Adsorption of Acid Yellow 23 Dye on Organobentonite/Alginate Hydrogel Beads in a Fixed-Bed Column

Abstract

This research evaluates the use of organoclay/alginate hydrogels in removing Acid Yellow 23 in a fixed-bed column and contributes to the application of these composites in the context of the adsorption of anionic dyes that are present in wastewater. An organobentonite (OBent) was synthesized and encapsulated in an alginate matrix, using Ca²⁺ ions as a crosslinking agent. Experiments in fixed-bed columns showed that breakthrough and exhaustion times were longer with increasing bed height, which decreased with increases in flow rate and initial dye concentration. The Thomas, Yoon–Nelson, and Adams–Bohart models were well fitted to the experimental data for the breakthrough curves with high Adj. R² correlation coefficients and low values of χ². The theoretical adsorption capacity of the organobentonite/alginate hydrogel calculated from the Thomas model was 0.50 ± 0.01 mg/g (equivalent to 30.97 mg/g OBent), and this was obtained by using a 15 cm (10.10 g) bed height, 1 mL/min flow rate, and a 45 mg/L input dye concentration. The bed was regenerated with a 0.5 M NaOH solution, and the reuse of the saturated column bed was studied for two adsorption–desorption cycles. The results obtained in this study suggest the potential use of an organoclay/alginate hydrogel for the adsorption of pollutants in continuous systems.

1. Introduction

The textile and dyeing industry generates 20% of the world’s wastewater, and it has become the second-largest polluter of freshwater globally [1,2]. Dyes that are incorporated into water sources reduce photosynthetic activity, decrease dissolved oxygen concentration, and increase chemical oxygen demand, thus generating adverse effects on aquatic ecosystems [3]. Dyes are highly toxic and potentially carcinogenic, and they are associated with various diseases in animals and humans, thus causing damage to the tissues of certain organs and the central nervous system [4,5,6].

Azo dyes are the most common type of synthetic dyes that are used in the food and textile industry, and they are characterized by the presence of one or more –N=N– functional groups, which are associated with one or more aromatic rings and may also contain sulfonic acid substituent groups [7]. Acid Yellow 23 (AY-23) is a widely used azo dye in the textile (as a dye for wool and silk), foodstuff, pharmaceutical, and cosmetic industries [8,9]. Although AY-23 is used in the food (also known as tartrazine or FD&C Yellow No. 5), drug, and cosmetic industries, the FDA requires all products containing this dye to identify it on their labels because certain people may suffer allergies when intaking this additive [10].

Different physical and chemical processes—such as coagulation–flocculation [11,12], polymer membranes [13], adsorption [14,15,16], ozonation [17], electrocoagulation [18], and Fenton and photo-Fenton processes [19], among others—have been applied to remove azo dyes from effluents. Due to its simplicity, high efficiency, and scale up over a wide range of concentrations, adsorption technology has received a great deal of attention among the available treatments [14]. Different adsorbent materials that can remove dyes from water—such as clays/zeolites and their composites, as well as agricultural solid wastes, biochar, and activated carbon (the latter being one of the most efficient, although its cost is high)—have been explored [20,21].

Clays and their modified derivatives are a group of adsorbent materials that can remove dyes from aqueous media [21,22]. The interest in clays results from their natural abundance, the specific surface area associated with their layered structure, and the possibility of physical or chemical modification to increase the affinity for particular adsorbates [23]. Organoclays are synthesized through exchange with organic cations, generally quaternary ammonium salts [24], which improve the adsorption capacity for anionic [25], acid [26,27], and reactive dyes [28]. Although organoclays have proven to be efficient in the adsorption of different pollutants [29,30,31,32], these materials are obtained as powder and, after adsorption, they must be separated by filtration (0.22–0.45 μm membrane) to avoid the formation of colloidal suspensions [23]. Polymer–clay nanocomposites (PCNCs), which are composed of clay particles dispersed in a polymeric matrix [33], have been proposed for applications in the biochemical [34], pharmaceutical, and environmental fields [35]. The polymer matrix can be formed by a variety of polymers, including homopolymers, copolymers, cross-linked polymers, networks of polymers, block copolymers, and by the mixtures of two or more polymers. Natural (such as bentonite and other smectites), modified (such as pillared clays and organoclays), and synthetic clay minerals are all suitable for producing PCNCs [33].

Alginate is a natural polysaccharide [36] that can form hydrogels through ionic cross-linking with metal ions such as Ca²⁺ [37,38]. By incorporating or encapsulating organoclay in an alginate matrix, it has been possible to form organoclay/alginate composite microspheres or hybrid materials [23,39,40]. Alginate–clay composite materials have been used for applications such as controlled drug delivery [41], pervaporation separation membranes [42], flame-retardant polyurethane foams [43], the adsorption of methylene blue and Congo red dyes [44], and the removal of pentachlorophenol and safranin from aqueous solutions [35].

For the specific case of organoclay–alginate hydrogels, these compounds have been used in the removal of cationic dyes such as methylene blue [23,39,45,46]; anionic dyes such as methyl orange [23], Allura red [47], acid green B, and direct pink 3B [40]; and ionic such as ClO₄⁻, Sr²⁺, Cu²⁺, and Cr⁶⁺ [48,49]. Results of the above studies indicate that organoclay-alginate composites are promising adsorbents for the remediation of the aqueous solutions contaminated with dyes and metal ions. In addition, reuse tests have also shown that the adsorption capacity persists between three and six successive adsorption–desorption cycles [39,49].

In the reviewed literature, the studies on removing cationic and anionic dyes with organoclay/alginate hydrogels were performed by batch discontinuous adsorption [23,39,40,45,46,47]. This is the first investigation focused on the adsorption of an anionic dye with organobentonite that is encapsulated in an alginate matrix on a fixed-bed column. Initially, the hydrogel beads were evaluated in batch adsorption tests of the Acid Yellow 23 dye, where the effect of the concentration of organoclay in the composite and the pH of the solution were studied. Subsequently, fixed-bed column experiments were performed to assess the influence of bed height, feed flow rate, and initial dye concentration. To analyze the dynamic adsorption behavior of AY-23, breakthrough curves were fitted using a nonlinear regression to the Thomas, Yoon–Nelson, and Adams–Bohart models. In addition, the regeneration of the bed was studied, and adsorbent reuse tests were carried out. Finally, at the best adsorption conditions of the dye in a fixed-bed column, tests were carried out to remove AY-23 contained in a real wastewater sample.

2. Materials and Methods

2.1. Reagents and Materials

The Acid Yellow 23 synthetic azo-dye (C₁₆H₉N₄Na₃O₉S₂, 534.36 g/mol, CAS No. 1934-21-0) with ≥85% purity was purchased from Sigma-Aldrich (Darmstadt, Germany). Cationic surfactant hexadecyltrimethylammonium bromide (HDTMA-Br, CH₃(CH₂)₁₅NBr(CH₃)₃, 364.46 g/mol, CAS No. 57-09-0, purity > 98.0%) was purchased from PanReac AppliChem (Barcelona, Spain). Sodium alginate (C₆H₉NaO₇, 216.12 g/mol, CAS No. 9005-38-3, >91% purity) was purchased from Loba Chemie Pvt. Ltd. (Mumbai, India).

