1 Introduction

Due to their propensity to gather in animals and plants and their toxicity to aquatic life, humans, and the ecosystem, heavy metal ions have been a major worry among many industrial branches related to environmental pollution. Pb (II) stands out as one of the most problematic heavy metals among all the others since it has a negative impact on the health of humans as well as animals and prevents water bodies from purifying themselves from it (L. Li et al., 2020). Lead is used in a variety of industrial processes, including those that create alloys, electrical products, chemical catalysis, polished metal surfaces, and batteries. Because lead has been found to cause damage to the brain in children even at low amounts, lead is at the top of the list among environmental dangers (Mishra & Patel, 2009). In addition to anemia, brain damage, mental deficiencies, and behavioral issues in people, it also kills plants and animals. In skeletal tissues, lead replaces calcium and builds up and lead within drinking water has been linked to several major health issues, including those that can be fatal in some circumstances (Low et al., 2000). So, acceptable limits have been set at 0.01 mg L−1 by the World Health Organization (WHO) and 0.05 mg L−1 by the US Environmental Protection Agency (USEPA) (Li et al., 2020).

Although these techniques have negative non-economic effects, it is said that liquid-based extraction (Nasu et al., 1997), Coprecipitation (Soylak & Erdogan, 2006), exchange of ions (Tao & Fang, 1998), adsorption, membrane filtration (Bessbousse et al., 2008), resin chelation (Atia et al., 2005), and electrochemical technologies are efficient at eliminating heavy metal ions from water bodies. On the other hand, improved enrichment factors, easy ease of use, protection with reference to noxious substances, great selectivity, decreased cost, quick recovery, and versatility to integrate it with many advanced detection strategies are only a few of the major advantages of solid-phase extraction (SPE) (Zhang et al., 2010). In fact, the SPE is regarded as a potent method for the extraction and enhancement of a number of inorganic and organic analytes (Bacalum et al., 2011; Radi et al., 2019).

Modified inorganic-organic composites have recently been used extensively as affordable adsorbents to extract heavy metal ions from water bodies due to their interesting qualities, including significant linking preferences, selectivity, excellent adsorption rates with regard to heavy metal ions, enormous areas of surface, and superior thermodynamic, chemical, and mechanical stabilities (Ahmed et al., 2020). There are a variety of modern natural and manufactured solid supports that, with just a few modifications or chemical alterations, can become effective sorbents. Although clays are a natural choice, synthesized silicas may have greater capabilities for significant surface modification to enhance adsorption sites and selectivity (Radi et al., 2014).

Silica has pore diameters that vary from microporous to mesoporous (5 to 500 A0) and huge surface areas (>600 m2 g−1). It comprises of siloxane (Si-O-Si) and silanol (Si-OH), two different types of functional groups. As an outcome, silica gel modification can take place as a result of a reaction involving a specific molecule that reacts either with siloxane (nucleophilic substitution at the Si) or silanol (direct interaction with the hydroxyl group) functionalities (Price et al., 2000). Research has revealed three primary techniques for attaching functional groups to silica surfaces: (i) by means of an interaction between organic or organosilane molecules and the functionalities of silica surfaces; (ii) first, the silica surface is chlorinated, and then the Si-Cl is reacted with the proper functional molecule or reactant; (iii) functional groups are included using the sol-gel technique, and then post modification is applied as needed (Price et al., 2000). However, it has been shown that the stable azomethine group, also known as an imine or Schiff base, has a significant impact on stabilizing metal complexes because it can act as a good interaction site for plenty of transition metals, resulting in steady coordination complexes (Ahmed et al., 2020). There have been several reports of extractants in this context that possess Schiff base functionality grafted on a porous silica substrate (Al-Wasidi et al., 2022a; Al-Wasidi et al., 2022b; Amarasekara et al., 2009a; Koorepazan Moftakhar et al., 2016; Radi et al., 2013; Radi et al., 2014; Radi et al., 2019; Song et al., 2023; Tighadouini et al., 2017; Zheng et al., 2021).

Although many literature reports are there based on excellent chelating ability and useful applications of 4-Acyl pyrazolone ligands, very few reports are there which include 4-acyl pyrazolone functionality grafted on solid support utilized for extraction of heavy metal ions (Pratiwi et al., 2017; Tong Akama & Tanaka, 1990), out of which some focuses on lanthanide extraction only (Amarasekara et al., 2009a; Amarasekara et al., 2009b; Tong Akama & Tanaka, 1990).

The aim of the present work is to develop an acyl pyrazolone containing Schiff base ligand anchored on porous silica substrate for the selective solid-liquid extraction of lead (II) since acyl pyrazolones are an intriguing family of beta-diketones with outstanding metal chelating abilities. To create a very effective and focused sorbent for the targeted extraction of Pb (II), we have here used a porous, high surface area silica support. Here, we have used the first method out of the three mentioned above for modification of the silica surface which involves the reaction of 3-aminopropyl trimethoxy silane with surface silanol (-OH) groups of silica. The conversion to a Schiff base derivative is an attractive modification of the 4-acyl pyrazolone system, which replaces one oxygen atom with a nitrogen donor atom and provides a convenient anchoring point to the ligand (Amarasekara et al., 2009b). An entirely new substance created by grafting (4-chlorophenyl) [5-hydroxy-3-methyl-1-(4-methylphenyl)-1H-pyrazol-4-yl] methanone can serve as a chelating arm on functionalized porous silica in an N, O-bidentate way creating six-membered chelating rings. To make it easier for the receiver and metal ion to make contact, this ligand was immobilized on silica gel using a long arm spacer (Radi et al., 2014). The solid-phase 13C-NMR, nitrogen adsorption-desorption isotherm, Fourier transformation of infrared spectroscopy, elemental examination, thermogravimetry measurements, BET surface area, BJH pore size measurement, scanning electron microscopy (SEM), and powder XRD studies were used to thoroughly evaluate the new chelating material. In the presence of other heavy metal ions, including Cd (II), Cu (II), Cr (II), Ni (II), and Zn (II), as well as alkali and alkaline earth metals, we evaluated its selectivity for adsorption of Pb (II) in aqueous samples using an atomic absorption spectrophotometer (AAS).

