Adsorptive Removal of Bisphenol A by Polyethylene Meshes Grafted with an Amino Group-Containing Monomer, 2-(Dimethylamino)ethyl Methacrylate

Abstract

The adsorptive removal of Bisphenol A (BPA) with the PE meshes photografted with 2-(dimethylamino)ethyl methacrylate (DMAEMA) was performed by varying the grafted amount, pH value, BPA concentration, and temperature, and the adsorption performance was correlated by the equilibrium, kinetic, and isotherm models. In addition, the regeneration of DMAEMA-grafted PE (PE-g-PDMAEMA) meshes was discussed from the repetitive adsorption/desorption process. The adsorption capacity had the maximum value at the grafted amount of 2.6 mmol/g and at the initial pH value of 8.0. The increase in the protonation of dimethylamino groups on grafted PDMAEMA chains and the dissociation of phenol groups of BPA present in the outer solution during the adsorption process results in the increase in BPA adsorption. The adsorption process followed the pseudo second-order equation. The BPA adsorption was enhanced by increasing the BPA concentration and the equilibrium data fit to Langmuir equation. The adsorption capacity stayed almost constant with the increase in the temperature, whereas the k₂ value increased against the temperature. These results comprehensively emphasized that BPA adsorption occurred through the chemical interaction or ionic bonding of a BPA anion to a terminal protonated dimethylamino group. Desorption of BPA increased by increasing the NaOH concentration and BPA was entirely desorbed at more than 20 mM. The cycle of adsorption at pH 8.0 and desorption in a NaOH solution at 100 mM was repeated five times without loss or structural damage. These results indicate PE-g-PDMAEMA meshes can be used as a regenerative adsorbent for BPA removal from aqueous medium.

1. Introduction

Water is an essential and vital resource for life and ecosystems, as well as for social and economic development. In many developed countries, 76–90% of wastewater is treated before being discharged into the aquatic environment. However, significant energy is required for wastewater treatment. About 24% of total whole greenhouse gas are emitted in the industrial processes, such as wastewater treatment [1]. Contrary to this, in developing countries, more than 50% is directly discharged into the aquatic environment without any, or with some, treatments [1,2]. Therefore, water resources are frequently contaminated and gradually deteriorated due to anthropogenic discharge of inorganic and organic chemicals.

The presence of organic contaminants, particularly endocrine disrupting chemicals (EDCs) is a growing concern also from the viewpoint of environmental protection. The EDCs have the potential to alter the hormonal and homeostatic systems of living organisms [3]. Among the EDCs, Bisphenol A (BPA) is most pervasive. BPA has been used as an additive or monomer in the manufacture of epoxy resins and polycarbonate plastics. Since the use of BPA provides essential physical properties, such as rigidity, transparency, and resistance to polymeric products, it has been used in food contact materials such as bottles, tableware, and cookware [4]. Also, BPA-based epoxy resins are used as protective linings for food and beverage cans. In addition, BPA has diverse non-food related applications, such as paints, medical devices, thermal paper, printing inks, and flame retardants. BPA can leach out from products through aminolysis, hydrolysis, as well as contact with acidic food, such as tomato products. Landfill leachate can be an emission source to migrate into the aquatic environment [5]. In fact, BPA has been detected in rivers, lakes, marshes, and sediment at concentrations ranging from ng/L to μg/L level [6,7,8].

The use of BPA in the manufacture of polycarbonate feeding bottles for infants was banned by the European Commission. The specific migration limit for BPA was decreased from 600 to 50 μg/kg [9]. In addition, no migration of BPA from materials specifically addressed to come into contact with infant formula, follow-on formula, and other products intended for infants and young children has been permitted [5,9,10,11]. Given the adverse effects on humans and environment described above, more appropriate and specific technologies are required to remove BPA from aqueous media. Many investigations have been performed on BPA removal by physical, chemical, and biological processes [12,13,14,15]. Of many removal methods, the adsorption process is more frequently used than other methods in terms of fast removal of pollutants with easy operation, simple design, and less production of harmful by-products.

