2 results
Nitrate Reduction by Redox-Activated, Polydiallyldimethylammonium-Exchanged Ferruginous Smectite
- Kai Su, Adi Radian, Yael Mishael, Lizhong Yang, Joseph W. Stucki
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- Journal:
- Clays and Clay Minerals / Volume 60 / Issue 5 / October 2012
- Published online by Cambridge University Press:
- 01 January 2024, pp. 464-472
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Nitrate is linked to chronic human illness and to a variety of environmental problems, and continues to be a contaminant of concern in soils and natural waters. Improved methods for nitrate abatement, thus, are still needed. The purpose of this study was to assess the potential for redox-modified, iron-bearing clay minerals to act as nitrate decontamination agents in natural environments. The model clay mineral tested was ferruginous smectite (sample SWa-1) exchanged with either sodium (Na+) or polydiallyldimethylammonium chloride (poly-DADMAC). Structural iron (Fe) in SWa-1 was in either the oxidized or reduced state. Little nitrate uptake was observed in the Na+-SWa-1, which was attributed to coulombic repulsion between the basal surfaces of the smectite and the nitrate anion. The addition of the DADMAC to the SWa-1 reversed the electrostatic charge manifested at the smectite surface from negative to positive, as measured by the zeta (ζ) potential. The positively charged poly-DADMAC-SWa-1 yielded high nitrate uptake due to coulombic attraction in both the oxidized and reduced states of the Fe in the SWa-1. The presence of reduced structural Fe(II) in the positively charged poly-DADMAC-SWa-1 enabled a chemical reduction reaction with the nitrate to produce nitrite. The amounts of nitrite found in solution, however, failed to account for all of the Fe(II) oxidized, so other N reduction products may also have formed or perhaps nitrite was also present in the adsorbed phase. The effects of other complexities, such as polymer configuration at the surface, also need further investigation. The results do clearly establish abiotic nitrate reduction to nitrite and possibly other reduction products. The combination of bacterial activity in soils and sediments, which is known to reduce structural Fe in smectites, and the abundance of organic cations in soil organic matter creates an environment where reversed-charge smectite could exist in nature. This represents a potentially effective system for mitigating harmful effects of nitrate in soils, sediments, groundwater, and surface water.
Nitrate Reduction by Redox-Modified Smectites Exchanged with Chitosan
- Martin Pentráková, Linda PentráKová, Adi Radian, Yael G. Mishael, Joseph W. Stucki
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- Journal:
- Clays and Clay Minerals / Volume 62 / Issue 5 / October 2014
- Published online by Cambridge University Press:
- 01 January 2024, pp. 403-414
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The presence of nitrate and other redox-active anionic contaminants in terrestrial ecosystems poses a significant risk to humans and other forms of life on Earth. The purpose of the present study was to test a potential in situ system, using poly-(D) glucosamine (chitosan) adsorbed to mineral surfaces under redox-active conditions in order to degrade nitrate to lower oxidation states. Chitosan is a linear polysaccharide derived from the chitin found in the shells of shrimp and other shellfish. Five different loadings of chitosan (0, 0.075, 0.25, 0.50, and 1.0 g/L; labeled C0, C1, C2, C3, and C4, respectively) were adsorbed to ferruginous smectite (SWa-1) to form chitosan-SWa-1 composites (CSC) in the pH range 5.8–4. The CSC was then reduced by Na2S2O4 in a citrate-bicarbonate buffered dispersion and washed free of excess salts under inert-atmosphere conditions. Upon addition of the nitrate, the solution pH remained slightly acidic, ranging from 5.5 to 4.7. Samples were analyzed for Fe(II) content, reacted with a NaNO3 solution, and then re-analyzed for structural Fe(II) content. Supernatant solutions were analyzed for nitrate, nitrite, and ammonium. In samples C1 to C4, extensive concentrations of nitrite were observed in the supernatants with a corresponding increase in the reoxidation of structural Fe(II), proving that a coupled redox reaction had occurred between the nitrate and the structural Fe in the clay mineral. The most efficient loading, defined as the largest percentage of adsorbed nitrate reduced to nitrite, occurred in sample C1. The total amount of nitrate reduced and Fe(II) reoxidized followed the trend 0 = C0 < C2 < C3 < C4 ≈ C1. Chitosan showed the potential to reverse the surface charge of constituent clay minerals, thereby enabling the CSC to remove nitrate anions from aqueous mineral systems via redox reactions with structural Fe(II) in clay minerals.