Influence of Co-Existing Ions

The adsorptive removal e fficiency of contaminant is typically influenced by the presence of co-existing ions in solution leading to competitive adsorption on the adsorbent surface [160]. The influence of various negatively charged anions such as chloride (Cl−), sulfate (SO4 <sup>2</sup>−), nitrate (NO3 −) and bicarbonate (HCO3 −) ions on the adsorption of fluoride by the three adsorbents (aluminum bentonite clay-malic acid chitosan (AlBC-A@CS), lanthanum bentonite clay-malic acid chitosan (LaBC-A@CS) and cerium bentonite clay-malic acid chitosan (CeBC-A@CS)) was examined in Figure 16d. The findings sugges<sup>t</sup> that the Cl− and NO3 − ions did not change the fluoride adsorption e fficiency, while the SO4 2− and HCO3 − ions had an adverse e ffect. This can be associated with high coulombic repulsion forces in existence, which reduced the mobility of fluoride ions during interaction with the active sites of the adsorbent. The larger degree of interference of the SO4 2− and HCO3 − ions is due to the arrival of OH− ions which induced the arrangemen<sup>t</sup> of sodium sulfate and sodium bicarbonate increasing the solution pH and thus made possible for these ions to compete with fluoride ions on the surface of the adsorbent [160].

Zhu et al. [161] studied the e ffect of calcined Mg/Al layered double hydroxides (Mg-Al LDH) as efficient adsorbents for polyhydroxy fullerenes (PHF) prepared by co-precipitation method. Naturally, PHF may co-exist with other inorganic anions, which may a ffect its adsorption on layered double oxide (LDO). The e ffect of selected various coexisting anions such as Cl<sup>−</sup>, CO3 2−, SO4 2−, and HPO4 2− at approximately pH of 10 was analyzed. Their results proved that Cl<sup>−</sup>, CO3 2−, and SO4 2− slightly improved the adsorption of the PHF on LDO. The adsorption capacity of PHF on LDO lessened considerably in the entire concentration range of PHF with the presence of HPO4 2− ions. The increase in PHF concentration implies that the negative e ffect of HPO4 2− declined noticeably due to the improved competitive e ffect of PHF. The e ffect of co-existing anions on the adsorption of LDH adsorbent may influence the surface property of the adsorbents resulting into two e ffects known as inhibiting e ffect and synergistic e ffect. These happen by occupying some of the adsorption vacant sites (an inhibiting e ffect) and providing additional adsorption sites (a synergistic e ffect) [161]. This means that the properties of the adsorbates are influenced by promoting the aggregation or dispersion of adsorbates. Ultimately, the co-existing ions may have various e ffects such as promoting, inhibiting, or no e ffect at all on the adsorption of adsorbates of the adsorbents.