2 Experimental

2.1 General Information

All solvents and other compounds were of analytical standard and utilized without additional processing (purity > 99.5%, Aldrich, SRL, and Loba Chemie). Silica gel was heated at 120 °C for a day to activate it before use (Loba Chemie, Mumbai, Maharashtra). The particle size range was 70–230 mesh, and the median pore diameter was 60 Å. Without any prior purification, the silylating agent 3-aminopropyltrimethoxtsilane (SRL, Mumbai, India) was utilized. Analytical grade Pb(NO3)2 was used for the lead extraction study.

2.2 Synthesis of 3-Aminopropylsilica (SiNH2)

The interaction between the silylating substance and the silanol moieties on the exterior of silica was the first stage in the preparation. The previously described process was followed in doing this (Radi et al., 2014). Under a nitrogen environment, activated silica gel (10 g, SiO2) was refluxed and physically agitated for 2 h in dried toluene (60 mL). Dropwise additions of 3-aminopropyltrimethoxysilane (4 mL) were made to this suspension, and the mixture was then maintained at reflux for a day. The filtered material was then cleaned with ethyl alcohol and toluene. The excess silylating agent was subsequently removed using a 12-h Soxhlet extraction with an equal mixture of ethanol and dichloromethane. The resulting product (SiNH2, 12.5g) was then dried under reduced pressure for 8 h at 100°C. SiNH2, the modified silica gel, was vacuum-dried at ambient temperature. The amine group coated on the surface of silica (1.6 mmol/g) was identified by reacting with an excess of 4×10−2 mol/L aqueous HCl, following a titration with 4×10−2 mol/L aqueous NaOH and utilizing phenolphthalein as an indicator.

2.3 Synthesis of (4-Chlorophenyl) [5-Hydroxy-3-Methyl-1-(4-Methylphenyl)-1H-Pyrazol-4-yl] Methanone (L 3 )

Acyl pyrazolone ligand (L3) was prepared according to a previously published literature by our research group (Shaikh et al., 2020; Shaikh et al., 2021) which was according to Jensen’s method (Jensen, 1959). (1) FT-IR (KBr, cm−1): 1552 (C=N, cyclic), 1592 (C=O, 4-chloro benzoyl group), and 1600 (m) (C=O, Pyrazolone ring); (2) 1H-NMR (CDCl3, ppm, 400 MHz): 2.13 (s, 3H), 2.41 (s, 3H), 7.29 (d, J = 8.4Hz, 2H), 7.51 (d, J = 8.4Hz, 2H), 7.61 (d, J = 8.4Hz, 2H), and 7.73 (d, J = 8.4Hz, 2H). Fig. S1 and S2 show 1H-NMR and FT-IR spectra for ligand L3, respectively.

2.4 Synthesis of (4-Chlorophenyl) [5-Hydroxy-3-Methyl-1-(4-Methylphenyl)-1H-Pyrazol-4-yl] Methanone Substituted Silica (SiNP-L3)

L3 ligand (8.0 g) and aminopropylsilica-SiNH2 (5.0 g) were mixed in 60 mL of dry methanol and agitated at 70–80°C for 48 h to generate SiNP-L3. Following filtering, the solid material was subjected to a 12-h Soxhlet treatment using acetonitrile, methyl alcohol, and dichloromethane, respectively. The product (SiNP-L3, 5.4g) was then vacuum-dried for 24 h at 70°C.

2.5 Batch Experiment Studies

The batch approach was used to investigate how the initial concentration, adsorbent dose, pH, and duration of stirring affected the uptake of Pb (II) by the prepared sorbent SiNP-L3. By agitating 20 mg of the modified silica gel with 10 mL of a lead solution with concentrations ranging from 50 to 250 mg/L, for different time intervals (1, 3, 5, 15, 30 min and 1, 2, and 24 h) and different pH values (1–8), the modified silica gel was brought to equilibrium. Compared to the capacity for sorption, the proportion of metal ions was excessive. 500 mg/L of aqueous stock solution of lead (II) ions was prepared by dissolving 0.799 mg of lead (II) nitrate (Pb(NO3)2) in 1 L of double distilled water. It was diluted to make solutions of 50, 100, 150, 200, 250 mg/L as required for optimization studies. The stock solution was stored in the refrigerator and was used for 45 days.

