Bisphenol A Adsorption from Aqueous Solution Using Graphene Oxide-Alginate Beads

In this study, the potential of graphene oxide-alginate beads (GO-AB) as an adsorbent for bisphenol A (BPA) removal from aqueous solution was investigated. GO was first prepared via modified Hummers’ techniques and an aerogel alginate bead with embedded GO was prepared using an extrusion dripping method, where calcium chloride was utilized as a curing agent. The physicochemical characteristics of GO-AB were investigated using XRD, FTIR, BET, TGA. The results revealed that crystal structure and the surface groups of GO and alginate were retained upon the formation of GO-AB. A batch adsorption testing was carried out as a function of pH (3, 7, and 9), contact time (up to 420 min) and initial concentration of BPA (50—200 mg·L−1). The adsorption rate was typically faster at the beginning of the adsorption process and started to level off after 180 min. AB and GO-AB had better adsorption performances at neutral condition (pH 7) than alkaline and acidic environments due to repulsive electrostatic interaction between BPA and the adsorbent surface’s charge. The sorption kinetic data was observed fitted to the pseudo-second-order kinetics model (R2 > 0.98) and obeyed the Freundlich isotherm model adsorption behaviour compared to Langmuir. However, the RL value of Langmuir model is between 0 and 1, which implies a favourable adsorption process. The maximum BPA adsorption capacity for AB and GO-AB was found to be 250.00 and 384.62 mg·g−1, respectively, indicating that GO-AB is a promising adsorbent for BPA removal from aqueous solution. In addition, the adsorption efficiency of GO-AB exhibited more than 80% in the six-consecutive adsorption–desorption cycles of experiments, further confirming the excellent reusability of GO-AB.


Introduction
Bisphenol A (BPA) is among the most utilized monomer in the production of various polymers such as epoxy resins, polycarbonates, polysulfone (PS) resins and other types of plastics [5,47,55]. It is a potentially harmful substance as it can cause endocrine disruption due to weak estrogenic, antiandrogenic, and antithyroid activities [37]. Besides industrial wastewater, the contamination of BPA has been found in other water sources such as surface water and groundwater [43]. As a result, it is essential to control BPA contamination in water resources.
Owing to the hydrophobic properties BPA, adsorption is considered the most appropriate technology to remove BPA from aqueous sources [6,40,54]. Adsorption is an adhesion process of atoms, ions, or molecules from liquid, gas or dissolved solid, which creates an adsorbate film on the surface of the adsorbent [48]. It is one of the water remediation technique that has been widely utilised due to the versatility of the process, the higher removal rate of chemical pollutants, simple setup, relatively low cost, simple operation, and produces minimal harmful secondary products [10]. However, the properties of adsorbent are vital in enhancing adsorption performances. The adsorbents should possess high adsorption capability, rapid adsorption rate, and specific surface reactivity.
Sodium alginate is a natural polymer or polysaccharide with abundance of carbonyl groups, hydroxyl groups and oxygen atom. As a result, alginate possesses many distinct characteristics such as highly hydrophilic, biodegradable, and gel-forming abilities making it suitable to be utilized as an adsorbent [35]. Many studies have confirmed that the high removal performance of alginate for chemical pollutants mainly depends on the high content of carboxyl acid groups [53]. Alginate is also known as an eco-friendly adsorbent that can be easily cross-linked and hardened with ions such as calcium ions [16]. However, it tends to have weak mechanical strength, particularly as a self-standing material and low thermostability. This has significantly restricted the application of alginate-based materials as adsorbent. Physical or chemical modification such as surface grafting and cross-linking are typically required to boost the applicability of alginate as an adsorbent.
Graphene oxide (GO) is a nanoscale carbon that was firstly isolated via a micromechanical cleavage. It can be synthesized through oxidation and exfoliation of cheap graphite, forming a monomolecular graphite layer with numerous oxygen-containing functionalities such as hydroxyl, carbonyl, carboxyl, and epoxide groups. As a result, GO exhibits many unusual and fascinating physical, chemical, thermal and mechanical properties [26]. Due to many oxygen functionalities on GO planes, it can be easily dispersed in various solvents for further modification or react with many chemical groups to enhance the properties of GO and create new functionalities [36]. GO have been widely investigated for various applications, including sensor, drug delivery and adsorbent for gas and water pollutants. In addition, due to its high theoretical specific surface area, excellent hydrophilicity and mechanical strength, GO is deemed suitable as an adsorbent. Recently, GO has been utilized to improve the performance of alginate-based material, particularly sodium alginate [16,58] and calcium-alginate [29,50] for wastewater treatment. It was observed that besides improving the thermal stability and mechanical properties, the adsorption capacity of GO-alginate-based adsorbent increased significantly [45]. This is because the alginate can act as a scaffold material and also a template for 3D porous structure, while GO acts as a reinforcing filler/linker to connect and strengthen the composite network. Yang et al. [56] prepared double network graphene oxide/sodium alginate(GO/SA) hydrogels and the adsorption of Mn(II) was noticed to be better (56.49 mg·g −1 ) as compared to other adsorbents. In additions, the GO/SA hydrogels have better thermal stability and biocompatibility due to GO and SA, respectively. Feng et al. [12] utilized sodium alginate as a template to create a 3D porous network structure with an aim to inhibit the restacking and agglomeration of rGO. The rGO-SA was then used as adsorbent for phenol, BPA and tetracycline. The adsorption of arsenic (As)) and tetracycline (TC) heavy metals on yttrium-immobilized-graphene oxide-alginate hydrogel (Y-GO-SA) was investigated by He et al. [19] and maximum adsorption capacities up to 273.39 mg·g −1 for arsenic and 477.9 mg·g −1 for tetracycline were observed.
These capacities were higher than other reported adsorbents due to electrostatic interaction, H-bonds, π − π EDA interaction, n-π EDA interaction, and cation-bonding bridge effects of pollutants with adsorbent.
The compatibility between adsorbent and sorbents is vital to enhance the adsorption performances. BPA comprises of hydrophilic hydroxyl groups and hydrophobic benzene rings, while GO consists of hydrophilic functional groups and a hydrophobic basal plane. Based on several computational molecular modelling investigations, the dominant force for adsorption of BPA is hydrophobic-stacking, while hydrogen bonding aids the binding of BPA on GO surface [9,24]. These computational studies support the use of GO as an effective adsorbent for the adsorption of BPA. In this work, sodium alginate bead (AB) and composite graphene oxide-alginate bead (GO-AB) were prepared and used to adsorb BPA from an aqueous solution. Although composite GO-alginate-based adsorbent development has been widely investigated, to the best of our knowledge, not many studies have utilized the GO-AB in beads form for BPA removal. The use of beads allows efficient recovery of the adsorbent for regeneration and reused in multiple adsorption cycles. The prepared AB and GO-AB adsorbents were first characterized followed by adsorption evaluation studies based on equilibrium and kinetics perspectives. In addition, the influence of pH, operating time, and initial concentration of BPA towards adsorption performances was investigated in this work.

