IN-SITU SPECTROSCOPIC INVESTIGATIONS OF ADSORPTION MECHANISMS OF NITROAROMATIC COMPOUNDS AT CLAY-MINERALS

Nitroaromatic compounds (NACs) including a number of priority pollute nts su ch as explosives (e.g., TNT) and herbicides (e.g., DNOC) have b een shown to adsorb strongly and specifically at natural clays. In sit u spectroscopic techniques (C-13-NMR, ATR-FTIR, UV/VIS, XRD) were appl ied to investigate the adsorption mechanism(s) of NACs at clays in aqu eous systems. Planar NACs with several electron-withdrawing substituen ts exhibited highest sorption and were used as model compounds. The co mbined experimental evidence suggests a n-pi electron donor-acceptor ( EDA) complex between oxygens of the siloxane surface(s) of the clays ( e(-) donors) and NACs (e(-) acceptors). Other adsorption mechanisms su ch as H-bonding or direct coordination of NO2 groups to surface sites were not important in aqueous environments, but contributed to the ads orption of NACs from apolar solvents. EDA complex formation took pla c e at both external and, to a lesser extent, interlamellar siloxane sur faces of expandable clays. Adsorbed NACs were oriented coplanar to the siloxane layers and exhibited a high degree of mobility consistent wi th fast and reversible sorption found in batch experiments. Significan t EDA complex formation at clays look place only in the presence of we akly hydrated exchangeable cations (e.g., K+, NH4+). Water coordinated to strongly hydrated exchangeable cations (e.g., Na+, Ca2+) strongly decreased the accessibility of siloxane sites for NACs. EDA complex fo rmation at clays may not only control the transport of NACs in the sub surface but may also affect their reactivity concerning reductive tran sformation processes.