3.1 Characterization of BC
Briefly, the characterization by Quansah et al. (2024) showed that the used BC has a Brunauer-Emmett-Teller (BET) surface area of 6.7 m²/g, total pore volume of 0.017 cm³/g, and a pHPZC of 4.02. It also has an irregular surface marked by apertures and cracks of diverse dimensions and forms, with carbon (C) and oxygen (O) as the primary elements. Moreover, it possesses oxygen-containing functional groups such as carbonyl (C = O), hydroxyl (O–H), aromatic C = C, methyl (C–H), Si–O–Si, Si-O-Al, and Al-OH-Al structures, which enhance its ability for cation adsorption. Mineral components including quartz (SiO2), calcium carbonate (CaCO₃), aluminium silicate hydroxide, and potassium sodium aluminium silicate were also identified in the BC. These minerals possess reactive sites that can also facilitate chemical interactions with metals.
3.2 Effect of initial solution pH on Mn2+ adsorption
The adsorption of metal ions is significantly influenced by the initial pH of the solution (Abollino et al., 2003). In view of this, experiments were performed under initial solution pH of 2–12 to identify the optimal pH for effective Mn2+ adsorption. Geochemical simulations using PHREEQC were also performed to enhance understanding of the mechanistic interactions between Mn2+ and the BC surface at different pH values. The experimental and simulation results are illustrated in Fig. 1a–c. From the experimental results (Fig. 1a), it can be seen that the amount of Mn2+ removed was low at pH 2 (i.e., about 17%) but increased precipitously to 94% when the pH was raised to 4. Subsequently, the adsorption rose gradually and stabilized with the maximum adsorption occurring at pH 6. Beyond pH 6 slight decline in adsorption was observed.
Two reasons are proffered to explain the low Mn2+ adsorption at low pH. First, H+ generally has greater affinity for many complexing and ion exchange sites than most metallic ions (Milicevic et al., 2012). Moreover, at low pH (pH 2) the concentration of H+ (1.1×10− 2 mol/L) exceeds that of Mn2+ (1.82 × 10− 4 mol/L), which is the dominant Mn species, constituting > 99% of the total Mn concentration. Due to these factors, H+ will predominantly occupy the available adsorption space/sites on the BC more readily than Mn2+ at low solution pH, resulting in low Mn2+ adsorption (Eq. 13). Second, protonation of BC surfaces occurs at pH < pHpzc whereas deprotonation occurs at pH > pHpzc (Jellali et al., 2021; Quansah et al., 2024). Quansah et al. (2024) noted that the surface of the used BC becomes positive at pH < 4.02. This means that the Mn2+ will be electrostatically repelled from the BC surface at pH < 4.02, which will further decrease the amount of Mn2+ adsorbed. The reverse of the above phenomena also explains the high adsorption of Mn2+ at high solution pH. As the solution pH increases, the H+ concentration declines (i.e., Mn2+ > conc. H+) (Fig. 1b). Consequently, the competition for adsorption sites between H+ and Mn2+ and the preferential adsorption of H+ becomes minimal, allowing for more Mn2+adsorption. Also, the surface charge on the BC gradually shifts from positive to negative through deprotonation (Eqs. 14 and 15) (Quansah et al., 2024). This shift will consequently result in higher removal efficiency as the impact of electrostatic repulsion diminishes, while attraction increases. Beyond pH 10, Mn(OH)3− and Mn(OH)42− complexes begin to form (Fig. 1b); this partly reduces the adsorption efficiency (declining trend) due to repulsion by the negative charges on the BC surface. Comparable observations have also been reported in other studies investigating the adsorption of heavy metals or cationic pollutants onto untreated or modified lignites and other adsorbents (e.g., Pehlivan et al., 2004; Mohan and Chander, 2006; Haddad et al., 2018).
