Eco-Friendly Anionic Surfactant for the Removal of Methyl Red from Aqueous Matrices

: This study aimed to evaluate the methyl red (MR) removal efﬁciency from aqueous matrices using an eco-friendly anionic surfactant (a calcium surfactant, or CaSF), obtained from frying oil residue. Data obtained by infrared spectroscopy revealed several functional groups that favor the capture of the dye by chemisorption by forming hydrogen bonds and covalent interactions. The kinetic testing results ﬁt the pseudo-second order model, reaching equilibrium in 30 min. Adsorption was greatly inﬂuenced by temperature. The Langmuir isotherm was the one best ﬁtting the process at 20 ◦ C, while the Dubinin–Radushkevich isotherm ﬁtted it better at higher temperatures. Under optimized conditions, the maximal MR adsorption capacity of CaSF reached 53.59 mg · g − 1 (a removal rate of 95.15%), proving that the adsorbent at hand can be an excellent alternative for the removal of undesirable levels of MR present in aqueous matrices.


Introduction
Colored effluents from the textile, paint, paper and leather industries are among the main sources of water pollution and can drastically affect the biota and the natural aquatic balance. The presence of dyes in the water can affect photosynthesis even at low concentrations (<1 mg·L −1 ), while many of them have a highly stable chemical structure, making their natural biodegradation difficult [1,2].
About 800,000 tons of synthetic dyes are produced annually and approximately half of that amount are azo dyes [3,4]. Azo dyes are resistant to traditional effluent treatment processes; they contribute to an increase in the total organic carbon content and turbidity, and aggravate the eutrophic processes taking place in the receiving water bodies. Furthermore, azo dyes pose a potential risk to public health, being associated with the occurrence of allergies, mutations and cancer even at trace levels [5,6]. Therefore, the removal of dyes from effluents before they are released into natural water bodies is critical.
Methyl red (MR, Figure 1) is an anionic azo dye, commonly used in the printing and dyeing industries due to its high color-fixing performance and mild fading [7]. Several techniques are used for the removal of this pollutant from effluents, among them are those based on the Fenton process [8], hydrogen peroxide-mediated oxidation [9], liquid-liquid extraction [10], anaerobic degradation [11], bacterial degradation [12,13] and adsorption [14]. Among these techniques, adsorption has gained significant notoriety due to its high efficiency and low cost. Several works have already studied the effectiveness of MR removal from water using different types of adsorbents, such as modified coconut shell [15], biochar [16], activated carbon [1], clay [17], chitosan [18], nanocomposites [19], eggshell [20], organosilicons [21] and sodium alginate flocs [22,23]. [15], biochar [16], activated carbon [1], clay [17], chitosan [18], nanocomposites [19], eggshell [20], organosilicons [21] and sodium alginate flocs [22,23]. A calcium surfactant (CaSF) is an amphiphilic compound with a hydrophobic anionic chain that facilitates the interaction with organic compounds of a similar nature. Therefore, CaSF can sequester/adsorb dyes present in liquid effluents, thus reducing their pollutant load. The use of surfactants to remove benzene [24], phenol [25], dioxins [26], antioxidants [27], pesticides [28], heavy metals [29] and aniline [30] has also been studied. The use of anionic surfactants to remove dyes is a recent technology and needs further research to improve the technique and design future pilot projects. With that in mind, this study aimed to evaluate the efficiency of CaSF as a sequestering agent for MR in a synthetic effluent, contributing new alternatives for the sound management of liquid discharges containing undesirable levels of this azo dye. It is also worth noting that the proposed method is in line with the United Nations' Sustainable Development Goals, contributing new ideas for waste minimization and the use of sustainable materials, as frying oil waste is used as raw material for the production of the surfactant.

