Nanoscale Profiling of 2D Surface Hydrophobicity Recognition of Environmental Media via AFM Measurements In Situ

Efficient Removal of Cationic Dye by Biomimetic Amorphous Calcium Carbonate: Behavior and Mechanisms

The search for efficient, environmentally friendly adsorbents is critical for purifying dye wastewater. In this study, we produced a first-of-its-kind effective biomimetic amorphous calcium carbonate (BACC) using bacterial processes and evaluated its capacity to adsorb a hazardous organic cationic dye-methylene blue (MB). BACC can adsorb a maximum of 494.86 mg/g of MB, and this excellent adsorption performance was maintained during different solution temperature (10-55 °C) and broad pH (3-12) conditions. The favorable adsorption characteristics of BACC can be attributable to its hydrophobic property, porosity, electronegativity, and perfect dispersity in aqueous solution. During adsorption, MB can form Cl-Ca, S-O, N-Ca, and H-bonds on the surface of BACC. Since BACC has excellent resistance to adsorption interference in different water bodies and in real dye wastewater, and can also be effectively recycled six times, our study is an important step forward in dye wastewater treatment applications.

Sluggish and Ion-Resilient Behavior of Interfacial Aqueous Layer on Single-Layer Graphene Oxide: Insights from In Situ Atomic Force Microscopy

Understanding interfacial interactions of graphene oxide (GO) is important to evaluate its colloidal behavior and environmental fate. Single-layer GO is the fundamental unit of GO colloids, and its interfacial aqueous layers critically dictate these interfacial interactions. However, conventional techniques like X-ray diffraction are limited to multilayer systems and are inapplicable to single-layer GO. Therefore, our study employed atomic force microscopy to precisely observe the in situ dynamic behaviors of interfacial aqueous layers on single-layer GO. The interfacial aqueous layer height was detected at the subnanometer level. In real-time monitoring, the single-layer height increased from 1.17 to 1.70 nm within 3 h immersion. This sluggish process is different from the rapid equilibration of multilayer GO in previous studies, underscoring a gradual transition in hydration kinetics. Ion strength exhibited negligible influence on the single-layer height, suggesting a resilient response of the interfacial aqueous layer to ion-related perturbations due to intricate ion interactions and electrical double-layer compression. Humic acid led to a substantial increase in the interfacial aqueous layers, improving the colloidal stability of GO and augmenting its potential for migration. These findings hold considerable significance regarding the environmental behaviors of the GO interfacial aqueous layer in ion- and organic-rich water and soil.

A comparative study of methylene blue adsorption and removal mechanisms by calcium carbonate from different sources

Efficient removal of organic dye pollution from contaminated water is a concern in the absorbent applications. In this study, a green biogenic calcium carbonate (BCC) absorbent was fabricated using Bacillus licheniformis for the removal of methylene blue (MB) from water. This was found to have superior adsorption capacity compared with abiotic calcium carbonate (ACC) and operate within a broad pH range from 3 to 9. MB adsorption on BCC was physical and exothermic. The hydrophobic features, rough nanoporous microstructure, and organic-inorganic mesoporous structure of the BCC may all be responsible for its favorable adsorption mass transfer. The adsorption energy of BCC had a more negative value than that of ACC, indicating a stronger MB interaction with BCC with a lower energy barrier. Hydrogen bonding and electrostatic attraction were involved in the adsorption process. Overall, the findings established a theoretical foundation for the application of BCC in remediation of MB-contaminated water.

Influence of microplastics on the transport of antibiotics in sand filtration investigated by AFM force spectroscopy

Microplastics and antibiotics were frequently detected in the effluent of sand filtration, while the presence of microplastics may change the interactions between the antibiotics and the quartz sands. However, the influence of microplastics on the transport of antibiotics in sand filtration has not been revealed. In this study, ciprofloxacin (CIP) and sulfamethoxazole (SMX) were respectively grafted on AFM probes to determine the adhesion forces to the representative microplastics (PS and PE) and the quartz sand. CIP and SMX exhibited low and high mobilities in the quartz sands, respectively. Compositional analysis of the adhesion forces indicated that the lower mobility of CIP in sand filtration columns could be attributed to the electrostatic attraction between the quartz sand and CIP compared with repulsion for SMX. Moreover, the significant hydrophobic interaction between the microplastics and the antibiotics could be responsible for the competitive adsorption of the antibiotics to the microplastics from the quartz sands; meanwhile, the π-π interaction further enhanced the adsorption of PS to the antibiotics. As a result of the high mobility of microplastics in the quartz sands, the carrying effect of microplastics enhanced the transport of antibiotics in the sand filtration columns regardless of their original mobilities. This study provided insights into the mechanism of the microplastics on enhancing the transport of antibiotics in sand filtration systems from the perspective of the molecular interaction.

