There are a range of definitions that describe nanoparticles and the nanoscale. However, in general they describe particles that are 100 nanometres (nm) or less in at least one dimension, and nanoscale may be used to describe processes that take place at that scale. The properties of materials can be different at the nanoscale for two main reasons. Firstly, nanomaterials have a relatively larger surface area which can make materials more chemically reactive. Secondly, particularly at lower size ranges, quantum effects can begin to affect electron behaviour in the material.
Nanoremediation is the use of nanoparticles in remediation. Nanoremediation processes generally involve reduction or oxidation. In some cases this may be facilitated by an embedded catalyst. There is also some evidence that sometimes nanoremediation can work in combination with the in situ biological processes of dehalorespiration (where contaminants such as solvents are biologically dechlorinated).
The redox processes mediated by nanoremediation may lead to contaminant degradation to simple compounds (for example in the treatment of chlorinated solvents), or contaminant immobilisation in insoluble forms with limited environmental accessibility (for example in the treatment of mobile arsenic) and/or reduced toxicity (for example in the conversion of chromium (VI) to chromium (III).
Nanoremediation is primarily deployed in situ in the subsurface, in saturated zones. This is partly because of the contamination problems they have been developed to treat (FAQ: What are the potential benefits of nanoremediation and its likely advantages over alternative technologies), and partly because the vast majority of nanoremediation has used nZVI which tends to be rapidly passivated in aerobic environments. In most cases nanoparticles are introduced via injection wells into an aquifer, typically requiring some form of active pumping. An emerging application is the placement of nanoparticles in engineered permeable reactive barriers emplaced in/across aquifers.
There are two modes of deployment of in situ remediation: control of the source term and control of the pathway. These may be applied individually or together to break source-pathway-receptor linkages. These modes of deployment relate to the general principles of how risks from contaminated land are assessed and managed for remediation in general see RBLM on COMMON FORUM web site.
- Pathway management describes the use of nanoremediation to degrade or immobilise contaminants migrating along pathways, typically being carried by groundwater flow from the source term.
- Source reduction describes the use of nanoremediation to treat a source term, for example a NAPL source within the saturated zone. The intention is usually to reduce the mass of the source term so that the duration of groundwater contamination is decreased. Note: In common with all in situ remediation techniques, in situ source term treatments may render residual sources more mobile and so lead to higher contamination fluxes to groundwater. Success is crucially dependent on the accessibility of the source term to the treatment being deployed, and this may limit applicability.
Remediation interventions for risk management
Direct Injection of nZVI in the Field at the Trenton Facility, New Jersey (Photo courtesy of Geosyntec Consultants)
Typical PRB Configuration (from www.readyreference.co.uk)
Nano-scale zero valent iron particles (nZVI) have been the most extensively used approach in nanoremediation projects to date. As produced, most nZVI tested falls into the 10-100 nm size range. See FAQ: "Where have nanoparticles been used in remediation?"
nZVI is a highly reactive reducing agent and as such degrades some organic compounds (e.g. chlorinated solvents) and changes the oxidation state of elements (e.g. chromium, uranium). This underlies its functionality in soil and groundwater remediation. nZVI is unstable in air and in the ground environment and is rapidly inactivated by several mechanisms. A number of modifications have been developed to improve stability and/or facilitate the migration of nZVI in aquifers (by coating and/or emulsification). nZVI particles have also been developed which are doped with low levels of other metals (in particular palladium) which are catalytic and increase their reactivity. However, typically there will always be a trade-off between reactivity and stability and between reactivity and mobility in the subsurface.
As well as nZVI there are emerging classes of nanoparticles these include:
- Oxidising agents, such as those based on oxides of iron
- Nanoparticles which combine sorption and redox, for example, to provide a more effective treatment solution of nonaqueous phase liquids (See WP 3 Description)
- Nanoparticles that are anchored or embedded in a matrix, for example, for use in permeable reactive barriers
A wide range of nanoparticles are being investigated by the NanoRem project (See WP2 and WP3 descriptions).
Related Thematic Pages
This information is drawn from t
he NanoRem report: 'A Risk/Benefit Appraisal for the Application of Nano-Scale Zero Valent Iron (nZVI) for the Remediation of Contaminated Sites'. The full report including additional information, detail and referencing can be downloaded from: www.nanorem.eu/Displaynews.aspx?ID=525.