Mamata Mohapatra

Review of fluoride removal from drinking water

Fluoride in drinking water has a profound effect on teeth and bones. Up to a small level (1-1.5mg/L) this strengthens the enamel. Concentrations in the range of 1.5-4 mg/L result in dental fluorosis whereas with prolonged exposure at still higher fluoride concentrations (4-10mg/L) dental fluorosis progresses to skeletal fluorosis. High fluoride concentrations in groundwater, up to more than 30 mg/L, occur widely, in many parts of the world. This review article is aimed at providing precise information on efforts made by various researchers in the field of fluoride removal for drinking water. The fluoride removal has been broadly divided in two sections dealing with membrane and adsorption techniques. Under the membrane techniques reverse osmosis, nanofiltration, dialysis and electro-dialysis have been discussed. Adsorption, which is a conventional technique, deals with adsorbents such as: alumina/aluminium based materials, clays and soils, calcium based minerals, synthetic compounds and carbon based materials. Studies on fluoride removal from aqueous solutions using various reversed zeolites, modified zeolites and ion exchange resins based on cross-linked polystyrene are reviewed. During the last few years, layered double oxides have been of interest as adsorbents for fluoride removal. Such recent developments have been briefly discussed.

Iron and aluminium based adsorption strategies for removing arsenic from water

Arsenic is a commonly occurring toxic metal in natural systems and is the root cause of many diseases and disorders. Occurrence of arsenic contaminated water is reported from several countries all over the world. A great deal of research over recent decades has been motivated by the requirement to lower the concentration of arsenic in drinking water and the need to develop low cost techniques which can be widely applied for arsenic removal from contaminated water. This review briefly presents iron and aluminium based adsorbents for arsenic removal. Studies carried out on oxidation of arsenic(III) to arsenic(V) employing various oxidising agents to facilitate arsenic removal are briefly mentioned. Effects of competing ions, As:Fe ratios, arsenic(V) vs. arsenic(III) removal using ferrihydrite as the adsorbent have been discussed. Recent efforts made for investigating arsenic adsorption on iron hydroxides/oxyhydroxides/oxides such as granular ferric hydroxide, goethite, akaganeite, magnetite and haematite have been reviewed. The adsorption behaviours of activated alumina, gibbsite, bauxite, activated bauxite, layered double hydroxides are discussed. Point-of-use adsorptive remediation methods indicate that Sono Arsenic filter and Kanchan™ Arsenic filter are in operation at various locations of Bangladesh and Nepal. The relative merits and demerits of such filters have been discussed. Evaluation of kits used for at-site arsenic estimation by various researchers also forms a part of this review.

Bimetallic nanoparticles for arsenic detection.

Effective and sensitive monitoring of heavy metal ions, particularly arsenic, in drinking water is very important to risk management of public health. Arsenic is one of the most serious natural pollutants in soil and water in more than 70 countries in the world. The need for very sensitive sensors to detect ultralow amounts of arsenic has attracted great research interest. Here, bimetallic FePt, FeAu, FePd, and AuPt nanoparticles (NPs) are electrochemically deposited on the Si(100) substrate, and their electrochemical properties are studied for As(III) detection. We show that trace amounts of As(III) in neutral pH could be determined by using anodic stripping voltammetry. The synergistic effect of alloying with Fe leads to better performance for Fe-noble metal NPs (Au, Pt, and Pd) than pristine noble metal NPs (without Fe alloying). Limit of detection and linear range are obtained for FePt, FeAu, and FePd NPs. The best performance is found for FePt NPs with a limit of detection of 0.8 ppb and a sensitivity of 0.42 μA ppb(-1). The selectivity of the sensor has also been tested in the presence of a large amount of Cu(II), as the most detrimental interferer ion for As detection. The bimetallic NPs therefore promise to be an effective, high-performance electrochemical sensor for the detection of ultratrace quantities of arsenic.

