## 3 Results
### Effect of pH  
pH of the solution is an important factor in the metal adsorption process, because if impacts the surface chemistry of the bioadsorbent, speciation, sequestration, and mobility of metal [39]; the effect of pH variation (3, 4, 5, and 6) on the Pb (II) removal process on eucalyptus sawdust was investigated. This pH range was selected because, according to  
Figure 2: Infrared (IR) spectrum of eucalyptus sawdust before and after Pb (II) adsorption.  
the speciation graph of Pb (II) in water [40], when pH \(>\) 6 the Pb + 2 species begins to convert into other species such as Pb(OH)\({}_{2}\), Pb(OH)\({}_{3}\)\({}^{-}\), Pb(OH)\({}_{4}\)\({}^{+2}\), PbNO\({}^{3+}\), PbOH, which are found in a low proportion between pH 3 to 6 [41]. Figure 3 shows the average of the pH data with respect to the adsorption capacity of Pb (II) on the eucalyptus sawdust, finding that the highest adsorption capacity is 4.80 mg/g obtained at pH 6, achieving a removal efficiency of 96.01%. This can be explained by the fact that as the surface charge of the biomass became more negative proportionally with the pH, the adsorption of the metal increased too, since the negative surface charge leads to the deprotonation of functional groups in the sawdust. Therefore, the deprotonated active sites, are easily accessible to the ions, increasing the adsorption rate [42]. At low pH's, the repulsion between the ions and positively charged exchange sites occur along with competition with H3O\({}^{+}\), which prevents the ions from being successfully captured [43]. Above pH 6, considering lead speciation, low solubility hydroxides such as Pb(OH)\({}_{2}\) are formed, so adsorption at higher pHs would be difficult, as reported [43].  
Similar results to this study have been obtained when using rice straw [44], oil palm fiber residues [45], avocado shells [30]. Studies report that by increasing the pH to 6, due to the fact that ion exchange is the process that dominates the removal of the metal and at low pHs the surface of the metal is protonated by H\({}_{3}\)O\({}^{+}\) ions, which compete for the active sites with the lead ions and by having greater mobility and preference for adsorption at the surface they decrease the adsorption capacity of the ion in the adsorbent [46]. As the pH increases, the negative charge density in the biosorbent increases due to the deprotonation of the metal-binding sites; this phenomenon increased the biosorption of the metal [47].  
Pb (II) adsorption capacities have been achieved between 3.9 and 8.9 mg/g on biomass of the fungus _Lepiota hystr_[48], 12.86 mg/g on a bio-adsorbent from _Lavan-dula pubescens Decne_[49], 82.24 mg/g, 91.12 mg/g, 99.19 mg/g, and 40.41 mg/g on yam, banana, cassava, and oil palm bagasse shells, respectively [32], and 0.112 mg/g on residual mushroom biomass [50]. It is observed that the results obtained in the present study are in the range of 0.1-99 mg/g reported in the literature, so it can be said that eucalyptus sawdust is a good source for the preparation of bioadsorbent material.  
Table 3 summarizes the results of the analysis of variance with a \(p\)-value of 0.05 reliability for adsorption capacity at a 95% confidence level, where it is shown that the pH has a significantly positive effect on the ability to remove Pb (II) on eucalyptus sawdust.  
Figure 3: Effect of pH variation on the adsorption capacity of Pb (II).