Co-adsorption characteristics of antibiotics with different functional groups and cadmium combined contamination on activated carbon

Due to the diversity of functional groups of antibiotics, antibiotics are easy to complex with metals, thereby changing their migration and transformation behavior. The adsorption/complexation mechanism of three antibiotics co-adsorbed with Cd(Ⅱ) and systematic comparison were studied using Raman, FTIR, XPS and Materials Studio (norfloxacin (NOR), sulfamethazine (SMT) and tetracycline (TC)). We concluded that two NORs complexed with Cd(Ⅱ) through oxygen atoms from carboxylic acid and carbonyl in ketone group to form a six-membered ring chelate. The Cd(II)(NOR±)2+ showed a stronger adsorption coefficient than other forms of NOR, Cd(II)(NOR±)2+, NOR and Cd(Ⅱ) would compete the same permanent charge sites, thus exhibiting an inhibitory effect on NOR adsorption. Contrarily, one SMT molecule complexed with Cd(Ⅱ) through the N atom on pyrimidine ring and the -NH attached to the pyrimidine ring, forming an AC-Cd(Ⅱ)-SMT ternary complex on activated carbon (AC), and Cd(Ⅱ) acted as bridge. The bridging effect of Cd(Ⅱ) and multi-layer adsorption of SMT provided more adsorption sites for SMT, thus promoting the SMT adsorption. There was no complexation between TC and Cd(Ⅱ), and the addition of Cd(Ⅱ) had no significant effect on TC adsorption. The adsorption capacity of AC-Cd(Ⅱ)-SMT complex (107.4 mg·g−1) was stronger than NOR-Cd(Ⅱ) complex (40.2 mg·g−1), and the adsorption capacity of antibiotics was SMT>TC>NOR in all systems. The correlation analysis and molecular orbital calculation further supported the strong effect of complexation between Cd(Ⅱ) and antibiotics with different function groups on the adsorption, and the environmental behavior of system was jointly determined by complex structure and adsorption mechanism.