient amount to obtain a primary alcohol/salt aqueous biphasic system comprising between 0.1 and 99.9 wt% of at least one alcohol and between 0.1 and 99.9 wt% of at least one salt;
   Separating a primary dye/alcohol-rich phase and a primary salt-rich phase of the primary alcohol/salt aqueous biphasic system;
   Adjusting the pH of the primary dye/alcohol-rich phase;
   Using the pH-adjusted primary dye/alcohol-rich phase in an alcohol-assisted dyeing process obtaining a secondary aqueous effluent comprising dyes;
   Mixing between 0.1% and 99.9% wt% of the primary salt-rich phase with the secondary aqueous effluent;
   Adding at least one short-alkyl chain alcohol, or at least one short-alkyl chain alcohol and at least one salt, to the previous mixture in sufficient amount to obtain a secondary alcohol/salt aqueous biphasic system with the same concentration of the primary alcohol/salt aqueous biphasic system;
   Separating a secondary dye/alcohol-rich phase and a secondary salt-rich phase of the secondary alcohol/salt aqueous biphasic system;
   Mixing the secondary dye/alcohol-rich phase with a primary effluent to obtain a tertiary alcohol/salt aqueous biphasic system with the same concentration of the previous alcohol/salt aqueous biphasic systems;
   Repeating the steps of separating the dye/alcohol-rich phase and the salt-rich phase, and using a pH-adjusted dye/alcohol-rich phase in subsequent alcohol-assisted dyeing process.
2. Process according to the previous claim, wherein at least one short-alkyl chain alcohol is selected from methanol, ethanol, propanol, isopropanol or isopropyl alcohol, butanol, isobutanol, sec-butanol, or tert-butanol.
3. Process according to any of the previous claims, wherein at least one salt is selected from triammonium citrate, diammonium sulfate, trisodium citrate, dipotassium hydrogen phosphate, tripotassium citrate, sodium dihydrogen phosphate, tripotassium phosphate, potassium carbonate, sodium chloride, sodium thiosulfate, sodium sulfate, sodium carbonate, disodium hydrogen citrate, disodium tartrate, diammonium hydrogen citrate, dipotassium oxalate, sodium acetate, potassium acetate, potassium chloride, ammonium acetate, calcium chloride, lithium sulfate, lithium acetate, or lithium nitrate.
4. Process according to any of the previous claims, wherein the alcohol/salt aqueous biphasic system comprises isopropyl alcohol between 5 and 50 wt% and sodium sulfate between 5 and 25 wt%.
5. Process according to any of the previous claims, wherein more alcohol and/or salt is added to the subsequent aqueous effluents in sufficient amount to obtain an alcohol/salt aqueous biphasic system with the same concentration of the primary alcohol/salt aqueous biphasic system.
6. Process according to any of the previous claims, wherein the primary aqueous effluent is an aqueous effluent obtained from an industrial process using dyes.
7. Process according to any of the previous claims, wherein the phases are separated using any processing operation able to separate two liquid immiscible phases.
8. Process according to any of the previous claims, wherein all steps of mixing of the aqueous effluent with the at least one short alkyl-chain alcohol, or the at least one short alkyl-chain alcohol and at least one salt, to obtain the alcohol/salt aqueous biphasic system, occur at a temperature between 10 and 90 ºC and a time between 30 seconds and 60 minutes.
9. Process according to any of the previous claims, wherein all steps of mixing of the salt-rich phase with the aqueous effluent occur at a temperature between 10 and 90 ºC and a time between 30 seconds and 60 minutes.
10. Process according to any of the previous claims, wherein the pH of the dye/alcohol-rich phases is adjusted between 2 and 13.