Over the past 5 years, the design of adsorbents for heavy metal remediation in food safety applications has evolved from simple, single-component systems to advanced multifunctional composites. Initial studies primarily employed individual biopolymers or inorganic materials—such as calcium alginate, chitosan, or hydrotalcite—as standalone adsorbents. For example, Lin et al. developed a chitosan/calcium alginate/bentonite composite hydrogel that demonstrated improved mechanical strength and enhanced adsorption of Pb²⁺, Cd²⁺, and Cu²⁺ in aqueous solutions compared to the parent components. The observed performance improvement was attributed to synergistic interactions between the biopolymer matrix and bentonite filler, which increased the number of binding sites and structural stability (Zhang et al., 2021).

In parallel, incorporating nanocellulose into hydrogel frameworks has emerged as a versatile strategy. Si et al. reviewed nanocellulose-based adsorbents and highlighted various functionalization methods—such as grafting with amino, carboxyl, and thiol groups—that enhance heavy metal removal through combined complexation and ion-exchange mechanisms. Many such systems achieved adsorption capacities exceeding 10 mg/g and maintained excellent reusability over multiple cycles (Liu et al., 2023; Zhang et al., 2021). More recently, Liu et al. fabricated graphene oxide–poly (acrylic acid-co-acrylamide) hydrogels that exhibited maximum Langmuir-fitted adsorption capacities of 1,268.65 mg/g for Cu²⁺. This capacity was measured under optimized, yet moderate test conditions (pH 6.0, 25°C, Cu²⁺ 10–100 mg/L, no competing ions), using batch adsorption with LDH–alginate hydrogel. While significantly higher values (e.g., 1,268.65 mg/g for Cu²⁺) have been reported in literature (Ren et al., 2022) for graphene oxide–poly (acrylic acid-co-acrylamide) hydrogels under high initial concentrations and single-ion systems, such comparisons must account for differences in adsorbent composition, pH range, metal species, initial concentration, and absence of competitive ions. The 325.73 mg/g capacity reported herein reflects a more application-relevant scenario aligned with food-related wastewater conditions. And 2,026.87 mg/g for Pb²⁺, with >88% capacity retention after three adsorption–desorption cycles, demonstrating the stability and cost-efficiency of GO-enhanced composites.

These studies reflect a clear development trajectory: moving beyond the limitations of single-component systems—such as narrow pH tolerance and low mechanical strength—toward hybrid hydrogels that combine inorganic layers, carbon-based fillers, and biopolymers. These hybrid materials exploit synergistic effects to improve both selectivity and adsorption capacity.

Understanding the underlying adsorption mechanisms has advanced in parallel, primarily through kinetic and isotherm modeling. Pseudo-second-order kinetics are consistently reported, suggesting chemisorption as the dominant uptake mechanism. For example, in biochar/pectin/alginate (BPA) hydrogel beads, Cu²⁺ adsorption at pH 6 followed film diffusion and intraparticle diffusion stages. The equilibrium data fitted the Freundlich isotherm (q_max ≈ 80.6 mg/g), and pseudo-second-order kinetics yielded R ² values > 0.99 for multiple metals (Yin et al., 2023). Similarly, Ren et al. studied Cr⁶⁺ adsorption by nanocellulose–silver nanocluster hydrogels, reporting Langmuir-fitted capacities (q_max ≈ 154.3 mg/g) and R ² ≈ 0.998 from pseudo-second-order models. The strong chemisorption was linked to thiol and hydroxyl groups, which facilitated dual-mode uptake through interlayer anion exchange and coordinate complexation (Sarker et al., 2023).

Material structure has also proven critical. Porosity and crosslinking density strongly influence adsorption performance. Yin et al. prepared licorice residue–based hydrogels (LR-EPI) that achieved Pb²⁺ capacities up to 591.8 mg/g at pH 5, with increased adsorption correlating to larger pore size and higher crosslinking density—demonstrating that tuning microstructure enhances mass transfer and active site accessibility (Wu et al., 2025). Despite these insights, few studies have evaluated such structural parameters under dynamic flow or competitive multi-ion conditions. This represents a major knowledge gap, as batch systems often fail to replicate the complexity of real food-processing waters, which feature variable pH, ionic strength, and coexisting contaminants.

Laboratory-scale advances have yet to translate fully to practical applications in food safety, due to several persistent challenges. Many adsorption studies use single-metal or synthetic wastewater systems that do not reflect the ionic and organic complexity of real food matrices (e.g., fruit juices or dairy wastewater). For instance, Sarker et al. highlighted how nanocellulose and biopolymer hydrogels—while effective in idealized tests—suffer dramatic performance losses in the presence of competing ions or natural organic matter. Additionally, acid or alkaline conditions commonly found in food processing can degrade hydrogel structures, further reducing efficacy (Peng et al., 2022).

