Preparation and Adsorption Performance on Congo Red of Sodium Alginate-Lanthanum Hydrogel Spheres

Sodium alginate-lanthanum (SA/Lan) hydrogel spheres were prepared by ion cross-linking method to remove Congo Red (CR) in an aqueous solution. Te adsorption performance was assessed through batch experiments. Te experiment revealed that the highest adsorption capacity of SA/Lan for CR was achieved when the mass concentration of La 3+ was 1%, the dosage was 2 g · L − 1 , the initial concentration of CR was 30 mg · L − 1 , the adsorption time was 60min, and the reaction temperature was 25 ° C. Te adsorption capacity of 10.81mg · g − 1 was the corresponding fgure. Te adsorption process was consistent with the Langmuir and pseudosecond-order models.


Introduction
Te use of dyestuf in textile and paper industries has led to the discharge of large amounts of dyestuf wastewater during their manufacturing and use [1].Te high chromaticity and difcult biodegradability of these dyestufs render their degradation challenging, and some dyestufs have been shown to produce carcinogenic and teratogenic substances upon contact with the human body, posing a threat to both the ecological environment and human health [2][3][4][5].Among the various treatment methods for dye wastewater, adsorption is a popular option due to its simplicity, economic efciency, and recyclability [6].However, the commonly used adsorbents such as activated carbon [7,8] and nanomaterials [9] are difcult to separate from wastewater, often causing secondary pollution, and hindering the overall environmental protection efort [10,11].
Hydrogels are a type of three-dimensional networkstructured gels with exceptionally high hydrophilicity and a vast specifc surface area that encompasses a large number of functional groups [12].Tis structure makes them ideal for removing diferent types of pollutants from wastewater.Sodium alginate (SA), as a biomass material, has outstanding qualities for efcient water treatment, such as biocompatibility, renewability, degradability, recyclability, and eco-friendliness.It is widely used as a biosorbent because it is rich in carboxyl and hydroxyl groups and has a high afnity for various multivalent metal ions and dyes [13].Han et al. developed stable porous hydrogels made of carboxymethyl chitosan/formic acid (CMCS-PA), where CMCS-PA (3 :1) composite hydrogels at pH � 7 at room temperature delivered excellent results on the removal of methyl orange [14].Meanwhile, Xu et al. produced a composite hydrogel using AOPIM/Alg that could efectively remove cationic dyes from aqueous solutions, and its adsorption capacity on neutral red could reach 182 mg•g −1 [15].Additionally, Li's study reported the preparation of a calcium alginate porous membrane with exceptional adsorption capacity, reaching 1679.52 mg•g −1 at the equilibrium concentration of methylene blue of 80 mg•L −1 at 15 °C.Tis membrane also has a reliable recycling and regeneration capacity, making it a realistic choice for the treatment of wastewater.
In the area of hydrogel-mediated adsorption of dyes, the majority of studies have focused on cationic dyes, with far fewer investigations into anionic dyes.Litefti et al. [16] adsorbed CR using by-products of the wood industry with a CR adsorption of 0.50 mg/g, which was too long for the CR removal cycle.Abbasi et al. [17] used lanthanum-doped bismuth ferrite calixarenes (BFOs) for their modifcation and adsorption of CR.However, due to the inherent problems such as the formation of a secondary crystalline phase from the preparation of the BFOs, their practical utilization was limited.
To address this research gap, this study aims to prepare and optimize SA/Lan hydrogel spheres through ionic crosslinking.Lanthanum doping optimizes its surface properties and increases its surface charge.We systematically investigate the key factors afecting the adsorption performance of these hydrogel spheres, including the initial concentration of Congo Red (CR), dosage, temperature, and time.Te synthesized SA/Lan adsorbs CR through abundant active sites and surface charge, and we aim to comprehend the process of adsorption by utilizing kinetic modeling and analyzing adsorption isotherms.Trough this systematic study, we aim to provide a reliable basis for the treatment of anionic printing and dyeing wastewater.Its simultaneous high cleanliness, low cost, and ease of preparation give it a good prospect for industrial adsorption of Congo Red dye.Except for the labeled, all are the analytical reagents, and water used in the experimental processes was deionized water.

