Surface Charge Character Research Articles

Identification of the degree of aging and adsorption behaviors of the naturally aged microplastics

Microplastics (MPs) inevitably experienced various aging processes in nature may exhibit varied and complex interfacial interactions with adjacent species. Therefore, clarifying the possible interfacial interactions between naturally aged MPs and organic pollutants is of great significance to assess the actual behaviors of MPs in the environment. Here several plastic packaging materials after use were employed as the raw materials and representatives of naturally aged MPs, the alteration of surface characteristics, especially the degree of aging and the adsorption properties of MPs for anionic and cationic dyes were investigated. The types and the degree of aging of MPs were identified, and the variation of oxygen-containing functional groups (carbonyl, hydroxyl, and ester groups), the hydrophilicity and surface charge character were characterized. The fitting results of kinetics and isotherm models indicated that the adsorption was mainly multi-layer on heterogeneous surfaces, with hydrogen bonding, electrostatic attraction, polar interaction, and hydrophobic partitioning possibly involving. The hydrogen bond interaction was further confirmed by FTIR spectra. The increased temperature promoted the adsorption of cationic dyes on MPs, and the increased salinity of the solution enhanced the uptake of most of the tested dyes by MPs. This research deepened the understanding on the aging degree of MPs and their interfacial interactions with hydrophilic pollutants, and provided vital information for MPs as pollutant carriers.

Structural/surficial dual regulated granular H2TiO3 lithium-ion sieves for lithium extraction from salt lake brine

H2TiO3 (HTO) is recognized as a highly promising lithium-ion sieve (LIS) material for lithium resources extraction from the liquid phase, attributed to its high theoretical capacity, stable structure, and excellent recyclability. However, when directly employed in continuous flow systems, the powder HTO-LIS encounters significant challenges, including elevated energy consumption and persistent material loss, impeding its practical application. Therefore, it is necessary to granulate the powder LIS material, which nevertheless results in the reduction of the specific capacity and Li extraction rate. Herein, a hybrid binder (cellulose acetate/sulfonated poly(ether ketone)/poly(vinyl chloride)) is employed for granulation and immobilization of the powder HTO-LIS. This approach regulates the porosity structure, hydrophilicity, surface charge character, and mechanical strength of the granulated LIS, thereby synergistically enhancing Li extraction capacity (26.54 mg g−1), recovery kinetics (19.02 mg g−1 within 4 h), and dynamic cycling stability (adsorption capacity retention of 94.02% after 40 cycles). This work proposes a feasible granulation strategy, contributing to advance the industrialization of LIS materials.

Enhanced photocatalytic activity of ZnO supported on Alumina and Antibacterial study

Al2O3 nanoparticles were synthesized by simple co-precipitation method using scrap aluminum foils. ZnO and ZnO supported on Alumina were also prepared. A detail structural characterization of prepared material was carried out using XRD and various other techniques. Gamma Alumina with particle size 4 nm was formed. A complete Photodegradation of Amido black dye was observed on the above materials under natural sunlight. 1:1 ZnO/Al2O3 showed increased photoactivity compared to ZnO alone. CO2 estimation and COD analysis were performed so as to elucidate the extent of mineralization of the dye. Surface properties of the photocatalyst were correlated with photocatalytic activity by studying the surface charge character of the catalysts. Antibacterial activity towards gram positive and gram negative bacteria was also studied which showed promising results.

Surface charge is a function of organic carbon content and mineralogical compositions of soil

A study of the distribution of the electric charges in the surface horizons of two highly weathered soils Fluvaquent and Haplaquept of West Bengal, India was made by direct measurement of adsorption of ions in the presence of varying concentration of electrolyte. The objective of this study was to evaluate charge properties of two highly weathered soil of India. The results show that pH 0 varies with soil according to the variation in organic carbon and sesquioxide/allophone content. Organic carbon strongly affects the variation of negative charge with pH, but sesquioxide/allophone is responsible for positive charge variation. Results used the difference between the soil pH values measured in 1M KCl and in water for estimating the point zero charge of the soil and ΔpH values estimating the net surface charge character. Surface charges is a function of organic carbon, clay content, composition of clay and amount of Fe, Al and there oxides.

Zeta potential: a case study of cationic, anionic, and neutral liposomes.

