| Nitroaromatic compounds (NACs) are a prominent class of environmental toxic contaminants that pose a threat to human and ecosystem health. Nitroaromatic compounds demonstrate a high affinity with smectite clays which are widely distributed in nature as components of soils and sediments, and in geological deposits. The magnitude of NAC sorption by smectites is highly dependent on the exchangeable cation. For example, smectites saturated with K+ have a very high affinity for NACs while those saturated with Ca2+ do not. In this study the ability of K+-saturated smectite to attenuate the bioavailability and hence toxicity of 2,4-dinitrotoluene (2,4-DNT) to the aquatic plant duckweeds (Lemna gibba and Lemna minor) was evaluated. 2,4-DNT at the concentration of 7.5 mg/L was highly toxic to duckweeds: However, the presence of small amounts of K-smectite in aqueous growth media reduced this toxicity substantially because the sorption of 2,4-DNT by the added smectite reduced bioavailability. This reduced bioavailability could be modulated by simple K+/Ca2+ cation exchange reaction occurring on mineral surfaces. In the subsequent experiments, CaCl2 was added and the Ca2+ replaced K+ associated with smectite surfaces. Such reaction released clay-sorbed NACs to aqueous solution, which is readily available to target plants. This study proves the concept that we can modulate the type and fraction of cations occupying the cation-exchange-capacity sites of smectite clays to control the sorption/desorption hence bioavailability of NACs in smectite-rich soils and sediments. This information is needed in developing strategies for control of NAC mobility and bioavailability to target organisms such as plants and microorganisms utilized in in-situ bioremediation technologies. |
