John J. Lenhart

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 and Dissolution of Silver Nanoparticles in Natural Surface Water

This study investigated aggregation and silver release of silver nanoparticles suspended in natural water in the absence and presence of artificial sun light. The influence of the capping layer was investigated using uncoated particles and particles coated with citrate or Tween 80. The experiments were conducted over 15 days in batch mode using a river water matrix. Silver release was monitored over this time while the aggregation state and morphological changes of the silver nanoparticles were tracked using dynamic light scattering and transmission electron microscopy. Results indicate sterically dispersed particles coated with Tween released silver quicker than did bare- and citrate-coated particles, which rapidly aggregated. A dissolved silver concentration of 40 μg/L was reached after just 6 h in the Tween-coated particle systems, accounting for ca. 3% of the total silver. Similar levels of dissolved silver were reached in the uncoated and citrate-coated systems at the end of the 15 days. Silver release was not significantly impacted by the artificial sun light; however, the light (and citrate) imparted significant morphological changes to the particles. Their impact was masked by aggregation, which seemed to be the controlling process in this study.

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.

Adsorption of C4-dicarboxylic acids at the hematite/water interface.

The adsorption at the hematite/water interface of a suite of C4-dicarboxylic acids (maleic, fumaric, and succinic acids) that differ with respect to the orientation of the carboxylic acid groups was studied using in situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy and batch adsorption experiments. Results of phthalic acid adsorption under similar experimental conditions from our previous study were compared with those observed for the C4-dicarboxylic acids. The batch adsorption experiments suggest that the orientation of the two carboxylic groups and pK(a) values of the organic acids noticeably influence the adsorption density as well as the position and characteristics of the pH adsorption edges. In particular, the adsorption capacity of fumaric acid was much lower than that observed for the other organic acids. The ATR-FTIR results also suggest that the adsorption modes of the organic acids on hematite depend on the orientation of the two carboxylic functional groups. Succinic acid with higher molecular flexibility, maleic acid with cis-configuration, and phthalic acid with ortho-position of the two carboxylic groups were similarly present as deprotonated bidentate inner- and outer-sphere complexes likely involving both carboxylic groups. The inner-sphere complex was more significant at acidic pH. Fumaric acid, however, with transconfiguration seems to bind to hematite primarily as a deprotonated outer-sphere complex utilizing only one carboxylic group.

Toxic cyanobacteria and drinking water: Impacts, detection, and treatment

Blooms of toxic cyanobacteria in water supply systems are a global issue affecting water supplies on every major continent except Antarctica. The occurrence of toxic cyanobacteria in freshwater is increasing in both frequency and distribution. The protection of water supplies has therefore become increasingly more challenging. To reduce the risk from toxic cyanobacterial blooms in drinking water, a multi-barrier approach is needed, consisting of prevention, source control, treatment optimization, and monitoring. In this paper, current research on some of the critical elements of this multi-barrier approach are reviewed and synthesized, with an emphasis on the effectiveness of water treatment technologies for removing cyanobacteria and related toxic compounds. This paper synthesizes and updates a number of previous review articles on various aspects of this multi-barrier approach in order to provide a holistic resource for researchers, water managers and engineers, as well as water treatment plant operators.

Investigation of Consolidation-Induced Solute Transport. I: Effect of Consolidation on Transport Parameters

This paper presents an experimental investigation of the effect of clay consolidation on parameters that govern the advective-dispersive transport of an inorganic solute. Batch, diffusion, dispersion, and solute transport tests were conducted using kaolinite clay and dilute solutions of potassium bromide (KBr). Batch tests produced the highest levels of K+ sorption and indicated that equilibrium sorption was achieved in approximately 10–30 min. The increase in sorption observed in the batch tests, as compared to the dispersion or solute transport tests, reflects the significantly lower solids-to-solution ratio and more efficient mixing process. By comparison, kaolinite consolidation had little effect on sorption due to the relatively small change in porosity. Values of hydrodynamic dispersion coefficient ( Dh ) , effective diffusion coefficient ( D∗ ) , and apparent tortuosity factor decreased with decreasing porosity. Values of D∗ obtained for Br− were generally larger than for K+ , whereas Dh values for...

Surface complexation modeling of dual-mode adsorption of organic acids: phthalic acid adsorption onto hematite.

