• Production of new types of nZVI particles with a complex inorganic-organic surface stabilisation.
• Production and improvement of nZVI based on grinding/milling.
• Modification with environmentally friendly stabilisers.
• Expansion of the range of potential applications.
Remediation in-situ with nano zero-valent iron (nZVI) has the potential to become an important method for the treatment of water polluted by a large spectrum of contaminants. nZVI particles show an extremely high reactivity, being reflected in the demonstrated effective transformation of more than 70 environmental contaminants such as polychlorinated hydrocarbons, highly toxic substances such as As(III), As(V), Cu(II), Co(II), Cr(VI), nitrite, amoxicillin and ampicillin, TNT, chemical warfare agents and cyanobacteria.
In general, production methods include wet chemical reduction of iron (II) or (III) solutions, dry chemical reduction from oxides, and crushing or milling of larger iron particles. nZVI products currently available on the world market (in a quantity sufficient for a full-scale remediation) are mainly produced by high temperature reduction of oxides in a hydrogen atmosphere. Because this production method is costly, an alternative milling/grinding method based on milling of micro-scale metallic iron particles (colloids) has been used. Production methods have been optimised.
Although there is high remedial potential, there are three key obstacles to market entry: relatively high cost of nZVI, nanoparticle performance (i.e. reactivity and mobility), and validated large scale performance data for optimisations. Previously used nanoparticles were not particularly stable and oxidised fairly rapidly, which affected particle storability, safety and transport. Moreover, nZVI particles tend to agglomerate and adhere to solid surfaces resulting in limited migration in groundwater.
Photograph 1 - nZVI particles injected into a column and Figure 1 - corresponding concentration profile (courtesy C. de Boer, VEGAS)
The first nZVI improvement being carried out by NanoRem is based on surface stabilisation through a targeted surface nZVI oxidation. Such nZVI can be stored in a dry form in air in the long term. These particles are being produced on a large scale, just as particles produced by dry/wet milling. In order to prevent the nZVI particles from agglomeration and sorption to aquifer materials, and so improve migration distance, a number of surface modification methods have been tested.
WP2 addresses improved performance and cheaper production of nZVI:
- Production of new types of nZVI particles with a complex inorganic-organic surface stabilisation. Solid-state reduction methods used to prepare air-stable nZVI with an inorganic shell are being combined with a secondary wet organic stabilisation to deliver an nZVI product with enhanced aggregation stability, reactivity and mobility compared to existing nanoparticles. These new air-stable products are more efficient, easier to handle, more storable and safer to transport, which significantly increases their application potential. Moreover, they provide a higher content of elementary iron per kg of product due to better surface stabilisation. This significantly improves the applicability of the nZVI for treatment of contaminated groundwater.
- Production and improvement of cheaper nZVI based on grinding/milling. Milling could provide cheaper nanoparticles, but currently, milled products tend to have relatively large particle sizes (>1000 nm) which tend to aggregate readily. WP2 is investigating routes for the production of smaller particle sizes with improved stability. Such nZVI could be a cheaper alternative to those produced by solid-state reduction, and also have a potential application in treating other types of water (wastewater, surface water etc.).
- Modification with environmentally friendly stabilisers. Prior to NanoRem work, surface stabilisers did not have optimal migration, toxicity and stability properties. The separation of nZVI treatment with an inorganic shell made during the production and organic stabilisation prior to application gives much broader potential for other types of stabilisers, including environmentally friendly compounds. This two-step modification improves nZVI migration properties (enabling stable aggregation and minimized adhesion to aquifer solids).
WP2 has also been responsible for the characterisation of nZVI particles (including modified) and testing its reactivity. Characteristic tests included structural analysis, and basic migration tests. During developmental stages, the thickness of the protective oxide shell was shown to be the most important parameter for reactivity and migration. This parameter was therefore included in characterisation of particles. Other parameters of importance are chemical composition, particle/aggregate size, and aggregate stability. Feasibility tests are being carried out using contaminated water from pilot sites.