Obtention of ZnO-Based Hybrid Pigments: Exploring Textile Dye Adsorption and Co-Adsorption with Copper Ion

Abstract

Annually, more than 10,000 synthetic dyes are produced worldwide, generating around 280,000 tons of waste, posing risks to human and aquatic life, and potentially creating even more toxic products than the dyes themselves. This study aims to immobilize organic dyes, forming hybrid pigments using ZnO as support obtained through starch combustion. ZnO was obtained by starch (sago) combustion and characterized by XRD, SEM and the BET method. It was then used for the adsorption of orange and green textile dyes, evaluating the adsorbent dosage, initial dye concentration, contact time, and selectivity with copper ions. The removal studies indicated up to 100% removal of both dyes at low concentrations. The co-adsorption system showed excellent performance, with removal percentages exceeding 90% for both textile dyes and Cu (II) ions. Hybrid pigments were assessed for solvent resistance and durability under extended white light exposure. ZnO immobilized the dyes, showing resistance to organic solvents and good stability under prolonged white light exposure.

1. Introduction

Population growth and industrialization have compromised water quality. Wastewater is the water that is thrown away after it is used in different processes. It can be classified as domestic or industrial, depending on where it came from. Dyes and textile products are the main cause of pollution. Each year, over 100,000 different synthetic dyes and pigments are made worldwide. Over 15% of these dyes are released into the environment, causing 280,000 tons of dye waste each year. Additionally, domestic wastewater is also a source of synthetic dyes [1,2,3,4]. Figure 1 presents the main sources of wastewater and their contaminants [5,6].

In Indonesia, the amount of gray water in the country’s wastewater is 1 to 4 times more than that of black water. The amount of gray water that is not treated is 3 to 6 times more than that of untreated black water. This is because gray water is much more polluted and is not thrown away properly. According to the 2024 United Nations report, of the 268 million m3 of domestic wastewater generated worldwide, only 58% is treated well. This is more than industrial wastewater generated, where industry throws away 35 billion m3 and 38% is treated [6,7]. In-home wastewater, dye residues, whether from food, cosmetics, printer inks, hair dyes, or dyes used for dyeing fabrics, are used and thrown away without any treatment.

Textile dyes are resistant to biodegradability, may undergo incomplete degradation or transformation through interaction with sediments, pollutants, or other intermediates, forming even more harmful compounds, directly impacting ecosystems and natural processes. They hinder photosynthesis, promote eutrophication, and affect water quality by increasing salinity, chemical and biological oxygen demand, suspended solids concentration, etc. For humans and animals, they present teratogenicity and mutagenicity and can be toxic to reproduction and cause other toxicological effects [1,2,8].

Methods for treating textile effluents include physical, chemical, and biological methods, such as advanced oxidation processes, coagulation/flocculation, filtration, ion exchange, activated sludge processes, photocatalysis, and adsorption, among others. However, degradation processes may not be effective for dyes due to their complex molecular structure, containing long carbon chains and high stability [8,9,10].

The adsorption method is among the most important due to its speed, low cost, high efficiency and selectivity, ease of application, and no sludge production. It is employed for the remediation of numerous pollutants, including dyes. This method allows the application of various adsorbent materials, such as activated carbon, metal oxides, polymers, which are easily synthesized, have low cost, high surface area, high efficiency, and are multifunctional [10].

Zinc oxide (ZnO) has been widely studied due to its bioavailability, low cost, non-toxicity, biocompatibility, and high surface area. It is a white crystalline solid that can present three crystalline phases: hexagonal wurtzite, cubic zinc blende, and rock salt, with the wurtzite structure being the most thermodynamically stable [11].

Several studies point to the use of zinc oxide in the remediation of effluents contaminated by dyes, involving different synthesis methods, such as the synthesis of ZnO through the hydrolysis of zinc nitrate with sodium hydroxide, used in the removal of the dye Violet Isnato 2R (IV2R) [12]. Synthesis of ZnO nanocomposites, Al/ZnO and W/Ag/ZnO for the removal of turquoise blue dye [13]. Green synthesis of ZnO and Mn/ZnO mediated by Phoenix dactylifera used in the adsorption of disperse orange acid dye [14]. Synthesis of ZnO/chitosan for the removal of Eriochrome Black T [15]. Sol–gel synthesis of Cr-doped ZnO for the removal of Congo red [16]. ZnO/activated carbon derived from wood sawdust was applied in the adsorption of methyl red and methyl orange [17]. Synthesis of ZnO using cassava starch in the removal of Congo red dye [18].

