When the adsorbent surface is negatively charged, Phe is adsorbed in the neutral form, with the phenyl ring oriented parallel to the surface and with both amino and carboxylic groups of the Phe molecule interacting with the surface and with other Phe molecules by hydrogen bonding ( Li, Chen, Roscoe& Lipkowski, 2001). At pH 8 and 10, there is a predominance of negative charges in both adsorbent and Phe selleck products molecules with the effect of electrostatic repulsion on the adsorption performance being observed prior to 4 h. Such effect is not as significant when adsorption equilibrium is reached and is attributed to a change in the dominant adsorption mechanism from one dependent
on the solution pH (e.g., interaction of ionized groups of Phe with groups at the adsorbent PD98059 solubility dmso surface) to one that is independent, such as hydrophobic interactions, after 8 h of adsorption. pH measurements after equilibrium were close to pHPZC for all values of initial solution pH between 4 and 8, with this variation explained by the H+ ions released by the ionized carboxylic groups of Phe molecules neutralizing negative charges at the adsorbent surface, partially restoring the charge balance to a value close to pHPZC. For the case of initial pH 2, the pH value remained unaltered during the entire adsorption period, corroborating the hypothesis of
a mechanism of hydrophobic interactions between Phe and the adsorbent
surface. Rajesh, Majumder, Mizuseki, and Kawazoe (2009) demonstrated that aromatic rings of amino acids tend to orient in parallel with respect to the planes of graphene sheets, favoring π-π type interactions, because this configuration is more energetically stable than others. The influence of adsorbent dosage on Phe adsorption can be viewed in Fig. 2c. Removal efficiency increased with the increase in adsorbent dosage, given the corresponding increase in number of available active adsorption sites resulting from the increase in adsorbent mass. However, the amount of Phe adsorbed per unit mass of adsorbent decreased with increasing adsorbent mass, due to the reduction in adsorbate/adsorbent ratio. Based on these results, the remaining experiments were conducted with an adsorbent 4��8C dosage of 10 g L−1, a choice based on the fact that higher dosages led to a significant decrease in Phe loading, whereas lower dosages did not present satisfactory Phe removal percentage. The data in Fig. 3a show that an increase in Phe initial concentration led to an increase in the total amount adsorbed, given the corresponding increase in driving force (concentration gradient). Results in Fig. 3a also show that a contact time of 3 h almost assured attainment of equilibrium conditions for all evaluated initial Phe concentrations.