Amberlite XAD-4 Functionalized with 4-(2-Pyridylazo) Resorcinol via Aryldiazonium Chemistry for Efficient Solid-Phase Extraction of Trace Metals from Groundwater Samples

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A new solid-phase extraction (SPE) sorbent was synthesized by covalently attaching the PAR chelating agent to Amberlite XAD-4 resin using aryldiazonium salt chemistry. The chelating resin (XAD-4-PAR) demonstrated significant selectivity and extraction efficacy for Co(II), Ni(II), and Cu(II) ions from actual groundwater samples. These results illustrate its potential utility as a reliable sorbent for the extraction of trace metals in environmental analysis.

Abstract

Aryl diazonium salt chemistry offers a robust and versatile approach for the modification of material surfaces via the covalent immobilization of reactive functional groups under mild conditions. In this study, this strategy was successfully applied to graft the chelating agent 4-(2-pyridylazo)resorcinol (PAR) onto Amberlite XAD-4 resin. Initially, 4-nitrobenzenediazonium tetrafluoroborate (NBDT) was covalently anchored onto the resin surface using hypophosphorous acid as a reducing catalyst to introduce aryl nitro groups. These nitro groups were subsequently reduced to aniline functionalities, enabling diazo coupling with PAR. The successful modification of the resin was confirmed by ATR-FTIR spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The synthesized chelating resin exhibited sorption capacities of 0.152, 0.167, and 0.172 mM g⁻¹ for Co(II), Ni(II), and Cu(II), respectively. The functionalized resin was packed into standard SPE cartridges and employed as a selective sorbent for the extraction and preconcentration of trace metals from groundwater samples collected in Dhalamah Valley, Al-Madinah Al-Munawwarah, prior to quantification by inductively coupled plasma mass spectrometry (ICP-MS). These results demonstrate the effectiveness of rapid diazonium-based surface functionalization for the preparation of selective polymeric metal chelators suitable for the extraction of trace metals from complex groundwater matrices.

1. Introduction

The growing demand to detect and quantify trace levels of metals in environmental monitoring, clinical diagnostics, and materials science has driven the advancement of highly sensitive analytical techniques. However, effective sample preparation remains crucial for concentrating analytes to detectable levels and eliminating matrix interferences that compromise measurement accuracy [1,2,3]. Various separation and enrichment methods, such as coprecipitation [4], ion exchange [5] electro-deposition [6], cloud point extraction [7], membrane filtration [8], solid-phase extraction (SPE) [9], and liquid–liquid extraction (LLE) [10], have been developed to address these challenges, depending on sample type, the concentration range, and the analytical technique to be used.

Among these techniques, SPE has been a widely adopted multi-element extraction and preconcentration method for over seven decades. Renowned for its high selectivity and efficiency, SPE enables the isolation and enrichment of target analytes from complex matrices, making it suitable for a broad range of sample types. Its cost-effectiveness, low solvent consumption, and compatibility with automation further reinforce its position as a leading sample preparation approach [9,11,12]. The technique’s versatility is further enhanced by the availability of a broad range of commercial SPE sorbents with tailored selectivity, as well as the potential for designing customized functionalized materials to meet specific analytical needs. Among these sorbents, chelating resins serve as a particularly effective substrate for extraction of trace metals due to their capacity to selectively bind metal ions through coordination chemistry, thereby forming stable and specific complexes. Chelating resins are typically synthesized through either physical impregnation or chemical immobilization of chelating moieties containing functional groups onto readily available natural or synthetic solid supports, such as activated carbon, alumina, cellulose, silica gel, or polymeric resins. While physical modification is simpler, covalent chemical bonding is generally preferred due to its superior stability, which prevents ligand leaching during operation, and its ability to precisely control functional group density—thereby enhancing selectivity and adsorption capacity. This combination of durability and customizability makes chemically modified resins more effective for consistent, high-performance sample preparation of trace metals from diverse matrices [13,14,15].

