Adsorption, aggregation and sedimentation of titanium dioxide nanoparticles and nanotubes in the presence of different sources of humic acids

Aptamer molecular gate functionalized mesoporous SiO2@MB controlled-release system for pollutant detection using Ti(Ⅲ) self-doped TiO2 NTs as active photoanode coupled with electrostatic modulation

Atrazine (ATZ) is a widely used herbicide that can cause serious harm to organisms and ecosystems. An immobilization-free photoelectrochemical (PEC) aptasensor has been herein developed for ATZ based on aptamer molecular gate functionalized mesoporous SiO2@MB controlled release system. Compared with traditional immobilization-based sensors, immobilization-free sensors (IFSs) avoid the modification of the recognition element on the electrode surface. Mesoporous SiO2 with large surface area and good biocompatibility can be used as nanocontainers to stably encapsulate the signal shuttle molecule methylene blue (MB). The bifunctional aptamer (APT) is used not only as the recognition element for ATZ but also as the signal switch to block or release MB. In the presence of ATZ, the specific recognition between ATZ and APT will cause the detachment of APT from the surface of SiO2, thus the molecular gate will open and release MB. Due to pH modulation, the positively charged MB can reach the surface of the negatively charged Ti(III) self-doped TiO2 NTs (Ti(III)-TiO2 NTs) electrode to act as an electron donor, which increases the photocurrent. The immobilization-free aptasensor has shown ultrasensitive detection of ATZ with a wide linear range from 1.0 pM to 100.0 nM and a low detection limit of 0.1 pM. In addition, the sensor has excellent selectivity, stability and anti-interference ability, and has been used in real water sample analysis successfully. This strategy has provided a new idea for the design of advanced immobilization-free PEC sensors for environmental pollutant detection.

Further study on particle size, stability, and complexation of silver nanoparticles under the composite effect of bovine serum protein and humic acid

Silver nanoparticles (AgNPs) are widely used due to their unique antibacterial properties and excellent photoelectric properties. Wastewater treatment plants form a pool of AgNPs due to the social cycle of wastewater. During biological treatment processes, the particle size and stability of AgNPs change. We studied the particle size changes and stability of silver nanoparticles in the presence of bovine serum albumin (BSA) and humic acid (HA). The experimental results indicated that silver nanoparticles can complex with the functional groups in BSA. For AgNP-BSA composites, as the BSA concentration increases, the size of the silver nanoparticles first decreases and then increases. AgNPs can combine with the amide, amino, and carboxyl groups in HA. As the concentration of HA increases, the particle size and large particle size distribution of AgNPs increase. This increasing trend is more obvious when the HA concentration is lower than 20 mg L-1. When HA and BSA exist at the same time, HA will occupy the adsorption sites of BSA on the surface of AgNPs, and the AgNP-HA complex will dominate the system. This study aims to provide key operational control strategies for the process operation of wastewater treatment plants containing AgNPs and theoretical support for promoting water environment improvement and economic development such as tourism.

A comparative study on aggregation and sedimentation of natural goethite and artificial Fe3O4 nanoparticles in synthetic and natural waters based on extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and molecular dynamics simulations

Natural iron oxides nanomaterials have important roles in biogeochemical processes. In this study, the effects of pH, natural organic matter, and cations on aggregation and sedimentation of natural goethite and artificial Fe₃O₄ nanoparticles in water were investigated to learn more about the environmental behaviors of engineered and natural nanomaterials and how they differ. In addition, a novel extended DLVO theory that considered steric, gravitational, and magnetic attraction forces concurrently was specifically developed to provide mechanisms explanations. Specifically, Fe₃O₄ NPs were more likely than bulk goethite to aggregate (because of magnetic attraction interactions) at low HA concentrations and disperse at high HA concentrations. Besides, goethite was less prone to settle with the same concentration of NaCl than Fe₃O₄ NPs, but the opposite trend was found for the same concentration of CaCl₂ because of the difference in maximum net energy (barrier) and strong Ca²⁺ bridging effectiveness of goethite in CaCl₂ solution. Statistical models were established to evaluate colloidal stability of the particles. XPS and molecular dynamics simulation results suggested that ions were adsorbed onto particles via ionic polarization and that the binding free energies at high coverage followed the order Ca²⁺ > Na⁺ > Cl^- and presence of cation bridging between particles.

