The intensive use of dyes in cosmetic, textile, pharmaceutical, leather, plastic and paper industries , makes them among the most abundant pollutants encountered in the environment. Because of their high content of organic matter which is difficult to biodegrade, of suspended matter and even of their coloring, dyes spilled into nature generate serious problems for the environment and especially for that of aqueous media  . Among these dyes, we can cite indigo carmine used as an additive in pharmacy and as a coloring agent in pastries, confectionery and cosmetics  . It belongs to the class of indigo dyes which is very toxic to the environment. Its presence in water reduces the penetration of light, affects photosynthesis of aquatic flora leading to decrease growth of bacteria and hence decreasing the bio-degradation of impurities in water  . According to WHO (World Health Organization), the amount of indigo carmine in water should not exceed 0.005 mg/L . Consumption of water with a concentration higher than the standard can cause damage in humans such as cancer, cardiovascular disease, affect the reproductive system and cause digestive disorders   .
Faced with the multiple dangers that indigo carmine can cause on the environment, several physicochemical and biological methods have been used for its elimination in aqueous media. We have among others flocculation-coagulation ; electrocoagulation ; ultra-filtration ; electrochemical  ; photodegradation  and adsorption  . Most of these techniques, despite being effective, are difficult to implement and expensive for industries. It is for this reason that adsorption is the most used method because it is simple, easy to implement, economical and non-polluting . On the other hand, the materials so far used in the adsorption of indigo carmine vary from one author to another and generally are activated carbon , magnesium oxide , zeolite  and nanofiber membranes . From economic point of view, the process of adsorption onto activated carbon is advantageous due to the plentiful accessibility of low-cost raw material. As a matter of fact, activated carbon, basically referred to as carbonaceous materials, has high physicochemical stability, porosity, sorption capacity and immense surface area . Given these different properties, our research group has for some time set its objective on the preparation of activated carbon using polyethylene terephthalate plastic bottles as well as polyethylene terephthalate/palm kernel waste composites and banana peel for the elimination of pollutants in aqueous media . However, there is little research on the use of plastic waste composite for the preparation of activated carbon used to decontaminate water due to coloring. As is the case, nearly a million plastic bottles are purchased every minute worldwide , a large part of which will end up in the ocean, then taking between 100 and 1000 years to degrade depending on the environment in which they are found . To this end, they can cause flooding or be ingested by aquatic animals causing adverse effects on the health of living beings .
Activated carbon prepared in combination with the pseudo-trunk of the banana pseudo stem of the genus “musa”, has a better yield than that based on PET alone , which made subject of our research. Activated carbons named TB1P1, TB1P1h and TB2P1 were characterized by FTIR, SEM/EDX, TGA/DTA, BET/BJH and XRD. Their performance on the adsorption of indigo carmine was systemically studied based on the contact time, initial concentration, mass of the adsorbent, isotherms and adsorption kinetics.
2. Experimental Section
2.1. Reagents and Materials
Polyethylene terephthalate plastic waste and “Musa” banana pseudo stem were collected from fields and municipal garbage cans, washed, dried, cut into small sizes and used as precursors. Sodium hydroxide (NaOH, ≥99%) and hydrochloric acid (HCl, 37%) came from Carlo ERBA. Indigo carmine (C16H8N2Na2O8S2 98%) provided by Reactive RAL. All the above chemicals were of analytical grade and used without further purification.
2.2. Preparation of Activated Carbon
The activated carbons were prepared by introducing in a 1:1 and 2:1 ratio of the banana tree trunk and plastic waste in tubular ovens, then the mixture was carbonized up to 500˚C with an evolution of 10˚C/min under nitrogen (0.15 mL/min) and a total residence time of 120 min. It was subsequently activated by increasing the temperature to 700˚C (evolution 5˚C/min) while sulmoultaneously spraying water vapor (0.1 mL/min) for a residence time of 60min. Let cool to room temperature. The compounds obtained for 1:1 and 2:1 ratios were respectively named TB1P1 and TB2P1. Similarly, the same procedure was followed for the preparation of TB1P1h (ratio 1:1) with the only difference of 700˚C (evolution of 10˚C/min) for 60 min for activation of the carbonized sample.
