Harold W. Walker

Dissolution-Accompanied Aggregation Kinetics of Silver Nanoparticles

Bare silver nanoparticles with diameters of 82 ± 1.3 nm were synthesized by the reduction of the Ag(NH(3))(2)(+) complex with D-maltose, and their morphology, crystalline structure, UV-vis spectrum, and electrophoretic mobilities were determined. Dynamic light scattering was employed to assess early stage aggregation kinetics by measuring the change in the average hydrodynamic diameter of the nanoparticles with time over a range of electrolyte types (NaCl, NaNO(3), and CaCl(2)) and concentrations. From this the critical coagulation concentration values were identified as 30, 40, and 2 mM for NaNO(3), NaCl, and CaCl(2), respectively. Although the silver nanoparticles were observed to dissolve in all three electrolyte solutions, the aggregation results were still consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The dissolution of the silver nanoparticles, which were coated with a layer of Ag(2)O, was highly dependent on the electrolyte type and concentration. In systems with Cl(-) a secondary precipitate, likely AgCl, also formed and produced a coating layer that incorporated the silver nanoparticles. Aggregation of the silver nanoparticles was also examined in the presence of Nordic aquatic fulvic acid and was little changed compared to that evaluated under identical fulvic acid-free conditions. These results provide a fundamental basis for further studies evaluating the environmental fate of silver nanoparticles in natural aquatic systems.

Aggregation Kinetics and Dissolution of Coated Silver Nanoparticles

Determining the fate of manufactured nanomaterials in the environment is contingent upon understanding how stabilizing agents influence the stability of nanoparticles in aqueous systems. In this study, the aggregation and dissolution tendencies of uncoated silver nanoparticles and the same particles coated with three common coating agents, trisodium citrate, sodium dodecyl sulfate (SDS), and Tween 80 (Tween), were evaluated. Early stage aggregation kinetics of the uncoated and coated silver nanoparticles were assessed by dynamic light scattering over a range of electrolyte types (NaCl, NaNO(3), and CaCl(2)) and concentrations that span those observed in natural waters. Although particle dissolution was observed, aggregation of all particle types was still consistent with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The aggregation of citrate-coated particles and SDS-coated particles were very similar to that for the uncoated particles, as the critical coagulation concentrations (CCC) of the particles in different electrolytes were all approximately the same (40 mM NaCl, 30 mM NaNO(3), and 2 mM CaCl(2)). The Tween-stabilized particles were significantly more stable than the other particles, however, and in NaNO(3) aggregation was not observed up to an electrolyte concentration of 1 M. Differences in the rate of aggregation under diffusion-limited aggregation conditions at high electrolyte concentrations for the SDS and Tween-coated particles, in combination with the moderation of their electrophoretic mobilities, suggest SDS and Tween imparted steric interactions to the particles. The dissolution of the silver nanoparticles was inhibited by the SDS and Tween coatings, but not by the citrate coating, and in chloride-containing electrolytes a secondary precipitate of AgCl was observed bridging the individual particles. These results indicate that coating agents could significant influence the fate of silver nanoparticles in aquatic systems, and in some cases these stabilizers may completely prevent particle aggregation.

Factors influencing the flocculation of colloidal particles by a model anionic polyelectrolyte

The flocculation of charged colloidal particles by oppositely charged polyelectrolytes is investigated, using as a model system, positively charged latex microspheres and negatively charged single-stranded DNA of varying chain length from 3-mers up through 1400-mers. For DNA molecules with chain lengths of eight or more monomers there is a single polymer mass (0.1 μg ml−1) which neutralizes the particle surface charge and destabilizes the particle suspension in the presence of 0.005 M NaCl at a particle concentration of 1.47 × 1010 particles per ml. The coagulation kinetics at this optimum polymer dose are diffusion-limited, even for the largest polymer tested. Hence, the electrostatic “patch” model for the flocculation of negatively charged particles by cationic polymers proposed by Gregory (Gregory, J. Colloid Interface Sci., 42 (1973) 448; 55 (1976) 35) does not apply to our system. The mechanism responsible for polymer flocculation in a given system appears to depend upon the relative spacing of charged groups on the polymer backbone and particle surface, the size and inherent flexibility of the polymer in question, and the nature of the initial polymer-particle interaction.

