## 3 Results and Discussion
### Adsorption Batch Experiments
#### 3.3.2 Effect of Initial pH  
The efficiency of heavy metal removal and adsorption processes is strongly influenced by the pH of the solution, which ultimately affects the ionization process and ionic strength of the adsorbate and adsorbent surface charge [54]. A diverse change in adsorption was noted under the influence of variable pH values (5, 6, 7, 8, and 9). A pH value of 7 was found to be the optimum pH for the adsorption of both Cd and Pb, and the highest adsorption was noted at this pH range (Figure 6A,B). Consistently lower adsorption of Cd and Pb was found in the acidic pH range, which could be due to fixation or complexation of metal ions. Similarly, a sharp decrease in adsorption of both heavy metals was noted in the alkaline pH range, which indicated precipitation of metal ions [55]. A similar trend of heavy metal removal was observed, and maximum removal efficiency of employed adsorbents was found at a pH of 7, which again endorsed the strong influence of pH on the stabilization of Cd and Pb in the contaminated soil (Figure 6C,D). At a pH value of 5, the removal efficiency of Cd ranged between 36 and 50% for all adsorbents, which was significantly increased to 85-90% at pH 7. A gradual decrease in the removal efficiency of Cd and Pb was observed by increasing the pH from 7 to 9. Similarly, for Pb, the highest removal  
Figure 5: Effect of adsorbent dose and particle size on removal efficiency of adsorbents for Cd (**A**,**B**) and Pb (**C**,**D**) removal from contaminated soil. NZ-M, natural zeolite with microparticle size; NZ-N, natural zeolite with nanoparticle size; LDH-M, LDH-zeolite with microparticle size; LDH-N, LDH-zeolite with nanoparticle size; HCI-M, HCI-zeolite with microparticle size; HCI-N, HCI-zeolite with nanoparticle size; NaOH-M, NaOH-zeolite with microparticle size; NaOH-N, NaOH-zeolite with nanoparticle size; NaCl-M, NaCl-zeolite in the microparticle size; NaCl-N, NaCl-zeolite in the nanoparticle size. (\(>\)88%) was found at pH 7, while at acidic pH, removal efficiency was recorded in the 35-39% range. Generally, at lower pH values, the zeolite surface is positively charged due to protonation of surface functional groups, which subsequently repel cationic Cd\({}^{+2}\) and Pb\({}^{+2}\) ions and hinder adsorption and removal of heavy metals [56]. Increasing pH resulted in the deprotonation of surface functional groups, which caused zeolite surfaces to become more electrostatically charged and enhanced adsorption of Cd and Pb with higher removal efficiency [57]. Also, the higher removal efficiency of adsorbent at increasing pH value could be due to the presence of fewer H\({}^{+}\) ions, which compete with Cd\({}^{+2}\) and Pb\({}^{+2}\) ions for sorption at surface active sites of zeolites [58]. The reduction in adsorption and lower removal efficiency of adsorbents for Cd could also be attributed to the formation of Cd(OH)\({}_{2}\) crystalline ionic compounds, which precipitate at higher pH [59]. Our findings are consistent with [PERSON] et al. [60], who found higher adsorption of Pb at a pH value of 6.5-7 using raw natural zeolites as an adsorbent. At high pH values, the formation of complex hydrated ions of Pb (Pb(OH)\({}_{2}\))) along with precipitation reactions caused a decline in Pb adsorption and removal efficiency [61]. Among all tested adsorbents, LDH-modified NZ showed maximum adsorption and removal efficiency. The presence of the Si-O functional group on the surfaces of the natural and modified zeolite surfaces was confirmed by the FTIR analysis (Figure 3). In this regard, previous research has indicated that the formation of silica (Si-O) results in an increase in the density of hydroxyl (-OH), hence increasing the negative charges on the surfaces of the adsorbent, as evidenced by the larger zeta potential, which enhances hydration lubrication and increases the sorption of hydrated cations on negatively charged surfaces [62].