Preparation of GO
GO was synthesized through oxidation and exfoliation of graphite powder based on our previous method [38,39]. The resulting GO solution was left to cool down after reaction termination before continually washed with diluted HCl and DI water. The neutral solution was then centrifuged at 10 000 rpm and 25 °C for 25 min. The supernatant obtained after centrifugation was filtered and dried in an oven at 60 °C for 24 h to collect GO powder (Fig. 1).

Synthesis of GO-AB Aerogel Beads
Extrusion dripping technique using CaCl 2 as a curing agent was utilized to produce alginate aerogel beads. First, 2.3 g of sodium alginate was added into 150 mL of distilled water and stirred vigorously for 3 h. Then, 1.5 wt% of GO suspensions was added to the sodium alginate solution and continually stirred until a homogenous solution of GO-alginate was obtained. After that, the alginate/GO solution was extruded through an 0.55 mm injection needle u into the solution of 0.1 wt % of CaCl 2 gelling solution using a syringe pump. The gap between the surface of the CaCl 2 solution and the surface of the needle was maintained at 10 cm to ensure the uniformity of the shape of the beads. After the aerogel GO-AB was formed, it was kept in the CaCl 2 solution with minimal agitation for 3 h (Fig. 2). In order to ensure complete gelation, the GO-AB was left overnight in CaCl 2 solution at 4 °C. The GO-AB was filtered, rinsed with DI water, and allowed to dry in an oven at 40 °C until constant weight. Characterization X-ray diffraction (XRD) was utilized for the study of structural characteristics of the synthesised AB and GO-AB. The XRD pattern was recorded using X'celerator detector (Rigaku) where Cu Kα was used as the radiation source. The equipment voltage and current were set at 40 kV and 40 mA, respectively. The sample was scanned within the scan range of 2θ of 5° to 90° at a scan step width of 0.05°. Fourier transform infrared spectroscopy (FTIR, Perkin Elmer) was utilized to identify organic functional groups where the scan was carried out from 600-4000 cm −1 . Thermogravimetric analysis (TGA) was performed to measure the thermal stability of adsorbents prepared using Mettler Toledo analyser. A heating rate of 10 °C·min −1 was used, and the weight loss profile was recorded at a temperature range of 25 to 1000 °C. The specific surface area and pore volume of adsorbents were analyzed using an automated gas sorption system through nitrogen adsorption and desorption system using Brunauer, Emmett, and Teller (BET) NOVA 1200e surface area and pore size analyzer (Quantachrome Instrument).