Zeta potential is often used to approximate a nanoparticle's surface charge, i.e., cationic, anionic, or neutral character, and has become a standard characterization technique to evaluate nanoparticle surfaces. While useful, zeta potential values provide only very general conclusions about surface charge character. Without a thorough understanding of the measurement parameters and limitations of the technique, these values can become meaningless. This case study attempts to explore the sensitivity of zeta potential measurement using specifically formulated cationic, anionic, and neutral liposomes. This study examines zeta potential dependence on pH and ionic strength, resolving power, and highlights the sensitivity of zeta potential to charged liposomes. Liposomes were prepared with cholesterol, 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC), and varying amounts of 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP) or 1,2-dioleoyl-sn-glycero-3-phospho-L-serine (DOPS). A strong linear relationship was noted between zeta potential values and the mole percentage of charged lipids within a liposome (e.g., cationic DOTAP or anionic DOPS). This finding could be used to formulate similar liposomes to a specific zeta potential, potentially of importance for systems sensitive to highly charged species. In addition, cationic and anionic liposomes were titrated with up to two mole percent of the neutral lipid 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (lipid-PEG; LP). Very small amounts of the lipid-PEG (<0.2mol%) were found to impart stability to the DOTAP- and DOPS-containing liposomes without significantly affecting other physicochemical properties of the formulation, providing a simple approach to making stable liposomes with cationic and anionic surface charge.

A novel type of polyelectrolyte complex/MWCNT hybrid nanofiltration membranes for water softening

A novel type of polyelectrolyte complex (PEC)/multi-walled carbon nanotube (MWCNT) hybrid nanofiltration (NF) membranes were prepared via in-situ ionic cross-linking between sodium alginate (SA), polyethyleneimine (PEI), and MWCNTs. Chemical structures and compositions of PECs and MWCNT hybrid PECs (HPECs) were characterized by Fourier transform infrared spectroscopy (FTIR). Morphologies of PEC NF membranes (PEC-NFMs) and HPEC NF membranes (HPEC-NFMs) were characterized by field emission scanning electron microscope (FESEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM); and their hydrophilicity was characterized by water contact angle (CA). Effects of PEI and MWCNT content on the surface zeta potential and NF performance were investigated thoroughly. It was found that the surface charge character of PEC-NFMs changed from negative to positive with increasing the PEI content. The water flux of HPEC-NFMs dramatically increased from 13.4 to 27.0Lm−2h−1 with increasing the mass ratio of MWCNTs to SA from 0 to 0.050, and the MgCl2 rejection maintained at around 93.5% (testing with 1gL−1 aqueous MgCl2 solution at 25°C and 0.6MPa). Moreover, the HPEC-NFM-3 exhibited a good Na+/Mg2+ selectivity. This study provided an effective approach for fabricating positively charged NF membranes with enhanced NF performance, which suits well for water softening.

Greatly improved electrochemical performance of lithium–oxygen batteries with a bimetallic platinum–copper alloy catalyst

Research on the cathode catalysts of lithium–oxygen (Li–O2) batteries is one of the most important branches to commercialize these batteries to overcome the sluggish kinetics during both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). In this study, a high performance catalyst based on a bimetallic Pt–Cu alloy is investigated for Li–O2 batteries using first-principles calculation. The theoretical prediction shows that the Pt–Cu alloy is much more effective than the pure Pt according to the electrochemical performance. In particular, the effectiveness of the catalytic property is maximized in the case of the PtCu (111) surface which greatly reduces the large overpotentials of the original Li–O2 batteries during the OER/ORR. It is identified for the first time that the charge overpotentials are affected mainly by the inherent surface charge character of the alloy catalyst. It is observed that the more negatively charged PtCu (111) surface can act as a weakly positively charged surface for the adsorption of Li–O intermediates and thus result in weak ionic bonding of the intermediates on the surface. As a result, the dominant factor improving the catalytic performance is clearly demonstrated, providing insight into the design of an efficient catalyst for Li–O2 battery technologies.