A way to avoid the formation of new waste after dye adsorption or the disposal of the dye by regenerating the adsorbent is by using the formed material as a hybrid pigment. Hybrid pigments are obtained by stabilizing dyes in inorganic matrices, such as clays, mesoporous materials, and zeolites, where dye molecules intercalate in confined spaces or absorb on the surfaces of the inorganic adsorbent. Hybrid pigments present improved color and stability, with the inorganic matrix offering thermal stability, mechanical resistance, and solvent resistance, while the organic component provides color and functionality [19,20].

Thus, the aim of this work is to obtain hybrid pigments using ZnO as an adsorbent, employing sago starch as fuel. Characterization of the adsorbent, and study of the best conditions for domestic dye removal in aqueous solution and selective ion, as well as testing the resistance of the hybrid pigment to interaction with solvents and prolonged exposure to white light.

2. Materials and Method

2.1. Synthesis of the ZnO-st

Zinc oxide was obtained through the adaptation of methodologies [21,22], using zinc nitrate hexahydrate ((Zn(NO₃)₂ 6H₂O), 98% purity), purchased from Neon Commercial Analytics Reagents, and sago, obtained from local commerce, as fuel in the synthesis. Both reagents were used without any previous treatment. The synthesis was carried out by adding 100 g of sago to 500 ml of distilled water, keeping it immersed for 24 h. Then, the sago was crushed in a blender, and 2% (wt.%) zinc nitrate related to the suspension was added, remaining under mechanical stirring at 600 rpm for 60 min. The ZnO precursor was calcined in air in a muffle furnace for 2 h at 600 °C, with a heating rate of 10 °C per minute, to obtain porous ZnO-st.

2.2. Characterization of ZnO-st

The X-ray powder diffraction profile was performed using a D2 Phaser instrument (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.541 Å, 2θ range from 20° to 70°, step rate of 0.2°/s) [22]. The Brunauer–Emmet–Teller (BET) surface area, porosity, and N₂ adsorption-desorption isotherm were measured using a Nova 800 instrument (Anton Paar, Graz, Austria) with N₂ gas at 77 K (P/P₀ = 0.015 to 0.95), analyzed using the Kaomi for NOVA Software (2.1.8) application.

2.3. Adsorption Experiments

For the study, two household textile dyes, sold in local stores, were chosen for adsorption studies based on early tests done with different textile dyes. The dyes chosen removed most water from the simulated wastewater. Additionally, their high color intensity made them particularly suitable for the development of hybrid pigments with improved visual properties. The dyes 05-orange and 20-green used in this study are commercial products from Guarany^® Tingecor (Itu, SP, Brazil). This dye is made for natural fibers. According to the supplier, the chemical composition consists of a solid mixture of sodium chloride (NaCl), direct dye, and a dispersant.

The effect of adsorbent dosage on the removal of the textile dyes was evaluated by varying the adsorbent dosage from 100 to 500 mg in 50 ml dye solution of 200 mg L⁻¹ concentration, pH 6.0, and under constant magnetic stirring (15 rpm) for 120 min at room temperature (25 °C). The initial concentration of dye was studied by stirring 400 mg of the adsorbent at room temperature and pH 6.0, varying initial concentrations from 25 to 500 mg L⁻¹, while keeping the same contact period [18]. After the predetermined contact time was achieved, the adsorbate solution was withdrawn and centrifuged to separate the adsorbent. The concentration was determined using a UV-1800 spectrophotometer (Shimadzu, Kyoto, Japan), at wavelengths of 493 nm (05-orange) and 622 nm (20-green). All the experiments were performed in duplicate.

