Specific Adsorption of Nitroaromatic Explosives and Pesticides to Clay Minerals

TLDR

The study shows that Kd values for a given nitroaromatic compound and clay can be predicted from other Kd data and discusses how these findings inform remediation strategies at contaminated sites. Specific adsorption of nitroaromatics to clays is explained by electron donor–acceptor complexation with surface siloxane oxygens. Nitroaromatic compounds adsorb specifically and reversibly to natural clays, with high Kd values (up to 60 000 L kg⁻¹) for polynitroaromatics when the clays contain K⁺ or NH₄⁺, negligible adsorption for homoionic Na⁺, Ca²⁺, Mg²⁺, or Al³⁺ clays, and an affinity order of kaolinite < illite < montmorillonite, implying clays can control their mobility and bioavailability.

Abstract

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 NH4+ but is negligibly small for homoionic Na+-, Ca2+-, Mg2+-, and Al3+-clays. Highest adsorption coefficients (Kd values up to 60 000 L kg-1) are found for polynitroaromatic compounds including some important contaminants such as explosives (e.g., trinitrotoluene, trinitrobenzene, dinitrotoluidines) and dinitrophenol herbicides (e.g., DNOC, DINOSEB). Nonaromatic nitro compounds (e.g., RDX) generally exhibit very low Kd 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. Kd values of a given NAC and clay mineral can be estimated from known Kd 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 NH4+) 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.