Batch BPA adsorption experiments
Adsorption capacity and BPA removal efficiency of composite GO-AB aerogels for the removal of BPA were examined using a batch experiment under a stirring speed of 150 rpm where the pH solution and BPA concentration were varied. The influence of pH was investigated by adding 0. 1 g of adsorbent with 200 mL (0.5 g·L −1 ) of 50 mg·L −1 of BPA concentration under constant stirring at 30 °C up to 420 min. The working solution of BPA was prepared by first dissolving solid BPA in ethanol due to BPA's low solubility in water. Then, it was diluted into the required concentrations using DI water in an Erlenmeyer flask. 0.1 M hydrochloric acid (HCl) or 0.1 M sodium hydroxide (NaOH) was then added to adjust the pH of BPA solutions.
The effects of initial BPA concentration were investigated using 50, 100, 150, and 200 mg·L −1 of BPA solutions. After adsorption, the supernatant was taken out by using a syringe and UV-visible spectrophotometer (Hach DR2800) at 276 nm was used to measure the concentration. The adsorption capacity, q t (mg·g −1 ) and removal efficiency (%) were determined using Eq. (1) and Eq. (2), respectively.
where the C 0 and C t are the BPA initial concentration and concentration at time t (mg·L −1 ), respectively, V is the volume of solution (L), and m is the mass of the adsorbent (g). The analysis was conducted in triplicates, and the results are depicted as mean values.
The adsorption kinetics offers vital knowledge on the adsorption rate and mechanism. This kinetic study can be further used to design and adsorption unit. Two kinetics models i.e.pseudo-first-order and pseudo-second-order were used to examine the mechanism of BPA adsorption. The linear form of the pseudo-first-order model is determined by Eq. (3): where K 1 is the first-order rate constant (min −1 ), and q e and q t are the quantities of adsorbed BPA (mg·g −1 ) at equilibrium and at time t (min). The straight-line plot of log (q e − q t ) against t gives log (q e ) as slope and intercept equal to K 1 /2.303. The linear form of the pseudo-second-order model is given by Eq. (4): where K 2 is the pseudo-second-order rate constant (g·mg −1 min −1 ) and can be determined from the intercept of the plot of t/q t versus t.

Regeneration Study
After the adsorption reaches the equilibrium condition, the regeneration study was carried out immediately. The GO-AB saturated with BPA was first collected and kept in ethanol solution to desorb the adsorbed BPA. Then, the GO-AB was washed with DI water to remove residual BPA, followed by drying at room temperature. 0.1 g of the regenerated GO-AB was then added to 200 mL of BPA solution with initial concentration fixed at 200 mg·L −1 and pH value of 7.
Adsorption study was performed for 420 min and the final BPA concentration was determined. The adsorption capacity was then calculated using Eq. (1). In order to investigate the regeneration performances of GO-AB the above procedure was repeated for five times. Figure 3 shows the resulting alginate bead (AB) and graphene oxide-alginate bead (GO-AB) formed through the extrusion process. In general, they have a uniform spherical shape where the size of GO-AB aerogel (3.20 ± 0.28 mm) was bigger than AB aerogel (~ 2.90 ± 0.25 mm). The extruded GO-AB beads were generally bigger than AB due to the high viscosity of GO-AB solution owing to the addition of GO. As the GO-AB solution was viscous, the attraction forces are stronger, forming bigger particles [15]. After drying overnight in an oven at 40 °C, the size of the aerogels decreased by 10% -20% while maintaining the spherical shape where GO-AB t has smaller particles than GO. The GO-AB shrunk to 2.35 ± 0.4 mm, while the AB shrunk to 2.6 ± 0.3 mm. The high shrunken rate of GO-AB was due to the moisture removal and water loss in GO and alginate beads and reduction of surface tension on the aerogel beads during oven-drying [44]. Typically, low drying temperature was utilised to avoid damage on the aerogel surface, shape, and nature.