The dye removal efficiency was calculated using Equation (1):

$$\%\mathrm{ }\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}=\mathrm{ }{\displaystyle \frac{\left({\mathrm{C}}_{\mathrm{i}}-\mathrm{ }{\mathrm{C}}_{\mathrm{f}}\right)}{{\mathrm{C}}_{\mathrm{i}}}}\times 100$$

(1)

where C_i is the initial concentration and C_f is the final concentration of the dye [23,24,25].

The amount of dye at equilibrium (q_e), which refers to how many milligrams of dye were removed per gram of adsorbent, was calculated using Equation (2):

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{ }{\displaystyle \frac{\left({\mathrm{C}}_{\mathrm{i}}-\mathrm{ }{\mathrm{C}}_{\mathrm{f}}\right)}{\mathrm{m}}}\times \mathrm{V}$$

(2)

where C_i is the initial concentration, C_f is the final concentration of the dye, V is the volume of the solution, and m is the mass of the adsorbent [23,24,25].

For the adsorption study, the contact time was evaluated at different time intervals (5, 10, 15, 30, 60, 90, 120, 240, and 300 min), containing 50 ml of solution with a concentration of 400 mg L⁻¹. Kinetic models were applied to evaluate the rate of the dye adsorption process by ZnO-st, and Equations (3) and (4) are shown below:

Pseudo-first order:

$${\mathrm{q}}_{\mathrm{t}}={\mathrm{q}}_{\mathrm{e}}\left(1-{\mathrm{e}}^{-{\mathrm{k}}_1\mathrm{t}}\right)$$

(3)

Pseudo-second order:

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2\mathrm{t}}{1+{\mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}\mathrm{t}}}$$

(4)

where qt represents the adsorption at the time of t (min) and the k₁ (h⁻¹) and k₂ (g mg⁻¹ h⁻¹) the corresponding the rates constant of pseudo-first-order and pseudo-second-order process [25,26].

2.4. Co-Adsorption Test

The co-adsorption test to evaluate the selectivity of ZnO-st removal, 50 ml of a solution containing 200 mg L⁻¹ of each dye (orange and green) and 20 mg L⁻¹ of Cu (II) solution was used. The removal process was carried out using 400 mg of ZnO-st under magnetic stirring for 2 h. The removal percentage and the amount of adsorbed dye were determined by monitoring the maximum absorption peaks of the dyes at 493 nm and 622 nm [27] for the orange and green dyes, respectively, using a UV–vis spectrophotometer (Shimadzu UV-1800). The removal of copper was determined using a multiparameter photometer (model HANNA HB8300), LED with a narrow-band interference filter at 575 nm, using an adaptation of the EPA method with a detection range of Cu (II) ions, resolution of 0.01 mg L⁻¹, and accuracy of ±0.02 mg L⁻¹ ± 4% of the reading at 25 °C.

2.5. Hybrid Pigments Preparation

The powders obtained from the study on dye concentration variation (as detailed in Section 2.3) were assessed as hybrid pigments. For this, only the powders derived from concentration adsorption studies exceeding 100 mg L⁻¹ were considered (Figure 2). Following the adsorption study, the adsorbent was filtered and washed to recover it for use as hybrid pigments (adsorbed dye/ZnO-st). The pigments were then dried in an oven at 70 °C for 48 h. For evaluation, the hybrid pigments were dispersed in white paint at a ratio of 0.1 g of pigment per 1 g of white paint, which had a composition of 45–55% solids, a volatile organic compound (VOC) content of 13–33 g L⁻¹, and a pH level ranging from 8 to 11.5 (Acrylic Standard matt, produced by Suvinil BASF SA, São Bernardo do Campo, Brazil). The resulting mixture was stirred and applied to gypsum blocks using a brush.

2.5.1. Solvent Interaction Tests

The hybrid pigments were subjected to interaction with different solvents to evaluate the stability of the pigment and its precursor dyes. For this purpose, 0.1 g of the hybrid pigment was wrapped in filter paper as a semi-permeable membrane and added to each tube with 15 ml of acetone (99%), ethanol (99%), and distilled water, remaining in contact for 24 h. The same procedure was carried out for the precursor dyes. Subsequently, the wavelengths of the dyes were monitored to quantify the release of the pure dye and the hybrid pigment [18,28].

