Electrokinetic Data Research Articles

Application of Potentiometric and Electrophoretic Measurements to Evaluate the Reversibility of Adsorption of Divalent Ions from a Solution on Titanium Dioxide

The adsorption of divalent ions on metal oxides is controlled by the pH of a solution. It is commonly assumed that this is a reversible process for pH changes. However, there are reports that the sorption of ions on oxides may not be reversible. To verify this, we used potentiometric titration, ion-selective electrodes (ISEs), and electrokinetic measurements to examine the reversibility of the adsorption of hydrogen ions and three metal ions (Ca2+, Cu2+, and Fe2+) on TiO2. The ferrous ion was used as a reference because its adsorption is entirely irreversible. The surface charge determined by potentiometric titration and the adsorption edges measured using ISE indicate that the adsorption of copper ions is reversible with changes in pH. In the case of calcium ions, the results suggest a certain degree of irreversibility. There are apparent differences in the electrokinetic potential data obtained during titration with base and acid, which suggests that the adsorption is irreversible. We have explained this contradiction by considering the complex and dynamic nature of electrophoretic mobility. In our opinion, potentiometric titration may be the simplest and most reliable method for assessing the reversibility of multivalent ion adsorption.

Modification of stability properties of yttrium(III) oxide particles by simultaneous adsorption of poly(acrylic acid) and poly(ethylene glycol) for possible applications in wastewater treatment

The adsorption, electrokinetic and stability properties of yttrium(III) oxide—Y2O3 in mixed solutions of macromolecular compounds were investigated. The interfacial behavior of poly(acrylic acid)-PAA and poly(ethylene glycol)—PEG in single and binary systems was examined in the pH range 3–10. The polymers used were characterized by different ionic nature—the PAA is an anionic polymer, whereas PEG belongs to the group of non-ionic polymers. Based on the results obtained, the most probable mechanisms for the binding of PAA and PEG macromolecules on the yttrium(III) oxide surface were proposed. In addition, the analysis of adsorption and electrokinetic data enabled explanation of the obtained changes in the stability of Y2O3 suspensions without and in the presence of PAA or/and PEG, as well as determination of the specific stabilization-destabilization mechanisms of the studied systems. It was shown that yttrium(III) oxide modification by mixed adsorption layers of both polymers with different ionic character changes considerably the surface and stability properties of the examined solid suspensions.Graphical

Evaluation of APC impact on controlling precursors properties in the sol for synthesizing meso porous ZrC nanopowder through sol-gel process

In this research, the dispersion characteristics of the suspension containing precursors were investigated in terms of charge, particle size, and rheological properties. Zirconium carbide nanopowder was prepared in the four-component system of alkoxide-resin-dispersant-catalyst based on the sol-gel chemical process under acidic conditions. Ammonium polycarboxylate (APC) was manipulated as a sol-dispersing agent to reach a particle size of less than 5 nm. Results depicted that the amount of surface charge of precursors was changed in the presence of ammonium polycarboxylate agent, and the PZC point was shifted towards acidic pHs less than 6.1. According to electrokinetic data, the interaction of surface sites and APC cause alteration of free specific energy for surface adsorption with a negative value. With regard to DLS analysis, the presence of APC stabilizes particles in the sol with precise control over their sizes in less than 10 nm. At intermediate pHs, the viscosity of the sol containing precursor increases which leads to a decrease in the particles stability. FTIR analysis of gel powder confirmed the proper formation of Zr–O–C bonds through hydrolysis and condensation reactions. The surface of the synthesized nanopowders is porous and in the meso range, so that their specific surface area is 165 m2/g. DTA analysis indicated that initial nuclei of ZrC were formed at around 1320 °C, which XRD patterns confirm this. SEM microstructure images showed that ZrC particles were created in the nanometer dimensions, and particle size distribution were reported in a narrow range with uniform morphology. TEM images and their diffraction patterns represented the synthesis of crystalline ZrC particles less than 50 nm.

