Binding of ciprofloxacin by humic substances: a molecular dynamics study

Assessing sorption of fluoroquinolone antibiotics in soils from a Kd compilation based on pure organic and mineral components

The presence of fluoroquinolone (FQ) antibiotics in soils may cause a threat to human health due to overexposure and the generation of antibiotic resistance genes. Understanding their sorption behavior in soils is important to predict subsequent FQ (bio) availability. Here, FQ sorption in pure soil organic (i.e., humic substances) and mineral (i.e., metal oxides; phyllosilicates) components is evaluated through a solid-liquid distribution coefficient (Kd (FQ)) dataset consisting of 243 entries originated from 80 different studies, to elucidate their respective contribution to the overall Kd (FQ) in bulk soils. First, different factors affecting FQ sorption and desorption in each of these soil phases are critically discussed. The strong role of pH in Kd (FQ), due to the simultaneous effect on both FQ speciation and surface charge changes, encouraged the derivation of normalized sorption coefficients for the cationic, zwitterionic and anionic FQ species in humic substances and in different phyllosilicates. Kd (FQ) in metal oxides revealed a key role of metal nature and material specific surface area due to complexation sorption mechanisms at neutral pH. Cumulative distribution functions (CDF) were applied to each dataset to establish a sorption affinity range for each phase and to derive best estimate Kd (FQ) values for those materials where normalized sorption coefficients to FQ species were unavailable. The data analysis conducted in the different soil phases set the basis for a Kd (FQ) prediction model, which combined the respective sorption affinity of each phase for FQ and phase abundance in soil to estimate Kd (FQ) in bulk soils. The model was subsequently validated with sorption data in well characterized soils compiled from the literature.

Adsorption characteristics of ciprofloxacin hydrochloride on polystyrene microplastics in freshwater

In order to reveal the adsorption mechanism of microplastics (MPs) on antibiotics, polystyrene (PS) was chosen as a typical microplastic, Fenton and high-temperature aging methods were used to obtain aged MPs particles. The adsorption behavior and mechanism of ciprofloxacin hydrochloride (CIP) on PS before and after aging were studied by batch adsorption experiments, and other influencing environmental conditions were evaluated concurrently. The results showed that the adsorption of CIP on PS was an exothermic reaction, the pseudo-second-order model and Freundlich isothermal models could fit the adsorption of CIP on PS. Aging treatment enhanced the adsorption capacity of PS to CIP, and Fenton aging for 7 days had the best effect. The highest adsorption was observed when the solution pH was 6. The adsorption capacity of microplastics gradually decreased with increasing ionic strength and the concentration of fulvic acid, while the aging microplastics changed little with the concentration of fulvic acid. The presence of both Cu (II) and CIP inhibits the adsorption of each other on microplastics. Based on the above findings, the adsorption of CIP on PS is dominated by physical adsorption, and electrostatic interactions and hydrogen bonding interactions are also important mechanisms for the adsorption of CIP on microplastics.

Antibiotics soil-solution chemistry: A review of environmental behavior and uptake and transformation by plants

The increased use of antibiotics by humans for various purposes has left the environment polluted. Antibiotic pollution remediation is challenging because antibiotics exist in trace amounts and only highly sensitive detection techniques could be used to quantify them. Nevertheless, their trace quantity is not a hindrance to their transfer along the food chain, causing sensitization and the development of antibiotic resistance. Despite an increase in the literature on antibiotic pollution and the development and transfer of antibiotic-resistant genes (ARGs), little attention has been given to the behavior of antibiotics at the soil-solution interface and how this affects antibiotic adsorption-desorption interactions and subsequent uptake and transformation by plants. Thus, this review critically examines the interactions and possible degradation mechanisms of antibiotics in soil and the link between antibiotic soil-solution chemistry and uptake by plants. Also, different factors influencing antibiotic mobility in soil and the transfer of ARGs from one organism to another were considered. The mechanistic and critical analyses revealed that: (a) the charge characteristics of antibiotics at the soil-root interface determine whether they are adsorbed to soil or taken up by plants; (b) antibiotics that avoid soil colloids and reach soil pore water can be absorbed by plant roots, but their translocation to the stem and leaves depends on the ionic state of the molecule; (c) few studies have explored how plants adapt to antibiotic pollution and the transformation of antibiotics in plants; and (d) the persistence of antibiotics in cropland soils can be influenced by the content of soil organic matter, coexisting ions, and fertilization practices. Future research should focus on the soil/solution-antibiotic-plant interactions to reveal detailed mechanisms of antibiotic transformation by plants and whether plant-transformed antibiotics could be of environmental risk.

The influence of pH and dissolved organic carbon on the ecotoxicity of ampicillin and clarithromycin

The impacts of water chemistry properties including pH and dissolved organic carbon (DOC) on the ecotoxicity of active pharmaceutical ingredients (APIs) are increasingly evident. These impacts are a result of alterations in API bioavailability: pH regulates the bioavailability of many ionizable APIs via chemical speciation, whereas DOC interacts with several APIs to inhibit the APIs from traversing the membrane system of organisms. In this study, we examined the influences of pH and DOC on the bioavailability of ampicillin (AMP) and clarithromycin (CLA) with the help of a bioavailability model. The effects on bioavailability were quantified by ecotoxicity observed in cyanobacteria growth inhibition tests with Microcystis aeruginosa PCC7806. The median effect concentration (96 h-EC50total) of AMP increased by 5-fold when pH raised from 7.4 to 9.0, suggesting the zwitterionic AMP+/- species being higher in bioavailability than the negatively charged AMP- species. CLA ecotoxicity showed no significant pH-dependency, suggesting CLA+ and CLA0 species to be equally bioavailable, albeit it correlated significantly with M. aeruginosa growth rate in negative controls. In addition, DOC demonstrated no significant effects on the ecotoxicity of AMP or CLA. Overall, together with earlier results on ciprofloxacin, our data show that bioavailability relations with pH and DOC are variable among different antibiotics. Factors other than chemical speciation alone could play a role in their bioavailability, such as their molecular size and polarity.

Quantitative structure-activity relationship models for the reaction rate coefficients between dissolved organic matter and PPCPs

To predict PPCPs' photolysis rate in natural aquatic environment, it is essential to grasp the reaction rates between DOM and PPCPs, yet there are few measured data and no prediction models for this important photochemical parameter. To address this, a reaction rate coefficient (α_DOM) was defined to describe the apparent rate of DOM-involved photoreaction for PPCPs. The measured α_DOM values for 40 PPCPs in 9 DOM samples varied dramatically, ranging from (-2.1 ± 0.1)× 10¹⁰ to (2.2 ± 0.1)× 10¹¹ M^-1 s^-1. Then the quantitative structure-activity relationship (QSAR) models were developed using chemical and water quality descriptors via the random forest method. We initially separated positive and negative values by a classifier with an AUC value of 0.965, followed by the construction of regression models for positive and negative values, respectively, using a regressor. Positive models achieved satisfactory goodness-of-fit and predictive ability (R²_adj=0.92 and Q²_ext=0.86), while negative models demonstrated acceptable performance (R²_adj=0.71 and Q²_ext=0.70). Finally, a comprehensive photolysis model that incorporates the QSAR models for α_DOM was established and the significance of water quality parameters was emphasized through sensitive analysis. This model enables more elaborate predictions of PPCPs' photolysis rates in various water samples, providing valuable assistance for forecasting PPCPs' environmental fate.

