Pink Hybrid Pigments Resulting from the Adsorption of Congo Red Dye by Zinc Oxide

Abstract

Hybrid pigments were obtained by combining zinc oxide with the anionic dye Congo red (CR), a breakthrough with significant environmental implications. By adjusting the ratio of solid mass to dye concentration, it is possible to obtain pigments with pink hues from a white solid (ZnO) through its adsorption of CR. The process involved using ZnO, prepared at 800 °C using cassava starch suspension as a suitable fuel. The oxide was characterized using XRD, SEM, and BET, and the results showed that the textural properties are typical of nanoparticles, with a size of 50.5 nm, a pore size of 3.48 nm, and a surface area of 3.03 nm, making it suitable for molecular dye removal. Controlling the adsorbent mass (in grams) and dye concentration (in mg L⁻¹) makes it possible to consistently produce hybrid pigments in various shades of pink that exhibit good thermal resistance. When dispersed in white waterborne paint, they are chemically stable in different solvents, have excellent painted surface coverage, and resist photochemical degradation. The results demonstrate technical feasibility and compatibility with the Sustainable Development Goals, particularly Goals 6, 11, 12, 14, 15, and 17, offering a promising solution for a more sustainable future.

1. Introduction

Synthetic dyes, as organic contaminants in wastewater, are not just significant, but cause massive environmental problems globally due to their poisonous nature, high solubility in water, limited degradability, and complex structural composition [1]. Dye effluents, originating from diverse industrial sectors such as food, pharmaceuticals, textiles, paper, leather, and plastics [2], are contributing to this large-scale problem. According to Hamad et al. (2024) [1], the amount of dye-polluted effluent released into water bodies is up to 15% and is estimated to be between 70 and 200 thousand tons annually. The presence of dyes in aquatic environments reduces the amount of dissolved oxygen due to the enhanced chemical oxygen demand and decreases the sunlight penetration in the water body, resulting in increased biochemical oxygen demand (BOD), impairing aquatic flora’s photosynthetic activities [1,3].

Congo red dye (CR) (C₃₂H₂₂N₆Na₂O₆S₂; MM = 696.68 g/mol) is an anionic acid dye containing a complex chemical structure, which includes auxochrome and chromophore functions like SO₃²⁻, NH₂, and a benzene ring, giving it a bright red color that is widely used in the textile, pharmaceutical, rubber, plastics, food, and paper industries, amongst others [3,4,5]. This synthetic dye is highly stable and water-soluble, and its aromatic structure makes it resistant to biodegradation, which leads to water pollution, compromising light penetration, interfering with photosynthetic activity, and affecting present species [1,5]. It is known by several names, such as CR 4B, CI22120, cotton red B, cotton red C, red 28, cosmos red, direct red Y, and direct red R [5]. Congo red is considered toxic for aquatic environments. Dwivedi et al. (2013) [6] conducted a study on the effects of CR dye concentrations using S. platensis, revealing that high concentrations of CR significantly suppressed the growth of the microalgae, suggesting that increased CR concentrations may reduce algal cell growth due to the potential toxicity of CR within the cell. In a study by Hernández-Zamora et al. (2016) [7], the impact of CR on the reproduction and survival of Ceriodaphnia dubia was analyzed. The researchers observed that the median lethal concentration (LC₅₀) over 48 h was 13.58 mg/L. When exposed to fresh algae (control) and algae previously exposed to CR, the fertility of C. dubia decreased to 40% and 70%, respectively, and survival decreased to 80% and 55%, respectively, compared to the control sample. The study concluded that CR significantly affects C. dubia, leading to decreased fertility and survival at concentrations exceeding 3 mg/L. Furthermore, benzidine, a degradation product of CR, is highly toxic to biological systems, and has been established as mutagenic [8] and carcinogenic [9]. Therefore, it is vital to eliminate the pollution load caused by releasing this dye contaminant through wastewater treatment. Several techniques can be employed to remove CR-laden effluents from wastewater, including coagulation [10], electrochemical processes [11], photocatalysis [12], advanced oxidation processes [13], and adsorption [14,15,16]. However, it is essential to select a cost-effective, simple, and feasible treatment process.

