Engineering of Calcium Alginate- PANI@Sawdust Wood hydrogel Bio-beads for the Removal of the Sulfonate Groups-Containing Orange G dye from Aqueous Solution

The aim of this work is to investigate the adsorption performance of orange G (OG) dye from aqueous solutions employing PANI@sawdust biocomposite enrobed by calcium-alginate biobeads (Alg-PANI@SD). The as-prepared adsorbent was characterized by scanning-electron-microscopy (SEM), X-ray energy dispersive spectroscopy (EDS) and Fourier transforms infrared (FT-IR) spectroscopy, and used to remove Orange G dye from water. batch tests were performed as a function of adsorbent dosage, pH, contact time, interfering ions and initial OG dye concentration. Experimental results show that the kinetic model of pseudo-rst-order (PFO) and Freundlich isotherm provided a good tting of the whole experimental data. The results revealed that the as-prepared tricomposite Alg-PANI@SD, has the potential to be applied as a low-cost adsorbent for the adsorption of OG dye from aqueous media.


Introduction
The rapid in ux of people growing creates a clean-water challenge, that is already being confronted in the world (Chang et al. 2016; Pradhan et al. 2020). The water pollution threat could be caused by industry such as the release of dyes into the environment in the form of wastewater (Salam et al. 2017;Yeamin et al. 2021). The occurrence of dyes, even at low concentrations, in water adversely affects human health and is toxic to the life of microorganisms and their biological systems (Zheng et al. 2019). Various dyes resist biodegradation due to their aromatic structure. Among the dyes, Orange G (OG) is generally used in textile tanneries. OG dye is one of the azo dyes and could be cause irritation to the skin, eyes and mucous membranes, respiratory organs and cancer ). Therefore, the elimination of dyes from contaminated water is necessary before their release into the natural environment. For this purpose, several techniques have been developed, such as chemical precipitation ( A large collection of biosorbents or biomaterials from natural sources like almond shells, walnut shells, bagasse sugarcane, sawdust, and biomass made up of living, or dead microorganisms, and biopolymers were employed as adsorbents to adsorb contaminants from aqueous solutions, owing of their low cost, availability and their adsorption capacity to treat wastewater (Hsini et al. 2020). However, all these materials provide a simple uptake of contaminants. In order to anhance the adsorption performance of biomaterials, conducting polymers have been increasingly used for their adsorption capability and the polyaniline (PANI) is the most e cient among them (Laabd et al. 2022). Because of its porous structure, regenerability and ion exchange capacity, as well as a large number of amine/imine groups.
Polysaccharide alginate is commonly obtained from brown seaweed. As a biopolymer, sodium alginate is largely applied in pharmaceuticals, cosmetic and hygienic products, textiles, food additives and dyes (Rocher et al. 2008). This polymer has very long-chain molecules with active carboxylic groups that can be straightened along with each other in a way to be in line with the capability to create a leaf material.
One of the tunable qualities of alginate is the capacity to build a hydrogel (Dąbrowski et al. 2005; Javanbakht and Sha ei 2020). An aqueous alginate solution is easily crystallized into a hydrogel, plus double metal cations such as Ca 2+ .
The main purpose of the current work is to the encapsulation of PANI@SD biocomposite in calcium alginate biobeads as a new eco-friendly adsorbent for OG dye adsorption from aqueous media. The engineered biobeads were characterized by SEM, EDX and FT-IR analyses. Batch testts were carried out for Orange G molecules removal experiments using the prepared biobeads. The effect of contact time, adsorbent doses, pH, initial OG dye concentration, and the co-interfernig ions, kinetics, isotherm studies were evaluated.

