Adsorption of Mn(II) from aqueous solution by silica-gel supported polyamidoamine dendrimers: Experimental and DFT study

The adsorption of Mn(II) from aqueous solution by silica-gel supported polyamidoamine (PAMAM) dendrimers (SiO2-G0 similar to SiO2 -G4.0) were investigated using experimental and DFT method. The effects of terminal group, dendrimer generation, pH, time, temperature, initial Mn(II) concentration, and adsorbent dosage on the adsorption were investigated. Results indicate the adsorption capacity of amino-terminated PAMAM dendrimers is higher than the corresponding ester-terminated ones. The adsorption capacity of ester- and amino-terminated PAMAM dendrimers follow the order of SiO2-G3.5 > SiO2-G2.5 > SiO2-G1.5 > SiO2-G0.5, and SiO2-G3.0 > SiO2 -G2.0 > SiO2 -G4.0 > SiO2-G1.0 > SiO2 -G0, respectively. Adsorption kinetic follows pseudo-second-order model and is controlled by film diffusion process. Adsorption isotherm can be well fitted by Langmuir model and proceeds by chemical mechanism. Thermodynamic parameters suggest the adsorption is spontaneous, endothermic, and entropy increased process. FTIR analysis and DFT simulation indicate ester-terminated PAMAM dendrimers mainly coordinate with Mn(II) by carbonyl O and secondary N atoms to form tetra-coordinated chelates, while that of amino-terminated ones tend to bind Mn(ll) with primary and secondary amine N atoms, and carbonyl O atoms in penta-coordinated manners. (C) 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.