Preparation of Hydrogel Spheres.
Here is a revised version of the paragraph with improved logical accuracy, detail, and academic style: to prepare SA/Lan hydrogel spheres of varying La 3+ concentrations, a solution was prepared by dissolving 2 g of sodium alginate in 98 mL of deionized water and stirred magnetically at 80 °C until fully dissolved.Te resulting solution was then slowly added dropwise to a constantly pressure-separated funnel containing 0.1 to 10 wt.% lanthanum chloride in an aqueous solution.Te mixture was left to rest and cross-link for 24 hours.Te resulting hydrogel spheres were named SA/ La3 (where "n" represents a 3.0 wt.% La 3+ concentration, for example).Next, the hydrogel spheres were rinsed with deionized water 3 to 5 times and dried by wiping with lens paper.

Absorption Experiments.
A certain quality of SA/Lan hydrogels (0.5 g•L −1 to 6.0 g•L −1 ) was placed in a 150 mL fask.50 mL of Congo Red dissolved at an initial concentration of 30-60 mg/L was added at pH 7 ± 0.5, which was put in a thermostatic shaker (25 °C, 35 °C, and 45 °C) at a speed of 200 rpm for some time (5 min to 180 min).Te residual solution of CR was measured by UV-Visible spectrophotometer, and adsorptive capacity was calculated using the following equation: where q is the adsorptive capacity (mg•g −1 ), c 0 , c are the initial concentration and the fnal concentration of CR solution (g•L −1 ), V is the volume of CR solution (mL), and m is the dosage of SA/Lan hydrogel spheres.

Material Characterization. Figures 1 and 2 indicate the ESEM images (Environmental Scanning Electron
Microscope, QUANTA200, Guangzhou Betop Scientifc Ltd.) of the outside surface and cross section of the SA/La1 hydrogel spheres, respectively.Figure 1 shows that the surface of the SA/La1 hydrogel spheres was rough, and the uneven crosslinking of La with hydroxyl groups led to the formation of a large number of grooves and wrinkles, which provided more active sites and efectively improved their adsorption capacity [18][19][20].As shown in Figure 2, the ESEM map presents a three-dimensional mesh structure with interconnected skeletal structures constituting numerous nanometer-and micrometer-scale pores, and this physically and chemically cross-linked network facilitates the exposure of a large number of binding sites, as well as the mass transfer of lanthanum and dyes to promote the removal process of pollutants [13].However, with an average pore size of 0.7718 nm, the access of dyes to the interior may be limited.Te SA/La1 hydrogel spheres have good adsorption space inside the sparse and porous interior, which provides the possibility for the next step of sparing and expanding the pores to improve the internal adsorption performance.Table 1 shows the specifc surface area data (BET, Mac2460) of SA/La1 hydrogel spheres.It is clear that the specifc surface area of the gel spheres is small and their superior adsorption performance depends on the abundance of functional groups on the external surface.
Figure 3 shows the infrared spectrum (FT-IR, Perkin Elmer Frontier, Shanghai) of SA/La1 hydrogel spheres.It indicates that 3415 cm −1 is the stretching vibration peak of the -OH group [21]; the peaks at 1615 cm −1 and 1426 cm −1 are the antisymmetric stretching vibration peak and the symmetric vibration absorption peak of carboxylic acid COO−, respectively [22]; the peaks at 1085 cm −1 and 1034 cm −1 are associated with C-O stretching in acids, phenols, ethers, and esters [23,24].Te wide range of functional groups on the surface of the hydrogel spheres provides a wealth of active sites for the adsorption of Congo Red.