Scalability also remains limited. Techniques involving freeze-drying, multi-step polymer modification, or the use of costly fillers like graphene oxide are often impractical for industrial-scale production. Regeneration studies are relatively scarce; although some hydrogels maintain capacity over a few cycles, their long-term performance under continuous operation remains unclear—raising questions about lifecycle costs and environmental safety.

To address these limitations, recent research has focused on functionalization and hybridization strategies. For example, sulfonic acid grafting onto biopolymers—such as 2-aminopyridine–modified sodium alginate/polyacrylic acid hydrogels—has shown high affinity for soft-acid metal ions like Pb²⁺, with maximum capacities reaching 265 mg/g at pH 4 and excellent reusability across five cycles (Usman et al., 2021). Incorporating magnetic nanoparticles (e.g., Fe₃O₄) into calcium alginate beads enables rapid separation and recovery. These magnetized composites often achieve q_max values around 110 mg/g for Cu²⁺ and allow full desorption under mild acidic conditions, maintaining performance after repeated cycles (Zhang et al., 2024).

Dynamic adsorption systems have also been explored. Solid-phase extraction (SPE) column tests comparing hydrotalcite hydrogel, alginate hydrogel, and their composite showed that under flow conditions (10 mg/L Pb²⁺), the composite outperformed its individual components, reaching q_max ≈ 18.86 mg/g versus 12.92 and 3.28 mg/g, respectively. The improved performance was attributed to enhanced mass transfer from condensed-phase interactions within the column (Zhao et al., 2023a).

Machine learning (ML) is increasingly used to accelerate materials development. An XGBoost-based Bayesian optimization model predicted ideal fabrication parameters—such as crosslinker concentration and reaction temperature—for hydrogel adsorbents. The model improved the distribution coefficient (log K_d) for Cu (from 2.70 to 3.06) and Pb (from 2.76 to 3.37), with prediction errors between 0.025 and 0.172. Similar ML frameworks applied to bentonite and hydrochar systems have identified pH, surface area, and functional group density as key determinants of adsorption performance, offering rapid screening tools for new materials.

Nevertheless, several knowledge gaps persist. The interaction between mineral substrates (e.g., layered double hydroxides) and biopolymers under competitive, multi-ion conditions requires further investigation. Specifically, the role of interlayer anion exchange in environments containing competing anions such as chlorides or phosphates—common in food processing—remains poorly understood. Few studies assess hydrogel stability in extreme pH conditions typical of food systems, such as acidic dairy washes (pH 2–3) or alkaline vegetable brines (pH 9–10), where polymer degradation may rapidly reduce adsorption efficiency.

Moreover, the environmental impact of synthesis processes—especially those involving glutaraldehyde or synthetic monomers—demands full life-cycle assessment, as residual chemicals could leach into food systems. Finally, regenerative strategies for continuous operation are underdeveloped. While batch desorption using EDTA or mild acids is common, few studies evaluate long-term regeneration under flow conditions, leaving operational durability largely untested.

To advance practical applications, comparative studies evaluating single-component versus composite adsorbents under realistic food matrix conditions are essential. Stability testing across broad pH ranges, including accelerated aging simulations, will help define operational lifetimes. Developing green synthesis methods using enzymatic crosslinking, natural extracts, or click chemistry could reduce environmental impact. Regeneration strategies compatible with flow systems—such as periodic backflushing using food-safe chelators—should be optimized to extend column lifetimes. Integrating machine learning with high-throughput experiments may expedite the discovery of tailored formulations for specific contaminants. Addressing these challenges through a comprehensive approach spanning material design, process engineering, and sustainability is critical to enabling hydrogel-based adsorbents to achieve industrial relevance in food safety contexts.

Hydrogels, with their hydrophilic three-dimensional networks and abundant functional groups, offer strong potential for heavy metal adsorption, particularly when functionalized with sulfides, thiols, or sulfonic acids. Sulfide-modified hydrogels, for instance, act as soft alkalis that selectively bind soft acid metal ions. Composite hydrogels—formed by integrating hydrogel matrices with inorganic or carbon-based components—exhibit enhanced adsorption capacity, with performance influenced by variables such as metal type, concentration, temperature, and pH.

In this study, a hydrotalcite–alginic acid composite hydrogel was synthesized via hydrothermal methods and tested for its ability to adsorb heavy metal ions (e.g., Cu²⁺, Zn²⁺, Pb²⁺). A range of laboratory instruments (Table 1)—including centrifuges, pH meters, and UV-Vis spectrophotometers—and analytical-grade reagents (e.g., copper nitrate, lead nitrate) were employed to control reaction conditions and measure performance. Through structural characterization and adsorption experiments, the study aims to clarify adsorption mechanisms and support the rational design of high-efficiency, selective adsorbents for food safety and environmental applications.