Interaction mechanism of heavy metal ions with the nanostructured zeolites surface – Adsorption, electrokinetic and XPS studies

The aim of the research was to compare the mechanisms of heavy metal ions (Pb(II) and Zn(II)) adsorption on the surfaces of nanostructured synthetic zeolites: Na-X, Na-P1, their carbon composites: Na-X (C), Na-P1 (C) and the precursor of these structures - high-carbon fly ash (HC FA). The research was carried out in single and mixed adsorbate systems. Heavy metals were selected on the basis of their toxicity and their occurrence in the natural environment. The adsorption–desorption and electrokinetic data were obtained by means of inductively coupled plasma-optical emission spectrometry, potentiometric titrations and zeta potential determination. Additionally, the X-ray photoelectron spectroscopy (XPS) was applied to specify the elemental composition of the solids surfaces and the chemical forms of adsorbed metals.

High current density microkinetic and electronic structure analysis of CO2 reduction using Co and Fe complexes on gas diffusion electrode

Reaction mechanisms of electrocatalytic CO 2 reduction into CO over Co or Fe complexes were examined using gas diffusion electrodes to meet the requirement of high current densities for industrial deployment. Our experimental and theoretical calculation results consistently revealed that the Fe-based molecular catalysts exhibited more positive redox potentials relevant to CO 2 electrocatalysis but disfavored the desorption of generated CO, especially at high overpotentials, failing to achieve appreciable reaction rates. Distinctively, the heterogenized Co-based molecular complexes were found to be tolerant to the high coverage of CO at steady state on the active site and achieved rates exceeding 100 mA cm −2 toward exclusive CO evolution. Density-functional theory calculations not only disclosed the redox non-innocent tetraphenylporphyrins and phthalocyanines during electrocatalytic CO 2 reduction but also corroborated the energetics, especially for CO 2 and CO adsorption, accounting for distinctive reaction pathways between Co and Fe complexes. • Microkinetic analysis was performed at high current densities • The redox non-innocent TPP and Pc were disclosed by electronic structure analysis • The distinctive performance between Co and Fe complexes was interpreted Molecular catalysts can electrocatalytically convert CO 2 reduction into CO at nearly 100% selectivity. However, the associated electrokinetic data and reaction mechanisms remain debatable, especially under higher current densities, hindering the establishment of design guidelines for effective catalysis. Herein, the reaction mechanism using Co and Fe complexes was examined using a gas diffusion electrode to meet the requirement of high current densities. The distinctive performance between Co and Fe complexes was detailed using microkinetic analysis and theoretical calculations. This work not only provides conclusive representation related to electronic structure and reaction mechanism of molecular complexes but also sheds light on the further development of immobilized molecular electrocatalysts at an industrial scale. The reactivity and energetics at each elementary step of immobilized Co and Fe complexes on gas diffusion electrode for the electrocatalytic CO 2 reduction reaction were examined through a combination of microkinetic assessment and theoretical DFT calculations. The experimental and theoretical calculation results disclosed distinctive performance between Co and Fe complexes resulting from their electronic structure and CO or CO 2 adsorption free energies on the metal center.

Structural and electrochemical corrosion studies of spin coated ZrO2 thin films over stainless steel alloy for bone defect applications.

