Permeation characteristics of electrolytes and neutral solutes through titania nanofiltration membranes at high temperatures.
Nanoporous titania membranes with controlled pore sizes ranging from 0.7 to 2.5 nm, which had molecular weight cutoffs (MWCO) ranging from 500 to 2000, were successfully prepared by sol-gel processing, and the transport characteristics were evaluated across a temperature range of 30-80 degrees C. With increasing temperature, the permeate flux increased 2- to 3-fold, depending on the pore size. The water permeation mechanism was found to be different from viscous flow and was explained by the state of the water (free water/bound water/nonfreezing water) inside confined pores. The rejection of neutral solutes such as raffinose, the separation mechanism of which is molecular sieving (steric hindrance), decreased with temperature whereas that of electrolytes (MgCl(2) and NaCl), the separation mechanism of which is the charge effect (Donnan exclusion), was approximately constant. The temperature dependence of neutral and electrolyte solutes was analyzed using the Spiegler-Kedem equation by combining the Arrhenius equations for diffusivity and viscosity, which we obtained DeltaE(m), the activation energy of diffusion, after eliminating the effect of viscosity. For large DeltaE(m), which corresponds to the rejection of neutral solutes on the basis of molecular sieving, rejection decreased with temperature but remained unchanged for small DeltaE(m), which corresponds to the rejection of electrolytes based on the charge effect.
Modelling the rejection of N-nitrosamines by a spiral-wound reverse osmosis system: Mathematical model development and validation
Abstract A mathematical model was developed based on the irreversible thermodynamic principle and hydrodynamic calculation to predict the rejection of N-nitrosamines by spiral-wound reverse osmosis (RO) membrane systems. The developed model is able to accurately describe the rejection of N-nitrosamines under a range of permeate flux and system recovery conditions. The modelled N-nitrosamine rejections were in good agreement with values obtained experimentally using a pilot-scale RO filtration system. Simulation from the model revealed that an increase in permeate flux from 10 to 30 L/m 2 h led to an increase in the rejection of low molecular weight N-nitrosamines such as N-nitrosodimethylamine (NDMA) (from 31% to 54%), which was validated by experimental results. The modelling results also revealed that an increase in recovery caused a decrease in the rejection of these N-nitrosamines, which is consistent with the experimental results. Further modelling investigations suggested that NDMA rejection by a spiral-wound system can drop from 49% to 35% when the overall recovery increased from 10% to 50%. The model developed from this study can be a useful tool for water utilities and regulators for system design and evaluating the removal of N-nitrosamine by RO membranes.
Titania membranes for liquid phase separation: effect of surface charge on flux
Titania membranes were prepared by sol-gel processes. Molecular weight cut-offs (MWCO) were successfully controlled in the range from 500 to 1000 with pure water permeabilities of 0.6-1.5x10 -11 m 3 /m 2 .s.Pa. Nanofiltration experiments were carried out for various types of electrolytes (NaCl, Na 2 SO 4 , MgCl 2 , MgSO 4 ) using several membranes having different MWCOs (400, 500, >1000). Rejection showed the minimum values near the isoelectric point (IEP) which were determined by streaming potential measurements. Larger rejections were obtained for the case where electrolytes having divalent co-ions were nanofiltrated, while low rejection were observed for the case of electrolytes having divalent counterions. On the other hand, permeate volume fluxes were maximized near IEP and the fluxes were almost the same irrespective of types of electrolytes. However, permeate volume fluxes decreased at pH higher than IEP. The dependency was pronounced for the case of divalent counterions and smaller pore diameters, probably because of larger hydrodynamic resistance by ionically-adsorbed counter ions.