Effect of cationic polymer additives on the adsorption of humic acid onto iron oxide particles

Abstract Improving the removal of natural organic matter (e.g. humic acid) during drinking water treatment is important in order to minimize the formation of disinfection by-products (DBPs). Although polymeric flocculants are often used to improve turbidity removal during the coagulation process, little information is available regarding the influence of these polymers on NOM removal. In this study, the adsorption of humic acid onto polymer-coated iron oxide particles was investigated as a way to improve humic acid removal during the coagulation process. Monodisperse iron oxide particles and well-characterized humic acid were used as a model system. The adsorption of humic acid onto bare iron oxide particles decreased as solution pH increased. When iron oxide particles were coated with a cationic polymer, adsorption doubled at high pH (∼9.5) and low salt concentration (0.001 M NaCl). The greater adsorption of humic acid on polymer-coated surfaces at high pH was largely due to changes in the electrostatic interactions between humic acid and the particle surface. Coating the iron oxide particles with cationic polymer resulted in the reversal of the negative particle surface charge, thus providing more favorable conditions for humic acid adsorption. Little change in humic acid adsorption was observed, however, for polymer-coated particles at near neutral pH values (∼6.8) or at high salt concentration. These data suggest that under certain conditions polymers may enhance humic acid adsorption onto iron oxide surfaces, a process which may improve DBP precursor removal during drinking water treatment.

Stability of particle flocs upon addition of natural organic matter under quiescent conditions

In this research, the influence of two natural organic polymers (polysaccharide and humic acid) on the stability of colloidal aggregates was examined. The primary objective of this research was to determine whether addition of organic matter to floc suspensions results in the fragmentation or stabilization of aggregates. A second objective was to determine how the size of aggregates and the composition of organic matter influence the floc breakup or stabilization process. It was found that the stability of aggregates depended on the type of organic material present as well as floc size. For example, humic acid increased the stability of aggregates more effectively than polysaccharides of larger size. It was also found that the addition of humic acid or polysaccharide generally decreased the rate of coagulation of small aggregates but had less influence on large aggregates. In no case did the addition of polysaccharide or humic acid result in the fragmentation of particle aggregates. The existence of strong interparticle forces within flocs prevented aggregate breakup upon adsorption of natural organic polymers. The results presented here provide important new information regarding the influence of NOM on the behavior of particles in aquatic systems.

Sonochemical reactions of dissolved organic matter

Property changes of Aldrich and Pahokee peat dissolved organic matter (DOM) at different ultrasonic frequencies and energy densities were systematically investigated. Exposure of DOM to ultrasound resulted in decreases in TOC, Color465, specific UV absorbance (SUVA), aromaticity and molecular weight, while DOM acidity increased. Compared to 20 kHz ultrasound, greater sonochemical transformation of DOM occurred at 354 kHz and at higher energy density, due to greater ·OH radical production. The changes to DOM properties suggest that ultrasound may significantly affect DOM-pollutant interactions (e.g.facilitate desorption of hydrophobic organics from DOM or promote complexation between metallic cations and DOM).

Enhanced adsorption of natural organic matter on calcium carbonate particles through surface charge modification

Abstract In this research, means to increase the removal of natural organic matter (NOM) during drinking water treatment were investigated. Specifically, the modification of calcium carbonate surfaces by cationic polyelectrolytes was tested as a way to improve humic acid adsorption to floc surfaces during lime softening. Results demonstrated that under high pH conditions (pH 9.5), coating the particles with high charge density cationic polyacrylamide (CPAM) significantly increased the adsorption of humic acid, while coating the particles with low charge density CPAM had no positive effect. The amount of humic acid adsorbed also depended on the molecular weight of CPAM, with high molecular weight coatings being more effective at enhancing adsorption than low molecular weight coatings. Electrophoretic mobility measurements demonstrated that the cationic polymer reduced repulsive electrostatic interactions between humic acid and the particle surface. Under low pH conditions (pH 7.5), adsorption of polyacrylamide was low, and coating the particles had less effect on the amount of humic acid adsorbed. The results presented here suggest that the modification of calcium carbonate floc surfaces using cationic polymers with high charge density may enhance natural organic matter removal during drinking water treatment.