CaSF Synthesis
CaSF was produced through the saponification of frying residue, using NaOH. The percentage of SB and PF in the residue was 95 wt% and 5 wt%, respectively. The proportion of the different fatty acids present in SB/PF, together with their structural properties, is summarized in Table 1. Subsequently, 10 g of the anionic surfactant was dissolved in 1.60 L of deionized water under stirring at 100 rpm at 50 °C. After its complete dissolution, 0.40 L of a 0.065 M CaCl2 solution was added, the heating was turned off and the stirring was reduced to 20 rpm for 10 min for the CaSF to form. The CaSF flocs were filtered and dried in a microwave oven for 5 min. After drying, the CaSF was macerated and sieved. CaSF granulometry was 40-50 mesh size. The generic structural formula of CaSF is represented in Figure 2. A calcium surfactant (CaSF) is an amphiphilic compound with a hydrophobic anionic chain that facilitates the interaction with organic compounds of a similar nature. Therefore, CaSF can sequester/adsorb dyes present in liquid effluents, thus reducing their pollutant load. The use of surfactants to remove benzene [24], phenol [25], dioxins [26], antioxidants [27], pesticides [28], heavy metals [29] and aniline [30] has also been studied. The use of anionic surfactants to remove dyes is a recent technology and needs further research to improve the technique and design future pilot projects. With that in mind, this study aimed to evaluate the efficiency of CaSF as a sequestering agent for MR in a synthetic effluent, contributing new alternatives for the sound management of liquid discharges containing undesirable levels of this azo dye. It is also worth noting that the proposed method is in line with the United Nations' Sustainable Development Goals, contributing new ideas for waste minimization and the use of sustainable materials, as frying oil waste is used as raw material for the production of the surfactant.

CaSF Synthesis
CaSF was produced through the saponification of frying residue, using NaOH. The percentage of SB and PF in the residue was 95 wt% and 5 wt%, respectively. The proportion of the different fatty acids present in SB/PF, together with their structural properties, is summarized in Table 1. Subsequently, 10 g of the anionic surfactant was dissolved in 1.60 L of deionized water under stirring at 100 rpm at 50 • C. After its complete dissolution, 0.40 L of a 0.065 M CaCl 2 solution was added, the heating was turned off and the stirring was reduced to 20 rpm for 10 min for the CaSF to form. The CaSF flocs were filtered and dried in a microwave oven for 5 min. After drying, the CaSF was macerated and sieved. CaSF granulometry was 40-50 mesh size. The generic structural formula of CaSF is represented in Figure 2.

CaSF Characterization
CaSF was analyzed by FTIR (PerkinElmer, Spectrum Two, Beaconsfield, United Kingdom) before and after MR adsorption in order to identify the surface functional groups responsible for dye capture. The analyzed spectra were in the range of 4000-450 cm −1 with a resolution of 4 cm −1 , using samples of the material dispersed in potassium bromide. To determine the point of zero charge (pHPZC), eleven NaCl solutions (0.1 M, 20 mL) were prepared within an initial pH range of 1 to 11. The pH of the solutions was adjusted with the addition of 0.01 M HCl or NaOH. These solutions were transferred to Erlenmeyer flasks containing 20 mg of CaSF and subsequently stirred at 100 rpm for 6 h. The initial and final pH of the solutions were measured using a pH meter (Orion 5−Star probe, Thermo Scientific, Beverly, USA). The [Initial pH-Final pH] was calculated and plotted vs. [Initial pH] to determine the pHPZC. The point where [Initial pH-Final pH] crosses the origin was established as the pHPZC [14].

Adsorption Experiments
A 100 mg.L −1 stock solution of MR was prepared by dissolving the MR in deionized water with vigorous stirring. Samples with different concentrations were prepared from the stock solution. The adsorption experiments were carried out in batches, by stirring the CaSF together with buffered (acetate buffer) MR solutions. Soon after, CaSF was separated from the samples by filtration. The MR concentration in the samples, before and after adsorption, was determined using a UV spectrophotometer (Shimadzu, UV 1800, Kyoto, Japan) adjusted to 525 nm. The amount of dye adsorbed on the CaSF (mg.g −1 ) and the removal efficiency were calculated using Equations (1)-(3): where Co, Ct and Ce are the dye concentrations at the initial time, at a given time and at equilibrium, respectively (mg.L −1 ), qt and qe are the adsorption capacities at a given time

