COMPLEX-FORMATION OF SOIL MINERALS WITH NITROAROMATIC EXPLOSIVES AND OTHER PI-ACCEPTORS

The ability of soil minerals to interact with organic solutes exhibiti ng pi-acceptor properties was studied using batch adsorption experimen ts. Among the major groups of naturally occurring minerals, only phyll osilicates were capable of forming strong electron donor-acceptor (EDA ) complexes with such solutes, including nitroaromatic explosives (e.g ., trinitrotoluene [TNT]) and other priority pollutants. Depending on the minerals and the solutes involved, adsorption constants due to suc h EDA interactions may exceed those caused by nonspecific interactions by several orders of magnitude. Two major factors controlled the abil ity of phyllosilicates to form EDA complexes: the n-donor properties o f their siloxane oxygens and the accessibility of such sites for pi-ac ceptors. The donor properties of siloxane oxygens are enhanced by isom orphic substitution, but their accessibility for pi-acceptors is restr icted by the steric effects of hydrated exchangeable cations. Relative adsorption constants (K-ad values) for a given set of pi-acceptors we re independent of the mineral structure, indicating that similar sites are involved in EDA complex formation on phyllosilicates. Thus, K-ad values measured on any type of model phyllosilicate may be used to est imate the relative extent of adsorption of pi-acceptors to natural sub surface matrices, irrespective of the types and abundance of phyllosil icates present. Cation exchange on phyllosilicates is a crucial geoche mical process that controls the accessibility of their siloxane sites to pi-acceptors due to the different sizes of the hydrated cations. In jection of electrolytes may be a promising but so far unexplored way t o control the adsorption and thus the bioavailability and transport of TNT and other contaminants with significant pi-acceptor properties in the subsurface.