$$\text{B}\text{C}-\text{C}\text{O}\text{O}\text{H}+ {\text{H}}^{+}\leftrightarrow \underset{\begin{array}{c}Protonation of BC surface\\ resulting in electrostactic \\ repulsion of {\text{M}\text{n}}^{2+}\end{array}}{\underset{⏟}{\text{B}\text{C}-{\text{C}\text{O}\text{O}\text{H}}_{2}^{+}}}$$
13
$$\text{B}\text{C}-\text{C}\text{O}\text{O}\text{H}+ {\text{O}\text{H}}^{-}\leftrightarrow \underset{\begin{array}{c}Deprotonation of BC surface\\ favouring adsorption\\ of {\text{M}\text{n}}^{2+} \end{array}}{\underset{⏟}{\text{B}\text{C}-{\text{C}\text{O}\text{O}}^{-}}}+{\text{H}}_{2}\text{O}$$
14
$${\text{n}\text{B}\text{C}-{\text{C}\text{O}\text{O}}^{-}+\text{M}\text{e}\text{t}\text{a}\text{l}}_{\left(\text{a}\text{q}\right)}^{\text{n}+}\leftrightarrow {\left(\text{B}\text{C}-\text{C}\text{O}\text{O}\right)}_{\text{n}}\text{M}\text{e}\text{t}\text{a}\text{l}$$
15
In adsorption studies, the pH must be maintained below the level at which the metal ion precipitates (Muntean et al., 2023). Thus, regarding the suitable pH for the adsorption experiments, geochemical simulations using PHREEQC were performed to study the precipitation dynamics of Mn minerals as the solution pH changes. Figure 1c is a plot of the saturation indices (SI) of the various minerals against solution pH, where the SI was calculated from the ion activity product and the solubility product at a given temperature. Generally, an SI of a mineral phase less than 0 indicates undersaturation with respect to that mineral; SI > 0 implies supersaturation with regard to that mineral while SI = 0 indicates equilibrium with respect to that mineral (Parkhurst and Appelo, 1999). Figure 1c shows that hausmannite (Mn3O4) and manganite (MnOOH) precipitates begin to form at pH 9, while pyrochroite (Mn(OH)2) and pyrolusite (MnO2:H2O) precipitates begin to appear at pH 10. These precipitates will reduce the concentration of Mn2+, but not necessarily its adsorption onto the BC. Since at pH 6.0 the highest adsorption was observed, it was chosen as the optimal pH for subsequent sorption experiments. This value falls within the range (i.e., pH 3.5–6.5) identified as the optimum pH range for the sorption of most divalent ions onto peat/lignite (Brown et al., 2000).
3.3 Effect of different BC dosage and Mn2+ concentration
The impact of BC dosage on Mn2+ adsorption was investigated under the initial conditions of 10 mg/L Mn2+ at 298 K and pH 6. Consistent with Jellali et al. (2021), the removal of Mn2+ increased from 42–98.09% with an increase in BC dosage from 0.2 to 1 g (Fig. 2a). This can be attributed to the increase in available adsorption sites as the BC dosage increased (Dragan et al., 2016). Beyond the adsorbent dosage of 1 g, no perceptible change was observed for the same Mn2+ concentration. The adsorption capacity, on the contrary, showed a decreasing trend with increasing BC dosage. This could be due to the occlusion of the adsorbent pores due to excess adsorbent dosage, constraining its surface area and consequently diminishing adsorption efficiency (e.g., Xu et al., 2017). Rawajfih and Nsour (2008) similarly explained that increasing the adsorbent dose at a constant volume of adsorbate triggers adsorbent aggregation, potentially reducing the overall surface area of the sorbent and elongating diffusional pathways.
The effects of different initial Mn2+ concentrations and different BC dosages were also assessed simultaneously. As shown in Fig. 2b the removal efficiency decreased as the initial Mn2+ concentration increased from 1 to 150 mg/L for all the three BC dosages studied. This is possibly because at a lower adsorbate concentration, the ratio of available adsorption sites to adsorbate concentration is higher, leading to rapid and high removal. Conversely, as the adsorbate concentration increases, this ratio decreases, leading to a decrease in removal efficiency (Awad et al., 2023). It can be seen that, while the influence of lignite mass on adsorption efficiency is noticeable, it appears to be less significant, particularly at lower concentrations. However, as the concentration of Mn2+ increases, the effect of dosage becomes more pronounced, emphasizing the importance of optimizing dosage levels for efficient adsorption processes.
