Environmental Science: Nano Blog

Graphene oxide (GO) is a precursor material in the preparation of graphene. Despite its name, on the surface of this material there are different types of functional groups including epoxy, hydroxyl and carbonyl groups. As a result, GO is hydrophilic and easily dispersed in water. This has led to a variety of investigations relating to GO as a potential pollutant, as well as a possible treatment to cancer.

The disrupted π-bond structure in GO enables the absorption of significant amounts of light from solar radiation. Therefore, environmental processing of GO can be expected to include photochemical processes. One important outcome of such processing is the generation of reactive oxygen species (ROS). These ROS can include singlet oxygen (¹O₂), superoxide anions (O₂^.–) and hydroxyl radicals (^.OH). Generation of ROS has a significant impact on ecological risks associated with GO and is critical in understanding the transformation pathways of carbon in the GO structure.

Therefore, Yingcan Zhao and Chad T. Jafvert at Purdue University (West Lafayette, USA) has investigated the ability of aqueous dispersions of single layered GO to generate ROS upon exposure to light within the solar spectrum (λ=300-410 nm). The generated ROS was detected using specific chemical probes, UV-vis spectroscopy and Raman spectroscopy.

The findings of this research highlighted that upon exposure to solar radiation there is electron transfer reactions occurring from GO to dissolved O₂, forming O₂^.– and significant quantities of H₂O₂.

Given the fact that these are reduction reactions, this resulted in an overall oxidation of GO. Some of the generated ROS reacted directly with the GO surface and therefore, the oxidation of GO was found to be non-stoichiometric.

The exposure to light also increased the chromophores content or the absorptivity of existing chromophores, as suggested by the increased darker colour of the GO suspensions. However, Raman spectroscopic analysis also indicated an increase in non-aromatic defects.

To access the full article, download a copy for free* by clicking the link below.

Environmental photochemistry of single layered graphene oxide in water
Yingcan Zhao and   Chad T. Jafvert
Environ. Sci.: Nano, 2015, 2, 136-142
DOI: 10.1039/C4EN00209a

—————-

About the webwriter

Imali Mudunkotuwa is a Postdoctoral Scholar and Research Assistant at The University of Iowa. She is interested in nanoscience, physical and surface chemistry. You can find more articles by Imali in her author archive .

—————-

* Access is free until the 21/06/2015 through a registered RSC account

Everyone loves and hates nanoparticle aggregation. Why? Because it is a fascinating phenomenon but it complicates all of our experiments, ranging from in vitro / in vivo studies, all the way to the environmental models. For ease of study, it can be broken down into homoaggregation and heteroaggregation. Investigating the former is relatively easier than the latter (relatively!), but in the real world we are forced to deal with the latter. Simply, heteroaggregation in the environment refers to attachment of ENPs to naturally occurring solids (e.g. soil particles, suspended sediments). This plays a key role in determining ENPs bioavailability and mobility in the environment.

Large-scale fate and transport models have been adapted for modelling of ENPs. Earlier models use mass balance as a framework with partition coefficients (K_d) to describe the soil-water distribution of ENPs, but recent studies argue that particle number-based kinetic models using attachment efficiency (α) are better at this, given that colloidal suspensions never reach an equilibrium state as well as ignore the size dependent properties. Yet no agreement has been established on the best way to treat heteroaggregation in these models.

Therefore, to shed light on this matter, Amy L. Dale (Engineering and Public Policy, Carnegie Mellon University, PA), Gregory V. Lowry (Civil and Environmental Engineering, Carnegie Mellon University, PA), and Elizabeth A. Casman (CEINT, Duke University, NC) discuss in detail the ongoing practical challenges in model formulation, parameterization, and calibration for ENPs in their perspective.

A couple of highlights in this article are:

– Particle balance models are more complex, making it challenging to parameterize ENPs in the environment. Therefore, many assumptions are needed that have been discussed in detail in the perspective.

