USE OF ADSORPTIVE SYSTEMS FOR WATER TREATMENT WITH THE PRESENCE OF AZO DYES

Objective: This study evaluated the adsorption capacity of activated carbon in relation to multicomponent solutions containing the azo dyes Blue Remazol, Yellow Remazol, and Red Remazol. These dyes were selected due to their extensive use across a wide range of industrial activities, including textiles, paper, plastics, and cosmetics. Theoretical Framework: Classical adsorption theory was applied, and mathematical studies were proposed and conducted based on the Langmuir, Freundlich, Langmuir-Freundlich, and Redlich-Peterson models, as well as knowledge of the chemical and molecular structure of the adsorbent and adsorbate for individual dyes and multicomponent mixtures. Method: The study involved varying parameters typically found in adsorption beds, such as adsorbent particle diameter, dye concentration, solution pH, bed temperature, and adsorbent mass to solution volume ratio to identify the point of maximum removal efficiency. Results


INTRODUCTION
In Brazil, the control of the quality of drinking water has traditionally been associated with the elimination of bacteria and other microorganisms, overlooking the real risk of chemical contamination, such as water contamination by dyes used in textile, food, automotive, and other industries (NEVES, 2015).
There are more than 100,000 types of commercially available dyes, with over 0,7 million tons of dye produced annually.Approximately 15% of these dyes are discharged into untreated effluents (ATMANI et al., 2009).Most dyes, depending on their concentration and consumption conditions, are toxic and carcinogenic.
Several studies have been conducted to find the best and most viable way to treat effluents with the most commonly methods being physicochemical and biological processes such as adsorption, coagulation, precipitation, aerobic and anaerobic treatment, activated sludge, and advanced oxidative processes (AOPs) (KASSAB et al., 2010;HASHIMOTO et al., 2014;ZHANG et al., 2016;ESTRADA-ARRIAGA et al., 2016).
Among the treatment technologies, the most efficient method for removing synthetic dyes from aqueous effluents is the adsorption process.This process transfers the dye from the effluent water to a solid phase while minimizing the effluent volume.Subsequently, the adsorbent can be regenerated or stored in a dry place without direct contact with the environment (ATMANI et al., 2009;MITTAL et al., 2010).Activated carbon is the most commonly used adsorbent in the treatment of textile wastewater.However, this process is still considered economically unfeasible due to the high cost of activated carbon.

THEORETICAL FRAMEWORK
In studying adsorption equilibrium in a solid adsorbent/solution system, understanding the structure and porosity of the solid is crucial for choosing the appropriate model (ABDEL-GHANI, 2014).Interactions between molecules in both the liquid and adsorbed phases are also key factors.Theories describing multicomponent adsorption equilibrium can be predictive, using only monocomponent data, or correlative, requiring binary system experimental data (NASCIMENTO et al., 2014).

EMPIRICAL REPRESENTATION OF MULTICOMPONENT EQUILIBRIUM
There are many such equations, some purely empirical and others semi-empirical, among which those presented in Table 1 are included.The Equation of Langmuir can be theoretically derived using the Ideal Adsorbed Solution Theory (IAST), assuming that the monocomponent equilibrium is described by the Langmuir equation and the parameters   ∞ are the same for all components.

ACTIVATED CARBON
Common carbon is produced from wood with greater mechanical strength, while activated carbon is made from materials with lower mechanical strength, which gives it higher porosity.Research on carbon is increasingly both as an energy generator and for the removal of undesirable compounds (RAHMAN et al., 2017).

AZO DYES
Azo dyes are those that have in their molecular structure one or more azo groups, -N=N-, linked to aromatic rings and functional groups such as -OH, -SO3.This group represents 60% to 70% of existing synthetic dyes.Figure 3 shows the structure (GOMES, 2009)."6

METHODOLOGY
Adsorptive systems essentially involve the interaction between the adsorbate, adsorbent, and equipment.Therefore, the starting point for the study of this type of operation is the understanding of all the components that make up the system.In this context, the experimental procedure consisted of the preparation and characterization of the adsorbent activated carbon, the study and understanding of the adsorbate's behavior with the variation of each operational parameter.

