Polymer Membranes as Innovative Means of Quality Restoring for Wastewater Bearing Heavy Metals

The problem that has aroused the interest of this review refers to the harmful effect of heavy metals on water sources due to industrial development. In this respect, the review is aimed at achieving a literature survey on the outstanding results and advancements in membranes and membrane technologies for the advanced treatment of heavy metal-loaded wastewaters. Particular attention is given to synthetic polymer membranes, for which the proper choice of precursor material can provide cost benefits while ensuring good decontamination activity. Furthermore, it was also found that better removal efficiencies of heavy metals are achieved by combining the membrane properties with the adsorption properties of inorganic powders. The membrane processes of interest from the perspective of industrial applications are also discussed. A noteworthy conclusion is the fact that the main differences between membranes, which refer mainly to the definition and density of the pore structure, are the prime factors that affect the separation process of heavy metals. Literature studies reveal that applying UF/MF approaches prior to RO leads to a better purification performance.

Robust nanocoatings based on ionic silicones

Two oppositely charged water-soluble oligosiloxanes with the same main chain were synthesized and used for the formation of multilayer nanocoatings. In spite of low molecular weight of the components, due to entropic reasons, linearly growing and robust films with a hydrophilic surface were formed for the first time. The multilayer films were found to be resistant to high temperature water treatment undergoing only reversible swelling and no surface recovery was observed after prolonged exposure to air indicating permanent water wettability of these silicone-based coatings. High flexibility of the silicone chains resulted in low glass transition temperature (ca. 27 °C) of both dry polyplexes and films as determined using calorimetry and spectroscopic ellipsometry, respectively. Moreover, the thin coating was applied on plasma-treated poly(dimethylsiloxane) preventing surface reconstruction in air and leading to long-lasting hydrophilization of the surface (water contact angles around 65°). Such water-borne systems may be used in common applications of silicones providing high flexibility and at the same time water wettability of the coatings.

Electrohydrodynamics of soft polyelectrolyte multilayers: point of zero-streaming current

We report a comprehensive formalism for the electrokinetics (streaming current, I(str)) at soft multilayered polyelectrolyte films. These assemblies generally consist of a succession of permeable diffuse layers that differ in charge density, thickness, and hydrodynamic softness. The model, which extends one that we recently reported for the electrokinetics of monolayered soft thin films (Langmuir 2010, 26, 18169-18181), is valid without any restriction in the number and thickness of layers, or in the degree of dissociation and density of ionizable groups they carry. It further covers the limiting cases of hard and free draining films and correctly compares to semianalytical expressions derived for I(str) under conditions where the Debye-Hückel approximation applies. The flexibility of the theory is illustrated by simulations of I(str) for a two-layer assembly of cationic and anionic polymers over a large range of pH values and electrolyte concentrations. On this basis, it is shown that the point of zero streaming current (PZSC) of soft multilayered interphases, defined by the pH value where I(str) = 0, generally depends on the concentration of the (indifferent) electrolyte. The magnitude and direction of the shift in PZSC with varying salinity are intrinsically governed by the dissymmetry in protolytic characteristics and density of dissociable groups within each layer constituting the film, together with the respective film thickness and hydrodynamic softness. The fundamental effects covered by the theory are illustrated by streaming current measurements performed on two practically relevant systems, a polyelectrolyte bilayer prepared from poly(ethylene imine) (PEI) and poly(acrylic acid) (PAA) and a polymer-cushioned (PEI) bilayer lipid membrane.