CaSF Characterization
CaSF was analyzed by FTIR (PerkinElmer, Spectrum Two, Beaconsfield, United Kingdom) before and after MR adsorption in order to identify the surface functional groups responsible for dye capture. The analyzed spectra were in the range of 4000-450 cm −1 with a resolution of 4 cm −1 , using samples of the material dispersed in potassium bromide. To determine the point of zero charge (pH PZC ), eleven NaCl solutions (0.1 M, 20 mL) were prepared within an initial pH range of 1 to 11. The pH of the solutions was adjusted with the addition of 0.01 M HCl or NaOH. These solutions were transferred to Erlenmeyer flasks containing 20 mg of CaSF and subsequently stirred at 100 rpm for 6 h. The initial and final pH of the solutions were measured using a pH meter (Orion 5−Star probe, Thermo Scientific, Beverly, MA, USA). The [Initial pH-Final pH] was calculated and plotted vs.
[Initial pH] to determine the pH PZC . The point where [Initial pH-Final pH] crosses the origin was established as the pH PZC [14].

Adsorption Experiments
A 100 mg·L −1 stock solution of MR was prepared by dissolving the MR in deionized water with vigorous stirring. Samples with different concentrations were prepared from the stock solution. The adsorption experiments were carried out in batches, by stirring the CaSF together with buffered (acetate buffer) MR solutions. Soon after, CaSF was separated from the samples by filtration. The MR concentration in the samples, before and after adsorption, was determined using a UV spectrophotometer (Shimadzu, UV 1800, Kyoto, Japan) adjusted to 525 nm. The amount of dye adsorbed on the CaSF (mg·g −1 ) and the removal efficiency were calculated using Equations (1)-(3): % Removal e f f iciency where C o , C t and C e are the dye concentrations at the initial time, at a given time and at equilibrium, respectively (mg·L −1 ), q t and q e are the adsorption capacities at a given time and at equilibrium, respectively (mg·g −1 ), m is the mass of CaSF (g) and V is the volume of the solution (L).
where k di is the mass transfer coefficient (mg·g −1 ·min −1/2 ), C is the constant associated with the thickness of the boundary layer (mg·g −1 ) and F is the fraction of MR adsorbed at any given time. B t (the mathematical function of F) can be calculated by Equations (9) and (10).
B t was calculated for each value of F and then a B t x F graph was plotted. The linear regression of the B t x F graph provides the angular coefficient and the Boyd constant (B). The diffusion coefficient, D (cm 2 ·s −1 ), can be calculated from B using Equation (11).
where d is the mean diameter of the particles (cm).

Equilibrium Study
In the equilibrium study, samples containing concentrations of 5-100 mg·L −1 of MR at a total volume of 25 mL were treated under ideal conditions (CaSF = 10 g·L −1 , pH = 4, t = 30 min, 100 rpm). The equilibrium results were analyzed using the Freundlich (Equation (12)), Langmuir (Equation (13)), Temkin (Equation (14)) and Dubinin-Radushkevich (DR) (Equation (15)) isotherms [31]. q e = k F C e 1/n (12) q e = q max k L C e 1 + k L C e (13) where k F , k L , k T and k DR are the constants of the Freundlich (mg·g −1 (mg·L −1 )), Langmuir (L·mg −1 ), Temkin (L·mg −1 ) and DR (mol 2 ·kJ −2 ) isotherms, respectively, n is the constant related to the heterogeneity of the CaSF surface, q max is the maximal adsorption capacity (mg·g −1 ), b is the constant related to the heat of adsorption (kJ·mol −1 ), R is the universal gas constant (8.314 J·mol −1 ·k −1 ), T is the process temperature (K) and q m is the maximal adsorption capacity of the monolayer (mg·g −1 ). The k DR constant is associated with the mean adsorption energy (E), which can be calculated by Equation (16).

Statistical Error Analysis
Using the adjusted determination coefficient (R adj 2 ) alone in order to adjust the experimental data to theoretical kinetics models and adsorption isotherms has its limitations and can lead to wrong conclusions. Therefore, other statistical error functions, such as the Chi-squared (X 2 ) test (Equation (17)) and the residual sum of squares (RSS) (Equation (18)), were used to validate the most appropriate kinetics and isothermal models. The corrected Akaike information criterion (AICc) (Equation (19)) was used for model comparison [31].
where q exp and q calc are the experimental and model-calculated adsorption capacities, respectively, z is the number of experimental points and k is the number of parameters of the model.