Fourier transform infrared (FTIR) analysis was performed using a mixture of 0.09 g KBr and 0.01 g sample which we analyzed on Vertex 70 brand wavelength device (4000 - 400 nm) with a resolution of 4 cm−1 (32 scans) to determine functional groups of surfaces. X-ray diffraction on XRD powder (RigakuGeigerflex, Cu Kα, λ = 1.5406 A) produced at 30 kV and 25 mA scanned the diffraction angles (2θ) between 10˚ and 80˚ with the step size of 0.002˚ 2θ per second. Elemental EDX analysis performed using EDAX TEAM, 125.9 ev of resolution, to know the composition of the elements present in the material coupled to SEM on a VEGA3 TESCAN brand device to know the surface morphology. Raman spectroscopy to determine the structural and electronic properties of materials performed with a Nano brand SP (Confotec MR-SOL instrument) with the 570 nm wavelength laser vert. The analysis of the specific surface, the pore volume as well as the particle size distribution was estimated using the BET equation employing the adsorption of N2 at 77.13 K, on a micrometric sorptometer model device (Thermo Electron Corporation, Sorptomatic Advanced Data Processing). After adsorption of N2, the sample was discharged at 307.13 K. The volume and the cumulative surface area of the pores were calculated using the BJH model coupled to the BET.
TGA/DTA was performed on a branded device (TGA/DTA, NETZSCH STA 409 C/CD). Approximately 10 mg of each dried sample was weighed in an aluminum mold and heated to a temperature ranging from 30˚C - 1500˚C (10 K/min) under nitrogen flow (2 mL/min). With the exception of the BET/BJH and TGA/DTA carried out at Freiburger Material for schungszentrum (Germany), all the other analyzes were carried out at the “Centre d’Analyse et de Characterization” Semlalia-Marrakech, Faculty of Sciences of Cadi Ayyad University (Morocco).
2.4. Adsorption of Indigo Carmine
The performance of the prepared activated carbon was evaluated on the adsorption of indigo carmine in aqueous solution. 20 mL of a solution with a concentration of 120 mg/L of the pollutant was introduced into a 50 mL bottleinto which an exact mass of the activated carbon is added to the solution and stirred for a given time. The mixture is filtered through filter paper and the concentration of the filtrate is measured using a SECOMAN brand UV-Vis spectrophotometer. The quantities adsorbed as well as the percentage of pollutant elimination are calculated using the formulas:
: Amount of adsorbate per gram of adsorbent (mg/g),
: Initial pollutant concentration (mg/L),
: Residual pollutant concentration (mg/L),
: Mass of activated carbon (g),
: Volume of the solution (L).
3. Results and Discussions
The FTIR spectra of the composite activated carbon are shown in Figure 1. The band around 3800 - 3386 cm−1 corresponds to the OH elongation vibrations . The peak around 3544.23 cm−1 (on the curve in black) represents the vibration of N-H elongation. We have a less intense peak around 1637 cm−1 and another more intense at 1403.7 cm−1 corresponding to elongation vibrations of C=O and C=C respectively. These bands move towards shorter wavelengths probably because of intermolecular hydrogen bonds. At 1134 cm−1 we have cellulose and lignite C-O-C present on the activated carbon. Between 999.37 - 615.73 cm−1 are vibrations out of the plane of the C-H bonds.
Figure 2 shows the Raman spectrum of the prepared activated carbons. All samples have two strong peaks at 1336.14 cm−1 and 1595.39 cm−1. The D band around 1336.14 cm−1 corresponds to the disordered graphitic network. The G band around 1595.39 cm−1 corresponds to the vibration of the C-C bonds of carbon atoms with a graphitic degree . The increase in the intensities of the G and D bands indicates an increase in the graphitic degree. Likewise, the decrease in the intensity ratio of the D band to the G band (ID/IG) indicates an increase in the graphitic degree. According to Figure 2, these ratios are 0.9568; 0.9879 and 0.9022 for the TB1P1, TB1P1h and TB2P1 respectively. These results clearly indicate that some materials have higher graphitic degrees and less disordered structure. However, increasing the graphitic degree of a material can dramatically increase electrical conductivity . So, we can confirm that the material TB2P1 is more disordered and has a higher electrical conductivity than TB1P1h and TB1P1.
Figure 1. FT-IR spectra of composite carbons TB1P1, TB1P1h and TB2P1 prepared from banana pseudo stems and plastic waste.
Figure 2. Raman spectrum of composite activated carbons TB1P1, TB1P1h and TB2P1.
The XRD spectrum of Figure 3 shows several bands, including two larger areas corresponding to the structure of amorphous carbon. The two peaks are at 2θ = 24.3˚ and 2θ = 44˚ respectively correspond to the 002 and 100 reflection planes. In addition to the amorphous character of carbonaceous materials, certain peaks indicate the presence of small amounts of impurity, such as silica at 2θ = 30˚. At 2θ = 33.88˚ we have potassium hydroxide and potassium chloride at 2θ = 40.72˚. These different bands are close to those obtained by Taer et al. in 2018 on activated carbon prepared from banana pseudo stems .