Atomic absorption measurements were used to estimate the lead ion concentration. The following algorithms were used to compute the adsorption amount (Qe, mg/g) and extraction (%) (Irfan et al., 2022).

$$Extraction\left(\%\right)=\left\{\left({C}_o-{C}_e\right)/{C}_o\right\}\times 100$$
(1)
$${Q}_e\left( mg/g\right)-\left\{\left({C}_o-{C}_e\right)/M\right\}\times V$$
(2)

where the metal ion amount on the adsorbent at equilibrium is indicated by Qe (mg/g), V is the abbreviation for the aqueous solution's volume (l), C0 represents the metal ion's starting amount (mg/L), the equilibrium concentration of the metal ion in solution is expressed by Ce (mg/L), and M is the weight (g) of the adsorbent utilized, SiNP-L3. The analyses were done in triplicate for each sample, and the mean outcomes are shown.

2.6 Physical Measurements

Atomic absorption measurements were made using a Perkin Elmer AAcle 500 Atomic Absorption Spectrometer to identify the quantity of adsorbed metal ions. Elemental analyses were performed by using Elementar Analysensysteme (GmbH, Germany by Unicube) at the Sophisticated Instrumentation Center for applied research and testing (SICART, Anand, Gujarat). At room temperature, FT-IR scans of SiNP-L3 as KBr discs were captured using a PerkinElmer IR spectrophotometer. SEM images were obtained on Hitachi SU1510 VP-SEM (Department of Geology, MSU, Baroda). With the nitrogen environment maintained during the measurement, the mass loss calculations were carried out in the TG-DTA 6300 INCARP EXSTAR 6000 using a temperature rise of 10 °C/min in the temperature interval between 25 and 600 °C. The 13C-NMR spectrum of the solid-state was obtained with a CP MAS ECZR series 600 MHz instrument (JEOL, Japan). Powder XRD was carried out using Rigaku smart lab SE 3 kW equipped with Cu-Kα(0.154nm) source (Department of Applied Physics, MSU, Baroda). FE-SEM analysis of the vacuum-dried Pb (II) adsorbed SiNP-L3-Ads was recorded by the model JSM7600F (Jeol) combined with an energy dispersive spectrometer (EDS). The precise surface area was calculated utilizing the BET equation. After the material had been immersed in a steady flow of dry nitrogen, the nitrogen adsorption-desorption was measured using a Micrometrics ASAP 2010 device at a temperature of 196 °C.

3 Results and Discussion

3.1 Sorbent Synthesis

Figure 1 provides a summary of the process for creating the novel chelating substance, SiNP-L3. In order to generate the sorbent (SiNP-L3), 3-aminopropyltrimethoxysilane was used to initially modify the surface of the activated silica gel through a reaction with exterior silanol groups in dry toluene as a solvent. In a dry methanol, acyl pyrazolone ligand-L3 was then reacted with 3-aminopropyl silica (SiNH2), and imine functionality was introduced. Figure 2

Fig. 1
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Synthetic procedure of new chelating material SiNP-L3

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Fig. 2
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Physical appearance of a free silica gel (SiO2), b SiNH2, c SiNP-L3

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3.2 Sorbent Characterization

3.2.1 Elemental Analysis

It is feasible to identify and emphasize the added organic functionalities on the exterior of silica owing to the elemental composition of aminopropyl-grafted silica SiNH2 and SiNP-L3 which contains carbon and nitrogen that were absent in the beginning activated silica. It is apparent that the 3-aminopropyl chain has successfully grafted on the surface of silica by replacing three silanol groups as suggested by the microanalysis results (%C =7.14%, %N =2.17 and %H = 2.594) and C/N ratio from practically and theoretically obtained formulas, which is given as supplementary data (Table S1). The percentage of C, N, and H in the final sorbent SiNP-L3 has increased (%C = 14.06, %N = 2.82, and %H = 2.664) as given in Table 1, indicating that the acyl pyrazolone units were successfully immobilized on the silica gel surface (El Abiad et al., 2017).

Table 1 Elemental analysis of free silica gel (SiO2), SiNH2, and SiNP-L3
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3.2.2 FT-IR Analysis

Figure 3 represents the FT-IR bands of free silica gel (SiO2), SiNH2, and ligand-L3 grafted silica (SiNP-L3). Appearance of some new bands and increase or decrease in intensity of others confirms the specific functionalization of silica surface. In the case of free silica, the O-H stretching and bending of the exterior silanol moieties and physically retained water (H2O) are responsible for the broadband between 3600 and 3300 cm−1 and the band at 1634 cm−1, respectively. Specific bands at 1089 cm−1 and 964 cm−1 are linked to Si-O-Si pulling and Si-O vibrations of exterior silanol groups, respectively (Han et al., 2005). In the FT-IR spectra of SiNH2, a new band corresponding to the CH stretching of the propyl chain developed at 2933 and 2877 cm−1 in contrast to free silica (Jiang et al., 2007). A band at 964 cm−1 vanished in comparison to free silica, while additional bands at 1723 cm−1 and 1634 cm−1 emerged as a result of the free amine group’s -NH bending (Radi et al., 2014). SiNP-L3 differs from SiNH2 in that the reaction of primary amine (-NH2) causes the adsorption band at 1634 cm−1 to vanish and a new characteristic band appears around 1500 cm−1 specifically at 1516 cm−1 and 1538 cm−1 as a result of the -C=N- vibrations of the cyclic and acyclic imine functionalities of Schiff base acyl pyrazolone, which supports the precise anchoring of the organic molecule onto the silica surface. Moreover, the C=O motions of the amide group located within the acyl pyrazolone framework can be offered a substantial adsorption band at 1611cm−1 (Amarasekara et al., 2009a; Amarasekara et al., 2009b). Fig. S3–S5 depict the FT-IR bands of all three materials.

