Insights into the mutual promotion effect of graphene oxide nanoparticles and tetracycline on their transport in saturated porous media

pH-dependent transport of tetracycline in saturated porous media: Single and combined effects of surfactants and iron oxide colloids

Herein, sodium dodecyl sulfate (SDS) and rhamnolipid (Rha) were employed to investigate their influences on TC mobility and ferrihydrite colloid-mediated transport of TC at variable pH values (5.0-9.0). In the binary system, surfactants suppressed TC transport because of surfactants' bridging effects; similarly, ferrihydrite colloids also restrained TC mobility stemming from the colloid-associated TC retention. Interestingly, the degree of the inhibitory effects of colloids/surfactants increased with decreasing pH values. Surprisingly, the mutual influences of surfactants and colloids on TC movement displayed a strong pH dependence. Concretely, surfactants strengthened the repressive impacts of ferrihydrite colloids on TC mobility at pH 5.0 caused by the enhanced TC deposition on colloids attached to sand surfaces through the linking effects of surfactants. Nevertheless, at pH 7.0, adding surfactants reduced the repressive effects due to increased TC-colloid mobility and enhanced electrostatic repulsion. Unexpectedly, colloids accelerated the transport of TC with surfactants at pH 9.0 owing to colloids acting as TC carriers, the enhanced TC2-/TC- species mobility, and competitive retention. Notably, SDS exhibited a greater effect on individual TC mobility or colloid-mediated TC transport than Rha at a certain pH, which was related to the different surfactant-binding abilities of sand grains/ferrihydrite colloids.

Key factors affecting transport/retention of graphene oxide nanoparticles in porous media: A systematic and critical review

The transport/retention of nanoparticles (NPs) within porous media is becoming a topic of growing global interest. This study presents a comprehensive investigation of the transport/retention and modeling of graphene oxide nanoparticles (GONPs) in saturated/unsaturated porous media as useful nanoparticles in different industries including electronics, biomedicine, and sensors. It conducts a systematic and critical review of 75 articles related to the implementation of GONPs transport/retention in porous media to achieve a comprehensive understanding of transport effective factors specially in unsaturated condition. For this purpose, the factors influencing GONPs transport in porous media are subsequently classified into three categories: (1) GONPs characteristics, (2) porous media characteristics, and (3) flow characteristics. Furthermore, the primary mechanisms including aggregation, straining, blocking, and ripening are crucial for the retention of GONPs. Ultimately, this study emphasizes the significance of GONPs transport within porous media and outlines potential objectives for subsequent studies. Several recommendations for future research directions are proposed based on the findings and aspects outlined in the reviewed literature to facilitate subsequent investigations.

Enhanced tetracycline removal from aqueous systems using starch-functionalized iron-graphene oxide nanocomposites: Synthesis, characterization, and mechanistic insights

The increasing presence of tetracycline antibiotics in aquatic ecosystems poses a critical environmental challenge, necessitating innovative remediation strategies. This study presents the development and characterization of starch-functionalized iron-graphene oxide (SFIGO) and starch-functionalized iron oxide (SFIO) nanocomposites adsorbents for tetracycline removal from water, with emphasis on sustainable synthesis and enhanced performance. Biluochun tea and cassava extract were used as renewable precursors in green synthesis to create a composite material that combines iron oxide's magnetic properties, graphene oxide's high surface area, and starch's biocompatibility. Comprehensive characterization using FTIR, XRD, SEM-EDX, TEM, and XPS revealed SFIGO's unique hierarchical architecture, featuring wrinkled graphene oxide sheets with well-dispersed iron-oxide nanoparticles. Batch adsorption studies demonstrated SFIGO's superior performance, achieving a maximum adsorption capacity of 865.79 mg/g at 298 K, significantly higher than SFIO's 634.83 mg/g. The adsorption process followed pseudo-second-order kinetics and showed endothermic behavior, with negative Gibbs free energy values, confirming process spontaneity. Multiple binding mechanisms, including π-π interactions, electrostatic attractions, and surface complexation, contributed to SFIGO's enhanced performance. The material demonstrated robust performance across various water matrices and maintained high removal efficiency. These findings advance our understanding of composite materials in environmental remediation and provide a sustainable solution for pharmaceutical pollutant removal.