Size-Selected TiO2 Nanocluster Catalysts for Efficient Photoelectrochemical Water Splitting

Nanoclusters (NCs) are of great interest because they provide the link between the distinct behavior of atoms and nanoparticles and that of bulk materials. Here, we report precisely controlled deposition of size-selected TiO2 NCs produced by gas-phase aggregation in a special magnetron sputtering system. Carefully optimized aggregation length and Ar gas flow are used to control the size distribution, while a quadrupole mass filter provides precise in situ size selection (from 2 to 15 nm). Transmission electron microscopy studies reveal that NCs larger than a critical size (∼8 nm) have a crystalline core with an amorphous shell, while those smaller than the critical size are all amorphous. The TiO2 NCs so produced exhibit remarkable photoelectrochemical water splitting performance in spite of a small amount of material loading. NCs of three different sizes (4, 6, and 8 nm) deposited on H-terminated Si(100) substrates are tested for the photoelectrochemical catalytic performance, and significant enhancement in photocurrent density (0.8 mA/cm(2)) with decreasing NC size is observed with a low saturation voltage of -0.22 V vs Ag/AgCl (0.78 V vs RHE). The enhanced photoconductivity could be attributed to the increase in the specific surface area and increase in the number of active (defect) sites in the amorphous NCs. The unique advantages of the present technique will be further exploited to develop applications based on tunable, size-selected NCs.

In situ synthesis of flowery-shaped α-FeOOH/Fe2O3 nanoparticles and their phase dependent supercapacitive behaviour

In situ, one-step, facile synthetic strategies for flowery-shaped iron oxide nanoparticles were developed. Herein, we report simplified controlled synthesis of 2-line ferrihydrite–goethite core–shell particles for the first time in a semi-aqueous-organic medium. The present route offered phase selectivity by controlling only the aqueous-to-organic phase ratio. The synthesised nanoparticles have high surface areas of 110 m2 g−1 and 185 m2 g−1 for 2-line ferrihydrite and core–shell goethite, respectively. Further, flowery-shaped hematite nanoparticles were obtained by annealing core–shell iron oxide nanoparticles at 400 °C. Phase purities were confirmed by XRD (X-ray diffraction), IR (infrared spectroscopy), and XPS (X-ray photo electron spectroscopy) analysis. Formation of the core–shell nanostructure for the iron oxide samples was confirmed by Mossbauer and selected area electron diffraction (SAED) studies. All the synthesized iron oxide materials were studied for their supercapacitor behaviour by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and chrono-potentiometry charge–discharge measurements. Specific capacitances for core–shell α-FeOOH and α-Fe2O3 were found to be 160 F g−1 and 200 F g−1, respectively. These values were much higher as compared to the previous reported values for pure phases of iron oxides. The chrono-potentiometric charge–discharge study for all the three samples revealed their self-discharging capacities. Moreover, these iron oxide composite electrodes exhibited excellent cycling performance with >99% capacitance retention over 500 cycles. Electrochemical performance of the two-electrode system was also studied. Furthermore, the electrochemical impedance spectroscopy (EIS) demonstrated that the electrochemical resistance of α-Fe2O3 was slightly reduced with the number of cycles, indicating easier access for intercalation/deintercalation of charges in the flowery-structured materials. Thus, the present material can be used as an electrochemical supercapacitor for high-performance energy storage devices in future.

Bimetallic FeNi concave nanocubes and nanocages.

Concave nanostructures are rare because of their thermodynamically unfavorable shapes. We prepared bimetallic FeNi concave nanocubes with high Miller index planes through controlled triggering of the different growth kinetics of Fe and Ni. Taking advantage of the higher activity of the high-index planes, we then fabricated monodispersed concave nanocages via a material-independent electroleaching process. With the high-index facets exposed, these concave nanocubes and nanocages are 10- and 100-fold more active, respectively, toward electrodetection of 4-aminophenol than cuboctahedrons, providing a label-free sensing approach for monitoring toxins in water and pharmaceutical wastes.

Solvent mediated surface engineering of α-Fe2O3 nanomaterials: facet sensitive energy storage materials

The surface chemical properties of iron oxide nanomaterials are keenly studied to explore their potential for many future applications. Therefore many synthetic strategies are now being pursued to develop unique morphologies with active surfaces and unusual crystal facets for advanced uses. Here, a novel process for the formation of an α-Fe2O3 phase has been established by a facile solvent mediated precipitation route. Ethylene glycol was used as the solvent and plays an active role in controlling the surface morphology and the orientation of facets during crystal growth. The effects of various parameters on the morphology, structure of the product, and electrochemical properties were studied. Mainly high surface energy facets were stabilized by a high concentration of EG in the reaction solution. The formation of (012) or (001) facets was observed in a reaction solution with a lower concentration of EG. Hematite with a flowery morphology and having (012) plane orientation was achieved by the assembly of pseudo cubes with (012) facets and a secondary growth process. The sample obtained at an Fe : EG ratio of 1 : 2 showed an ultra-high pseudo capacitance value of 450 F g−1 related to its high surface area. The present study can be further extended for the preparation of other functional oxides with new active facets for energy storage applications.