Humic substances derived from unconventional resources: extraction, properties, environmental impacts, and prospects

Humic substances comprise up to 70% of the total organic matter in soils, between 50 and 80% of the dissolved organic matter in water, and about 25% of dissolved organic matter in groundwater. Elucidation of the complex structure and properties of humic substances requires advanced analytical tools; however, they are of fundamental importance in medicine, agriculture, technology, and the environment, at large. Although they are naturally occurring, significant efforts are now being directed into their extraction owing to their relevance in improving soil properties and other environmental applications. In the present review, the different fractions of humic substances were elucidated, underlying the mechanisms by which they function in soils. Furthermore, the extraction processes of humic substances from various feedstock were illustrated, with the alkali extraction technique being the most widely used. In addition, the functional group and elemental composition of humic substances were discussed. The similarities and/or variations in the properties of humic substances as influenced by the source and origin of feedstock were highlighted. Finally, the environmental impacts of humic substances were discussed while highlighting prospects of humic acid production. This review offers enormous potential in identifying these knowledge gaps while recommending the need for inter- and multidisciplinary studies in making extensive efforts toward the sustainable production of humic substances.

Nano/microplastics: Fragmentation, interaction with co-existing pollutants and their removal from wastewater using membrane processes

NANO: and microplastic (NP/MP) is one of the most challenging types of micropollutants, coming from either direct release or degradation of plastic items into ecosystems. NP/MP can adsorb hazardous pollutants (such as heavy metals and pharmaceutical compounds) and pathogens onto their surface that are consumed by humans, animals, and aquatic living organisms. This paper presents the interaction of NP/MP with other pollutants in the water environment and mechanisms involved to enable the ultimate fate of NP/MP as well as the effectiveness of metal-organic frame (MOF)-based membrane over conventional membrane processes for NP/MP removal. It is found that conventional membranes could remove MPs when their size is usually more than 1000 nm, but they are ineffective in removing NPs. These NPs have potentially greater health impacts due to their greater surface area. MOF-based membrane could effectively remove both NP and MP due to its large porous structure, high adsorption capacity, and low density. This paper also discusses some challenges associated with MOF-based membranes for NP/MP removal. Finally, we conclude a specific MOF-based ultrafiltration membrane (ED-MIL-101 (Cr)) that can potentially remove both negative and positive charged NP/MP from wastewater by electrostatic attraction and repulsion force with efficient water permeability.

Antibiotics in wastewater of Guangdong, China: distribution patterns, and their environmental risk due to incomplete removal

In this study, the occurrence, removal, spatial distribution and environmental risks of ten antibiotics in 38 wastewater treatment plants (WWTPs) which located in 15 communities (in 12 cities) of the Guangdong province, China. The results indicated that amoxicillin (AMO, 381.00-5230.00 ng·L^-1) and ofloxacin (OFX, 7.01-836.00 ng·L^-1) were predominant in the province with high detection rates (100 %) and high concentrations. The concentration of norfloxacin (NFX) was high in some influents (2070.00 ng·L^-1), but its detection rate was only 47.4 %. In addition, the average daily mass load per 10³ capita (ADMLpc) of AMO (383.34 mg·(10³ capita)^-1·d^-1) was significantly higher than the other antibiotics. The comparison between the ADMLpc and social-economic factors indicated that adult (age between 18 and 60) was the age group mainly consume antibiotics in the province. Furthermore, the ADMLpc of human antibiotics (e.g., AMO and OFX) were positively affected by the economic development of the region. Comparatively, the ADMLpc veterinary antibiotic (e.g., SMM) was positively related to the livestock related factors. In general, no significant difference (p > 0.05) in antibiotic removal efficiencies among four treatment processes applied in all WWTPs. Nevertheless, the low removal rates of sulfamethazine (SMZ, 44.6 %) and OFX (26.3 %) indicated that they were more difficult to be removed than other antibiotics. Furthermore, high ADMLpc of antibiotics in the influent would cause high ADMLpc in effluents. Therefore, heavy consumption of antibiotics could pose high risk to the ecosystem, especially when multiple antibiotics appeared in the effluent of the WWTPs.

A Bioavailability Model to Predict the Impact of pH and Dissolved Organic Carbon on Ciprofloxacin Ecotoxicity to the Cyanobacterium Microcystis aeruginosa

Ciprofloxacin (CIP) is a pseudopersistent antibiotic detected in freshwater worldwide. As an ionizable chemical, its fate in freshwater is influenced by water chemistry factors such as pH, hardness, and dissolved organic carbon (DOC) content. We investigated the effect of pH, DOC, and Ca²⁺ levels on the toxicity of CIP to Microcystis aeruginosa and developed a bioavailability model on the basis of these experimental results. We found that the zwitterion (CIP^+/- ) is the most bioavailable species of CIP to M. aeruginosa, whereas DOC is the most dominant factor reducing CIP toxicity, possibly via binding of both CIP^+/- and CIP⁺ to DOC. pH likely also regulates CIP-DOC binding indirectly through its influence on CIP speciation. In addition, higher tolerance to CIP by M. aeruginosa was observed at pH < 7.2, but the underlying mechanism is yet unclear. Calcium was identified as an insignificant factor in CIP bioavailability. When parameterized with the data obtained from toxicity experiments, our bioavailability model is able to provide accurate predictions of CIP toxicity because the observed and predicted total median effective concentrations deviated by <28% from each other. Our model predicts that changes in pH and DOC conditions can affect CIP toxicity by up to 10-fold, suggesting that CIP in many natural environments is likely less toxic than in standard laboratory toxicity experiments. Environ Toxicol Chem 2022;41:2835-2847. © 2022 SETAC.

Sorption of pharmaceuticals over microplastics' surfaces: interaction mechanisms and governing factors

Microplastics are one of the emerging and ubiquitous environmental pollutants. Recent studies have proven their co-existence with pharmaceuticals in the environment wherein microplastics act as a potential vector for the transportation of pharmaceuticals. Both microplastics and pharmaceuticals are charged moieties enriched with diverse functional groups resulting in the possibility of multiple interactions. Major interactions could be electrostatic, hydrogen bonding, and hydrophobic, while minor interactions may occur through π-π interaction, cationic bridging mechanism, van der Waals interaction, partition, and pore-filling mechanism. These interactions have both short- and long-term effects over pharmaceutical sorption on microplastics and possibly, ensuing toxicity. This review analyses and summarises the currently reported interactions between microplastic particles and pharmaceuticals as well as establishes the link to various factors affecting the process, viz. pH, salinity, dissolved organic matter, and physiochemical properties of microplastics.

Fate and transport of pharmaceuticals in water systems: A processes review

Pharmaceuticals are globally consumed by humans and animals to support daily health and to treat disease. Following consumption, they may reach the aquatic environment either directly through the discharge of untreated wastewater to water bodies, or indirectly via treated wastewater as a result of their incomplete removal from wastewater treatment plants. This paper reviews the processes that control the occurrence and fate of pharmaceuticals in water systems, including sorption, photodegradation, hydrolysis and biodegradation. The degree to which these four processes occur is influenced by pharmaceutical types and their chemical structure as well as environmental factors such as sunlight, water depth, organic matter content, water chemistry, sediment properties, and type and abundance of microorganisms. Depending on the complex interactions of these factors, pharmaceutical compounds may be mineralized, partially degraded, or remain intact because they are resistant to degradation. Kinetic rate parameters and the half-life of a variety of pharmaceutical products are provided herein for the above processes under different environmental conditions. Usually, photodegradation and biodegradation represent dominant reaction processes, while hydrolysis only affects some pharmaceuticals, particularly antibiotics. The identified sorption and reaction rate parameters can be incorporated into a concise modeling framework to assess and predict longitudinal concentration profiles of pharmaceutical products in the manmade and natural systems, particularly when large amounts of pharmaceuticals are discharged during abnormal events such as a virus outbreak. Finally, future research is suggested, including the fate of transformed products (intermediates) in water systems.

