A Critical Look at Colloid Generation, Stability, and Transport in Redox-Dynamic Environments: Challenges and Perspectives.

Colloid generation, stability, and transport are important processes that can significantly influence the fate and transport of nutrients and contaminants in environmental systems. Here, we critically review the existing literature on colloids in redox-dynamic environments and summarize the current state of knowledge regarding the mechanisms of colloid generation and the chemical controls over colloidal behavior in such environments. We also identify critical gaps, such as the lack of universally accepted cross-discipline definition and modeling infrastructure that hamper an in-depth understanding of colloid generation, behavior, and transport potential. We propose to go beyond a size-based operational definition of colloids and consider the functional differences between colloids and dissolved species. We argue that to predict colloidal transport in redox-dynamic environments, more empirical data are needed to parametrize and validate models. We propose that colloids are critical components of element budgets in redox-dynamic systems and must urgently be considered in field as well as lab experiments and reactive transport models. We intend to bring further clarity and openness in reporting colloidal measurements and fate to improve consistency. Additionally, we suggest a methodological toolbox for examining impacts of redox dynamics on colloids in field and lab experiments.

Structure and composition of natural ferrihydrite nano-colloids in anoxic groundwater.

Fe-rich mobile colloids play vital yet poorly understood roles in the biogeochemical cycling of Fe in groundwater by influencing organic matter (OM) preservation and fluxes of Fe, OM, and other essential (micro-)nutrients. Yet, few studies have provided molecular detail on the structures and compositions of Fe-rich mobile colloids and factors controlling their persistence in natural groundwater. Here, we provide comprehensive new information on the sizes, molecular structures, and compositions of Fe-rich mobile colloids that accounted for up to 72% of aqueous Fe in anoxic groundwater from a redox-active floodplain. The mobile colloids are multi-phase assemblages consisting of Si-coated ferrihydrite nanoparticles and Fe(II)-OM complexes. Ferrihydrite nanoparticles persisted under both oxic and anoxic conditions, which we attribute to passivation by Si and OM. These findings suggest that mobile Fe-rich colloids generated in floodplains can persist during transport through redox-variable soils and could be discharged to surface waters. These results shed new light on their potential to transport Fe, OM, and nutrients across terrestrial-aquatic interfaces.

Nitrate Controls on the Extent and Type of Metal Retention in Fine-Grained Sediments of a Simulated Aquifer.

Aquifer groundwater quality is largely controlled by sediment composition and physical heterogeneity, which commonly sustains a unique redox gradient pattern. Attenuation of heavy metals within these heterogeneous aquifers is reliant on multiple factors, including redox conditions and redox-active species that can further influence biogeochemical cycling. Here, we simulated an alluvial aquifer system using columns filled with natural coarse-grained sediments and two domains of fine-grained sediment lenses. Our goal was to examine heavy metal (Ni and Zn) attenuation within a complex aquifer network and further explore nitrate-rich groundwater conditions. The fine-grained sediment lenses sustained reducing conditions and served as a sink for Ni sequestration─in the form of Ni-silicates, Ni-organic matter, and a dominant Ni-sulfide phase. The silicate clay and sulfide pools were also important retention mechanisms for Zn; however, Ni was associated more extensively with organic matter compared to Zn, which formed layered double hydroxides. Nitrate-rich conditions promoted denitrification within the lenses that was coupled to the oxidation of Fe(II) and the concomitant precipitation of an Fe(III) phase with higher structural distortion. A decreased metal sulfide pool also resulted, where nitrate-rich conditions generated an average 20% decrease in solid-phase Ni, Zn, and Fe. Ultimately, nitrate plays a significant role in the aquifer's biogeochemical cycling and the capacity to retain heavy metals.

Export of Organic Carbon from Reduced Fine-Grained Zones Governs Biogeochemical Reactivity in a Simulated Aquifer.

Sediment interfaces in alluvial aquifers have a disproportionately large influence on biogeochemical activity and, therefore, on groundwater quality. Previous work showed that exports from fine-grained, organic-rich zones sustain reducing conditions in downstream coarse-grained aquifers beyond the influence of reduced aqueous products alone. Here, we show that sustained anaerobic activity can be attributed to the export of organic carbon, including live microorganisms, from fine-grained zones. We used a dual-domain column system with ferrihydrite-coated sand and embedded reduced, fine-grained lenses from Slate River (Crested Butte, CO) and Wind River (Riverton, WY) floodplains. After 50 d of groundwater flow, 8.8 ± 0.7% and 14.8 ± 3.1% of the total organic carbon exported from the Slate and Wind River lenses, respectively, had accumulated in the sand downstream. Furthermore, higher concentrations of dissolved Fe(II) and lower concentrations of dissolved organic carbon in the sand compared to total aqueous transport from the lenses suggest that Fe(II) was produced in situ by microbial oxidation of organic carbon coupled to iron reduction. This was further supported by an elevated abundance of 16S rRNA and iron-reducing (gltA) gene copies. These findings suggest that organic carbon transport across interfaces contributes to downstream biogeochemical reactions in natural alluvial aquifers.

