Metal-Doped Carbon Aerogels. New Materials for Water Treatments

The aims of the present study were (i) to prepare and characterize carbon aerogels doped with Ag(I), Co(II), Ti(IV), and Mn(II), (ii) to study their behavior during the photooxidation or ozonation processes of organic micropollutants (para-chlorobenzoic acid (pCBA) and 1,3,6-naphthalentrisulphonic acid, NTS) and, (iii) in the case of Ag-doped carbon aerogel, to analyze its efficiency to selectively remove halide anions from drinking water. Results obtained indicate that the addition of a transition metal (Ag, Co, Mn, Ti) does not have a marked influence on aerogel textural and chemical properties of aerogels. The capacity of metal-doped carbon aerogels to accelerate the transformation of ozone into ·OH radicals is related to the metal on their surface. Thus, only the presence of metals susceptible to oxidation by ozone accelerates this transformation (A-Mn). Thus, 10% of the surface Mn was present in Mn(II) form, 4% in Mn(III) form, and 2% in Mn(IV) form after its use in the pCBA oxidation process. In the remaining cases (samples A-Co and A-Ti), the oxidation state of the metal was unchanged at +2 and +4 for Co and Ti, respectively. The results obtained from photodegradation of NTS in the presence of the different aerogels have shown that the presence of A-Na, A-Co, and A-Ti aerogels did not increase the NTS degradation rate. However, the presence of sample A-Mn increased the oxidation kinetics of NTS. Since, there was no adsorption of NTS on these aerogel samples, the increased elimination of NTS, due to the presence of Mn doped aerogel, would indicate that this sample has photocatalytic activity, enhancing the generation of highly oxidant species in the medium. Results obtained have also shown that the photooxidation rate of NTS in the presence of TiO 2 is much higher than that detected in the presence of sample A-Mn. This can largely be attributed to the larger absorption of UV radiation by TiO 2 . Analysis of the adsorption of chloride, bromide, and iodide on Ag-doped carbon aerogels indicate that these aerogels present a high adsorption capacity. This is due to the presence of Ag(I) on its surface, with the formation of the corresponding halides. Consequently, the corresponding halide anions dissolved in the water are retained on the aerogel surface by a chemisorption process. The regeneration of the Ag carbon aerogel beds is based on substitution of the halide ions with ammonia to form a silver ammonia complex. In fact, the characteristics of the column remained practically constant after three adsorption/regeneration cycles, both for bromide and iodide. These results could indicate that these aerogels are highly promising materials for the removal of the above species from drinking waters.