Understanding the Effect of pH on the Solubility and Aggregation Extent of Humic Acid in Solution by Combining Simulation and the Experiment

Molecular dynamics (MD) simulations were performed to investigate the dynamics of humic acid (HA) in an aqueous solution and the influence of pH, temperature, and HA concentration. The HA model employed in MD simulations was chosen and validated using experimental chemical composition data and Fourier transform infrared (FTIR) spectra. The simulations showed that the HA molecule has a strong propensity to adopt a compact conformation in water independent of pH, while the aggregation of HA was found to be pH-dependent. At high pH, the ionized HAs assembled into a thread-like structure, maximizing contact with water. At low pH, the neutral HAs formed a droplet-like aggregate, minimizing contact with the solvent. The simulation results are consistent with experimental data from dynamic light scattering (DLS) measurements and transmission electron microscopy (TEM) imaging. This work provides new insight into the folding and aggregation of HA as a function of pH and a molecular-level understanding of the relationship between the acidity and the structure, solubility, and aggregation of HA, with direct implications for HA-based remediation strategies of contaminated sites.

Molecular Docking as a Tool to Examine Organic Cation Sorption to Organic Matter

Molecular docking simulations were performed to examine the structural effects of organic cations on their sorption to organic matter. A set of benzylamine compounds was used to assess the sorption trends arising from the systematic structural differences between ring or nitrogen substituents. Binding simulations were performed using AutoDock 4.2 with Schulten's proposed soil organic matter as a representative organic matter structure. The calculated binding energies for the sorbate compounds correlated strongly with the measured sorption energies for Pahokee peat, indicating that the simulated binding energies and their associated sorbate orientations were representative of the experimental conditions. Graphical docking orientations showed primary, secondary, and tertiary aminium compounds to form hydrogen-bond interactions with deprotonated carboxylic acid groups in a pocket of the organic matter structure. Quaternary ammonium compounds formed pi-pi or cation-pi interactions with the aromatic groups elsewhere in the same organic matter pocket. Ring substituents showed no clear trends in sorption energies with the substituent group type for primary aminium compounds. Rather, substituent groups altered the simulated van der Waals, electrostatic, hydrogen-bond, and desolvation energy contributions to the overall sorption energies, in part because of the variations in docking orientations between compounds. Increasing methyl substitution of the aminium nitrogen group was associated with an increase in van der Waals energy contributions and a decrease in electrostatic energy contributions to the overall compound sorption energies because of aminium charge delocalization into methyl substituents and steric hindrance from methyl substituents to form specific interactions. The findings illustrate how molecular docking can be used to explore the effects of organic cation structure on sorption interactions with organic matter.

Adsorption of cationic dyes, drugs and metal from aqueous solutions using a polymer composite of magnetic/β-cyclodextrin/activated charcoal/Na alginate: Isotherm, kinetics and regeneration studies

In this work, we successfully synthesized novel polymer gel beads based on functionalized iron oxide (Fe3O4), activated charcoal (AC) particles with β-cyclodextrin (CD) and sodium alginate (SA) polymer (Fe3O4/CD/AC/SA), by a simple, reproducible and inexpensive method. These beads proved to be versatile and strong adsorbents with magnetic properties and high adsorption capacity. The composites were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, vibrating sample magnetometry, adsorption at -196 °C, high resolution transmission electron microscopy, thermogravimetric analysis and point of zero charge measurements. Two dyes, two drugs and one metal were used to test the adsorption capability of the prepared polymer nanocomposite. The adsorbent showed good removal efficiencies for the studied pollutants, especially the cationic dyes and the metal, when compared to other low-cost adsorbents. The saturated adsorption capacity of Fe3O4/CD/AC/SA reached 5.882 mg g-1 for methyl violet (MV), 2.283 mg g-1 for brilliant green (BG), 2.551 mg g-1 for norfloxacin (NOX), 3.125 mg g-1 for ciprofloxacin (CPX), 10.10 mg g-1 for copper metal ion (Cu(II)). The adsorption isotherm studies showed that data fitted well with Langmuir and Temkin isotherms models. The kinetic data showed good correlation coefficient with low error function for the pseudo-second order kinetic model. The data analysis was carried out using error and regression coefficient functions for the estimation of best-fitting isotherm and kinetic models, namely: chi-square test (χ2) and sum of the squares of errors (SSE). The activation energy was found to be 47.68 kJ mol-1 for BG, 29.09 kJ mol-1 for MV, 28.93 kJ mol-1 for NOX, 4.53 kJ mol-1 for CPX and 17.08 kJ mol-1 for Cu(II), which represent chemisorption and physisorption behavior of sorbent molecules. The polymer composites can be regenerated and easily separated from aqueous solution without any weight loss. After regeneration, the Fe3O4/CD/AC/SA beads still have good adsorption capacities up to four cycles of desorption and adsorption. The results indicate that the polymer gel beads are promising adsorbents for the removal of different categories of toxicants (like dyes, drugs and metal) in single adsorbate aqueous systems. Thus, the novel Fe3O4/CD/AC/SA beads can be effectively employed for a large-scale applications as environmentally compatible materials for the adsorption of different categories of pollutants.

Significant role of iron on the fate and photodegradation of enrofloxacin

Enrofloxacin (ENR) belongs to the fluoroquinolone (FQ) antibiotics family, which are contaminants of emerging concern frequently found in effluents. Although many works studying photo-Fenton process for FQ degradation have been reported, there are no reports analysing in deep the effect of iron complexation, as well as other metals, towards FQs' photolysis, which, evidently, also contributes in the overall degradation of the pollutant. Therefore, in this work, we report a comparative study between the photochemical fate of ENR and its complex with Fe(III) under simulated sunlight irradiation. In addition, the effect of dissolved oxygen, self-sensitization process, and H₂O₂ addition on the studied photochemical systems are also investigated. Results indicate that, for free and iron-complexed ENR, singlet oxygen (¹O₂) is generated from the interaction of its triplet state with ground state oxygen. Half-life time (t_1/2) of ENR under sun simulated conditions is estimated to be around 22 min, while complexation with iron enhances its photostability, leading to a t_1/2 of 2.1 h. Such finding indicates that at least the presence of iron, might notably increase the residence time of these pollutants in the environment. Eventually, only with the addition of H₂O₂, the FQ-iron complex is efficiently degraded due to photo-Fenton process even at circumneutral pH values due to the high stability of the formed complex. Finally, after LC/FT-ICR MS analysis, 39 photoproducts are detected, of which the 14 most abundant ones are identified. Results indicate that photoproducts formation is pH and iron dependent.

Disentangling Mechanisms in Natural Toxin Sorption to Soil Organic Carbon

Natural toxins are multifunctional, often ionizable organic compounds increasingly detected in the environment. Surprisingly little is known about their interactions with soil organic carbon, although sorption largely controls transport, bioavailability, and dissipation. For a set of 117 natural toxins from 36 compound classes the pH-dependent organic carbon-water distribution coefficient (D_oc) was quantified using a soil column chromatography approach under changing conditions with regards to pH, ionic strength, and the major inorganic cation in solution. Natural toxins could be assigned to groups with either hydrophobic partitioning or specific interactions (complexation reactions, cation exchange) as dominating sorption mechanisms. The complex interplay of interactions in the sorption of natural toxins was equally influenced by sorbate, sorbent, and solution specific characteristics. High variability in sorption was particularly observed in the presence of Ca²⁺ resulting in D_oc being enhanced by a factor of 10 when the pH was increased from 4.5 to 6. Sorbates following this trend contain either functional groups able to form ternary complexes via Ca²⁺ or aromatic moieties adjacent to protonated N presumably stabilizing cation exchange reactions. Although sorption was often stronger than predicted, investigated natural toxins were highly mobile under all considered conditions.

