Exploring the electrostatic interaction mechanism of polyaspartic acid in improving the rejection of tetramethylammonium hydroxide by reverse osmosis membranes

Abstract

Antiscalants are widely used to control scaling in applications of reverse osmosis (RO) membranes. However, whether charged antiscalants can enhance the rejection of oppositely charged contaminants by RO membranes remains unclear. We unexpectedly found that a negatively charged antiscalant, polyaspartic acid (PASP), can enhance the rejection of positively charged tetramethylammonium hydroxide (TMAH) by RO membranes. PASP significantly enhanced the rejection efficiency of relatively loose RO membranes more than the relatively dense ones. Using a quartz crystal microbalance with dissipation (QCM-D), Raman spectroscopy, and molecular dynamics (MD) simulations, we unraveled that the electrostatic interaction between TMAH and PASP played a critical role in TMAH rejection enhancement. The complex of PASP and TMAH had an impact on the association of TMAH with functional groups of the RO membrane, which imposed additional energy barriers and hindered the ion diffusion. This study explores the unexpected positive role of antiscalants and provides new insights into contaminant removal by RO membranes.