Overview

The long-term purpose of this project was to devise strategies for on-site or in-stream nitrate remediation using common, natural materials called clay minerals. The clay minerals have a very large surface area and are the most chemically active inorganic fraction of natural soils and sediments. They are also mined from pure deposits. This project studied their potential as a resource to eliminate nitrate from drinking water and lakes, rivers, and oceans. The unique contribution of this study was that it taught us how to attract nitrate to the surfaces of the clay minerals, which are normally not attracted due to like-charge repulsion (both are negatively charged), and how to convert them to less harmful forms once they were adsorbed at the surface. The conversion occurred by a redox reaction with the iron that is located in the clay crystal structure, in which the iron in its 2+ form, labeled iron(II), reacted with nitrate, converting it to nitrite, ammonium, and/or other products, and the iron changed to its 3+ form, labeled iron(III). An intermediate step was required to attract the nitrate, which was to reverse the charge on the clay mineral from negative to positive. This was done by adsorbing a large organic cation (positively charged ion) to the clay surfaces, which created a strong electrostatic attraction for the nitrate. We tested two large cations, one synthetic, named poly-diallyldimethylammonium (pDADMAC), and one natural, named chitosan. Both were very effective at attracting nitrate, but chitosan was preferred because it attracted more nitrate and it is derived from naturally occurring organisms.

Intellectual Merit

The specific objectives of the project were to: (1) Establish the baseline level of nitrate reduction by iron redox-modified clay minerals with no organic cation present. (2) Determine the effects of the organic cation (charge reversal) at the clay mineral surface on the reactivity of the redox-modified clay minerals toward nitrate. And (3) determine the distribution of nitrogen species between solid and solution phases. We analyzed both the solution and solid phases of and around the clay mineral after reaction with nitrate. We found nitrite in solution and nitrate, nitrite, and ammonium adsorbed to the solid phase. Further reaction would probably remove all of the nitrite from solution. We also analyzed the clay mineral crystals for iron(II) and iron(III) and confirmed that the nitrate had indeed converted some of the iron(II) to iron(III), confirming that a redox reaction had occurred.

Broader Impact

These results will have a broad impact on at least three branches of the water treatment industry. First, drinking water treatment plants current remove nitrate from the water, but instead of changing it they just transfer it back to the river or put it into a municipal waste water stream. The results from the present study indicate a way whereby the nitrate can be removed and eliminated. Second, agricultural fields receiving nitrate fertilizer generally are unable to retain and use all of it, so a good portion of it runs off the field into ditches and field tile drains. This water finds its way to rivers, lakes, and oceans, causing algae blooms and other unhealthy and unsightly consequences. The results from the present study will help us construct filters that can intercept that water in the ditches and streams and eliminate the nitrate from it. And third, water reclamation districts deal with municipal and industrial waste, but often do not do anything to remove nitrate from their discharge water. This could change with a straight-forward and inexpensive method for doing it. Results from this project suggest that it can be done with a filtration system based on reversed-charge, redox-activated clay minerals.

Last Modified: 10/13/2016
Modified by: Joseph W Stucki