Interactions between microplastics, pharmaceuticals and personal care products: Implications for vector transport

Microplastics are well known for vector transport of hydrophobic organic contaminants, and there are growing concerns regarding their potential adverse effects on ecosystems and human health. However, recent studies focussing on hydrophilic compounds, such as pharmaceuticals and personal care products (PPCPs), have shown that the compounds ability to be adsorbed onto plastic surfaces. The extensive use of PPCPs has led to their ubiquitous presence in the environment resulting in their cooccurrence with microplastics. The partitioning between plastics and PPCPs and their fate through vector transport are determined by various physicochemical characteristics and environmental conditions of specific matrices. Although the sorption capacities of microplastics for different PPCP compounds have been investigated extensively, these findings have not yet been synthesized and analyzed critically. The specific objectives of this review were to synthesize and critically assess the various factors that affect the adsorption of hydrophilic compounds such as PPCPs on microplastic surfaces and their fate and transport in the environment. The review also focuses on environmental factors such as pH, salinity, and dissolved organics, and properties of polymers and PPCP compounds, and the relationships with sorption dynamics and mechanisms. Furthermore, the ecotoxicological effects of PPCP-sorbed microplastics on biota and human health are also discussed.

Occurrence and risk assessment of pharmaceutically active compounds in water supply systems in Brazil

The presence of pharmaceutically active compounds (PhACs) in water supply systems has been generating great concern about their effects on the environment and human health. Twenty-eight PhACs were monitored during one year in four Brazilian water sources, aiming to understand the factors that influence their occurrence and removal in conventional drinking water treatment plants (DWTPs) and to assess the environmental and human health risks. Trace levels of PhACs were detected in surface and drinking water in all assessed water sources. Effects of seasonality and socioeconomic aspects were observed in PhACs occurrence, like their higher concentrations during winter and in locales with higher values of gross domestic product per capita and human development index. Betamethasone, prednisone, and fluconazole were the most commonly detected PhACs, and also presented the highest concentrations. However, they were not related to toxicological risks. Nonetheless, all surface waters were subject to toxicological risk owing to at least one PhAC. PhACs related to the highest toxicological risks were loratadine, atorvastatin, norfloxacin, caffeine, and ranitidine, however, all these PhACs presented low quantification frequency. DWTPs capacity to remove PhACs was only partial, so treated water was still contaminated with these compounds. Furthermore, atorvastatin presented a margin of exposure below 100, indicating possible risk for public health. Thus, additional advanced treatment steps should be considered to improve PhACs removal during drinking water treatment.

Sorption of ionized dyes on high-salinity microalgal residue derived biochar: Electron acceptor-donor and metal-organic bridging mechanisms

Biochar (BC) has attracted much attention owing to its superior sorption capacity towards ionized organic contaminants. However, the mechanism of ionized organics sorption occurring within BC containing large amounts of minerals is still controversial. In this study, we demonstrate the physicochemical structure of high-salinity microalgal residue derived biochar (HSBC) and elucidate the corresponding sorption mechanisms for four ionized dyes along with determining the crucial role of involved minerals. The results indicate that sodium and calcium minerals mainly exist within HSBCs, and the pyrolysis temperature can dramatically regulate the phases and interfacial property of both carbon matrix and minerals. As a result, the HSBC shows a higher sorption potential, benefiting from abundant functional groups and high content of inorganic minerals. Using theoretical calculations, the activities of electron donor-acceptor interaction between HSBCs and different dyes are clearly illustrated, thereby identifying the critical role of Ca²⁺ in enhancing the removal of ionized dyes in HSBCs. In addition, Ca-containing minerals facilitate the sorption of ionized dyes in HSBCs by forming ternary complexes through metal-bridging mechanism. These results of mineral-induced dye sorption mechanisms help to better understand the sorption of ionized organics in high-salt containing BC and provide a new disposal strategy for hazardous microalgal residue, as well as provide a breakthrough in making the remediation of ionized organic contaminated microalgal residue derived absorbent feasible.

Effect of biosolids characteristics on retention and release behavior of azithromycin and ciprofloxacin

Azithromycin (AZ) and ciprofloxacin (CIP) are commonly prescribed antibiotics frequently detected in municipal biosolids and identified by the USEPA as contaminants of emerging concern. The land application of municipal biosolids is an agronomically beneficial practice but is also a potential pathway of CIP and AZ release into the environment. Understanding retention-release behavior is crucial for assessing the environmental fate of and risks from land-applied biosolids-borne target antibiotics. Here, we used batch equilibrations to assess retention and release of environmentally relevant concentrations of CIP and AZ in ten different biosolids. The biosolids included Class A and Class B materials with a range of physiochemical characteristics (e.g. pH, cation exchange capacity (CEC), organic matter content (OM), and iron (Fe) and aluminum (Al)) expected to influence retention and release of AZ and CIP. Retention was linear (R² > 0.99 for AZ and >0.96 for CIP) and sorption coefficients (K_d) ranged from 52 to 370 L kg^-1 for AZ and 430-2300 L kg^-1 for CIP. Desorption also varied but was highly hysteretic, with hysteresis coefficients (H) ranging 0.01 to 0.15 for AZ and ≤0.01 for CIP, suggesting limited bioaccessibility. The penalized and shrinkage method least absolute shrinkage and selection operator (LASSO) was used to produce models describing AZ and CIP sorption behavior based on any given biosolids physiochemical characteristics. Multiple linear regression analysis linked AZ sorption behavior to total Fe content, likely due to a predisposition of AZ to participate in reactions with in situ Fe species. CIP sorption behavior was linked to oxalate extractable Al and total phosphorus (P) content, suggesting CIP bonding with amorphous forms of Al and a potential relationship between CIP sorption to biosolids and biosolids production processes, as manifested by correlation of CIP sorption with total P content.

Evaluation of Existing Models to Estimate Sorption Coefficients for Ionisable Pharmaceuticals in Soils and Sludge

In order to assess the environmental risk of a pharmaceutical, information is needed on the sorption of the compound to solids. Here we use a high-quality database of measured sorption coefficients, all determined following internationally recognised protocols, to evaluate models that have been proposed for estimating sorption of pharmaceuticals from chemical structure, some of which are already being used for environmental risk assessment and prioritization purposes. Our analyses demonstrate that octanol-water partition coefficient (K_ow) alone is not an effective predictor of ionisable pharmaceutical sorption in soils. Polyparameter models based on pharmaceutical characteristics in combination with key soil properties, such as cation exchange capacity, increase model complexity but yield an improvement in the predictive capability of soil sorption models. Nevertheless, as the models included in this analysis were only able to predict a maximum of 71% and 67% of the sorption coefficients for the compounds to within one log unit of the corresponding measured value in soils and sludge, respectively, there is a need for new models to be developed to better predict the sorption of ionisable pharmaceuticals in soil and sludge systems. The variation in sorption coefficients, even for a single pharmaceutical across different solid types, makes this an inherently difficult task, and therefore requires a broad understanding of both chemical and sorbent properties driving the sorption process.

