Clay soils are effective in filtering pollutants due to their ability to adsorb, exchange ions, and physically trap contaminants through their small pores and charged surfaces. The layered structure of clay molecules, composed of silica tetrahedra and alumina octahedra, enhances these properties, allowing them to retain pollutants effectively.

Because the pores in clay are very small, it can retain pollutants and prevent them from passing through. This is a common practice and is even being applied in some countries, where people use clay pottery to filter contaminated drinking water.

Clay is considered a natural water filter.

Clay is a relative term used by soil scientists to describe the diameter of the particle, i.e., less than 0.002 mm. For the purposes of filtration and sorption, we need to know more than particle size, but also particle shape, e.g., platy versus round, and electric charge (as mentioned). For example, bentonite is a good candidate for a waste site liner given it is an extremely absorbent clay that strongly attracts positively charged water particles. This means bentonite rapidly hydrates when exposed to water and other leachate liquids with a net positive charge. As the clay hydrates it swells, sealing in wastes landfill and other waste sites. In addition to naturally formed clays like bentonite, geosynthetic clay liner (GCL) systems can enhance these processes. For example, in one system, bentonite is affixed between synthetic materials for a three-phase liner system.