Sustainable Solid-Phase Extractant Based on Spent Coffee Waste-Derived Activated Carbon Functionalized with 1,10-Phenanthroline-5-Amine for Trace Metals from Groundwater Samples

Abstract

In this work, spent coffee grounds, an abundant agro-waste, were transformed into activated carbon, providing a sustainable substrate for immobilizing 1,10-phenanthroline-5-amine chelating agent, to develop a solid-phase extractant for trace metals. ATR-IR, TGA, and XPS analyses confirmed successful functionalization and revealed the material’s physicochemical properties. Sorption studies showed optimal uptake at pH 6.0–6.5, enabling rapid extraction of Mn(II), Cd(II), Ni(II), and Pb(II) within 30 min, with capacities of 13.5, 8.4, 13.3, and 8.5 mg g⁻¹, respectively. The prepared chelator was employed as a packed sorbent in standard SPE cartridges operated with a conventional SPE apparatus, achieving efficient extraction and preconcentration of the studied ions from both certified reference material (BCR-609) and real groundwater. The results obtained closely matched certified values, while spiked recoveries ranged from 96.00% to 106.80%. These findings highlight the effective valorization of agricultural waste into a reusable, high-performance SPE sorbent with strong potential for water purification and trace metal recovery.

1. Introduction

Although highly sensitive analytical instruments are now capable of detecting numerous analytes at femtomole levels or lower [1], the accurate quantification of many substances within complex matrices still necessitates the use of effective sample preparation to preconcentrate target analytes and minimize matrix interferences [2]. Over the years, a wide range of sample preparation techniques have been developed for the extraction of trace metals from diverse matrices [3]. These methods include liquid–liquid extraction (solvent extraction) [4,5], membrane extraction [6], chemical precipitation [7], and solid-phase extraction (SPE) [8,9].

SPE remains the most extensively employed sample preparation approach in the analysis of trace metals, owing to its high efficiency, operational simplicity, and broad adaptability. The overall extraction efficiency is primarily governed by the sorbent’s selectivity toward specific target metals in the presence of complex interferences in sample matrices. Among various sorbent types, chelating resins are the most effective selective materials. Typically, these resins are prepared by covalently grafting specific ligands onto readily available porous substrates such as silica, synthetic polymers, cellulose, ion-exchange resins, or activated carbon. These functionalized materials exhibit strong metal-binding affinities and have been widely utilized for the extraction and preconcentration of trace metals [10,11]. Numerous surface functionalization strategies, along with their applications for the isolation and enrichment of trace metals, have been comprehensively documented in the literature [12,13,14,15].

Activated carbon (AC) is widely recognized as a high-performance adsorbent owing to its large specific surface area, hierarchical porosity, strong adsorption capacity, and excellent chemical stability. In recent years, agricultural and food processing residues have gained increasing attention as sustainable precursors for AC, driven by the need for cost-effective materials and waste minimization [16,17]. Among them, spent coffee waste (SCW), a carbon-rich and inherently porous biomass generated in large quantities worldwide, has emerged as a particularly promising feedstock. Converting SCW into AC not only provides an economical and renewable route for adsorbent synthesis but also mitigates the environmental burden of landfill disposal and greenhouse gas emissions [18,19]. Compared with conventional sources such as coal, wood, and coconut shells, SCW-derived AC exhibits comparable or even superior adsorption performance, particularly in environmental remediation and heavy metal removal from aqueous solutions [20]. Reports have documented high uptake capacities for metals such as Pb²⁺, Cd²⁺, and Cu²⁺, reaching up to 120 mg/g under optimized conditions, largely attributed to abundant surface oxygen-containing functional groups that enhance metal-binding affinity [21,22].

