David M. Cwiertny

Adsorption of Organic Acids on TiO2 Nanoparticles: Effects of pH, Nanoparticle Size, and Nanoparticle Aggregation

In this study, the adsorption of two organic acids, oxalic acid and adipic acid, on TiO2 nanoparticles was investigated at room temperature, 298 K. Solution-phase measurements were used to quantify the extent and reversibility of oxalic acid and adipic acid adsorption on anatase nanoparticles with primary particle sizes of 5 and 32 nm. At all pH values considered, there were minimal differences in measured Langmuir adsorption constants, K ads, or surface-area-normalized maximum adsorbate-surface coverages, Gamma max, between 5 and 32 nm particles. Although macroscopic differences in the reactivity of these organic acids as a function of nanoparticle size were not observed, ATR-FTIR spectroscopy showed some distinct differences in the absorption bands present for oxalic acid adsorbed on 5 nm particles compared to 32 nm particles, suggesting different adsorption sites or a different distribution of adsorption sites for oxalic acid on the 5 nm particles. These results illustrate that molecular-level differences in nanoparticle reactivity can still exist even when macroscopic differences are not observed from solution phase measurements. Our results also allowed the impact of nanoparticle aggregation on acid uptake to be assessed. It is clear that particle aggregation occurs at all pH values and that organic acids can destabilize nanoparticle suspensions. Furthermore, 5 nm particles can form larger aggregates compared to 32 nm particles under the same conditions of pH and solid concentrations. The relative reactivity of 5 and 32 nm particles as determined from Langmuir adsorption parameters did not appear to vary greatly despite differences that occur in nanoparticle aggregation for these two different size nanoparticles. Although this potentially suggests that aggregation does not impact organic acid uptake on anatase particles, these data clearly show that challenges remain in assessing the available surface area for adsorption in nanoparticle aqueous suspensions because of aggregation.

Chemistry and Photochemistry of Mineral Dust Aerosol

It has become increasingly clear that heterogeneous and multiphase chemistry of tropospheric aerosols can change the chemical balance of the atmosphere. In this review, we focus on recent laboratory studies of the heterogeneous and multiphase chemistry and photochemistry of mineral dust aerosol, a large mass fraction of the tropospheric aerosol. Mineral dust aerosol contains a mixture of oxides, clays, and carbonates. Molecular-based studies of reactions of these dust components provide insights into the chemistry of Earths atmosphere. We discuss several different types of heterogeneous and multiphase reactions, including (a) ozone decomposition, (b) nitrogen dioxide and nitrate photochemistry, and (c) the dissolution and redox chemistry of Fe-containing dust. We also review some of the important chemical concepts that have recently emerged.

Combined factors influencing the aggregation and deposition of nano-TiO2 in the presence of humic acid and bacteria.

This study investigates the contributions of natural organic matter (NOM) and bacteria to the aggregation and deposition of TiO(2) nanoparticles (TNPs) in aquatic environments. Transport experiments with TNPs were conducted in a microscopic parallel plate system and a macroscopic packed-bed column using fluorescently tagged E. coli as a model organism and Suwannee River Humic Acid as a representative NOM. Notably, TNPs were labeled with fluorescein isothiocyanate allowing particles and cells to be simultaneously visualized with a fluorescent microscope. Results from both experimental systems revealed that interactions among TNPs, NOM, and bacteria exhibited a significant dependence on solution chemistry (pH 5 and 7) and ion valence (K(+) and Ca(2+)), and that these interactions subsequently affect TNPs deposition. NOM and E. coli significantly reduced deposition of TNPs, with NOM having a greater stabilizing influence than bacteria. Ca(2+) ions played a significant role in these interactions, promoting formation of large clusters of TNPs, NOM, and bacteria. TNPs transport in the presence of both NOM and E. coli resulted in much less deposition than in the presence of NOM or E. coli alone, indicating a complex combination of interactions involved in stabilization. Generally, over the aquatic conditions considered, the extent of TNPs deposition follows: without NOM or bacteria > with bacteria only > with NOM only > combined bacteria and NOM. This trend should allow better prediction of the fate of TNPs in complex aquatic systems.

Influence of Anionic Cosolutes and pH on Nanoscale Zerovalent Iron Longevity: Time Scales and Mechanisms of Reactivity Loss toward 1,1,1,2-Tetrachloroethane and Cr(VI)

Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl(-), SO(4)(2-), and ClO(4)(-) suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO(3)(-) and HCO(3)(-) concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO(3)(-) suspensions. Markedly different trends in Cr(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr(VI) removal capacities and rates of Fe(II) production measured in the absence of Cr(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe(II) species are primarily responsible for Cr(VI) reduction.

