Daniela S. Tavares

Efficient sorbents based on magnetite coated with siliceous hybrid shells for removal of mercury ions

New sorbents based on magnetite particles coated with siliceous shells were synthesized and tested in the removal of mercury ions from aqueous solutions by magnetic separation. Atomic fluorescence spectroscopy studies performed on water samples treated with these sorbents have shown that the metal ion concentration decreased to values lower than the guideline values for drinking water. The high adsorption affinity for mercury ions in these materials was attributed to dithiocarbamate groups grafted over the siliceous domains using a new surface modification method. Hence, the magnetic particles have been coated with amorphous silica shells that include thiolate moieties via a hydrolytic co-condensation process using tetraethoxysilane (TEOS) and siloxydithiocarbamate (SiDTC) as the sol–gel precursors. This one-step method for the surface modification of magnetite particles led to sorbents with extensive thiolate moieties that can be then collected by applying an external magnetic field.

Ferromagnetic sorbents based on nickel nanowires for efficient uptake of mercury from water.

This work reports the preparation of ferro-magnetic nickel nanowires (NiNW) coated with dithiocarbamate-functionalized siliceous shells and its application for the uptake of aqueous Hg(II) ions by magnetic separation. NiNW with an average diameter and length of 35 nm and 5 μm, respectively, were firstly prepared by Ni electrodeposition in an anodic aluminum oxide template. The NiNW surfaces were then coated with siliceous shells containing dithiocarbamate groups via a one-step procedure consisting in the alkaline hydrolytic co-condensation of tetraethoxysilane (TEOS) and a siloxydithiocarbamate precursor (SiDTC). A small amount of these new nanoadsorbents (2.5 mg·L(-1)) removed 99.8% of mercury ions from aqueous solutions with concentration 50 μg·L(-1) and in less than 24 h of contact time. This outstanding removal ability is attributed to the high affinity of the sulfur donor ligands to Hg(II) species combined with the high surface area-to-volume ratio of the NiNW.

Competitive effects on mercury removal by an agricultural waste: application to synthetic and natural spiked waters

In this work, the efficiency of a local and highly available agricultural waste, the raw rice husk, was used to remove mercury (Hg) from synthetic and natural waters, spiked with concentrations that reflect the contamination problems found in the environment. Different operating conditions were tested, including initial pH, ionic strength, the presence of co-ions (cadmium) and organic matter. The sorption efficiency of rice husk was slightly affected by the presence H+ ions (pH range between 3 and 9), but in the presence of NaNO3 and NaCl electrolytes and in binary solutions containing Cd2+ and Hg2+, the sorption efficiency was dependent on the nature and levels of the interfering ion and on the initial concentration of Hg2+ used. Nevertheless, in a situation of equilibrium the effect of those ions was negligible and the removal efficiency ranged between 82% and 94% and between 90% and 96% for an initial Hg2+ concentration of 0.05 mg L−1 and 0.50 mg L−1, respectively. In more complex matrices, i.e. in the presence of humic substances and in natural river waters, the speciation and dynamics of Hg was changed and a fraction of the metal becomes unavailable in solution. Even then, the values obtained for Hg removal were satisfactory, i.e. between 59% and 76% and 81% and 85% for an initial concentration of Hg2+ of 0.05 and 0.50 mg L−1, respectively.

Fluorescent Magnetic Bioprobes by Surface Modification of Magnetite Nanoparticles

Bimodal nanoprobes comprising both magnetic and optical functionalities have been prepared via a sequential two-step process. Firstly, magnetite nanoparticles (MNPs) with well-defined cubic shape and an average dimension of 80 nm were produced by hydrolysis of iron sulfate and were then surface modified with silica shells by using the sol-gel method. The Fe3O4@SiO2 particles were then functionalized with the fluorophore, fluorescein isothiocyanate (FITC), mediated by assembled shells of the cationic polyelectrolyte, polyethyleneimine (PEI). The Fe3O4 functionalized particles were then preliminary evaluated as fluorescent and magnetic probes by performing studies in which neuroblast cells have been contacted with these nanomaterials.

Remediation of mercury contaminated saltwater with functionalized silica coated magnetite nanoparticles

The study aimed to evaluate the efficiency of dithiocarbamate functionalized silica coated magnetite nanoparticles (NPs) for Hg decontamination of saltwater either contaminated with Hg alone or with As and Cd. For this, the residual levels of Hg in seawater were assessed and Hg-contaminated or Hg+As+Cd-contaminated seawater toxicity to aquatic biota, before and after the sorption process, was compared. The results showed that under highly competitive conditions (water salts, Cd and As), the removal of Hg from seawater, by using these magnetic NPs, for the lowest concentration (50μg/L) was superior to 98% and for the highest concentration (500μg/L) ranged between 61% to 67%. Despite the great affinity of the magnetic NPs for Hg, they were not effective at removing As and Cd from seawater. In relation to the ecotoxicity endpoints after remediation, the mixture with lower Hg concentration exhibited no toxicity to rotifer Brachionus plicatilis and bacteria Vibrio fischeri ; however, the mixture with higher concentration revealed toxicity. In addition, the toxicity of bacteria V. fischeri, rotifer B. plicatilis and algae Phaeodactylum tricornutum, whose responses where inhibited during its exposure to the non-remediate sample was considerably reduced after treatment with NPs. Furthermore, microalgae P. tricornutum appears to be most sensitive species while Artemia franciscana showed no toxic effects to the tested solutions. Both chemical and ecotoxicological approaches revealed a high efficiency for the remediation of Hg-contaminated saltwater.

Soil-pore water distribution of silver and gold engineered nanoparticles in undisturbed soils under unsaturated conditions

Release of engineered nanoparticles (ENPs) to soil is well documented but little is known on the subsequent soil-pore water distribution of ENPs once present in soil. In this study, the availability and mobility of silver (Ag) and gold (Au) ENPs added to agricultural soils were assessed in two separate pot experiments. Pore water samples collected from pots from day 1 to 45 using porous (<0.17 μm) membrane samplers suggest that both Ag and Au are retained almost completely within 24 h with less than 13% of the total added amount present in pore water on day 1. UV-Vis and TEM results showed that AuENPs in pore water were present as both homoaggregates and heteroaggregates until day 3 after which the concentration in pore water was too low to detect the presence of aggregates. A close relation between the concentration of Au and Fe in pore water suggests that the short term solubility of Au is partly controlled by natural soil colloids. Results suggest that under normal aerated soil conditions the actual availability of Ag and AuENPs is low which is relevant in view of risk assessment even though the impact of environmental conditions and soil properties on the reactivity of ENPs (and/or large ENPs aggregates) retained in the solid matrix need to be addressed further.