The role of clinoptilolite in organo-zeolitic-soil systems used for phytoremediation.

The present work is an extension of earlier research [Leggo, P. (2000) An investigation of plant growth in an organo-zeolitic substrate and its ecological significance. Plant and Soil, 219: 135-146.] in which zeolitic tuffaceous rock containing clinoptilolite, a commonly occurring natural zeolite mineral, was composted with organic waste to produce a very effective bio-fertilizer thought to be activated by a very large increase in the population of nitrifiers. This material has now been modified by the addition of extra untreated zeolitic tuff (i.e. un-ammoniated clinoptilolite). Comparing plant growth in a series of substrates containing increasing amounts of zeolitic tuff the limit of growth enhancement has been established. Aqueous leachate analysis has demonstrated a correlation between shoot growth and the mobilization of cations in the soil pore water. Measurement of soil water suction pressure has shown that soil moisture is directly related to the amount of the zeolitic tuff amendment. It has also been found that the zeolite component of the soil system supports biofilm formation and this behaviour is thought to account for the additional plant growth in substrates containing extra untreated tuff. Similar trends are found for plants growing in clean and metal polluted soils. It is now clear that organo-zeolitic-soil systems offer an opportunity to re-vegetate land made barren by metal pollution and as a consequence, erosion and dissemination of contaminants are reduced. By harvesting metal tolerant plants it would appear feasible to recover metals and clean the rhizosphere simultaneously.