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Increasing attention is being placed on capping as a relatively new option in managing both contaminated sediments and dredged materials, due to its economic and environmental benefits. Capping denotes the placement of a cover onto potentially hazardous sediments or dredged material dumps to inhibit the transfer of contaminants into the water column. Retention of divalent iron and manganese cations using sandy capping layers containing natural zeoiites as a reactive additive (active barrier systems, ABS) is evaluated in this study. Three different natural zeolite (clinoptilolite) rocks, two from deposits in Australia and one from a North-American deposit, were investigated and compared with respect to their mineralogical, physical and chemical properties. In particular, results from batch and column experiments show that ABS based on these materials can efficiently demobilise iron and manganese from percolating, anoxic pore water by cation exchange under favourable conditions. The retention, however, may be reduced strongly where competitive exchange with divalent cations such as calcium prevails or where mobile colloidal pore water constituents such as clay minerals or humic substances bind fractions of the dissolved iron or manganese. Therefore, the potential of ABS as a means for in situ remediation has to be evaluated diligently with particular regard to the pore water composition of the sediment to be capped. (C) 2003 Elsevier Ltd. All rights reserved.