PARTICLE SIZE SEPARATION OF ACTIVATED CARBON
The activated carbon Carbomafra 141-S, in its commercial form, has an excessively large grain size for conducting adsorption experiments.To reduce the particle size and ensure homogeneity, particle size reduction and granulometric separation were performed.
The technique involved taking an approximate 1,0 kg aliquot of the activated carbon and grinding it in a knife mill.Subsequently, size segregation was performed using a set of TYLER-type sieves with the aid of an electromagnetic sieve shaker, as shown in Figure 4.

TEXTURAL CHARACTERIZATION BY BET
A BET characterization of the material aims to quantify the surface area per unit mass of the adsorbent material.This information is crucial as it directly identifies the adsorbent's capacity to retain the adsorbate on its surface.In this study, materials with diameters of 0,30 mm, 0,21 mm, and 0,16 mm were selected for investigation.The specific surface area of the materials was determined using the nitrogen adsorption method at 77 K, employing a QUANTACHROME NOVA 1000e apparatus in the Laboratory of Catalytic Processes (LPC) at UFPE.

ZERO CHARGE POINT pHPCZ
The methodology to determine the zero charge point pH (pHZCP) involves placing a 0.2 g sample in distilled water with 11 initial pH conditions, adjusted between pH 2 to 12 using HCl and NaOH solutions, and measured with a Mettler-Toledo pH meter.The solutions are agitated for 24 hours at 200 rpm and 30°C, after which the final pH is recorded.The pHPCZ is found by averaging final pH values that converge.A graph of (final pH -initial pH) vs. initial pH is then plotted, with the curve's intersection on the initial pH axis indicating the pHPCZ, where the material's surface charge is neutral.

ANALYSIS BY INFRARED SPECTROSCOPY (FT-IR)
The thermogravimetric analysis of the sample was carried out following the standard procedure conducted at LATECLIM/UFPE.Essentially, this involved using a Perkin Elmer STA 6000 thermobalance with a heating rate of 20°C.min - under a nitrogen (N2) flow rate of 20 mL.min -1 .The mass of the material placed in the platinum crucibles was fixed at 10 mg, and they were heated from 30°C to 800°C.The results obtained were processed using Pyris Data Analysis software, version 11.

DYE SOLUTION PREPARATION
A standard solution was prepared at a concentration of 500 mg.L -1 for each dye using a mass-based approach.For each Remazol dye, 0,25 g was weighed on an analytical balance, dissolved in distilled water, and the volume was brought up to the mark in a 500 mL volumetric flask.From this solution, dilutions were made for the concentrations used both in the preparation of analytical curves and in the solutions used in each experiment.

DEFINITIONS
In this section, the variables necessary for the evaluation of adsorptive processes will be defined.All of them will be based on the mass of adsorbate (dye) in the solution.Equation 1was used to evaluate the removal efficiency.
For the determination of the amount adsorbed the equation 2 was used.

SAMPLE QUANTIFICATION
To prepare analytical curves for each dye, solutions of 5 to 50 mg/L were made and absorbance was measured using a Varian Cary 50 Bio UV-Vis Spectrophotometer with a 10 mm quartz cuvette.Linear regression was used to relate concentration to absorbance.For mixtures of dyes, measurements were taken to find the optimal wavelength and construct curves correlating absorbance with concentration.

Figure 5
Absorbance as a function of wavelength for the dyes individually and for the multicomponent solution.
Source: Author's own work (October 2020) As shown in Figure 5 and highlighted in Table 1, it is noticeable that the values found are within the expected range and are consistent with those found in the literature for the values of the dyes when analyzed individually.When the dyes were mixed, there was a noticeable intensification in the maximum absorbance peak for the Remazol Yellow and Remazol Red dyes.However, there was no change in the characteristic wavelength of each component since the absorbance of each occurs at a well-defined wavelength.For medium preparation, pH was adjusted using 2.0M NaOH for basic and 2.0M H2SO4 for acidic conditions.Temperature was controlled with a thermostatic bath.After singlecomponent experiments, multicomponent mixtures were tested under conditions that maximized adsorption in initial trials.For adsorption kinetics, 750 mL of dye solution was mixed with 1.5 g of activated carbon, and samples were taken at specified intervals over 150 minutes to monitor adsorption.