The adsorption process has been widely investigated for removal of BPA. Various materials have been applied as adsorbents, including clay and modified one, natural and synthetic zeolite and their modified forms, natural polymer electrolyte, activated carbon from agricultural waste, graphene, molecularly imprinted polymer, metal organic framework (MOF), nanomaterials, and composite materials [16]. Some of these adsorbents have shortcomings to be solved in practical applications, such as low adsorption capacity, low adsorption rate, difficult solid-liquid separation, and poor regeneration. In addition, compared with activated carbon and MOF, only a few studies have been published on the development of polymeric adsorbents for BPA removal [17,18,19,20].

Polyethylene (PE) can be mentioned as a suitable substrate of adsorbents, in terms of the fact that PE is insoluble in water and many organic solvents, and can be readily modified by graft polymerization. In addition, since one end of a grafted polymer chain is tethered to a substrate through a covalent bond, a grafted polymer chain is highly mobile in a good solvent. To be specific, poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) is a water-soluble polyelectrolyte with positively chargeable functional groups, and the grafted layer consisting of grafted PDMAEMA chins has a good water absorptivity. These characteristics are beneficial for the adsorption process.

We have reported some findings on removal of BPA with oxidoreductases, such as tyrosinase [21], peroxidase [22], and laccase [23]. However, since BPA undergoes the conversion into the corresponding quinone [21,22] or phenoxy radicals [23] through the enzymatic reactions, it is impossible to retrieve BPA in the original form. Therefore, we focused adsorptive removal of BPA with polymeric adsorbent materials, based on the fact that polymeric adsorbents for Cr(VI) ions were developed by the grafting 2-(dimethyl-amino)ethyl methacrylate (DMAEMA) onto the PE in our previous studies [24,25].

In this study, the DMAEMA-grafted PE (PE-g-PDMAEMA) meshes were applied to remove BPA from aqueous media. The adsorption of BPA on the PE-g-PDMAEMA meshes was investigated by varying some experimental factors, including the grafted amount, initial pH, initial BPA concentration, and temperature. Kinetic and isotherm analyses of Cr(VI) ion adsorption were conducted under variable experimental conditions. Finally, the desorption behavior was also estimated for regenerating and reusing the PE-g-PDMAEMA meshes.

2. Materials and Methods

2.1. Materials

A PE mesh (density = 0.924 g/cm³) was used as a polymer substrate. A photograph of the PE mesh used and its main characteristics are shown in Figure 1 and Table 1 in Ref. [25]. DMAEMA (monomer for grafting) and benzophenone (BP) (sensitizer) were purchased from FUJIFILM Wako Pure Chemical Corp. (Osaka, Japan). All chemicals were also used without further purification.

2.2. Photografting

The photografting was performed according to the procedure described in Ref. [25]. The PE meshes (70 mm long × 30 mm wide) were immersed in an acetone solution of BP at 0.50 w/v%. Then, acetone was evaporated to coat the strand surfaces with BP. The pH value of an aqueous DMAEMA solution at 1.0 M was adjusted to 8.0 with conc. HCl to increase the solubility of PDMAMEA. After a BP-coated PE mesh was put into a DMAEMA solution in a Pyrex glass tube, UV rays were irradiated for different times at 60 °C with a 400 W high-pressure mercury lamp. The wavelength of the high-pressure mercury lamp used ranged from 300–450 nm, including main ones at 310, 365, and 405 nm. The grafted amount was calculated using Equation (1):

$$\mathrm{Grafted} \mathrm{amount} (\mathrm{mmol}/\mathrm{g})={\displaystyle \frac{({{\displaystyle \mathrm{W}}}_{\mathrm{g}}-{{\displaystyle \mathrm{W}}}_0)/{{\displaystyle \mathrm{M}}}_{\mathrm{D}\mathrm{M}\mathrm{A}\mathrm{E}\mathrm{M}\mathrm{A}}}{{{\displaystyle \mathrm{W}}}_0}}\cdot {{\displaystyle 10}}^3$$

(1)

2.3. Determination of BPA Concentration

An aqueous stock solution of BPA at 0.50 mM was diluted to 0.05–0.25 mM. And then, the pH value was adjusted to 3.5–11 with HCl or NaOH. The BPA concentration was determined by measuring the absorbance at 278.0 nm, which is the isosbestic point of BPA. The absorbance at this wavelength depends on the concentration but is independent from the pH value. The calibration curve had a good linearity (R₂= 0.9996, log ε = 3.475 L/mol·cm).

