John T. Mayo

The effect of nanocrystalline magnetite size on arsenic removal

Abstract Higher environmental standards have made the removal of arsenic from water an important problem for environmental engineering. Iron oxide is a particularly interesting sorbent to consider for this application. Its magnetic properties allow relatively routine dispersal and recovery of the adsorbent into and from groundwater or industrial processing facilities; in addition, iron oxide has strong and specific interactions with both As(III) and As(V). Finally, this material can be produced with nanoscale dimensions, which enhance both its capacity and removal. The objective of this study is to evaluate the potential arsenic adsorption by nanoscale iron oxides, specifically magnetite (Fe3O4) nanoparticles. We focus on the effect of Fe3O4 particle size on the adsorption and desorption behavior of As(III) and As(V). The results show that the nanoparticle size has a dramatic effect on the adsorption and desorption of arsenic. As particle size is decreased from 300 to 12 nm the adsorption capacities for both As(III) and As(V) increase nearly 200 times. Interestingly, such an increase is more than expected from simple considerations of surface area and suggests that nanoscale iron oxide materials sorb arsenic through different means than bulk systems. The desorption process, however, exhibits some hysteresis with the effect becoming more pronounced with small nanoparticles. This hysteresis most likely results from a higher arsenic affinity for Fe3O4 nanoparticles. This work suggests that Fe3O4 nanocrystals and magnetic separations offer a promising method for arsenic removal.

Size-dependent sedimentation properties of nanocrystals.

Centrifugation is an increasingly important technique for nanomaterial processing. Here, we examine this process for gold, cadmium selenide, and iron oxide nanocrystals using an analytical ultracentrifuge. Such data provide an accurate measure of the sedimentation coefficients for these materials, and we find that this parameter has a significant dependence on the size and surface coating. Conventional models for particle sedimentation cannot capture the behavior of these nanocrystals unless the density of the nanocrystals is described by a size-dependent term that accounts for both the inorganic core and the organic coating. Using this modification in the particle sedimentation framework, it is possible to estimate sedimentation coefficients from information about the nanocrystal core and surface coating dimensions. Such data are useful in choosing the speeds for a centrifugation process and are particularly important when bimodal nanocrystal distributions are present.

Pollution magnet: nano-magnetite for arsenic removal from drinking water.

Arsenic contamination in groundwater is a severe global problem, most notably in Southeast Asia where millions suffer from acute and chronic arsenic poisoning. Removing arsenic from groundwater in impoverished rural or urban areas without electricity and with no manufacturing infrastructure remains a significant challenge. Magnetite nanocrystals have proven to be useful in arsenic remediation and could feasibly be synthesized by a thermal decomposition method that employs refluxing of FeOOH and oleic acid in 1-octadecene in a laboratory setup. To reduce the initial cost of production,

Applying analytical ultracentrifugation to nanocrystal suspensions.

US 2600/kg, and make this nanomaterial widely available, we suggest that inexpensive and accessible “everyday” chemicals be used. Here we show that it is possible to create functional and high-quality nanocrystals using methods appropriate for manufacturing in diverse and minimal infrastructure, even those without electricity. We suggest that the transfer of this knowledge is best achieved using an open source concept.

Engineered superparamagnetic iron oxide nanoparticles for ultra-enhanced uranium separation and sensing

While applied frequently in physical biochemistry to the study of protein complexes, the quantitative use of analytical ultracentrifugation (AUC) for nanocrystal analysis is relatively rare. Its application in nanoscience is potentially very powerful as it provides a measure of nanocrystal density, size and structure directly in the solution phase. Towards that end, this paper examines the best practices for applying data collection and analysis methods for AUC, geared towards the study of biomolecules, to the unique problems of nanoparticle analysis. Using uniform nanocrystals of cadmium selenide, we compared several schemes for analyzing raw sedimentation data. Comparable values of the mean sedimentation coefficients (s-value) were found using several popular analytical approaches; however, the distribution in sample s-values is best captured using the van Holde-Weischt algorithm. Measured s-values could be reproducibly collected if sample temperature and concentration were controlled; under these circumstances, the variability for average sedimentation values was typically 5%. The full shape of the distribution in s-values, however, is not easily subjected to quantitative interpretation. Moreover, the selection of the appropriate sedimentation speed is crucial for AUC of nanocrystals as the density of inorganic nanocrystals is much larger than that of solvents. Quantitative analysis of sedimentation properties will allow for better agreement between experimental and theoretical models of nanocrystal solution behavior, as well as providing deeper insight into the hydrodynamic size and solution properties of nanomaterials.

LOW-FIELD MAGNETIC SEPARATION OF MONODISPERSE FE3O4 NANOCRYSTALS

Rapid separation and analysis of radionuclides in the environment remains a challenge despite broad needs, particularly for ultra-sensitive field detection. Ionizing radiation detection/counting can be limited by sample matrix shielding and long integration times, while more sensitive spectrometry requires extensive sample preparation and advanced instrumentation. In this work we have designed, synthesized, and demonstrated optimized iron oxide nanoparticles (IONPs) for low-energy, high-efficiency separation and concentration for ultra low-level uranium (as a model actinide) sensing in dilute (environmental) applications. Monodispersed single crystalline, IONPs with an ordered, oleic acid bilayer coating, are demonstrated to bind ca. 50% wt U/wt Fe, under optimal conditions, which is the highest reported for any iron-based sorbent materials to date. Superparamagnetic material properties allow for subsequent low-field magnetic separations from heterogeneous and relatively large (dilute) aqueous volumes resulting in highly concentrated residues. Through a final filtration step, high particle (aqueous) stability gives rise to self-assembling, homogenous, sub-micron films, arranged to minimize α-particle self-shielding, thus allowing for optimized sensitivity/detection with a handheld Gieger counter. Taken together, we demonstrate a ca. 10 000-fold increase in uranium detection sensitivity when compared to commercially available nanoscale IONPs (combining sorption and detection/counting enhancements). Lastly, these advantages are demonstrated for real world samples.