Raw bentonite was collected from a mine operated by Gea Minerales S.A.S. in Armero-Guayabal (Tolima, Colombia). This bentonite’s chemical and mineralogical composition has been previously published [50].

2.2. Preparation of the Organobentonite/Sodium Alginate Beads

The clay fraction (≤2 μm) was obtained by a conventional sedimentation following Stokes’ law, and this was, subsequently, exchanged with a NaCl solution [50,51]. Purified sodium bentonite (symbolized as Na-Bent), with a cation exchange capacity (CEC) of 63.02 cmol(+)/kg, was used for the synthesis of the organoclay.

The intercalation of the Na-Bent with HDTMA-Br was carried out according to a previous study [52] and, for the modification, an amount of surfactant that was 1.5 times the cation exchange capacity of clay was used.

The preparation diagram of the hydrogel beads is shown in Figure 1. For this, six solutions of sodium alginate (Na-Alg) were prepared in distilled water at 60 °C (1 g/100 mL), and the mixtures were magnetically shaken for 1 h at 250 rpm. Then, the Na-Alg solutions were cooled to room temperature, whereupon a specific amount of organoclay (OBent) was added, such that the final suspensions had concentrations of 0 (blank), 2, 4, 6, 8, and 10 wt.%. After mixing the organoclay with a Na-Alg solution for 1 h, the suspension was dropwise added into a CaCl₂ solution (4 wt.% at 20 ± 1 °C) via a Pasteur pipette. The hydrogel beads were immersed in a CaCl₂ solution for 3 h to ensure Ca²⁺ cross-linking. Then, they were repeatedly washed with distilled water to remove excess chloride ions. The hydrogel beads were labeled Obent(wt.%)/Alg, where wt.% corresponded to the mass percentage of the organobentonite used to synthesize the material. The blank corresponds to the alginate-only hydrogel beads, which were labeled as Alg-Hydrogel.

2.3. Characterization of the Adsorbents

The X-ray diffraction patterns for Na-Bent, Obent, Na-Alg, and the Obent/Alg hydrogel were taken on a Rigaku Mini-flex II Diffractometer when under Cu-Kα radiation of 30 kV and 15 mA, as well as a 0.05° 2θ step and 2 s/step. For this analysis, the hydrogel beads were dried at 40 °C until a constant weight was achieved [35], then crushed and sieved in a 100 mesh (<149 μm).

The mean diameters and bead-size distribution of the hydrogel beads were obtained from digitized photographs captured with ImageJ software (Version 1.53t), which was developed at the National Institute of Health (Bethesda, MD, USA). For each image, the diameter of 80 hydrogel beads was measured.

The water content in the Obent/Alg hydrogel was determined by drying beads at 40 °C up to a constant weight, and this was achieved following the procedure that used alginate-encapsulated pillared clays [35]. Water content (WC) was calculated with Equation (1):

$$Water content (\%)=\frac{W_w-W_d}{W_w}\times 100$$

(1)

where $W_w$ and $W_d$ are, respectively, the weight of hydrogel beads before and after drying.

Swelling studies were performed using dried hydrogel beads at 40 °C as the starting material. A known hydrogel mass was immersed in 100 mL of distilled water for 12 h, the water was then removed by decantation, and the mass of wet beads was determined [35]. The swelling capacity of hydrogel beads was determined from Equation (2):

$$Swelling capacity (\%)=\frac{W_s-W_d}{W_d}\times 100$$

(2)

where $W_d$ is the hydrogel’s initial (dry) weight and $W_s$ is the weight of the hydrogel when swollen.

The zero-point charge (pH_zpc) of the organoclay/alginate hydrogel was determined by introducing a fixed mass of 2.0 g of hydrogel into 50 mL solutions with an initial pH (pHi) between 3 and 11. Suspensions were then stirred for 24 h at room temperature (18 ± 2 °C), and the final pH was measured again. The pH_zpc value was determined from the plot of the ΔpH vs. pH_i [53].

2.4. Batch Adsorption Tests

The batch adsorption tests of AY-23 on hydrogel beads were carried out to establish the operating conditions for the column tests. The effect of organoclay concentration on the Obent(wt.%)/Alg hydrogel beads (2–10 wt.%) and pH (4–8) was analyzed. Tests were carried out in Erlenmeyer flasks containing 100 mL of a 30 mg/L aqueous dye solution at a fixed pH to which a given mass of the hydrogel beads was added. The pH of the solutions was measured with an SI Analytics Lab 845 pH-meter, and it was adjusted with 0.1 M of HCl and 0.1 M of NaOH. All tests were performed in ambient conditions (20 ± 1 °C and 78 kPa atmospheric pressure) for 4 h, and the stirring speed was kept at 250 rpm.

The concentration of AY-23 in an aqueous solution was determined in a UV-Vis spectrophotometer (Genesys 150, Thermo Scientific, Madison, WI, USA) by measuring the absorbance of the samples (0.75 mL) at 428 nm. The dye removal efficiency was calculated using Equation (3):

$$Removal (\%)=\frac{C_0-C_t}{C_0}\times 100$$

(3)

where $C_0$ and $C_t$ represent the initial and remaining AY-23 concentrations at a given time (t), respectively.

2.5. Fixed-Bed Column Adsorption Tests

The continuous adsorption experiments of AY-23 were performed in a fixed bed in a glass column with a length of 25 cm and diameter of 1.2 cm, which was packed with a specified number of OBent/Alg hydrogel beads (Figure 2). A peristaltic pump with a silicone hose was connected to the bottom of the column to feed the dye solution. To guarantee a continuous flow rate at the inlet and outlet of the column, 1.0 cm diameter glass beads were placed at both ends of the hydrogel bed. Before each experiment, water was pumped through the column to remove air bubbles from the fixed bed. All tests were performed in ambient conditions (20 ± 1 °C) while the initial pH of the dye solution was adjusted to a specific value. Samples were taken at the top of the column at time intervals (until bed saturation), and the dye concentration was quantified by measuring the absorbance at 428 nm.

The parameters evaluated in the adsorption of AY-23 in a fixed bed were as follows: the effect of the bed height ($h$ = 10, 15, and 20 cm of the hydrogel beads), feed flow rate ($Q$ = 1, 2, and 5 mL/min), and initial dye concentration in the feed ($C_0$ = 15, 30, and 45 mg/L). The breakthrough curves were obtained by plotting $C_t/C_0$ (mg/L) vs. t (min), where $C_t$ is the effluent dye concentration, $C_0$ is the influent dye concentration, and t is the service time [54].

