Efficient adsorption of tartrazine from an aqueous solution using a low-cost orange peel powder

Several industrial processes like textiles, rubber, plastics, food products, paper, and leather employ a variety of synthetic product dyes for various applications. These industries produced about 800,000 tons of synthetic dyes annually and 50% of these are azo dyes (Greluk & Hubicki 2011). The azo dyes are molecules with a complex aromatic structure, resistant to temperature, light, and oxidizers (Pearce et al. 2003). Tartrazine (E102) is a synthetic azo dye used as a food additive by many industries, especially in the manufacture of food products (juices, ice creams, jams, sauces, yogurt, and cakes), pharmaceuticals (syrup, capsules, and pills), and cosmetics (scent, nail polish, soaps, toothpastes, and shampoos) (Scotter & Castle 2004; Ollemberg 2005; Pereira et al. 2006; Kraemer et al. 2022). Excessive consumption of this dye beyond 0–7.5 mg/kg which is the acceptable daily dose leads to cancer, asthma, infertility, visual disturbances, and rhinitis in the long term (Panel & Chain 2009). This dye used by the industries is subsequently rejected in the water collection basin (Ramuthai et al. 2009). Many works have been achieved in order to remove the color and other pollutants by ozonation, the addition of reducing agents, membrane filtration, ion-exchange, the Fenton method, and adsorption methods. The latter has many advantages like simplicity and efficacy but its utilization is limited due to its expensive cost. For this, many adsorbents have been prepared like activated carbon¹³, clay, alumina, silicon oxide, and food residues were used for the removal of color by activating it in three ways for adsorption of E102. Among various adsorbents, one of the most explored studied adsorbents is activated carbon. Many researchers have explored the feasibility of low-cost substitutes such as orange peel, rice husk, and used coffee leaves for the elimination of dyes from aqueous solutions (Arami et al. 2006).

The present study addresses the use of orange peel powder as an adsorbent for the elimination of tartrazine, an azo dye from an aqueous solution. The raw material (orange peel) is a solid by-product or waste produced in large quantities in many countries. A thorough literature search revealed that there is presently no research on the use of orange peel powder for the elimination of tartrazine. According to the literature, the chemical modification of raw materials can significantly increase the elimination capacity of dyes in an aqueous solution (Malik et al. 2007). Therefore, in this study, the potential of thermally and chemically modified orange peel powder for the elimination of tartrazine in aqueous solutions is studied. The prepared material was characterized to get information on the morphology and then the effect of operating parameters on the elimination of the azo dyes was studied.

Orange peels were collected from Ngaoundere, Cameroon, washed using distilled water, and then dried at 45 °C (Benaïssa & Elouchdi 2011) in an oven. After drying, peels were crushed and sieved (1 mm), and a part of them is subjected to thermal activation (POAT), chemical activation (sulfuric acid (POAA)), and soda activation (POAS). For thermal activation, these powders were calcined at 500 °C in a muffle oven for 1 h. For acid and soda activation, respectively, the as-prepared orange peel powders were impregnated with a solution of sulfuric acid (H₂SO₄) and soda in an oven at 105 °C and at ambient temperature, respectively, and then washed at neutral pH with distilled water before being dried between 40–50 °C and at 105 °C, at a constant weight.

The dry matter provides information on the real mass of material that is brought in contact with the solution, while the water content makes it possible to determine the residual humidity of our material samples. As for the ash tenor, it allows us to know the mineral, inert, and unusable portion of the orange peel.

Iodine Index is a relative indicator of the porosity of adsorbents. It measures the micropores contained and accessible to small molecules; it was identified by the method used by Benamraoui (2014). Specific surface area determines the area available by methylene blue (MB) for the adsorption of a monomolecular layer from the outer and inner surfaces of fine particles of a solid suspended in the aqueous medium.