Sol-Gel-based reaction mixture sols have been long used to fabricate dense and uniform bioactive coatings with superior mechanical stability over metallic implants. On account of precise control over synthesis, fabrication, formed and low temperature of processing, this technology is one of the most feasible routes to produce bio-ceramic coatings. The study aims to develop a physical barrier over metal implants in form of bioinert Zirconia coatings, phase-stabilized using Dysprosium. The metallic substrates were cut into 10 mm × 10 mm samples and diamond polished after being polished with a 1000 grade emery sheet. Novel spin-coated zirconia films were fabricated over 316L Stainless steel substrates and were sintered at 600°C to obtain firm and uniform crack-free coatings. The thickness of the coatings was determined by ELCA-D meter thermal analysis was performed using TGA-DTA. Phase determination was performed using X-Ray diffraction followed by morphological investigations using Scanning electron microscopy. The corrosion resistance was evaluated with Polarization studies and electrokinetic data was derived using Tafel extrapolation. Biocompatibility evaluation was performed against MG-63 cell lines and RBCs along with bone-forming ability in vitro in SBF. Stable crack-free 3 Layer coatings fabricated at 2000 rpm for 3 s with a thickness of around 1 μm were found to be optimal for corrosion resistance behavior of steel implants at a low ICorr value of 0.501 µA/cm2 and adhesion strength of 40.93 MPa when untreated falling down to 39.92 MPa when immersed in SBF. The study concludes that medium rpm coatings sustain enough sol to produce crack-free coatings that form a strong physical barrier between body fluid and implant surface thereby reducing the attack of corrosive ions and protecting the implant surface without participating in any form of bioactivity but supporting native bone regeneration capabilities.

C−C Coupling Is Unlikely to Be the Rate‐Determining Step in the Formation of C2+ Products in the Copper‐Catalyzed Electrochemical Reduction of CO

AbstractThe identity of the rate‐determining step (RDS) in the electrochemical CO reduction reaction (CORR) on Cu catalysts remains unresolved because: 1) the presence of mass transport limitation of CO; and 2) the absence of quantitative correlation between CO partial pressure (pCO) and surface CO coverage. In this work, we determined CO adsorption isotherms on Cu in a broad pH range of 7.2–12.9. Together with electrokinetic data, we demonstrate that the reaction orders of adsorbed CO at pCO <0.4 and >0.6 atm are 1st and 0th, respectively, for multi‐carbon (C2+) products on three Cu catalysts. These results indicate that the C−C coupling is unlikely to be the RDS in the formation of C2+ products in the CORR. We propose that the hydrogenation of CO with adsorbed water is the RDS, and the site competition between CO and water leads to the observed transition of the CO reaction order.

C-C Coupling Is Unlikely to Be the Rate-Determining Step in the Formation of C2+ Products in the Copper-Catalyzed Electrochemical Reduction of CO.

The identity of the rate-determining step (RDS) in the electrochemical CO reduction reaction (CORR) on Cu catalysts remains unresolved because: 1) the presence of mass transport limitation of CO; and 2) the absence of quantitative correlation between CO partial pressure (pCO ) and surface CO coverage. In this work, we determined CO adsorption isotherms on Cu in a broad pH range of 7.2-12.9. Together with electrokinetic data, we demonstrate that the reaction orders of adsorbed CO at pCO <0.4 and >0.6 atm are 1st and 0th , respectively, for multi-carbon (C2+ ) products on three Cu catalysts. These results indicate that the C-C coupling is unlikely to be the RDS in the formation of C2+ products in the CORR. We propose that the hydrogenation of CO with adsorbed water is the RDS, and the site competition between CO and water leads to the observed transition of the CO reaction order.

(Invited) Identifying the Rate-Determining Step in the Electrochemical CO Reduction Reaction

Electrocatalysis powered by renewable energy is expected to play a pivotal role in the global decarbonization process. In particular, electrocatalytic CO2 and CO reduction reactions (CO(2)RR) are widely considered as indispensable components in the valorization of CO2. Much recent research effort has been devoted to the development of efficient and selective catalysts for valuable C2+ products on Cu surfaces, however, many open questions regarding how species in the electrochemical interfaces impact the activity and selectivity remain. In this lecture, we discuss recent efforts in understanding the impact of the surface speciation, adsorbed reaction intermediates and near surface species on the CO(2)RR at the molecular level. The identification of the rate-determining step in the CO(2)RR has been complicated by the lack of rigorous electrokinetic data, from which key mechanistic variables such as the Tafel slopes and reaction orders are determined. Rigorous electrokinetic studies without mass transport limitation show that all C2+ products in the CORR have the common Tafel slope of ~120 mV/dec, and the formation rates are largely invariant with respect to the electrolyte pH on the absolute potential scale. In addition, CO adsorption isotherms determined by in-situ surface enhanced infrared absorption spectroscopy shows that the reaction orders for adsorbed CO are 1st and 0th at low and high partial pressures of CO. Based on these results, we assign the first hydrogenation step of adsorbed CO with adsorbed water, rather than the C-C coupling, as the rate-determining step in the CORR, and propose a reaction mechanism capable of reconciling all experimental observations.