Understanding effect of interaction of nanoparticles and antibiotics on bacteria survival under aquatic conditions: Knowns and unknowns

The review provides a comprehensive overview of the available state-of-the-art of nanoparticles (NPs) and antibiotics (ABs) occurrence and their fate in the natural aquatic settings by addressing different research questions and the challenges faced while addressing those questions. Firstly, understand the interaction of NPs and ABs with themselves in addition to other matrix components (presence of natural organic matter, bacteria, biofilms, other anthropogenic pollutants and metals from natural sources). Secondly, summarize the bactericidal activity of NP and AB due to reactive oxygen species (ROS) production. The complete information was gathered from database and analysed as per the conjectured questions under laboratory versus environmental-relevant conditions (1. Fate of NPs and ABs, and 2. Will the presence of NPs and ABs alone and their mixtures influence the ROS concentration and antibacterial activity), and proposed six reactions to describe the fate of NP and AB in natural aquatic settings. However, laboratory-based studies revealed that NP and AB fate largely depend on the ionic strength, organic matter content and pH of the matrix whereas field based information is missing about this. The former was performed at sterile conditions using sophisticated instruments and standard protocol as compared to latter and can't be replicated under natural aquatic settings due to lack of: (i) accurate environmental concentration of NPs and ABs, (ii) knowledge of bacterial type and their concentration, (iii) optimized protocol and tracking systems. The author's recommendation is to verify the proposed reactions experimentally by using the frequently found pairs of NPs and ABs in the natural aquatic settings. Further, ranked them on their decreasing order of toxicity and informed regulatory bodies for further action. Overall research is needed in the suggested directions to reduce uncertainty behind the impacts of NPs and ABs on the aquatic settings and their role in bactericidal activity.

Excited Triplet State Interactions of Fluoroquinolone Norfloxacin with Natural Organic Matter: A Laser Spectroscopy Study

In sunlit waters, the fate of fluoroquinolone antibiotics is significantly impacted by photodegradation. The mechanism of how natural organic matter (NOM) participates in the reaction has been frequently studied but still remains unclear. In this work, the interactions between the excited triplet state of the fluoroquinolone antibiotic norfloxacin (³NOR*) and a variety of NOM extracts were investigated using time-resolved laser spectroscopy. The observed transient absorption spectrum of ³NOR* showed a maximum at ca. 600 nm, and global fitting gave a lifetime of 1.0 μs for ³NOR* in phosphate buffer at pH = 7.5. Quenching of ³NOR* by Suwannee River hydrophobic acids (HPO), Beaufort River HPO, and Gartempe River HPO yielded rate constants of 1.8, 2.6, and 4.5 (×10⁷ mol_C^-1 s^-1) respectively, whereas HPO from South Platte River unexpectedly increased the lifetime of ³NOR* with an as yet unknown mechanism. Concurrent photodegradation experiments of NOR (5 μM) in the presence of these NOM were also performed using a sunlight simulator. In general, the effects of NOM on the photodegradation rate of NOR were in agreement with observations from transient absorption studies. We suggest that adsorption of NOR to NOM is one of the major factors contributing to the observed quenching. These results yield a new insight into the likely role of NOM in sunlight-induced degradation of micropollutants.

Ciprofloxacin-metal complexes -stability and toxicity tests in the presence of humic substances

The co-contamination of ciprofloxacin (CIP) with metal ions results in alteration of CIP mobility, antimicrobial activity and distribution/development of the antibiotic-resistance genes. In this study, the stability of five CIP-Me complexes [Me = Al(III), Co(II), Cu(II), Fe(III), Mg] was investigated in the presence of humic substances (HS) at two temperatures 18 ± 2 °C and 4 ± 1 °C for seven days period. The most stable complexes were CIP-Al, CIP-Cu, and CIP-Co with the stability constants (K) at 18 °C 35.5 ± 1.4 11.5 ± 1.5 and 11.7 ± 1.5 respectively. At lower temperature (4 °C), the stability constants decreased: 1-fold for CIP-Al, 14-fold for CIP-Co and 2-fold for CIP-Cu. The presence of humic substances decreased the stability of complexes. The chemical reactions of Fe³⁺ in water at circumneutral pH resulted in stability alteration. The formation of CIP-Mg complexes at lower temperatures and in the presence of HS was limited. In ultrapure water, CIP-Me complexes exhibit higher toxicity towards Gram-negative Enterobacter aeruginosa (ranged between 0.125 and 0.5 μg/ml). However, the presence of HS reduced the antimicrobial activity of CIP-Me complexes by at least 2-fold. Gram-positive representative, Bacillus subtilis was not affected by the presence of metal ions and/or HS. The toxicity toward B. subtilis for the complexes was equal to toxicity of CIP alone (MIC = 0.25 μg/ml). This suggested the different susceptibility to CIP and its complexes.

Ultrahigh selective adsorption of zwitterionic PPCPs both in the absence and presence of humic acid: Performance and mechanism

Since zwitterionic PPCPs could be combined with humic acid (HA) leading to certain contaminants residue in aqueous solution, adsorbents with much stronger binding with zwitterionic PPCPs were needed to avoid this phenomenon. Through comparison of serial magnetic carboxyl modified hypercrosslinked resins including MA10, MA30, MA40 and MA70, MA50 was found to exhibit ultrahigh selective adsorption of zwitterionic PPCPs including tetracycline and quinolone antibiotics due to the remarkable synergistic effects generated from cation exchange interaction and non-ion exchange interaction. To figure out the effect of HA, other five adsorbents including hypercrosslinked resin Q100, aminated hypercrosslinked resin GMA50, anion exchange resin MIEX, wood-based activated carbon F400D and coal-based activated carbon Norit were chosen as comparison to MA50 in adsorption performance of tetracycline (TC) and ciprofloxacin (CPX). It turned out that the percentage of CPX or TC combined with HA in aqueous solution varied greatly for studied adsorbents. There existed serious false-positive adsorption during the adsorption process by some commercial adsorbents such as MIEX and F400D, while MA50 exhibited relatively lower false-positive adsorption, ensuring the maximum safety of effluents. Breakthrough tests showed that MA50 had significant advantages in PPCPs removal at environment concentration, indicating its potential in application for real water.

Dissipation of antibiotics in three different agricultural soils after repeated application of biosolids

Application of biosolids to agricultural soils is one of the pathways by which antibiotics can be introduced into agricultural ecosystems. A pot experiment was conducted with repeated soil amendment with biosolids to examine the concentrations of four classes of antibiotics (tetracyclines, sulfonamides, fluoroquinolones, and macrolides) and their dissipation in three different soil types in wheat-rice rotations. Antibiotics accumulate in the soils after repeated application of biosolids. Fluoroquinolones showed stronger accumulation and persistence in the test soils than the other three classes of antibiotics. The maximum residual antibiotic concentration was that of norfloxacin at 155 ± 16 μg kg^-1 in the Typic Hapli-Stagnic Anthrosols (paddy soil). Predicted half-lives were up to 3.69 years, a much longer period than that between biosolid applications (twice each year on average). Antibiotic accumulation followed the rough order fluoroquinolones > tetracyclines > macrolides > sulfonamides, and the sulfonamides were seldom encountered. When biosolid application was suspended, the dissipation rate accelerated. Antibiotic dissipation was slightly slower when biosolids with high heavy metal concentrations were applied and microbial degradation may have been the main mechanism of dissipation. Norfloxacin persistence was positively correlated with its soil adsorption capacity. Cation exchange capacity and soil organic matter content may have vital roles in the soil adsorption of fluoroquinolones. Because of their persistence, the fluoroquinolones must be taken into account in the planning of biosolid applications in agricultural practice.

