Simão P. Cardoso

Competitive Removal of Cd2+ and Hg2+ Ions from Water Using Titanosilicate ETS-4: Kinetic Behaviour and Selectivity

Mercury and cadmium are priority hazardous substances. Some titanosilicates have been tested for the removal of Cd2+ and Hg2+ from single solutions by ion exchange. In this work, the competition between both contaminants for the exchanger binding sites of titanosilicate Engelhard titanosilicate material number 4 (ETS-4) was studied by performing batch experiments with aqueous solutions containing the two counter ions. The results evidenced the large capacity of ETS-4 and shown that the cadmium(II) diffusivity through the sorbent is higher than that of mercury(II). Furthermore, the ETS-4 exhibited higher kinetic and equilibrium selectivities for Cd2+, which attained values in the ranges 8.9–12.5 and 7.9–12.8, respectively. With respect to modelling, the pseudo-second-order equation described successfully the competitive removal of Cd2+ and Hg2+.

Ion Exchange Equilibria and Kinetics

The accurate modelling of equilibrium and kinetics of ion exchange is fundamental for economic and safe design of industrial units, particularly to carry out the delicate scale-up studies and simulations.

Metal Recovery, Separation and/or Pre-concentration

Metals are essential for the existence of life. Due to their chemical, physical, electrical and mechanical properties, they found a large number of applications, their use being intrinsically associated with the development of society. Besides their natural occurrence in the ecosystem, the application of metallic compounds in several industrial and agricultural sectors gives an inevitable rise to their release and dispersion into the environment. Accordingly, metallic ions must be separated from water and industrial water or effluents prior to final discharge whenever toxic or radioactive, or recovered in the case of valuable and precious species.

Kinetic Modeling of Pure and Multicomponent Gas Permeation Through Microporous Membranes: Diffusion Mechanisms and Influence of Isotherm Type

The main transport mechanisms involved in pure and multicomponent gas permeation through real microporous membranes are reviewed in this article. They include viscous flow, Knudsen diffusion, bulk diffusion (in mixtures), surface diffusion, and activated gaseous diffusion. The individual contribution of each mechanism may be discriminated from permeation experiments, and can be used to detect the occurrence of defects in the membrane structure. In the case of multicomponent mixtures, the milestone theory of Maxwell–Stefan can be advantageously applied to model the transfer mechanisms embodied. The separation of mixtures can be predicted from data measured for pure gases; here, computer simulations may provide relevant information concerning the loading influence upon diffusivities. With respect to surface diffusion, equilibrium plays a major role in the process, which requires accurate isotherms to compute the corresponding Maxwell–Stefan thermodynamic factors. New single/multicomponent factors are derived here for the first time for Freundlich, Dual-site Langmuir, and Dual-site Langmuir–Freundlich isotherms. The influence of loading upon the surface diffusivities is also addressed, and the most significant theories and approaches adopted to model the phenomenon are discussed.

Inorganic Membranes for Hydrogen Separation

Hydrogen, one of the most promising energy carriers for the future, is currently produced mainly by natural gas reforming or coal gasification, where mixtures containing H2, CO2 and contaminants like CO, H2S and CH4 are obtained. Among other methods, membrane technology has received special attention due to its potential efficiency for hydrogen separation, simplicity of operation, low energy consumption, and because it is environmentally friendly. For this application, the inorganic membranes can be essentially divided into five main families: metallic and proton conducting (dense phases), and silica, zeolite and carbon molecular sieve (porous solids). Over the past 20 years, palladium-based membranes have been the most studied and implemented at industrial level; however, recent advances in other membrane types have received a great deal of attention. This article critically reviews more than 520 publications, highlighting the latest research developments on inorganic membranes for the recovery and purification of hydrogen, with emphasis on their structural characteristics, synthesis, commercial application, drawbacks and challenges. Furthermore, a large compilation of data is provided in Supplementary Material divided according to membrane type.

Inorganic Ion Exchangers for Cesium Removal from Radioactive Wastewater

Ion exchange is a proven process for radioactive wastewater decontamination, where inorganic sorbents are ideal due to their thermal, chemical and radiation stability. This review focuses on the removal of Cs+ by inorganic exchangers, viz. zeolites, titanosilicates, hexacyanoferrates metal oxides and hydrous metal oxides, bentonite/clays and the key family of ammonium phosphomolybdates (AMPs). The design of new selective composites is also addressed focusing on those based on AMPs, hexacyanoferrates and titanosilicates/zeolites. Future inorganic Cs+ exchangers will encompass promising solids, like lanthanide silicates, sodium titanates and metal sulfides. The sensing ability derived from the photoluminescence properties of lanthanide silicates and the efficiency of layered gallium-antimony-sulfide materials in acidic and basic solutions disclose considerable potential for real applications. The ion exchange systems are discussed in terms of sorbent capacity and selectivity (with competitors), pH, temperature and solution salinity. The microscopic features of the exchangers and the associated mechanisms (e.g., pore size, counterions radii, dehydration energy of the ions, coordination environments in the solid exchanger, and site accessibility) are always used for interpreting the ion exchange behavior. On the whole, more than 250 publications were reviewed and a large compilation of data is provided in Supplemental Material.