Ultrafiltration behavior of major ions (Na, Ca, Mg, F, Cl, and SO4) in natural waters

Aquatic colloids, including macromolecules and microparticles, with sizes r anging between 1 nm to 1 mum, play important roles in the mobility and bioa vailability of heavy metals and other contaminants in natural waters. Cross -flow ultrafiltration has become one of the most commonly used techniques f or isolating aquatic colloids. However, the ultrafiltration behavior of che mical species remains poorly understood. We report here the permeation beha vior of major ions (Na, Ca, Mg, F, Cl, and SO4) in natural waters during ul trafiltration using an Amicon 1 kDa ultrafiltration membrane (S10N1). Water samples across a salinity gradient of 0-20 parts per thousand were collect ed from the Trinity River and Galveston Bay. The permeation behavior of maj or ions was well predicted by a permeation model, resulting in a constant p ermeation coefficient for each ion. The value of the model-derived permeati on coefficient (P-c) was 0.99 for Na, 0.97 for Cl, and 0.95 for F, respecti vely, in Trinity River waters. Values of P-c close to 1 indicate that reten tion of Na, CI, and F by the 1 kDa membrane during ultrafiltration was inde ed minimal (<1-5%). In contrast, significant (14-36%) retention was observe d for SO4, Ca, and Mg in Trinity River waters, with a P-c value of 0.64, 0. 82, and 0.86 for SO4, Ca and Mg, respectively. However, these retained majo r ions can further permeate through the 1 kDa membrane during diafiltration with ultrapure water. The selective retention of major ions during ultrafi ltration may have important implications for the measurement of chemical an d physical speciation of trace elements when using cross-How ultrafiltratio n membranes to separate colloidal species from natural waters. Our results also demonstrate that the percent retention of major ions during ultrafiltr ation decreases with increasing salinity or ionic strength. This retention is largely attributed to electrostatic repulsion by the negatively charged cartridge membrane. :<(c)> 2001 Elsevier Science Ltd. All rights reserved.