Silicate surface coverage controls quinolone transport in saturated porous media

Nalidixic Acid and Fe(II)/Cu(II) Coadsorption at Goethite and Akaganéite Surfaces

Interactions between aqueous Fe(II) and solid Fe(III) oxy(hydr)oxide surfaces play determining roles in the fate of organic contaminants in nature. In this study, the adsorption of nalidixic acid (NA), a representative redox-inactive quinolone antibiotic, on synthetic goethite (α-FeOOH) and akaganéite (β-FeOOH) was examined under varying conditions of pH and cation type and concentration, by means of adsorption experiments, attenuated total reflectance-Fourier transform infrared spectroscopy, surface complexation modeling (SCM), and powder X-ray diffraction. Batch adsorption experiments showed that Fe(II) had marginal effects on NA adsorption onto akaganéite but enhanced NA adsorption on goethite. This enhancement is attributed to the formation of goethite-Fe(II)-NA ternary complexes, without the need for heterogeneous Fe(II)-Fe(III) electron transfer at low Fe(II) loadings (2 Fe/nm2), as confirmed by SCM. However, higher Fe(II) loadings required a goethite-magnetite composite in the SCM to explain Fe(II)-driven recrystallization and its impact on NA binding. The use of a surface ternary complex by SCM was supported further in experiments involving Cu(II), a prevalent environmental metal incapable of transforming Fe(III) oxy(hydr)oxides, which was observed to enhance NA loadings on goethite. However, Cu(II)-NA aqueous complexation and potential Cu(OH)2 precipitates counteracted the formation of ternary surface complexes, leading to decreased NA loadings on akaganéite. These results have direct implications for the fate of organic contaminants, especially those at oxic-anoxic boundaries.

Biosurfactant-affected mobility of oxytetracycline and its variations with surface chemical heterogeneity in saturated porous media

Herein, the influences of rhamnolipid (a typical biosurfactant) on oxytetracycline (OTC) transport in the porous media and their variations with the surface heterogeneities of the media (uncoated sand, goethite (Goe)-, and humic acid (HA)-coated sands) were explored. Compared to uncoated sand, goethite and HA coatings suppressed OTC mobility by increasing deposition sites. Interestingly, rhamnolipid-affected OTC transport strongly depended on the chemical heterogeneities of aquifers and biosurfactant concentrations. Concretely, adding rhamnolipid (1-3 mg/L) inhibited OTC mobility through sand columns because of the bridging effect of biosurfactant between sand and OTC. Unexpectedly, rhamnolipid of 10 mg/L did not further improve the inhibition of OTC transport owing to the fact that the deposition capacity of rhamnolipid reached its maximum. OTC mobility in Goe-coated sand columns was inhibited by 1 mg/L rhamnolipid. However, the inhibitory effect decreased with the increasing rhamnolipid concentration (3 mg/L) and exhibited a promoted effect at 10 mg/L rhamnolipid. This surprising observation was that the increased rhamnolipid molecules gradually occupied the favorable deposition sites (i.e., the positively charged sites). In comparison, rhamnolipid facilitated OTC transport in the HA-coated sand column. The promotion effects positively correlated with rhamnolipid concentrations because of the high electrostatic repulsion and deposition site competition induced by the deposited rhamnolipid. Another interesting phenomenon was that rhamnolipid's enhanced or inhibitory effects on OTC transport declined with the increasing solution pH because of the decreased rhamnolipid deposition on porous media surfaces. These findings benefit our understanding of the environmental behaviors of antibiotics in complex soil-water systems containing biosurfactants.