Engineering of Alginate-PANI@SD biobeads
The Sawdust was washed and then air-dried for 4 days. The dried sawdust was crushed and sieved. This SD powder was washed with distilled water and then dried in an oven at 80°C for 24h. For the fabrication of the Alg-PANI@SD biobeads, the 1:1 weight ratio of the synthesized PANI@SD biocomposite and sodium alginate was added to 50 ml distilled water and kept under stirring for 3 h until the formation of a viscose gel. In order to form the biobeads, we added dropwise using a syringe with a 4 cm distance between the needle and the solution surface, the gel to the solution of 0.5 M calcium chloride 0.5 M as cross-linker. The prepared alginate-PANI@SD biobeads were stirred for 30 min, then the nal biobeads were washed with distilled water.
The point zero of charge (pH PZC ) was de ned by a conventional technique, which involves preparing 60 ml solutions of KNO 3 (0.03 M) and modifying their pH to accurate values (2 -10). Next, 0.45 g of biobeads were added to every solution. The mix was kept under stirring at ambient temperature for 24 h before calculating the pH nal . The pH PZC was computed from the curve of pH nal -pH initia l = f(pH initial ).

Batch experimental studies
In a series of 100 ml beakers, 50 ml of aqueous solutions of OG with speci ed concentrations were added into a de ned amount of biobeads in order to conduct the adsorption measurements. The mix was stirred at 120 rpm and under a constant temperature of 298K for 180 min. At a xed time, the stirring was stopped, and the samples were centrifuged. The OG dye residual concentration in the reaction mix was evaluated via UV2300 spectrophotometer. Adsorption tests were performed by changing initial solution pH, contact time, adsorbent dose, co-interfering ions and initial OG concentration for adsorption kinetics and adsorption isotherm.
Furthermore, the dye elimination e ciency, i.e., percent of adsorption, was examined as: C 0 is the initial OG dye concentration (mg/L), C t is the concentration of OG dye at time t, V is the volume of OG dye containing-solution (L), and m is the mass of adsorbents in g. Experiments at the equilibrium were carried out via the above process with a broad array of initial dye concentration. The time of contact between solid-liquid was 180 min, which was higher than the equilibrium time.

Characterization of Alg-PANI@SD biobeads
3.1.1. Texture morphology SEM investigation associated with EDX was gured out the morphologies and chemical composition of SD, PANI@SD and Alg-PANI@SD biobeads as shown in Fig. 1 (a-c). As shown in Fig. 1 (a-c), the SEM images presente that the SD particle has a heterogeneous and porous texture, while the SEM image of the PANI@SD biocomposite suggests that the sawdust surface has been successfully enrobed by PANI ( ) ( ) particles, conducting to the generation of pores within the PANI@SD composite structure. These pores may ease the diffusion of sulfonate groups (-SO 3 Na)-containing OG molecules into the PANI@SD surface. Therefore, SEM image of the Alg-PANI@SD biobeads depicts that the PANI@SD particles were inside a layer formed by alginate and calcium alginate ions allowing the construction of spherical biobeads.
The elemental compositions obtained from the EDX analysis are presented in g. (d-e). The EDS analysis spectrum of SD (Figure 1 (d)) depicts that the sawdust contains mainly carbon and oxygen elements.

In uence of adsorbent dosage and Effect of initial pH
To clear up the adsorption process, it is crucial to investigate the parameters in uencing the adsorbateadsorbent interface. The adsorbent dose effect of Alg-PANI@SD biobeads on the removal of OG dyestuff was studied and presented in Fig. 3. From the Fig. 3, it is obvious that the OG dye adsorption e ciency depends on the amount of Alg-PANI@SD biobeads. The percentage of OG dye removal increases as the adsorbent dosage ratio (from 0.5 to 4 g L −1 ) increases, which is due to the raise in the amount of available binding sites ( to 10 and illustrated in Fig. 3(b-c), respectively. The PZC point was found to be 5.5, this nding means that the Alg-PANI@SD biobeads surface charged positively at pH values less than the PZC point (pH<PZC) and vice versa. Mainly, the maximum OG dye removal e ciency (94.7%) was reached at pH=2. As far, the OG dye removal favorability at pH<PZC could be suggested as the result of electrostatic attractions between sulfonate groups (-SO 3 Na)-containing-OG molecules and the Alg-PANI@SD biobeads surface. However, at pH values above the PZC value, the removal of the OG dye gradually decreases as pH increases. This lower a nity for the OG dye may be related to repellency forces. In addition, the pH value 5.0 was selected as an optimal value for further adsorption experiments.