Fig. 3
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Fourier transform infrared spectrum of free silica (SiO2), SiNH2, and SiNP-L3

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3.2.3 Thermogravimetry Measurements and Thermal Stability

To determine the organic content and to assess the possible thermal strength of the created sorbent SiNP-L3, TG analysis was performed. As shown in Fig. 4, the thermogravimetric curves for all surfaces allowed the establishment of the data on thermal stability and the confirmation of the quantity of the chemicals immobilized. The fact that the created materials degrade between 131 and 550°C confirmed its high thermal stability. Due to physically captured water and the combination of the unbound silanol groups into siloxane (Si-O-Si) functionality, there was a first mass loss of 3.8% between rt and 110°C and a second mass loss of 2.01% within 110°C and 550°C (Sales & Airoldi, 2005), respectively, were the important features of free silica gel (SiO2). Again, SiNH2 shows two specific mass loss regions: (1) the silanol moisture retention that is still available due to the use of these molecules in the binding reaction initially causes a slight mass reduction of 22.2% in the temperature range of rt to 180 °C and (2) a considerable mass loss increase of 11.6% was seen during the second step between 208 and 550°C as a result of the organic content that was added to the outer layer during grafting, and it continued after that. Two noticeable mass loss regions were observed for final sorbent SiNP-L3. According to the previous elucidation, the initial mass reduction of 5.1% within rt to 180.37°C is attributed to consumed water, and the second mass loss of 14.82% between 200 and 550°C which also shows sudden decrease after 550°C is assigned to the acylpyrazolone units anchored on the outer layer and the combination of the remained silyl alcohol groups (Tighadouini et al., 2017). The increase in organic chain that is covalently linked to the silica surface and the rising amount of anchoring organic functionalities are consistent with the rise in mass loss, as represented in Fig. S8–S10.

Fig. 4
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Thermogravimetric analysis of free silica (SiO2), SiNH2, and SiNP-L3

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3.2.4 Solid-State 13C-NMR

13C NMR peaks in the stationary state can be utilized to detect significant characteristics of the binding of suspended groups on the inorganic skeleton of the blend produced, as shown in Fig. 5. The signals for propyl skeleton Si-CH2, -CH2-, and N-CH2 observed at δ = 9.72, 21.72, and 42.55 ppm, respectively, in NMR spectra of SiNH2. The most downfield signal at 165.28 is due to imine C and other signals at 146.58, 136.59, 128.61, 117.77, 99.25, 51.26, and 12.95 ppm region occurred due to the anchoring of the L3 ligand; these signals demonstrate the alteration of the support surface and are indicative of the groups found in the framework of the pyrazolone derivative (Tighadouini et al., 2017). Fig. S11 and Fig. S12 show 13C-NMR spectra of SiNH2 and SiNP-L3, respectively.

Fig. 5
figure 5

13C NMR spectra of a 3-aminopropyl grafted silica (SiNH2) and b acylpyrazolone-grafted silica (SiNP-L3)

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3.2.5 Chemical Stability

To illustrate the chemical sustainability of the final sorbent SiNP-L3, several acidic and buffered mixtures (pH 1–7) were utilized. Even after 24 h of interaction, there was no evident alteration in the material’s extraction properties. We have measured its weight and investigated its extraction capacity by doing extraction experiments maintaining the optimized condition using the AAS (atomic absorption spectrophotometer) instrument. Also, we have taken its FT-IR spectra (in the case of both acidic and basic pH) to confirm structural changes if any, but there was no change. S6 and S7 depict FT-IR spectra of prepared adsorbent SiNP-L3 after 24 h of interaction with acidic media (pH=2) and basic media (pH=10). The pendant group is most likely responsible for the connected organofunctional group’s high level of stability. Because prolonged chains lack an effective grip that can undergo Si cation b-elimination, the breakage of the Si-C bond in a mineral acid medium was shown to be prevented when the span of the hydrocarbon link was more than two methylene groups (Mureseanu et al., 2010; Roumeliotis et al., 1983).

3.3 Surface Analysis

3.3.1 Scanning Electron Micrographs

In order to identify alterations to the surface morphology of free silica and chemically transformed silicas, SEM analyses were performed on all four materials. SEM images of free silica gel, SiNH2, SiNP-L3, and Pb (II) adsorbed material depicted as SiNP-L3-ads shown in Fig. 6 were taken at 100–1000 magnifications. Due to the higher number of organic components than the initial non-functionalized silica, which was smooth, the external layer of functionalized particles becomes rougher and has a tendency to partially agglomerate. That organic moieties were successfully caught on the surface is confirmed by the roughness. It is well-recognized that a rough surface typically offers favorable characteristics for usage as an adsorbent. It was clear that the organic functionalities were spread throughout the exterior of the product SiNP-L3, which had a uniformly rough surface due to the attachment of a big pyrazolone ligand (Tighadouini et al., 2017). Furthermore, the exterior of lead-adsorbed material (SiNP-L3-ads) features a regular triangle pattern covering the entire surface, which can be attributed to the consistent complex formation of Pb (II) with binding units anchored on the surface.

Fig. 6
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SEM images of a unmodified silica (SiO2), b SiNH2, c and d SiNP-L3, and e SiNP-L3-ads

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3.3.2 FE-SEM and EDX Analysis

The elemental composition and surface morphology of synthesized adsorbent SiNP-L3 was confirmed using field emission scanning electron microscope coupled with energy dispersive spectrometer. This technique is of great importance for analyzing surface morphology and composition. The morphology of Pb (II) loaded adsorbent is presented in Fig. 7. According to EDS analysis of SiNP-L3-ads, the carbon and oxygen dominate the composition, and the presence of nitrogen confirms the imine linkage with silica surface (Yadav et al., 2022).