Highly Efficient Removal of Cu(II) Ions from Acidic Aqueous Solution Using ZnO Nanoparticles as Nano-Adsorbents

Water pollution by heavy metals has significant effects on aquatic ecosystems. Copper is one of the heavy metals that can cause environmental pollution and toxic effects in natural waters. This encourages the development of better technological alternatives for the removal of this pollutant. This work explores the application of ZnO nanoparticles (ZnO-NPs) for the removal of Cu(II) ions from acidic waters. ZnO NPs were characterized and adsorption experiments were performed under different acidic pHs to evaluate the removal of Cu(II) ions with ZnO NPs. The ZnO NPs were chemically stable under acidic conditions. The adsorption capacity of ZnO NPs for Cu(II) was up to 47.5 and 40.2 mg·g−1 at pH 4.8 and pH 4.0, respectively. The results revealed that qmax (47.5 mg·g−1) and maximum removal efficiency of Cu(II) (98.4%) are achieved at pH = 4.8. In addition, the surface roughness of ZnO NPs decreases approximately 70% after adsorption of Cu(II) at pH 4. The Cu(II) adsorption behavior was more adequately explained by Temkin isotherm model. Additionally, adsorption kinetics were efficiently explained with the pseudo-second-order kinetic model. These results show that ZnO NPs can be an efficient alternative for the removal of Cu(II) from acidic waters and the adsorption process was more efficient under pH = 4.8. This study provides new information about the potential application of ZnO NPs as an effective adsorbent for the remediation and treatment of acidic waters contaminated with Cu(II).

Impact of montmorillonite clay on the homo- and heteroaggregation of titanium dioxide nanoparticles (nTiO2) in synthetic and natural waters

The homoaggregation of titanium dioxide nanoparticles (nTiO₂) and their heteroaggregation with ubiquitous natural clay colloids are crucial processes affecting the environmental transport and fate of nTiO₂, whereas the latter has received less attention. In this study, the effects of pH, electrolytes, natural organic matter (NOM), and montmorillonite on the homo- and heteroaggregation of nTiO₂ were systematically investigated. The isoelectric point of bare nTiO₂ was 6.98, whereas TiO₂-montmorillonite mixtures remained negative charged due to the reduced particle surface potential by heteroaggregation. Homoaggregation of nTiO₂ was mainly affected by anions, whereas heteroaggregation in TiO₂-montmorillonite mixtures was mainly affected by cations. Heteroaggregation between nTiO₂ and montmorillonite involved the adsorption of CC/CH. Intensive aggregation of nTiO₂ was observed with 4 mg/L montmorillonite, whereas with 20 mg/L montmorillonite, the aggregation was significantly inhibited by the over-coverage of montmorillonite. NOM was attached to the surface of nTiO₂ with the adsorption of functional groups involving CC/CH and OCO. The addition of NOM effectively reduced the homo- and heteroaggregation of nTiO₂, and the stabilizing effect was enhanced with the increased molecular weight and aromatic/aliphatic fraction in NOM. Besides electrostatic repulsion, steric repulsion could also be one of the main stabilization mechanisms of NOM. With Ca²⁺ in the solutions, the stabilizing effect of NOM was significantly weakened through cation bridging. The addition of montmorillonite could facilitate the aggregation of nTiO₂ in the presence of NOM. The homo- and heteroaggregation of nTiO₂ were also observed in 7 different types of natural waters. Homoaggregation predominated in waters with high ionic strength and low NOM contents (seawater and groundwater), whereas heteroaggregation predominated in surface freshwater and wastewater systems. The results reflect the instability of nTiO₂ in natural aquatic environments and the potential risk they pose to benthic organisms.