Use of Dithionite to Extend the Reactive Lifetime of Nanoscale Zero-Valent Iron Treatment Systems

Nanoscale zero-valent iron (NZVI) represents a promising approach for source zone control, but concerns over its reactive lifetime might limit application. Here, we demonstrate that dithionite (S₂O₄²⁻), a reducing agent for in situ redox manipulation, can restore the reducing capacity of passivated NZVI. Slurries of NZVI were aged in the presence (3 days) and absence (60 days) of dissolved oxygen over a range of pH values (6-8). Upon loss of reactivity toward model pollutants{1,1,1,2-tetrachloroethane, hexavalent chromium [Cr(VI)], nitrobenzene}, aged suspensions were reacted with dithionite, and the composition and reactivity of the dithionite-treated materials were determined. NZVI aging products generally depended on pH and the presence of oxygen, whereas the amount of dithionite influenced the nature and reducing capacity of products generated from reaction with aged NZVI suspensions. Notably, air oxidation at pH ≥ 8 quickly exhausted NZVI reactivity despite preservation of significant Fe(0) in the particle core. Under these conditions, formation of a passive surface layer hindered the complete transformation of NZVI particles into iron(III) oxides, which occurred at lower pH. Reduction of this passive layer by low dithionite concentrations( 1 g/g of NZVI) restored suspension reactivity to levels equal to, and occasionally greater than, that of unaged NZVI. Multiple dithionite additions further improved pollutant removal, allowing at least a 15-fold increase in Cr(VI) removal [∼300 mg of Cr(VI)/g of NZVI] relative to that of as-received NZVI [∼20 mg of Cr(VI)/g of NZVI].

Tailored Synthesis of Photoactive TiO2 Nanofibers and Au/TiO2 Nanofiber Composites: Structure and Reactivity Optimization for Water Treatment Applications

Titanium dioxide (TiO2) nanofibers with tailored structure and composition were synthesized by electrospinning to optimize photocatalytic treatment efficiency. Nanofibers of controlled diameter (30-210 nm), crystal structure (anatase, rutile, mixed phases), and grain size (20-50 nm) were developed along with composite nanofibers with either surface-deposited or bulk-integrated Au nanoparticle cocatalysts. Their reactivity was then examined in batch suspensions toward model (phenol) and emerging (pharmaceuticals, personal care products) pollutants across various water qualities. Optimized TiO2 nanofibers meet or exceed the performance of traditional nanoparticulate photocatalysts (e.g., Aeroxide P25) with the greatest reactivity enhancements arising from (i) decreasing diameter (i.e., increasing surface area), (ii) mixed phase composition [74/26 (±0.5) % anatase/rutile], and (iii) small amounts (1.5 wt %) of surface-deposited, more so than bulk-integrated, Au nanoparticles. Surface Au deposition consistently enhanced photoactivity by 5- to 10-fold across our micropollutant suite independent of their solution concentration, behavior that we attribute to higher photocatalytic efficiency from improved charge separation. However, the practical value of Au/TiO2 nanofibers was limited by their greater degree of inhibition by solution-phase radical scavengers and higher rate of reactivity loss from surface fouling in nonidealized matrixes (e.g., partially treated surface water). Ultimately, unmodified TiO2 nanofibers appear most promising for use as reactive filtration materials because their performance was less influenced by water quality, although future efforts must increase the strength of TiO2 nanofiber mats to realize such applications.

Product-to-Parent Reversion of Trenbolone: Unrecognized Risks for Endocrine Disruption

Return of the Steroid Trace levels of organic contaminants enter aquatic ecosystems from a variety of sources, including runoff of from agricultural lands. When these compounds and their metabolites break down, it is generally assumed that they become inert and pose less ecological risk. Qu et al. (p. 347, published online 26 September) tracked the sunlight-mediated transformation of metabolites of trenbolone acetate (TBA)—a common growth-promoting steroid given to beef cattle—across a number of conditions in the laboratory and in the field. When the degradation products were exposed to dark conditions following photodegradation, they surprisingly reverted back to TBA metabolites, including analog steroidal compounds similar to TBA with unknown biological effects. Phototransformation of growth steroid metabolites is readily reversible in aquatic environments. Trenbolone acetate (TBA) is a high-value steroidal growth promoter often administered to beef cattle, whose metabolites are potent endocrine-disrupting compounds. We performed laboratory and field phototransformation experiments to assess the fate of TBA metabolites and their photoproducts. Unexpectedly, we observed that the rapid photohydration of TBA metabolites is reversible under conditions representative of those in surface waters (pH 7, 25°C). This product-to-parent reversion mechanism results in diurnal cycling and substantial regeneration of TBA metabolites at rates that are strongly temperature- and pH-dependent. Photoproducts can also react to produce structural analogs of TBA metabolites. These reactions also occur in structurally similar steroids, including human pharmaceuticals, which suggests that predictive fate models and regulatory risk assessment paradigms must account for transformation products of high-risk environmental contaminants such as endocrine-disrupting steroids.

