Adsorption of acid yellow dye 17 on activated carbon prepared from Euterpe oleracea: kinetic and thermodynamic studies

Environmental pollution has been a point of discussion in the international community and an object of investigation by research groups, which focus on the development of remediation methods. In the current study, the bunch of açaí (Euterpe oleracea) was used as a precursor for the preparation of low-cost activated carbon in order to remove the dye 17 AY 17 from the aqueous solution. The synthesis was carried out at temperatures of 500, 600 and 700 °C, for 2.0 h in a muffle furnace. The kinetic and thermodynamic mechanism of the adsorption process of the acid yellow dye 17, and the effects of pH, contact time and initial concentration were investigated. Activated carbon carbonized at 700 °C had the highest adsorption capacity, about of 99.9% of removal of the AY. The adsorption capacity of AY 17 was slightly pH dependent with a maximum value at pH 2.0. The kinetic data show that the equilibrium time was 200 min, and the adsorption capacity of activated carbon was 99.9% at 50 mg L and 67.0% at 150 mg L of adsorbate, suggesting high adsorption capacity of the material, even in the presence of high dye concentration. The adsorption process of AY 17 is described by the pseudo-second order kinetic model, and the experimental adsorption isotherms are adjusted to the Freundlich model, indicating that the adsorption of AY 17 on activated carbon occurs with the formation of multilayers. The present study shows that this low-cost material has great potential for remediation of textile effluents.


Introduction
The contamination and degradation of the environment by polluting gases, residues from agricultural activities, organic waste, toxic chemical products and other polluting sources has been the subject of debate in today's society (Chen et al., 2020;Khattab et al., 2020;Kishor et al., 2021;Nambela et al., 2020;Sarkar et al., 2017;Ignachewski et al., 2010). These pollutants promote an imbalance in the environment, and have reduced drastically the quality of available drinking water (Kishor et al., 2021;Munagapati et al., 2021;Hynes et al., 2020;Berradi et al., 2019;Sarkar et al., 2017;Ignachewski et al., 2010). Within the industrial sector, textiles are responsible for a large part of the economy of developed and some emerging countries (Haseeb et al., 2020). However, it is estimated that for the processing of about 12-20 tons of raw material are discarded daily between 1000 and 3000 m 3 of wastewater, thus promoting significant impacts on the environment (Ghaly, 2014). In addition, wastewater from activities in this sector has around 20% of dyes (Hynes et al., 2020;Khattab et al., 2020;Berradi et al., 2019), which are potentially toxic (Hynes et al., 2020;Benkhaya et al., 2020;Berradi et al., 2019;Verma et al., 2019;Obaid et al., 2016) generating negative impacts on the environment in terms of salinity, biological oxygen demand (BOD), chemical oxygen demand (COD), pH, temperature and others (Hynes et al., 2020;Berradi et al., 2019;Parveen & Rafique, 2018;Roy et al., 2018;Sarkar et al., 2017).
Several studies have shown that the treatment of these residues in effluents represents one of the great challenges of the textile sector, since these chemical substances have high chemical and biological stability, making their degradation difficult (Hynes et al., 2020;Khattab et al., 2020;Berradi et al., 2019;Obaid et al., 2016). In addition, the presence of color significantly contributes to the pollution of water resources by drastically reducing the penetration of sunlight, and consequently the photosynthesis capacity of aquatic plants and algae (Hynes et al., 2020;Benkhaya et al., 2020). On the other hand, the chemical activity of these substances in the human body can cause breathing difficulties, eye irritation, impairment of the cardiovascular system, mutations, tumors and affect the nervous system ( Shindhal et al., 2021;Yusop et al., 2021;Bulca et al., 2021;Achour et al., 2021;Khatri et al., 2018;Hassaan et al., 2016;Vacchi et al., 2013). Therefore, many research groups have focused on the development of efficient and low-cost methods that enable the removal and remediation of these pollutants, in order to alleviate possible environmental impacts and maintain the integrity of human health.
On the other hand, dyes are chemical substances that play an essential role in various industry sectors, such as textiles, printing, food, plastics, cosmetics and pharmaceutical industries, whose main objective is to add color to various products (Benkhaya et al., 2020;Berradi et al., 2019;Verma et al., 2019;Castro et al., 2018). These compounds are classified according to several criteria, including the chemical structure, which is made up of chromophore and auxochrome groups (Benkhaya et al., 2020;Berradi et al., 2019;Verma et al., 2019;Castro et al., 2018;Raman & Kanmani, 2016). Azo dyes, for example, which have the azo chromophore group (-N=N-) is the predominant class in the textile industry, representing 70% of the dyes used in this sector (Benkhaya et al., 2020;Berradi et al., 2019;Castro et al., 2018;Sarkar et al., 2017;Lang et al., 2013). This class of textile dye is highly water soluble and easily reacts with cellulosic, protein, polyester, acrylic and polyamide fibers (Castro et al., 2018;Guaratini & Zanoni, 2000).
However, due to the high costs of commercial activated carbon, alternative adsorbents that guarantee the same adsorption efficiency have been studied, with emphasis on those produced from residues from agricultural and extractive activities (Achour et al., 2021;Alam et al., 2020;Alkathiri et al., 2020;Panwar and Pawar, 2020;Kannaujiya et al., 2021;Zoroufchi Benis et al., 2020;Reza et al., 2020). Several studies report the use of these materials in the synthesis of activated carbon for remediation of effluents, such as the study by Njoku et al. (2014) who studied the efficiency of activated carbon obtained from rambutan husk (Nephelium lappaceum) in the adsorption of acid yellow dye 17 (Reza et al., 2020). Other works address the removal of acid yellow dye 17 using activated carbon obtained from eggplant residues, achieving maximum removal of 99.58 % (Kannaujiya et al., 2021), and the removal of anionic acid yellow dye 17 using avocado seed powder, showing excellent removal over a wide pH (Munagapati et al., 2021), range and adsorption study of acid yellow dye 17 using activated carbon obtained from the rice husks, providing a maximum removal of 99.98% (Patil et al., 2015). However, most of these works use synthesis routes with inert gases, such as nitrogen and argon, which increase the cost of the process (Reza et al., 2020;.Patil et al., 2015).

