The Adsorption Of Tetracycline Interactions With Organic Modified Montmorillonites

Tetracycline antibiotics (TCs) are one of the most commonly veterinary pharmaceuticals. Misuse of TC, TC has been a potential pollutant in soil, sediment and aquatic environments. Therefore, the behavior and fate of tetracycline are always the hot topic in the field of soil science and environmental science. Montmorillonite is one of the most common clay minerals in soils, and has always been used as materials for environmental pollution treatment, due to its high specific surface area (SSA), swelling ability and high cation exchange capacity. In this study, a series of organo-montmorillonites (organo-monts) have been synthesized by using the sodium montmorillonite (Na-mont), which is modified by three different alkyl chain quaternary ammonium surfactants (QACs), respectively. The crystalline structure and interlayer spaces of the tested samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared adsorption spectrum (FTIR), thermal gravimetric (TG) and differential thermal analysis (DTA), pore size and microscope scanning methods (SEM). The adsorption properties and mechanisms of tetracycline organo-monts were investigated by using the adsorption kinetic and isotherm adsorption experiments. The main results were listed as follows:1. The basal spacing and organic carbon contents of the organo-monts modified by QACs (tetramethylammonium bromide, TMAB; dodecyltrimethylammonium bromide, DDTMAB and hexadecyltrimethylammonium bromide, HDTMAB) increased with respect to Na-mont. The SSAs of organo-monts were decreased, and the main pores were mesoporous. The hydrophilic of organo-monts surfaces decreased, and the hydrophobic properties enhanced. The TG-DTA results suggested that the water contents of organo-monts and its interlayer decreased, and the temperatures of dehydroxylation decreased with increasing amount of QACs (0.2CEC and1.0CEC). At low loading (0.2CEC), the TMA cations were single, isolated with each other in the interlayer, and the DDTMA and HDTMA cations were lateral monolayer in the interlayer. While at high loading (1.0CEC), the TMA cations were interaction with each other, and it makes the silicate sheet closely together; DDTMA cations were lateral bilayer in the interlayer and HDTMA cations were lateral monolayer and paraffin-type monolayer between the silicates.2. The initial adsorption rates of all organo-monts were higher than Na-mont. The adsorption kinetics reactions of organo-monts and Na-mont were rapid and80%of adsorption capacities of the tested samples were achieved in the first2h and the adsorption reaction reached equilibrium state after24h. The adsorption equilibrium times of high loading organo-monts were short than that of low loading. The processes of TC adsorption on the organo-monts could be well fitted by the pseudo-second-order equation.3. Both the Langmuir and Freundlich model could well fit the adsorption isotherms. The maximum adsorption capacities of TC on the organo-monts (1000-2000mmol/kg) were considerably higher than that on Na-mont (769mmol/kg). The maximum sorption capacities (Qmax) of TC on organo-monts were increased as the QACs loading increased, and it was influenced by the types of QACs.4. The Freundlich parameters (n and KF) values of organo-monts were higher than that of Na-mont, and the parameter of n values increased with the loading increasing. The promotion of the TC adsorption on the surface of montmorillonite was caused by a more homogeneous sorbent. The values of organic carbon normalized partition coefficient (Koc) for each organo-monts decreased with increasing organic carbon contents. The arrangement of QACs in the interlayers of silicates is one of the possible factors affected the sorption properties.5. The adsorption partition coefficient (KD) was relatively high at low pH (3-4), and all the organo-monts with high QACs loading have higher KD than those with low loading. The adsorption partition coefficient decreased significantly at pH4-5.5and remained nearly constant between pH5.5and7, the pH has almost no influence on organo-monts adsorption of TC. The adsorption partition coefficient (KD) could well reveal the effect of pH on adsorption on organo-monts.