Environmental Science: Nano Blog

Selenium (Se) is a metalloid element found in trace amounts in the earth’s crust and which has found extensive application due to its semiconducting properties. The use in photocopiers, microelectronic circuits and other applications has created a demand which makes selenium a valuable element.

Selenium also shows biological activity with a strong dependence on concentration: it is essential in low doses for mammalian organisms but becomes strongly toxic to humans over a certain intake threshold. Efficient removal of selenium from wastewater being discharged in the environment is imperative and the development of cost-effective procedures to achieve this needs to be addressed.

Under typical environmental conditions Se can be found in a variety of oxidation states (-II, 0, IV, and VI). The former two are insoluble and give rise to little toxicity on account of their low mobility in aqueous phases. The latter two however are found as highly mobile oxyanions which are the principal targets for Se removal.

Finding the right reagent

Ling et al have used an established strategy involving the reduction of Se(IV) to the insoluble Se(0) form, but their choice of nanoscale zero-valent iron (nZVI) as the reagent has led to a superior method of wastewater decontamination being developed. As little as 0.2 g L^-1 nZVI can achieve over 99% removal of high levels of Se(IV) within 5 hours. Additionally, on account of the magnetic properties of the nZVI its recovery could be achieved simply with the use of a magnet, leaving pure elemental selenium as the product. The potential for elemental selenium recovery and recycling provides grounding for the method becoming cost-neutral or even profitable.

Furthermore, in depth studies were conducted to elucidate the pathway taken by the decontamination process, with attention focused on the nano- and microstructure of the resulting Se particles and of the nZVI before and after reaction.

The nZVI particles consist of a metallic iron core surrounded by an oxide layer which under aqueous conditions is capable of performing adsorption of Se oxyanions, thus paving the way for their reduction by the metallic core. Two types of Se structures result following the reductive process: almost perfectly spherical nanoparticles and nano-needles, both being attributed to known forms of elemental Se: amorphous and trigonal, respectively.

A complete account of the Se(IV) reduction and Se(0) structure formation mechanisms operating in this process is available in the full article, free to view for a limited time:*

Genesis of pure Se(0) nano- and micro-structures in wastewater with nanoscale zero-valent iron (nZVI)

Environ. Sci.: Nano, 2016, Advance Article
DOI: 10.1039/C6EN00231E

Jean-Pierre Sauvage, Fraser Stoddart and Ben Feringa have just been awarded the Nobel Prize in Chemistry for the design and synthesis of molecular machines. Ranging from artificial muscles to micromotors, these nanomachines can perform different tasks and present a myriad of applications. In 1983, Sauvage linked two-ring-shaped molecules by a freer mechanical bond. He was then followed by Stoddart who developed, among other things, a molecule-based computer chip, and more recently Ben Feringa who designed a nanocar. These three remarkable scientists were pioneers in the field and many others now benefit from their contributions to science.

Taking advantage of these findings, Uygun and colleagues developed unique micromotors that offer high-speed metal remediation. Microscale machines have been used for accelerated isolation and degradation of toxins and clean-up of oil-contaminated water, among other uses. The continuous autonomous movement of functionalised nanomachines around a contaminated sample leads to enhanced transport of the remediation agent resulting in greatly accelerated decontamination. However, most of these require external fuel, such as hydrogen peroxide, and expensive catalysts, which then prevents their widespread use. Only recently have new fuel-free Mg-based microparticles been developed. They are highly biocompatible as they use water as their sole fuel. Using these new nanomachines, Uygun describes a Mg Janus-micromotor that is functionalised with meso-2,3-dimercaptosuccinic acid (DMSA), which has been recognised as an excellent chelating agent for heavy metals (Figure 1).

Figure 1. Micromotor “on the fly” removing Zn, Cd and Pb.

The micromotors were prepared by half-coating magnesium microparticles with Ti and Au layers and the external gold surface was modified by incubation in DMSA. The removal of its passivation layer will expose the Mg surface allowing an Mg-water redox reaction that generates hydrogen microbubbles leading to an efficient water-propulsion. Moreover, a small number of micromotors can lead to a nearly complete removal of heavy metals within a short period of time and they are not impacted by co-existing metal ions present in complex samples, making these nanomachines an interesting and cost effective option for fast removal of heavy metal pollutants.

By developing these self-propelled water-driven molecular machines, Uygun and colleagues are corroborating the Nobel Prize laureates and spreading the belief of the Swedish Academy of Science: “we are at the dawn of a new industrial revolution of the 21^st century and the future will show how molecular machinery can become an integral part of our lives”.

To read the full article for free* click the link below:

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About the webwriter

Luiza Cruz is a PhD student in the Barrett Group at Imperial College London. Her work is towards the development of new medicines, using medicinal and natural products chemistry.