2.4. Adsorption of BPA

The PE-g-DMAEMA meshes were cut into 2.0 cm long and 2.0 cm wide and immersed in HCl or NaOH solutions at pH 2–11 for 12 h. Then, a PE-g-PDMAEMA mesh was placed in an aqueous BPA solution with mild stirring. Unless otherwise described, the adsorption experiments were performed under the conditions where a PE-g-DMAEMA mesh was immersed in 50 mL of an aqueous BPA solution of 0.10 mM and pH 8.0 at 30 °C. The adsorbed amount, in mmol/g-grafted PE and in mmol/g-PDMAEMA, was calculated using Equations (2) and (3), respectively, based on the absorbance at 278.0 nm.

$$\mathrm{adsorbed} \mathrm{amount} (\mathrm{mmol}/\mathrm{g}-\mathrm{grafted} \mathrm{PE})={\displaystyle \frac{({{\displaystyle \mathrm{C}}}_0-{{\displaystyle \mathrm{C}}}_{\mathrm{t}})\cdot \mathrm{V}}{{{\displaystyle \mathrm{W}}}_{\mathrm{g}}^{\mathrm{cut}}/1000}}$$

(2)

$$\mathrm{adsorbed} \mathrm{amount} (\mathrm{mmol}/\mathrm{g}-\mathrm{PDMAEMA})={\displaystyle \frac{({{\displaystyle \mathrm{C}}}_0-{{\displaystyle \mathrm{C}}}_{\mathrm{t}})\cdot \mathrm{V}}{\left({{\displaystyle \mathrm{W}}}_{\mathrm{g}}^{\mathrm{cut}}-{{\displaystyle \mathrm{W}}}_{\mathrm{g}}^{\mathrm{cut}}\cdot \frac{{{\displaystyle \mathrm{W}}}_{\mathrm{g}}}{{{\displaystyle \mathrm{W}}}_0}\right)/1000}}$$

(3)

2.5. Kinetic Analysis

The BPA adsorption data were kinetically analyzed using the linear forms of the pseudo first- and pseudo second-order models [26,27,28,29], shown in Equations (4) and (5), respectively. The rate constants, k₁ and k₂, were calculated from the slope and intercept of the linearity. In addition, the value of the adsorption capacity calculated from Equation (5) was compared with the experimental value.

$$\ln ({{\displaystyle \mathrm{Q}}}_{\mathrm{eq}}-{{\displaystyle \mathrm{Q}}}_{\mathrm{t}})=\ln {{\displaystyle \mathrm{Q}}}_{\mathrm{eq}}-{{\displaystyle \mathrm{k}}}_1 \mathrm{t}$$

(4)

$${\displaystyle \frac{\mathrm{t}}{{{\displaystyle \mathrm{Q}}}_{\mathrm{t}}}}={\displaystyle \frac{1}{{{\displaystyle \mathrm{k}}}_2{{\displaystyle \mathrm{Q}}}_{\mathrm{eq}}^2}}+{\displaystyle \frac{\mathrm{t}}{{{\displaystyle \mathrm{Q}}}_{\mathrm{eq}}}}$$

(5)

2.6. Isotherm Analysis

The adsorption experiments were performed in the BPA concentration range of 0.05–0.20 mM with a PE-g-PDMAEMA mesh at the grafted amount of 2.6 mmol/g, and the equilibrium data obtained were applied to Langmuir and Freundlich equations, shown in Equations (6) and (7), respectively [30,31,32,33,34].