Thermodynamic Analysis of the Temperature Effect on Calcite Surface Reactions in Aqueous Environments

Reactive transport models for subsurface applications, particularly those for modified salinity waterflooding (MSW) in carbonates, have to be equipped with thermodynamic models that can reliably predict/describe adsorption phenomena at variable temperature conditions. To advance the understanding of the calcite reactivity at high temperatures, combining experimental and modeling studies is required. Therefore, we used a surface complexation model (SCM) to describe the reactivity at the calcite–water interface and obtained the enthalpies of the surface reactions by fitting the SCM either to high-temperature electrokinetic data obtained through streaming potential measurements on limestone samples or to the effluent concentration from single-phase flooding experiments. We assumed that the equilibrium constants of the surface reactions follow a temperature dependence according to van’t Hoff equation. Our calculations suggest that calcite protonation is more exothermic compared to other minerals (e.g., silica, hematite) and that the adsorption of Ca2+, Mg2+, and SO42– are all endothermic, with SO42– having the highest reaction enthalpy. We further show that the inferred enthalpies were in agreement with enthalpy changes from published microcalorimetry experiments. Despite the consistency with microcalorimetry data, the obtained enthalpies are not undisputable, and additional data are necessary to unequivocally constrain the temperature effect on the calcite surface reactivity.

Aspects of albendazole metabolism in western lowland gorillas (Gorilla gorilla gorilla) compared to humans and other species assessed by HPLC, LC‐MS, and chiral electrokinetic chromatography

AbstractThe benzimidazole anthelmintic drug albendazole becomes metabolized to chiral albendazole sulfoxide, albendazole sulfone, albendazole 2‐aminosulfone, and other metabolites. High‐performance liquid chromatography with UV absorbance and fluorescence detection, liquid chromatography with mass spectrometry, and chiral electrokinetic chromatography were used to analyze albendazole metabolites in extracts of western lowland gorilla (Gorilla gorilla gorilla) plasma of patients with alveolar echinococcosis and the data are compared to those obtained with human patient samples and to those reported for other species in the literature. The data revealed that the albendazole sulfoxide to albendazole sulfone concentration ratio in gorilla plasma is significantly smaller compared to that in samples of men, rats, dogs, and cattle. It is, however, similar to what was observed in sheep and goats. The (+)‐albendazole sulfoxide to (−)‐albendazole sulfoxide enantiomeric ratio in gorilla blood determined by chiral electrokinetic chromatography was found to be &gt;1 which is comparable to what was observed in humans, dogs, sheep, goats, and cattle but different to mice and rats. Furthermore, the electrokinetic chromatography data suggest that small amounts of albendazole 2‐aminosulfone may be present in plasma of gorillas and humans. The data gathered with the various separation‐based assays illustrate the value of technology featuring different analyte separation and detection principles. This is the first account describing aspects of the albendazole metabolism in gorillas.

