Abstract

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.

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