Tuesday, 8 November 2005
7

This presentation is part of: Chemistry of Organic Compounds on Mineral Surfaces

Manipulation of Aquatic Plant Toxicity of Nitroaromatic Compounds by K+/ Ca2+ Exchange on Smectite Clay.

Michael Roberts, Hui Li, Brian Teppen, Clayton Rugh, and Stephen Boyd.

Nitroaromatic compounds (NACs) are prominent soil contaminants and potent mutagens. We recently conducted studies on the adsorption mechanisms of NACs by K+-saturated smectite clays using macroscopic sorption measurements and X-ray diffraction. Our results indicate that the strong adsorption of NACs on K-clays results from the complexation of –NO2 groups with the interlayer K+ ions. Adsorption by Ca2+-saturated smectite clays was less favourable, presumably because of the greater hydration of Ca2+ (vs. K+). We hypothesize that the adjustment of K+ - Ca2+-saturation status of clays can be used as geochemical control on the bioavailability/toxicity of NACs during remediation. To test this hypothesis, we measured the aquatic toxicities of 2,4-dinitrotoluene (2,4-DNT) to two duckweed species Lemna minor and Lemna gibba in a 1/5-strength Hoagland solution supplemented with K- SWy-2 and utilized K+/ Ca2+ exchange process for manipulating 2,4-DNT adsorption/release. The strong retention of NACs by K-SWy-2 significantly reduced the availability/aquatic toxicity of initially added solutes to Duckweeds. The addition of CaCl2 replaced K+ associated with K-SWy-2, thereby releasing the clay-adsorbed NACs into aqueous solution. This is illustrated by a higher toxicity to Duckweeds in terms of frond numbers and biomass. We concluded that we can manage the base saturation status of smectite clays to control the sorption/desorption, and hence bioavailability of NACs in smectite-rich soils and sediments. Nitroaromatic contaminants can be sequestered in K-saturated smectite interlayer, therefore not available to plants. The sequestration process for NACs also permits the robust establishment of phytoremediative plants and microorganisms. The subsequent Ca-introduction is to release NACs into an active rhizosphere for biodegradation and plant uptake. Overall, the application of this simple geochemical modulation is a very promising protocol in developing a sound and environmental–friendly technique to control sorption, mobility, bioavailability and toxicity of NACs in smectite-rich soils.

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