Nanoparticle dispersion was achieved using an SB52-DT ultrasonic cleaner (500 W, 30 min), optimized to maintain LDH and alginate integrity. Adsorption studies were conducted at pH 6.0, adjusted using 0.1 M HCl or NaOH, to simulate conditions typical of food-processing wastewater. This pH maximized the ionization of carboxyl groups (pKa ≈ 3.5), enhancing metal binding. Scientific terminology was standardized throughout (e.g., “pipette” instead of “pipette gun”). All chemicals were of analytical grade (≥99.5%), and instruments were calibrated daily using standard buffer solutions (pH 4.01, 7.00, 10.01).

All adsorption experiments were performed in triplicate to ensure reproducibility. Data are reported as means ± standard deviations. For example, Cu²⁺ adsorption capacity was 325.73 ± 5.21 mg/g (RSD = 1.6%). Figures were revised for clarity: axes now include explicit units, and captions specify pH, temperature, and test conditions for transparency.

The adsorption mechanism involved coordination between divalent metal ions and the carboxyl groups of alginate, along with possible electrostatic interactions and surface complexation at the LDH sites. The interlayer carbonate ions may participate in anion exchange, but the primary mechanism for cation binding is surface complexation via hydroxyl groups on LDH layers and alginate’s carboxylate moieties.

Spectroscopic analyses confirmed that adsorption was driven by synergistic interactions between alginate’s carboxyl groups and LDH interlayers. XPS showed binding energy shifts (+0.8 eV for Cu 2p₃/₂; +1.2 eV for Pb 4f₇/₂), indicating strong metal coordination. FTIR analysis revealed new peaks (1,242, 1,022 cm⁻¹) post-adsorption, consistent with metal–carboxylate complexation.

Competitive adsorption studies revealed that metal binding affinity correlated with ionic radius and charge density. Cu²⁺ and Zn²⁺ demonstrated the highest uptake due to stronger electrostatic and coordination interactions. The observed selectivity trend was: Cu²⁺ > Zn²⁺ > Pb²⁺ > Cd²⁺ > Mn²⁺. This trend highlights the hydrogel’s preference for metals with favorable physicochemical properties.

The adsorption performance of the composite hydrogel was systematically evaluated through kinetic, isothermal, and structural analyses. Initial kinetic experiments focused on the removal of Pb²⁺ and Cu²⁺ ions. The hydrogel demonstrated rapid adsorption within the first 30 min, achieving over 90% removal for Pb²⁺ and approximately 50% for Cu²⁺. The corresponding maximum adsorption capacities from these experiments were 13 mg/g for Pb²⁺ and 66 mg/g for Cu²⁺. The adsorption followed the pseudo-second-order kinetic model, indicating that chemisorption was the predominant mechanism.

Equilibrium data were then fitted to Langmuir and Freundlich isotherm models. The Langmuir model provided a better fit for both ions, suggesting monolayer adsorption on a homogeneous surface. Based on Langmuir fitting, the maximum adsorption capacities were 284.78 mg/g for Pb²⁺ and 325.73 mg/g for Cu²⁺, highlighting the hydrogel’s high adsorption potential.

Post-adsorption structural analysis using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FT-IR) revealed further insights. XRD results showed preserved crystalline structure with a slight decrease in basal spacing (d₀₀₁), suggesting coordination between metal ions and interlayer sulfide anions, likely forming [M(MoS₄)₂]²⁻ complexes.

XPS data showed no significant chemical shift in O1s and S2p peaks but revealed decreased intensity, indicating surface coordination between metal ions and functional groups such as OH–M(II), O–M(II), SO₄–M(II), and MoS₄–M(II). These findings were supported by FT-IR, which revealed a new band at 1,397 cm⁻¹—attributed to electrostatic interactions—and intensified bands at 1,242 and 1,022 cm⁻¹, corresponding to NFL–S–M(II) and MoS₄–M(II) complexes.

Scanning electron microscopy (SEM) confirmed that the hydrogel retained its three-dimensional flower-like microstructure after adsorption. Elemental mapping showed uniform Pb²⁺ distribution across the surface, validating both structural integrity and functional performance.

A schematic illustration of the proposed adsorption mechanism is presented in Figure 1.

The adsorption behavior of the composite hydrogel toward Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, and Mn²⁺ was thoroughly investigated under two different concentration levels: 10 mg/L and 100 mg/L. Relevant data are provided in Tables 2 and 3 as well as in Figure 2. The results clearly indicate two main patterns. One is the decrease in adsorption efficiency as concentration increases. The other is the stable order of selectivity among the metal ions.