For each fixed-bed adsorption test, the respective breakthrough curve was processed, and the following parameters were determined:

Breakthrough time ($t_b$, min): the time that elapsed for the AY-23 concentration in the effluent to reach a 10% concentration in the influent ($C_t/C_0$ = 0.1) [55].

Exhaustion time ($t_e$, min): the time elapsed for the AY-23 concentration in the effluent to reach a 95% concentration in the influent ($C_t/C_0$ = 0.95) [56,57].

The volume of treated effluent ($V_t$, mL): calculated by Equation (4):

$$V_t=Q\times t_e$$

(4)

where $Q$ is the volumetric flow rate (mL/min) and $t_e$ is the exhaustion time (min) [53,54].

The total column capacity ($q_{total}$, mg): obtained from the area under the curve of the adsorbed concentration ($C_{ad}$, mg/L) versus time (t, min). Calculated by Equation (5):

$$q_{total}=\frac{Q}{1000}{{\displaystyle \int }}_{t=0}^{t=t_{total}}{C}_{ad}dt$$

(5)

where $C_{ad}$ is calculated from the difference between dye concentrations in the influent and effluent, i.e., $C_{ad}=C_0-C_t$. The total time ($t_{total}$, min) corresponds to the total continuous adsorption time [53,54,58].

The maximum adsorption capacity of the hydrogel beads ($q_m$, mg/g): corresponds to the amount of dye that was adsorbed ($q_{total}$) per mass (m, g) of the adsorbent at the end of the total flow time [53,54]. $q_m$ was obtained from Equation (6):

$$q_m=\frac{q_{total}}{m}$$

(6)

The total amount of adsorbate retained in the column ($W_{total}$, mg): calculated from Equation (7) [54]:

$$W_{total}=\frac{C_o\times Q\times t_{total}}{1000}$$

(7)

The concentration of adsorbate remaining in the solution when equilibrium was reached ($C_e$, mg/L): obtained from Equation (8) [57]:

$$C_e=\frac{W_{total}-q_{total}}{V_t}$$

(8)

The total amount of adsorbate removed in the column ($R$, %): calculated by Equation (9) [53,54]:

$$R (\%)=\frac{q_{total}}{W_{total}}\times 100$$

(9)

To analyze the dynamic behavior of AY-23 adsorption, the experimental data of $C_t/C_0$ vs. t (min) were fitted to the Thomas, Yoon–Nelson, and Adams–Bohart models (

Table 1. Models for the analysis of the breakthrough curves.

Model–Equation	Parameters	Ref.
Thomas $\frac{C_t}{C_0}=\frac{1}{1+exp\left[\left(\frac{K_{Th}{q}_{0}m}{Q}\right)-K_{Th}{C}_{0}t\right]}$ Equation (10)	$C_0$ = influent dye concentration, mg/L $C_t$ = effluent dye concentration (mg/L) at time t $t=$ time, min $K_{Th}$ = Thomas rate constant, mL/mg min $q_0$ = adsorption capacity, mg/g $Q$ = volumetric flow rate, mL/min $m$= adsorbent amount, g	[59]
Yoon–Nelson $\frac{C_t}{C_0}=\frac{1}{1+exp\left(K_{YN}\left(\tau -t\right)\right)}$ Equation (11)	$K_{YN}$ = Yoon–Nelson rate constant, 1/min $\tau$ = time required for $C_t/C_o$ = 0.5, min	[60]
Adams–Bohart $\frac{C_t}{C_0}=exp\left(K_{AB}{C}_{0}t-K_{AB}{N}_0\frac{h}{v}\right)$ Equation (12)	$K_{AB}$ = Adams–Bohart rate constant, L/mg min $N_0$ = adsorption capacity per unit volume of the bed, mg/L $h$ = height of the bed, cm $v$ = linear flow velocity, cm/min	[60]

). Nonlinear data fitting to the models was performed with OriginLab^®-Pro-8.1 software (OriginLab Corporation, Northampton, MA, USA).

2.6. Desorption Studies and Reuse of the Adsorbent

The AY-23 desorption experiments on hydrogel beads recovered from the column adsorption were performed in batch mode under continuous stirring (250 rpm). The solvents used for the dye desorption were 0.5 M of HCl and 0.5 M of NaOH. After stirring the suspension (200 g hydrogel/L solvent) for 2 h, hydrogel beads were separated by filtration, washed several times with distilled water, and reused in new continuous dye adsorption. The percentage of dye desorption was calculated by Equation (13):

$$Desorption (\%)=\frac{m_{des}}{m_{ads}}\times 100$$

(13)

where $m_{des}$ (mg) and $m_{ads}$ (mg) are the amounts of desorbed and adsorbed AY-23, respectively.

Under the best batch desorption conditions, fixed-bed tests and two continuous adsorption cycles were performed.

2.7. Adsorption in Real Wastewater Matrix

The real wastewater sample was taken from effluent from a food company in Manizales (Caldas, Colombia). This sample was provided under confidentiality, and the presence of the dye and other compounds such as gelatin, citric acid, sodium citrate, sodium benzoate, sucralose, and acesulfame-K were guaranteed. The integrated sample was collected from a wash tank and stored at 4 °C before use. The physicochemical characteristics of the sample were determined by following the Standard Methods (SM) for the Examination of Water and Wastewater [61]. Fixed-bed column adsorption tests with the real wastewater matrix were performed under the best adsorption conditions obtained with the aqueous dye solution prepared in the laboratory (synthetic sample of AY-23).

3. Results and Discussion

3.1. Adsorbent Characterization

The XRD patterns of sodium bentonite, organobentonite, sodium alginate powder, and the organoclay/alginate hydrogels are shown in Figure 3. The structural modification of the clay by cation exchange with the surfactant was confirmed by the change in reflection 001 from a d-value of 14.8 Å (2θ = 5.48°) in Na-Bent to 19.3 Å (2θ = 4.62°) in Obent. The basal spacing value of 19.3 Å in the organoclay suggests a structural arrangement of the surfactant alkyl chains between the bilayer (at 17.7 Å) and pseudo-trimolecular layer (at 21.7 Å) [62,63]. The diffraction pattern of sodium alginate exhibited three diffraction peaks at 2θ = 12.7, 22.8, and 39.6°, which were assigned to the (110) plane from the polyguluronate unit, the 200 plane from polymannuronate, and the other from an amorphous halo [64,65]. The characteristic signal of organobentonite was preserved in the OBent(2%)/Alg hydrogel. However, the intensity decrease in this signal was associated with the semi-crystalline nature of the alginate.

The size distribution of the hydrogel beads is shown in Figure 4. Although all beads were prepared by dripping the alginate or organoclay/alginate suspension with the same Pasteur pipette, the size of hydrogels varied between 2.0 and 5.1 mm. A total of 46% of the beads measured a diameter between 3.1 and 3.7 mm, regardless of the amount of organoclay contained in them.