Characterization of Graphene Oxide (GO), Alginate Bead (AB) and Graphene Oxide-Alginate Bead (GO-AB)
The XRD patterns of graphene oxide (GO), alginate bead (AB) and graphene oxide-alginate bead (GO-AB) are shown in Fig. 4. The characteristic diffraction peak of GO was observed at 2θ = ~ 10°, which corresponded to the 002 plane and consistent with others finding [22,36,51]. The peaks indicate the existence of an oxygen-containing group and further confirmed that the graphite was fully oxidized into GO. Sodium alginate is typically amorphous with a distinct diffraction peak at 2θ = ~ 13°. As the alginate beads (AB) has been crosslinked with calcium chloride, the crystallinity of AB was observed to increase, which further confirms the interaction between sodium alginate and calcium chloride [17]. As for GO-AB, the diffraction pattern was found to be consistent with the XRD pattern of AB where no distinct difference for alginate bead (AB) and after GO addition (GO-AB). This confirms that GO was evenly dispersed and the addition of GO has minimal impact on the crystallinity of GO-AB [49]. This also indicates that excellent intermolecular interaction between GO and AB, which provides good miscibility for the preparation of GO-AB aerogels [23].
The FTIR spectra of GO, AB, and GO-AB are presented in Fig. 5 and the spectra are typically comparable to the ones previously reported [22,23,36,44]. In general, the wide band spectra in the range of 3600-3100 cm −1 indicate the presence of hydroxyl group peaks, and the lower wavelengths present oxygen-containing groups, such as carbonyl and epoxides [8]. The characteristic peaks of GO were observed at 3353, 1714, 1623, and 1094 cm −1 , which validates the existence of hydroxyl (O-H) stretching vibration, carboxylic group (C=O) stretching vibration, C=C stretching mode of sp 2 network, and C-O group stretching vibration, respectively. This confirmed the successful of oxidation and exfoliation of graphite to GO [36].   [22]. The C-H stretching vibration observed for AB at 939 and 884 cm −1 signifies uronic acid and mannuronic acid, respectively. The FTIR spectra of GO-AB shows the presence of both characteristics peaks of AB and GO, with no apparent changes as compared to AB. This further confirms that GO-AB was successfully prepared in this work. Figure 6 displays the thermal stability of GO, AB, and GO-AB at room temperature until 500 °C. GO had better stability than AB and GO-AB, particularly at the beginning of the heating process. GO lost only 10% of its mass at a temperature below 180 °C and up to 60% from 180 to 200 °C. In comparison, AB and GO-AB were not stable and started to lose mass rapidly at the beginning of the heating up process up to 80% and 60%, respectively. The weight losses at the beginning of analysis was mainly due to the evaporation of free water from the AB and GO-AB [11]. As the temperature increased up to 220 °C, the observed weight losses were probably due to the removal of hydrated water. Then, the alginate chains start to break and possible disintegration of GO fractured and GO disintegrated. At approximately 300 °C, the mass loss of AB and GO-AB started to stabilise, giving a total mass loss of 83% and 74%, respectively. It can be concluded that GO-AB was more stable than AB, which suggests that the movement of AB chains was hindered by strong electrostatic interaction with GO leading to better thermal stability of GO-AB.
The N 2 adsorption-desorption isotherm and measurement of AB and GO-AB and summarises are shown in Fig. 7 and Table 1. The adsorption and desorption of N 2 increased rapidly, and both the AB and GO-AB isotherms can be classified as type IV isotherm based on the International Union of Pure and Applied Chemistry (IUPAC) classification [21]. Type IV isotherm is normally for the ordered mesoporous materials i.e. materials with pore diameter between 2 and 50 nm, which is consistent with the average pore diameter obtained in the work [11]. The BET surface areas of AB and GO-AB obtained in this work were 51.28 m 2 ·g −1 and 7.19 m 2 ·g −1 , respectively. The BET surface area of GO-AB was comparatively higher than previous reported graphene oxide-sodium alginate beads (0.2 m 2 ·g −1 -2.6 m 2 ·g −1 ) [56] and others composite-sodium alginate-based beads such as activated carbon-AB (2.56 m 2 ·g −1 ) [27] and carboxymethyl cellulose-AB (2.163 m 2 ·g −1 ) [46].
Although the surface area of adsorbent reduced with the addition of GO, the pore diameter shows a different trend. A bigger pore diameter was observed for GO-AB (24.52 nm) as compared to AB (12.14 nm), which was differed with the findings by Fei et al. [11] where he observed a decrement of pore diameter after the addition of GO. Typically, adsorbent with larger pore size is preferable as it allows the capturing of larger adsorbates [7]. As the pore size of GO-AB is larger than AB, GO-AB might have better performances than AB. The increased of pore diameter of GO-AB beads might be caused by several factors such as pore architecture and threedimensional structure of GO-AB.

