Solution pH affects single, sequential and binary systems of sulfamethoxazole and cadmium adsorption by self-assembled cellulose: Promotion or inhibition?

ABSTRACT

A new self-assembled cellulose (SACS) containing multi-functional amine, carboxyl and hydroxyl groups was successfully obtained through etherification, cross-linking and grafting processes. Then, the adsorption of sulfamethoxazole (SMZ) and Cd(II) onto SACS at pH values of 3, 5.7 and 7.5 was systematically investigated by batch experiments of single, sequential and binary systems, characterization and density functional theory (DFT) calculations. The presence of Cd(II) decreased the adsorption of SMZ because of hydrophilic site competition, while SMZ inversely increased the adsorption of Cd(II), which was attributed to bridging and especially to electrostatic shielding effects; moreover, both the inhibitory and synergistic effects were more obvious in the binary system and at a pH of 7.5. There was a dynamic balance between the inhibitory and synergistic effects that depended on the system, pH value and concentration ratio. DFT results further indicated that SMZ- more easily coordinated with Cd(II) at sulfonyl oxygen and nitrogen sites, and the cationic bridge of Cd(II) with SMZ- mainly occurred in the sequential system. Moreover, a complexation-decomplexation-complexation balance of SMZ- and Cd(II) probably occurred in the binary system.