Table 2. Average mass, water content, and swelling capacity of the organobentonite/alginate hydrogel beads.

Parameter a	OBent(2%)/Alg	OBent(4%)/Alg	OBent(6%)/Alg	OBent(10%)/Alg
Mass, mg	38.8 ± 0.4	42.7 ± 0.4	48.3 ± 0.3	63.9 ± 0.5
Water content, %	94.3 ± 0.5	93.6 ± 0.5	92.1 ± 0.6	91.8 ± 0.9
Swelling capacity, %	133.3 ± 0.7	133.3 ± 0.6	133.3 ± 0.8	133.3 ± 0.5

^a For these analyzes, 100 hydrogel beads with a diameter between 3.1–3.7 mm were used.

shows data for the average bead mass, moisture content, and swelling capacity for the OBent/Alg hydrogel, which was prepared with 2, 4, 6, and 10 wt.% organobentonite. The average mass of a bead with a diameter between 3.1 and 3.7 mm increased as the amount of organoclay in the hydrogel increased, such as rising from 38.8 ± 0.4 mg in the OBent(2%)/Alg to 63.9 ± 0.5 mg in the OBent(10%)/Alg. The average water content in the beads decreased from 94.2 ± 0.5 to 91.8 ± 0.9% when increasing the organoclay concentration from 2 to 10 wt.%. However, the swelling capacity remained practically constant in all hydrogel beads, with an average of 133.32 ± 0.65%, and this was independent of the organoclay concentration. The high swelling capacity of the hydrogels results from these polymeric material’s ability to absorb high amounts of water in their three-dimensional structure [66].

The zero-point charge (pH_zpc) in the OBent/Alg hydrogel beads (Figure 5) slightly varied with the concentration of the organoclay, whereby it increased from 5.5 in OBent(2%)/Alg to 5.7 in OBent(10%)/Alg, with an average of 5.6 ± 0.1. The zero-point charge was defined as the pH of the solution, at which the charge of the positive surface sites was equal to that of the negative ones, i.e., the adsorbent net surface charge had zero value [67]. Acid Yellow 23 dye is a trisodium salt that dissociates in an aqueous solution to form the C₁₆H₉N₄O₉S₂³⁻ anion. Therefore, the AY-23 anions were electrostatically adsorbed when pH < pH_zpc.

3.2. Batch Adsorption Tests

In order to establish certain operating conditions for the column tests, batch experiments were performed on dye removal to evaluate the effect of the organobentonite concentration in the hydrogel beads (Figure 6a) and pH solution (Figure 6b). The effect of the concentration of the organoclay in the beads was studied under the optimal adsorption conditions of AY-23 on a HDTMA-Br-modified Colombian bentonite that was found in a previous study (30 mg/L of dye, pH = 6.0, and 38.04 g of adsorbent) [52].

As the amount of organobentonite in the hydrogel beads increases, the dye adsorption capacity decreases considerably, such as decreasing from 37.76% in OBent(2%)/Alg to 24.79% in OBent(10%)/Alg (Figure 6a). This result can be attributed to diffusional limitations since dye molecules must pass through the hydrogel surface to be electrostatically adsorbed by the organoclay. At a higher concentration of organoclay within the hydrogel, there is a possibility that the modified clay mineral particles agglomerate, and that the adsorption capacity of the dye decreases. Alginate beads without organoclay (blank) showed a limited removal of the dye (<2.0%), suggesting that the adsorption was carried out by the organoclay and not by the matrix in which it was encapsulated. It is important to highlight that the bentonite was modified with a cationic surfactant so as to incorporate positively charged sites for electrostatic anionic dye adsorption, and that the alginate matrix cannot adsorb negatively charged species. A similar result was found in the removal of methyl orange in an aqueous solution on organobentonite/alginate beads since the adsorption capacity of the dye decreased with a growing amount of organobentonite in the encapsulated beads [23].

The effect of pH on the removal of AY-23 was measured in the hydrogel beads with 2 wt.% organobentonite since they showed the highest efficiency in dye adsorption. Batch tests were carried out by adjusting the initial pH of the dye solution to 4.0, 5.0, 6.0, 7.0, and 8.0 units, and the results of these are shown in Figure 6b. The highest dye adsorption occurred at pH 4.0 and 5.0, and since the pHzpc of OBent(2%)/Alg hydrogel was 5.5, the pH values < pH_zpc favored the removal of AY-23 anions (see Figure 5). In general, the dye adsorption decreased with increased pH, whereby it decreased from 53.74% at pH = 4.0 to 28.55% at pH = 8.0. Similar results in the effect of pH were obtained in the removal of other cationic (methylene blue) and anionic (Congo red) dyes when using alginate/natural bentonite beads [44], as well as in the selective adsorption of fluoride ions on nanohydroxyapatite that were encapsulated in alginate [68].

3.3. Fixed-Bed Column Adsorption Tests

In the batch tests with OBent(2%)/Alg beads at pH 4.0 and 5.0, the highest dye removals were obtained (see Figure 6b). At the above pH values and at an intermediate one (pH = 4.5), adsorption tests in a fixed-bed column were carried out (Figure 7) under the following conditions: $h$ = 15 cm, $Q$ = 1 mL/min and $C_0$ = 30 mg/L. Breakthrough curves at pH 4.0, 4.5 and 5.0 were processed, yielding AY-23 removals of 40.53, 39.72, and 37.96%, respectively.

The following AY-23 adsorption tests on the fixed-bed column were performed with OBent (2%)/Alg hydrogel beads, and the pH of the dye solution was adjusted to 4.5 since, for this pH, the dye removal in the column only decreased by 2.0% when concerning the value obtained at pH = 4.0. The effects of bed height, solution flow rate, and initial dye concentration were studied using fixed-bed columns under different conditions. Breakthrough curves for the different test conditions were obtained (Figure 8), and the adsorption parameters are summarized in

Table 3. Adsorption parameters of the AY-23 on OBent(2%)/Alg hydrogel beads in a fixed-bed.

$\mathit{h}$ cm	$\mathit{m}$ g	$\mathit{Q}$ mL/min	${\mathit{C}}_{\mathit{o}}$ mg/L	${\mathit{t}}_{\mathit{b}}$ min	${\mathit{t}}_{\mathit{e}}$ min	${\mathit{V}}_{\mathit{t}}$ mL	${\mathit{t}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ min	${\mathit{q}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mg	${\mathit{q}}_{\mathit{m}}$ mg/g	${\mathit{W}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mg	${\mathit{C}}_{\mathit{e}}$ mg/L	$\mathit{R}$ %
10	6.55	1	30	45.030	274.843	274.843	320	3.280	0.416	9.625	22.994	34.168
15	10.10	1	30	88.333	337.909	337.909	390	4.648	0.460	11.737	20.871	39.724
20	13.65	1	30	147.500	395.004	395.004	450	6.343	0.485	13.501	18.120	46.981
15	10.10	1	30	88.333	337.909	337.909	390	4.648	0.460	11.701	20.871	39.724
15	10.10	2	30	40.000	204.934	409.868	250	3.864	0.383	15.003	27.171	25.758
15	10.10	5	30	5.512	125.699	628.494	150	0.530	0.053	22.505	34.967	2.355
15	10.10	1	15	138.483	449.401	449.401	560	3.483	0.345	8.404	10.940	41.469
15	10.10	1	30	88.333	337.909	337.909	390	4.648	0.460	11.707	20.871	39.724
15	10.10	1	45	69.057	287.910	287.910	340	5.622	0.557	15.306	33.615	36.745

.

