Study On Endocrine-Disrupting Compounds Removal Using Low Pressure Membranes

With the rapid development of organic industry, more and more hardly-naturally-degraded persistent bioaccumulative toxic pollutants have been released into the natural waters directly without any effective control, which leads to a highly-polluted water environment around the world. The endocrine disrupting chemicals (EDCs), which have been widely detected in the raw and drinking water, do a tremendous threat to the health of human beings by endangering the security of water quality. Being related most to the human's life, Bisphenol A (BPA) has been recognized as one of the typical EDCs.The low-pressure membrane filtration is one of membrane technologies applied to the potable water treatment according with the conventional regulations of drinking water quality. Numerous overseas studies have shown that low-pressure membrane filtration, such as ultrafiltration(UF), has a potential for satisfying the increasingly stringent water quality standards because of its effective removal of various endocrine disrupting compounds. In this dissertation, it has been first studied that the removal of BPA in the drinking water by ultrafiltration and microfiltration(MF). The experiment investigated that the effect of various factors on removal efficiency and membrane flux with respect to BPA initial concentration, membrane properties(e.g., molecular weight cut offs (MWCOs), materials), solution conditions(e.g., pH, ionic strength ) and organic matter in the waters. There are further researches for the characteristics of BPA adsorption to the membrane surface and its impact factors according to the static adsorption experiments. Models were set up to describe the kinetics of BPA adsorption to the membranes tested. Based on the data, the results give a new idea to the studies on removal of EDCs in the drinking water by membrane filtrations.It was indicated that UF could remove BPA in the drinking water effectively. All the tested UF membranes with different MWCOs have high removal efficiency of BPA. With the initial concentration of BPA of about 100μg/L and the operation pressure of 100kPa, the removal percentages of BPA by UF membranes of 10000, 6000, 2000 were 93.0%, 88.9% and 97.7%, respectively. There was a negligible effect of initial concentration of BPA on its removal, adsorption and the UF flux.With the continuous adsorption of BPA to the membranes in the drinking water, the removal of BPA decreased gradually from 95% to 20% at the end of the microfiltration process. It showed that under the condition of 0.1-μm microfiltration membranes and operation pressure of 100kPa, the constant initial concentration of BPA ranging from 0～500μg/L has little effect on the BPA removal and MF flux. During the successive filtration of BPA solutions of changing initial concentration, a negative retention of BPA occurred due to the desorption of BPA from the membrane when the influent concentration reduced and a positive retention could be observed because of the readsorption of BPA to the membrane when the influent concentration increased. It was investigated that the MF flux influenced negligibly the adsorption equilibrium content and the removal of BPA. A higher flux could increase the rate of BPA diffusion in the solutions.As pH of solutions approached the pKa of BPA (9.6～11.3), the removal and adsorption efficiency of BPA dropped significantly. In the acidic and alkaline solutions, the UF flux decreased compared with its pure flux, however, the MF flux has no obvious variations. The effects of ionic strength on both BPA removal and the flux of the two membranes were negligible.The humic acid in solutions has some effect on BPA removal by UF. The interaction between BPA and humic acid would be enhanced when the content of humic acid reached enough, which improved the hydrophobicity of BPA and accordingly benefited the more adsorption of BPA to the UF membranes. Due to the adsorption characteristics of MF membranes, the influence of humic acid on the removal of BPA by MF was minor. The sieving action of the MF membranes covered by the humic acid of high concentration resulted in increasing the BPA removal.It can be concluded that adsorption may play a significant role in low-pressure membranes with large pore size for removing the small molecular BPA. The adsorption content of BPA to the membranes became more with the increasing initial concentration of BPA. It was showed that the simulated isotherms of adsorption of BPA to the membrane surfaces fits Freundlich adsorption model better, which indicated that many kinds of adsorption sites on the membrane surface could adsorb BPA simultaneously according to the proportion of 1:1. The adsorption efficiency of the PVDF membranes tested was less than the one of PES membranes tested.The whole adsorption process of BPA to the membranes was controlled by the adsorption action. The pseudo-first-order kinetic model fits the experimental data of the kinetics of BPA adsorption well. Both the decreasing initial concentration and increasing membrane area could make the rate of BPA absorption to the UF membrane higher. It was different that the absorption rate of MF membrane could be enhanced only by the increasing membrane area.The adsorption of BPA to the MF membrane surface may not be completely reversible. When the BPA concentration in the solution was lower than the equilibrium concentration corresponding to the surface concentration, the accumulated BPA would desorb from the membrane. The BPA desorption process was also satisfactorily described by pseudo-first-order kinetic model.The backwashing could improve the BPA removal by refreshing some adsorption sites on the MF membrane surface, resulting in the partial recovery of the removal efficiency of BPA by MF.Last but not least, the problems requiring further researches on this subject were brought forward.