541 IGC-2004 DEVELOPMENT OF REACTIVE BARRIER MATERIAL FOR USE IN LANDFILL LINERS AND IN-SITU BARRIERS TO IMMOBILIZE CHROMIUM M.M. Sethy and R.C. Vaishya Department Civil Engineering, Motilal Nehru National Institute of Technology, Allahabad K.R. Reddy Department of Civil and Materials Engineering, University of Illinois at Chicago, Chicago, Illinois, USA ABSTRACT : This paper presents the development of a reactive media that consists of treatment mixtures of iron oxide coated sand (IOCS) and kaolin (K), known as Reactive Barrier Material (RBM), to use as liner or barrier material for effective containment of toxic hexavalent chromium (Cr(VI)). A series of batch kinetic experiments show that at equilibrium time, the percentage removal of Cr(VI) varies from 65 to 70% tested at initial pH of 8.61 ± 0.1. The percentage removal of Cr(VI) was decreased from 49 to 54% as sorbate concentrations in the solute were increased from 10mg/L to 50 mg/L keeping other experimental conditions similar. The results of isothermal studies conducted at different pH values show that adsorption data satisfied both the Langmuir and Freundlich isotherm models. The effect of pH on the Cr removal was negligible by RBM; however, the maximum sorption occurred at about pH 5.0 - 6.0 under the experimental conditions. The effective removal of Cr(VI) by RBM under dynamic flow conditions (column studies) suggests that RBM may be a viable alternative material to use in landfill liners to effectively contain Cr(VI) in leachate and in in-situ barriers to treat groundwater contaminated by Cr(VI). INTRODUCTION Chromium is a common groundwater contaminant in industrial regions throughout the world. Chromium contamination has been caused mainly by past industrial activities such as electroplating, wood preservation, leather tanning, and metal finishing. Chromium exists in two valance states, trivalent, Cr(III), and hexavalent, Cr(VI). Cr(VI) is more toxic and mobile as compared with Cr(III). It exists primarily in two anion forms, HCrO 4 - and CrO 4 2- (Fendorf and Zasoski, 1992; Chintamreddy and Reddy, 1998). Pump-and-treat groundwater remediation methods have proved to be expensive and in many cases ineffective at achieving the proposed level of clean up (Mackay, 1989). This approach can be ineffective. A new technology being developed for the remediation of contaminated groundwater involves placing reactive barrier material below the ground surface in the path of flowing groundwater. These engineered barriers contain reactive solids by various in-situ transformations. For many dissolved solutes, these transformations include reduction or oxidation reactions and subsequent precipitation of sparingly soluble solids. Generally, landfill leachate contains both organic compounds and heavy metals (such as chromium). Currently, clay materials are selected as liner materials primarily based on their low hydraulic conductivity to prevent advective transport of leachate constituents. However, research studies (Astrop et al., 2000; Lo, 2003; Edil et al., 1991; Blowes et al., 1997; Bright et al., 2000; Abu-El-Sha'r et al, 2003) have shown that contaminants migrate through the liner due to diffusion. If there is a significant diffusive flux, potential for underlying groundwater contamination exists. The present study assesses the ability of reactive barrier material to remove dissolved Cr(VI) from synthetic solution under dynamic flow conditions over a sustained period of time. The attenuating reactive barrier material (RBM) studied contains treatment mixtures of iron oxide coated sand (IOCS), and kaolin. Geochemical calculations and batch tests were conducted to prescreen potential candidates for further study using column tests. EXPERIMENTAL METHODOLOGY Materials: All glass and plastic equipment was soaked in 2N HNO 3 for at least 24 h and then rinsed several times in distilled water before use. Analytical reagent grade chemicals (Merck) and distilled water were used for all solutions. Stock Cr(VI) solution was prepared as per standard methods (Chesceri et al., 1989). Quartz sand of geometrical mean (GM) size of 505 mm by IS sieves was procured from the bank of river Yamuna at Allahabad (India) and coating was applied as per Edwards and Benjamin (1989). Three different types of reactive barrier material (RBM), i.e. 75% IOCS and 25% Kaolin (RBM I); 80% IOCS and 20% Kaolin (RBM II); and 90% IOCS and 10% Kaolin (RBM III) were prepared.