Amberlite XAD resins—highly porous crosslinked polystyrene-divinylbenzene (PS-DVB) copolymers—are widely recognized adsorbents, owing to their large surface area, tunable porosity, and robust chemical stability [16,17,18,19]. However, in their plain form, these hydrophobic resins lack selectivity and are hence unsuitable for the sorption of metals. This limitation can be overcome by functionalizing their surfaces with specific chelating ligands to tailor their selectivity and binding capacity for specific metal ions [20]. For instance, iminodiacetate-functionalized XAD resins exhibit high affinity for uranium and transition metals [21], whereas dithiocarbonate-modified variants exhibit superior efficiency in capturing heavy metals such as Pb(II) and Cd(II) [19]. Such modifications leverage the resin’s inherent porosity and surface area to empower efficient mass transfer and high capacity for the retention of analytes [22]. Moreover, the robust structural integrity of PS-DVB resins under operational conditions is critical for their regeneration and reuse, thereby contributing to their cost-effectiveness in applications such as metal recovery, analytical preconcentration, and wastewater treatment [23,24].

Polystyrene-based resins, such as Amberlite, are frequently functionalized via chloromethylation and nitration to introduce reactive sites for subsequent derivatization. Chloromethylation incorporates chloromethyl (–CH₂Cl) groups, which act as electrophilic centers facilitating nucleophilic substitution reactions, thereby enabling the covalent attachment of diverse ligands and functional groups. This modification is widely employed in the preparation of ion-exchange resins, catalyst supports, and SPE materials [19,25,26]. Nitration introduces nitro groups that can be selectively reduced to primary amines, which are exploited in subsequent functionalization with chelating agents or catalytic moieties, thus expanding the material’s application in adsorption and catalysis [25]. However, these methods have significant limitations. Chloromethylation typically requires hazardous reagents, such as chloromethyl methyl ether (a known carcinogen), and must be carefully controlled to avoid undesirable cross-linking or polymer degradation [19,25,26]. Nitration, meanwhile, uses strongly acidic conditions that can compromise the integrity of the polymer backbone, which could lead to reduced mechanical stability and limited long-term usability [27,28]. These limitations hinder reproducibility and limit the versatility of these methods [29]. While advanced techniques such as surface electro-initiated emulsion polymerization (SEEP) offer a more sustainable alternative by enabling water-based electroreduction and radical polymerization [28], this approach is limited to conductive substrates and hence its usage to modify non-conductive materials like Amberlite resins is delimited.

These challenges have driven the development of redox-based surface modification strategies, expanding the chemical functionalization of a wider range of materials, including non-conductive substrates. One prominent method is aryldiazonium salt chemistry, which utilizes hypophosphorous acid reduction to generate highly reactive aryl radicals in situ. This approach has emerged as a versatile and effective surface modification technique, gaining increasing adoption in recent years. Early research demonstrated successful grafting of aryldiazonium radicals onto carbon-based materials (e.g., activated carbon), polymers (e.g., polymethylmethacrylate, PMMA), and natural substrates such as cocoa shells [29,30,31]. However, more recent reports highlight the method’s versatility, showcasing its application to a broad range of substrates, including boron nitride, cellulose, clay, silica, titanium dioxide, zeolites, carbon nanotubes, graphene, and various metal and metal oxide nanomaterials [32,33]. These advancements underscore the method’s growing potential for tailoring the surface properties across diverse material classes.

Interestingly, despite the remarkable versatility of diazonium salt chemistry for modifications of materials in many applications, its use to fabricate metal chelators remains scarcely reported, except for two studies from our laboratory. In these articles, AC [34] and Amberlite XAD-4 resin [35] were functionalized with 8-hydroxyquinoline (8-HQ) agent to produce chelating resins as specific SPE sorbents for trace metals, e.g., Cd(II), Ni(II), Mn(II), Zn(II), Pb(II), Co(II), and Cu(II) from groundwater matrices prior to their analysis by ICP-MS.

