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Colloid-mediated contaminant mobility is absolutely critical for the environmental behavior of contaminants such as antibiotics in water resources. In this study, the influences of phosphate (a commonly inorganic ligand in the environment) on the ferrihydrite colloid-mediated transport of tetracycline (TC, a typical antibiotic) in porous media were investigated. In the absence of colloids, phosphate promoted TC mobility due to the competitive deposition of phosphate and TC on the sand surface as well as the electrostatic repulsion. Interestingly, ferrihydrite colloids could inhibit TC transport; however, the inhibitory effect of the colloids was weakened by the addition of phosphate. This phenomenon stemmed from colloid-associated TC mobility, the increased electrostatic repulsion induced by adsorbed phosphate, and deposition site competition effect. Another interesting finding was that the impacts of phosphate on the colloid-mediated mobility of TC were pH-dependent. That is, phosphate exhibited a weaker effect on the inhibitory role of ferrihydrite colloids in TC mobility at pH 5.0 than that at pH 7.0; specially, ferrihydrite colloids acted as possible carriers of TC and facilitated antibiotic transport at pH 9.0. The observations were ascribed to different influences of phosphate on the binding affinity of ferrihydrite toward TC and the mobility of free TC under different pH conditions. Therefore, the findings of this study provide useful information about the fate and co-transport of antibiotics and natural mineral colloids in the presence of inorganic ligands in the aquatic environment., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

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.

• Phosphate significantly enhanced GO transport in saturated porous media. • Enhanced role was due to electrostatic repulsion and deposition site competition. • Enhancement effect in goethite-coated sand was to a larger extent than that in sand. • Cation-bridging promoted the enhancement effects in the presence of Ca2+. In this study, transport behaviors of graphene oxide (GO) in saturated uncoated (i.e., clean sand) and goethite-coated sand porous media were examined as a function of the phosphate. We found that phosphate enhanced the transport of GO over a wide range of solution chemistry (i.e., pH 5.0–9.0 and the presence of 10 mmol/L Na+ or 0.5 mmol/L Ca2+). The results were mainly ascribed to the increase of electrostatic repulsion between nanoparticles and porous media. Meanwhile, deposition site competition induced by the retained phosphate was another important mechanism leading to promote GO transport. Interestingly, when the phosphate concentration increased from 0.1 to 1.0 mmol/L, the transport-enhancement effect of phosphate in goethite-coated sand was to a much larger extent than that in clean sand. The observations were primarily related to the difference in the total mass of retained phosphate between the iron oxide-coated sand and clean sand columns, which resulted in different degrees of the electrostatic repulsion and competitive effect of phosphate. When the background solution contained 0.5 mmol/L Ca2+, phosphate could be bind to sand/ goethite-coated sand surface by cation bridging; and consequently, promoted competition between phosphate and nanoparticles for deposition sites, which was an important mechanism for the enhanced effect of phosphate. Moreover, the DLVO theory was applicable to describe GO transport behaviors in porous media in the absence or presence of phosphate. Taken together, these findings highlight the important status and role of phosphate on the transport and fate of colloidal graphene oxide in the subsurface environment. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, batch and column tests were performed to investigate the co-transport of graphene oxide (GO) nanoparticles and tetracycline in saturated porous media under various solution chemistry conditions. Research indicated that GO and tetracycline had mutual promotion effect on their transport in the porous media under all the tested conditions, which was ascribed to the high adsorption capacity of tetracycline onto GO and the increased electrostatic repulsion as well as their competition for deposition sites on sand surfaces. Interestingly, the mutually promoting function of GO and tetracycline under acidic conditions was greater than that under alkaline conditions, the dominant mechanism was that the increased solution pH decreased the sorption of tetracycline onto GO and weakened the deposition site competition. Furthermore, the mutually promoting effect of GO and tetracycline was Na+ or Ca2+ concentration-dependent. Specially, increased Ca2+ concentration weakened the promoting effect of GO on tetracycline transport but magnified the promoting effect of tetracycline on GO transport. This is because higher Ca2+ concentration could cause a decrease in the adsorption of tetracycline on GO and facilitate more tetracycline molecules to occupy the deposition sites on sand surfaces. Additionally, sodium dodecyl sulfate had enhancement effect on co-transport of GO and tetracycline. Findings from this study clearly indicated that antibiotics and carbon based nanomaterials may transport together under various solution chemistry conditions, and consequently affect their fates in aquatic environments. Image 1 • GO and tetracycline have mutual promotion function on co-transport. • Sorption and deposition site competition contribute to mutually promoting effects. • Extent of tetracycline-mobilizing capability of GO depends on solution pH. • Different co-deposition mechanisms are involved in the presence of Ca2+ vs. Na+. • SDS has enhancement effect on co-transport of GO and tetracycline. GO and tetracycline had mutual promotion effect on their transport in the porous media under various solution chemistry conditions. [ABSTRACT FROM AUTHOR]