$${\displaystyle \frac{{{\displaystyle \mathrm{C}}}_{\mathrm{eq}}}{{{\displaystyle \mathrm{Q}}}_{\mathrm{eq}}}}={\displaystyle \frac{1}{{{\displaystyle \mathrm{K}}}_{\mathrm{L}}{{\displaystyle \mathrm{Q}}}_{\max }}}+{\displaystyle \frac{{{\displaystyle \mathrm{C}}}_{\mathrm{eq}}}{{{\displaystyle \mathrm{Q}}}_{\max }}}$$

(6)

$$\log {{\displaystyle \mathrm{Q}}}_{\mathrm{eq}}=\log {{\displaystyle \mathrm{K}}}_{\mathrm{F}}+{\displaystyle \frac{1}{\mathrm{n}}}\log {{\displaystyle \mathrm{C}}}_{\mathrm{eq}}$$

(7)

2.7. Desorption and Regeneration

A PE-g-PDMAEMA mesh (grafted amount = 2.6 mmol/g) loaded with BPA was immersed in aqueous NaOH solutions at different concentrations. The absorbance was measured to calculate the desorbed amount at time t. After the desorption experiments at a NaOH concentration of 100 M, the PE-g-PDMAEMA mesh was incubated at pH 4–5, and then immersed in an aqueous BPA solution at pH 8.0 to adsorb BPA. Each adsorbed amount remained almost 0.137 mmol/g by the adsorption process for about 5 h. The cyclic process of adsorption and desorption was repeated five times.

3. Results and Discussion

3.1. Characterization of PE-g-PDMAEMA Meshes

The photografting of DMAEMA on the PE mesh was performed at pH 8.0 and 60 °C. The amount of grafted DMAEMA increased with the irradiation time and went up to approximately 4 mmol/g by the UV irradiation for 5 h. Based on this result, the grafted amount was adjusted by varying the irradiation time [25]. The FT-IR and XPS analysis of the PE-g-PDMAEMA meshes was performed in our previous article [25]. Here, the instrumental characterization is briefly described. The FT-IR spectra of the original PE mesh, PDMAEMA, and PE-g-PDMAEMA mesh can be seen in Figure 2 in Ref. [25]. The PE mesh had some distinctive peaks at 2916 cm⁻¹ attributed to –CH₂– asymmetric stretching, at 2848 cm⁻¹ attributed to –CH₂– symmetric stretching, and at 1462 cm⁻¹ attributed to –CH₂– bending [35]. In addition, the peaks of the doublet 719 and 729 cm⁻¹ were observed, assigning to –CH₂– rocking deformation in amorphous and crystalline domains, respectively [36,37,38]. The FT-IR spectrum of PDMAEMA has some characteristic peaks of –CH₂– stretching vibration at 2941 cm⁻¹, C–H stretching vibration of the –N(CH₃)₂ groups at 2775 and 2821 cm⁻¹, –C=O stretching vibration at 1723 cm⁻¹, –C–N– stretching vibration at 1153 cm⁻¹, and –O=C–O– stretching vibration at 1263 cm⁻¹ [39,40,41]. The PE-g-PDMAEMA mesh has a spectrum containing the characteristic peaks of both PE and PDMAMEA.

Figure S1 shows the C_1s, O_1s, N_1s, and Cl_2p core level spectra of a PE-g-PDMAEMA mesh immersed in HCl and NaOH. The PE-g-PDMAEMA mesh immersed in HCl had a N_1s peak corresponding to protonated dimethylamino groups at about 403 eV. Also, a Cl_2p peak corresponding to chloride counter ions was observed. When immersed in NaOH, a N_1s peak shifted to 399 eV due to deprotonation and the Cl_2p peak disappeared [25]. These instrumental results explain the grafting of DMAEMA on the PE mesh and the protonation-deprotonation behavior of dimethylamino groups. The water absorptivity increased with increasing grafted amount, reaching 0.43 g/g at the grafted amount of 4 mmol/g, with a sharp increase in the range of 2–3 mmol/g (See Figure 5 in Ref. [25]).

