The bulk elemental composition (Carbon, Hydrogen, Nitrogen, Sulfur) of the prepared NSWR powder was determined using a Perkin Elmer 2400 SERIES-II CHNS/O analyzer. Oxygen content was typically calculated by differences.

To establish baseline levels, the inherent concentrations of the target HMs [(Cd), (Cu), (Pb), and (Zn)] in the raw NSWR sample were quantified. This analysis was performed using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Horiba Jobin Yvon Activa-M) following sample digestion.

The phase composition and degree of crystallinity of the NSWR were examined using X-Ray Diffraction (XRD). Analyses were carried out on a Bruker D4 Endeavor X-Ray Diffractometer (Germany). The instrument was configured with CuKα radiation (λ = 1.54056 Å) and operated at a voltage of 40 kV and a current of 30 mA (Power = 1.8 kW). Diffractograms were recorded over a 2θ angular range from 5 to 70°, utilizing a step size of 0.02° and a data acquisition time of 1 s per step.

The identification of surface functional groups presents on the NSWR, which may serve as binding sites for HM ions, was achieved using Fourier Transform Infrared (FTIR) spectroscopy. Spectra were acquired using a Bruker Vertex 70v spectrometer equipped with an Attenuated Total Reflectance (ATR) sampling accessory. Scans were performed over the wavenumber range of 4,000–400 cm⁻¹ with a resolution of 4 cm⁻¹ and accumulating 32 scans for signal averaging.

The surface topography and morphological features of the NSWR particles, both in their initial state and after being loaded with HMs during adsorption experiments, were visualized using Scanning Electron Microscopy (SEM). Thermo Scientific Phenom XL G2 Desktop SEM was employed for this purpose. Images were typically captured using a Backscattered Electron Detector (BSD) under low vacuum conditions (approximately 10 Pa) with an electron beam accelerating voltage of 10 kV at various magnifications as required (e.g., 650x, 1,000x). Concurrently, semi-quantitative elemental analysis of the sample surface was performed using energy-dispersive X-ray Spectroscopy (EDS) integrated with the SEM system.

The specific surface area, total pore volume, and average pore width of the NSWR biosorbent were determined based on nitrogen gas adsorption–desorption isotherms measured at 77 K. The analysis was performed using a Micromeritics Gemini VII 2390 t Surface Area and Porosity Analyzer. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method from the adsorption data in the relative pressure (P/P₀) range of 0.05–0.35. Total pore volume was estimated from the amount of nitrogen adsorbed at a relative pressure close to unity (e.g., P/P₀ ≈ 0.99).

The point of zero charge (pH_pzc), representing the pH value at which the net surface charge of the NSWR material is neutral, was determined using the pH drift method. Briefly, fixed amounts of NSWR were added to a series of solutions with known initial pH values (typically ranging from 2 to 10), adjusted using dilute NaOH. The suspensions were agitated until equilibrium was reached (e.g., 24–48 h), and the final equilibrium pH was measured. The pH_pzc was identified as the point where the plot of final pH versus initial pH intersects the line of final pH = initial pH or where the plot of ΔpH (final pH - initial pH) versus initial pH crosses the zero ΔpH axis.

The thermal stability of NSWR sample and the corresponding changes in its mass were carried out by (TGA) and (DSC) analysis using Netzsch-Gerätebau GmbH - STA 409 PC Luxx Simultaneous thermal analyzer, with dynamic nitrogen atmosphere (gas flow of 50 ml min⁻¹), between 80 and 6,497°C with Heating and cooling rates: 5 K/min to 20 K/min, 5.2 mg ± 0.1 mg sample.

NSWR, as a lignocellulosic biomass, derives its properties from its chemical makeup and physical structure (Bhardwaj et al., 2025). Chemically, it is primarily organic, composed mainly of cellulose, hemicellulose, and lignin (Miranda et al., 2012; Basit et al., 2024; Yi et al., 2024). This was reflected in the elemental analysis conducted in this study, which showed high proportions of Carbon (43.46%), Oxygen (50.86%), and Hydrogen (5.52%). Proximate analysis of Spruce (Picea abies) further characterized the material with a volatile content of 80.2 ± 1.11%, fixed carbon content (17.63 ± 0.45%), moisture content (8.60 ± 0.2) and ash content (0.39 ± 0.07) as reported by Awan et al. (2024) and Matin et al. (2024).