Effects of Initial pH Solution Towards BPA Removal
The pH of the aqueous solution can significantly affect the adsorption performances as it could alter the charge density of the adsorbent surface and the concentration of dissolved ions in the solution [57]. The effect of pH towards the removal rate and adsorption capacity of 50 mg·L −1 BPA was investigated at 30 °C. Figure 8 shows the adsorption performances of BPA at different pH values for AB and GO-AB. The removal efficiency of BPA increased with time and nearly reached a plateau after approximately 180 min. This is assumed as equilibrium time at which adsorption occurred. At the start of adsorption, the removal efficiency was greater owing to the high active sites' availability of the adsorbent. Then, it decreases with time because of saturation of BPA and reduction of the adsorbent active sites, which caused removal efficiency declination [1]. By comparing the removal efficiency of BPA for AB and GO-AB at each pH value, GO-AB adsorbed more BPA than AB, and consequently leads to higher BPA removal rate up to 85% after 420 min. This is because GO helps to increase the interactions with the benzene rings in the BPA structure through π-π stacking, n-π stacking, and hydrogen bonding (see Fig. 1). In addition, the high BET surface area and pore volume of GO-AB not only enhance the transport of BPA into the porous GO-AB, but also provide more active sites to adsorb BPA.
The adsorption capacity of GO-AB was greater than AB, where the maximum adsorption capacity was observed at pH = 7 (83.33 mg·g −1 ) and reduced to 72 mg·g −1 and 65.87 mg·g −1 at acidic (pH = 3) and alkaline (pH = 9) environments, respectively. Low adsorption capacities were observed at low and high pH values due to the competitive adsorption between H + , OH − ions, and BPA molecules. At low pH, the protons (H +) will be integrated with the carboxyl groups (COO −) of the GO, hindering the n-π stacking and hydrogen bonding interactions between the GO-AB and BPA. In contrast, the rapid decrease of adsorption capacity at a pH value of 9 might be caused by the adsorption inhibition in the strong alkaline environment.
BPA is a neutral molecule and forms divalent anions at around pH 9.56 [6]. The pKa of BPA is 9.6 -10.2, and therefore, BPA can be ionized in pH from 9-10, which increases the formation of bisphenolate anions [31]. Li et al. [30] investigated the effects of pH towards adsorption of BPA using HTAB-bentonite and observed a slight increase of BPA between pH 4-7, while at pH > 7, a significant decrease of BPA removal was observed. Similar findings have been observed by Guo et al. [18] where the adsorption capacity of BPA reduced significantly when the pH value is greater than 7. According Liu et al. [32], the molecular form of BPA started to deprotonated at pH 8.0-9.0, which caused the ionization of BPA molecules into mono-or divalent anions. As a result, a repulsive electrostatic interaction of organic oxygen-containing functional group or negatively charged adsorbent surface was formed with bisphenolate anion [6,42]. In addition, the alkaline solution might contain more sodium cations [11], which can affect the surface properties of AB and GO-AB and therefore, reduces the adsorption capacity. At lower pH, there might also be an excess of H + ions competing for adsorption sites. Hence, this indicates that the adsorption of BPA should be conducted in a neutral environment. The fast adsorption rate and high adsorption capacity demonstrated that GO-AB was an efficient adsorbent for the adsorption of BPA.

