The IAT-GO design strategy and Hg(II) adsorptionGO synthesis utilized a reported method involving graphite oxidation with H3PO4/H2SO4/KMnO416. A novel adsorbent IAT-GO, for capturing heavy metals, leverages the potent chelating ligand IAT covalently grafted onto the surface of graphene oxide (GO). This two-step approach creates numerous active sites for efficient metal adsorption. Step 1: AT, with its ethoxy groups hydrolyzed to Si–OH, condenses with hydroxyl groups on GO, forming stable (O–Si–O) bonds22,23. Step 2: Primary amine functionalities on AT-GO act as nucleophiles, attacking alkyl halides in CAA under a base, introducing additional chelating sites19,24. This facile and cost-effective functionalization strategy, unique in the literature, yields IAT-GO with exceptional performance. It boasts high capacity, rapid adsorption kinetics, and efficient removal of trace-level Pb(II) from water, making it a promising candidate for heavy metal-contaminated water treatment.IAT-GO adsorbents capture lead from water through four main mechanisms as in Fig. 1: (1) Ion exchange: Pb(II) ions swap places with protons on the adsorbent, becoming trapped while releasing protons back into the water. (2) Surface adsorption: Pb(II) ions are physically attracted to the adsorbent’s surface via weak forces like van der Waals. (3) Chemisorption: Stronger chemical bonds form between Pb(II) and functional groups on the surface, like carboxyl or hydroxyl groups. (4) Adsorption-complexation: Pb(II) forms complexes with the adsorbent’s surface groups, which are then adsorbed. These processes happen simultaneously, with their relative importance depending on the specific adsorbent and water conditions. The capture process involves three steps: migration of Pb(II) towards the adsorbent, diffusion through a surrounding water layer, and finally, binding to active sites on the surface through one of the mentioned mechanisms25.Figure 1Synthesis of IAT-GO via functionalization of GO nanosheets with AT ligand for efficient Pb(II) capture.The IAT-GO characterizationsThe UV–Vis absorption spectra of GO and IAT-GO nanosheets dispersed in water are presented in Fig. S1. Two prominent bands are observed: a main peak at λmax = 220 nm for GO and 200 nm for IAT-GO, attributed to the π–π* transition of aromatic (C=C) bonds, and a shoulder at 300 nm assigned to n–π* transitions of (C–N, C=O) and (C–O) bonds26,27. Notably, the IAT-GO sample exhibits a significantly enhanced absorption peak at 200 nm, indicating increased conjugation (potentially due to higher concentration of aromatic and carbonyl groups). Furthermore, the red shift of the π–π* transition of aromatic (C=C) bonds to 200 nm in IAT-GO suggests partial reduction of GO and restoration of some (C=C) bonds in the IAT-GO sheets, consistent with the observed color change from brown GO to black IAT-GO19,27.XRD analysis (Fig. 2a) reveals distinct structural differences between GO and IAT-GO. While GO exhibits a sharp peak at 2θ = 10.55°, corresponding to an interlayer distance of 0.801 nm, attributed to oxygen-containing functional groups, IAT-GO presents a new, prominent peak at 2θ = 5.2°, indicative of a significantly larger interlayer spacing (d-spacing) of 1.31 nm. This increase is likely due to the presence of the bulky functional groups (–O–SiCH2CH2CH2N-(CH2-COOH)2) attached to the GO sheets. Additionally, a broad peak observed at 2θ = 22.43°, corresponding to an interlayer spacing of 0.35 nm, suggests the presence of stacked IAT-GO graphene platelets19,28.Figure 2(a) XRD pattern and (b) Raman spectra of GO and IAT-GO.Figure 2b shows the Raman spectra of both GO and IAT-GO sheets. Both spectra reveal the characteristic D band (around 1350/cm) and G band (around 1600/cm). The G band originates from the stretching of sp2 C=C bonds, while the D band indicates defects within this sp2 network. To quantify the level of disorder introduced by the chemical modification, the intensity ratio of D-band to G-band (ID/IG) was calculated. This ratio increased from 0.93 for GO to 1.04 for IAT-GO, suggesting a higher