Sorption Characteristics Of Sulfamethoxazole On Functionalized Carbon Nanotubes

The environmental behavior and risk assessment of antibiotics have attracted special research attention due to their wide occurrence in the environment and toxicity to aquatic organisms and human beings. The presence of engineered nanomaterials lead to more complicated environmental behavior of antibiotics because of the strong interactions between engineered nanomaterials and various antibiotics. As an important member of engineered nanomaterials, carbon nanotubes (CNTs) are seemed to be a good model adsorbent because of their definite structures and uniform surfaces. Thus, this study used functionalized carbon nanotubes, namely hydroxylized (MH), carboxylized (MC), and graphitized (MG) multi-walled CNTs, as adsorbents and sulfamethoxazole (SMX) as an adsorbate to study the adsorption mechanisms of antibiotics on solid particles. Various interactions simultaneously play roles for the adsorption of SMX on carbon nanotubes while these adsorption mechanisms are still unclear. Identifying the contributions of different mechanisms is vital in predicting antibiotic environmental. behavior and consequently understanding their environmental risks. According our study, at pH around 3.7, SMX always showed the highest adsorption on different CNTs and the adsorption followed the order of MH> MG> MC. Combining the results on SMX specie analysis, the pH-dependent adsorption is well explained by hydrophobic and electron-donor-acceptor interactions. The adsorption of neutral SMX is always dominant by contributing generally over 80% to the overall adsorption. A significant contribution of cationic SMX at pH< 3.5 suggested significant contribution of hydrogen bonds to the adsorption of SMX. The strength of bisphenol A (BPA) inhibiting SMX adsorption was dependent on the ratio of BPA concentration to neutral SMX concentration, instead of BPA concentration to overall SMX concentration. This result emphasized the importance to identify the dominant mechanisms or species for the adsorption of antibiotics.Competitive sorption between different chemicals is nearly always observed as indicated by decreased sorption of the primary sorbate. However, the relationship between the sorption inhibition of primary sorbate (Δqpri) and sorption of secondary sorbate (Qsec) could provide a new angle to understand coadsorption of different chemicals. Therefore, bisphenol A (BPA) is used as the primary adsorbate and sulfamethoxazole (SMX) as the competitor to study the complementary and competitive adsorption mechanisms between chemicals with different properties. At low BPA concentrations, the sorption of SMX (Qsec) exceeded BPA sorption inhibition (Δqpri), indicating that these two chemicals complementarily adsorbed on their respectively preferred sorption sites on CNTs. The extent of SMX and BPA shared sorption sites increased with increasing BPA concentration, which resulted in increasedΔqpri. The site energy distribution analysis well illustrated competitive sorption between BPA and SMX in the binary system. At high BPA concentrations, higherΔqpri was observed in comparison to Qsec, which may be resulted from the process that SMX sorption disrupted the "orderliness" of BPA on CNT surface. This result emphasized that both competitive and complementary sorption should be discussed in binary sorption system. Importantly, competitive sorption may be explained by different mechanisms and should be recognized separately. The effects of co-existent ions on organic chemicals adsorption on solid particles are always observed. The presence of anions and/or cations makes the environmental behaviors of antibiotics more complicated. The effects of metal ions on organic chemicals adsorption are still unclear. In addition, most of studies focused on heavy metals with strong abilities to form complexes with organic ligands. The study for alkaline and earth metal ions with weak complexing properties is still limited. In addition, few studies focused on anionic effects on organic chemicals adsorption, especially antibiotics adsorption. Therefore, cations (Ca2+, Cs+) and anions (phosphate) were selected as co-existent ions to investigate their effects on SMX adsorption on CNTs.Cations (Ca2+, Cs+) and anions (phosphate) could "wedge into" SMX adsorption mechanisms and thus alter the adsorption of SMX. This study emphasized that both increased and decreased the adsorption of SMX could be observed with the addition of cations/anions, depending on environmental conditions (such as pH in this current study). The net effect is the balance between the increased and decreased effects. The contribution of different mechanisms to the overall antibiotic adsorption on solid particles should be identified to accurately predict the apparent effect by cations and anions.