Role of solution chemistry in the attachment of graphene oxide nanoparticles onto iron oxide minerals with different characteristics

Effects of surfactants on the adsorption of norfloxacin onto ferrihydrite: comparison between anionic and cationic surfactants

There is little information on how widespread surfactants affect the adsorption of norfloxacin (NOR) onto iron oxide minerals. In order to elucidate the effects of various surfactants on the adsorption characteristics of NOR onto typical iron oxides, we have explored the different influences of sodium dodecylbenzene sulfonate (SDBS), an anionic surfactant, and didodecyldimethylammonium bromide (DDAB), a cationic surfactant, on the interactions between NOR and ferrihydrite under different solution chemistry conditions. Interestingly, SDBS facilitated NOR adsorption, whereas DDAB inhibited NOR adsorption. The adsorption-enhancement effect of SDBS was ascribed to the enhanced electrostatic attraction, the interactions between the adsorbed SDBS on ferrihydrite surfaces and NOR molecules, and the bridging effect of SDBS between NOR and iron oxide. In comparison, the adsorption-inhibition effect of DDAB owning to the adsorption site competitive adsorption between NOR and DDAB for the effective sites as well as the steric hindrance between NOR-DDAB complexes and the adsorbed DDAB on ferrihydrite surfaces. Additionally, the magnitude of the effects of surfactants on NOR adsorption declined with increasing pH values from 5.0 to 9.0, which was related to the amounts of surfactant binding to ferrihydrite surfaces. Moreover, when the background electrolyte was Ca2+, the enhanced effect of SDBS on NOR adsorption was caused by the formation of NOR-Ca2+-SDBS complexes. The inhibitory effect of DDAB was due to the DDAB coating on ferrihydrite, which undermined the cation-bridging effect. Together, the findings from this work emphasize the essential roles of widely existing surfactants in controlling the environmental fate of quinolone antibiotics.

Adsorption of fluoroquinolone antibiotics onto ferrihydrite under different anionic surfactants and solution pH

To date, little information is available regarding the impacts of the widespread anionic surfactants on the adsorption behaviors of antibiotics onto typical iron oxides. Herein, we have investigated the effects of two typical surfactants (sodium dodecyl sulfate (SDS) and sodium dodecylbenzene sulfonate (SDBS)) on the adsorption of two widely used antibiotics (i.e., levofloxacin (LEV) and ciprofloxacin (CIP)) onto ferrihydrite. Results of kinetic experiments showed that the adsorption of antibiotics was well fitted by the pseudo-second-order kinetic models, indicating that the adsorption process might be controlled by chemisorption. The affinity of ferrihydrite toward CIP was greater than that toward LEV, which was ascribed to the higher hydrophobicity of CIP than LEV. Both surfactants enhanced antibiotic adsorption owing to SDS or SDBS molecules as bridge agents between ferrihydrite particles and antibiotics. Interestingly, the extent of the enhanced effects of surfactants on antibiotic adsorption declined as the background solution pH increased from 5.0 to 9.0, which was mainly due to the weaker hydrophobic interactions between antibiotics and the adsorbed surfactants on the iron oxide surfaces as well as the greater electrostatic repulsion between the anionic species of antibiotics and the negatively charged ferrihydrite particles at higher pH. Together, these findings emphasize the importance of widespread surfactants for illustrating the interactions between fluoroquinolone antibiotics and iron oxide minerals in the natural environment.

Surfactant-mediated mobility of carbon dots in saturated soil: comparison between anionic and cationic surfactants