3.2. BPA Adsorption Performance

3.2.1. Effect of Grafted Amount

The effect of grafted amount on the BPA adsorption was estimated at pH 8.0 and 30 °C. Figure 1 shows the changes in the adsorption capacity, initial rate of adsorption, and adsorption half-time with the grafted amount. The adsorption capacity increased with an increase in the grafted amount. The adsorption capacity in mmol/g-grafted PE had the maximum value at 2.6 mmol/g, and then gradually decreased when the grafted amount further increased. It was found that the photografting of DMAEMA makes the surfaces of the PE strands hydrophilic [42], forming a grafted layer with good water absorptivity [25]. However, since the water absorptivity was pretty low at less than 1 mmol/g, little adsorption of BPA was observed. On the other hand, the adsorbed amount was suppressed at higher grafted amounts due to the formation of grafted layers with highly dense grafted polymer structures. A similar decrease in the adsorption was observed for Cr(VI) ion adsorption on quaternized PE-g-PDMAEMA meshes [25].

The protonation of PDMAEMA was characterized by colloid titration. The pH dependence of the degree of protonation of PDMAEMA is shown in Figure 1 in Ref. [43]. On the other hand, the adsorption capacity in mmol/g-PDMAEMA had the maximum value at 2.4 mmol/g, as shown in Figure 2. The maximum value of the adsorption ratio calculated from this value was 0.097. For the adsorption experiment at pH 8.0, the pH value decreased to 6.74 at the adsorption equilibrium. At this pH value, the degree of protonation is 0.34, as seen in Figure 1 in Ref. [43]. This indicates that 28.5% of the protonated dimethylamino groups on the grafted PDMAEMA chains are involved in BPA adsorption. The mechanism of adsorption of BPA will be discussed in Section 3.2.5.

We have published investigations on removal of BPA with different oxidoreductases [21,22,23]. For treatment with tyrosinase and polyphenol oxidase, BPA is removed by the enzymatic conversion into the corresponding quinone compound and the subsequent quinone adsorption on chitosan beads [21]. For peroxidase and laccase, BPA is removed by the generation of water-insoluble oligomers through the enzymatic radical formation and nonenzymatic coupling reaction [22,23]. Although these enzymatic treatments are effective for removal of BPA, it is impossible to retrieve BPA. Since the method in this study is able to directly recover BPA, the desorption process will be discussed in Section 3.4.

3.2.2. Effect of pH Value

In general, both the pH value and temperature affect the adsorption performance. First, the effect of the pH value on BPA adsorption was investigated for a PE-g-PDMAEMA mesh with the grafted amount of 2.6 mmol/g. Figure 3 shows the changes in the adsorption capacity, initial rate, and half-time with the initial pH value. The adsorption capacity increased with increasing the pH value, and then had a high value in the range of pH 5–10, with the maximum value at pH 8. However, when the pH value further increased from 9, the adsorption capacity sharply dropped. The initial rate had a high value in the pH range of 6–9. When the pH value deviated lower or higher than this range, the initial rate sharply decreased. On the other hand, the half-time increased with an increase in the pH value and had the maximum value at pH 9.

Figure 4 shows the change in the pH value at equilibrium with the initial pH value. The pH value remained almost unchanged during the adsorption process in the pH range from 3 to 6. In the pH range of 6–10.5, the pH value at equilibrium gradually increased from 6 to 7.5. The pH dependence of the adsorption capacity can be described in terms of the protonation behavior of dimethyl amino groups and the pK_a (pK_a1 and pK_a2) values of BPA. Although different pK_a values of BPA have been published [44,45,46,47], in this study, the fraction of BPA (molecule, monoanion, and dianion) was calculated using pK_a1 = 9.78 and pK_a2 = 10.39 [44,45] and shown as a function of the pH value in Figure 5. As the pH value increases, the protonation of dimethylamino groups decreases [44], whereas the dissociation of BPA increases, as shown in Figure 5. Only 0.20% of BPA is dissociated at pH 7.0. In other words, most or all of BPA is present in the molecular (BPA) form in the pH range lower than 6. On the other hand, dimethylamino groups are present in the free amine form in the pH values higher than 10, and BPA is present even in the dianion (BPA²⁻) form through the dissociation of both phenol groups at higher than pH 8. The portion of BPA dianions increases with increasing the pH value. Therefore, the ionic repulsion between a dissociated phenol group of BPA molecules adsorbed on the PE-g-PDMAEMA meshes and a BPA anion (BPA⁻) or a BPA dianion present in the outer solution can suppress BPA adsorption on the PE-g-PDMAEMA meshes. The maximum value of BPA adsorption at pH 8.0 can be explained in terms of the above reasons.