Preparation of dye stock solution Acid yellow 17
The acid yellow dye 17, namely (AY17) (C16H10Cl2N4Na2O7S2), CAS number: 6359-98-4, ID 329751987, was purchased from Sigma-Aldrich company, Saint Louis, USA, at 60% content. This dye has an intense yellow color, being characterized by an absorption maximum around 402 nm, molar absorption coefficient 14,000.00 mol -1 L cm -1 and molecular weight equal to 551.29 g mol -1 . Ultra-pure water with a resistivity of 18 MΩ cm, purified in a purifier (Milli-Q®, Merck Millipore), was used to prepare the solutions. The 1000 mg L -1 standard stock solution was prepared by dissolving 1.6 g of the dye in 1000 mL of ultrapure water. Then, the solution was properly stored for the studies.

Synthesis of adsorbents prepared from açaí bunch
The raw material (açaí bunches) collected in the municipality of Marabá-PA, was initially washed in running water to remove unwanted solid residues and subjected to a drying process at a temperature of 80 ºC in an (SSD-11L, Solidsteel) oven for 48 h. Then, the material was crushed in a knife mill model NL-226/02 (NewLab, Brazil) and classified in a Tyler-type sieve (Bertel, Brazil) coupled to a stirrer in the pass-through mesh range above 28 (0.25 mm). After classification, 50 g samples of biomass were carbonized in a Magnus brand muffle oven at temperatures of 500, 600, 700 ºC for 2.0 h and heating rate of 10 ºC min -1 . After the carbonization process, all samples were again classified in 325 mesh (0.044 mm) opening sieves and reserved for adsorption tests. The carbonized adsorbents at temperatures of 500, 600 and 700 ºC were named CA-CA500, CA-CA600 and CA-CA700 respectively.

