Gamma radiation-induced synthesis of a high-performance alginate/poly(acrylic acid-vinyl sulfonic acid) adsorbent for Cu(II) and Zn(II) ions

High-purity chemicals and solvents were used for the synthesis and analysis, including sodium alginate (Alg) and vinyl sulfonic acid sodium salt (VSA) (Sigma-Aldrich, Germany), acrylic acid monomer (AA), N,N’-methylenebisacrylamide (NMBA), hydrochloric acid (HCl), sodium hydroxide (NaOH), zinc(II) sulphate heptahydrate (ZnSO₄.7H₂O), and copper(II) sulphate pentahydrate (CuSO₄.5H₂O) (Merck, Germany), and methanol (ADWIC, Egypt).

Alg-P(AA-co-VSA) graft copolymer was synthesized under optimized conditions. Briefly, 0.75 g of Alg was dispersed in 15 mL of distilled water (DW) and ultrasonicated at 50 °C for 30 min to obtain a homogeneous solution. Subsequently, 8.4 g of AA, 3.6 g of VSA, and 0.24 g of NMBA were added sequentially under continuous stirring, and the total volume was adjusted to 30 mL. Nitrogen gas was bubbled through the mixture to remove dissolved oxygen, and the solution was irradiated at room temperature with a 25 kGy dose using a Cobalt-60 gamma irradiator (⁶⁰Co–γ-ray cell MC-20, Cyclotron Project, Inshas, Egypt). After irradiation, the grafted material was cut into small pieces, washed with a water-methanol mixture (50:50 v/v), filtered, and dried at 75 °C to constant weight for further characterization [9, 12]. Grafting percentage and grafting efficiency were calculated using Eqs. (1) and (2) [29].

Where W_o, W_g, and W_m are the weights of the polymer backbone (Alg), the grafted polymer (Alg-P(AA-co-VSA), and the monomers (AA and VSA), respectively.

The structural characteristics of the Alg-P(AA-co-VSA) graft copolymer were analysed using Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and differential thermal analysis (DTA). In addition, the point of zero charge (\(\:{pH}_{pzc}\)) of the graft copolymer was determined. The FTIR was employed to identify the chemical functionalities of the grafted copolymer within the 4000–400 cm^− 1 range at a resolution of 4.0 cm^− 1 using the KBr disc method on a Shimadzu infrared spectrometer (BOMEM, FTIR, Japan). Thermal properties were examined using TGA and DTA by heating 13.702 mg of the graft copolymer from 30 to 700 °C at a rate of 20 °C/min with a Shimadzu DTG-60 thermal analyzer (Japan). The \(\:{pH}_{pzc}\) of the Alg-P(AA-co-VSA) graft copolymer was determined by adding 10.0 mg of the graft copolymer to an array of 50 mL stoppered bottles containing 10 mL of buffer solutions with pH ranging from 2.0 to 12.0. The suspensions were shaken at 150 rpm at room temperature for 24 h. The final pH was measured, and \(\:\varDelta\:\:pH\:({pH}_{Initial\:}-\:{pH}_{Final})\) was plotted against \(\:{pH}_{Initial\:}\). The \(\:{pH}_{pzc}\) was identified at the point where \(\:\varDelta\:\:pH\) = 0 [30]. On the other hand, zeta potential measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were employed to characterize the structural and compositional features of the composite before and after metal ions adsorption. The zeta potential of both the unloaded and metal-loaded composites was measured in water using a Malvern Zetasizer Nano ZSP (ZEN5600). XRD analysis was performed to examine the crystallographic and amorphous features of the synthesized Alg-P-(AA-co-VSA) composite before and after adsorption, using a Rigaku SmartLab SE diffractometer (Japan). Measurements were carried out with Cu Kα radiation (Kβ filter) operated at 40 kV and 50 mA. Diffraction patterns were collected over a 2θ range of 5–80° at a scan speed of 5°/min and a step size of 0.02°. Samples were dried, finely powdered, and mounted on a silicon low-background holder. XPS analysis for the unloaded and loaded composite samples was conducted at room temperature using a Thermo Fisher Scientific ESCALAB 250Xi spectrometer with Al Kα radiation (hν = 1486.6 eV). Survey and high-resolution spectra were recorded at a pass energy of 20 eV with a 500 μm spot size. Binding energies were calibrated to the C 1s peak at 284.6 eV, and all spectra were processed using Origin software. SEM analysis was performed to investigate the surface morphology of the synthesized composite and metal-loaded samples. Micrographs were obtained using a JEOL JSM-5610LV (Japan) operated at 10 kV. Prior to imaging, the samples were sputter-coated with a thin conductive platinum layer using a JEOL JFC-1600 coater at 20 mA for 3 min to prevent charging and enhance image quality. All samples were mounted on aluminum stubs using conductive carbon tape.