Background levels of the target HMs (Cd, Cu, Pb, Zn) were measured in the sample prepared to assess potential interference. As detailed in Table 2, these inherent concentrations were found to be low.

The XRD diffractogram of unmodified (Figure 1) confirms its predominantly amorphous nature, characterized by a broad diffraction halo spanning 2θ ≈ 15–22°, consistent with the disordered structure of cellulose and lignocellulosic materials (Ivanovska et al., 2021). Minor crystalline phases are evident as sharp peaks at 2θ = 26.6° and 36.5°. The peak at 26.6° is indexed to the (101) plane of quartz (SiO₂), aligning with JCPDS No. 00–046-1045 (Deka and Maji, 2013), a common impurity observed in wood-derived materials. The peak at 36.5° is tentatively attributed to calcite (CaCO₃), corresponding to the (104) plane (JCPDS No. 00-005-0586; Zhang et al., 2022). These findings corroborate with prior studies reporting similar diffraction patterns in aged lignocellulose fibers and carbonized wood, underscoring the material’s inherent heterogeneity. The presented pattern shows a well-resolved characteristic peak with the highest intensity obtained at a value of around 21° corresponding to the (200) plane for cellulose, thus indicating an amorphous cellulose type I. The obtained peak was broad and not sharp, which is typical for amorphous substances since they do not possess a long range of Bragg peaks in X-ray diffraction order. This data further confirmed FTIR analysis that showed cellulose as the main constituent of the used NSWR biosorbent (Figure 1), with functional groups having bonding affinity toward HM ions in aqueous solutions. The same findings were obtained from Šehović et al. (2022) on lignocellulosic biomass of Citrus limon peel as a biosorbent for the removal of Cr³⁺, Cu^2+, and Pb²⁺ ions. Overall, the XRD analysis validates NSWR’s suitability as a low-crystallinity adsorbent for HM ions while highlighting the need for deeper characterization of trace mineral phases.

The FTIR spectrum of NSWR (Figure 2) exhibits several absorption bands indicative of its lignocellulosic composition. A broad band centered around 3,330 cm⁻¹ corresponds to O-H stretching vibrations from hydroxyl groups (in cellulose, lignin, and adsorbed water). Peaks at approximately 2,920 and 2,854 cm⁻¹ are attributed to C-H stretching vibrations in aliphatic chains. The prominent peak at 1,605 cm⁻¹ can be assigned to C=C aromatic ring vibrations in the lignin and possibly overlapping contributions from adsorbed water-bending vibrations. The band at 1513 cm⁻¹ is also characteristic of lignin aromatic skeletal vibrations. The region between 1,450 cm⁻¹ and 1,200 cm⁻¹ < contains multiple peaks related to C-H bending and C-O stretching vibrations (Abou-Elyazed et al., 2024; Yi et al., 2024). A very strong and broad absorption band dominating the spectrum is centered near 1,025 cm⁻¹, characteristic of C-O stretching and C-O-C ether linkages prevalent in cellulose and hemicellulose polysaccharides (Armaya’u et al., 2024; Uddin et al., 2024). The presence of these diverse oxygen-containing functional groups (hydroxyl, carboxyl/carbonyl - implied by C=O near 1,605 or potential weak shoulder near 1730, ether suggests multiple potential binding sites for heavy metal cations.

SEM micrographs reveal the surface morphology and the porous structure of NSWR before and after HMs adsorption (Figures 3). Prior to adsorption (Figure 3A, 1,000×), the material displays a heterogeneous and irregular surface structure, characterized by rough textures, fragmented particles, and evidence of a relatively porous feature. After exposure to the HMs solution (Figure 3B, 650×), the holes were filled with the HM ions, the surface heterogeneity declined, and the pores appeared to be smooth. These characteristics indicate a high ability for biosorption of HMs. The same findings were obtained by applying Meretrix lusoria biosorbent for removing Cd²⁺ and Pb²⁺ ions (Al-Mur, 2024).