The chelating agent 4-(2-pyridylazo)-resorcinol (PAR) is particularly remarkable due to its capability to form stable complexes with various transition and heavy metals over a wide pH range [36,37]. Its multifunctional reactive groups (heterocyclic nitrogen, azo, and o-hydroxyl moieties) enable selective coordination with metals. Therefore, it has been extensively immobilized onto various substrates by numerous chemical and physical methods [38,39,40,41]. In this study, the eco-friendly diazonium salt grafting approach was employed to anchor PAR onto Amberlite XAD-4, and the resulting chelating resin has been evaluated as a SPE sorbent for the extraction of trace metals from groundwater samples [39,42,43,44].

2. Materials and Methods

2.1. Chemical Reagents

All chemicals and reagents used were of analytical grade. Working solutions were prepared using ultrapure water from a Millipore Milli-Q Advantage A10 system (Deutschland, Germany). Amberlite XAD-4 resin (polystyrene-divinylbenzene, 20–60 mesh, 750 m²/g surface area), 4-nitrobenzene diazonium tetrafluoroborate (NBDT), ammonium hydroxide, ammonium acetate, acetic acid, hypophosphorous acid, and 4-(2-pyridylazo)resorcinol (PAR) were purchased from Sigma-Aldrich Chemie GmbH (Darmstadt, Germany). Sodium hydrosulfite (Na₂S₂O₄) and ICP-grade metal stock solutions (1 mg/mL) were obtained from Acros Organics (Geel, Belgium). Sodium nitrite (NaNO₂), hydrochloric acid (HCl), and nitric acid (HNO₃) were supplied by Loba Chemie (Mumbai, India), and ethanol from United Beta Industries (Dammam, Saudi Arabia). Chelex-100 resin was sourced from Bio-Rad (Hercules, CA, USA).

Metal standards were prepared by diluting stock solutions. Buffer solutions were made with ammonium acetate and purified using Chelex-100 beads (column). pH adjustments were performed with HNO₃ or acetic acid. All glassware was cleaned with detergent, soaked overnight in 5% (v/v) HNO₃, and rinsed with Milli-Q water before use.

2.2. Preparation of PAR-XAD-4 Chelating Resin

Amberlite XAD-4 resin was chemically modified with PAR chelator following a sequential diazotization–reduction–coupling approach. Initially, 10 g of the resin was dispersed in 50 mL of a 30 mM solution of 4-nitrobenzenediazonium tetrafluoroborate (NBDT) under constant stirring. Then, 30 mL of hypophosphorous acid was added dropwise to the suspension, and the reaction was maintained at 5 °C for 30 min with frequent stirring every 5 min. The nitro-functionalized resin (AXAD-4-NO₂) was collected by vacuum filtration, washed repeatedly with distilled water and acetonitrile to remove residual reagents, and dried under vacuum for 24 h [30,35,43]. The nitro groups were reduced to amines by treating the AXAD-4-NO₂ derivative with 50 mL of 5% sodium hydrosulfite (Na₂S₂O₄) in a sealed container at 45 °C for 24 h. The resulting aminated resin (AXAD-4-NH₂) was filtered and thoroughly washed with cold water to remove residual salts [44]. Next, AXAD-4-NH₂ was diazotized with 50 mL of 2% (w/v) sodium nitrite in 1% acetic acid at 0–5 °C for 1.5 h. The product was filtered and washed with cold water, then dispersed in an ethanolic solution of 4-(2-pyridylazo)resorcinol (PAR) (50 mL, 1% w/v) and stirred for 4 h to complete the diazo coupling. The final PAR-functionalized resin (AXAD-4-PAR) was sequentially washed with ethanol, deionized water, and 2% HCl until the washings were colorless, then vacuum-dried and stored in a desiccator until use [35].

2.3. Sorption Studies and Solid-Phase Extraction Procedure

2.3.1. Sorption Experiments

The effects of solution pH, contact time, and sorption capacity of the synthesized AXAD-4-PAR chelating resin for Co(II), Ni(II), and Cu(II) ions were evaluated using batch-mode experiments. For pH optimization, 0.1 g of dry resin was dispersed in 10 mL of a 100 ng/mL metal ion solution prepared in 0.2 M ammonium acetate buffer, with the pH adjusted across the range of 2–10 using 0.1 M nitric acid, acetic acid, or ammonia solutions. The suspension was equilibrated overnight on a mechanical shaker, followed by centrifugation (4000 rpm, 5 min) to separate the solid phase. The residual metal concentrations in the supernatant were then quantified by ICP-MS.