Effects of Initial Concentration of Bisphenol A (BPA) Solution
The effect of initial BPA concentration on the BPA removal was evaluated using 0.1 g of adsorbent in 200 ml of BPA solution and the solution was kept at pH of 7. In Fig. 9, it can be seen that as the initial concentration of BPA increased, the removal efficiency decreases as more active sites of adsorbent is used to adsorb high concentration of BPA. At 50 mg·L −1 , the removal efficiency was the highest due to the existence of more accessible surfaceactive sites in AB and GO-AB as compared to the quantity of BPA that needs to be absorbed. As the BPA concentration increases, the active sites or number of pores in AB and GO-AB adsorbent were insufficient to adsorb more BPA compound and became saturated. As a result, the BPA could not be taken by the adsorbent, thus, reducing the removal efficiency of the adsorbent [20]. These results were in accordance with other previous works [31,34]. It should be noted that when the BPA concentration was higher than 100 mg·L −1 , the removal efficiency of BPA (%) was not substantially influenced by the initial BPA concentration. This might indicate that the adsorbent surface reaches the saturation point. In terms of the adsorbent's equilibrium capacity, it can be seen that it is positively correlated to the initial BPA concentration in the aqueous solution. The adsorption capacity of BPA for AB and GO-AB increased from 66 to 203 mg·L −1 , and 85.33 to 267.33 mg·L −1 , respectively, with a rise in the initial BPA concentration from 50 to 250 mg·L −1 . The key reason for this is the increase in driving force to overcome the mass transfer resistance between aqueous and solid phases [3,13,41]. Overall, the removal efficiency of GO-AB was better than that of AB due to the porosity of GO-AB adsorbent and availability of oxygen-containing functional group on the surface of GO, which offered additional strong surface complexion of BPA [4].

Adsorption Mechanism of BPA onto GO-AB
From the FTIR study, GO-AB contains hydrophobic graphene basal planes with aromatic rings and hydrophilic groups (carboxyl (O=C-O) and hydroxyl (-OH)). Owing to the hydrophobic phenyl groups and hydrophilic hydroxyl groups on BPA structure, the interaction of adsorbate BPA and GO-AB adsorbent can affect the overall performances of adsorption. Figure 10 illustrates the adsorption mechanism of BPA onto GO-AB based on the characterization and adsorption performances. There are several probable mechanisms for BPA adsorption such as pore filling, formation of hydrogen bond, electrostatic attraction, π-π interaction, n-π interaction and hydrophobic interaction [14]. According to the BET analysis, the adsorption of BPA into the micropores of GO-AB can occur through the pore filling mechanism as the pore size of GO-AB is in the mesoporous range. In addition, the abundance of -OH functional group on the surface of GO-AB can lead to the formation of hydrogen bonds with the C-H and -OH on BPA molecules. The π-π interaction could also be established between the benzene rings contained in GO-AB and BPA [25]. Based on the pH study, the adsorption performances were also observed to be significantly affected by the pH of aqueous BPA. At low pH, the phenolic hydroxyl group's in the structure of BPA can be protonized, causing electrostatic repulsion with the positively charged surfaces of GO-AB. Under the alkaline condition, the acidic functional groups (-COOH, C=O) on GO-AB could also behave as electron acceptors and forms π-π electron donor-acceptor (EDA) interactions [52]. The surface of GO-AB is then transformed into a negative charge to maintain the electrostatic repulsion. Hydrophobic interaction could also leads to the adsorption of BPA onto GO-AB due to the hydrophobic nature of BPA.

Adsorption Kinetics
The results of pseudo-first-order (PFO) and pseudo-secondorder (PSO) adsorption kinetics data fitting are listed in Fig. 11 and Table 2. The linear form of the PFO model is generally appropriate for lower concentrations of solute. It can be seen that the values of R 2 for PFO model are lowered for most of the adsorption data, which indicates that the BPA adsorption onto AB and GO-AB does not obey the PFO kinetic model. PSO model includes three main stages in adsorption i.e., film diffusion, intraparticle diffusion, and equilibrium stage. The rate of PSO reaction is dependent on the amount of solute adsorbed on the adsorbent surface and the amount adsorbed at equilibrium. This indicates that the rate-limiting step in PSO is the chemisorption-surface adsorption and the adsorbate removal rate is highly dependent on the physicochemical interactions between the two phases. In general, the correlation coefficients show that the adsorption kinetic data for BPA onto AB and GO-AB were better associated with the PSO model, where the value of R 2 is greater than 0.99. Several works on the adsorption of BPA onto various adsorbents noticed a similar trend [2,52], [28]. According to the PSO model, boundary layer resistance is not the rate-limiting step. The external resistance model cannot satisfactorily explain the adsorption mechanism. Thus, it can be assumed that the overall mechanism of BPA adsorption was chemisorption, and the rate-controlling step involves valence forces via the exchange and share of electrons between BPA molecules and the adsorbent.