Batch experiments were performed to evaluate the efficiency of the Alg-P(AA-co-VSA) graft copolymer in removing Cu(II) and Zn(II) individually. The effects of pH, initial metal concentration (\(\:{\text{C}}_{\text{o}}\)), contact time (t), adsorbent dosage (m), agitation speed (A), and adsorption temperature (T) were investigated. A summary of the batch adsorption experiments is provided in Table 1.

A Cu(II) stock solution of 10 mg/L was prepared by dissolving 39.29 mg of CuSO₄.5H₂O in DW and diluting to a one liter in a volumetric flask. Similarly, a 10 mg/L of Zn(II) stock solution was prepared by dissolving 43.98 mg of ZnSO₄.7H₂O in DW and diluting to one liter. The pH of the Cu(II) and Zn(II) solutions was adjusted within the range of (3.0–7.0) using 0.2 M HCl and 0.2 M NaOH. The remaining concentrations of Cu(II) and Zn(II) ions after adsorption were measured by inductively coupled plasma-optical emission spectrometry (ICP-OES; Agilent 5110 VDV, Germany).

The Alg-P(AA-co-VSA) graft copolymer was synthesized through γ-radiation-induced graft copolymerization of AA and VSA onto the surface of Alg, using NMBA as the crosslinker. The synthesis followed optimized conditions that maximized grafting percentage (G% ~ 810.5%), and grafting efficiency (GE% ~ 77.0%). The optimal composition for the Alg-P(AA-co-VSA) graft copolymer consisted of 2.5 w% Alg, 40 w% comonomer, 0.8 w% NMBA, and a 70/30 w% AA/VSA ratio, irradiated at an absorbed dose of 25 kGy [12, 27].

The functional and microstructural properties of the native Alg polymer, characterized by FTIR and SEM, are provided in the supplementary data (Figure S1). The FTIR spectrum of the Alg-P(AA-co-VSA) graft copolymer (Fig. 1a) displays characteristic absorption bands assigned to the functional groups originating from the Alg backbone, AA, VSA, and the NMBA crosslinker. A broad band observed at 3150–3550 cm⁻¹ corresponds to O-H stretching vibrations, indicative of hydrogen bonding among hydroxyl and carboxyl groups of Alg, and AA. Absorption peaks in the 2875–2950 cm⁻¹ range are attributed to C-H stretching of methylene (-CH₂-) groups in the graft copolymer network. A strong band at 1723 cm^− 1 is due to the C = O stretching of carboxylic acid groups, mainly from AA. The absorption band in the 1530–1630 cm^− 1 range suggests the presence of amide bonds (amide II), resulting from the interaction between NMBA’s amide groups and the carboxyl groups from AA or Alg, in addition to the asymmetric stretching of the carboxylate (COO⁻) group. Other characteristic bands include 1452 cm^− 1 (C-H bending), 1390–1430 cm⁻¹ (COO⁻ symmetric stretching), and 1175–1325 cm⁻¹, likely due to the stretching vibration of C-O (Alg) and the sulfonyl (S = O) group in VSA. The region 1005–1050 cm^− 1 corresponds to C-O-C stretching, confirming the polysaccharide backbone of Alg.

