John C. Crittenden

Stability of commercial metal oxide nanoparticles in water

The fate of commercial nanoparticles in water is of significant interest to health and regulatory authorities. This research investigated the dispersion and stability of metal oxide nanoparticles in water as well as their removal by potable water treatment processes. Commercial nanoparticles were received as powder aggregates, and in water neither ultrasound nor chemical dispersants could break them up into primary nanoparticles. Lab-synthesized hematite was prepared as a primary nanoparticle (85 nm) suspension; upon drying and 1-month storage, however, hematite formed aggregates that could not be dispersed completely as primary nanoparticles in water. This observation may explain why it is difficult to disperse dry commercial nanoparticles. Except for silica, other nanoparticles rapidly aggregated in tap water due to electric double layer (EDL) compression. The stability of silica in tap water is related to its low Hamaker constant. For all these nanoparticles, at an alum dosage of 60 mg/L, coagulation followed by sedimentation could remove 20-60% of the total nanoparticle mass. Filtration using a 0.45 microm filter was required to remove more than 90% of the nanoparticle mass.

Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles.

The stability of nanoparticles in aquatic environment plays an important role in determining their environmental implication and potential risk to human health. This research studied the impact of natural organic matter (NOM) and divalent cations (Ca(2+)) on the stability of engineered metal oxide nanoparticles (e.g. ZnO, NiO, TiO(2), Fe(2)O(3) and SiO(2)). When nanoparticles were present in neutral water, a relatively weak electrolyte concentration (0.01 M KCl) could result in their aggregation; however, with the addition of 1 mg/L NOM, the negative surface charge of nanoparticles increased significantly and therefore their propensity to aggregate is reduced. 4 mg/L NOM stabilized most nanoparticles by producing -30 mV or higher zeta potentials. On the other hand, the negative charge that NOM imparted to nanoparticles could be neutralized by divalent cations (calcium ions). 0.04 M-0.06 M Ca(2+) induced the aggregation of NOM-coated nanoparticles. It should be noted that among all the studied nanoparticles, SiO(2) exhibited the unique stability due to its low NOM adsorption capacity and small Hamaker constant. SiO(2) remained stable no matter whether the solution contained NOM or Ca(2+).

A kinetic model for H2O2/UV process in a completely mixed batch reactor

Abstract A dynamic kinetic model for the advanced oxidation process (AOP) using hydrogen peroxide and ultraviolet irradiation (H 2 O 2 /UV) in a completely mixed batch reactor (CMBR) is developed. The model includes the known elementary chemical and photochemical reactions, and literature reported photochemical parameters and chemical reaction rate constants are used in this model to predict organic contaminant destruction. Unlike most other kinetic models of H 2 O 2 /UV oxidation process, the model does not assume that the net formation rate of free radical species is zero (pseudo-steady state assumption). In addition, the model considers the solution pH decrease during the process as mineral acids and carbon dioxide are formed. The model is tested by predicting the destruction of a probe compound, 1,2-dibromo-3-chloropropane (DBCP) in distilled water with the addition of inorganic carbon. The new model developed in this work gives better predictions of the destruction of the target organic compound than the model based on the pseudo-steady state assumption. The model provides a comprehensive understanding of the impact of design and operational variables on process performance. Accordingly the ability of the model to select optimum process variables, such as hydrogen peroxide dosage, is illustrated.

Prediction of multicomponent adsorption equilibria using ideal adsorbed solution theory

Test de cette theorie pour la prevision des interactions entre les composes suivants: chloroforme, bromoforme, trichloroethylene, tetrachloroethylene, 1-2 dibromoethane et chlorodibromomethane

Oxidation of organics in retentates from reverse osmosis wastewater reuse facilities.

The use of membrane processes for wastewater treatment and reuse is rapidly expanding. Organic, inorganic, and biological constituents are effectively removed by reverse osmosis (RO) membrane processes, but concentrate in membrane retentates Disposal of membrane concentrates is a growing concern. Applying advanced oxidation processes (AOPs) to RO retentate is logical because extensive treatment and energy inputs were expended to concentrate the organics, and it is cheaper to treat smaller flowstreams. AOPs (e.g., UV irradiation in the presence of titanium dioxide; UV/TiO(2)) can remove a high percentage of organic matter from RO retentates. The combination of AOPs and a simple biological system (e.g., sand filter) can remove higher levels of organic matter at lower UV dosages because AOPs produce biologically degradable material (e.g., organic acids) that have low hydroxyl radical rate constants, meaning that their oxidation, rather than that of the primary organic matter in the RO retentate, dictates the required UV energy inputs. At the highest applied UV dose (10 kWh m(-)3), the dissolved organic carbon (DOC) in the RO retentate decreased from approximately 40 to 8 mg L(-)1, of which approximately 6 mg L(-)1 were readily biologically degradable. Therefore, after combined UV treatment and biodegradation, the final DOC concentration was 2 mg L(-)1, representing a 91% removal. These results suggest that UV/TiO(2) plus biodegradation of RO retentates is feasible and would significantly reduce the organic pollutant loading into the environment from wastewater reuse facilities.

Influence of titanium dioxide nanoparticles on speciation and bioavailability of arsenite.