Breakdown time ($t_b$, min) and exhaustion time ($t_e$, min) were obtained by the interpolation of the $C_t/C_0$ vs. t plot using OriginLab^®-Pro-8.1 software (Interpolar/Extrapolar Y of X tool). With the breakthrough time ($t_b$) and volumetric flow rate ($Q$, mL/min), the volume of treated effluent ($V_t$, mL) was calculated via Equation (4).

To determine the total adsorption capacity of the column ($q_{total}$, mg/g), Equation (5) was used. First, $C_{ads}$ vs. t was calculated and plotted in TableCurve 2Dv5.01 software (SYSTAT Software Inc., Chicago, IL, USA), and the “Curve-Fill All Equation” option was selected. Then, an equation with a coefficient of determination R² > 0.9993 was selected, and the area under the curve (integral of Equation (5)) was obtained. The maximum adsorption capacity ($q_m$, mg/g), the total amount of dye retained in the column ($W_{total}$, mg), the equilibrium adsorbate concentration ($C_e$, mg/L), and the total amount of dye removed in the column (R, %) were calculated through Equations (6), (7), (8), and (9), respectively.

The adsorption capacity ($q_m$) increased from 0.416 to 0.485 mg/g when the bed height increased from 10 to 20 cm (Figure 8a). The higher amount of adsorbent and adsorption sites available explained the increase in AY-23 adsorption capacity with the bed height. In the adsorption of methylene blue on a citrus peel that was encapsulated in calcium alginate, similar results were obtained in removing the dye as a function of bed height [53].

When the flow rate increased from 1 to 5 mL/min (Figure 8b), the adsorption capacity decreased from 0.460 to 0.053 mg/g. This was due to reduced contact time between the dye and hydrogel beads at higher flow rates. Since the adsorption rate was diffusion controlled, an early breakdown occurred, causing the saturation to be reached faster, thus leading to a low adsorption capacity of the bed [69].

Increasing the dye concentration in the influent generated a higher driving force for mass transfer such that adsorbent saturation occurred more rapidly. As a result, both exhaust time ($t_e$) and treated volume ($V_t$) were reduced [57]. By increasing the dye concentration from 15 mg/L to 45 mg/L (Figure 8c), the adsorption capacity increased from 0.345 to 0.557 mg/g. A similar tendency was found in the fixed-bed adsorption study of the Drimarine Black CL-B dye in an aqueous solution when using a lignocellulosic residue (peanut husk) immobilized on sodium alginate [70].

3.4. Breakthrough Curve Modelling

The breakthrough curves were analyzed under different conditions and fitted to the Thomas, Yoon–Nelson, and Adams–Bohart models. The kinetic parameters of each model were determined with a nonlinear regression of the data using OriginLab^®-Pro-8.1 software (

Table 4. Parameters of the Thomas model under different conditions.

$\mathit{h}$ cm	$\mathit{Q}$ mL/min	${\mathit{C}}_0$ mg/mL	${\mathit{K}}_{\mathit{Th}}$ mL/mg min	${\mathit{q}}_0$ mg/g	${\mathbf{R}}^2$	$\mathbf{A}\mathbf{d}\mathbf{j}. {\mathbf{R}}^2$	${\mathit{\chi }}^2$
10	1	0.030	0.9684 ± 0.0758	0.3640 ± 0.0114	0.9655	0.9665	0.0044
15	1	0.030	0.8748 ± 0.0671	0.4169 ± 0.0098	0.9676	0.9685	0.0047
20	1	0.030	0.8738 ± 0.0540	0.4558 ± 0.0061	0.9811	0.9815	0.0031
15	1	0.030	0.8748 ± 0.0671	0.4169 ± 0.0098	0.9676	0.9685	0.0047
15	2	0.030	1.8612 ± 0.2300	03675 ± 0.0150	0.9558	0.9575	0.0059
15	5	0.030	1.8962 ± 0.4603	0.3163 ± 0.0616	0.8055	0.8177	0.0171
15	1	0.015	1.2320 ± 0.0567	0.3273 ± 0.0042	0.9843	0.9846	0.0024
15	1	0.030	0.8748 ± 0.0671	0.4169 ± 0.0098	0.9676	0.9685	0.0047
15	1	0.045	0.7548 ± 0.0671	0.4998 ± 0.0121	0.9717	0.9725	0.0042

, Table 5 and Table 6).

The adjusted coefficient of determination (Adj. R²) and the reduced chi-square value (χ²) were selected as measures of the fit quality in the breakthrough curves [71,72]. These two statistical parameters eliminated the adverse effects of the degrees of freedom [72] since the Thomas, Yoon–Nelson, and Adams–Bohart models included different numbers of parameters. High Adj. R² (close to 1.0) and low χ² (close to zero) values were associated with the fact that the models’ curves were a good fit of the experimental data [72].

The Adj. R² value obtained by fitting the experimental data of the breakthrough curves to the Thomas nonlinear model were equal to those found in the Yoon–Nelson model, i.e., the Adj. R² was greater than 0.9575, except for the test with a flow rate of 5 mL/min where the Adj. R² was 0.8177 (Table 4 and Table 5). For the Adams–Bohart model, an Adj. R² between 0.9555 and 0.9999 and a χ² < 1.36 × 10⁻³ was found (Table 6), indicating a good fit of the experimental data to this model.

Both external (fluid-film) and intraparticle (solid) mass-transfer resistance were ignored in the Thomas model. This indicates that the surface reaction between the adsorbate and the unused capacity of the adsorbent controlled the adsorption rate [73]. The kinetic constant values, $K_{Th}$, reduced as the bed height increased because the adsorbent–adsorbate interactions could occur faster in a greater amount of adsorbent. Therefore, the adsorption capacity increased from 0.3640 to 0.4558 mg/g when the bed height increased from 10 to 20 cm.

For the highest flow rate (5 mL/min), high normalized concentration ($C_t/C_0$) values were observed at the beginning of the continuous adsorption. Consequently, the dye concentration in the effluent increased rapidly, and the bed became saturated. As the flow rate increased, so too did the Thomas constant, $K_{Th}$. This behavior may be attributed to the insufficient residence time of the dye in the column.

