## 3 Results and Discussion
### Adsorption Efficiency of LPD Biomass
#### 3.2.3 Effect of Solution pH  
The pH of solution is a crucial parameter that affects the adsorption process, by influencing both the solubility of the metal ions and the surface characteristics of adsorbent in the aqueous solution [33]. The effect of pH on the biosorption of Pb(II) ions by \(L\). _pubescens_ biomass was studied  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{\(q_{e}\). Exp. (mg/g)} & \multicolumn{3}{c}{**Pseudo-First-Order**} & \multicolumn{3}{c}{**Pseudo-Second-Order**} \\ \cline{2-9}  & \(q_{e}\) (mg/g) & \(k_{1}\) (1/min) & \(R^{2}\) & \(q_{e}\) (mg/g) & \(k_{2}\) (g/(mg\(\cdot\)min)) & \(R^{2}\) & \(h\) (mg/(g\(\cdot\)min)) \\ \hline
4.12 & 15.10 & 0.063 & 0.946 & 6.91 & 0.0020 & 0.969 & 0.095 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Kinetic parameters of pseudo-first and pseudo-second order equations for lead(II) adsorption onto LPD.  
Figure 3: Pseudo-first and pseudo-second order kinetic models for adsorption of Pb\({}^{2+}\) onto LPD. Conditions: 55 ppm Pb\({}^{2+}\) initial concentration, 50 mL Pb\({}^{2+}\) solution volume, 0.2 g LPD adsorbent dose, pH 6, 25 \({}^{\circ}\)C adsorption temperature, and 150 rpm agitation speed.  
in the pH range from 3 to 7. In Figure 4a, the maximum adsorption capacity was at pH = 5 (\(q_{e}\) = 12.86 mg/g). However, a slight difference with pH 7 (12.55 mg/g) was observed. At pH lower than 5 (i.e., acidic medium), the additional H+ compete with the metal cations for active sites on the biomass surface [33], and also repel the metal ions from the surface by electrostatic force. Hence, the adsorption capacity is reduced. In contrast, with pH increase, negatively charged functional groups on the LPD surface (such as carboxyl, phosphate, phosphodies, sulphonate, and imidazole) attract the positively charged metal ions, and enhance their adsorption [30]. As the pH value increases above 7, however, the OH ions increase in solution and react with the metal ions to form hydroxide precipitates and, therefore, adsorption experiments could not be performed well [34]. From these observations, it can be concluded that the adsorption efficiency of Pb2+ ions by \(L\). _pubescens_ biomaterial increases as the pH value approaches pH 5. According to literature [29; 30], before precipitation occurrence of Pb(OH)2 at pH > 7, the slight decrease in biosorption of lead(II) at pH between 5 and 7 is due to the formation of soluble hydroxide complexes of the lead ions, which compete for the active sites and, as a consequence, decrease the adsorption efficiency. Such competition between biosorption, lead-hydroxyl complexation, and precipitation of the lead ions commonly occurs at pHs close to neutrality (-pH 7) [30]. The chromatography and FTIR analysis indicate that the LPD biosorbent contains dominant organic-based polar or negatively-charged functional groups, such as hydroxyl, carboxyl, ester, amide, and amine; thus, ionic interaction, as one possible adsorption mechanism, with LPD is facilitated by complex formation between metal ions and functional groups on the adsorbent. Moreover, the chemisorption mechanism is supposed to be the dominant mechanism, at least in the first stage of adsorption.  
Figure 4: Effect of (**a**) pH value (C0 = 55 ppm, 0.2 g/50 mL adsorbent dose, temperature = 25 \({}^{\circ}\)C, and 150 rpm agitation speed), (**b**) initial lead(II) concentration (0.2 g/50 mL adsorbent dose, pH 6, temperature = 25 \({}^{\circ}\)C, and 150 rpm agitation speed), (**c**) adsorbent dosage (C0 = 55 ppm, 50 mL adsorbent volume, pH 6, temperature = 25 \({}^{\circ}\)C, and 150 rpm agitation speed), and (**d**) adsorption temperature (C0 = 55 ppm, 0.2 g/50 mL adsorbent dose, pH 7, and 150 rpm agitation speed) on the adsorption capacity of _Lavandula pubescens_ Decne for removal of lead(II) ions from aqueous media.