degree of defect formation in the IAT-GO structure. Interestingly, new bands appeared in the IAT-GO spectrum at 450 and 798/cm. These additional peaks are likely attributed to the covalent interactions between the grafted Iminodiacetic Acid Triethoxy(propyl)silane molecules and the GO sheets, as documented in previous studies19,29.The surface functional groups of GO, AT-GO, andIAT-GOwas investigated by FTIR spectroscopy and the results are depicted in (Fig. 3a and b). The FT-IR spectrum of GO revealed eight characteristic peaks at 948, 1038, 1215, 1380, 1620, 1735, 2540, and 3252/cm. These peaks correspond to (C–C), (C–O hydroxy), (C–O carboxyl), (C–O epoxy), (C=C), (C=O), and (OH) bonds, respectively30,31. In contrast, the spectra of AT-GO and IAT-GO displayed additional peaks at 690, 755, 912, 1015, 1112, 1200, 1380, 1450, 1530, 1550, 1655, 2860, 2930, and 2950/cm. These peaks indicate the presence of (C–H), (Si–C, C–O and Si–O), (Si–O), (V–H), (C–CH3), (C–CH2), (C–N), (C–H), and (CH3 and CH2) functional groups32,33. Furthermore, the IAT-GO spectrum exhibited a strong peak at 3274/cm, signifying the stretching vibration of OH in the carboxylic acid group. Additionally, peaks in the range of 1550–1350/cm were attributed to asymmetric and symmetric (COO) stretches19. These FT-IR spectra confirm the successful grafting of desired functional groups onto the surface of GO, particularly in the case of IAT. The presence of these peaks provides unambiguous evidence for the chemical modification of GO achieved through the IAT chelating ligand.Figure 3FTIR spectra of as-prepared GO and IAT-GO adsorbents. (a) Full spectrum (600–3800/cm). (b) Specific region (600–1900/cm).XPS analysis confirmed the presence of functional groups on the surface of the IAT-GO hybrid material as well as lead adsorption. The survey spectrum (Fig. 4a) revealed five characteristic peaks at 220, 550, 440, 343, and 345 eV, corresponding to C 1s, O 1s, N 1s, Pb 4f, and Si 2p, respectively. After Pb2+ removal, high resolution XPS of Pb 4f exhibited two distinct peaks allocated at 138.8 and 143.8 eV assigned for Pb 4f7/2 and Pb 4f5/2, which indicates the adsorption of lead on the surface of IAT-GO by complexation adsorption mechanism (Fig. 4b)34. This implies that functional groups containing nitrogen and oxygen play a role in lead adsorption13. Additionally, These peaks were also shifted to higher binding energies compared to free lead atoms, indicating strong interactions between lead ions and IAT-GO’s functional groups, likely forming lead complexes. Therefore, the XPS analysis provided strong evidence for the successful incorporation of the IAT chelating ligand onto the GO surface, leading to the formation of the IAT-GO hybrid material that has strong ability to chelate lead ions from the solution.Figure 4XPS spectra of the as-prepared IAT-GO nanocomposite after lead adsorption. (a) Shows the survey scan, (b) focuses on the Pb 4f peaks.This study further employs SEM and TEM microscopy (Fig. 5) to investigate the morphological changes induced by chemically modifying GO with bulky functional groups (–O-SiCH2CH2CH2N(CH2-COOH)2). The pristine GO exhibits a typical sheet-like morphology with numerous layered nanosheets, as evident in the SEM image (Fig. 5a) and TEM images (Fig. 5c). However, upon introduction of the bulky functional groups, the SEM and TEM image of the modified IAT-GO (Fig. 5b and d) reveals a significant change. The surface becomes rough due to the stacking of GO nanosheets into a framework structure. This framework likely arises from the formation of covalent bonds between the GO nanosheets and the added functional groups (–O-SiCH2CH2CH2N(CH2-COOH)2). Notably, this crosslinking of GO nanosheets has been documented by various research groups, further corroborating our observations19,35.Figure 5SEM (a, b) and TEM (c, d) images of the GO and IAT-GO, respectivelly.Adsorption and desorption resultspH InfluenceFigure 6a illustrates how the initial pH impacts the removal of Pb(II) using GO and IAT-GO nanosheets. As pH rises, so does the adsorption