3.2.3. Analysis by Kinetic Equations

The adsorption data at pH 8.0 and 30 °C were subjected to analysis using both pseudo first-order and pseudo second-order equations for a PE-g-PDMAEMA mesh with the grafted amount of 2.6 mmol/g. Although the experimental data fit to the pseudo first-order equation only for the first 60 min, thereafter the deviation between the experimental and theoretical data gradually increased [48]. On the other hand, the experimental data followed the pseudo second-order equation for more than 6 h (see Figure S2). In addition, the pseudo second-order kinetics model showed a good match between the calculated and experimental values of adsorption capacity and provided the k₂ value of 4.52 g/μmol·h.

Figure 6 shows the change in the k₂ value with the grafted amount. The k₂ value sharply decreased with an increase in the grafted amount, with a mild decrease at higher than 2 mmol/g. As shown in Figure 1, the increase in BPA adsorption led to the increase in the initial rate. However, the initial rate increased only by about three times for the increase in the grafted amount from 0.7 to 2.4 mmol/g. This means that it took longer to reach the adsorption equilibrium. Therefore, the half-time increased at higher grafted amounts. Consequently, the k₂ value sharply decreased with the increase in the grafted amount, as shown in Figure 6. The fitting to the pseudo second-order kinetics demonstrates that the overall process of adsorption is dependent on the amounts of both adsorbate in the outer solution and adsorption sites on the adsorbent dosed [49,50]. Therefore, an effect of the BPA concentration on the adsorption was investigated at the grafted amount of 2.6 mmol/g.

Both the adsorption capacity and the initial rate increased with increasing the initial BPA concentration, as shown in Figure 7. On the other hand, the half-time was shortened with increasing the BPA concentration. The k₂ values of these adsorption processes were also calculated using the pseudo second-order equation. Figure 8 shows the change in the k₂ value with the initial BPA concentration. The k₂ value decreased with an increase in the initial BPA concentration. The increase in the BPA concentration resulted in the increase in the initial rate. However, the half-time gradually decreased. This means that it takes longer for the adsorption process to reach equilibrium. The multiple involvement of these matters successfully demonstrates the decrease in the k₂ value as shown in Figure 8. In order to emphasize the involvement of chemical interaction to the adsorption process, the temperature dependence of BPA adsorption was estimated.

3.2.4. Effect of Temperature

Figure 9 shows the changes in the adsorption capacity, initial rate, and half-time with the temperature. The initial rate increased, and the half-time shortened with an increase in the temperature, whereas the adsorption capacity was almost independent of temperature. This means that the adsorption came to the equilibrium for shorter times at higher temperatures. The k₂ value rose with the temperature due to the increase in the initial rate and the decrease in the half-time. For physical adsorption, in general, the adsorption will decrease at higher temperatures. However, the decrease in the adsorption capacity was not observed in this study. This also suggests that BPA adsorption occurs through a chemical mechanism. The adsorption mechanism will be more deeply discussed in the next section.

In addition, when the ln k₂ value was plotted against the reciprocal temperature according to Equation (8), the linear relationship was obtained, as shown in Figure 10, and the activation energy was calculated.

$${{\displaystyle \ln \mathrm{k}}}_2=-{\displaystyle \frac{\mathrm{E}}{\mathrm{RT}}}+\ln \mathrm{A}$$

(8)

The activation energy of 17.6 kJ/mol was calculated from the obtained slope. The type of adsorption can be defined from the magnitude of activation energy. The activation energy of no higher than 4.18 kJ/mol is usually obtained for the physical adsorption due to weak attractive interaction [51,52,53]. Chemical adsorption involves stronger forces than the physical adsorption. Chemical adsorption can be explained by the principle that the adsorption rate is varied by the temperature with activation energy between 8.4 and 83.7 kJ/mol [53]. A high magnitude of the activated energy obtained also supports adsorption through chemical interaction.

