Deposition of MnO2 nanoparticles on the magnetic halloysite nanotubes by hydrothermal method for lead(II) removal from aqueous solutions

Recent Progress on the Adsorption of Heavy Metal Ions Pb(II) and Cu(II) from Wastewater

With the processes of industrialization and urbanization, heavy metal ion pollution has become a thorny problem in water systems. Among the various technologies developed for the removal of heavy metal ions, the adsorption method is widely studied by researchers and various nanomaterials with good adsorption performances have been prepared during the past decades. In this paper, a variety of novel nanomaterials with excellent adsorption performances for Pb(II) and Cu(II) reported in recent years are reviewed, such as carbon-based materials, clay mineral materials, zero-valent iron and their derivatives, MOFs, nanocomposites, etc. The novel nanomaterials with extremely high adsorption capacity, selectivity and particular nanostructures are summarized and introduced, along with their advantages and disadvantages. And, some future research priorities for the treatment of wastewater are also prospected.

Employing Pd nanoparticles decorated on halloysite nanotube/carbon composite for electrochemical aptasensing of HER2 in breast cancer patients

An effective biosensing platform is described based on halloysite nanotube/carbon composite decorated with Pd nanoparticles (HNT/C@Pd NPs). A novel electrochemical aptasensor was designed using the proposed nano-platform to determine human epidermal growth factor receptor 2 (HER2), a breast cancer biomarker. Inherently, aptasensing interfaces provide high sensitivity and selectivity for tumor markers owing to the high specific surface area of HNT/C and good conductivity stems from deposition of Pd NPs into HNT/C composite. With a correlation coefficient of 0.996, the electrochemical aptasensor demonstrated a wide linear range from 0.03 ng/mL to 9 ng/mL. The limit of detection (LOD) of the established assay was 8 pg/mL based on S/N = 3 method. Further, the designed biosensor demonstrated acceptable selectivity, good reproducibility, and high stability. The applicability of the impedimetric sensor in human serum samples was also examined and compared to enzyme-linked immunosorbent assay (ELISA) assay (p-value >0.05). Based on the results, it was found that the proposed methodology can be used in quantification of breast cancer markers for early diagnosis and treatment.

Halloysite-TiO2 Nanocomposites for Water Treatment: A Review

Halloysite nanotubes (HNTs) are clay minerals with a tubular structure that can be used for many different applications in place of carbon nanotubes. Indeed, HNTs display low/non-toxicity, are biocompatible, and can be easily prepared. Moreover, the aluminum and silica groups present on HNTs' inner and outer surfaces facilitate the interaction with various functional agents, such as alkalis, organosilanes, polymers, surfactants, and nanomaterials. This allows the deposition of different materials, for instance, metal and non-metal oxides, on different substrate types. This review article first briefly presents HNTs' general structure and the various applications described in the last 20 years (e.g., drug delivery, medical implants, and energy storage). Then, it discusses in detail HNT applications for water purification (inorganic and organic pollutants). It focuses particularly on HNT-TiO₂ composites that are considered very promising photocatalysts due to their high specific surface area and adsorption capacity, large pore volume, good stability, and mechanical features.

Cadmium adsorption performance and mechanism from aqueous solution using red mud modified with amorphous MnO2

In this study, red mud modified by manganese dioxide(MRM) was utilized as an adsorbent to effectively remove Cd²⁺ from aqueous solution. The characteristics were analysed by SEM-EDS, XRD, BET, FTIR and XPS. Different factors that affected the Cd²⁺ removal on MRM, such as dosage, initial pH, initial Cd²⁺ concentration, were investigated using batch adsorption experiments. Simultaneously, the adsorption kinetics, adsorption isotherms and adsorption thermodynamics of Cd²⁺ were also investigated using adsorption experiments data. The characterization results showed that MRM had a rougher, larger specific surface area and pore volume (38.91 m² g^-1, 0.02 cm³ g^-1) than RM (10.22 m² g^-1, 0.73 cm³ g^-1). The adsorption experiments found that the equilibrium adsorption capacity of MRM for Cd²⁺ was significantly increased to 46.36 mg g^-1, which was almost three times that of RM. According to the fitting results, the pseudo-second-order kinetic model described the adsorption process better than the pseudo-first-order kinetic model. The Langmuir model fitted the adsorption isotherms well, indicating that the adsorption process was unimolecular layer adsorption and the maximum capacity was 103.59 mg g^-1. The thermodynamic parameters indicated that the adsorption process was heat-trapping and spontaneous. Finally, combined XPS and FTIR studies, it was speculated that the adsorption mechanisms should be electrostatic attachment, specific adsorption (i.e., Cd-O or hydroxyl binding) and ion exchange. Therefore, manganese dioxide modified red mud can be an effective and economical alternative to the removal of Cd²⁺ in the wastewater treatment process.

