Removal of bromide and iodide anions from drinking water by silver-activated carbon aerogels
The aim of this study is to analyze the use of Ag-doped activated carbon aerogels for bromide and iodide removal from drinking water and to study how the activation of Ag-doped aerogels affects their behavior. It has been observed that the carbonization treatment and activation process of Ag-doped aerogels increased the surface area value (S-N2), whereas the volume of meso- (V-2) and macropores (V-3) decreased slightly. Chemical characterization of the materials revealed that carbonization and especially activation process considerably increased the surface basicity of the sample. Original sample (A) presented acidic surface properties (pH(PZC) = 4.5) with 21% surface oxygen, whereas the sample that underwent activation showed mainly basic surface chemical properties (pH(PZC) = 9.5) with only 6% of surface oxygen. Carbonization and especially, activation process considerable increased the adsorption capacity of bromide and iodide ions. This would mainly be produced by (i) an increase in the microporosity of the sample, which increases Ag-adsorption sites available to halide anions, and (ii) a rise of the basicity of the sample, which produces an increase in attractive electrostatic interactions between the aerogel surface, positively charged at the working pH (pH(solution) < pH(PZC)), and the corresponding halide. To test the applicability of these aerogels in water treatment, adsorption of bromide and iodide was studied under dynamic conditions using waters from Lake Zurich. Results obtained showed that the carbonization and activation processes increased the adsorptive capacity of the aerogel sample. However, results showed that the adsorption capacity of the aerogel samples studied was considerably lower in water from Lake Zurich. Results showed X-0.02 (amount adsorbed to initial breakthrough) values of 0.1 and 4.3 mg/g for chloride anion and dissolved organic carbon (DOC), respectively, during bromide adsorption process in water from Lake Zurich. This indicates that the adsorptive capacity reduction observed may be due to (i) blocking of the porosity, caused by adsorption of dissolved organic matter on the aerogel surface, that would impede the access of bromide and iodide ions to Ag-adsorption sites, and (ii) the competition of chloride anions for the same adsorption sites. Bromide- and iodide-saturated columns were regenerated with NH3 (0.02 M), observing little change in column characteristics. Moreover, the organic polymer precursors were not dissolved and the concentration of surface Ag-adsorption sites is not significantly changed after two adsorption/regeneration cycles. According to these results, Ag-doped activated carbon aerogels could be a very promising agents to remove bromide and iodide from drinking water. (c) 2006 Elsevier Inc. All rights reserved.