Inhomogeneity of polylysine adsorption layers on lipid membranes revealed by theoretical analysis of electrokinetic data and molecular dynamics simulations

The adsorption of large polycations on a charged lipid membrane is qualitatively different from the small inorganic cations, which almost uniformly populate the membrane surface. We assume that the polycationic adsorption layer might be laterally inhomogeneous starting from a certain polymer length, and this effect can be more visible for membranes with low anionic lipid content. To study systems with inhomogeneous adsorption layers, we carried out electrokinetic measurements of mobility of liposomes containing anionic and neutral phospholipids in the presence of polylysine molecules. Some of these systems were simulated by all-atom molecular dynamics. Here we proposed a theoretical approach accounting for the formation of separated regions at the membrane surface, which differ in charge density and surface potential. Our model allowed us to determine the adsorption layer’s geometric parameters such as surface coverage and surface-bound monomer fraction of polymer, which correlate with the molecular dynamics (MD) simulations. We demonstrated that the configuration polylysine adopts on the membrane surface (tall or planar) depends on the polymer/membrane charge ratio. Both theory and MD indicate a decrease in the anionic lipid content, alongside with a decrease in the bound monomer fraction and corresponding increase in the extension length of the adsorbed polymers.

Investigations of mechanism of Ca2+ adsorption on silica and alumina based on Ca-ISE monitoring, potentiometric titration, electrokinetic measurements and surface complexation modeling

Research on Ca2+ adsorption onto the mineral surface is of significant importance with regard to geochemical processes. Sverjensky (Geochim Cosmochim Acta 70(10), 2427–2453, 2006) assumed that alkaline earths form two types of surface species on oxides: tetranuclear (> SOH)2(> SO−)2_M(OH)+ and mononuclear > SO−_M(OH)+. To look into the above assumption we investigated calcium adsorption on SiO2 and Al2O3 because they are the most widespread minerals in the environment. We have determined the proton surface charge, electrokinetic potential and metal adsorption as a function of pH. The Ca2+ uptake and concentration in the system were monitored by the calcium ion-selective electrode (Ca-ISE). The Ca-ISE measurements indicated a similar affinity of Ca2+ for both materials despite their differently charged surface, negative for silica and mainly positive for alumina. This may suggest that simple electrostatic interactions are not the primary driving force for calcium adsorption, and that solvation of calcium ions at the surface may be crucial. We have analyzed our experimental data using the 2-pK triple-layer model (2-pK TLM). Three calcium complexes on the mineral surface were reported. Two of them were the same for both oxides, i.e. the tetranuclear (> SOH)2(> SO−)2_Ca2+ and mononuclear complexes > SO−_CaOH+. Additionally, minor contribution from >SOH…Ca2+ for silica was assumed. In the case of Al2O3 the hydrolyzed tetranuclear complexes (> SOH)2(> SO−)2_CaOH+ at pH > 7.5 occurred based on the modeling results. Two types of surface complexes suggested by Sverjensky allowed for the correct description of proton and calcium uptake for alumina. However, the electrokinetic data excluded hydrolyzed tetranuclear surface species for this oxide.

Electrolytic conductivity of ionic polymers in a nonpolar solvent.

The electrolytic conductivity of two electrolytes as solutions in the nonpolar solvent, n -dodecane, as a function of concentration has been studied. One was a small molecule electrolyte (tetraalkyl cation and a highly fluorinated tetraphenylborate anion), and the other was a macromolecular electrolyte (cation-containing poly(alkyl methacrylate) chain with the same anion). Two series of the macromolecular cation were prepared: one with entirely cation-containing molecules and the other with a small proportion (10%) cation-containing and the rest nonionic. The conductivity data were qualitatively similar for all systems, which formed both single ions and triple ions. The data from the two series of macromolecular electrolytes were particularly informative to understand some recent and counterintuitive electrokinetic data for particles that were stabilized by these polymers. Reducing the proportion of cationic chains in the stabilizer of the particles was found to increase their electrophoretic mobility. In the conductivity data in this study, reducing the proportion of cationic chains in solution was found to increase the magnitude of the single-ion equilibrium constant and suppress the formation of triple ions. These data should support the development of models to understand these electrokinetic results.

Mechanism of Oxygen Evolution Catalyzed by Cobalt Oxyhydroxide: Cobalt Superoxide Species as a Key Intermediate and Dioxygen Release as a Rate-Determining Step.