Biosurfactant-mediated transport of tetracycline antibiotics in saturated porous media: Combined effects of the chemical properties of contaminants and solution chemistry conditions

The mobility of tetracycline antibiotics (TCs) in saturated aquifers is possibly affected by the presence of biosurfactants, which are widespread in the aquatic/soil environments. This study investigated the mobility characteristics of various tetracyclines-specifically tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC)-within quartz sand columns in the presence of rhamnolipid, a common biosurfactant. Exogenous rhamnolipid significantly inhibited the transport of the three TCs over the pH range of 5.0-9.0 (e.g., the mass of retained TC, OTC, and CTC increased from 32.6 %, 26.9 %, and 39.2 % (in the absence rhamnolipid) to 39.4 %, 38.9 %, and 51.7 % (in the presence of rhamnolipid), respectively). This observation could be attributed to the bridging effects of this biosurfactant. Specifically, the hydrophilic head of rhamnolipid molecules is likely associated with the surfaces of sand grains through surface complexation and/or hydrogen bonding interactions. Accordingly, the hydrophobic moieties of the deposited rhamnolipid molecules (i.e., the aliphatic chains) interact with the hydrophobic groups of TCs molecules via hydrophobic interactions. Interestingly, the extent of the inhibitory effect on CTC mobility was greater than that on OTC and TC, which was related to the different hydrophobic characteristics of the three antibiotics. Furthermore, the inhibitory effect of rhamnolipid on the transport of TCs diminished as the pH of the background solution increased. This observation was attributed to the weakened bridging effects, resulting from the reduced deposition of the biosurfactant on the sand surfaces. Additionally, the cation-bridging mechanism involved in the retention of TCs in the addition of rhamnolipid when the background electrolyte was Ca2+ (i.e., Ca2+ ions served as bridging agents between the deposited rhamnolipid molecules and TCs). The insightful findings enhance our understanding of the critical roles of biosurfactants in influencing the environmental dynamics and ultimate fate of conventional antibiotic pollutants within groundwater systems.

Significance of non-DLVO interactions on the co-transport of levofloxacin and titanium dioxide nanoparticles in porous media

With the application of engineered nanomaterials and antibiotics in the fields of medicine, aerospace, new energy and agriculture, the associated contamination is detected widely in soil-groundwater systems. It is of great scientific and practical significance to deeply explore the environmental interface process between nanoparticles and antibiotics for the scientific assessment of environmental fate and ecological environmental risks, as well as the development of new composite pollution control technologies. In this study, the co-transport behaviors of positively charged titanium dioxide nanoparticles (TiO2-NPs) and negatively charged levofloxacin (LEV) in quartz sand (QS) are investigated in this study. The results show that TiO2-NPs hardly flow out when transported alone in the column because of its positive charge, which creates a strong attraction with the negatively charged quartz sand on the surface. When TiO2-NPs co-migrate with LEV in porous media, the presence of LEV promotes the transport of TiO2-NPs, while the presence of TiO2-NPs inhibits LEV transport. Non-XDLVO interactions based on molecular dynamics (MD) simulations can help explain the observed promotion and inhibition phenomena as well as the correlation between TiO2-NPs and LEV. The results indicate that TiO2-LEV complexes or aggregates can be formed during the co-transportation process of TiO2-NPs and LEV in porous media. As flow velocity increases from 0.204 cm min-1 to 1.630 cm min-1, both the transport capacities of TiO2-NPs and LEV are enhanced significantly. Under the condition of high citric acid (CA) concentration (15 mmol L-1), the transport capacity of TiO2-NPs is slightly inhibited, while the transport capacity of LEV is enhanced. This study provides new insights into the transport of nanometallic oxides and antibiotics in porous media, which suggests that non-XDLVO interactions should be considered together when assessing the environmental risks and fate of nanometallic oxides and antibiotics in soil-groundwater systems.