Fourier Transform Infra-Red (FTIR) was utilized to determine functional groups present in the materials. Using a mixture of potassium bromide and orange peel powders, a ball was made and was scanned in a wave-number scale of 400–4,000 cm⁻¹. Field emission scanning electron microscopy (FE-SEM) characterization was carried out to acquire the structure and morphology of POAT, POAA, and POAS. A Carl Zeiss Supra 55VP microscope equipped with an accelerating voltage of 0.1–30 kV was used for this purpose.

A laser Raman microscope (LabRam HR, Horiba) with 600-nm wavelength was used to carry out Raman measurements. The spectra were recorded from 1,000 and 3,000 cm⁻¹. Powder X-ray diffraction (XRD) characterization has been used to determine the phase purity, grain size, and lattice space increment and decrease in the concentration of adsorbents. It is also often used to study the crystal defect of POAT, POAA, and POAS. The XRD patterns of all the samples were measured using a Bruker D8 Advance X-ray diffractometer with a rate of 5°/min at λ = 1.55 Ả in the interval from 5° to 80°.

The role of the pH_PZC is to know the exact charge carried by the area of the adsorbent while adsorbing the dye. This corresponds to the pH value of the medium and the resultant charges (positive and negative) of the surface are zero. For this, 0.2 g of the adsorbent was added to 50 mL of 0.01 M NaCl for a pH range from 2 to 12 which will be adjusted by adding NaOH or HCl according to the Mahmood method (Mahmood et al. 2011). The bottle was closed and stirred at room temperature for 48 h in order to increase their final pH. The pattern of interception of the final pH (pH_f) according to the initial pH (pH_i) with bisector determines the pH_PZC.

For the equilibrium pH, 1 g of the adsorbent is dissolved in 100 mL of distilled water (pH = 5.2 ± 0.2), stirred for 24 h and then allowed to stabilize and finally the measurements are taken after 25 min with a pH-meter (VOLTCRAFT).

Various adsorption samples were prepared by batch systems by mixing 0.01 g of adsorbent with 50 mL of dye solutions at desired concentrations, and stirred at 300 rpm at given time intervals at ambient temperature (25 ± 2 °C), followed by filtration via Whatman filter paper to measure the residual tartrazine concentration by UV–Visible spectrophotometer (RAYLEIGH). The experiments were carried out at different times (5–40 min), masses (0.01–0.1 g), concentrations (10–40 mg/L), temperatures (298, 308, 318, and 328 K), and at different pH values (3–11).

The peaks at 2,856 and 1,326 cm⁻¹ correspond to the deformation of aliphatic C–H and C–O, respectively. The difference between the thermally treated adsorbent (POAT) and chemically treated adsorbents (POAS and POAA) is the reduction of the C–O bond of ester, ether, carboxylic acids, or alcohol in POAT while we have this bond in POAS and POAA. This is due to the activation temperature which has weakened the bonds present. Similar peaks are observed by Khormaei et al. (2007).

It appears from the table that POAT has the highest dry matter content while its moisture and ash contents are low for the POAA and POAS. Our three adsorbents are well chosen because their ash content is low compared to those obtained by Torre Chauvin in 2015, and a low ash content indicates a good adsorbent and the high moisture content in POAA and POAS is due to the nature of activations (Table 1).

The specific surface of chemically activated adsorbents such as POAA (65.74 m²/g) and POAS (49.70 m²/g) is greater than that of thermally activated adsorbents, POAT (35.47 m²/g). With regard to the values of a specific surface, we can say that chemical modification seems to improve the adsorption capacity of material more than thermal activation.

The values of the iodine number increase significantly as the activation temperature increases because the increase in temperature makes the micropores more available; hence, POAT contains more micropores than POAA and POAS. The value of the equilibrium pH of POAT is basic, whereas for POAA and POAS it is acidic; this is due to the way each support has been made (Table 2).

