Fecal Indicator Bacteria Transport and Deposition in Saturated and Unsaturated Porous Media

Beach sediment and sand are recognized as nonpoint fecal indicator bacteria (FIB) sources capable of causing water quality and health risks for beach-goers. A comprehensive understanding of the key factors and mechanisms governing the migration and exchange of FIB between beach water column and sediment is desired to better predict FIB concentration variations and assess the associated risk. The transport and retention behavior of two model FIB <i>Enterococcus faecalis</i> (<i>E. faecalis</i>) and <i>Escherichia coli</i> (<i>E. coli</i>) was examined using packed-bed columns in both saturated and unsaturated porous media to evaluate FIB migration potentials at conditions simulating the coastal aquatic environment. Additionally, complementary cell characterization techniques were conducted to better understand the migration behaviors of both FIB strains observed in the column experiments. The mobility of the gram-positive species <i>E. faecalis</i> was much more sensitive to solution chemistry and column saturation level than that of the gram-negative species <i>E. coli</i>. Interaction energy calculations suggest that <i>E. faecalis</i> retention was largely governed by the combination of DLVO (Derjaguin–Landau–Verwey–Overbeek) and non-DLVO (most likely hydrophobic and/or polymer bridging) interactions in saturated porous media, while the combination of DLVO and steric interactions controlled the deposition of <i>E. coli</i> cells. The measured surface properties of the two FIB strains supported the distinct bacteria transport behaviors and the differences of the identified mechanisms for each strain. As a result, <i>E. faecalis</i> showed the least affinity to sand in freshwater and appeared to be irreversibly attached in primary energy minima at elevated salt conditions; whereas the retained <i>E. coli</i> cells were reversibly attached and mostly associated with the secondary energy minima at both freshwater and seawater conditions. In unsaturated porous media, <i>E. faecalis</i> cells seemed to prefer to attachment at air/water interface rather than sand surface, while <i>E. coli</i> showed a similar affinity to the two interfaces. It was proposed that the different surface characteristics of the two FIB strains resulted in the distinct transport and retention behavior in porous media. These results highlight the need for FIB management to consider variations in transport behavior between model FIB when assessing water quality and associated risks

Distinct Effects of Humic Acid on Transport and Retention of TiO₂ Rutile Nanoparticles in Saturated Sand Columns

The distinct effects of humic acid (HA, 0–10 mg L^–1) on the transport of titanium dioxide (rutile) nanoparticles (nTiO₂) through saturated sand columns were observed under conditions of environmental relevance (ionic strength 3–200 mM NaCl, pH 5.7 and 9.0). Specifically, the transport of nTiO₂ was dramatically enhanced in the presence of HA at pH 5.7, even at a low HA concentration of 1 mg L^–1. The mobility of nTiO₂ was further increased with greater concentrations of HA. In contrast, this enhancement of the nTiO₂ transportability due to the presence of HA was limited at pH 9.0 because of the negligible adsorption of HA onto nTiO₂, regardless of the concentrations of HA examined in this study. The distinct effects can be explained by the adsorption behaviors of HA to nTiO₂ and sand surfaces and the resulting interactions between nTiO₂ and sand surfaces under different conditions, which resulted in a large variation of the nTiO₂ transport and deposition behaviors at various conditions. In addition, theoretical interaction energy calculations and additional elution experiments indicate that the secondary energy minimum played an important role in controlling the nTiO₂ transport and deposition in porous media observed in this study. Moreover, the interaction energy calculations suggest that at pH 5.7, HA affected nTiO₂ transport by increasing the negative surface charge of nTiO₂ at low HA adsorption densities; whereas, combinations of increased electrostatic and steric interactions due to the presence of HA were the main mechanisms of enhanced transportability of nTiO₂ at high HA adsorption densities. Overall, results from this study suggest that natural organic matter and solution pH are likely key factors that govern the stability and mobility of nTiO₂ in the natural aquatic environment

Transport and Retention of TiO₂ Rutile Nanoparticles in Saturated Porous Media under Low-Ionic-Strength Conditions: Measurements and Mechanisms