CaSF Characterization
The main functional groups on the CaSF surface were analyzed by FTIR before and after MR adsorption ( Figure 3). The FTIR spectra ( Figure 3) showed several absorption peaks, indicating the presence of different functional groups at different wavenumbers, namely 3417 cm −1 , associated with the elongation of the OH − functional group, which indicates the presence of hydrogen bonds; 3011 cm −1 , typical of simple unsaturated olefinic compounds (CH=CH); and 2921-2850 cm −1 , corresponding to the C-H stretch in longchain linear aliphatic compounds. The good resolution and high intensity of these peaks also suggest that CaSF does not have impurities in its molecular structure [33,34].
The peak at 1693 cm −1 indicates alkenyl stretching (CH=CH). The sequences of peaks at 1578-1541 cm −1 and 1432-1420 cm −1 suggest the presence of a carboxylic acid salt (COO-). The peak at 1113 cm −1 indicates the presence of saturated esters (COO). Finally, the peaks at 722-675 cm −1 indicate out-of-plane olefin deformations (CH=CH). Most functional groups were affected after CaSF came into contact with the dye. The disappearance of certain bands and the reduction in the intensity of some peaks point to the occurrence of MR adsorption on the CaSF surface. The pHPZC analysis enables finding the optimal pH for MR adsorption. Adsorption of anionic dyes is favored at pH values below the pHPZC, while adsorption of cationic dyes is favored at pH values above the pHPZC. Figure 4 presents the pHPZC results for CaSF, showing that the difference between the initial pH and the final pH crosses the origin at pH = 6, which is the pHPZC of CaSF. MR adsorption is favored when the pH of the medium is lower than the pHPZC, as the CaSF surface is positively charged due to protonation. The ideal pH determined for the process was 4, as it is below CaSF´s pHPZC and provides for the electrostatic attraction between the positively charged CaSF surface and the negatively charged MR molecules. A pH of 4 was also considered optimal for the adsorption of MR on other adsorbents, such as activated carbon [1] and iron oxide nanoparticles [35]. The pH PZC analysis enables finding the optimal pH for MR adsorption. Adsorption of anionic dyes is favored at pH values below the pH PZC , while adsorption of cationic dyes is favored at pH values above the pH PZC . Figure 4 presents the pH PZC results for CaSF, showing that the difference between the initial pH and the final pH crosses the origin at pH = 6, which is the pH PZC of CaSF. MR adsorption is favored when the pH of the medium is lower than the pH PZC , as the CaSF surface is positively charged due to protonation. The ideal pH determined for the process was 4, as it is below CaSF's pH PZC and provides for the electrostatic attraction between the positively charged CaSF surface and the negatively charged MR molecules. A pH of 4 was also considered optimal for the adsorption of MR on other adsorbents, such as activated carbon [1] and iron oxide nanoparticles [35].

CaSF Dosage
An adsorbent dosage analysis is essential to measure the amount of adsorbent necessary to attain the maximal MR removal efficiency. As shown in Figure 5 concentrations of CaSF below 2 g.L −1 are inefficient in removing MR molecules in the solution (<1%). A CaSF dosage of 10 g.L −1 lead to the best MR removal, reaching a removal

CaSF Dosage
An adsorbent dosage analysis is essential to measure the amount of adsorbent necessary to attain the maximal MR removal efficiency. As shown in Figure 5, concentrations of CaSF below 2 g·L −1 are inefficient in removing MR molecules in the solution (<1%). A CaSF dosage of 10 g·L −1 lead to the best MR removal, reaching a removal efficiency of 95.15%. Dosages above 10 g·L −1 did not show any significant increase in MR removal; therefore, it was considered the optimal dosage for the process. CaSF dosages of 1 and 2 g·L −1 were inefficient for removing MR, possibly due to the low number of active sites available to adsorb dye molecules and the consequent increased affinity to water of the MR molecules. The number of active sites available for adsorption was directly proportional to CaSF dosage, starting from 4 g·L −1 . With an increase in dosage up to 10 g·L −1 , the number of active sites and removal efficiency increased. Similar results have been found for the adsorption of MR by eggshell [20] and a metal-organic network [36].

