Abstract:The feasibility of an in-situ remediation of a shallow aquifer underneath a production building contaminated with carbon disulphide (CS2) up to residual saturation was investigated under contact of an international chemical company and its consultant. Two different methods were investigated on a lab scale to determine the remediation capacity: alcohol flushing and hot water flushing. Additionally the treatment and destruction of the effluent contaminant as well as the regeneration of the applied alcohol was investigated under the aspects of efficiency and costs.
Based on column experiments it was found that a mixture of ispropanol and water (70/30, v/v) was adequate to remove more than 99% of the residual contaminant mass during less than 5 pore volumes of flushed alcohol mixture. The density of the initial CS2 was significantly reduced and resulted in forming a single phase LNAPL mixture. No co-solvent alcohol was required.
The 2-D box experiments with original soil from the contaminated site verified the high remediation capacity of the flushing technology. During the box experiments the investigations were focused on the design of an adequate hydraulic system to remove CS2 from the bottom of the aquifer using horizontal wells. The hydraulic system was designed by numerical modeling. After flushing the soil with 7 pore volumes of the alcohol mixture, only 0.2 % of the initial CS2 contamination remained in the soil.
For financial reasons a recycling of the alcohol is required. Concerning the separation process (alcohol from CS2), the efficiency of chemical adsorption on activated carbon and stripping was investigated. For the concentration process of the used alcohol mixture rectification, distillation, and vacuum evaporation were examined. The cost of recycling the isopropanol and the oxidation process to destruct CS2 were estimated to be half of the price of isopropanol of industrial quality.
Hot Water Flooding
The 2-D box experiment applying hot water flooding (75°C) demonstrated the applicability of this technology to remediate a carbon disulphide contaminated aquifer using soil material from the site. Due to the physical properties of CS2 the remediation process is based on evaporation (soil vapor extraction), mobilization of free product and thermal destruction. During the thermal remediation prozess 32 % of the initial contamination were removed as gaseous phase. The extracted masses of free phase and dissolved CS2 summed up to 40%. It was found that a thermal transformation of carbon disulphide to CO and CO2 takes place leading to an extracted mass of 15% of the initial contaminant mass. A low amount of CS2 remained in the soil after the remediation experiment was finished(< 0.4%). Considering the mass extraction curves a total remediation time of 15 – 20 pore volumes of hot water flooding was required.
The heat propagation was horizontally oriented but the upper part of the saturated zone was significantly hotter than the lower part. This was caused by the vertical density driven flow of hot water. Nevertheless the required temperature for an evaporation of the contaminant was achieved.
For both options it was advised to conduct a pilot study on a larger scale (container experiments) to verify the positive results of the 2-D scale experiments and to develop the technology for a full scale field application using horizontal injection and extraction wells. Due to political reasons (change of legislation) the customer decided to change the remediation concept, so unfortunately the large scale investigations weren't conducted.