Factors influencing reverse osmosis rejection of inorganic solutes from aqueous solution

Reverse osmosis (RO) rejection is strongly influenced by the distribution of solute between the membrane and solvent phases. For this reason, we examined the partition coefficients of inorganic compounds between water and cellulose acetate (CA) membranes. Cation and anion partition coefficients were determined by independent analyses. Effects of fixed (negative) membrane charges on CA are clearly apparent at low solute concentrations. The mean cation/anion partition coefficients decrease with the product of the cation and anion valence, and increase with increasing ionic size. Un-ionized inorganic compounds, HgC1₂ and HAuC1₄, are strongly sorbed by CA membranes. All of these observations are consistent with electrostatic theory.Experimental membrane/water partition coefficients are influenced by temperature, pH, and ion-pairing. CA membranes exhibit swelling and shrinkage when exposed to certain aqueous solutions. Swelling and shrinkage influence solute partition and diffusion coefficients, the water content of the membranes, and their RO rejection.The present results provide a comprehensive experimental basis for understanding the mechanism of RO rejection by CA membranes. Moreover, these results can be used to predict RO behavior under a wide variety of experimental conditions. The potential use of reverse osmosis in a variety of wastewater applications is considered in some detail.     

Effect of solution chemistry on the surface charge of polymeric reverse osmosis and nanofiltration membranes

A streaming potential analyzer has been used to investigate the effect of solution chemistry on the surface charge of four commercial reverse osmosis and nanofiltration membranes. Zeta potentials of these membranes were analyzed for aqueous solutions of various chemical compositions over a pH range of 2 to 9. In the presence of an indifferent electrolyte (NaCl), the isoelectric points of these membranes range from 3.0 to 5.2. The curves of zeta potential versus solution pH for all membranes display a shape characteristic of amphoteric surfaces with acidic and basic functional groups. Results with salts containing divalent ions (CaCl₂, Na₂SO₄, and MgSO₄) indicate that divalent cations more readily adsorb to the membrane surface than divalent anions, especially in the higher pH range. Three sources of humic acid, Suwannee River humic acid, peat humic acid, and Aldrich humic acid, were used to investigate the effect of dissolved natural organic matter on membrane surface charge. Other solution chemistries involved in this investigation include an anionic surfactant (sodium dodecyl sulfate) and a cationic surfactant (dodecyltrimethylammonium bromide). Results show that humic substances and surfactants readily adsorb to the membrane surface and markedly influence the membrane surface charge.     

Molecular dynamics simulations of osmosis and reverse osmosis in solutions

Computer simulation studies using the method of molecular dynamics have been carried out to investigate osmosis and reverse osmosis in solutions separated by semi-permeable membranes. The method has been used to study the dynamic approach to equilibrium in such systems from their initial nonequilibrium state. In addition density profiles of both the solute and solvent molecules have been investigated, especially near the walls for adsorption effects. Finally the diffusion coefficients and osmotic pressure have also been measured.Our results show both osmosis and reverse osmosis, as well as a smooth transition between the two when either the solution concentration is changed, or the density (pressure) difference between the solvent and solution compartments is varied. We believe this new method can be used to improve our understanding of these two important phenomena at the molecular level.     

Produced water treatment by nanofiltration and reverse osmosis membranes

Produced water, water that is co-produced during oil and gas manufacturing, represents the largest source of oily wastewaters. Given high oil and gas prices, oil and gas production from non-conventional sources such as tar sands, oil shale and coal bed methane will continue to expand resulting in large quantities of impaired produced water. Treatment of this produced water could improve the economic viability of these oil and gas fields and lead to a new source of water for beneficial use.Two nanofiltration and one low-pressure reverse osmosis membrane have been tested using three produced waters from Colorado, USA. The membranes were analyzed before and after produced water filtration using field emission scanning electron microscopy (FESEM), attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). In addition, membrane–water contact angles have been measured. XPS data indicate adsorption of organic and inorganic species during filtration. FESEM and ATR-FTIR data support theses findings. Water contact angles indicate the effect of membrane hydrophilicity on fouling. Our results highlight the value of using multiple surface characterization methods with different depths of penetration in order to determine membrane fouling. Depending on the quality of the produced water and the water quality requirements for the beneficial uses being considered, nanofiltration may be a viable process for produced water treatment.     

Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.     