CaSF Dosage
An adsorbent dosage analysis is essential to measure the amount of adsorbent necessary to attain the maximal MR removal efficiency. As shown in Figure 5, concentrations of CaSF below 2 g.L −1 are inefficient in removing MR molecules in the solution (<1%). A CaSF dosage of 10 g.L −1 lead to the best MR removal, reaching a removal efficiency of 95.15%. Dosages above 10 g.L −1 did not show any significant increase in MR removal; therefore, it was considered the optimal dosage for the process. CaSF dosages of 1 and 2 g.L −1 were inefficient for removing MR, possibly due to the low number of active sites available to adsorb dye molecules and the consequent increased affinity to water of the MR molecules. The number of active sites available for adsorption was directly proportional to CaSF dosage, starting from 4 g.L −1 . With an increase in dosage up to 10 g.L −1 , the number of active sites and removal efficiency increased. Similar results have been found for the adsorption of MR by eggshell [20] and a metal-organic network [36].

Adsorption Kinetics
Kinetic adsorption models were used to study the mass transfer of free MR molecules in solution to the CaSF surface over time, as well as the time required for the process to reach a state of equilibrium. Figure 6 shows that the adsorption of MR occurs very quickly

Adsorption Kinetics
Kinetic adsorption models were used to study the mass transfer of free MR molecules in solution to the CaSF surface over time, as well as the time required for the process to reach a state of equilibrium. Figure 6 shows that the adsorption of MR occurs very quickly at the beginning of the treatment and then slows down until it reaches equilibrium after 30 min. The higher adsorption capacity in the first few minutes is the result of the high availability of free active sites on the CaSF surface. After the first 30 min, adsorption occurred with a low adsorption capacity due to the saturation of the CaSF active sites, in addition to the repulsive effect between the MR molecules already adsorbed on the CaSF and those free in the solution [37]. Tables 2 and 3 show that the kinetic model that best fits the experimental data was the PSO, with the highest R adj 2 value (R adj 2 = 0.99) and the lowest error values in the statistical analysis (RSS = 0.01; X 2 = 7.78 × 10 −4 ; AICc = −110.46). Furthermore, this model retrieved the value of q e closest to the adsorption capacity measured at experimental equilibrium (q e_exp ). The fact that the PSO model is the one that best fits the MR adsorption is a strong indication that chemisorption is the mechanism governing the process. Furthermore, it indicates that the adsorption rate is strongly dependent on MR concentration and on the availability of CaSF active sites. Similar results have been obtained using chitosan [18,38] and organosilicons [21].
at the beginning of the treatment and then slows down until it reaches equilibrium after 30 min. The higher adsorption capacity in the first few minutes is the result of the high availability of free active sites on the CaSF surface. After the first 30 min, adsorption occurred with a low adsorption capacity due to the saturation of the CaSF active sites, in addition to the repulsive effect between the MR molecules already adsorbed on the CaSF and those free in the solution [37].  Tables 2 and 3 show that the kinetic model that best fits the experimental data was the PSO, with the highest Radj 2 value (Radj 2 = 0.99) and the lowest error values in the statistical analysis (RSS = 0.01; X 2 = 7.78×10 −4 ; AICc = −110.46). Furthermore, this mode retrieved the value of qe closest to the adsorption capacity measured at experimenta equilibrium (qe_exp). The fact that the PSO model is the one that best fits the MR adsorption is a strong indication that chemisorption is the mechanism governing the process Furthermore, it indicates that the adsorption rate is strongly dependent on MR concentration and on the availability of CaSF active sites. Similar results have been obtained using chitosan [18,38] and organosilicons [21].