In this study, the influence of the co-existence of TiO(2) nanoparticles on the speciation of arsenite [As(III)] was studied by observing its adsorption and valence changing. Moreover, the influence of TiO(2) nanoparticles on the bioavailability of As(III) was examined by bioaccumulation test using carp (Cyprinus carpio). The results showed that TiO(2) nanoparticles have a significant adsorption capacity for As (III). Equilibrium was established within 30 min, with about 30% of the initial As (III) being adsorbed onto TiO(2) nanoparticles. Most of aqueous As (III) was oxidized to As(V) in the presence of TiO(2) nanoparticles under sunlight. The carp accumulated considerably more As in the presence of TiO(2) nanoparticles than in the absence of TiO(2) nanoparticles, and after 25-day exposure, As concentration in carp increased by 44%. Accumulation of As in viscera, gills and muscle of the carp was significantly enhanced by the presence of TiO(2) nanoparticles.

Prediction of multicomponent adsorption equilibria in background mixtures of unknown composition

Abstract In the past, most predictive mathematical modeling work has focused on mixtures of known composition. Unfortunately, drinking water and wastewater contain many competing organic solutes and most of them may never be identified. Accordingly, a technique has been developed to predict the adsorption equilibria of known organic solutes onto granular activated carbon (GAC) in mixtures of unknown composition. Ideal adsorbed solution theory (IAST) was used to describe the competitive interactions between adsorbates. Theoretical components (TCs) were used in IAST calculations to account for the competitive effects of the unknown mixture. The TC isotherm parameters and concentrations were determined by conducting a multicomponent isotherm of a tracer component which is added to the unknown mixture or singled out of the unknown mixture. Once the TC parameters were determined, the identical parameters were used to predict the competitive interactions between any known component and the unknown mixture. This procedure was verified experimentally for two activated carbons, three synthetic mixtures and a contaminated groundwater. The organic solutes were halogenated one and two carbon aliphatics which are common groundwater contaminants.

Attachment efficiency of nanoparticle aggregation in aqueous dispersions: modeling and experimental validation.

To describe the aggregation kinetics of nanoparticles (NPs) in aqueous dispersions, a new equation for predicting the attachment efficiency is presented. The rationale is that at nanoscale, random kinetic motion may supersede the role of interaction energy in governing the aggregation kinetics of NPs, and aggregation could occur exclusively among the fraction of NPs with the minimum kinetic energy that exceeds the interaction energy barrier (E(b)). To justify this rationale, we examined the evolution of particle size distribution (PSD) and frequency distribution during aggregation, and further derived the new equation of attachment efficiency on the basis of the Maxwell-Boltzmann distribution and Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The new equation was evaluated through aggregation experiments with CeO(2) NPs using time-resolved-dynamic light scattering (TR-DLS). Our results show that the prediction of the attachment efficiencies agreed remarkably well with experimental data and also correctly described the effects of ionic strength, natural organic matter (NOM), and temperature on attachment efficiency. Furthermore, the new equation was used to describe the attachment efficiencies of different types of engineered NPs selected from the literature and most of the fits showed good agreement with the inverse stability ratios (1/W) and experimentally derived results, although some minor discrepancies were present. Overall, the new equation provides an alternative theoretical approach in addition to 1/W for predicting attachment efficiency.

THE INFLUENCE OF MASS TRANSFER ON SOLUTE TRANSPORT IN COLUMN EXPERIMENTS WITH AN AGGREGATED SOIL

Abstract The spreading of concentration fronts in dynamic column experiments conducted with a porous, aggregated soil is analyzed by means of a previously documented transport model (DFPSDM) that accounts for longitudinal dispersion, external mass transfer in the boundary layer surrounding the aggregate particles, and diffusion in the intra-aggregate pores. The data are drawn from a previous report on the transport of tritiated water, chloride, and calcium ion in a column filled with Ione soil having an average aggregate particle diameter of 0.34 cm, at pore water velocities from 3 to 143 cm/h. The parameters for dispersion, external mass transfer, and internal diffusion were predicted for the experimental conditions by means of generalized correlations, independent of the column data. The predicted degree of solute front-spreading agreed well with the experimental observations. Consistent with the aggregate porosity of 45%, the tortuosity factor for internal pore diffusion was approximately equal to 2. Quantitative criteria for the spreading influence of the three mechanisms are evaluated with respect to the column data. Hydrodynamic dispersion is thought to have governed the front shape in the experiments at low velocity, and internal pore diffusion is believed to have dominated at high velocity; the external mass transfer resistance played a minor role under all conditions. A transport model such as DFPSDM is useful for interpreting column data with regard to the mechanisms controlling concentration front dynamics, but care must be exercised to avoid confounding the effects of the relevant processes.

A comparison of pilot-scale photocatalysis and enhanced coagulation for disinfection byproduct mitigation

This study evaluated pilot-scale photocatalysis and enhanced coagulation for their ability to remove or destroy disinfection byproduct (DBP) precursors, trihalomethane (THM) formation potential (FP), and THMs in two Arizona surface waters. Limited photocatalysis (<5 kWh/m(3)) achieved reductions in most of the DBP precursor parameters (e.g., DOC, UV(254), and bromide) but led to increased chlorine demand and THMFP. In contrast, enhanced coagulation achieved reductions in the DBP precursors and THMFP. Extended photocatalysis (<320 kWh/m(3)) decreased THMFP once the energy consumption exceeded 20 kWh/m(3). The photocatalytic energy requirements for THM destruction were considerably lower (EEO=20-60 kWh/m(3)) than when focusing on precursor destruction and THMFP. However, rechlorination increased the total THM (TTHM) concentration well beyond the raw value, thereby negating the energy benefits of this application. Enhanced coagulation achieved consistent 20-30% removals of preformed THMs. Outstanding issues need to be addressed before TiO(2) photocatalysis is considered feasible for DBP mitigation; traditional strategies, including enhanced coagulation, may be more appropriate.