## 3 Results and Discussion
### Methylene Blue Removal
#### 3.2.3 Adsorption Temperature and Initial MB Concentrations  
The impact of temperature was investigated for the natural biosorbents at different temperatures (20-40 \({}^{\circ}\)C) for a 200 ppm concentration MB solution for 24 h (Figure 4a). Due to the decreasing viscosity of the solution, increasing temperature will increase the rate of diffusion of the adsorbate molecules across the external boundary layer as well as in the internal pores of the adsorbent particle [104]. Furthermore, changes in temperature will also change the equilibrium capacity of the adsorbent. Figure 4a shows that higher temperatures exhibited higher uptakes for HH and CH, indicating an endothermic adsorption [69], which may be due to an increase in the mobility of the dye with increasing temperature. However, higher adsorptions rates were noted for LD and RS at lower temperatures, which indicates the process is exothermic [6], which reduces the diffusional resistance and  
Figure 3: The impact of initial pH on the dye removal and adsorption capacity of CH, HH, LD, RS, and WW for a 200 ppm initial MB concentration at 30 \({}^{\circ}\)C for 24 h.  
increases the intraparticle diffusion [105]. Increasing temperature had a negligible impact on the MB uptake of WW. Detailed thermodynamic analysis and further discussion of these phenomena is provided in Section 3.3. Among these five biosorbents, LD had the highest MB removal (~90 wt.%, ~180 mg/g), followed by HH (~71 wt.%, ~140 mg/g), CH (~60 wt.%, ~120 mg/g), and RS (~45 wt.%, ~90 mg/g). WW had the lowest MB removal (~15 wt.%, ~30 mg/g) despite having the highest surface area (27.3 m\({}^{2}\)/g) and micro-pore volume (0.01 m\({}^{3}\)/g) (Table 1). HH had the second highest micro-pore volume (0.0098 m\({}^{3}\)/g) but had a slower MB uptake rate compared with LD, CH, and RS (Figure 4c,d). This study does not show a direct relationship between surface area/pore volume of the natural biosorbent and MB adsorption. Although surface area and pore volume are key parameters in dye adsorption, they do not dominate the adsorption process for these natural biosorbents. The study indicates that, in addition to their accessibility and availability, the surface functional groups that can absorb MB (via electrostatic attraction, cation exchange, and surface complex mechanisms [106]) can also have an dramatic impact on MB removal efficiency. The influence of initial MB concentration on the uptake for the different biosorbents was explored for initial concentrations of 50 ppm, 100 ppm, and 200 ppm (Figure 5). LD had relatively high MB removal efficiency (>89 wt.%) for all initial MB concentrations. Figure 4: (**a**) Impact of temperature (20–40 \({}^{\circ}\)C) on MB removal efficiencies of a 200 ppm initial MB concentration over 24 h and the adsorption capacity isotherms in the first 60 min at 40 \({}^{\circ}\)C (**b**), 30 \({}^{\circ}\)C (**c**), and 20 \({}^{\circ}\)C (**d**). Similarly, HH and CH provided relatively high MB removal efficiency (>89 wt.%) at the initial MB concentrations of 50 and 100 ppm, while MB removal efficiency drastically decreased (to 60-65 wt.%) at 200 ppm. RS had large differences in uptake, ranging between ~87 wt.% at 50 ppm and ~40 wt.% at 200 ppm (Figure 5a), lower uptakes for higher solution concentrations for all samples apart from LD (Figure 5a). However, Figure 5b,c illustrates that higher absorption capacities were observed in the higher MB concentration tests. This could potentially be due to the driving force of mass transfer [69]. Both the temperature and initial MB concentration tests showed that MB adsorption capacity and removal efficiency are rapidly enhanced at a very early stage of MB and biosorbent interaction: ~2.5 min for 200 ppm, ~3 min for 100 ppm, and ~6 min for 50 ppm of MB solution. This is potentially due to the presence and availability of large quantities of binding sites [95]. At 200 ppm of MB solution, MB adsorption capacity and removal efficiency research equilibrium was 30-60 min for LD, RS, and WW and 60-120 min for CH and HH (Figures 4b and 5b).