For the optimization of pH and the determination of the sorption capacity, 0.25 g of resin was added to 25 mL of a 50 mg/L metal ion solution and agitated on a shaker overnight. The remaining metal concentrations in the supernatant were analyzed by ICP-MS, and the sorption capacity millimole/g(mM/g) was calculated based on the difference from the initial concentration. Meanwhile, in kinetic studies, 0.1 g of resin was dispersed in 100 mL of a 10 mg L⁻¹ metal ion solution under magnetic stirring. Aliquots were withdrawn at predetermined time intervals (2–120 min), filtered, and analyzed to determine the residual metal ion concentrations.

2.3.2. Solid-Phase Extraction Protocol

SPE experiments were performed using a 12-port vacuum manifold (Ato Science, Shanghai, China) connected to a vacuum pump (AP-9950). SPE cartridges (Agilent, Santa Clara, CA, USA) were filled with 0.5 g of AXAD-4-PAR resin, pressed into 5 mm discs between two porous Teflon filters. The SPE process followed these steps: First, the cartridges were conditioned by passing 5 mL of buffer solution through them at 1 mL/min to activate the chelator. Next, 50 mL of the sample was loaded at 0.5 mL/min, followed by a 5 mL rinse with Milli-Q water (at 1 mL/min flow rate) to clear out any remaining matrix components. The trapped metal ions were then eluted using 5 mL of 1.5 M nitric acid at 2 mL/min. The eluate was collected in 10 mL PTFE vials and diluted with 5 mL of Milli-Q water. This SPE method was used for calibration standards, certified groundwater reference materials, and real samples before analysis with ICP-MS [35,41]. For better match matrix of groundwater, the calibration standards were prepared in synthetic groundwater following a reported method with slight modifications [45].

2.4. Instrumentation and Measurements

Appropriate analytical techniques were used to characterize the synthesized metal chelator and quantify metal ions after sample preparation using the developed SPE processing. Fourier-transform infrared (FTIR) scan was performed with a Thermo Nicolet 380 FTIR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) equipped with an ATR accessory, and the surface functional groups were identified by recording spectra from 400 to 4000 cm⁻¹. X-ray photoelectron spectroscopy (XPS) analysis was accomplished using a Thermo Scientific instrument (Waltham, MA, USA) with an Al Kα X-ray source (pass energy: 150.0 eV, step size: 1 eV) to analyze the elemental composition and chemical states of C1s, N1s, and O1s core-level regions. Thermogravimetric analysis (TGA) on a TA Instruments SDT Q600 thermal analyzer (Tokyo, Japan) under a nitrogen atmosphere at a 10 °C/min heating rate was used to evaluate thermal stability and immobilized organic content. A HI 2211 pH/ORP meter (HANNA Instruments, Carrollton, TX, USA) was used to monitor pH during synthesis and extraction. Quantification of metals in standards and samples was conducted using inductively coupled plasma mass spectrometry (ICP-MS, 7500 Series, Agilent Technologies, Santa Clara, CA, USA) under standard conditions for optimal trace metal sensitivity and accuracy.

3. Results and Discussion

3.1. Chemical Modification Process

The immobilization of PAR onto Amberlite XAD-4 was carried out via a diazonium-mediated sequence (Figure 1). First, an aryl diazonium salt was reduced with hypophosphorous acid (H₃PO₂) to generate aryl radicals that graft covalently onto the aromatic PS–DVB framework, introducing nitroaryl handles on the resin surface. This grafting proceeds through surface-mediated electron transfer and is well documented for polymeric and carbonaceous substrates; the radical pathway of H₃PO₂ reductions of aryl diazonium salts is also well established [46,47]. In the second step, the pendant nitro groups were chemo-selectively reduced to anilines using sodium dithionite (Na₂S₂O₄), providing nucleophilic sites for the final transformation. Lastly, the surface anilines were diazotized at low temperature and coupled with PAR chelator, affording XAD-4–PAR through stable azo (–N=N–) linkages formed by electrophilic azo coupling [33,38,46].