Facile synthesis of Vulcan XC-72 nanoparticles-decorated halloysite nanotubes for the highly sensitive electrochemical determination of niclosamide

We report a scalable and controllable ultrasound-assisted strategy for the preparation of Vulcan XC-72 nanoparticles-decorated halloysite nanotubes (HNTs@VXC-72), which was applied to modify glassy carbon electrode (GCE) for the highly sensitive electrochemical determination of niclosamide (NA). For the HNTs@VXC-72 nanocomposite, VXC-72 nanoparticles with excellent electrical conductivity and good dispersing property contributed to the formation of the interconnected conductive network; HNTs possessed good adsorption performance and promoted the electrochemical redox reaction. The research results showed that the combination of VXC-72 nanoparticles and HNTs produced the effect of synergistic enhancement. The HNTs@VXC-72/GCE sensor could show a relatively low detection limit of 3.28 nM in the great linear NA concentration range of 0.01-1 µM. When used for the NA determination in food samples, the HNTs@VXC-72/GCE sensor exhibited good practical feasibility with low RSD and acceptable recoveries, which provided a promising NA determination approach to ensure food safety.

Facile in situ synthesis of cellulose microcrystalline-manganese dioxide nanocomposite for effective removal of Pb(II) and Cd(II) from water

A novel cellulose microcrystalline-manganese dioxide nanocomposite (CMC-NMO) was synthesized by the redox reaction between potassium permanganate and ethanol based on cellulose microcrystalline. The cellulose microcrystalline (CMC) as support providing growth sites for the manganese dioxide nanowhiskers produced by the redox reaction and its application for Pb(II) and Cd(II) removal from aqueous was investigated. The characterization of as-synthesized material was revealed by various spectroscopic and microscopic techniques. Infrared-transform infrared (FITR) indicates that the incorporation of manganese oxide to CMC does not change the initial structure of it. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) data show that the manganese dioxide nanowhiskers with a few nanometers are uniformly dispersed on the surface of cellulose. Kinetics experiments reveal that Pb(II) and Cd(II) adsorption on CMC-NMO is a fast process and pseudo-second-order model fits the adsorption better. The maximum adsorption capacities of Pb(II) and Cd(II) obtained from the Langmuir model are 290.8 mg/g and 67.4 mg/g, respectively. The mechanism is mainly attributed to surface complexation and electrostatic attraction by energy dispersive analysis of X-rays (EDAX), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) analysis. In addition, depth removal experiments show that the residual concentrations of Pb(II) and Cd(II) in natural water after adsorption are lower than 0.01 mg/L. The regeneration and cyclic utilizing studies indicate that CMC-NMO has good adsorption stability. Therefore, the results indicate that this material can be employed as a potential adsorbent for current serious Pb(II) and Cd(II) pollution caused by industrial emissions. Graphical abstract.

Synthesis of novel sepiolite-iron oxide-manganese dioxide nanocomposite and application for lead(II) removal from aqueous solutions

In this study, the sepiolite-iron oxide-manganese dioxide (Sep-Fe₃O₄-MnO₂) nanocomposite was synthesized and applied as a magnetically separable adsorbent for removal of Pb(II) ions from water in a batch system. The effects of initial Pb(II) concentration, adsorbent dosage, contact time, pH value, and temperature were investigated to optimize the conditions for maximum adsorption. The equilibrium adsorption data were analyzed with the Langmuir, Freundlich, and Temkin models. The adsorption process closely agreed with the Langmuir adsorption isotherm, and the monolayer saturation adsorption value was achieved as 131.58 mg g^-1. The adsorption kinetics follow the pseudo-second-order (PSO) model that illustrated the rate controlling step might be chemisorption. Thermodynamic investigations for the removal process were conducted by determining the values of ∆G°, ∆H°, and ∆S°. The adsorption behavior of Pb(II) on the Sep-Fe₃O₄-MnO₂ was a spontaneous and endothermic process. Several consecutive adsorption-desorption cycles confirmed that the proposed Sep-Fe₃O₄-MnO₂ nanocomposite could be reused after successive lead removal. Furthermore, the practical application of the adsorbent was successfully realized by the treatment of real Pb-contaminated water samples.