Characterization of adsorbents by BE and BJH and IR
The textural properties of activated carbon were determined by adsorption-desorption of N2 at 77.35 K using a surface analyzer (QUANTACHROME model NOVA 2200e) with liquid nitrogen of density 0.808 g cm -3 . Before taking the measurements, the sample was subjected to a thermal pre-treatment at 423 K for 2.0 h. The adsorption of N2 in the sample was used to calculate the specific surface area (SBET) by the BET method (Brunauer -Emmett -Teller), while the diameter (Dp) and pore volume (Vp) were determined by the BJH method (Barrett -Joyner -Halenda).
The infrared spectra of the in natura and carbonized samples and CA-CA700, both in a particle size of 325 meshes, were obtained by attenuated total reflectance (ATR), using a Thermo brand spectrometer, model Nicolet iS50 FT-IR, in the spectral region 4000-400 cm -1 , at 100 scans and 4 cm -1 resolution. Data acquisition was performed using the OMNIC program, and the treatment using the origin program, version 8.0. As a pre-treatment, the samples were dried at 105 °C for 24 hours.

Analytical curve
From the 1000 mg L -1 stock solution, solutions were prepared with a volume of 2.0 mL of the dye in the concentration range of 0 to 22 mg L -1 for the construction of the analytical curve. The UV-Vis experiments were carried out in a Bel Spectro S05 spectrophotometer, at the maximum absorption wavelength (λmax) 400 nm. Sample absorbances were measured in a 1.0 mL quartz cuvette and 1.0 cm optical path.

Adsorption experiments
The adsorption measures were carried out in a 250 mL erlenmeyer flask, varying the mass of the adsorbents in the range of 0.1 to 1.0 g, in 100 mL of dye solution AY 17 in the concentration range of 25 to 300 mg L -1 , under stirring at 200 rpm at 20 °C.
Initially, measurements were taken to evaluate the most efficient adsorbent, in which 0.20 g of each synthesized material was exposed to 100 mL of the dye at 25 mg L -1 , under agitation in an orbital shaking incubator (SL-223) for 200 min, at pH 7.0. Adsorbent removal capacity (%) was measured using Equation 1: Where C0 (mg L -1 ) and Ce (mg L -1 ) are the dye concentrations initially and at equilibrium, respectively (Munagapati et al., 2021).
In a second step, measurements were carried out to evaluate the effect of the adsorbent mass in the range ( 25 to 300 mg L -1 , to improve the acquisition of equilibrium isotherms. The ability to remove AY17 at equilibrium qe (mg g -1 ) was calculated from Equation 2: where C0 (mg L -1 ) and Ce (mg L -1 ) are the concentrations of the initial and equilibrium AY17 respectively, V (L) is the volume of solution and m (g) is the mass of adsorbent (Munagapati et al., 2021). Table 1 shows the adsorption capacity of the three adsorbents synthesized from the açaí bunch as a function of carbonization temperature. Under these conditions, the data show that the increase in temperature significantly contributes to the adsorption efficiency, with removal rates of 38.0, 61.7 and 99.9 % being observed for CA-CA500, CA-CA600 and CA-CA700, respectively (Table 1). The increase of 200 ºC granted an increase of 61.9 % in the AY17 removal (Table 1). This removal index obtained for the CA-CA700 is very similar to other studies such as Kannaujiya et al., 2021 andPatil et al., 2015 show that the increase in carbonization temperature produces two main effects, the first is the significant increase in the surface area of the carbonized material, and consequently a greater adsorption efficiency, and second, a significant loss of synthesis yield (Ani et al., 2020;Machrouhi et al., 2018), corroborating the data reported in the present study.

Adsorption efficiency of materials
Thus, the following experiments were carried out only with the adsorbent with the highest adsorption capacity, CA-CA700. Figure 1 shows the adsorption-desorption isotherm of N2 at 77 K for activated carbon from the açaí bunch (CA-CA700).

