Rejection Of Trace Organics And Theirmass Transfer Properties In Osmotic Membranes

Osmotic membrane is a promising technology for trace organic compound removal during wastewater reclamation and drinking water treatment processes. However, few studies have been conducted to compare the rejection performance of different osmotic membranes. In addition, the mass transfer properties of trace organics in osmotic membranes was not very clear. The rejection of nine haloacetic acids(HAAs) and 24 pharmaceuticals(PhACs) were comprehensively investigated by using six osmotic membranes including three nanofiltration membranes(HL, NF270 and NF90), one low-pressure reverse osmosis(RO) membrane(ESPA1) and two forward osmosis(FO) membranes(ES and NW). The influence of membrane fouling on the rejection performance was also investigated. The solution-diffusion model and the DSPM&DE(Donnan Steric Pore Model & Dielectric Effect) model were employed to predict the rejection ratios and mass transfer properties of HAAs and PhACs by selected membranes.The rejection ratios for HAAs and PhACs by different osmotic membranes were measured. Results showed that the rejection ratios for HAAs and PhACs of small molecular weight(including carbamazepine, chloramphenicol and propranolol) were typically less than 80% and 90% for HL and NF270 membranes, respectively. The rejection ratios by the NF90 and ESPA1 membranes were typically very high(>90%) even under low applied pressures.When the concentration of draw solution(NaCl) was higher than 1 mol/L, the rejection ratios for most of compounds can be higher than 90% for ES and NW membranes, although there are some exceptions for ES membrane(nalidixic acid(82.5%), gemfibrozil(83.0%), carbamazepine(84.7%) and sulfamethoxazole(88.2%)). Under the same water flux, the orders of the rejection ratios for HAAs and PhACs by the six membranes were not consistent with that of water flux and the rejection ratios of NaCl.The interactions between fouled layer and trace organics were qualitatively measured by using quartz crystal microbalance with dissipation, which indicated that sulfadiazine and ranitidine could interact with alginate(as fouling layer). Notwithstanding the interactions between trace organics and alginate,concentration polarization played a more important role in altering the rejection ratios.The solution-diffusion model was used to predict the rejection ratios by the ES and NW membranes with the permeability coefficient obtained both by using the RO mode approach and the diffusion cell approach. It indicated that the rejection ratios for most HAAs and PhACs can be very well predicted, apart from some small molecules with negative charge. It is argued that ion exchange between the PhAC and the draw solute can potentially facilitate their transport. The diffusion cell was employed to measure the permeability coefficient for each compound. The data obtained from the diffusion cell can be well used in the solution-diffusion model for the prediction of the rejection ratios by the FO membranes. The solution—diffusion model was not suitable for NF270 membrane due to the neglect of convection in mass transport.The DSPM&DE model was used to predict the rejection performance of HL and NF270 membranes. The DSPM&DE model can be well used to predict the rejection ratios of HAAs, which are similar to NaCl. Most of the PhACS can not be well predicted. With the help of diffusion cell, it was found that the DSPM&DE model cannot really model the mass transport in the membrane. The values of hindrance parameters may not be accurate enough for accurate prediction of the rejection ratios.The results obtained are useful for better understanding of the rejection and mass transfer properties of osmotic membranes and to improve the accuracy of the prediction by using mathematical models.