## 3 Results and Discussion
### Biosorption Characteristics of 1 M NaOH-Treated \(H\). fusiformis
#### 3.4.2 Effect of Initial Heavy Metal Concentration  
Equilibrium biosorption experiments were performed to quantify the maximum biosorption capacity of 1 M NaOH-treated \(H\). _fusiformis_ (initial metal concentration: 10-1000 mg/L; pH: 4.0; adsorbent dosage: 1.67 g/L; reaction time: 1440 min; agitation speed: 100 rpm; temperature: 25 \(\pm\) 0.5 \({}^{\circ}\)C). Figure 6 shows that the biosorption of different metal ions increased with an increase in the initial heavy metal concentration and reached the maximum adsorbed amount at \(>\)300 mg/L of initial heavy metal concentration. The equilibrium parameters derived from the fitting of the Langmuir and Freundlich models to the experimental data are presented in Table 4. The Freundlich model provided a good fit for the equilibrium data of biosorption with a high correlation coefficient (\(R^{2}\) = 0.965 for Cd\({}^{2+}\), \(R^{2}\) = 0.956 for Cu\({}^{2+}\), and \(R^{2}\) = 0.970 for Ni\({}^{2+}\)). Similar results were obtained by [PERSON] and [PERSON] [52], who reported that Cd\({}^{2+}\), Cu\({}^{2+}\), and Ni\({}^{2+}\) biosorption by sepiolite was  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Metals**} & \multicolumn{3}{c}{**Pseudo-First-Order Model**} & \multicolumn{3}{c}{**Pseudo-Second-Order Model**} & \multicolumn{3}{c}{**Elovich Model**} \\ \cline{2-13}  & \(q_{z}\) **(mg/g)** & \(k_{1}\) **(1/min)** & \(R^{2}\) & **SSE** & \(q_{z}\) **(mg/g)** & \(k_{2}\) **(g/mg-min)** & \(R^{2}\  
well fitted to the Freundlich model. In contrast, the fit of the equilibrium data for Pb\({}^{2+}\) ion biosorption by the Langmuir model was better (\(R^{2}\) = 0.967, \(SSE\) = 1054.9) than the Freundlich model. These results indicate that the biosorption of Cd\({}^{2+}\), Cu\({}^{2+}\), and Ni\({}^{2+}\) adsorbed on the 1 M NaOH-treated _H. fusiformis_ via the formation of a multilayer but Pb\({}^{2+}\) via monolayer [21]. The maximum biosorption of the treated _H. fusiformis_, obtained from the Langmuir model, was in the order of Pb\({}^{2+}\) >> Cu\({}^{2+}\)\(\approx\) Ni\({}^{2+}\) > Cd\({}^{2+}\). The affinity of these heavy metals for _H. fusiformis_ can be explained by the electronegativity of metals, the hydrolysis constant, and the charge-to-radius ratio [21; 53]. Among them, electronegativity was most suitable to explain our experimental data. The electronegativities of the four heavy metals, in decreasing order, were Pb\({}^{2+}\) (2.33) > Ni\({}^{2+}\) (1.91) > Cu\({}^{2+}\) (1.90) > Cd\({}^{2+}\) (1.69) [54; 55]. Electronegativity can influence heavy metal adsorption because metals with higher electronegativities can build stronger covalent bonds with O atoms present on the surface of the adsorbent [54]. Pb\({}^{2+}\), which had the highest electronegative value, showed the highest biosorption, whereas Cd\({}^{2+}\), which had the lowest electronegative value, presented the lowest biosorption. Ni\({}^{2+}\) and Cu\({}^{2+}\) had similar electronegativity and biosorption amounts similar to those of treated _H. fusiformis_. This relatively superior Pb\({}^{2+}\) adsorption derived from electronegativity has also been reported similarly in other algae (_Codium vermilara_, _Spioyyn insignisis_, _Asparagopsis armata_, _Chondrus crispus_, _Fucus spiralis_, _Ascopphyllum nodosum_) [56]. The adsorption of metals onto adsorbents increased as the hydrolysis constant (pK) decreased. The pK values of the metals, in increasing order, were Cu\({}^{2+}\) (pK = 7.7) = Pb\({}^{2+}\) (pK = 7.7) < Ni\({}^{2+}\) (pK = 9.9) < Cd\({}^{2+}\) (pK = 10.1) [57]. The least amount of Cd\({}^{2+}\) with pKa was removed by _H. fusiformis_, but the hydrolysis constants for other metals were not consistent with their uptake by _H. fusiformis_. The larger ionic radius led to the lower charge density and columbic attraction between ions and the surface of adsorbents, and the ionic radius of metals, in decreasing order, was Pb\({}^{2+}\) (1.20 A) > Cd\({}^{2+}\) (0.97 A) > Cu\({}^{2+}\) (0.73 A) > Ni\({}^{2+}\) (0.69 A) [58].