SPECIFIC ADSORPTION OF NITROAROMATIC - EXPLOSIVES AND PESTICIDES TO CLAY-MINERALS

It is demonstrated that nitroaromatic compounds (NACs) may adsorb spec ifically 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 compound s including some important contaminants such as explosives (e.g., trin itrotoluene, trinitrobenzene, dinitrotoluidines) and dinitrophenol her bicides (e.g., DNOC, DINOSEB). Nonaromatic nitro compounds (e.g., RDX) generally exhibit very low K-d values. The specific adsorption of NAC s can be rationalized by electron donor-acceptor (EDA) complex formati on with oxygens present at the external siloxane surface(s) of clay mi nerals. K-d values of a given NAC and clay mineral can be estimated fr om known K-d values of other NACs, even when measured at other clay mi nerals. The affinity and the adsorption capacity of the clays for NACs increase in the order kaolinite ( illite < montmorillonite. Thus, cla y minerals, depending on their abundance and degree of K+- (or NH4+) s aturation, may control the phase distribution and thus the mobility an d (bio)availability of NACs in soils and aquifers. Implications of the results with respect to remediation measures at contaminated sites ar e discussed.