CA-CA700 characterization
A The isotherm obtained presents a characteristic profile of the type IV isotherm, according to the classification of the International Union of Pure and Applied Chemistry (IUPAC), (Foo and Hameed, 2012;Sing, 1982) with an H4-type hysteresis cycle in the P/P0 range from 0.4 to 0.99 , suggesting that the surface of CA-CA700 is predominantly constituted by mesopores (Munagapati et al., 2021;Sing, 1982;Shoaib et al., 2020;Hamza et al., 2018). The textural properties obtained by the BET and BJH methods, indicate that the material has a surface area of 353 m 2 g -1 , average pore diameter of 3.628 nm and pore volume of 0.046 cm 3 g -1 corroborating the presence of mesopores in the structure, since for this type of isotherm the diameter range for mesoporous material is between 2.0 and 50 nm. (Munagapati et al., 2021;Shoaib et al., 2020;Mahmoud et al., 2020;Berradi et al., 2019;Jedynak et al., 2019;Hamza et al., 2018). The infrared spectra in the range from 4000 to 450 cm -1 for in natura sample of the açaí bunch and for the CA-CA700 adsorbent are shown in Figure 2. The FTIR spectrum of the açaí bunch biomass is characterized by the presence of a band of greater intensity at 3,385 cm -1 associated with the stretching of the hydroxyl group with hydrogen bonds in the cellulose (-OH).
Other less intense bands centered at 2920 cm -1 , due to axial CH2 deformation characteristic of the methyl group, 1740 cm -1 associated with the presence of the natural carbonyl group (C=O) of hemicellulose, 1610 cm -1 attributed to the stretching of the C bond = C of aromatic compounds, 1518 cm -1 contribution of primary and secondary amines, 1,160 and 1,056 cm -1 contribution of COC binding in hexoses, are observed. Carbonization promotes drastic changes in the functional groups of the biomass, characterized by an intense reduction in the contributions of oxygenated groups, which suggest the dehydration of the material.
The CA-CA700 spectrum presents bands at 1420, 1040 and 870 cm -1 that are associated with the cellulose structure and indicate the partial degradation of the material in pyrolysis.

Figure 2 -Infrared spectra for samples of açaí biomass (Euterpe oleracea) in natura and carbonized at 700 °C (CA-CA700).
Source: Authors (2021).  Research, Society and Development, v. 11, n. 2, e16511225556, 2022 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v11i2.25556 8 The rapid adsorption of the dye at the beginning of the process is associated with the large availability of empty sites and the reduction in the rate with increasing interaction time indicates the increase in repulsive forces between the adsorbate molecules as the sites are occupied, as well as the saturation of the sites available for adsorption (Munagapati et al., 2021;Reza et al., 2020;Patil et al., 2015;Srivastava et al., 2006). The adsorption capacity of CA-CA700 for acid yellow dye 17 increased from 47.9 ± 0.1 mg g -1 to 100.2 ± 0.2 mg g -1 when the initial concentration of dye increased from 50 to 150 mg L -1 , suggesting that this material is suitable for the treatment of textile effluents.

Effect of pH on adsorption
The effect of pH on the removal capacity of CA-CA700 is shown in Figure 4. The result shows that the AY 17 removal index decreases from 99 % at pH 2.0 to 92% at pH 9.0, indicating a promising applicability of the material under real conditions for textile effluents. According to the literature, adsorption is favored in an acidic medium for anionic dyes, because the surface protonation favors the interaction with the sulfonic groups of the dye through electrostatic forces (Bouhadjra et al., 2021;Felista et al., 2020;Jain et al., 2020;Cardoso et al., 2011). This phenomenon is due to the presence of sulfonate groups (-SO3Na) in the structure of the acid yellow dye 17, which, in an aqueous medium, makes available the sulfonic groups (-SO3 -), allowing the occurrence of electrostatic interactions with the functional groups present on the surface of the CA-CA700. At pH below 7.0, the initially negative functional groups of CA-CA700 are partially neutralized by the addition of protons (H + ), enabling an electrostatically favorable interaction of the adsorbent with the anionic form of the dye in an aqueous medium, thus increasing the index of removal (Bouhadjra et al., 2021;Bhomick et al., 2018;Li et al., 2018). Higher removal rates of the acid yellow dye 17 at lower pH values is in accordance with the study by Patil et al., 2015, using rice husk as an adsorbent, in which a removal of 72.1% was obtained at pH 2, value lower than that found in this study, being 99.8% at the same pH.
In addition, Reza et al., 2020 andMunagapati et al., 2021 studied the adsorption of AY 17 on activated carbon, with a high removal capacity being reported in the pH range between 2.0 and 7.0.