3.2.5. Adsorption Mechanism

A possible mechanism of BPA adsorption is illustrated in Figure 11. The pH dependence of BPA adsorption can be explained in terms of the ionic characteristics of a BPA molecule and a dimethylamino group on the PDMAEMA chains. The degree of protonation of dimethylamino groups is about 15% at pH 8. At this pH value, only 1.2% of BPA is present in the monoanion form (BPA⁻), as shown in Figure 5 according to Chemical Reac-tion (1). The BPA adsorption occurs by the electrostatic binding of BPA anions to protonated dimethylamino groups on the grafted PDMAEMA chains. Simultaneously, counter ions (Cl⁻ ion or OH⁻ ions) are released into the outer solution. In addition, since the degree of protonation of dimethylamino groups increases with the decrease in the pH value during the adsorption, the ionic binding shown in Chemical Reaction (4) proceeds. Since the concentration of BPA anions in the outer solution decreases by BPA adsorption, further dissociation of BPA occurs (Chemical Reaction (1)) to keep the dissociation equilibrium. BPA is considered to adsorb on the PE-g-PDMAEMA meshes through the above-described multiple steps.

Since the concentration of BPA anions in the outer solution is low pH values lower than 8.0 (See Figure 5), BPA adsorption decreases. On the other hand, at the initial pH range higher than 8.0, the deprotonation of protonated dimethylamino groups results in the decrease in BPA adsorption. In addition, since the difference between the pK₁ and pK₂ values is small for BPA (ΔpK_a = 0.61) [44,45], not only BPA dianions (BPA²⁻) but also BPA monoanions are present on the PE-g-PDMAEMA mesh and in the outer solution, as shown in Figure 5. The presence of BPA dianions causes the ionic repulsion between BPA anions electrostatically bound to the grafted PDMAEMA chains and ones present in the outer solution. The maximum adsorption at the initial pH value of 8.0 is considered to be obtained by the comprehensive involvement of the processes and phenomena described here.

3.3. Comparison with Other Adsorbents

The maximum adsorption capacity of 0.270 mmol/g-grafted PE was obtained at 0.20 mM, corresponding to 61.73 mg/g-grafted PE. The BPA adsorption is much sensitive to the experimental factors, such as the initial pH value, temperature, ratio of adsorbent to adsorbate, and BPA concentration. Although the range of these experimental factors varies depending on the study, our adsorption capacity was compared with those obtained in other studies. The values of the adsorption capacity in other articles are listed in

Table 1. The comparison with adsorption capacities of other materials.

Adsorbent	pH (-)	Temp. (°C)	Dose (g/L)	BPA conc. (mg/L)	Qexp (mg/g)	Qmax (mg/g)	Ref.
PE-g-PDMAEMA mesh	8	30	0.135	22.83	31.43	94.75	this study
			0.145	45.66	61.73
polyvinylpyridine/SiO2/APTMS particles	9.0	unclear	1.25	10	19.97		[17]
				700	160	184.4
6FDA-based polyimides	unclear	22	1.0	20	17.6	67	[18]
PP-g-MAH-β-CD nonwoven fabric	7.1	25	2	50	16.02		[19]
imine(TAPB)-based covalent organic frameworks	4–7	30	0.2	50	145	149.25	[45]
N-doped mesoporous activated pyrocarbons	5–9	30	1	50	51.038		[46]
				400	285.5	270.6
multi-walled carbon nanotubes coated with CoFe2O4	3.0	25	0.8	25	30.84	294.1	[54]
				100	121.7
carboxymethyl cellulose-lignin composite beads	6	25	1	25	14.6		[55]
				400	95.6	98.9
PP-g-PGMA-βCD nonwoven fabric	7.0	25	2	75	27.72	123.79	[56]
				300	65.59
bentonite functionalized with dialkyl	7.0	25	1.28	14.6	10.27	64.606	[57]
-dimethylammonium				168	42.69
CuZnFe2O4-supported biochar composite	7.0	25	0.2	20	101.5	263.2	[58]
β-CD functionalized mesoporous magnetic clusters	6.5–7.0	30	1.0	130	49.1	52.7	[59]
Fe/Mn/N co-doped biochar	6.9	25	0.2	20	48.64		[60]
				200	78.51	80.32