Despite its inherently high adsorption capacity, raw AC typically exhibits poor selectivity toward specific metal ions. To address this limitation, surface functionalization strategies are employed, taking advantage of the intrinsic surface chemistry of AC, which is rich in carboxyl, hydroxyl, and aromatic groups. These functionalities provide versatile binding sites that can be chemically modified to introduce chelating moieties such as amines, thiols, or carboxylates, thereby enhancing both the affinity and selectivity for target metals. For example, carboxyl groups can be tailored via amidation or esterification, hydroxyl groups can be modified through silylation, and aromatic domains may be exploited for π–π interactions or electrophilic substitution, to enable more efficient and selective metal ion capture [18,19]. These chemical modifications enable the covalent attachment of ligands such as tetraethylenepentamine [23], poly(N,N-dimethylaminoethyl methacrylate) [24], ethylenediamine [25], and tris(2-aminoethyl)amine [26] with extensions to carbon nanotubes [26,27,28] and activated carbon cloths [29]. A recent comprehensive review by Sultana et al. [30] highlights the advancements in chemically modified activated carbons and their proven efficacy in the removal of heavy metals and dyes from aqueous media.

1,10-Phenanthroline derivatives are robust bidentate ligands, widely applied for trace metal analysis [31,32,33,34], forming stable complexes with metals such as Cd, Cu, Ni, Co, Fe and Pb [35,36]. Ligand immobilization improves reusability, as demonstrated by 5-amino-1,10-phenanthroline (PTA) grafted to graphene oxide for selective Pb removal via acyl chloride coupling [37]. Our group previously functionalized porous silica monolith [38] and mesoporous silica nanoparticles [39] with phenanthroline derivatives for SPE and dSPE of trace metals.

Herein, AC obtained from SCW was chemically functionalized with PTA chelator via an acyl chloride intermediate in an anhydrous toluene to preserve the ligand’s bidentate nitrogen donor sites. This simple and low-cost modification approach enhances selectivity for soft and borderline metal ions and offers a sustainable alternative to conventional sorbents. The efficiency of the prepared AC–PTA sorbent was evaluated for the simultaneous SPE of Cd, Pb, Mn, Ni, and Zn from groundwater prior to their analysis by ICP–MS.

2. Materials and Methods

2.1. Chemicals and Reagents

All reagents were of analytical grade and used without further purification. Spent coffee waste (SCW) was collected from local cafés in Al-Madinah Al-Munawwarah, Saudi Arabia, and employed as the precursor for activated carbon (AC) synthesis. The chelating ligand, 1,10-phenanthroline-5-amine (PTA), and thionyl chloride (SOCl₂) were obtained from Sigma-Aldrich (Gillingham, UK). Elemental standard solutions (1.0 mg·mL⁻¹) for Mn, Cd, Ni, and Pb were purchased from Acros Organics (Geel, Belgium). Toluene, nitric acid (HNO₃), hydrochloric acid (HCl), potassium hydroxide (KOH), ethanol, and chloroform were supplied by Scharlau (Barcelona, Spain). Acetate buffer solutions were prepared from ammonium acetate (Chem-Lab NV, Zedelgem, Belgium) and further purified through a Chelex-100 cation-exchange resin column (Bio-Rad, Berkeley, CA, USA). Buffer pH was adjusted using either glacial acetic acid or aqueous ammonia. Ultrapure water (18.2 MΩ·cm) obtained from a Milli-Q purification system (Merck, Darmstadt, Germany) was used in all experiments.

2.2. Instrumentation and Apparatuses

ATR–FTIR spectra of AC and AC–PTA were recorded over the range 200–4000 cm⁻¹ using a PerkinElmer 100 Series spectrometer (Beaconsfield, UK). X-ray photoelectron spectroscopy (XPS) analysis of C1s, N1s, and O1s was performed with a VG ESCALab250 spectrometer (Thermo Scientific, Waltham, MA, USA) using an Al Kα source (1486.68 eV), a pass energy of 20 eV, and a step size of 0.05 eV. Thermogravimetric analysis (TGA) was conducted using a TA Instruments SDT600 analyzer (New Castle, UK) under a nitrogen atmosphere at a heating rate of 10 °C·min⁻¹. Metal ion concentrations were quantified using an Agilent 7500 Series inductively coupled plasma–mass spectrometer (ICP–MS; Davis, CA, USA). pH values were measured with an HI2211 pH/ORP meter (Hanna Instruments, Bedfordshire, UK). Furnace tube model BR-12SNT-40/300 from Brother Furnace (Zhengzhou, China) used for spent Coffee wastes carbonization.

