New Technologies to Remove Halides from Water: An Overview

Abstract

The increasing demand for hydric resources inevitably requires the greater use of alternative sources for the supply of drinking water. Stricter drinking water quality guidelines can also be expected, such as those proposed by the World Health Organization (WHO). Consequently, there is a need to implement effective technologies for halide removal from water to avoid the formation of disinfection by-products (DBPs). There are innumerable individual DBP species that cannot plausibly be controlled and regulated, and the removal of DBP precursors offers the key advantage of minimizing the formation of all brominated and/or iodized DBPs, including known or unknown and regulated or nonregulated products, in a simple and effective manner. Bromide and iodine removal methods are classified into three categories: membrane, electrochemical, and adsorption techniques. Membrane techniques (reverse osmosis, nanofiltration, electrodialysis) have demonstrated excellent effectiveness to remove halides but are costly and energetically inefficient. Electrochemical techniques (electrolysis, capacitive deionization [CDI], and membrane CDI [MCDI]) have also shown good halide removal capacity but, unlike membrane techniques, they do not effectively remove natural organic matter (NOM), essential for the control of DBP formation. After further technological development, CDI and/or MCDI may prove suitable for application in drinking water treatments. Variable results have been obtained for bromide and/or iodine removal using adsorption techniques (hydrated oxides, activated carbons, carbon aerogels, ion-exchange resins, aluminum coagulation, and flocculation), which are limited by interference with halide adsorption from competitor anions and NOM. Nevertheless, the adsorption approach is a promising research area, given its relatively low cost and easy application. Water treatment companies continuously improve coagulation processes or add nonselective adsorbents to reduce the presence of DBP precursors and achieve disinfection with minimum DBP generation. The search for new approaches has been stimulated by more restrictive legislation on maximum DBP concentrations in water intended for human consumption. The development of nanotechnology has been responsible for novel approaches to water treatment and disinfection and other environmental problems based on the intrinsic characteristics of nanoparticles, including their large surface area, high reactivity, and surface plasmon resonance, among others.