As the initial dye concentration increased, the value of $K_{Th}$ decreased because the driving force for the adsorption is the concentration difference of the AY-23 between the adsorbents and solution. The highest adsorption capacity ($q_0$ = 0.4998 mg/g) obtained with the Thomas model was for a bed height of 15 cm, with the lowest flow rate (1 mL/min) and the highest initial dye concentration (45 mg/L).

Considering that the OBent(2%)/Alg swollen hydrogel contains approx. 1.50 ± 0.03% OBent, and that for a bed height of 15 cm (10.10 g), the amount of active phase is 0.1515 g, the adsorption capacity of the organoclay on the hydrogel is given by Equation (14):

$$Adsortion Capacity=0.4998 \frac{mg dye}{g hydrogel}\times \left(\frac{10.10 g hydrogel}{0.1515 g OBent}\right)=33.32 \frac{mg dye}{g OBent}$$

(14)

The maximum adsorption capacity obtained for the organoclay powder when using the Langmuir model was 40.79 mg/g [52]. Therefore, the OBent(2%)/Alg hydrogel retained 81.69% of the adsorption capacity of the organobentonite powder, although the experimental conditions of both tests were different. The advantage of using OBent(2%)/Alg hydrogel beads was that no filtration step was required, which in the case of powdered organobentonite, was performed with 0.45 μm of cellulose filters. Furthermore, using hydrogels as an adsorbent material allows for their use in continuous processes [74].

The maximum dye adsorption capacity of the hydrogel prepared in this study was compared with other alginate composites, and the results are shown in Table 5. There are no adsorption studies of Acid Yellow 23 on organobentonite/alginate hydrogels, and most of the research has focused on the adsorption of cationic dyes on other types of alginate composites. Only one investigation has studied the adsorption of an anionic dye (Allura red) on an organoclay/alginate composite in a batch system [47]. In the adsorption kinetic study of Allura red, the adsorption capacities were 0.478 and 0.509 mg/g for the wet and dry composite, respectively. The maximum adsorption capacity obtained with the Langmuir model for batch experiments with the dry composite was 7.0 mg/L [47]. The results of this study are similar to those obtained in the Allura red adsorption, although the $q_m$ values in the batch process were higher than those for the fixed-bed column process [53].

Table 5. Comparison of the adsorption capacity of the dyes on alginate composites.

Adsorbent	Dye	Type	Process	${\mathit{q}}_{\mathit{m}}$, mg/g	Ref.
Alginate encapsulated pillared clay	Safranine	Cationic	Batch	268.5	[35]
Alginate/natural bentonite (1/3)	Methylene blue	Cationic	Batch	550.92	[44]
	Congo red	Anionic	Batch	111.07
Alginate–organobentonite composite (1/3)	Methylene blue	Cationic	Batch	670.54	[45]
Organoclay–alginate matrix	Allura red	Anionic	Batch	5.7–7.0	[47]
Citrus peels–calcium alginate	Methylene blue	Cationic	Batch	964.54	[53]
			Column	31.45
Cellulose nanocrystal–alginate	Methylene blue	Cationic	Column	255.5	[54]
Activated bentonite/alginate composite	Methylene blue	Cationic	Batch	780.59	[75]
	Crystal violet	Cationic		546.53
Activated carbon–bentonite–alginate	Methylene blue	Cationic	Batch	756.97	[76]
Organobentonite/alginate hydrogel	Acid Yellow 23	Anionic	Column	0.50	This study

Table 5. Comparison of the adsorption capacity of the dyes on alginate composites.

Adsorbent	Dye	Type	Process	${\mathit{q}}_{\mathit{m}}$, mg/g	Ref.
Alginate encapsulated pillared clay	Safranine	Cationic	Batch	268.5	[35]
Alginate/natural bentonite (1/3)	Methylene blue	Cationic	Batch	550.92	[44]
	Congo red	Anionic	Batch	111.07
Alginate–organobentonite composite (1/3)	Methylene blue	Cationic	Batch	670.54	[45]
Organoclay–alginate matrix	Allura red	Anionic	Batch	5.7–7.0	[47]
Citrus peels–calcium alginate	Methylene blue	Cationic	Batch	964.54	[53]
			Column	31.45
Cellulose nanocrystal–alginate	Methylene blue	Cationic	Column	255.5	[54]
Activated bentonite/alginate composite	Methylene blue	Cationic	Batch	780.59	[75]
	Crystal violet	Cationic		546.53
Activated carbon–bentonite–alginate	Methylene blue	Cationic	Batch	756.97	[76]
Organobentonite/alginate hydrogel	Acid Yellow 23	Anionic	Column	0.50	This study

The Yoon–Nelson model assumes that the decrease in the probability of each adsorbate is proportional to the probability of its adsorption and breakthrough on the adsorbent [57,58,77]. As shown in Table 6, the rate constant Yoon–Nelson ($K_{YN}$) decreased while the τ increased as the bed height was augmented. As the bed height (and thus adsorbent mass) increases, more adsorbent particles will be available in the column to interact with the solute, thus requiring a longer period to reach a 50% breakthrough.

Table 6. Parameters of the Yoon–Nelson model under different conditions.

$\mathit{h}$ cm	$\mathit{Q}$ mL/min	${\mathit{C}}_0$ mg/mL	${\mathit{K}}_{\mathit{YN}}$ 1/min	$\mathbf{\tau }$ min	${\mathbf{R}}^2$	$\mathbf{A}\mathbf{d}\mathbf{j}. {\mathbf{R}}^2$	${\mathit{\chi }}^2$
10	1	0.030	0.0291 ± 0.0023	95.7365 ± 2.9960	0.9665	0.9655	0.0044
15	1	0.030	0.0263 ± 0.0020	140.2932 ± 3.3000	0.9685	0.9676	0.0047
20	1	0.030	0.0262 ± 0.0016	198.8837 ± 2.6672	0.9815	0.9811	0.0031
15	1	0.030	0.0263 ± 0.0020	140.2932 ± 3.3000	0.9685	0.9676	0.0047
15	2	0.030	0.0558 ± 0.0069	61.8320 ± 2.5204	0.9575	0.9558	0.0059
15	5	0.030	0.0568 ± 0.0138	21.3060 ± 4.1485	0.8177	0.8055	0.0171
15	1	0.015	0.0185 ± 0.0008	220.2752 ± 2.8097	0.9846	0.9843	0.0024
15	1	0.030	0.0263 ± 0.0020	140.2932 ± 3.3000	0.9685	0.9676	0.0047
15	1	0.045	0.0340 ± 0.0028	112.1453 ± 2.7188	0.9725	0.9717	0.0042

Table 6. Parameters of the Yoon–Nelson model under different conditions.