Effect of interfering ions
To gure out the proportional aspects of various competing ions on sulfonate groups (-SO 3 Na)containing OG adsorption on Alg-PANI@SD biobeads, the in uence of co-existing Cl − , SO 4 2− , CO 3 2− and NO 3 − interfering ions were assessed at initial concentrations (100mg L −1 ) (Abdellaoui et al. 2021). Fig. 4 depicts the interfering ions effect on the OG dye removal percentage by Alg-PANI@SD biobeads. It was assigned that the elimination of OG dye was not in uenced by the presence of SO 4 2− , Cl − and NO 3 − ions.
In contrast, the presence of CO 3 2− ions affects the removal e ciency of OG dye. This carbonate competing trend could be attributed to the producing of hydroxide ions by hydrolysis, which in turn contest for SO 3 Na-containing OG binding sites or decrease the solution acidity (Vickers 2017).

Adsorption kinetics
The adsorption kinetic of OG dye onto Alg-PANI@SD biobeads was performed to investigate the relationship between contact time and adsorption capacity for insight into the OG molecule's adsorption process and to provide relevant conditions for the e cient application of Alg-PANI@SD biobeads adsorbent (Mahi et al. 2021). As can be seen in Fig. 5, the OG dye starts to adsorb rapidly onto Alg-PANI@SD biobeads within the rst contact hour owing to the availability of OG adsorption vacant sites.
Thereafter, the OG dye removal rate becomes slower until attaining the equilibrium adsorption after 180 min, as a result of the reduction in the number of sulfonate groups (-SO 3 Na)-containing OG dye-binding sites. Pseudo-rst-order (PFO) and pseudo second-order (PSO) kinetic models were empolyed to assess the reaction kinetics (Zheng et al. 2012;Amjlef et al. 2021). The nonlinear expressions and the tting parameters data of PFO and PSO kinetic models are presented in Table 1 and Fig. 5 , was found to be close to the experimentally-determined one (4.301 mg g −1 ), con rming that the OG dye adsorption process was better described by this kinetic model.

Adsorption equilibrium
The equilibrium adsorption isotherm is indispensable to gure out the distribution of OG dye molecules at the Alg-PANI@SD surface, when the adsorption gets to the equilibrium (Feng et al. 2020). Therefore, the analysis by Langmuir and Freundlich isothermal models was carried out (Salam et al. 2017), their nonlinear equations and equilibrium data were displayed in Table 2, and their tting curves are plotting in Fig. 5(b). According to the correlation values, this gure discloses that the OG dye removal onto the Alg-PANI@SD biobeads, was well tted with the Freundlich model (R 2 =0.991), more than the Langmuir one (R 2 =0.949). This assumes that the OG dye removal onto the heterogeneous as-synthesized biobeads surface was carried out in multilayer.

Conclusion
In this current paper, Alg-PANI@SD biobeads were prepared and applied as were e cient adsorbent to remove OG dyestuff from the aqueous phase. The adsorbent was characterized for the analysis of the surface properties. It was concluded that OG adsorption was affected by several operating factors like adsorbent dosage, pH, contact time and initial OG dye concentration. The adsorption equilibrium was achieved within 180 min at pH 5.0. The OG dye adsorption process on the Alg-PANI@SD biobeads was followed by pseudo-rst order kinetics. The isotherm modeling revealed an appropriate tting of the Freundlich isotherm. The electrostatic interactions between sulfonate groups (-SO 3 Na)-containing OG dye molecules and Alg-PANI@SD biobeads surface played a primordial role in the adsorption prcocess. In view of the strong adsorption ability of the adsorbent Alg-PANI@SD biobeads over OG dye, easy separability, the present study submits that Alg-PANI@SD biobeads is a novel, effective, and economically feasible for the adsorption of dyes from wastewater containing OG dyestuff.

Declarations Ethics approval
This study was approved by the research ethics committee of Ibn zohr university.

Consent to participate
All the authors participed in this article     Effect of interfering ions on OG dye removal by Alg-PANI@SD biobeads: adsorbent dose = 2.5 g L-1; pH= 5.0; 10 mg L-1 OG dye concentration; T = 298 K. Figure 5