3.4 Effect of competing ions
The impact of background cations (Ca2+, Na+, K+, Cu2+, and Zn2+) on Mn2+ adsorption in binary systems was explored using 10 mg/L of each cation. As shown in Fig. 3, in the Mn2+/Na+ system, Mn2+ decreased from 10 mg/L to 2.99 mg/L, while the concentration of Na+ increased from 10 mg/L to 15.2 mg/L, indicating the release of Na+ from the BC into solution. A similar trend was observed for Ca2+, where Ca2+ concentration increased from 10 mg/L to 43.5 mg/L. Conversely, K+ concentration in the Mn2+/K+ system exhibited a slight decrease. However, the concentration of Mn2+ in the three binary systems at the end of the experiment was nearly identical, suggesting that the effect of Ca2+, Na+, K+ on Mn2+ uptake was similar. It is unclear what the effect of higher concentrations of these mono- and di-valent cations on the sorption of Mn2+ would be. However, Merdy et al. (2002) found that Mn2+adsorption is significantly lowered in the presence of Ca2+ at a concentration of 200 mg/L. For the HMs, concentrations of Zn2+ and Cu2+ decreased from 10 mg/L to 8.72 mg/L and 2.48 mg/L, respectively. Although Mn2+ concentration decreased in the presence of both metals, Cu exhibited a more substantial competitive effect than zinc and suggested that it will be preferentially retained compared to Zn. Pehlivan et al. (2004) also noted similar trends, suggesting that BC has a higher affinity for adsorbing Cu compared to Zn. Spinti et al. (1995) found the following order of affinity for immobilized lignite/peat beads: Fe > Al > Cu > Cd > Zn > Ca > Mn > Mg > Na, which is consistent with our study. They further noted that all the metal ions tested follow a pattern of the higher charged ion having the higher affinity. Ong and Swanson (1966) also explained that metals with the greatest ionic potential (ratio of charge to ionic radius) would be removed first, and if the sites were still undersaturated, then those having lower ionic potential would be removed in sequence. Other researchers have also suggested that the order of adsorption of HMs is based on ionic properties such as electronegativity and ionic radius (Pentari et al., 2009).
3.5 Mn2+ adsorption thermodynamics
The thermodynamic parameters–ΔH°, ΔG°, and ΔS°– analysed from Fig. 4 to determine the spontaneity, feasibility and mechanisms of the adsorption are presented in Table 1. The values of ∆G○ were negative, indicating that Mn2+ adsorption on BC was spontaneous and favourable (Hao et al., 2010; Sahmoune, 2019). The ∆Gº values were also low (< -10 kJ/mol), suggesting that Mn2+adsorption was primarily driven by physical forces (physisorption) (Apea et al., 2023). It can also be seen that as the temperature increased, the ∆G° values became more negative. This signifies that, higher temperatures enhance the adsorption of Mn2+ (Hao et al., 2010). This is because, as temperature increases, the kinetic energy of Mn molecules also increases, leading to increased motion and collisions towards and with the BC surface, resulting in adsorption (Dragan et al., 2014). The obtained ΔH° value was positive and below 40 kJ/mol (i.e., 15.16 kJ/mol), indicating that the adsorption was endothermic and principally governed by weak van der Waals forces (physisorption) (e.g., Sahmoune, 2019; Apea et al., 2023). These findings contrasted with Mohan and Chander (2006), who reported that the removal of Mn2+ by lignite was exothermic in nature. The positive value of ∆S○ indicates that the system becomes more disordered as the adsorption process unfolds; however, this does not lead to any structural modification of the BC (Hao et al., 2010; Sahmoune, 2019).
Table 1
Thermodynamic parameters for Mn2+ adsorption on BC
Temperature (K) | ∆G○ (kJ/mol) | ∆H○ (kJ/mol) | ∆S○ (kJ/mol-K) |
298 | -4.86 | 15.16 | 0.067 |
323 | -6.71 | | |
348 | -8.23 | | |
3.6 Sorption isotherms
Understanding the interactions between adsorbents and adsorbates is crucial for the large-scale application of adsorbents in adsorption processes. In our study, we utilized the Freundlich, Langmuir, and Temkin equilibrium isotherms to quantify Mn2+ adsorption onto BC, with the isotherm parameters provided in Table 2. The Freundlich isotherm (Fig. 5a) yielded 1/n and KF values of 0.85 and 0.023 (L/g), respectively, and a coefficient of determination, R2 of 0.993. The 1/n of 0.85 suggests that the adsorption process is favourable, which corroborates the results from the thermodynamic studies (Chantawong et al., 2003). Furthermore, the deviation of 1/n from unity signifies nonlinear sorption, implying that the sorption energy barrier intensifies exponentially as the fraction of occupied sites on the BC rises. The values of Qmax and KL determined from the Langmuir isotherm (Fig. 5b) were of 1.19 mg/g and 0.018 L/mg, respectively, with a high R2 of 0.994. The separation factor (RL), which indicates the favourability of the adsorption process, was computed from Eq. 16 below (Shoukat et al., 2017). also showed that the adsorption process is favourable. The obtained RL values ranged from 0 to 1 (i.e., 0.53–0.98), confirming the favourability of Mn2+ adsorption onto the BC.