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Efect of La 3+ Concentration of SA/Lan Hydrogel Spheres.
A series of SA/Lan hydrogel spheres were prepared by sodium alginate cross-linking with lanthanum chloride solution using La 3+ concentration of 0.1 to 10 wt.%. 2 g•L −1 SA/ Lan hydrogel spheres were added to 30 mg•L −1 of CR solution shaking for 4 h.As shown in Figure 4, the doping of La ions is useful for the removal of CR [25].Te adsorption of CR is very low when not doped with La.Te adsorption capacity of SA/Lan hydrogel spheres increased dramatically with the rise of the La 3+ concentration before 1.0 wt.% of La 3+ concentration.It is because La ions cross-link with carboxyl groups to optimize the surface properties and increase the surface positive charge of the hydrogel, enhancing the adsorption of CR.Te La cross-linking saturated and then the adsorption capacity remained stable after the adsorption capacity reached 11.63 mg•g −1 .So, 1 wt.% of La 3+ concentration was selected for the preparation of hydrogel spheres in subsequent experiments.

3.3.
Efect of Adsorption Time. 2 g•L −1 of SA/La1 hydrogel spheres was placed in 30 mg•L −1 of CR solution at 25 °C for 5 min to 180 min.As shown in Figure 5, the adsorption capacity of SA/La1 hydrogel spheres increased rapidly before 60 min and reached adsorption equilibrium after 60 min, which is of great importance in such a short time to achieve equilibrium in practical applications.Te equilibrium adsorption capacity of SA/La1 hydrogel spheres attained 10.81 mg•g −1 at 60 min.

Efect of Adsorbent Dosage.
A certain dosage of SA/La1 hydrogel spheres was placed in 30 mg•L −1 of CR solution at 25 °C for 60 min.As shown in Figure 6, when the initial concentration was 30 mg•L −1 , the dosage of SA/La1 hydrogel spheres increased from 0.5 g•L −1 to 2 g•L −1 .Te adsorption capacity saw an increase from 5.32 mg g −1 to 9.17 mg g −1 , which is likely attributed to the addition of the adsorbent.Tis addition resulted in an increase in active adsorption   sites, thereby enhancing the adsorption capacity.Followed by, the adsorption capacity decreased as adsorbent was continuously added.Te possible reason is that the adsorption of CR solution reaches equilibrium [22].Unsaturated sites during adsorption lead to an increase in adsorption capacity with decreasing dosage as well as excess adsorbent required for the adsorption of dyes leading to a decrease in adsorption [26].Terefore, the optimal input of the adsorbent was 2 g•L −1 .

Adsorption Kinetics.
Te interaction between dye molecules and the adsorbent was accompanied by studying the kinetics isotherms of adsorption behavior on the SA/La1 hydrogel spheres.Te pseudofrst-and pseudosecond-order kinetic models were used to describe the kinetics isotherms of CR adsorption on the adsorbent respectively [27].
Te pseudofrst-order kinetic equation is ln q e -q t  � ln q e -K 1 t. ( Te pseudosecond-order kinetic equation is where q e is the adsorption capacity at equilibrium (mg g −1 ), q t is the adsorption capacity at time (mg g −1 ), K 1 is the rate constant of the pseudofrst-order kinetic model at equilibrium (min −1 ), t is the adsorption time (min), and K 2 is the rate constant of the pseudosecond-order kinetic model at equilibrium (g•mg −1 •min −1 ).Te entire adsorption process can be divided into two stages: during the initial stage, the adsorption rate is rapid.Te adsorption capacity of CR on SA/La1 hydrogel spheres has reached 95% of the equilibrium adsorption capacity until adsorbing for 35 min.As the extension of adsorption time, the adsorption capacity gradually reaches the equilibrium state after 60 min.Tis may be attributed to the abundance of available active sites on the surface of SA/La1 hydrogel spheres during the initial stage.Moreover, the diference of concentration between the liquid phase and solid phase surface CR is large, meaning the adsorption driving force is big, which makes CR difuse on the surface of the hydrogel sphere for reaction rapidly resulting in the adsorption rate increasing obviously.As the adsorption progresses, the active sites on the surface of the adsorbent gradually decrease, which causes a decrease in the adsorption rate until equilibrium.Figure 7 and Table 2 display that the pseudosecondorder kinetic model is more suitable for describing the CR adsorption on SA/La1 hydrogel spheres as the R 2 of this model is 0.9999.It indicates that the chemisorption dominates the adsorption process.
To further illustrate the difusion mechanism, we used an intraparticle difusion model and we tried to study the difusion mechanism of CR adsorption on SA/La1 hydrogel spheres as shown in Figure 8. Attributed to the rough surface formed by the association of La with hydroxyl groups, which provides more active sites and promotes the difusion of dyes on the surface, the CR solution difuses rapidly on the surface of SA/La1 hydrogel.Te adsorption capacity rises rapidly and then reaches the adsorption equilibrium, which is consistent with the data obtained from the characterization of SA/La1 hydrogel spheres.
Te ion difusion equation is where q t is the adsorption capacity at time (mg g −1 ), t is the adsorption time (min), and K ipd is intraparticle difusion model constants (mg•g −1 •min −0.5 ).

