Olive oil mills waste waters and clay minerals interactions: organics transformation and clay particles aggregation

Agro-industry produces an increasing amount of organic wastes. Olive mills waste waters (OMWW) in the Mediterranean area can constitute a serious organic pollution risk for superficial and underground waters. OMWW have been spread on cultivated soils to reduce the environmental impact, and for exploiting their amendment properties. Positive effects on chemical fertility have been generally reported, but less attention has been paid to the effect of OMWW on soil physical properties. The aim of this study is to investigate the organic transformations, the particles aggregation and their water stability in two different clays incubated with fresh OMWW. Studies of interactions between clay particles (considered the more active soil mineral fraction) and these organic substances can provide an assessment of the physical fertility improvement when OMWW are used as soil amendment. Therefore, OMWW have been added to a standard kaolinite and montmorillonite, incubated for 90 days and subjected to wetting and drying cycles. The organic matter transformation was characterized by CP/MAS 13C NMR spectroscopy. Aggregation was assessed by water-aggregate stability measurements on size-selected clay aggregates obtained after incubation. Organic matter from incubated OMWW-clay samples has shown an enrichment of alkyl groups and a decrease of O-alkyl in both clays. Incubation of OMWW with kaolinite promoted a sensible increase of aggregation and water stability. These findings revealed that the mineralogy of clays can substantially influence both the transformation of OM and the aggregation processes that, in turns, could affect the amendment properties of OMWW applied to the soil.