Removal of organics from produced water by reverse osmosis using MFI-type zeolite membranes

Separation of an organics/water mixture was carried out by reverse osmosis using an α-alumina-supported MFI-type zeolite membrane. The organic rejection performance is strongly dependent on the ionic species and dynamic size of dissolved organics. The membrane showed high rejection efficiency for electrolytes such as pentanoic acid. An organic rejection of 96.5% with a water flux of 0.33 kg m⁻² h⁻¹ was obtained for 100 ppm pentanoic acid solution at an operation pressure of 2.76 MPa. For non-electrolyte organics, separation efficiency is governed by the molecular dynamic size; the organics with larger molecular dynamic size show higher separation efficiency. The zeolite membrane gives an organic rejection of 99.5% and 17% for 100 ppm toluene and 100 ppm ethanol, respectively, with a water flux of 0.03 kg m⁻² h⁻¹, 0.31 kg m⁻² h⁻¹ at an operation pressure of 2.76 MPa. It was observed that organic rejection and water flux were affected by the organic concentration. As pentanoic acid concentration increased from 100 ppm to 500 ppm, both organic rejection and water flux decreased slightly.     

Separation of linear hydrocarbons and carboxylic acids from ethanol and hexane solutions by reverse osmosis

In this study, asymmetric cellulose acetate membranes with moderate NaCl rejection (85.5%) were prepared and used to study the influence of the chemical nature of organic solutes in different organic solvents. The solute rejection and the solvent flux of linear hydrocarbons (M_w=226–563 g/mol) and linear carboxylic acids (M_w=228–340 g/mol) in ethanol and hexane were studied as a function of the molecular weight, the feed concentration and the transmembrane pressure.The ethanol flux was three times higher than the hexane flux. The rejection coefficients for both types of solute were quire acceptable (R=60–90%), when ethanol was the solvent. In hexane the linear hydrocarbons showed a rejection of 40–60%, while all carboxylic acids reached a negative rejection of −40 to −20%. This negative “observed” rejection can be attributed to accumulation of carboxylic acid at the membrane; the solute concentration at the membrane becomes much higher than in the bulk solution, due to a higher affinity of the solute with the membrane in hexane than in ethanol. Sorption experiments support this hypothesis.Furthermore, it was found that the rejection increases with increasing molecular weight and the rejection and flux are hardly affected by the feed concentration.     

Hydraulic permeation of NaCl solution through sulfonated polysulfone-polyvinyl alcohol polysulfone composite reverse osmosis membrane

The hydraulic permeation of NaCl solution through sulfonated polysulfone-polyvinyl alcohol/polysulfone composite reverse osmosis membrane was systematically investigated. It was found that the transport of water in a sulfonated polysulfone-polyvinyl alcohol/polysulfone composite reverse osmosis membrane follows the modified solution diffusion equation developed by Yang and Chu. Moreover, the equation between salt rejection and applied pressure was proposed for describing the hydraulic permeation of salt solution through a sulfonated polysulfone-polyvinyl alcohol/polysulfone composite membrane. The salt rejection was found to fit the greater part of separation transport mechanism. The experimental data and the salt rejection equation appear to be in excellent agreement.     

Gravimetric analysis of tantalocolumbites by precipitation from homogeneous solution

An analysis of complex tantalocolumbites has been carried out by precipitation from homogeneous solutions. A homogeneous precipitation of tungsten, titanium, tantalum and niobium by thermal decomposition of the soluble peroxytungstates, described in previous papers, is used. Corrections for incomplete precipitation and coprecipitation phenomena are applied on the basis of the experimentally found values. Silicon and tin are separated by volatilisation as fluoride and iodide, respectively. Iron is extracted by means of isopropyl ether and the rare earth metals are precipitated homogeneously from an oxalate solution. Manganese is precipitated as the ammonium phosphate. The results are in good agreement with an independent method, the standard deviations being within 1 % for the major constituents.     

Gelation and precipitation of copolymers from solution

There are several reports in the literature to the effect that when isotactic polystyrene crystallizes from moderately dilute solution, lamellas (precipitate) form at high crystallization temperatures and gels at low temperatures. We report here the occurrence of a very similar phenomenon during the crystallization of ethylene-butene-1 copolymers. The observations for the latter system can be given a straightforward explanation based on the demonstrated compositional fractionation that occurs on crystallization. Low co-unit content copolymers are associated with lamellas. Gels invariably contain much higher co-unit content. A generalization of these findings is invoked to explain the previous polystyrene results. A new resonance detected by ¹³C NMR experiments in this polymer can be ascribed to head–head sequences.     

Highly ordered metal oxide nanopatterns prepared by template-assisted chemical solution deposition

TiO₂, Al₂O₃, and ZrO₂ patterns composed of ordered nano motifs of various morphologies (i.e. perforations (craters), rings, canyons, wires, dots, or channels) with typical lateral dimensions of less than 40 nm and thickness below 15 nm are presented. Simple chemical solution deposition (CSD) of molecular inorganic precursors and commercial block copolymers was used to create patterns on several substrate surfaces (bare, hydrophobized or gold covered silicon wafers and ITO). Self-assembly during evaporation and subsequent stabilization at 500 °C leads to the various nanostructures. Compared to other techniques for surface nano patterning, the present method has the advantage of being cheap, reproducible and easy to scale up and does not require specialized equipment. The type, dimension, and organization of these motifs were assessed by AFM, FE-SEM, spectroscopic ellipsometry, and GI-SAXS and are shown to depend on the conditions of preparation. Usage as model surfaces for modelling of wetting properties and as nanoelectrode arrays were investigated.     