Fig. 7
figure 7

a FE-SEM analysis and b EDX analysis of Pb (II) adsorbed material (SiNP-L3-ads)

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3.3.3 Powder XRD

Finally, the X-ray diffraction was used to study the inner skeleton of the sorbent and to compare it with that of the non-functionalized silica. The XRD patterns in Fig. 8 show two distinct peaks indexed at around 100 and 110, demonstrating that the materials maintained their mesoporous structure afterward modification without experiencing any considerable change. Furthermore, a well-organized material type is indicated by the distinctive broad peak at 2Ө (around 5°), which is in accordance with the pore family (100). Since there are no peaks associated with a crystalline phase, the other little broad peak (110), which is at a high 2Ө angle (at about 23°), is typical of amorphous silica. The loss of the long-range porosity order during the functionalization process can be explained by the drop in intensity at (100), which also shows that the mesoporous channels were where the functionalization took place most frequently (Miao et al., 2009; Radi et al., 2018). Also, the powder XRD pattern of Pb (II) adsorbed material SiNP-L3 shows the change in structure and incorporates some new sharp peaks which can be due to the complex formation with Pb (II).

Fig. 8
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X-ray diffraction patterns of SiO2, SiNH2, SiNP-L3, and SiNP-L3-ads

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3.3.4 BET Surface Area and BJH Pore Size Analysis

To demonstrate the porosity alterations of the silica brought about by the introduction of the acyl-pyrazolone unit, we measured the exterior area SBET (Brunauer-Emmett-Teller) (George & Stephen Brunauer, 1938), pore volumes, and pore dimensions of both silica and its derived product by employing nitrogen adsorption-desorption isotherms (Fig. 9) and by Barrett-Joyner-Halenda (BJH) pore diameter measurements. The density of the hooked moieties covalently bonded to the inorganic silica framework changes the surface’s original characteristics. As seen in Table 2, as encapsulation proceeds, the original specific surface area SBET, which was 601 m2 g−1 and had a pore volume of 0.58 cm3 g−1, drops to 298 m2 g−1 and a pore size of 0.28 cm3 g−1. Organic elements that may block nitrogen from reaching the silica base are the primary factor causing a decrease in SBET (Radi et al., 2014). Additionally, the nitrogen-induced adsorption-desorption isotherm for silica derivative in Fig. 9 demonstrates a considerable hysteresis for partial pressures 1> P/P0 > 0.4 and is categorized by the IUPAC as type IV (Miao et al., 2009; Sing & Williams, 2004) which suggested a capillary condensation of the mesoporous substrate (Zheng et al., 2021).

Fig. 9
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Nitrogen adsorption-desorption isotherm and plot of pore diameter vs pore volume of SiNP-L3

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Table 2 Physical measurement values of surface area and pore volume of silica and its derivative SiNP-L3
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3.4 Solid-Liquid Adsorption of Lead (II) by SiNP-L3

3.4.1 Effect of Starting Concentration (C0)

In order to determine the ideal adsorbate concentration, solutions with Pb (II) concentrations of 50–250 mg/L were examined using an adsorbent dosage of 10 mg as shown in Fig. 10a. It was discovered that the sorbent capacity does not significantly increase as compared to concentration at concentrations greater than 50 mg/L.

Fig. 10
figure 10

a Effect of initial Pb2+ content on SiNP-L3 adsorption capability and extraction efficiency at 25°C (10 ml, 20mg of sorbent, pH=5). b Influence of SiNP-L3 amount on the percent elimination of Pb2+ from 10ml of 50 mg/L Pb+2 solution at pH=5. c Influence of pH on extraction efficiency of Pb2+ by SiNP-L3 at 25°C (10 ml, 50 mg/L Pb+2, 20 mg of sorbent). d Impact of interaction time on the adsorption capability and extraction efficiency of sorbent SiNP-L3 (10 ml, 50 mg/L Pb+2, 20 mg of sorbent, pH=6)

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3.4.2 Influence of Sorbent Dosage (mg)

The sorbent quantity ranged from 10 to 60 mg in a 10 ml solution having 50 mg/L of Pb (II) in order to optimize the quantity of sorbent needed for significant extraction of Pb (II) as presented in Fig. 10b. As we can see in Fig. 10b, if we increase the dose after 20 mg (30, 40, 50 mg, etc.), extraction capacity (mg/g) does not increase much compared to that in the case of changing the dose from 10 to 20 mg. Also, if we consider a dose of more than 50 mg, we have to compromise its extraction efficiency which will not be convenient. Thus, as the best sorption was shown to occur at a sorbent dose of 20 mg, we considered it as an optimum dose for further studies.

