Does hindered transport theory apply to desalination membranes?

As reverse osmosis (RO) and nanofiltration polyamide membranes become increasingly used for water purification, prediction of pollutant transport is required for membrane development and process engineering. Many popular models use hindered transport theory (HTT), which considers a spherical solute moving through an array of fluid-filled rigid cylindrical pores. Experiments and molecular dynamic simulations, however, reveal that polyamide membranes have a distinctly different structure of a "molecular sponge", a network of randomly connected voids widely distributed in size. In view of this disagreement, this study critically examined the validity of HTT by directly measuring diffusivities of several alcohols within a polyamide film of commercial RO membrane using attenuated total reflection-FTIR. It is found that measured diffusivities deviate from HTT predictions by as much as 2-3 orders of magnitude. This result indicates that HTT does not adequately describe solute transport in desalination membranes. As a more adequate alternative, the concept of random resistor networks is suggested, with resistances described by models of activated transport in "soft" polymers without a sharp size cutoff and with a proper address of solute partitioning.

Enhanced partitioning and transport of phenolic micropollutants within polyamide composite membranes.

Aromatic phenols represent an important class of endocrine-disrupting and toxic pollutants, many of which (e.g., bisphenol A and substituted phenols) are known to be insufficiently removed by reverse osmosis (RO) and nanofiltration polyamide membranes that are widely used for water purification. In this study, the mechanism of phenol transport across the polyamide layer of RO membranes is studied using model phenolic compounds hydroquinone (HQ) and its oxidized counterpart benzoquinone (BQ). The study employs filtration experiments and two electrochemical techniques, impedance spectroscopy (EIS) and chronoamperometry (CA), to evaluate the permeability of an RO membrane SWC1 to these solutes in the concentration range 0.1-10 mM. In addition, combination of the permeability data with EIS results allows separately estimating the average diffusivity and partitioning of BQ and HQ. All methods produced permeability of the order 10(-7) to 10(-6) m s(-1) that decreased with solute concentration, even though the permeability obtained from filtration was consistently lower. The decrease of permeability with concentration could be related to the nonlinear convex partitioning isotherm, in agreement with earlier measurements by FTIR. The diffusivity of HQ and BQ was estimated to be of the order 10(-15) m(2) s(-1) and partitioning coefficient of the order 10. The high affinity of phenols toward polyamide and their high uptake may change membrane characteristics at high concentration of the solute. EIS results and hydraulic permeability indeed showed that permeability to ions and water significantly decreases with increasing concentration of organic solute.

Effect of water matrices on removal of veterinary pharmaceuticals by nanofiltration and reverse osmosis membranes.

This study explored the removal of five veterinary pharmaceuticals (VPs) (sulfamethoxazole (SMETOX), trimethoprim (TMP), ciprofloxacin (CIPRO), dexamethasone (DEXA) and febantel (FEBA)) from different water matrices (Milli-Q water, model water, tap water and real pharmaceutical wastewater using four types of nanofiltration (NF) membranes (NF90, NF270, NF and HL) and two reverse osmosis (RO) membranes (LFC-1 and XLE). All VPs were added to different water matrices at a concentration of 10 mg/L. Rejections of VPs and water flux were measured. The rejection increased with increase of molecular weight. The highest rejections were obtained with RO membranes (LFC-1, XLE) and tight NF (NF90) membrane. In general, the rejection of VPs was higher in model water and tap water than in Milli-Q water, but the water flux was lower. This was mainly explained by ion adsorption inside the membranes pores. Narrower pore size counteracted the effect of presence of low concentration of natural organic matter (NOM) in tap water. The NOM was assumed to enhance the adsorption of VPs onto membrane surface, increased the size exclusion and electrostatic repulsion also appeared during the transport. Investigated water matrices had influence on water flux decline due to their complexity.

Effect of operating conditions on the performances of multichannel ceramic UF membranes for textile mercerization wastewater treatment.