The adsorption method offers advantages such as a low cost, simplicity, and the renewability of the adsorbent, making it a sustainable solution for water treatment. It can also be used for a wide range of target contaminants and provides the possibility of using numerous low-cost materials as adsorbents [3,14,17]. Different adsorbents have been reported to eliminate CR dye from wastewater [1,18,19,20,21,22,23,24]. Zinc oxide (ZnO) is a material suitable for the adsorption of anionic dyes from aqueous solutions at a circumneutral pH [24], playing a significant role in water treatment. In adsorption processes, it possesses an unsaturated surface and excellent behavior in the adsorption of organic and inorganic pollutants in aqueous matrices [25], removing and degrading dyes, toxic metal ions, and other pollutants [25]. Different studies have evaluated ZnO as an adsorbent for the adsorption of CR dye from aqueous solutions [24,26,27,28], contributing to environmental remediation. However, a sustainable solution to the problem of adequately disposing of and reusing used adsorbents involves making the dye removal process cost-effective and eco-friendly. Thus, in addition to using ZnO as an adsorbent, it can be explored as an inorganic matrix after its environmental remediation of CR dye, such as for obtaining hybrid pigments due to its biocompatibility, non-toxic nature, and chemical and thermal stability [25].

Synthetic hybrid pigments can be prepared by the adsorption of dyes onto inorganic materials [29]. Stabilizing organic dyes through adsorption or intercalation with inorganic supports forms hybrid pigments, or organic–inorganic pigments, which generally exhibit improved thermal stability and enhanced physicochemical and light properties [30]. A wide range of adsorbents are used in the adsorption processes of selected dyes, from silica [29] to TiO₂ [31] and zeolites [32]. These pigments combine the flexibility and color of organic dyes with the high thermal and chemical resistance of inorganic supports, making them suitable for various applications, i.e., chemical sensors, colored filters, and luminescent sunlight collectors [33,34]. Hybrid pigments, which combine the mechanical, ionic, electronic, and optical properties of solids and biomaterials, can potentially give new pigments innovative and improved properties [35]. Therefore, the objective of this work is to obtain zinc oxide nanoparticles using cassava peels (Manihot esculenta Crantz) as a fuel, use them to remove different concentrations of Congo red dye through the adsorption method, and subsequently use the post-adsorption material as a hybrid pigment for waterborne paints.

2. Materials and Methods

All reagents used, including zinc nitrate and Congo red (CR), were of analytical grade and obtained from Dynamic without any additional purification. The fuel used was natural cassava starch in the form of a colloidal suspension. The cassava plant was harvested in the Palmital region of Parana, Brazil. Distilled water was used for all synthesis and treatment processes.

2.1. Synthesis of Porous Zinc Oxide

Porous ZnO was prepared according to a previous report [36]. During solution combustion synthesis, 100 g of natural cassava starch was mixed into 500 mL of distilled water for 10 min using an industrial blender. Then, 46.43 g of zinc nitrate (Zn(NO₃)₂·6H₂O) was added to the colloidal starch suspension and subjected to mechanical stirring for 60 min. The ZnO precursor was calcined in air for 2 h at 800 °C, with a heating rate of 10 °C min⁻¹, to obtain porous ZnO-starch.

2.2. Preparation of the Hybrid Pigments (Hy-P)

2.2.1. Equilibrium Adsorption Experiments

CR adsorption isotherms were obtained by adding a certain amount (1 g) of the porous ZnO-starch to a series of 100 mL beakers filled with CR solutions (25 mL: 25, 50, 100, 200, 400, and 500 mg L⁻¹) at pH 7.0–7.5. The beakers were sealed and agitated at 50 rpm for 3 h at 25 °C. At the end of the agitation, the aqueous solutions were separated from the samples through centrifugation. The residual concentration of CR was measured based on the maximum adsorption peak of CR (at 498 nm) by using a UV/Vis spectrophotometer (Shimadzu UV/Vis 1800 spectrophotometer, Kyoto, Japan). The amount of CR adsorbed by the sample at equilibrium qe (mg/g) was calculated using the following Equation (1):

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{\mathrm{W}}}$$

(1)

where C₀ and C_e (mg L⁻¹) are the initial and equilibrium concentrations of CR, respectively; V (L) is the volume of the CR solution; and W (g) is the mass of adsorbent used.