$\mathit{h}$ cm	$\mathit{Q}$ mL/min	${\mathit{C}}_0$ mg/mL	${\mathit{K}}_{\mathit{YN}}$ 1/min	$\mathbf{\tau }$ min	${\mathbf{R}}^2$	$\mathbf{A}\mathbf{d}\mathbf{j}. {\mathbf{R}}^2$	${\mathit{\chi }}^2$
10	1	0.030	0.0291 ± 0.0023	95.7365 ± 2.9960	0.9665	0.9655	0.0044
15	1	0.030	0.0263 ± 0.0020	140.2932 ± 3.3000	0.9685	0.9676	0.0047
20	1	0.030	0.0262 ± 0.0016	198.8837 ± 2.6672	0.9815	0.9811	0.0031
15	1	0.030	0.0263 ± 0.0020	140.2932 ± 3.3000	0.9685	0.9676	0.0047
15	2	0.030	0.0558 ± 0.0069	61.8320 ± 2.5204	0.9575	0.9558	0.0059
15	5	0.030	0.0568 ± 0.0138	21.3060 ± 4.1485	0.8177	0.8055	0.0171
15	1	0.015	0.0185 ± 0.0008	220.2752 ± 2.8097	0.9846	0.9843	0.0024
15	1	0.030	0.0263 ± 0.0020	140.2932 ± 3.3000	0.9685	0.9676	0.0047
15	1	0.045	0.0340 ± 0.0028	112.1453 ± 2.7188	0.9725	0.9717	0.0042

The constant $K_{YN}$ increased and the τ (time required for $C_t/C_o$ = 0.5) decreased with the increasing flow rate and the dye’s initial concentration. The increase in $K_{YN}$ with a growing flow rate or initial dye concentration indicated a decrease in the mass-transfer resistance, and the decrease in the τ indicated a faster saturation of the adsorption bed. The mass-transfer resistance was proportional to the axial dispersion and thickness of the liquid film on the particle surface [78]. Considering that the flow rates were so low, it is reasonable to assume that the axial dispersion was minimal in a such situation [78,79]. Therefore, increasing the $Q$ and $C_o$ allowed for the driving force of the mass transfer in the liquid film to rise.

The Adams–Bohart model assumes that the adsorption rate is proportional to both the adsorbate concentration and the residual capacity of the adsorbent in the initial part ($C_t/C_0$ < 0.5) of the breakthrough curve [73,80]. The results of modeling the experimental rupture curves with the Adams–Bohart model are presented in Figure 8. This approach was focused on the estimation of characteristic parameters, such as the adsorption capacity per unit bed volume ($N_0$) and the kinetic constant ($K_{AB}$) from the Adams–Bohart model (

Table 7. Parameters of the Adams–Bohart model under different conditions.

$\mathit{h}$ cm	$\mathit{v}$ cm/min	$\mathit{Q}$ mL/min	${\mathit{C}}_0$ mg/mL	${\mathit{K}}_{\mathit{A}\mathit{B}}$ mL/mg min	${\mathit{N}}_0$ mg/L	${\mathbf{R}}^2$	$\mathbf{A}\mathbf{d}\mathbf{j}. {\mathbf{R}}^2$	${\mathit{\chi }}^2$
10	0.884	1	0.030	0.0014 ± 9.6 × 10−5	256.0382 ± 4.8154	0.9851	0.9833	4.58 × 10−4
15	0.884	1	0.030	0.0012 ± 1.3 × 10−4	257.1605 ± 4.4398	0.9661	0.9635	1.17 × 10−3
20	0.884	1	0.030	0.0011 ± 6.4 × 10−5	275.0142 ± 2.5432	0.9851	0.9768	5.57 × 10−4
15	0.884	1	0.030	0.0012 ± 1.0 × 10−4	257.1605 ± 4.4398	0.9661	0.9635	1.17 × 10−3
15	1.768	2	0.030	0.0022 ± 2.5 × 10−4	238.7197 ± 5.2533	0.9743	0.9701	1.36 × 10−3
15	4.421	5	0.030	0.0179 ± 4.6 × 10−4	96.5801 ± 0.2316	0.9999	0.9999	8.19 × 10−6
15	0.884	1	0.015	0.0016 ± 1.3 × 10−4	197.8971 ± 3.1616	0.9576	0.9555	1.23 × 10−3
15	0.884	1	0.030	0.0012 ± 1.3 × 10−4	257.1605 ± 4.4398	0.9661	0.9635	1.17 × 10−3
15	0.884	1	0.045	0.0086 ± 7.4 × 10−5	323.3084 ± 5.4846	0.9716	0.9690	1.26 × 10−3

).

The $N_0$ and $K_{AB}$ values increased and decreased, respectively, when increasing the bed height and initial dye concentration due to the saturation of the adsorbent sites [81]. The values of $K_{AB}$ were affected by the flow rate, and they increased with an augmented flow rate. This indicated that the overall system kinetics was dominated by external mass transfer in the initial part of adsorption in the fixed bed [82]. The adsorption mechanism of AY-23 on the organobentonite/alginate hydrogels may occur due to the electrostatic interactions between the cationic groups of the organoclay and the dye anions (Figure 9). In addition, the removal due to the interactions between the hydrophobic alkyl chain of the organoclay with the hydrophobic part of the dye molecule has been suggested previously [40,47,83,84].

3.5. Desorption Studies and Adsorbent Reuse

Regeneration is an essential factor that impacts an adsorbent’s reusability and efficiency [85]. Hydrogel beads from the fixed-bed test with $h$ = 15 cm, $Q$ = 1 cm/min, and $C_0$ = 30 mg/L were used for the desorption tests, which exhibited a high adsorption capacity ($q_m$ = 0.460 mg/L, see Table 3).

The highest desorption of AY-23 in the batch tests was obtained with the aqueous solution of NaOH (Figure 10). The aqueous solution’s pH affected the desorption, going from 76.61% with 0.5 M of NaOH (pH = 13.7) to 17.45% with 0.5 M of HCl (pH = 0.3). It is important to consider that the desorption conditions are opposite to those for the adsorption of an anionic dye. Desorption was favored when the pH of the solution was higher than the pH_zpc of the adsorbent, i.e., at pH > 5.5.

In addition to the batch-desorption tests, a desorption cycle was performed in the fixed-bed column with 0.5 M of NaOH. After desorption, distilled water was passed through the bed, and two new adsorption cycles were performed. Breakthrough curves for the initial adsorption and reuse of the adsorbent are shown in Figure 11, and the adsorption parameters are summarized in

Table 8. Adsorption parameters of the AY-23 on the OBent(2%)/Alg hydrogel beads. The initial adsorption and adsorbent reuses.

Test	${\mathit{t}}_{\mathit{b}}$ min	${\mathit{t}}_{\mathit{e}}$ min	${\mathit{V}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mL	${\mathit{t}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ min	${\mathit{q}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mg	${\mathit{q}}_{\mathit{m}}$ mg/g	${\mathit{W}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mg	${\mathit{C}}_{\mathit{e}}$ mg/L	$\mathit{R}$ %
Initial adsorption	88.333	337.909	337.909	390	4.648	0.460	11.737	20.871	39.724
Reuse–Cycle 1	41.402	252.871	252.873	310	3.046	0.302	9.300	24.734	31.750
Reuse–Cycle 2	38.588	128.599	128.599	200	1.539	0.152	6.001	34.691	25.645

. The adsorbent in its first use in the fixed bed achieved a dye removal of 39.72% and, with the first and second reuse cycles, the removal became 31.75 and 25.65%, which corresponds to approximately 79.93 and 64.56% of the initially removed adsorbate.

