The Impacts Of Cations On Dissolved Organic Matter-drugs Interaction

In recent decades, a mass of organic contaminants (especially drugs) were imported into the environment. Conventional methods applied in wastewater treatment plant may not be able to completely remove drug. The long-term contact between drugs and microorganisms provides a training condition for the microorganisms to adapt to the environment with drugs and thus drug-resistant genes were developed. Drug-resistant human bacterial pathogens were significantly increased and the transfer of resistance genes has occurred on a global scale. The development of drug resistance results in the failure of medical functioning of antibiotics, which seriously threatened human health. Therefore, it is urgent to reveal the fundamentals of drug transport in the environment. So the risks and fates of drug can be prognosticated, and the dependable methods of removing drug will be established. The drugs in aqueous environments were mainly binding with colloid (the main component is dissolved organic matter). The interactions between dissolved organic matter (DOM) and drug could alter antibiotic solubility, sorption, mobility, bioavailability and risks. The DOM used in this study was prepared using a humic acid (HA) extracted from a Dianchi sediment. This study using dialysis equilibrium system to compare the binding phenomena of four different kinds of drugs:ofloxacin (OFL), sulfamethoxazole (SMX), cefotaxime sodium (CTX) and carbamazepine (CBZ). To find out which kind of property (such as polarity) of DOM, will affect the character of drugs binding. Combining the views of cation behavior and drug behavior could be a useful method to study the roles of ionic strength on drug binding behavior, and the mechanism of DOM-drug interaction. Cu(II) and Mg(Ⅱ) were applied as model cations because they are very common in the environment. This study applied dialysis equilibrium system to investigate the roles of Cu(II) and Mg(Ⅱ) in DOM-OFL interactions. Both the behaviors of cation and drug are observed.The main research results and conclusions are as follows:(1) Dialysis equilibrium system can separate freely-dissolved drug and cations, and DOM-bound drug and cations, effectively.(2) Mass balance calculation was avoided in this study. OFL concentrations inside and outside of the dialysis membrane were detected. So, the adsorption loss of drug will not affect the equilibrium. The weights of the solutions inside and outside the dialysis membrane were monitored before and after the equilibrium. The obtained KDOC was based on direct determination of Cb, Cf and the real DOM concentrations. Therefore, change of DOM concentration, drug degradation and error of instruments could be excluded in this current experiment.(3) The observed nonlinear binding between OFL and DOM was resulted from their real interactions. The binding of OFL and DOM was described well with Freundlich equation. The very low n values of OFL-DOM interaction indicated highly heterogeneous binding sites for OFL in DOM. DOM-OFL interactions involved site-specific interactions, such as H-bonding, electrostatic interactions and cation exchange.(4) SMX, CTX and CBZ have different reasons on weak binding behavior with DOM. SMX is hard to associate with DOM, as both SMX and DOM are negatively charged at pH7.0. CTX exhibits its strong hydrophilicity because of the high solubility in water. DOM can not enhance its solubility. Photolysis of CBZ may affect the binding capacity with DOM. To compare functional groups of four kinds of drugs, carboxyl groups and phenolic C-OH stretches only exist in OFL. The result shows that only OFL could bind with DOM from the four drugs. H-bonding between DOM and OFL maybe stronger than others because of the carboxyl groups or phenolic C-OH stretches.(5) Clearly, the addition of Cu(Ⅱ) ions could enhance the association of OFL with DOM and this enhancement became more distinct at higher Cu(Ⅱ) concentrations. On the other hand, after adding Mg(Ⅱ) ions, the association of OFL with DOM was restrained, especially at high applied Mg(Ⅱ) concentrations.(6) Significant binding between Mg(Ⅱ) or Cu(Ⅱ) and DOM was observed for both metal ions in the dialysis equilibrium system. The presence of OFL enhanced Cu-DOM binding, but decreased Mg-DOM interaction.(7) Cu(Ⅱ) may act as a bridge between the negatively charged DOM and OFL, forming DOM-Cu-OFL ternary complex. The DOM-bound OFL may be able to bind Cu(Ⅱ), forming DOM-OFL-Cu(Ⅱ) ternary complex. DOM-OFL-Cu complex may have higher contribution over DOM-Cu-OFL complex at high Cu(Ⅱ) concentrations. (8) The addition of Mg(Ⅱ) decreased OFL binding. Mg(Ⅱ) ion and OFL may compete the same binding sites on DOM. The hydration shells of the DOM-bound Mg(Ⅱ) may shield the sites of DOM (such as oxygen-containing functional groups for hydrogen bonding and cation exchange), leading to the inhibition of adsorption of OFL around the Mg-complexed moieties.(9) FTIR spectra were collected for mechanistic discussion. OFL may preferentially interact with DOM carboxyl groups. It was also widely accepted that Cu(Ⅱ) may form stable complex with carboxyl groups and thus Cu(Ⅱ) may form bridging OFL and DOM through their carboxyl groups. Mg(Ⅱ) may interact with both carboxyl and phenolic groups through cation exchange or electrostatic interactions. Mg(Ⅱ) may compete with OFL for carboxyl site and complementarily interact with phenolic group in DOM.