Adsorption of BS-18 Amphoterically Modified Bentonite to Tetracycline and Norfloxacin Combined Pollutants

Abstract

Antibiotic pollution in the environment has become a hot topic. The amphoteric surfactant octadecyl dimethyl betaine (BS-18) was adopted to modify bentonite to investigate the effects and mechanisms of the composite adsorption of different types of antibiotics. Under the different modification ratios, temperatures, pH values, and ionic strength conditions, the adsorption of tetracycline (TC) and norfloxacin (NOR) by bentonite was studied under single and compound conditions, and the adsorption mechanism was analyzed and discussed in combination with the surface properties of amphoterically modified bentonite. The results showed that compared with those of CK, the CEC and specific surface area of the soil samples modified by BS-18 decreased, whereas the total carbon and total nitrogen contents increased. The adsorption order of BS-18 amphoterically modified bentonite to TC was CK > 100BS > 25BS > 50BS, which was in accordance with the Langmuir model; the adsorption order of NOR was 25BS > 50BS > CK > 100BS, which was consistent with the Henry model. The adsorption capacity of TC and NOR in the TC and NOR composite system was higher than that in the single system. With the increase in temperature, the adsorption of amphoterically modified bentonite to TC showed a positive warming effect, whereas the adsorption of NOR declined as the temperature increased. When the ionic strength increased from 0.001 mol·L-1 to 0.500 mol·L-1, the adsorption of TC and NOR on each soil sample was inhibited. The pH of the solution can affect the existing forms of TC and NOR, and the adsorption capacity showed different trends as the pH increased. The adsorption of TC by BS-18-modified bentonite was mainly caused by electric charge attraction, whereas the adsorption of NOR was mainly caused by the combination of electric charge attraction and the hydrophobic effect. The different values of the octanol/water partition coefficient and the difference in structure resulted in different adsorption modes. In the TC and NOR composite system, a TC+NOR mixture was formed to promote the adsorption of soil samples.