Influence of organic matter on the reactivity of clay minerals in highly alkaline environments

Soil organic matter has important implications in dissolution reactions of clay minerals and their transformation. Under acidic conditions increased dissolution rates are generally reported. However, in high pH environments, organic matter (OM) might have a passivation effect on the mineral surface, thereby hindering dissolution and transformation substantially. The reduced reactivity of clay minerals is an important consideration when clays are used as backfill materials in nuclear waste disposals or when lime is used for soil stabilization. Here the outcome of a comparative study is presented using soil, containing common clay minerals such as smectite, interstratified illite-smectite, illite and kaolinite, that was treated with highly concentrated alkaline solutions in the presence and absence of naturally occurring organic matter. The results clearly show that clay mineral dissolution and the formation of new zeolitic phases at room T are delayed in the presence of OM. The soils' OM content is, thus, an important parameter to be considered in systems where clay minerals are exposed to highly alkaline environments.