(430b) Arsenic Rejection By Nanofiltration Membranes: Experimental Results and Model Prediction

Nanofiltration (NF) membranes, NF270 and TFC-SR-2, were selected for arsenic rejection in dead-end stirred cell filtration equipment. The membranes were characterized using Atomic Force Microscopy, Attenuate total reflection-Fourier transform infrared spectroscopy, potentiometric titration, and contact angle measurements. The results showed that NF270 was rougher than TFC-SR-2, and both membranes contained typical carboxylic and amide groups. NF270 had a contact angle of 30.7^o, and TFC-SR-2 was more hydrophilic with a contact angle at 20^o. Potentiometric titration results showed that both membranes were negatively charged while pH>3.0, and carried more negative charges with increasing pH. The pore sizes of NF270 and TFC-SR2 were 0.49 and 0.46 nm, respectively, which were determined by fitting rejection data of uncharged solutes (glucose and maltose) into Spiegler-Kedem-Steric Hindrance Pore Model.

Arsenic rejection experiments showed that rejection of As(V) by NF270 and TFC-SR-2 membranes increased with increasing initial concentrations due to dilution effect and the rejection rate could be higher than 90%. The rejection of As(V) increased with increasing pH. As(III) rejection was not affected by pH while less than 9.2 because of its existence in neutral form. Arsenic rejection increased with increasing operating pressure and stirring speed. Ionic strength has influences on arsenic rejection due to shielding phenomenon. Co-occurring ions, such as sulfate, phosphate, and calcium, could reduce the arsenic rejection. As(V) rejection rate decreased with increasing concentration of co-occurring ions in the range of 0.0001M to 0.01M. Rejection rate of As(III) also reduced with the presence of co-occurring ions but was not affected by their concentration.

The As(III) rejection by NF270 and TFC-SR-2 membranes was fitted into Spiegler-Kedem-SHP model.  The model calculated rejection was much higher than the experimental data. The poor fitting was possibly due to the affinity interaction between arsenite molecules and membranes. The formed hydrogen bond rendered the easy entry of arsenite into membrane structure, consequently, lowered the rejection of arsenite. The rejections of As(V) as a function of permeate flux by NF270 and TFC-270 were modeled using Donnan Steric Pore Model (DSPM) model combined with concentration polarization theory. The extended Nernst-Planck equation was solved numerically using a fourth order Runge-Kutta method. Membrane charge density and pore size are two important factors which have a major impact on membrane performance. The calculated rejections were in good agreement with the experimental results.