Influence of Adsorbent Dosage

As matter of principle, the degree of adsorption of a solute increase with the increase in the content of an adsorbent. This can be attributed to the increase in adsorbent dosage, which indicates the increased active exchangeable adsorption surface vacancy sites. Nevertheless, the total solute adsorption per unit weight of an adsorbent can decline subsequent to the upsurge in adsorbent dosage because of meddling initiated by the interaction of active sites of an adsorbent [156]. In Figure 16b, the results showed the most favorable dose achieved as 25 mg for the adsorption of fluoride ions via three bentonite clay-based adsorbents. The resultant adsorption capacities of 87%, 90% and 92% were reported for 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) adsorbents, respectively. Beyond 25 mg, the results showed no significant increase in the fluoride removal limit due to the lower availability of active adsorption sites [157].

Li and co-authors [158] studied Mg–Al layered double hydroxides/MnO2(Mg–Al LDHs/MnO2) adsorbents for removal of Pb(II) from aqueous solutions synthesized by one-pot hydrothermal method. It was reported that the adsorbent dosage of Mg–Al LDHs/MnO2 considerably influenced the adsorptive removal of contaminants like lead ions. It was also observed that the percentage adsorptive removal of Pb(II) contaminant increased fivefold from 18.48% to 99.56% with the adsorbent dosage increasing by a factor of 9 from 0.01 to 0.09 g. In addition, the higher adsorbent dose results in a reduced adsorption capacity of Mg–Al LDHs/MnO2 at Pb(II) concentration of 50 mg/L. This observation can probably be associated with the low adsorbent dosage leading to the dispersion of Mg–Al LDHs/MnO2 particles in aqueous solutions. The maximum adsorption efficiency and performance of LDH-based adsorbent and bentonite clay-based adsorbent was achieved at 99.56% and 92%, respectively. The LDH-based adsorbent achieved higher percentage removal efficiency of Pb(II) contaminant than bentonite clay-based adsorbent. This is attributed to the increase in the concentration of adsorption sites in aqueous solution, which enables the contaminants adsorption on a larger number of actives sites.

#### Influence of Initial Ion Concentration

The influence of the initial ion concentration of the contaminant on the adsorption is one of the most important factors to be studied. It can be seen from Figure 16c that the adsorption capacity of fluoride ions increased with increase in initial concentration. The initial concentrations improved from 2.0 mg per liter to 10 mg per liter where the adsorption capacity/proficiency of the 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) adsorbents moved from 70.1% to 98% [151]. Thus, the adsorption limit was observed to be straight forward undertaking related to the adsorption of fluoride ions. Mostafa et al. [159] investigated the effect of different Fe(II) concentrations on adsorption capacity of Co/Mo-LDH with carbonate (CO3)<sup>2</sup>− as an interlayer anion prepared through co-precipitation method. Their results revealed that the Co/Mo-LDH seemingly removed a significant amount of Fe(II) contaminant from the aqueous solutions. The maximum adsorption e fficiency improved to a 99.74%, and the saturation occurred when no more metal ions could be adsorbed on the surface of Co/Mo-LDH. A high e fficiency for ferrous adsorption was obtained through a relatively short period of time up to 60 min at initial concentrations of 1.0, 2.0, 3.0 and 5.0 mg/L. The LDH-based adsorbent has 99.74% higher maximum adsorption capacity than bentonite clay-based adsorbent with 98%.
