Viscoelastic gels of guar and xanthan gum mixtures provide long-term stabilization of iron micro- and nanoparticles

Iron micro- and nanoparticles used for groundwater remediation and medical applications are prone to fast aggregation and sedimentation. Diluted single biopolymer water solutions of guar gum (GG) or xanthan gum (XG) can stabilize these particles for few hours providing steric repulsion and by increasing the viscosity of the suspension. The goal of the study is to demonstrate that amending GG solutions with small amounts of XG (XG/GG weight ratio 1:19; 3 g/L of total biopolymer concentration) can significantly improve the capability of the biopolymer to stabilize highly concentrated iron micro- and nanoparticle suspensions. The synergistic effect between GG and XG generates a viscoelastic gel that can maintain 20 g/L iron particles suspended for over 24 h. This is attributed to (i) an increase in the static viscosity, (ii) a combined polymer structure the yield stress of which contrasts the downward stress exerted by the iron particles, and (iii) the adsorption of the polymers to the iron surface having an anchoring effect on the particles. The XG/GG viscoelastic gel is characterized by a marked shear thinning behavior. This property, coupled with the low biopolymer concentration, determines small viscosity values at high shear rates, facilitating the injection in porous media. Furthermore, the thermosensitivity of the soft elastic polymeric network promotes higher stability and longer storage times at low temperatures and rapid decrease of viscosity at higher temperatures. This feature can be exploited in order to improve the flowability and the delivery of the suspensions to the target as well as to effectively tune and control the release of the iron particle

Al/Fe isomorphic substitution versus Fe2O3 clusters formation in Fe-doped aluminosilicate nanotubes (imogolite)

Textural, magnetic and spectroscopic properties are reported of Fe-doped aluminosilicate nanotubes (NTs) of the imogolite type, IMO, with nominal composition (OH)3Al2−x Fe x O3SiOH (x = 0, 0.025, 0.050). Samples were obtained by either direct synthesis (Fe-0.025-IMO, Fe-0.050-IMO) or post-synthesis loading (Fe-L-IMO). The Fe content was either 1.4 wt% (both Fe-0.050-IMO and Fe-L-IMO) or 0.7 wt% (Fe-0.025-IMO). Textural properties were characterized by High-Resolution Transmission Electron Microscopy, X-ray diffraction and N2 adsorption/desorption isotherms at 77 K. The presence of different iron species was studied by magnetic moment measurements and three spectroscopies: Mössbauer, UV-Vis and electron paramagnetic resonance, respectively. Fe3+/Al3+ isomorphic substitution (IS) at octahedral sites at the external surface of NTs is the main process occurring by direct synthesis at low Fe loadings, giving rise to the formation of isolated high-spin Fe3+ sites. Higher loadings give rise, besides IS, to the formation of Fe2O3 clusters. IS occurs up to a limit of Al/Fe atomic ratio of ca. 60 (corresponding to x = 0.032). A fraction of the magnetism related to NCs is pinned by the surface anisotropy; also, clusters are magnetically interacting with each other. Post-synthesis loading leads to a system rather close to that obtained by direct synthesis, involving both IS and cluster formations. Slightly larger clusters than with direct synthesis samples, however, are formed. The occurrence of IS indicates a facile cleavage/sealing of Al-O-Al bonds: this opens the possibility to exchange Al3+ ions in pre-formed IMO NTs, a much simpler procedure compared with direct synthesi

Magnetic properties of pure and Eu-doped hematite nanoparticles

Nanoparticles (NPs) of pure and Eu-doped hematite were prepared by a wet chemical technique; their structure, size and morphology were determined by XRD and transmission electron microscopy. The magnetic properties were measured in the 6-320 K temperature range by vibrating-sample magnetometry. Pure hematite NPs exhibit a pseudo-cubic shape with a size of about 74 nm; addition of trivalent Eu cations in different amounts (Eu/Fe atomic ratios 2.4 %) brings about a definite change in particle morphology with the development of rice-grain like NPs with aspect ratios of about 2.8. Trivalent Eu cations act as magnetic defects making the overall antiferromagnetic arrangement of hematite host less robust. A large defect magnetism arises below the Morin transition temperature. The resulting uncompensated moments on NPs (mostly arising from the NP cores) behave superparamagnetically before undergoing single-particle blocking at about 40 K. Instead, uncompensated moments in pure hematite NPs mostly occur at the NP surface and lead to standard defect ferromagnetism. Above the Morin temperature, the usual spin-canted ferromagnetism is observed in all samples, although it is somewhat inhibited by Eu additio

Photocatalysis with Nanoparticles for Environmental Applications: Reactor Design Issues

The scale-up of photochemical or photo-catalytic processes is a hard task, requir-ing the correct definition of light distribution across the device. After a collection of examples of different photoreactor layouts adopted for water treatment, the main modelling issues are reviewed. Alternative radiation modelling approaches are compared. The reaction rate ex-pressions are presented, considering the dependence on light, catalyst and reactants distribution and including possible mass transfer limitations

Colloidal Semiconductor Nanocrystals for Artificial Photosynthesis

Light-driven reactions for solar fuels have been receiving tremendous interest, leading of the possibility to store solar energy, our biggest and cleanest renewable energy source. Efficient solar to fuel conversion needs photosynthetic materials with strong absorption and high photocatalytic properties. Colloidal semiconductor nanocrystals are cutting-edge materials for this application, thanks to their tunable optical and electronic properties through size, composition, morphology, and assembly. In this chapter, some insights on the challenges to improve photocatalytic performance are reported, followed by an overview of different parameters that can be controlled to cope with these limitations. Finally, some devices at the forefront are illustrated