The mechanisms governing the transport and retention kinetics of titanium dioxide (TiO2, rutile) nanoparticle (NP) aggregates were investigated in saturated porous media. Experiments were carried out under a range of well-controlled ionic strength (from DI water up to 1 mM) and ion valence (NaCl vs CaCl2) comparable to the low end of environmentally relevant solution chemistry conditions. Solution chemistry was found to have a marked effect on the electrokinetic properties of NP aggregates and the sand and on the resulting extent of NP aggregate transport and retention in the porous media. Comparable transport and retention patterns were observed for NP aggregates in both NaCl and CaCl2 solutions but at much lower ionic strength with CaCl2. Transport experimental results showed temporal and spatial variations of NP aggregate deposition in the column. Specifically, the breakthrough curves displayed a transition from blocking to ripening shapes, and the NP retention profiles exhibited a shift of the maximum NP retention segment from the end toward the entrance of the column gradually with increasing ionic strength. Additionally, the deposition rates of the NP aggregates in both KCl and CaCl2 solutions increased with ionic strength, a trend consistent with traditional Derjaguin−Landau−Verwey−Overbeek (DLVO) theory. Upon close examination of the results, it was found that the characteristics of the obtained transport breakthrough curves closely followed the general trends predicted by the DLVO interaction-energy calculations. However, the obtained NP retention profiles were found to deviate severely from the theory. We propose that a NP aggregate reconformation through collision between NP aggregates and sand grains reduced the repulsive interaction energies of NP−NP and NP−sand surfaces, consequently accelerating NP deposition with transport distance and facilitating approaching NP deposition onto NPs that had already been deposited. It is further suggested that TiO2 NP transport and retention are determined by the combined influence of NP aggregate reconformation associated with solution chemistry, travel distance, and DLVO interactions of the system

Influence of Collector Surface Composition and Water Chemistry on the Deposition of Cerium Dioxide Nanoparticles: QCM-D and Column Experiment Approaches

The deposition behavior of cerium dioxide (CeO₂) nanoparticles (NPs) in dilute NaCl solutions was investigated as a function of collector surface composition, pH, ionic strength, and organic matter (OM). Sensors coated separately with silica, iron oxide, and alumina were applied in quartz crystal microbalance with dissipation (QCM-D) to examine the effect of these mineral phases on CeO₂ deposition in NaCl solution (1–200 mM). Frequency and dissipation shift followed the order: silica > iron oxide > alumina in 10 mM NaCl at pH 4.0. No significant deposition was observed at pH 6.0 and 8.5 on any of the tested sensors. However, ≥ 94.3% of CeO₂ NPs deposited onto Ottawa sand in columns in 10 mM NaCl at pH 6.0 and 8.5. The inconsistency in the different experimental approaches can be mainly attributed to NP aggregation, surface heterogeneity of Ottawa sand, and flow geometry. In QCM-D experiments, the deposition kinetics was found to be qualitatively consistent with the predictions based on the classical colloidal stability theory. The presence of low levels (1–6 mg/L) of Suwannee River humic acid, fulvic acid, alginate, citric acid, and carboxymethyl cellulose greatly enhanced the stability and mobility of CeO₂ NPs in 1 mM NaCl at pH 6.5. The poor correlation between the transport behavior and electrophoretic mobility of CeO₂ NPs implies that the electrosteric effect of OM was involved

Transport and Retention of Colloids in Porous Media: Does Shape Really Matter?

The effect of particle shape on its transport and retention in porous media was evaluated by stretching carboxylate-modified fluorescent polystyrene spheres into rod shapes with aspect ratios of 2:1 and 4:1. Quartz crystal microbalance with dissipation (QCM-D) experiments were conducted to measure the deposition rates of spherical and rod-shaped nanoparticles to the collector (poly-l-lysine coated silica sensor) surface under favorable conditions. The spherical particles displayed a significantly higher deposition rate compared with that of the rod-shaped particles. Theoretical analysis based on Smoluchowski–Levich approximation indicated that the rod-shaped particles largely counterbalance the attractive energies due to higher hydrodynamic forces and torques experienced during their transport and rotation. Under unfavorable conditions, the retention of nanoparticles in a microfluidic flow cell packed with glass beads was studied with the use of laser scanning cytometry (LSC). Significantly more attachment was observed for rod-shaped particles than spherical particles, and the attachment rate of the rod-shaped particles showed an increasing trend with the increase in injection volume. Rod-shaped particles were found to be less sensitive to the surface charge heterogeneity change than spherical particles. Increased attachment rate of rod-shaped particles was attributed to surface heterogeneity and possibly enhanced hydrophobicity during the stretching process

Heterogeneous Esterification from α‑Hydroxy Ketone and Alcohols through a Tandem Oxidation Process over a Hydrotalcite-Supported Bimetallic Catalyst

Heterogeneous aerobic oxidative esterification between α-hydroxy ketone and alcohols catalyzed by a hydrotalcite-supported bimetallic catalyst (CuMn/HT) using O2 as a green oxidant was achieved. Recyclable CuMn/HT exhibits high catalytic activity due to increased content of oxygen vacancies and a newly generated CuMn2O4 crystal phase. This clean esterification proceeds through a tandem oxidation process in the absence of any additives and ligands and affords α-keto esters in good to excellent yields. Moreover, the catalytic system tolerates complicated bioactive molecules as raw materials and can be performed on a multigram scale