capacity, with a peak at pH 5. For GO and IAT-GO, respectively, the amount of Pb(II) adsorbed at equilibrium (qe) increases from 0 to 22.4 mg/g and 6.0 to 45.9 mg/g as pH goes from 1 to 6. This pH dependence is due to the presence of various oxygen-containing functional groups on the adsorbents. These include carboxyl and hydroxyl groups in GO, and additionally amide groups in IAT-GO. The electrostatic and ionic interactions between Pb(II) ions and the adsorbents at different pH values play a crucial role in the adsorption process. At low pH values, the removal efficiency of lead ions is reduced due to competition between Pb(II) ions and hydrogen ions for adsorption sites. Hydrogen ions, being smaller, are strong competitors. Conversely, at high pH values, repulsion forces weaken, and electrostatic attraction between the negatively charged surface and Pb(II) ions draws them towards the adsorbent surface. The high maximum adsorption at pH 6 is attributed to the decreased solubility of lead caused by extensive hydrolysis, resulting in an increase in HgClOH and Hg(OH)2 species19,36.Figure 6Effects of (a) solution pH and (b) adsorbent dose on Pb(II) removal by GO and IAT-GO.Adsorbent dose influenceFigure 6b demonstrates how the effectiveness of Pb(II) ion removal depends on the amount of GO and ITAGO used. For IAT-GO nanosheets, there’s a nearly linear relationship between the amount of sorbent used and the percentage of Pb(II) removed. When the sorbent dose increases from 0.01 to 0.035 g, the lead removal percentage increases from 40.6 to 92.3% for GO and from 48.0 to 64.8% for IAT-GO. This is likely due to the increased availability of sorption sites and the overall growth in surface area with more sorbent. However, at higher doses, the sorbent particles clump together, reducing the total surface area available for adsorption and hence the removal capacity. This happens with GO when the dose reaches 0.035 g, where the Pb(II) removal percentage plateaus at 65.8% and 54.60% for initial Pb(II) concentrations of 50 and 300 ppm, respectively. Interestingly, for IAT-OG at the same dose (0.035 g) and an initial Pb(II) concentration of 600 ppm, the removal percentage doesn’t plateau, suggesting no saturation of the available binding sites.Concentration influence and adsorption isothermFigure 7a demonstrates how the initial concentration of Pb(II) ions affects their removal using GO and IAT-GO at a pH of 6 and room temperature. As the initial concentration increases, the amount of Pb(II) adsorbed onto the sorbents also increases. This is because a higher concentration creates a stronger driving force, pulling more Pb(II) ions towards the sorbent’s surface8,19. For GO, the adsorbed amount jumps from 8.6 to 24.0 mg/g as the initial concentration goes from 10.0 to 250.0 mg/L. Similarly, IAT-GO’s adsorption increases from 39.6 to 124.0 mg/g with a concentration rise from 50.0 to 400.0 mg/L. However, while the adsorbed amount increases, the removal efficiency (percentage of Pb(II) removed) actually decreases at higher concentrations. For GO, it drops from 86.0 to 9.2% between 250.0 and 400.0 mg/L, and for IAT-GO, it falls from 97.2 to 31.3%. This is likely due to the saturation of active sites on the sorbents. Once these sites are full, excess Pb(II) ions remain in the solution, leading to a lower removal efficiency. As expected, Fig. 7a shows that the adsorption efficiency generally increases with the initial Pb(II) concentration. More lead ions mean more sites are engaged in the adsorption process, but eventually, all the available sites get filled, and the efficiency plateaus14,19. Scientists were able to understand the observed trend by using a model called the Langmuir adsorption isotherm12,37. This model perfectly described the data, as shown in (Fig. 7b and Table 1). The strong agreement between the model and data was confirmed by correlation coefficients (R2) ranging from 0.9866 to 0.9984 and RL values between 0 and 0.1, which signify favorable adsorption. This implies a uniform adsorption process where all active sites on the