Iron oxide nanoparticles are not just a laboratory curiosity but a major presence within the whole of the natural world. Ferrihydrite is the predominant iron oxide nanoparticle found under abiotic conditions owing to its very low surface energy. The essential function of iron storage and transport within living organisms is carried out by the protein ferritin, made possible through the incorporation of iron nanoparticles inside the protein’s internal cavity.

The ability of ferritin to template the formation of iron oxide nanoparticles of defined size has been exploited in the production of systems used in targeted drug delivery, magnetic resonance imaging and nano-electronics. The ability of the iron core in ferritin to adsorb phosphate anions has also been put to use in removing this nutrient down to levels which prevent the development of bacteria on water purification membranes.

Understanding nanoparticles

In spite of being recognised for their essential function and with much work dedicated to the development of exciting applications surrounding them, iron oxide nanoparticles have been so far poorly understood at a fundamental level regarding their structure and reactivity models. Their implicitly small size and low symmetry has made imaging difficult using conventional crystallographic techniques. The properties of nanoparticles are generally dependent on their size and any model attempting to quantify the reactivity displayed by the surface must take this into account. The nature of the surface itself is dependent on the chemical properties of the surrounding environment.

Hiemstra and Zhao have conducted both an experimental and a computational study in order to generate a valid reactivity model for the adsorption of phosphate and arsenate by ferrihydrite and by the ferritin core. Ferrihydrite was modelled and essential properties such as surface area, density of surface reactive groups such as O(H), and surface charge were calculated as a function of the particle size.

The experimental study followed the adsorption of phosphate anions onto freshly prepared ferrihydrite and the effect of phosphate concentration on the formation and properties of iron oxide nanoparticles inside ferritin was analysed. Corroboration of theoretical with experimental data allowed for the development of an anion adsorption model with account for surface reactivity and generated new understanding concerning the formation, growth and aggregation of iron oxide nanoparticles under conditions relevant to environmental applications.

The full article is free to access* for a limited time only:

Reactivity of ferrihydrite and ferritin in relation to surface structure, size, and nanoparticle formation studied for phosphate and arsenate

Tjisse Hiemstra and Wei Zhao

Environ. Sci.: Nano, 2016, Advance Article
DOI: 10.1039/C6EN00061D

The answer is nanomaterials. Particularly Carbon Black (CB) and silica nanomaterials which reinforce rubber increasing its durability, road grip and mileage. Other materials, such as carbon nanotubes (CNTs), offer better performance and ecological benefits via fuel savings. However, these are more expensive therefore their use is still very limited.

Nanomaterial fragments are generally released into the environment, constituting a rather uncontrollable source of emission of nanocomposites. In some countries where release quantification is already required, these emissions have been estimated between 4,000 and 7,000 tons of microplastic fragments. In spite of this immense environmental impact, little is known regarding the release from nanocomposites under mechanical and chemical stresses combined. Challenging the hypothesis of release being induced by a synergy of stresses, Wohlleben and co-workers bring a new sequence to test degradation pathways (Figure 1) as well as a fresh look at appropriate analytical techniques.

Figure 1. Synergetic degradation pathways by combined mechanical and chemical stresses.

In both cases shown in Figure 1, only chemical degradation or mechanical shear does not induce appreciable release of fragments. This happens only when the second stress is introduced, showing that synergetic degradation occurs on the diagonal of the scheme shown above.
In the first case, polyurethane (PU) with different single fillers (blue pathway in Figure 1) was first aged under standard conditions then put under mechanical stress simulating rain conditions (immersion, shaking or sonicating). Ultraviolet spectroscopy (UV-Vis), transmission electron microscopy (TEM) and analytical ultracentrifugation (AUC) or field flow fractionation were used to analyse the results. Images from X-ray photoelectron spectroscopy (XPS) showed that nanofillers remain on the surface after UV and rain weathering, accumulated into dense agglomerates as the polymer matrix was removed by the combined photolysis and hydrolysis.

Moreover, by creating an extended, highly reproducible, very low scatter semi-quantitative method to analyse turbidity of the released fragments, Wohlleben and co-workers were able to affirm that the release was reduced when CNTs were used (Figure 2a). Considering fragments below 150 nm diameter, PU filled with CNTs also showed reduced release (Figure 2b). More importantly, fragments coming from PU with CNTs were mostly organic, showing that the release of nanofiller fragments was suppressed.

Figure 2. a) Turbidity assessment of the released fragments from aged PU surfaces after UV and rain, with increasing mechanical shear: 24 h immersion (light grey), 24 h shaking (dark grey) and 1 h sonicating (black). b) Size-selective analysis (AUC) of fragments in the size range of 5 nm to 150 nm released from aged PU surfaces after UV and rain, with increasing mechanical shear: 24 h immersion (light grey), 24 h shaking (dark grey) and 1 h sonicating (black).

