In-situ reduction technology of groundwater remediation with the use of Fe(0) nanoparticles
In-situ technology utilizing of zero-valent iron nanoparticles for treatment of groundwater and rock environment represents a modern method of treatment of environment polluted by chlorinated hydrocarbons, heavy metals, radionuclides and other contaminants. This technology started to be used tentatively in U.S.A. several years ago and the treatment works already take place in the various world sites at present - see our latest presentation (2012).
PCE degradation in time on site
Industrial dispersing unit DI 50
Groundwater remediation method is based on injection of Fe0 nanoparticles into the groundwater through application wells. This technology is environmentally friendly and cost-effective compared to methods like pump&treat or gas extraction (venting). Nanoiron is applied through small injection wells with minimal negative environmental impact. Due to nanodimensional character and suitable surface stabilization, the injected nanoparticles migrate through the rock environment, where they induce oxidation-reduction (redox) reactions with pollutants dissolved in the ground water.
From a wide range of pollutants that can be successfully removed by the use of Fe(0) nanoparticles, we can mention chlorinated hydrocarbons (e.g. carbon tetrachloride, tetrachloroethylene, trichloroethylene, dichloroethylene, vinyl chloride, chloroform, polychlorinated biphenyls) and other organic substances (e.g. nitrobenzene, dioxins) that are decomposed into simpler non-toxic phases. As a consequence of reduction and coprecipitation processes, which occur after application of nanoiron, it is possible to transform a wide variety of heavy metals and also highly toxic arsenic into an insoluble form, which remains tightly bounded in the rock environment. Thus, these pollutants can not migrate any more, which leads to a significant reduction of their ecological impacts. Full list of contaminants is available here.
Reducing potential of zero-valent iron nanoparticles is very high, therefore nZVI can reduce another matters contained in the water like dissolved oxygen, suplhates, nitrates, etc. Reaction of nZVI and oxygen in water:
Fe0 + 1/2 O2 + 2 H+ ↔ Fe2+ + H2O
Oxygen present in the environment is reduced which leads to decrease of redox potential and reduction of monitored contaminants proceeds.
The oxidation of Fe0 nanoparticles stored in water is another important process: zero-valent iron nanoparticles tend to significantly affect the redox potential, reduce water to hydrogen and hydroxide and increase pH.
Fe0 + 2 H+ ↔ Fe2+ + H2
Oxidation and reactivity reduction of nanoparticles is an undesirable phenomenon in the time since slurry manufacturing till its application. This side effect can be suppressed by fresh slurry manufacturing from a dry nanopowder just priot to the application. Aging of nZVI stored in water (NANOFER 25 and NANOFER 25S) during time is showed in following fugures. More information regarding slurry products degradation and recommended handling and storage is available in distribution and storage section.
Groundwater remediation by NANOFER products:
For in-situ groundwater remediation we recommend to use stabilized product NANOFER 25S or a combination of products NANOFER 25 and NANOFER 25S. For large-scale pilot projects and full-scale applications we strongly recommend fresh slurry manufacturing from the dry nanopowder by industrial dispersing unit – please contact us for the offer. Presentations of groundwater remediation pilot experiments and applications using NANOFER products are available here (references).
Degradation pathways of selected contaminants
Reduction of nitrates - the reactions between nanoFe and nitrates run to the nitrogen (gas) eventually to the ammonia. The mechanism of nitrates reduction is mediated by free electrons, these are released directly or indirectly by corroding process of elemental Fe0.
An example of reduction nitrates into nitrites: Fe0 + NO3– + 2 H+ ↔ Fe2+ + NO2– + H2O
An example of reduction nitrates into ammonia: 4 Fe0 + NO3– + 10 H+ ↔ 4 Fe2+ + NH4 + 3 H2O
Reduction of chromiun – the reduction of hexavalent chromium is based on change of the Cr6+ into Cr3+, that is less soluble, less toxic and less mobile. The reduction of hexavalent chromium is described by the equation:
3 Fe0 + 2 Cr6+ ↔ 3 Fe2+ + 2 Cr3+
Fe0 + CrO42– + 8 H+ ↔ Cr3+ + Fe3+ + 4 H2O
Reduction of arsenic – the principle of method of using elemental Fe0 is spontaneous adsorption and coprecipitation of As3+ and As5+ with oxides and hydroxides Fe2+, Fe3+, which are developed by oxidation of Fe0 in groundwater environment. The reduction of arsenic with hydrated oxides is described by following equations:
Fe(OH)3 (s) + H3AsO4 ↔ FeAsO4 . 2 H2O (s) + H2O
≡ Fe(OH)0 + AsO43- + 3H+ ↔ FeH2AsO4(s) + H2O
≡ Fe(OH)0 + AsO43- + 2H+ ↔ FeHAsO4–(s) + H2O
Reduction of chlorinated hydrocarbons - Chlorinated hydrocarbons are the most widely known and most common pollutants of groundwater, they occur in compounds (PCE, TCE, DCE and VC). NZVI react with these contaminants very fast. TCE and PCE are transformed according to these reactions:
C2HCl3 + 4 Fe0 + 5 H+ ↔ C2H6 + 4 Fe2+ + 3 Cl–
C2Cl4 + 5 Fe0 + 6 H+ ↔ C2H6 + 5 Fe2+ + 4 Cl–