Figure 4 shows the adsorption/desorption isotherm of N2 by the different activated carbons. These curves are all the shape of the type IV isotherm according to the IUPAC classification, and the hysteresis loops are in the relative pressure range of 0.4 and 1, which are characteristic of mesoporous materials . In such a medium, capillary condensation also occurs and the phenomenon is not reversible. The specific surfaces according to BET of the various activated carbons are 424.37 m2/g; 385.45 m2/g and 338.84 m2/g for TB1P1, TB1P1h and TB2P1 respectively.
For the three samples, a first very low mass loss is observed in Figure 5 which varies from 5.19% to 6.14% in a range of 23.52˚C to 37.04˚C which corresponds to humidity rate of these samples. After this phase of dehydration, mass losses of 18.8%; 19.58% and 20.23% are observed respectively for TB1P1; TB2P1 and TB1P1h for temperatures between 173.10˚C and 196.82˚C. Then significant losses are between 35.53 and 41.59 % for temperatures between 794.70˚C and 961˚C. The mass losses at moderate temperatures would correspond to the decomposition of carboxyl groups (150˚C and 400˚C), lactone functions between 350˚C and 600˚C and phenol functions between 600˚C and 700˚C .
For DTA, the heat flux values are all negative. This is proof that reactions at the surface of activated carbons are endothermic.
Figure 3. XRD spectrum of composite carbons TB1P1, TB1P1h and TB2P1.
Figure 4. Isotherms of adsorption/desorption of N2 by composite carbons TB1P1, TB1P1h and TB2P1.
3.2. Study of the Adsorption of Indigo Carmine by Different Activated Carbons
3.2.1. Effect of Contact Time on Adsorption
The time-dependent pollutant removal study in Figure 6 indicates that the curves for TB1P1, TB2P1 and TB1P1h are almost similar. Indeed, the adsorption increases as a function of the stirring time with a rapid effect during the first 60 minutes, before reaching equilibrium at t ≥ 60 min. To this, equilibrium corresponds to an adsorbed quantity of dye of 94.71%; 86.18% and 84.17% for TB1P1, TB1P1h and TB2P1 respectively. It should be noted that the pollutants are adsorbed in the first place on easily accessible sites, with diffusion to less accessible adsorption sites taking place as the stirring time increases until a balance is achieved. As such, at different times of 15, 45 and 75 minutes, we had adsorption rates of 42.28%; 84% and 94.64% for TB1P1; 48.33%; 62.88% and 81.97% for TB1P1h and 39.87%; 66.89% and 83.23% for TB2P1.
3.2.2. Effect of Adsorbent Mass
The effects of mass-dependent indigo carmine removal are shown in Figure 7. In this figure, the mass increase promotes uptake of indigo carmine for all three activated carbons samples. Indeed, for masses 0.2, 0.4, 0.6, 0.8 g and 1.2 g of the activated carbons samples, we have the reduction rates of 13.6%, 40.98%, 74.94%. 94.64% and 94.7% for TB1P1; 2.1%, 8.89%, 17.56%, 45.66% and 86.18% for TB1P1h and 2.1%, 3.28%, 17.8%, 46.37% and 89.46% for the TB2P1. This increase in adsorption depending on the mass of activated carbon is probably due to the availability of active sites as the mass increases . However, the maximum adsorption is that of sample TB1P1 which has a larger specific surface area than samples TB1P1h and TB2P1. The more we increase the mass, the more adsorption sites are available for the same amount of pollutant and the greater the absorption.
Figure 5. TGA/DTA curves for composite activated carbons TB1P1, TB1P1h and TB2P1.
Figure 6. The effect of the contact time on the retention of indigo carmine (V = 20 mL pH = 4.7 mass = 0.6 g).
Figure 7. The effect of the mass of the adsorbent on the retention of indigo carmine dye.
3.2.3. Effect of Initial Pollutant Concentration
In Figure 8 the adsorption capacity of activated carbon increases with the increase in the concentration of indigo carmine before reaching an equilibrium with maximum adsorbed amounts of 78.3, 67.7 and 77.51 mg/g respectively for TB1P1, TB1P1h and TB2P1. This can be explained by the fact that the diffusion of dye molecules to the surface of the adsorbent is accelerated as the dye concentration increases. Also, if the initial concentration of dye is low, the molecules adsorb only on the surface to form a monolayer. On the other hand, when the initial concentration is high, there will consequently be more molecules which will diffuse towards the surface of the sites of the particles of the adsorbents  .
3.3. Study of the Isotherm and the Retention Kinetics of the Dye
The isothermal models and kinetic equilibria used are grouped together in the Table 1 .