After the experiments, the Hy-P were then centrifugated, washed with deionized water, and dried at 70 °C for 24 h (Figure 1), and each was labeled in relation to the concentration of CR used in the adsorption study.

2.2.2. Solvent Resistance

The solvent resistance of the Hy-P was evaluated in accordance with the PN-C-04406/1998 standard. Each pigment sample (0.1 g) was immersed in a conical flask with solvent (acetone, or ethyl alcohol) or water for 24 h, at room temperature. Their solvent resistance was estimated (on a scale of 1/5) based on the degree of decolorization.

2.2.3. Application of Hybrid Pigments in White Paint

A white waterborne paint with a solid content by weight of 45–55%, a VOC (volatile organic compounds) of 13–33 g L⁻¹, and a pH of 8–11.5 (Acrylic Standard matt, prod. Suvinil BASF S.A., São Bernardo do Campo, Brazil) was used for the dispersion of the Hy-P. In a proportion of 10 wt.%, Hy-P particles were dispersed in the white paint and mixed to obtain a uniform dispersion for 10 min using magnetic stirring. Next, the well-dispersed mixtures were painted on paster blocks (PC) using a brush (Figure 1).

2.2.4. Light-Induced Aging of Prepared Hybrid Pigment in White Paint

Light-induced tests were performed, involving the exposure of the painted plates containing the dispersed solid pigments in white paint to white light irradiation for 96 h, using an LED lamp set to provide 92 Klx of illumination intensity.

Light-induced aging was evaluated by means of colorimetric measurements, and the CIELAB data were calculated according to the “Commission Internationale of l’Eclairage” (CIE). The L*, a*, and b* coordinates were obtained for the ΔE* determination, which refers to changes in the pigment color over time spent exposed to light. The total color difference (ΔE*) was obtained using Equation (2):

$${\left({\left({\mathrm{a}}_{\mathrm{i}+\mathrm{j}}^{\mathrm{*}}-{\mathrm{a}}_{\mathrm{i}}^{\mathrm{*}}\right)}^2+{\left({\mathrm{b}}_{\mathrm{i}+\mathrm{j}}^{\mathrm{*}}-{\mathrm{b}}_{\mathrm{i}}^{\mathrm{*}}\right)}^2+{\left({\mathrm{L}}_{\mathrm{i}+\mathrm{j}}^{\mathrm{*}}-{\mathrm{L}}_{\mathrm{i}}^{\mathrm{*}}\right)}^2\right)}^{{\displaystyle \frac{1}{2}}}$$

(2)

where i and j refer to measurements taken at the beginning and the end of the experiment.

2.3. Characterization

The crystalline structure of the synthesized ZnO was identified using X-ray diffraction (XRD) performed on a D2 Phaser (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 1.5418 Å). The morphology of the particulate samples was examined using a scanning electron microscope (VEGA 3, TESCAN, Brno, Czech Republic) operated at 10 kV. To determine the Brunauer–Emmett–Teller (BET) surface area and the porosity properties, characterization of each sample’s surface area and N2 adsorption–desorption measurements was performed at 77 K using a Quantachrome gas adsorption analyzer, model 2000, by NovaWin software (Boynton Beach, FL, USA). The surface charge of each tested sample was evaluated with a Zeta potential analyzer (Malvern, Zetasizer Nano, ZS90). The electronic spectra of the powdered pigment samples were measured in the range of 450–700 nm using a UV–Vis Ocean Optics spectrophotometer, model USB-2000 (Orlando, FL, USA).

3. Results and Discussion

3.1. Characterization of ZnO-Starch

The crystallographic structure, morphology, and adsorption isotherms of the synthesized ZnO-starch particles were investigated. The XRD pattern of the ZnO-starch, shown in Figure 2a, follows the ICDD card 01-075-9742 of the hexagonal phase with the Wurtzite structure [37], with the lattice planes at (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), and (2 0 1), corresponding to 31.76°, 34.41°, 36.24°, 47.53°, 56.58°, 62.85°, 66.36°, 67.93°, and 69.07° 2θ values. The average crystallite size of the ZnO-starch particles was 50.5 nm, estimated from the widening of the full width at half of the maximum (101) diffraction peak via Scherrer’s equation [38].