3.6. Fixed-Bed Column Adsorption of a Real Wastewater Matrix

The presence of the AY-23 dye in the wastewater sample was verified with the visible spectrum (Figure 12). The λ_max for the AY-23 solution was 428 nm, and the sample presented a broad absorption band centered at this value. The initial concentration of AY-23 in the wastewater sample was 11.37 mg/L, obtained from the absorbance vs. concentration calibration curve.

The results of the physicochemical characterization of the wastewater matrix are summarized in

Table 9. Physicochemical characteristics of real wastewater matrix.

Parameter	Standard Method (SM)	Wastewater Sample (before Adsorption)	Wastewater Sample (after Adsorption)
pH	4500-H+ B	7.4 ± 0.1	7.5 ± 0.2
Conductivity (μS/cm)	2510-B	510 ± 10	303 ± 8
COD (mg O2/L)	5220-D	825 ± 35	527 ± 14
BOD5 (mg O2/L)	5210-B	348 ± 21	269 ± 11
TOC, mg C/L	5310-B	318 ± 12	235 ± 8
Acid Yellow 23, mg/L		11.37 ± 0.03	<0.13 *

* This concentration is reached at less than 130 min of continuous adsorption.

. The total organic carbon (TOC) concentration in the sample was very high compared to the carbon content contributed by the dye (11.37 mg/L AY-23, equivalent to 4.1 mg/L TOC). The TOC content was mainly due to the presence of gelatin, citric acid, sodium citrate, sodium benzoate, sucralose, and acesulfame-K, which also contribute to the chemical and biological oxygen demand of the sample.

The initial pH of the wastewater sample was 7.4, and for the adsorption tests, it was acidified with 0.1 M of HCl solution. Adsorption tests were performed at pH = 4.5, using a bed height of 15 cm and a flow rate of 1 mL/min (Figure 13). The adsorption parameters for the breakthrough curves are summarized in

Table 10. Adsorption parameters of AY-23 present in the wastewater sample using OBent(2%)/Alg hydrogel beads.

Test	${\mathit{t}}_{\mathit{b}}$ min	${\mathit{t}}_{\mathit{e}}$ min	${\mathit{V}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mL	${\mathit{q}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mg	${\mathit{q}}_{\mathit{m}}$ mg/g	${\mathit{W}}_{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$ mg	${\mathit{C}}_{\mathit{e}}$ mg/L	$\mathit{R}$ %
Initial	170.498	503.768	503.768	3.451	0.342	7.618	8.272	45.301
Duplicate	169.337	498.462	498.462	3.383	0.335	7.595	8.499	44.543
Triplicate	169.139	496.838	496.838	3.355	0.332	7.583	8.509	44.244

. The mean $q_m$ value for the adsorption of the dye present in the wastewater sample using the OBent(2%)/Alg hydrogel was 0.336 ± 0.005 mg/g. The $q_m$ value was lower than that obtained in tests with the same bed height and flow rate at different dye concentrations (15, 30, and 45 mg/L); this is because the wastewater matrix has a lower dye concentration ($C_0$ = 11.37 mg/L), and the driving force for mass transfer decreases. Furthermore, at pH 4.5, the dissociation of citric acid, sodium citrate, and sodium benzoate generates anions that could adsorb on the active sites of the OBent(2%)/Alg hydrogel, decreasing the adsorption capacity of the dye. The total amount of dye removed in the column ($R$, %) for the three continuous adsorption tests with the wastewater matrix was 44.696 ± 0.545%, and this value is in the upper range of the $R$ calculated in this study for the synthetic dye solutions (See Table 3).

The adsorption results obtained with the wastewater sample were compared with those obtained in a fixed-bed column study using free and immobilized Nelumbo nucifera leaf adsorbent to remove Congo red from industrial effluent [86]. The decolorization efficiencies of Congo red adsorption were 76.25% and 62.18%, higher values than in this study. The average COD and BOD removals were 86.91 and 91.96%, respectively [86]. In this study, the COD and BOD reductions were 36.09 + 1.02% and 22.64 + 1.51%, indicating that, in addition to AY-23 dye adsorption, the removal of other organic compounds present in the actual wastewater sample was achieved.

4. Conclusions

The modification of the bentonite with the cationic surfactant hexadecyltrimethylammonium bromide (HDTMA-Br) changed the value of the basal spacing d₀₀₁ from 14.8 to 19.3 Å, which suggests there is a structural arrangement in the surfactant alkyl chains between the bilayer and pseudo-trimolecular layer. The encapsulation of the organoclay in the alginate matrix did not change the basal spacing in the hydrogel. However, there was a decrease in the intensity of this signal in the XRD pattern, and this was associated with the semi-crystalline nature of the alginate.

Evaluating some of adsorption parameters in the batch mode of Acid Yellow 23 on the OBent/Alg hydrogel allowed for an establishment of the operating ranges for the fixed-bed column tests. The dye removal decreased as the concentration of the organoclay in the hydrogel increased from 2 to 10 wt.%. The highest dye adsorption occurred at pH 4.0–5.0 with the hydrogel containing 2 wt.% organoclay.

The experimental data of the breakthrough curves were well-fitted to the Yoon–Nelson, Thomas, and Adams–Bohart models, and the parameters of each model were determined by using nonlinear fitting. When using the Thomas model, the maximum adsorption capacity ($q_0$ = 0.4998 mg/g) was obtained for an average bed height (15 cm), the lowest flow rate (1 mL/min), and the highest initial dye concentration (45 mg/L). The concentration of organoclay in the hydrogel, $q_0$, was equivalent to 33.32 mg/g of OBent. Therefore, the OBent(2%)/Alg hydrogel was retained at 81.69% of the adsorption capacity of the powdered organoclay. However, the experimental conditions of both tests differed (i.e., the batch adsorption for the powdered OBent and continuous adsorption for the hydrogel).

The Acid Yellow 23 dye adsorbed in the OBent(2%)/Alg hydrogel can be desorbed by using a 0.5 M NaOH solution as a solvent. After two cycles of continuous reuse of the adsorbent material, it retained 64.56% of the initial dye removal capacity, guaranteeing the adsorbent material’s reusability. The results obtained in this study suggest the potential of organoclay/alginate hydrogel for the removal of anionic dyes in a fixed-bed column.

The OBent(2%)/Alg hydrogel removed the AY-23 dye contained in a real wastewater sample, suggesting the applicability of the composite for the adsorption of pollutants in continuous systems.