Biochar colloids are likely to control the environmental fate of heavy metals because of the widespread application of this carbon-based material in environmental remediation. Low-molecular-weight aromatic acids (LWMAAs), a class of common organic substances in the environment, are critical in affecting biochar colloid-mediated transport of heavy metals in aquifer media. Nevertheless, thus far, little is known about the effects of LWMAAs on their co-mobility behaviors. Herein, the influences of two aromatic acids, including cinnamic acid and caffeic acid, on the biochar colloid-assisted mobility of Cd2+ in quartz sand were examined. Generally, LMWAAs promoted biochar colloid mobility because of the enhanced electrostatic repulsion, competitive retention between LMWAAs and colloids, and steric hindrance. More importantly, biochar colloids could act as carriers for contaminants to considerably facilitate Cd2+ transport, mainly due to the strong binding affinities of colloids toward Cd2+ and the high mobility of colloids. Intriguingly, although the presence of LMWAAs inhibited Cd2+ adsorption onto biochar colloids, the two aromatic acids enhanced the contaminant-mobilizing transport capacity of colloids, owing to the increased colloid mobility and the formation of stable Cd-LMWAA complexes. Furthermore, the promotion effects of caffeic acid on colloid mobility or colloid-mediated transport of Cd2+ were greater than those of cinnamic acid. This pattern was related to the different chemical properties (e.g., molecular size and/or hydrophobicity) of the two LMWAAs. The observations of this paper provide useful information for understating the effects of coexisting LMWAAs on the co-mobility behaviors of carbonaceous materials and heavy metals in the groundwater environment. [Display omitted] • LMWAAs considerably enhance the transport of biochar colloids. • Steric repulsion and deposition site competition are critical mechanisms. • Highly mobile biochar colloids serve as carriers to enhance Cd2+ mobility markedly. • LMWAAs promote the colloid-facilitated transport of Cd2+ via multi-mechanisms. • The improved effect on colloid-assisted mobility was LMWAA-type dependent. [ABSTRACT FROM AUTHOR]

Since iron oxide minerals are ubiquitous in natural environments, the release of graphene oxide (GO) into environmental ecosystems can potentially interact with iron oxide particles and thus alter their surface properties, resulting in the change of their transport behaviors in subsurface systems. Column experiments were performed in this study to investigate the co-transport of GO nanoparticles and hematite colloids (a model representative of iron oxides) in saturated sand. The results demonstrated that the presence of hematite inhibited GO transport in quartz sand columns due to the formation of less negatively charged GO-hematite heteroaggregates and additional deposition sites provided by the adsorbed hematite on sand surfaces. Contrarily, GO co-present in suspensions significantly enhanced the transport of hematite colloids through different mechanisms such as the increase of electrostatic repulsion, decreased physical straining, GO-facilitated transport of hematite (i.e., highly mobile GO nanoparticles served as a mobile carrier for hematite). We also found that the co-transport behaviors of GO and hematite depended on solution chemistry (e.g., pH, ionic strength, and divalent cation (i.e., Ca2+)), which affected the electrostatic interaction as well as heteroaggregation behaviors between GO nanoparticles and hematite colloids. The findings provide an insight into the potential fate of carbon nanomaterials affected by mineral colloids existing in natural waters and soils. Image 1 • Hematite inhibited GO mobility by heteroaggregation and providing retention sites. • GO significantly enhanced hematite transport through multi-mechanisms. • Highly mobile GO acted as a carrier of hematite colloids. • Low pH or high ionic strength decreased the mobility of both particles. • Ca2+ magnified transport-enhancement effect of GO on hematite transport. [ABSTRACT FROM AUTHOR]