Mineral Nanoparticle Aggregation Alters Contaminant Transport under Flow

Iron oxyhydroxide nanoparticle reactivity has been widely investigated, yet little is still known on how particle aggregation controls the mobility and transport of environmental compounds. Here, we examine how aggregates of goethite (α-FeOOH) nanoparticle deposited on 100-300 μm quartz particles (G_agCS) alter the transport of two emerging contaminants and two naturally occurring inorganic ligands-silicates and phosphates. Bromide tracer experiments showed no water fractionation into mobile and immobile water zones in an individual goethite-coated sand (GCS) column, whereas around 10% of the total water was immobile in a G_agCS column. Reactive compounds were, in contrast, considerably more mobile and affected by diffusion-limited processes. A new simulation approach coupling the mobile-immobile equation with surface complexation reactions to surface reactive sites suggests that ∼90% of the binding sites were likely within the intra-aggregate zones, and that the mass transfer between mobile and immobile fractions was the rate-limited step. The diffusion-controlled processes also affected synergetic and competitive binding, which have otherwise been observed for organic and inorganic compounds at goethite surfaces. These results thereby call for more attention on transport studies, where tracer or conservative tests are often used to describe the reactive transport of environmentally relevant molecules.

Fate and Transport of Pharmaceuticals in Iron and Manganese Binary Oxide Coated Sand Columns

Predicting the fate and transport of pharmaceuticals in terrestrial environments requires knowledge of their interactions with complex mineral assemblages. To advance knowledge along this front, we examined the reactivity of pipemidic acid (PIP), a typical quinolone antibiotic, with quartz particles coated with a mixture of manganese oxide (MnO₂) and goethite (α-FeOOH) under static and dynamic flow conditions. Batch and dynamic column experiments showed that PIP binding to MnO₂ proceeded through a heterogeneous redox reaction, while binding to goethite was not redox-reactive. Mixed columns of aggregated goethite-manganese particles however enhanced redox reactivity because (i) goethite facilitated the transport of dissolved Mn(II) ion and increased the retention of PIP oxidation products, and (ii) MnO₂ was protected from passivation. This mobility behavior was predicted using transport models accounting for adsorption and transformation kinetics of PIP on both goethite and MnO₂. This work sheds new light on reactivity changes of mixtures of Fe and Mn oxides under flow-through conditions and will have important implications in predicting the fate and transport of redox-active organic compounds as well as development of new geomedia filters for environmental remediation.

Coupling Molecular-Scale Spectroscopy with Stable Isotope Analyses to Investigate the Effect of Si on the Mechanisms of Zn-Al LDH Formation on Al Oxide

While silicate has been known to affect metal sorption on mineral surfaces, the mechanisms remain poorly understood. We investigated the effects of silicate on Zn sorption onto Al oxide at pH 7.5 and elucidated the mechanisms using a combination of X-ray absorption fine structure (XAFS) spectroscopy, Zn stable isotope analysis, and scanning transmission electron microscopy (STEM). XAFS analysis revealed that Zn-Al layered double hydroxide (LDH) precipitates were formed in the absence of silicate or at low Si concentrations (≤0.4 mM), whereas the formation of Zn-Al LDH was inhibited at high silicate concentrations (≥0.64 mM) due to surface-induced Si oligomerization. Significant Zn isotope fractionation (Δ⁶⁶Zn_{sorbed-aqueous} = 0.63 ± 0.03‰) was determined at silicate concentrations ≥0.64 mM, larger than that induced by sorption of Zn on Al oxide (0.47 ± 0.03‰) but closer to that caused by Zn bonding to the surface of Si oxides (0.60-0.94‰), suggesting a presence of Zn-Si bonding environment. STEM showed that the sorbed silicates had a close spatial coupling with γ-Al₂O₃, indicating that >Si-Zn inner-sphere complexes (">" denotes surface) likely bond to the γ-Al₂O₃ surface to form >Al-Si-Zn ternary inner-sphere complexes. This study not only demonstrates that dissolved silicate in the natural environment plays an important role in the fate and bioavailability of Zn but also highlights the potential of coupled spectroscopic and isotopic methods in probing complex environmental processes.

