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.