Study on the Application of Floating Beds of Macrophites (Vetiveria zizanioides and Phragmites australis), in Pilot Scale, for the Removal of Heavy Metals from Água Forte Stream (Alentejo-Portugal)

The surrounding area of the Roxo stream sub-basin (basin of the Sado River, Portugal) has completely sterile sections, jeopardizing the productivity of the agricultural activities practiced there. This may be due to the inflow of the Água Forte stream, which has characteristics of Acid Mining Drainage (AMD). The objective of this study was to test the efficiency of heavy metal removal from the Água Forte stream using the macrophyte floating bed technology (Vetiveria zizanioides and Phragmites australis) in a pilot plant, monitoring and evaluating the water quality and performance of macrophytes. Two experiments were carried out in 2019 over 6 months (January to June). Both experiments were performed in polyvinyl chloride (PVC) tanks with the nominal capacity of 1 m³ each. The tanks were filled with about 0.8 m³ of water coming from the Água Forte stream, which was renewed monthly. The floating beds consisted in a high-density polyethylene floating system and an organic plant support mat filled with a plant density of 285 plants m^-2. The heavy metal removal rates obtained from the Vetiveria zizanioides and Phragmites australis floating bed were Fe = 40%; Zn = 33%; Cu = 23%; Mn = 14% and Fe = 27%; Zn = 19%; Mn = 17%; Cu = 14%; respectively. The order of heavy metals accumulation in Vetiveria zizanioides and Phragmites australis in plant biomass was Fe > Zn > Cu > Mn and Fe > Zn > Mn > Cu, respectively. The growth of Vetiveria zizanioides and Phragmites australis in leaf biomass was 7.1 ± 0.3 cm/month and 2.5 ± 0.0 cm/month and in root biomass 3.8 ± 0.1 cm/month and 4.1 ± 0.1 cm/month, respectively. The growth of macrophytes showed the ability to survive in the AMD-containing waters without severe damage in their external and anatomical morphology, although their growth suffered inhibition. The results suggest that the floating bed technology may be an environmentally sustainable alternative, allowing long-term heavy metal removal.