3.4.3 Effect of pH

It is widely acknowledged that the main factors that influence the metal ion binding to the chelate compounds, whether in solution or applied to solid supports, are the type, charge, dimensions (Myers, 1978; Pearson & Busch, 1963), form and interaction characteristics of the donor atoms, and the buffering conditions. These parameters have been extensively studied in both solution chemistry and in the solid-liquid extraction of specific metals by organic chelates anchored on the exterior of solid supports like silica gel, nanocomposites, or polymeric moieties. Thus, as one of the most important governing factors in such a procedure, we investigated the impact of the pH of the solution containing Pb (II) on the metal extraction capacity in order to assess the feasibility of the synthesized material SiNP-L3 for Pb (II) separation and its bonding. pH range of 1–8 was used for the estimation of the adsorption qualities of SiNP-L3 as shown in Fig. 10c. The findings show that the metal ion intake of the adsorbent varies significantly as the pH fluctuates. At lower pH levels, the consumption of Pb (II) ions by the altered silica SiNP-L3 is minimal because the ligand must be almost entirely in its protonated form (Radi et al., 2014). The protonation weakens as the pH rises, enhancing the coordinate interaction and uptake of Pb (II) ions. It is challenging to tell if the lead (II) in Fig. 10c is adsorbed or hydrolyzed because lead ions get hydrolyzed at pH values higher than 6.5, which results in the formation of Pb (II) hydroxides: Pb (OH)+ and Pb (OH)2. So, pH=6 was the most suitable pH for the highest possible sorption of Pb (II). Also, we have determined the pH of the solution after the sorption study. We observed that pH of solution increases in case of acidic pH while pH of solution decreases in case of basic pH after sorption. This could be due to the fact that adsorbent will acquire positive charge on their surface in acidic pH due to the protonation of N and O donor atoms in the ligand (extraction (%) decreases), and this protonation becomes weak in basic pH (Extraction (%) increases).

3.4.4 Effect of Contact Time and Adsorption Kinetics

The duration of stirring required to achieve equilibrium conditions and the Pb (II) adsorption by the ligand L3 functionalized silica gel is crucial. Batch experiment studies were used to examine the impact of interaction time on the uptake of Pb (II) by SiNP-L3. The Pb (II) kinetic charts demonstrated that the sorption was fast, and a saturation point was reached after roughly 60 min of contact, as can be shown in Fig. 10d, but to examine the effect of extended time on the extraction percentage (%), the study was kept for 24 h, and the change was insignificant. The rapid Pb (II) ion adsorption demonstrates that the engaged donor atoms, oxygen and nitrogen, on the exterior of the improved silica gel are oriented in a manner that does not restrict their affordability, allowing for rapid contact with the Pb (II) ions in the solution. Rapid kinetics is extremely useful since it enables reduced reactor sizes while maintaining efficiency and economics.

Extraction kinetics is essential for practical applications, operation control, and design because it provides significant insights regarding the reaction route and the internal workings of the procedure of the uptake. Kinetics may now be used to explain the consumption of solutes by controlling the degree of adsorbate acceptance at the junction between solid and solution. As we are focusing on developing acyl pyrazolone–incorporated adsorbent material for extraction of heavy metal ions which was not reported before, in order to prove the exact suitable kinetic model, rate determining step, and solute uptake phenomenon, we have applied experimental data to all basic kinetic models. The findings from experiments have been analyzed using a variety of kinetic models, including zero-order, first-order, second-order, third-order, pseudo-first-order, and pseudo-second-order models. The correlation co-efficient led to the conclusion that the current readings suited the pseudo-second-order model nicely compared to others, as shown in Fig. 11. qe is the equilibrium amount of lead adsorbed (mg/g) in this instance. The amount of lead that has been adsorbed at time t (min) is symbolized by the abbreviation qt (mg/g), and the rate constants for the pseudo-first-order and pseudo-second-order kinetic equations are k1 and k2, respectively, in mg min g−1. With a correlation coefficient of 0.985, the experimental data satisfies a pseudo-second-order model (Yadav et al., 2022). Table 3 presents the kinetic parameters of Pb2+ adsorption. This suggests that the chemical interactions between Pb+2 and the adsorbent determine the rate of chemisorption (Yadav et al., 2019).

Fig. 11
figure 11

a Zero-order, b first-order, c second-order, d third-order, e pseudo first-order, f pseudo-second-order kinetic curves for adsorption of Pb+2 on SiNP-L3

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Table 3 Coefficients of pseudo-first-order and pseudo-second-order kinetic models for adsorption
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3.4.5 Adsorption Isotherms

Isotherm is a vital element feature in the procedure of adsorption. Typically, the performance of an adsorbent is estimated using the adsorption isotherm which depends on adsorption equilibrium. At constant temperature, the isotherm for adsorption is defined via the connection between the optimal adsorption concentration (Ce) and the equilibrium rate of adsorption per gram of mass (qe) (Hu et al., 2017). The variable is determined by the bulk content and the pollutant that has been adsorbed at the interface. Adsorption isotherms provide a comprehensive description of the nature of interaction and are essential for developing an adsorption system. Pb2+ adsorption capacities onto SiNP-L3 were examined in this context employing Langmuir (Eq. 1) and Freundlich (Eq. 2) isotherm models for adsorption (Fig. 12).

$$\frac{C_e}{q_e}=\frac{1}{K_L}+\frac{C_e}{q_m}$$
(1)
$$\log {q}_e=\log {K}_F+\frac{1}{n}\log {C}_e$$
(2)

wherein Ce (mg L−1) is the Pb (II) equilibrium amount, the potential for adsorption of Pb (II) at a state of equilibrium is defined by qe (mg g−1); the highest potential for adsorption (mg g−1) is symbolized by qm, 1/n is a constant affiliated with the adsorption intensity fluctuation owing to adsorbent heterogeneity, and the Langmuir and Freundlich retention constants at equilibrium are denoted by KL (L/mg) and KF (L/mg), respectively. Under the conditions of 0<KL<1, KL>1, and KL=1, respectively, the value of KL reflects whether the adsorption is favorable, unfavorable, or linear in nature (Das et al., 2020). Table 4 displays the computed data from the Langmuir and Freundlich equilibrium models. Both adsorption isotherm models accurately predicted the experimental results. The Langmuir model, on the other hand, outperforms the Freundlich model based on the coefficient of correlation, confirming a monolayer adsorption process. In the monolayer formation, specific homogeneous areas on the exterior of adsorbent enhance the adsorption; all of these areas are similar in terms of energy, and no adsorbate molecule migration takes place on the surface’s plane (Yadav et al., 2022).