Environmental designer drugs: when transformation may not eliminate risk.

Environmental transformation processes, including those occurring in natural and engineered systems, do not necessarily drastically alter molecular structures of bioactive organic contaminants. While the majority of generated transformation products are likely benign, substantial conservation of structure in transformation products can imply conservation or even creation of bioactivity across multiple biological end points and thus incomplete mitigation of ecological risk. Therefore, focusing solely on parent compound removal for contaminants of higher relative risk, the most common approach to fate characterization, provides no mechanistic relationship to potential biological effects and is inadequate as a comprehensive metric for reduction of ecological risks. Here, we explore these phenomena for endocrine-active steroid hormones, focusing on examples of conserved bioactivity and related implications for fate assessment, regulatory approaches, and research opportunities.

Dissolution of Hematite Nanoparticle Aggregates: Influence of Primary Particle Size, Dissolution Mechanism, and Solution pH

The size-dependent dissolution of nanoscale hematite (8 and 40 nm α-Fe(2)O(3)) was examined across a broad range of pH (pH 1-7) and mechanisms including proton- and ligand- (oxalate-) promoted dissolution and dark (ascorbic acid) and photochemical (oxalate) reductive dissolution. Empirical relationships between dissolution rate and pH revealed that suspensions of 8 nm hematite exhibit between 3.3- and 10-fold greater reactivity per unit mass than suspensions of 40 nm particles across all dissolution modes and pH, including circumneutral. Complementary suspension characterization (i.e., sedimentation studies and dynamic light scattering) indicated extensive aggregation, with steady-state aggregate sizes increasing with pH but being roughly equivalent for both primary particles. Thus, while the reactivity difference between 8 and 40 nm suspensions is generally greater than expected from specific surface areas measured via N(2)-BET or estimated from primary particle geometry, loss of reactive surface area during aggregation limits the certainty of such comparisons. We propose that the relative reactivity of 8 and 40 nm hematite suspensions is best explained by differences in the fraction of aggregate surface area that is reactive. This scenario is consistent with TEM images revealing uniform dissolution of aggregated 8 nm particles, whereas 40 nm particles within aggregates undergo preferential etching at edges and structural defects. Ultimately, we show that comparably sized hematite aggregates can exhibit vastly different dissolution activity depending on the nature of the primary nanoparticles from which they are constructed, a result with wide-ranging implications for iron redox cycling.

Hydroxyl Radical Formation during Ozonation of Multiwalled Carbon Nanotubes: Performance Optimization and Demonstration of a Reactive CNT Filter

We explored factors influencing hydroxyl radical (•OH) formation during ozonation of multiwalled carbon nanotubes (MWCNTs) and assessed this systems viability as a next-generation advanced oxidation process (AOP). Using standard reactivity metrics for ozone-based AOPs (RCT values), MWCNTs promoted •OH formation during ozonation to levels exceeding ozone (both alone and with activated carbon) and equivalent to ozone with hydrogen peroxide. MWCNTs oxidized with nitric acid exhibited vastly greater rates of ozone consumption and •OH formation relative to as-received MWCNTs. While some of this enhancement reflects their greater suspension stability, a strong correlation between RCT values and surface oxygen concentrations from X-ray photoelectron spectroscopy suggests that surface sites generated during MWCNT oxidation promote •OH exposure. Removal of several ozone-recalcitrant species [para-chlorobenzoic acid (p-CBA), atrazine, DEET, and ibuprofen] was not significantly inhibited in the presence of radical scavengers (humic acid, carbonate), in complex aquatic matrices (Iowa River water) and after 12 h of continuous exposure of MWCNTs to concentrated ozone solutions. As a proof-of-concept, oxidized MWCNTs deposited on a ceramic membrane chemically oxidized p-CBA in a flow through system, with removal increasing with influent ozone concentration and mass of deposited MWCNTs (in mg/cm2). This hybrid membrane platform, which integrates adsorption, oxidation, and filtration via an immobilized MWCNT layer, may serve as the basis for future novel nanomaterial-enabled technologies, although long-term performance trials under representative treatment scenarios remain necessary.