Diffusion Mechanism
To understand the diffusion mechanism, the data were initially evaluated by the Weber-Morris adsorption model (intraparticle model). Figure 7 shows the formation of three different linear regions, representing different adsorption stages. Furthermore, the graph does not cross the origin, suggesting that intraparticle diffusion is not the only

Diffusion Mechanism
To understand the diffusion mechanism, the data were initially evaluated by the Weber-Morris adsorption model (intraparticle model). Figure 7 shows the formation of three different linear regions, representing different adsorption stages. Furthermore, the graph does not cross the origin, suggesting that intraparticle diffusion is not the only factor limiting the rate of MR adsorption. The first stage is attributed to a fast diffusion in the outer layer (film), the second is associated with intraparticle diffusion and the third represents the adsorption equilibrium [39]. Table 4 presents values of C that are not zero, indicating that the boundary layer influences the process. This finding corroborates the fact that intraparticle diffusion is not the only limiting step in the MR adsorption rate. The linear adjustment of the second stage showed a low R adj 2 value, which could directly influence the value of K d and generate complex and sometimes wrong interpretations of the adsorption mechanism.
Given the abovementioned limitation, the Boyd diffusion model (Figure 8) was applied in order to obtain more detailed information on the limiting step of MR adsorption on CaSF. The studied relationship does not cross the origin in the graph, indicating that the adsorption of MR on CaSF was mainly governed by a mass transfer in the outer layer (liquid film). Furthermore, the obtained value of D (5.28 × 10 −9 cm 2 ·s −1 ) indicates a diffusional behavior with multiple controls, influenced by factors such as stirring rate, initial dye concentration, CaSF particle shape, number of pores in the adsorbent and adsorption/desorption rate until the equilibrium state is reached. Figure 9 shows the adsorption mechanism suggested in this work. The free MR molecules in solution are believed to interact with the hydrophobic tails (covalent interactions) and hydrophilic heads (hydrogen bonds) of the CaSF surface.
Textiles 2023, 3, FOR PEER REVIEW 9 factor limiting the rate of MR adsorption. The first stage is attributed to a fast diffusion in the outer layer (film), the second is associated with intraparticle diffusion and the third represents the adsorption equilibrium [39]. Table 4 presents values of C that are not zero, indicating that the boundary layer influences the process. This finding corroborates the fact that intraparticle diffusion is not the only limiting step in the MR adsorption rate. The linear adjustment of the second stage showed a low Radj 2 value, which could directly influence the value of Kd and generate complex and sometimes wrong interpretations of the adsorption mechanism.  Given the abovementioned limitation, the Boyd diffusion model (Figure 8) was applied in order to obtain more detailed information on the limiting step of MR adsorption on CaSF. The studied relationship does not cross the origin in the graph, indicating that the adsorption of MR on CaSF was mainly governed by a mass transfer in the outer layer (liquid film). Furthermore, the obtained value of D (5.28 × 10 −9 cm².s −1 ) indicates a diffusional behavior with multiple controls, influenced by factors such as stirring rate, initial dye concentration, CaSF particle shape, number of pores in the adsorbent and adsorption/desorption rate until the equilibrium state is reached. Figure 9 shows the adsorption mechanism suggested in this work. The free MR molecules in solution are believed to interact with the hydrophobic tails (covalent interactions) and hydrophilic heads (hydrogen bonds) of the CaSF surface.

Adsorption Equilibrium
Adjusting the experimental data with the theoretical adsorption isotherm models makes it possible to estimate qmax for the CaSF dosage used, in addition to the type of adsorption taking place in the process (physical adsorption, chemisorption or both). Figure 10 shows the experimental data with the theoretical models at three different temperatures. The adsorption capacity of CaSF is extremely sensitive to temperature under all the dye concentrations studied. A temperature of 20 °C shows an increasing linear behavior with increasing MR concentrations, and does not reach a saturation stage (equilibrium). At 25 °C and 30 °C, however, saturation is reached for the last three tested concentrations, along with a sudden drop in CaSF adsorption capacity. This type of behavior can be attributed to the combination of the following factors: an exothermic MR adsorption process, pH range and CaSF´s Krafft temperature.

