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The use of clinoptilolite as permeable reactive barrier substrate for acid rock drainage Lai, Ranee Wan Man

Abstract

This study investigated the use of clinoptilolite as a permeable reactive barrier (PRB) substrate for retaining heavy metals from Acid Rock Drainage (ARD). PRBs are one of the options for retaining metals from ARD, allowing a cleaned groundwater plume to the receiving water bodies. The mineral clinoptilolite, a molecular sieve which has high cation exchange capacity, can retain heavy metals. Clinoptilolite is available in many locations and is inexpensive (~CDN$100-200/tonne). The suitability of clinoptilolite for the use as a PRB substrate was evaluated based on its chemical stability and metal retention in acidic environments. Results showed that clinoptilolite is chemically stable in ARD environment, the clinoptilolite structure is stable at pH >1.5. Clinoptilolite was found to retain 130.6mg Cu/kg soil (63.8% of Cu), 22.65mg Fe/kg soil (82.1% of Fe), 158mg Zn/kg soil (39.5% of Zn) and 215.4mg Al/kg soil (89.7% of Al) from the Britannia Mine natural ARD (pH 3.28) in batch equilibrium adsorption tests. Pretreatment of clinoptilolite with NaCl solution helped improve the retention of metals and reduced the leaching of Mn from the clinoptilolite. The performance of the clinoptilolite was evaluated using column leaching tests to simulate on-site conditions. Breakthrough curves were obtained at various flowrates and influent metal concentrations. Copper and zinc were the major contaminants of concern in the ARD. The breakthrough of copper occurs at 40 pore volumes (pv), manganese at 13 pv, zinc at 45 pv and aluminum at 38 pv, whereas iron precipitated once introduced to the leaching cell. Metal retention was found to be dependant on the flow rate. Selective extractions of metals on the clinoptilolite was conducted. Results indicated that partitioning was dependent on the flow conditions and the chemical characteristics of the leachate (pH and chemical composition). An algorithm was developed in the geochemical model PHREEQC for the design of the clinoptilolite barrier. The model helps to predict the performance and the transport of contaminants based on the amount of exchange sites, influent composition and concentration within the clinoptilolite PRB system, which are useful for estimating the service life and thickness required in the design of clinoptilolite PRB systems.

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