2.5.2. Light Exposure Test

The gypsum blocks containing hybrid pigments dispersed in white paint were exposed to white light irradiation using an LED lamp with an illumination intensity of 92 Klx for a period of 7 days [29,30].

2.5.3. Colorimetric Analysis

For the analysis of these tests, the colorimetric method with CIELAB measurements was employed. The coordinates L*, a*, and b* were obtained, allowing the calculation of color variation before and after dispersion in white paint and after prolonged exposure to white light [31]. The color variations were calculated using Equation (5):

$$\mathsf{\Delta }\mathrm{E}=\sqrt{{\left({\mathrm{a}}_{\mathrm{f}}-{\mathrm{a}}_{\mathrm{i}}\right)}^2+{\left({\mathrm{b}}_{\mathrm{f}}-{\mathrm{b}}_{\mathrm{i}}\right)}^2+{\left({\mathrm{L}}_{\mathrm{f}}-{\mathrm{L}}_{\mathrm{i}}\right)}^2}$$

(5)

3. Results and Discussion

3.1. Zinc Oxide Characterization

The obtained zinc oxide was characterized using X-ray diffraction (XRD), SEM, and BET method, as described in the experimental section, and the results are shown in Figure 3. The crystalline phase exhibits characteristic peaks of the wurtzite phase of zinc oxide, with a hexagonal close-packed structure and space group P63mc, as per the crystallographic chart [JCPDS, #PDF01-089-7102]. No additional peaks were detected, indicating that the obtained ZnO is monophasic. The absence of impurities confirms the complete decomposition of the precursors. The XRD patterns allowed for the calculation of the average crystallite size of the ZnO particles, resulting in a value of 30.84 nm [22,32].

Figure 3b presents the N₂ desorption/adsorption isotherm of the ZnO synthesized at 600 °C. As observed in Figure 3b., the results show an isotherm of type IV.a with a type H3 hysteresis loop. This is characteristic of a mesoporous material (2 < pore size < 50 nm) [33]. The H3 loop indicates that the synthesized ZnO is given by non-rigid aggregates of plate-like particles [33]. The textural properties, such as BET specific surface area (28.846 m² g⁻¹) and pore volume (0.0578 cm³ g⁻¹) were determined from the N₂ adsorption–desorption isotherm, and pore diameter (3.22 nm) was determined by BJH. The scanning electron microscopy images of the ZnO-st sample are shown in Figure 3c. The SEM image reveals a material composed of spherical particles with a high degree of aggregation, forming densely packed clusters. Due to the small size of the particles and their proximity within these aggregates, distinguishing individual spheres is challenging.

3.2. Adsorption Study

The dye solutions were analyzed using ultraviolet–visible spectrophotometry (Figure 4). The maximum absorption band for the orange dye was identified at 493 nm. For the green dye, two characteristic bands were observed: one at 381 nm and another at 622 nm. For the adsorption study, only the band in the visible region was monitored.

Figure 5 shows the results of the adsorbent dosage study, where q_e represents the amount of dye adsorbed per gram of ZnO-st at equilibrium (in mg g⁻¹), and % removal represents the percentage of dye removed from the solution. The best result for both dyes was achieved with a dosage of 400 mg of ZnO-st. The green dye exhibited the highest removal percentage, reaching 99.13% dye removal, with an efficiency of 26.56 mg of dye per gram of ZnO-st. For the orange dye, 96.93% removal was achieved, with an efficiency of 19.48 mg g⁻¹. Increasing the adsorbent dosage at fixed adsorbate concentrations enhances dye removal performance, as it provides greater availability of binding sites on the adsorbent surface and a larger active surface area [24]. However, as the mass of ZnO-st increases, the removal efficiency of the dyes decreases. This is because excessive adsorbent concentration leads to particle agglomeration, reducing the total adsorption area and the amount of adsorbate per unit mass of adsorbent [24,34,35].

