Recovery of iodide as triiodide from thin-film transistor liquid crystal display wastewater by forward osmosis

Abstract

Triiodide, a larger charged molecule compared to iodide, is thermodynamically favored with the presence of both iodide and iodine, and is easier to be retained by membrane processes. For the first time, iodide was recovered in the form of triiodide by forward osmosis (FO) for thin-film transistor liquid crystal display industries by preoxidation of iodide to triiodide. Partial oxidation by NaOCl was used to convert the iodide to iodine and then to form triiodide. Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), a commonly used chelating agent in the industry, was used as the draw solute because of its low reverse salt flux. The results revealed that the ideal efficiency of iodide recovery was at pH 3 with a preoxidation (adding 0.0150 M NaClO) for the 0.048 M iodide wastewater with a recovery of 98.5%. Additionally, the Pourbaix diagram and starch indicator were used to verify the formation of triiodide. Membrane distillation was demonstrated to recover the EDTA-2Na draw solute, and more than 99% of recoveries for the draw solutes with initial water flux of 12.0 L/m² h were achieved, indicating that simultaneous recovery of the EDTA-2Na draw solute and water is feasible.

Keywords: Draw solution; Forward osmosis; Iodide recovery; Membrane distillation; Triiodide.