The thermal analysis of the Alg-P(AA-co-VSA) graft copolymer reveals four distinct degradation stages, as shown in Fig. 1b. The first stage (13.96%) occurs between 80 and 225 °C, attributed to moisture and adsorbed water loss. The second stage (12.34%) between 225 and 335 °C involves the degradation of volatile functional groups and the Alg backbone. The third stage (12.31%) between 335 and 420 °C reflects the breakdown of grafted AA-co-VSA chains. The final major stage (31.79%) between 420 and 520 °C represents the copolymer’s thermal degradation.

Above 520 °C, the remaining residual mass is is attributed to thermally stable inorganic salts (e.g., sodium oxide or sulfate), carbonaceous char and nitrogen-containing residues from the NMBA cross-linking framework.

The point of zero charge experiment as a function of initial pH values is presented in Fig. 1c. The plot displays fluctuations in ΔpH which can be attributed to several factors, including surface heterogeneity, ion exchange, and the buffering capacity of the Alg-P(AA-co-VSA) graft copolymer [31, 32]. The graft copolymer contains various functional groups with distinct pKa values, which interact with protons in a non-linear manner, leading to saturation or deprotonation effects that influence ΔpH. In this study, the isoelectric point (\(\:{pH}_{pzc}\)) of the Alg-P(AA-co-VSA) graft copolymer was determined to be at pH = 6.63. Notably, at pH = 2.0, both functional groups (-COOH and -SO₃H) are highly protonated and readily make proton exchange with solution, leading to a less negative ΔpH value of -0.43. At pH = 3.0 and pH = 5.0, both sulfonate and carboxylate groups exist predominantly in their deprotonated forms, resulting in strong negative ΔpH values of -1.131 and − 1.249, respectively, associated with the strong proton uptake from the solution. However, at pH 4.0, the sulfonic groups are deprotonated, while the carboxylic groups remain protonated, thus the carboxyl groups act as buffers, releasing protons to stabilize pH fluctuations, leading to a smaller ΔpH of -0.354. At pH values between 5.0 and 6.63 (\(\:{pH}_{pzc}\)), the graft copolymer acts as a buffer due to the predominance of deprotonated groups. Beyond \(\:{pH}_{pzc}\), the graft copolymer surface becomes negatively charged, indicated by positive ΔpH values.

The zeta potential measurements of the Alg-P(AA-co-VSA) composite and the metal-loaded samples (Zn(II) and Cu(II)) were carried out in water to evaluate the surface charge characteristics and to gain insight into the colloidal stability and metal–polymer interactions (Figure S2; Supporting data). The pristine composite showed a strongly negative zeta potential (–37.6 mV), indicating high colloidal stability due to abundant anionic surface groups. After Zn(II) loading, the zeta potential increased to − 26.8 mV, while Cu(II) loading resulted in − 29.2 mV. In both cases, the decrease in negative charge reflects partial neutralization as the divalent metal ions bind to surface functional groups. Zn(II) caused slightly greater charge reduction than Cu(II), suggesting stronger or more extensive interaction. Despite these changes, all samples remained within the range of good colloidal stability, confirming successful metal binding without inducing aggregation.

The XRD patterns of the Alg-P(AA-co-VSA) composite and the metal-loaded samples (Composite@Zn and Composite@Cu) exhibit broad humps centered at approximately 7.5°, 20.9–22.4°, and 39.4–40.6°, characteristic of the predominantly amorphous nature of alginate-based polymers (Figure S3; Supporting data). The main broad peak near 21–22° corresponds to the disordered polysaccharide backbone, while the humps at low (≈ 7.5°) and high angles (≈ 39–41°) reflect short-range structural ordering within the grafted networks. After Cu(II) and Zn(II) adsorption, a slight decrease in peak intensity and minor shifts in the broad features are observed, particularly for Composite@Cu. These changes indicate reduced structural regularity and partial disruption of the polymer matrix due to metal–ligand interactions with carboxylate and sulfonate groups. The absence of sharp crystalline peaks for Cu or Zn species suggests that the ions are uniformly dispersed and coordinated within the composite rather than forming separate crystalline phases. Thus, the XRD results confirm that metal uptake occurs through binding within the amorphous polymer network without inducing crystallization.