A facile, single-step synthesis of flowery shaped, pure/lithium-doped 3D iron oxides

The shape-dependent surface properties of iron oxides are being paid increasing attention for their many advanced and synergistic applications. The present investigation deals with the preparation of pure and lithiated 3D iron oxide through a simple and single-step synthesis route. The nano-hierarchical flowers were synthesized by adopting a semi-aqueous ligated system. Here, the reagent played a double role for ligation as well as for precipitation. In the absence of lithium, goethite and ferrihydrite phases were formed, whereas formation of a mixture of hematite and ferrihydrite was observed in its presence, confirming participation of Li in phase transformation of goethite/ferrihydrite to hematite. With the progress of time, flowery shaped nanoparticles developed. Mossbauer spectroscopy revealed Li ion-induced formation of an α-Fe2O3 phase. Single-phase hematite was formed on annealing at 500 °C. The Li-doped iron oxide sample has high surface area and has a sharp distribution peak centered at 19.13 nm, showing homogeneity of the pores. On calcination of the sample at 400 °C, the surface area decreased; however, pore size distribution remained unchanged, which was an unusual trend. The annealed sample (500 °C) possessed bimodal (small and large) mesopore distribution. The fluoride adsorption behaviour and magnetic properties of the as-synthesized and annealed Li-doped samples are discussed. Magnetic properties of the samples suggest that incorporation of Li resulted in an increase of coercivity due to stabilization of the domain. The unique surface behaviour of the present samples can be further examined for other high end applications. The present synthesis strategy has the advantage of producing shape-controlled hierarchical materials with tunable surface properties, which promises the further development of other functional materials.

Metal doped mesoporous FeOOH nanorods for high performance supercapacitors

In the present study, the effect of doping of foreign atoms on the parent atoms and the application of the resultant material for energy storage are successfully investigated. A facile method is reported for successful incorporation of cobalt into the regular crystal lattice of iron oxide in ethylene glycol media. As iron oxides are reasonable, the Co doped nano-goethite is expected to be of potential use for supercapacitor application with a high specific capacitance value of 463.18 F g−1 at 0.1 A g−1 current density. It shows a cycling stability of 1000 at 1 A g−1 with 96.36% of initial capacitance. The doped goethite nanorod with a band gap of 2.82 eV and high surface area (159.74 m2 g−1) was found to be a superior electrode material for supercapacitors in terms of specific capacitance and cycling capability at a particular percentage of doping. The high discharge capacitance and its retention are attributed to high surface area and porosity of the doped iron oxide.

In situ hybridization of superparamagnetic iron-biomolecule nanoparticles.

The increase in interest in the integration of organic-inorganic nanostructures in recent years has promoted the use of hybrid nanoparticles (HNPs) in medicine, energy conversion, and other applications. Conventional hybridization methods are, however, often long, complicated, and multistepped, and they involve biomolecules and discrete nanostructures as separate entities, all of which hinder the practical use of the resulting HNPs. Here, we present a novel, in situ approach to synthesizing size-specific HNPs using Fe-biomolecule complexes as the building blocks. We choose an anticancer peptide (p53p, MW 1.8 kDa) and an enzyme (GOx, MW 160 kDa) as model molecules to demonstrate the versatility of the method toward different types of molecules over a large size range. We show that electrostatic interaction for complex formation of metal hydroxide ion with the partially charged side of biomolecule in the solution is the key to hybridization of metal-biomolecule materials. Electrochemical deposition is then used to produce hybrid NPs from these complexes. These HNPs with controllable sizes ranging from 30 nm to 3.5 μm are found to exhibit superparamagnetic behavior, which is a big challenge for particles in this size regime. As an example of greatly improved properties and functionality of the new hybrid material, in vitro toxicity assessment of Fe-GOx HNPs shows no adverse effect, and the Fe-p53p HNPs are found to selectively bind to cancer cells. The superparamagnetic nature of these HNPs (superparamagnetic even above the size regime of 15-20 nm!), their biocompatibility, and the direct integration approach are fundamentally important to biomineralization and general synthesis strategy for bioinspired functional materials.