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The PHREEQC geochemical modelling code version 3.7.3–15,968 (2021)." href="/articles/s41598-023-38564-1#ref-CR26" id="ref-link-section-d67686656e1091"26 was used to model and simulate the adsorption of fluoride onto the modified alumina at various conditions. PHREEQC can be used to determine the concentration of adsorbate in an aqueous solution, uptake, and percent removal of an adsorbent. When all the necessary information is included in the input script, the interaction of the adsorbate and the adsorbent can be precisely determined. The input script used in the simulation is given in Table 1. The “Alum_al” denotes the AlOH functional group on the modified alumina. PHREEQC also allows the user to specify other parameters such as the number of moles surface sites (mol), specific surface area (m2/g), and dosage (g) of the adsorbent. These three parameters are necessary for defining the properties of the adsorbent. Other parameters such as temperature, feed water quality, the volume of feed, etc. are used to define the solution used in the simulation. All conditions used in the static adsorption process were used to calibrate the model. The built-in WATEQ4F database was chosen because it has all the relevant analytes and the laboratory settings that serve as a good representation of field parameters./p> 1 it shows that the adsorption process is unfavourable; 0 < 1/n < 1 means favourable adsorption process. 1/n = 0 and 1/n = 1 mean irreversible and linear adsorption processes respectively. The 1/n values are used to envisage the shape of the isotherms30. A better fit of adsorption equilibrium data to this model indicates that the sorption of the adsorbates involving multilayer adsorption on the surface of the sorbent is heterogeneous./p> pHpzc)./p>

Fluoride aqueous speciation was calculated for a solution with a total fluoride of 5 mg/L. The speciation was computed by using PHREEQC interactive geochemical modelling code version 3.7.3–15,968 (2021)." href="/articles/s41598-023-38564-1#ref-CR26" id="ref-link-section-d67686656e1796"26 with WATEQ4F thermodynamic database./p> 5./p> A3 (9.8 mg/g) > A1 (5.75 mg/g). On the contrary, a different pattern was observed in the fluoride percentage removal from an initial concentration of 1 mg/L to 30 mg/L. This is true because, at higher concentrations, the active sites on the adsorbents become saturated owing to the existence of more adsorbates than the adsorption capacity of the adsorbents. The higher ratio of the adsorbates at constant adsorbent dosage over the readily available active sites with increasing initial adsorbate concentrations saturate the surfaces which reduces the sorption capacity hence the reduction in percent removal40. At low adsorbate concentrations, there are more readily available active sites on the adsorbent than the adsorbate and hence most of the adsorbates interact with these active sites during the sorption process. The percent removal increases until equilibrium is reached. Shimelis et al.41, Gomoro et al.42 and Wambu et al.43 reported a similar trend in their adsorption experiment, pointing out that as the initial concentrations of the adsorbate were increased, the percentage removal of fluoride by the adsorbent decreased./p> pHpzc, the surface of the sorbent became negatively charged and was characterized by the presence of OH- ions. Beyond the pHpzc of the sorbents, the adsorptive capacity decreased because of the electrostatic repulsion between the F- ions and the OH- ion. The formation of HF, which reduced the coulombic attraction between fluoride and the adsorbent surface, is thought to be responsible for the low fluoride removal capacity at acidic pH as shown in Fig. 4. Tabi et al.45 studied the removal of fluoride from simulated water using zeolite modified with alum and obtained a maximum percent removal of about 98 at a pH of 6. In a defluoridation process by Zhao et al.46 using Fe3O4@Al (OH)3 magnetic nanoparticles, maximum adsorption of fluoride was achieved in a pH range of 5 to 7./p>

(2021)./p>