RESULTS AND DISCUSSIONS
Adsorption experiments optimized conditions for Remazol Blue, assessed kinetics and equilibrium, and applied these to multicomponent mixtures.A mathematical model predicted the process, and materials were characterized to link their properties to adsorption.The adsorption isotherms for activated carbon 141-S exhibit mixed characteristics of Type I and Type IV isotherms.It is possible to observe mixed isotherms of Type I and Type IV, suggesting a transition state from microporosity to mesoporosity.This transition state is associated with the hysteresis phenomenon, which occurs when the evaporation behavior follows a different path than the condensation behavior, as can be seen in Figure 7 ( DABBAWALA et al., 2018).

Figure 7
Experimental Setup for Adsorption Tests.

pH AT THE ZERO POINT OF CHARGE -pHPCZ
The pHPCZ (pH at the zero point of charge) of the commercial activated carbon 141-S is 7.2.Below this pH, the material has a positive surface charge, favoring anionic compound adsorption, while above this pH, it has a negative surface charge, favoring cationic compounds.
Knowing the pHPCZ helps understand the adsorption behavior of the material.For the anionic azo dyes studied, adsorption at pH 6.0, which is below the pHPCZ, is expected.

Figure 8
Graph of the pH at the Zero Point of Charge of commercial activated carbon 141-S.

THERMOGRAVIMETRIC ANALYSIS
The thermogravimetric analysis (TGA) of the commercial activated carbon shows standard thermal stability.A mass loss of about 10.50% up to 100°C indicates the removal of water and adsorbed substances.Beyond 100°C, the sample remains stable until it starts to degrade and form ash at temperatures above 500°C.

INFRARED SPECTROSCOPY ANALYSIS (FT-IR)
The FT-IR analysis of commercial Carbomafra S/A activated carbon 141-S (Figure 10) reveals carbon-carbon triple bonds with weak absorption peaks between 2,100 and 2,260 cm⁻¹.
Peaks at approximately 2,362 cm⁻¹ and 1,570 cm⁻¹ indicate lignocellulosic material (SOLOMONS;FRYHLE, 2009).Bands between 2,400 and 2,300 cm⁻¹ are associated with CH/CH₂/CH₃ groups, while bands around 2,500-3,000 cm⁻¹ suggest the presence of carboxylic acids with hydrogen (SOLOMONS; FRYHLE, 2009).The presence of lignocellulosic material can enhance dye adsorption efficiency due to its complex structure and affinity for dyes through various chemical interactions and functional groups.

EFFECT OF INITIAL DYE CONCENTRATION
Figure 11 shows that as the initial concentration of Remazol Blue increased from 20 to 60 mg.L⁻¹, the amount adsorbed per unit mass of activated carbon 141-S rose from 4.40 to 11.45 mg.g⁻¹.However, the removal efficiency decreased from 97% to 66% under constant conditions of 1.5 g of carbon, pH 6.0, and approximately 30°C.

Figure 11
Effect of the initial concentration of the dye on the removal efficiency of Remazol Blue dye.T = 30°C, pH 6,0, Adsorbent mass: 1,5g, V = 750 mL, contact time = 150 min.
Source: Author's own work (October 2020) The initial dye concentration is key for overcoming mass transfer resistance, leading to increased qe (adsorbed dye) at higher concentrations.However, dye removal efficiency decreases as the adsorbent saturates faster, reaching adsorption equilibrium.ZHOU et al. (2019) and others observed similar trends across various adsorbents.DAHLAN et al. (2012) andLAKSHMI et al. (2009) found that removal efficiency increases to an equilibrium point, beyond which no significant gains occur, indicating an optimal mass ratio of adsorbent to adsorbate.