X-ray photoelectron spectroscopy (XPS) was employed to examine the surface chemical states and elemental composition of the pristine composite and the metal-ion-adsorbed samples [33]. All binding energies were calibrated with respect to the C 1s reference peak at 284.6 eV [34]. The survey scans (0–1100 eV) revealed signals characteristic of C, N, and O in all samples (Fig. 2a), while sulphur appeared only as a very weak or noisy feature due to its low abundance and the limited sensitivity of the Al Kα excitation source. Nevertheless, S-related peaks were confirmed in the high-resolution spectra provided in the supplementary data (Figure S4a-f). The detected Auger and core-level features were consistent with values reported in established XPS databases and literature sources [35‐38]. After adsorption, clear Zn and Cu signals appeared, demonstrating successful metal uptake. The Zn 2p_3/2 peak at 1021.6 eV and the Cu 2p_3/2 peak at 933.1 eV indicated that both metals were present in the + 2 oxidation state (Fig. 2b, c). This confirms that Zn(II) and Cu(II) are stabilized on the composite surface through coordination with oxygen-containing groups [35, 38, 39]. The C 1s spectra showed the expected components (C–C, C–O/C–N, and O–C = O). Following adsorption, only slight increases in the C–O peak were observed, indicating interaction between metal ions and oxygen donor sites but no formation of new carbon chemical states (Fig. 2d-f) [40, 41]. The O 1s spectra provided clear evidence of metal–oxygen coordination. An additional low-binding-energy peak at 530.5–530.7 eV, corresponding to M–O bonds, appeared only in the Zn- and Cu-loaded samples. The increased intensity of this component confirms the formation of Zn–O and Cu–O bonds on the composite surface (Fig. 2g-i) [42, 43]. Analyses of the N 1s and S 2p core-level spectra are provided in the supplementary material (Figure S4 a-f). Thus, the XPS analysis verifies the expected elemental composition and demonstrates effective adsorption of Zn(II) and Cu(II) through oxygen-mediated coordination on the composite.

The microstructure SEM morphology of the unloaded Alg-P-(AA-co-VSA) composite shows an irregular, highly rough surface composed of aggregated clusters with a sponge-like texture (Fig. 3a). The particles appear interconnected, forming a porous structure with numerous micro-cavities. This SEM morphology is advantageous for adsorption because it increases the available surface area and provides multiple binding sites for metal ions. After Zn(II) or Cu(II) adsorption, noticeable morphological changes are observed. The surface becomes more compact and less porous compared to the unloaded sample (Fig. 3b, c). The aggregated clusters appear smoother, and some pores seem partially filled, indicating successful metal ion uptake. The reduced surface roughness and denser morphology suggest that Zn(II) and Cu(II) ions were deposited on the composite surface or within the pores, forming a more consolidated structure.

The evaluation and analysis of the experimental trials were conducted using JMP^® software (SAS Institute). A one-factor-at-a-time approach within a factorial design was employed to optimize preparation conditions and enhance the efficiency of heavy metal removal. This approach involved varying one factor while keeping other factors constant, allowing for the assessment of the impact of each individual factor on the \(\:\%R\) and the \(\:{Q}_{e}\) for Cu(II) and Zn(II).

Table 5 summarizes 40 experimental runs performed using six variables: pH, incubation time (min), initial concentration of the heavy metal (mg/L), temperature (°C), adsorbent dose (g/L), and agitation speed (rpm). These variables were selected based on existing literature and preliminary experiments. To estimate the effect of each factor, Logworth values were calculated. These values are the negative logarithm of the p-value of each factor’s effect, indicating the significance of the model’s parameters. At pH 6–7 (Runs 4 and 5, Table 5), the removal of Cu(II) drops to 0% because Cu²⁺ no longer remains in soluble form. Near neutral pH, Cu(II) undergoes hydrolysis and precipitates mainly as Cu(OH)₂(s), greatly reducing the amount of free Cu²⁺ available for adsorption. Thus, the observed loss in removal efficiency is due to precipitation rather than poor adsorbent performance. In contrast, Zn(II) remains soluble in this pH range, allowing continued adsorption [57]. Figure 7 presents a summary of the effects of the investigated factors. The Logworth values for both Cu(II) and Zn(II) removal studies highlighted that the removal efficiency is predominantly influenced by three factors: initial concentration, pH, and time, which were identified as the most critical parameters for \(\:\%R\) and \(\:{Q}_{e}\).