Fluoroquinolones (FQs) in the environment: A review on their abundance, sorption and toxicity in soil

The use of fluoroquinolones (FQs) antibiotics as therapeutic agents and growth promoters is increasing worldwide; however their extensive uses are also resulting in antibiotic resistance among world communities. FQs have also become one of the major contaminants in the waste water bodies, which are not even completely removed during the treatment processes. Furthermore, their abundance in agricultural resources, such as the irrigation water, the bio-solids and the livestock manure can also affect the soil micro-environment. These antibiotics in soil tend to interact in several different ways to affect soil flora and fauna. The current review endeavors to highlight the some critical aspects of FQs prevalence in the environment. The review presents a detailed discussion on the pathways and abundance of FQs in soil. The discussion further spans the issue of sorption and FQs transformation into the soil better understand of their behavior and their toxicity to soil flora and fauna.

A Fluorescence Quenching Analysis of the Binding of Fluoroquinolones to Humic Acid

Fluorescence quenching was used to investigate the interaction of six fluoroquinolones with humic acid. Static quenching was observed for the binding of ciprofloxacin, enoxacin, fleroxacin, levofloxacin, norfloxacin, and ofloxacin to humic acid. The equilibrium binding constants were found from Stern-Volmer plots of the data. The quenching experiments were repeated over a temperature range of 25-45 ℃ and van't Hoff plots were generated. From these linear plots, thermodynamic values were calculated for Δ H, Δ G, and Δ S for each of the fluoroquinolones. The equilibrium binding constants were found to be <1 for all the antibiotics studied. The calculated ΔH values were all negative and ranged from -9.5 to -27.6 kJ/mol. The high water solubility of the antibiotics and low ΔH of binding suggests that the antibiotics will be transported easily through the environment. Finally, whether the fluoroquinolones are in a protonated, deprotonated, or partially protonated state is found to correlate to the strength of binding to humic acid.

Use of nuclear receptor luciferase-based bioassays to detect endocrine active chemicals in a biosolids-biochar amended soil

Biosolids are a potentially valuable source of carbon and nutrients for agricultural soils; however, potential unintended impacts on human health and the environment must be considered. Virtually all biosolids contain trace amounts endocrine-disrupting chemicals derived from human use of pharmaceuticals and personal care products (PPCPs). One potential way to reduce the bioavailability of PPCPs is to co-apply biosolids with biochar to soil, because biochar's chemical (e.g., aromaticity) and physical properties (e.g., surface area) give it a high affinity to bind many organic chemicals in the environment. We developed a soil-specific extraction method and utilized a luciferase-based bioassay (CALUX) to detect endocrine active chemicals in a biosolids-biochar co-amendment soil greenhouse study. Both biochar (walnut shell, 900 °C) and biosolids had positive impacts on carrot and lettuce biomass accumulation over our study period. However, the walnut shell biochar stimulated aryl hydrocarbon receptor activity, suggesting the presence of potential endocrine active chemicals in the biochar. Since the biochar rate tested (100 t ha^-1) is above the average agronomic rate (10-20 t ha^-1), endocrine effects would not be expected in most environmental applications. The effect of high temperature biochars on endocrine system pathways must be explored further, using both quantitative analytical tools to identify potential endocrine active chemicals and highly sensitive bioanalytical assays such as CALUX to measure the resulting biological activity of such compounds.

Sorption mechanism of enrofloxacin on humic acids extracted from Brazilian soils

Veterinary antimicrobials are emerging environmental contaminants of concern. In this study, the sorption of enrofloxacin (ENR) onto humic acids (HAs) extracted from three Brazilian soils was evaluated. HAs were characterized by elemental analysis and solid ¹³C nuclear magnetic resonance spectroscopy. The sorption of ENR onto HAs was at least 20-fold higher than onto the soils from which they were separated. Ionic and cation bridging are the primary interactions involved. The interactions driven by cation exchange are predominant on HAs, which appear to have abundant carboxylic groups and a relatively high proportion of H-bond donor moieties with carbohydrate-like structures. Interactions explained by cation bridging and/or surface complexation on HAs are facilitated by moieties containing conjugated ligands, significant content of oxygen-containing functional groups, such as phenolic-OH or lignin-like structures. HAs containing electron-donating phenolic moieties and carboxylic acid ligand groups exhibit a sorption mechanism that is primarily driven by strong metal binding, favoring the formation of ternary complexes between functional groups of the organic matter and drugs.

Transformation and toxicity evaluation of tetracycline in humic acid solution by laccase coupled with 1-hydroxybenzotriazole

Enzyme-based catalyzed oxidative coupling reactions (E-COCRs) are considered as viable technologies to transform a variety of pharmaceutical antibiotics. This study indicated that the extracellular fungal laccase from Pleurotus ostreatus was effective in transforming tetracycline (TC) with 1-hydroxybenzotriazole (HBT) present at varying conditions during E-COCRs. The presence of humic acid (HA) showed suppressive effect on the transformation rate constants (k) of TC, and the k values for TC decreased as HA concentration increased. It was ascribed primarily to the covalent binding between TC and HA, which reduced the apparent concentration and availability of TC in water. It is noted that TC molecules from the cross-coupling products were likely re-released under extreme conditions (pH<2.0). The intermediate products were identified regardless of HA presence by high-resolution mass spectrometry (HRMS). A possible reaction pathway of TC in HA solution including electron transfer, hydroxylation, dehydrogenation, oxidation, radical reaction, decomposition, and covalent binding was proposed. The growth inhibition assays of Escherichia coli (E. coli) confirmed that the antimicrobial activity of TC was remarkably reduced with an increasing reaction time. These findings provide novel insights into the decomposition and cross-coupling of TC in a multi-solute natural aquatic environment by laccase-based catalyzed oxidative processes.

Characterization of extracellular polymeric substances in biofilms under long-term exposure to ciprofloxacin antibiotic using fluorescence excitation-emission matrix and parallel factor analysis

The presence of antibiotic residues in the environment has been regarded as an emerging concern due to their potential adverse environmental consequences such as antibiotic resistance. However, the interaction between antibiotics and extracellular polymeric substances (EPSs) of biofilms in wastewater treatment systems is not entirely clear. In this study, the effect of ciprofloxacin (CIP) antibiotic on biofilm EPS matrix was investigated and characterized using fluorescence excitation-emission matrix (EEM) and parallel factor (PARAFAC) analysis. Physicochemical analysis showed that the proteins were the major EPS fraction, and their contents increased gradually with an increase in CIP concentration (0-300 μg/L). Based on the characterization of biofilm tightly bound EPS (TB-EPS) by EEM, three fluorescent components were identified by PARAFAC analysis. Component C1 was associated with protein-like substances, and components C2 and C3 belonged to humic-like substances. Component C1 exhibited an increasing trend as the CIP addition increased. Pearson's correlation results showed that CIP correlated significantly with the protein contents and component C1, while strong correlations were also found among UV₂₅₄, dissolved organic carbon, humic acids, and component C3. A combined use of EEM-PARAFAC analysis and chemical measurements was demonstrated as a favorable approach for the characterization of variations in biofilm EPS in the presence of CIP antibiotic.

Adsorption of sulfamethoxazole (SMZ) and ciprofloxacin (CIP) by humic acid (HA): characteristics and mechanism

The adsorption behavior and mechanisms of single adsorption and co-adsorption of ciprofloxacin and sulfamethoxazole with HA were studied in detail.

