Fluoride Adsorption onto Soil Adsorbents: The Role of pH and Other Solution Parameters

Abstract

Soil adsorbents continue to attract increasingly high numbers of researchers in water defluoridation studies. An aspect of solution parameters, that is the aqueous adsorption of fluoride onto soil adsorbents in defluoridation studies, has been reviewed and reported. The pH was found to be the main factor controlling fluoride adsorption on the popular soil adsorbents including: aluminosilicates, iron (hydr)oxides, aluminum (hydr) oxides, apatites, carbonaceous minerals, calcareous soils and zeolites and the other key parameters being temperature, time of contact, and co-existent ions. Fluoride adsorption onto metal-exchanged zeolites and hydroxyapatites (optimum pH = 4-10), iron (hydro) oxide minerals (pH = 2-7), and carbonaceous minerals (pH = 4-12) is relatively pHindependent, and high amounts of fluoride are able to sorb upon the surfaces of these minerals in a wide range of pH values. However, montmorillonites (optimum pH = 5-6), aluminum (hydro)oxide minerals (pH = 5-7), and calcareous minerals (pH = 5-6) only sorb significant amount of fluoride in a narrow range of pH values. The fluoride adsorption onto the latter class of minerals, also generally occurring at slightly above room temperatures, appears to be highly specific and not strongly affected by the presence of coexistent anions including: PO 4 3− , SO 4 2− , Cl − , and NO 3 − .

Preparation and Adsorption Behavior of Ce(III)-MOF for Phosphate and Fluoride Ion Removal from Aqueous Solutions

“…0.2 g of phosphate and 0.35 g of fluoride ions of the adsorbent (optimum values) were loaded into a 50 mL Erlenmeyer flask and 25 mL of each of the initial phosphate and fluoride ion solutions was then added, respectively, keeping all other parameters at the optimum values. The dependency of phosphate and fluoride ion adsorption by the adsorbent in the presence of competing anions such as SO 4 2− , HCO 3 1− , and Cl – (10 and 15 mg L –1 each prepared from Na 2 SO 4 , NaHCO 3 , and NaCl, respectively) was also evaluated. , Furthermore, the pH pzc (point of zero charge) of the as-synthesized adsorbent was determined in a separate experiment following a procedure described elsewhere. , …”

“…The dependency of phosphate and fluoride ion adsorption by the adsorbent in the presence of competing anions such as SO 4 2− , HCO 3 1− , and Cl – (10 and 15 mg L –1 each prepared from Na 2 SO 4 , NaHCO 3 , and NaCl, respectively) was also evaluated. 37 , 38 Furthermore, the pH pzc (point of zero charge) of the as-synthesized adsorbent was determined in a separate experiment following a procedure described elsewhere. 39 , 40 …”

“…To assess the effect of adsorbate initial concentration, the experiment was conducted using different initial phosphate and fluoride ion concentrations (10, 15, 20, 30, 40, , and Cl − (10 and 15 mg L −1 each prepared from Na 2 SO 4 , NaHCO 3 , and NaCl, respectively) was also evaluated. 37,38 Furthermore, the pH pzc (point of zero charge) of the as-synthesized adsorbent was determined in a separate experiment following a procedure described elsewhere. 39,40 2.6.…”

Remediation of soils contaminated with fluoride using seaweed-derived materials: case of slopes of mount Meru

“…The defluoridation behavior manifested by SB was analogous to the behavior of hydrous zirconium oxide which dropped from 12 mg/g at pH 5 to 2.7 mg/g at pH 7 to 0 mg/g at pH 9 (Das et al, 2003). The observed fluoride adsorption by SB is non-specific, such that, its adsorption process is dependent on pH (Wambu & Kurui, 2018). At pH 3 fluoride could not be adsorbed because it exists mainly as 𝐻𝐹.…”

“…On the contrary, HSB exhibits both specific and nonspecific fluoride adsorption. Specific fluoride adsorption is observed through ion-exchange between 𝐹 − and 𝑂𝐻 − in the hydroxyapatite, and the precipitation reaction between 𝐹 − and 𝐶𝑎 2+ released from the dissolution of hydroxyapatite (Wambu & Kurui, 2018). At pH 4 -12, hydroxyapatite exists as a stable calcium phosphate salt and therefore its 𝑂𝐻 − can easily be exchanged with the soil anions.…”