Prediction models were developed using JMP^® as shown in Fig. 8. The model is defined as: Response = Error + pH + Time + Initial concentration + Dose + Temperature + Agitation.

Figure 8 provides a comprehensive comparison between the predicted and experimental values for both \(\:\%R\) and \(\:{Q}_{e}\) of Cu(II) and Zn(II). The plots demonstrate that the DoE models describe the experimental data well, particularly for Zn(II) and for the \(\:{Q}_{e}\) of both metals, where strong linearity and very high coefficients of determination (R² = 0.86–0.97) are evident. The narrow confidence bands and low RMSE values further confirm the robustness and predictive reliability of these models. In contrast, the \(\:\%R\) model for Cu(II) shows noticeably higher scatter and a lower R² value, which reflects the greater experimental variability associated with Cu(II) behavior, especially near its hydrolysis region. Despite this, the model retains statistical significance and correctly captures the overall trend. Taken together, the results in Fig. 8 indicate that the DoE approach successfully explains the dominant factors influencing metal removal, while also highlighting the unique sensitivity of Cu(II) to small changes in operating conditions.

To determine the maximum removal of Cu(II) and Zn(II), regression equations for the prediction models were evaluated using the analysis of variance (ANOVA) method with least-square fit, see Table 6. The robustness of the established models was confirmed by a high F-ratio.

Table 6 summarizes the predicted conditions for achieving maximum removal efficiencies for Cu(II) and Zn(II), including both percentage removal and the adsorption capacity (\(\:{Q}_{e}\)) for each metal under optimal conditions. The results predicted the maximum \(\:\%R\) for Cu(II) is 99.04% with \(\:{Q}_{e}\) of 67.05 mg/g at optimal conditions of pH 5.91, 300 min, and 100 mg/L. For Zn(II), the removal is 99.54% with \(\:{Q}_{e}\) of 61.47 mg/g at pH 6.12, 295.6 min, and 100 mg/L. These findings suggest that minor pH adjustments can be employed to maximize the removal efficiency of each specific metal.

Alginate-based adsorbents show a wide range of metal-uptake performance depending on their structural modifications, functional groups, and incorporation of hybrid components such as graphene, nanoparticles, cyclodextrin, or synthetic copolymers. As summarized in Table 7, reported capacities span low to exceptionally high values for both Cu(II) and Zn(II), reflecting differences in cross-linking density, ligand availability, and accessible surface area. In the present study, the Alg-P(AA-co-VSA) composite achieved adsorption capacities of 65.78 mg/g for Cu(II) and 60.97 mg/g for Zn(II). These values place the material within the mid-performance range compared to other alginate-based adsorbents reported in the literature. For Cu(II), the current capacity (65.78 mg/g) is higher than that of several traditional alginate systems such as mesoporous alginate/β-cyclodextrin beads (15.54 mg/g) [25], post-crosslinked sodium alginate beads (54.9 mg/g) [16] and Cu(II)-imprinted alginate hydrogel (50.21 mg/g) [28]. It also remains comparable to Ca²⁺-Alg immobilized activated carbon/yeast (64.90 mg/g) [14]. Although some modified composites demonstrate superior performance such as alginate/reduced graphene double-network beads (169.5 mg/g) [18], ethylenediamine-modified Ca-Alg aerogels (87.83 mg/g) [22], and modified microspheres (124.1 mg/g) [24], these materials often involve more complex functionalization, multi-network structures, or high-surface-area nanofillers that substantially enhance metal binding. For Zn(II), the current study’s capacity (60.97 mg/g) slightly lower than the Ca-Alginate–nZVI–biochar systems for Zn(II) uptake (71.77 mg/g) [26]. Many literature examples focus more extensively on Pb(II), Cd(II), or Cu(II) rather than Zn(II).