Kinetic models
The characterization of the adsorption kinetics provides important information about the adsorption mechanism, which is essential to evaluate the adsorption efficiency (Munagapati et al., 2021). Experimental data were fitted using pseudo-first-order and pseudo-second-order models. The linearized mathematical expression for the pseudo-first order model is shown in Equation 3 (Veit et al., 2014).
where, qt (mg g -1 ) and qe (mg g -1 ) are the amount of solute adsorbed over time t and the amount adsorbed when equilibrium is reached, respectively, k1 (min -1 ) is the pseudo-first order adsorption rate constant and e t (min) is the contact time. Likewise, the linearized mathematical expression for pseudo-second order adsorption is shown in Equation 4.

= +
(4) where k2 (g mg -1 min -1 ) is the pseudo-second order adsorption rate constant. Figure 5 shows the fits using Equations 3 and 4 of the experimental data for the pseudo-first order (Figure 5a) pseudosecond order (Figure 5b) models. The kinetic parameters obtained from the adjustments are shown in Table 2.
From Figure 5a and the parameters presented in Table 2, the pseudo first order kinetic model is not adequate to describe the adsorption process of dye AY 17 in CA-CA700. Considering the linear regression coefficients (R 2 adj) for all concentrations in this study, the pseudo-second order model better describes the adsorption process of AY 17 and suggests that the dye adsorption rate on the surface of CA-CA700 is strongly dependent on the number of species adsorbed on the surface of the adsorbent (availability of free sites), than the concentration of dye in the solution (Table 2). This behavior is evidenced in the low dependence on the velocity constant (k2), Table 2, as a function of concentration, in which almost no significant change is observed as a function of the concentration in the range from 75 to 150 mg L -1 . Another parameter used to identify the kinetic controlling mechanism of the adsorption process is the proximity between the values of qe calculated by the theoretical (qe, cal) and experimental (qe, exp) model, in this case, the closer the proximity between these parameters, the greater it is the applicability of the model (Munagapati et al., 2021;Habibi et al., 2018). Table 2 shows that the values of qe1, cal obtained by the pseudo-first model are much lower than the values of qe, exp for all concentrations, Table 2 -Kinetic parameters for pseudo-first and pseudo-second orders models for acid yellow dye 17 (AY 17), 0.5 g of CA-CA700 at 20 °C, pH 6.0.
pseudo-first order pseudo-second order k1 (min -1 ) x 10 -3 qe1, cal (mg g -1 ) that the adsorption process follows pseudo-second order kinetics. Studies in the literature for different dyes indicate that the adsorption process on activated carbon is described by the pseudo-second order model (Karthik et al., 2019). Njoku et al., 2014, studied the activated carbon adsorption kinetics of AY 17 in a concentration range of 50 to 400 mg L -1 and reported that the pseudo-second order model best describes the kinetic process of AY 17. Likewise, recent studies carried out by Munagapati et al., 2021 show that the adsorption of AY17 obeys the pseudo-second order model, with R 2 adj greater than 0.99 for six study temperatures. Other works involving adsorption of AY 17 on activated carbon also portray better adjustments to this model (Jedynak et al., 2019;Patil et al., 2015;Ahmad et al., 2014).

Adsorption isotherms
To evaluate the adsorption capacity of CA-CA700, adsorption isotherms obtained by the graphical relationship between qe and Ce were used. The experimental data were adjusted according to the Langmuir, Freundlich and Temkin models, in order to obtain information regarding the specific surface properties and nature of the interactions between the adsorbate and the adsorbent (Munagapati et al., 2021;Njoku et al., 2014;Karthik et al., 2019). The Langmuir model (1918) suggests that the adsorption process occurs in monolayers and that the energy of the available sites is homogeneous (Njoku et al., 2014;Srivastava et al., 2006;Karthik et al., 2019). The mathematical expression for the linearized form of this model is shown in Equations 5 (Langmuir, 1918;Al-Ghouti & Da'ana, 2020).