2.3. Preparation of AC from Spent Coffee Wastes

Activated carbon was produced from SCW using a modified literature procedure [20,40]. Briefly, the SCW was washed thoroughly with distilled water to remove adhering impurities, oven-dried at 105 °C for 24 h, and carbonized in a tubular furnace at 500 °C for 2 h under continuous nitrogen flow. The resulting biochar was chemically activated with KOH at a 1:2 (w/v) ratio, stirred at 85 °C for 4 h, and then dried oven. This was followed by a thermal activation at 800 °C for 1 h under nitrogen. The final product was washed sequentially with 0.1 M HCl and distilled water until neutral pH, dried at 110 °C overnight, and stored in a desiccator until further use.

2.4. Preparation of AC-PTA Chelator

The AC–PTA chelator was synthesized through covalent immobilization of PTA onto oxidized AC via a two-step process. First, 10 g of dried AC was oxidized under reflux with 50 mL of 5.0 M HNO₃ for 24 h, filtered, and washed with deionized water until the filtrate reached pH 5. The material was then dried at 100 °C for 24 h (adapted from [41]). In the second step, 5 g of the oxidized AC was refluxed with 20 mL of SOCl₂: Toluene (1:1 v/v) for 24 h, filtered, and washed thoroughly with chloroform to remove residual SOCl₂. The AC–OCl₂ intermediate was air-dried in a fume hood, then reacted with 9 × 10⁻³ M PTA in chloroform under reflux for 48 h. The resulting AC–PTA material was recovered by centrifugation, washed with chloroform, dried, and stored in a desiccator until use.

Sorption and Solid-Phase Extraction (SPE) Procedure

The effect of pH on metal ion sorption was optimized using a static batch method. Standard solutions containing 5 mg·L⁻¹ of Mn(II), Ni(II), Pb(II), and Cd(II) were used to evaluate the performance of the AC–PTA chelating resin under varying pH conditions. Subsequently, the sorption capacity of the resin for each metal ion was determined using a static batch method, as described in the literature [42], with individual standard solutions containing 50 mg·L⁻¹ of each metal ion. The sorption capacity of chelating resin was determined for the studied ions following a static batch method as described in the literature using standard solutions containing 50 mg L⁻¹ of each metal separately.

The SPE experiments were conducted using a 12-position manifold as described in our previous work [42]. The manifold was assembled from a 12-way standard SPE head from Ato Science (Shanghai, China) and a Welch^® Gardner Denver vacuum pump (Thomas GmbH, Furstenfeld Bruck, Germany). A 12 mL SPE cartridge was packed with 200 mg of AC–PTA and kept in place between two porous PTFE filters (Agilent Technologies, CA, USA), forming a bed height of 5–10 mm. Cartridges were conditioned by passing 5 mL of buffer solution at a flow rate of 2 mL·min⁻¹, then the desired volume of standard solution or sample solution was loaded at 1 mL·min⁻¹. Subsequently, unbound species were removed by washing with 2.5 mL of ultrapure water, and the chelated metals were eluted with 5 mL of 1.5 M HNO₃. The eluates were collected in PTFE tubes, diluted threefold with ultrapure water, and analyzed by ICP–MS. For pH optimization studies, a mixed-metal standard containing 10 µg·mL⁻¹ of Mn(II), Ni(II), Pb(II), and Cd(II) ions was employed.

3. Results and Discussion

3.1. Preparation of AC Substrate and Its Modification with PTA Chelator

The SEM image of AC derived from spent coffee waste (Figure 1) shows a rough, heterogeneous surface morphology characterized by numerous pores, cracks, and cavities of different sizes. Such structural features confirm the effectiveness of the activation process in developing micro- and mesoporous networks, which enhance the surface area and facilitate adsorption. These observations are in line with previous studies that reported highly porous and irregular morphologies of activated carbon prepared from lignocellulosic biomass [43].

