AbstractThe studies that have been carried out in the present PhD thesis Project are based on the development of an improved adsorption process for oxoanions removal, specifically arsenic and antimony due to their toxicity and commercial value. The upgrades are focused on the possibility of adsorbent reuse (As), and on synthetic methods that could endow materials scientists with tools to precisely tailor their structures/pores and have accurate control of adsorption (Sb). For this, adsorption-desorption studies of arsenic and antimony have been performed using a commercial polymeric adsorbent, Metalzorb® sponge, and its modification by SuperParamagnetic Iron Oxide Nanoparticles (SPION).
Adsorption-desorption studies will be performed in batch mode. Analytical techniques (ICP-MS, UV-Vis spectrometry) were used to obtain information regarding the Sb and As content in solution. Microscopy techniques (TEM and SEM) were applied to characterise the nanoparticles. Spectroscopic techniques (FTIR and XAS) were used to characterise the adsorption process mechanisms.
Arsenic desorption process has been performed by applying an electrochemical potential to the solution containing the adsorbent loaded with As(V) to achieve its reduction to As(III) and its desorption from Metalzorb® sponge. This reduction process is not possible without a chemical reagent. The use of inert electrodes does not produce any As(V) reduction. Whereas a combination of Sn and Sn coating on stainless steel mesh as working and counter electrode, respectively, present the best results (60% of As(V) reduction), also forming a white precipitate (As-Sn compound properly characterised), which indicate that Sn plays a key role in As removal.
Concerning the Metalzorb® sponge, a new application of enhanced adsorbent material for Sb removal is envisaged. The pathway used for SPION loaded into sponge influences the physicochemical properties of the adsorbent and the sorption process. Among the different synthesis evaluated, direct synthesis shows the best characteristics, producing SPION nanoparticles diffusion inside of the matrix of the sponge, increasing the adsorbent stability and their sorption properties. Furthermore, it is the fastest adsorbent due to the NP placement on the external sponge surface, as well as the presence of smaller NPs internally (≈ 4.75 nm). There is an increase of the adsorption sites, creating a larger contact area between loaded-SPION/target solution, and enhancing the adsorption kinetics and producing a decrease in the diffusion layer.
Comparison of this material with Metalzorb® sponge suggest that both are appropriate adsorbents for antimony (Sb(III) and Sb(V)). However, the presence of SPION improves the removal process enhancing bare sponge adsorption properties. Sb adsorption on the sponge is influenced by pH, contact time, initial concentration and temperature. The strong pH influence, S-type isotherm profiles, the strong effect of interfering anions, and the easy chemical desorption using ionic and complexation stripping agents indicate that Sb(III) and Sb(V) are adsorbed to the sponge by weak interactions. The presence of SPION reduce the pH influence and the influence of interfering anions and shows an L-type isotherm. These evidence, together with almost inexistent desorption with the stripping agent used indicates that Sb(III) and Sb(V) are adsorbed to the sponge+SPION system by strong interactions. XAS and FTIR measurements confirm these results. With a lower affinity, Sb(III) and Sb(V) are absorbed into the bare sponge through H-bonding and electrostatic interaction, which indicates the formation of outer-sphere complexes. The presence of SPION facilitates the formation of the Fe-O-Sb bonds. For Sb(III), adsorption is independent of the pH, indicating that inner-sphere complexes are formed with a partial Sb(III) oxidation. On the contrary, Sb(V) adsorption depends on the pH, reducing its sorption capacity up to 40 % when the pH increases from 8 to 9. This pH dependency indicates that inner- and outer-sphere complexes are generated during Sb(V) adsorption.
|Date of Award||11 Jul 2019|
|Supervisor||Cristina Palet Ballus (Director) & Manuel Valiente Malmagro (Tutor)|