Functionalized SBA-15 mesoporous silica with Schiff base ligands for the adsorption of Cr(VI), Cd(II), and Pb(II) cations from aqueous solution

ABSTRACT The Schiff base ligands (E)-2-(3,3-dimethoxy-2-oxa-7,10-diaza-3-silaundec-10-en-11-yl)phenol (L1), (E)-N-(2-((pyridine-2ylmethylene)amino)ethyl)-3-(trimethoxysilyl)propan-1-amine (L2), and (E)-N-(2-((thiophen-2-ylmethylene)amino)ethyl)-3-(trimethoxysilyl)propan-1-amine (L3) were immobilized onto mesoporous SBA-15 silica. The resultant adsorbents, 1–3, were investigated as potential adsorbents for Cr(VI), Cd(II), and Pb(II) cations from aqueous solution. Various parameters, such as pH, were varied to determine their influence on the metal cation removal capacity of adsorbent 1, where the highest removal efficiencies recorded were at pH 3 for Cr(VI) and Cd(II) cations of 64 and 62%, respectively, and pH 9 for Pb(II) at 99%. Compound 2 recorded efficiencies of 58, 66, and 93% and Compound 3 recorded 20, 88, and 55% for Cr(VI), Cd(II), and Pb(II), respectively. The adsorption nature of the metal cations was studied using 1 by employing several adsorption kinetic models, where the pseudo-second-order model recorded the highest R2 values for all the metal cations (R2 > 0.98) and adsorption capacities of 9.66, 9.25, and 74.7 mg/g for Cr(VI), Cd(II), and Pb(II) cations, respectively. To further study the nature of adsorption, various isotherms were utilized, where the Langmuir isotherm yielded the best fit for Pb(II) and Cr(VI) cations, recording adsorption capacities of 125 and 36.11 mg/g, respectively, while the Redlich–Peterson isotherm yielded the best fit for Cd(II) ion with an adsorption capacity of 16.63 mg/g.