EFFECT OF PARTICLE DIAMETER DP.
The study of adsorbent particle diameter shows that smaller diameters enhance adsorption efficiency.This is due to increased surface area, reduced intraparticle diffusion, and better contact with the adsorbate.These factors improve the adsorption process, offering significant economic benefits at the industrial level.
Source: Author's own work (October 2020) The study of pH is crucial for adsorption, affecting adsorbent charge and adsorbate ionization.Removal efficiencies were 93% at pH 3.0, 89% at pH 6.0, and 84% at pH 9.0, with the best results at pH 3.0.However, pH 6.0 was selected for kinetics to observe slower rates.
Source: Author's own work (October 2020) Figure 13 shows that pH 3.0 is optimal for dye adsorption with activated carbon 141-S, which has a pHPZC of about 7.2.This aligns with PELOSI, et al., (2014), who observed similar results with Salvinia Natans at low pH.

EFFECT OF TEMPERATURE
14 shows that removal efficiency decreased from 89% at 30°C to 58% at 50°C for Remazol Blue, with 1.5g of activated carbon and a solution volume of 750 mL at pH 6.0.

Figure 14
Effect of temperature on the removal efficiency of Remazol Blue dye.pH 6,0, Adsorbent mass: 17 Temperature affects dye adsorption by influencing diffusion and interaction types, making it exothermic and physical, as shown by decreased efficiency at higher temperatures (ROSA, 2009;LAKSHMI et al., 2009).).Remazol Red also had higher removal efficiency over Yellow in their binary system (c).In the ternary system, Remazol Blue had the highest removal efficiency, though the exact reasons for this preference are unclear due to unknown molecular structures and various influencing factors (d).

Figure 15
Removal efficiency in the multicomponent system.

Figure 2
Figure 2Types of isotherms

Figure 3
Figure 3Basic molecular structure of azo dyes

Figure 4 (
Figure 4 (a) Knife mill and (b) Model of an electromagnetic sieve shaker with TYLER sieve set.

3. 8
ISOTHERMS AND ADSORPTION KINETICSAdsorption isotherms and kinetics were tested in a batch stirred tank reactor.Initially, individual components were studied with varying concentrations, carbon masses, pH, and temperatures.Later, experiments involved multicomponent mixtures.

Figure 6
Figure 6Experimental setup for adsorption tests ___________________________________________________________________________ Rev. Gest.Soc.Ambient.| Miami | v.18.n.8 | p.1-19 | e08518 | 2024.11 4.1 TEXTURAL CHARACTERIZATION OF ACTIVATED CARBON BY BET In Table3, we have the results of the BET method for quantifying the surface area of the adsorbent activated carbon 141-S.
et al, 2018    A mixed-characteristics isotherm exhibits behaviors of both Type I and Type IV isotherms as per the Brunauer, Emmett, and Teller (BET) classification.Type I isotherms, seen in microporous adsorbents, show a continuously increasing adsorption curve with increasing adsorbate pressure.Type IV isotherms, typical of mesoporous materials, feature an inverted "J"-shaped curve with low initial adsorption that increases rapidly.A mixed-characteristics isotherm, combining these behaviors, suggests the adsorbent has a wide pore size distribution, including both micropores and mesopores, enabling efficient adsorption of various molecule sizes.

4. 9
ADSORPTION IN MULTICOMPONENT SYSTEMS In multicomponent experiments, optimal conditions were C = 40 mg/L, mad = 1.5 g, dp = 0.16 mm, V = 750 mL, T = 30°C, pH = 6.0, 200 rpm, and 150 minutes contact time.Remazol Blue consistently showed higher removal efficiency compared to other dyes in binary systems with Yellow and Red (Figures 15 (a) and (b) The adsorption tests confirmed the technical feasibility of removing Remazol Blue, Yellow, and Red dyes.Smaller particle sizes increased adsorption efficiency.Optimal dye removal occurred at pH 3.0 due to the anionic nature of the dyes and interactions with the adsorbent's surface, though pH 6.0 was used for kinetic studies.Remazol Blue achieved the

Table 1
Mathematical equations representing multicomponent equilibrium

Table 1
Wavelengths of max.absorption.

Table 2
Detailed experimental conditions for multicomponent analysis