Understanding the mobility, retention, and fate of carbon dots (CDs) is critical for the risk management of this emerging carbon material. However, the influences of surfactants on CDs' transport through subsurface media are still poorly understood. Herein, column experiments were conducted to explore the different influences of an anionic surfactant, sodium dodecylbenzene sulfonate (SDBS), and a cationic surfactant, cetyltrimethylammonium bromide (CTAB), on the CDs' transport in water-saturated soil. In the Na⁺ background electrolyte, both surfactants facilitated the transport of CDs at pH 7.0. The trend stemmed from steric hindrance, a decline in the straining effect, and competitive deposition between CDs and surfactant molecules. Additionally, SDBS increased the electrostatic repulsion of CDs and soil. Interestingly, in the divalent cation background electrolytes (i.e., Ca²⁺ or Cu²⁺), SDBS suppressed CDs' mobility, whereas CTAB had the opposite effect. The transport-inhibited effect of SDBS was mainly due to anionic surfactant ion (DBS^-) precipitation with metal cations and the formation of adsorbed SDBS-Cu²⁺/Ca²⁺-CDs complexes. The enhanced effect of CTAB resulted from the CTAB coating on soil grains, which suppressed the cation bridging between CDs and soil. Furthermore, the magnitude of the SDBS promotion effect was pH-dependent. Surprisingly, CTAB could inhibit CDs' mobility at pH 9.0, owing to the binding cationic surfactant's strong hydrophobicity effect on the soil surface. Moreover, the experimental breakthrough curves of CDs were well described using a two-site transport model. Overall, the observations obtained from this study shed light on the relative mobility of CDs with different surfactants in typical groundwater conditions.

Insight into the effect of phosphate on ferrihydrite colloid-mediated transport of tetracycline in saturated porous media

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.

Insight into the inhibitory mechanism of inorganic ligands on the adsorption of tetracycline onto hematite

Inorganic ligands, ubiquitous in the natural environment, can interact with iron oxide minerals. To date, our knowledge regarding the effects of inorganic ligands on the adsorption properties of antibiotics onto iron oxides is still limited. In this work, the influences of different inorganic ligands (chosen iodate, silicate, and phosphate as the model ligands) on the adsorption of tetracycline (TC) onto hematite were examined. Adsorption isotherms indicated that inorganic ligands inhibited TC adsorption. The observations were attributed to the increase of electrostatic repulsion between anionic species (i.e., TC^-) and negatively charged hematite particles as well as the competition between TC^- species and inorganic ligand anions for the adsorption sites on hematite surfaces. Interestingly, the inhibitory effects of the three inorganic ligands were in the order of phosphate > silicate > iodate; the trend was stemmed from their differences in the binding affinities to hematite and the molecular size. When the background solutions contained divalent cations (e.g., Ca²⁺), surface precipitation of Ca-inorganic ligand compounds on hematite was another important mechanism for the inhibitory effects. Furthermore, adsorption of TC onto hematite with or without inorganic ligands was strongly affected by solution pH, which was due to the combination of the amphoteric behavior of TC and highly pH-dependent surface charges of the hematite mineral. Current results highlight the critical roles of ubiquitous inorganic ligands in revealing the fate of tetracycline antibiotics in natural systems.

Colloid-mediated transport of tetracycline in saturated porous media: Comparison between ferrihydrite and montmorillonite

Given the ubiquitous mineral (e.g., clays and iron oxides) playing critical roles in impacting the fate of antibiotics in the subsurface environment, the effects of two mineral colloids (i.e., ferrihydrite and montmorillonite) on tetracycline (TC, a representative of antibiotic) transport in sand columns were investigated in this study. Interestingly, the results clearly showed that ferrihydrite colloids inhibited TC transport, while montmorillonite colloids enhanced TC mobility under neutral conditions (pH 7.0). This phenomenon resulted from the positively charged ferrihydrite colloids with weak mobility, which assisted TC deposition; besides, providing additional deposition sites for TC by the deposited ferrihydrite colloids was another important mechanism. In contrast, the transport-enhancement effect of montmorillonite on TC was attributed to the strong binding affinity of TC to clay particles as well as the competition between colloids and TC for deposition sites on sand surfaces. Moreover, the transport-inhibition effect of ferrihydrite at pH 7.0 was greater than that at pH 5.0, mainly due to more colloid-associated TC under neutral conditions. Surprisingly, ferrihydrite colloids could act as carriers of antibiotics and enhanced TC transport at pH 9.0. Because the surface charge of colloids was altered to negative and could break through the column. Meanwhile, the transport-enhancement effect of montmorillonite decreased with increasing pH from 5.0 to 9.0, resulting from the decrease of colloid-adsorbed TC. Furthermore, colloid-mediated transport of TC was influenced by ionic strength, which affected the aggregation characteristics of colloids and the binding affinities of TC to minerals. These findings provide critical information for assessing the risks of antibiotics in aquatic ecosystems where abundant natural minerals are present.