Specific Adsorption of Nitroaromatic Explosives and Pesticides to Clay Minerals STEFAN B. HADERLEIN,* KENNETH W. WEISSMAHR, AND RENE Ä P. SCHWARZENBACH Swiss Federal Institute for Environmental Science and Technology (EAWAG) and Swiss Federal Institute of Technology (ETH), CH-8600 Du ¨ bendorf, Switzerland It is demonstrated that nitroaromatic compounds (NACs) may adsorb specifically and reversibly to natural clay minerals in aqueous suspension. Adsorption of NACs to clays is high when the exchangeable cations at the clays include K + or NH 4 + but is negligibly small for homoionic Na + -, Ca 2+ -, Mg 2+ -, and Al 3+ -clays. Highest adsorption coefficients (K d values up to 60 000 L kg -1 ) are found for polynitroaromatic compounds including some important contaminants such as explo- sives (e.g., trinitrotoluene, trinitrobenzene, dinitro- toluidines) and dinitrophenol herbicides (e.g., DNOC, DINOSEB). Nonaromatic nitro compounds (e.g., RDX) generally exhibit very low K d values. The specific adsorption of NACs can be rationalized by electron donor-acceptor (EDA) complex formation with oxygens present at the external siloxane surface(s) of clay minerals. K d values of a given NAC and clay mineral can be estimated from known K d values of other NACs, even when measured at other clay minerals. The affinity and the adsorption capacity of the clays for NACs increase in the order kaolinite < illite < montmorillonite. Thus, clay minerals, depending on their abundance and degree of K + - (or NH 4 + ) saturation, may control the phase distribution and thus the mobility and (bio)availability of NACs in soils and aquifers. Implications of the results with respect to remediation measures at contaminated sites are discussed. Introduction Nitroaromatic compounds are widely used as pesticides, explosives, solvents, and intermediates in the synthesis of dyes and other high volume chemicals (1, 2). Many of these compounds and their transformation products are of significant toxicological concern (3, 4). NACs are ubiquitous environmental pollutants, particularly in subsurface en- vironments (5). Contamination of soils and groundwaters with nitroaromatic munitions residues such as 2,4,6- trinitrotoluene (TNT) and other nitro- and aminonitro- toluenes has recently drawn considerable public attention due to the high number of contaminated sites and the substantial efforts necessary for ongoing and future re- mediation measures (6, 7). In order to assess the fate of NACs in the subsurface and to control their mobility and reactivity during remediation processes, the sorption behavior of these compounds must be understood. Partitioning of organic pollutants into particulate organic matter is commonly assumed to be the major sorption mechanism for solid matrices exhibiting an appreciable fraction of organic matter (i.e., f om > 10 -3 kg om / kg solid ), particularly when dealing with hydrophobic com- pounds (8-12). In such cases, predictive sorption and transport models have been applied with reasonable success (8, 13, 14). There are, however, situations where adsorption to other natural surfaces may also become important or may even dominate the overall sorption process. This is the case for environments where very little organic material is present, e.g., in many aquifers (15-19), and for com- pounds that may specifically adsorb to surface sites of natural minerals (20-22). In previous work (23, 24), we have demonstrated that NACs, particularly those exhibiting several nitro groups or other electron-withdrawing substituents that are in reso- nance with the aromatic ring, may adsorb specifically and reversibly to the siloxane surface of the clay mineral kaolinite, but not to other natural minerals including aluminum and iron (hydr)oxides, carbonates, and quartz. It was found that the affinity of a given NAC strongly depends on the type of exchangeable cations(s) adsorbed to the siloxane surface of kaolinite. Significant adsorption of NACs was observed in the presence of weakly hydrated cations (i.e., Cs + , Rb + ,K + , or NH 4 + , while strongly hydrated cations such as H + , Na + , Ca 2+ , Mg 2+ , or Al 3+ prevented any specific interaction. The specific adsorption of NACs to kaolinite was interpreted in terms of coplanar electron donor-acceptor (EDA) formation (i.e., η f π-complex interactions) with oxygen ligands at the external siloxane surface of kaolinite as e - -donors and the π-system of the NACs as e - -acceptor. The strong impact of exchangeable cations on the adsorption of NACs indicates that these sites are only accessible for NACs in the presence of weakly hydrated cations that fit into the ditrigonal cavities of the siloxane surface. We have concluded that in the presence of adsorbed K + or NH 4 + , such sites may represent important surfaces for the adsorption of NACs in soils and aquifers. In the study presented in this paper, the work on NAC adsorption to kaolinite has been extended to other clay minerals and to environmentally more relevant nitroaro- matic compounds. The major goals were to evaluate to what extent the postulated EDA complex formation is a general phenomenon and to assess the significance of this sorption mechanism for a variety of important NACs. To this end, the sorption of a series of nitroaromatic explosives and pesticides from aqueous solution to various natural clay minerals has been investigated in batch experiments in the presence of K + -, Na + -, or Ca 2+ -electrolytes. The clay minerals used were kaolinite, illite, and montmorillonite. The compounds investigated included neutral and ionizable NACs, e.g., TNT and related compounds (i.e., dinitrotolu- enes and various aminonitrotoluenes), alkylated dinitro- * To whom correspondence should be addressed: Fax: +41 1 8235471. E-mail: haderlein@eawag.ch. Environ. Sci. Technol. 1996, 30, 612-622 612 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 2, 1996 0013-936X/96/0930-0612$12.00/0  1996 American Chemical Society