3.2. Characterization of AXAD-4-PAR Chelating Resin

Covalent immobilization of the PAR chelating agent onto the Amberlite XAD-4 surface was first evidenced by the distinct color change characteristic of azo dyes. Confirmation was obtained by attenuated total reflectance infrared spectroscopy (ATR-IR), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS), which together elucidated the structural, thermal, and surface-chemical features of the modified resin.

The ATR-IR spectra of bare XAD-4 and PAR-functionalized resins, shown in Figure 2, revealed significant differences. As clear from Figure 2a, the polymer XAD-4 exhibits the polystyrene matrix bands: aromatic C–H stretch at 3031 cm⁻¹, aliphatic C–H at 2925 and 2851 cm⁻¹, aromatic ring C=C at 1605 cm⁻¹, and ring modes at 1485–1453 cm⁻¹ with out-of-plane C–H bends at 738 and 680 cm⁻¹—positions consistent with certified polystyrene standards and polymer IR assignments [48]. Figure 2b shows that XAD-4-PAR chelator retains these backbone features but shows additional diagnostic absorptions: 1506 cm⁻¹ (azo N=N stretch), 1360 cm⁻¹ (aryl C–N stretch), and 1247 cm⁻¹ (phenolic C–O). The N=N band near ~1510 cm⁻¹ and the blue-shift of C–O into ~1201–1240 cm⁻¹ are characteristic of PAR and its coordinated/immobilized forms, while aryl C–N stretches typically lie in the 1335–1250 cm⁻¹ region [49].

TGA was used to validate the surface functionalization by comparing the thermal profiles of unmodified XAD-4 and PAR-grafted XAD-4–PAR (Figure 3). The bare resin exhibits a broad, monotonic mass loss from ≈230 to 895 °C, characteristic of PS–DVB decomposition. In contrast, XAD-4–PAR shows an early mass-loss step of ~11% below ≈370 °C, attributable to desorption/decomposition of the surface-bound PAR moieties, followed by a major high-temperature mass loss (~89%) associated with degradation of the polymer matrix. The two-step pattern is consistent with incorporation of PAR onto the resin surface [49,50].

XPS provided independent confirmation of PAR immobilization on the XAD-4 surface by probing the resin’s near-surface elemental composition (Figure 4). The survey spectrum of XAD-4–PAR reveals nitrogen signals that are absent in native XAD-4, and the high-resolution N 1s envelope can be deconvoluted into components at 398.08 eV (pyridinic N) and 400.08 eV (azo –N=N–), consistent with covalent attachment of PAR through diazonium coupling. Taken together with the vibrational data, these XPS features verify successful surface functionalization of the resin with the PAR moiety [50].

3.3. Optimization of Sorption Process

The systematic optimization of the key parameters influencing the sorption process, such as solution pH and contact time, is crucial for adjusting the sorption performance of chelating resins toward the targeted metal ions. These parameters have a fundamental impact on metal–ligand complexation and thus the overall efficiency of the SPE process. It is also decisive to predict the effective preconcentration and quantitative recovery of the target metal ions using a metal chelator. The systematic optimization of the key parameters influencing the sorption process, such as solution pH and contact time, is crucial for adjusting the sorption performance of chelating resins toward the targeted metal ions. These parameters have a fundamental influence on metal–ligand complexation and thus the overall efficiency of the SPE process. It is also decisive to predict the effective preconcentration and quantitative recovery of the target metal ions using metal chelator.