$${\text{R}}_{\text{L}}=\frac{1}{\left[1+\left({\text{K}}_{\text{L}}\times {\text{C}}_{0}\right)\right]}$$
16
where RL < 1 indicates favourable adsorption; RL = 0 indicates irreversible adsorption; RL = 1 is linear adsorption; and RL > 1 indicates unfavourable adsorption (Ademiluyi and Ujile, 2013).
The KT and bT, determined from the Temkin isotherm (Fig. 5c), were of 0.67 L/mg and 18.65 kJ/mol, respectively. The positive bT value confirmed that heat is required for adsorption to occur (endothermic process), and the process is favoured at higher temperatures. The R2 values obtained for the different isotherms (Table 2) show that the Freundlich and Langmuir equations exhibit excellent fits with the experimental data although the former gave a slightly higher R2 value. This suggests a complex adsorption process that involves multiple mechanisms. The Langmuir isotherm postulates a monolayer adsorption mechanism occurring on a uniform surface, while the Freundlich isotherm hypothesises multiple-layer adsorption on diverse surfaces. Thus, the ability of both models to fit the experimental data indicates that different adsorption sites on the BC surface are involved in the process, and various layers of adsorbate molecules contribute to the overall adsorption. Pentari et al. (2009) also noted that the Langmuir isotherm gave slightly better results compared to the Freundlich equations. The Qm, for the BC was compared to that of other adsorbents. The results presented in Table 3 demonstrate that the Qm for BC is comparable or even higher than those of other natural adsorbents.
Table 2
Estimated parameters of the Langmuir, Freundlich and Temkin isotherms.
Langmuir | | Freundlich | Temkin |
Qmax (mg/g) | \({K}_{L}\) (L/mg) | \({R}^{2}\) | | \({K}_{F}\) (L/g) | \(1/n\) | \({R}^{2}\) | \({b}_{T}\) (kJ/mol) | \({K}_{T}\) (L/mg) | \({R}^{2}\) |
1.19 | 0.018 | 0.994 | | 0.023 | 0.85 | 0.993 | 18.65 | 0.67 | 0.865 |
Table 3 Comparison of Mn2+adsorption capacities for different adsorbents.
Adsorbent | Qm (mg/g) | Particle size | pH | Temperature (°C) | Reference |
Zeolite Y | 0.015 | 0.75 µm | 6.50 | 25 | (Kwakye-Awuah et al., 2019) |
Natural zeolite | 0.080 | 850 µm | 7.00 | 25 | (Shavandi et al., 2012) |
Palm fruit bunch | 2.21 | 215–300 µm | NA | 22 ± 2 | (Nassar et al., 2004) |
Maize Cob | 2.28 | 215–300 µm | NA | 22 ± 2 | (Nassar et al., 2004) |
BC | 1.74 | 75 µm | 6.65 | 25 | This study |
Qm: maximum adsorption capacity; NA = not available |
3.7 Sorption kinetics
In Fig. 6a, the time-dependent adsorption of Mn2+ is illustrated. The graph shows a rapid linear increase in Mn2+ adsorption within the first 30 min, followed by gradual increase until saturation at 120 min. The initial rapid adsorption can be attributed to instant utilization of the most readily accessible adsorption sites on the surface of the BC (Kumar et al., 2010; Wen et al., 2014), while the slower adsorption thereafter is attributed to adsorption to limited readily accessible adsorption sites, complexation or diffusion of Mn2+ ions into BC micropores, a process typically associated with some resistance (Kaveeshwar et al., 2018). The kinetics of metal binding to BC has been previously studied by other researchers. Jellali et al. (2021) reported of a much lower equilibrium time compared to our studies while reaction times reported by Havelcova et al. (2009) and Pentari et al. (2019) were much higher. Most authors, however, have reported equilibrium times in the range 20–60 min for most divalent ions (Ong and Swanson, 1966; Gossett et al. 1986). Differences in these values have mostly been attributed to the type of metal ion, type of lignite, and experimental conditions (Yang et al., 2019).