The effect of initial dye concentration variation can be observed in Figure 6. This factor indirectly affects the efficiency of the adsorption process by either decreasing or increasing the availability of binding sites on the adsorbent surface [24,34]. The highest removal percentages were observed at concentrations of 25 and 50 mg L⁻¹ for the orange dye, while for the green dye, the removal percentage remained high up to 100 mg L⁻¹. As the dye concentration increases, the removal percentage decreases due to greater competition among dye molecules for the active sites on the adsorbent. The availability of these active sites becomes saturated as a certain amount of dye is adsorbed, preventing additional adsorption and promoting the dispersion of more dye molecules into the solution [24,34,35].

An inverse effect is observed in removal efficiency, which increases proportionally with the increase in dye concentration, reaching 47.45 mg g⁻¹ for the orange dye at 500 mg L⁻¹ and 51.88 mg g⁻¹ for the green dye at the same concentration. Removal efficiency increases with higher initial dye concentrations because, as the dye concentration rises, more molecules are adsorbed per gram of adsorbent due to the increased number of collisions between dye molecules and adsorbent particles. The increase in dye concentration acts as a driving force for the adsorption process, favoring diffusion and mass transfer from the solution to the adsorbent surface [24,34].

The effect of contact time is demonstrated in Figure 7. The adsorption rate of both dyes increases rapidly within a contact time of 60 min for the orange dye and 120 min for the green dye, where the probability of having more available and accessible active sites is higher. As the species are adsorbed onto the sites, a new equilibrium is established, allowing more molecules of the species to settle, enabling greater adsorption [18]. Thus, an equilibrium phase is established when the curve levels off, becoming stable after 240 min for both dyes.

The summary of parameters and the regression coefficient (R²) for the two kinetic models is presented in

Table 1. Kinect parameters for absorption of orange and green dyes onto ZnO-st.

		Pseudo-First Order			Pseudo-Second Order
Sample	qexp (mg g−1)	k1 (h−1)	qcal (mg g−1)	R2	k2 (g.mg−1 h−1)	qcal (mg g−1)	R2
Orange dye	91.23	42.13	1.66	0.8394	1.78 × 10−5	91.32	0.9999
Green dye	78.12	2.63	11.12	0.9622	0.23	84.74	0.9997

. The pseudo-second-order models for both dyes provided the best fit for the data. For the orange dye, the R² value was close to 1.00, and the calculated adsorption capacity was 91.32 mg g⁻¹, which is close to the experimental value (91.23 mg g⁻¹). For the green dye, the R² value was 0.9997, with the calculated adsorption capacity from the model being close to the experimental value of 78.12 mg g⁻¹. This kinetic model suggests the existence of chemisorption. However, since the adsorption phenomenon is a complex process, it is difficult to distinguish between physisorption and chemisorption based solely on this factor [36,37].

3.2.1. Statistical Analysis

A one-way analysis of variance (ANOVA) was conducted to compare two groups of dyes. Following this, Tukey’s post hoc test was employed for pairwise comparisons between the groups, as the ANOVA results demonstrated statistical significance at the alpha level (p < 0.005). The ANOVA findings for the 05-orange and 20-green textile dyes, concerning initial dye concentration and contact time, are detailed in Tables S1 and S2, respectively. The ANOVA results revealed that both initial dye concentration and contact time had highly significant effects (p < 0.05). As a result, the outcomes of the Tukey HSD test are summarized in Tables S3 and S4. Table S3 compares the efficacy of ZnO-st as an adsorbent across different initial concentrations of the dyes studied. The percentage removal of 05-orange and 20-green textile dyes using ZnO-st as an adsorbent exhibited significant differences at higher concentrations, with these differences becoming more pronounced as the concentration increased. However, at lower concentrations, no significant differences were observed. Furthermore, the results from the Tukey HSD test in Table S4 indicate that, as contact time increased, there were no significant differences in the percentage removal of the dyes at a concentration of 200 mg L⁻¹ using ZnO-st as an adsorbent.