Facile synthesis of EDTA-functionalized halloysite nanotubes for the removal of methylene blue from aqueous phase

EDTA-functionalized halloysite nanotubes (EDTA-HNTs) were synthesized by a two-step method. The structures of the EDTA-HNTs were analyzed by Fourier-transform infrared spectra, X-ray diffraction, thermogravimetric analysis, and transmission electron spectroscopy. Then, the EDTA-HNTs were applied as adsorbents for the removal of methylene blue from aqueous phase. Adsorption of methylene blue onto EDTA-HNTs was investigated with respect to adsorbent dose, contact time, initial methylene blue concentration, and temperature. A maximum adsorption capacity of 115.8 mg/g for methylene blue onto EDTA-HNTs at room temperature was achieved. The adsorption kinetics could be described by the pseudo-second-order model. Both the Langmuir and Freundlich models were employed to describe the adsorption isotherms, and the results presented that the equilibrium data obeyed the Langmuir model. Thermodynamic parameters of ΔG0 and ΔH0 verified the spontaneous and exothermic nature of the methylene blue adsorption onto EDTA-HNTs. Moreover, the EDTA-HNTs could be facilely regenerated and efficiently reused, exhibiting a good prospect for the treatment of dye effluents.

Surface modified halloysite nanotubes: A flexible interface for biological, environmental and catalytic applications

Halloysite Nanotubes (HNTs) are clay minerals that possess unique chemical composition and a tubular structure due to which, they have recently emerged as a potential nanomaterial for umpteen applications. Over the years, the myriad applications of HNT have been realized through the surface modification of HNT, which involves the modification of nanotube's inner lumen and the outer surface with different functional compounds. The presence of aluminum and silica groups on the inner and outer surface of HNT enhance the interfacial relationship of the nanotube with different functional agents. Compounds such as alkalis, organosilanes, polymers, compounds of biological origin, surfactants and nanomaterials have been used for the modification of the inner lumen and the outer surface of HNT. The strategies change the constitution of HNT's surface either through micro-disintegration of the surface or by introducing additional functional groups on the surface, which further enhances their potential to be used as a flexible interface for different applications. In this review, the different surface modification strategies of the outer surface and the inner lumen that have been employed over the years have been discussed. The biological, environmental and catalytic applications of these surface modified HNTs with such versatile interface in the past two years have been elaborately discussed as well.

Halloysite nanotubes in analytical sciences and in drug delivery: A review

Halloysite (HNT) is a natural inorganic mineral that has many applications in manufacturing. This review (with 192 references) covers (a) the chemical properties of halloysites, (b) the effects of alkali and acid etching on the loading capacity and the release behavior of halloysites, (c) the use of halloysite nanotubes in analytical sciences and drug delivery, and (d) recent trends in the preparation of magnetic HNTs. Synthetic methods such as co-precipitation, thermal decomposition, and solvothermal method are discussed, with emphasis on optimal magnetization. In the analytical field, recent advancements are summarized in terms of applications of HNT-nanocomposites for extraction and detection of heavy metal ions, dyes, organic pollutants, and biomolecules. The review also covers methods for synthesizing molecularly imprinted polymer-modified HNTs and magnetic HNTs. With respect to drug delivery, the toxicity, techniques for drug loading and the various classes of drug-halloysite nanocomposites are discussed. This review gives a general insight on the utilization of HNT in analytical determination and drug delivery systems which may be useful for researchers to generate new ideas. Graphical abstract Schematic presentation of the structure of halloysite nanotubes, selected examples of modifications and functionalization, and represetative field of applications.

Capturing Cd2+ ions from wastewater using PVA/α-MnO2–oleic acid nanocomposites

A poly(vinyl alcohol) nanocomposite containing α-MnO₂–oleic acid has been fabricated as an efficient adsorbent for capturing Cd²⁺ ions from aqueous solution.

Assessment of TiO2 photoactivity on the lead removal: kinetic and mechanistic processing

Removal of lead ions from aqueous solutions, in the presence and in absence of commercial TiO₂, under UV-light was studied. The influence of catalyst mass, concentration of Pb(II) ions and of citric acid in the starting solution as well as the impact of illumination conditions on the removal rate were also investigated. The results were fitted on the pseudo-first order, pseudo-second order and Elovich kinetic models, Weber-Morris intraparticle and liquid film diffusion models, in order to establish the photoreduction mechanism. The Pb removal rate increased in time by a combined linear-exponential mechanism. The catalyst had a positive influence on the removal rate at the start of the photoreduction. At a low concentration of Pb(II) ions (20 mg/L), the amount of photoreduced Pb(II) ions was proportional to the concentration of the citric acid solution, but at high concentrations (240 mg/L), the correlation was inversely proportional. The rate limiting steps in the removal of lead were both the photoreduction on the TiO₂ surface and the diffusion through the film surrounding the catalyst particle. The lead removal was significantly influenced by the mobility and by the concentration of the species in solution.

Preparation of a polyvinylidene fluoride tree-like nanofiber mat loaded with manganese dioxide for highly efficient lead adsorption

A novel polyvinylidene fluoride tree-like structure nanofiber mat loaded with manganese dioxide as a highly efficient lead adsorbent was fabricated.