A review on nanofiltration membrane fabrication and modification using polyelectrolytes: Effective ways to develop membrane selective barriers and rejection capability.
ABSTRACT The performance of nanofiltration (NF) processes is mainly governed by factors such as the sieving effect (also known as size exclusion) and the Donnan effect (which depends on membrane surface charges). This has encouraged the development of new types of NF membranes using various kinds of polyelectrolytes as they have good pore-sealing effects and are able to improve the membrane surface charge density. Manipulation of the pH, supporting electrolyte concentration, type and concentration of polyelectrolyte solutions can significantly vary the characteristics of polyelectrolyte molecules thus improving their electrostatic interactions with the surrounding compounds. This is highly desired and useful when polyelectrolytes are to be incorporated in membrane surface modification as the charges formed can increase the membrane surface charge density, membrane surface coating stability and membrane selectivity. Most of the research discussed in this paper employed the special features of polyelectrolyte molecules to improve the performance of NF membranes in various applications. Various methods have been used to incorporate polyelectrolytes in order to improve NF membrane performance, such as static deposition, dynamic deposition, single layer coating, layer-by-layer (LbL) coating, and so forth. Some of the suitable devices or instruments used for polyelectrolyte-modified membranes are recommended and evaluated. In conclusion, polyelectrolyte-modified membranes offer significant improvements, can be produced in a short period of time, require less energy during membrane modification or fabrication and incur lower production costs. Thus, a full understanding of the factors affecting polyelectrolyte-modified membranes is very much desired and worth further detailed investigation in the near future.
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ABSTRACT: Alternate adsorption of polyanions and polycations on a polyethersulfone (PES) membrane was studied by the tangential streaming potential method using a parallel-plate channel to investigate the properties of the outer surface of the membrane. These streaming potential data were complemented by diffusion experiments and by membrane potential measurements in order to characterize the inner surface of the membrane. Tangential streaming potential measurements demonstrated that after completing a bilayer of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS), periodic variations in the zeta potential between positive and negative values appeared for multilayer films obtained from membrane dipped in polyelectrolyte (PE) solutions at 10 g/L. On the contrary, the zeta potential was always positive when multilayer films were obtained from 1 and 5 g/L polyelectrolyte concentration solutions. Diffusion experiments carried out with unmodified and modified membranes showed a decrease in the solute flux after functionalization of the membrane by several polyelectrolyte bilayers, indicating that the diffusional resistance of the PE layers contributes significantly to the overall resistance to diffusion of the modified membrane. By means of membrane potential measurements the pore walls of the membrane were functionalized since the charge of its pore walls was modified even if it is difficult to discriminate between the contribution of the membrane and that of the multilayer buildup.Journal of Colloid and Interface Science 04/2010; 344(1):221-7. · 3.17 Impact Factor
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ABSTRACT: Flocculant blocks are commonly used as a component of (passive) water treatment systems to reduce suspended sediment loads in the water column. This study investigated the potential for aquatic biological impacts of a flocculant block formulation that contained an anionic polyacrylamide (PAM) active ingredient and a polyethylene glycol (PEG) based carrier. The toxicity of the whole flocculant block was assessed and the individual components of the block were also tested separately. Five Northern Australian tropical freshwater species (i.e. Chlorella sp. Lemna aequinoctialis, Hydra viridissima, Moinodaphnia macleayi and Mogurnda mogurnda) were exposed to a range of concentrations of the whole flocculant block, and of the individual PAM and PEG components. The concentration of Total Organic Carbon (TOC) in solution was used to provide a measure of the total amount of PAM and PEG present. An extremely wide range of toxic responses were found, with the flocculant blocks being essentially non-toxic to the duckweed, fish and algae (IC(50)>1880mgl(-1)CTOC, IC(10)>460mgl(-1)CTOC), slightly toxic to the hydra (IC(50)=610-2180mgl(-1)CTOC, IC(10)=80-60mgl(-1)CTOC) and significantly more toxic to the cladoceran (IC(50)=10mgl(-1)CTOC, IC(10)=4mgl(-1)CTOC). More detailed investigation of the two components indicated that the PAM was the primary "toxicant" in the flocculant blocks. Derived Protective Concentrations (PCs) for the flocculant blocks, expressed as equivalent TOC concentrations, were found to be lower than typically measured natural environmental concentrations of TOC. It will thus be possible to use TOC as measure of the concentration of PAM only in those situations where lower levels of ecosystem protection (i.e. higher PCs) are applicable.Water Research 12/2011; 45(19):6393-402. · 4.66 Impact Factor
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ABSTRACT: Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K+/Mg2+ selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections. Under MgCl2 concentration gradients across either (PSS/PAH)4- or (PSS/PAH)4PSS-coated alumina, transmembrane potentials indicate Mg2+ transference numbers approaching 0. The low Mg2+ transference numbers with both polycation and polyanion-terminated films likely stem from exclusion of Mg2+ due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nanofiltration, the high asymmetry of membrane permeabilities to Mg2+ and Cl- creates transmembrane diffusion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as -200% for trace monovalent cations such as K+ and Cs+. Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of mixed salts.Langmuir 01/2013; · 4.38 Impact Factor