The oxygen evolution reaction (OER) is the performance-limiting half reaction of water splitting, which can be used to produce hydrogen fuel using renewable energies. Whereas a number of transition metal oxides and oxyhydroxides have been developed as promising OER catalysts in alkaline medium, the mechanisms of OER on these catalysts are not well understood. Here we combine electrochemical and in situ spectroscopic methods, particularly operando X-ray absorption and Raman spectroscopy, to study the mechanism of OER on cobalt oxyhydroxide (CoOOH), an archetypical unary OER catalyst. We find the dominating resting state of the catalyst as a Co(IV) species CoO2. Through oxygen isotope exchange experiments, we discover a cobalt superoxide species as an active intermediate in the OER. This intermediate is formed concurrently to the oxidation of CoOOH to CoO2. Combing spectroscopic and electrokinetic data, we identify the rate-determining step of the OER as the release of dioxygen from the superoxide intermediate. The work provides important experimental fingerprints and new mechanistic perspectives for OER catalysts.

Investigation of adsorption mechanism of phosphate(V) ions on the nanostructured Na-A zeolite surface modified with ionic polyacrylamide with regard to their removal from aqueous solution

The sorption properties of Na-A-type synthetic zeolite, obtained from fly ash with regard to the phosphate(V) ions, were determined. The Na-A zeolite was prepared using hydrothermal conversion of fly ash with aqueous sodium hydroxide. The effects of solution pH and zeolite surface modification by ionic polyacrylamide (PAM) were examined. Both anionic and cationic forms of PAM were applied. The adsorption and electrokinetic data were obtained by means of spectrophotometric measurements, potentiometric titrations and zeta potential determination. It was shown that the presence of ionic polyacrylamide adsorption layers modifies the surface properties of the solid. The resulting composite material (zeolite/polymer) can be applied for effective removal of phosphate(V) ions from the liquid medium. Its regeneration possibilities are also considerable. Additionally, the use of fly ash for zeolite preparation leads to management of hazardous waste material.

Peat-based activated carbons as adsorbents for simultaneous separation of organic molecules from mixed solution of poly(acrylic acid) polymer and sodium dodecyl sulfate surfactant

The activated carbons obtained from peat were applied for simultaneous removal of low-molecular weight poly(acrylic acid) – PAA (2 000 Da) and sodium dodecyl sulfate (SDS) from their mixed solution. There were applied three samples of activated carbons: N – obtained by peat pyrolysis followed by the steam activation, N9C – obtained from N by means of physical activation with CO2 and N9K – obtained from N by chemical activation with K2CO3. The final products are characterized by mesoporous structure, well-developed surface area and specific acidic-basic properties of the surface. The effect of solution pH, influencing PAA chains conformation (changes in polymer carboxyl groups dissociation) and solid surface charge density were examined. The applied adsorbents were characterized using the low-temperature nitrogen adsorption-desorption method, Fourier-transform infrared spectroscopy (FTIR), elemental analysis and back-titration method. Moreover, the adsorption affinity of PAA and SDS for the activated carbon surface was characterized based on the adsorption and electrokinetic data obtained from the spectrophotometric experiments, potentiometric titrations and zeta potential measurements. Additionally, the sizes of poly(acrylic acid) chains (hydrodynamic radius) in the solution as a function of pH were determined based on viscosity measurements. It was demonstrated that under specific pH conditions the prepared activated carbons exhibit high adsorptive affinity for PAA chains (reaching about 265 mg/g). Furthermore, it was also proved that these adsorbents can be successfully regenerated (using an appropriate desorbing agent) and reused. Therefore the low-cost activated carbons can be used for simultaneous removal of undesirable low-molecular polymers and surfactants from wastewaters.