Hydroxyapatite of platelet morphology synthesized by ultrasonic precipitation from solution

Hydroxyapatite (HA) synthesis by precipitation with urea from aqueous solutions of calcium nitrate and ammonium hydrogenphosphate is studied. Ultrasonication during the synthesis decreases the size of platelet HA crystals from several micrometers to 200–300 nm. At low calcium concentrations in solution, the crystallizing phase is carbonate-hydroxyapatite, whereas at high calcium concentrations, octacalcium phosphate (OCP) precedes hydroxyapatite crystallization.     

Use of a standard system to evaluate the matrix effect on the treatment of a solution from atrazine contaminated soils

The influence of humic substances on the electrochemical treatment of solutions resulting from the remediation of atrazine contaminated soil has been investigated. In particular the effect on the hydrogen peroxide production stage and the effect on the hydroxyl radical oxidation treatment have been separately studied. In order to quickly assess the possible inhibition of the removal, an inorganic system has been adopted as a standard system. The results confirm the validity of electrochemical technologies also for the treatment of a real effluent where a matrix effect is expected since the organic matter content affects only the hydrogen peroxide production.     

Separation and determination of tantalum and niobium by precipitation from homogeneous solution

An attempt to separate niobium and tantalum by precipitation from homogeneous solution by thermal decomposition of their peroxy complexes, in the presence of tannin and oxalate, has been only moderately successful. A more satisfactory separation of tantalum and niobium for ratios from 50:1 to 1:30 is obtained by extracting the bisulphate melt with ammonium oxalate before adding hydrogen peroxide, hydrochloric acid and tannin. For a tantalum/niobium ratio of 1:1 the niobium coprecipitation is reduced to 5 %. Furthermore, two alternative possibilities are presented: (1) a quantitative recovery of a tantalum precipitate at small oxalate and high tannin concentration, leaving 90% of the tantalum-free niobium in solution; (2) an 85 % recovery of niobium-free tantalum at high oxalate and small tannin concentration. A study of the coprecipitation process of niobium shows that the distribution coefficients follow a logarithmic pattern, true homogeneous mixed crystals being formed.     

Pilot scale membrane separation of electroplating waste water by reverse osmosis

Electroplating waste water containing copper was treated by means of reverse osmosis (RO) membrane separation on a pilot scale. The copper concentration in the untreated waste water was 340 ppm. After the treatment, the concentration in the treated water was below 4 ppm which is the Hong Kong government discharge limit. It is shown that, by increasing transmembrane pressure drop, metal concentrations in the treated water can be further reduced. This study suggests that larger scale operations on treating electroplating waste water by RO membrane separation is possible and effective. Effects of operating variables including transmembrane pressure drop and temperature on membrane separation performance were studied and explained based on the solution-diffusion model. The present study is part of the recent investigation of industrial waste water management sponsored by the Hong Kong Government. The purpose of this project is to provide guidelines to the local industries for waste minimization which is closely monitored by the Hong Kong legislature.     

Nonuniqueness of solution of reverse problem of chemical kinetics for first-order reactions

The question of nonuniqueness of the solution of the reverse problem of chemical kinetics has been investigated. The subject of investigation is the situation in which some of the substances participating in the reaction are not accessible for measurement (intermediate substances). The investigation was performed on models of first-order reactions, using Laplace transforms. Direct mathematical formulas have been derived, through which an answer can be obtained to the question of how many independent functional combinations of constants, and which combinations, and will specifically admit an unambiguous evaluation on the basis of kinetic information on the initial reactants and the reaction products. The reaction of ethylene chlorination is examined as an illustrative example.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 701–708, November–December, 1985.     

Evaporation of a reverse osmosis discharge studied by Pitzer model and solubility phase diagrams

Water desalination by reverse osmosis necessarily leads to the production of concentrates which generates a serious challenge to operators. Indeed, disposal of concentrates in natural environment cannot be considered because of the nature of the chemicals contained and their high concentrations. Thus, a specific treatment has to be achieved, which increases the cost of softened water. To prevent this drawback it is proposed the enhancement of the discharge by extracting several valuable salts such as sulfates or chlorides salts of sodium, magnesium and potassium which may counterbalance the treatment additional costs. For this purpose isothermal evaporation at 25 °C is suggested. In the present study, in addition to X-ray diffraction used to identify the various salts recovered, quinary and hexary diagrams were establish to draw the experimental crystallization paths. In addition, because the tested solutions can exhibit very high ionic strength, Pitzer model has been applied to describe the multicomponent systems for calculation of solubility products of the various electrolytes. The results obtained show on one hand good agreement between theoretical and experimental studies and, on the other hand that most salts recovered from the brine are of a high potential use.