New redox-active material for permeable water remediation systems

Abstract

Reduction of structural iron in Fe-bearing clays could be used for reductively treating contaminated waters. When used in permeable water treatment systems, clays are generally mixed with sand to satisfy required hydraulic properties. Depending on compaction and pore size, clay particles can migrate within the clay/sand mix resulting in progressive clogging and failure of the system. In this study, a novel Fe-bearing clay-material suitable for permeable water treatment systems has been developed. Particles of NAu-2 and SWy-2 smectite were attached to the surface of sand grains using a polymer binding agent, with optimum coating levels of 62 and 51mg clay/g sand, respectively. The quality and extent of the clay coating are strongly affected by the type of sand being used, very poor coatings being obtained with smooth quartz grains compared to rougher and more porous surfaces. The clay-coatings are stable under most pH and redox conditions and are reducible at room temperature with obtained Fe(II)/Total Fe ratio of 55% to 83%. The CEC of the two smectites decreases when coated on sand but the chemically reduced samples are able to reduce Cr6+ to Cr3+ at room temperature. At a compaction density of 1300kg/m3, the permeability of NAu-2 clay-coated sand remains at ~1×10−4m/s after multiple reduction and reoxidation cycles.