Comparative study of the adsorption mechanisms of caffeine and Remazol brilliant blue R dye on activated carbon from alpaca fiber residue

This work reports the preparation of a new activated carbon (AC), using alpaca fiber residue as carbon precursor and H₃PO₄ as chemical activating agent, for studies of organic compounds in aqueous medium. The obtained AC was properly characterized from analysis techniques and methodologies and then applied in adsorption studies of single aqueous solutions of caffeine (CFN) and Remazol brilliant blue R dye (RBBR). The AC presented high yield (35.06%), BET surface area (S_BET) of 256 m² g⁻¹ and predominance of acid groups on the its surface (pH_ZPC = 3.0). The kinetic models of pseudo-first order, pseudo-second order, Elovich, and diffusions of Knudsen and intraparticle were fitted to the experimental data, and they indicated stronger interaction of RBBR with the AC surface and greater diffusion of CFN through its pores. The equilibrium models showed the Langmuir fitted better for system CFN-AC and Freundlich for the system RBBR-AC, suggesting monolayer adsorption for CFN and multilayer for RBBR, with Q_max values of 38.4 and 20.1 mg g⁻¹, respectively. Additionally, the value of E determined from this model for the RBBR-AC system (E = 24.1 kJ mol⁻¹) was greater than that for CFN-AC (E = 9.13 kJ mol⁻¹). This study shows that AC from alpaca fiber residue is a potential adsorbent for removal of organic pollutants in aqueous media, and that adsorption mechanisms for CFN and RBBR differ mainly in the diffusion of their molecules through the pores of the AC.