• Goe- and HA-coatings both inhibit the transport of OTC. • Rhamnolipid inhibits OTC mobility in quartz sand due to the hydrophobic interaction. • High concentration resists the rhamnolipid-inhibited OTC mobility in Goe-coating. • The enhanced OTC mobility in HA-coated sand is linear to rhamnolipid concentration. • The extent of the enhanced or inhibitory effect of rhamnolipid is pH-dependent. 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. [Display omitted] [ABSTRACT FROM AUTHOR]

Biomass-pyrogenic smoke-derived dissolved organic matter (SDOMs) percolating into the underground environment profoundly impacts the transport and fate of environmental pollutants in groundwater systems. Herein, SDOMs were produced by pyrolyzing wheat straw at 300–900 °C to explore their transport properties and effects on Cu2+ mobility in quartz sand porous media. The results indicated that SDOMs exhibited high mobility in saturated sand. Meanwhile, the mobility of SDOMs was enhanced at a higher pyrolysis temperature due to the decrease in their molecular sizes and the declined H-bonding interactions between SDOM molecules and sand grains. Furthermore, the transport of SDOMs was elevated as pH values were raised from 5.0 to 9.0, which resulted from the strengthened electrostatic repulsion between SDOMs and quartz sand particles. More importantly, SDOMs could facilitate Cu2+ transport in the quartz sand, which stemmed from forming soluble Cu–SDOM complexes. Intriguingly, the promotional function of SDOMs for the mobility of Cu2+ was strongly dependent on the pyrolysis temperature. Generally, SDOMs generated at higher temperatures exhibited superior effects. The phenomenon was mainly due to the differences in the Cu-binding capacities of various SDOMs (e.g., cation-π attractive interactions). Our findings highlight that the high-mobility SDOM can considerably affect heavy metal ions' environmental fate and transport. [Display omitted] • SDOMs exhibited excellent mobility in quartz sand over the pH range of 5.0–9.0 • The molecular sizes of SDOMs and H-bonding interactions contributed to the mobility. • SDOMs facilitated Cu2+ mobility owing to the formation of soluble SDOM-Cu complexes. • The promotion effects of SDOMs strongly depended on the pyrolysis temperature. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mobility of TCs through the column follows the order of OTC > TC > CTC. • SDS significantly enhances TC transport at pH 7.0 through multi-mechanisms. • SDS inhibits OTC and CTC mobility due to the strong hydrophobic interaction. • The extent of the effects of SDS on the mobility of TCs is pH-dependent. Tetracyclines (TCs), a class of antibiotics that are most widely used veterinary and human medicine, possessed an ability to produce irreversible damage towards humans and environment. Antibiotics transport and deposition are critical processes in their environmental fate. In this study, the mobility properties of various TCs such as oxytetracycline (OTC), chlortetracycline (CTC) and tetracycline (TC), in quartz sand columns with sodium dodecylsulfate (SDS), a typical anionic surface-active agent over the pH array of 5.0∼9.0 were investigated. The mobility of TCs through the column was in the order: OTC > TC > CTC. The difference in the molecular structures of various TC antibiotics could explain the trend. Interestingly, SDS enhanced TC transport via deposition site competition between surfactant and TC molecules, increased electrostatic interaction, and steric hindrance. SDS, on the other hand, inhibited OTC and CTC mobility primarily owing to the strong aquaphobic collaboration between antibiotics and the absorbed surfactant on the sand surface (i.e., SDS molecules as bridge agents between antibiotics and sand grains). Furthermore, at higher pH conditions (5.0∼9.0), the enhanced or inhibitory effect of surfactant on the mobility of TCs declined. The observed decrease in the steric hindrance effect, competitive deposition, or hydrophobic interaction was attributed to the decreased deposition of SDS. Innovative information obtained from this work provides insight into the important role of an anionic surfactant on the mobility and deposition of various tetracycline antibiotics in aquifers. [ABSTRACT FROM AUTHOR]