3.2.2. Co-Adsorption

The selectivity of dye adsorption was evaluated using a solution containing 20 mg L⁻¹ of Cu²⁺ and 200 mg L⁻¹ of the respective dyes. For the orange dye, 98.13% removal was achieved, with an efficiency of 19.72 mg g⁻¹. The significant reduction in the orange dye band due to adsorption can be observed in Figure 8a, as well as the decolorization of the solution (Figure 8b) corresponding to dye adsorption. The detection of copper ions (Figure 8c) using the photometric method (adaptation of the EPA method) described in the experimental Section 2.4 resulted in a 97% reduction of copper ions in the solution, with a residual concentration of 1.94 mg L⁻¹.

The same co-adsorption test was applied using green dye. The reduction in the dye band (Figure 9a) and the decolorization of the solution due to removal (Figure 9b) resulted in 100% dye removal, with an efficiency of 21.5 mg g⁻¹. The reduction of copper ions in the solution was 98.42% (Figure 9c), with a residual concentration of 1.56 mg L⁻¹ of copper II ions. The selectivity studies indicate that ZnO-st is an efficient adsorbent for both dyes and copper ions in solution when in contact with both species simultaneously. Copper II ions are easily adsorbed by ZnO due to the strong metal-oxygen interaction, Cu²⁺ has a relatively low ionization energy and high electron affinity, allowing it to interact with the electrons in the ZnO substrate, promoting the formation of chemisorbed states. Furthermore, ZnO provides stable support for copper, preventing aggregation or sintering during the reaction, which explains the ease of adsorption not only for the metal but also for the combined adsorption of metal/dyes [38].

3.3. Study of the Hybrid Pigments

The CIELAB color model uses three main components: luminosity (L*), which ranges from 0 to 100, where 0 represents black and 100 represents white, and the chromatic components a* and b*. The a* component represents the distance between the red-green axes, with positive values indicating red and negative values indicating green. The b* component defines distances between the yellow-blue axes, with positive values indicating yellow and negative values indicating blue. The center is achromatic [39,40]. C* indicates color saturation. The total color variation within these parameters was calculated using ΔE (Equation (5)).

Table 2. The colorimetric parameters of the hybrid pigment in powder form (Hy-Pi) from the orange dye adsorption and the Hy-Pi dispersed in white paint.

Sample	Medium	L*	a*	b*	C*	ΔE	Photo
Orange-100	Hy-Pi	81.98	12.10	13.98	18.49	14.80
	Hy-Pi in white paint	91.65	4.09	6.14	7.38
Orange-200	Hy-Pi	84.83	14.68	16.73	22.26	14.10
	Hy-Pi in white paint	90.75	5.42	7.83	9.52
Orange-400	Hy-Pi	74.32	21.29	27.89	35.09	23.50
	Hy-Pi in white paint	86.25	9.63	11.34	14.88
Orange-500	Hy-Pi	73.45	23.89	29.75	38.15	24.55
	Hy-Pi in white paint	85.89	11.13	12.86	17.01

presents the colorimetric parameters for the hybrid pigment obtained from the removal of the orange dye and after its dispersion in white paint, as well as the comparison of the total color variation (ΔE) after dispersion. The hybrid pigments for the orange dye exhibit a decrease in luminosity (L*) as the concentration of removed dye increases, indicating the presence of the adsorbed dye through the darkening of the obtained pigments. The positive a* and b* quadrants indicate a red/yellow color direction, with values increasing proportionally with dye concentration, indicating higher color intensity and greater saturation (C*). The high L* values for the pigments dispersed in paint reflect the influence of the white paint on color intensity, increasing light reflectance, which causes the chroma values to become lower than those of the undispersed pigment. Similarly, the ΔE values increase with higher dye concentrations in the pigments, indicating a greater variation in the color parameters of the hybrid pigment before and after dispersion in white paint as the concentration of adsorbed dye increases [18,41].

Table 3. The colorimetric parameters of the hybrid pigment in powder form (Hy-Pi) from the green dye adsorption and the Hy-Pi dispersed in white paint.