EDS analysis was conducted to identify the elements present on the NSWR surface, particularly after adsorption. A representative spectrum of the metal-loaded NSWR is shown in Figure 4. The spectrum confirms the presence of distinct peaks corresponding to the targeted HMs (Pb), (Cu), (Cd), and (Zn) are clearly visible, confirming their successful adsorption onto the biosorbent surface. The corresponding values of the wt. % of the HM ions reflect their affinities arrangement to NSWR. Additionally, elements such as Calcium (Ca), Sulfur or Sulphur (S), Phosphorus (P), and Aluminum (Al) were detected, likely to originate from the inherent composition of the wood biomass (Figure 4), which probably originate from substances absorbed by the tree in the soil (Salamat et al., 2017). Semi-quantitative results from EDS analysis are presented in Table 2. The presence of cations (especially Ca²⁺) reinforces the potential role of ion exchange in the adsorption and hydrogen bond formation mechanisms. This interesting role of such ions in adsorption is consistent with literature findings (Šehović et al., 2022).

Quantitative surface analysis using N₂ adsorption provided the parameters detailed in Table 3.

The NSWR exhibited a BET-specific surface area of 4.5630 m²/g and a total pore volume of 0.00532 cm³/g. While the surface area is modest, the average pore width of 46.655 Å suggests a predominantly mesoporous character, potentially facilitating access to internal binding sites. Likewise, similar findings were obtained using Abies bornmulleriana cones for the removal of Pb, Cu, Cd, Ni, and Co ions. The BET surface area and pore volume of the biosorbent were defined to be 5.05 m²/g and 0.0018 cm³/g. The average pore width of the biosorbent used in that study was calculated to be 9.34 (Oguz, 2020).

The point of zero charge (PZC) is almost obligatory in the characterization of adsorbents (Kosmulski, 2023) because the surface charge behavior of NSWR is crucial for electrostatic interactions. The point of zero charge (pH_pzc) was determined to be 5.7 (Figure 5). This indicates the pH at which the net surface charge is neutral; above (pH_pzc), electrostatic repulsion occurs, reducing metal cation adsorption above this (pH_pzc), the surface becomes increasingly negative, enhances electrostatic attraction thus, favoring and facilitating cation adsorption (Al-Labadi et al., 2023).

Both DSC and TGA analyses were conducted on the adsorbent material (NSWR). The results are shown in Figures 6A,B. The DSC thermogram (Figure 6A) reveals a significant exothermic event, indicating thermal decomposition or a phase transition in the sample. The process begins at an onset temperature of 145.0°C, suggesting the thermal stability is up to this point. The maximum thermal activity occurs at (369.7°C), marking the peak of decomposition or structural transformation, and the process concludes at (298.2°C). The broad peak width of 210.7°C, spanning 37% of the temperature range, suggests a complex reaction mechanism, potentially involving overlapping transitions or multiple degradation steps. As indicated by the peak area (228.2 μV·s/mg), the total energy involved reflects a high-energy reaction. These results provide insight into the material’s thermal behavior, highlighting its stability range and the energy dynamics of its decomposition process, which are critical for applications requiring thermal stability or controlled degradation.

The TGA thermogram (Figure 6B) shows a slight weight loss was detected between (30 to 110°C) which was attributed to the removal of moisture content and light volatile compounds, while a mass loss of 43.05%, indicating a significant thermal decomposition process occurring between 145 and 369.7°C. This range aligns closely with the exothermic event observed in the DSC thermogram, confirming that the mass loss is due to decomposition or structural breakdown of the material.

The stable region before (145°C) reflects the material’s thermal stability, consistent with the onset of the DSC peak. The correlation between the TGA and DSC results highlights that the observed energy release in the DSC is directly related to the decomposition process evidenced by the mass loss in the TGA, suggesting a strong link between the thermal stability and degradation mechanisms of the material. This thermal stability can be considered a good property of NSFR as an efficient candidate adsorbent for HM removal.

The mass of the biosorbent is an important determinant of the absorption measurements for a particular ion concentration. It accurately determines the ideal absorbent mass required to absorb the most metal cations under a certain set of configurable parameters.

