4.1. Materials
In this work, three dyes with different characteristics were used to test the method when there is a different polarity of the dye and the dyeing conditions are different. An anionic dye of the direct type called direct blue 199 CI 74180 (DB199), another cationic dye classified as basic with the denomination Basic Yellow 2 (BY2) C.I. 41000 and finally a non-ionic dye without polarity of the disperse class referred to as Disperse Red 1 CI 11110 (DR1) were used. Their structures are shown in
Figure 9.
Hydrotalcite Mg
6Al
2(CO
3)(OH)
16·4(H
2O) [
66,
67,
68] of Sigma Aldrich Gillingham (U.K), was the adsorbent used. There are different methods that can be used to achieve the adsorption of dyes in this kind of nanoclay. For this study, calcination was selected following the method described by Dos Santos R.M.M. [
27]. This method, called calcination, consists of introducing the clay in a kiln at 600 °C for 3 h in order to destroy the H structure and reduce the presence of certain anions such as CO
32− carbonate, which will allow for the incorporation of new negatively charged compounds in a later reconstruction phase during hydration thanks to the shape memory of this mineral. After this calcination process, it is considered to be calcined hydrotalcite (HC). Its lamellar structure will allow it to adsorb and fix other elements that are not anionic, so this material is not exclusive for adsorbing negatively charged pollutants.
Figure 2 shows the change in the structure of the H in the three cases described, prior to calcination, after being subjected to 600 °C for 3 h and after hydration and reconstruction. It is worth noting that in the last SEM and TEM images (
Figure 10), it can be seen how the clay structure has been reconstructed [
69]. Within
Figure 10, image “b” shows how the structure has been destroyed by calcination and in figure “c” it can be seen how the structure has been recovered, being very similar to the original “a”. In addition, in image “e” it can be seen how the layers are further apart after calcination, which improves the adsorbent capacity as it is more likely that the adsorbate can be introduced between these layers.
4.2. Synthesis Methods
After a dyeing process by depletion, the dye remains in the dye bath; just as after the adsorption effect, the dye that has not been adsorbed may remain [
47,
70]. In order to determine the amount of dye in the form of g·L
−1 concentration in each of these cases, simple regression models by Lambert-Beer [
71] are used beforehand. Starting from various dilutions of the dyes at controlled concentrations, the adsorbance can be measured using a transmission spectrophotometer and the equations given in
Table 6 can be obtained.
The first objective to be met in the study is to achieve the maximum possible dye adsorption in order to leave the water completely clean. To assess the adsorption capacity of the hydrotalcite, 4 L of a solution of each of the dyes to be studied was prepared at a concentration of 1 g·L
−1. The next step was to introduce the nanoclay into these solutions. The amount introduced was 3 g·L
−1. Once the mixture is prepared, it is subjected to agitation using a magnetic stirring system in which the maximum possible speed is applied at 1600 r.p.m. for 2 h. The speed is then changed to 500 r.p.m. for a further 22 h [
72]. In the first 2 h, the aim is to achieve penetration of the dye with maximum centrifugal force, but then the speed is lowered to ensure that the dye does not come out of the clay again and remains as stable as possible, allowing its accommodation in the structure of the reconstructed nanoclay.
The hybrid formed by the hydrotalcite and the dye is then separated from the water. To do this, the solution is filtered using filter paper and all the aqueous part is separated from the solid part by gravity so that the hybrid can be collected in solid form after 48 h. Samples are then taken from the water that has fallen by gravity. This water is taken to the transmission spectrophotometer where, with the absorbance reading and using the equations in
Table 5, the concentration of dye that still remains in the solution and has not been adsorbed is calculated [
73,
74]. On the other hand, the solid hybrid is freeze-dried [
26,
27] in order to extract all the water and avoid agglutinations that could occur during drying in the oven. In this way, the hybrids identified as HDB199, HBY2 and HDR1 are obtained for the union of the dyes DB199, BY2 and DR1, respectively.