Simulated Aquifer Heterogeneity Leads to Enhanced Attenuation and Multiple Retention Processes of Zinc.

Alluvial aquifers serve as one of the main water sources for domestic, agricultural, and industrial purposes globally. Groundwater quality, however, can be threatened by naturally occurring and anthropogenic metal contaminants. Differing hydrologic and biogeochemical conditions between predominantly coarse-grained aquifer sediments and embedded layers or lenses of fine-grained materials lead to variation in metal behavior. Here, we examine processes controlling Zn partitioning within a dual-pore domain-reconstructed alluvial aquifer. Natural coarse aquifer sediments from the Wind River-Little Wind River floodplain near Riverton, WY, were used in columns with or without fine-grained lenses to examine biogeochemical controls on Zn concentrations, retention mechanisms, and transport. Following the introduction of Zn to the groundwater source, Zn preferentially accumulated in the fine-grained lenses, despite their small volumetric contributions. While the clay fraction dominated Zn retention in the sandy aquifer, the lenses supported additional reaction pathways of retention-the reducing conditions within the lenses resulted in ZnS precipitation, overriding the contribution of organic matter. Zinc concentration in the groundwater controlled the formation of Zn-clays and Zn-layered double hydroxides, whereas the extent of sulfide production controlled precipitation of ZnS. Our findings illustrate how both spatial and compositional heterogeneities govern the extent and mechanisms of Zn retention in intricate groundwater systems, with implications for plume behavior and groundwater quality.

Organic compounds alter the preference and rates of heavy metal adsorption on ferrihydrite.

The availability of heavy metals in terrestrial environments is largely controlled by their interactions with minerals and organic matter, with iron minerals having a particularly strong role in heavy metal fate. Because soil organic matter contains a variety of compounds that differ in their chemical properties, the underlying impact organic matter-soil mineral associations bestow on heavy metal binding is still unresolved. Here, we systematically examine the binding of Cd, Zn and Ni by a suite of organic-ferrihydrite assemblages, chosen to account for various compound chemistries within soil organic matter. We posited that organic compound functionality would dictate the extent of association with the organic-ferrihydrite assemblages. Increased heavy metal binding to the assemblages was observed and attributed to the introduction of additional binding sites by the organic functional groups with differing metal affinities. The relative increase depended on the metal's Lewis acidity and followed the order Cd > Zn > Ni, whereas the reverse order was obtained for metal binding by pristine ferrihydrite (Ni > Zn > Cd). Citric acid-, aspartic acid- and cysteine-ferrihydrite assemblages also enhanced the metal binding rate. X-ray absorption spectroscopy revealed that the organic coating contributed significantly to Zn binding by the assemblages, despite relatively low organic surface coverage. Our findings provide valuable information on the nature of heavy metal-organic-mineral interactions and metal adsorption processes regulating their bioavailability and transport.

The missing link between carbon nanotubes, dissolved organic matter and organic pollutants.

Ternary interactions between carbon nanotubes (CNTs), dissolved organic matter (DOM) and small organic molecules (namely low molecular mass organic pollutants) are of great importance since they can affect the reactivity and fate of all involved compartments in the environment. This review thoroughly assesses existing knowledge on the adsorption of DOM and small organic molecules by CNTs, while giving special attention to (i) the complex nature of DOM, (ii) the ternary rather than binary interactions between CNTs, DOM and the small organic molecules and (iii) the DOM-organic molecule interactions. We discuss in detail the main factors influencing DOM adsorption by CNTs and attempt to differentiate between the role of DOM composition and conformation. We then outline how the presence of DOM influences the adsorption of small organic molecules by CNTs, considering the introduction stage of DOM and the impact of the organic molecule's properties. DOM adsorption by CNTs is highly dependent on its composition and is governed by the size, hydrophobicity and aromaticity of DOM. DOM adsorption was found to alter the assembly of the CNTs, resulting in changes in the distribution of adsorption sites. Small organic molecules may adsorb to residual surface area on the CNTs, to DOM-coating the CNTs or remain in solution, possibly complexed with DOM. This results in their suppressed or enhanced adsorption in comparison to DOM-free media. The physicochemical properties of the organic molecules (hydrophobicity, size, structure and charge) also play a major role in this process. We present knowledge gaps that need clarification such as the extent of DOM desorption from CNTs, the amount of co-adsorbed DOM during competition with small organic molecules for adsorption sites on the CNTs and the behavior of CNTs under realistic conditions. More data generated from experiments using natural DOM rather than dissolved humic substances are required to improve our understanding of the interactions between CNTs and small organic molecules in realistic environmental scenarios. This review provides conclusions and research directions needed to evaluate the nature of interactions between CNTs, DOM and organic pollutants in aquatic systems affected by anthropogenic activities.