Adjusting the mass of the biosorbent is necessary to choose the most economical amount to be employed. (Figure 7) illustrates the absorption efficiency (R%) of HMs for Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions, concerning the mass of the biosorbent. As the adsorbent dosage increased from 0.005 to 0.1 g, the percentage of removal efficiency (R%) for Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions each rose from 42.65 to 88.51%, 60.34 to 94.87%, 64.57 to 99.11, and 45.93 to 86.46%, respectively, in an orderly and accordingly in direct proportion. The increase in adsorption percentage with a dosage of adsorbent can be ascribed to the addition of more sorbent mass, namely a higher ratio of metal cations to binding sites, which leads to a greater number of accessible unsaturated or vacant active sites in the biosorbent. Consequently, the surface area of the sorbent also improves. Moreover, the highest level of biosorption for Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions of the HMs ions was achieved at a biosorbent mass of 0.1 g/0.01 L, equivalent to 1 g/0.1 L. Further rises in the adsorbent mass above 0.1 g were found to have an inverse effect on the biosorption proportion. In the current investigation, a biomass dosage of 0.1 g/0.001 L was deemed optimal for further trials in this work.

The impact of the duration time on the absorption of Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ ions was examined. Figure 8 shows that the sorption process occurred rapidly within the first 20 min, resulting in the deletion of approximately 65.54% Cd²⁺, 94.93% Cu²⁺, 97.64% Pb²⁺, and 64.38% Zn²⁺ ions. Following 20 min, the clearance percentage for Pb²⁺ and Cu²⁺ ions remained almost constant, and no noticeable alteration in biosorption was observed.

Nevertheless, the absorption rate of Cd²⁺ and Zn²⁺ ions decreased after the initial 20 min, although it persisted until reaching a plateau at 200 min. The sluggish rate may be attributed to the mechanism of metal cation dispersal into the permeable structure of the biosorbent. The decline in removal effectiveness over time may also be attributed to biosorption occurring at active spots within a monolayer. Once a plateau phase (equilibrium state) was reached at 200 min, a relation of equilibrium between sorption and desorption procedures was detected. This offered repulsive forces and a drop in the HM adsorption after 200 min and was connected to the saturation of solid surfaces since the HM ions should be able to travel very long distances via the small-sized pores with very high concentrations. Thus, mass transfer between the solid and liquid phases is declining over time. Notably, there was no significant improvement in biosorption for 4 ions after 200 min. Hence, a contact duration of 200 min was selected as the equilibrium time for subsequent studies. Nevertheless, it is important to note that time, as an influencing factor, is inadequate alone to give a full picture regarding the adsorption process, as the other parameters simultaneously and collectively affect the process in different ways and percentages.

The absorption of HMs is significantly influenced by pH. It impacts the processes of hydrolysis, precipitation, dissociation, speciation, and accessibility of the HMs ions that collectively impact the biosorption procedure (Cruz-Lopes et al., 2021). Furthermore, the pH level is a crucial factor in the biosorption of HMs-ions as it influences the binding between the adsorbent and adsorbate compounds, as well as the charges of the functional groups on the biosorbent (Loh et al., 2020). In selecting the mechanism of metal removal (adsorption or precipitation), pH is paramount. The pH level decides both the surface charge and the chemical composition of the cell walls of the biosorbent. Indeed, it is crucial to initially adjust a solution’s pH value before adjusting the other parameters.

This work aimed to examine the impact of pH on the biosorption of Pb²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ ions via NSWR at five different pH values: (2, 3, 4, 5, and 6). According to Figure 9, the percentages of Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺ cations absorbed by NSWR were seen to be low at pH 2 with values of 24.560, 16.533, 20.053, and 20.890, respectively. This phenomenon arises from the competition between HMs’ cations and H⁺ for the limited biosorption sites. Moreover, acidic conditions chemically facilitate the process of protonation of the functional groups on the NSWR, thereby producing repulsive interactions on the surface of the biosorbent and metal cations. Although the absorption percentage of the metals indicated above increased dramatically when the pH value went from 2 to 5, the specific scales of the upsurges were as follows: 52.28% for Cd²⁺, 78.37% for Cu²⁺, 79.017% for Pb²⁺, and 56.65% for Zn²⁺. This happening may be ascribed to the expanded presence of readily available negative charge surfaces on the adsorbent, which facilitated the capture of positively charged HMs cations. Furthermore, with an upsurge in pH, the adsorption surface exhibited a decline in positivity, resulting in an associated rise in electrostatic attraction between the HM ions and the NSWR surface. Within the pH range of 5–6, two relatively distinct behaviors were shown. For Cu²⁺ and Pb²⁺, the enhancements in the uptake percentages were not significant, with values of only 0.817 and 0.2%. However, for Cd²⁺ and Zn²⁺, the observed increases were 10.030 and 7.090%, respectively. The highest sorption effectiveness seen at pH 5.5–6 can be attributed to the interaction between Mi⁺, Mi(OH)⁺, and Mi(OH)₂, where Mi denotes the metal ions and surface functional groups that are present on (NSWR). This value, pH 5.5–6, corroborates the obtained magnitude of pH_pzc 5.7. The potential reduction in biosorption efficiency at higher pH levels (pH > 6) is not only due to the production of soluble hydroxylated complexes of HM ions (copper ions as Cu(OH)₂, cadmium ions as Cd(OH)₂, and lead ions as Pb(OH)₂) but also to the ionized state of the cell wall surface of the wood powder at the pH level under investigation (Munagapati et al., 2010). Hence, the experiment did not consider pH values above 6, as higher pH has been shown to induce precipitation of both Pb and Cu ions (Kariuki et al., 2017). The results of our study align with a review paper authored by Bhattacharjee and colleagues, which states that the ideal pH range for the absorption of HMs utilizing various cellulosic-based materials as biosorbents is mostly between 5 and 6 (Bhattacharjee et al., 2020).