Fig. 12
figure 12

a, b Langmuir and c Freundlich coefficients of adsorption for Pb+2 deposition on SiNP-L3

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Table 4 Coefficients of Langmuir and Freundlich isotherms
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3.4.6 Thermodynamic Study

The thermodynamics of adsorption is an important technique for determining whether an adsorption response is natural or not. Adsorption mechanism is also determined by the thermodynamic study. Thermodynamic algorithms were employed for estimating the data gathered during adsorption investigations at different temperatures, notably 35 °C, 45 °C, and 55 °C. Several parameters, comprising Gibbs free energy (ΔG°), alteration of entropy (ΔS°), and variation in enthalpy (ΔH°), were computed from the thermodynamic equilibrium parameter (Kc) using the following equations:

$${K}_c=\frac{a_s}{a_e}=\frac{\vartheta_s}{\vartheta_e}\ \frac{q_e}{C_e}$$
(3)

Here, the movement of the Pb+2 ions tied on the surface of the adsorbent is signified by as and its equilibrium activity contained within the solution is symbolized by ae. The activity factor of adsorbate taken up in solution is signified by ϑs, while the activity factor of adsorbate molecules at equilibrium is signified by ϑe. Whenever the concentration of the adsorbate solution falls and approaches zero, the activity coefficient ϑ eventually hits unity. In such a scenario, the aforementioned equation could be expressed as follows;

$$\underset{C_e\to 0}{\lim}\frac{a_s}{a_e}=\frac{q_e}{C_e}={K}_c$$
(4)

The intercept and slope of a Van’t Hof plot of data obtained from experiments (lnKc vs 1/T) at a trio of temperatures, 308 K, 318 K, and 328 K, were taken into account to figure out the values of ΔH° and ΔS°. Then, implementing Eq. 5, the Gibbs free energy was computed.

$$\Delta {G}^0=- RT\ln {K}_c$$
(5)
$$\ln {K}_c=\frac{\Delta {S}^0}{R}-\frac{\Delta {H}^0}{R}\ \frac{1}{T}$$
(6)

The negative numbers of ΔG° at varying temperatures highlight the natural pattern of Pb2+ adhesion on SiNP-L3. Higher negative numbers with increasing temperature suggest a more energy-efficient technique for adsorption. An increment in the negative numbers of ∆G° at elevated temperature is revealed by the present study as shown in Table 5. The positive value of ΔS° implies a rise in randomness through the adsorption process at the solid-solution junction. A positive reading of ΔH° explains the endothermic character of the adsorption procedure employing adsorbent SiNP-L3. Furthermore, the physisorption mechanism is illustrated by ΔH° values that span 2.1–20.9 kJ mol−1, and the chemisorption technique is outlined by values spanning from 80–200 kJ mol−1. Because the results of the computation exceed 20.9, chemisorption is accompanied by SiNP-L3 for extraction of Pb (II). This validates the kinetics modeling that had been identified (Yadav et al., 2022).

Table 5 Thermodynamic parameters for the adsorption of Pb+2 on SiNP-L3
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3.4.7 Desorption Study

To make the practice of adsorption economical and profitable, the adsorbent must have properties such as greater capacity to adsorb, easy renewal, and replicated reuse abilities. The desorption study is a vital indicator for renewing the adsorbent and recuperating the Pb+2 ions. The efficiency of the adsorbent is also explained by the desorption experiments. For the act of desorption to be successful, a compatible solvent arrangement that corresponds to the characteristics of adsorbent and adsorbate interplay is required (Yadav et al., 2022). To explore the reusability of SiNP-L3, repeated adsorption/desorption rounds have been conducted with 0.01N, 0.1N, 2N, and 6N HCl and 0.01N, 0.1N NaOH as eluents (Fig. 13a). The outcomes establish that 2N HCl (5-10 ml per g of support followed by washing with deionized water) is a more beneficial desorption eluent rather than NaOH. The rapid desorption efficiency with 2N HCl could be attributed to the robust relationship of Pb (II) ions with the external functionalities present on the SiNP-L3 adsorbent, boosting desorption. Ninety-eight percent of extraction efficiency was found with 2N HCl as eluent (Tighadouini et al., 2021). Results indicate that extraction efficiency gradually declines after each cycle (Fig. 13b). The characteristic spectra of SiNP-L3 after regeneration are preserved as suggested by FT-IR investigation, as shown in Fig. 13c. This implies that SiNP-L3 preserves its structure after effective Pb2+ adsorption, increasing the economic feasibility of the process (Radi et al., 2016; Tighadouini et al., 2017).