The functionalization of AC to fabricate the AC-PTA chelating resin was achieved through the chemical transformation schematically illustrated in Figure 2. The procedure entailed the activation of the carboxyl functional group via nucleophilic substitution to form a more reactive acyl chloride intermediate, thereby facilitating the covalent attachment of the 1,10-phenanthroline-5-amine moiety through an amide linkage. The resultant metal-chelating resin was subsequently characterized using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-IR), X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA).

3.2. ATR–FTIR

The Attenuated Total Reflection–Fourier Transform Infrared (ATR–FTIR) spectra in Figure 3 provide unambiguous evidence for covalent immobilization of PTA onto AC. In the pristine AC (Figure 3a), only a broad O–H stretch (≈3373 cm⁻¹), faint aromatic C=C vibrational signatures (≈1600 cm⁻¹), and a C–O stretch near 1204 cm⁻¹ are observed—consistent with its graphitic surface. Upon functionalization, the PTA–AC spectrum (Figure 3b) displays several new and diagnostic absorption bands: N–H stretching at 3180 cm⁻¹ (indicative of amide or amine formation) and a distinct C=O stretching mode around 1600 cm⁻¹ characteristic of amides [44,45,46]. These bands, absent in pristine AC, along with aromatic C–H stretching (~3080 cm⁻¹), bending (~1560 cm⁻¹), C=N (1497 cm⁻¹), C–N (~1090 cm⁻¹), and out-of-plane aromatic C–H (800 and 725 cm⁻¹) modes, collectively confirm successful PTA grafting.

3.3. XPS

X-ray photoelectron spectroscopy (XPS) is a powerful technique for characterizing the surface chemistry of modified materials, providing detailed insights into elemental composition and chemical bonding states. It was employed to investigate the surface chemical composition and bonding states of the functionalized material, owing to its high surface sensitivity and ability to quantitatively resolve chemical environments at the nanometer scale. The survey spectrum of AC–PTA resin (Figure 4) displays characteristic photoemission features attributable to carbon (C 1s), nitrogen (N 1s), and oxygen (O 1s) core levels, confirming the presence of heteroatoms introduced via surface modification. High-resolution deconvolution of the N 1s region reveals two distinct components centered at binding energies (BE) of 399.3 eV and 405.5 eV. Based on literature assignments [43,44,45,46,47] the lower-BE component corresponds to pyridinic nitrogen atoms within the aromatic heterocycle of the PTA ligand, whereas the higher-BE component is attributed to nitrogen in protonated or electron-withdrawing environments, consistent with amide-group amines. Quantitative analysis yields a C:N atomic ratio of 13:3, which closely matches the stoichiometric value predicted from the proposed AC–PTA structure. This strong agreement between experimental and theoretical ratios provides compelling evidence that the PTA chelating moiety is covalently grafted onto the activated carbon (AC) framework rather than being physically adsorbed.

3.4. TGA

Thermogravimetric analysis (TGA) is an effective method for evaluating the thermal stability and compositional changes in modified materials, providing critical insights into their behavior under elevated temperatures. The TGA of raw AC and AC-PTA chelator were conducted over a thermal range from room temperature to 900 °C at a heating rate of 10 °C/min under a constant flow of N2 gas. As depicted in Figure 5, pristine AC exhibits an initial weight loss at ~40 °C, attributed to desorption of physiosorbed moisture, followed by a major decomposition event between 450 and 650 °C associated with carbon matrix degradation. In contrast, AC–PTA shows an early mass loss at ~50 °C due to volatilization of residual solvent and surface-bound water [47,48,49], followed by an additional ~13.0% reduction corresponding to thermal cleavage of covalently anchored PTA moieties Beyond 450 °C, AC–PTA undergoes an extensive 84.99% weight loss, consistent with volatilization of unreacted carboxyl groups and progressive breakdown of the carbonaceous framework.