The Role of Concentration and Solvent Character in the Molecular Organization of Humic Acids

The molecular organization of humic acids in different aqueous solutions was studied over a wide concentration range (0.01-10 g·dm^-3). Solutions of humic acids were prepared in three different media: NaOH, NaCl, and NaOH neutralized by HCl after dissolution of the humic sample. Potentiometry, conductometry, densitometry, and high resolution ultrasound spectrometry were used in order to investigate conformational changes in the humic systems. The molecular organization of humic acids in the studied systems could be divided into three concentration ranges. The rearrangements were observed at concentrations of ~0.02 g·dm^-3 and ~1 g·dm^-3. The first "switch-over point" was connected with changes in the hydration shells of humic particles resulting in changes in their elasticity. The compressibility of water in the hydration shells is less than the compressibility of bulk water. The transfer of hydration water into bulk water increased the total compressibility of the solution, reducing the ultrasonic velocity. The aggregation of humic particles and the formation of rigid structures in systems with concentrations higher than 1 g·dm^-3 was detected.

Fluorescence quenching effects of antibiotics on the main components of dissolved organic matter

Dissolved organic matter (DOM) in wastewater can be characterized using fluorescence excitation-emission matrix and parallel factor (EEM-PARAFAC) analysis. Wastewater from animal farms or pharmaceutical plants usually contains high concentration of antibiotics. In this study, the quenching effect of antibiotics on the typical components of DOM was explored using fluorescence EEM-PARAFAC analysis. Four antibiotics (roxarsone, sulfaquinoxaline sodium, oxytetracycline, and erythromycin) at the concentration of 0.5∼4.0 mg/L and three typical components of DOM (tyrosine, tryptophan, and humic acid) were selected. Fluorescence quenching effects were observed with the addition of antibiotics. Among these four antibiotics, roxarsone (2.9∼20.2 %), sulfaquinoxaline sodium (0∼32.0 %), and oxytetracycline (0∼41.8 %) led to a stronger quenching effect than erythromycin (0∼8.0 %). From the side of DOM, tyrosine and tryptophan (0.5∼41.8 %) exhibited a similar quenching effect, but they were higher than humic acids (0∼20.2 %) at the same concentration of antibiotics. For humic acid, a significant quenching effect was observed only with the addition of roxarsone. This might be the first report about the fluorescence quenching effect caused by antibiotics. The results from this study confirmed the interference of antibiotics on the fluorescence intensity of the main components of DOM and highlighted the importance of correcting fluorescence data in the wastewater containing antibiotics.

Electrochemical evaluation of ciprofloxacin adsorption on soil organic matter

In order to evaluate humic acid–ciprofloxacin adsorption, a new electrochemical method was tested and applied to adsorption isotherm studies, which were well fitted by the Langmuir–Freundlich model.

Biodegradation screening of chemicals in an artificial matrix simulating the water-sediment interface

Biodegradation is the most important attenuation process for most of organic chemicals in the environment. This process decides whether the organic substance itself or its degradation products rests in the environment and should be considered for a further risk assessment. This work presents the development of a water sediment screening test, based on OECD guideline 308, with a high significance to environmental conditions and with a good reproducibility and consistency of results. The increased reproducibility was achieved by creating an artificial and standardized medium, based on the existing OECD guidelines OECD 302C, 301D and 218. Each test consisted of five different series: blank, quality control, test, toxicity control and abiotic control. Biodegradation was assessed by measurement of pressure difference in closed vessels using the OxiTop(®) system. Aniline, diethylene glycol and sodium acetate were used to optimize and validate test conditions. Additionally, two pharmaceuticals: Acetaminophen and ciprofloxacin (CIP) were tested as an example of possible test application. Acetaminophen was mainly removed from the system by biodegradation whereas CIP was removed from water phase by sorption onto sediment. Water sediment test proved to be a promising tool for the biodegradation investigation of chemicals in the water-sediment interface.

Sorption of norfloxacin in soils: analytical method, kinetics and Freundlich isotherms

Fluoroquinolones are potent antibacterial agents that are active against a wide range of pathogenic organisms and are widely used in veterinary medicine. Fluoroquinolones and their metabolites may reach the soil through animal excreta or manure and may contaminate water and soils. The degree of sorption of these antimicrobials to soils varies widely, as does the mobility of these drugs. In the present study, sorption of norfloxacin in four soils of the state of São Paulo was investigated with batch equilibrium experiments. A strong matrix effect on the chromatographic determination of norfloxacin was verified. Sorption kinetics were best fit by a pseudo second-order model (r>0.99), and sorption/desorption isotherms were well fit by the Freundlich model in log form (r>0.97). Norfloxacin showed high affinity for soil particles, with KF sorption values ranging from 643 to 2410 μg(1-1/n)(cm(3))(1/n)g(-1) and KF desorption values ranging from 686 to 2468 μg(1-1/n)(cm(3))(1/n)g(-1). The high desorption KF values indicate that norfloxacin is highly immobile in the evaluated soils.

Investigation of mono/competitive adsorption of environmentally relevant ionized weak acids on graphite: impact of molecular properties and thermodynamics

The thermodynamics of adsorption and competitive interactions of five weak acids on a graphite surface was assessed in alkaline solutions. Adsorption of the acids in mono- and multicompound solutions followed their Freundlich isotherms which suggest a diversity of graphite adsorption sites as confirmed by the presence of carboxylic and phenolic groups observed on graphite surfaces. Thermodynamic calculations assigned the formation of the negatively charged assisted hydrogen bond (-CAHB) between ionized solutes and adsorbent surface groups as the possible adsorption mechanism. However, the similar pKa values of current acids resulted in comparable free energies for -CAHB formation (ΔG(-CAHB)) being less than solvation free energies (ΔGSolv). Thus, additional ΔG is supplemented by increased hydrophobicity due to proton exchange of ionized acids with water (ΔΔG Hydrophobicity). Adsorption capacities and competition coefficients indicated that ΔΔG Hydrophobicity values depend on the neutral and ionized acid Kow. Competitive adsorption implies that multilayer adsorption may occur via hydrophobic bonding with the CH3 ends of the self-assembled layer which affects the acid adsorption capacities in mixtures as compared to monocompound solutions. The determination of adsorption mechanisms will assist in understanding of the fate and bioavailability of emerging and classical weak acids released into natural waters.

New insight into adsorption mechanism of ionizable compounds on carbon nanotubes

We studied the pH-dependent adsorption of benzoic acid (BA), phthalic acid (PA), and 2,6-dichloro-4-nitrophenol (DCNP) by hydroxylated, carboxylated, and graphitized carbon nanotubes (CNTs). Adsorption is contributed by formation of a negative charge-assisted H-bond (-)CAHB between a carboxyl group on the solute and a phenolate or carboxylate group on the surface having a comparable pKa. This exceptionally strong H-bond is depicted as (RCO2···H···O-CNTs)(-). Over a limited pH range the free anion undergoes proton exchange with water concurrent with adsorption, releasing hydroxide ion in a stoichiometry of up to 1.0 for BA, 1.7 for PA, and 0.5 for DCNP. Little hydroxide is released upon adsorption by the O-sparse graphitized CNTs. Anion exchange and ligand exchange reactions as a source of hydroxide release were ruled out. The higher stoichiometry for PA indicates involvement of both carboxyl groups with adjacent surface oxyl groups. The lower stoichiometry for DCNP is consistent with steric inhibition of H-bonding by the ortho chlorines. Formation of (-)CAHB helps overcome the unfavorable free energy of proton exchange with water, and results in an upward shift in the pKa in the adsorbed state compared to the dissolved state from 0.9 to 3.1 units. The proposed mechanism is further supported by additional structure-activity considerations. The findings provide new understanding of the interactions between ionizable organic compounds and carbonaceous surfaces, which has implications for noncovalent derivatization of CNTs, fate of ionizable pollutants, and associations of natural organic matter with CNTs and other carbonaceous materials in the environment.