Characterization of dissolved organic matter for understanding the adsorption on nanomaterials in aquatic environment: A review

Nanomaterials (NMs) have received tremendous attention as emerging adsorbents for environmental applications. The ever-increasing release into aquatic systems and the potential use in water treatment processes heighten the likelihood of the interactions of NMs with aquatic dissolved organic matter (DOM). Once DOM is adsorbed on NMs, it substantially modifies the surface properties, thus altering the fate and transport of NMs, as well as their toxic effects on (micro)organisms in natural and engineered systems. The environmental consequences of DOM-NMs interaction have been widely studied in the literature. In contrast, a comprehensive understanding of DOM-NM complexes, particularly regarding the controlling factors, is still lacking, and its significance has been largely overlooked. This gap in the knowledge mainly arises from the complex and heterogeneous structures of the DOM, which prompts the urgent need to further characterize the DOM properties to deepen the understanding associated with the adsorption processes on NMs. This review aims to provide in-depth insights into the complex DOM adsorption behavior onto NMs, whether they are metal- or carbon-based materials. First, we summarize the up-to-date analytical methods to characterize the DOM to unravel the underlying adsorption mechanisms. Second, the key DOM characteristics governing the adsorption processes are discussed. Next, the environmental factors, such as the nature of adsorbents and solution chemistry, affecting the DOM-NM interactions, are identified and discussed. Finally, future studies are recommended to fully understand the chemical traits of DOM upon its adsorption onto NMs.

Hybrid humic acid/titanium dioxide nanomaterials as highly effective antimicrobial agents against gram(-) pathogens and antibiotic contaminants in wastewater

Humic acids (HAs) provide an important bio-source for redox-active materials. Their functional chemical groups are responsible for several properties, such as metal ion chelating activity, adsorption ability towards small molecules and antibacterial activity, through reactive oxygen species (ROS) generation. However, the poor selectivity and instability of HAs in solution hinder their application. A promising strategy for overcoming these disadvantages is conjugation with an inorganic phase, which leads to more stable hybrid nanomaterials with tuneable functionalities. In this study, we demonstrate that hybrid humic acid/titanium dioxide nanostructured materials that are prepared via a versatile in situ hydrothermal strategy display promising antibacterial activity against various pathogens and behave as selective sequestering agents of amoxicillin and tetracycline antibiotics from wastewater. A physicochemical investigation in which a combination of techniques were utilized, which included TEM, BET, ¹³C-CPMAS-NMR, EPR, DLS and SANS, shed light on the structure-property-function relationships of the nanohybrids. The proposed approach traces a technological path for the exploitation of organic biowaste in the design at the molecular scale of multifunctional nanomaterials, which is useful for addressing environmental and health problems that are related to water contamination by antibiotics and pathogens.

Preparation of sonoactivated TiO2-DVDMS nanocomposite for enhanced antibacterial activity

Titanium dioxide (TiO₂) nanoparticle has good photo-/sono-catalytic features, the reunion of this particle in solution-phase generally limits the extensive biomedical application. In the present study, the aggregation of TiO₂ nanoparticles was alleviated by facile fabrication under different pH conditions. A novel TiO₂ nanocomposite was further synthesized by properly conjugation with trace amount of DVDMS sensitizer (named DFT). The characterization, sonoactivity, as well as the antibacterial efficiency were specially evaluated. The results showed that the sonochemical activity of DFT was greatly improved as compared with the simple surface modification of TiO₂ (F-TiO₂) and free DVDMS, regarding to the hydroxyl radicals and singlet oxygen yields using the same ultrasound exposure. Moreover, ultrasonic stimulation of DFT exhibited excellent bacterial eradication, with up to 92.41% of killing efficiency in S. aureus. The flow cytometry analysis indicated an increased intracellular ROS and membrane disturbance by combination of DFT and ultrasound. The findings suggest that the proper fabrication and DVDMS incorporation greatly improved the sonocatalytic process of TiO₂, and the ultrasound based biomedical applications of DFT deserve future deep investigation.