The solution temperature is an influential operational cause that impacts the transport and kinetic mechanisms of HM ion adsorption. This characteristic is conditional on the mobility of HM ions and the swelling effect inside the inner structure of the biomaterials, which in turn impacts the adsorption capabilities.

In this study, the impact of temperature was examined within the temperature range of 25–45°C. It may be inferred that the impact of temperature on the proportion of deletion of 4 HM ions under consideration was directly proportional to the uptake proportion. Specifically, as the temperature rose from 25 to 45°C, the elimination uptake also expanded. This observation suggests that the sorption process is endothermic. The observed rise can be recognized by the enhanced mobility of HM cations, which in turn leads to a rise in the number of active spots. The impact of temperature rise on the removal% % of the studied HMs is not remarkable. The same result was obtained from Tounsadi et al. (2015) using Diplotaxis harra and Glebionis coronaria L. biomasses for the removal of Cd (II) and Co (II) (Figure 10)

The primary factor responsible for physisorption is the intermolecular or electrostatic binding between HM ions and the adsorbent. This attraction could be strengthened by increasing the specific surface area of the adsorbent (Kumar et al., 2019). Though biosorption is a surface phenomenon that depends to some degree on the specific area unprotected from biosorption, only a limited number of studies have investigated the impact of altering the size of adsorbent particles on adsorption, especially in the case of non-living LCB.

The present work has demonstrated the impact of the particle size of NWSR on the ratio of uptake of the investigated HMs. The phenomenon was investigated in 5 distinct particle size classifications, as elucidated in Figure 11. Based on the flaky shape and relatively porous structure of NSWR particles, it can be inferred that they have acquired both an external and an internal surface that is readily accessible for the aqueous phase to penetrate, therefore facilitating the adsorption of HMs-ions to the active ligation adsorption sites (Božić et al., 2009). Importantly, the ratio at which 4 divalent cations (Cd²⁺, Cu²⁺, Pb²⁺, and Zn²⁺) are removed slightly increases when the particle size is reduced from 1,000 to 250 μm. Consequently, it may be inferred that a smaller particle size of the adsorbent results in a greater number of binding sites and a higher specific surface area, hence improving the effectiveness of removing bivalent hazardous metals. Meanwhile, fine particles also enhance the rapid adsorption rate and decrease the desired reaction time. As the particle size grows, the contact time required to achieve equilibrium also rises. Furthermore, the distance that the cations travel along the diffusion pathway across the bivalent cations of the sorbent is significantly reduced. In contrast, within the range of finer particles, specifically those smaller than 90 μm, the deletion percentages of Cd²⁺ and Cu²⁺ cations showed no increase, while the uptake rates of Cd²⁺ and Pb²⁺ cations somewhat decreased.