Textile wastewaters are rated as one of the most polluting in all industrial sectors, and membrane separation is the most promising technology for their treatment and reuse of auxiliary chemicals. This study evaluates the performance of three types of tubular ceramic ultrafiltration membranes differing by mean pore size (1, 2 and 500 kDa) treating textile mercerization wastewater from a textile mill at different operating conditions: cross-flow velocity (CFV) and temperature. Acceptable results were obtained with 1 kDa ceramic membrane, with rejection efficiencies 92% for suspended solids, 98% for turbidity, 98% for color and 53% for total organic carbon at 20°C and 3 m s-1 CFV. Highest fouling effect was observed for 500 kDa membrane and lowest CFV. According to the observed results, 1 kDa membrane could be used for the treatment of wastewater from the textile mercerization process in terms of permeate quality.

Permeability of uncharged organic molecules in reverse osmosis desalination membranes.

Reverse osmosis (RO) membranes are primarily designed for removal of salts i.e. for desalination of brackish and seawater, but they have also found applications in removal of organic molecules. While it is clear that steric exclusion is the dominant removal mechanism, the fundamental explanation for how and why the separation occurs remains elusive. Until recently there was no strong microscopic evidences elucidating the structure of the active polyamide layers of RO membranes, and thus they have been conceived as "black boxes"; or as an array of straight capillaries with a distribution of radii; or as polymers with a small amount of polymer free domains. The knowledge of diffusion and sorption coefficients is a prerequisite for understanding the intrinsic permeability of any organic solute in any polymer. At the same time, it is technically challenging to accurately measure these two fundamental parameters in very thin (20-300 nm) water-swollen active layers. In this work we have measured partition and diffusion coefficients and RO permeabilities of ten organic solutes in water-swollen active layers of two types of RO membranes, low (SWC4+) and high flux (XLE). We deduced from our results and recent microscopic studies that the solute flux of organic molecules in polyamide layer of RO membranes occurs in two domains, dense polymer (the key barrier layer) and the water filled domains.

Optimization of coagulation with ferric chloride as a pretreatment for fouling reduction during nanofiltration of rendering plant secondary effluent.

The treatment and reuse of rendering plant wastewater with membrane processes is a poorly investigated area that could result in substantial water savings. Membrane fouling is still the main obstacle when treating secondary effluents (SEs) with high content of effluent organic matter (EfOM). Thus, the optimization of coagulation with ferric(III) chloride (FeCl3) as a pretreatment for nanofiltration was performed to reduce membrane fouling and achieve higher permeate quality. Coagulation was modeled (total carbon, inorganic carbon, dissolved organic carbon (DOC), turbidity, conductivity, and resulting pH) and optimized with response surface methodology (RSM) to remove DOC and turbidity with a pH close to neutral. The effluent after coagulation at optimal conditions (5.58 pH and 26.38 mg L-1 of Fe3+) and sand filtration (SF) was subjected to nanofiltration (NF270, NF, and NF90 membranes). The fouling was compared to evaluate the efficiency of each pretreatment. Coagulation with FeCl3 reduced the flux decline of nanofiltration membranes 4.2 to 19.3 times while SF barely reduced the fouling. Coagulation increased the flux recovery and chemical cleanliness after the membrane washing. In addition to fouling reduction, higher permeate quality was achieved.

Permeability and selectivity of reverse osmosis membranes: correlation to swelling revisited.

Membrane swelling governs both rejection of solutes and permeability of polymeric membranes, however very few data have been available on swelling in water of salt-rejecting reverse osmosis (RO) membranes. This study assesses swelling, thickness and their relation to water permeability for four commercial polyamide (PA) RO membranes (SWC4+, ESPA1, XLE and BW30) using atomic force microscopy (AFM) and attenuated total reflection Fourier transform IR spectroscopy (ATR-FTIR). ATR-FTIR offered a significantly improved estimate of the actual barrier thickness of PA, given AFM is biased by porosity ("fluffy parts") or wiggling of the active layer or presence of a coating layer. Thus obtained intrinsic permeability (permeability times thickness) and selectivity of aromatic polyamides plotted versus swelling falls well on a general trend, along with previously reported data on several common materials showing RO and NF selectivity. The observed general trend may be rationalized by viewing the polymers as a random composite medium containing molecularly small pores. The results suggest that the combination of a rigid low dielectric matrix, limiting the pore size, with multiple hydrophilic H-bonding sites may be a common feature of RO/NF membranes, allowing both high permeability and selectivity.