Cotransport of nanoplastics and plastic additive bisphenol AF (BPAF) in unsaturated hyporheic zone: Coupling effects of surface functionalization and protein corona

The ecological risk of combined pollution from microplastics (MPs) and associated contaminants usually depends on their interactions and environmental behavior, which was also disturbed by varying surface modifications of MPs. In this study, the significance of surface functionalization and protein-corona on the cotransport of nanoplastics (NPs; 100 nm) and the related additive bisphenol AF (BPAF) was examined in simulated unsaturated hyporheic zone (quartz sand; 250-425 μm). The electronegative bovine serum albumin (BSA) and electropositive trypsin were chosen as representative proteins, while pristine (PNPs), amino-modified (ANPs), and carboxyl-modified NPs (CNPs) were representative NPs with different charges. The presence of BPAF inhibited the mobility of PNPs/CNPs, but enhanced the release of ANPs in hyporheic zone, which was mainly related to their hydrophobicity changes and electrostatic interactions. Meanwhile, the NPs with high mobility and strong affinity to BPAF became effective carriers, promoting the cotransport of BPAF by 16.4 %-26.4 %. The formation of protein-coronas altered the mobility of NPs alone and their cotransport with BPAF, exhibiting a coupling effect with functional groups. BSA-corona promoted the transport of PNPs/CNPs, but this promoting effect was weakened by the presence of BPAF via increasing particle aggregation and hydrophobicity. Inversely, trypsin-corona aggravated the deposition of PNPs/CNPs, but competition deposition sites and increased energy barrier caused by coexisting BPAF reversed this effect, facilitating the cotransport of trypsin-PNPs/CNPs in hyporheic zone. However, BPAF and protein-coronas synergistically promoted the mobility of ANPs, owing to competition deposition sites and decreased electrostatic attraction. Although all of the NPs with two protein-coronas reduced dissolved BPAF in the effluents via providing deposition sites, the cotransport of total BPAF was improved by the NPs with high mobility (BSA-PNPs/CNPs) or high affinity to BPAF (BSA/trypsin-ANPs). However, the trypsin-PNPs/CNPs inhibited the transport of BPAF due to their weak mobility and adsorption with BPAF. The results provide new insights into the role of varying surface modifications on NPs in the vertical cotransport of NPs and associated contaminants in unsaturated hyporheic zone.

Influence of surfactant molecular features on tetracycline transport in saturated porous media of varied surface heterogeneities

This study aims to understand how surfactants affect the mobility of tetracycline (TC), an antibiotic, through different aquifer media. Two anionic and cationic surfactants, sodium dodecylbenzene sulfonate (SDBS) and cetyltrimethyl ammonium bromide (CTAB), were used to study their influence on TC mobility through clean sand and humic acid (HA)-coated sand. HA coating inhibits TC mobility due to its strong interaction with TC. Both surfactants promoted TC mobility at pH 7.0 due to competitive deposition, steric effect, and increased hydrophilicity of TC. CTAB had a more substantial effect than SDBS, related to the surfactants' molecular properties. Each surfactant's promotion effects were greater in HA-coated sand than in quartz sand due to differences in surfactant retention. CTAB inhibited TC transport at pH 9.0 due to its significant hydrophobicity effect. Furthermore, in the presence of Ca2+, SDBS enhanced TC transport by forming deposited SDBS-Ca2+-TC complexes. On the other hand, CTAB increased TC mobility due to its inhibition of cation bridging between TC and porous media. The findings highlight surfactants' crucial role in influencing the environmental behaviors of tetracycline antibiotics in varied aquifers.