Recent spectroscopic and theoretical investigations provide evidence suggesting organic acids bind to mineral surfaces simultaneously as inner- and outer-sphere complexes and that the relative importance of these species varies with solution chemistry (e.g., pH), mineral type, and organic acid. Constraining surface complexation models (SCMs) with this information has proven difficult; however, as efforts to tune the surface complex stoichiometry within the SCM to match the structures and speciation trends observed in the spectra are seldom successful. In this study, the adsorption of phthalic acid on hematite was investigated using the Extended Constant Capacitance Model (ECCM) in a manner that was consistent with reported spectroscopic results. The influence of adjustable model parameters (capacitance, site density, and charge distribution) on model fit and the corresponding determination of surface speciation was tested. Although nearly equivalent fits were determined with a model comprised of a single outer-sphere complex and a dual-mode model that included one inner-sphere complex and one outer-sphere complex, results with the dual-mode model were more consistent with spectroscopic results from the literature. The surface speciation predicted by the dual-mode model was dependent on model parameter choice, with the concentration of the inner-sphere complex at low pH significantly increasing with decreasing inner-layer capacitance and site density. These results suggest to reliably use SCMs to describe adsorption over a wide range of experimental conditions requires direct knowledge of surface speciation to constrain the selection of surface complexation reactions and adjustable model parameters.

Coupled Consolidation and Contaminant Transport in Compressible Porous Media

This paper presents an experimental and numerical investigation of coupled consolidation and contaminant transport in compressible porous media. Numerical simulations were performed using the CST2 computational model, in which a dual-Lagrangian framework is used to separately follow the motions of fluid and solid phases during consolidation. Diffusion and large strain consolidation-induced transport tests were conducted on composite specimens of kaolinite slurry consisting of an upper uncontaminated layer and a lower layer contaminated with potassium bromide. Assessment of the importance of the consolidation process on solute transport is based on measured and simulated solute breakthrough curves and final contaminant concentration profiles. CST2 simulations closely match the experimental data for three different loading conditions. Diffusion and consolidation-induced advection made important contributions to solute transport and mass outflow in this study. Additional simulations indicate that consolidati...

The influence of dicarboxylic acid structure on the stability of colloidal hematite

Low molecular weight organic acids comprise an important pool of reactive ligands in aquatic systems. These acids readily bind to nano-sized mineral particles and thereby strongly influence a particles physicochemical behavior. Predicting this influence requires the integration of molecular-level details that control surface complexation mechanisms and structures with macro-scale observations of mineral colloid behavior. We report on the aggregation kinetics of nano-sized hematite in the presence of fumaric acid and maleic acid, which are naturally occurring dicarboxylic acids of similar size and structure. Our results indicate that the structure and orientation of the adsorbed dianion at the hematite surface, not the adsorption mechanism, defines the resulting effect. Maleate, which directs both carboxyl groups to the surface in the form of inner- and outer-sphere surface complexes, enhances colloidal stability. Fumarate, however, which binds to the hematite surface as an outer-sphere complex with just one carboxyl group only slightly influenced particle stability. This outcome suggests that subtle differences in the structure of adsorbed acids produce important differences in the physicochemical behavior of particles in dilute aquatic systems.

Dicarboxylic acid transport through hematite-coated sand.

To better understand the behavior of low molecular weight organic acids in subsurface environments, the transport of three dicarboxylic acids (phthalic, maleic, and fumaric acid) in water-saturated columns packed with a hematite-coated sand was investigated in single and binary organic acid systems. Experiments were conducted at a single ionic strength (0.1M) and at two pH values (4.1 and 5.3). In single-acid systems, the order of breakthrough at both pH values was fumaric acid, followed by maleic acid, and then phthalic acid. The shape of the breakthrough curves for the acids at the two pH values were similar except at pH 5.3 phthalic acid showed two adsorption fronts. The initial front only partially broke through, whereas the second front proceeded to complete breakthrough. This behavior resulted from a marked pH increase during phthalic acid adsorption and suggests that the single-acid systems behaved as dual-component systems with the organic acid and hydrogen ion as variables. The breakthrough curves for the binary organic acid systems showed organic acids with a higher adsorption affinity (e.g., phthalic acid) competitively displace organic acids with a lower adsorption affinity (e.g., fumaric acid). The dual-component effect observed for phthalic acid in the single-acid systems was suppressed in the mixed acid systems, perhaps reflecting pH changes that accompanied the desorption of the weakly-binding acids. These results may provide an important step toward further elucidating the processes controlling organic acid fractionation in the subsurface.