Novel Multicharge Hydroacid Complexes That Effectively Remove Heavy Metal Ions from Water in Forward Osmosis Processes

Abstract

Suitable draw solutes are crucial to maximize the potential of forward osmosis (FO) in water treatment. In this study, a novel three-dimensional, multicharge draw solute of oxalic acid (OA) chromium complex in a tetramethylammonium salt of [N(CH₃)₄]₃[Cr(C₂O₄)₃] (NMe₄-Cr-OA) was developed for electroplating wastewater treatment using FO processes. The newly designed NMe₄-Cr-OA has a Cr-centered octahedral structure and can ionize a multicharge anion [Cr(C₂O₄)₃]^3- and three cations [N(CH₃)₄]^- in its aqueous solution. Such characteristics are conducive to not only generating a high osmotic pressure but also minimizing reverse solute flux in an FO process, thus resulting in good FO performance. A high water recovery with a negligible reverse solute flux was achieved for NMe₄-Cr-OA in electroplating wastewater treatment via the FO process. Furthermore, a significant increase of 77% in FO efficiency was obtained when NMe₄-Cr-OA replaced the conventional draw solute of NH₄HCO₃ for FO electroplating wastewater treatment with a copper rejection higher than 99.9%, which surpasses the best performance of other technologies reported for copper ion removal.