Complexes of the antimicrobial ciprofloxacin with soil, peat, and aquatic humic substances

Natural organic matter (NOM) is implicated in the binding of antibiotics by particles in soils and waters. The authors' previous computational study revealed structural rearrangement of both hydrophilic and hydrophobic moieties of NOM to favor H-bonding and other intermolecular interactions, as well as both competition with ion-exchange reactions and bridging interactions by NOM-bound divalent cations. The importance of these interactions was investigated using fluorescence-quenching spectroscopy to study the adsorption of ciprofloxacin (Cipro), a fluoroquinolone antibiotic, on 4 reference humic substances (HSs): Elliott soil humic acid (HA), Pahokee peat HA, and Suwannee river HA and fulvic acid. A simple affinity spectrum HS model was developed to characterize the cation-exchange capacity and the amount of H-bond donor moieties as a function of pH. The adsorption results stress the influence of both pH conditions and the type of HS: both soil HA and peat HA exhibited up to 3 times higher sorption capacity than the aquatic HS at pH ≥ 6, normalizing to the aromatic C content accounted for the differences among the terrestrial HS, and increasing the concentration of divalent cations led to a decrease in adsorption on aquatic HA but not on soil HA. In addition, the pH-dependent speciation models of the Cipro-HS complexes illustrate an increase in complexation due to an increase in deprotonation of HS ligands with increasing pH and, at circumneutral and alkaline pH, enhanced complexation of zwitterionic Cipro only in the presence of soil HA and peat HA. The findings of the present study imply that, in addition to electrostatic interactions, van der Waals interactions as facilitated by aromatic structures and H-bond donating moieties in terrestrial HS may facilitate a favorable binding environment. Environ Toxicol Chem 2013;32:1467-1478. © 2013 SETAC.

Occurrence and fate of eleven classes of antibiotics in two typical wastewater treatment plants in South China

Wastewater treatment plants (WWTPs) are regarded as one of the most important sources of antibiotics in the environment. Two sampling campaigns over a period of one year in two wastewater treatment plants (plant A: activated sludge with chlorination, and plant B: oxidation ditch with UV) of Guangdong Province, China were carried out to assess the occurrence and fate of 11 classes of 50 antibiotics. The wastewater samples were extracted by Oasis HLB cartridges (6mL, 500 mg), while the solid samples (sludge and suspended solid matter) were extracted by ultrasonic-assisted extraction with solvents (acetonitrile and citric acid buffer), followed by an enrichment and clean-up step with solid-phase extraction using SAX-HLB cartridges in tandem. The results showed the presence of 20 and 17 target compounds in the influents and effluents, respectively, at the concentrations ranging from low ng/L to a few μg/L. Sulfamethoxazole, norfloxacin, ofloxacin, anhydro erythromycin and trimethoprim were most frequently detected in the WWTPs wastewaters. Twenty-one antibiotics were found in the sewage sludge from the two WWTPs at the concentrations up to 5800ng/g, with tetracycline, oxytetracycline, norfloxacin and ofloxacin being the predominant antibiotics. The total mass loads of antibiotics per capita in the two plants ranged from 494 to 901 μg/d/inhabitant (672±182 μg/d/inhabitant) in the influents, from 130 to 238 μg/d/inhabitant (175±45 μg/d/inhabitant) in the effluents and from 107 to 307 μg/d/inhabitant (191±87.9 μg/d/inhabitant) in the dewatered sludge, respectively. The aqueous removals for sulfonamides, macrolides, trimethoprim, lincomycin and chloramphenicol in the WWTPs were mainly attributed to the degradation processes, while those for tetracyclines and fluoroquinolones were mainly due to the adsorption onto sludge.

Interstratification patterns from the pH-dependent intercalation of a tetracycline antibiotic within montmorillonite layers

Little is known about the distribution of the intercalated molecules within the interstratified layers resulting from the pH-dependent interlayer adsorption of ionizable organic molecules, including antibiotics, within smectite-type clay minerals. Here we employed experimental and simulated X-ray diffraction (XRD) to characterize interstratification (or mixed layering) from the intercalation of oxytetracycline (OTC), a commonly used tetracycline antibiotic, within Na-montmorillonite layers at pHs 4, 5, 6, and 8. Our XRD data reveal that OTC is distributed nonrandomly in the interlayers such that Na- and OTC-saturated interlayers coexist. The profile of the full width at half-maximum intensity (fwhm), monitored as a function of increasing layer-to-layer distance (d001), resulting from an increasing amount of intercalated OTC, reflects such mixed-layer crystals under the acidic pH conditions. A minimum in fwhm occurs at a d spacing of about 1.8 nm, which is to be the optimal d001 for OTC-saturated layers, in agreement with molecular modeling results. Using the coordinates of the thermodynamically favorable configuration of the adsorptives in a model montmorillonite interlayer, we simulated XRD profiles to unravel the different patterns of interstratification from the experimental data. At both pHs 4 and 5, Na- and OTC-interlayers are randomly interstratified, whereas at pH 6, these layers are clustered, as characterized by a segregated interstratification pattern. The theoretical layer stacking sequences of the simulated XRD illustrate, as pH increases, the clustering of similar layer types with the exclusion of OTC intercalation from clay populations enriched in Na. At pH 8, both fwhm and d001 indicate OTC adsorption primarily on external surface sites, not within interlayers. Our findings imply that, in addition to chemical speciation, a pH-dependent formation of montmorillonite crystallites with unexfoliated layers may be responsible both for the decreased OTC intercalation and for the increased binding on external sites, thus resulting in the different patterns of interstratification as a function of increasing pH.

Risk assessment on the presence of pharmaceuticals in sediments, soils and waters of the Pego-Oliva Marshlands (Valencia, eastern Spain)

This study is focused on the occurrence of 17 pharmaceuticals in waters (34 samples), sediments (16 samples) and soils (23 samples, at two different depths) in a typical Mediterranean coastal wetland (Pego-Oliva marsh, Spain). Soil and sediment samples were extracted by pressurized liquid extraction (PLE). Aqueous extracts from PLE and water samples were concentrated by solid-phase extraction (SPE) and determined by liquid-chromatography tandem mass spectrometry (LC-MS/MS). Pharmaceuticals were detected in concentrations up to 112 ng/L in water samples, up to 15.1 ng/g sediments and up to 8.4 ng/g in soil. In surface waters, ibuprofen and codeine were the compounds more frequently detected (up to 59 ng/L and 63 ng/L, respectively). Ground and tap water samples analyzed were also contaminated with pharmaceuticals. The 94% of sediments and the 80% of agricultural soils were polluted (mostly by carbamazepine and acetaminophen). Diffusion of codeine and fluoroquinolones to deeper soil horizons was observed. Possible relationships between variables were established by Pearson correlations and principal components analysis (PCA). An environmental risk assessment based on the available long-term data was performed. Results showed actual risk for the lowest trophic level, and for fishes, due to the presence of fluoroquinolones and ibuprofen. Nevertheless, the presence of pharmaceuticals in the environment is not limited only to an ecological problem since contamination also affects drinking water, being a potential risk to human health.