FAQ: What are the potential benefits of nZVI nanoremediation and its likely advantages over alternative technologies?
Owing to their small particle size, nanoparticles (NPs) may exhibit unique properties, which in turn may allow utilisation of nanoparticles in novel applications. Nanoremediation has a potential niche in remediation projects where excavation is not possible and an in situ, process based remediation technique is required.
NPs have the potential to offer a range of benefits over competing technologies. These potential benefits can be broadly outlined as:
- Improving the speed of contaminant destruction
- Improving the extent of contaminant destruction
- Extending the treatable range of contaminants
- Limited longevity of action
- Compatibility with other treatments
NPs have a high specific surface area as a result of their small size, which may result in higher reactivity relative to their micro-sized counterparts. This reactivity is the key to many of the benefits offered by nanotechnology, including a potentially improved rate of contaminant destruction. Rapid contaminant destruction has many benefits, including reduced costs, shorter site stand-still periods, more rapid site redevelopment and a reduction in the amount of time that workers are exposed to a contaminated site during its treatment.
NPs may be able to achieve greater penetration into the subsurface, allowing greater access (and therefore treatability) of contaminants that may be inaccessible to micro-sized chemical treatments. However, this is a potential benefit that requires further investigation to ascertain the veracity of the claim. Nonetheless, it has been suggested that certain NPs, such as nano-scale zero valent iron (nZVI) can rapidly and completely degrade chlorinated solvents, such as perchloroethene (PCE) and trichloroethene (TCE), without the production or accumulation of toxic intermediate products, including vinyl chloride (VC). This has been compared with the field scale performance of in situ bioremediation for treating chlorinated solvents, where there are instances of the accumulation of intermediate products dichloroethene (DCE) and/or vinyl chloride (VC). Furthermore, NPs may extend the range of treatable contaminants to include types traditionally seen as recalcitrant, for example, PCBs and lindane. Laboratory scale experiments have demonstrated the ability of nZVI in particular to effectively treat a wide variety of contaminants, including: PCBs and PCPS; uranium and radionuclides; and potentially toxic elements such as As, Cu, Pb and Cr (vi). These findings suggest that there is scope to employ one NP treatment to successfully treat multiple contaminants where mixed contamination is present. However, many of the contaminants successfully treated at laboratory scale have not yet been demonstrably treated at field scale. This knowledge gap has led to uncertainty over the true extent contaminants treatable by nZVI. Further field trials are required to confirm laboratory findings and reduce this uncertainty. Non-nZVI NPs, including newly developed NP types, may also demonstrate an extension of the treatable range of contaminants.
As a result of their high reactivity, NPs may react quickly with contaminants, or be passivated by elements of the soil/groundwater matrix to which it is applied, decreasing the longevity of action of NPs. This limited longevity of action, can be a benefit or a drawback of NPs depending on the context of the remediation site. This is useful for some remediation interventions where the treatment agent does not persist or where limited mobility is desired, e.g. source depletion. Indeed, due to their high reactivity, it has been suggested that NPs may be injected as an in situ source management application in saturated zones, capable of destroying NAPL. However, in trials a lack of effective injection technique has constrained successful NAPL remediation.
Finally, there has been a notable amount of evidence to suggest NP treatments may be compatible with other remediation technologies, including bioremediation, possibly even obtaining a synergistic effect (see Issues Paper). Although a number of field studies exist which demonstrate the potential of nanoparticles for remediation uses, the technology is relatively new and there are associated uncertainties. The NanoRem project will yield carefully designed field studies to improve our understanding of the nanoremediation process and address some of the uncertainties that may be perceived as barriers to the use of this promising technology.
This information is drawn from
the 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.