The PFKM and PSKM were utilized to evaluate the kinetic data, with the results summarized in Table 4. The PFKM assumes that the adsorption rate is proportional to the number of unoccupied sites on the adsorbent surface, while the PSKM suggests that the rate of adsorption is influenced by chemical interaction between the adsorbate molecules and the adsorbent surface (Hao et al., 2010; Adekola et al., 2016; Wang and Guo, 2020). The R2 values show that the experimental data can be effectively described by both kinetic models. However, the experimental equilibrium adsorption capacity closely aligned with that estimated from the PSKM (0.268 mg/g). This suggests that, the PSKM is more fitting for describing the adsorption process.
The sorption of an adsorbate on an adsorbent comprises multiple steps, with two being particularly crucial: the movement of the adsorbate through the boundary layer to reach the adsorbent surface and the subsequent transfer of the adsorbate from the surface into the pores, known as intra-particle diffusion (IPD) or inner diffusion (ID), which is generally a slow process. Both the PFKM and PSKM are unable to predict any of these diffusion mechanisms. Consequently, the intra-particle diffusion model (IPDM) was employed to further help to understand the diffusion of Mn2+ into the internal pores of the BC. The model allows to identify whether IPD is the sole controlling factor or if other mechanisms also contribute to the overall adsorption behaviour. The IPDM assumes that IPD (i.e., diffusion of adsorbate into the pores or internal structure of the adsorbent) is the sole rate-limiting step if the plot of qt versus t1/2 is linear and passes through the origin (Weber and Morris, 1963). The IPD is influenced by factors such as the porosity and structure of the adsorbent material. Deviations from linearity or portions of the plot not passing through the origin may indicate the influence of other processes, such as film diffusion or external mass transfer (i.e., the movement of solute molecules through a thin layer or film of liquid surrounding the adsorbent particles). During the adsorption process, solute molecules must traverse this boundary layer before reaching the adsorbent surface where adsorption takes place. The rate of film diffusion is influenced by factors such as the thickness of the boundary layer, the concentration gradient of the solute between the bulk liquid and the solid surface, and the properties of the solute-solvent system. In situations where film diffusion occurs, the intercept represented by C, will provide insight into the thickness of the boundary layer. Film diffusion can be a rate-limiting step, particularly when the boundary layer is thick (Wang and Guo, 2020). As shown in Fig. 6b, the plot of qt against t1/2 consisted of two linear portions, none of which passed through the origin. This confirms that the adsorption process is intricate and encompasses multiple stages, rather than being solely controlled by IPD (Hao et al., 2010). Similar results were reported by Di et al. (2022) when they studied the adsorption of Cu2+, Zn2+ and Pb2+ by lignite.
Table 4
Parameters of the PFKM, PSKM and IPDM
PFKM | | PSKM | | IPDM |
\({\text{q}}_{\text{e}}\) (mg/g) | \({\text{k}}_{1}\) (1/h) | \({\text{R}}^{2}\) | \({\text{q}}_{\text{e}}\) (mg/g) | \({\text{k}}_{2}\) (mg/g h) | \({\text{R}}^{2}\) | | kp1 (mg/g min1/2) | R2 | kp2 (mg/g min1/2) | R2 |
0.259 | 0.070 | 0.988 | 0.268 | 0.673 | 0.996 | | 0.005 | 0.913 | 0.0008 | 0.722 |
3.8 Reusability and environmental benignity of the BC
Results of experiments conducted to investigate the ability of the BC to be regenerated and reused are shown in Fig. 7a. There was a decline in adsorption efficiency from 91.99% during the first cycle to 84.61% after the fourth cycle. This is possibly due to the loss of adsorption sites as a result of incomplete desorption of Mn2+ from the BC surface as well as the loss of solid material due to its fine nature (Quansah et al., 2024). These results suggest that chemical ion-exchange dominated the adsorption process.
Evaluating the environmental compatibility of natural adsorbents intended for use as adsorbents in environmental remediation is imperative due to the potential for pollution swapping. In broad terms, pollution swapping refers to the unintended increase or release of one pollutant while attempting to mitigate another; essentially, one pollutant is exchanged for another (Stevens and Quinton, 2009). In this study, we investigated pollution swapping by assessing the concentrations of HMs, including Cu, Zn, Cd, Pb, Cr, and Fe, both before and after the addition of BC to Mn-contaminated groundwater. Figure 7b illustrates that the initial concentrations of Cu, Zn, Cd, Pb, Cr, and Fe in the untreated water were low. Following the introduction of BC, the concentration of Zn exhibited a slight increase, while those of Cd and Fe decreased. The concentrations of Zn leached into the solution, however, remained within the recommended limits set by the World Health Organization (WHO). The concentrations of Pb, Cu, and Cr, on the other hand, remained unaltered. These results suggest that the utilization of BC as an adsorbent will not result in pollution swapping, signifying its environmental benignity and reinforcing its potential suitability for environmental remediation.