Sample	Medium	L*	a*	b*	C*	ΔE	Photo
Green-100	Hy-Pi	83.11	−0.50	5.29	5.31	10.31
	Hy-Pi in white paint	93.17	−064	3.01	3.08
Green-200	Hy-Pi	77.49	−2.88	3.92	4.87	12.62
	Hy-Pi in white paint	89.97	−1.60	2.56	3.02
Green-400	Hy-Pig	75.12	−4.66	1.47	4.89	14.07
	HyPi in white paint	89.15	−3.57	1.33	3.81
Green-500	Hy-Pi	65.78	−7.90	−3.57	8.67	20.94
	Hy-Pi in white paint	86.0	−4.64	0.79	4.70

presents the colorimetric parameters of the hybrid pigments obtained from the removal of the green dye, after dispersion in white paint, and the total color variation (ΔE) after dispersion. Similar to the hybrid pigments of the orange dye, the hybrid pigments of the green dye exhibit a reduction in luminosity (L*) as the concentration increases. The negative a* and positive b* quadrants indicate a green/yellow color direction. As the dye concentration increases, the b* parameter becomes more negative, shifting toward a green/blue color direction at higher concentrations, indicating the presence of a higher dye concentration and resulting in more intense chroma. The high L* values for the pigments dispersed in paint reflect the influence of the white paint on color intensity. The ΔE value also increases with higher dye concentrations in the pigments, indicating a greater variation in the color parameters of the hybrid pigment before and after dispersion in white paint [18,42,43].

The stability of the precursor dyes and hybrid pigments was tested in different solvents: water, ethanol, and acetone (Figure 10). Absorbance values were measured after 24 h of contact with the solvents using an ultraviolet/visible spectrophotometer, monitoring the maximum absorption bands of each dye. The absorbance values after 24 h of contact are presented in

Table 4. Stability of hybrid pigments compared to the dyes in different solvents.

Sample	Solvent	Medium	Abs. (nm)
Orange Dye	Water	Commercial Dye	0.017
		Hy-Pi	0.002
	Ethanol	Commercial Dye	0.652
		Hy-Pi	0.002
	Acetone	Commercial Dye	0.045
		Hy-Pi	0.005
Green Dye	Water	Commercial Dye	0.009
		Hy-Pi	0
	Ethanol	Commercial Dye	1.435
		Hy-Pi	0
	Acetone	Commercial Dye	0.455
		Hy-Pi	0.002

.

The degree of discoloration after 24 h of exposure to solvents was evaluated on a scale of 1/5, where 5 indicates total insolubility [28]. The hybrid pigments, as shown in Figure 10, in all solvents scored 5, except for the orange pigment in acetone, which scored 4. While the precursor dyes in water scored 3, the orange dye in acetone scored 3, the green dye in acetone and the orange dye in ethanol scored 2, and the green dye in ethanol scored 1, indicating the highest solubility [28].

The orange and green dyes showed greater resistance to water than to other solvents. The orange dye exhibited low resistance to ethanol, as did the green dye, which also showed low resistance to acetone. In general, the dyes have both polar and nonpolar parts in their structure, which explains their better interaction with ethanol and acetone than with water. Water is a highly polar molecule, while the other solvents have intermediate polarity, allowing better interaction with the dye molecules [44].

The hybrid pigments of both dyes demonstrated high stability in all solvents, with only the hybrid pigment of the orange dye showing a slight release in contact with acetone. The stabilization of the dye chromophores on the ZnO-st support significantly reduced the release of the dye into the solvent, as evidenced by the colorless solution and the reduction or absence of absorbance signals at the dye wavelengths. This proves the stabilization of the dye on the inorganic carrier and the improved chemical resistance due to metal-dye and electrostatic interactions [45,46,47].

The hybrid pigments were exposed to white light for 7 days to evaluate the effects of discoloration caused by light exposure. The total color difference before and after exposure to white light is represented by the ΔE values.

Table 5. Colorimetric parameters of the hybrid pigment from the orange dye dispersed in white paint, before and after prolonged exposure to white light.