Unveiling combined ecotoxicity: Interactions and impacts of engineered nanoparticles and PPCPs

Emerging contaminants such as engineered nanoparticles (ENPs), pharmaceuticals and personal care products (PPCPs) are of great concern because of their wide distribution and incomplete removal in conventional wastewater and soil treatment processes. The production and usage of ENPs and PPCPs inevitably result in their coexistence in different environmental media, thus posing various risks to organisms in aquatic and terrestrial ecosystems. However, the existing literature on the physicochemical interactions between ENPs and PPCPs and their effects on organisms is rather limited. Therefore, this paper summarized the ecotoxicity of combined ENPs and PPCPs by discussing: (1) the interactions between ENPs and PPCPs, including processes such as aggregation, adsorption, transformation, and desorption, considering the influence of environmental factors like pH, ionic strength, dissolved organic matter, and temperature; (2) the effects of these interactions on bioaccumulation, bioavailability and biotoxicity in organisms at different trophic levels; (3) the impacted of ENPs and PPCPs on cellular-level biological process. This review elucidated the potential ecological hazards associated with the interaction of ENPs and PPCPs, and serves as a foundation for future investigations into the ecotoxicity and mode of action of ENPs, PPCPs, and their co-occurring metabolites.

Transport and retention of ciprofloxacin with presence of multi-walled carbon nanotubes in the saturated porous media: impacts of ionic strength and cation types

The environmental fate and risks of ciprofloxacin (CIP) in the subsurface have raised intensive concerns. Herein, the transport behaviors of CIP in both saturated quartz sand and sand/multi-walled carbon nanotubes (MWCNTs) mixtures under different solution ionic strength of the solution and coexisting cation types were investigated. Batch adsorption experiments highlighted growing adsorptive capacity for CIP with the increasing content of MWCNTs in the MWCNTs-quartz sand mixtures (from 0.5% to 1.5%, w/w). Breakthrough curves (BTCs) of CIP in the MWCNTs-quartz sand mixtures were well fitted by the two-site chemical nonequilibrium model (R2 > 0.833). The estimated retardation factors for CIP increased from 9.68 to 282 with growing content of MWCNTs in the sand column, suggesting the presence of MWCNTs significantly inhibited the transport of CIP in saturated porous media. Moreover, the values of retardation factors are negatively correlated with the ionic strength and higher ionic strength could facilitate the transport of CIP in the saturated porous media. Compared with monovalent cations (Na+), the presence of divalent cations (Ca2+) significantly facilitated the transport of CIP in the columns due to the complexation between CIP and Ca2+ as well as deposition of MWCNTs aggregates on the sand surface. Results regarding CIP retention in columns indicated that MWCNTs could enhance the accumulation of CIP in the layers close to the influent of sand columns, while they could hinder upward transport of CIP to the effluent. This study improves our understanding for transport behaviors and environmental risk assessments of CIP in the saturated porous media with MWCNTs.

Statuses, shortcomings, and outlooks in studying the fate of nanoplastics and engineered nanoparticles in porous media respectively and borrowable sections from engineered nanoparticles for nanoplastics

This review discussed the research statuses, shortcomings, and outlooks for the fate of nanoplastics (NPs) and engineered nanoparticles (ENPs) in porous media and borrowable sections from ENPs for NPs. Firstly, the most important section was that we reviewed the research statuses on the fate of NPs in porous media and the main influencing factors, and explained the influencing mechanisms. Secondly, in order to give NPs a reference of research ideas and influence mechanisms, we also reviewed the research statuses on the fate of ENPs in porous media and the factors and mechanisms influencing the fate. The main mechanisms affecting the transport of ENPs were summarized (Retention or transport modes: advection, diffusion, dispersion, deposition, adsorption, blocking, ripening, and straining; Main forces and actions: Brownian motion, gravity, electrostatic forces, van der Waals forces, hydration, filtration, bridging; Affecting elements of the forces and actions: the ENP and media grain surface functional groups, size, shape, zeta potential, density, hydrophobicity, and roughness). Instead of using the findings of ENPs, thorough study on NPs was required because NPs and ENPs differed greatly. Based on the limited existing studies on the NP transport in porous media, we found that although the conclusions of ENPs could not be applied to NPs, most of the influencing mechanisms summarized from ENPs were applicable to NPs. Combining the research thoughts of ENPs, the research statuses of NPs, and some of our experiences and reflections, we reviewed the shortcomings of the current studies on the NP fate in porous media as well as the outlooks of future research. This review is very meaningful for clarifying the research statuses and influence mechanisms for the NP fate in porous media, as well as providing a great deal of inspiration for future research directions about the NP fate in porous media.

