Complex Formation of Soil Minerals with Nitroaromatic Explosives and other π-Acceptors

The ability of soil minerals to interact with organic solutes exhibiting π-acceptor properties was studied using batch adsorption experiments. Among the major groups of naturally occurring minerals, only phyllosilicates 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 such EDA interactions may exceed those caused by nonspecific interactions by several orders of magnitude. Two major factors controlled the ability of phyllosilicates to form EDA complexes: the n-donor properties of their siloxane oxygens and the accessibility of such sites for π-acceptors. The donor properties of siloxane oxygens are enhanced by isomorphic substitution, but their accessibility for π-acceptors is restricted by the steric effects of hydrated exchangeable cations. Relative adsorption constants (Kₐd values) for a given set of π-acceptors were independent of the mineral structure, indicating that similar sites are involved in EDA complex formation on phyllosilicates. Thus, Kₐd values measured on any type of model phyllosilicate may be used to estimate the relative extent of adsorption of π-acceptors to natural subsurface matrices, irrespective of the types and abundance of phyllosilicates present. Cation exchange on phyllosilicates is a crucial geochemical process that controls the accessibility of their siloxane sites to π-acceptors due to the different sizes of the hydrated cations. Injection of electrolytes may be a promising but so far unexplored way to control the adsorption and thus the bioavailability and transport of TNT and other contaminants with significant π-acceptor properties in the subsurface.