Characterization of pulverized Marula seed husk and its potential for the sequestration of methylene blue from aqueous solution (2024)

Characterization of the adsorbent

The results from the Fourier transform infra-red showed a broad peak at 3303cm⁻¹ with a high transmittance frequency (Fig.2a), which can be attributed to the presence of hydroxyl group [27, 28]. The band detected at 2866cm⁻¹ is due to C–H stretching vibrations of alkanes. A medium, weak band recorded at 1731cm⁻¹ corresponds to C=O stretch of carbonyl group [27]. The bands observed at 1234 and 1026cm⁻¹ can be attributed to C–O–C stretching vibrations of ether. The XRD spectra (Fig.2b) shows a major peak at 22.3° (2θ) with other minor peaks which can be attributed to the presence of cellulosic content of the adsorbent. This is expected as the material is basically of plant origin.

Figure3 shows the results from the thermogravimetry analysis (TGA) and differential scanning colorimetry (DSC) trend of the adsorbent. The initial loss in weight of the adsorbent was recorded at approximately 195°C, this can be attributed to evaporation of bound water and moisture in the adsorbent [27]. A subsequent loss in weight was detected at 250°C, due to the thermal degradation of cellulose and hemicellulose in the plant-based material. The final weight loss occured at approximately 370°C, and could be ascribed to the degradation of lignin, which has a much higher thermal stability than either cellulose or hemicellulose polymers [29]. The corresponding DSC degradation pattern of the adsorbent is also presented in Fig.3.

The surface morphologies of the unused (Fig.4a) and spent (Fig.4b) adsorbent is presented in Fig.4. The raw adsorbent has a lot of cracks and voids with coarse surface suitable for the adsorption of contaminants. The spent adsorbent showed a reduction in the heterogeneous nature of the adsorbent which is a reflection that adsorption had occurred, with the adsorbate attached to the adsorbent.

Effects of adsorbent modification by acid and base

Figure5 shows the comparative removal efficiency of the natural, acid-treated and base-treated pulverized Marula seed husk for the sequestration of MB. The base-treated Marula seed husk recorded a higher MB removal efficiency than the untreated and acid treated forms. Low adsorbent dosage of the base-adsorbent recorded a higher removal efficiency of MB compared to others. However, at a slightly higher dosage no significant difference was obtained for the various forms of the adsorbents.

Effect of contact time

Time of equilibration usually plays a major role in adsorption experiments. In this study, there was an initial rapid uptake of MB by Marula seed husk within 5min of equilibration; 1.25g/L recorded 66% removal while 2.5g/L recorded 94% removal (Fig.6). There was however, a slight increase in MB removal efficiency with increased time from 71% at 20min to 83% at 30min with 1.25g/L. Similarly, a slight increase was also recorded with 2.5g/L from 94% (20min) to 96% (30min). After 30min, only a slight increase was observed for both dosages.

The initial rapid uptake of MB by pulverized Marula seed husk can be attributed to the presence of more surface areaon the adsorbent available for dye adsorption. After the filling of the surfaces, only a few uptakes of the dye removal were observed due to few active sites on the surface of sorbent. This continued until equilibrium was reached where no further increase was recorded. The findings obtained in this study are in consonance with results obtained by other scholars [30,31,32].

Effect of adsorbent dosage and concentration

Figure7 shows the effects of adsorbent dosage and initial MB concentration on the uptake of MB (30, 50 and 70mg/L) by 0.25g/L–4.0g/L of Marula seed husk. The percentage removal of MB increases with increased adsorbent dosage. From 45% (0.25g/L) to 98% (4.0g/L) for 30mg/L, 47% (0.25g/L) to 97% (4.0g/L) for 50mg/L and 57% (0.25g/L) to 96% (4.0g/L) for 70mg/L, respectively. Generally, higher removal of MB as expected was recorded for 30mg/L compared to 50mg/L and 70mg/L but from 2.5g/L similar levels of MB were removed by the adsorbent irrespective of the dye initial concentration.

The initial increase in performance with increase in adsorbent dosage is due to the corresponding increase in surface area available for adsorption. The subsequent little additional MB removal recorded could result from either aggregation or overlapping of adsorption sites. No removal was recorded after attainment of equilibrium.