Adsorption Isotherms.
Te study of adsorption isotherms provided a more lucid comprehension of the distribution of dye molecules between the liquid phase and the adsorbent surface when the adsorption system is in equilibrium.To understand the CR adsorption on SA/La1 hydrogel spheres, Figure 9 shows the adsorption isotherms at 25 °C, 35 °C, and 45 °C.Te Langmuir isotherm model ( 4) and the Freundlich isotherm model ( 5) were used to correspond to the data [26].

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Advances in Materials Science and Engineering Te Langmuir isotherm equation is Te Freundlich isotherm equation is where c e is the equilibrium concentration of CR in solution (mg•L −1 ), q e is the adsorption capacity at equilibrium (mg•g −1 ), q m is the maximum monolayer adsorption capacity (mg•g −1 ), K L is Langmuir adsorption equilibrium constant (L•mg −1 ), K F is Freundlich adsorption equilibrium constant (mg•L −1 ), and n is the characteristic constant related to adsorption temperature.As indicated in Table 3, the CR adsorption on SA/La1 hydrogel spheres at diferent temperatures can be described by Freundlich and Langmuir isotherm models.Te R 2 value of the Langmuir model is closer to 1, and the maximum

Adsorbent
Experimental value Pseudofrst-order kinetic model Pseudosecond-order kinetic model q e (mg•g −1 ) q e (mg•g  The pseudo-first-order kinetic equation The pseudo-second-order kinetic equation Advances in Materials Science and Engineering adsorption capacity is 20.08 mg•g −1 .Tese results indicated that the Langmuir model more suitably described this system and its adsorption process belongs to single molecular layer adsorption.

Conclusions
SA/Lan hydrogel spheres were synthesized via ionic crosslinking, and their adsorption capacity for CR was investigated at various La 3+ concentrations, with the highest performance observed for spheres containing 1 wt.% La 3+ .Te efects of adsorption time, dosage, and temperature on CR adsorption by SA/Lan hydrogel spheres were examined, with optimal performance achieved using 2.0 g•L −1 of spheres, at an initial CR concentration of 30 mg•L −1 , and reaction temperature of 25 °C, for a 60 min adsorption time with an adsorption capacity of 10.81 mg•g −1 .Te adsorption process of SA/Lan hydrogel spheres was observed to follow both the pseudosecond-order kinetic model and the Langmuir adsorption isotherm model.By this, we try to explain a possible chemisorption mechanism.Te Langmuir model better fts the experimental data than the Freundlich model.SA/Lan hydrogel spheres have a uniform surface structure but a loose and porous interior.Terefore, further investigation of the expansion of surface pore size is necessary to enhance their adsorption performance.Tere is limited research on materials for adsorbing CR, and compared to other studies, SA/Lan's low cost, environmental friendliness, and enormous potential make it a suitable dye adsorbent for the industry.

Figure 1 :
Figure 1: Te outside surface of the SA/La1 hydrogel sphere.

Figure 5 :
Figure 5: Efect of adsorption time on adsorption capacity of SA/La1 hydrogel spheres.

Table 2 :
Fitting parameters of adsorption kinetics simulation.