3.5. Influence of pH on Metals Sorption

Optimizing solution pH is a critical step in the sorption of metals by chelating resins, as it governs both the ionization state of the functional groups and the aqueous speciation of target metals, and hence influences the binding affinity, selectivity, and overall extraction efficiency. The sorption of Mn(II), Ni(II), Pb(II), and Cd(II) onto AC-PTA resin exhibited pronounced pH dependence, consistent with the predictions of the Hard and Soft Acids and Bases (HSAB) principle [50]. As shown in Figure 6, increasing the solution pH from 2.0 to 6.5 significantly enhanced metal uptake, primarily due to reduced competition from protons and increased deprotonation of donor atoms, which facilitated stronger coordination with metal ions. Cd(II), Ni(II), and Pb(II) achieved maximum sorption efficiencies within the pH range of 6.0–6.5, beyond which the uptake was plateaued or slightly declined—likely due to surface site saturation or partial precipitation of metal hydroxides. Pb(II) displayed consistently high retention across the studied pH range, with only minor ecreases above pH 7.0, in agreement with previous reports [51,52]. In contrast, Ni(II) uptake decreased at pH 8.0, potentially reflecting slower complexation kinetics, in contrast to the enhanced performance observed for 1,10-phenanthroline on C18 sorbents, where solvent and hydrophobic interactions predominate [53]. Mn (II) uptake increases progressively with pH, peaking at pH 8.0, indicating weaker proton competition and slower complexation. Considering all target metals, a pH range of 5.5–6.5 was identified as optimal for multi-metal sorption and employed in subsequent SPE optimization.

3.6. Effect of Contact Time on Sorption

Contact time is a decisive factor in SPE, as it dictates both the extent and rate of metal ion uptake. Optimizing this parameter is essential for the development of sorbents capable of rapid and efficient extraction at trace concentrations, which are common in environmental matrices.

As shown in Figure 7, the AC-PTA chelating resin demonstrated distinct kinetic profiles for Cd(II), Pb(II), Mn(II), and Ni(II) when tested using a 50 mL solution containing 5 mg·L⁻¹ of each ion. Mn(II) and Ni(II) displayed rapid uptake, reaching near-maximum removal efficiencies (~80.5% and 57.5%, respectively) within 20–30 min. The early saturation suggests fast surface complexation and rapid occupancy of the available chelating sites—behavior typical of sorbents functionalized with high-affinity ligands for soft metal ions [53,54]. In contrast, Pb(II) and Cd(II) revealed a more gradual sorption trend, with final removal efficiencies of 69% and 92%, respectively, after 60 min. The slower uptake is likely attributable to weaker initial interactions or intraparticle diffusion limitations. Nevertheless, the high ultimate Pb(II) removal implies the formation of stable complexes with the nitrogen and oxygen donor atoms of the PTA moiety [55].

Despite these kinetic variations, AC-PTA was able to uptake trace concentration of the studied metals within short contact times. Even at trace concentrations, appreciable removal (≥20%) occurred within the first 10 min, particularly for Pb(II) and Cd(II) ions, which reflect the abundance of accessible binding sites. These results confirm the resin’s suitability as a preconcentration medium in SPE workflows for the determination of metals at trace levels in environmental samples [8,56].

3.7. Sorption Capacity

Sorption capacity is a key parameter in SPE, as it defines the amount of sorbent required to efficiently extract or preconcentrate target analytes from a given sample volume. The sorption capacity was determined as described in the experimental section, using 25 mL of a 50 mg L⁻¹ solution of each metal ion individually. The AC-PTA chelator prepared in this work exhibited sorption capacities of 13.5, 8.4, 13.3, and 8.5 mg g⁻¹ for Mn, Cd, Ni, and Pb ions, respectively.

Comparative data from similar chelating resins prepared by immobilization of phenanthroline derivatives on different substrates indicate that sorption performance varies with both the support material and the target ion. AC demonstrated a well-balanced and consistent capacity for the studied metal ions, outperforming silica monoliths, which exhibited lower uptake (e.g., 8.22 mg g⁻¹ for Ni) [38]. Although graphene oxide showed an exceptionally high capacity for Pb (548 mg g⁻¹), data for other metals are not available [37]. On the other hand, mesoporous silica nanoparticles achieved the highest recorded capacities for Cd (116), Ni (132) and Pb (121) ions mg g⁻¹ which is likely due to their large surface area and pore accessibility [39].