The desorption process is described as a phenomenon in which there is a transfer of the dye from the adsorbate in the solid to the liquid phase [
75,
76]. Several models [
77,
78,
79] explain different theories involving isotherms on how this desorption occurs, describing them as non-ideal and reversible adsorption/desorption systems. Another describes a system in which there is not always an interaction between neighbouring active sites due to the non-homogeneity of the nanoadsorbate, and therefore, no homogeneous adsorption. All these theories give an insight into the true nature of the adsorption/desorption process. The authors Momina, Shahadat Mohammad, and SuzylawatiIsamil [
80] explain a desorption model for methylene blue (MB) by first subjecting the hybrid to high temperatures to weaken the bonds and then using various solvents such as HCl, ethanol, and nitric acid or acetone. They argue that any one of these phases alone is not sufficient to produce good desorption results.
In this study, a simultaneous desorption-dyeing process is proposed, in which, based on the theory that temperature weakens the bond between the clay and the dye [
80], subsequently taking advantage of the dye-fibre affinity and the dyeing process commonly used so that the dye migrates completely from the hydrotalcite to the textile fibre (
Figure 11). Furthermore, in the model of this work, heat is applied by convection and not by radiation as in the model of the authors Momina, Shahadat Mohammad, and SuzylawatiIsamil, as this heat is more effective at reaching more areas of the clay and is also more energetic.
The clay-dye hybrid is then used as a dyeing material for dyeing by exhaustion, using a bath ratio of 1/40. For the dyeing of a 100% cotton (CO) openwork fabric with a grammage of 135 g·m−2, 25 yarns·cm−1, 22 weft·cm−1 of plane weave openwork fabric, the direct dye hybrid HDB199 is used, 40% s.p.f. of clay + dye, 20 g·L−1 of sodium sulphate and three drops of a wetting agent are added to the dyeing bath to submit it to the dyeing process for 60 min at 100 °C, obtaining the dyed fabric referenced as TDB199. For the dyeing of a 100% polyester fabric (PES) 200 g·m−2, 13 yarns·cm−1, 52 weft·cm−1 of plain weave was introduced in a bath containing 40% s.p.f. of clay + HDR1 dye, for 60 min at 140 °C in a closed machine with 1 g·L−1 ammonium sulphate, 0.5 g·L−1 Dekol SN dispersant after previously adjusting the pH to 4.5–5 with acetic acid, thus obtaining the sample with reference TDR1. The last dyeing was on a fabric of 100% acrylic composition (PAN) a weft knitted fabric with eight rows per centimetre and nine columns per centimetre forming an English knitted weave in whose bath 40% s.p.f. of clay + HBY2 dye, acetic acid 2% s.p.f., 20 g·L−1 of sodium sulphate was added and processed for 40 min at 100 °C, thus obtaining the dyed fabric referenced as TBY2. The dyeing of the polyester fabric was carried out in a closed machine due to the temperatures above 100 °C that must be used. The apparatus used was the Testherm type 9S from the manufacturer Talcatex S.A, San Sebastian de los Reyes (Spain). Conversely, the dyeing of acrylic and cotton fibre was carried out in the open machine referenced as Open Bath dye Master from the manufacturer Paramount S.A, Geneva (Switzerland). All the fabrics were washed after dyeing to eliminate any remaining dye that was not fixed to the fibres.
After the dyeing process described above, the clay that was in the dye baths was collected again to assess the desorption that it has undergone. For this purpose, the dye baths are separated from the dyed fabrics and are again filtered by gravity with filter paper, as was conducted in the previous process. The clay is analysed again after dyeing to assess its colour change and other characteristics that may have been altered after this process. From each of the hybrid samples HDB199, HBY2 and HDR1, new clay-dye hybrids are obtained from the remainder collected after the dyeing, respectively, referred to as H2DB199, H2BY2 and H2DR1.