In the second case in Figure 1, natural rubber (NR) nanocomposite were filled with 40% CB and 4% CNT representing an innovative reinforced tire tread and it was compared to NR with 40% CB representing a conventionally reinforced tread and also to neat NB. The test focused on a sequence of mechanical-chemical-mechanical stresses, enabling the simulation of dust aging on dry roads and also the direct run-off into surface waters of the secondary fragments using UV irradiation.

During sanding, there was no noticeable difference between the particle concentration of the three rubber specimen. After aging, the structural differences of the fragments were minimal between wet and dry aging. Moreover, being CB and CNT both relatively more inert to UV degradation, they seem to have accumulated on the surface (less oxidised organic structures were quantified).

Fragments could potentially release smaller fragments and even free nanomaterial. Hence, Wohlleben and colleagues also analysed this scenario and indeed, smaller fragments were formed when a second sanding process was introduced, with no significant differences between NB with both fillers and only CB. However, it clearly showed that dry aging induces stronger secondary fragmentation than submersed aging, these results being in contradiction with the expected combined effect of hydrolysis and photolysis being more aggressive than photolysis only.

In summary, regarding analytical techniques, simple UV-Vis was shown to be the most sensitive technique. Qualitative identification by TEM is essential and analysis of XPS images was also important for a plausibility check.

This study is the first to analyse the combined forces of mechanical fragmentation, environmental aging and again mechanical stresses, showing a stepwise sequence that could continue ad infinitum and be tailored to simulate specific scenarios and provide useful estimates of release rates, enabling more reliable modelling and risk assessments.

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Release from nanomaterials during their use phase: combined mechanical and chemical stresses applied to simple and multi-filler nanocomposites mimicking wear of nano-reinforced tires
Wendel Wohlleben, Jessica Meyer, Julie Muller, Philipp Müller, Klaus Vilsmeier, Burkard Stahlmecke and Thomas A. J. Kuhlbusch

Environ. Sci.: Nano, 2016,3, 1036-1051
DOI: 10.1039/C6EN00094K, Paper

Significant quantities of radioactive compounds are contaminating our environment. Due to nuclear weapon production during the Cold War, large quantities of radioactive Technetium-99 (⁹⁹Tc) has been released into the environment. Although the Cold War ended 25 years ago, the total Tc inventory continues to rise as it is a major product during the generation of nuclear power resulting from the nuclear fission of uranium and plutonium.

Radioactive Tc is long lived and predominantly found in nuclear waste, in the highly mobile form of the water-soluble pertechnetate anion TcO₄^–. The high mobility possessed by TcO₄^– causes concerns regarding its distribution within the environment and makes storage impracticable within cement-based materials, a widely employed medium for the storage of nuclear waste.

Managing nuclear waste
Reduction of TcO₄^– from Tc(VII) to Tc(IV)  is one possible solution. This is a challenging solution as Tc(IV) tends to convert back to TcO₄^– if exposed to oxygen. The reduction process requires a proton source to proceed and is disfavoured under the strongly alkaline conditions found in the waste media. The nuclear waste tanks also contain a large amount of both spectator ions, which make TcO₄^– more difficult to reduce, and high-valent metal species such as Cr(VI). This can interfere with the reduction process by consuming the available Sn(II) and/or by converting any formed Tc(IV) back to TcO₄^–.

To overcome these obstacles, Eric D. Walter and colleagues have devised a novel Tin(Sn)-based material which displays selective and efficient removal of Tc under conditions similar to those found in nuclear waste tanks. The approach taken relies on the inclusion of both Sn(II) and Sn(IV) within an inert aluminophosphate matrix. The low-valent form of tin is known for its ability of reducing Tc(VII) to Tc(IV), whereas the high-valent Sn(IV) was chosen for its ability to form SnO₂ phases capable of accommodating the resulting Tc(IV) and thus facilitate its sequestration.

Representative SEM images of the Sn–Al–PO4 composite (A) before and (B) after exposure to TcO4 − .

The material labelled Sn-Al-PO₄ could be easily made from common laboratory chemicals and in this study was characterised by a number of analytical techniques investigating both the bulk and surface properties. It consists of two distinct phases:

1. An amorphous Sn-based matrix containing the majority of the aluminium.  This material has a large surface area and contains predominantly Sn(II), a combination of properties expected to yield efficient reduction of pertechnetate from the solution medium.
2. Small embedded fibres of crystalline material with the composition dominated by Sn and phosphate.

The full article is free to access for a limited time only:

Inorganic tin aluminophosphate nanocomposite for reductive separation of pertechnetate Environ. Sci.: Nano, 2016, Advance Article
DOI: 10.1039/C6EN00130K

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About the webwriter

Dan Mercea is a PhD student in the Fuchter group at Imperial College London. He is working on developing enantioselective FLP catalysis.

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