(Corrosion Division H.H. Uhlig Award) Perspectives on Passivity and Its Breakdown

Aluminum, stainless steels, and many other metals and alloys are protected by a passive film that protects the surface from corrosion. However, in environments that contain aggressive anions such as chloride, Cl‾, the passive film becomes unstable and degrades locally causing oxide film breakdown and pitting corrosion. This presentation will discuss two aspects related to passivity and its breakdown that have been a focus of our research; the surface charge character of the oxide film (1-2) and chloride adsorption and incorporation into the oxide film (3-6). Most of the discussion will be related to passivity and its breakdown of aluminum, Al, in Cl‾ solutions.The first step in the initiation of metastable or stable pitting corrosion of Al is the adsorption of Cl‾ on the surface of the passive oxide. Chloride adsorption by the oxide film has been primarily discussed in terms of surface attractive forces. The isoelectric point (IEP) or pH of zero charge (pHpzc) of an oxide defines the surface charge character as a function of pH. The pHpzc of Al is ~ 9 in aqueous solutions. Thus, at pH values less than 9, which is the case for most natural water systems, the surface charge of Al is positive and this leads to the attraction of negative ions such as Cl‾ to the surface. The relationship between the pitting potential value of oxide-covered Al and the pHpzc, of its oxide was studied by ion implanting aluminum (99.999% purity) with elements whose oxides were of known pHpzc (1-2). The results showed that all the implanted ions chosen because of the low pHpzc of their oxides, i.e. lower than that of Al, (Si, Cr, Zr, Nb, Ta, and Mo) produced binary surface alloys that had higher (more positive) pitting potential values than Al. Conversely, the implanted ions chosen because of the high pHpzc of their oxides relative to Al (Li and Zn) produced binary surface alloys that had pitting potential values that were lower than that of Al. The implantation of Al into Al had no effect on the pitting potential, indicating that the increased and decreased pitting potential values observed for the other implant ions were chemical in nature and not a result of the ion implantation process.The second aspect of this presentation will discuss Cl‾ adsorption and incorporation into the oxide film. X-ray absorption spectroscopy (XAS), specifically X-ray absorption near edge spectra (XANES), and X-ray photoelectron spectroscopy (XPS) were used to investigate Cl‾ adsorption and incorporation into the oxide film on aluminum (3-6). Samples were polarized at selected potentials below the pitting potential in deaerated 0.1M NaCl solutions. XANES and XPS spectra both showed the presence of two distinct Cl‾ species associated with the oxide film on Al for samples that were polarized below the pitting potential. Chloride was found to be present as an adsorbed species at the surface, as well as an incorporated species within the passive oxide. Acknowledgements The author gratefully acknowledges the Naval Research Laboratory and the Office of Naval Research for financial support of this work. The author also gratefully acknowledges his colleagues especially the co-authors of the cited papers without whose contributions this work would not have been possible.

Predicting isosteric heats for gas adsorption

A method for predicting the isosteric heat of gas adsorption on solid materials is developed which requires the measurement of a single isotherm - where previous methods, such as the Clausius-Clapeyron approach, require either multiple isotherms or complex calorimetric measurement. The Tóth potential function, stemming from the Polanyi potential function, is evaluated using the Langmuir and Tóth isotherm equations to generate new equations for the isosteric heat. These new isosteric heat equations share common parameters with the isotherm equations and are determined from isotherm fitting. This method is demonstrated in the literature for gas adsorption onto solid adsorbates including zeolites of various surface charge character and non-porous rutile phase titanium dioxide. Predictions are made using the new isosteric heat equations and then compared to calorimetric data.

Dependence of the direct electron transfer activity and adsorption kinetics of cytochrome c on interfacial charge properties

With the advantages of in situ analysis and high surface sensitivity, surface-enhanced infrared absorption spectroscopy in attenuated total reflection mode (ATR-SEIRAS) combined with electrochemical methods has been employed to examine the interfacial direct electron transfer activity and adsorption kinetics of cytochrome c (cyt c). This work presents data on cyt c adsorption onto negatively charged mercaptohexanoic acid (MHA) and positively charged 6-amino-1-hexanethiol (MHN) self-assembled monolayers (SAMs) on gold nanofilm surfaces. The adsorbed cyt c displays a higher apparent electron transfer rate constant (33.5 ± 2.4 s(-1)) and apparent binding rate constant (73.1 ± 5.2 M(-1) s(-1)) at the MHA SAMs surface than those on the MHN SAMs surface. The results demonstrate that the surface charge density determines the protein adsorption kinetics, while the surface charge character determines the conformation and orientation of proteins assembled which in turn affects the direct electron transfer activity.