4.3. Characterisation
The colour measurement of the obtained hybrids was studied using the Jasco V-670 double UV-VIS/NIR spectrophotometer. Measurements were carried out in the range of 2700–190 nm at a frequency of 0.5 nm. The Jasco V-670 is equipped with a double-grating monochromator. The first grating monochromator is used for the UV-VIS region serving 1200 grids-mm
−1 which is equipped with detectors based on a photomultiplier tube. On the other hand, the second grating is used for the rest of the spectrum studied, i.e., for the IR infrared region, but this time with 300 grids-mm
−1 and using a PbS detector. Both gratings are equipped with an automatic system that allows them to adapt to changes in wavelength. The light sources were a halogen lamp (330–2700 nm) and a deuterium lamp (190–350 nm). The CIE-1964 observer was used under the D65 illuminant, reflectance factors were also applied to obtain optical values for comparison [
81].
A scanning electron microscope (SEM) model PHENOM (FEI Company, Eindhoven, The Netherlands) was used to perform the topographical analysis of the surface of the samples. It was operated at an acceleration of 5 kV. The previous sample preparation consists of sputtering with a palladium/gold alloy with an EMITECH sputter coater mod. SC7620 (Quorum Technologies Ltd., East Sussex, UK). As the coating thickness is only 5–7 nm it will not alter the readings. For TEM imaging, a JEOL model JEM-2010 transmission electron microscope was used. The image acquisition camera is a GATAN model ORIUS SC600. It is mounted on an axis with the microscope at the bottom and is integrated into the image acquisition and processing software GATAN DigitalMicrograph 1.80.70 for GMS 1.8.0.
The clay-dye hybrids were subjected to infrared spectrophotometer analysis in order to calculate the Fourier transform (FTIR). Due to the characteristics of the material to be analysed, the horizontal attenuated total reflection technique (FTIR-ATR) was used using a ZnSe prism. The instrumentation used for the readings was the Jasco FTIR 4700 IRT 5200 spectrophotometer with a DTGS detector sensor. It was necessary to use a pressure accessory to obtain a uniform reading on each of the samples. The spectrophotometer worked at a resolution of 4 cm−1 and scanned 64 scans.
Continuing with the characterisation of the hybrids obtained from the clay-dye, these samples were subjected to X-ray diffraction (XRD) tests [
82,
83] in order to analyse their behaviour, especially the changes in their lamellar structure during the calcination process and reconstruction during rehydration. Special attention is paid to the basal space between the hydrotalcite lamellae, which produces the adsorption of both ions and other non-ionic substances. For this purpose, the RD bruker D8-Advance (Bruker, Billerica, MA, USA) with a Göebel mirror (power: 3000 W, voltage: 20–60 kV and current: 5–80 mA) was used. The analysis was performed in an oxidising atmosphere at an angular velocity of 1°/min, STEP 0.05°, and an angular sweep of 2.7–70°. The diffraction patterns were indexed by making a comparison with the JCPDS files.
The dyeing samples of the three dyes were subjected to different fastness tests to assess their correct dyeing and subsequent behaviour. In order to check their fastness to washing, each of the samples was subjected to washing according to the UNE-EN ISO 105-C06:1994 standard using the Linitest described in this standard. The test carried out was the A1S test described in the standard at a temperature of 40 °C for 30 min and with a bath volume of 150 mL. The pH was not adjusted and 10 steel balls were added to generate an abrasive action. Tests for colour fastness to ironing were carried out according to UNE-EN ISO 105-X11 using a pressure plate. The tests were carried out in wet, damp and dry conditions as stated in the standard. The ironing time for all samples was 15 s at a temperature of 200 °C for PAN and PES fabrics, although for CO fabrics it was conducted at 150 °C, as cotton may yellow at higher temperatures. To assess the colour fastness to rubbing, the Crockmeter was used according to the UNE-EN ISO 105-X12 standard. This test was carried out wet and dry as described in the standard.
Colour degradation and discharge were measured instrumentally using a Minolta CM-3600d reflection spectrophotometer in the range 360–740 nm with a step of 10 nm according to UNE-EN ISO 105-A05 for degradation and UNE-EN ISO 105-A04 for discharge. The results are expressed according to the grey scale as stated in the aforementioned standards.
BET analysis was performed to measure the surface area, pore volume and pore size using nitrogen adsorption and desorption values at −196 °C on a Micromeritics ASAP-2020. The samples are first degassed in a vacuum atmosphere at temperatures between 150 °C and 200 °C so as not to carbonise any elements in the sample [
64,
65].