Adsorption Equilibrium
Adjusting the experimental data with the theoretical adsorption isotherm models makes it possible to estimate q max for the CaSF dosage used, in addition to the type of adsorption taking place in the process (physical adsorption, chemisorption or both). Figure 10 shows the experimental data with the theoretical models at three different temperatures. The adsorption capacity of CaSF is extremely sensitive to temperature under all the dye concentrations studied. A temperature of 20 • C shows an increasing linear behavior with increasing MR concentrations, and does not reach a saturation stage (equilibrium). At 25 • C and 30 • C, however, saturation is reached for the last three tested concentrations, along with a sudden drop in CaSF adsorption capacity. This type of behavior can be attributed to the combination of the following factors: an exothermic MR adsorption process, pH range and CaSF's Krafft temperature. Regarding the nature of the process, Table 5 clearly shows the decrease both in the maximal experimental adsorption capacity (qmax_exp) and in the adsorption capacities calculated by the isotherm models (qmax and qm), which indicate an exothermic process. In this case, the increase in temperature promotes the agitation of the MR molecules, disfavoring their adsorption on the CaSF surface. The pH of the solution also influences the structure of CaSF, since it was synthesized from an anionic surfactant that undergoes deprotonation at pH values of < 8 [40,41]. The data also showed evidence that an acidic Regarding the nature of the process, Table 5 clearly shows the decrease both in the maximal experimental adsorption capacity (q max_exp ) and in the adsorption capacities calculated by the isotherm models (q max and q m ), which indicate an exothermic process. In this case, the increase in temperature promotes the agitation of the MR molecules, disfavoring their adsorption on the CaSF surface. The pH of the solution also influences the structure of CaSF, since it was synthesized from an anionic surfactant that undergoes deprotonation at pH values of <8 [40,41]. The data also showed evidence that an acidic pH can decrease the Krafft temperature of the CaSF, changing its stability, making it more soluble and hindering the adsorption of MR molecules. The increase in temperature reduces the efficiency of the process because the solubility of CaSF increases [42]. E values of > 16 kJ mol −1 in the DR model indicated that dye adsorption occurred predominantly by chemisorption [32]. Table 5. Parameters of the isothermal adsorption models at different temperatures.  Table 6 presents the statistical analysis of the theoretical models using the experimental data. The best model is the one retrieving the highest value of R adj 2 and the lowest values of X 2 , RSS and AICc. For a temperature of 20 • C, the Langmuir model had the best fit to the experimental data. Therefore, it is assumed that: (i) adsorption occurred with a formation of a monolayer, (ii) there is a defined number of sites with an equivalent energy, (iii) the adsorbed MR molecules do not interact with each other and (iv) each site can only hold one MR molecule [31]. However, the DR model fitted better to the experimental data at higher temperatures. Therefore, the adsorption of MR by CaSF occurred heterogeneously and with the formation of multilayers [43,44]. Table 7 compares the CaSF's q max with those of other reported adsorbents for MR removal, proving that CaSF has an MR removal capacity similar or superior to those of other adsorbents already reported in the literature, showing its potential efficiency for the removal of MR from aquatic environments polluted with this dye.  The reuse of the material after adsorption has not yet been tested, but considering that is is a nitrogen source, it could be used as a fertilizer. Another future goal is to study the regenerative capacity of the adsorbent, in order to improve the cost-benefit relationship for large-scale applications, in addition to using it as a precursor to synthesize carbon allotropes, to develop catalysts or as a template to produce nanoparticles.

Conclusions
Anionic surfactants can be used as adsorbents for the removal of azo dyes from aqueous matrices, as proven by the present study in which CaSF successfully removed 95.15% of the MR content. The achieved performance strongly depended on the controlled parameters, which included a pH of 4 and a CaSF dosage of 10 g·L −1 , which contribute to the greater availability of active sites for the capture of the azo dye molecules. Adsorption equilibrium was reached upon 30 min of treatment, and the PSO kinetic model was the one best fitting the experimental data. The adsorption mechanism was governed by mass transfer in the outermost layer due to hydrophobic interactions and hydrogen bonds established between the dye and the adsorbent. Temperature radically influenced the adsorption process and the increase in this variable hinders the efficiency of the process, a fact that points to its exothermic nature. As for the equilibrium conditions studied at 20 • C, the experimental data better fitted the Langmuir model (q max = 53.59 mg·g −1 ). However, when the process took place at higher temperatures, it was better defined by the DR model. Since the obtained adsorption capacity was equivalent to those observed with other natural and synthetic adsorbents, it can be stated that CaSF is a promising material for the removal of MR from matrices polluted with this azo dye. It should be noted that the use of CaSF as an adsorbent also has a sustainability aspect, as it can be produced from used frying oil, thus providing an adequate final destination for this waste, stimulating at the same time the circular economy.