Sample	Quite	L*	a*	b*	C*	ΔE
Orange-100	White paint	91.65	4.09	6.14	7.38	11.82
	Light exposition	79.86	3.31	6.39	7.20
Orange-200	White paint	90.75	5.42	7.83	9.52	12.16
	Light exposition	78.66	4.71	8.88	10.5
Orange-400	White paint	86.25	9.63	11.34	14.88	10.69
	Light exposition	75.61	8.63	11.72	14.55
Orange-500	White paint	85.89	11.13	12.86	17.01	10.42
	Light exposition	75.56	9.80	12.90	16.20

corresponds to the data collected for the orange dye, and

Table 6. Colorimetric parameters of the hybrid pigment from the green dye dispersed in white paint, before and after prolonged exposure to white light.

Sample	Medium	L*	a*	b*	C*	ΔE
Green-100	White paint	93,17	−0.64	3.01	3.08	12.88
	Light exposition	80.30	−1.13	3.44	3.62
Green-200	White paint	89.97	−1.60	2.56	3.02	10.94
	Light exposition	79.05	−2.22	2.93	3.67
Green-400	White paint	89.15	−3.57	1.33	3.81	12.91
	Light exposition	76.26	−4.24	1.56.	4.52
Green-500	White paint	86.0	−4.64	0.79	4.70	10.74
	Light exposition	75.31	−5.67	0.87	5.73

corresponds to the data for the green dye.

The ΔE values for both the orange and green dye samples decrease as the concentration of dye in the hybrid pigment increases. In other words, the higher the concentration of dye immobilized on the inorganic support, the greater its stability under prolonged exposure to white light [14]. The fading test provided a total dose of 15,456 Klux.h (92 Klux over 168 h, 7 days), which is equivalent to 21 years in a museum gallery illuminated at 200 lux with 10 h of exposure per day [29,30,47].

4. Conclusions

The zinc oxide synthesized through the combustion of sago starch demonstrated efficient production of ZnO in the wurtzite phase, characterized by a hexagonal close-packed structure and high purity, as well as mesoporous material exhibiting a relatively high specific surface area. Scanning electron microscopy (SEM) images revealed the presence of spherical particles with a significant degree of aggregation.

Adsorption studies indicated that the optimal dosage of the adsorbent was 400 mg of ZnO-st, resulting in a removal efficiency of 99.13% for the green dye and 96.93% for the orange dye. By varying the initial concentrations of the dyes, complete removal (100%) was achieved for the orange dye at concentrations of 25 and 50 mg L⁻¹, and for the green dye at concentrations of 25, 50, and 100 mg L⁻¹. A decrease in removal percentages was observed at higher concentrations, whereas the removal efficiency improved at a concentration of 500 mg L⁻¹, yielding 47.45 mg g⁻¹ for the orange dye and 51.88 mg g⁻¹ for the green dye. The most favorable adsorption results were obtained at 60 and 120 min, consistent with the pseudo-second-order kinetic model. The results of the Tukey test conducted in this study indicated significant differences in the percentage removal of textile dyes at higher concentrations for the as-synthesized ZnO. Furthermore, contact times exceeding 240 min for both dyes did not yield significant differences in removal efficiency (%Removal > 90%).

Co-adsorption studies demonstrated the efficiency and selectivity of ZnO-st in the simultaneous adsorption of copper ions and dyes, achieving a removal of 98.2% for copper ions and 100% for the green dye, alongside a removal of 97% for copper ions and 98.13% for the orange dye. ZnO serves as a stable support for copper, effectively preventing aggregation or sintering of the adsorbent, thereby facilitating efficient adsorption. This capability to concurrently remove dyes and toxic metals does not compromise the adsorbent’s efficacy. However, further studies are warranted to investigate additional metal ions and dyes to enhance our understanding of this behavior. Adsorption and colorimetric analyses of the resulting pigments confirmed the formation of hybrid pigments, with the dye concentration in the hybrid pigments decreasing as the initial dye concentration increased. This observation indicates that the dyes are effectively adsorbed and darken with increasing concentration, contributing to enhanced stability under prolonged exposure to white light. The hybrid pigments also exhibited significantly improved resistance to organic solvents compared to their predecessors, indicating the stability of the dye on the inorganic support. Thus, ZnO-st has proven to be an excellent inorganic support for stabilizing dyes and copper ions from domestic gray water or industrial wastewater.