Interplay of compound pollutants with microplastics transported in saturated porous media: Effect of co-existing graphene oxide and tetracycline

Co-existence of microplastics, nanomaterials, and antibiotics may lead to intensified multifaceted pollution, which may influence their fate in soils. This study investigated the co-transport behavior of polystyrene microplastics (PS) and compound pollutants of graphene oxide (GO) and tetracycline (TC). Packed column experiments for microplastic with or without combined pollutants were performed in KCl (10 and 30 mM) and CaCl2 solutions (0.3 and 1 mM). The results showed transport of PS was facilitated at low ionic strengths and inhibited at high ionic strengths by GO with or without TC under examined conditions. Carrier effect of GO as well as the aggregation of PS in the presence of co-exiting GO or GO-TC could be the contributor. Although the existence of TC relieved the ripening phenomenon of PS and GO deposition due to enhanced electronegativity of sand media, the effect of GO on the PS transport has not been significantly impacted, indicating the dominant role of GO during cotransport process. Furthermore, the transport of PS was increased by TC owing to competition for deposition sites on sand surfaces. In turn, the transport of TC was mainly affected by PS whether graphene was present or not. The increase in electrostatic repulsive force (transport-promoting) and addition adsorption sites (transport-inhibiting) may be responsible for the observations. Our findings could improve understandings of complex environmental behaviors of microplastics and provide insight into investigation on cotransport of emerging contaminants under various conditions relevant to the subsurface environment.

Effects of Antibiotic Resistance Genes and Antibiotics on the Transport and Deposition Behaviors of Bacteria in Porous Media

Antibiotics present in the natural environment would induce the generation of antibiotic-resistant bacteria (ARB), causing great environmental risks. The effects of antibiotic resistance genes (ARGs) and antibiotics on bacterial transport/deposition in porous media yet are unclear. By using E. coli without ARGs as antibiotic-susceptible bacteria (ASB) and their corresponding isogenic mutants with ARGs in plasmids as ARB, the effects of ARGs and antibiotics on bacterial transport in porous media were examined under different conditions (1-4 m/d flow rates and 5-100 mM NaCl solutions). The transport behaviors of ARB were comparable with those of ASB under antibiotic-free conditions, indicating that ARGs present within cells had negligible influence on bacterial transport in antibiotic-free solutions. Interestingly, antibiotics (5-1000 μg/L gentamicin) present in solutions increased the transport of both ARB and ASB with more significant enhancement for ASB. This changed bacterial transport induced by antibiotics held true in solution with humic acid, in river water and groundwater samples. Antibiotics enhanced the transport of ARB and ASB in porous media via different mechanisms (ARB: competition of deposition sites; ASB: enhanced motility and chemotaxis effects). Clearly, since ASB are likely to escape sites containing antibiotics, these locations are more likely to accumulate ARB and their environmental risks would increase.

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.

Biosurfactant-mediated mobility of graphene oxide nanoparticles in saturated porous media