At the lower concentration of 10 mg/L, Cu²⁺ reached a removal efficiency of 99.81%. Zn²⁺ followed closely at 99.11%. These high values suggest a strong interaction between the hydrogel and these two metal ions under mild adsorption conditions. Pb²⁺ achieved a removal rate of 58.67%. Cd²⁺ was adsorbed at a rate of 43.29%. Mn²⁺ had the lowest value, with only 28.61% of ions removed from solution. These differences can be explained by variations in ionic radius and charge density, which influence the ability of each ion to interact with functional groups in the hydrogel matrix.

When the concentration increased to 100 mg/L, the removal efficiency of each ion dropped. Cu²⁺ declined to 78.68%, which is a decrease of 21.13%. Zn²⁺ dropped by 23.69% and reached 75.42%. Pb²⁺, Cd²⁺, and Mn²⁺ showed more significant declines. These decreases suggest that the higher concentration intensified the differences in binding behavior among the ions.

Three mechanisms account for the drop in Cu²⁺ removal. The first mechanism involves the saturation of binding sites. The hydrogel contains a limited number of functional groups, such as carboxyl groups and the interlayer spaces in the layered double hydroxide structure. At 100 mg/L, the total amount of Cu²⁺ in solution exceeds the number of available sites. The Langmuir isotherm model shows a maximum adsorption capacity (qmax) of 325.73 mg/g, and the strong linear fit (R ² > 0.98) supports the interpretation that a monolayer coverage was reached.

The second mechanism is the effect of ion competition. A high total concentration causes stronger electrostatic shielding. This reduces the effective charge between Cu²⁺ and the hydrogel surface. In addition, Zn²⁺ and Pb²⁺ can reach the binding sites more quickly due to their higher diffusion rates. These ions may block Cu²⁺ from binding immediately. Over time, Cu²⁺ displaces them due to its stronger affinity, but the final removal percentage remains lower than at 10 mg/L. This dynamic behavior follows the pseudo-second-order kinetic model.

The third factor is mass transfer resistance. At 100 mg/L, the gradient between the bulk solution and the interior of the hydrogel becomes steeper. As more ions try to diffuse into the hydrogel matrix, the porous network starts to limit the rate of transport. This creates uneven ion distribution. SEM-EDX analysis shows that Cu²⁺ is more uniformly distributed at 10 mg/L than at 100 mg/L, where regions of lower uptake are observed.

The influence of hydrogel dosage was evaluated using concentrations of 1 and 2 mg/mL. As shown in Figure 3, increasing the dosage significantly improved removal efficiencies for Pb²⁺ and Zn²⁺. Cu²⁺ was nearly completely removed in both cases, highlighting the hydrogel’s strong affinity and selectivity.

To assess the synergistic effect of the composite formulation, its performance was compared with hydrotalcite-only and alginic acid-only hydrogels at an initial ion concentration of 10 mg/L. The composite hydrogel consistently outperformed the individual components for all tested metal ions, as shown in Figure 4.

The enhanced performance is attributed to synergistic interaction between hydrotalcite and alginate, particularly the interlayer carbonate and hydroxide anions in LDH, which facilitated ion-exchange and electrostatic attraction, while alginate contributed carboxyl groups for metal coordination. The observed selectivity followed the trend: Cu²⁺ > Zn²⁺ > Pb²⁺ > Cd²⁺ > Mn²⁺, which is consistent across all hydrogel types.

Detailed kinetic and isothermal studies were conducted for Cu²⁺ and Pb²⁺—two common environmental and food contaminants. As shown in Figure 5, Cu²⁺ reached equilibrium faster than Pb²⁺ and achieved a higher final adsorption capacity (112.8 vs. 65.9 mg/g).

Kinetic data fit the pseudo-second-order model, indicating chemisorption as the dominant mechanism. Isothermal adsorption, modeled using the Langmuir equation, yielded maximum theoretical capacities of 325.73 mg/g for Cu²⁺ and 284.78 mg/g for Pb²⁺, with Langmuir parameters (q_max and K) confirming strong monolayer binding (Figures 6, 7).

Cu²⁺’s higher adsorption is attributed to its smaller ionic radius and higher charge density, which promote stronger interactions with functional groups in the hydrogel network.

Dynamic adsorption tests using solid-phase extraction (SPE) columns further confirmed the composite’s superior performance. As shown in Figure 8, the composite hydrogel achieved a dynamic adsorption capacity of 18.86 mg/g for Pb²⁺, surpassing hydrotalcite (12.92 mg/g) and alginate (3.28 mg/g). This improvement is due to increased surface contact and enhanced mass transfer within the SPE column.