Remoção de aminoácidos aromáticos de soluções aquosas por adsorvente preparado de resíduo agrícola

Abstract

Phenylalanine (Phe) removal is a necessary step for production of protein hydrolysates appropriate for consumption by Phenylketonuria patients. Given the need to reduce production costs, in this study, the feasibility of employing corn cobs, an agricultural residue generated in high amounts worldwide, as precursor material in the production of adsorbents for Phe removal from model solutions was studied. Ideal conditions for preparation of the adsorbent, considering maximum adsorption capacity and yield, were treatment of the residue with H3PO4 and activation at 400oC. The prepared activated carbon presented meso and micropores, and its surface was predominantly acid. A contact time of 6 hours was enough to ensure equilibrium conditions. Maximum uptake capacity of phenylananine was deemed satisfactory, being higher comparable to values encountered in the literature for other adsorbents. Electrostatic interactions were favoured at pH values below 3 and above 8. Interactions between Phe and the adsorbent were evaluated by Infrared analysis. Results demonstrated a tendency of the amino functional groups and the aromatic rings of the Phe molecule to interact with the graphene structures and phosphate groups at the surface of the adsorbent. Adsorption of tyrosine (Tyr) and tryptophan (Trp) was also investigated, given that removal of such aromatic aminoacids in protein hydrolysates by adsorption can occur by the same mechanisms as those of Phe, and probably in larger quantities due to their higher hydrophobicity. An increase in temperature led to a decrease in adsorption capacity of all evaluated aminoacids, evidencing the exothermic characteristic of the process, corroborated by negative enthalpy values. Thermodynamic calculations and isotherm shapes indicated that the process is favorable and spontaneous. Adsorption kinetics were satisfactorily described by a pseudo-second-order model, whereas Langmuir-Freundlich , Langmuir and Temkin models provided the best fits for equilibrium description. Such models were also indicative of chemisorption, corroborated by high values of activation energy. Competitive adsorption of Phe-Tyr by the prepared adsorbent was also investigated, given the usual need to supplement the Phe-depleted protein hydrolysate with Tyr, since it is an essential aminoacid and it is usually simultaneously removed with Phe by adsorption. Results indicated Phe to have higher affinity for the adsorbent surface and thus Phe was predominantly adsorbed when present in solution with Tyr. Such results show that agricultural residues present a potential as materials for production of adsorbents for removal of phenylalanine from aqueous solutions.