Different approaches to the development and use of carbon materials for water treatment purposes

Tese de doutoramento, Química (Química Tecnológica), Universidade de Lisboa, Faculdade de Ciências, 2018The work presented in this thesis consisted in the preparation of porous materials and in their application in adsorption processes of organic compounds in aqueous phase, and bacteria adhesion. After the theoretical introduction (Chapters II to VI), the results presented in Chapter VII correspond to the study centred in the preparation of activated carbons from an apple tree branches char by chemical activation with potassium carbonate and hydroxide, under different experimental conditions. The employed methodology allowed to obtain materials with a high global preparation yield and to reduce the volatile content of the precursor. The activated carbons synthesized with potassium carbonate and hydroxide present a highly developed porosity, with high micropore volumes and specific surface areas (values up to 1920 cm3 g-1 and 2445 cm3 g-1, respectively). The impact of activation on the morphology of the carbons was evaluated by scanning electron microscopy and it was observed that the more severe activation with KOH led to a higher consumption of the carbonaceous matrix and an homogenization of the particle size, in comparison to the activation with carbonate. The performance of the materials in the adsorption of the pharmaceutical compounds atenolol and tiamulin hydrogen fumarate was comparable and better than that of the commercial samples tested. The carbons with higher adsorption capacity were the KOH activated samples, which allowed for the removal of 556 mg g-1 of atenolol and of approximately 250 mg g-1 of tiamulin, in equilibrium conditions. The supermicropores volume was important for atenolol adsorption and some impact of the carbons’s surface chemistry on the process was observed. The adsorption of the antibiotic tiamulin also occurred on the supermicropores of the solids and the mesopore volume of carbon NS facilitated its diffusion. In Chapter VIII the performance of activated carbons with different morphologies in the adsorption of two organic pollutants was assessed, and the role of morphology in thermal treatment regeneration processes was evaluated. The activated carbon cloth was characterized in terms of texture, morphology and surface chemistry, together with commercial samples in the granular and powdered forms. The carbon felt presents an open structure composed by fibres of high diameter. The porosity of this solid revealed to be mostly composed by supermicropores easily accessible at the surface of the material fibres. The carbons were tested in liquid phase adsorption experiments for the removal of paracetamol and clofibric acid. The diffusion of these species was facilitated in the cloth because the micropores are at the matrix surface, and in carbon NS due to the high mesopore volume. Both pollutants were efficiently adsorbed by the solids up to maximum adsorbed quantities of 323 mg g-1 and 568 mg g-1 of paracetamol and clofibric acid, respectively. Regeneration by thermal treatment was studied in cyclic saturation-regeneration assays. The more moderate conditions of regeneration (heating at 400 ºC during 1 h) were more efficient in recovering the porosity of the samples. The regeneration treatment was more efficient in the case of the cloth and granular carbon that retained some porosity and approximately 50 % of the initial adsorption capacity up to the third reuse.In regards to the study of bacteria adhesion of Escherichia coli cells in activated carbons, the results in Chapter IX reveal that the powdered carbon had a clear better performance removing up to 4log of cells per cm3 of suspension after 48 h of contact time. The adhesion mechanism in this solid seems to occur in two stages: an initial adhesion favoured by attractive electrostatic interactions, and immobilization of bacteria cells due to the formation of carbon-cell-cell carbon agglomerates. No adhesion of microorganisms occurred on the oxidized sample, suggesting that the establishment of initial repulsive interactions was sufficient to avoid colonization of the sample. The formation of cell + carbon structures was more difficult when the carbon particles were saturated with caffeine. At last, in the study reported in Chapter X, tannin polymers obtained from Mimosa tree bark were characterized, and the potential of the materials as adsorbents of several organic compounds in aqueous phase was evaluated. The use of higher surfactant amounts promoted the formation of wider mesopores and lower phenolic content. One of the samples was washed with a hydrochloric acid solution, and the treatment caused a slight widening of the mesopores with some porous volume loss, and without loss in phenolic content. A detailed study of methylene blue adsorption in aqueous phase was performed, and then the materials were tested for the removal of a set of pharmaceutical compounds and a plasticizer, highly consumed worldwide. The two tested samples showed adsorbent properties removing up to a maximum amount of 152 mg g-1 of pollutant, in equilibrium conditions. The good performance of the polymers was attributed at a great extent to the aromatic character of the materials. The molecular species with smaller molar volume, and with lower width were adsorbed in higher amounts suggesting a packing of the species in the polymers mesopores