Although the AC–PTA chelator does not match the maximum capacities observed with advanced nanomaterials, it offers a practical balance of multi-metal sorption efficiency, sustainability, and cost-effectiveness for SPE applications (

Table 1. Comparison of sorption capacities (mg g⁻¹) of 1,10-phenanthroline derivatives immobilized on different substrates for Mn(II), Cd(II), Ni(II), and Pb(II) ions, determined using a sample volume of 25 mL at a concentration of 50 mg L⁻¹ for each metal ion.

Substrate	Analyte				Ref.
	Mn(II)	Cd(II)	Ni(II)	Pb(II)
Activated Carbon	13.5	8.4	13.3	8.5	This work
Graphene Oxide	-	-	-	548	[37]
Silica Monolith	-	-	8.22	-	[38]
Mesoporous silica NPs	-	116	132	121	[39]

).

3.8. Method Validation: Analysis of Groundwater Reference Materials

The validation of the developed analytical methods using certified reference materials (CRMs) is a valuable practice to ensure the accuracy and reliability of protocols for trace metal determination. Thus, the synthesized chelating resin was employed as a SPE adsorbent to isolate trace metals from groundwater samples prior to ICP-MS analysis. The calibration data in

Table 2. Figures of merit for the determination of Pb(II), Mn(II), Cd(II), and Ni(II) following SPE using AC–PTA resin.

Calibration Parameters	Metal Ions (ng mL−1)
	Cd(II)	Pb(II)	Mn(II)	Ni(II)
Conc. range (ng mL−1) (n = 7)	0–10	0–10	0–25	0–25
RSD at 2 ng mL−1 (n = 7)	1.14	1.22	1.18	1.92
RSD at 10 ng mL−1 (n = 7)	1.73	1.29	1.57	2.13
Correlation coefficient, R2	0.9990	0.9950	0.9988	0.9980
Sensitivity, CPS ratio/ng mL−1	3144.1	35,119.0	20,038.0	283.95
LOD/ng mL−1	0.013	0.033	0.086	0.072

demonstrate high precision and accuracy, along with very low limits of detection for the studied metal ions.

The performance of the developed sample preparation method exploiting the synthesized chelating resin as SPE sorbent has been assessed using groundwater CRM BCR-609. The SPE protocol described in the experimental section applied for sample preparation. The results, presented in

Table 3. Analysis of reference materials BCR-609 after SPE using AC-PTA resin (mean ± RSD ng mL⁻¹).

	Metals
	Cd(II)	Pb(II)	Mn(II)	Ni(II)
Certified	0.164	1.67	-	9.11
Fund	0.153 ± 0.018	1.71 ± 0.05	2.31 ± 0.16	9.97 ± 1.67
Recovery%	93.29%	102.39%	-	109.44%

, demonstrate a close agreement between the measured concentrations and the certified values, and hence confirm the accuracy of the SPE method and its suitability for processing groundwater and similar matrices.

3.9. Method Applications: Analysis of Real Groundwater Samples

The validated sample-preparation workflow was applied to groundwater (TDS: 534–742 mg L⁻¹) collected from three narrow-mouth wells (80–120 m depth) in the Al-Senayea district of Al-Madinah Al-Munawwarah, Saudi Arabia (Figure 8). The sites, previously characterized in an earlier study [57], were deliberately re-sampled to ensure comparable hydrogeological conditions and to enable a reliable assessment of performance under realistic environmental conditions. All samples were field-filtered and acidified to pH 2 during collection, then transported at 4 °C in pre-cleaned 1 L PET bottles to ensure preservation. Samples were processed by the optimized SPE protocol and analyzed for dissolved Cd(II), Mn(II), Ni(II), and Pb(II).

Table 4. Dissolved Metal Concentrations (ng mL⁻¹) and Spike Recoveries (%) in Groundwater after SPE with AC-PTA (Spiked samples contained +5 ng mL⁻¹ of each ion. Values are mean ± SD (n as per method); recovery (%) = (C_spiked − C_unspiked)/5 × 100).