Effect of particle size

Particle size is known to influence the adsorption rate of many adsorption systems. The small adsorbent particles usually have higher surface area than the larger ones, and this often contributes to more available adsorption active sites, hence resulting in higher adsorption [33]. In this study (Fig.8), the smaller particle size (< 125µm) achieved a better adsorption of MB than the larger size (> 125µm). This could also be due to a reduction in the limitation of internal diffusion and mass transfer of the adsorbate into the adsorbent with smaller particle sizes [34].

Effect of pH

The effect of pH on MB removal was examined over a range of pH values from 2 to 10 and the results are presented in Fig.9a. MB removal was minimum at pH of 2 for both dosages of Marula seed husk. There was a significant increase of MB uptake up to pH 6, after which slight increases were recorded up to pH 10. This could be due to increased electrostatic interaction between the dye molecules and the adsorbent at higher pH values [35]. The results imply that percentage removal of methylene blue by Marula seed husk was lower in acidic medium. This might be due to the presence of positively charged hydrogen ions which compete and/or interfere with dye cations for the available adsorption sites [7, 36]. A similar pH trend has been reported by Oden and Ozdemir[37] but disagrees with the study conducted by Jirekar et al.[38] which showed a maximum removal of MB in the acidic pH range.

The point of zero charge of 5.8 was determined for the adsorbent (Fig.9b). At pH < pH_PZC, the adsorbent surface is positively charged with high concentrations of H⁺ capable of competing with MB cations for the unadsorbed sites leading to a decrease in the uptake of MB. But when pH > pH_PZC, the adsorbent surface becomes negatively charged and favors the adsorption of MB due to increased electrostatic force of attraction and decreased H⁺. This result clearly supports the data obtained for the effects of change in pH where low amount of MB was adsorbed at low pH but as the pH increased, significant increase in the uptake of MB was recorded. The pH_PHZ obtained in this study (5.8) is slightly lower than that reported for Aleutites Moluccane seeds (5.84) [29] but was higher than the pH_PHZ determined for modified celery (4.7) [39] and acid washed black cumin seed (2.0) [40].

Effect of matrix

This experiment was performed to determine if change in water chemistry had any influence on the adsorption process. It was established that the adsorption process performed better in natural surface water than in de-ionized water (Fig.10). This can be attributed to the catalyzing effects of some natural materials present in the natural surface water. The characteristics of natural surface water used in the study are presented in Table1.

Adsorption isotherm

In this study, the Langmuir isotherm [41] was used to correlate the adsorption equilibrium data obtained. The isotherm is often used to estimate the maximum adsorption capacity corresponding to complete monolayer coverage on the adsorbent surface and is expressed by Eq.(3).

where C_e is the equilibrium concentration of MB (mg/L), q_e is the quantity of MB adsorbed at equilibrium (mg/g), q_max is the maximum amount adsorbed (mg/g) and b is the adsorption constant (L/mg). The values of b and q_max were obtained from the slope and the intercept of the plots of 1/C_e versus 1/q_e.

The Freundlich isotherm [42] was also used to correlate the adsorption equilibrium data obtained in this work. The linearized form of the Freundlich equation is expressed by Eq.(4).

where \(q_{e}\) (mg/g) is the adsorption density, \(C_{e}\) is the concentration of MB in solution at equilibrium (mg/l) andK_f is the Freundlich constant which relates to the sorption capacity of the adsorbent. Also, the value of \(\frac{1}{n}\) indicates the affinity of the adsorbate towards the adsorbent. The experimental data were fitted into Eq.(4) by plotting \(logC_{e}\) against \(logq_{e} .\) The value of \(\frac{1}{n}\) and \(logK_{f }\) were determined from the slope and intercept of the plots, respectively.

From Eq.3, a plot of 1/Ce versus 1/q_e (Fig.11) gave a straight line with linearized coefficients of 0.91 (313K), 0.95 (333K) and 0.94 (343K), respectively. This implies that the equilibrium data can be described using Langmuir isotherm. The values b and q_max, are presented in Table2.

Separation factor (a dimensionless constant) which is an expression of the Langmuir isotherm can also be used to predict if an adsorption system is “favorable” or “unfavorable” by the Langmuir isotherm [43]. This can be evaluated from the relation in Eq.(5);

where C_o is the initial MB concentration (mg/L) and b the Langmuir constant (L/mg). R_L > 1 indicates an unfavorable monolayer adsorption process, if R_L = 1, the relationship is linear, the process is favorable when 0 < R_L < 1 and if R_L = 0 the process is irreversible. The results obtained from this study has an R_L value between zero and one, indicating a favorable adsorption process (Table2).

