ABSTRACT
Constructed wetlands (CWs) are systems designed to maximize pollutants removal by various mechanisms, most of which are associated with the presence of plants. However, the substances secreted by plants to defend themselves against external aggressions during their growth are very little studied in these systems. This study aimed to characterize the chemical constituents of Pennisetum purpureum extracts used in an experimental mesocosm filled with shale and laterite treating domestic wastewater. Above-ground biomass, strain diameter and secondary metabolites of P. purpureum plants grown on the different substrates (shale and laterite) were monitored, as were those grown on the experimental site (control). In addition, the removal performance of chemical oxygen demand (COD), biochemical oxygen demand (BOD5), total Kjedahl nitrogen (TKN) and Total Phosphorus (TP) was determined at the outlet of CWs. Plant biomass measured on the shale bed (13.7 ± 0.5 kg m-2) was higher than on the laterite bed (12.5 ± 0.1 kg m-2), both lower than the biomass obtained in the natural environment (14.9 ± 0.6 kg m-2). Performances ranged from 83 ± 5.4 to 76.9 ± 7 % (COD), 84.7 ± 6.8 to 78 ± 8.1 % (BOD5), 72.2 ± 10.7 to 55.5 ± 16.4 % (NTK) and 72.4 ± 4.9 to 58.4 ± 3.4 % (TP), with higher efficiencies in the shale-filled bed. Plant extracts from the experimental site were richer in secondary metabolites (total polyphenol [73.5 mgEAG/gMS], total flavonoids [18.1 mgEQ/gMS] and condensed tannin [13.3 mgEC/gMS]) than those from plants grown in CWs. However, plants in the shale-filled bed secreted more total polyphenol (57.7 mgEAG/gMS), total flavonoids (12.1 mgEQ/gMS) and condensed tannin (12 mgEC/gMS) than those in the laterite-filled bed. In short, wastewater and filtration materials have an influence on the secretion of secondary plant metabolites. However, of the two materials, shale seems to be better suited to CWs, as it promotes an environment close to the natural environment.
