# Adsorption of Hexavalent Chromium Ions Using Pine Sawdust Cellulose Fibres
## 3 Results and Discussion
### Adsorption Studies
#### 3.8.5 Effect of Temperature  
Figure 15a,b depict the adsorption capacity and removal percentage, respectively, of untreated pine sawdust and cellulose fibres for Cr (VI). As shown in Figure 14, the adsorption capacity and removal percentage revealed a rapid rise with increasing temperature but subsequently decreased upon a further increase in temperature. As the temperature increased, the adsorption capacity of Cr (VI) ions increased from 8.26 to 9.59 mg/L, and the removal percentage increased from 58.1% to 89.6%. However, when the temperature exceeded 313 K, there was a slight decrease in both the adsorption capacity and removal percentage. Thus, the optimum temperature for the cellulose fibres (CF) was 313 K, at which the removal efficiency and adsorption capacity were found to be 89.6% and 9.59 mg/L, respectively. [PERSON] et al. [50] reported that the removal percentage and adsorption capacity of chromium (VI) ions on groundnut shells increased with temperature, indicating an endothermic process. Consequently, the rise in temperature led to a minor alteration in the reaction or bond properties involved in the adsorption process by promoting the mobility and diffusion of both the Cr (VI) ions and the cellulose fibres (CF) [50; 55]. Figure 14: Effect of concentration on hexavalent chromium ions’ adsorption by pine sawdust (adsorbent dose of 2 g/L, agitation speed of 200 rpm, and adsorbate concentration 100 mg/L) and cellulose fibres (CF) (adsorbent dose of 2 g/L, pH of 6, and agitation speed of 200 rpm). coefficient [50], showed contrasting values in this study. A higher K\({}_{\rm L}\) value indicated a stronger affinity between the solute and the adsorbent. However, this study observed contrasting K\({}_{\rm L}\) values when comparing the linear and nonlinear Langmuir plots. The K\({}_{\rm L}\) value of \(2.9\times 10^{-3}\) in the linear Langmuir plot indicated a relatively weak affinity between the solute and the adsorbent. Conversely, in the nonlinear Langmuir plot, the K\({}_{\rm L}\) value was 171.589, suggesting a significantly higher affinity between the solute and the adsorbent. The low K\({}_{\rm L}\) value from the linear plot may be indicative of factors like incomplete monolayer formation or heterogeneity in the adsorbent surface, which weaken the solute's affinity to the adsorbent. Figure 16: Isotherm plots of (**a**) linear and (**b**) nonlinear Langmuir and Freundlich isotherms for the adsorption of hexavalent chromium ions on cellulose fibres (CF). In contrast, the high K\({}_{\rm L}\) value obtained from the nonlinear plot suggests a strong affinity between the solute and the adsorbent. This was possibly due to the nonlinear plot accommodating deviations from the ideal assumptions, resulting in a more accurate representation of the adsorption process and indicating a higher affinity between the solute and the adsorbent [56]. Additionally, the Langmuir isotherm can be mathematically expressed using a dimensionless constant, the separation factor (R\({}_{\rm L}\)), represented by Equation (6). \[\rm R_{L}=\frac{1}{1+K_{L}C_{o}} \tag{6}\]  
where the R\({}_{\rm L}\) value indicates the characteristics of the biosorption process. Suppose the R\({}_{\rm L}\) values range from 0 to 1; in this case, they indicate a favourable biosorption process: R\({}_{\rm L}\) greater than 1 represents the unfavourable biosorption, R\({}_{\rm L}\) equal to 1 represents the linear biosorption, and R\({}_{\rm L}\) equal to 0 specifies the irreversible biosorption process but is favourable when 0 \(<\) R\({}_{\rm L}\)\(<\) 1 [57; 58]. The linear and nonlinear Langmuir R\({}_{\rm L}\) was 0.476 and 6.5 \(\times\) 10\({}^{-2}\), respectively (Table 5), suggesting favourable biosorption. The nonlinear Langmuir isotherm model better describes the adsorption data points than the linear Langmuir isotherm model, as indicated by the R\({}_{\rm L}\) values. A lower R\({}_{\rm L}\) value suggests a stronger affinity between the adsorbate (chromium (VI)) and the adsorbent (cellulose fibres) at various equilibrium concentrations.