– Attachment efficiency is agreed to be the most appropriate fate descriptor for small-scale models, but at large-scale these models it is claimed to be relative insensitive to the particulate nature of ENPs.

Given the lack of scientific understanding on ENP heteroaggregation it was suggested that balance models to be the better choice, at least in the short term. Overall rates of sorption and desorption has been suggested to be used in the place of equilibrium partition coefficients to overcome concerns arising from using the equilibrium K_d.

To access the full article, download a copy for free* by clicking the link below.

Much ado about α: reframing the debate over appropriate fate descriptors in nanoparticle environmental risk modeling
Amy L. Dale, Gregory V. Lowry, Elizabeth A. Casman
Environ. Sci.: Nano, 2015, Perspective
DOI: 10.1039/C4EN00170B

—————-

About the webwriter

Imali Mudunkotuwa is a Postdoctoral Scholar and Research Assistant at The University of Iowa. She is interested in nanoscience, physical and surface chemistry. You can find more articles by Imali in her author archive .

—————-

* Access is free through a registered RSC account

Graphene and graphene oxide are among the recent attractions in novel nanomaterials for electronic and composite applications. Structure of graphene can simply be viewed as taking a single walled carbon nanotubes and unrolling it to give a sheet.

History of graphene runs all the way back to the 19th century but the very first single layer graphene was synthesized in 2004, by Andre Geim and Kostya Novoselov at The University of Manchester, UK. Graphene can be synthesized to contain a single layer or multiple layers. Nanoscience and nanotechnology community has always been very excited about the remarkable electrical and heat conductivity of these materials and a significant influx of graphene into the composite and electronic markets are being anticipated.

There are raising concerns about environmental and health risks of these materials with this increasing usage as there are no established methods for their extraction from complex matrices and quantification. Therefore, Kyle Doudrick and co-workers from the University of Notre Dame and Arizona State University have successfully developed an extraction and quantification method for graphene and graphene oxide from biomass using in situ reduction method followed by detection with programmed thermal analysis (PTA).

PTA is known to determine the carbon mass based on the thermal stability. In this study, the separation was achieved by subjecting the samples to a time dependent temperature ramp program where the less thermally stable carbon compounds evolved early in the program and the high thermally stable compounds were evolved later. Graphene, which has a high thermal stability was therefore easily separated from rest of the components in the biomass. In order to obtain similar separation for graphene oxide that has a higher oxygen content, which limits the separation, an in situ reduction was conducted using sodium borohydride.

Since this makes the graphene oxides more hydrophobic forcing aggregation an efficient separation and extraction was expected. The results of the study has proved that this technique is capable of recovering 52 ± 8% and 80 ± 6% of few layer graphene (FLG) and graphene oxide (GO) from dried biomass, respectively. Therefore, this technique can be applied to the extraction of graphene from complex organic matrices used in nanotoxicological studies.

To access the full article, download a copy for free* by clicking the link below:

Quantification of graphene and graphene oxide in complex organic matrices
Kyle Doudrick, Takayuki Nosaka, Pierre Herckes, Paul Westerhoff
Environ. Sci.: Nano, 2015,2, 60-67
DOI: 10.1039/C4EN00134F

—————-

About the webwriter

Imali Mudunkotuwa is a Postdoctoral Scholar and Research Assistant at The University of Iowa. She is interested in nanoscience, physical and surface chemistry. You can find more articles by Imali in her author archive .

—————-

* Access is free through a registered RSC account

Multi-walled carbon nanotubes (MWCNT) are the preferred choice of nanotubes for many applications as they has a lower cost than the single walled carbon nanotubes (SWCNT). Therefore the chances of them getting into air, natural water systems and soil is extremely probable. Although many beneficial effects are postulated for MWCNTs direct applications such as incorporation into fertilizer to enhance water uptake, seed germination and cell growth can increase their levels in the environment, especially in soil. Since there is evidence of some negative affects on soil microbial communities as well as plants it is always better to have means of monitoring and controlling their levels in the environment. However, methods to detect and quantify MWCNTs in soil and sediments are still not well established. Therefore, Alexander Gogos and co-workers from the Agroscope, Institute for Sustainability Sciences in Switzerland have developed and evaluated a novel approach using asymmetric field flow fractionation (A4F) coupled with multi-angle light scattering (MALS) to differentiate MWCNTs in soil.