Effect of Fouling Conditions and Cake Layer Structure on the Ultrasonic Cleaning of Ceramic Membranes

Abstract Homogeneous alumina membranes fouled by polystyrene latex particles at different pH values and ionic strengths were subjected to ultrasonic cleaning. Cleaning was more effective at high and low pH than at neutral pH. At low pH values, less repulsive particle‐particle interactions resulted in the removal of millimeter‐scale aggregates and highly effective cleaning. At near‐neutral pH, stronger repulsive particle‐particle interactions caused detachment to occur as individual particles from the cake layer rather than as flocs, which was a slightly less effective cleaning mechanism. Ultrasonic cleaning of cake layers formed at high ionic strength (>0.3 M KCl) was less effective than cleaning at lower ionic strength (<0.3 M KCl). High ionic strength caused particles to coagulate in solution and deposit as flocs on the membrane surface forming a highly permeable fouling layer. This fouling layer was resistant to ultrasound at the sub‐optimal cleaning conditions used in this study, perhaps due to particle attachment occurring within a primary energy minimum. Membrane cleaning experiments performed with particles of varying size showed that particle size was less important than the surface potential of the particles. For a given mass, particles that possessed the largest surface potential formed the thickest fouling layer, irrespective of particle size, and showed the greatest improvement in flux with ultrasonic cleaning. These results demonstrate that solution conditions influence ultrasonic cleaning of membranes primarily by modifying particle‐particle and particle‐membrane interactions as well as cake layer structure, rather than by impacting the extent or magnitude of cavitation events.

INFLUENCE OF SURFACE CHARGE AND PARTICLE SIZE ON THE STABILIZATION OF COLLOIDAL PARTICLES BY MODEL POLYELECTROLYTES

Abstract In this study, a homologous series of single-stranded DNA homopolymers is used to investigate the effect of polymer molecular weight on the ability of polyelectrolytes to stabilize suspensions of polystyrene latex particles. The critical polymer length (CPL) required for stabilization depends strongly on the average diameter of the latex particles, and is not influenced by the surface charge of the bare particles, at least at 1 M NaCl. The CPL for ∼500 nm latex particles is approximately 80 nucleotides, or greater than two persistence lengths, indicating that a high degree of polymer flexibility is required for the stabilization of these particles. The CPL for ∼100 nm latex particles, on the other hand, is approximately ten nucleotides, or less than 0.3 persistence lengths, suggesting that relatively rigid polymers are capable of stabilizing suspensions of the smaller latex particles. This relationship between CPL and particle size is apparently a result of the larger van der Waals interactions between larger particles. Changes in polymer conformation with particle size appear to be of secondary importance in the stabilization of these lattices.

Effect of natural organic coatings on the polymer-induced coagulation of colloidal particles

Abstract In this research, we investigate the effect of natural organic coatings of humic acid on the flocculation of well-characterized amidine polystyrene latex microspheres by a polymeric flocculent (polyvinyl alcohol, or ‘PVA’). Our results demonstrate that in the absence of humic acid, PVA effectively coagulates the particles as a result of polymer bridging interactions. When the particle surface is saturated with humic acid, however, the latex particles are stable at all dosages of PVA tested, presumably as a result of repulsive electrostatic interactions between primary particles and a decrease in the number of active sites available for bridging. At low humic acid surface coverage, on the other hand, an increase in PVA-induced coagulation is observed compared with when PVA is the sole polymer in the system. Here, both polymer bridging and electrostatic interactions appear to play a role in controlling the coagulation rate of these particles. Our results suggest that the predominance of polymer bridging in natural systems may depend on the preferential displacement of low molecular humic and fulvic acids and/or favorable interactions between large macromolecules and particle surfaces coated with humic materials.