Modification of Ion Permeability Utilizing pH-Dependent Selective Swelling of Polyelectrolyte Multilayer Films

The selective swelling behavior of polyelectrolyte multilayer (PEM) films prepared by layer-by-layer (L-b-L) assembly influences the ion-permeability in contrast to surface charge density of the films. The cation terminated polyethylene amine (PEI) and anion terminated polyacrylic acid (PAA) were dissolved in DI water, and the pH was adjusted to 10 and 4, respectively, exemplifies thick denser film with good layering structure. The layered polyelectrolyte films has selective swelling behavior at pH 4 (PEI) or pH 10 (PAA), influences the permeability of both Ru(NH3)6(2+) and Fe(CN)6(3-) rather than surface charge character or film charge density. The swollen top most layer shows "on" character, whereas, shrunken top most layer shows "off" character for the ion-permeability. Such "on-off" character can be used for the pH-dependent switches based on surface morphology.

High Functional Adsorbent Alumina Nanofibers Prepared by a Hydrolysis of Al Nanopowder

The alumina (Al 2 O 3 ) nanofibers with high specific surface area, and chemical stability were prepared by a simple hydrolysis and a subsequent dehydration of Al nanopowder. The performance of these nanofibers as an adsorbent of radio isotope (RI) and a filter for bacteria and heavy metal was examined using Nal scintillation counter and UV-spectrophotometer. The observed filtering ability of the alumina nanofibers was remarkably high, which was 197 milligram per a gram of alumina (mg/g) for the RI of Mo-99 under pH 5.1 and 20 °C, and 21, 8, and 37 mg/g for Zn, Cr and Cu, respectively. According to the experiment of removal of bacteria, strain of Bacillus subtilis, the aluminium hydroxide nanofiber coated on the micron-sized glass fibers removed the bacteria completely with an effective removal rate of 99.999%. The prominent filtering effect of the alumina nanofibers is attributed to a very high surface reaction area induced by ultrafine nanofibrous structure with the typical diameters of 2 nm to 3 nm, and an intrinsic electropositive surface charge character in aqueous solution.

The role of reactive surface sites and complexation by humic acids in the interaction of clay mineral and iron oxide particles

Aggregation and dispersion of mineral particles spontaneously take place under changing environmental conditions in natural systems. The structure of particle network in soils, the retardation or release of colloidal particles, and their mobility and transport are inherently influenced by natural organic matter bound to the mineral matrix. Since the surface properties of clay mineral and metal oxide particles, and the electrified mineral–water interfaces play a major role in formation, structure and strength of aggregates, any surface modification, especially by polyanionic organic complexants such as humic substances, has a significant affect on particle interaction in a mineral assemblage. The permanently and/or conditionally charged clay minerals (montmorillonite and kaolinite) and iron oxides (hematite and magnetite), as known major mineral components in natural waters, were selected for studying their surface charge characteristics and pH dependent interactions. We discuss how the surface charge correlates with particle aggregation through some characteristic examples for homo and heterocoagulation of similar and dissimilar mineral particles under acidic condition (at pH ∼4) in the dilute and concentrated systems studied by means of light scattering and rheology, respectively. The adsorption of a brown coal derived humic acid, and its influence on the surface charge character and particle aggregation of clay and iron oxide particles were also studied in dilute and concentrated suspensions. Humic acids can be bound to the most reactive surface sites of clay and oxide particles, i.e. to Al -OH mainly at the edges of clay lamellae, and to Fe -OH on iron oxides, in surface complexation reaction, therefore their role in particle aggregation is specific. Relations between surface complexation, surface charge modification, and particle aggregation in pure and mixed montmorillonite–iron oxide systems are explained.

Particle aggregation in complex aquatic systems

The influence of humate, gallate and salicylate on the surface charge character and particle aggregation of permanently and/or conditionally charged clay and oxide minerals (montmorillonite, kaolinite and aluminium-oxide) was investigated at pH 7–8 in the presence of an indifferent electrolyte. pH-dependent surface charging of minerals were determined by acid–base titration. The pH- and ionic strength dependent particle aggregation was measured in both the absence and presence of organic anions by means of dynamic light scattering (DLS). Relatively small amount of organic anions, less than one species per nm 2, adsorbed on the Al-OH surface sites of minerals induces a significant increase in the colloidal stability of dilute suspensions. Bound organic anions smear out the heterogeneity of surface charge and the differences between the clay and oxide minerals.