Sample ID	Metals Concentrations (Recovery %)
	Pb(II)	Ni(II)	Mn(II)	Cd(II)
G1	1.420 ± 0.35	3.02 ± 0.76	8.37 ± 1.67	1.36 ± 0.14
G1 Spike	6.290 ± 0.82 (97.40%)	8.04 ± 1.82 (99.40%)	12.74 ± 1.74 (87.40%)	6.18 ± 0.16 (96.40%)
G2	1.75 ± 0.62	2.66 ± 1.03	6.25 ± 1.34	1.50 ± 0.28
G2 Spike	6.64 ± 0.62 (97.80%)	7.96 ± 1.25 (106.00%)	11.67 ± 1.13 (108.40%)	6.35 ± 0.32 (97.00%)
G3	1.53 ± 0.38	1.04 ± 1.21	8.17 ± 1.67	1.43 ± 0.27
G3 Spike	6. 87 ± 0.52 (106.80%)	6.16 ± 1.19 (101.4%)	13.81 ± 1.42 (112.80%)	6.50 ± 0.40 (101.34%)

reports dissolved concentrations together with recoveries from matrix-spiked aliquots (+5 ng mL⁻¹ of each ion). Across sites, recoveries (min–max) were: Pb(II) 97.40–106.80%, Ni(II) 99.40–106.00%, Mn(II) 87.40–112.80%, and Cd(II) 96.40–101.34%. These values indicate that the AC-PTA chelator is a reliable SPE sorbent for extraction/preconcentration of the target ions in this matrix, with most recoveries falling within the commonly accepted 90–110% range (and a few slightly > 110% likely reflecting local matrix effects).

The concentrations of studied metals were broadly consistent across locations and align with prior data for the same wells [57]. This is expected due to the absence of anthropogonic pollutant ion sources in the area. The determined ion concentrations, on the other hand, appear to be higher than those obtained from Swary Valley groundwater samples using a comparable method [58] plausibly due to differences in lithology and geochemical conditions. Possible diffuse inputs (e.g., legacy automobile yards or municipal dumping) merit further investigation to evaluate their contribution to trace-metal loading.

3.10. Reusability and Stability of AC-PTA Chelating Resin

The long-term stability and reusability of SPE sorbent are essential for its sustainability and cost-effectiveness. In this study, the chemical stability of the synthesized chelating resin was evaluated following previous protocols [39,59]. The resin was exposed to acidic media (HNO₃, up to 4 M) and strong alkaline conditions (pH 12), after which its sorption capacity for the target metal ions was measured. Treatment with nitric acid up to 3 M was found to results in a slight reduction (<3.2%) in sorption capacity; however, the suspension in 4 M led to a ~7% decline and solution discoloration was observed, which indicate partial leaching of chelating groups. In contrast, alkaline exposure at pH 12 had no effect, highlighting excellent base resistance. The resin maintained stable performance across 20 sorption–desorption cycles, consistent with literature on the acid stability of activated carbons [60].

4. Conclusions

The work reported herein demonstrates a simple synthetic method for the production of a sustainable chelating resin by covalently grafting a PTA chelator onto AC derived from spent coffee waste. The produced AC–PTA composite chelator demonstrated high extraction efficiency for Cd(II), Mn(II), Ni(II), and Pb(II), with optimal uptake at pH 5.5–6.5 and maximum sorption capacities of 13.5, 8.4, 13.3, and 8.5 mg g⁻¹, respectively. These values were comparable to phenanthroline-based sorbents on synthetic supports. The sorbent demonstrated excellent efficiency when applied as SPE sorbent for the extraction/preconcentration of the studied metals from certified reference materials (CRM BCR-609) and groundwater samples prior their analysis by ICP–MS. The metal chelator remained stable over 20 sorption–desorption cycles and offers a cost-effective, biomass-derived alternative for selective SPE applications. Future work will address the feasibility of scaling up its applications in water treatment and the quantitative recovery of dissolved metals from waste streams.