3.3.1. Effect of pH

Solution pH is a primary control on sorption by chelating resins because it governs (i) competition between protons (H⁺) and metal cations for donor sites and (ii) hydrolysis/precipitation of metal ions at elevated pH. In this work, equilibrium sorption capacity q_e (mM g⁻¹) was plotted against pH (Figure 5). Capacity increased with pH and reached clear maxima at pH 8 for Co(II) and Ni(II) ions, whereas Cu(II) ions peaked earlier at pH 6 and then declined slightly. The low uptake under strongly acidic conditions is attributed to protonation of the PAR phenolic and N-donor groups and competitive binding by H⁺; the rise toward mildly acidic–neutral pH reflects progressive deprotonation and stronger metal–ligand coordination. This trend agrees with previous studies on chelating XAD resins, including PAR- and amine-functionalized analogues [51,52].

At higher pH, partial conversion of dissolved ions to hydroxy complexes and/or metal-hydroxide solids (e.g., Cu(OH)₂, Ni(OH)₂, Co(OH)₂) competes with ligand complexation, reducing the concentration of free M²⁺ available for chelation and thus lowering apparent removals. Such an effect is widely documented for transition-metal systems. This could explain the slight drop in the recovered percentage of metals perceived at pH = 10 [52,53].

3.3.2. Effect of Contact Time

Contact time between the XAD-4–PAR resin and dissolved metal ions governs the approach to sorption equilibrium. Batch tests were performed with 50.0 mL multi-element solutions of Co(II), Ni(II), and Cu(II) (each 10 mg L⁻¹), and percent extraction versus time was used to construct the kinetic profiles. As shown in Figure 6, uptake increases monotonically with minimal fluctuation; all three metals reach ~50% of their ultimate extraction within the first 20 min. Co(II) attains apparent equilibrium at ~40 min, whereas Ni(II) and Cu(II) require longer contact times to maximize extraction. The fast initial uptake followed by a slower approach to equilibrium is consistent with surface complexation controlling the early stage and intraparticle diffusion contributing thereafter; such behavior is frequently described by pseudo-second-order kinetics and Weber–Morris diffusion analysis [54,55]. The observed performance at trace levels supports the use of XAD-4–PAR in dynamic SPE formats (e.g., cartridges/columns) where available contact time is limited by flow [56,57].

3.4. Sorption Capacity

Sorption capacity (q_e_max) quantifies the maximum amount of analyte retained per unit mass of sorbent. In this work, q_e_max for the AXAD-4–PAR chelating resin was determined by a batch method following Nelms et al. [28] (see Section 2.3.1).

Table 1. Sorption capacities (mM g⁻¹) of AXAD-4–PAR for Co(II), Ni(II), and Cu(II) compared with PAR supported on other substrates; (nd = not determined).

Sorbent Support	Ions			Reference
	Co(II)	Ni(II)	Cu(II)
AXAD-4-PAR	0.152	0.167	0.172	(this study)
PAR-Silica	nd	0.121	0.132	[41]
PAR-impregnated AC	0.015	0.023	0.007	[42]
PAR-Magnetic nano sorbent	nd	nd	0.07	[39]

compiles the capacities measured here alongside representative values reported for PAR-based sorbents on alternative supports.

The XAD-4–PAR sorbent exhibited superior sorption capacities for Co(II), Ni(II), and Cu(II), with values of 0.152, 0.167, and 0.172 mM g⁻¹ respectively. These capacities surpass those reported for PAR immobilized on silica gel, which are 0.121 mM g⁻¹) for Ni(II) and 0.132 mmol/g for Cu(II) [41]. It also outperformed PAR physically impregnated on activated carbon (Co(II) 0.015, Ni(II) 0.023, Cu(II) 0.007 mM g⁻¹) [42] and PAR immobilized on magnetic particles (Cu(II) 0.07 mmol g⁻¹) [39]. These trends are consistent with the higher surface area of the XAD-4 matrix and, critically, with covalent PAR attachment, which provides a more favorable interfacial microenvironment for metal binding than physical adsorption/impregnation on inorganic or carbon supports.

3.5. Analytical Figure of Merits

The analytical performance of the optimized SPE method using XAD-4–PAR chelating resin was evaluated for Co(II), Ni(II), and Cu(II) in groundwater using matrix-matched multi-element standards. As seen in

Table 2. Figures of merit for Co(II), Ni(II), and Cu(II) in synthetic groundwater after SPE with XAD-4–PAR resin.