Host-Endosymbiont Relationship Impacts the Retention of Bacteria-Containing Amoeba Spores in Porous Media

Amoebae are protists that are commonly found in water, soil, and other habitats around the world and have complex interactions with other microorganisms. In this work, we investigated how host-endosymbiont interactions between amoebae and bacteria impacted the retention behavior of amoeba spores in porous media. A model amoeba species, Dictyostelium discoideum, and a representative bacterium, Burkholderia agricolaris B1qs70, were used to prepare amoeba spores that carried bacteria. After interacting with B. agricolaris, the retention of D. discoideum spores was enhanced compared to noninfected spores. Diverse proteins, especially proteins contributing to the looser exosporium structure and cell adhesion functionality, are secreted in higher quantities on the exosporium surface of infected spores compared to that of noninfected ones. Comprehensive examinations using a quartz crystal microbalance with dissipation (QCM-D), a parallel plate chamber, and a single-cell force microscope present coherent evidence that changes in the exosporium of D. discoideum spores due to infection by B. agricolaris enhance the connections between spores in the suspension and the spores that were previously deposited on the collector surface, thus resulting in more retention compared to the uninfected ones in porous media. This work provides novel insight into the retention of amoeba spores after bacterial infection in porous media and suggests that the host-endosymbiont relationship regulates the fate of biocolloids in drinking water systems, groundwater, and other porous environments.

In situ scrutinize the adsorption of sulfamethoxazole in water using AFM force spectroscopy: Molecular adhesion force determination and fractionation

The interaction force of a typical antibiotic molecule during adsorption has never been experimentally determined and fractionated, which hindered the evolution of removal strategies. In this study, sulfamethoxazole (SMX) as a typical antibiotic was stably immobilized onto an atomic force microscopy (AFM) tip without affecting original properties. The SMX modified AFM tip visualized the potential adsorption sites on a graphene oxide (GO) nanosheet for the first time by mapping the SMX adhesion force distribution. Moreover, the interaction force of a single SMX molecule to GO was determined at 38.6 pN which was subsequently fractionated into the hydrophobic (17.9 pN) and π-π (160.0 pN) attractions as well as the electrostatic repulsion (- 139.3 pN) at pH: 5.7. As compared with highly-ordered pyrolytic graphite (HOPG), the introduced oxygen containing groups on GO not only reduced the hydrophobic interaction but also generated an opposite electrostatic repulsion force to SMX. This study experimentally and theoretically revealed the adhesion mechanisms of SMX and potentially other sulfonamide antibiotics in molecular level, which may contribute to the study of antibiotic environmental transportation and the development of next-generation antibiotic remediation protocols.

Adhesion force evolution of protein on the surfaces with varied hydration extent: Quantitative determination via atomic force microscopy

The adhesion force evolution of protein on surfaces with continuously varied hydrophobicity/hydration layer has not been completely clarified yet, limiting the further development of environmental applications such as membrane anti-biofouling and selective adsorption of the functional surfaces. Herein, chemical force spectroscopy using atomic force microscopy (AFM) was utilized to quantify the evolution of the adhesion forces of protein on hydration surfaces in water, where bovine serum albumin (BSA) was immobilized on an AFM tip as the representative protein. The stiffness, roughness and charge properties of the substrate surfaces were kept constant and the hydrophobicity was the only variant to monitor the role of hydrated water layers in protein adhesion. The adhesion force increased non-monotonically as a function of hydrophobicity of substrate surfaces, which was related to the concentration of humic acid, and independent of pH values and ionic strength. The non-monotonic variation occurred in the range of contact angle at 60-80° due to the mutual restriction between solid-liquid interface energy and solid-solid interface energy. Hydrophobic attraction was the dominant force that drove adhesion of BSA to these model substrate surfaces, but the passivation of hydration layers at the interface could weaken the hydrophobic attraction. In contrast to the measurements in water, the adhesion forces decreased as a function of surface hydrophobicity when measured in air, because capillary forces from condensation water dominated adhesion forces. The passivation of hydration layers of protein was revealed by quantitatively determining the evolution of adhesion forces on the hydration surfaces of varying hydrophobicity, which was ignored by traditional adhesion theory.