= +
where qm (mg g -1 ) and qe (mg g -1 ) are the maximum adsorption capacity per monolayer and the amount of solute adsorbed at equilibrium respectively, Ce (mg L -1 ) is the concentration of adsorbate at equilibrium and kb (L mg -1 ) is the Langmuir constant associated with the free energy of the adsorption process.
The Temkin isotherm model, on the other hand, assumes that the heat of adsorption of all molecules in the layer decreases linearly with surface coverage and that the adsorbent sites are heterogeneous (Njoku et al., 2014;Inyinbor et al., 2016;Mane et al., 2007;Kataria et al., 2016). The mathematical expression of this model is shown in equation 07 (Saadi et al., 2015).
In linearized form, the equation that describes the Temkin isotherm is commonly expressed as: where B = RT/b and b (J mol -1 ) is the Temkin constant related to the heat of adsorption, A (L g -1 ) is the Temkin isotherm constant, R (8.314 J mol -1 K -1 ) is the universal gas constant and T (K) is the absolute temperature (Njoku et al., 2014;Piccin et al., 2011;Inyinbor et al., 2016;Kataria et al., 2016).
The adjustments using the Langmuir, Freundlich and Temkin (Saadi et al., 2015) models for the adsorption of AY 17 at 20 ºC, obtained from Equations 5, 6 and 8, are shown in Figure 6. The parameters obtained from the adjustments are shown in Table 3. The results show that the adsorption equilibrium data of AY17 is better described by the Freundlich model, whose R 2 adj of the adjustments was 0.9873 and the adsorption capacity constant kf equal to 47.9 (mg g -1 ) (L mg -1 ) 1/n ( Table 3). The value of the Freundlich constant (n) equal to 5.95 suggests that the adsorption process between AY 17 and CA-CA700 is favorable, because the higher the value of n (smaller the 1/n ratio), the stronger it is the interaction between the adsorbate and the adsorbent (Obaid et al., 2016;Lang et al., 2013;Al-Ghouti and Da'ana, 2020;Piccin et al., 2011;Inyinbor et al., 2016;Saadi et al., 2015;Delle Site, 2001). In addition, this model assumes that the process of adsorption of AY17 occurs mostly in multilayers (Njoku et al., 2014;Patil et al., 2015;Ahmad et al., 2014;Ashraf et al., 2013), a non-homogeneous surface, where the energy distribution in the sites is heterogeneous and strictly exponential (Al-Ghouti and Da'ana, 2020;Piccin et al., 2011;Inyinbor et al., 2016;Delle Site, 2001;Febrianto et al., 2009;Ahmad et al., 2014). Patil et al., 2015 when studying the models of Langmuir and Freundlich in the adsorption of dye AY 17, they also identified that the Freundlich model, in its linearized form, better adjusted to the experimental data, obtaining n equal to 1.85 and R 2 adj equal to 0.996 confirming that the adsorption of AY 17 occurs preferentially in multilayers. Ashraf et al., 2013 studied the adsorption of AY 17 on activated carbon produced from T. Angustata L. finding excellent fits to the Freundlich model.

Conclusion
The data reported in this study indicated that the charcoal produced from the açaí bunch in an open atmosphere, without the use of inert gases that make the synthesis route more expensive, is promising for the treatment of effluents and, therefore, is an important alternative for the reduction of these waste in the environment. CA-CA700 is effective for the removal of acid yellow 17, showing high adsorptive capacity under the conditions studied. The equilibrium time is reached with 200 min and the adsorption process of AY 17 in the CA-CA700 is described by the pseudo-second order kinetic model while the Freundlich isotherm model indicates that the interaction between adsorbent and adsorbate is favorable and occurs in multilayers.
Our results represent a nice contribution and an important and necessary step in the low-cost materials adsorbents for removal dyes in the aqueous solutions. In future publications, the authors propose the use of adsorbent materials obtained from the plant biomass of the Amazon biome, mainly from extractivism, and composite materials for the treatment of textile effluents, heavy metals, drugs and other contaminants in water bodies and soil.