Síntese e caracterização de compósitos ETS-4/PS para tratamento de águas

abstract

Water purification, particularly the removal of heavy metals during tertiary treatments is of major importance and a challenge for the science and engineering. To tackle it novel sorbents need to be synthesized, characterized and tested. Furthermore, new equipments and adequate processes have to be devised, modeled and optimized. A clear example of the relevance of this issue is the European Union directive that imposes the total elimination of cadmium from industrial effluents in twenty years which has encouraged the scientific investigation to reach that goal. This thesis has emerged in this framework: it aims at studying the preparation of ETS-4/polystyrene composites and its potential application as ion exchanger for the decontamination (purification) of cadmium contaminated waters. Some studies have already been reported in the literature regarding the high capacity and selectivity of ETS-4. However they report the use of ETS-4 crystals whose reduced dimensions (micrometers) hinder their industrial application and processing. Therefore, particles with bigger dimensions are required in order to minimize the effects on the mass transport kinetics. Initially, a series of studies was carried out to assess, and select the parameters that affect the preparation of crosslinked polystyrene (PS) beads by suspension polymerization. This included the type of diluent and the concentration of initiator and of suspension agent. The obtained materials were characterized with dynamic light scattering (DLS) and scanning electron microscopy (SEM). Then, the surface modification of ETS-4 particles was studied in order to improve their dispersion in the PS matrix. Particular attention was given to the surface area of the fillers and to the concentration of the coupling agent 3-metacriloxipropiltrimetoxisilane (MPS). The resulting material was characterized by thermo gravimetric analysis (TGA). The information collected was combined and used in the preparation of the composite material. The obtained composites were characterized by SEM and the one with the best characteristics was characterized by SEM, energy-dispersive X-ray spectroscopy (EDS) and TGA (calcination). The presence of metallic elements was confirmed by EDS and calcinations revealed a 2,22% (m/m) content. The average particle diameter is 350 μm. The best composite was then used to carry out ion exchange tests in a batch stirred vessel. For that, fixed volumes of solutions of known concentration of Cd2+ with accurate masses of the composite prepared in this work were putted in contact. The concentration evolution of Cd2+ over the time was measured by mass spectroscopy with inductively coupled plasma. The results showed that: i) the poly(styrene-co-divinylbenzene) matrix doesn’t have any ion exchange capacity; ii) the surface treatment with MPS increases diffusion limitations to mass transfer in the crystals of ETS- 4; iii) the registered removal velocity with the composite material was inferior to that of the treated ETS-4 crystals, due to the biggest dimensions of the former (ca. 150x superior). Finally, a mathematical model for the exchange process was developed based on the Nernst-Planck equations which combines the internal and external resistances to mass transfer. The self diffusion coefficients of the counter-ions, Cd2+ and Na+, were the parameters to optimize. The programs were elaborated on Matlab R2007a. The model provided good results: the removal curves of the surface treated ETS- 4 and composite material were represented with an average absolute relative deviation of 5,40% e 0,52%, respectively. The determined diffusion coefficients confirmed the introduction of diffusional limitations assigned to the MPS. The calculated concentration profiles of cadmium(II) for the particles of surface treated ETS-4 and composite material showed that the internal resistance to mass transfer is fundamental in both cases. The gradients near the surface are significant and they just fade for long times of the process.

authors

Bruno Reis Figueiredo

supervisors

Carlos Manuel Santos da Silva; Ana Margarida Madeira Viegas de Barros-Timmons; Zhi Lin

Groups