Sorption to soils and sediments of nitroaromatic explosives may be due to interactions with natural organic matter (NOM) or complex formation with clays, which strongly depends on the type of exchangeable cations, i.e., the base saturation of the clays. We examined the relative importance of these two processes for NAC sorption at aquifer material and evaluated the potential of decreasing or enhancing the mobility of NACs in contaminated aquifers by stimulated cation exchange. Generally, sorption on NOM of 2,4,6-trinitrotoluene (TNT) and related nitroaromatic compounds (NACs) was low compared to sorption at clays, and no evidence for specific interactions with NOM was found. Adsorbed NOM hardly affected the complex formation of NACs with clays. NAC sorption at pure clays and at aquifer material depended on the K +saturation of these materials. Typical aquifer material containing e1% NOM and 3-5% clays exhibited similar sorption features than pure clay minerals, suggesting that NAC sorption to the bulk aquifer matrix was dominated by complex formation at clays. We applied these laboratory findings to a two-step field test designed to control the groundwater transport of NACs by the injection of electrolytes. 4-Nitrotoluene (4-NT) and KCl were injected into a sandy aquifer, and their groundwater transport was monitored at an extraction well. Subsequent injection of CaCl 2 remobilized the previously adsorbed 4-NT due to ion exchange of Ca 2+ for K + at clays present in the aquifer matrix. The susceptibility of NAC sorption to the composition of exchangeable cations at clays was confirmed for TNT and related NACs. Controlling the base saturation of the aquifer matrix by electrolyte injections thus opens new perspectives for gentle remediation of sites contaminated with nitroaromatic explosives.
Keywords: Ammunition Wastes, Nitro Compounds, Sorption, Biodegradation, Transport, REV-tests, Soil-column Tests Summary: Leaching the munition residues from the former explosive production site Elsnig in the Upper Elbe Valley (Saxony, Germany) resulted in an undefined plume of groundwater contaminated by nitroaromatics and nitroamines approaching important drinking water resources. Laboratory experiments were carried out to investigate transport and fate phenomena of such substances in aquifer materials. Specific solute storage and migration parameters for modelling the subsurface migration processes were obtained from steady state experiments in soil cores used as 0-dimensional reactors and from dynamic breakthrough curves in soil columns. Using the 0-dimensional reactor tests we focused on isotherm estimation. Sorption was found to be reflected best by Freundlich isotherms for concentrations of nitroaromatics less than 10 mg L-' and low organic carbon content in the tested subsurface material. TNT-adsorption was slow and strongly correlated with soil permeability. Preliminary kinetic measurements revealed sorption equilibrium after two days. RDX-adsorption was low. All sorption experiments were conducted under nonsterile and aerobic conditions. Microbial activity was controlled by measuring the enzyme activity and the biomass in water and soil samples. After steady state experiments in the 0-dimensional reactors, products initiated by biodegradation of explosives such as aminonitrotoluenes were found. Based on literature, degradation was estimated and correlated with soil texture. For five components, different retardation was observed depending on soil texture by using native groundwater samples in the columns. Specially designed reactor facilities and soil column installations with temperature and flux control as well as on-line measurements of pH, pE, and conductivity were applied. Concentrations of contaminants were analysed both by high performance liquid chromatography and thin layer chromatography. Photolytic reactions have been prevented. Based on all these laboratory experiments, sorption, degradation, and retardation parameters of trinitrotoluene (TNT), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), dinitrobenzene (DNB), dinitrotoluene (DNT), and mononitrotoluene (MNT) in Elsnig sandy aquifers were estimated.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.