Adsorption Isotherm
Adsorption isotherm is crucial to establish the adsorption behaviour and to forecast the favourability of the adsorption system. The adsorption data were then analyzed by fitting to Langmuir and Freundlich isotherm models. Langmuir isotherm model presumes that maximum adsorption is corresponded to the formation of monolayer adsorbate on the adsorbent surface due to chemisorption. However, when adsorption reaches equilibrium, no further adsorption can happen due to limited adsorption sites. At this point, the energy of adsorption is constant, and no transmigration of adsorbate occurs on the surface. The linearized form of the Langmuir isotherm model can be expressed in Eq. (5): where C e (mg·L −1 ) is the concentration at equilibrium, Q e (mg·g −1 ) is the amount of adsorption at equilibrium, Q m (mg·g −1 ) is the maximum monolayer capacity of adsorbent, and K L (L·mg −1 ) is the Langmuir adsorption constant. A plot of 1/q e against 1/C e should be linear if the adsorption follows Langmuir behaviour, with a slope of 1/K L Q m and an intercept of 1/ Q m . Dimensionless separation factor, R L is also an essential parameter in Langmuir isotherm that can be calculated as Eq. (6): where R L implies the Langmuir isotherm to be either irreversible (R L = 0), favourable (0 < R L < 1), linear (R L = 1) or unfavourable (R L > 1). The Freundlich isotherm model is an empirical correlation depicting the adsorption of solutes from liquid to solid surface and presumes that multilayer adsorption happens on a heterogeneous surface through   (7): where K f and n are Freundlich constant and heterogeneity factor, respectively, which are determined by the intercept and slope of the linear plot (Fig. 12). It can be seen that as the BPA concentration increases, the equilibrium adsorption capacity of both AB and GO-AB adsorbents rises gradually owing to an escalation of BPA chemical potential. The estimated model parameters are summarised in Table 3. Owing to the high correlation coefficient (R 2 > 0.98), the Freundlich isotherm seems to be a favourable model to describe the adsorption activity, which is initiated by the multi-molecular layer adsorption process. However, the R L values of AB and GO-AB obtained through Langmuir model are in the range of 0 to 1, which indicated that the adsorption of BPA on both adsorbents is preferential adsorption. In addition, based on the Q max obtained from the Langmuir isotherm, GO-AB fabricated in this work possessed higher adsorbability (384.62 mg/g), exceeding other adsorbents investigated, making it a potential candidate to remove BPA.

Regeneration Study
Regeneration and recycling of adsorbent are essential for practical applications and commercialization feasibility. Figure 13 shows the adsorption capacity of GO-AB after six adsorption − desorption cycles. In general, the adsorption capacity of GO-AB was observed to decrease with the increasing number of cycles. This might be due to the incomplete removal of BPA from the pores of the GO-AB aerogels, which cause a reduction of the adsorption active (7) log q e = 1 n f log C e + log K f sites [33]. After six cycles, the adsorption of BPA was observed to reduce from 268.22 mg·g −1 to 211.11 mg·g −1 , a decrease of 21.30%. However, the adsorption capacity is still considered as high as compared with other adsorbents. The use of ethanol solution for desorption of BPA from saturated GO-AB also might help to maintain the performance of GO-AB adsorbent by assisting the removal of BPA from GO-AB adsorbent surface.

Conclusion
In this work, alginate-based adsorbents i.e., alginate bed (AB) and graphene oxide-alginate bead (GO-AB), were successfully prepared and characterized. In general, the addition of GO into AB significantly improved the physicochemical properties of the alginate-based adsorbent through composite networking reinforcement, where GO acts as a crosslinker to strengthen the GO-AB structure. The thermal stability of GO-AB was found to improve due to the suppression of AB chain's mobility by strong electrostatic interaction with GO. The BET result shows that  the adsorbent prepared is mesoporous, with an increase of pore diameter for GO-AB. The adsorption of BPA on AB occurred rapidly through the electrostatic interaction and highly depending on the pH of the aqueous solution.
The adsorption behaviour fitted the pseudo-second-order kinetics model and Freundlich model, respectively. The maximum adsorption capacity of AB and GO-AB were 250.00 and 384.62 mg•g −1 , respectively. Owing to the high adsorption properties, easy separation of aerogel beads, and excellent recyclability, GO-AB is considered a promising adsorbent for removing BPA from an aqueous solution.