(446x) Design of Metal Based Nanofiber Scaffolds for the Removal of Heavy Metal Ions in Harsh Effluents

Wastewater containing heavy metals may be generated from fertilizers with the use of arsenical pesticides, from dumping products containing heavy metals with household waste such as Cd-Li batteries, from lead soil contamination resulting from airborne deposition due to lead-based petrol additives, or from mining effluents such as the release of mercury (estimated to 1,000 ton per year) during the gold extraction process [1, 2]. Membrane filtration and especially nano-filtration has been suggested as a preferable treatment to remove heavy metals from contaminated streams as it is capable of achieving strict discharge criteria while providing high efficiency, and has higher fluxes at lower pressures compared to reverse osmosis, for a relative process cost-efficiency and minimized environmental impact compared to electro-chemical techniques [3, 4]. However, it is confronted to some limitations including the reduction degree, the usually required de-complexation, and the material or membrane stability towards abrasion and pH variation [3]. Thus, new techniques able to stand the harsh water conditions are required.

Adsorption is an outstanding technique as it can potentially achieve high yields of heavy metal removal even for low concentration effluents. It can be a more cost-effective process to remove and recover heavy metals, based on the choice of the adsorbent material [4, 5]. Among the investigated adsorbents for heavy metal ions, metal oxide nanoparticles showed high adsorption capacities towards a wide range of heavy metal contaminants, however their application is hindered by their tendency to agglomerate in liquid streams due to attraction forces such as Van der Waals between the oxide layers [5].

Metal-based nanofiber scaffolds are a class of materials combining the enhanced specific surface area and controlled flow pattern of nanofiber mats with metal unique surface properties [6]. While single metals such as Cu, Ti, Ni, Zn and Fe have been investigated, metal alloys such as Cr and Cu can offer new opportunities with the combination of several metals useful properties (such as corrosion resistance and electrical conductivity) in one membrane.

Electro-spinning has been chosen as a fabrication route by using a precursor composed of metal salt or metal salts mixture to form a sol-gel network with a polymer template, followed by an annealing post-treatment to remove the polymer and induce the nucleation and coalescence of metal and metal oxide grains [7, 8]. The particle coarsening process has been imaged in situ by TEM and an Ostwald ripening type phenomenon was observed. The materials were further treated under a 15% H₂ in N₂ reductive atmosphere to lower the oxidation degree of the structure in order to improve the adsorption capacity with the increase of oxygen vacancies at the surface of the nanofiber web. A reduction of 6.2 At % of oxygen in the global composition of a Cu/CuO nanofiber web has been achieved after one hour treatment at 400 ⁰C, according to x-ray photo-spectroscopy measurements.

Compared to the as-spun fiber mat size, a shrinkage from 55% and up to 85% has been obtained during thermal treatment due to the polymer removal and the fiber crystallization. However, the homogeneity of fiber diameter and pore size distributions resulting from the electro-spinning process has been conserved.

For an electro-spun copper salt in polyvinyl alcohol, it has been observed from TEMs that when the temperature is higher than 700°C, the fiber morphology is lost as the coalescence of bigger grains forms necklaces. This resulted in a loss of mechanical resistance and excessive brittleness. In-situ SAXS annealing and TEM diffraction were used to observe the reorganization of microstructure and it was noticed that the monoclinic structure of cupric oxide crystals were formed from 120 ⁰C. The diffraction patterns showed that the reactivity of the exposed crystallographic plans of copper crystals obtained after reduction increased with the temperature, resulting in the need of a compromise between sufficient mechanical strength (2 to 5 bar in compression for adsorptive micro-filters) and higher surface reactivity. The nano-scale texture of metal nanofibers - measured by AFM - with high angle grain boundaries enhance the surface reactivity even for low specific surface areas (of 7.6 m²/g and 11.8 m²/g for an as-spun and annealed copper in PVA fiber mat), while the nanofiber mat porosity and sub-micrometer pore size provides microfiltration efficiency with low liquid flow resistance for ease of operation with liquid streams. In this study the synergy between the fiber microstructure and the nanofiber web surface reactivity will be explored to improve the heavy metal ion adsorption performance of single metal and metal alloy nanofiber mats.

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7. Pomogailo, K., Metallopolymer Nanocomposites. Materials Science, ed. Springer. Vol. 81. 2005.

8. Barakat, N.A.M., et al., Spider-net within the N6, PVA and PU electrospun nanofiber mats using salt addition: Novel strategy in the electrospinning process. Polymer, 2009. 50(18): p. 4389-4396.