There is currently a lack of scientific understanding regarding how bio-surfactants influence the mobility of graphene oxide (GO) through saturated porous media. In this study, the transport characteristics of GO through porous media with different heterogeneities (i.e., quartz sand and goethite-coated sand) after the addition of saponin (a representative bio-surfactant) were investigated. The results demonstrated that saponin (3-10 mg L^-1) promoted GO mobility in both types of porous media at pH 7.0. This trend was attributed to the competitive deposition between nanoparticles and bio-surfactant molecules for attachment sites, the enhanced electrostatic repulsion, the decreased strain, the presence of steric effects induced by the adsorbed saponin, and the increase in the hydrophilicity of nanoparticles. Intriguingly, saponin promoted GO mobility in goethite-coated sand (i.e., chemically heterogeneous porous media) to a greater extent than that in sand (i.e., relatively homogeneous porous media) when saponin concentrations increased, which stemmed from the differences in the extent of the deposition site competition for saponin on the two porous media and the electrostatic repulsion between GO and the porous media. Furthermore, a cation-bridging mechanism was also involved in the ability of saponin to increase GO mobility when the electrolyte solution was 0.1 mM Cu²⁺. Moreover, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and the colloid transport model were applicable to elucidate the mobility properties of GO with or without saponin in porous media. The findings from this work highlight the important status of bio-surfactants in the fate of colloidal carbon-based nanomaterials in subsurface systems.

Graphene oxide/MIL 101(Cr) (GO/MOF) nano-composite for adsorptive removal of 2,4-dichlorophenoxyacetic acid (2,4 D) from aqueous media: synthesis, characterization, kinetic and isotherm studies

Contamination of water resources with various pollutants and therefore lack of clean water resources are major problems that threaten many human societies. The need to develop efficient methods and materials to decontaminate water resource is an undeniable fact. Metal-organic frameworks (MOFs), as new class of highly crystalline porous solids, have attracted a great deal of attention in different research fields, especially in adsorptive removal and purification. In this study, MIL 101(Cr) MOF decorated with graphene oxide nano-layers (GO/MOF) was synthesized by a simple one-pot hydrothermal method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and electron dispersion energy (EDS) were utilized to approve the growing of Cr-MOF on graphene oxide nano-layer. The synthesized nano-composite was used as a potential adsorbent for the removal of a pesticide, 2, 4-dichlorophenoxyacetic acid (2,4 D). The adsorption performance, kinetic and mechanism of 2,4 D adsorption onto GO/MOF were studied. The highest adsorption capacities of 476.9 mg g^-1 was obtained at room temperature, pH 6.0 using 0.6 gL^-1 of GO/MOF which was 34% higher than that of pristine Cr-MOF. The kinetics and isotherm data fitted well with pseudo-second kinetic and Langmuir isotherm model, respectively. The reusability and stability analyses showed that the synthesized GO/MOF nanocomposite kept 89% of sorption capacities for 2,4 D after four adsorption-desorption cycles. GO/MOF nano-composite was successfully applied to remove 2,4 D from agricultural waste. The results approved that the synthesized nano-composite could introduce as a stable and high performance adsorbent for adsorptive removal of selected pesticide.

Surfactants-mediated the enhanced mobility of tetracycline in saturated porous media and its variation with aqueous chemistry

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.

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.

The detection and characterization techniques for the interaction between graphene oxide and natural colloids: A review

The high dispersibility of graphene oxide (GO) and the universality of natural colloids (clay minerals, (hydr)oxides of Al, Fe, silica, etc.) make them interact easily. Many kinds of analytical methods have been used to study the interaction between GO and natural colloids. This review provides a comprehensive overview of analytical methods for the detection and quantification of interaction process. We highlighted the influence of the most relevant environmental factors (ionic strength, pH, etc.) on batch experiment, quartz crystal microbalance with dissipation monitoring measurements, and column experiments. Besides, the benefits and drawbacks of spectroscopic, microscopic techniques, theoretical models, calculation and time-resolved dynamic light scattering methods also have discussed in this work. This review can give some guidance to researchers in their selection and combination of the technique for the research of the interaction between GO and natural colloids.

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.