Fig. 13
figure 13

a Screening of various eluents for recycling, c FT-IR spectra comparison of SiNP-L3 with recycled SiNP-L3, b extraction efficiency after every adsorption-desorption cycle using 2N HCl as eluent

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3.4.8 Effect of Coexisting Ions

Several more common metal ions (Li(I), Na(I), K(I), Ca (II), Mg (II), Cd II), Cr (III), Cu (II), Ni (II), and Zn (II)) in aqueous solution were utilized to conduct studies for Pb (II) selectivity. At pH 5, 100 mg of SiNP-L3 was added to 50 mL of a watery solution containing about 50 mg/L of each of the metal ions claimed before. The adsorption fraction of the Pb (II) ions on the SiNP-L3 approached over 85%, while lower than 15% other metal ions in which is not affected by the presence of large excess of several common alkali and alkaline earth metal ions (see Fig. 14). This illustrates that SiNP-L3 remains capable to capture Pb (II) ions in almost measurable concentrations even when multiple competing metal ions are present in solution, making SiNP-L3 a perfect option for selective lead adsorption. We assumed that the differing affinities between SiNP-L3 and Pb (II) ions or other metal ions were the cause of the adsorption selectivity of SiNP-L3 for Pb (II) ions. As a result, we inquired about the previously reported data about the electronegativity and ionic diameter values of metal ions, as shown in Table 6 (Li et al., 2015). Pb (II) has the next biggest ionic radius, while a variety of metal ions have considerably fewer radii than it. In summary, a robust steric barrier preventing extra metal ions from physically reaching the adsorption sites appeared unlikely to exist. Fortunately, Pb (II) ions exhibited the highest absolute electronegativity of any other metal ion used in this investigation, indicating that Pb (II) has the strongest attraction to the isolated pairs of electrons on oxygen and nitrogen atoms and can thus form extremely resilient complexes. Owing to the hypothesis of hard and soft acids and bases (HSAB), Schiff ligands operate as Lewis soft bases, while Pb (II) ions may be designated a middle-soft acid. As a result of the attraction of electrostatic charge and chelating interaction, the significant affection of SiNP-L3 toward the desired Pb (II) could be observed (Zheng et al., 2021). These two are probably what caused Pb (II) ions to preferentially bind to SiNP-L3 over other metal ions. Additionally, it has been determined that practically all metal ions have absolute electronegativity values lower than Pb (II) ions; therefore, this would not be a barrier to the use of SiNP-L3 for the selective capturing of Pb (II) ions in real water samples.

Fig. 14
figure 14

Selectivity for Pb (II) in the presence of other metal ions

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Table 6 Physical characteristics of several metal ions
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3.4.9 Adsorption Mechanism

The probable interaction of SiNP-L3 with Pb2+ is shown in Fig. 15. The pore volume, surface area, active sites, and surface chemical groups of the adsorbent, as well as the structural and functional behavior of the adsorbate molecules, all influence the adsorption mechanism (Zhou et al., 2019). Cationic pollutant is involved in this system, i.e., heavy metal (Pb2+). Host-guest interaction is initially thought to be the process of adsorption dependent on significant selectivity for Pb+2 ions. The adsorbate molecules get attracted to the oxygen and nitrogen donor atoms where they form six-member chelation rings with their lone pair of electrons. The number of active sites and the microstructure of the adsorbent have an impact on both the adsorption capacity and the rate of adsorption. Therefore, it is believed that Pb (II) adsorption involves electrostatic interaction (Al-Wasidi et al., 2022b; Tighadouini et al., 2017).

Fig. 15
figure 15

Probable adsorption mechanism for SiNP-L3

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3.4.10 Comparison with Alternate Adsorbents (Table 7)

Table 7 Comparison study of present work with previously reported literatures
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4 Conclusion

In this study, novel silica functionalized with acyl pyrazolone Schiff base ligand (L3) was successfully prepared. 3-Aminopropyl trimethoxy silane was used to initially modify the surface of the activated silica gel through a reaction with exterior silanol groups and then simple Schiff base reaction with (4-chlorophenyl) [5-hydroxy-3-methyl-1-(4-methylphenyl)-1H-pyrazol-4-yl] methanone was carried out to prepare introduced sorbent SiNP-L3. The new material was well characterized by scanning electron microscopy, the Fourier transformation of infrared spectroscopy, elemental examination, BET surface measurement, Barrett-Joyner-Halenda pore dimension, solid-state 13C-NMR, powder XRD, and thermogravimetry measurements. The newly synthesized eco-friendly adsorbent displayed good thermal stability and selective adsorption capacity from aqueous solutions towards Pb (II) ions. The adsorption isotherms of Pb (II) fitted well with the Langmuir Isotherm Model (R2=0.98), indicating that the adsorption mechanism followed Langmuir monolayer adsorption, and the adsorption kinetics of Pb (II) followed the pseudo-second-order kinetic model (R2=0.98). In addition, the newly synthesized material was proven to be a cost-effective adsorbent and could be used for the selective extraction of Pb (II) from contaminated water even in the presence of other metal ions having an adsorption capacity of 16.8 mg g−1. Also, as the adsorption process of Pb (II) on a synthetic adsorbent SiNP-L3 is endothermic, we may consider optimizing the conditions for large-scale experiments. According to Le Chatelier’s principle, in endothermic processes, if we increase the temperature, the reaction rate (here extraction rate) also increases. So we can increase the temperature up to 65°C (as we have done in thermodynamic study) as water is the solvent. Also, during adsorption, we have to use efficient insulation to minimize heat loss and explore ways to enhance the kinetics of the endothermic process. Additionally, monitoring other factors such as pH, concentration, and contact time is crucial for successful large-scale implementation.