Water-soluble aerosol organic matters (WSAOMs) produced by biomass pyrolysis/burning can penetrate subsurface environment, and are anticipated to have a profound effect on the fate of contaminants in aquatic ecosystems. Herein, WSAOMs derived from corn straw (CS-WSAOMs) and pinewood sawdust (PW-WSAOMs) pyrolysis at 300–900 °C were utilized to investigate their mobility characteristics and impacts on the transport of heavy metal ions (i.e., Cd2+) in saturated quartz sand with or without soil colloids. This study clearly demonstrated that WSAOMs in subsurface systems exhibited high mobility, which increased as WSAOMs molecular sizes decreased and hydrogen-bond interactions between WSAOMs and sand grains declined. WSAOMs significantly improved heavy metal (i.e., Cd2+) and soil colloid-mediated Cd2+ mobility in the porous media, which stemmed from the increased binding affinities of colloids toward metal ions and the high mobility of WSAOMs. Interestingly, in terms of the mobility and colloid-facilitated transport of Cd2+, WSAOMs from higher pyrolysis temperatures exhibited enhanced effects; meanwhile, the PW-WSAOMs demonstrated stronger effects than the CS-WSAOMs. The trends were mainly attributed to the differences in the metal-binding affinities (e.g., cation-π interactions) and transport abilities of WSAOMs, as well as diverse Cd2+ adsorption capacities of colloids induced by various WSAOMs. [Display omitted] • WSAOMs exhibited high mobility in saturated quartz sand porous media. • The components and molecular sizes of WSAOMs strongly affect their transport. • WSAOMs enhanced Cd2+ mobility due to the formation of WSAOMs-metal complexes. • WSAOMs promoted soil colloid-facilitated transport of Cd2+ through multi-mechanisms. • Enhanced effects of WSAOMs depended on the biomass type and pyrolysis temperature. [ABSTRACT FROM AUTHOR]

Knowledge of the mobility of tetracycline (TC) antibiotics in porous media is critical to understand their potential environmental influences. The transport characteristics of TC in sand columns with three different surfactants, including Tween 80, sodium dodecylbenzene sulfonate (SDBS), and didodecyldimethylammonium bromide (DDAB) under various conditions were investigated in this study. Results demonstrated that all surfactants enhanced TC transport under neutral conditions (10 mM NaCl at pH 7.0). The observation was attributed mostly to deposition site competition, higher electrostatic repulsion between TC molecules and sand grains, steric hindrance, and the increase of TC hydrophilicity. Furthermore, the order of the transport-enhancement effects was generally observed as follows: DDAB > SDBS > Tween 80. The trend was controlled by the variation in the physicochemical properties of surfactants. It was noticed that the presence of Cu2+ (a model divalent cation) in the background solution, the cation-bridging contributed to the promotion effects of DDAB or Tween 80 on TC mobility. Interestingly, SDBS considerably suppressed TC transport due to the precipitation of SDBS–Cu2+ complexes onto sand surfaces. Moreover, the enhancement order of surfactants at pH 5.0 was similar to that pH 7.0. However, DDAB could inhibit TC transport in sand columns at pH 9.0, which were mainly caused by the decrease of electrostatic repulsion and the hydrophobicity induced by the binding cationic surfactant. Findings from this work provide novel insight into involvement of surfactants in antibiotic transport behaviors in the subsurface environment. [Display omitted] • Surfactants significantly enhance TC transport at pH 7.0 through multi-mechanisms. • The transport-enhancement effects followed the order: DDAB > SDBS > Tween 80. • SDBS inhibits the TC mobility in the presence of Cu2+due to surface precipitation. • Hydrophobicity interaction contributes to the inhibitory effects of DDAB at pH 9.0 [ABSTRACT FROM AUTHOR]