Here the high aspect ratios of MWCNT’s have been exploited to differentiate between MWCNTs, soot and native soil particles. The shape factors (ρ) for these materials were calculated by taking the ratio between the radius of gyration (rg) and the hydrodynamic radius (rh). Simply, the rg corresponds to the weighted average of all possible radii of a particle from its center of mass and rh is approximated for non-spherical particles as the radius of a sphere with same diffusion behavior. Elaborately, presence of MWCNTs in the mixtures resulted in increased ρ-values. The fractions of MWCNTs in the mixtures were calculated using the ρ-values obtained from A4F-MALS. They were cross-validated by comparing with the results obtained from automated electron microscopy analysis and were found to be in reasonable agreement. Since natural soils exhibited lower ρ-values consistently this method can be used in specific identification of MWCNTs as well as other high aspect ratio nanomaterials in soil.

To access the full article, download a copy for free* by clicking the link below.

Capabilities of asymmetric flow field-flow fractionation coupled to multi-angle light scattering to detect carbon nanotubes in soot and soil.
Alexander Gogos, Ralf Kaegi, Renato Zenobi, Thomas D. Bucheli
DOI: 10.1039/C4EN00070F

*Access is free through a registered RSC account – click here to register

Groundwater contamination is becoming a bigger problem than in the past as a result of ever increasing industries and agricultural practices. Furthermore, a significant percentage of drinking water supply relies on clean ground water such that its efficient and effective remediation is a timely need. Specifically chlorinated solvents and nitroaromatic compounds are oxidized organics, which are frequently found as persistent contaminants in groundwater. As a result of their stability in oxic environments these contaminants can act as transporters to a variety of other pollutants between soil and ground waters phases in addition to endangering humans and wildlife.

The oxidized functional groups in these contaminants can be reduced by Fe(II) associated with iron minerals thus providing means for their degradation that can be incorporated into engineered remediation schemes. Ferrihydrite, goethite, magnetite, hematite and lepidocrocite are examples of some ubiquitous iron containing minerals. In these degradation reactions the number of reactive sites on the minerals are directly related to the specific surface area and thereforethe nanoparticles of these minerals, which inherently has large surface areas hold the greatest potential towards degrading these contaminants. Nevertheless, nanoparticles are highly susceptible to aggregation, which can significantly hinder the efficiency of the reduction process. Therefore, Amanda M. Stemig and co-workers from the University of Minnesota, have conducted an extensive investigation using 4-Chloronitrobenzene (4-ClNB) as a model compound to elucidate the link between the aggregation state of iron oxide nanoparticles and their reactivity.

The study was conducted by using well-characterized goethite nanoparticles  as the iron containing minerals. The results revealed that the size of the goethite nanoparticles are significantly reduced upon the adsorption of transition metals. This is, of course no surprise as the adsorption of transition metals introduces additional surface charge. Furthermore, a comparison between the pseudo first order rate constants of 4-ClNB degradation in a variety of buffers indicated that the buffer type affected the reaction kinetics by controlling the aggregation state and thereby changing the available surface area. It was clearly demonstrated that zwitterionic buffers with spatial charge separations are better at preventing aggregation, giving better degradation rates. In addition buffer concentration also affected the degradation kinetics as higher buffer concentrations resulted in more densely packed aggregates with lowered surface area.

To access the full article, download a copy for free* by clicking the link below.

Goethite nanoparticle aggregation: effects of buffers, metal ions, and 4-chloronitrobenzene reduction

Amanda M. Stemig, Tram Anh Do, Virany M. Yuwono, William A. Arnold and R. Lee Penn

DOI: 10.1039/c3en00063j

*Access is free through a registered RSC account – click here to register