The shape and size of the particles in the ZnO-starch sample were examined by SEM (Figure 2b). SEM revealed that using the starch as a fuel in the synthesis at 800 °C results in particles in a quasi-sphere that tend to be hexagons. In addition, the ZnO-starch particles have an irregular size with the formation of aggregated nanocrystallite, with particle sizes ranging from 200 to 650 nm. BET was carried out to determine the specific surface area of the synthesized ZnO-starch particles, using a N₂ adsorption temperature of 77 K, Figure 2c. The synthesized ZnO-starch exhibits a surface area of 3.03 m²/g. The Barrett–Joyner–Halenda (BJH) average pore size was found to be 3.48 nm for the sample, the nitrogen adsorption–desorption isotherms of ZnO particles obtained from the BET analysis. The porous nature of the sample is further confirmed through the BJH isotherm (adsorption–desorption), which is of the type IV category (Figure 2c), and the average pore size range shows that the materials are mesoporous [39].

3.2. Hybrid Pigments

The modification of ZnO-starch by CR dye resulted in a series of hybrid pigments. The hybrid pigments were varying shades of pink, depending on the concentration of CR dye in ZnO, as observed in Figure 1. During the process of producing hybrid pigments using ZnO-starch and CR, the ζ-potential was determined at the pH conditions used in the adsorption experiment (pH 7–7.5), and the ZnO surface charge shows a zeta potential of ~22.5–22.8 mW, which demonstrates that the synthesized ZnO-starch plays a significant role in the adsorption of CR, a common anionic dye.

3.2.1. Study of Adsorption and Equilibrium

The effect of contact time on the removal of CR dye by ZnO-starch is shown in Figure 3. The adsorption rate of the CR dye increased rapidly with a contact time of up to 60 min, where more active sites were available and accessible. As the adsorbed species occupied the sites, new equilibrium conditions were established, and the CR molecules reorganized themselves to accommodate as many species as possible [40]. It became stable after 120 min of contact, where the curve leveled out, implying that the system had reached equilibrium.

The influences of adsorption time were investigated, and kinetic analyses were performed. The Kinect models were applied to evaluate the rate of the adsorption process of CR by ZnO-starch, and the Equations (3) and (4) were used, as follows [41,42,43]:

pseudo-first-order:

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

(3)

pseudo-second-order:

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

(4)

where qt represents the adsorption at the time of t (min), and k₁ (h⁻¹) and k₂ (g mg⁻¹ h⁻¹) are the corresponding rate constants of the pseudo-first-order and pseudo-second-order processes.

Table 1. Kinect parameters for absorption of Congo red onto ZnO-starch.

	Pseudo-First-Order			Pseudo-Second-Order
qexp (mg g−1)	102 k1 (h−1)	qcal (mg g−1)	R2	k2 (g mg−1 h−1)	qcal (mg g−1)	R2
10.06	2.34	0.82	0.9624	8.60	10.10	1.0000

summarizes the parameters and the regression coefficient, R², of the two Kinect models. Among the two models, the pseudo-second-order has the best fit, R² is greater than 1.00, and the fitted equilibrium adsorption capacity, which is 10.10 mg g⁻¹, is similar to the actual experimental results of 10.06 mg g⁻¹ (25 °C). This Kinect model indicates the existence of chemisorption; however, since the adsorption phenomenon is a complex process, it is difficult to differentiate between physisorption and chemisorption based on this factor alone [44,45].

The influence of the initial concentration on the adsorption performance of ZnO-starch has also been investigated. As shown in Figure 4, as the initial concentration increases, the adsorption capacity of ZnO-starch gradually increases, and the removal percentage remains stable. When the initial concentration was 200 mg L⁻¹, an adsorption capacity of 10.06 mg g⁻¹ was achieved, and this tended to increase as the concentration rose. The increase in the amount adsorbed results in an increase in the driving force associated with the availability of active sites on the adsorbent [40,44]. On the other hand, a decrease in the percentage of removal reflects the compromise of the adsorbate absorption, due to competition between more species for a limited number of active sites and the interaction between these species once they have been adsorbed [40].

3.2.2. Color Study of the Hybrid Pigments

Table 2. Colorimetric parameters of pigments in powder form and after dispersion in white paint.

