In SituSpectroscopic Investigations of Adsorption Mechanisms of Nitroaromatic Compounds at Clay Minerals

Nitroaromatic compounds (NACs) including a number of priority pollutants such as explosives (e.g., TNT) and herbicides (e.g., DNOC) have been shown to adsorb strongly and specifically at natural clays. In situ spectroscopic techniques ( 13 C-NMR, ATR-FTIR, UV/VIS, XRD) were applied to investigate the adsorption mechanism(s) of NACs at clays in aqueous systems. Planar NACs with several electron-withdrawing substituents exhibited highest sorption and were used as model compounds. The combined experimental evidence suggests a n−π electron donor−acceptor (EDA) complex between oxygens of the siloxane surface(s) of the clays (e - donors) and NACs (e - acceptors). Other adsorption mechanisms such as H-bonding or direct coordination of NO 2 groups to surface sites were not important in aqueous environments, but contributed to the adsorption of NACs from apolar solvents. EDA complex formation took place at both external and, to a lesser extent, interlamellar siloxane surfaces of expandable clays. Adsorbed NACs were oriented coplanar to the siloxane layers and exhibited a high degree of mobility consistent with fast and reversible sorption found in batch experiments. Significant EDA complex formation at clays took place only in the presence of weakly hydrated exchangeable cations (e.g., K +, NH 4 + ). Water coordinated to strongly hydrated exchangeable cations (e.g., Na +, Ca 2+ ) strongly decreased the accessibility of siloxane sites for NACs. EDA complex formation at clays may not only control the transport of NACs in the subsurface but may also affect their reactivity concerning reductive transformation processes.