Multicomponent adsorption exhibits a lower removal effectiveness of manifold HM pollutants compared to the single HMs elimination efficacy achieved by a similar adsorbent. Furthermore, the inconsistency in the sequence of selectivity is attributed to variations in physicochemical characteristics, including electronegativity, hydrated radius, ionic radius, and covalent indexes (Table 5). This behavior may also be attributed to the polarizability of HM ions and, to a certain degree, to the hard-soft acid–base (HSAB) theory: typically, hard acids exhibit stronger coordination with hard bases and soft acids with soft bases (Bulin et al., 2022). The adsorption efficiency of NSWR for HMs ions in water can be affected by several factors, including the type and quantity of functional groups, specific surface area, surface shape, and structural features. Our results demonstrate a selective adsorption pattern, with Pb having the greatest adsorption capacity, followed by Cu, then Cd. Finally, Zn is shown in Figure 12. A recent review article by Amar et al. (2021) has concluded that throughout modest biosorption from multi-metal systems, most biosorbents can efficiently eliminate Pb²⁺ even when other HMs are exhibited. Nonetheless, the incidence of Pb²⁺ ions significantly lowered the adsorption of Cu²⁺, Cd²⁺, Zn²⁺, Ni²⁺, and Cr³⁺. Investigation of the adsorption of HM ions in a quaternary system utilizing Kappaphycus sp. illustrates the adsorption ranking of Pb²⁺, Cu²⁺, Fe²⁺, and Zn²⁺ as reported by Natrayan et al. (2022). Furthermore, when Pine bark was applied, the order observed was Pb²⁺ > Cu²⁺ > Fe²⁺ > Zn²⁺ (Cutillas-Barreiro et al., 2014). Similarly, Anastopoulos et al. (2019) reported Pb²⁺ > Cu²⁺ > Zn²⁺ arrangement on watermelon rinds. A metal ion’s covalent index is determined by its ionic radius and electronegativity. A higher covalent index indicates a better potential for the metal ion to form covalent bonds with the functional groups of the biosorbent (Batool et al., 2022). The covalent index of Pb is 6.41, while Cu, Cd, and Zn have covalent index values of 2.98, 2.71, and 2.04, respectively. Furthermore, the level of toxicity often rises as the covalent index magnitude increases. The data presented in Table 5 demonstrates a positive correlation between electronegativity, covalent index, and ion exchange. Consequently, the sorption capacity also increases. These results align with the works of Baool and coworkers (Batool et al., 2022). The adsorption kinetics of HMs-cations on particle surfaces, particularly in multi-ion solutions, are intricate and cannot be fully elucidated only based on electrostatic or ion-exchange adsorption, cation size, or hydration. It is necessary to consider all possible mechanisms comprehensively and succinctly. Overall, the variability in the sequence of selectivity can be associated with variations in physicochemical characteristics, including electronegativity, hydrated radius, ionic radius, and covalent indexes, and can be potentially connected to the “shielding effect” or “screening effect” of HM cations (Table 5). Furthermore, (Table 6) lists several types of biomass together with their related q_max values. Among these, unmodified NSWR is a highly favorable material for bio-sequestration when compared to other lean bio-adsorbents.

The speed of agitation is a salient aspect in biosorption investigations, particularly in the context of porous materials such as LCB. However, scholars have not adequately taken it into account. Indeed, effective agitation potentially promotes the infiltration of HM ions into the internal framework of the biosorbent, accelerates the mobility of HM ions in the solutions containing the metal, decreases the duration of the reaction, and optimizes the transfer of mass resistance, thereby improving the biosorption process.

To investigate the impact of agitation speed on HM bio-absorption, we examined 10 distinct mixing speeds ranging from 40 to 400 rpm, as detailed in Figure 13. At agitation speeds below 150 rpm, the elimination uptake was quite low. It is probable that a portion of the adsorbent settled down and did not participate in adsorption, resulting in a reduction in biosorption. As the agitation speed increases within 150–200 rpm, the turbulence in the liquid phase intensifies, leading to higher external mass transfer coefficients and decreased boundary layer effect. Thus, the decrease in external resistance results in accelerated adsorption (Sreedhar and Reddy, 2019). On the other hand, whenever the stirring speed exceeds 200 rpm, the adsorbent may not have sufficient time to bind the HM cations, therefore decreasing its effectiveness. It is vital to note that the shaking ratio should be appropriate and adequate to guarantee the easy availability of all surface binding spots for the absorption of HM ions. The results of this study have demonstrated that an agitation speed of around 200 rpm is sufficient and appropriate for the absorption of the four ions under investigation. Similar outcomes were achieved when Moringa stenopetala bark was used for the removal of Cd²⁺, Pb²⁺, and Cu²⁺ (Kebede et al., 2018).