Antibiotics And Cu(Ⅱ):Co-sorption And Acetylcholin Esterase Activity Inhibition

The presence of multiple pollutants is common in contaminated environments. Both antibiotics and heavy metals (HMs) were detected in aquatic and soil circumstances. Their interactions can influence the environmental behavior and risks of both antibiotics and HMs. Sorption characteristics and bio-toxicity are the major focuses on contaminant behavior studies, largely because these two processes are ubiquitous in all environmental media.Soil is one of the important mediums of pollutants environmental behavior. Organic matter of soil as an important component is directly exposed to the outer surface of soil particles and related to the sorption of pollutants. In this study, soils before and after organic matter removal were used as sorbates to simulate the co-sorption of antibiotics (ofloxacin, OFL and sulfamethoxazole, SMX) and HMs (Cu2+) on soils. In this study, both increased and decreased antibiotics and Cu2+ sorption in their co-sorption system were observed. The comparison of the sorption coefficients of primary adsorbate (Kdpri) and co-adsorbate (Kdco) suggested that enhanced sorption occurred at high Kdpri region (low primary adsorbate concentration). Competitive sorption was observed when Kdpri was decreased to a certain value depending on solid particle properties. We thus summarized that if the adsorbates were introduced with low concentrations, antibiotics (such as hydrophobic region in solid particles) and Cu2+(such as inner-sphere complexation sites) occupied their unique high-energy sorption sites. Cu2+ complexed with the adsorbed antibiotics, and antibiotics bridged by the adsorbed Cu2+ promoted the sorption for both chemicals. With the increased concentrations, the adsorbates spread to some common sorption sites with low sorption energy, such as cation exchange and electrostatic attraction region. The overlapping of Cu2+ and antibiotics on these sorption sites resulted in competitive sorption at high concentrations.These results indicated that the ionizable oxygen-containing functional groups (IOFGs) in organic matter of soil surface are the possible sorption sites of Cu2+ because sorption of Cu2+ decreased solution pH. This may due to the cation exchange between Cu2+ and protonated hydrions of oxygen-containing functional groups. Besides, Cu2+ promoted more antibiotics sorption on higher organic matter-contented soil. In order to verify the influence of hydrophobicity and IOFGs on co-sorption of antibiotics and Cu2+, graphitized (MG) and hydroxylized (MH) carbon nano-tubes (MWCNTs) were applied as model sorbents. The variation of solution pH was also included. The results indicated that hydrophobic region and hydroxy of MH were the primary sorption sites of antibiotics and Cu2+, respectively. Antibiotics adsorption was increased at high Cu2+ concentrations, while Cu2+ adsorption was increased at low Cu2+ concentrations at pH 6.5. Moreover, Cu2+ promoted more sorption of antibiotics on MH than on MG It illustrated that besides the complexation of antibiotics and Cu2+, IOFGs of organic matter was also one of the important reasons for co-sorption of antibiotics and Cu2+ on soils.On the other hand, both antibiotics and heavy metal ions have biological toxicity. Their interactions may result in the increased joint toxicity to organism. In this study, experiments of enzyme (acetylcholin esterase, AChE) activity inhibition by coexisting heavy metal ions (Cu2+) and antibiotics (OFL) were conducted to investigate their joint toxicity. Besides, pH effect was also considered. Ellman assay was used to measure activity of AChE. Circular dichroism spectrum (CD) and attenuated total reflectance Fourier transform infrared infrared spectrum (ATR-FTIR) were selected to characterize the secondary structure of AChE. The results showed that both Cu2+ and OFL could inhibit activity of AChE. Inhibition abilities of Cu2+ and OFL were decreased when solution pH was increased form 6.0 to 7.0. Moreover, Cu2+ and OFL had the antagonistic and synergistic effect on AChE activity inhibition at pH 6.0 and 7.0, respectively. Charged states of Cu2+, OFL and Cu-OFL complex were the major influence factor. This illustrated that the coexisting Cu2+ and OFL may be of great joint biological toxicity. Moreover, the joint biological toxicity depended on environmental pH.Based on the systematic analysis of "co-sorption mechanism of antibiotics and Cu2+ on soils" and "inhibition mechanism of AChE activity by the coexisting OFL and Cu2+", the results indicated that the co-existence of antibiotics and Cu2+ could lead to the change of environmental behavior and risk of both antibiotics and Cu2+. Studies on environmental behavior and risk of coexisting contaminants should get more and more attention.