3.9 Mn2+ sorption mechanisms
The thermodynamic, equilibrium, and kinetic studies have suggested that multiple mechanisms are involved in Mn2+ removal by BC. Many studies have also reported a similar observation during the adsorption of other metal ions onto BC (e.g., Hanzlík et al., 2004). Among these mechanisms, however, complexation and ion exchange are commonly reported, with ion exchange considered the most prevalent (Brown et al., 2000; Mohand and Chander, 2006; Pehlivan and Arslan, 2007; Milicevic et al., 2012). The ion exchange and complexation mechanisms are often not straight forward to distinguish due to their intricate and often overlapping processes. FTIR analysis is an effective technique for scrutinizing modifications in the surface properties of adsorbents subsequent to adsorption via complexation with oxygen-containing functional groups. Complexation results in distinct shifts in the peak intensities and configurations of the functional group moieties in the BC. This phenomenon is accompanied by a decrease in solution pH (Cao et al., 2009; Milicevic et al., 2012; Cui et al., 2016; Jellali et al., 2021). Mohan and Chander (2006) noted that the decrease of pH at low initial solution pH is due to dissociation of carboxylic functional groups because their dissociation constant (pKa) is generally between 4 and 6. Conversely, the reduction of pH at higher initial solution pH is attributed to the dissociation of phenolic functional groups, as these groups release their H+ at pH levels higher than 9 (Pehlivan and Arslan, 2007). To investigate the contribution of complexation, FTIR analyses were carried out on the BC samples before and after Mn2+ loading. As illustrated in Fig. 8a, there was minimal change in the nature of the peaks of the functional groups in the 4000 − 1800 cm− 1 region, while peaks of functional groups like C = O, C–H, P–O–C/C–H, Al-OH-Al/vinyl or aromatic C-H, and Si-O-Al/C-I in the 1735 − 400 cm− 1 range changed slightly or markedly after Mn2+ uptake, indicating the potential involvement of these functional groups in Mn2+ complexation. Contrary to what has been commonly reported in the literature, an increase in solution pH after Mn2+ adsorption was observed. This likely resulted from the release from mineral components of the BC. This has commonly been reported when there is no prior demineralization of the used adsorbent, which was the case in our study (Cui et al., 2016).
The release of mono- and divalent cations (Ca2+, Mg2+, Na+, H+, and K+) from the BC into solution is an indication of the occurrence of ion exchange. These cations are complexed with functional groups (through inner and outer sphere complexation) or fixed in silicate minerals in the BC (Chen et al., 1990). In a pure ion exchange system, the number of cations released and adsorbed are equal. To ascertain the contribution of ion exchange in this study, the amount of Ca2+, Mg2+, Na+, and K+ released into solution and the amount of Mn2+ removed from solution were measured at different solution pHs. As shown in Fig. 8b, Ca2+, Mg2+, Na+, and K+ were released into solution during Mn2+ adsorption, indicating the occurrence of ion exchange. From pH 2 to 4, the total amount of mono- and di-valent cations released were more than the total amount of Mn2+ removed, suggesting that other ions, possibly H+ were also taken up in exchange for the released cations. This is consistent with the findings of Quansah et al. (2024). From pH 6 and beyond, however, the amount of Mn2+ removed was more than the total amount of Ca2+, Mg2+, Na+, and K+, suggesting that other mechanisms in addition to ion exchange were involved in Mn2+ removal. Consistent with Mohand and Chander (2006) and Quansah et al. (2024), the amount of Ca2+ released exceeded that of Mg2+, K+ and Na+. Figure 8c further illustrates a notable release of these basic cations within the initial 30 minutes, followed by subsequent declines in their concentrations. This trend is consistent with the results of the kinetic studies, which demonstrated significant removal of Mn2+ within the first 30 minutes of adsorption which was rapid followed by a gradual removal. The initial removal could possibly be attributed to ion exchange while the gradual removal could be due to complexation or other diffusional mechanisms.