Transport of tetracycline in saturated porous media: combined functions of inorganic ligands and solution pH

To date, there is still very little knowledge about the combined effects of typical inorganic ligands and solution pH values on mobility characteristics of tetracycline (TC) through saturated aquifer media. In this work, three typical inorganic ligands (i.e., phosphate, silicate, and iodate) were employed in the transport experiments. Generally, all the ligands promoted TC mobility over the pH range of 5.0-9.0 owing to the enhanced electrostatic repulsion between sand grains and TC anionic forms (i.e., TC^- and TC^2-) as well as the competitive deposition between ligands and antibiotic molecules for attachment sites. Furthermore, the transport-enhancement effects of ligands on TC intensively depended on ligand type and followed the sequence of phosphate > silicate > iodate. This phenomenon was ascribed to their different molecular sizes and binding abilities to sand grains. Interestingly, the differences in extents of enhanced effects of various inorganic ligands on TC transport varied with background solution pH due to pH-induced different extents of deposition site competition effects. Moreover, the two-site nonequilibrium model (which accounts for an equilibrium site and a kinetic site) as well as adsorption and kinetic studies were performed to help interpret the controlling mechanisms for the synergistic effects of inorganic ligands and solution pH on TC transport in saturated quartz sand. The findings of our study clearly demonstrate that inorganic ligands may be critical factors in assessing the fate and transport of antibiotics in groundwater systems.

Interactions of Anti-Inflammatory and Antibiotic Drugs at Mineral Surfaces Can Control Environmental Fate and Transport

Various pharmaceutical compounds often coexist in contaminated soils, yet little is known about how their interactions impact their mobility. We here show that two typical antibiotic and anti-inflammatory agents (nalidixic acid (NA) and niflumic acid (NFA)) commonly form dimers at several representative soil- and sediment-building minerals of contrasting composition and structure. Cobinding occurs in the form of a NFA-NA dimer stabilized by hydrogen bonding and van der Waals interactions. Using dynamic column experiments containing goethite-coated sand, we then demonstrated that presorbed NA effectively captured the otherwise weakly binding NFA from solution. Simultaneously injecting NA and NFA to presorbed NA enhanced even further both NA and NFA loadings, thereby altering their transport under flow-through conditions. We also showed that environmental level amounts of natural organic matter can reduce the overall retention in column experiments, yet it does not suppress dimer formation. These environmentally relevant scenarios can be predicted using a new transport model that accounts for kinetics and cobinding reactions of NFA onto NA bound to goethite through metal-bonded, hydrogen-bonded, and outer-sphere complexes. These findings have important implications on assessing the fate of coexisting pharmaceutical compounds under dynamic flow conditions in contaminated soils.

Competitive Carboxylate-Silicate Binding at Iron Oxyhydroxide Surfaces

Dissolved silicate ions in wet and dry soils can determine the fate of organic contaminants via competitive binding. While fundamental surface science studies have advanced knowledge of binding in competitive systems, little is still known about the ranges of solution conditions, the time dependence, and the molecular processes controlling competitive silicate-organic binding on minerals. Here we address these issues by describing the competitive adsorption of dissolved silicate and of phthalic acid (PA), a model carboxylate-bearing organic contaminant, onto goethite, a representative natural iron oxyhydroxide nanomineral. Using surface complexation thermodynamic modeling of batch adsorption data and chemometric analyses of vibrational spectra, we find that silicate concentrations representative of natural waters (50-1000 μM) can displace PA bound at goethite surfaces. Below pH ∼8, where PA binds, every bound Si atom removes ∼0.3 PA molecule by competing with reactive singly coordinated hydroxo groups (-OH) on goethite. Long-term (30 days) reaction time and a high silicate concentration (1000 μM) favored silicate polymer formation, and increased silicate while decreasing PA loadings. The multisite complexation model predicted PA and silicate binding in terms of the competition for -OH groups without involving PA/silicate interactions, and in terms of a lowering of outer-Helmholtz potentials of the goethite surface by these anions. The model predicted that silicate binding lowered loadings of PA species, and whose two carboxylate groups are hydrogen- (HB) and metal-bonded (MB) with goethite. Vibrational spectra of dried samples revealed that the loss of water favored greater proportions of MB over HB species, and these coexisted with predominantly monomeric silicate species. These findings underscored the need to develop models for a wider range of organic contaminants in soils exposed to silicate species and undergoing wet-dry cycles.