Calibration Parameters	Metal Ions (ng/mL)
	Co	Ni	Cu
Conc. range (ng mL−1) (n = 4)	0–15	0–15	0–20
RSD at 2 ng mL−1 (n = 4)	1.13	1.15	1.78
RSD at 10 ng mL−1 (n = 4)	1.61	1.07	2.21
Correlation coefficient, R2	0.997	0.998	0.999
Sensitivity, CPS ratio/ng mL−1	20038.0	283.95	3144.1
LOD/ng mL−1	0.053	0.012	0.061

, the calibration was linear over 0–15 ng mL⁻¹ for Co(II) and Ni(II) and 0–20 ng mL⁻¹ for Cu(II) with R² = 0.997–0.999. Intra-day precision (RSD, n = 4) was 1.07–2.21% at 2 and 10 ng mL⁻¹, indicating excellent repeatability. Instrumental sensitivity (slope, CPS per ng mL⁻¹) decreased in the order Co(II) (20,038) > Cu(II) (3144) > Ni(II) (284). Limits of detection (LOD) were 0.012–0.061 ng mL⁻¹, supporting trace-level quantification in complex aqueous matrices.

3.6. Method Validation

Method performance was evaluated with the certified reference material SLRS-4 to validate the SPE–ICP-MS procedure using the XAD-4–PAR sorbent for sample preparation. For Co, Cu, and Ni, five independent preparations were analyzed, and results are reported as mean ± 90% confidence interval (CI).

Table 3. Method validation using SLRS-4 (n = 5; mean ± 90% CI): measured concentrations (µg L⁻¹) of Co, Cu, and Ni after SPE using XAD-4–PAR chelator.

CRM	Metals (Concentration in µg/L)
	Co		Cu		Ni
	Found	Certified	Found	Certified	Found	Certified
SLRS-4	0.032 ± 0.005	0.033 ± 0.006	1.79 ± 0.08	1.81 ± 0.08	0.64 ± 0.07	0.67 ± 0.08
Recovery (%)	96.90%		98.89%		95.52%

compares measured concentrations with certified values and lists percent recoveries, calculated as (measured/certified) ×100.

Measured concentrations agreed closely with the certified values, giving recoveries of 96.90% (Co), 98.89% (Cu), and 95.52% (Ni). The narrow CIs across replicates indicate good repeatability, supporting the method’s accuracy for trace-level metals in aqueous matrices and its suitability for routine groundwater monitoring.

3.7. Application to Groundwater Samples

The optimized SPE protocol was applied to groundwater collected from five deep wells in Dhalamah Valley (Al-Madinah Al-Munawwarah Province, Saudi Arabia; sampling locations in Figure 7). Field preservation included pre-acidification, on-site filtration, and cold transport on ice to the laboratory; samples were stored at 4 °C until analysis. Immediately prior to SPE, the pH was adjusted to the optimized range (6.0–6.5) using 0.2 M ammonium acetate buffer.

The groundwater samples (GW1–GW5) were processed with SPE using the synthesized AXAD-4-PAR chelating resin prior to the analysis of their content of Co(II), Cu(II), and Ni(II) ions by ICPMS. To assess the method’s accuracy and robustness, recovery tests were performed by spiking the samples with 5 ng/mL of each ion. The results in

Table 4. Native concentrations (ng mL⁻¹, mean ± SD) and spike recoveries (%) for Co(II), Ni(II), and Cu(II) in GW1–GW5 after SPE with XAD-4–PAR (5 ng mL⁻¹ spike).