Sample	Medium	Colorimetric Parameters					Photo
		L*	a*	b*	C*	∆E
Zn-25	Powder	85.4	10.8	6.1	12.5	8.7
	White Paint	91.7	5.2	3.9	6.5
Zn-50	Powder	79.8	16.0	7.2	17.6	11.3
	White Paint	88.8	9.7	4.6	10.7
Zn-100	Powder	75.4	19.1	5.8	19.9	13.5
	White Paint	87.0	12.3	4.4	13.1
Zn-200	Powder	66.8	25.2	4.7	25.6	21.9
	White Paint	85.2	13.9	2.7	14.1
Zn-400	Powder	64.3	28.9	6.3	29.6	15.2
	White Paint	76.9	21.0	3.2	21.2
Zn-500	Powder	63.9	30.2	8.8	31.5	15.8
	White Paint	77.6	23.5	5.1	24.0

presents the colorimetric parameters of the samples in powder form and after application in white paint. Through the total color variation (∆E), it is possible to compare the coloration in the different forms; all the differences observed are considered strong to very strong to the human eye [46], highlighting the loss of saturation due to the presence of the white matrix of the paint that tends to increase the reflectance of light [47]. It is possible to verify that all the pigments are in the red/yellow color quadrant with positive a* and b* parameters, with the red coming from the Congo red dye and the yellowish nuance resulting from ZnO. With the increase in the concentration of Congo red dye in the sample, it is possible to observe the decrease in luminosity (L*) and the increase in color saturation (C*), associating the presence of the dye directly with the final coloration obtained in the pigment.

Figure 5a shows the absorption spectra of the Hy-P in powder obtained using different concentrations of CR. The absorption spectrum of the pigments shows a structured band, with a maximum at 523 nm. The UV–vis spectrum of the CR has a λ_max at 498 nm [48], revealing a significant bathochromic shift of the maximum peak (from 498 to 523 nm) for CR in the presence of the ZnO host. This may be related to the conformational changes of CR and result from dye-ZnO interactions.

Figure 5b presents digital photos of the color alterations of Hy-P powders heated in an oven for 30 min at either 250 or 300 °C. The CIE parameters (

Table 3. Colorimetric parameters of hybrid pigments and corresponding heated pigments.

Sample	Heating Temperature (°C)	Colorimetric Parameters
		L*	a*	b*	C*	∆E
Zn-25	Without heating	85.39	10.83	6.18	12.49	--
	250	86.17	6.33	6.95	9.40	4.63
	300	89.58	3.91	8.53	9.38	8.42
Zn-50	Without heating	79.87	16.05	7.16	17.58	--
	250	84.29	9.58	8.51	12.81	7.95
	300	87.57	6.08	9.26	11.08	12.77
Zn-100	Without heating	75.39	19.07	5.84	19.94	--
	250	72.70	17.37	9.72	19.90	5.08
	300	82.39	9.42	6.45	11.42	11.94
Zn-200	Without heating	66.79	25.20	4.71	25.63	--
	250	71.03	18.94	8.39	20.72	8.41
	300	76.17	14.23	8.82	16.75	15.01
Zn-400	Without heating	64.33	28.91	6.27	29.58	--
	250	64.34	22.23	9.18	24.05	7.28
	300	70.57	19.32	11.83	22.65	12.72
Zn-500	Without heating	63.86	30.24	8.78	31.49	--
	250	62.24	23.65	11.22	26.18	7.21
	300	68.64	20.11	12.75	23.82	11.88

) of the Hy-P after heating show that after heating to 300 °C, the brightness increased with the L* value for all pigments, demonstrating a shift to brighter shades; for the temperature of 250 °C, the brightness did not change significantly. In addition, the smaller positive a* and larger positive b* values of Hy-P heated at both temperatures prove that the color changes to less red and more yellow, with significant color differences. ΔE* quantifies the deviation in color change of the pigments heated at different temperatures (Table 3). According to [49], an ΔE* value between 3.5 and 5 signifies a noticeable difference, and a value above 5 suggests a discernible contrast in colors, showing that Zn-25 heated at 250 °C presents a noticeable color difference, presenting a better thermal stability, while with an increase in the CR dye concentration, the color change is more discernible.