Insights into the molecular mechanism of tetracycline transport in saturated porous media affected by low-molecular-weight organic acids: Role of the functional groups and molecular size

The transport of tetracycline possessed a great challenge in its environmental applications. This study looked at how various low-molecular-weight organic acids (LMWOAs) affect the transport of tetracycline in environments. To that end, four LMWOAs were employed in experiments; acetic acid, malonic acid, malic acid, and citric acid. It was observed that LMWOAs promoted the tetracycline passage in presence of various experimental environments. The LMWOAs steric hindrance and deposition competition facilitated tetracycline transport at pH 5.0. The other deposition mechanism for tetracycline was the electrostatic repulsion between tetracycline and sand enhanced by deprotonated LMWOAs at pH 7.0. Moreover, the enhanced effects of LMWOAs on tetracycline mobility were intensively dependent on LMWOA type with more functional groups (e.g. carboxyl and hydroxyl groups) and larger molecular size supported stronger deposition competition, steric hindrance as well as electrostatic repulsion. Additionally, cation-bridging played a vital role for the enhanced effects of LMWOAs on tetracycline transport with divalent cations (e.g., Ca²⁺ and Pb²⁺). Interestingly, tetracycline exhibited a higher mobility in the presence of Ca²⁺ relative to Pb²⁺ regardless of LMWOAs-free or LMWOAs-addition. This phenomenon was attributed to the fact that Pb²⁺ has a greater affinity with tetracycline and LMWOAs than Ca²⁺. Furthermore, under the shadow of numerous LMWOAs, the non-equilibrium two site transportation model was employed to investigate the movement of tetracycline in porous saturated media. The present study suggests that LMWOAs may be important considerations in assessing the antibiotic passage in soil as well as groundwater.

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.

Phototransformation of Graphene Oxide on the Removal of Sulfamethazine in a Water Environment

Graphene oxide (GO) is widely used in various fields and has raised concerns regarding its potential environmental fate and effect. However, there are few studies on its influence on coexisting pollutants. In this study, the phototransformation of GO and coexisting sulfamethazine (SMZ) under UV irradiation was investigated, with a focus on the role of reactive oxygen species. The results demonstrated that GO promoted the degradation of SMZ under UV irradiation. The higher the concentration of GO, the higher the degradation rate of SMZ, and the faster the first-order reaction rate. Two main radicals, ∙OH and ¹O₂, both contributed greatly in terms of regulating the removal of SMZ. Cl⁻, SO₄²⁻, and pH mainly promoted SMZ degradation by increasing the generation of ∙OH, while humic acid inhibited SMZ degradation due to the reduction of ∙OH. Moreover, after UV illumination, the GO suspension changed from light yellow to dark brown with increasing absorbance at a wavelength of 225 nm. Raman spectra revealed that the I_D/I_G ratio slightly decreased, indicating that some of the functional groups on the surface of GO were removed under low-intensity UV illumination. This study revealed that GO plays important roles in the photochemical transformation of environmental pollutants, which is helpful for understanding the environmental behaviors and risks of nanoparticles in aquatic environments.

Transport of nanoparticles in porous media and its effects on the co-existing pollutants

Nanomaterials are widely used in daily life owing to their superior characteristics. The release and transport of nanoparticles (NPs) in the environment is inevitable during their entire life cycle, posing a risk to the aquatic environment. Thus, considerable attention has been focused on the fate and behavior of NPs in porous media, as well as the co-transport of NPs with other pollutants. In this review, current knowledge about the retention and transport behavior of NPs in porous media is summarized. NP transport in porous media is dominated by various internal and external factors, including the characteristics of NPs, porous media, and water flow. Generally, NPs with high density, small particle size, and surface coating are easily transported in porous media with the characteristics of large size, smooth surface, and low water saturation. Meanwhile, high pH and velocity, low temperature, and natural organic matter-containing fluids are also conducive to NP transport. Aggregation, adsorption, straining, and blocking are the primary mechanisms by which NPs affect the transport of co-existing pollutants in porous media. Current research on NP transport has been performed predominantly using modal porous media (e.g., sand and glass beads); however, there is a large gap between simulated and natural porous media. Further studies should focus on the transport, fate, and interaction of NPs and coexistent pollutants in natural porous media, as well as the coupling mechanisms under actual environmental conditions.

Effects of phosphate on the transport of graphene oxide nanoparticles in saturated clean and iron oxide-coated sand columns

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.