Sample ID	Metal Concentrations (Recovery %)
	Co(II)	Ni(II)	Cu(II)
G1	1.07 ± 1.08	0.89 ± 0.03	2.00 ± 0.29
G1 Spike	6.48 ± 1.27 (108.2%)	6.13 ± 1.03 (104.8%)	7.22 ± 1.16 (104.4%)
G2	1.30 ± 0.44	1.05 ± 1.10	2.25 ± 0.53
G2 Spike	6.53 ± 1.18 (104.6%)	6.14 ± 1.28 (101.8%)	7.09 ± 1.73 (96.8%)
G3	1.23 ± 1.41	1.05 ± 1.11	1.96 ± 0.21
G3 Spike	6.35 ± 1.45 (102.4%)	6.12 ± 1.19 (101.4%)	7.62 ± 1.15(113.2%)
G4	1.15 ± 1.01	1.06 ± 0.02	1.98 ± 0.87
G4 Spike	6.33 ± 1.27 (103.6%)	6.13 ± 1.28 (101.4%)	7.34 ± 1.24(107.2%)
G5	1.13 ± 1.02	1.09 ± 1.12	2.00 ± 0.29
G5 Spike	6.37 ± 1.18 (104.8%)	5.99 ± 1.29 (98%)	7.33 ± 1.16 (106.6%)

demonstrate the method’s effectiveness for trace-level determination of these metals in groundwater.

Obviously, the measured concentrations in the unspiked samples revealed detectable native levels of the test ions, suggesting a natural geochemical origin, as (apart from some agricultural activities) there are no local anthropogenic sources known to contribute to metal contamination.

Spike recoveries ranged from 96.8 to 113.2% with relative standard deviations (RSDs) generally < 2%, indicating high accuracy and excellent repeatability. Element-specific recoveries were 102.4–108.2% for Co(II), 98.0–104.8% for Ni(II), and 96.8–113.2% for Cu(II). These values fall within commonly accepted criteria for trace-metal analysis in groundwater, imply negligible matrix effects under the tested conditions, and support the robustness of the SPE–ICP-MS workflow for routine monitoring. Slightly greater variability observed for Cu(II) is plausibly attributable to stronger complexation or interactions with groundwater constituents, yet overall performance remains consistent with (or better than) previously reported datasets from Al-Madinah Al-Munwwarah [41,58] and Swary Valley [59].

3.8. Resin Stability and Reusability

The synthesized XAD-4-PAR chelating resin demonstrated remarkable chemical and mechanical stability when employed as a packed sorbent in SPE cartridges for sample preparation in the analysis of trace metals. It retained its sorption efficiency after more than 30 consecutive extraction and regeneration cycles involving both metal ion solutions and 1.5 M nitric acid, with no significant reduction in sorption capacity [41]. The resin’s resistance was further assessed under extreme pH conditions by its resistance to a prolonged exposure (3 h) to acidic (1 M and 3 M HNO₃) and basic (1 M NH₄OH, pH 11–12) environments. While no physical changes or deterioration in performance was observed when subjected to acidic media (up to 3 M HNO₃), a light-yellow coloration appeared when it was dispersed in strongly basic conditions (pH 12), which could indicate a partial cleavage of the chelating ligand from the polymer backbone [60]. Despite this, the resin maintained its metal-binding capacity after long-term storage for over one year under ambient laboratory conditions, suggesting excellent shelf-life stability.

4. Conclusions

This work demonstrated a rapid, scalable diazonium-based route to immobilize PAR chelator onto Amberlite XAD-4, yielding a robust chelating resin (XAD-4–PAR) verified by ATR-IR, TGA, and XPS. Batch adsorption studies established an operational pH window of 6–8 with high recoveries from 50 mL of 100 ng mL⁻¹ multielement solutions, 96.69% for Co(II) and 98.20% for Ni(II) at pH 8, and 90.86% for Cu(II) at pH 6, with equilibration achieved within 60 min for Co(II)/Cu(II) and 80 min for Ni(II). Despite these kinetics, the material performed effectively in dynamic mode on a standard SPE manifold, particularly for the analyte at low levels. The resin retained capacity over ≥20 adsorption–desorption cycles and after >1 year of ambient storage, indicating excellent durability and shelf life. Collectively, these results support diazonium grafting as a versatile platform for engineering high-performance, polystyrene-based chelating sorbents. XAD-4–PAR shows strong promise for routine sample preparation in environmental monitoring and, with further optimization of flow conditions and matrix tolerance, for scale-up to water treatment and metal recovery applications.