Simultaneous removal of lead(II) ions and poly(acrylic acid) macromolecules from liquid phase using of biocarbons obtained from corncob and peanut shell precursors

The activated biocarbons obtained from corncobs and peanut shells were applied for simultaneous removal of poly(acrylic acid) – PAA macromolecules and lead(II) ions from their mixed solution. The biocarbons were prepared via the two-step chemical activation, including pyrolysis of the precursors at 500 °C in nitrogen atmosphere, followed by impregnation with K2CO3 and heat treatment at 800 °C. The obtained materials were characterized using the low-temperature nitrogen adsorption-desorption method, elemental analysis and back-titration method. The activation products were micro/mesoporous biocarbons of medium developed surface area (862 and 654 m2/g, respectively), showing slightly acidic character of the surface. The average molecular weight of PAA ensured penetration of polymer coils into the adsorbent pores under specific pH conditions. The nanostructure of PAA adsorption layer without and with Pb(II) cations on the activated carbons surface was characterized based on the adsorption, electrokinetic, stability and microscopic data obtained from the spectrophotometric experiments, both potentiometric titration and zeta potential measurements, turbidimetric experiments and SEM imaging, respectively. It was shown that the micro/mesoporous biocarbons are effective and cheap adsorbents for simultaneous removal of polymeric macromolecules and heavy metal ions from the liquid medium (the presence of both adsorbates has an extremely favorable effect on their adsorbed amounts).

Specific ion effects on the aggregation behavior of aquatic natural organic matter

Specific ion effects on the aggregation behavior of a reference aquatic natural organic matter (NOM), Suwannee River NOM (SRNOM), were investigated using kinetic, titration, calorimetric, and surface tension methods. Monovalent cations induced structural compacting of SRNOM, but not its aggregation. Their ability to induce structural compacting follows the order: Cs+ > Rb+ > K+ > Na+ > Li+. Divalent cations except Mg2+ can readily induce SRNOM aggregation. Their critical coagulation concentrations (CCC) follow the order: CCCSr > CCCCa > CCCBa. Electrokinetic, titration, and calorimetric data suggest that monovalent cations have weak interactions with SRNOM, while divalent cations strongly interact with SRNOM. Overall, the cation specificity in aggregation is determined by cation-NOM interactions and their ability to modulate surface tension. Specific ion effects of monovalent cations correlate to their hydration free energy, while that of divalent cations correlate to the ratio of the hydration entropy of cation to the enthalpy change of cation-NOM interactions. The cation specificity is consistent with the extended Derjaguin-Landau-Verwey-Overbeek theory, and the intermolecular interaction energy is dominated by the Lewis acid-base interactions. Our results suggest that specific cations should be targeted to predict or manipulate intermolecular interactions of aquatic NOM in natural and engineered settings.

Prediction of nanofiltration rejection performance in brackish water reverse osmosis brine treatment processes

A predictive nanofiltration model was built in Python to be used in reverse osmosis brine treatment processes. The model was fitted to rejection data obtained from NF trains functioning as a brine concentrator upstream of a gypsum precipitation reactor at a full-scale mine water treatment plant in Ahafo, Ghana. Over the six month operational period considered (September 2017 – March 2018) the rejection capability of the installed elements deteriorated considerably. This was reflected by a 13% increase in membrane pore radius, 40% decrease in effective active layer thickness and an 18% decrease in absolute value of the feed-membrane Donnan potential. Performance of elements from other manufacturers was simulated by loading their respective properties into the model. Cases modelled included a tightly wound Dow NF 90, a loosely wound Desal DS-5 DL and a Koch TFC SR-2 element. Rejections obtained from the installed MDS elements most closely approximated the performance of a loose NF element. These modelling studies have shown that the NF model built is capable of modelling nanofiltration in brackish water treatment processes. The current disadvantage of the model is the number of membrane—specific input parameters which need to be verified with independent experimentation. As a start, it is recommended that electro-kinetic data be obtained for similar solutions to enable a membrane charge density sub-module to be incorporated into the model.