Sorption of d-metal cations by keratin from aqueous solutions

Abstract

The Cu2+, Cd2+, Ni2+ and Zn2+ metal ions sorption by wool keratin was evaluated from the thermodynamic and kinetic points of view. The experimental data had a very good fit with Langmuir, Freundlich and Dubinin-Radushkevich isotherm models, and the best coefficient of determination was deduced by Langmuir isotherm having value 0.99. The process kinetics was evaluated by intraparticle diffusion model, pseudo-first and pseudo-second order kinetic models, the latter showing the highest correlation with the experimental data. The temperature effect on sorption of Cu2+, Ni2+, Cd2+ and Zn2+ ions from aqueous solutions was determined within a range of adsorption temperatures (283–353 K). The endothermic effect of heavy metal ions sorption by zwitter-ionic form of wool keratin (+H3N-)W(−COO−) indicates that there is a superposition of the sorption process which runs with participation of amino- and carboxyl groups. Acidic keratin groups in the anionic form are involved in the sorption of metal cations, and the amino groups being in the cationic H-form are participated in the sorption of anions to form salt structures [SO42−(+H3N-)2]W[(−COO)2M]. A competitive sorption between doubly- and singly-charged metal cations (M2+/2M+) according to the ion exchange mechanism on the same acid sorption centers of the sorbent was observed. FTIR analysis demonstrated the formation of complexes between Cu2+ and keratin sorption centers that involve terminal free carboxyl groups of amino acids of wool. The chelate structures with metal complexes on acidic sorption sites were computed with the methods of quantum chemistry on modeled compounds, by adhering to chemistry’s best practices.