It is apparent from the figure that independent of the adsorbent used, the adsorbed amount of tartrazine decreases with increased mass (24.94–2.25 mg/g; 25.20–2.13 mg/g; and 25.58–2.23 mg/g, respectively), for POAA, POAT, and POAS due to the unsaturation of adsorption sites and the reciprocal influence between the molecules of the adsorbent which causes the desorption of the adsorbant from the small sites of the adsorbent (Wu et al. 2018). The same results are obtained by Harouna et al. (2020) and Abia et al. (2019). It can be concluded that POAS adsorbed more and the optimum mass of our three adsorbents is 0.01 g, and this mass was used for the rest of the study (Figure 7).

In order to determine the adsorbate–adsorbent interaction, four suitable model isotherms were used, namely Langmuir, Freundlich, Temkin, and Dubinin-Redushkevic.

The plots of log q_e against log C_e should give a linear graph n = slope; K_F is the intercept of the graph.

This model considers a non-uniformity of surface and a preferential occupation of the most adsorbent sites.

It is based on the volume occupied using the application of Polanyi's potential theory, which assumes that the interactions between the adsorbate and the adsorbent are established by a potential field and the adsorbate volume is one of the function of the potential field ε.

These results (Table 3) show that only Freudlich and Temkin isotherm models best explain the phenomenon of E102 adsorption on POAT, POAA, and POAS.

Pseudo-first- and second-order and intra-particle diffusion were employed to explain the kinetic adsorption process.

The values of K₁ and q_e are calculated and listed in Table 4.

K₂ (mg/g.min) is the second-order rate constant. The values of q_e and K₂ are listed in Table 4.

The parameters of the kinetic model of intraparticle scattering are shown in Table 4.

From the following results, we can conclude that only the pseudo-second model best describes the adsorption of tartrazine because the values of correlation coefficients R² are greater than 0.90 and its mechanism is two steps: the diffusion of E102 to the area and the interaction of the E102 molecules to the area of adsorbents.

Table 5 shows the comparison of the adsorption removal capacity of various orange peel powders treated differently for E102 with other reported materials. It is evident that POAT, POAA, and POAS displayed a better affinity for tartrazine than most of the materials utilized for tartrazine uptake such as bio-adsorbent, activated carbon, and ion-exchange resin. This demonstrates the potential application of orange peel powder as a low-cost material for the efficient depollution of wastewater contaminated with tartrazine dye.

According to Table 6, the enthalpies ΔH° (kJ/K.mol) are shown as negative values, which implies that the elimination of E102 is exothermic and this reaction is spontaneous of physical type with all negative ΔrG° values. As for the values of ΔrS°, we can say that there is a reduction at the interface of POAA and POAS which leads to a good organization of the E102 molecules at the level of the adsorption sites while this order increases in POAT.

This work investigated the utilization of low-cost adsorbents derived from POAT, POAA, and POAS for the elimination of tartrazine (E102) azo dyes from an aqueous solution. The surface characterization of adsorbents revealed the presence of functional groups required for the removal of E102 by POAT, POAA, and POAS. The FE-SEM micrographs of adsorbents displayed porous structures which have specific chemical functions on their surface according to their nature of activation. During the study of different parameters like initial concentration, mass, temperature, and pH over time, the quantity adsorbed increases with concentration, whereas it decreases with mass, temperature, and pH. The optimal conditions for adsorption are as follows: concentration = 40 mg/L; mass = 0.01 g; temperature = 25 °C; pH = 3. The E102 adsorption process was better described by Freundlich and Temkin isotherm than Langmuir and Dubinin-Radushkevich, as well as the pseudo-second-order model. Furthermore, thermodynamic studies confirmed that the adsorption process was spontaneous, physical reaction, and exothermic. It can be concluded that the findings of this study show an efficient removal of E102 azo dyes using low-cost adsorbents (POAT, POAA, and POAS). This study can be extended for the removal of other dyes from various aqueous solutions. To complete this research, further work will be conducted to determine the mechanisms of tartrazine adsorption onto the adsorbents and investigate their adsorption/desorption isotherms.

All relevant data are available from an online repository or repositories.

The authors declare there is no conflict.