3.2.3. Light-Induced Aging

Table 3 presents the chroma (C*) values of the pigments dispersed in white paint before and after exposure to 96 h of UV light. The test performed on the Hy-P gave each one a total light dose of 8832 Klux.h (92 Klux 96 h). This light dose is equivalent to about 12 years in a museum gallery illuminated at 200 lux (10 h of light exposure per day). Similar calculations can be found in the literature [50]. After this exposure, a loss of color saturation was observed, with a decrease in all chroma (C*) values. Statistically, the coefficient of variation (CV) was calculated, which is a measure that expresses the variability of the distribution in relation to its mean. The CV is calculated by dividing the standard deviation by the mean, multiplied by 100 to express the result in percentage terms ($\mathrm{C}\mathrm{V}=\left({\displaystyle \frac{\mathrm{s}\tan \mathrm{d}\mathrm{a}\mathrm{r}\mathrm{d}\mathrm{ }\mathrm{d}\mathrm{e}\mathrm{v}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{ }}{\mathrm{m}\mathrm{e}\mathrm{a}\mathrm{n}}}\right)\times 100$) [51]. The CV can be used to monitor color stability over time. In general, a CV of less than 15% indicates low variability, a CV between 15% and 30% indicates moderate variability, and a CV greater than 30% indicates high variability [51]. In this study (

Table 4. Variation in chroma of samples before and after exposure to UV light.

Sample	${\mathbf{C}}_{\mathbf{i}}^{\mathbf{*}}$	${\mathbf{C}}_{\mathbf{f}}^{\mathbf{*}}$	$\mathsf{\Delta }{\mathbf{C}}^{\mathbf{*}}$
Zn-25	6.5	4.7	1.8
Zn-50	10.7	5.7	5.0
Zn-100	13.1	6.9	6.2
Zn-200	14.1	7.2	6.9
Zn-400	21.2	12.4	8.8
Zn-500	24.0	15.6	8.4

), the samples Zn-25 and Zn-500 presented moderate variability, with values lower than 30%. Thus, it is possible to suggest that light-sensitive defects or impurities are not significant in the samples, since the decrease in chroma (C*) after exposure indicates that there is no formation of new chromophore compounds in the samples, pointing to the photostability of these pigments.

3.2.4. Solvent Resistance Analysis

The stability of the prepared Hy-P was assessed by examining their resistance to selected organic solvents and water. Since Congo red dye is soluble in both water and organic solvents, prolonged washing with these solvents should remove any excess or weakly physiosorbed dye from the ZnO surface. The degree of discoloration was evaluated after 24 h on a scale from 1 to 5, where 5 indicates total insolubility (colorless solvent) and 1 denotes high solubility (intensive solvent coloration) [52]. The solubility of the Zn-500 pigment varied depending on the solvent (see Figure 5c). The Zn-500 sample exhibited the best resistance to dissolution in water and acetone, as these solvents remained colorless after 24 h of immersion. In contrast, the Zn-500 pigment showed a higher degree of discoloration in ethanol (rating of 4, see Figure 5c), which can be attributed to weaker dye–host interactions caused by an excess of Congo red dye. The higher resistance of the Hy-P in water demonstrates the successful synthesis of the soluble dye into a solvent-resistant hybrid pigment.

4. Conclusions

This study explored the potential of creating hybrid pigments using Congo red (CR) on solid-state zinc oxide. ZnO, synthesized through Green Solution Combustion and utilizing starch as a fuel, was used as an inorganic host to produce pink pigments though adsorption remediation. This method involves using any surfactant or organic solvent at a low calcination temperature, resulting in ZnO with a crystalline structure and a significant surface area. The results from the dye adsorption using ZnO-starch as an adsorbent demonstrated that it is possible to obtain inorganic–organic pigments (Hy-P) through adsorption. The thermal stability and light-induced aging tests revealed that the pigments obtained were less stable with increased CR concentrations. However, the Zn-500 pigment, when tested in different solvents, exhibited high resistance in water and acetone. The ZnO developed in this study demonstrates significant promise as an inorganic matrix for obtaining hybrid pigments through the adsorption of CR. However, aquatic toxicity assays were not performed in this work. This test should be considered in any future research to assess ZnO’s potential environmental impact and evaluate CR’s bioavailability when combined with ZnO.

Finally, based on the findings of this study, the CR/ZnO hybrid pigment is not only stable and easy to produce, but also affordable. This makes it a promising option for the production of new pigments using oxides in adsorption methods, offering a cost-effective solution for water treatment and pigment production.