Removal of triazine-based pollutants from water by carbon nanotubes: Impact of dissolved organic matter (DOM) and solution chemistry.

Adsorption of organic pollutants by carbon nanotubes (CNTs) in the environment or removal of pollutants during water purification require deep understanding of the impacts of the presence of dissolved organic matter (DOM). DOM is an integral part of environmental systems and plays a key role affecting the behavior of organic pollutants. In this study, the effects of solution chemistry (pH and ionic strength) and the presence of DOM on the removal of atrazine and lamotrigine by single-walled CNTs (SWCNTs) was investigated. The solubility of atrazine slightly decreased (∼5%) in the presence of DOM, whereas that of lamotrigine was significantly enhanced (by up to ∼70%). Simultaneous introduction of DOM and pollutant resulted in suppression of removal of both atrazine and lamotrigine, which was attributed to DOM-pollutant competition or blockage of adsorption sites by DOM. However the decrease in removal of lamotrigine was also a result of its complexation with DOM. Pre-introduction of DOM significantly reduced pollutant adsorption by the SWCNTs, whereas introduction of DOM after the pollutant resulted in the release of adsorbed atrazine and lamotrigine from the SWCNTs. These data imply that DOM exhibits higher affinity for the adsorption sites than the triazine-based pollutants. In the absence of DOM atrazine was a more effective competitor than lamotrigine for adsorption sites in SWCNTs. However, competition between pollutants in the presence of DOM revealed lamotrigine as the better competitor. Our findings help unravel the complex DOM-organic pollutant-CNT system and will aid in CNT-implementation in water-purification technologies.

Adsorption and desorption of dissolved organic matter by carbon nanotubes: Effects of solution chemistry.

Increasing use of carbon nanotubes (CNTs) has led to their introduction into the environment where they can interact with dissolved organic matter (DOM). This study focuses on solution chemistry effects on DOM adsorption/desorption processes by single-walled CNTs (SWCNTs). Our data show that DOM adsorption is controlled by the attachment of DOM molecules to the SWCNTs, and that the initial adsorption rate is dependent on solution parameters. Adsorbed amount of DOM at high ionic strength was limited, possibly due to alterations in SWCNT bundling. Desorption of DOM performed at low pH resulted in additional DOM adsorption, whereas at high pH, adsorbed DOM amount decreased. The extent of desorption conducted at increased ionic strength was dependent on pre-adsorbed DOM concentration: low DOM loading stimulated additional adsorption of DOM, whereas high DOM loading facilitated release of adsorbed DOM. Elevated ionic strength and increased adsorbed amount of DOM reduced the oxidation temperature of the SWCNTs, suggesting that changes in the assembly of the SWCNTs had occurred. Moreover, DOM-coated SWCNTs at increased ionic strength provided fewer sites for atrazine adsorption. This study enhances our understanding of DOM-SWCNT interactions in aqueous systems influenced by rapid changes in salinity, and facilitates potential use of SWCNTs in water-purification technologies.

Adsorptive fractionation of dissolved organic matter (DOM) by carbon nanotubes.

Dissolved organic matter (DOM) and carbon nanotubes are introduced into aquatic environments. Thus, it is important to elucidate whether their interaction affects DOM amount and composition. In this study, the composition of DOM, before and after interactions with single-walled carbon nanotubes (SWCNTs), was measured and the adsorption affinity of the individual structural fractions of DOM to SWCNTs was investigated. Adsorption of DOM to SWCNTs was dominated by the hydrophobic acid fraction, resulting in relative enhancement of the hydrophilic character of non-adsorbed DOM. The preferential adsorption of the HoA fraction was concentration-dependent, increasing with increasing concentration. Adsorption affinities of bulk DOM calculated as the normalized sum of affinities of the individual structural fractions were similar to the measured affinities, suggesting that the structural fractions of DOM act as independent adsorbates. The altered DOM composition may affect the nature and reactivity of DOM in aquatic environments polluted with carbon nanotubes.