material have equal affinity for the metals, resulting in a single layer of metal atoms covering the surface.Figure 7(a) Effects of solution concentration, (b) the Langmuir adsorption isotherm module, (c) effects of contact time, and (d) The pseudo-second-order kinetic module on Pb2+ removal by GO and IAT-GO.Table 1 Langmuir parameters for the adsorption of phosphate ions by the GO and IAT-GO.Time influence and adsorption kineticsFigure 7c effectively depicts the influence of contact time on the adsorption capacity of GO and IAT-GO for Pb(II) ions at varying concentrations (50 and 300 ppm) and a consistent pH of 6. The observed trend highlights the distinct adsorption behaviors of the two materials. Both GO and IAT-GO demonstrate a remarkable initial surge in removal efficiency, achieving over 43.0% and 59.9% of their maximum adsorption capacity, respectively, within just 5 min. This rapid uptake signifies the presence of an abundance of readily accessible adsorption sites on the adsorbent surfaces at the beginning of the process. Following the initial burst, the adsorption rate progressively slows down, eventually reaching equilibrium. GO attains maximum Pb(II) removal within 30 min, while IAT-GO requires 120 min. This gradual decline reflects the diminishing availability of vacant sites as adsorption progresses. At equilibrium, the quantity of Pb(II) ions adsorbed onto the material balances the amount desorbed, indicating a dynamic equilibrium state. This suggests that the adsorbent has reached its maximum loading capacity under the given conditions. The researchers employed a mathematical model, called the pseudo-second-order kinetic model (SSOKM)11,16, to understand how the adsorption process worked. This model proved highly accurate, as demonstrated by the very strong correlations (R2 > 99%) seen in Fig. 7d. Additionally, the model’s predictions regarding the maximum adsorption capacity (qt) closely aligned with the experimental results (Table 2). The SSOKM assumes a chemical reaction (chemisorption) between the adsorbent and the heavy metals, involving the sharing and exchange of electrons. This aligns with the presence of nitrogen and oxygen atoms within IAT-GO, which are known for forming strong bonds with metals.Table 2 Kinetic Parameters for the Adsorption of Lead ions by the GO and IAT-GO.Competing ions influenceIAT-GO;s selective adsorption of Pb(II) in the presence of Hg(II), Cu(II), Co(II), Zn(IV), and Cd(II)was demonstrated (Fig. 8a). The high Pb(II) selectivity is attributed to the strong interaction leads to the formation of stable Pb(II) complexes, resulting in superior selectivity compared to other metal ions12,38. These properties make IAT-GO a promising material for Pb(II) removal from wastewater and other contaminated solutions containing multiple heavy metals.Figure 8(a) Impact of coexisting anions [Conditions: Co = 100 mg/L, T = 273 K, dose = 0.01 g/10 mL, and t = 300 min, and pH = 5]. and (b) Reusability for lead removal by IAT-GO adsorbent [Co = 500 mg/L, T = 273 K, dose = 0.01 g/10 mL, and t = 300 min, and pH = 5].Desorption and reusabilityLead ions were efficiently desorbed from GO and IAT-GO adsorbents, achieving recoveries above 99.0% using 0.4 M HNO3 (Table S1). Interestingly, the IAT-GO regenerated with HNO3 required further treatment with NaOH to restore its negative charge before reuse. Despite this additional step, the adsorbent maintained excellent stability over multiple cycles (Fig. 8b). Even after five adsorption–desorption cycles, the regeneration efficiency remained above 94.0%, demonstrating the reusability and chemical robustness of IAT-GO for Pb(II) removal.Comparison with different adsorbentsTable 3 showcases the impressive performance of our chemically modified GO adsorbents (ITA-GO) in capturing Pb(II) from water. Compared to existing methods, ITA-GO boasts a significantly higher adsorption capacity, positioning them as effective and potentially commercially viable solutions for Pb(II) removal.Table 3 Comparison of different adsorbents on the adsorption performance for Pb(II).