[17,18,19,45,46,54,55,56,57,58,59,60]. The values of the maximum adsorption capacity obtained from Langmuir equation were also shown for some studies. The values of adsorption capacity obtained at BPA concentrations (10–75 mg/L) close to those performed in this study were also added.

The adsorption capacity of 61.73 mg/g in this study was higher than the values obtained at BPA concentrations lower than 75 mg/L in other articles. Since an ungrafted PE mesh adsorbs little BPA, the above value corresponds to 228.3 mg/g-PDMAEMA. This value is greater than other adsorption capacity values. Therefore, the PE-g-PDMAEMA mesh prepared in this study can be used as an adsorbent for BPA removal and shows good adsorption performance.

3.4. Desorption of BPA and Repetitive Use

PE-g-PDMAEMA meshes with the grafted amount of 2.6 mmol/g loaded with BPA at pH 8.0 were transferred into NaOH solutions at different concentrations. The change in the desorption percent and desorption time with the NaOH concentration is shown in Figure 12. Here, the adsorbed amount was kept almost constant (0.137 mmol/g). As the NaOH concentration increased, the desorption percent value increased and the desorption time was shortened. BPA was completely liberated at concentrations higher than 100 mM.

Therefore, the cycle of adsorption at pH 8.0 and desorption in a NaOH solution at 100 mM was repeated five times [61,62,63]. As shown in Figure 13, for five repetitive cycles, BPA was successfully desorbed from the PE-g-PDMAEMA meshes at each desorption process. It was found that the PE-g-PDMAEMA mesh was regenerated without any loss and structural damage. This is an appreciable note obtained in this study. In the future, we will investigate the adsorptive removal of other Bisphenol Analogues with different alkyl chain lengths, such as bisphenol B, bisphenol E, and bisphenol F, and, in addition, bisphenol S (BPS), with the PE-g-PDMAEMA meshes to increase their versatility because removal of BPS was low for the enzymatic treatment with oxidoreductases [23,24,25].

4. Conclusions

We systematically investigated adsorptive removal of BPA from aqueous medium with the PE-g-PDMAEMA meshes. The adsorption behavior toward BPA was estimated by varying the grafted amount, pH value, temperature, and BPA concentration, and some important findings were found from the equilibrium, kinetic study, and isotherm analysis. The adsorption capacity in mmol/g-grafted PE and in mmol/g-PDMAEMA had the maximum values at the grafted amount of 2.6 and 2.4, respectively. BPA adsorption successfully took place in the pH range of 5–10, with the maximum value at pH 8. The adsorption data followed the pseudo second-order model. The initial rate increased with increasing temperature, and the adsorption capacity increased with increasing BPA concentration. In addition, the equilibrium data obtained at different concentrations obeyed the linear form of Langmuir isotherm. This emphasizes that the adsorption process occurs through the chemical interaction, or ionic bonding, of BPA anions to terminal protonated dimethylamino groups on the grafted PDMAEMA chains. The adsorption capacity shown in this study is higher than the values shown in other articles. Therefore, our results successfully demonstrate that the PE-g-PDMAEMA mesh can be applied as an adsorbent for removal of BPA from aqueous medium. From on now, we will investigate adsorption of BPA analogues, such as BPB, BPE, and BPF, in the batch mode and removal of BPA and its analogue in the column mode, in order to prove valuable insight into the practical application of PE-g-PDMAEMA meshes.