A plot of log C_e against log q_e from Eq.4 also gave a straight line with linearized coefficients of 0.80 (313K), 0.84 (333K) and 0.88 (343K), respectively (Fig.12). This also implies that the equilibrium data can also be described by the Freundlich isotherm. However, the Langmuir plot best described the equilibrium data. Tables2 shows the values of 1/n and K_f derived from the slope and intercept of the plot. The values of 1/n were between 0 and 1 indicating that the adsorption of the MB onto the adsorbent used was favorable at the studied conditions [44].

The comparison of the various pulverized adsorbents on MB removal is presented in Table3. Several factors such as pH, temperature, initial adsorbent dosage, initial MB concentration, nature of the adsorbent and the time of equilibration affect the performance of the adsorbent for the sequestration of MB.

Thermodynamics of the adsorption processes

The thermodynamic feasibility and the thermal effects of the sorption process were determined by estimating the standard Gibbs free energy change (\(\Delta G^{ \circ }\)), the standard entropy change \(\left( {\Delta S^{ \circ } } \right)\) and the standard enthalpy change (\(\Delta H^{ \circ }\)). The value of \(\Delta G^{ \circ }\) determines if a process occurs spontaneously or not. For a given temperature, a phenomenon is considered to be spontaneous if the \(\Delta G^{ \circ }\) has a negative value. Moreover, if \(\Delta H^{ \circ }\) is positive, the process is endothermic and if it is negative, the process is exothermic. \(\Delta G^{ \circ }\) was determined using the relation in Eq.6

where \(K_{0}\) is the equilibrium constant (m³mol⁻¹) determined from the Langmuir constant b. \(\Delta S^{ \circ }\) and \(\Delta H^{0}\) were determined using the Vant Hoff equation (Eq.7) [50, 51]:

where T is the absolute temperature (K) and R is the gas constant, (8.314Jmol⁻¹K⁻¹). The plot of \(InK_{0}\) as a function of 1/T should give a linear relationship with slope of \(\Delta H^{ \circ }\)/R and an intercept of \(\Delta S^{ \circ }\)/R. The values calculated for \(\Delta G^{ \circ }\) are presented in Table4. Figure13 shows the plot of \(InK_{0}\) versus of 1/T; \(\Delta S^{ \circ }\) and \(\Delta H^{ \circ }\) calculated from the plot are also presented in Table4.

The change in enthalpy \((\Delta H^{ \circ } )\) of the process has a positive value which confirms that the adsorption process is endothermic in nature with the absorption of heat during the sorption process. The negative values of \(\Delta G^{ \circ }\) indicate that the process is feasible and spontaneous. The positive values of \(\Delta S^{ \circ }\) reflect the affinity of the adsorbents for MB and also suggest an increase in the randomness at the solid/liquid interface during the adsorption of MB onto pulverized Marula seed husk [50, 52]. Similar findings have been reported by Siddiqui et al. [40] and Nayak and Pal [53]. However, a different trend where ΔH and ΔS are negative have also been reported in various studies on the sequestration of MB from aqueous solution [29, 39].

Kinetic study of the adsorption processes

The experimental data obtained under the effects of change in time were subjected to three kinetic models (pseudo-first order, pseudo-second order and Elovich kinetic models). Equations8–10, show the linearized mathematical representation of the models [54, 55], respectively.

where \(q_{e } \left( {mg/l} \right)\;{\text{and}}\;q_{t} \left( {mg/l} \right)\) are the adsorption capacities at equilibrium and at time “t” respectively; \(k_{1} \left( {l/min} \right)\) is the pseudo-first-order rate constant. \(k_{1} \left( {l/min} \right)\) and \(q_{e } \left( {mg/l} \right)\) can then be determined from the slope and the intercept of the plot, respectively.

A plot of \(t/q_{t}\) against “t” using Eq.(9) would give a linear relationship from which \(q_{e}\) and \(k_{2}\) can be determined from the slope and intercept, respectively.

Thus, the plot of \(q_{t}\) against \(In\left( t \right)\), should give a straight line if adsorption process conforms to Elovich model, where α is the initial adsorption rate (mg/gmin); β is the desorption constant (g/mg).

Figure14 shows the kinetic plots of the three models. The Pseudo second order kinetic best described the kinetics of the adsorption process based on the linearized coefficient. The kinetic constants obtained from the three models are presented in Table5. Similar results have been reported for MB uptake by various plant materials [52, 56,57,58].