3.3.1. Kinetic Models
In order to understand the rate mechanism that controls the adsorption of indigo carmine by prepared activated carbons, pseudo-first and pseudo-second order kinetic models were exploited.
Figure 8. Effect of the initial concentration on the quantity of indigo carmine dye adsorbed (m = 0.6 g; pH = 4.7; V = 20 mL; Time = 30 min).
Table 1. Data on isotherms and kinetic equilibria used.
The results in Figure 9 and Table 2 show correlation coefficients greater than 0.9 (R2 > 0.9). These results are in agreement with a multilayer adsorption on the surface of the different materials. This involves van der Waal-type interactions between the dye and the various carbons, thus demonstrating a physical adsorption between the adsorbate and the adsorbent.
As for pseudo-second order kinetic models (Figure 10), the hypothesis is that adsorption is proportional to the number of occupied sites. Results indicating correlation coefficients R2 > 0.95; shows that this model applies well in the adsorbent/dye systems studied. It is also noted that the values of the constants K2 are almost identical for the three adsorbents. Likewise, the quantities calculated at equilibrium are close to the experimental values in Table 2. The low values of K2describe a competition between the active sites . In view of these results, we can say that the pseudo-second order model is best suited to model the adsorption of indigo carmine on adsorbents. This model highlights the chemical interactions (covalent bond) between the adsorbate and the adsorbent. The adsorption is monolayer and we can say that chemical adsorption dominates the process of adsorption of indigo carmine on the different activated carbons.
Figure 9. Pseudo-first kinetic order kinetics of indigo carmine adsorption (mass of adsorbent 0.6 g; 20 mL, pH = 4.7).
Table 2. Calculated and experimental values of pseudo-first and second order.
Figure 10. Pseudo-second kinetic order kinetics indigo carmine adsorption (mass of adsorbent = 0.6 g; 20 mL, pH = 4.7).
Although both processes agree with adsorption, the one that best defines it is the characteristic pseudo second-order pattern of chemisorption at the surfaces of activated carbon samples.
3.3.2. Adsorption Isotherms
The Freundlich and Langmuir models were used to interpret the surface heterogeneity of different activated carbons during the adsorption of indigo carmine. Actually, the empirical Freunlich equation with two parameters KF and 1/n (Freundlich coefficients), takes into account an exponential distribution of the energies of the adsorption sites on the surface of the solid and an adsorption in localized sites . The adsorption isotherms of the three adsorbents are shown in Figure 11 and Figure 12. The 1/n values obtained are less than 1, which indicates that the adsorption is normal on these materials. These values of 1/n support the hypothesis of the heterogeneity of the surface of the different adsorbents. The constant KF is a rough indicator of the adsorption capacity while 1/n is a function of the adsorption affinity in the process.
However, it can be seen that the values of R2 for TB1P1 and TB2P1 are less than 0.96, which limits the use of the Freundlich model for the description of the adsorption of indigo carmine on the two adsorbents . On the other hand, the model is more favorable for the sample TB1P1h (R2 = 0.98).
However, Langmuir’s thermodynamic model is theoretically applicable only in the case of single-layer, localized, homogeneous adsorption sites of equal energies and without lateral interaction between the adsorbed particles . The Langmuir isotherm describes the adsorption of indigo carmine on different carbons with the correlation coefficients R2 > 0.97. The essential characteristic of this isotherm is its separation factor which is a dimensionless constant expressed by:
Figure 11. Linear transformation of Langmuir.
Figure 12. Linear transformation of Freundlich.
For adsorption to be favourable, the separation factor RL should provide values ranging between 0 < RL < 1. The values obtained for these factors are: 0.10; 0.11 and 0.18 respectively for TB1P1; TB1P1h and TB2P1. These values show that the quantity adsorbed Qm for TB2P1 is higher than that of TB1P1 which is in turn higher than that of TB1P1h (Table 2). Therefore, the sample TB2P1 shows a greater affinity to adsorb the dye on a single layer relative to TB1P1; TB1P1h respectively. In view of the different experimental values obtained, we can say that Langmuir’s model aptly describes the phenomenon of dye adsorption on the 3 adsorbents.
Hence strong chemical adsorption dominates the process on a single layer surface.
Plastic waste and the banana stem were used in the preparation of activated charcoals by steam activation, for the removal of indigo carmine in aqueous media. The materials obtained had non-negligible specific surfaces, and various surface groups. This allows them to adsorb indigo carmine with a percentage of 94% as well as a maximum amount adsorbed of 127.55 mg/g according to Langmuir. The pseudo-second order kinetic model best defined the absorption phenomenon. That is, adsorption is chemical in nature, single-layered on homogeneous surface.
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